APPLIED PHYSICS LETTERS 96, 202512 共2010兲
Atomically resolved scanning tunneling microscopy on perovskite manganite single crystals Sahana Rößler,1 B. Padmanabhan,2 Suja Elizabeth,2 H. L. Bhat,2 F. Steglich,1 and S. Wirth1,a兲 1
Max Planck Institute for Chemical Physics of Solids, 01187 Dresden, Germany Department of Physics, Indian Institute of Science, Bangalore 560012, India
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共Received 19 March 2010; accepted 4 May 2010; published online 21 May 2010兲 Atomically resolved scanning tunneling microscopy was conducted on cleaved single crystals of the cubic perovskite Pr0.68Pb0.32MnO3. Several different surface configurations could be resolved including a frequent square arrangement with atomic distances in excellent agreement to the bulk lattice constant of the cubic structure. We also observed stripe formation and a surface reconstruction. The latter is likely related to a polar rare earth-oxygen terminated surface. © 2010 American Institute of Physics. 关doi:10.1063/1.3432753兴 The mixed valence manganites of perovskite type R1−xAxMnO3 共R–rare earth, A–divalent or tetravalent ion兲 demonstrate a wealth of remarkable and complex physical properties1,2 due to strong electronic correlations. These correlations involve structural, charge, orbital as well as spin degrees of freedom leading to a vast variety of different types of order that, in turn, may result in competition, or even coexistence, of different phases.3 The balance between these phases can be subtle such that small changes in a tuning parameter 共e.g., chemical composition or magnetic field兲 prompt a significant response of the material’s properties. Examples here are the metal-insulator transition and the colossal magnetoresistance 共CMR兲.2 The extent of the involved phases can range from a very few to several hundreds of nanometers. Because of the complexity of the underlying physics of manganites a comprehensive investigation is called for. Here, scanning tunneling microscopy/spectroscopy 共STM/S兲 is of special interest as it provides local information on the electronic states close to the Fermi level. As such, STM/S has been utilized early on, specifically in order to shed light on the electronic phase separation between the insulating paramagnetic and the conducting ferromagnetic phase existing in some manganites close to the metal-insulator transition temperature, TMI.4,5 In addition, the strong Jahn–Teller interaction in many manganites is an essential ingredient in the understanding of their transport properties and, in particular, the CMR effect.6 In contrast to other techniques, STM/S is capable of directly imaging the resulting high-temperature polaronic state in real space.7 Moreover, charge ordering could be observed in the paramagnetic state of two different manganites.8,9 A prerequisite of major concern for STM/S is a proper surface preparation. Typically, samples are cleaved in situ and kept under vacuum conditions. However, for the perovskitelike manganites, it has been pointed out8,10,11 that such an approach is exceedingly difficult. Therefore, research by means of STM 共Ref. 7兲 or by other surface-sensitive techniques12,13 reverted to the bilayer manganites which easily cleave between the rare-earth rock-salt oxide layers.14 A different approach relied on in situ epitaxially grown thin a兲
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films of 共La5/8−xPrx兲Ca3/8MnO3 which exhibited charge ordering at the nanometer scale.8 Charge ordering was also found for highly doped Bi0.24Ca0.76MnO3, the only report of atomically resolved STM on perovskitelike bulk manganites.9 Other reports on spatially resolved STM/S did not yield atomic resolution.4,5,15–19 Here, we report on STM measurements on single crystals of Pr0.68Pb0.32MnO3 at room temperature, i.e., in its paramagnetic insulating regime. Our atomically resolved images provide evidence for different types of surfaces: 共i兲 an undisturbed square arrangement in accord with the Mn–O plane of the cubic perovskite structure, 共ii兲 stripe formation, and 共iii兲 a reconstruction possibly related to the polar interactions on rare-earth terminated surfaces. The growth and properties of Pr0.68Pb0.32MnO3 single crystals have been reported before.20,21 In particular, the crystal structure of this compound exhibits cubic symmetry 共lattice constant a = 3.87 Å兲 as confirmed by x-ray studies. While being immersed in isopropanol, the sample was scratched by means of a scalpel, and loaded into our ultrahigh vacuum STM 共Omicron Nanotechnology, equipped with an additional active vibration isolation control system兲. This procedure yielded frequently large terraces 共similar to the terraces reported in Ref. 17兲 separated by steps of heights close to a or its multiples. On such terraces, atomically resolved topography was obtained, Fig. 1共a兲. Here, a tunneling gap voltage Vg = 0.2 V at a current set point of Isp = 0.5 nA was applied. The crystal orientation as well as the step heights is consistent with a 具100典 surface of the cubic perovskite crystal. The exponential dependence of the tunneling current I on the tip-sample distance z over two decades in I confirmed excellent vacuum tunneling with typical work functions of ⬃ 1.5 eV. The topography and its fast Fourier transform 共FFT兲, Figs. 1共a兲 and 1共c兲, respectively, clearly reveal a square arrangement of the prominent surface atoms with a distance of d = 3.7⫾ 0.3 Å, in excellent agreement with the lattice constant. This, however, prevents a clear assignment of the atoms observed in the surface layer. Nonetheless, the small value of Vg along with the calculated bulk band structure22 suggests that only Mn ions are seen. Such an assignment would imply a Mn–O termination of the surface, in accord with findings on thin films.8
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lateral position (nm) FIG. 1. 共Color兲 共a兲 Atomically resolved STM topography taken on a cleaved Pr0.68Pb0.32MnO3 single crystal 共the area shown is 3.0⫻ 3.0 nm2兲. 共b兲 The line profile reflects the height along the blue line in the topography image. 共c兲 The FFT of the topographic image 共a兲 indicates a square arrangement of the surface atoms with average distance of 3.7 Å.
The periodicity and height scales of the topography in Fig. 1 match well with results on layered manganites7 and thin films.8 Assuming a Mn–O surface termination there is, in contrast to Ref. 8, no indication for having different Mn ions at the surface; we likely observed Mn3+ ions only. In our case of a doped manganite the Mn–O surface is polar but there is a possibility of electronic reconstruction at the surface. In addition, several factors may account for our finding as follows: 共i兲 Mn3+ and Mn4+ ions may not necessarily be statistically distributed in the bulk sample. 共ii兲 The surface may not reflect the bulk distribution of Mn3+ and Mn4+ ions. In addition to the broken symmetry, the rough cleaving process may contribute to differences between surface and bulk. 共iii兲 Some parts of the surface did not exhibit atomic resolution as in Fig. 1共a兲. Specifically, the formation of stripes 关Fig. 2共a兲兴 was occasionally observed 共see below兲. One could speculate that Mn4+ ions are restricted to these areas. 共iv兲 As outlined in Ref. 8, in case of a Mn–O terminated surface the hole doping stems from a subsurface layer and 共v兲 the holes at Mn4+ 共i.e., the “missing” eg electrons with respect to Mn3+兲 are delocalized. In manganites, the formation of stripes resulting from ordered polarons in the insulating regime is controversially discussed.23–25 Earlier STM investigations revealed trapped small polarons,7 or indeed pointed toward stripe formation26 similar to Fig. 2共a兲. When probing the unoccupied electronic density of states by applying a positive Vg, the doped holes at the Mn4+ sites appear as bright spots in the STM image, whereas electron tunneling from a metallic STM-tip into a Mn3+ state is difficult and produces dark spots.7,8 Hence, the contrast indicates polaron formation, yet the missing atomic resolution in Ref. 26 and Fig. 2共a兲 impedes a direct evaluation. However, occasionally we observed atomically resolved
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lateral position (nm) FIG. 2. 共Color兲 共a兲 STM topography over an area of 2.5⫻ 2.5 nm2 on a Pr0.68Pb0.32MnO3 single crystal indicating stripe formation. 共b兲 Atomic resolution observed on a different area 共size of 5.0⫻ 4.0 nm2兲 suggests a surface reconstruction. 共c兲 The line profile taken along the blue line in 共b兲 clearly reveals two different distances between the surface atoms 共indicated by arrows兲 of approximately 3.6 Å and 5.1 Å, respectively. The tunneling parameters are the same as for Fig. 1.
topography again resembling a stripelike order of the surface atoms. As becomes immediately obvious from Fig. 2共b兲 the surface atoms are arranged differently when compared to Fig. 1共a兲. The line profile of Fig. 2共c兲 taken perpendicular to this stripelike pattern 关blue line in Fig. 2共b兲兴 reflects alternating longer and shorter distances between the surface atoms 关arrows in Fig. 2共c兲兴. Here, the average shorter distance of d1 ⬇ 3.6 Å is still close to the lattice constant whereas the longer one d2 ⬇ 5.1 Å corresponds to 冑2d1. Such alternating distances with a length ratio of 冑2 have been related to charge modulation9 and polaron formation. However, from Fig. 2共b兲 we find an average atomic distance along the stripes of d3 ⬇ 2.7 Å, a length scale close to d / 冑2. These average distances suggest a surface reconstruction in the atomically resolved topography of Fig. 2共b兲 as an alternative scenario to the stripe formation of Fig. 2共a兲. We note that the two topographic images Figs. 1共a兲 and 2共b兲 represent different areas of the same single crystal, i.e., the underlying crystallographic orientation 关as revealed by Fig. 1共a兲兴 is identical. So far, all our images of a potential reconstruction like Fig. 2共b兲 point to an orientation of the bright stripes of about 25° with respect to the 关001兴 direction. Identical tunneling parameters were used to obtain Figs. 1共a兲 and 2共b兲. Clearly, Figs. 2共a兲 and 2共b兲 present conflicting results obtained on different locations of the same single crystal. A possible interpretation is based on the fact that the parent manganite LaMnO3 exhibits polar surfaces.27 As shown for SrTiO3, such polar surfaces are prone to reconstructions.28 In addition, a R / A – O surface termination becomes possible for doped manganites29 and in view of the very rough cleaving process. Consequently, the surveyed surface areas could be differently terminated. Likely, polaron formation and stripe order similar to Fig. 2共a兲 is observed for a more frequent Mn–O terminated surface. The atomically resolved stripelike features of Fig. 2共b兲 may result from a reconstruction, possibly of a R / A – O terminated surface. So far, we could not
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FIG. 3. 共Color online兲 共a兲 Topography 共5.0⫻ 5.0 nm2, Vg = −0.2 V, Isp = 0.5 nA兲 and 共b兲 its FFT exhibiting a nearly hexagonal arrangement of the manganite surface atoms. The prominent line in the FFT is an artifact caused by the horizontal fast scanning direction.
directly confirm this conjecture by observing a step height of a / 2 on the surface. Rarely, we also observed a nearly hexagonal arrangement of the manganite surface atoms as shown in Fig. 3. A very similar arrangement, considered an impurity phase, has been observed30 on charge ordered Bi0.24Ca0.76MnO3 and layered La2−2xSr1+2xMn2O7. In conclusion, atomic resolution on cleaved single crystals of the cubic perovskite Pr0.68Pb0.32MnO3 has been achieved. Beside the expected and frequent square arrangement of the surface atoms stripe formation, a possible surface reconstruction and, rarely, a nearly hexagonal arrangement were observed. The reconstruction may be caused by a polar R / A – O terminated surface. We are grateful for fruitful discussions with G. A. Sawatzky and L. H. Tjeng. This work was supported by the DAAD 共ID 50726385兲 and the Department of Science and Technology 共DST:INT/FRG/DAAD/P-196/2009兲 India. J. M. D. Coey, M. Viret, and S. von Molnár, Adv. Phys. 48, 167 共1999兲. Colossal Magnetoresistive Oxides, edited by Y. Tokura 共Gordon and Breach, Amsterdam, 2000兲. 3 E. Dagotto, Nanoscale Phase Separation and Colossal Magnetoresistance 共Springer, New York, 2003兲. 4 M. Fäth, S. Freisem, A. A. Menovsky, Y. Tomioka, J. Aarts, and J. A. Mydosh, Science 285, 1540 共1999兲. 5 T. Becker, C. Streng, Y. Luo, V. Moshnyaga, B. Damaschke, N. Shannon, and K. Samwer, Phys. Rev. Lett. 89, 237203 共2002兲. 1 2
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