superstripes 2017

SU PE RS T RI P E S 2017

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Antonio Bianconi superstripes press

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S U PE RS T RI P E S 20 1 7 Qu a n tu m in C omp lex M a tter : S upe r c o n d u c tiv ity, M a gn e ti s m & Fer r oe le c tr i c i ty

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Antonio Bianconi

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Science Series No.11 Title: Superstripes 2017 Published on June 2017 by Superstripes Press, Rome, Italy http://www.superstripes.net/science/science.htm

© 2017 Superstripes Press © 2017 Multiple authors

ISBN 978-88-6683-069-6 ISBN-A 10.978.886683/0696

This work is licensed under the Creative Commons Attribution-ShareAlike 4.0 International License. To view a copy of this license, visit http://creativecommons.org/licenses/by-sa/4.0/ or send a letter to Creative Commons, PO Box 1866, Mountain View, CA 94042, USA.

Authors: Abbamonte P., Aeppli G., Alarco J.A., Albertini R., Aoki D., Attanasio C., Avigo I., Babaev E., Bachar N., Badoux S., Balakirev F.F., Balicas L., Bao W., Barbiellini B., Berciu M., Berthod C., Bianconi A., Billinge S.J.L., Bonca J., Boris A., Borisenko S., Borzenets I., Bozin E.S., Brazovskii S., Brun C., Bussmann-Holder A., Capone M., Carlström J., Chang J., Chávez I., Chu P.C.W., Conradson S.D., Coslovich G., Crisan A., Daghero D., de Llano M., de’ Medici L., Dean M. P. M., Degiorgi L., Destraz D., Deutscher G., Di Gioacchino M., Di Giorgio C., Dobrosavljevic V., Drechsler S.L., Efremov D., Egami T., Einaga M., Eremets M.I., Eremin I., Eremin M., Fanfarillo L., Farina D., Feng S., Fine B.V., Fink J., Flammia L., Frésard R., Fujimori A., Fujita M., Galda A., Giannetti C., Glatz A., Goncharov A.F., Goto H., Goto Y., Gray A.X., Grilli M., Grochala W., Guguchia Z., Guidi T., Guo H., Guzman-Verri G.G., Hanaguri T., Hayden S., Hess C., Huecker M., Iavarone M., Ideue T., Imada M., Irizawa A., Ivanov A.A., Jackeli G., Joon E., Jurkutat M., Kanazawa I., Kapcia K.J., Kataev V., Kato R., Khaliullin G., Khomski D.I., Kimura A., Kimura T., Kirova N., Knebel G., Kokalj J., Kolodziej J.J., Komarek A.C., Kontani H., Kresin V., Kruger F., Krzton-Maziopa A., Ksenofontov V., Kubozono Y., Langerome B., Larkin T.I., Leridon B., Littlewood P., Liu C., Lorenzana J., Louca D., Luo J., Lychkovskiy O., Madan I., Maeno Y., Marcelli A., Markiewicz R.S., Marsiglio F., Massarotti D., Mazziotti M.V., Mazzoli C., McNally D.E., Mertelj T., Mesaros A., Miletto Granozio F., Mironov A. Yu., Miyasaka S., Mizuguchi Y., Mizukami Y., Moewes A., Momono N., Monney C., Moreo A., Moroni M., Moskvin A.S., Mukhin S.I., Mustre de León J., Nattermann T., Neilson D., Nissinen J., Oda M., Oles A.M., Orgad D., Orth P.P., Ovchinnikov S.G., Pelliciari J., Peng Y., Perali A., Perring T., Pomarico E., Ponomarenko L.A., Popović D., Prassides K., Ptok A., Pudalov V., Purans J., Puzniak R., Quader K., Raimondi R., Rajasekaran S., Renner Ch., Reznik D., Robinson I.K., Roy P., Sanna S., Sato M., Schmitt T., Schneider W.-D., Seibold G., Semenov A.G., Shen D., Shengelaya A., Shi M., Shimizu K., Shimojima T., Silaev M., Silhanek A.V., Soh Y.-Ah, Soldatov A.V., Spera M., Spivak B., Steppke A., Stornaiuolo D., Strinati Calvanese G., Sumida K., Sunko V., Sushkov O.P., Tafuri F., Tallon J., Tanatar B., Tanner D.B., Teitel’baum G., Terao T., Timusk T., Toda Y., Tortello M., Tripathi V., Truccato M., Tsuchiizu M., Uemura Y.J., van der Marel D., van Wezel J., Vanacore G.M., Vargas-Paredes A.A., Vinokur V.M., von Rohr F., Wahl P., Wall S., Wirth S., Wohlfeld K., Wysokiński K.I., Wysokinski M.M., Yanagisawa T., Yoshida Y., Zaanen J., Zaikin A.D., Zhigadlo N.D., Zhuang J., Zwicknagl G.

*These authors presented the scientific reports collected in this book at the Superstripes 2017 conference held in Ischia (It) on June 4-10, 2017

Papers presented at the international conference Superstripes 2017 Ischia Italy June 4-10, 2017 Organized by Non profit organization for scientific research Superstripes onlus Rome International Center for Materials Science Superstripes - RICMASS

Chairman Prof. Antonio Bianconi, RICMASS, Rome, I Organizing Committee Gabriel Aeppli, Paul Scherrer Institute, CH Bernd Büchner, IFW Dresden, D Takeshi Egami, University of Tennessee, USA Vladimir Kresin, University of California, USA P.B. Littlewood, Argonne National Laboratory, UK Despina Louca, University of Virginia, Charlottesville, USA Andrea Perali, University of Camerino, I Kosmas Prassides, Durham University, UK Valerii Vinokur, Argonne National Laboratory, USA Jan Zaanen, University of Leiden, NL

Superstripes 2017, Ischia June 4-10, 2017

Superstripes 2017

Book of Abstracts


Table of Contents Guy Deutscher: Multi-level Kondo effect and enhanced critical temperature in nanoscale granular Al............................................................................................ 12 Steven Conradson: The Fröhlich-type, non-equilibrium, polaronic condensate in the Mott system UO2(+x) ........................................................................................... 13 Leonardo Degiorgi: Investigation of the optical anisotropy in the nematic phase of FeSe ..................................................................................................................... 14 Kosmas Prassides: New candidate quantum spin liquids in ionic polyaromatic hydrocarbons ........................................................................................................ 16 Dmitry Reznik: Dynamic charge stripe spectrum in nickelate perovskites .............. 17 Yoshiteru Maeno: Mott transition and strong diamagnetism in Ca2RuO4 tuned by electric field/current .............................................................................................. 18 Annette Bussmann-Holder: The road map toward room temperature superconductivity .................................................................................................. 20 Takeshi Egami: d9 Nickelates under Pressure ........................................................ 22 Yasutomo Uemura: Three novel features in the correlations between Tc and the superfluid density: local phase coherence below the Bose gas temperature TB and multiple ratios of Tc/TF .......................................................................................... 24 Vladimir Kresin: Electron-lattice and electron-electron interactions in novel systems: paths to room temperature superconductivity......................................................... 27 Hiroshi Kontani: Strong interplay between high-Tc superconductivity, nematicity, and magnetism in Fe-based Superconductors......................................................... 28 Daniel Khomskii: Strong covalency and ligand holes, or how to make magnetic gold?..................................................................................................................... 29 Stephen Hayden: Spin and charge-density-wave fluctuations in cuprates ............... 30 Johan Chang: Electronics of high-temperature cuprate superconductors ................ 32 Naoki Momono: 1/8 Anomaly and Charge Order in Dy-doped Bi2212 .................. 33 Masaki Fujita: Neutron-Scattering Study of Magnetic Excitations in Electron-Doped Cuprate ................................................................................................................. 35 Dirk van der Marel: Superconductivity and electronic structure of SrTiO3 ............ 37 Yoshihiro Kubozono: Pressure-driven high-Tc superconductivity in carbon-based and inorganic materials ......................................................................................... 39 Akio Kimura: Non-equilibrium Surface Dirac Fermion Dynamics of Topological Insulators Probed By Time Resolved ARPES ........................................................ 41 Dragana Popović: Charge dynamics near the onset of charge-density-wave order in La-214 cuprates .................................................................................................... 43 Dror Orgad: Dimensional Crossover of Charge-Density Wave Correlations in the Cuprates ............................................................................................................... 45

Superstripes 2017, Ischia June 4-10, 2017 Marco Truccato: Modifications induced by synchrotron radiation in Bi2Sr2CaCu2O8+delta and YBa2Cu3O7-x: a novel non-destructive patterning method.................................................................................................................. 46 Michael Jurkutat: Local charges and charge order in the cuprates revealed by NMR ............................................................................................................................. 48 Nikolai Zhigadlo: Crystal Growth and Advanced Synthesis of Contemporary Functional and Superconducting Materials: From Simplicity to Complexity .......... 49 Alexander Shengelaia: Rapid Synthesis of Superconducting and Magnetic oxides by Light Irradiation .................................................................................................... 51 Jose Alarco: A complete and accurate description of superconductivity of AlB2 – type structures from phonon dispersion calculations .............................................. 52 Wolf-Dieter Schneider: Atomic structure of supported ultrathin Germania films.... 54 Mauro Tortello: Nanoscale Characterization of the Thermal Conductivity of Supported Graphite Nanoplates, Graphene and Few-layer Graphene ..................... 55 Yoshikazu Mizuguchi: Material design strategies for BiCh2-based layered superconductors .................................................................................................... 56 Yosuke Goto: Enhanced thermoelectric performance in BiS 2-based layered compound LaOBiS2-xSex ....................................................................................... 57 Wojciech Grochala: Ag/F vs. Cu/O: powerful analogy with far-reaching implications ............................................................................................................................. 59 Timofei Larkin: Exciton-phonon complexes and giant exciton Fano resonances in Ta2NiSe5.............................................................................................................. 61 Jianlin Luo: Pressure induced superconductivity in Cr- and Mn- based materials ... 62 Yeong-Ah Soh: Crystal structure and epitaxy of topological insulator films grown on Si and SrTiO3 ....................................................................................................... 63 Despina Louca: Elastic and Electronic Tuning of Magnetoresistance in MoTe2 ..... 65 Robert S. Markiewicz: Entropia cupratesque cano… [A new model of the cuprate pseudogap] ........................................................................................................... 67 Dario Daghero: Effects of a pressure-induced topological Fermi-surface transition on the order parameter of CaFe2As2 ..................................................................... 69 Brigitte Leridon: Ionic .......................................................................................... 70 Wei Bao: Simultaneous occurrence of multiferroism and short-range magnetic order in DyFeO3 ............................................................................................................ 72 Gian Giacomo Guzman Verri: Random Electric Field Instabilities of Relaxor Ferroelectrics ........................................................................................................ 73 Tsuyoshi Kimura: Multiple order parameters and their domain control in magnetoelectric multiferroics ................................................................................ 74 Benjamin Langerome: High Pressure – Low Temperature Set-up for Infrared Spectroscopy of H3S at the AILES Beamline ........................................................ 75

Superstripes 2017, Ischia June 4-10, 2017 Takahito Terao: Preparation of new metal-intercalated FeSe superconductors and their pressure dependence ..................................................................................... 77 Jonathan Pelliciari: Resonant Inelastic X-ray Scattering on Iron Pnictides ............. 79 Luca de’ Medici: Hund’s correlated metals ........................................................... 81 Boris Spivak: Macroscopic character of composite high temperature superconducting wires........................................................................................... 82 Jacek Kolodziej: Effect of a skin-deep surface zone on the formation of a twodimensional electron gas at a semiconductor surface ............................................. 83 Masahisa Tsuchiizu: Functional-Renormalization-Group Analysis on Electron Nematic State and Charge-Density-Wave State in Cuprate Superconductors.......... 84 Johan Carlström: Spontaneous breakdown of time-reversal symmetry induced by thermal fluctuations .............................................................................................. 85 Stefan-Ludwig Drechsler: Constraints on the total coupling strength to low-energy bosons in iron based superconductors .................................................................... 86 Dmitri Efremov: Multiband Eliashberg approach – a way to the realistic description of iron based superconductors ............................................................................... 89 Takashi Yanagisawa: Crossover-induced spin fluctuation and electron pairing in strongly correlated electrons.................................................................................. 90 Bilal Tanatar: Drag Effect in Bilayer Systems of Dipolar Bosons and Fermions .... 92 Donato Farina: Raman of YBCO out-of-equilibrium ............................................. 93 Paul C. W. Chu: A Possible Paradigm Shift in the Search for Higher Tc ................ 94 Atsushi Fujimori: Multiple component Fermi surfaces of high-Tc cuprates revealed by ARPES ............................................................................................................ 95 Gregory Teitel’baum: On the phenomenological two- component physics for cuprates ................................................................................................................ 97 Masatoshi Imada: Frontiers of high-Tc studies and spin liquids ............................. 99 Simon Billinge: Orbital degeneracy lifting, broken local symmetries and properties in correlated electron materials ............................................................................ 100 Claudio Mazzoli: On the time correlation of long range ordered CDW state in LBCO ........................................................................................................................... 101 Ian Robinson: Stripe Pinning in LBCO................................................................ 102 Mark P. M. Dean: Precursor Charge Density Wave in La2-xBaxCuO4 ................ 104 David Tanner: Optical spectroscopy of La2-xBaxCuO4 single crystals: influence of stripe order.......................................................................................................... 106 Markus Huecker: High Pressure 3D to 2D Tuning of Magnetism in Cuprates ...... 107 Andrej Mesaros: Building blocks of cuprate charge density modulations ............. 108

Superstripes 2017, Ischia June 4-10, 2017 Shiping Feng: Pseudogap-generated a coexistence of Fermi arcs and Fermi pockets in cuprate superconductors .................................................................................. 109 Toby Perring: Magnetic field induced magnon decay in the spin ½ square lattice Heisenberg antiferromagnet ................................................................................ 110 Vladimir Dobrosavljevic: Disorder-Driven Metal-Insulator Transitions in Deformable Lattices ............................................................................................ 111 Oleg Sushkov: Multiple universalities in order-disorder magnetic phase transitions ........................................................................................................................... 112 Jaakko Nissinen: Three-dimensional quantum liquid crystals and dislocation worldsheet condensation ..................................................................................... 113 Alexander Boris: Proximity of superconductivity and magnetism in δ-doped La2CuO4 heterostructures................................................................................... 114 Alexander Gray: Emerging X-ray Techniques for Probing Matter with Depth and Time Resolution ................................................................................................. 115 Götz Seibold: The inverse Edelstein effect at oxide interfaces ............................. 116 Yasuo Yoshida: Emergence of surface orbital ordering in the heavy fermion superconductor CeCoIn5 ..................................................................................... 117 Tetsuo Hanaguri: STM studies of superconductivity and nematicity in Fe(Se,S) .. 118 Shigeki Miyasaka: Change of phase diagram of 1111-type iron pnictide by varying of rare-earth element, solid solution of pnictogens and electron doping................ 119 Mona Berciu: One-hole and two-holes low-energy states in a cuprate layer ......... 121 Sven Badoux: Signature of the pseudogap critical point in cuprate superconductors ........................................................................................................................... 122 Peter P. Orth: Enhanced nematic fluctuations near the Mott insulating phase of highTc cuprates ......................................................................................................... 124 Zurab Guguchia: Cooperative coupling of static magnetism and bulk superconductivity in the stripe phase of La2-xBaxCuO4: Pressure- and dopingdependent studies ................................................................................................ 126 Adrian Crisan: Non-centrosymmetric vortices in multi-component superconductors ........................................................................................................................... 128 Roman Puzniak: Anisotropy, phase separation, and superconductivity in iron-based superconductors .................................................................................................. 130 Maria Iavarone: Low temperature STM/STS of FeSe .......................................... 132 Migaku Oda: STM/STS studies on the spatial dependence of energy gap in high-Tc cuprate Bi2Sr2CaCu2O8+δ ..................................................................................... 133 Isabella Avigo: Ultrafast doublon dynamics in photo-excited 1T-Tas2 ................ 135 Natasha Kirova: Dynamical phase transition to a charge-transfer state ................. 136 Claudio Giannetti: Ultrafast orbital manipulation in copper oxides ...................... 138

Superstripes 2017, Ischia June 4-10, 2017 Tomaz Mertelj: Collective electronic orders under strong optical drive studied by means of time-resolved multipulse optical spectroscopy ...................................... 139 Giovanni Maria Vanacore: Unraveling the ultrafast dynamics of spatially confined phonons and plasmons in low-dimensional nanosystems ..................................... 140 Yasunori Toda: Nonequilibrium quasiparticle dynamics in Bi-based superconductors measured by modulation photoexcitation spectroscopy ........................................ 141 Janez Bonca: Delocalized charge carriers in strongly disordered t–J model.......... 143 Matteo Moroni: Superconductivity emerging from an electronic phase separation in the charge ordered phase of RbFe2As2 ................................................................. 144 Mikhail Eremets: High temperature superconductivity in hydrides at high pressures ........................................................................................................................... 146 Alexander Goncharov: Stable high-pressure phases in the H-S system determined by chemically reacting hydrogen and sulfur up to 140 GPa ...................................... 147 Katsuya Shimizu: Search for Pressure-Induced Superconductivity in Other Hydrides ........................................................................................................................... 148 Thomas Timusk: Optical Spectroscopy of H3S: Evidence of a new Energy Scale for superconductivity ................................................................................................ 149 Antonio Bianconi: Fano Resonances at Lifshitz transitions driving high Tc superconductivity: from iron based superconductors to the case of H3S and pTerphenyl ........................................................................................................... 151 Jeff Tallon: Compressed H2S, superfluid density and the quest for room-temperature superconductivity ................................................................................................ 153 Frank Marsiglio: High Temperature Superconductivity in H3S --- why so high? . 155 Mari Einaga: Formation Process of High-T c Phase of Sulfur Hydride .................. 156 Luis Balicas: Impurity dependent superconductivity, Berry phase and bulk Fermi surface of the Weyl type-II semi-metal candidate MoTe2 ..................................... 157 Jose Mustre de León: Resonant X-ray Inelastic Scattering and nanoscale inhomogeneity in FeSe1-xTex ............................................................................... 159 Serguei Brazovskii: Phenomenological theory of switching of electronic phases by optical, current, voltage and STM pulses in TaS2. ................................................ 161 Jasper van Wezel: Collective modes of the excitonic condensate in 1T-TiSe2...... 162 Marco Grilli: Intrinsic inhomogeneity of 2D crystalline superconductors............. 163 Massimo Capone: Exploiting multiorbital physics to achieve high-temperature superconductivity: Fullerene and beyond............................................................. 165 Sergei Mukhin: Eliashberg equations with antiferromagnetic ‘hidden order’- induced pairing boson in cuprates..................................................................................... 167 Alexander Moskvin: Topological structures in a model cuprate ........................... 169 Enno Joon: Spin-Peierls dimerization caused by Jahn-Teller effect in NaTiSi2O6 171

Superstripes 2017, Ischia June 4-10, 2017 Reizo Kato: Quantum spin liquid in a molecular Mott system based on Pd(dmit)2 173 Jure Kokalj: Half-filled anizotropic triangular Hubbard model and organic charge transfer salts based on BEDT-TTF ...................................................................... 175 Takahiro Shimojima: Nematic electronic structure of the iron-based superconductor FeSe ................................................................................................................... 176 Sergey Borisenko: ARPES of iron-based superconductors ................................... 177 Jörg Fink: Non-Fermi-liquid behavior, Lifshitz transitions, and Hund’s metal behavior of iron-based superconductors and related compounds from ARPES ..... 178 Ming Shi: Electron-electron correlation in iron-Pnictides, revealed by ARPES .... 179 Francesco Tafuri: The new frontiers of the Josephson effect in novel unconventional nano-scale and magnetic systems ........................................................................ 180 Alejandro Silhanek: Superconducting weak links created by electromigration ..... 181 Andrei Zaikin: Quantum decay of supercurrent in transparent nanojunctions ....... 183 Claude Monney: Resonant inelastic x-ray scattering to measure short-range magnetic order .................................................................................................... 184 Alexander Moewes: Studying Silicene mono- and multilayers with soft X-ray spectroscopy and DFT ........................................................................................ 185 Hidenori Goto: Doping effects on electronic properties of bilayer graphene......... 186 Canhua Liu: Superconductivity of K-doped FeSe ultra-thin films on SrTiO3(001) substrate ............................................................................................................. 187 David Neilson: Multicomponent electron-hole superfluidity and the BCS-BEC crossover in double bilayer graphene................................................................... 188 Leonid Ponomarenko: High temperature quantum oscillations in graphene superlattices ........................................................................................................ 190 Ivan Borzenets: Governing energies of graphene Josephson junctions from dirty to ultra-clean regimes .............................................................................................. 191 Daniel Destraz: Superconducting fluctuations in a thin NbN film probed by the Hall effect .................................................................................................................. 193 Fabian von Rohr: Electron count and chemical complexity in high-entropy alloy superconductors .................................................................................................. 194 Samuele Sanna: Extensive study of the superfluid density of oxypnictides........... 195 Yuta Mizukami: Quantum critical points in iron-based superconductors .............. 197 Krzysztof Wohlfeld: How different are the iridates from the cuprates? Insights from the RIXS and ARPES spectroscopies .................................................................. 199 Jan Zaanen: Principles of holographic duality in the laboratory ........................... 200 Valerii Vinokur: Gauge theory of the BKT transition in disordered systems ........ 201 Adriana Moreo: Multi-orbital Hamiltonian for Iron Chalcogenides ...................... 202

Superstripes 2017, Ischia June 4-10, 2017 Gabriel Aeppli: Non-destructive high-resolution three-dimensional imaging of intelligent matter ................................................................................................. 204 Peter Littlewood: Elastic Effects at the Mott Transition ....................................... 205 Sergei Ovchinnikov: Effect of electron-phonon interaction on the doping and temperature dependent spectral function in cuprates ............................................ 206 Ilya Eremin: Robust determination of the superconducting gap sign structure via quasiparticle interference .................................................................................... 207 Fedor Balakirev: Magnetotransport Signatures of Competing Ground States and Critical Scaling in Strongly Correlated Superconductors ..................................... 208 Christoph Renner: Conventional vortices in the high temperature superconductor YBa2Cu3O7-δ ....................................................................................................... 209 Roberto Raimondi: SU(2) Gauge Theory Description of the Current-Induced Spin Polarizations in an Electron Gas .......................................................................... 210 Vladislav Kataev: Insights into the spin-orbital entanglement in complex iridium oxides from high-field ESR spectroscopy ............................................................ 211 Khandker Quader: Electronic Structure Study of Filled Skutterudites .................. 212 Vladimir Pudalov: Spin susceptibility of the correlated 2D electron system ......... 214 Tatiana Guidi: INS study of single and entangled rings ....................................... 215 Enrico Pomarico: Enhancement of electron-phonon interaction in bilayer graphene with optically-driven lattice................................................................................. 216 Nimrod Bachar: Detailed optical spectroscopy of hybridization gap and hidden-order transition in high-quality URu2Si2 single crystals ............................................... 218 Davide Massarotti: Phase dynamics and macroscopic quantum phenomena in unconventional Josephson junctions .................................................................... 220 Andrew Semenov: Voltage noise in short superconducting bridges...................... 221 Carmine Attanasio: Small NbN superconducting nanonetwork fabricated using porous silicon templates ...................................................................................... 222 Raymond Frésard: On superconducting stripes of the two-dimensional Hubbard model ................................................................................................................. 223 Hanjie Guo: Magnetic ground state of the pyrochlore iridate Nd2Ir2O7............... 224 George Jackeli: Spin-orbital frustration in Mott insulators ................................... 225 Giacomo Coslovich: Observing Interactions As They Happen: Ultrafast X-ray and THz Studies of Charge Ordered Materials ........................................................... 226 Marcin Wysokinski: Topological Kondo semi-metals.......................................... 227 Konrad J. Kapcia: Magnetic Lifshitz transition in iron-based superconductors ..... 229 Andrzej Ptok: The ab initio study of unconventional superconductivity in CeCoIn5 and FeSe ............................................................................................................. 231

Superstripes 2017, Ischia June 4-10, 2017 Boris Fine: Modeling low-energy electronic excitations in the background of spinvortex checkerboard ............................................................................................ 232 Peter Wahl: Local probing of magnetic order and excitations in iron-based superconductors .................................................................................................. 234 Christophe Berthod: Local spectroscopy of vortices in the presence of disorder: application to YBCO........................................................................................... 236 Yingying Peng: RIXS study on charge order and magnetic excitations in superconductor Bi2201 ....................................................................................... 237 José Lorenzana: Berezinskii–Kosterlitz–Thouless in curved space....................... 239 Alexey Mironov: Effect of Nanoperforation on critically disordered NbTiN films 240 Thomas Nattermann: BKT phases in disordered systems, past and present .......... 241 Vikram Tripathi: Disordered BKT criticality and superinsulation ........................ 242 Manuel de Llano: BCS-Bose Crossover Extended with Hole Cooper Pairs .......... 244 Israel Chávez: Dimensionless Coupling Constants in Superconductivity.............. 245 Luca Flammia: Quantum-size effects in superconducting nanostripes with step-edge ........................................................................................................................... 246 Alfredo Vargas-Paredes: Competition between Intraband Pairing and Crosspairing ........................................................................................................................... 247 Masatoshi Sato: Topological Crystalline Materials .............................................. 249 Egor Babaev: Dirty multi-band superconductors: time reversal symmetry breaking, multiple coherence lengths, type-1.5 superconductivity and vortex matter ........... 251 Karol Izydor Wysokiński: Low temperature thermoelectric power of strongly correlated quantum dot ........................................................................................ 252 Andrea Perali: Enhancement of Superconductivity by Shape Resonances: from Nano-films to Nano-stripes ................................................................................. 253 Andrzej M. Oles: Topological phases emerging from spin-orbital physics ........... 255 Thorsten Schmitt: Ground-state oxygen holes and the metal–insulator transition in the negative charge-transfer rare-earth nickelates ................................................ 256 Juris Purans: In-situ EXAFS studies of metal to insulator transition in ReO3-WO3 and related electrochromic materials ................................................................... 257 Pascale Roy: Room-temperature ferroelectricity in SrTiO3 ultrathin films on silicon: Infrared and ab initio study ................................................................................. 259 Peter Abbamonte: Bose condensation of excitons in a transition metal dichalcogenide .................................................................................................... 261 Dai Aoki: Unconventional superconductivity in uranium compounds .................. 262 Christophe Brun: Two-dimensional topological superconductivity in Pb/Co/Si(111) ........................................................................................................................... 263

Superstripes 2017, Ischia June 4-10, 2017 Kazuki Sumida: Ultrafast Surface Dirac Fermion Dynamics of Sb2Te3-based Topological Insulators......................................................................................... 265 Steffen Wirth: STS studies on correlated f-electron systems: Kondo lattice, quantum criticality and topological Kondo insulator .......................................................... 267 Georg Knebel: Field induced Lifshitz transitions in heavy fermion systems ......... 268 Gertrud Zwicknagl: Transport spectroscopy and Electronic Topological Transitions in heavy fermion materials .................................................................................. 269 Alexander Komarek: Physical properties of LaNiO3 single crystals..................... 270 Emil Bozin: Orbital Degeneracy Lifting and Short Range Orbital Order in CuIr2S4 ........................................................................................................................... 271 Veronika Sunko: Spin-split surface states of transition-metal delafossite oxides .. 273 Anna Krzton-Maziopa: Electrochemical intercalation of alkali metal ions into the layered structure of iron chalcogenides ................................................................ 275 Mikhail Silaev: Structure and dynamics of composite vortex states in multiband superconductors .................................................................................................. 276 Ikuzo Kanazawa: Quantized Massive Gauge Fields and Anomalous Angle-resoluved Photoemission Spectra in HIgh-Tc Cuprates........................................................ 278 Mikhail Eremin: Asymmetry of critical temperatures in e- and h- doped cuprates 279 Andreas Glatz: Detailed Simulation of Vortex Crossing ...................................... 281 Fabio Miletto Granozio: Engineering the functional properties of 2-dimensional electron gases at oxide interfaces......................................................................... 283 Daniela Stornaiuolo: Josephson junctions based on LAO/STO 2DEG.................. 285 Laura Fanfarillo: Orbital selectivity and Hund's physics in Iron-based superconductors .................................................................................................. 286 Christian Hess: Temperature dependent quasiparticle interference of LiFeAs ...... 287 Toshiya Ideue: Superconductivity in noncentrosymmteric low-dimensional materials ........................................................................................................................... 288 Alexander Steppke: Strong peak in Tc of Sr2RuO4 under uniaxial pressure........... 290 Frank Kruger: Topological Excitations and Bound States in a Quantum Dimer Antiferromagnet .................................................................................................. 292 Simon Wall: Imaging the insulator-metal phase transition with resonant soft X-ray holography.......................................................................................................... 294 Daniel McNally: Thickness and Temperature-Driven Metal-Insulator Transitions in CaVO3: A Resonant Inelastic X-ray Scattering Study ......................................... 296 Alexey Galda: Spin-transfer torque-induces parity-time symmetry-breaking........ 297 Ivan Madan: Optical control of skyrmions in helimagnetic FeGe. ........................ 298

Superstripes 2017, Ischia June 4-10, 2017 Srivats Rajasekaran: Nonlinear Terahertz Spectroscopy of Cuprates – A Probe of the ........................................................................................................................... 299 Marcello Spera: Remarkable energy dependent domain formation in the CDW of 1T-CuxTiSe2 ...................................................................................................... 300 Andrey Ivanov: X-ray magnetic circular dichroism at the K edge of Cu in Bi2Sr2CaCu2O8+x ............................................................................................. 302 Alexander Soldatov: Picometer-scale determination of 3D local atomic structure parameters in nanostructured materials ................................................................ 304 Augusto Marcelli: Nanoscale phase separation and lattice complexity in VO2, a complex multiphase correlated electron systems .................................................. 305 Akinori Irizawa: Metal-Insulator transitions in calcium ferrite compounds .......... 308 Bernardo Barbiellini: Metal-Insulator Transitions in Complex Oxides Probed by xray inelastic scattering ......................................................................................... 309 Giancarlo Strinati Calvanese: Non-local equation for the superconducting gap parameter ............................................................................................................ 311 Giniyat Khaliullin: Soft spins and Higgs mode in ruthenates ............................... 312 Vadim Ksenofontov: Interplay of superconductivity and magnetism in Fe-based superconductors under high pressure ................................................................... 313 Jincheng Zhuang: STM study of germanene ........................................................ 314 Ruben Albertini: Micro-XAS measures of the local structure changes in BaPb1xBixO3 as a function of temperature ..................................................................... 315 Michael Di Gioacchino: Levy flight distribution of fluctuation supramolecular structure of myelin .............................................................................................. 317 Cinzia Di Giorgio: Low temperature scanning probe microscopy investigation of FeSe single crystals ............................................................................................. 319 Oleg Lychkovskiy: Spin excitations in cuprates from spin cluster calculations..... 320 Maria Vittoria Mazziotti: High Tc in the organic superconductor Kx p-Terphenyl by Fano resonances in superconducting gaps at the Lifshitz transition ...................... 321

Superstripes 2017, Ischia June 4-10, 2017 1.1

Multi-level Kondo effect and enhanced critical temperature in nanoscale granular Al. Aviv Moshe1, Nimrod Bachar1,2, Y. Lereah3 and G. Deutscher1 1 School of Physics and Astronomy, 2 Faculty of Engineering, Tel Aviv University, Israel 3 Department of Physics, University of Geneva, Switzerland The normal state conductance of high resistivity granular Al films, having an enhanced critical temperature above 3K, shows a non-monotonous temperature dependence. Upon cooling the conductance first increases, reaches a broad maximum, then decreases, and finally increases sharply again at low temperatures. This behavior bears a striking resemblance to that predicted for the conductance of quantum dots in a regime where the broadening of discrete energy levels, due to coupling of the dot to the leads reservoirs, is larger than the level separation. In this regime a multi-level Kondo temperature can be larger than the effective Coulomb charging energy, leading to a metallic-like behavior at high temperatures. This is followed by a Coulomb dominated regime, while at yet lower temperatures the single level Kondo behavior can be recovered (1). We believe that this multi-level Kondo resonance model for single dots may apply to our 3D network of nanoscale grains. The multi-level resonance prevents the insulating state from setting -in already at high temperatures, which is favorable for superconductivity. This interpretation of the experimental conductance data is in line with the previously reported presence of magnetic moments in these films (2). High Resolution Electron Microscopy shows that intergrain coupling can take place through atomic size Sharvin contacts, which is consistent with the typical values of the intergrain resistance of the films as the Metal to Insulator transition is approached. N. Bachar and G. Deutscher are indebted to Antoine Georges for illuminating discussions on the multi-level Kondo resonance effect.

References 1. S. Florens et al., Phys. Rev. B68, 245311 (2003). 2. N. Bachar et al., Phys. Rev. B91, 041123 (2015).

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The Fröhlich-type, non-equilibrium, polaronic condensate in the Mott system UO2(+x) Steven D. Conradson Institut Jozef Stefan, Washington State University, NEqCST Corporation Email: [email protected] Keywords: condensation, non-equilibrium condensate, Fano-Feshbach resonance Results from a large number of experiments on O- and photo-doped UO2(+x), a 5f Mott insulator, are best and perhaps only interpreted as demonstrating that the polarons aggregate and self organize into a Bose-Einstein condensate. The basis for this is Fröhlich's prediction of a non-equilibrium condensate composed of specific phonons that become coherent because of dipole interactions that are enhanced by anharmonicity. The charge-transfer that we have observed in UO2 would be an extreme case of that. An evaluation of the electronic density-of-states also shows that the polaronic quantum phase meets the conditions for stabilization by a Fano-Feshbach resonance. Evidence will be presented for typical condensate properties as well as several that have not been predicted.

References 1. H. Frohlich, Phys. Lett. A A 26, 402 (1968). 2. A. Bianconi, Nature Phys. 9, 536 (2013). 3. S. D. Conradson, et al., Phys. Rev. B 88, 115135 (2013). 4. S. D. Conradson, et al., Sci. Rep. 5, 15278 (2015). 5. S. D. Conradson, et al. Phys. Rev. B, (2017) submitted.

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Investigation of the optical anisotropy in the nematic phase of FeSe L. Degiorgi ETH Zurich

Email: [email protected] Keywords: optical properties, iron-pnictide superconductors The iron-pnictide superconductors provide the most recent playground in which to address the competition between structural, magnetic and superconducting phases. In the well-known 122 materials, the non-superconducting parent compounds undergo an antiferromagnetic transition into a broken-symmetry spin-density-wave (SDW) ground state at TN, which is always preceded by or coincident with a tetragonal-toorthorhombic structural distortion at Ts ≥TN. Their relevant resistivity anisotropy at Tce due to the SDW collective state in the orthorhombic state. Understanding the effects of the structural transition on the charge dynamics and the electronic bands by studying the optical properties of the system is an important step in order to develop a comprehensive description of these materials. We use our technique [1] that allows in-situ variation of uniaxial stress to probe the polarization dependence of the optical reflectivity of FeSe through the tetragonal-toorthorhombic structural transition and with respect to the electronic nematic phase (Fig. 1). We will compare our newest results with our early data on the representative Co-underdoped 122-iron-arsenide [1]. These measurements reveal a different hysteretic behavior of the anisotropic optical response to uniaxial stress (i.e., degree of sample detwinning) in the orthorhombic state for the two classes of materials, which may reveal important peculiarities of their own electronic structure and their sensitivity to the nematic phase.

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(a) Optical reflectivity of FeSe at 10 K in the fully detwinned orthorhombic phase and along both axes. The main panel emphasizes the mid-infrared spectral range, while the inset shows the full measurement from the far-infrared up to the UV. (b) Reflectivity ratio between data along both axes at 10 K with increasing uniaxial stress (p) after a zero pressure-cooling protocol [1].

References 1. C. Mirri et al., Phys. Rev. B 93, 085114 (2016) and references therein.

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New candidate quantum spin liquids in ionic polyaromatic hydrocarbons Kosmas Prassides WPI-AIMR, Tohoku University

Email: [email protected] Keywords: ionic polyaromatic hydrocarbons; Mott localization; frustrated topology; quantum magnetism Molecular solids whose cooperative electronic properties are based purely on pielectrons from carbon atoms offer a fertile ground in the search for exotic states of matter, including unconventional superconductivity and quantum magnetism. The field was ignited by reports of high-temperature superconductivity in materials obtained by reaction of alkali metals with polyaromatic hydrocarbons (PAHs) such as phenanthrene and picene. However, the results have not been reproduced and the compound identities have remained unknown. Recently we have been successful in devising new reproducible synthetic routes of ionic salts of PAHs under mild conditions; crystalline single-phase materials of ionic PAHs (phenanthrene, pentacene, picene) are available for structural and electronic characterization for the first time [1,2]. The binary phenanthrene salts, CsPhen and Cs2Phen are multi-orbital strongly-correlated Mott insulators. Cs2Phen is diamagnetic due to orbital polarisation while CsPhen is a Heisenberg antiferromagnet with a gapped spin-liquid ground state. The absence of long-range magnetic order renders the compound an excellent candidate of a spin-½ quantum spin liquid arising purely from carbon pi-electrons. References 1. Y. Takabayashi, M. Menelaou, H. Tamura, N. Takemori, T. Koretsune, A. Štefančič, G. Klupp, A. J. C. Buurma, Y. Nomura, R. Arita, D. Arčon, M. J. Rosseinsky, and K. Prassides, Nature Chem. 9, in press (2017). 2. F. D. Romero, M. J. Pitcher, C. I. Hiley, G. F. S. Whitehead, S. Kar, A. Y. Ganin, D. Antypov, C. Collins, M. S. Dyer, G. Klupp, R. H. Colman, K. Prassides, and M. J. Rosseinsky, Nature Chem. 9, in press (2017).

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Dynamic charge stripe spectrum in nickelate perovskites Dmitry Reznik University of Colorado-Boulder

Email: [email protected] Keywords: Dynamic charge stripes, nickelates The insulator-to-metal transition continues to be a challenging subject, especially when electronic correlations are strong. In layered compounds, such as La2–xSrxNiO4 and La2–xBaxCuO4, the doped charge carriers can segregate into periodically spaced charge stripes separating narrow domains of antiferromagnetic order. Although there have been theoretical proposals of dynamically fluctuating stripes, direct spectroscopic evidence of charge-stripe fluctuations has been lacking until recently. I will discuss our measurements of critical lattice fluctuations, driven by charge-stripe correlations, in La2–xSrxNiO4 using inelastic neutron scattering. This scattering is detected at large momentum transfers where the magnetic form factor suppresses the spin fluctuation signal. The lattice fluctuations associated with the dynamic charge stripes are narrow in q and broad in energy. They are strongest near the charge-stripe melting temperature. Recent work established that the stripe dispersion is anisotropic and strongly depends on temperature between the charge and magnetic ordering transitions. Our results open the way towards the quantitative theory of dynamic stripes and for directly detecting dynamical charge stripes in other strongly correlated systems, including hightemperature superconductors such as La2–xSrxCuO4.

References 1. R. Zhong, B.L. Winn, G. Gu, D. Reznik, J.M. Tranquada, arXiv:1608.04799 (2016). 2. S. Anissimova, D. Parshall, G. D. Gu, K. Marty, M. D. Lumsden, Songxue Chi, J. A. Fernandez-Baca, D.L. Abernathy, D. Lamago, J.M. Tranquada, D. Reznik, Nature Communications 5, 3467 (2014).

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Mott transition and strong diamagnetism in Ca2RuO4 tuned by electric field/current Yoshiteru Maeno Department of Physics, Kyoto University, Kyoto 606-8502, Japan.

Email: [email protected] Keywords: Mott transition; diamagnetism; non-equilibrium; Ca2RuO4. The Mott insulator is considered as an electron “solid” frozen due to strong electron correlations. It has a potential to become a good metal if the electron solid melts by suitable stimuli. In this talk, we will describe novel phenomena we found in the layered ruthenium oxide Ca2RuO4, for which non-equilibrium conditions introduced by DC electric field and current trigger and maintain the charge “liquid” state down to low temperatures [1]. When the electric current is not very strong, the Mott-gap can be tuned to disappear gradually. In such a condition, Ca2RuO4 exhibits a semi-metallic conduction and giant diamagnetism [2]. We will discuss how the partial Mott-gap closing leads to such diamagnetic behavior. The important implication of this study is that simple DC current may be used as a useful control parameter to induce new states from some Mott insulators.

Figure 1: Schematic crystal structure of undistorted Ca2RuO4. In reality, the ground-state Mottinsulating phase has strong distortions with RuO6 octahedra flattening, as well as tilting and rotation.

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Superstripes 2017, Ischia June 4-10, 2017 This work is done mainly in collaboration with F. Nakamura, C. Sow, S. Yonezawa, T. Oka, S. Kitamura, and K. Kuroki. This work was supported by JSPS KAKENHI Nos. JP26247060 and JP15H05852 (Topological Materials Science).

References 1. “Electric-field-induced metal maintained by current of the Mott insulator Ca2RuO4”, F. Nakamura, M. Sakaki, Y. Yamanaka, S. Tamaru, T. Suzuki, and Y. Maeno, Sci. Rep. 3, 2536 (2013). 2. “Current-induced giant diamagnetism in the Mott insulator Ca2RuO4”, C. Sow, S. Yonezawa, S. Kitamura, T. Oka, K. Kuroki, F. Nakamura, and Y. Maeno, arXiv. 1610.02222.

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The road map toward room temperature superconductivity Annette Bussmann-Holder MPI FKF

Email: [email protected] Keywords: multiband Superconductivity, isotope effects Combining real space and momentum pairing corresponds to the coexistence of BEC and BCS physics. While typically the crossover between both is discussed and analyzed, we consider here their coexistence within a weak coupling approximation. The most profound results for this scenario are: a substantial increase in the superconducting transition temperature which easily reaches values of 300K (Figure 1a). The isotope exponent α deviates substantially from the BCS value to decrease to vanishingly small numbers with increasing Tc (Figure 1b). Further consequences related to the gap to Tc ratio and the variation of the gaps with the parameters are discussed and brought into relation with real materials.

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a) The superconducting transition temperature Tc as a function of the polaronic coupling λ1 for three different parameter sets as indicated in the figure. b) The isotope exponent α as a function of Tc where the color code is identical to figure 1a).

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d9 Nickelates under Pressure T. Egami1,2,3 D.C. Mitchell1,2 and K.A. Lokshin1,3 1 Shull Wollan Center – Joint Institute for Neutron Sciences, University of Tennessee and Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA 2 Department of Physics and Astronomy, University of Tennessee, Knoxville, TN 37996, USA. 3 Department of Materials Science and Engineering, University of Tennessee, Knoxville, TN 37996, USA. Email: [email protected] Keywords: nickelates; pressure; cuprate analog; metallic conductivity.

Nickel ions are commonly seen in Ni3+ (d7) or Ni2+ (d8) state. However, by strong reduction it is possible to produce nickelates in the Ni+ (d9) state in the square-planar structure as in the cuprates. Because of the charge transfer gap larger than that in the cuprates d9 nickelates are insulating in the ambient condition, but it can be made metallic under pressure. Various interesting properties have been observed for La4Ni3O8 [1] and related compounds. Here we discuss the physical properties of Nd4Ni3O8 (d8.67) and Nd3Ni2O6 (d8.5) under pressure. The insulator to metal transition was observed, but so far superconductivity has not been observed. We discuss why superconductivity does not occur in this system. The measurements were made by home-made pressure apparatus compatible with the PPMS. The contacts were deposited on the sample by micro-fabrication technique. We also discuss the possibility of superconductivity in undoped cuprates. In Fe-pnictides isoelectronic doping, such as As to P, suppresses AFM order and induces superconductivity. Thus the role of AFM order is simply to compete and suppress superconductivity, and superconductivity can occur without spin fluctuations. Recent results [2,3] suggest that the phase diagram of the cuprates is more similar to that of the pnictides than considered earlier. We argue that a similar logic applies also to the cuprates to some extent.

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Nd4Ni3O8 ZFC 4GPa 15GPa 28GPa 37GPa-1 37GPa-2 51GPa 60GPa

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T (K) Figure 1: Resistivity of Nd4Ni3O8 as a function of temperature and pressure.

References 1. V.V. Poltavets, K.A. Lokshin, A.H. Nevidomskyy, M. Croft, T.A. Tyson, J. Hadermann, G.V. Tendeloo, T. Egami, G. Kotliar, N. ApRoberts-Warren, A.P. Dioguardi, N.J. Curro and M. Greenblatt, Phys. Rev. Lett. 104, 206403 (2010). 2. M. Brinkmann, T. Rex, H. Bach and K. Westerholt, Phys. Rev. Lett. 74, 4927 (1995). 3. M. Horio, T. Adachi, Y. Mori, A. Takahashi, T. Yoshida, H. Suzuki, L.C.C. Ambolode II, K. Okazaki, K. Ono, H. Kumigashira, H. Anzai, M. Arita, H. Namatame, M. Taniguchi, D. Ootsuki, K. Sawada, M. Takahashi, T. Mizokawa, Y. Koike and A. Fujimori, Nature Commun. 7, 10567 (2015).

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Three novel features in the correlations between Tc and the superfluid density: local phase coherence below the Bose gas temperature TB and multiple ratios of Tc/TF Yasutomo J. Uemura1 1 Department of Physics, Columbia University, New York, NY 10027, USA

Email:[email protected] Key words: superfluid density, Nernst effect, photo-induced transient response, BoseEinstein condensation Since 1988, the present author performed muon spin relaxation (MuSR) measurements of the magnetic field penetration depth, and reported nearly linear relationship between Tc and the superfluid density in high-Tc cuprate, Fe-based, A3C60, organic-BEDT and heavy-fermion superconductors [1-3]. These results have been shown in a plot of Tc versus effective charge energy scale TF derived from the superfluid density, as shown in Fig. 1. The strong correlations between Tc and the carrier density cannot be expected in BCS theory, while Bose Einstein Condensation (BEC) of preformed pairs give the linear relationship, yet with significantly higher transition temperature TB if one calculates the condensation temperature of corresponding number (ns/2) and mass (2m*) of tightly-bound bosons forming a non-interacting Bose gas. In this talk, I would like to point out three new features by including new results from Nernst effect, transient optical responses and additional MuSR measurements. (1) Signatures of dynamic or transient superconductivity have been seen in Nernst effect and diamagnetic susceptibility above Tc, pioneered by Ong [4], and optical responses to photo-excited state reported in recent measurements of Cavalleri et al. [5]. When we plot the onset temperature of Nernst effect TNER:on of LSCO, URu2Si2 and CeCoIn5 against the equilibrium value of TF, the points come very close to the TB line, as demonstrated in Fig. 1 for an underdoped LSCO. Furthermore, the onset point of the photo-excited transient response may also come close to the TB line, for the cases of LBCO, YBCO, and K3C60. These features can be understood as representing the formation of local phase coherence of pre-formed pairs below TB, yet the actual equilibrium condensation temperature Tc is significantly (factor of 4 or more) lowered due to the competition with the antiferromagnetic spin/charge order. In the photoexcited states, this competition may not play a destructive role for the gapped transient optical responses.

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(2) The lowering of actual Tc from the hypothetical Bose gas temperature TB can be explained by the existence of inelastic resonance mode originating from the spin and charge correlations of the competing state, analogous to rotons in superfluid He [2,3]. Indeed we find the resonance mode features in YBCO, LSCO, URu2Si2 and CeCoIn5, which has the mode energy proportional to Tc with the same proportionality constant as that of roton energy versus Lambda temperature in superfluid 4He. (3) Recently, MuSR results on the superfluid density have been added for (a) Fe(Se,Te), single-layer FeSe, (b) electron-doped cuprates, and (c) MoTe2 under pressure and NbSe2 [6]. The ratio of Tc/TF in (a) is similar to those of hole doped cuprates, while about 4 times reduced in (b) and about 16 times reduced in (c), as shown in Fig. 2. This demonstrates that even for systems with a rather small Tc/TF, linear correlations between Tc and the superfluid density (reminiscent of BEC) survive within a given family, presenting a new challenge in interpreting these features with BEC-BCS crossover phenomenology.

(left) Figure 1. Plot of the transition temperature Tc versus effective Fermi temperature TF derived from the superfluid density ns/m* [1-3]. N:on denotes the Nernst onset temperature in an underdoped LSCO. TB denotes the Bose condensation temperature of an ideal noninteracting gas of tightly bound bosons with the density ns/2 and mass 2m*. The onset temperature of the gapped response measured by Cavalleri et al. [5] in the transient photoinduced state in K3C60 comes close to the TB line. (right) Figure 2. Plot of the transition temperature Tc versus the superfluid density ns/m* proportional to the inverse square of the magnetic field penetration depth λ, in various superconductors. Linear relationship can be found with three proportionality constants for three different groups of materials: (a) hole-doped cuprates, Fe(Se,Te). 1111 FeAs, 122 FeAs with the largest ratios of Tc/TF; (b) electron-doped cuprates with the ratio about 4 times smaller than that of the group (a); and (c) transition-metal dichalcogenides (TMDC) MoTe2 under pressure and NbSe2 in ambient pressure with the ratio 16 times reduced from that of the group (a). From Guguchia et al. [6].

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References 1. Y.J. Uemura et al., Phys. Rev. Lett. 62 (1989) 2317; 66 (1991) 2665; Nature 352 (1991) 605. 2. Y.J. Uemura, J. Phys. Condens. Matter 16 (2004) S4515; Physica B404 (2009) 3195. 3. Y.J. Uemura, Nature Materials 8 (2009) 253. 4. Y. Wang, N.P. Ong et al., Phys. Rev. B73 (2006) 024510; Phys. Rev. Lett. 95 (2005) 247002. 5. M. Mitrano, A. Cavalleri et al., Nature 530 (2016) 461, and references therein. 6. Z. Guguchia, Y.J. Uemura et al., arXiv:1704.05185

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Electron-lattice and electron-electron interactions in novel systems: paths to room temperature superconductivity Vladimir Kresin* and Lev Gor’kov** *Lawrence Berkeley Laboratory, University of California, Berkeley, CA 94720, USA **NHMFL, Florida State University, Tallahassee, Florida, 32310, USA Email: [email protected] Keywords: strong coupling; nano; carbon According to microscopic theory, the room temperature superconductivity is a perfectly realistic phenomenon. Recent progress in the area of high Tc has attracted a lot of interest. The record value of Tc observed recently in sulfur hydrides is provided by strong electron-lattice coupling with high frequency optical modes. However, one should go beyond the usual approach to describe such a state. This is caused by complex nature of the phonon spectrum, which is broad and contains various branches. The proposed two-coupling constants model and the coupling re-distribution concept can be used to describe the non-trivial phase diagram, that is the pressure dependence of T¬c. The future tunneling spectroscopy will allow one to observe the two-gap picture for the highest Tc phase. The strengthening of the interaction caused by polaronic effects and by drastic increase in the density of states for nano-systems (nano-clusters and quantum dots) will be also discussed. High Tc superconducting state can be caused by electron-electron interaction between two electronic groups, for example, between two specially arranged carbon nanotubes. Superconducting state of biologically active systems and organic molecules is caused by the electron-lattice interaction and merit a more detailed study.

References 1. A. Drozdov, M. Eremets, et al., Nature 525,73 (2015). 2. A. Bianconi, T. Jarlborg, Novel Supercond. Mater. 1,15(2015). 3. L. Gor’kov and V. Kresin, Nature, Sci. Rep. 6, 25608 (2016). 4. A. Hamo et al., Nature 535,395 (2016). 5. M. Rosseinsky and K. Prassides, Nature,464,39 (2010). 6. V. Kresin et al., Superconducting State, Sec.14.6, Oxford Univ. Press (2014).

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Strong interplay between high-Tc superconductivity, nematicity, and magnetism in Fe-based Superconductors Hiroshi Kontani Nagoya Univerisity

Email: [email protected] Keywords: Fe-based superconductors, electronic nematic state, pairing mechanism The interplay between the nematicity, magnetism and high-Tc superconductivity in Febased superconductors is one of the key unsolved problems [1,2]. To understand this issue, the plane s-wave superconducting state in heavily electron-doped FeSe, in which the Fermi surfaces are composed of only two electron-pockets, attracts great attention. We attacked this issue by focusing on the higher-order many-body effect called the vertex correction (VC) that has been neglected in conventional Migdal-Eliashberg (ME) formalism [3]. Due to the VC, the dressed effective Coulomb interaction possesses nontrivial spin and orbitaldependences. We find in FeSe that (i) the orbital-fluctuation-mediated pairing interaction is strongly magnified by the Aslamazov-Larkin type VC due to the strong modification of the electron-boson coupling constants. In addition, (ii) sizable pairing glue is caused by the multi-fluctuation exchange processes. Due to both important beyond-ME effects, which are caused by the interplay between orbital and spin fluctuations, the anisotropic plane s-wave state in heavily electron-doped FeSe is satisfactorily explained. The proposed hole-pocketless pairing mechanism would be important for various Fe-based superconductors. We also study the rich electronic phase diagram in FeSe under pressure [4]. Based on the first-principles calculations, we find that the pressure-induced xy orbital Fermi pocket appears. Then, the spin fluctuations on the xy orbital are enhanced, whereas those on other orbitals are reduced. For this reason, the orbital order , which is caused by the spin fluctuations on xz and yz orbitals via the Aslamazov-Larkin VC, is suppressed and replaced with the magnetism of xy-electrons. The nodal s-wave state at ambient pressure in the nematic state and the enhancement of Tc under pressure are driven by the cooperation between spin and orbital fluctuations. References 1. Y. Yamakawa, S. Onari, and H. Kontani, Phys. Rev. X 6, 021032 (2016). 2. S. Onari, Y. Yamakawa, and H. Kontani, Phys. Rev. Lett. 116, 227001 (2016). 3. Y. Yamakawa and H. Kontani, arXiv:1611.05375. 4. Y. Yamakawa and H. Kontani, arXiv:1609.09618.

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Strong covalency and ligand holes, or how to make magnetic gold? D.I. Khomskii II.Physikalisches Institut, Universitaet zu Koeln, Germany

Email: [email protected] Keywords: negative charge transfer gaps, self-doping, gold telluride AuTe2. In this talk I will discuss some effects occurring in transition metal compounds with small or negative charge transfer gap and with large contribution of ligand (e.g. oxygen) holes. Special attention will be paid to the compounds of 4d and 5d elements. The apparent inversion of crystal field levels, as well as the tendency to spontaneous charge disproportionation will be discussed. Specifically, some systems containing gold, such as Cs2Au2Cl6 and AuTe2 will be discussed, and the question formulated in the title will be addressed.

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Spin and charge-density-wave fluctuations in cuprates Stephen Hayden1 1 H. H. Wills Physics Laboratory, University of Bristol, Tyndall Ave., Bristol, BS8 1TL, UK

Email: [email protected] Keywords: high temperature superconductivity, spin fluctuations, charge and spin order. We discuss recent investigations of spin fluctuations and charge density wave correlations (CDW) in La2-xSrxCuO4 (LSCO) and YBa2Cu3O6+x (YBCO) [1-5]. YBCO shows CDW order near p=1/8. Here we present our recent determination of the atomic displacements associated with the CDW [4]. Specifically, we find that the mirror symmetry of the CuO2 is broken by the CDW. LSCO is a canonical layered cuprate system which may be continuously doped from parent antiferromagnet, through hightemperature superconductor to an overdoped metal. We have recently shown that near p=1/8 the system shows charge density wave order [2] together with the well known incommensurate spin freezing. In theories of magnetically mediated superconductivity, it is important to know the evolution of the Fig. 1 Atomic displacements of the spin fluctuations with doping across the CDW in YBa2Cu3O6+x [4]. phase diagram. Thus here we report a systematic neutron scattering and x-ray studies of the evolution of the magnetic excitations and low-energy spin freezing with doping. Specifically, we have measured the magnetic excitations for the charge ordered composition p~1/8. We find that pseudogap and optimally doped compositions show a strong commensurate (π,π) excitation which forms at high temperature and

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Superstripes 2017, Ischia June 4-10, 2017 persists over an energy range 40-100 meV. The similar features are present in other cuprate systems. At lower energies near p=1/8 we show how the charge and spin order are coupled. References 1. J. Chang, E. Blackburn, A. T. Holmes, N. B. Christensen, J. Larsen, J. Mesot, R. Liang, D. A. Bonn, W. N. Hardy, A. Watenphul, M. v. Zimmermann, E. M. Forgan, and S. M. Hayden, Nat Phys 8, 871 (2012). 2. T. P. Croft, C. Lester, M. S. Senn, A. Bombardi, and S. M. Hayden, Phys. Rev. B 89, 224513 (2014). 3. M. Hücker, N. B. Christensen, A. T. Holmes, E. Blackburn, E. M. Forgan, R. Liang, D. A. Bonn, W. N. Hardy, O. Gutowski, M. v. Zimmermann, S. M. Hayden, and J. Chang, Phys. Rev. B 90, 054514 (2014). 4. E. M. Forgan, E. Blackburn, A. T. Holmes, A. K. R. Briffa, J. Chang, L. Bouchenoire, S. D. Brown, R. Liang, D. Bonn, W. N. Hardy, N. B. Christensen, M. V. Zimmermann, M. Hucker, and S. M. Hayden, Nat Commun 6, 10064 (2015). 5. J. Chang, E. Blackburn, O. Ivashko, A. T. Holmes, N. B. Christensen, M. Hucker, R. Liang, D. A. Bonn, W. N. Hardy, U. Rutt, M. v. Zimmermann, E. M. Forgan, and S. M. Hayden, Nat Commun 7, 11494 (2016).

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Electronics of high-temperature cuprate superconductors C. E. Matt1, D. Sutter1, Y. Sassa2, A. Cook1, L. Das1, N.C. Plumb3, M. Shi3, M. Månsson4, S. M. Hayden5, T. Neupert, Johan Chang1 1 Physik-Institut, Universität Zürich, Winterthurerstr 190, CH8057 Zürich, Switzerland 2 Department of Physics and Astronomy, Uppsala University, S75121 Uppsala, Sweden 3 Swiss Light Source, Paul Scherrer Institut, CH-5232 Villigen PSI, Switzerland 4 KTH Royal Institute of Technology, Materials Physics, S-164 40 Kista, Sweden 5 H. H. Wills Physics Laboratory, University of Bristol, Bristol BS8 1TL, United Kingdom Email: [email protected] Keywords: Cuprates, Superconductivity, Electronic structure The minimal ingredients to explain the rich phenomenology of cuprate superconductors are still heavily debated. In this talk, recent photoemission experiments aiming to address the electronic structure of these materials will be presented. It will be discussed how these new results (C.E. Matt et al. In preparation) impact superconductivity and the pseudogap phase.

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1/8 Anomaly and Charge Order in Dy-doped Bi2212 N. Momono1, K. Kawamura1, C. Kobashi1, N. Kaimai1, S. Takahashi1, Y. Kawai1, T. Kurosawa2, M. Oda2, M. Ido2 1 Applied Physics Course, Muroran Institute of Technology, Mizumoto-cho 27-1, Muroran 050-8585, Japan 2 Department of Physics, Hokkaido University, Sapporo 060-0810, Japan

Email: [email protected] Keywords: 1/8 anomaly, checker board charge order, STM/STS In high-Tc cuprate superconductors, it is considered that the superconductivity and various competing orders coexist in the underdoped region. It is important to clarify the anomalous electronic states in the underdoped region. In Bi2Sr2Ca1-xDyxCu2O8+d (Dy-Bi2212), we found an anomalous "dip" feature near the hole concentration p~1/8 on the Tc-p curve, which is not seen in the Tc-p curve of pure Bi2212. In STM/STS experiments on Dy-Bi2212 with p~1/8, the so-called ‘checkerboard charge order’ with a wave vector q~0.3 r.l.u. is observed. The STS spectra for the sample with p~1/8 exihibit no well-defined coherent peaks, which is similar to the results reported in ARPES experimens on Dy-Bi2212 [1]. The dip of the Tc-p curve induced by the substitution of Dy in Bi2212 was similar to that observed in Zn-doped Bi2212 [2], suggesting that the suppression of Tc near p~1/8 in Dy-Bi2212 is related to the pinning of the charge order.

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Figure 1: Doping dependence of Tc for Bi2Sr2Ca1-xDyxCu2O8+d (Dy-Bi2212).

References 1. J. Zhao et al., PNAS 110, 17774 (2013). 2. M. Akoshima et al., Phys. Rev. B 57, 7491 (1998).

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Neutron-Scattering Study of Magnetic Excitations in Electron-Doped Cuprate Masaki Fujita1 , Shun Asano2 , Kensuke Suzuki1 Institute for Materials Research, Tohoku Univsersity Sendai 9808577, Japan 2 Department of Physics, Tohoku University, Sendai 980-8578, Japan 1

Email: fujita @imr.tohoku.ac.jp Keywords: electron-doped cuprate, magnetic excitations, neutron scattering The doping-evolution of spin and charge dynamics in a doped Mott insulator is an important issue in the research field of strongly correlated electron system. The entire excitation spectra have been intensively and continuously studied with the development of spectroscopic techniques after the discovery of high-transitiontemperature (high-TC) superconductivity in a lamellar cuprate oxide. The recent stateof-the-art spectrometers at large research facilities such as J-PARC enables us to study the details of composite dynamics in the energy-momentum space arising from the interacting degrees of freedom [1, 2]. Although the comprehensive study on both holeand electron-doped systems are indispensable to extract the universal mechanism of high-TC superconductivity, the majority of spectroscopic measurements was done on the hole-doped system. Here, we introduce the results of systematic inelastic neutron scattering measurements done on the electron-doped copper oxide to explore the entire spin excitations. First, we confirmed that the spin excitation in Pr1.4La0.6CuO4, which is the parent compound of electron-doped superconductor, is consistent with the spin-wave excitation expected from the s=1/2 two-dimensional Heisenberg model. The evaluated nearest neighbor exchange coupling is 140±5 meV, comparable to the value for the parent compound of hole-doped superconductor, La2CuO4. It was found that the spin excitation tends to elongate toward the higher energy region upon electron-doping in Pr1.4-xLa0.6CexCuO4 and the zone boundary energy exceeds 300 meV in the sample with x ≥ 0.08, suggesting the steeper dispersion in the highly doped samples [3]. This dopingevolution is in stark contrast with a negligible doping effect on the high-energy spin excitation in the hole-doped La2-xSrxCuO4, Therefore, the electron-hole asymmetry exists in the observed spin excitation against the doping. We furthremore studied the annealing effect and the thermal effect on the spin excitations of Pr1.4La0.6CuO4. The results will be discussed in connection with the possibility of “undopedsuperconductivity” in T’-structured system.

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References 1. M. Matsuura, S. Kawamura, M. Fujita, R. Kajimoto, and K. Yamada, “Development of spin-wave-like dispersive excitations below the pseudogap temperature in the high-temperature superconductor La2-xSrxCuO4” Phys. Rev. B 95, 024504 (2017). DOI: https://doi.org/10.1103/PhysRevB.95.024504 2. K. Sato, M. Matsuura, M. Fujita, R. Kajimoto, S. Ji, K. Ikeuchi, M. Nakamura, Y. Inamura, M. Arai, M. Enoki, and K. Yamada, ”Temperature Dependence of Spin Fluctuations in Underdoped La1.90Sr0.10CuO4”, Proc. Int. Conf. Strongly Correlated Electron Systems (SCES2013), JPS Conf. Proc. 3, 017010 (2014), DOI: http://dx.doi.org/10.7566/JPSCP.3.017010 3. K. Ishii, M. Fujita, T. Sasaki, M. Minola, G. Dellea, C. Mazzoli, K. Kummer, G. Ghiringhelli, L. Braicovich, T. Tohyama, K. Tsutsumi, K. Sato, R. Kajimoto, K. Ikeuchi, K. Yamada, M. Yoshida, M. Kurooka, and J. Mizuki, ”High-Energy Spin and Charge Excitations in Electron-Doped Copper Oxide Superconductors”, Nature Communications 5, 4714 (2014). DOI:10.1038/ncomms4714

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Superconductivity and electronic structure of SrTiO3 Dirk van der Marel A. Stucky, G. Scheerer, Z. Ren, D. Jaccard, J.-M. Poumirol, C. Barreteau, E. Giannini University of Geneva

Email: [email protected] Keywords: superconductivity, electron-phonon interaction, ferro-electricity, quantum criticality, isotope effect, polaron, SrTiO3 Doped SrTiO3 is a very intriguing superconductor. In principle it is a garden variety semiconductor, but the pristine material can be made ferroelectric by substition of 18O on the oxygen sites. The RPA electronic structure calculations predict that, depending on the level of doping, one, two or three bands get occupied, at critical doping levels that have been confirmed by quantum oscillation experiments. Doping with less than 1 electron per 10'000 formula units makes the materials superconducting. Moreover, the maximum value of Tc is of the order 0.5 Kelvin regardless whether obtained in 2D interfaces or in bulk SrTiO3. The doped electrons are coupled to the lattice parameters, and from a wealth of optical and ARPES experiments it is known that this causes a factor of two mass enhancement, corresponding to the limit of large -and highly mobile- polarons. With regards to superconductivity there exists universal agreement that it pairing is mediated by electron-phonon coupling, but it is not clear which phonons are the most important ones. Recently it has been suggested that the optical phonons that soften at the para-ferro electric transition are the main culprits, on the other hand optics and ARPES data suggest strong coupling to the highest energy branch of optical phonons. Our isotope studies demonstrates a factor 1.5 enhancement of Tc, with a sign opposite to standard BCS. The effect can be due to polaronic bandnarrowing, coupling to ferro-electric fluctuations or a combination of these two.

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Polarons in SrTiO3 as demonstrated by optical conductivity data (J.L.M. van Mechelen et al.; PRL 100, 226403 (2008); T. Devreese et al.; PRB 81, 125119 (2010)).

References 1. K.A. Mueller, and H. Burkard, Phys. Rev. B, 19, 3593-3602 (1979). 2. S. E. Rowley, L. J. Spalek, R. P. Smith, M. P. M. Dean, M. Itoh, J. F. Scott, G. G. Lonzarich, and S. S. Saxena, Nat. Phys. 10, 367-372 (2014). 3. A. S. Alexandrov, Phys. Rev. B 46, 14932 (1992). 4. J. M. Edge, Y. Kedem, U. Aschauer, N. A. Spaldin, and A.V. Balatsky, Phys. Rev. Lett. 115, 247002 (2016). 5. D. van der Marel, J.L.M. van Mechelen, and I.I. Mazin, Phys. Rev. B 84, 25111 (2011). 6. A. Stucky, G. Scheerer, Z. Ren, D. Jaccard, J.-M. Poumirol, C. Barreteau, E. Giannini, and D. van der Marel; Scientific Reports 6, 37582 (2016).

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Pressure-driven high-Tc superconductivity in carbon-based and inorganic materials Yoshihiro Kubozono1, Saki Nishiyama1, Xiao Miao1, Takahiro Terao1, Xiao Fang Yang1, Hidenori Fujita2, Masatoshi Hoshi2, Hidenori Goto1, Takafumi Miyazaki3, Tomoko Kagayama 2, Katsuya Shimizu2, Hitoshi Yamaoka4, Hirofumi Ishii5, Yen-Fa Liao5 1 Research Institute for Interdisiplinary Science, Okayama Univertsity, Okayama 700-8530, Japan; 2 Center for Science and Technology under Extreme Conditions, Osaka University, Osaka 560-8531, Japan; 3 Research Laboratory for Surface Science, Okayama University, Okayama 700-8530, Japan; 4 Riken Harima Branch, Hyogo 679-5198, Japan; 5 National Synchrotron Radiation Research Center, Hsinchu 30076, Taiwan Email: [email protected] Keywords: pressure, high-Tc, superconductivity, carbon-based materials, transition metal dichalcogenides, Fe-based superconductors, topological materials Pressure dependence of superconductivity in diverse two-dimensional (2D) materials, is investigated in a wide pressure range from their temperature dependence of magnetic susceptibility (M / H) and resistance (R). All superconducting materials were prepared by doping of 2D layered materials (graphite, MoSe2, FeSe1-zTez and Bi2Se3). The preparation of superconductors was achieved using a liquid metal alloy for metalintercalated graphite (graphite intercalation compound (GIC)), and a liquid NH3 (or ND3) and liquid organic-solvents for other inorganic compounds. Superconducting transition temperature, Tc, of Ca0.5(2)Sr0.5(2)C6, which takes a SrC6-type structure (hexagonal, space group No. 194, P63/mmc), was 3.2 K at ambient pressure (0 GPa). The Tc increased rapidly with increasing pressure up to 8.3 GPa; the maximum Tc was 5.4 K. With further increasing pressure, the Tc suddenly dropped. This behavior is similar to those of CaC6 and Ca0.6K0.4C8 (KC8-type structure) [1,2]. The increase in Tc against pressure for CaC6 was assigned to the softening of Ca - Ca phonon (hardening of Ca – C phonon), and the decrease in Tc attributed to the order-disorder transition originating from random off-center displacement of Ca atoms in the plane which accompanies the lattice softening. The same transition may take place in Ca0.5(2)Sr0.5(2)C6. In Ca0.5(2)Sr0.5(2)C6 (SrC6-type structure), the lattice shrank monotonically up to 20 GPa, but 112 Bragg peak disappeared at around 10 GPa. Here, it should be noticed that the graphene-graphene distance, dGG (= c / 2), of Ca0.5(2)Sr0.5(2)C6 at around 10 GPa is close to that (dGG = c / 3) of CaC6 (CaC6-type structure). In the case of Ca0.6K0.4C8 (KC8-type structure), the graphite – non-graphite transition emerged

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Superstripes 2017, Ischia June 4-10, 2017 when the dGG (= c /4) was close to that of CaC6 [2]. Therefore, the disappearance of 112 Bragg peak may be associated with graphite – non-graphite transition. To sum up, the GIC superconductors show an exciting positive pressure-dependence of Tc. Furthermore, we succeeded in preparation of metal-doped FeSe1-zTez and MoSe2 using NH3 (ND3) and organic solvents such as ethylenediamine (EDA). The metaldoped FeSe1-zTez compounds showed a pressure-driven high-Tc superconducting phase, Tc of which reached ~50 K. The (NH3)yNaxMoSe2 compound also provided two different superconducting phases above / below 2 GPa. In this conference, we will show new metal-doped 2D layered superconductors and their exciting pressure dependence of superconductivity. References 1. A. Gauzzi et al. Phys. Rev. Lett. 98, 067002 (2007). 2. T.L.H Nguyen et al. Carbon 100, 641-646 (2016).

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Non-equilibrium Surface Dirac Fermion Dynamics of Topological Insulators Probed By Time Resolved ARPES Akio Kimura Graduate School of Science, Hiroshima University, 1-3-1 Kagamiyama, Higashi-hiroshima 739-8526, Japan

Email: [email protected] Keywords: Topological insulators; Nonequilibrium surface Dirac fermion dynamics

Topological insulators (TIs) have attracted a great deal of attention in spin-electronics field. Gapless and massless edge or surface states appear where the topological number changes, while their bulk is insulating. One of the most remarkable properties of the topological surface states (TSS) are their helical spin textures in momentum space where the backscattering is forbidden. It would lead to a dissipationless spin transport on the surface of TIs, which is very promising for spintronic applications. Here, the surface Dirac fermion dynamics in the “carrier tuned” topological insulator (Sb1xBix)2Te3 have been explored by time- and angle- resolved photoelectron spectroscopy implementing pump-probe method that enables us to observe not only occupied/unoccupied electronic states but also carrier dynamics. The experiment was carried out with linearly polarized pump (hν=1.48 eV) and probe (5.92 eV) pulses generated by Ti:sapphire laser system operating at a repetition rate of 250 kHz [1].

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Pump 1.5 eV

Probe 5.9 eV

4 ∝s

(a)

n -type

Γ Γ

-1.33 ps

(b)

n -type

(c)

+1.33 ps n -type

Γ Γ

Figure 1: Time resolved ARPES images recorded without pump (a), and with pump ∆t=1.33ps (=∼4µs) (b) and ∆t =1.33ps(c). Pump-probe configurations are also illustrated.

A prolonged duration of the excited surface carriers has been observed for the sample with its Fermi energy tuned at the Dirac point of the TSS [2]. We have also observed a downward (an upward) surface photovoltage shifts in n-type (p-type) sample of Bi2Te3 [Figure 1]. This finding paves a pathway to ambipolar optical control of spin current generation on the surface of TIs in the next generation opto-spinelectronic devices [3].

References 1. Y. Ishida et al., Rev. Sci. Instrum. 85, 123904 (2014). 2. K. Sumida et al., submitted. 3. T. Yoshikawa et al., submitted.

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Charge dynamics near the onset of charge-density-wave order in La214 cuprates Dragana Popović* National High Magnetic Field Laboratory, Florida State University, 1800 E. Paul Dirac Drive, Tallahassee, FL 32310, USA

Email: [email protected] Keywords: cuprates; stripes; charge dynamics; phase transitions. The dynamics of charge-ordered states is one of the central unresolved issues in underdoped cuprate high-temperature superconductors. Static short-range chargedensity-wave (CDW) domains have been detected in almost all cuprates. The key questions that arise are: (1) Do the observed static domain structures correspond to a ground state or a long-lived metastable state? (2) What is the role of disorder? We introduce a new method for detecting and probing domain dynamics in cuprates. We focus first on the regime across the CDW (and structural) transition in La1.48Nd0.4Sr0.12CuO4. By employing two different nonequilibrium protocols, a response to a change in temperature T and magnetic field H, we find evidence for metastable states in the CDW-ordered phase, including slow, nonexponential relaxations of resistance R, hysteresis and memory in the magnetoresistance (MR) (Fig. 1), and avalanches with a power-law distribution of sizes.[1] A picture emerges of interacting domains that, although strongly pinned by disorder, are not static, but

Figure 1: Negative out-of-plane magnetoresistance in La1.48Nd0.4Sr0.12CuO4 just below the onset of CDW order.[1] The arrows and numbers show the direction and the order of field sweeps. Inset: subloops shifted vertically for comparison.

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Superstripes 2017, Ischia June 4-10, 2017 trapped in long-lived metastable states. Surprisingly, nonequilibrium effects are revealed only when the transition is approached from the charge-ordered phase. In order to clarify precisely the interplay of charge and lattice degrees of freedom, we have performed similar measurements on La1.7Eu0.2Sr0.1CuO4, in which CDW ordering at T=TCO does not coincide with the structural transition at T=TLTT. We find that, for H||c axis, both materials exhibit a similar onset of negative magnetoresistance near TCO. In contrast, the MR is positive for all field orientations near TLTT in La1.7Eu0.2Sr0.1CuO4, indicating that H||c cannot drive the structural transition.[2] Therefore, these results show that our observations in La1.48Nd0.4Sr0.12CuO4 [1] are related to the onset of charge order, and not to the structural transition. Other similarities and differences in charge transport of the two materials will also be discussed. By unveiling the asymmetry of the transition to a CDW-ordered phase [1], our work points a way to detecting fluctuating CDWs in the cuprates using also other experimental techniques. In addition, nonequilibrium protocols in charge transport can be extended to other correlated-electron systems, such as iron pnictides, to probe charge domain dynamics. This work was partially supported by NSF grant No. DMR-1307075 and the NHMFL via NSF Cooperative Agreement DMR-1157490 and the State of Florida. References 1. P. G. Baity, T. Sasagawa, and D. Popović, (under review); arXiv:1609.02591v2 (2016). https://arxiv.org/abs/1609.02591 2. P. G. Baity, T. Sasagawa, and D. Popović (unpublished).

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Dimensional Crossover of Charge-Density Wave Correlations in the Cuprates Dror Orgad, Yosef Caplan The Hebrew University

Email: [email protected] Keywords: CDW correlations, underdoped cuprates, competing orders Short-range charge-density wave correlations are ubiquitous in underdoped cuprates. They are largely confined to the copper-oxygen planes and oscillate out of phase from one unit cell to the next in the c-direction. Recently, it was found that a considerably longer-range charge-density wave order develops in YBCO above a critical magnetic field. This order is more three-dimensional and is in-phase along the c-axis. Here, we show that such behavior is a consequence of the tension between the conflicting ordering tendencies induced by the disorder potential and the Coulomb interaction, where the magnetic field acts to tip the scales from the former to the latter. We base our conclusion on analytic large-N analysis and Monte-Carlo simulations of a nonlinear sigma model of competing superconducting and charge-density wave orders. Our results are in agreement with the observed phenomenology in the cuprates, and we discuss their implications to other members of this family, which have not been measured yet at high magnetic fields.

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Modifications induced by synchrotron radiation in Bi2Sr2CaCu2O8+delta and YBa2Cu3O7-x: a novel non-destructive patterning method Marco Truccato1, Valentina Bonino1, Lorenzo Mino1, Angelo Agostino2, Carmelo Prestipino3, Elisa Borfecchia2, and Carlo Lamberti2,4 1 Department of Physics, Interdepartmental Centre NIS, University of Torino, via P.Giuria 1,I-10125Torino, Italy 2 Department of Chemistry, Interdepartmental Centre NIS, University of Torino, via P.Giuria 7,I-10125 Torino, Italy 3 Institut Sciences Chimiques de Rennes, UMR-CNRS 6226, Campus de Beaulieu, Université de Rennes 1, 35042 Rennes Cedex, France 4 IRC “Smart Materials”, Southern Federal University, ul. Andreya Sladkova 178/24, 344090 ,Rostov-on-Don, Russia Email: [email protected] Keywords: synchrotron X-rays nano-beam, direct-write patterning, high-temperature superconductors, Bi-2212, intrinsic Josephson junctions Changes in the material properties induced by synchrotron nanobeams are typically neglected or considered as unwanted side-effects implied by sample characterization. At the same time, conventional X-rays nano-lithography is based on the traditional approach of photoresist impression and development, which suffers from the difficulty of producing masks with sufficient contrast at the nanoscale. On the other hand, in our direct-write approach we have successfully controlled material changes over selected areas and exploited them to produce Josephson devices out of high-Tc superconducting crystals without the use of any photoresist or etching process. Here we report on the doping change of Bi2Sr2CaCu2O8+δ (Bi-2212) obtained by means of synchrotron radiation [1] and on the fabrication of devices based on the instrinsic Josephson junction structure of Bi-2212 and YBa2Cu3O7-x (Y-123) that achieve spatial resolution in the nanometric domain [2]. We have used highly focused synchrotron radiation with energy of about 13 or 17 keV, with minimum feature size of 50 nm and online monitoring of the device resistance during irradiation, which enabled us to tune the desired material condition into a non-superconducting state. Although actual microscopic mechanisms have not been clarified yet and include both local heating and knock-on interstitial oxygen atoms by photoelectrons as possible processes, X-ray nano-diffraction patterns show that the crystal structure has not been remarkably perturbed in the irradiated regions in spite of their electrical changes.

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Superstripes 2017, Ischia June 4-10, 2017 These results prove that a conceptually new patterning method is actually possible for oxides, based on the local change of electrical properties, with remarkable potential in terms of heat dissipation, chemical contamination, miniaturization and high aspect ratio of the devices.

Figure 1: View of a Josephson device patterned by means of a synchrotron radiation nanobeam. Violet boxes delimit the regions acting like trenches in the device.

References 1. A. Pagliero et al, Nano Lett. 14, 1583-1589 (2014). 2. M. Truccato et al, Nano Lett. 16, 1669-1674 (2016).

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Local charges and charge order in the cuprates revealed by NMR Michael Jurkutat, Steven Reichardt, Jürgen Haase University of Leipzig, Faculty of Physics and Earth Sciences, Linnéstr. 5, 04103 Leipzig, Germany Email: [email protected] Keywords: superconducting cuprates, charge order, high pressure NMR, phase diagram Quantitative determination of the local charges in the ubiquitous CuO2 plane with NMR reveals unsuspected differences in terms of material chemistry between the various cuprate families. It turns out that the material specific charge distribution between Cu and O reflects - depending on the model - the charge transfer gap, the covalency of the bond, or orbital contributions to electronic bands, and has been found to set many electronic properties, e.g., the maximum Tc and superfluid density. Interestingly, substantial charge density variations within the CuO2 plane were found for most cuprate materials, although it has long been debated whether this reflects merely chemical inhomogeneity or actually interesting electronic features. Using high pressure NMR that only affects the electronic properties, but leaves the chemistry unchanged, optimally doped YBCO is found to exhibit charge order. The spectral changes observed for planar Cu and O NMR cannot be accounted for structurally, and combined with field and temperature dependent NMR linewidths, we can conclude on the universality of charge order in the YBCO family. This can likely even be extended to all cuprates if charge order is perturbed, differently aligned or pinned in some fashion by chemical inhomogeneities, magnetic field, or some other form of symmetry breaking in the CuO2 plane.

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Crystal Growth and Advanced Synthesis of Contemporary Functional and Superconducting Materials: From Simplicity to Complexity Nikolai D. Zhigadlo1, Mitra Iranmanesh1, John R. Kirtley2, Wilfried Assenmacher3, Werner Mader3, Jürg Hulliger 1 1 Department of Chemistry and Biochemistry, University of Bern, 3012 Bern, Switzerland 2 Geballe Laboratory for Advanced Materials, Stanford University, Palo Alto, California, 94305, USA 3 Institute of Inorganic Chemistry, University of Bonn, D-53117 Bonn, Germany Email: [email protected] or [email protected] Keywords: superconductors; helical magnets; van der Waals; intermetallics; oxides. In this talk we will provide new insights to the materials synthesis and characterization of contemporary functional and superconducting materials. Mainly, two different approaches such as the high-pressure, high-temperature method and ceramic combinatorial solid state chemistry will be presented with application to several typical compound classes. First, the high-pressure phase diagram of the Mg-B-N system will be explored. Here, we discovered the simultaneous growth of completely different types of crystals: a rare two-band superconductor MgB2 and a wide-band semiconductor hBN [1]. Besides interesting physics, both of these materials hold great potential for practical applications. Then, we further emphasize the beneficial role of the high-pressure, high-temperature conditions in exploring the crystal growth of various intermetallic superconductors, such as MgCNi3 [2], Mo3Al2C, SrPt3P [3], pyrochlores, helical magnets MnP [4], CrAs [5], magnetocaloric MnAs compound, and 2D van der Waals semiconductors (hBN, black P, P1-xAsx). The underlying correlations and the general trends between composition, structure, magnetism and superconductivity in these materials will be discussed. Successively, we will highlight the key role of extreme conditions in the growth of Fe-based superconductors, where a careful control of the composition-structural relations is vital for understanding of the physical behavior [6]. The availability of sizable, high-quality LnFeAsO (Ln: lanthanide) single crystals with substitution of O by F or H, Sm by Th, Fe by Co, and As by P, allowed us to measure intrinsic and direction-dependent superconducting properties [6-12]. Finally, we will demonstrate that combinatorial ceramic solid state chemistry is an efficient way to search for new superconducting compounds, while the problem of determining which compositions are strongly diamagnetic in a mixed-phase sample is challenging. A single sample synthesis concept based on multi-element ceramic samples can produce a variety of local products. When applied to cuprate superconductors (SC), statistical modeling predicts the occurrence of possible compounds in a concentration in the order of 50 ppm. A sample with such a low

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Superstripes 2017, Ischia June 4-10, 2017 concentration needs local probe analytical or separation techniques to identify compounds of interest. We will report the results obtained from random mixtures of Ca, Sr, Ba, La, Zr, Pb and Cu oxides reacted at different conditions under ambient pressure [13]. The bulk state displays superconductivity up to about 125 K. By magnetic separation technique many SC grains in the range of 50 to 300 µm were collected for further analysis. Scanning SQUID microscopy applied to single grains has detected local µm sized areas of SC up to 115 K. Transmission electron microscopy applied to some grains shows evidence for the formation of Sr-Ca-Ba-Cu based phases, containing a small amount of La and Pb. According to the literature, in this phase system known superconducting phases were obtained so far only at high pressure. Here, we report a new synthetic approach for either known or unknown compounds attaining a high Tc without containing Bi, Tl, Hg or the need for a high pressure synthesis [13]. References 1. N. D. Zhigadlo, J. Cryst. Growth 402, 308 (2014). https://dx.doi.org/10.1016/j.jcrysgro.2014.06.038. 2. R. T. Gordon, N. D. Zhigadlo, S. Weyeneth, S. Katrych, R. Prozorov, Phys. Rev. B 87, 094520 (2013). https://doi.org/10.1103/PhysRevB.87.094520. 3. N. D. Zhigadlo, J. Cryst. Growth 455, 94 (2016). https://dx.doi.org/10.1016/j.jcrysgro.2016.10.003 4. R. Khasanov, et al., Phys. Rev. B 93, 180509 (2016) https://doi.org/10.1103/PhysRevB.93.180509 5. R. Khasanov, et al., Sci. Rep. 5, 13788 82015). https://doi:10.1038/srep13788 6. N. D. Zhigadlo, et al., Phys. Rev. B 86, 214509 (2012). https://doi.org/10.1103/PhysRevB.86.214509 7. N. D. Zhigadlo, et al., Phys. Rev. B 84, 134526 (2011). https://doi.org/10.1103/PhysRevB.84.134526 8. L. Fang, et al., Nature Communications 4, 2655 (2013). https://doi:10.1038/ncomms3655 9. T. Mertelj, et al., Phys. Rev. B 87, 174525 (2013). https://doi.org/10.1103/PhysRevB.87.174525 10. P. J. W. Moll, et al., Phys. Rev. Lett. 113, 186402 (2014). https://doi.org/10.1103/PhysRevLett.113.186402 11. A. Charnukha, et al., Sci. Rep. 5, 10392 (2015); ibid. 5, 18273 (2015). https://doi:10.1038/srep10392 ; https://doi:10.1038/srep18273 12. S. V. Borisenko, et al., Nature Physics 12, 311 (2016). https://doi:10.1038/nphys3594 13. N. D. Zhigadlo, et al., J. Supercond. Nov. Magn. 30, 79 (2017). https://doi:10.1007/s10948-016-3800-z

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Rapid Synthesis of Superconducting and Magnetic oxides by Light Irradiation A. Shengelaya1, D. Daraselia1, D. Japaridze1, Z. Jibuti1, K. A. Müller2 1 Department of Physics, Tbilisi State University, GE-0179, Tbilisi, Georgia 2 Physik-Institut der Universität Zürich, CH-8057, Zürich, Switzerland

Email: [email protected] Keywords: oxide materials, light effects Most of the technologically important oxide materials are usually obtained through solid state reaction. This implies long-term (for tens of hours) heating of reactants in powder form at high temperatures in furnace, which is a highly time and energy consuming process. Moreover, the long-term high temperature synthesis may result in deviation from stoichiometry and other unwanted side effects. Therefore there is a significant worldwide effort to develop technologies to considerably reduce the solid state reaction temperature and time. We report a novel kind of synthesis of oxide materials, which involves the irradiation of the mixture of starting oxides by light in a broad spectral range from infrared to ultraviolet with intensity sufficient for starting the solid state reaction between the reagents contained in the powder mixture [1, 2]. We synthesized different superconducting and magnetic oxides using this method. It was demonstrated that light irradiation leads to a dramatic increase of the solid state reaction speed and a lowering of the reaction temperature. The rate of the resulting reaction exceeds the conventional thermal solid state reaction rate in furnace by about two-three orders of magnitude. The photostimulated solid-state reaction (PSSR) method demonstrated is quite general and opens up the possibility of fast synthesis of a wide range of technologically important bulk and thin-film oxide materials. References 1. D. Daraselia, D. Japaridze, Z. Jibuti, A. Shengelaya and K. A. Müller, Journal of Superconductivity and Novel Magnetism 26, 2987 (2013). 2. D. Daraselia, D. Japaridze, Z. Jibuti, A. Shengelaya, and K. A. Müller, “Rapid solid-state reaction of oxides with ultraviolet radiation”, European Patent Application PCT/EP2013/050664.

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A complete and accurate description of superconductivity of AlB2 – type structures from phonon dispersion calculations Jose A Alarco, Peter C Talbot and Ian D. R. Mackinnon Institute for Future Environments, and Science and Engineering Faculty, Queensland University of Technology

Email: [email protected] Keywords: phonon dispersions; anomalies; thermal energy; isotope; substitution, pressure and temperature effects A predictive tool for the design of new, higher temperature superconductors requires a simple, first-principles technique, based on well-established axioms as embodied in Density Functional Theory without post-calculation corrections or increased levels of complexity. We show that anomalies in the calculated phonon dispersions of compounds with AlB2-type structures are good descriptors of their superconducting transition temperatures. This ab initio methodology has proven robust for descriptions of: the superconductivity of MgB2 and other related AlB2-type structures [1], the different isotopic forms of MgB2 [2], a series of substituted MgB2 compositions with Al [3] and transition metals [4], the pressure dependence of the superconductivity of MgB2 [5] and various temperature effects. Calculated values are well correlated with experimentally determined values within experimental errors, when sufficiently fine k-grids are used to resolve Fermi surface details [1-5]. This methodology provides invaluable insight on the mechanisms of superconductivity can be extended to other crystal systems and has been used to predict superconducting Tc for new compounds [3, 4].

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Figure 1: Comparison of calculated and experimentally measured superconducting transition temperatures (Tc) for Al substituted MgB2 compositions (left) and MgB2 under hydrostatic pressure (right).

References 1. J. A. Alarco, A. Chou, P. C. Talbot and I. D. R. Mackinnon, “Phonon modes of MgB2: super-lattice structures and spectral response”, Physical Chemistry Chemical Physics 16 (2014) 24443-24456. 2. J. A. Alarco, P. C. Talbot and I. D. R. Mackinnon, “Coherent phonon decay and the boron isotope effect for MgB2”, Physical Chemistry Chemical Physics 16 (2014) 25386-25392. 3. J. A. Alarco, P. C. Talbot and I. D. R. Mackinnon, “Phonon anomalies predict superconducting Tc for AlB2 type structures”, Physical Chemistry Chemical Physics 17 (2015) 25090-25099. 4. I. D. R. Mackinnon, P. C. Talbot and J. A. Alarco “Phonon dispersion anomalies and superconductivity in metal substituted MgB2” Computational Material Science 130 (2017) 191-203. 5. J. A. Alarco, P. C. Talbot and I. D. R. Mackinnon, “Phonon dispersion models for MgB2 with application of pressure”, submittted to Physica C, 2017.

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Atomic structure of supported ultrathin Germania films A. Lewandowski, P. Schlexer, K. Burson, C. Büchner, W.-D. Schneider, M. Heyde, G. Pacchioni, H.-J. Freund Fritz-Haber-Institute of the MPG, Berlin, Germany, Dipartimento di Scienza dei Materiali, Uni di Milano-Bicocca

Email: [email protected] Keywords: oxides, 2D-ultrathin films, 2D network structures, glass formers, The high refractive index of germanium dioxide makes it an important material for optical fibers, lenses and windows [1]. Such applications benefit from well-defined surface structures, which illustrates the need for a fundamental understanding of germania surfaces. However, to our knowledge, no surface structure for GeO2 has been experimentally established up to now. This situation was the motivation for the present study. GeO2 thin films were grown epitaxially on Ru(0001) by physical vapor deposition and subsequent annealing in oxygen. They were characterized by low energy electron diffraction and scanning tunneling microscopy. Layer-by-layer growth of atomically flat films is observed and the coverage tuned by varying the deposition time. Atomic resolution images reveal a hexagonal lattice with a variety of line defects. Different imaging contrasts show either the oxygen or the germanium atoms in the network. Two layers of corner-sharing GeO4 tetrahedral building blocks are connected by oxygen bridges to form a bilayer structure. Three in-plane oxygen bridges and one out-of-plane bridge per tetrahedron result in a chemically saturated flat sheet that interacts weakly with the substrate. Density functional theory proposes a low-energy structure in agreement with experimentally observed atomic arrangements. Moreover, a theoretical investigation of different building blocks sheds light on the most common defects found in germania films. These defect structures, similar to observations for SiO2 films [2-4], are analyzed and discussed as possible precursors for a glassy GeO2 phase. References 1. F. Mitschke, Fiber Optics, Springer Berlin Heidelberg, Berlin, Heidelberg, 2010. 2. L. Lichtenstein, M. Heyde, H.-J. Freund, Atomic Arrangement in Two-Dimensional Silica: From Crystalline to Vitreous Structures, J. Phys. Chem. C. 116 (2012) 20426. 3. L. Lichtenstein, C. Büchner, B. Yang, S. Shaikhutdinov, M. Heyde, M. Sierka, et al., The Atomic Structure of a Metal-Supported Vitreous Thin Silica Film, Angew. Chemie, Int. Ed. 51 (2012) 404. 4. L. Lichtenstein, M. Heyde, H.-J. Freund, Crystalline-Vitreous Interface in Two Dimensional Silica, Phys. Rev. Lett. 109 (2012) 106101.

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Nanoscale Characterization of the Thermal Conductivity of Supported Graphite Nanoplates, Graphene and Few-layer Graphene Mauro Tortello1, Samuele Colonna2, Julio Gomez3, Iwona Pasternak4, Wlodek Strupinski4, Fabrizio Giorgis1, Guido Saracco1, Renato S. Gonnelli1, Alberto Fina2 1. Dipartimento di Scienza Applicata e Tecnologia, Politecnico di Torino, 10129 Torino, Italy. 2. Dipartimento di Scienza Applicata e Tecnologia, Politecnico di Torino, 15121 Alessandria, Italy. 3. AVANZARE Innovacion Tecnologica S.L., 26370 Navarrete, Spain. 4. Institute of Electronic Materials Technology, 01-919 Warsaw, Poland. Email: [email protected] Keywords: Graphene, Scanning Thermal Microscopy, SThM Scanning Thermal Microscopy (SThM) [1,2] is a powerful technique for the thermal characterization and can reach a spatial resolution on the order of a few tens of nanometers while recording nanoscale topography at the same time. Here we show that [3] i) annealing in vacuum at 1700 °C for 1 h strongly reduces the amount of defects in reduced graphite oxide (RGO) nanoplates, as shown by Raman, XRD, XPS and TGA measurements. ii) As a consequence, their thermal conductivity considerably increases, as revealed by high-resolution SThM results on individual RGO flakes supported by SiO2/Si. iii) This fact is more clearly observed when the RGO nanoplates are supported by a less conducting substrate (PET). iv) Lumped parameter models and finite element analysis are discussed in order to interpret the results and try to determine the thermal conductivity and the effect of the substrate. Moreover, SThM results are also presented for a case study of multilayer (1 to 4) CVD graphene [4] supported by different substrates, i.e. SiO2/Si, PET, Al2O3. We found that a) the thermal conduction of multilayer graphene supported by SiO2/Si improves with increasing number of layers. b) In the SThM maps, the thermal contrast observed between the supported graphene and the bare substrate changes depending on the thermal conductivity of the substrate itself.

References 1. A. Majumdar, Annu. Rev. Mater. Sci. 29, 505 (1999). 2. S. Gomes, A. Assy, P.-O. Chapuis, Phys. Status Solidi A 212, 477 (2015). 3. M. Tortello et al., Carbon 109, 390 (2016). 4. I. Pasternak et al., AIP Advances 4, 097133 (2014).

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Material design strategies for BiCh2-based layered superconductors Yoshikazu Mizuguchi Tokyo Metropolitan University Email: [email protected] Keywords: new superconductor; BiCh2-based layered superconductor; local structure In 2012, we have discovered new layered superconductors with BiS2-type conducting layers, such as Bi4O4S3 [1] and REO1-xFxBiS2 [2]. Since the layered crystal structure resembles to those of cuprates and Fe-based superconductors, many researchers have explored new BiS2-based superconductors and the possibility of higher transition temperature (Tc) and unconventional nature of the superconductivity mechanisms in the system. To address the mechanisms of BiS2-based superconductivity, we have focused on the relationship between the crystal structure and superconducting properties. We revealed that, in the REOBiCh2 system (RE: rare earth; Ch: calcogen), the emergence of bulk superconductivity in BiS2-based compounds correlates with the in-plane chemical pressure, which can be manipulated by systematic isovalent substitutions at the blocking and/or conducting layers [3,4]. In addition, the importance of the suppression of in-plane local disorder has been proposed in the BiS2-xSex layers [5]. In this presentation, I will review the crystal structure and physical properties of the BiCh2-based compounds. Then, the material design strategies will be discussed on the basis of the latest results on local structure analysis and revealed intrinsic phase diagram. References 1. Y. Mizuguchi et al., Phys. Rev. B 86, 220510(1-5) (2012). 2. Y. Mizuguchi et al., J. Phys. Soc. Jpn. 81, 114725(1-5) (2012). 3. Y. Mizuguchi et al., Sci. Rep. 5, 14968(1-8) (2015). 4. Y. Mizuguchi et al., Phys. Chem. Chem. Phys. 17, 22090-22096 (2015). 5. K. Nagasaka, Y. Mizuguchi et al., arXiv: 1701.07575.

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Enhanced thermoelectric compound LaOBiS2-xSex

performance

in BiS2-based layered

Yosuke Goto1, Atsuhiro Nishida1, Osuke Miura1, Chul-Ho Lee2, Yoshikazu Mizuguchi1 1

Tokyo Metropolitan University, 1-1, Minami-osawa, Hachioji, Japan. National Institute of Advanced Industrial Science and Technology (AIST), 1-1-1, Umezono, Tsukuba, Ibaraki 305-8568 Japan

2

Email: [email protected] Keywords: new thermoelectric material; layered compound; BiS2-based compound

We have studied the thermoelectric properties of LaOBiS2-based compounds, which have drawn much attention as a new layered superconductor [1], because the layered structure and relatively narrow band gap are preferable for thermoelectric materials. With an aim to enhance dimensionless figure-of-merit (ZT), we have demonstrated systematic substitutions of S by Se, which should result in chemical pressure effects. We have investigated crystal structure and thermoelectric properties (electrical resistivity, Seebeck coefficient, and thermal conductivity) of LaOBiS 2-xSex [2-4]. Crystal structure analysis clarified that the doped Se selectively occupy the in-plane site. Then, with increasing Se concentration, the packing density of ions at the conducting plane is enhanced, which can be regarded as a positive in-plane chemical pressure effect [3]. With increasing in-plane chemical pressure, electrical resistivity largely decreased, while the absolute Seebeck coefficient exhibited large value [4]. From Hall measurements, we revealed that the low electrical conductivity was induced by the large enhancement of carrier mobility [4]. As a result, high ZT of 0.36 (at 650 K) was observed for x = 1 (LaOBiSSe) [5]. In this presentation, we discuss the origins of the enhancement of ZT in LaOBiS2-xSex in detail, and the recent advances on the exploration of new thermoelectric materials with BiS2-related structure.

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Figure 1: (a) Schematic image of the crystal structure of LaOBiS2. (b) Temperature dependences of dimensionless figure-of-merit ZT for LaOBiS2-xSex.

References 1. Y. Mizuguchi, J. Phys. Chem. Solids, 84, 34-48 (2015). 2. Y. Mizuguchi et al., J. Appl. Phys. 116, 163915 (2014). 3. Y. Mizuguchi, A. Nishida et al., J. Appl. Phys. 119, 155103 (2016). 4. A. Nishida et al., J. Phys. Soc. Jpn. 85, 074702 (2016). 5. A. Nishida et al., Appl. Phys. Express 8, 111801 (2015).

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Ag/F vs. Cu/O: powerful analogy with far-reaching implications Wojciech Grochala1*, Zoran Mazej2 1 Centre of New Technologies, University of Warsaw, Zwirki i Wigury 93, 02089 Warsaw, Poland 2 Department of Inorganic Chemistry and Technology, Jožef Stefan Institute, Jamova 39, SI-1000 Ljubljana, Slovenia.

Email: [email protected] Keywords: electronic structure; hybridization; magnetic properties; superconductivity The ongoing quest for quantum systems showing high temperature superconductivity and/or strong magnetic interactions seems nowadays to be largely serendipitous. Regretfully, physical theories describing these complex quantum phenomena do not offer a basis for steady rational material design or improvement of desired observables, since they introduce parameters which are difficult to be translated to language of synthetic chemists. Here I will briefly describe the 17-year lasting attempts to design a novel family of high-TC superconductors using chemical intuition and the ramifications of the Molecular Orbital theory (Figure 1) translated from molecules to solids [1,2].

Figure 1: Ideographic comparison of Ag/F vs. Cu/O analogy within the framework of spinunpolarized MO theory applied for solids.

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The materials in focus are fluoride (F–) connections of Ag(2+) which are isoelectronic to undoped Cu(2+) oxides (i.e. both cation and anion have the same valence electron count). [3] Both families show many striking similarities, including but not limited to: structural features,[4] pronounced hybridization of metal/nonmetal valence states,[5] similar character of electronic bands, comparable energy scale for phonons,[2] as well as ultra-strong magnetic superexchange, [6] – and all that despite the seemingly “ionic” character of chemical bonding in metal fluorides. The recent high-level density functional theory calculations show that the magnetic superexchange constant, J, may reach over 300 meV in certain one-dimensional silver(2+) fluorides at ambient (p,T) conditions, thus exceeding the corresponding record value of ~250 meV for analogous copper(2+) oxides.[7] The most recent experimental confirmation of giant superexchange constant will be presented. References 1. W. Grochala, R. Hoffmann, J. Phys. Chem. A 104, 9740 (2000). DOI: 10.1021/jp0017745 2. W. Grochala, R. Hoffmann, Angew. Chem. Int. Ed. 40, 2742 (2001). DOI: 10.1002/1521-3773(20010803)40:153.0.CO;2-X 3. W. Grochala, Z. Mazej, Phil. Trans. A 373, 20140179 (2015). DOI: 10.1098/rsta.2014.0179 4. W. Grochala, Nature Mater. 5, 513 (2006). DOI: 10.1038/nmat1678 5. W. Grochala, R. G. Egdell, P. P. Edwards, Z. Mazej, B. Žemva, ChemPhysChem 4, 997 (2003). DOI: 10.1002/cphc.200300777 6. T. Jaroń, W. Grochala, Phys. Stat. Sol. RRL 2, 71 (2008). DOI: 10.1002/pssr.200701286 7. D. Kurzydłowski, W. Grochala, Angew. Chem. Int. Ed., submitted (2017).

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Exciton-phonon complexes and giant exciton Fano resonances in Ta2NiSe5 T. I. Larkin 1, A. N. Yaresko 1, D. Pröpper 1, K. A. Kikoin 2, Y.-F. Lu 3, T. Takayama 1, Y.-L. Mathis 4, A. W. Rost 1,5, H. Takagi 1,3,5, B. Keimer 1 & A. V. Boris 1 1 Max Planck Institute for Solid State Research, Heisenbergstraße 1, 70569 Stuttgart, Germany 2 School of Physics and Astronomy, Tel Aviv University, 69978 Tel Aviv, Israel 3 Department of Physics, The University of Tokyo, Hongo, Tokyo 113-0033, Japan 4 Synchrotron Facility ANKA, Karlsruhe Institute of Technology, 76344 Eggenstein-Leopoldshafen, Germany 5 Institute for Functional Materials and Quantum Technology, University of Stuttgart, Pfaffenwaldring 57, 70550 Stuttgart, Germany Email: [email protected] Keywords: excitonic-insulator, exciton-phonon-complexes, fano-resonances The excitonic insulator (EI) is a long-conjectured electronic state of matter that may arise in narrow-gap semiconductors or small-overlap semimetals. Though several EI candidates have been studied by means of transport measurements or photoemission spectroscopy, unambiguous evidence of the EI state is so far lacking. Furthermore, excitons have not yet been directly observed in any EI candidate. In this study we employ spectroscopic ellipsometry to detect doublets of Fano resonances in the EI candidate Ta2NiSe5 [1] and the related Ta2NiS5. We develop a Green function based approach to model a coupling between discrete excitations of an excitonic nature and a broad continuum of single-electron excitations, enabling us to extract the parameters of the Fano resonances. The spectral weight of the excitonic Fano resonances is found to be at a very large value of ~1 e-/u.c. in Ta2NiSe5 and a more modest, but still appreciable 0.1 e-/u.c. in Ta2NiS5. We interpret these large spectral weights in the framework of Rashba's theory for exciton-phonon complexes [2]. This leads us to conclude that we are observing spatially extended complexes of excitons weakly bound to the self-induced lattice distortions. The larger spectral weight of the excitonic resonances in Ta2NiSe5 suggests a weaker binding with larger spatial extent of the complex and corroborates the EI scenario in this material.

References 1. Phys. Rev. Lett. 103, 026402 (2009). 2. Soviet Physics Semiconductors 8, 807–816 (1975).

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Pressure induced superconductivity in Cr- and Mn- based materials Jianlin Luo Institute of Physics, Chinese Academy of Sciences

Email: [email protected] Keywords: CrAs, MnP, new superconductors Transition-metal pnictides, CrAs and MnP, have been studied over fifty years ago due to the presence of interesting magnetic properties: CrAs forms a double-helical magnetic structure below 270 K accompanied with a strong first-order structural transition, while MnP first undergoes a ferromagnetic transition at 290 K and then adopts a similar double-helical order below 50 K. Both compounds are correlated metals and exhibit distinct anomalies at these characteristic magnetic transitions. By application of high pressure, we recently observed superconductivity with a maximum superconducting transition temperature of Tc ~ 2 K and 1 K when their helimagnetic orders are suppressed under a critical pressure of Pc ~ 0.8 and 8 GPa for CrAs and MnP, respectively. Despite of a relatively low Tc, CrAs and MnP are respectively the first superconductor among the Cr- and Mn-based compounds in that the electronic density of states at Fermi energy are dominated by the Cr/Mn-3d electrons. In this talk, I will summarize the current progresses achieved about the superconductivity in CrAs and MnP. Work done in collaboration with Wei Wu, Jinguang Cheng, F. K. Lin and K. Matsubayashi, Y. Uwatoko. References 1. W. Wu, J.-G. Cheng, K. Matsubayashi, P. P. Kong, F. K. Lin, C. Q. Jin, N. L. Wang, Y. Uwatoko, and J. L. Luo, Nat. Comm. 5, 5508 (2014). 2. J.-G. Cheng, K. Matsubayashi, W. Wu, J. P. Sun, F. K. Lin, J. L. Luo, and Y. Uwatoko, Phys. Rev. Lett. 114, 117001 (2015).

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Crystal structure and epitaxy of topological insulator films grown on Si and SrTiO3 Yeong-Ah Soh1 Hanover, NH, USA

1

Email: * [email protected] Key words: topological insulator, crystal structure, epitaxy, capping. New quantum materials called topological insulators have attracted considerable attention due to fascinating properties such as almost dissipationless surface transport and potential for “spintronic” applications. For electronic device research and applications it is required to fabricate low-dimensional nanostructures. Therefore, growth of high quality thin films of topological insulators is essential and has been attempted using various methods. The surfaces of topological insulators are prone to oxidation and environmental doping by exposure to air resulting in degradation of the surface characteristics. One method used widely for the protection of the topological insulator surface is the deposition of a capping layer. Since not only the substrate, but also the capping layer determines the electrical properties of the films it is important to characterize the capping layer as well. It has been, in particular, reported that the capping layer hinders the occurrence of sensitive effects such as the quantum anomalous Hall effect [1]. In this talk, I will present comprehensive x-ray diffraction studies of the crystal structure and epitaxy of thin films of the topological insulator Bi2Te3 grown on Si (1 1 1) [2] and the ferromagnetic topological insulator Crx(BiySb1-y)2-xTe3 grown on SrTiO3 (1 1 1) with and without a Te capping layer [3]. Our studies show that the films are single crystals with the crystal quality being substantially higher for the films grown on Si substrates than those grown on SrTiO3 substrates even though the most promising electrical transport data have been found for SrTiO3 substrates. Furthermore, we found that the Te capping layer grows epitaxially and the deposition of the capping layer does not degrade the crystallinity of the Crx(BiySb1-y)2-xTe3 thin film even though it hinders the observation of the anomalous quantum Hall effect.

References 1. “Experimental observation of the quantum anomalous Hall effect in a magnetic topological insulator”, Chang, C.-Z. et al., Science 340, 167-170 (2013).

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Superstripes 2017, Ischia June 4-10, 2017 2. “Crystal structure and epitaxy of Bi2Te3 films grown on Si”, Jihwey Park, Yeong-Ah Soh, G. Aeppli, S. R. Bland, Xie-Gang Zhu, Xi Chen, Qi-Kun Xue, and Francois Grey, Applied Physics Letters 101, 221910 (2012) http://dx.doi.org/10.1063/1.4768259 3. “Crystallinity of tellurium capping and epitaxy of ferromagnetic topological insulator films on SrTiO3”, Jihwey Park, Yeong-Ah Soh, Gabriel Aeppli, Xiao Feng, Yunbo Ou, Ke He, Qi-Kun Xue, Scientific Reports 5,11595 (2015). http://dx.doi.org/10.1038/srep11595

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Elastic and Electronic Tuning of Magnetoresistance in MoTe2 Despina Louca1,*, J. Yang1, J. Liu1, G.-W. Chern1 1 University of Virginia, Department of Physics, 382 McCormick Road, Charlottesville, VA 22904, USA.

Email: [email protected] Keywords: tensile strain, electron cyclotron mass, anisotropy. The transition metal dichalcogenide (TMD) semiconductors exhibit many versatile physical features and have become a new paradigm for optoelectronic applications [111], based on exfoliated single layer molecular structures [3-6]. Fueled by intense interest on new device concepts, TMDs provide a platform from which optoelectronic properties such as spin-valley coupled physics and two-dimensional valley excitons can be pursued [12]. They have a desirable optical band gap in the 1-2 eV energy range, important for visible and near-infrared technologies. Manipulation of the band gap either by reducing the sample thickness down to a monolayer or applications of strain can lead to distinct changes in their physical characteristics. The observation of an extremely large magnetoresistance (MR) in the layered TMDs of WTe₂ and MoTe₂ has led to a surge of interest in this field [4]. The layered crystal structure consists of strong in-plane covalent bonding and weaker van der Waals type interactions between planes [13]. MoTe₂ in particular [14] can exist in several crystal configurations that includes the high temperature 2H, the intermediate temperature 1T' and the low temperature Td structures. β-MoTe₂ (1T') is metastable at room temperature and is metallic with a monoclinic (P2₁/m) structure [13-15]. Upon cooling from room temperature, an anomaly appears in the transport data around 240 K that has been linked to a first order structural transition from the 1T' of β-MoTe₂ to the orthorhombic Td phase (Pmn2₁). The Td phase exhibits the extreme MR effect and the host of a Weyl semimetal state, a new state of matter in which collective excitations known as Weyl fermions may exist. It has been suggested that the band structure of MoTe₂ is highly sensitive even to small changes in the lattice constants either brought upon by strain or as a function of temperature. The anomalously large MR observed under high magnetic fields in MoTe₂ can be reversibly controlled under tensile strain. The MR is enhanced by as much as ∼ 30 % at low temperatures and high magnetic fields, when uniaxial strain is applied along the a-crystallographic direction and reduced by the same amount when strain is applied along the b-direction. The large in-plane electronic anisotropy sets in at the transition from the 1T' monoclinic to the Td orthorhombic Weyl phase. Ab-initio calculations of

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Superstripes 2017, Ischia June 4-10, 2017 the electronic structure under strain show a comparable change in the electron cyclotron mass closely related to the MR under high magnetic field. The sensitivity of the cyclotron mass to tensile strain could have its origin to the presence of Weyl points between electron and hole pockets.

References 1. Q. H. Wang et al., Nat. Nanotechnol. 7, 699 (2012). 2. D. H. Keum et al., Nat. Phys. 11, 482 (2015). 3. M. Chhowalla et al., Nat. Chem. 5, 263 (2013). 4. B. Radisavljevic, A. Radenovic, J. Brivio, V. Giacometti, A. Kis, Nat. Nanotechnol. 6, 147 (2011). 5. M. N. Ali et al., Nature 514, 205 (2014). 6. S. Song et al., Nano Lett. 16, 188 (2016). 7. Y. Qi et al., Nat. Commun. 7, 11038 (2016). 8. X. Qian, J. Liu, L. Fu, J. Li, Science 80, 346 (2014). 9. M. Rifliková, R. Martoňák, E. Tosatti, Phys. Rev. B 90, 35108 (2014). 10. K.-A. N. Duerloo et al., Nat. Commun. 5, 10451 (2014). 11. H. Zeng, J. Dai, W. Yao, D. Xiao, X. Cui, Nat. Nanotechnol. 7, 490 (2012). 12. K. S. Novoselov et al., Proc. Natl. Acad. Sci. U. S. A. 102, 10451 (2005). 13. B. E. Brown, Acta Cryst. 20, 268 (1966). 14. H. P. Hughes et al., J. Phys. C Solid State Phys. 11, L103 (1978). 15. R. Clarke, E. Marseglia. H. P. Hughes, Philos. Mag. B 38, 121 (1978).

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Entropia cupratesque cano… [A new model of the cuprate pseudogap] R.S. Markiewicz, I.G. Buda, P. Mistark, C. Lane, and A. Bansil Northeastern University, 360 Huntington Ave., Boston MA 02459 USA.

Email: [email protected] Keywords: pseudogap, Mott-Slater transition; bosonic entropy We extend our earlier density-functional theory + quasiparticle-GW (DFT+QPGW) approach to correlated materials by incorporating vertex corrections via Moriya’s mode-coupling theory[1]. We find[2] that the cuprate pseudogap can be understood as a regime of short-range magnetic order caused by competition between many different density-wave modes trying to soften at the same time. This ‘bosonic entropy’ effect is closely akin to McMillan’s phonon entropy effect in strongly coupled charge-density wave systems[3], and similarly suppresses transition temperatures, leading to anomalously large values of 2∆/kBTN, wher ∆ is the magnetic gap and TN the Neel temperature. We find that the entropy can diverge at a crossover between commensurate and incommensurate magnetic order near (π,π) in the Brillouin zone, signaling a transition between Mott physics at (π,π) and Slater physics associated with Fermi surface nesting at an incommensurate wave vector. The driving force for (π,π) nesting is found to be Van Hove nesting, with a strong temperature dependence associated with Pauli unblocking. We find that cuprates differ in their degree of correlation, and by introducing a concept of reference families, we are able to tune between different band structures. While most cuprates fall on the Slater-side of the transition, La2-xSrxCuO4 (LSCO) lies just on the Mott side. Remarkably, just at the transition there is an emergent spin-liquid phase, which may play a role in the LSCO phase diagram.

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Figure 1: Left: At the commensurate-incommensurate transition, the susceptibility has a very flat peak. Right: This corresponds to a very large entropy, measured by the susceptibility density of states, N-, leading to anomalously small correlation lengths consistent with a spin liquid.

References 1. T. Moriya, Spin Fluctuations in Itinerant Electron Magnetism, (Springer, Berlin, 1985).

2. T. Das, R.S. Markiewicz, and A. Bansil, “Intermediate coupling model of the cuprates”, Advances in Physics 63, 151-266 (2014). 3. W.L. McMillan, “Microscopic model of charge-density waves in 2H-TaSe2”, Phys. Rev. B16, 643 (1977).

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Effects of a pressure-induced topological Fermi-surface transition on the order parameter of CaFe2As2 D. Daghero 1, R.S. Gonnelli 1, M. Tortello 1, G. A. Ummarino 1, Z. Bukowski 2, J. Karpinski 3, P. G. Reuvekamp 4, R. K. Kremer 4, G. Profeta 5, K. Suzuki 6, and K. Kuroki 6 1 Dipartimento di Scienza Applicata e Tecnologia, Politecnico di Torino, 10129 Italy 2 Polish Academy of Sciences, 50-950 Wrocław, Poland 3 Ecole Polytechnique Fédérale de Lausanne, CH-1015 Lausanne, Switzerland 4 Max Planck Institute for Solid State Research, Stuttgart, Germany 5 Dipartimento di Scienze Fisiche e Chimiche, Università dell’Aquila, L’Aquila, Italy 6 Department of Physics, Osaka University, Toyonaka, Osaka 560-0043, Japan Email: [email protected] Keywords: Iron-based superconductors, Lifshitz transition, poit-cntact spectroscopy, order-parameter symmetry Iron-based compounds (IBS) display a surprising variety of superconducting properties that seems to arise from the strong sensitivity of these systems to tiny details of the lattice structure. In this respect, systems that become superconducting under pressure, like CaFe2As2 [1,2,3] are of particular interest. Here we report on the first directional point-contact Andreev-reflection spectroscopy (PCARS) measurements on CaFe2As2 crystals under quasi-hydrostatic pressure [4] and on the interpretation of the results using a 3D model for Andreev reflection combined with ab-initio calculations of the Fermi surface (within the density functional theory) and of the order parameter symmetry (within a random-phase-approximation approach in a ten-orbital model). This combined experimental/theoretical approach shows that, on the verge of the pressure-induced structural transition between the orthorhombic and the collapsed tetragonal phase [4], that corresponds to a topological 2D-3D transition in the holelike Fermi surface sheet, i) a horizontal line node emerges in the relevant order parameter, ii) the critical temperature increases, iii) the amplitudes of the gaps increase even more, which suggests a considerable enhancement of the electron-boson coupling. References 1. M. S. Torikachvili, S. L. Bud’ko, N. Ni and P. Canfield, Phys Rev. Lett. 101, 057006 (2008). 2. T. Park, et al. J. Phys.: Condens. Matter 20, 322204 (2008). 3. K. Prokeš et al., Phys. Rev. B 81, 180506(R) (2010). 4. R.S. Gonnelli et al., Scientific Reports 6, 26394 (2016). 5. A. Sanna, G. Profeta, S. Massidda, and E. K. U. Gross, Phys. Rev. B 86, 014507 (2012).

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Ionic «ferroelectricity» and colossal dielectric constant: when a dielectric is not an insulator Brigitte Leridon LPEM-ESPCI Paris, PSL Research University, CNRS, Sorbonne Universités, UPMC, 10 rue Vauquelin, F-75005 Paris, France Email: [email protected] Keywords: ferroelectricity; titanium oxide; colossal dielectric constant; superionic conductor. Electrical conductivity and high dielectric constant are in principle self-excluding, which makes the terms insulator and dielectric usually synonymous. This is certainly true when the electrical carriers are electrons, but not necessarily in a material where ions are extremely mobile, electronic conduction is negligible and the charge transfer at the interface is immaterial. In this case, although the material conducts charge internally, it may possess huge polarization and dielectric constant. The electrical properties of a newly synthetized two-dimensional perovskite titanate are explored. This material is shown to exhibit ferroelectric-like I-V cycles together with extremely high (around 109, so well above state-of-the-art) dielectric constant at low frequency. Detailed investigations of dielectric constant behavior allow to demonstrate that it is due to ion migration and accumulation that this material behaves like a giant dipole, exhibiting colossal ferroelectric-like polarization (of the order of 0.1 C.cm-2). This material may therefore be considered as an «ionic ferroelectrics» or a strictly speaking a «ferro-ionet» and is extremely promising in terms of applications.

Figure 1: Typical I-V curves. Inset: polarization obtained from time-integration of the current as function of voltage.

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References 1. B. Leridon, R. Federicci and S. Holé “Super-condensateur à électrolyte perfectionné» FR 16 59322 patent (2016). 2. R. Federicci, et al., “Rb2Ti2O5−δ: A superionic conductor with colossal dielectric constant”, to be published.

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Simultaneous occurrence of multiferroism and short-range magnetic order in DyFeO3 Wei Bao, Jinchen Wang, Juanjuan Liu, Jieming Sheng Renmin Univ. of China

Email: [email protected] Keywords: short-range magnetic order, multiferroics We report a combined neutron scattering and magnetization study on the multiferroic DyFeO3, which shows a very strong magnetoelectric effect [1]. Applying magnetic field along the c axis, the weak ferromagnetic order of the Fe ions is quickly recovered from a spin reorientation transition, and the long-range antiferromagnetic order of Dy becomes a short-range one. We found that the short-range order concurs with the multiferroic phase and is responsible for its sizable hysteresis. OurH−Tphase diagram suggests that the strong magnetoelectric effect in DyFeO3 has to be understood with not only the weak ferromagnetism of Fe but also the short-range antiferromagnetic order of Dy [2].

References 1. Y. Tokunaga, S. Iguchi, T. Arima, and Y. Tokura, Phys. Rev. Lett. 101, 097205 (2008). 2. Jinchen Wang et al., Phys. Rev. B 93, 140403(R) (2016).

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Random Electric Field Instabilities of Relaxor Ferroelectrics G. G. Guzman-Verri U of Costa Rica and Argonne Natl. Lab

Email: [email protected] Keywords: relaxor ferroelectrics; theory; random electric fields Relaxor ferroelectrics are complex oxide materials which are rather unique to study the effects of random field disorder on phase transitions. Unlike the mostly studied random field magnets where the order parameter is uniaxial or isotropic, the polarization of typical relaxors lives in a cubic environment. Moreover, the exchange interaction that drives the magnetic transition is short-ranged and isotropic, while the relevant interaction in ferroelectrics is the highly anisotropic and long-ranged dipolar force. This puts relaxors in a different universality class from that of disordered magnets making the standard model of random field disorder inadequate to describe their unusual properties. Here, we study the effects of cubic random electric fields on the lattice instabilities that lead to the ferroelectric transition and show that, within a microscopic model and a statistical mechanical solution, even weak compositional disorder can prohibit the development of long-range order and that a random field state with anisotropic and power-law correlations of polarization emerges from the combined effect of dipole forces and their inherent charge disorder. We compare and reproduce several key experimental observations in the well-studied relaxor PbMg1/3Nb2/3O3-PbTiO3. [1]

References 1. J. R Arce-Gamboa and G. G. Guzman-Verri, arXiv: 1612.07667

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Multiple order parameters magnetoelectric multiferroics

and

their

domain

control

in

Tsuyoshi Kimura Osaka University Email: [email protected] Keywords: magnetoelectric, multiferroics, noncollinear magnetic order One of the most important concepts in condensed matter physics is the spontaneous breakdown of symmetry in a solid, which bears the ordered phase and domains in its consequence. In magnetoelectric multiferroics, multiple order parameters coexist in a system, sometimes couple with each other, and exhibit nontrivial crossed phenomena. In this presentation, we deal with magnetoelectric multiferroics in which a symmetry breaking due to the orderings of various order parameters such as electric dipole, magnetic dipole, and magnetic quadrupole moments as well as chirality originating from these multipole moments. We show our recent research activity on exploration for new magnetoelectric multiferroics and manipulations of their multiple order parameters as well as domains.

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High Pressure – Low Temperature Set-up for Infrared Spectroscopy of H3S at the AILES Beamline B. Langerome1,*, F. Capitani1, J.-B. Brubach1, A. Drozdov2, M.I. Eremets2, E. J. Nicol3, J. P. Carbotte4,5, T. Timusk4,5 and P. Roy1 1 Synchrotron SOLEIL, AILES Beamline, Saint-Aubin, 91190, France 2 Biogeochemistry Department, Max Planck Institute for Chemistry, PO Box 3060, 55020 Mainz, Germany 3 Department of Physics, University of Guelph, Guelph, N1G 2W1 ON Canada 4 Department of Physics and Astronomy, McMaster University, Hamilton, ON L8S 4M1, Canada 5 The Canadian Institute for Advanced Research, Toronto, ON M5G 1Z8 Canada Email: [email protected] Keywords: high pressure – low temperature, infrared reflectance, synchrotron radiation, H3S In solid state physics, it is important to explore the phase diagram of materials through a fine control of physical parameters such as pressure and temperature. Such control when combined with spectroscopic measurements provides rich information on the fundamental properties of a system. We described here an optical setup which has yielded the first spectroscopic data on the superconducting phase in sulfur hydride [1]. The discovery of superconductivity in H3S with a record transition temperature above 200K under pressure of 150GPa is a breakthrough for the physicist community. Although electric and magnetic measurements clearly demonstrate the presence of superconductivity, the mechanism at the origin still remains to be determined. This spectroscopic study, done at the AILES beamline of the synchrotron SOLEIL [2], demonstrates that superconductivity in H3S is driven by the electron–phonon interaction making this material a conventional BCS superconductor. Infrared measurements of the H3S system are challenging for the following reasons: • Size of the sample is 50 µm or less • Expected spectroscopic features represent less than 4% of total intensity • Measurements have to be performed in the reflectivity mode with a diamond anvil cell. • Temperature of the sample has to be controlled from RT down to 80K To overcome these technical problems, we exploited the high brilliance synchrotron radiation source, which allows measurements in a wide spectral range (60 – 600 meV), even on such tiny samples. The reflectance measurements were carried out with a setup specifically designed for infrared spectroscopy on samples at high pressure and low temperature [3].

75

Superstripes 2017, Ischia June 4-10, 2017 The samples (synthetized in high pressure laboratory in Mainz) were placed in a diamond anvil cell, which itself was in thermal contact with the cold tip of a helium flow cryostat. A camera was used to view the radiation beam spot on the sample and to maintain optimal alignment. The reproducibility of the signal from one measurement to another was better than 1%. In order to extract superconducting features, data are presented as ratio of the signal from the sample in superconducting state (~150K) to that of the normal metallic state (~220K). In addition, H3S phonons were observed at 300 K with the use of the NaCl gasket or the front surface of the diamond anvil as reference surface. This presentation will provide the experimental details complementing the talk about the optical properties of H3S given by Pr. Timusk in this conference.

Left panel: Schematic view of the optical HPLT setup. The right chamber is a part of the interferometer while the left chamber contains optics designed for reflectivity measurements. Right panel: Camera picture of the visible synchrotron radiation focused on and reflected from the sample (orange spot in the center). The dashed circles correspond approximately to the spot size in the mid infrared. A yellow light passing through the NaCl gasket allows to visualize the cell

References 1. A. Drozdov, M. Eremets, I. Troyan, V. Ksenofontov and S. Shylin. Nature 525, 7376 (2015). 2. P. Roy, M. Rouzières, Z. Qi and O. Chubar. Infrared Physics and Technology 49, 139-146 (2006). 3. A. Voute, M. Deutsch, A. Kalinko, F. Alabarse, J.-B. Brubach, F. Capitani, M. Chapuis, V. Ta Phuoc, R. Sopracase and P. Roy. Vibrational Spectroscopy 86, 17-23 (2016).

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Preparation of new metal-intercalated FeSe superconductors and their pressure dependence Takahiro Terao, Xiao Miao. Hidenori Goto, Takahumi Miyazaki and Yoshihiro kubozono

Email: [email protected] Keywords: FeSe, pressure, superconductivity, structure, XRD A pressure driven-superconducting phase has recently attracted much attention from viewpoint of pursuing new materials with high superconducting transition temperature (Tc). Application of pressure higher than 10 GPa for (NH3)yCsxFeSe produced the high-Tc superconducting phase, Tc of which reached ~50 K at 21 GPa [1]. Such pressure-driven high-Tc phases were also found in other metal-intercalated FeSe compounds [2]. In this study, we investigated the superconducting properties of (NH3)yNaxFeSe under high pressure. This material provides two different superconducting phases (Tc = 46 K and Tc = 33 – 36 K at 0 GPa) depending on x; the 46 K or 33 - 36 K phase is independently realized at ≥ 0.8 or ≤ 0.4. These phases are termed ‘high-Tc phase’ and ‘low-Tc phase’. Figure 1 shows the pressure dependence of Tc in the low-Tc phase (Tc = 36 K) of (NH3)yNaxFeSe, suggesting a presence of two distiguished superconducting phases (SC-I and SC-II). Contrary to the pressure dependence of Tc in (NH3)yCsxFeSe, the maximum Tc value achived in the high-pressure range (≥ 10 GPa) was at most 12 K at 17.5 GPa (see Figure 1). The reason why the pressure-driven high-Tc phase did not emerge in the low-Tc phase of (NH3)yNaxFeSe is still unclear. The study on pressure dependence of Tc in the high-Tc phase of (NH3)yNaxFeSe is now in progress. The powder X-ray diffraction (XRD) pattern was measured for both phases under pressure of 0 – 30 GPa, and the monotonical shrinkage of lattice was observed for both phases. In the conference, more detailed study on superconductivity and structure under high pressure will be reported.

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Figure 1: Pressure dependence of Tc in the low-Tc phase.

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Resonant Inelastic X-ray Scattering on Iron Pnictides J. Pelliciari1,2*, Y. Huang1, 3, K. Ishii3, M. Dantz1, V. Strocov1, X. Wang3, L. Xing3, C. Q. Jin3, X. Lu1, D. McNally1, T. Watashige5, S. Kasahara5, H. S. Jeevan6, P. Gegenwart6, Y. Matsuda5, T. Shibauchi5,7, T. Das8, and T. Schmitt1 1 Swiss Light Source, Paul Scherrer Institut, Switzerland 2 Massachusetts Institute of Technology, Cambridge, USA 3 Beijing National Lab for Condensed Matter Physics, Institute of Physics, China 4 Spring-8, Japan Atomic Energy Agency, Japan 5 Department of Physics, Kyoto University, Japan 6 Center for Electronic Correlations and Magnetism, University of Augsburg, Germany 7 Department of Advanced Materials Science, Tokyo University, Japan 8 Department of Physics, Indian Institute of Science, India Email: [email protected] Keywords: iron pnictides, magnetism, scattering, unconventional superconductivity Superconductivity in iron pnictides was discovered in 2008 [1], and since then, a lot of effort has been devoted in order to explain their unconventionality. As in other high temperature superconductors (HTSCs), magnetism and superconductivity (SC) exhibit proximity, competition and / or coexistence along the phase diagram, indicating strong connections between them [2-4]. In this context, the experimental characterization of static and dynamic magnetism is of vital importance in constraining advanced theoretical models. I will describe Resonant Inelastic X-Ray Scattering (RIXS) experiments on NaFe1xCoxAs. RIXS has proven to be a powerful spectroscopic tool for probing high energy spin fluctuations in HTSCs [5-7]. The NaFe1-xCoxAs series of pnictide superconductors contrasts with the most studied BaFe2-xCoxAs2 because of their much lower magnetic moment (ca 0.1 µB for NaFeAs vs. ca 1.3 µB for BaFe2As2) even though TC of Co optimal doped materials is similar (21 K vs. 22 K) [2,4]. I will present a high resolution Fe L3 RIXS study of parent and doped NaFe1-xCoxAs spanning optimal and overdoped regimes. Spectral shape decomposition reveals the persistence of broad dispersive magnetic excitations for all doping levels. In contrast to previous RIXS experiments on hole-doped BaFe2As2 compounds [6], the energy of such modes is not strongly affected by doping and the magnetic weight per iron atom of magnons / paramagnons remains constant. However, renormalized per formula unit the magnetic weight slightly decreases with doping. We argue that cobalt-doping is mainly tuning the electronic correlations. In the second part of my talk, I will discuss the work carried out on the BaFe2(As1xPx)2 series. The BaFe2(As1-xPx)2 series is an interesting case because SC appears

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Superstripes 2017, Ischia June 4-10, 2017 with isovalent doping without changing the number of carriers [2,4]. I present a combined Fe L3 RIXS and Kβ X-rays emission spectroscopy (XES) study of parent and doped BaFe2(As1-xPx)2 spanning a large portion of the phase diagram. RIXS measurements reveal the persistence of broad dispersive magnetic excitations for all doping levels. Remarkably, the energy of such modes is strongly hardened by doping contrasting with the case of hole-doped BaFe2As2 [6]. Moreover their spectral weight is conserved along the phase diagram. XES experiments show a gradual quenching of the local magnetic moment, intriguing if compared to the behavior of spin correlations. Employing intermediate coupling calculations (DFT-GW), we link the unconventional evolution of magnetism to the shift from 2- to 3-dimensional electronic structure of the system, hand in hand with the warping of the Fermi surface. This speaks for a picture where unconventional SC emerges from a balance of local and correlated magnetism, further demonstrating that magnetism is not detrimental for SC. References 1. Y. Kamihara et al, J. Am. Chem. Soc. 130, 3296 (2008). 2. G. R. Stewart, Rev. Mod. Phy., 83, 1589 (2011). 3. D. J. Scalapino, Rev. Mod. Phy., 84, 1383 (2012). 4. D. C. Johnston, Advances in Physics Vol. 59, No. 6, 803 (2010). 5. L. J. P. Ament et al, Rev. Mod. Phys. 83, 705 (2011). 6. K. J. Zhou et al, Nat. Comm., 4, 1470 (2013). 7. M. P. M. Dean et al, Nat. Mat. 12, 1019 (2013).

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Hund’s correlated metals Luca de’ Medici Ecole Supérieure de Physique et Chimie Industrielles de la Ville de Paris (ESPCI) Email: [email protected] Metallic phases with Hund’s correlations (“Hund’s metals”) are presently the focus of intensive research, in particular in relation to unconventional Fe-based superconductors. Recent experiments validate this emerging theoretical picture of the normal phase with evidences of large local paramagnetic moments, large and orbital-selective mass renormalizations and orbitally-selective pairing in the superconducting state. Further theoretical insight shows that Hund’s coupling can also alter the quasiparticle interactions in some regimes, thus potentially renormalizing the pairing strength. This is shown to correlate with experimental high-Tc superconductivity in Fe-based pnictides and FeSe. References 1. L. de’ Medici, Phys. Rev. Lett. 118, 167003 (2017) 2. P. Villar-Arribi and L. de’ Medici, unpublished (2017)

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Macroscopic character superconducting wires

of

composite

high

temperature

Boris Spivak University of Washington

Email: [email protected] “d-wave” symmetry of the superconducting order in the cuprate high temperature superconductors is a well established fact, and one which identiﬁes them as “unconventional.” However, in macroscopic contexts – including many potential applications (i.e. superconducting “wires”) – the material is a composite of randomly oriented superconducting grains in a metallic matrix, in which Josephson coupling between grains mediates the onset of long-range phase coherence. Here, we analyze the physics at length scales large compared to the size of such grains, and in particular the macroscopic character of the long-range order that emerges. While XY-glass order and macroscopic d-wave superconductivity may be possible, we show that under many circumstances – especially when the d-wave superconducting grains are embedded in a metallic matrix – the most likely order has global s-wave symmetry. We also show that magnetic field may enhance superfluid density in the wires, and more generally, in composite D-wave superconductors.

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Effect of a skin-deep surface zone on the formation of a twodimensional electron gas at a semiconductor surface Jacek J. Kolodziej (1), Natalia Olszowska (1), Jakub Lis (1), Piotr Ciochon (1), Lukasz Walczak (2), Enrique G. Michel (2) (1) Faculty of Physics, Astronomy, and Applied Computer Science, Jagiellonian University. (2) Departmento de Fısica de la Materia Condensada and Condensed Matter Physics Center Universidad Autonoma de Madrid

Email: [email protected] Keywords: 2DEG, two-dimensional electron gas, semiconductor surface, angleresolved photoemission

Two-dimensional electron gases (2DEGs) at surfaces and interfaces of semiconductors are described straightforwardly with a one-dimensional (1D) self-consistent PoissonSchroedinger scheme. However, their band energies have not been modeled correctly in this way. Using angle-resolved photoelectron spectroscopy we study the band structures of 2DEGs formed at sulfur-passivated surfaces of InAs(001) as a model system. Electronic properties of these surfaces are tuned by changing the S coverage, while keeping a high-quality interface, free of defects and with a constant doping density. In contrast to earlier studies we show that the Poisson-Schroedinger scheme predicts the 2DEG band energies correctly but it is indispensable to take into account the existence of the physical surface. The surface substantially influences the band energies beyond simple electrostatics, by setting nontrivial boundary conditions for 2DEG wave functions.

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Functional-Renormalization-Group Analysis on Electron Nematic State and Charge-Density-Wave State in Cuprate Superconductors Masahisa Tsuchiizu Department of Physics, Nagoya University Email: [email protected] Keywords: electron nematic state; charge-density-wave state; cuprate superconductors The discovery of the charge-density-wave state in the cuprate superconductors has activated intensive theoretical studies for the pseudogap states. To elucidate charge instabilities in the cuprate superconductors, we analyze the charge susceptibilities theoretically by utilizing the improved functional-renormalization-group method [1,2] to the d-p Hubbard model [3]. We reveal that the charge fluctuation of the uniform (q = 0) modulation on the px and py orbitals with antiphase (d-symmetry) form factor develops owing to the strong spin fluctuations. The spontaneous symmetry breaking with respect to the occupation of px and py orbitals with the wavevector q = 0 accounts for the recently-observed electronic nematic phase transition in cuprates. In addition, we find that the p-orbital density wave instability at the wavevectors Q ~ (0.3 pi, 0) and (0, 0.3 pi) also develops in the strong spin-fluctuation region, and is further enhanced when the q=0 nematic ordering is present. We predict that the main driving force of these charge fluctuations is the Aslamazov-Larkin vertex correction that becomes singular near the magnetic quantum-critical point. This work has been done in collaboration with Dr. H. Kontani and Dr. Y. Yamakawa. References 1. M. Tsuchiizu, Y. Ohno, S. Onari, and H. Kontani, Phys. Rev. Lett. 111, 057003 (2013). 2. M. Tsuchiizu, Y. Yamakawa, S. Onari, Y. Ohno, and H. Kontani, Phys. Rev. B 91, 155103 (2015). 3. M. Tsuchiizu, Y. Yamakawa, and H. Kontani, Phys. Rev. B 93, 155148 (2016).

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Spontaneous breakdown of time-reversal symmetry induced by thermal fluctuations Johan Carlström, Egor Babaev University of Massachusetts

Email: [email protected] Keywords: Frustrated superconductivity, broken time-reversal symmetry In systems with broken U(1) symmetry, such as superfluids, superconductors or magnets, the symmetry restoration is driven by proliferation of topological defects in the form of vortex loops. Here we discuss that in certain systems the proliferation of topological defects can, by contrast, lead to the breakdown of an additional symmetry. As a particular example we demonstrate that this effect should take place in s+is superconductors, which are widely discussed in connection with the Iron-based materials. In these systems a vortex excitation can create a "bubble" of fluctuating Z2 order parameter. Thermal excitation of vortices then leads to breakdown of timereversal symmetry when the temperature is increased. References Phys. Rev. B 91 140504(R) (Rapid Communication) (2015). Phys. Rev. B 84, 134518 (2011).

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Constraints on the total coupling strength to low-energy bosons in iron based superconductors Stefan-Ludwig Drechsler1, S. Borisenko1, H. Rosner2, V. Grinenko 1,2, J. Tomczak4, and S. Johnston4 1 Leibniz Inst. f. Solid State and Materials Research IFWDresden, D-01169 Dresden, Helmholtz. Str. 20, Germany 2 MPI for Chem. Phys. of Solids, Dresden, Germany 3 Dpmt. of Physics. TU Dresden, Dresden Germany 4 Vienna University of Technology, Wien, Austria 5 Dpmt. of Physics and Astron.omy, University of Tennesee, Knoxville 37996 USA Email: [email protected] Keywords: mass renormalization, band shifts, VAN HOVE singularities, pairing strength. Although the Fe-based superconductors (FeSC) were discovered ten years ago, there is still no consensus on the microscopic pairing mechanism(s) and main interactions involved. Also a consistent interpretation of their normal state pro-perties is still lacking. Especially the strength of the el-el interaction and correla-tion effects are under debate. Here, we examine several materials and illustrate various problems and concepts that are generic for all FeSC. Based on empirical observations and qualitative insight from DFT, we show that the super-conduc-ting (SC) and normal state properties of the FeSC can be described semi-quantitatively within multiband MIGDAL-ELIASHBERG theory. We account for the large high-energy mass renormalization (MR) phenomenologically and a mode-rate low-energy bosonic MR, in accord with constraints provided by thermodyna-mic, optical, and ARPES data. Then, all FeSCs with Tc < 55~K, studied so far, are found to belong to an intermediate coupling regime at odds with strong coup-ling suggested in the early period of the FeSC history. Support of SC by intraband el-ph or el-orbital fluctuations couplings [11 and phonon anomalies detected by point contact [2] and EXAFS [3] measurements are briefly addressed, too.

We discuss band shifts [4] as counter parts of the MR measured conveniently by the positions of VAN HOVE singularities (VHS), and the nature of a suggested quantum critical point (QCP) [5] in the h-overdoped systems AFe2As2 (A=K, Rb, Cs). Using high-precision full relativistic GGA-calculations for the total DOS at EF,, we arrive at a milder MR for Cs122 and the same MR for K122 and Rb122 at variance with other studies. The importance of spin-orbit coupling is supported by GW-calculations [4]. From the calculated mass anisotropies of all Fermi surface sheets, only the ε-pocket near the corner of the BZ is compatible with the observed anisotropy of the upper critical field Hc2 [6], 86

Superstripes 2017, Ischia June 4-10, 2017 pointing to its dominant role in the SC of these three systems. At high fields only that band survives as evidenced by a single-band elliptical angular mass anisotropy [7]. Finally, a general doping phase diagram shown in Fig. 1 is proposed. The QCP slightly be-low 0.5 hole doping is ascribed to the vicinity of an orthorhombic stripe-phase triggered by the dxz /dyz derived VAN HOVE singularity close to EF (at -14.5 meV for KFe2 As2 (K122 ) according to ARPES) in qualitative accord with DMFT and GW calculations and 75 As NMR data [7]. Its puzzling absence in a STT study for CsFe2 As2 [8] is ascribed to critical stripe fluctuation causing a local splitting of the tetragonal symmetry and yielding an additional MR seen in the large SOMMERFELD constant γ, but being detrimental for dx2-y2 pairing in contrast to Sr2RuO4 where the VHS approaching EF strengthens also the SC pairing [9]. Here, it enhances the MR, only, detrimentally for SC and explains the lowest Tc.

Figure 1: Suggested Fe pnictides phase diagram. Blue (red): magnetic (SC) regions, resp.,. Phase I - a combined charge, orbital, and spin ordered phase near the QCP responsible for the non-Fermi liquid in Cs122 [7]. Yellow line: isovalent/no doping for such systems as Li(Na)FeAs, P-doped Ba(Sr)-122 and bulk FeSe where the competing magnetic SDW magnetic stripe-phase is absent or strongly suppressed. Phhase II - observed but not yet characterized experimentally. The hypothetical SDW phase around Fe+ is our suggestion. Bright (dark red) regions: 122 and H doped La-1111 (under pressure) [10] FeSC , respectively.

References 1. S. Johnston, M. Abdel-Hafiez, L. Harnagea, et al., PRB 89, 134507 (2014). 2. V. Ivanov, A. Ivanov, et al. J. Supercond. Nov. Mat. 29, 3035 (20016). 3. Y. Naidyuk, O. Kvitnitskaya, N. Gamayunova et al , PRB 90, 094505 (2014). 4. J. Tomczak, M. van Schilfgaarde, et al. Phys. Rev. Lett. 109, 237010 (2012).

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Superstripes 2017, Ischia June 4-10, 2017 5. F. Eilers, K. Grube, D. Zocco, et al., Phys. Rev. Lett. 116, 237003 (2016). 6. F. Eilers, PhD, Thesis (2014): S. Khim, phys. stat. s.( b) 254, 1600214 (2016). 7. T. Terashima, M. Kimata, et al, J. Phys. Soc. Jpn. 78, 060504 (2009). 8. Z. T. Zhang, D. Dmytrieva, S. Molatta, et al., arXiv:1703.00780 (2017). 9. H. Yang, J. Xing, Z. Du, X.Yang, H. Lin, et al., PRB 93, 224516 (2016). 10. A. Stepke, L. Zhao, M.E. Barber et al., Science 355 (6321) eaaf9398 (2017). 11. N. Kawaguchi, Fujiwara, S. Iimura, et al. PRB 94, 161104 (2016)

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Multiband Eliashberg approach – a way to the realistic description of iron based superconductors Dmitri Efremov1, Micha Benjamin Schilling2, Andreas Baumgartner2, Boris Gorshunov2,4,5, Elena Zhukova2,4,5, Valery Dravin6, Kiril Mitsen6, Kazumasa Iida1,3, Oleg Dolgov6,7, Martin Dressel2, Sina Zapf2 1 IFW-Dresden, Institute for Solid State Research, D-01171 Dresden, Germany 2 Phylikashches Institut, Universität Stuttgart, Pfaffenwaldring 57, 70569 Stuttgart, Germany 3 Department of Crystalline Materials Science, Graduate School of Engineering, Nagoya University, Nagoya 464-8603, Japan 4 Moscow Institute of Physics and Technology (State University), 141700, Dolgoprudny, Moscow Region, Russia 5 P.N. Lebedev Physical Institute, Moscow, 119991 Russia Email: [email protected] Keywords: multiband superconductivity, iron-based superconductors, Eliashbergtheory Since the discovery of the Fe based superconductors (FeSC) a lot of efforts has been applied to elucidate the microscopic mechanism behind cooper pairing and the symmetry of the order parameter. Here we propose an experimental approach to investigate the order parameter symmetry of unconventional multiband superconductors, which is based on a disorder-induced change from sign-reversed (s±) to sign-preserved (s++) symmetry [1,2]. We present an investigation of a Ba(Fe0.9Co0.1)2As2 thin film by THz spectroscopy and stepwise proton irradiation [3]. With increase of the irradiation, the low-energy superconducting gap first vanishes but recovers at higher irradiation doses. At the same time, the decrease of the superfluid density with disorder comes to a halt. The behavior is explained by the change from sign-reversed (s±) to sign-preserved (s++) symmetry and consequently by s± symmetry in the pristine sample.

References 1. Efremov, D.V.; Korshunov, M. M; Dolgov, O.V.; Golubov, A.A.; Hirschfeld, PJ, Phys Rev B, 84, 180512 (2011). 2. Efremov, D. V.; Golubov, A. A.; Dolgov, O. V., NJP, 15,013002 (2013). 3. M. B. Schilling, A. Baumgartner, B. Gorshunov, E. S. Zhukova, V. A. Dravin, K. V. Mitsen, D. V. Efremov, O. V. Dolgov, K. Iida, M. Dressel, and S. Zapf, Phys. Rev. B 93, 174515 (2016).

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Crossover-induced spin fluctuation and electron pairing in strongly correlated electrons Takashi Yanagisawa National Institute of Advanced Industrial Science and Technology

Email: [email protected] Keywords: strong correlation; mechanism of superconductivity; crossover; spin fluctuation; Hubbard model; d-p model The mechanism of high-temperature superconductivity has been studied intensively since the discovery of cuprate high-temperature superconductors. The electron correlation plays an important role in cup rate superconductors because parent compounds without doped carriers are insulators. It is important to clarify the phase diagram of electronic states in the CuO2 plane. The ground state of the two-dimensional single-band Hubbard model and three-band dp model is investigated by adopting improved wave functions that take into account intersite electron correlation beyond the Gutzwiller ansatz. The ground-state energy is lowered considerably, which gives the best estimate of the ground-state energy for the two-dimensional Hubbard model. We argue that there is a crossover from weakly to strongly correlated regions as the on-site Coulomb repulsion U increases when holes are doped. The antiferromagnetic (AF) correlation function increases as U increases in weakly correlated region, and has a peak at the intermediate value of U being of the order of the bandwidth. The large U, greater than the bandwidth, suppresses the AF correlation to lower the ground-state energy, by increasing the kinetic energy gain. Large spin and charge correlations are induced in the strongly correlated region. This results in electron pairing and would lead to high-temperature superconductivity. The conventional spin fluctuation in weakly correlated region should be distinguished from that in strongly correlated region. It is just the spin fluctuation in strongly correlated region that would induce high-temperature superconductivity.

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Superconducting condensation energy as a function of the gap function for ∆dp = εp−εd = 2, 4, 8 in units of tdp for the d-p model. Numerical calculations were carried out on 8x8 lattice with 76 holes. The band parameters are tpp = 0.4, Ud = 10 and Up = 0.

References 1. T. Yanagisawa, J. Phys. Soc. Jpn. 85, 114707 (2016). 2. T. Yanagisawa, S. Koike and K. Yamaji, J. Phys. Soc. Jpn. 67, 3867 (1998). 3. T. Yanagisawa and M. Miyazaki, EPL 107, 27004 (2014). 4. T. Yanagisawa, New J. Phys. 15, 033012 (2013). 5. K. Yamaji, T. Yanagisawa, T. Nakanishi and S. Koike, Physica C304, 225 (1998). 6. T. Yanagisawa, S. Koike and K. Yamajji, Phys. Rev. B64, 184509 (2001).

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Drag Effect in Bilayer Systems of Dipolar Bosons and Fermions Bilal Tanatar1,* 1 Department of Physics, Bilkent University, Bilkent, Ankara, 06800, Turkey.

Email: [email protected] Keywords: drag effect, dipolar gases. We consider two parallel layers of two-dimensional, ultracold spin-polarized dipolar Fermi or dipolar Bose gases, without any tunneling between the layers. The effective interactions describing screening and correlation effects between the dipoles in a single layer (intra-layer) and across the layers (inter-layer) are modeled within the Hubbard approximation. We calculate the rate of momentum transfer between the layers when the gas in one layer has a steady flow. The momentum transfer induces a steady flow in the second layer which is assumed initially at rest. This is the drag effect familiar from double-layer semiconductor and graphene structures. Our calculations show that the momentum relaxation time has temperature dependence similar to that in layers with charged particles and and it is enhanced by the contributions from the collective modes of the bilayer system.

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Raman of YBCO out-of-equilibrium Donato Farina1, G. De Filippis2 and V. Cataudella2 1 Scuola Normale Superiore, Piazza dei Cavalieri 7, 56126, Pisa, Italy. 2 SPIN-CNR and Dip. di Scienze Fisiche - Università di Napoli Federico II - I-80126 Napoli, Italy Email: [email protected] Key words: Cuprates; Magnetism; Raman scattering; non-equilibrium; electronphonon interaction. It is widely accepted that the electronic correlations play an important role in mediating Cooper pairing in high critical temperature superconducting cuprates [1] . For this reason the accurate characterization of collective electronic excitations in these materials assumes crucial importance. We studied numerically electronic Raman scattering on insulating YBCO in B1g symmetry - the most sensitive configuration to the electronic excitations - showing a resonant peak at 2900 cm-1, originated by twomagnon bound states [2]. Our analysis is based on the exact diagonalization of the Hubbard model of small lattices [3] both at thermal equilibrium [4,5] and out-ofequilibrium [6] to simulate a pump and probe experiment. The comparison with experimental data for half filled YBCO and predictions on the effect of hole doping will be discussed together with the effects of electron-phonon interaction.

References 1. Keimer, B., et al. "From quantum matter to high-temperature superconductivity in copper oxides." Nature 518.7538 (2015): 179-186. 2. Blumberg, G., et al. "Resonant two-magnon Raman scattering in cuprate antiferromagnetic insulators." Physical Review B 53.18 (1996): R11930. 3. Dagotto, Elbio, "Strongly correlated electronic systems with one hole: Dynamical properties." Rev. Mod. Phys 66 (1994): 763. 4. Shastry, B. Sriram, and Boris I. Shraiman. "Theory of Raman scattering in MottHubbard systems." Phys. Rev. Lett. B 65 (1990): 1068. 5. Thomas P. Devereaux and Rudi Hackl, “Inelastic light scattering from correlated electrons”, Rev. Mod. Phys. 79 (2007):175. 6. G. De Filippis, et al., “Quantum Dynamics of the Hubbard-Holstein Model in Equilibrium and Nonequilibrium: Application to Pump-Probe Phenomena”, Phys. Rev. Lett. 109 (2012): 176402

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A Possible Paradigm Shift in the Search for Higher Tc Paul C. W. Chu,1,2 1 Department of Physics and Texas Center for Superconductivity, University of Houston, Houston, Texas 77204-5002, USA 2 Lawrence Berkeley National Laboratory, 1 Cyclotron Road, Berkeley, California 94720, USA.

Email: [email protected] Keywords: cuprates, iron-based superconductors, interfacial superconductivity. In the last 30 years, great progress has been made in all areas of high temperature superconductivity (HTS) research and development from raising the transition temperature Tc, discovering new HTS compounds, and developing theoretical models of HTS, to fabricating HTS prototype devices. For example, the Tc has been increased to 134 K in the stable cuprate HgBa2Ca2Cu3Ox at ambient by the ETH group and 164 K at 30 GPa achieved by the Houston group in 1993 and to 203 K in the unstable H3S above 200 GPa by Eremets et al. in 2015; more than 200 cuprate compounds stable at ambient and four superconducting hydrides under ultrahigh pressures have been found; numerous theoretical models have been developed; and many HTS prototype devices have been tested to display superior performance to that of their non-superconducting counterparts. However, several questions concerning the occurrence of HTS remain, for example: 1) Why do all Tcs above 77 K occur in cuprates until very recently? 2) Are the strong electron correlation and the two-dimensional feature characteristic of HTS cuprates necessary and sufficient for high Tc as previously suggested by many? 3) What is the role of interfaces in the enhancement of Tc? 4) Will there be a paradigmshift needed for our understanding of high temperature superconductivity and for the search for higher Tc, especially in view of the recently reported Tc of 203 K? A brief review of recent experimental results relevant to the above questions will be presented and discussed.

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Multiple component Fermi surfaces of high-Tc cuprates revealed by ARPES Atsushi Fujimori Department of Physics, University of Tokyo

Email: [email protected] Keywords: fermi surface, multi-layer cuprate, electron-doped cuprate, charge order, antiferromagnetism The physical properties of cuprate superconductors are often discussed based on a large single hole Fermi surface centered at the Brillouin zone corner. However, the large Fermi surface consists of Fermi arc and pseudogap regions (with a van Hove singularity) (Fig. 1(a), (b)), giving rise to two-component behaviors [1]. Long-range and short-range magnetic or charge order induces band folding and creates small multiple Fermi surfaces in electron-doped cuprates (Fig. 1(c)) [2]. Furthermore, more complex crystal structures such as the co-existence of Cu-O chains in YBCO [3] and the neighboring CuO2 layers in multi-layer cuprates [4] indeed lead to multiple Fermi surfaces. In this talk, overview is given on ARPES studies of the complex Fermi surfaces in relation to quantum oscillation studies [5] and their implications for phase competition, enhancement of Tc, and other new phenomena. I thank my collaborators M. Horio, S. Ideta, K. Okazaki, T. Mizokawa, T. Yoshida, K. Tanaka, A. Ino, H. Namatame, M. Taniguchi, S. Shin, K. Horiba, S. Minohara, H. Kumigashira, M. Hashimoto, D. Lu, Z.-X. Shen, Y. Ando, H. Eisaki, K. Kojima, S. Uchida, Y. Krockenberger, H. Yamamoto, T. Adachi, and Y. Koike.

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Fig. 1: Fermi surfaces of single-layer cuprates and band dispersion around the van Hove singularity (VHS) at k = (p,0). (a) Hole-doped cuprates with VHS below the Fermi level (EF). (b) The same as (a) with VHS above EF. (c) Electron-doped cuprates showing antiferromagnetic zone folding. q’s are nesting vectors for possible charge and/or spin ordering.

References 1. L. P. Gor’kov and G. B. Teitel’baum, Phys. Rev. Lett. 97, 247003 (2006) 2. M. Horio et al., Nat. Commun. 7, 10567 (2016). 3. Y. Sassa et al., Phys. Rev. B 83, 140511 (2011). 4. S. Ideta et al., Phys. Rev. Lett. 104, 227001 (2010). 5. S. E. Sebastian et al., Nature 511, 61 (2014).

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On the phenomenological two- component physics for cuprates Gregory Teitel’baum,1 1 E.K. Zavoiskii Institute for Technical Physics of the Russian Academy of Sciences, Sibirskii trakt, 10/7, 420029 Kazan, Russia

Email: [email protected] Keywords: cuprates; pseudogap state, electronic propertiess. At the very beginning of high-T c boom Gor’kov and Sokol suggested [1] that the strong lattice distortions induced by hole doping of the cuprates CuO2 planes give rise to microscopic phase separation into the metal and magnetic sub-phases. Later it was shown [2] that the most favorable type of such separation, at least for LSCO is the creation of incommensurate stripes. The observation of stripe structures by EXAFS and diffraction [3], by neutron scattering [4] and elucidation of its local properties was followed by numerous papers on various aspects of coexistence of the localized and itinerant states in high-Tc cuprates. Later it has been argued that the combined analysis of the ARPES and transport data together unequivocally gives evidence in favor of the two- component physics for cuprates also in the momentum representation. We report an attempt to further comprehend these findings unifying them with the more recent data on the energy spectrum, quantum oscillations and charge ordering in cuprates into a self-consistent phenomenological picture. We address details of the electronic spectrum in the pseudogap (PG) phase critical for understanding mechanisms of high-temperature superconductivity (SC) in cuprates. The angle-resolved photoemission spectroscopy finds coherent excitations only at socalled “Fermi arcs” (FAs). Another branch - small electronic pocket is seen in the quantum oscillations [5]. With tendency to a charge ordering (CO) revealed in few recent X-rays experiments the view became popular that pockets emerge via reconstruction of the Fermi surface (FS) in vicinity of the nodal points in a CO transition. However the residual metallic contribution into the specific heat deep in the SC phase of YBCO observed in [6] contradicts the reconstruction scenario, as SC suppressing the CO would thereby destroy such pocket. Recently it was suggested [7] that at doped hole concentrations x>0.08-0.10 the experimental Hall coefficient identifies the pocket as a permanent feature, in contrast to the idea of FS reconstruction at the charge ordering phase transition. To reveal the origin of the electron pocket we analyze the impact of the lattice structural changes on the energy spectrum in cuprates. It is important that the quantum oscillation which may be considered as the manifestation of the electron pocket were observed in the doping range corresponding to the so-called low temperature tetragonal

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Superstripes 2017, Ischia June 4-10, 2017 (LTT) phase. In this respect it is necessary to take into account, that the well-known LTO-LTT structural transition influences not only the lattice degrees of freedom but also the charge ones. We suggest that the corresponding changes of the electronic dispersion may result in opening of the small electron pocket at the center of Brillouin zone.

References 1. L. P. Gor’kov and A. V. Sokol, JETP Lett. 46, 420 (1987). 2. J. Zaanen and O. Gunnarson, Phys. Rev. B 40, 7391(R)1989. 3. A. Bianconi, M. Missori, J. Phys.I (France) 4, 361 (1994); Solid State Commun. 91, 287 (1994). 4. J. M. Tranquada et al., Nature 375, 561 (1995). 5. N. Doiron-Leyraud et al., Nature 447, 565-568 (2007). 6. S. C. Riggs, et al., Nature Phys. 7, 332-335 (2011). 7. L. P. Gor’kov & G. B. Teitel’baum, G.B., Sci. Rep. 5, 8524 (2015).

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Frontiers of high-Tc studies and spin liquids Masatoshi Imada University of Tokyo

Email: [email protected] Keywords: organic materials, Ir compounds We focus on frontiers of high-temperature superconductivity. The first subject is the mechanism for the bulk cuprates, which attracts renewed interest in the light of the possible existence of hidden fermions. The hidden fermions boost up the superconducting transition temperature high and is an electronic state that is bistable with quasiparticles. This bistability or fractionalization of electrons is a side of the same coin opposite to the charge inhomogeneity or phase separation ubiquitous in the cuprates and iron-based superconductors. The high critical temperature of superconductivity requires strong effective attraction of electrons while it also induces the charge inhomogeneity such as charge ordering and electronic phase separation. The understanding of the bistability has inspired ideas for pioneering at frontiers of high-Tc superconductivity beyond conventional bulk equilibrium. One is interfaces and thin films, which have unique functions of self-optimizing strong-coupling superconductivity. The second is to utilize the nonequilibrium state that is not possible in equilibrium because of the instability to charge inhomogeneity. We also discuss possibilities of realizing quantum spin liquids, driven by geometrical frustration effects. References 1. S. Sakai, M. Civelli and M. Imada, Phys. Rev. Lett. 116 (2016) 057003. 2. T. Misawa and M. Imada, Phys. Rev. B 90 (2014) 115137. 3. T. Misawa and M. Imada, Nat. Commun. 5 (2014) 5738. 4. T. Misawa, Y. Nomura, S. Biermann and M. Imada, Sci. Adv. 2 (2016) e1600664. 5. R. Kaneko, S. Morita and M. Imada, J. Phys. Soc. Jpn. 5, 5738 (2014). 6. S. Morita, R. Kaneko and M. Imada, J. Phys. Soc. Jpn. 84, 024720 (2015).

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Orbital degeneracy lifting, broken local symmetries and properties in correlated electron materials

Simon J. L. Billinge1,2 Department of Applied Mathematics and Applied Physics, Columbia University, New York, NY 10027, US. 2 Condensed Matter Physics and Materials Science Department, Brookhaven National Laboratory, Upton, NY 11973 1

Email: [email protected] Keywords: orbital order, nematicity, local structure, PDF analysis, orbital degeneracy. We now understand the broken symmetry states are widespread in strongly correlated electron materials, including charge, spin and orbital orders. All these involve arranging local objects in patterns that break the average structure, for example, antiferromagnetic arrangements of spins, and striped arrangements of charges and spins. In the case of orbital order, less focus has been placed on the exact nature of the local objects that arrange themselves and more on how they arrange; the order. However, when considering what happens when the order goes away, by melting for example, this distinction becomes important. Local probes such as atomic pair distribution function analysis (PDF) allow us to study disordered and short-range ordered states of matter. To help understand our observations I will introduce language of orbital degeneracy lifting (ODL) as a general concept that can span all the way from disordered locally symmetry broken orbital states through short-range ordered domain states all the way to long-range orbital order. We are finding ODL states in a wide range of materials evident in the local structure, that plays an important role in understanding the properties of correlated electron materials.

Figure 1: Schematic of how orbital degeneracy lifting comes about

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On the time correlation of long range ordered CDW state in LBCO X. M. Chen, V. Thampy, C. Mazzoli, A. M. Barbour, H. Miao, G. D. Gu, Y. Cao, J. M. Tranquada, I. Robinson, M. P. M. Dean and S. B. Wilkins Brookhaven National Lab, Upton, USA Email: [email protected] Keywords: CDW, HTSC, time correlation The occurrence of charge-density-wave (CDW) order in underdoped cuprates is now well established, although its nature and its relationship with superconductivity is not. Theoretical proposals include contrasting ideas such as that pairing may be driven by CDW fluctuations or that static CDWs may intertwine with a spatially modulated superconducting wave function. We report on the CDW order dynamics in LBCO by using x-ray photon correlation spectroscopy at its wave vector, detected by resonant soft x-ray diffraction at the Cu L3 edge. The long time stability of the measured CDW signal is discussed in the view of its varying correlation length with temperature. References 1. Phys. Rev. Lett. 117, 167001 (2016) and references therein.

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Stripe Pinning in LBCO I. K. Robinson, X. M. Chen, V. Thampy, H. Miao, Y. Cao, M. P. M. Dean, C. Mazzoli, A. M. Barbour, W. Hu, S. B. Wilkins, G. D. Gu, and J. M. Tranquada Soft X-ray coherent diffraction at NSLS-II has been used to address the pinning of stripes in the cuprate HTSC material La1.825Ba0.125CuO4 (LBCO) [1]. LBCO’s relatively long stripe correlation length of 26nm [2], compared with only 5.5nm for the YBCO, meant that the resonant signal level at CSX-1 was sufficient to record coherent diffraction at the charge-density wave (CDW) vector (0.24,0,1.5) on resonance at the Cu L3-edge. The CDW domains were found to be surprisingly static, with no evidence of significant fluctuations up to the 2.75 hour duration of the measurements from 15K up to 45K, approaching the stripe melting temperature [3]. To ensure consistent illumination of the same region, we introduced a fabricated pinhole array physically attached to the sample and attempted to depin the CDW domains using temperature excursions. We found that the observed pattern of stripe pinning is extremely robust and survives temperature sweeps as high as 220K, but not beyond. Since this temperature is close to the 235K “LTO-HTT” structural transition, we postulate that the pinning sites are the orthorhombic grain boundaries which are rearranged by crossing it. A possibly related “return point memory” effect has been studied before in magnetic systems [4,5], but never for CDWs. We are presently attempting to image the pattern of domains by inversion of the Bragg coherent diffraction pattern [6].

Coherent X-ray diffraction pattern of the cuprate La1.825Ba 0.125CuO4 (LBCO) recorded at the charge-density wave vector (0.24,0,1.5) on resonance at the Cu L3-edge, using the CSX-1 beamline of NSLS-II.

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References 1. J. M. Tranquada, B. J. Sternlieb, J. D. Axe, Y. Nakamura, and S. Uchida, Nature 375 561 (1995). 2. V. Thampy, S. Blanco-Canosa, M. Garcıa-Fernandez, M. P. M. Dean, G. D. Gu, M. F ̈orst, T. Loew, B. Keimer, M. Le Tacon, S. B. Wilkins, and J. P. Hill, “Comparison of charge modulations in La1.875Ba0.125CuO4 and YBa2Cu3O6.6.”, Phys. Rev. B 88 024505 (2013). 3. X. M. Chen, V. Thampy, C. Mazzoli, A. M. Barbour, H. Miao, G. D. Gu, Y. Cao, J. M. Tranquada, M. P. M. Dean and S. B. Wilkins, “Remarkable Stability of Charge Density Wave Order in La1.875Ba0.125CuO4”, Phys. Rev. Letts. 117 167001 (2016). 4. M. S. Pierce, C. R. Buechler, L. B. Sorensen, J. J. Turner, S. D. Kevan, E. A. Jagla, J. M. Deutsch, T. Mai, O. Narayan, J. E. Davies, K. Liu, J. Hunter Dunn, K. M. Chesnel, J. B. Kortright, O. Hellwig, and E. E. Fullerton, “Disorder-Induced Microscopic Magnetic Memory”, Phys. Rev. Letts. 94 017202 (2005). 5. K. A. Seu, R. Su, S. Roy, D. Parks, E. Shipton, E. E. Fullerton and S. D. Kevan, “Microscopic return point memory in Co/Pd multilayer films”, New Journal of Physics 12 035009 (2010). 6. Ian Robinson and Ross Harder, “Coherent Diffraction Imaging of Strains on the Nanoscale”, Nature Materials 8 291-298 (2009).

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Precursor Charge Density Wave in La2-xBaxCuO4 Mark P. M. Dean Brookhaven National Laboratory Email: [email protected] Keywords: Cuprates, CDW, stripes, superconductivity, x-rays It has long been hypothesized that long-range charge density wave (CDW) order is an intrinsic property of the cuprates that arises from the pinning of precursor hightemperature CDW fluctuations. Many years after this initial discovery of CDW order in 214 cuprates, the idea of a universal tendency towards CDW order was bolstered by the observation of CDW correlations in various systems such as YBa2Cu3O6+x (YBCO). However, the CDW in YBCO has a very different wavevector (~0.3 rather than 0.24) and is seemingly unrelated to the low-energy spin correlations (which are gapped in YBCO) impeding efforts to understand the cuprates within a universal framework. Much of the difficulty is that although precursor spin density wave (SDW) correlations have been studied in detail, the corresponding transition between longrange ordered and precursor CDW correlations has never been observed. Here we report the discovery of precursor CDW correlations in La1.875Ba0.125CuO4 [1]. As shown in Fig 1., the precursor CDW has a correlation length of a few unit cells and exists at a different wavevector from the low temperature CDW decoupled from the SDW correlations. We find that the CDW and SDW correlations lock together at low temperature to form a phase with meandering partially ordered CDW correlations, reconciling the apparently different properties of the charge correlations in different cuprates.

Figure 1: Decoupling of the CDW and SDW in the precursor phase [1]. The results of fitting the quasi-elastic intensity showing: a the full width at half maximum, b the incommensurability.

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Superstripes 2017, Ischia June 4-10, 2017 The black dashed line at 54 K corresponds to the low temperature LTT-LTO structural phase transition which is depicted in b, blue and yellow code temperatures below and above this threshold [2]. The behavior of the SDW, taken from inelastic neutron scattering results at 3 and 6 meV energy transfer from Ref. [3] are included on panels a and b. We see that the CDW and SDW incommensurabilities evolve in different directions above 54 K, which indicates a decoupling of the charge and spin degrees of freedom.

References 1. H. Miao, J. Lorenzana, G. Seibold, Y.Y. Peng, A. Amorese, F. Yakhou-Harris, K. Kummer, N. B. Brookes, R. M. Konik, V. Thampy, G. D. Gu, G. Ghiringhelli, L. Braicovich, M. P. M. Dean, arXiv:1701.00022 (2017) 2. S. B. Wilkins, M. P. M. Dean, Jörg Fink, Markus Hücker, J. Geck, V. Soltwisch, E. Schierle, E. Weschke, G. Gu, S. Uchida, N. Ichikawa, J. M. Tranquada, and J. P. Hill, Phys. Rev. B 84, 195101 (2011) 3. M. Fujita, H. Goka, K. Yamada, J. M. Tranquada, L. P. Regnault, Phys. Rev. B 70, 104517 (2004)

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Optical spectroscopy of La2-xBaxCuO4 single crystals: influence of stripe order D.B. Tanner, Luyi Yan, and Genda Gu University of Florida and Brookhaven National Laboratory Email: [email protected] Keywords: high Tc, infrared, cuprate, stripes The ab-plane and c-axis reflectance spectra of ten La2-xBaxCuO4 single crystals, with x ranging from undoped to optimally doped, have been measured over a wide frequency range and at temperatures from 10 to 300 K. The influence of stripe order around x=0.125 appears in the spectra below T = 50 K, observed both as a reduction in the free-carrier (normal state) and superfluid (superconducting state) density and by the appearance of a relatively narrow conductivity band near 25 meV. The superfluid density is estimated from the real part of the dielectric function and the f-sum rule. The c-axis spectra are those of an insulator or very bad metal, with very little doping or temperature dependence. The Josephson plasma edge is not observed in any of these spectra.

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High Pressure 3D to 2D Tuning of Magnetism in Cuprates Markus Huecker Department of Condensed Matter Physics, Weizmann Institute of Science, 234 Herzl St., Rehovot 76100, ISRAEL

Email: [email protected] Keywords: Cuprates, Magnetism, Stripes, Superconductivity Broken lattice symmetries often play an integral part in the selection process of the electronic ground state. A prime example is found in the La-214 cuprates, where lattice distortions result in a complex relationship between superconductivity, charge and magnetic orders. In an attempt to dissect this problem into its various parts, here we highlight the impact of lattice distortions on the pristine magnetism of a cuprate parent compound.

Figure 1: High pressure phase diagram of magnetic order and crystal structure in a cuprate parent compound.

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Building blocks of cuprate charge density modulations Andrej Mesaros, Kelvin Chang, Ehsan Khatami, Michael J. Lawler, J.C. Seamus Davis, Eun-Ah Kim Cornell University, San Jose State University

Email: [email protected] Keywords: high Tc, cuprates, charge density wave, machine learning, STM Charge modulations in cuprates have been described from opposing theoretical standpoints, predicting either commensurate or incommensurate charge order wave vectors. A recently introduced method for analyzing STM data on Bi2Sr2CaCu2O8+x has shown[1] that the underlying charge wave vector is commensurate, in contrast to conventional diffraction measurements on some other cuprates which find incommensurate wave vectors. To understand the universality of charge order in cuprates it is necessary to relate the conventionally averaged wave vector value to the underlying wave vector, which is challenging due to disorder at different lengthscales. To illuminate the nanoscale configuration and organization of the disordered charge modulations in cuprates, here we for the first time apply methods beyond Fourier analysis to the STM data on cuprates. I will describe the quantitative views which the methods of machine learning and wavelet analysis provide into the nanoscale patterns of charge modulations. I will also discuss how different types of randomness in the charge modulations relate the nanoscale to the larger scale averaged observables relevant for diffraction techniques. References 1. A. Mesaros, K. Fujita, S.D. Edkins, M.H. Hamidian, H. Eisaki, S.-i. Uchida, J.C. Seamus Davis, M.J. Lawler, E.-A. Kim, Proc Natl Acad Sci USA 113, 12661 (2016).

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Pseudogap-generated a coexistence of Fermi arcs and Fermi pockets in cuprate superconductors Shiping Feng, Huaisong Zhao, and Deheng Gao Department of Physics, Beijing Normal University, Beijing 100875, China

Email: [email protected] Keywords: Pseudogap; Fermi arc; Fermi pocket; Electron spectrum; Cuprate superconductor One of the most intriguing puzzle is why there is a coexistence of Fermi arcs and Fermi pockets in the pseudogap phase of cuprate superconductors? This puzzle is calling for an explanation. Based on the t-J model in the fermion-spin representation, the coexistence of the Fermi arcs and Fermi pockets in cuprate superconductors is studied by taking into account the pseudogap effect. It is shown that the pseudogap induces an energy band splitting, and then the poles of the electron Green's function at zero energy form two contours in momentum space, however, the electron spectral weight on these two contours around the antinodal region is gapped out by the pseudogap, leaving behind the low-energy electron spectral weight only located at the disconnected segments around the nodal region. In particular, the tips of these disconnected segments converge on the hot spots to form the closed Fermi pockets, generating a coexistence of the Fermi arcs and Fermi pockets. Moreover, the single-particle coherent weight is directly related to the pseudogap, and grows linearly with doping. The calculated result of the overall dispersion of the electron excitations is in qualitative agreement with the experimental data. The theory also predicts that the pseudogap-induced peak-dip-hump structure in the electron spectrum is absent from the hot-spot directions.

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Magnetic field induced magnon decay in the spin ½ square lattice Heisenberg antiferromagnet (5CAP)2CuCl4 Toby Perring1*, Paul Gosuly1,2, Martin Mourigal3, Niels Christensen4, Goran Nilsen1, Henrik Rønnow5, Des McMorrow2 1 ISIS Facility, STFC Rutherford Appleton Laboratory, Didcot, OX11 0QX, UK 2 London Centre for Nanotechnology, UCL, London WD1H 0AH, UK 3 School of PhysicsGeorgia Institute of Technology, Atlanta, GA 30332, USA 4 Department of Physics, Technical University of Denmark, DK2800 Kgs. Lyngby, Denmark 5 Laboratory for Quantum Magnetism, École Plytechnique Fédérale de Lausanne (EPFL), CH-1015, Switzerland Email: [email protected] Keywords: Quantum magnetism, magnon decay, cuprates The concept of quasi-particles is ubiquitous in condensed matter physics, as it offers a major simplification of the strongly interacting many body problem by which the low energy excited states can be characterised by weakly interacting long-lived particles. Correspondingly, the circumstances when this picture breaks down are also important. In ordered magnetic systems in two or three dimensions the elementary excitations are magnons. Here we report the breakdown of magnons in the spin ½ quasi twodimensional (2D) square lattice Heisenberg antiferromagnet (5CAP)2CuCl4 in a magnetic field applied perpendicular to the plane of the ordered moments in zero field, using the results of high-resolution time-of-flight inelastic neutron scattering experiments. Above a critical field HS the moments are fully aligned, and we obtained the magnetic exchange parameters in the Hamiltonian from the measured magnon dispersion relation. At H=0.85HS we observe marked damping of the magnons around (0.5,0.5) and (0.5,0) and the formation of a continuum of magnon scattering, as well as a small renormalisation of the magnon energies. We find a good quantitative agreement between the data and predictions of renormalised 1/S spin-wave theory using the intra- and inter-plane exchange parameters extracted from the polarised phase. Our results provide the first experimental evidence for spontaneous magnon decay in the quantum limit of the Heisenberg antiferromagnet on a square lattice in applied magnetic field.

References 1. M. Mourigal et al, Phys Rev B 82 144402 (2010). 2. W.T. Fuhrman et al., Phys Rev B 85 184405 (2012).

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Disorder-Driven Metal-Insulator Transitions in Deformable Lattices Vladimir Dobrosavljevic Florida State University and MagLab

Email: [email protected] Keywords: Anderson localization, polarons We show that, in the presence of a deformable lattice potential, the nature of the disorder-driven metal-insulator transition is fundamentally changed with respect to the noninteracting (Anderson) scenario. For strong disorder, even a modest electronphonon interaction is found to dramatically renormalize the random potential, opening a mobility gap at the Fermi energy. This process, which reflects disorder-enhanced polaron formation, is here given a microscopic basis by treating the lattice deformations and Anderson localization effects on the same footing. We identify an intermediate “bad insulator” transport regime which displays resistivity values exceeding the Mott-Ioffe-Regel limit and with a negative temperature coefficient, as often observed in strongly disordered metals. Our calculations reveal that this behavior originates from significant temperature-induced rearrangements of electronic states due to enhanced interaction effects close to the disorder-driven metal-insulator transition. References 1. Domenico Di Sante, Simone Fratini, Vladimir Dobrosavljević, and Sergio Ciuchi Phys. Rev. Lett. 118, 036602 (2017).

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Multiple universalities in order-disorder magnetic phase transitions H. D. Scammell, O. P. Sushkov University of New South Wales Email: [email protected] Keywords: Quantum phase transition

Phase transitions in isotropic quantum antiferromagnets are associated with the condensation of bosonic triplet excitations. In three dimensional quantum antiferromagnets, such as TlCuCl3, condensation can be either pressure or magnetic field induced. The corresponding magnetic order obeys universal scaling with thermal critical exponent ∅. Employing a relativistic quantum field theory, the present work predicts the emergence of multiple (three) universalities under combined pressure and field tuning. Changes of universality are signalled by changes of the critical exponent ∅. Explicitly, we predict the existence of two new exponents ∅ = 1 and 1/2 as well as recovering the known exponent ∅ = 3/2. We also predict logarithmic corrections to the power law scaling.

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Three-dimensional quantum worldsheet condensation

liquid

crystals

and

dislocation

Jaakko Nissinen, Aron J. Beekman, Kai Wu, Jan Zaanen Lorentz-Institute for Theoretical Physics, the Netherlands and Low Temperature Laboratory, Aalto University, Finland; Keio University, Japan; Standford, USA; Leiden University, the Netherlands

Email: [email protected] Keywords: Liquid crystals, vortices, two-form gauge fields, duality A crystalline quantum solid can partially melt into a liquid crystal, where spontaneous rotational symmetry breaking is maintained while translational symmetry is (partially) restored. The melting can be understood via the unbinding of dislocations, the topological defects of translational order. Through a duality mapping, phonons turn into flavored dual gauge fields mediating interactions between dislocations. The phenomenological Landau-Ginzburg theory for this dual gauge theory allows one to study liquid crystal phases in a unified fashion. Upon condensation of dislocations, the dual gauge fields undergo the Anderson--Higgs mechanism and become gapped, signaling the loss of shear rigidity. We recently provided a comprehensive review of quantum dislocation-mediated melting in 2D (arXiv:1603.04254, Phys. Rep. to be published). Here we extend this theory to three dimensions. Dislocations are now linelike objects, strings, tracing out worldsheets in imaginary time, while the dual gauge-fields become two-form (Kalb--Ramond) fields. We obtain the Higgs phase of these two-form gauge fields. Translational symmetry can be restored in three, two or one directions leading to nematic, smectic or columnar quantum liquid crystals. We derive the spectrum of low-energy excitations and its linear response. Goldstone modes due to broken rotational symmetry as well as superconductivity emerge whenever translational symmetry is restored. The peculiar features of liquid-crystalline order can be probed by finite-momentum spectroscopy.

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Proximity of superconductivity and magnetism in δ-doped La2CuO4 heterostructures Alexander Boris Max Planck Institute for Solid State Research, Stuttgart, Germany Email: [email protected] Keywords: superlattices; superconductivity; magnetism; proximity effects Proximity and concomitant emergence of superconductivity (SC) and magnetism bear much fundamental interest and potential for applications. A striking manifestation of this phenomenon occurs in the (La,Sr)2CuO4+δ family, where SC and magnetism appear to have the same onset temperature [1-3]. To gain insight on the intimate connection between the SC and magnetic ground states, we follow a novel approach to dope La2CuO4. Instead of random La/Sr substitution, we replace single planes of LaO2 with SrO2 dopant planes by means of atomic layer-by-layer oxide-MBE [4]. We investigate superlattices (SLs) with the composition [(LaO-SrO-CuO2) + N × (LaOLaO-CuO2)] (δSr-LCON), N = 3…12. These heterogeneously δ-doped SLs have inherent broken inversion symmetry and substantially reduced Sr disorder, compared with homogeneously doped bulk La2CuO4. Utilizing the low-energy muon spin rotation technique, we find up to 30-percent enhancement in the magnetic volume fraction in δSr-LCON right below Tc, ranging from 18 K to 30 K. While resembling phase-separated bulk (La,Sr)2CuO4+δ, the close proximity of the magnetic and SC ground states in δSr-LCON is leading to a nontrivial interplay between these two orders. Our THz spectroscopy study confirms that the SC state in this system is essentially two-dimensional. The upper critical field is significantly reduced in the Faraday geometry - the SC gap closes at HC2 ≈ 1.5 T, whereas an external magnetic field parallel to the SC layers is much less effective for the pair breaking. The experimental optical spectra reveal the thermally activated charge transfer between the SC and magnetic phases - while effective at high temperatures it becomes no longer noticeable when approaching the SC and magnetic transition temperature. References 1. Y. S. Lee et al., Phys Rev B 60, 3643 (1999). 2. L. Udby et al., Phys Rev Lett 111, 227001 (2013). 3. H. E. Mohottala et al., Nature Matt. 5, 377 (2006). 4. F. Baiutti et al., Nature Commun.6, 8586 (2015).

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Emerging X-ray Techniques for Probing Matter with Depth and Time Resolution Alexander X. Gray Department of Physics, Temple University, Philadelphia, Pennsylvania, PA 19122, USA

Email: [email protected] Keywords: hard x-ray photoemission spectroscopy, standing-wave photoemission spectroscopy, oxide interfaces, 2DEG In this talk I will describe several new directions in the fields of x-ray spectroscopy and imaging, made possible with the advent of third-generation synchrotron light sources and free-electron lasers, and enabling investigations of fundamental physical processes in novel complex materials and technologically-relevant nanostructures and interfaces with depth- and time resolution. I will present the first results of hard x-ray angleresolved photoemission measurements (HARPES) at excitation energies of up to 6 keV [1,2]. Compared to the traditional ARPES, carried out in the UPS regime (6-120 eV), this new technique enables one to probe up to 40 times deeper below the surface, thus allowing for more bulk-sensitive momentum-resolved electronic structure determination. Furthermore, I will introduce a new x-ray photoemission technique (SWARPES) which combines soft x-ray ARPES with standing-wave (SW) excited photoelectron spectroscopy, wherein the intensity profile of the exciting x-ray radiation is tailored within the sample in order to provide a depth-selective probe of the electronic structure of buried layers and interfaces [3,4]. Finally, I will discuss the latest applications of the above-mentioned techniques to the studies of superconducting/magnetic heterostructures [5] and two-dimensional electron gas at oxide interfaces [6].

References 1. A. X. Gray et al., Nature Materials 10, 759 (2011). 2. A. X. Gray et al., Nature Materials 11, 957 (2012). 3. A. X. Gray et al., Europhys. Lett. 104, 17004 (2013). 4. A. X. Gray et al., J. Electron Spectrosc. Relat. Phenom. 195, 399 (2014). 5. B. A. Gray et al., Scientific Reports 6, 33184 (2016). 6. S. Nemsak et al., Phys. Rev. B 93, 245103 (2016).

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The inverse Edelstein effect at oxide interfaces G. Seibold(1), S. Caprara(2), M. Grilli(2), R. Raimondi(3) (1) Institut f. Physik, BTU Cottbus-Senftenberg, PBox 101344, 03013 Cottbus, Germany; (2) Dipartimento di Fisica, Universita di Roma 'La Sapienza', piazzale Aldo Moro 5, 00185 Roma, Italy; (3) Dipartimento di Matematica e Fisica, Universita Roma Tre, Via della Vasca Navale 84, 00146 Roma, Italy Email: [email protected] Keywords: oxide interfaces, spintronics, Edelstein effect The manipulation of spin degrees of freedom in order to generate a charge current and the inverse process are at the heart of spintronics devices. One of the most prominent examples is the spin-Hall effect (SHE) where a charge current induced by an electric field along the x-direction produces a z-polarized spin current flowing along the ydirection. Another related phenomenon is the Edelstein effect where a charge current is converted into a non-equilibrium spin polarization. Both, SHE and Edelstein effect occur in systems with strong spin-orbit coupling, in particular two dimensional electron gases which lack inversion symmetry perpendicular to the gas plane and which are usually described with the so-called Rashba hamiltonian. The situation is more complex in LaAlO3/SrTiO3 interfaces where the interplay between inversion asymmetry and atomic spin orbit coupling is at the heart of strong Rashba interactions. Recently, two experiments [1,2] have demonstrated a strong inverse Edelstein effect at such interfaces by generating a strong non-equilibrium spinpolarization at the interface and detecting the resulting charge current. The reported spin-to-charge efficiency is more than order of magnitude larger than in conventional metallic layers which suggests the LAO/STO interface as a promising system for spintronic devices. Within linear response theory we investigate the inverse Edelstein effect in oxide interfaces by generalizing the approach of Raimondi et al. [3] to a multiband model which involves the 3d t2g bands of the Ti ions. Consistently with experiment we find a gate-tunable inverse Edelstein effect which changes sign depending on the occupation of and ⁄ orbitals.

References 1. E. Lesne et al., Nature Materials 15, 1261 (2016). 2. Q. Song et al., Nature Communications, 10.1038 (2016). 3. K. Shen, G. Vignale and R. Raimondi, Phys. Rev. Lett. 112, 096601 (2014).

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Emergence of surface orbital ordering in the heavy fermion superconductor CeCoIn5 Yasuo Yoshida University of Tokyo Email: [email protected] Keywords: orbital order, STM, heavy fermion

The newfangled orbital-mediated quantum phenomena have proved over the past decade to be far-reaching and complex as exemplified in exotic orbital orders, nontrivial orbital-fluctuation-mediated superconductivity, orbital Kondo effect, and multipole-moment ordering. To understand electronic, spin, and orbital correlations in these phenomena, it is crucial to have a direct access to real-space orbital texture, but so far orbital-sensitive probes have shown rather limited functionality. Recent progress of a scanning tunneling microscope (STM) has enabled orbital-selective tunneling by fine-tuning the tip-sample distance (TSD). We exploit the orbital sensitivity of STM to unveil a surface-assisted cobalt d- orbital order in the heavy fermion compound CeCoIn5. We find that at a small TSD, cobalt atoms in STM topographies take on dumbbell shapes alternatingly aligned in the [100] and [010] directions on a cleaved (001) surface. A domain boundary of this ordered structure, which is localized within a terrace, denotes two-dimensionality of the ordered structure. First-principles calculations show that the structure is a consequence of a staggered dxz-dyz orbital order assisted by surface termination. This novel surface-assisted orbital ordering seems to be ubiquitous in transition metal oxides, heavy fermion superconductors and other materials, but has been overlooked until now.

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STM studies of superconductivity and nematicity in Fe(Se,S) Tetsuo Hanaguri Email: [email protected] Spontaneous breaking of lattice rotational symmetry in the electronic state, which is known as electronic nematicity, has been observed in various materials including unconventional superconductors such as cuprates and iron-based materials. In order to make clear the relationship between superconductivity and nematicity, we have performed spectroscopic-imaging STM on FeSe1-xSx. The parent material FeSe undergoes tetragonal-to-orthorhombic transition at 90 K, which is a manifestation of electronic nematic order. Superconductivity sets in at lower temperature of 9 K. The electronic nematic order is suppressed with increasing sulfur content x and disappears above x ~ 0.17, whereas superconducting transition temperature remains intact1. We have investigated the evolution of the band structure as a function of x by analyzing the quasiparticle interference patterns. We have found that anisotropy of the in-plane band structure diminishes with increasing x but there is little change in the band structure at x = 0.17. Superconducting gap is hardly affected by sulfur doping in the nematic phase but is suddenly smeared once the nematic phase diminishes. This result indicates that superconductivity and nematicity are strongly interrelated. References 1. S. Hosoi et al., Proc. Natl. Acad. Sci. USA 113, 8139 (2016).

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Change of phase diagram of 1111-type iron pnictide by varying of rare-earth element, solid solution of pnictogens and electron doping S. Miyasaka, T. Yamamoto, M. Uekubo, H. Tsuji, K. T. Lai, A. Takemori, M. Nakajima, S. Tajima Dept. of Phys., Osaka Univ., Osaka 560-0043, Japan

Email: [email protected] Keywords: Iron based superconductor, Fermi surface nesting. In spite of many experimental and theoretical studies on iron-based superconductors, their superconducting mechanism has not clarified yet. Just after the discovery of these superconductors [1], it has been reported that the system can be described within a moderated electron correlation regime where the band theory works well. Then, antiferromagnetic spin fluctuation due to Fermi surface nesting was considered as a strong candidate for the pairing interaction of superconductivity. [2,3] Recently, three different antiferromagnetic states were found in LaFeAsO system, which is called La1111 system. In this system, the first antiferromagnetic phase (AFM1) exists around LaFeAsO. By P substitution for As, AFM1 disappears and superconducting phase (SC1) appears around x=0.6-0.8 in LaFeP1-xAsxO. The further P substitution induces the appearance of another antiferromagnetic phase (AFM2) between x=0.3 and 0.6. Below x=0.3, the different superconducting phase (SC2) exists. [4,5] On the other hand, the heavily H doping induces third antiferromagnetic phase (AFM3) and superconducting one (SC3) in LaFeAsO. [6] In the present work, we have investigated the change of electronic phase diagram of 1111-type systems by varying of rare-earth element (R), solid solution of pnictogens (Pn) and electron doping. The R change, Pn solid solution and electron doping induce the variety of Fermi surface topology, and resultantly the 1111-type systems show a complicated phase diagram. For example, RFeP1-xAsxO shows the systematic but complicated change of phase diagram, when R is changed from La to Nd. Figure 1 presents the electronic phase diagram of RFeP 1-xAsxO with R=La, Pr and Nd. As described above, LaFeP1-xAsxO has two antiferromagnetic and superconducting phases (AFM1, AFM2, SC1 and SC2). The antiferromagnetic states of AFM1 and AFM2 are caused by the Fermi surface nesting in FeAs-type and FeP-type Fermi surface states, respectively. [7] With the change of R from La to Nd, the Pn height from Fe layer increases, and xy hole Fermi surface around zone corner are expanded. The Fermi surface nesting related with xy Fermi surface is enhanced and the AFM1 is stabilized. As shown in Fig. 1, the AFM1 survives in lower As concentration (x) regions in PrFeP1-xAsxO and NdFeP1-xAsxO. Another

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Superstripes 2017, Ischia June 4-10, 2017 antiferromagnetic phase (AFM2) exists at x~0.4 in these systems. As a result, AFM1 and AFM2 merge, and the antiferromagnetic phase is observed in wider x-region in PrFeP1-xAsxO and NdFeP 1-xAsxO.

150 (a) LaFeP1-xAsxO TN

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Figure 1: Electronic phase diagrams of (a) LaFeP1-xAsxO, (b) PrFeP1-xAsxO and (c) NdFeP1xAsxO. Red and blue dots indicate superconducting transition temperature Tc and magnetic transition one TN.

References 1. Y. Kamihara et al., J. Am. Chem. Soc. 128, 10012 (2006). 2. K. Kuroki et al., Phys. Rev. B 79, 224511 (2009). 3. I. I. Mazin et al., Phys. Rev. Lett. 101, 057003 (2008). 4. K. T. Lai et al., Phys. Rev. B 90, 064504 (2014). 5. H. Mukuda et al., J. Phys. Soc. Jpn. 83, 083702 (2014). 6. M. Hiraishi et al., Nat. Phys. 10, 300 (2014). 7. H. Usui et al., Sci. Rep. 5, 11399 (2015).

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One-hole and two-holes low-energy states in a cuprate layer Mona Berciu, Clemens Adophs, Mirko Moeller, Hadi Ebrahimnejad, George Sawatzky University of British Columbia Email: [email protected] Keywords: three-band models, quasiparticle dispersion, spin-polarons, effective magnon-mediated interactions We use a variational method to study the strongly-correlated limit of the three-band Emery model, which has spins at Cu sites and doped holes on the O sublattice. For an infinite layer with one doped hole, we find a quasiparticle with the correct dispersion for reasonable values of the Emery parameters. However, unlike in one-band models, the quasiparticle dispersion is not controlled by the spin fluctuations of the antiferromagnetic background; this is due to the hierarchy of the energy scales. We also study two holes doped in an infinite layer, and characterize the effective magnonmediated interactions that arise between them. References 1. "The dynamics of a doped hole in cuprates is not controlled by spin fluctuations", Hadi Ebrahimnejad, George A. Sawatzky and Mona Berciu, Nature Physics 10, 951955 (2014). 2. "Differences in the quasiparticle dynamics for one-band and three-band cuprate models", Hadi Ebrahimnejad, George A. Sawatzky and Mona Berciu, J. Phys.: Cond. Mat. 28, 105603 (2016).

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Signature of the pseudogap critical point in cuprate superconductors Sven Badoux Département de physique & RQMP, Université de Sherbrooke, Sherbrooke, Québec J1K 2R1, Canada

Email: [email protected] Keywords: superconductivity, pseudogap, cuprates Understanding the mechanism of the superconductivity in cuprates will go through the understanding of the different phases that composed these materials and the link between them. One of the biggest mysteries of the cuprates remains the nature of the pseudogap phase. In order to study this phase at low temperature, high magnetic fields are required to suppress superconductivity. I will present measurements of the resistivity, Hall, Seebeck coefficients and thermal conductivity on three cuprate materials, YBCO [1] and LSCO [2,3] and Nd-LSCO[4], performed in magnetic fields large enough to suppress superconductivity at low temperature. These measurements lead to two main findings. First, the pseudogap critical doping p* and the onset of the charge-density-wave order occur at different doping values. So the two phenomena are separate. Secondly, the carrier density n is observed to drop sharply at p*, going from n = 1+ p above p* to n = p below p* (fig. 1). This signature imposes strong constraints on the possible nature of the pseudogap phase.

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Doping dependence of the Hall number, nH = V / e RH. A sharp drop of nH is observed to occur at p*.

References 1. S. Badoux et al, Nature 531, 210–214 (2016). 2. S. Badoux et al, PRX 6, 021004 (2016). 3. F. Laliberté et al, arXiv:1606.04491 (2016). 4. C. Collignon et al, arXiv:1607.05693 (2016).

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Enhanced nematic fluctuations near the Mott insulating phase of high-Tc cuprates Peter P. Orth, Bhilahari Jeevanesan, Joerg Schmalian, Rafael M. Fernandes Iowa State University, Karlsruhe Institute of Technology, Karlsruhe Institute of Technology, University of Minnesota

Email: [email protected] Keywords: high Tc superconductors, cuprates, nematicity, strong-coupling Mott physics The complexity of the phase diagram of the cuprates goes well beyond its unique highTc superconducting state, as it hosts a variety of dierent electronic phenomena, such as the pseudogap, nematic order, charge order, and strange metallic behavior. The parent compound, however, is well understood as a Mott insulator, displaying quenched charge degrees of freedom and low-energy antiferromagnetic excitations described by the Heisenberg exchange coupling J. Here we show that doping holes in the oxygen orbitals inevitably generates another spin interaction a biquadratic coupling that must be included in the celebrated t ∇ J model. While this additional interaction does not modify the linear spin wave spectrum, it promotes an enhanced nematic susceptibility that is peaked at a temperature scale determined by J. Our results explain several puzzling features of underdoped YBCO, such as the proximity of nematic and antiferromagnetic order, the anisotropic magnetic incommensurability, and the in-plane resistivity anisotropy. Furthermore, it naturally accounts for the absence of nematicity in electron-doped cuprates, and supports the idea that the pseudogap temperature is related to strong local antiferromagnetism.

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Nematic uctuations induce a short-ranged magnetic stripe ordered region (light red) within a Néel ordered background (green), as seen in our Monte Carlo simulations.

References 1. V. Hinkov et al., Science 319, 597 (2008). 2. D. Haug et al., New J. Phys. 12, 105006 (2010).

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Cooperative coupling of static magnetism and bulk superconductivity in the stripe phase of La2-xBaxCuO4: Pressure- and dopingdependent studies Z. Guguchia,1,2, R. Khasanov,1 A. Shengelaya,3, 4 E. Pomjakushina,5 S.J.L. Billinge,6 Y.J. Uemura,2, A. Amato,1 E. Morenzoni,1 and H. Keller7 1 Laboratory for Muon Spin Spectroscopy, Paul Scherrer Institute, CH-5232 Villigen PSI, Switzerland 2 Department of Physics, Columbia University, New York, NY 10027, USA 3 Department of Physics, Tbilisi State University, Chavchavadze 3, GE-0128 Tbilisi, Georgia 4 Andronikashvili Institute of Physics of I.Javakhishvili Tbilisi State University, Tamarashvili str. 6, 0177 Tbilisi, Georgia 5 Laboratory for Developments and Methods, Paul Scherrer Institut, CH-5232 Villigen PSI, Switzerland 6 Condensed Matter Physics and Materials Science Department, Brookhaven National Laboratory, Upton, NY 11973, USA 7 Physik-Institut der Universit at Zurich, Winterthurerstrasse 190, CH-8057 Zurich, Switzerland Email: [email protected] Keywords: Pressure effects, Static stripe order, Cuprate superconductors Static spin-stripe order and superconductivity were systematically studied in La2−xBaxCuO4 (0.11 ≤ x ≤ 0.17) at ambient pressure by means of magnetization and µSR experiments [1,2]. We find that all the investigated La2−xBaxCuO4 samples exhibit static spin-stripe order and that the quasi two-dimensional superconducting (SC) transition temperature Tc1 and the static spin-stripe order temperature Tso have very similar values throughout the phase diagram. Moreover, the magnetic and the SC properties of the x = 0.155 (LBCO-0.155) and x = 0.17 (LBCO-0.17) samples were studied under hydrostatic pressure. As a remarkable result, in these bulk cuprate superconductors the three-dimensional SC transition temperature Tc and Tso nearly coincide [Tc(p) = Tso(p)] (Fig. 1) at all pressure investigated (0 ≤ p ≤ 2.3 GPa). We also observed a pressure induced transition from long-range spin stripe order to a disordered magnetic state at p = 1.6 GPa in LBCO-0.155, coexisting with a SC state with substantial superfluid density. In LBCO-0.17 a disordered magnetic state is present at all p. The present results indicate that static magnetic order and SC pairing correlations develop in a cooperative fashion in La2−xBaxCuO4, and provide a new route of understanding the complex interplay between static magnetism and superconductivity in the stripe phase of cuprates.

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The superconducting transition temperature TC and the magnetic ordering temperature TSO of LBCO-0.155, obtained from DC susceptibility and muSR experiments, are plotted as a function of pressure. The magnetic volume fraction Vm is shown as a colormap.

References 1. R. Khasanov, Z. Guguchia et. al., High Pressure Research 36, 140 (2016). 2. Z. Guguchia et. al., Phys. Rev. B 94, 214511 (2016).

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Non-centrosymmetric vortices in multi-component superconductors Adrian Crisan National Institute of Materials Physics Bucharest, 405A, Atomistilor Str., 077125 Magurele, Romania

Email: [email protected] Keywords: multicomponent superconductors; ac susceptibility; vortex dynamics; noncentrosymmetric vortices Multicomponent superconductivity is a novel quantum phenomenon in many different superconducting materials, such as multiband ones in which different superconducting gaps open in different Fermi surfaces, films engineered at the atomic scale to enter the quantum confined regime, multilayers, two- dimensional electron gases at the oxide interfaces, and complex materials in which different electronic orbitals or different carriers participate in the formation of the superconducting condensate [1]. (Cu,C)-, Tland Hg-based multilayer cuprate superconductors are outstanding multiband superconductors, where the interband interaction is very weak and they can be considered as multicomponent superconductors. One of the consequences of two-gap superconductivity could be the presence, in certain conditions, of non-Abrikosov (noncentrosymmetric) vortices, which may be similar (or close) from the point of view of topology, to those proposed theoretically [2-4]. In a certain, but large range of applied DC field, the out-of-phase susceptibility of (Cu,C)Ba2Ca2Cu3Oy with aligned crystallites has two dissipation peaks [5]. Due to the characteristics of the sample, the shape of the second peak and its position’s field dependence, the possibility of inter-grain dissipation was ruled out. The anomalous intra-grain second peak was explained by a resonant rotational motion of a noncentrosymmetric vortex molecule, composed of two fractional flux quanta from the two condensates, glued by a soliton [6]. At that time, this (and subsequent) work has attracted rather little interest due to the rather unconventional cuprates in discussion, the fact that they could be produced only by high-pressure synthesis, and to the impossibility to obtain large single crystals. In the last few years, the discovery of MgB2 and of iron-based superconductors ignited a huge interest in multi-component superconductors. In this context, we have investigated the ac susceptibility of a high-quality single crystal of isovalently substituted iron pnictide BaF2(As0.68P0.32)2 with Tc=28.8K. At the maximum frequency of 10 kHz provided by the PPMS equipment, we have clearly detected a second dissipation peak, which is absent at lower frequencies. An example is shown in

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Superstripes 2017, Ischia June 4-10, 2017 Fig. 1. Further investigation on such single crystals are on the way. Taking into account that now we have investigated a single crystal, we are confident to claim that the ac susceptibility measurement are clearly an experimental evidence of noncentrosymmetric vortices in multi-component superconductors.

Figure 1: Temperature dependence of the out-of-phase susceptibility of the BaF2(As0.68P0.32)2 single crystal in a DC field of 5 T, ac field amplitude of 1 Oe, and frequencies of 6, 8 and 10 kHz

References 1. M.V. Milosevic and A. Perali, Supercond. Sci. Technol., 28 (2015) 060201. 2. V. Stanev and Z. Tesanovic, Phys. Rev. B 81 (2010) 134522. 3. R. Geurts, M.V. Milosevic, F.M. Petters, Phys. Rev. B 81 (2010) 214514. 4. J. Garaud, J. Carlstrom, E. Babaev, Phys. Rev. Lett. 107 (2011) 197001. 5. A. Crisan, Y. Tanaka, et al., Japn. J. Appl. Phys., 46, 19 (2007) L451-L453. 6. Y. Tanaka, A. Crisan, et al., Japn. J. Appl. Phys., 46, 1 (2007) 134-145.

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Anisotropy, phase separation, and superconductivity in iron-based superconductors Roman Puzniak Institute of Physics, Polish Academy of Sciences, PL-02-668 Warsaw, Poland

Email: [email protected] Keywords: high-Tc superconductors, iron based superconductors, anisotropy, nanoscale phase separation The iron chalcogenide superconductors attract a lot of attention due to flexible solid state chemistry and very high critical temperature, Tc, up 100 K for single layer of FeSe. The iron chalcogenides FeSe1−x superconductors are composed of two dimensional sheets held together by van der Waals interactions, which enables their intercalation with various species and interestingly leads to an enhancement of T c. It was suggested that an inhomogeneous spatial distribution of ions and small inclusions of hexagonal phase chalcogenides with nanoscale phase separation seems to enhance the superconductivity [1]. The pnictides, with constituting layers held by stronger ionic forces, do not provide such a possibility of material modification through intercalation chemistry. Nevertheless, it seems that they are excellent material to introduce fine changes in the microstructure of the crystals. Furthermore, they exhibit strong anisotropy of superconducting state properties being a common ingredient for all high-Tc superconductors. Comprehensive review of methods of synthesis and crystal growth, structural and superconducting properties of alkali metal intercalated iron selenides superconductors AxFe2−ySe2 (A = K, Rb, Cs, Tl) was published recently by Krzton-Maziopa et al. [2]. It was concluded that magnetic order is mutually combined with defined iron vacancy order in the structure. It was shown that magnetic order does not hinder an appearance of superconductivity in these materials below ~ 30 K and both magnetism and superconductivity can coexist, as a phase separation occurs in apparently single crystals. The majority phase of the composition A2Fe4Se5 (245) is insulating, magnetic and shows ordered pattern of Fe-vacancies in the structure. The second, minority 122-phase is conducting/semiconducting and becomes superconducting below Tc ~ 30 K. The minority phase, which fills about 10–15% of the crystal volume, is uniformly distributed within the volume of the crystal and it looks to be impossible to increase significantly the minority phase fraction by doping or annealing. The superconducting state is characterized by a very small lower critical field and a large

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Superstripes 2017, Ischia June 4-10, 2017 superconducting penetration depth of the order of 1.8 µm. Superconducting properties were found to strongly depend on exact stoichiometry and post-synthesis annealing. Magnetic and transport measurements of RbxFe2−ySe2 single crystals showed that after annealing at the temperature of phase separation, Tp, a significant rise of Tc is observed and the transition to the superconducting state becomes narrower. The muon spin rotation and relaxation (µSR) and scanning transition electron microscopy (STEM) measurements showed that nonmagnetic regions of the crystal reorder after annealing at Tp = 488 K. It was found that regions size decreases, however, their number increases, hence, in consequence total volume remains the same. It was concluded that annealing of RbxFe2−ySe2 is an effective tool to vary the microstructure of the crystals resulting from mesoscopic phase separation and to improve their superconducting properties, in particular Tc. The aim of presented studies is to clarify the nature of Fe sites of RbxFe2−ySe2 and to determine the impact of annealing at phase separation temperature, leading to more homogenous phase distribution in mesoscopically phase-separated superconductors. The mechanism leading to appearance of superconducting state is proposed. This work was partially supported by the National Science Centre of Poland based on decision No. DEC-2013/08/M/ST3/00927.

References 1. A. Wittlin, P. Aleshkevych, H. Przybylinska, D. J. Gawryluk, P. Dluzewski, M. Berkowski, R. Puzniak, M. U. Gutowska, and A Wisniewski, “Microstructural magnetic phases in superconducting FeTe0.65Se0.35”, Supercond. Sci. Technol. 25, 065019 (2012) http://iopscience.iop.org/0953-2048/25/6/065019 2. A. Krzton-Maziopa, V. Svitlyk, E. Pomjakushina, R. Puzniak, and K. Conder, “Superconductivity in alkali metal intercalated iron selenides”, J. Phys.: Condens. Matter 28, 293002 (2016) https://doi.org/10.1088/0953-8984/28/29/293002

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Low temperature STM/STS of FeSe C. Di Giorgio 1, A. Putilov 1, D. Trainer 1, E. Lechner 1, A. Aladishkin2, A. Melnikov2, O.S. Volkova4,5A. N. Vasiliev, 4,5,6 D. A. Chareev, 7 G. Karapetrov, 2 and M. Iavarone1 1 Department of Physics, Temple University, Philadelphia, Pennsylvania 19122, USA 2 Institute for Physics of Microstructures, Russian Academy of Sciences, Nizhni Novgorod (Russia) 3 Department of Physics, Drexel University, Philadelphia, Pennsylvania 19104, USA 4 Physics Faculty, M.V. Lomonosov Moscow State University, Moscow 119991, Russia 5 Theoretical Physics and Applied Mathematics Department, Ural Federal University, 620002 Ekaterinburg, Russia 6 National University of Science and Technology “MISiS”, Moscow 119049, Russia 7 Institute of Experimental Mineralogy, Russian Academy of Sciences, 142432 Chernogolovka, Moscow District, Russia Email: [email protected] Keywords: low temperature STM, Fe based superconductors, multigap superconductivity, vortex matter FeSe superconductors and their related systems have attracted much attention in the study of iron-based superconductors owing to their simple crystal structure and peculiar electronic and physical properties. The bulk FeSe superconductor has a superconducting transition temperature (Tc) of ~8 K and it can be dramatically enhanced both by applying pressure and by chemical substitution. We will present low temperature scanning tunneling microscopy and spectroscopy measurements of high quality FeSe single crystals. Our results show the multigap nature of superconductivity in this material from the vortex lattice and the change of tunneling spectra as a function of chemical substitution. The role of disorder and twin boundaries in this material will be discussed as well.

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STM/STS studies on the spatial dependence of energy gap in high-Tc cuprate Bi2Sr2CaCu2O8+δδ M. Oda,1 Kurosawa,1 S. Mizuta,1 T. N. Momono,2 K. Takeyama,3 H. Yoshida1 and M. Ido1 1 Department of Physics, Hokkaido Univ., Sapporo, Japan 2 Department of Applied Science, Muroran Institute of Technology, Muroran, Japan 3 Department of Physics, Asahikawa Medical Univ., Asahikawa, Japan Email: [email protected] Keywords: STM/STS; 1d-superlattices; superconducting gap; pseudogap; charge order. One of the interesting features of Bi-based high-Tc cuprates such as Bi2Sr2CaCu2O8+δ (Bi2212) is the one-dimensional (1D) superlattice structure along the b-axis (forming a slant angle of 45° from the Cu-Oinplane bond direction), modulating the bond length between the Cu and the apical O (Oapical) atoms of CuO5 pyramids [1]. This has been considered to cause a periodic modulation of the antiferromagnetic (AFM) coupling between Cu-spins J(r) [2]. The period of the 1D superlattice, ~26 Å, is similar to the superconducting (SC) coherence length of this system; therefore, it may affect local properties of the superconductivity [2, 3]. In this study, to clarify the effect of such a superlattice structure on the high-Tc superconductivity, we performed STM/STS experiments at 8 K on cleaved surfaces of underdoped (UD) and optimal (OP) Bi2212 crystals and examined the intrinsic spatial dependence of the SC gap (SCG). In many cases of STM/STS experiments on UD Bi2212 crystals, we observe STS spectra consistent with a two-gap structure consisting of a d-wave SCG and a spatially inhomogeneous pseudogap (PG), the so-called “large PG,” whose size ∆PG varies in nanometer scale over a wide range from an energy of the d-wave SCG amplitude ∆0 to several times larger one [6,8]. In such UD crystals, images of the local density of states (LDOS) exhibit a nanostripe Cu-O-Cu bond-centered modulation at higher energies around ∆PG and a checkerboard modulation at lower energies around ∆0 [4~8]; therefore, it is unclear whether the overall SCG on the entire Fermi surface correlates with the 1D supperlattice. On some cleaved surfaces of OP Bi2212, however, no nanostripe and checkerboard modulations are observed, and the STS spectrum exhibits a single d-wave like SCG structure within the areas examined; it is characterized by sharp peaks at bias voltages Vs corresponding to the gap edges Vs = ±∆0/e and a V-shaped bottom at lower voltages around Vs=0. This enabled us to demonstrate the intrinsic spatial dependence of SCG in Bi2212. Thus, it has been demonstrated that the d-wave like SCG changes along the b-axis with an amplitude of ~5% of its average value and the same period as in the 1D superlattice or J(r). This finding probably suggests a correlation between the SCG and the AFM coupling of Cu-spins.

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References 1. A. Bianconi et al., Phys. Rev. B 54, 4310 (1996). https://doi.org/10.1103/PhysRevB.54.4310 2. M. Mori et al., Phys. Rev. Lett. 101, 247003 (2008). https://doi.org/10.1103/PhysRevLett.101.247003 3. J. A. Slezak et al., PNAS 105, 3203 (2008). https://doi.org/10.1073/pnas.0706795105 4. Y. Kohsaka et al., Science 315, 1380 (2007). https://doi.org/10.1126/science.1138584 5. Y. Kohsaka et al., Nature 454, 1072 (2008). https://doi.org/10.1038/nature07243 6. T. Kurosawa et al., Phys. Rev. B 81, 094519 (2010). https://doi.org/10.1103/PhysRevB.81.094519 7. T. Machida et al., Nature Communications 7, 11747 (2016). http://doi.org/10.1038/ncomms11747 8. T. Kurosawa et al., J. Phys. Soc. Jpn. 85, 0447091 (2016). http://doi.org/10.7566/JPSJ.85.044709

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Ultrafast doublon dynamics in photo-excited 1T-Tas2 Isabella Avigo1, Manuel Ligges1, Denis Golež 2, Hugo U. R. Strand2, Ljupka Stoichevska1, Matthias Kalläne3, Kai Rossnagel3, Martin Eckstein4, Philipp Werner2, Uwe Bovensiepen1 1 Faculty of Physics, University of Duisburg-Essen, 47048 Duisburg, Germany 2 Department of Physics, University of Fribourg, 1700 Fribourg, Switzerland 3 Institute of Experimental and Applied Phisics, University of Kiel, 24098 Kiel, Germany 4 Max Plank Research Departement of Structural Dynamics, University of HamburgCFEL, 22761 Hamburg, Germany Email: [email protected] Keywords: Ultrafast spectroscopy; Charge-density-wave; Mott physics; Metalinsulator transition Complex matter is characterized by a rich interplay among different microscopic degrees of freedom. This competition or coexistence often occurs on comparable energy scales and, thus, might be hard to disentangle in the spectral domain. Analyzing the dynamics of such systems, driven out of equilibrium, has the potential to shed new light on the underlying short- and long-range interactions because different coupling mechanisms can result in the time domain on experimentally distinguishable femto- to picosecond time scales[1]. 1T-TaS2 is a quasi-2 dimensional Mott-Insulator that also exhibits strong electron-phonon interaction, making it a suitable model system to address such a problem. Using femtosecond time-resolved photoemission spectroscopy we monitor the transient population of the upper Hubbard band, showing that doublon dynamics occur on a time scale of one or few hopping cycle, ruling out any interaction with the lattice. This finding leads to a reduction in complexity in the formulation of theoretical modeling, as, at least at early stages after photo-perturbation, only the electronic part of the system can be considered, while the phononic part is still frozen. From results of a theoretical modeling obtained with a nonequilibrium dynamical mean field theory approach, we concluded that the dynamics of the transient population of the doublon states are governed by the effective band filling and our results are best reproduced in the case of a slightly hole-doped system. Financial support by the Deutsche Forschungsgemeinschaft through SFB 616, SPP 1458, SFB 1242 and from ERC Starting Grant No. 278023 are gratefully acknowledged. References 1. C. Giannetti et al., Advance Physics 65, 58 (2016) http://dx.doi.org/10.1080/00018732.2016.1194044

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Dynamical phase transition to a charge-transfer state N. Kirova1 and S. Brazovskii2 1 LPS, CNRS, Univ. Paris Sud, Université Paris–Saclay 91405, Orsay, France 2 LPTMS, CNRS, Univ. Paris Sud, Université Paris–Saclay 91405, Orsay, France

Email: [email protected] Keywords: optical pumping, excitons, excitonic insulator, pump induces phase transition A dynamical phase transition can be provoked by a short optical pumping to excitons. We consider a system prone to a thermodynamic instability towards a charge-ordered state of electrons like in neutral-ionic transitions of donor-acceptor structures. We notice that here the density of pumped excitons contributes additively to the thermodynamic order. To describe both thermodynamic and dynamical effects on equal footing, we adopt for the phase transition a view of the “excitonic insulator” and suggest a formation of the macroscopic quantum state for the pumped excitons. The double nature of the ensemble of excitons leads to an intricate time evolution: the dynamical transition between number–preserved and phase–locked regimes, macroscopic quantum oscillations from interference between the Bose condensate of excitons and the ground state of the excitonic insulator. Modelling of an extended sample shows also stratification in domains of low and high densities which evolve through local dynamical phase transitions and a sequence of domains’ merging.

Formation of domains of the new phase.

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References 1. S. Brazovskii and N. Kirova, “The excitonic insulator rout trough a dynamical phase transition induced by an optical pulse”, JETP, 122, (2016) 412; arxiv.org/abs/1512.06200 2. S. Brazovskii and N. Kirova “Dynamical phase transitions and patterns formation induced by pulse pumping of excitons to a system near a thermodynamic instability”, Phys. Rev. B 94 (2016) 054302.

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Ultrafast orbital manipulation in copper oxides Claudio Giannetti Email: [email protected] The short-range interactions among the electrons occupying the copper and oxygen orbitals represent the universal underlying mechanism of many exotic properties of copper oxides, such as the intrinsic nanoscale electronic inhomogeneity, the ubiquitous antinodal pseudogap and the onset of high-temperature superconductivity. Recently, the use of ultrashort light pulses has been introduced as a "non-conventional" control parameter to transiently manipulate the electronic occupation of the Cu-3d and O-2p orbitals and investigate the fundamental interactions that drive the relaxation towards the correlated ground state. Here I will review the most recent discoveries obtained via the ultrafast manipulation of the orbital occupation in cuprates. I will discuss the existence of a high-temperature crossover at optimal doping between the physics of a doped Mott insulator to that of a more coherent metal. The underdoped correlated ground state constitutes the fertile ground for the onset of the low-temperature symmetry-breaking instabilities, such as the onset of charge-order.

References 1. S. Peli et al. The room temperature prodrome of charge-order in copper oxides. arXiv:1508.03075 (2016).

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Collective electronic orders under strong optical drive studied by means of time-resolved multipulse optical spectroscopy A. Pogrebna (1,2), I. Madan (1), P. Kusar (1), M. Naseska (1), V. V. Kabanov (1), T. Mertelj (1,3), Z. A. Xu (4), M. Oda (5), D. Mihailovic (1,3) (1) Complex Matter Department, Jozef Stefan Institute, Jamova 39, SI-1000 Ljubljana, Slovenia (2) Radboud University, Institute for Molecules and Materials, Nijmegen 6525 AJ, The Netherlands (3) Center of Excellence on Nanoscience and Nanotechnology – Nanocenter (CENN Nanocenter), Jamova 39, SI-1000 Ljubljana, Slovenia (4) Department of Physics, Zhejiang University, Hangzhou 310027, People’s Republic of China (5) Department of Physics, Hokkaido University, Sapporo 060-0810, Japan Email: [email protected] Keywords: ultrafast nonequilibrium phase transitions, cuprates, iron based superconductors, superconductivity, spin density wave Ultrafast phase transitions from and into electronically ordered states, which occur during the quench following a strong femtosecond laser excitation, have become a rather hot research topic during the last decade. Experimentally various electronic orders were investigated with emphasis on ferromagnetism[1], charge and orbital ordering[2], charge density waves[3] as well as superconductivity[4]. Despite a significant occurrence frequency of antiferromagnets the collinear antiferromagnetism and the related spin density wave (SDW) order were among the less studied orders[5] in this context, perhaps due to the absence of the linear coupling of the order parameter to photons. Despite the lack of momentum sensitivity and in general complicated response functions all-optical time resolved spectroscopy can give some insight into ultrafast optical quenches of electronic order parameters. I will present some of our recent efforts reaching beyond the standard 2-pulse pump-probe technique applied to the cuprate superconductors and the antiferromagnetic spin density wave in undoped iron based pnictides. References 1. A. Kirilyuk, A. V. Kimel, T. Rasing, Rev. Mod. Phys. 82, 2731 (2010). 2. T. Ogasawara, T. Kimura, T. Ishikawa, M. Kuwata-Gonokami, and Y. Tokura, Phys. Rev. B 63, 113105 (2001). 3. P. Kusar, V. V. Kabanov, J. Demsar, T. Mertelj, S. Sugai, and D. Mihailovic, Phys. Rev. Lett. 101, 22700 (2008). 4. F. Schmitt et al., Science 321, 1649 (2008). 5. A. V. Kimel, R. V. Pisarev, J. Hohlfeld, and Th. Rasing, Phys. Rev. Lett. 89, 287401 (2002).

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Unraveling the ultrafast dynamics of spatially confined phonons and plasmons in low-dimensional nanosystems Giovanni Maria Vanacore and Fabrizio Carbone Institute of Physics, Laboratory for Ultrafast Microscopy and Electron Scattering (LUMES), Ecole Polytechnique Fédérale de Lausanne (EPFL), Lausanne, Switzerland Email: [email protected] Keywords: Ultrafast electron microscopy; quantum materials; ultrafast dynamics; phononics; plasmonics Understanding the ultrafast evolution of low-dimensional materials under nonequilibrium conditions plays a fundamental role in deciphering the mechanism governing chemical and physical functions. With direct visualization, the technological development of new generation nanoscale devices would become feasible. Although an enormous effort has been devoted to the comprehension and improvement of these materials and devices, the capability of investigating their dynamic behavior is hindered by the difficulty of simultaneously studying their evolution in space and time at the appropriate scales. Instead, a novel approach for visualization of matter with high temporal and spatial resolutions, together with momentum and energy selection, is indispensable to fully exploit their potential. Ultrafast electron microscopy (UEM) has been recently developed with the capability of performing time-resolved imaging, diffraction and electron-spectroscopy [1], making this technique a unique tool for the dynamic investigation of surfaces, interfaces and nanostructures. In this contribution, we will address several recent applications to the investigation of elementary excitations, such as plasmons and phonons, in low-dimensional nanosystems. In particular, it is now possible to visualize and control the dynamics of the plasmonic near-field optically created in the vicinities of single nanostructures or arrays of nanocavities with nanometer spatial and femtosecond temporal resolutions [2,3]. Also, ultrafast diffraction is used to unveil the effect of the reduced dimensionality on the non-equilibrium dynamics of lattice vibrations (phonons) and their transport regimes [4,5].

References 1. G.M. Vanacore, et al., Nano Today 11, 228-249 (2016). 2. L. Piazza, et al., Nat. Commun. 6, 6407 (2015). 3. T.T.A. Lummen, et al., Nat. Commun. 7, 13156 (2016). 4. G.M. Vanacore, et al., Nano Lett. 14, 6148-6154 (2014). 5. J. Hu, G. M. Vanacore, et al., Proc. Natl. Acad. Sci. USA 113, E6555-E6561 (2016).

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Nonequilibrium quasiparticle dynamics in Bi-based superconductors measured by modulation photoexcitation spectroscopy Y. Toda1, S. Tsuchiya1, T. Kurosawa2, M. Oda2, T. Mertelj3, I. Madan3, D. Mihailovic3 1 Department of Applied Physics, Hokkaido University, Sapporo, 060-8628, Japan 2 Department of Physics, Hokkaido University, Sapporo, 060-0810, Japan 3 Complex Matter Department, Jozef Stefan Institute, Ljubljana, SI-1000, Slovenia. Email: [email protected] Keywords: time-resolved spectroscopy; polarimetry; pseudogap. Optical pump-probe (Pp) spectroscopy using femtosecond pulse laser is a unique method to separate the quasiparticle (QP) dynamics associated with superconducting (SC) and pseudogap (PG) states in the time-domain, where the nonequilibrium QPs excited by the pump are characterized by the reflectivity changes of the probe pulse with variable delay times between the two pulses [1]. The method can also clarify the condensation dynamics of superconductors within the photoexcited volume when the pump fluence is strong enough to destroy the SC state. In the previous studies on underdoped Bi2212, we found that the QP dynamics shows a delay of the SC recovery whose duration time increases with increasing the fluence. Since the observed delay time is comparable to the relaxation time of the PG QPs, we concluded that the PG is responsible for the formation of high-Tc superconductivity [2]. We also performed the polarized Pp experiments, which can separate the QP dynamics into isotropic and anisotropic modes with respect to the probe polarization [3]. Since the pump polarization shows almost no distinct anisotropies, the probe anisotropies can be associated with the presence of a spontaneous breaking of the rotational symmetry. Here the anisotropic SC and PG signals are identified to be polarized along the crystalline axes and Cu-O bond directions, respectively, which are associated with B1g and B2g like symmetries of the dielectric tensor in analogy with Raman spectroscopy. The B2g symmetry breaking can originate in the weak orthorhombicity of the crystal (BiO chain ordering) while the B1g symmetry breaking can be associated with the softness of the CuO2 planes towards stripe ordering or similar textures. The conventional Pp spectroscopy including the above employs lock-in detection by chopping the pump pulse, where we can detect the differences of the probe reflectivity with and without the pump pulse (upper part of Fig. 1 (a)). In this work, we employ the modulation of the pump polarization/chirality for the lock-in detection (lower part of Fig. 1 (a)). The modulation of the pump polarization was realized by using a liquid crystal variable retarder. The combination with a vortex waveplate can provide additional modulations of the chirality. Because of the nearly constant pump fluence, improvements of the sensitivity to the local/global anisotropies induced by the pump

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Superstripes 2017, Ischia June 4-10, 2017 can be expected. In the weak excitation condition, no additional change to the QP dynamics was detected while anomalous transients were observed above the saturation condition of the SC (Fig. 1 (b)). We will discuss the results from the viewpoint of the symmetry breaking in the ground states of Bi-based superconductors.

Figure 1: (a) schematic illustrations of the lock-in detections for Pp spectroscopy, (b) DR/R obtained by polarization-modulation Pp spectrocopy with various pump fluences. The data were measured for UD Bi2212 at 10K.

References 1. YH. Liu et al., Phys. Rev. Lett. 101, 137003 (2008). 2. Y. Toda et al., Phys. Rev. B 84, 174516 (2011). 3. Y. Toda et al., Phys. Rev. B. 90, 094513 (2014).

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Delocalized charge carriers in strongly disordered t–J model Janez Bonca (1, 2) and Marcin Mierzejewski (3) 1) Jozef Stefan Institute, SI-1000 Ljubljana, Slovenia 2) Faculty of Mathematics and Physics, University of Ljubljana, SI-1000 Ljubljana, Slovenia 3) Institute of Physics, University of Silesia, 40-007 Katowice, Poland

Email: [email protected] Keywords: many body localisation, sub-diffusion, t-J model We show that electron-magnon interaction delocalizes the particle in a strongly disordered system. The analysis is based on results obtained for a single hole in the one–dimensional t–J model. Unless there exists a mechanism that localizes spin excitations, the charge carrier remains delocalized even for a very strong disorder and shows subdiffusive motion up to the longest accessible times. Moreover, upon inspection of the propagation times between neighboring sites as well as a careful finite–size scaling we conjecture that the anomalous subdiffusive transport may be transient and should eventually evolve into a normal diffusive motion.

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Superconductivity emerging from an electronic phase separation in the charge ordered phase of RbFe2As2 M. Moroni1, E. Civardi1, M. Babij2, Z. Bukowski2, P. Carretta1 Dipartimento di Fisica ed Unità CNISM di Pavia, I-27100 Pavia, Italy 2 Institute of Low Temperature, Polish Academy of Sciences, 50422 Wroclaw, Poland 1

Email: [email protected] Keywords: charge order, iron based superconductor, orbital selective Mott transition. Half band filling, a necessary requisite for Mott insulators, can be approached in BaFe2As2 iron based superconductors (IBS) by replacing Ba with an alkali atom A=K, Rb or Cs, resulting in 5.5 electrons per Fe atom [3]. Transport measurements show that AFe2As2 compounds are still metallic [5] but with sizable electronic correlations and with a low Tc superconducting transition [2] (for RbFe2As2 Tc = 2.7 K). Nuclear quadrupole resonance (NQR) is a powerful tool to probe the local charge distribution. Upon cooling the sample below T0≃140 K the 75As NQR [1] spectrum is observed to progressively broaden with decreasing temperature and below 50 K one clearly observes that the spectrum is actually formed by two peaks nearly symmetrically shifted with respect to the center. This shape recalls the one expected for an incommensurate charge density wave, which causes a periodic modulation of the electric field gradient (EFG) at the nuclei and gives rise to two symmetrically shifted peaks in the spectrum. However the EFG modulation could also be due to the onset of an orbital order. A single exponential recovery function describes very well the recovery of the 75As NQR nuclear magnetization at T ≥ 20K. Below T∗≃ 20 K one observes the appearance of a second component characterized by much longer relaxation times. The appearance of different relaxation rates below T∗ arise from a phase separation causing marked differentiation in the low-energy excitations which starts to be significant at low temperature once the effect of electronic correlations is relevant. This behavior could possibly be explained in terms of the orbital selective Mott transition predicted for IBS [4]. The observation of a phase separation in the hole-doped IBS is also supported by recent theoretical works [6].

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Figure 1: 75As NQR spectrum in RbFe2As2 at different temperatures between 5 K and 300 K.

References 1. E. Civardi et al., Phys.Rev.Lett. 117, 217001 (2016). 2. Y. P. Wu et al., Phys.Rev.Lett. 116, 147001 (2016). 3. F. F. Tafti et al., Phys. Rev. B 91, 054511 (2015). 4. L. de’ Medici et al., Phys.Rev.Lett. 102, 126401 (2009). 5. F. Eilers et al., Phys. Rev. Lett. 116, 237003 (2016). 6. L. de’ Medici, arXiv:1609.01303v1

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High temperature superconductivity in hydrides at high pressures M. I. Eremets, A. Drozdov Max Planck Institute for Chemistry

Email: [email protected] Keywords: high tempearture superconductivity, high pressure We will present our recent results on the high temperature superconductivity up to 203 K[1] in different hydrides. The superconductivity has been proved by observation of zero resistance, Meissner effect, isotope effect, and X-ray diffraction studies [2]. Recent results on infrared and Raman studies will be presented. Pure hydrogen also will be discussed. The observed apparently conventional superconductivity will be discussed in view of numerous theoretical works. Recent proposals of new superconducting materials and prospects for achieving higher critical temperatures of superconducting transition will be discussed too.

References 1. Drozdov, A.P., et al., Conventional superconductivity at 203 K at high pressures. Nature 2015. 525: p. 73-77. 2. Einaga, M., et al., Crys

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Stable high-pressure phases in the H-S system determined by chemically reacting hydrogen and sulfur up to 140 GPa Alexander F. Goncharov 1 Key Laboratory of Materials Physics, Institute of Solid State Physics CAS, Hefei 230031, China 2 Geophysical Laboratory, Carnegie Institution of Washington, Washington, DC 20015, USA

Email: [email protected] Keywords: H-S system, structure, high pressure, superconductivity Synchrotron X-ray diffraction and Raman spectroscopy have been used to study chemical reactions of molecular hydrogen with sulfur at high pressures. We find theoretically predicted Cccm and Im-3m H3S to be the reaction products at 50 and 140 GPa, respectively. Im-3m H3S is a stable crystalline phase above 140 GPa and it transforms to R3m H3S on pressure release below 140 GPa. The latter phase is (meta)stable down to at least 70 GPa where it transforms to Cccm H3S upon annealing (T

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Search for Pressure-Induced Superconductivity in Other Hydrides Katsuya Shimizu1, Harushige Nakao1, Akiyoshi Masuda1, Mari Einaga1, Masafumi Sakata1, Naohisa Hirao2, Saori Kawaguchi2, Yasuo Ohishi2 1 KYOKUGEN, Grad. Sch. Eng. Sci., Osaka University, 1-3 Machikaneyama, Toyonaka, Osaka 560-8531, Japan 2 JASRI, 1-1-1, Sayo-cho, Sayo-gun, Hyogo 679-5198, Japan Email: [email protected] Keywords: high-Tc; H2S; pressure Superconductivity above 200 K was recently reported in the highly compressed hydrogen sulfide (H2S) [1]. The crystal structure of the superconductor was studied by using the synchrotron x-ray diffraction at room temperature and the superconducting temperature [2]. The creation of the superconductor from pure H2S was experimentally confirmed. The sample was compressed in a diamond-anvil cell (DAC) with the same process with the ref.1, and cooled down to 10 K in the cryostat in SPring-8. The critical temperature and zero resistivity were observed around 180 K. Recently other candidates of the high-Tc superconductor in other hydrides were theoretically predicted [3]. We conducted the synthesis of other/new hydrides by compression of some elements in pure hydrogen under low temperature. This work was supported by JSPS KAKENHI Grant Number 26000006.

References 1. A. Drozdov et al., Nature 525, 73 (2015). http://dx.doi.org/10.1038/nature14964. 2. M. Einaga et al., Nature Physics 12, 835 (2016). http://dx.doi.org/10.1038/nphys3760. 3. Y. Li et al., Scientific Reports 5, 9948 (2015). http://dx.doi.org/10.1038/srep09948.

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Optical Spectroscopy of H3S: Evidence of a new Energy Scale for superconductivity Thomas Timusk4,5, F. Capitani1, B. Langerome1, J.-B. Brubach1, A. Drozdov2, M.I. Eremets2, E. J. Nicol3, J. P. Carbotte4,5, and P.Roy1 1 Synchrotron SOLEIL, AILES Beamline, Saint-Aubin, 91190, France 2 Biogeochemistry Department, Max Planck Institute for Chemistry, PO Box 3060, 55020 Mainz, Germany 3 Department of Physics, University of Guelph, Guelph, N1G 2W1 ON Canada 4 Department of Physics and Astronomy, McMaster University, Hamilton, ON L8S 4M1, Canada 5 The Canadian Institute for Advanced Research, Toronto, ON M5G 1Z8 Canada Email: [email protected] Keywords: superconductors, high pressure, infrared. reflectance, H3S.

The discovery of a superconducting phase in sulfur hydride under high pressure with a critical temperature above 200 K by Drozdov et al. [1] has provided a new impetus to the search for even higher Tc. Theory predicted and experiment confirmed that the phase involved is H3S with Im-3m crystal structure. The observation of a sharp drop in resistance to zero at Tc, its downward shift with magnetic field and a Meissner effect confirm superconductivity but the mechanism involved remains to be determined. Using the AILES beamline at Soleil, we provide a first optical spectroscopy study of this new superconductor.[2] Experimental results for the optical reflectivity of H3S, under high pressure, for several temperatures and over the range 60 to 600 meV of photon energies, are compared with theoretical calculations based on Eliashberg theory using DFT results for the electron-phonon spectral density. Two significant features stand out: some remarkably strong infrared active phonons at approximately 160 meV and a band with a depressed reflectance in the superconducting state in the region from 450 meV to 600 meV. In this energy range, as predicted by theory, H3S is found to become a better reflector with increasing temperature. This temperature evolution is traced to superconductivity originating from the electron-phonon interaction. The shape, magnitude, and energy dependence of this band at 150 K agrees with our calculations. This provides strong evidence of a conventional mechanism. However, the unusually strong optical phonon suggests a contribution of electronic degrees of freedom.

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Figure 1: Reflectance ratio R(T)/R(200 K) of H3S in a diamond anvil cell. Solid blue curve is the measured ratio, normalized at 600 meV. The dashed curves are calculated ratios. The strong between 100 and 600 meV are structures are due to bosons. The step at 175 meV is due to the energy gap.

References 1. A.P. Drozdov, M.I. Eremets, I.A. Troyan, V. Ksenofontov, and  S.I. Shylin, “Conventional superconductivity at 203 Kelvin at high pressures in the sulfur hydride system,” Nature 525, 73–76 (2015). http://dx.doi.org/10.1038/nature14964. 2. F.Capitani, B. Langerome, J.-B. Brubach, P. Roy, A. Drozdov, M.I. Eremets, E. J. Nicol, J. P. Carbotte, and T. Timusk, Spectroscopy of H3S: evidence of a new energy scale for superconductivity, arXiv:1612.0673v2

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Fano Resonances at Lifshitz transitions driving high Tc superconductivity: from iron based superconductors to the case of H3S and p-Terphenyl Antonio Bianconi 1,2,3 RICMASS, Rome International Centre for Material Science Superstripes, via dei Sabelli 119A, 00185 Rome, Italy 2 Institute of Crystallography, CNR, via Salaria Km 29.300, Roma, I-00015, Italy 3 National Research Nuclear University MEPhI 115409 Moscow Russia 1

Email: [email protected] Keywords: superlattices of atomic layers or atomic wires; Lifshitz transitions, Fano resonances, Shape resonances, Feshbach resonances In 1993 it has been proposed that particular metallic heterostructures at the atomic limit made of superconducting units (layers, or stripes, or wires, or spheres) having a nanoscale size of the order the wavelength of electrons at the Fermi level (see panel A in Fig.1) could show high temperature superconductivity (HTS) if the chemical potential EF is tuned near a Lifshitz transition (see panel A in Fig.2)

i)

ii)

Figure Panel:i) the drawing of the heterostructure in the patent [1] and the structure or pTerphenyl Panel ii) the drawing of DOS for HTS from ref.[1], and the tuning of the chemical potential in K dopecd p-Terphenyl ref. [2]

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Superstripes 2017, Ischia June 4-10, 2017 This scenario has been called the superstripes scenario where a striped landscape promotes HTS where the chemical potential is tuned by chemical doping, strain or pressure [2]. The maximum Tc amplification is driven by "Fano resonance" between the pairs in the BCS-BEC crossover in the n-th new appearing band with low Fermi energy and BCS like pairs in the other (n-1) bands with large Fermi energies. This scenario has been recently proposed for plastic superconductors like p-terphenyl [3] and for H3S [4]. Fig. 1 shows structure (a superlattice of stripes) of p-terphenyl and the tuning the chemical potential at a Lifshitz transition in the conduction band of p-terphenyl by K doping. However the synthesis of HST tuned at a Lifshitz transition is very difficult task since in strongly correlated multiband systems tuning the chemical potential at a Lifshitz transition gives a frustrated or arrested multiscale phase separation from nano-scale to micron-scale [5] . This is the driving force for the emerging of granular matter with percolating pathways at the interphase between multiple puddles where a complex non Euclidean geometry promotes quantum coherence at high temperature at optimum doping [6]. Therefore a similar complex structural landscape is expected for pTerphenyl at optimum doping.

References 1. A. Bianconi, “Process of increasing the critical temperature Tc of a Bulk Superconductor by Making Metal Heterostructures at the Atomic Limit” US Patent 6,265,019 (2001) priority date Dec 7, 1993. 2. A. Bianconi, Nature. Phys. 9, 536 (2013) 3. M.V. Mazziotti, A. Valletta, G. Campi, D. Innocenti, A. Perali, A. Bianconi (May 26, 2017) Arxiv: arXiv:1705.09690 4. A. Bianconi, T. Jarlborg EPL (Europhysics Letters 112, 37001 (2015) 5. N. Poccia, et al., Proc. Nat. Acad. Sci. 109, 15685 (2012). 6. G. Campi, A. et al., Nature 525, 359 (2015).

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Compressed H2S, superfluid density and the quest for roomtemperature superconductivity Jeff Tallon1,2, Evgeny Talantsev1, Wayne Crump1, James Storey1 Robinson Research Institute, Victoria University of Wellington, P.O. Box 33436, Lower Hutt 5046, New Zealand. 2 MacDiarmid Institute for Advanced Materials and Nanotechnology, Victoria University of Wellington, P.O. Box 33436, Lower Hutt 5046, New Zealand. 1

Email: [email protected] Keywords: high pressure, fluctuations, H2S, room-temperature superconductivity, superfluid density. Fifty years ago New Zealand physicist Neil Ashcroft predicted that hydrogen at very high-pressure would superconduct at room temperature [1]. Now that prediction is close to being realized. Recently sulphur hydride when compressed to 1.5 million atmospheres was shown to superconduct with Tc = 203 K, the current record [2]. But at such temperatures thermal fluctuations might be expected to break up Cooper pairs. For example in the cuprates fluctuations reduce T c by 30% or more below the meanfield value which in Bi2212 is as high as 150 K [3]. Similar effects are found in iron pnictides. How does superconductivity survive in sulphur hydride at such balmy temperatures?

Figure 1: The penetration depth and superfluid density of highly compressed H3S is calculated from the critical current density. It implies a very high phase fluctuation temperature scale (1470 K) arising from the 3D character of H3S.

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Superstripes 2017, Ischia June 4-10, 2017 The key parameter underlying fluctuations in these materials is the superfluid density [4]. We show how this can be measured in any superconductor from the self-field critical current density [5]. This leads to a remarkable scaling behaviour observed for all superconductors of any type or symmetry or anisotropy, including single atomiclayer superconductors. In this way we measure the superfluid density in compressed sulphur hydride in order to determine the fluctuation temperature scale Tfluc. We find that Tfluc for phase fluctuations exceeds 1400 K [6] and it is shown that dimensionality plays a key role in suppressing or enhancing thermal fluctuations to the benefit of hydrogen sulphide and the detriment of its more layered 2D competitors. At the same time we find that Tfluc for amplitude fluctuations is around 300 K and this has important implications for the quest for room temperature superconductivity. Appealing to the way in which superfluid density, Tfluc and Tc each scale it seems that room temperature superconductivity is nearly ruled out – but not quite.

References 1. N.W. Ashcroft, Phys. Rev. Lett. 21, 1748 (1968). https://doi.org/10.1103/PhysRevLett.21.1748. 2. A.P. Drozdov et al., Nature 525, 73 (2015). http://dx.doi.org/10.1038/nature14964. 3. J.L. Tallon, J.G. Storey and J.W. Loram, Phys. Rev. B 83, 092502 (2011). https://doi.org/10.1103/PhysRevB.83.092502. 4. V.J. Emery and S.A. Kivelsen, Nature 374, 434 (1995). http://dx.doi.org/10.1038/374434a0. 5. E.F. Talantsev, J.L. Tallon, Nature Commun., 6, 7820 (2105). https://dx.doi.org/10.1038%2Fncomms8820. 6. E.F. Talantsev, W.P. Crump, J.G. Storey, J.L. Tallon, Annalen der Physik, 529 (3),1600390 (2017). http://dx.doi.org/10.1002/andp.201600390.

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High Temperature Superconductivity in H3S --- why so high? Frank Marsiglio1* 1 University of Alberta Department of Physics

Email: [email protected] Keywords: high Tc, superconductivity, microscopic mechanism H3S superconducts at unprecedented high temperature under extreme pressures. It is important to understand why, particularly if we are to optimize the possibility of achieving the necessary conditions through chemical pressure by way of substitution. In this talk we will discuss possible mechanisms and the role of the electronic density of states in achieving high Tc.

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Formation Process of High-Tc Phase of Sulfur Hydride Mari Einaga1, Masafumi Sakata1, Katsuya Shimizu1, Alexander Drozdov2, Mikhail Eremets2, Saori Kawaguchi3, Naohisa Hirao3, and Yasuo Ohishi3. 1 KYOKUGEN, Graduate School of Engineering Science, Osaka University, Machikaneyamacho 1-3, Toyonaka, Osaka, 560-8531, Japan. 2 Max-Planck Institute for Chemistry, Hahn-Meitner-Weg 1, 55128 Mainz, Germany. 3 JASRI, 1-1-1, Sayo-cho, Sayo-gun, Hyogo 679-5198. Email: [email protected] Keywords: Superconductivity; Synchrotron X-ray Diffraction; Sulfur Hydride. The cooperation between theoretical and experimental investigation broke the record for superconducting critical temperature Tc in hydrogen sulfide under high pressure at the end of 2014[1, 2]. The material improved the highest-T c by more than 30 K and shows the conventional superconductivity. By compression at low temperature 200 K, a superconducting phase named low-T c phase appears above 100 GPa, and Tc increases up to 150 K with increase pressure to 200 GPa. High-T c phase showing Tc over 200 K is obtained after the low-Tc phase is annealed at near room temperature. Several theoretical groups proposed the crystal structures and the value of Tc in some sulfur hydrides HxSy under pressure [3-6]. According to the results of their theoretical calculation, it is considered that H2S dissociates into H3S and elemental sulfur through other stoichiometric compounds HxSy, and the H3S which has cubic structure shows the high-Tc over 200 K [2, 3]. However, the phase boundary and the crystal structures above 50 GPa were not determined experimentally yet. Here we report our recent results of synchrotron X-ray diffraction studies on formation process of the superconducting phases in sulfur hydride and deuteride. Our results suggest that H2S dissociates into H3S and elemental sulfur under high pressure through metastable phases, and the high-Tc phase corresponds to theoretically predicted cubic-H3S. References 1. A. P. Drozdov et al., Nature, 525, 73 (2015). 2. Y. Li et al., J. Chem. Phys., 140, 174712 (2014). 3. D. Duan et al., Sci. Rep., 4, 6968 (2014). 4. I. Errea et al., Phys. Rev. Lett., 114, 157004 (2015). 5. T. Ishikawa et al., Sci. Rep., 6, 23160 (2016). 6. R. Akashi et al., Phys. Rev. Lett., 117, 075503 (2016). 7. M. Einaga et al., Nature Phys. 12, 835 (2016).

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Impurity dependent superconductivity, Berry phase and bulk Fermi surface of the Weyl type-II semi-metal candidate MoTe2 D. Rhodes†,1, 2 Q. Zhou†,1, 2 R. Schoenemann,1, 3 Q. R. Zhang,1, 2 E. Kampert,3 Y.-c. Chiu,1, 2 Y. Lai,1, 2 Y. Shimura,1, 4 G. T. McCandless,5 J. Y. Chan,5 J. Lee,6 J. P. C. Ruff,6 S. Das,1, 2 E. Manousakis,2, 1 M. D. Johannes,7 and L. Balicas1,∗ 1 Rome International Centre for Material Science Superstripes, RICMASS, via dei Sabelli 119A, 00185 Rome, Italy 2 Institute of Crystallography, CNR, via Salaria Km 29.300, Monterotondo Roma, I00015, Italy. Email: [email protected] Keywords: Weyl type II semimetals; Fermi surface; Berry phase.

The electronic structure of semi-metallic transition-metal dichalcogenides, such as WTe2 and orthorhombic MoTe2, are claimed to be characterized by a non-trivial Z2 topological invariant [1]. In addition, their Fermi surfaces are predicted to contain pairs of linearly touching electron- and hole-pockets each associated with a non-trivial Chern number [2]. For this reason, these compounds were recently claimed to conform to a new class, deemed type II, of Weyl semi-metalic systems [2]. A series of very recent angle resolved photoemission experiments (ARPES) seem to display a broad agreement with these last predictions detecting, for example, topologically non-trivial Fermi arcs [3]. In an attempt to validate these predictions, through measurements of their bulk Fermi surface (FS) via quantum oscillatory phenomena, we synthesized high quality single-crystals of semi-metallic MoTe2 [4]. We find that its superconducting transition temperature depends on disorder as quantified by the ratio between the roomand low-temperature resistivities, suggesting the possibility of a non-trivial superconducting pairing symmetry. Similarly to WTe2, its magnetoresistivity does not saturate at high magnetic fields and can easily surpass 106 %. An analysis of the quantum oscillatory signal superimposed onto the magnetic susceptibility extracted from the measurements of the magnetic torque, indicates a non-trivial Berry phase quite close to the value of ≃ π predicted for Weyl type-II systems. Quite surprisingly, the geometry of the Fermi surface (FS) as extracted from the quantum oscillatory phenomena, is markedly distinct from the calculated one and therefore from the FS recently revealed by ARPES. A broad anomaly seen in the heat capacity and in the Hall-effect suggests that the crystallographic and the electronic structures might evolve upon cooling below 100 K, perhaps explaining the discrepancy between predictions, ARPES and our experimental observations.

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c #1; RRR = 385 #2; RRR = 518 #3; RRR = 1020

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Figure 1: (a) Resistivity ρ, for currents flowing along the a−axis, as a function of the temperature T for three representative single crystals displaying resistivity ratios ρ(300 K)/ρ(2 K) between 380 and ∼ 1000. (b) ρ as a function of T for each single-crystal indicating that Tc depends on sample quality. The apparent hysteresis is due to a non-ideal thermal coupling between the single crystals, the heater and the thermometer. (c) ρ as a function of the field H applied along the c-axis at a temperature T = 1.7 K. Notice i) the nonsaturation of ρ(H) and ii) that ∆ρ (µ0H)/ρ0 = (ρ(µ0H) − ρ0)/ρ0, where ρ0 = ρ(µ0H = 0 T; T = 2 K) surpasses 1.4 × 106 % at µ0H = 60 T. (d) ρ as a function of µ0H applied along the b−axis also at T = 1.7 K and for the same single-crystal. (e) Shubnikov de Haas signal superimposed onto the magnetoresistivity for µ0H∥c-axis and for three temperatures, T = 8 K (blue line), 4.2 K (red line) and 1.7 K (black line), respectively. (f) Oscillatory signal (black-line) superimposed onto the magnetic susceptibility ∆χ= ∂(τ/µ0H)=∂(µ0H), where τ is the magnetic torque. Red line is a fit to four Lifshitz-Kosevich oscillatory components, i.e. two fundamental frequencies plus their harmonics, from which one extracts the respective Berry phases.

References 1. X. F. Qian, J. W. Liu, L. Fu, and J. Li, Quantum spin Hall effect in two-dimensional transition metal dichalcogenides, Science 346, 1344 (2014). 2. A. A. Soluyanov, D. Gresch, Z. Wang, Q. Wu, M. Troyer, X. Dai and B. A. Bernevig, A New Type of Weyl Semimetals, Nature 527, 495-498 (2015). 3. L. Huang, et al, Spectroscopic evidence for type II Weyl semimetal state in MoTe2, Nat. Mater. 15, 1155-1160 (2016); K. Deng et al, Experimental observation of topological Fermi arcs in type-II Weyl semimetal MoTe2, Nat. Phys. 12, 1105 (2016). 4. D. Rhodes et al., Impurity dependent superconductivity, Berry phase and bulk Fermi surface of the Weyl type-II semi-metal candidate MoTe2, arXiv:1605.09065 (2016).

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Resonant X-ray Inelastic Scattering and nanoscale inhomogeneity in FeSe1-xTex Jose Mustre de León, Diego Mulato Departamento de Física Aplicada, Cinvestav-Mérida, Mérida, Yucatán, México 1

Email: [email protected] Keywords: Fe-based calchogenide superconductors, density of states, x-ray absorption, FeSe monolayers Reports of superconductivity in few layers of FeSe deposited in SrTiO3 substrates, with critical temperatures reaching Tc = 65 K [1,3] have renewed the interest in Fechalcogenide superconductors. We report real-space calculations of electronic density of states of 1- and 2-layers of FeSe deposited on SrTiO3. For these calculations we have used a Density Functional Theory approach, based on a one-electron approximation, spherically symmetric self-consistent pseudopotentials, previously used in X-ray absoption spectroscopy calculations in Fe-chalcogenide superconductors[4]. We have used structural models based on experimental reports,[1] which assume a I4/mcm crystal structure for the substrate, which leads to a 1% expansion of the structural parameters of the FeSe layers. We compare the electronic density of states, in the vicinity of the Fermi energy, EF, assuming a nonmagnetic configuration of the FeSe monolayer and two different antiferromagnetic (AF) configurations. We find that the density of states at the Fermi level ρ(EF), is dominated by the Fe-3d states contributions, although the Ti-3d states contribute up to 10%. For the AF order configurations, we find that ρ (EF) exhibits a maximum value for the 1-layer case, with a decrease for the two-layer and bulk values, consistent with a possible increase in Tc, with respect to the bulk case (see Fig. 1).

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Figure 1. Electronic density of states for the system FeSe/SrTiO3 using a Density Functional Theory approach.

References 1. D. Liu, et al, Nature Communications 3, 931 (2012) http://nature.com/dx.doi.org/10.1038/ncomms1946. 2. W. Qing-Yan, et al, Chinese Physics Lett 29 037402 (2013) stacks.iop.org/0256307X/29/i=3/a=037402. 3. S. He, et al, Nature Materials 12,605 (2013), http://dx.doi.org/10.1038//nmat 3648. 4. A. Vega-Flick, J. Mustre de Leon, N.L. Saini, Journal of Superconductivity and Novel Magnetism 28, 1355 (2015). DOI: 10.1007/s10948-015-2955-3.

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Phenomenological theory of switching of electronic phases by optical, current, voltage and STM pulses in TaS2. Serguei Brazovskii1,2* LPTMS-CNRS, University Paris-Sud, Orsay, France 2 Jozef Stefan Institute, Ljubljana, Slovenia 1

Email: [email protected] Keywords: polaron, Wigner crystal, Mott insulator, hidden state, switching The recent mainstream in strongly correlated electronic systems is a quest for so called “hidden states”. A success came recently from observations of ultrafast (~ps) switching by means of optical [1] and current or voltage [1,2] pulses, as well by local manipulations by the STM tip [3,1,4]. These observations have been done upon the most popular layered material 1T-TaS2 which is an enigmatic “polaronic Wignercrystalline Mott insulator”. The presented phenomenological theory focuses upon equilibration among electrons and holes as mobile charge carriers, and the crystallized electrons modifiable by intrinsic defects (anti-polaronic voids and their walls). The dynamical exchange among these reservoirs proceeds by formation of a network of charged domain walls [5] originated by the intrinsic Coulomb instability of the super-lattice of self-trapped electrons – the polarons. References 1. L. Stojchevska, et al., Science, 344, 177 (2014); I. Vaskivskyi, et al., Science Advances, 1, 1500168 (2015); I. Vaskivskyi, et al., Nature Comm., 7, 11442 (2016). 2. M. Yoshida, et al., Science Reports, 4, 7302 (2014). 3. D. Cho, et al., Nature Comms 7, 10453 (2016). 4. L. Ma, et al., Nature Comms 7, 10956 (2016). 5. P. Karpov and S. B., Phys. Rev. B 94, 125108 (2016)

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Collective modes of the excitonic condensate in 1T-TiSe2 Jasper van Wezel University of Amsterdam

Email: [email protected] Keywords: dichalcogenides, excitons, charge order Light absorbed by a semiconductor can create an electron-hole bound state called an exciton. In the 1960s it was realized that, if the exciton binding energy were larger than the semiconductor band gap, excitons would spontaneously proliferate. The resulting “excitonic insulator” is a macroscopic condensate of electron-hole pairs with non-zero centre-of-mass momentum, or in other words, a charge-density wave. For 50 years, no experimental technique has been able to unambiguously identify an excitonic insulator phase in any material, despite many candidate materials being investigated. The reason is that its only tell-tale signature—an electronic “soft mode” with non-zero momentum—could not be detected with any technique. In this talk I will describe how momentum-resolved EELS (electron-energy loss spectroscopy) was recently used to demonstrate the existence of an electronic soft mode in the transition metal dichalcogenide TiSe2. This study represents the first observation of a soft electronic mode in any material, and the first unambiguous evidence for the existence of an excitonic phase.

References arXiv, 1611.04217 (2016)

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Intrinsic inhomogeneity of 2D crystalline superconductors Sergio Caprara, Gianluca Dezi, Niccolò Scopigno, Marco Grilli* Department of Physics University of Rome “Sapienza”

Email: [email protected] Keywords: twodimensional superconductors, transition metal dichalcogenides, electronic phase separation Recent progress in the fabrication of 2D highly ordered thin films and in increasing their electron density both by chemical doping or ionic gating has opened a new field with a wealth of interesting physical effects. These range from superconductivity in monolayers, possibly with an Ising-like order parameter, sizable spin-orbit coupling, competition with spatially ordered phases like CDW, and so on. In this framework, we analyzed transport properties in terms of a phenomenological model of Random Resistor Network representing an inhomogeneous system, where superconducting regions are embedded in a normal metal matrix. Fitting the resistance curves of several systems like TiSe2, MoS2, ZrNCl, we find that these systems are electronically inhomogeneous despite their high electron mobility and nearly perfect crystalline structure [1]. This finding, not only is relevant per se in systems of such broad interest, but naturally explains the quantum metallic phase taking place, e.g., in ZrNCl. Moreover, this raises the crucial question of the general mechanism(s) leading to this electronic inhomogeneity. We propose two general, possibly cooperative, mechanisms based on the electrostatic confinement and/or the density dependence of the superconducting critical temperature [1]. While similar mechanisms have been proposed in the past in the context of superconducting oxide interfaces [2,3], we find that they have a more general character and may be adjusted to account for the inhomogeneity formation in 2D superconducting films in general.

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(Left) Experimental (solid lines) and calculated (points) resistivity of a ionic gated ZrNCl film. (Right) Same as the left panel, but for the resistivity of ionic gated TiSe2 film [1]

References 1. S. Caprara, G. Dezi, N. Scopigno, and M. Grilli, preprint. 2. N. Scopigno, D. Bucheli, S. Caprara, J. Biscaras, N. Bergeal, J. Lesueur, and M. Grilli, Phys. Rev. Lett. 116, 026804 (2016). 3. N. Bovenzi, et al., J. Superc. Nov. Magn. 28, 1273 (2015), DOI 10.1007/s10948014-2903-7, 2014.

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Exploiting multiorbital physics to achieve superconductivity: Fullerene and beyond

high-temperature

M. Capone International School for Advanced Studies (SISSA) and CNR-IOM, Trieste

Email: [email protected] Keywords: Fullerides, multiorbital physics, Hund's coupling, electron-phonon interaction Alkali-metal doped fullerides are surprising materials, where electron-phonon driven superconductivity survives and lives prosper close to a transition to a Mott insulator, in constrast with intuition. We show that this surprising result relies on the multi orbital electronic structure of these molecular crystals, which allows to form local s-wave pairs despite a strong short-ranged Coulomb repulsion. The Jahn-Teller phonons give rise to an inverted Hund’s coupling which favors lowspin states which are not harmed by Coulomb repulsion. This, together with the reduction of the kinetic energy due to strong correlations, leads to an enhanced superconducting order parameter with respect to a system without Coulomb repulsion. We briefly discuss several anomalies of these superconductors which descend from strong correlations. This suggest that one can define a broader class of strongly correlated superconductors regardless of the pairing mechanism. In this light, we discuss how this mechanism can be relevant for other classes of superconductors including the cuprates and the iron-based superconductors and its relation with non-equilibrium properties of superconductors.

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Theoretical and experimental phase diagram for alkali-doped fullerides as a function of the lattice spacing

References 1. M. Capone, M. Fabrizio, C. Castellani and E. Tosatti, Science 296, 2364 (2002). 2. M. Capone, M. Fabrizio, C. Castellani and E. Tosatti, Rev. Mod. Phys. 81, 943 (2009). 3. Y. Nomura, S. Sakai, M. Capone and R. Arita, Science Advances, 1 e1500568 (2015). 4. L. de' Medici, G. Giovannetti and M. Capone, Phys. Rev. Lett. 112, 177001 (2014).

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Eliashberg equations with antiferromagnetic ‘hidden order’- induced pairing boson in cuprates Sergei I. Mukhin Department for Theoretical physics and quantum technologies, Moscow Institute for Steel and Alloys, Moscow, Russia

Email: [email protected] Keywords: Eliashberg equations for antiferromagnetic ‘hidden order’ induced pairing boson in cuprates An emergence of the ‘magnetic boson’ providing Cooper-pairing ‘glue’ is considered in the model of superconducting cuprates with antiferromagnetically fluctuating electron-hole quasi-classical condensate - an instantonic “crystal”. The “crystal” manifests broken Matsubara time translational symmetry of the correlated Fermisystem and plays the role of its ‘hidden order’. Usually unstable [1], the ‘soft mode’ of the “crystal” is shown to be stabilized in the presence of superconducting condensate of Cooper pairs. This ‘soft mode boson’ is considered as an origin of the Cooper pairing “glue” in the initially repulsive Fermi-system. Thus, the two competing orders: antiferromagnetic and superconducting coexist (below some Tc) in the form of superconductor with ‘hidden order’. The ‘hidden order’ scenario based on the Euclidean “crystallization” was proposed earlier in [2]-[4], where a spin density wave (SDW) with Matsubara time-periodic amplitude was considered. It was demonstrated analytically in [2], that the SDW with an amplitude that behaves as a snoidal Jacobi function of the Matsubara time, leads to zero scattering cross section [2] for weakly perturbing external probes, like neutrons, etc., thus representing ‘hidden order’. In the present work the set of extended Eliashberg-like equations with bosonic ‘glue’ in the presence of instantonic “crystal” is derived and solved self-consistently for the case of a single lowest “crystal” vibration band. This new picture is discussed in relation with the different experiments in high- Tc superconductors. References 1. A.M. Polyakov, “Guage fields and strings”, Harwood Academic Pub., 1987. 2. S. I. Mukhin, «Spontaneously broken Matsubara’s time invariance in fermionic system: macroscopic quantum ordered state of matter», J. Supercond. Nov. Magn., vol. 24, 1165-1171 (2011).

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Superstripes 2017, Ischia June 4-10, 2017 3. S. I. Mukhin, «Euclidean action of fermi-system with “hidden order”, Physica B: Physics of Condensed Matter, vol. 460, 264 (2015). 4. S. I. Mukhin, «Euclidian Crystals in Many-Body Systems: Breakdown of Goldstone’s Theorem», J. Supercond. Nov. Magn., vol.27, 945-950 (2014).

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Topological structures in a model cuprate A.S. Moskvin, Yu.D. Panov Ural Federal University,620083, Ekaterinburg, Russia

Email: [email protected] Keywords: cuprates, pseudospin formalism, topological structures, skyrmions The origin of high-Tc superconductivity and other unconventional properties of cuprates is presently still a matter of great controversy mainly due to a complex interplay of charge, orbital, spin, and lattice degrees of freedom. Recently we have introduced a minimal model to describe the charge degree of freedom with the on site Hilbert space reduced to only the three effective valence centers [CuO4]7-,6-,5(nominally Cu1+;2+;3+), where the electronic and lattice degrees of freedom get strongly locked together, and made use of the S=1 pseudospin formalism [1]. Effective pseudospin Hamiltonian does incorporate all the on-site and inter-site couplings with a charge density constraint: , where ∆n is a deviation from the halffilling. Here we make use of conventional spin operators, T±={Sz, S±}, 2D=Ueff is a correlation parameter, Vij intersite electron-hole attraction, nij intersite coupling correction, m chemical potential, t,t’,t” are the three types of the the correlated single particle transfer integrals, tb is a two-particle transfer integral. The Hamiltonian implies further possible simplifications, in particular, so-called negative-U model given large negative D [2]. The 2D pseudospin system is prone to a creation of different topological structures from domain walls, in-plane and out-of-plane vortices to singlecentered and multi-centered skyrmions which form topologically protected inhomogeneous distributions of the eight local S=1 pseudospin order parameters including charge density and superfluid order parameters. Different nontrivial skyrmion-like topological defects for 2D (pseudo)spin S=1 systems were considered in Ref [3]. Hereafter, we focus on the so-called quadrupole skyrmion [3] which is believed to be a candidate for a topological charge excitation in parent or underdoped cuprates. Fig.1 demonstrates the radial distribution of different order parameters for such a skyrmion. Puzzlingly, such an unconventional structure is characterized by an uniform distribution of the mean on-site charge (=0), that is the quadrupole skyrmionic structure and the bare parent Cu2+ monovalent (insulating) phase have absolutely the same distribution of the mean on-site charges that makes the quadrupole skyrmion texture to be invisible for X-rays.

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Superstripes 2017, Ischia June 4-10, 2017 We suppose that the parent insulating antiferromagnetic cuprates may be unstable with regard to nucleation of topological defects in the unconventional form of the one- or multi-center skyrmion-like object with ring-shaped superfluid regions. *Supported by Act 211 Government of the Russian Federation, agreement № 02.A03.21.0006 and by the Ministry of Education and Science, projects 2277 and 5719.

Figure 1 Radial distribution of the local order parameters for a quadrupole pseudospin skyrmion: (1- ) is the density of the Cu2+ centers, the superfluid order parameter, the T-type order parameter.We see a circular layered structure with clearly visible anticorrelation effects due to a pseudospin kinematics. At the center (r = 0) and far from the center (r® ∞) for such a skyrmion we deal with a parent Cu2+ monovalent (insulating) state while for the domain wall center (r = l) we arrive at a fully disproportionated "superconducting" Cu1+-Cu3+ superposition whose weight diminishes with moving away from the center. The | | parameter turns into zero at the domain wall center r = l, at the skyrmion center r = 0 and at the infinity r® ∞, with the two extremuma at r±=λ⁄((√2±1) ). In other words, the ring shaped domain wall is an area with a circular distribution of the superconducting order parameter, or circular "bosonic" supercurrent with a d-wave angular symmetry. Nonzero T-type order parameter distribution points to a circular "fermionic" current with a puzzlingly opposite sign of the T-type order parameter for "internal" (0

References 1. A.S. Moskvin, Low Temp. Phys. 33, 234 (2007); Phys. Rev. B 84, 075116 (2011); J. Phys.: Condens. Matter 25, 085601 (2013); J. Phys: Conf. Ser. 592, 012076 (2015); JETP, 121, 477 (2015). 2. A.S. Moskvin, Yu.D. Panov, F.N. Rybakov, A.B. Borisov, J. Supercond. Nov. Magn., 30(1), 43 (2017). 3. N.A. Mikushina, A.S. Moskvin, Phys. Lett. A 302/1, 8 (2002).

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Spin-Peierls dimerization caused by Jahn-Teller effect in NaTiSi2O6 E. Joon, I. Heinmaa, R. Stern, R. Rästa* National Institute of Chemical Physics and Biophysics Email: [email protected] Keywords: Mott insulator, Spin-Peierls dimerization, Jahn-Teller effect, Quantum magnetism The Spin-Peierls (SP) effect is the crystal lattice dimerization with the formation of a singlet ground state from the paramagnetic high temperature state. It takes place in low dimensional crystals, like one dimensional (1D) Heisenberg ½ spin chains, with antiferromagnetic (AF) exchange coupling. The essence of Jahn-Teller (JT) effect is the lifting of degeneracy of the symmetric state by lowering of crystal lattice symmetry. Both phenomena are dynamic at high temperature T≥ Tc and become static in low temperature phase T≤ Tc, where Tc is the structural phase transition temperature. According to the existing point of view these effects act separately and do not coexist. In this report we demonstrate the opposite; the JT effect causes SP dimerization in 1D Ti3+ magnetic chains of NaTiSi2O6. NaTiSi2O6 is a ½ spin member of quasi 1D magnets of pyroxene family. It is a Mott insulator, which crystallizes in the monoclinic space group C2/c at high temperatures [1,2,3]. Its crystal lattice consists of slightly distorted TiO6 octahedra, which are skew-edge connected into zigzag chains, bridged by SiO4 tetrahedra. Using local coordinates, defined in [4], the threefold t2g level of Ti3+ splits due to the crystal field into low-lying doublet with dxy and dyz orbitals and a single irrelevant dxz orbital removed to higher energy. This orbital degeneracy causes cooperative JT effect through strong electron-lattice coupling [4]. At Tc= 210 K the lattice undergoes structural phase transition into triclinic P1Ī space group [2,3] with the dimerization of Ti chains and the forming of the spin singlet ground state. As a result a spin-gap of order 500-700 K [1, 5] and 53 meV [6] opens and the magnetic susceptibility starts rapidly to decrease. In order to gain more insight on local spin structure and its dynamics, we have performed 29Si and 23Na NMR studies of NaTiSi2O6 [7, 8]. At low temperatures we found a high amount of undimerized Ti3+ ions, whose concentration c(T) follows activation type temperature dependence: , with the energy . We relate Ea with soliton energy Ea =2/π*∆, where ∆=470K is the amplitude of the spin-gap in our case. As a result solitons act as paramagnetic impurities and spins between them form SP phase. The detailed analysis of the NMR line shapes and their temperature dependencies allows us to elucidate the distribution of singlets and solitons. We also demonstrate that JT distortions of the neighbour TiO6 octahedra lead directly to the dimerization of Ti spins and SP state. References 1. M. Isobe et al., Novel phase transition in spin-1/2 linear chain systems: NaTiSi2O6 and LiTiSi2O6, J. Phys. Soc. Japan 71, 1423 (2002).

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Superstripes 2017, Ischia June 4-10, 2017 2. E. Ninomiya et al., Observation of lattice dimerization in spin-singlet low temperature phase of NaTiSi2O6, Physica B, 329-333, 884 (2003). 3. G. J. Redhammer, et al., Single-crystal structure refinement of NaTiSi2O6 clinopyrexene at low temperatures (298 ent of the spin gap in a quasi-one-dimensional clinopyroxene: NaTiSi2O6, Phys. Rev. B90, 140402(R) (2014). 4. R. Rästa, I. Heinmaa, E. Joon, R. Stern, 29Si NMR study of NaTiSi2O6 (unpublished) 5. E. Joon, R. Rästa, I. Heinmaa, R. Stern, Two-level behavior of Ti3+ d-orbitals in NaTiSi2O6 (unpublished).

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Quantum spin liquid in a molecular Mott system based on Pd(dmit)2 Reizo Kato Condensed Molecular Materials Laboratory, RIKEN, Wako-shi, Saitama 351-0198, Japan Email: [email protected] Keywords: quantum spin liquid; Mott insulator; dimer; charge disproportionation. A series of anion radical salts of a metal complex Pd(dmit)2 (dmit = 1,3-dithiol-2thione-4,5-dithiolate), X[Pd(dmit)2]2 (X: monovalent cation), belong to a Mott system with a quasi triangular lattice of [Pd(dmit)2]2– dimers. We found that β'EtMe3Sb[Pd(dmit)2]2 is a promising candidate for quantum spin liquid (QSL) [1, 2]. The ground state of the Pd(dmit)2 salts is classified by the anisotropy of the triangular lattice that can be tuned by the choice of the counter cation X. The cation effect on the degree of frustration is associated with the arch-shaped distortion of the Pd(dmit)2 molecule [3].

Figure 1: Crystal structure of β'-EtMe3Sb[Pd(dmit)2]2 (left) and phase diagram of the Pd(dmit)2 salts (right).

The QSL phase is near the Mott transition and is situated between the antiferromagnetic long-range order (AFLO) phase and the charge order (CO) phase. The AFLO phase in the vicinity of the QSL phase exhibits very small magnetic moment. The AFLO state in the Pd(dmit)2 salts is accompanied by intramolecular antiparallel spin configuration and charge disproportionation.

173

Superstripes 2017, Ischia June 4-10, 2017 On the other hand, the CO state in the Et2Me2Sb salt is understood in terms of the formation of a closed shell octamer composed of two charge-rich dimers and two charge-poor dimers [4]. In connection with this, the valence bond order (VBO) state observed in the EtMe3P (monoclinic and triclinic) salts is characterized by the formation of a closed shell tetramer composed of two equivalent dimers with intradimer charge disproportionation [5]. Vibrational spectra of the QSL salt indicate competition between the tetramerization and octamerization, suggesting that the charge degree of freedom is important for the emergence of the QSL state. We will also discuss an incommensurate modulated phase in the Et2Me2Sb salt that appears above the CO transition temperature. The modulation is coupled to the archshaped distortion of the Pd(dmit)2 molecule and thus to the anisotropy of the triangular lattice. References 1. (a) K. Kanoda and R. Kato, Annu. Rev. Condens. Matter Phys., 2, 167 (2011). http://www.annualreviews.org/doi/10.1146/annurev-conmatphys-062910-140521 (b) R. Kato, Bull. Chem. Soc. Jpn., 87, 355 (2014). http://www.journal.csj.jp/doi/10.1246/bcsj.20130290 2. (a) T. Itou, et al., Phys. Rev. B, 77, 104413 (2008). http://journals.aps.org/prb/abstract/10.1103/PhysRevB.77.104413 (b) M. Yamashita, et al., Science, 328, 1246 (2010). http://science.sciencemag.org/content/328/5983/1246 (c) T. Itou, et al., Nature Phys., 6, 673 (2010). http://www.nature.com/nphys/journal/v6/n9/full/nphys1715.html (d) S. Yamashita, et al., Nature Commun., 2, 275 (2011). http://www.nature.com/articles/ncomms1274 (e) D. Watanabe, et al., Nature Commun., 3, 1090 (2012). http://www.nature.com/articles/ncomms2082 3. R. Kato and H. Cui, Crystals, 2, 861 (2012). http://www.mdpi.com/20734352/2/3/861 4. T. Yamamoto et al., J. Phys. Soc. Jpn., 85, 104711 (2016). http://journals.jps.jp/doi/10.7566/JPSJ.85.104711 5. T. Yamamoto et al., J. Phys. Soc. Jpn., 83, 053703 (2014). http://journals.jps.jp/doi/10.7566/JPSJ.83.053703

174


Half-filled anizotropic triangular Hubbard model and organic charge transfer salts based on BEDT-TTF Jure Kokalj1 and Ross H. McKenzie2 1 Jozef Stefan Institute, Ljubljana, Slovenia and Faculty for Civil and Geodetic Engineering, University of Ljubljana, Slovenia 2 University of Queensland, Brisbane, Australia Email: [email protected] We numerically calculated several quantities for the half-filled anisotropic triangular Hubbard model with the aim to describe the behavior of organic charge transfer salts based on BEDT-TTF. In particular, the Mott-metal insulator transition is observed via charge susceptibility, the bad-metallic regime is characterized with large entropy and local moment, while below the relatively low coherence temperature the Fermi liquid phase is strongly renormalized as seen, e.g., in specific heat and spin susceptibility. Further the DMRG results suggest possible gapless spin liquid phase between the metallic and 120 degree ordered AFM Neel state. The calculated results will be discusses in the light of experiments on the organic charge transfer salts. References 1. J. Kokalj and R. H. McKenzie, Phys. Rev. Lett. 110, 206402 (2013). 2. T. Shirakawa, T. Tohyama, J. Kokalj, S. Sota and S. Yunoki, arXiv:1606.06814. 3. J. Kokalj and R. H. McKenzie, Phys. Rev. B 91, 125143 (2015). 4. J. Kokalj and R. H. McKenzie, Phys. Rev. B 91, 205121 (2015). 5. P. Prelovsek, J. Kokalj, Z. Lenarcic and R. H. McKenzie, Phys. Rev. B 92, 235155 (2015).

175


Nematic electronic structure of the iron-based superconductor FeSe Takahiro Shimojima RIKEN Center for Emergent Matter Science (CEMS), Wako 3510198, Japan

Email: [email protected] Keywords: electronic structure; iron-based superconductor; angle-resolved photoemission. Most of the parent compounds of the iron-based superconductors exhibit the tetragonal-orthorhombic structural transition at Ts and the stripe-type antiferromagnetic order below TN (toemission spectroscopy (ARPES). Experimental and theoretical studies suggested that the tetragonal-orthorhombic phase transition (nematic order) is caused by the spin or orbital degrees of freedoms1. FeSe is a good example to examine the role of orbital degrees of freedom, since it shows the structural and superconducting transitions at Ts ≈ 90 K and Tc ≈ 9 K without any magnetic order2. In order to understand the role of orbital degrees of freedom on the nematicity, it is required to clarify how the electronic structure breaks four-fold symmetry in the entire Brillouin zone. In this talk, we report the electronic reconstruction across Ts of FeSe by employing ARPES with several photon sources on detwinned single crystals. We observed highly two-fold symmetric Fermi surfaces (FSs) below 90 K around both the G and M points. The polarization between xz and yz orbitals at M point was found to be more than five times larger than that expected from the orthorhombic lattice distortion, thus indicative of the orbital-driven nematicity3. Polarization-dependent ARPES further revealed that the xz orbital shifts upward at the G point, while it moves downward at the M point. Such momentum-dependent sign inversion of orbital polarization modifies the FSs at the G and M point into elliptical shapes elongating along ky and kx, respectively4. We further investigated the dynamics of the nematic electronic structure by employing time-resolved ARPES. We observed the complete suppression and recovery of the ellipticity in the nematic FS after a femtosecond photoexcitation. We will also discuss the orbital dependence in the relaxation process of the photoexcited carriers. References 1. R. M. Fernandes et al., Nature Physics 10, 97 (2014). 2. A. E. Böhmer et al., Phys. Rev. B 87, 180505(R) (2013). 3. T. Shimojima et al., Phys. Rev. B 90, 121111(R) (2014). 4. Y. Suzuki et al., Phys. Rev. B 92, 205117 (2015).

176


ARPES of iron-based superconductors Sergey Borisenko IFW-Dresden Email: [email protected] Keywords: ARPES, Iron-Based Superconductors, Gap Functions I will overview our most recent ARPES results on iron-based superconductors. In particular, we have determined both the 3D electronic structure and superconducting energy gaps with a new precision. Multiorbital nature of these superconductors makes itself evident in the gap functions and provides new constraints for the successful theoretical models of high-Tc superconductivity.

177


Non-Fermi-liquid behavior, Lifshitz transitions, and Hund’s metal behavior of iron-based superconductors and related compounds from ARPES Jörg Fink1, 2,3 Leibnitz Institute Dresden, Germany 2 Technische Universität Dresden, Germany 3 Max Planck Institute for Chemical Physics of Solids, Dresden, Germany 1

Email: [email protected] Keywords: high Tc, ferropnictides, ferrochalcogenides, Lifshitz transitions, correlation effects Using ARPES, we have investigated the electronic band structure and many-body properties of iron-based high-Tc superconductors and related compounds in a large range of the occupation of the 3d shell, starting from Cr compounds via the ferropnictides and ferrochalcogenides until Cu pnictide. For those compounds having the highest superconducting transition temperatures, we find Lifshitz transitions for those bands exhibiting the highest superconducting gaps. Furthermore, from the measured scattering rates we observe the strongest correlation effects for compounds near a half filled 3d shell (Fe-based high Tc superconductors), while the correlation effects are strongly reduced for smaller 3d count (Cr compounds) or larger 3d count (Co, Ni, Cu compounds). This indicates that the correlation effects in these multi-orbital systems are strongly influenced by Hund’s exchange interaction. From these results we conclude that the high superconducting transition temperatures and the strange normal state properties are influenced both by band structure properties and by correlation effects [1-3]. References 1. J. Fink, E. D. L. Rienks, S. Thirupathaiah, J. Nayak, A. van Roekeghem, S. Biermann, T. Wolf, P. Adelmann, H. S. Jeevan, P. Gegenwart, S. Wurmehl, C. Felser, and B. Büchner, Phys. Rev. 95, 144513 (2017). 2. J. Fink, A. Charnukha, E. D. L. Rienks, Z. H. Liu, S. Thirupathaiah, I. Avigo, F. Roth, H. S. Jeevan, P. Gegenwart, M. Roslova, I. Morozov, S. Wurmehl, U. Bovensiepen, S. Borisenko, M. Vojta, and B. Büchner, Phys. Rev. 92, 201106 (R) (2015). 3. I. Avigo, S. Thirupathaiah, E. D. L. Rienks, L. Rettig, A. Charnukha, M. Ligges, R. Cortes, J. Nayak, H. S. Jeevan, T. Wolf, Y. Huang, S. Wurmehl, M. I. Sturza, P. Gegenwart, M. S. Golden, L. X. Yang, K. Rossnagel, M. Bauer, B. Büchner, M. Vojta, M. Wolf, C. Felser, J. Fink, and U. Bovensiepen, Phys. Status Solidi B 254, 1600382 (2016).

178


Electron-electron correlation in iron-Pnictides, revealed by ARPES Ming Shi Paul Scherrer Institute, CH-5232 Villigen, Switzerland Email: [email protected] Keywords: superconductivity, Pnictides, AFM insulator, correlation, ARPES The effects of electron-electron correlations on the low-energy electronic structure and their relationship with unconventional superconductivity are important aspects in the research on the iron-based pnictide superconductors. Here we use angle-resolved photoemission spectroscopy (ARPES) to investigate how electronic correlations evolve in different chemically substituted iron pnictides. We revealed 1) the electronic structure of the antiferromagnetic iron-pnictide insulator is very similar to that of the parent compound of cuprates [1], 2) in transition-metal pnictides electron-electron correlations are intrinsically related to the effective filling of the correlated orbitals, rather than to the filling obtained by valence counting [2,3], and 3) modifying the spacer layer between FeAs layers, the common building block of iron-pnicides, could result in a staggered combination of a quantum spin Hall insulator and a hightemperature superconductor.

References 1. C. E. Matt et al., Phys. Rev. Lett. 117, 097001 (2016). 2. E. Razzoli et al., Phys. Rev. B. 91, 214502 (2015). 3. E. Razzoli et al., Phys. Rev. Lett. 108, 257005 (2012).

179


The new frontiers of the Josephson effect in novel unconventional nano-scale and magnetic systems F. Tafuri 1,2*, R. Caruso 1,2, D. Massarotti 1,2,3, D. Stornaiuolo 1,2, G.P. Pepe 1,2, L. Longobardi 3,4, F. Lombardi 5, P. Lucignano 1,2, G. Campagnano 1, G. Rotoli 3, and A. Tagliacozzo1,2 1 Dipartimento di Fisica “E. Pancini”, Università di Napoli Federico II, Napoli Italy 2 CNR-SPIN UOS Napoli, Italy 3 Dipartimento di Ingegneria Industriale e dell’Informazione, Università della Campania L. Vanvitelli, Aversa (CE) Italy 4 American Physical Society, 1 Research Road, Ridge, New York 11961, USA 5 Chalmers University of Technology, Gotenborg, Sweden Email: [email protected] Keywords: Hybrid Josephson junctions, quantum devices, nanoscale Josephson Junctions (JJs) provide unique solutions to frontier fundamental problems and very advanced applications in quantum technologies. We have investigated various unconventional systems including junctions with graphene or topological insulator or ferromagnets as barriers and found significant anomalous behaviors. This evidence partly invalidates standard approaches to the Josephson effect in these systems and confirm the presence of competing mechanisms induced by the special nature of the superconductors or of the barriers or of their interfaces. Effects due to nano-engineering of the junctions and weak links contribute to enhance specific effects, manifesting themselves through special fingerprints in the phenomenology of the Josephson effect. All this has to be taken into account to use these junctions as quantum sensors in a variety of experiments.

180


Superconducting weak links created by electromigration X. Baumans, J. Lombardo, V. Zharinov, J. Scheerder, D. Massarotti, R. Caruso, J. Yuan, B.Y. Zhu, K. Jin, R. B.G. Kramer, F. Tafuri, V. V. Moshchalkov, J. Van de Vondel, and A. V. Silhanek Experimental Physics of Nanostructured Materials, Q-MAT, CESAM, Université de Liège, B-4000 Sart Tilman, Belgium INPAC – Institute for Nanoscale Physics and Chemistry, Nanoscale Superconductivity, and Magnetism Group, K.U. Leuven, Celestijnenlaan 200D, B–3001 Leuven, Belgium Dipartimento di Fisica, Università degli Studi di Napoli ’Federico II’, Monte S. Angelo, I-80126 Napoli, Italy Dipartimento di Ingegneria Industriale e dell´Informazione, Seconda Università degli Studi di Napoli, I-81031 Aversa (Ce), Italy CNR-SPIN UOS Napoli, Monte S. Angelo-via Cinthia, I-80126 Napoli, Italy Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China Université Grenoble Alpes, Institut NEEL, F-38000 Grenoble, France and CNRS, Institut NEEL, F-38000 Grenoble, France Email: [email protected] Keywords: electromigration, phase slips, weak links

In this presentation, we explore in-situ controlled electromigration to fabricate superconducting weak links. We show evidence that in Al a transition from thermally assisted phase slips (TAPS) to quantum phase slips may takes place when the effective cross section becomes smaller than ~150 nm2. In the regime dominated by quantum phase slips the nanowire loses completely its capacity to carry current without dissipation, even at the lowest possible temperature. We also discuss the origin of negative magnetoresistance at low magnetic fields in the bow-tie shaped constrictions. Strikingly, the detrimental effect caused by the repeated electromigration can be healed by simply inverting the current direction. These findings reveal perspectives of the proposed fabrication method for exploring various fascinating superconducting phenomena in atomic size constrictions.

181


Al nanoconstriction as fabricated by electron beam lithography (virgin) and after two consecutive electromigration procedures (EM1 and EM2).

References 1. Baumans, X. D. A., Cerbu, D., Adami, O.-A., Zharinov, V. S., Verellen, N., Papari, G., Scheerder, J. E., Zhang, G., Moshchalkov, V. V., Silhanek A. V., and Van de Vondel. J. Nat. Commun. 7, 10560 (2016).

182


Quantum decay of supercurrent in transparent nanojunctions Andrei D. Zaikin Institute of Nanotechnology, Karlsruhe Institute of Technology (KIT), 76021 Karlsruhe, Germany

Email: [email protected] Keywords: Macroscopic quantum tunneling, dissipation, quantum phase slips Making use of the effective action theory [1,2] we demonstrate that the problem of macroscopic quantum tunneling of the superconducting phase in highly transparent superconducting nanojunctions can be exactly mapped onto that of a quantum particle in a dissipative environment formed by a collection of harmonic oscillators with parameters directly related to those of subgap Andreev levels inside the junction. We evaluate both quantum and thermally activated supercurrent decay rates in such nanojunctions and identify crossover conditions between these regimes. We also predict the possibility for non-monotonous dependence of the switching current distributions on temperature and elucidate the physics behind this non-trivial effect. References 1. A.V. Galaktionov and A.D. Zaikin, Phys. Rev. B 82, 134508 (2010). 2. A.V. Galaktionov and A.D. Zaikin, Phys. Rev. B 92, 214511 (2015).

183


Resonant inelastic x-ray scattering to measure short-range magnetic order Claude Monney University of Zurich

Email: [email protected] Keywords: RIXS, spin chain, cuprate I will present resonant inelastic x-ray scattering (RIXS) data obtained at O K-edge on the quasi-one-dimensional spin-chains Li2CuO2 and CuGeO3. In these edge-sharing cuprates having a standard Cu d9 electronic configuration, RIXS can create a particular excitation called a Zhang-Rice singlet (ZRS) in its final state. The ZRS consists in a double-hole occupation on a CuO4 plaquette, which occurs via an effective hole transfer from one plaquette to another. The intensity of this excitation depends on the ground state spin configuration and is a measure of the nearest-neighbour spin arrangement. This effect is used here to monitor the intrachain short-range spin correlations in Li2CuO2 and CuGeO3 as a function of temperature [1]. I will also present results on Li2CuO2 showing that not only the intrachain, but also the interchain short-range spin correlations can be extracted via the chemical- and siteselectivity of RIXS by tuning the x-ray incident energy [2]. I will finally argue that the charge-transfer energy obtained from this material is not only due to a purely electronic effect, but has a large contribution due to the deformation of the underlying lattice [3].

References 1. C. Monney et al., PRL 110, 087403 (2013). 2. C. Monney et al., PRB 94, 165118 (2016). 3. S. Johnston, C. Monney et al., Nature Commun. 10653 (2016).

184


Studying Silicene monospectroscopy and DFT

and

multilayers

with

soft

X-ray

Alexander Moewes and Neil Johnson Department of Physics, University of Saskatchewan, Canada

Email: [email protected] Keywords: Silicene, band gap, X-ray absorption and emission spectroscopy, DFT We study the hexagonal honeycomb of Si atoms – referred to as Silicene [1] – deposited on Ag(111) with synchrotron-based soft X-ray spectroscopy (XAS & XES) at the Si L-edge. When compared to our electron density functional theory calculations, we unambiguously show that the Si valence and conduction states are continuous across the Fermi energy; i.e. that the silicene overlayer was indeed metallic [2]. If Simonolayers are to come into use, they must be all at once isolated from their substrates. One suggested way of achieving these characteristics is to produce a multilayer of silicene on the Ag(111) surface. Our DFT calculations predict a stable, AA-stacked silicene bilayer on Ag(111) that corresponds nicely to the scanning tunnelling microscopy (STM) bilayer observations. Unfortunately, these same DFT calculations predict a similar electronic structure as that of the monolayers, namely metallic and bound to the Ag(111). Our measurements indicate a transition to bulk Si beginning shortly after the completion of a monolayer [3]. When studying the oxidation of Silicene, our calculations indicate that moderate levels of oxidation do not cause a significant bandgap opening in the epitaxial silicene monolayer. In addition, moderate oxidation is calculated to strongly distort the hexagonal Si lattice, causing it to cluster in regions of highest oxygen adatom concentration but retain its 2D sheet structure. Our experiments reveal that beaminduced oxidation is consistent with the formation of islands of bulk-like SiO2 [4].

References 1. P. Vogt, P. De Padova, C. Quaresima, J. Avila, E. Frantzeskakis, M.C. Asesnsio, A. Resta, B. Ealet and G. Le Lay, Phys. Rev. Lett. 108, 155501 (2012). 2. N.W. Johnson, P. Vogt, A. Resta, P. De Padova, I. Perez, D. Muir, E. Z. Kurmaev, G. Le Lay and A. Moewes, Adv. Funct. Mater. 24, 5253 (2014). 3. N.W. Johnson, D. Muir, E.Z. Kurmaev, and A. Moewes Adv. Funct. Mat. 25, 4083 (2015). 4. N.W. Johnson, D.I. Muir and A. Moewes, Sci. Rep. 6, 22510 (2016).

185


Doping effects on electronic properties of bilayer graphene Hidenori Goto1, Takaki Uchiyama1, Yoko Nakashima1, Hidehiko Akiyoshi1, Ritsuko Eguchi1, Hiroshi Osada2, Takao Nishikawa2, Yoshihiro Kubozono1 1 Research Institute for Interdisciplinary Science, Okayama University, Japan 2 Iwate University, Japan Email: [email protected] Keywords: bilayer graphene, energy gap, electric field effect, doping effect Graphene exhibits unique electronic properties resulting from the linear dispersion relation, whereas zero energy gap between the valence and conduction bands makes it difficult to develop practical graphene devices. One solution to this problem is to apply electric field perpendicular to bilayer graphene (BLG). Two methods are known to produce the electric field. One is a conventional gating method using double-gate structure. The other is a doping method which utilizes electron transfer between graphene and surface adsorbates. However, it is not yet conclusive whether the doping method can actually produce uniform electric field[1,2]. In this study, we investigated the presence of band gap of doped BLG by measuring temperature dependence of the conductivity. BLG devices were prepared on self-assembled monolayers (NH2-SAMs), on which 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ) molecules were deposited. NH2-SAMs donate electrons, while F4TCNQ molecules donate holes, which can generate effective electric field through the BLG. As a result, some devices showed an energy gap, while the others no gap. Even in the former sample, the energy gap was reduced after subsequent deposition of F4TCNQ molecules. It is inferred that random doping from NH2-SAMs and F4TCNQ molecules does not always lead to uniform electric field. The potential fluctuation by adsorbed molecules decreases scattering length and prevents the energy gap from opening.

References 1. S. Xiao et al., Phys. Rev. B 82, 041406(R) (2010). 2. J. Park et al., Adv. Mater. 24, 407 (2012).

186


Superconductivity of K-doped FeSe ultra-thin films on SrTiO3(001) substrate Canhua Liu Shanghai Jiao Tong University

Email: [email protected] Keywords: superconductive phase diagram, FeSe thin film, electron doping Single unit-cell layer of FeSe film grown on a SrTiO3 substrate was found to have a superconductive transition at a temperature much higher than FeSe bulk crystal, which stimulated a great deal of research interest in revealing the mechanism of the interface enhanced superconductivity. One of the key ingredients for the superconductivity enhancement is charge transfer from SrTiO3 substrate to the FeSe thin film, i.e., electrons doped to the FeSe thin film due to oxygen vacancies in the SrTiO3 substrate. Inspired by this idea, researchers found in both ARPES and STM experiments that Kdoped FeSe thin films also exhibit K-coverage dependent superconductive energy gaps (∆), which are much larger than that of FeSe bulk crystal and thus indicate much higher Tc. However, the superconductivity of K-doped FeSe thin films haven’t been confirmed in experiments yet, since neither its zero resistance nor diamagnetism could have been measured. We recently invented a piezo scanner tube with four electrodes for a special scanning tunneling microscope (STM), based on which, we developed a four-point probe and a double-coil mutual inductance head for in situ measurements of electrical and magnetic properties of a sample in an ultra-high vacuum chamber, respectively. The original STM function is fully sustained, which enables us to investigate the surface morphology, surface atomic arrangement and surface electronic structure while doing electrical and magnetic property measurement of a same sample. Using this setup, we succeeded for the first time in measuring the diamagnetism of K-doped FeSe thin films. By comparing the evolution of Tc and ∆ as function of K coverages, we found that the Tc is not determined by the pair potential in the overdoped region. This work was in cooperation with Ming-Chao Duan, Gang Yao, Yan-Fu We, Yao-Yi Li, Dong Qian, and Jin-Feng Jia.

187


Multicomponent electron-hole superfluidity and the BCS-BEC crossover in double bilayer graphene David Neilson, Sara Conti, and Andrea Perali Dipartimenti di Fisica e di Farmacia, Università di Camerino, 62032 Camerino (MC), Italy.

Email: [email protected] Keywords: Multicomponent electron-hole superfluidity; BCS-BEC crossover; double bilayer graphene The recent fabrication of a pair of very close, but electrically independent, conducting bilayer graphene sheets, one containing electrons and the other holes [1],raises exciting possibilities of observing high-temperature superfluidity [2]. In bilayer graphene, by decreasing the densities, the system can be moved from the region of weak interactions to the region where the Coulomb interactions dominate over kinetic energies [3]. In addition, an electric field applied perpendicular to the bilayer graphene sheets opens up a tuneable energy band gap between its conduction and valence bands [4]. We investigate the effect of the two bands of the bilayer graphene sheets on the BCSBEC crossover regime and the BEC regime of the multicomponent superfluid in the conduction and valence bands. We find that the crossover properties depend sensitively not only on the densities of the carriers but also on the tuneable energy band gap. The regimes of the crossover phenomena are characterized by the superfluid condensate fraction c, the fraction of carriers in pairs relative to the total number of carriers [5]. We recall the usual classification, c > 0.8 for the BEC regime, 0.2

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