ISSN 10628738, Bulletin of the Russian Academy of Sciences: Physics, 2010, Vol. 74, No. 3, pp. 330–334. © Allerton Press, Inc., 2010. Original Russian Text © M.G. Kozin, I.L. Romashkina, 2010, published in Izvestiya Rossiiskoi Akademii Nauk. Seriya Fizicheskaya, 2010, Vol. 74, No. 3, pp. 360–364.
Will the Iron Age of Superconductivity Become the Mössbauer Age? M. G. Kozin and I. L. Romashkina Skobeltsyn Institute of Nuclear Physics, Moscow State University, Moscow, 119991 Russia email:
[email protected] Abstract—The discovery of new ironcontaining hightemperature superconductors is considered in relation to their studies by means of the Mössbauer effect. The importance of the relevant results for the problem of the coexistence of magnetism and superconductivity and the stability of superconductivity against to disorder is shown by analyzing the available literature data. DOI: 10.3103/S1062873810030093
INTRODUCTION The problem of hightemperature superconductiv ity (HTSC), one of the most important problems fac ing physics of the 21st century, has a long history. The state of this problem at the turn of the century was described in Maksimov’s work [1]. There is no need to demonstrate that solving this problem is important not only for physics. A new wave of interest to HTSC arose at the begin ning of 2008 after the discovery, by a group of Japanese researchers headed by Hosono [3], of the transition to the superconducting state at the rather high tempera ture of 26 K in lanthanum oxypnictide of Fe doped with fluorine La(O1 – xFx)FeAs. As often happens, this discovery was made as a result of a comprehensive study with entirely different aims [3]. On the wave of this interest, several classes of ironcontaining com pounds already possessing superconducting properties (not necessarily at high temperature) or capable of becoming superconducting due to substitution (dop ing), nonstoichiometry, applied of pressure, and so on, were found. At the time of this writing (August, 2009), (Gd0.8Th0.2)OFeAs and Sm(O0.9F0.1)FeAs hold the record for transition temperature (about 56 K) [4, 5]. For the sake of brevity, we use the following notations below: 1111—quaternary compounds of the LnOFeAs type, where Ln is lanthanide; 122—ternary compounds of the AFe2As2 type, where A is alkaline or alkalineearth metals; 111—ternary compounds of the AFeAs type, where A = Li, Na; 011—double compounds of the FeCh type, where Ch = S, Se, Te (iron chalcogenides). The great interest displayed by researchers and the application of the experience accumulated while studying HTSCs on the basis of copper oxides have allowed scientists to obtain a large amount of actual
data in less than a year. Their interpretation, however, is not always unequivocal, as can be seen in three reviews published in the last 2008 issue of Advances in Physics (PhysicsUspekhi)[6]. Nevertheless it is uncontroversial that superconductivity in the new superconductors is observed in the layers of iron atoms associated with pnictogen or chalcogen. The crystal structure of the first two systems is shown in Fig. 1. It consists of layers of iron atoms in a tetrahedral environment of arsenic atoms separated by LaO or Ba layers, respectively. In the 111 structure, the FeAs layers are separated by the Li or Na atoms, while in the 011 structure the FeCh layers are not separated by anything. In nonsuperconducting structures, the iron in these compounds has low magnetic moment (in comparison with metallic iron) and is ordered anti ferromagnetically. “Iron” superconductivity is sur prising, since in classical superconductors even a small amount of Fe and other magnetic element impurities leads to the rapid suppression of superconductivity,
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La Fe As O
Ba Fe As
Fig. 1. Crystal structure of the 1111 and 122 systems [26]. A common feature of the layered crystal structure in the new HTSC is the presence of square nets of iron atoms in tetrahedral coordination with the arsenic atoms located above and below them.
WILL THE IRON AGE OF SUPERCONDUCTIVITY BECOME THE MÖSSBAUER AGE?
due to Cooper pair breaking upon scattering on the impurity magnetic moment [7, p. 165]. On the other hand, iron under sufficiently high pressure (~20 GPa) undergoes transition to the superconducting state at temperatures below 2 K [8]. It is quite natural that Fe57 Mössbauer spectros copy (MS) has been used to study the new ironcon taining compounds in both the superconducting and nonsuperconducting states. The results of these stud ies were summarized in the recent work [9]. In this review, the contribution of MS to the study of the superconducting materials known earlier, using (in addition to Fe57) such Mössbauer isotopes as Ru 99, Sn119, and Eu151, was briefly considered as well. The authors concluded that in all of these studies, MS was sensitive only to the magnetic state and little information about the superconducting state was obtained. In the present paper, we note only several works that were not mentioned in [9]. On the basis of the available data, we consider two questions: the compe tition or coexistence of magnetism and superconduc tivity, and the role of structural disorder. Both are closely associated with the problem of the homogene ity of the systems under study and with phase separa tion and its characteristic scale and features (not the presence of foreign phases of, e.g., the FeAs, Fe2As, or FeAs2 type, but heterogeneity within the limits of the considered composition). The first attempt to apply the Mössbauer effect (ME) to the investigation of superconductors was made in [10]. The temperature dependence of ME on iron impurity in indium was studied. In [11], the anomaly of the ME probability at the point of the superconducting transition of tin was found. Both studies were performed in the emission MS mode. We used this MS mode when studying copper oxide HTSCs. Analysis of the angular dependence of the spectra of Y123 and Bi2212 single crystals [12, 13] allowed us to identify the positions of the iron impurity atoms in the crystal lattices of these superconductors. Study of a ceramic sample of the Y123 system [14] showed that the number and relative weight of dou blets in the spectrum depend on the thermal prehis tory of the sample [14]. This should be kept in mind when studying the new ironcontaining superconduc tors. Different methods of synthesis and sample prep aration can, along with usually uncontrollable param eters (e.g., atmospheric humidity), be important for the obtained results [15]. The spectra discussed below were obtained on powders. The first Mössbauer study of new supercon ductors to come to our attention was [16] (system Nd1111; F content, 0.18; Тс ≈ 50 K). The singleline spectra (Fig. 2) do not vary as the temperature passes the superconducting transition. We do not discuss the small impurity of a foreign phase. The isomer shift is
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T, % 295 K 200 K 150 K 78 K 60 K 50 K 45 K 40 K 35 K 4.2 K Doublet 2% Singlet −1
0
1
2 υ, mm s−1
Fig. 2. Mössbauer spectra (single line) of a Nd1111 system of superconducting composition (18% of the oxygen is replaced by fluorine) at different temperatures. The form of the spectrum does not change upon the transition to the superconducting state [16] (T = relative transmission).
within the systematics of the isomer shifts for Fe2+ in the lowspin state. The temperature line shift is described in the Debye approximation and has no remarkable features at Тс. The effect probability was not discussed by authors of [16]. Similar behavior was observed for the 1111 system in studies by other groups using MS as well [17–20], and is characteristic for other classes of the new HTSC systems: the form of the spectra on both sides of Тс does not differ visibly in the simple simulation treatments used in [21–23]. The only distinction being that in 111 and 011 it is a qua drupolar doublet, and in 1111 and 122 it is a singlet. The quadrupolar splitting is evidently due to the closer arrangement of the neighboring atoms around the iron in 111 and 011 (i.e., the compression and distortion of the corresponding tetrahedra). The spectra of the nonsuperconducting parent compounds and those of the nonoptimally doped structures of the 1111 and 122 systems exhibit mag
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KOZIN, ROMASHKINA T, %
120
2%
4%
Ts
2%
T, K 150
1%
90
2%
2%
SDW
102 K
Tc
110 K
0.6
0.8
1.0 x
Fig. 3. Structure–temperature phase diagram of the (Ba1 ⎯ xKx)Fe2As2 system [27], showing the coincidence of the structural and magnetic transitions (SDW = spin density wave), and the transition to the superconducting state (SC) (compare to the phase diagram in Fig. 6 of [26]).
netic splitting; this allows to speak of possible types of magnetic ordering and of the iron magnetic moment, and to compare the results with data obtained by the diffraction of neutrons, the spin precession of muons, and other techniques. It is necessary to keep in mind that the different groups use different coefficients to transform the magnetic splitting of the spectra to the magnetic moment. A common feature, however, is the low value (less than 1 μB) of the magnetic moment. The system investigated in the most detail with regard to composition and temperature by means of MS and a number of other methods is the (Ba1 ⎯ xKx)Fe2As2 system [21, 24–26]. The close values of the ionic radii of Ba2+ (1.42 Å) and K+ (1.51 Å) allowed study of the dependence of the structure and superconductivity over the hole range of substitutions with a step of 0.1 on х [26]. The phase diagram of structural tetra–ortho phase transition Ts (from the Xray data) and the transition to superconducting state Tc (from the resistance) [26] virtually coincides with the structure–temperature phase diagram obtained by means of synchrotron radiation, the dif fraction of neutrons, and the resistance data in [27] (Fig. 3). The structural phase transition in this system coincides with the magnetic phase transition (Ts = TN). The phase diagrams show the region of the coex istence of magnetic ordering and superconductivity for compositions lower than optimum doping х = 0.4. The temperature evolution of the spectra for com position х = 0.2, demonstrating the presence of a sex tet and the absence of the paramagnetic component at the helium temperature (i.e., the coexistence of the
2%
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−2 −1 0
1
2
30 K
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0
50 K 2%
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100 K
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77 K 1%
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60
94 K
104 K
2%
4%
298 K
4.2 K
96 K
−2 −1 0
1
2
−2 −1 0 1 2 υ, mm s−1
Fig. 4. Mössbauer spectra of Ba0.8K0.2Fe2As2 (Tc = 23.6 К). The magnetic sextet at 4.2 K indicates the coex istence of magnetic ordering and superconductivity [21] (T = relative transmission). For х = 0.1 (Tc = 5 К), the paramagnetic component at 4.2 K is also absent.
magnetic ordering and superconductivity), is given in Fig. 4 [21]. The spectra of the initial nondoped BaFe2As2 below temperature TN = 155 K are described by a mag netic sextet. The saturation field at 4.2 K is 5.47 T, to which the authors relate the magnetic moment of 0.4– 0.5 μB on the Fe atom. In the region of the magnetic phase transition, the spectra are temperaturedepen dent superpositions of the magnetic sextet and the nonmagnetic singlet. This behavior the authors associ ate with the heterogeneity of the potassium distribu tion in the samples. The spectra of the samples with х = 0 – 0.2 exhibit complete splitting by the hyperfine magnetic field at 4.2 K, indicating magnetic ordering. In samples with х ≥ 0.3, this magnetic splitting is completely absent. Magnetic ordering and superconductivity thus coexist in the underdoped samples of this system, while mag netic ordering is not observed in the optimally doped and overdoped samples, and the spectra are single line. M. Rotter’s results are supported by the μSR data on single crystals [28–30] with regard to the coexist ence of magnetism and superconductivity, but they do not agree in the composition of samples for which this occurs. The Mössbauer data also indicate the homo geneous coexistence of magnetism and superconduc tivity, while the μSR data are interpreted in terms of the separation into magnetic and superconducting phases.
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WILL THE IRON AGE OF SUPERCONDUCTIVITY BECOME THE MÖSSBAUER AGE?
The μSR method alone does not allow one to determine how the nonmagnetic (superconducting) and magnetic (nonsuperconducting) regions are dis tributed in the sample. The researchers in [30] used magnetic force microscopy (MFM) to visualize the spatial distribution of the magnetically ordered regions on a cleaved surface of (Ba, K)122 single crystal with Tc = 26 K. Magnetic contrast that the authors relate to the borders of the antiferromagnetic domains in the cleaved plane was observed with resolution better than 50 nm and was reproducible. On the basis of Fourier analysis of the MFM image, the scale of the mesos copic separation to the phases of the magnetically ordered and nonmagnetic states in the considered plane was determined as 65 ± 10 nm. In the opinion of the authors of [30], such phase separation is an inter nal property of the material, since Xray analysis and susceptibility measurements indicate the absence of chemical heterogeneities in the samples. The various known forms of the coexistence of magnetic ordering and superconductivity can be found in [7]. From a theoretical point of view, one form of their coexistence is a modulated magnetic structure [31]. It is interesting and unexpected that magnetism and superconductivity in the new super conductors coexist in one system of atoms (iron atoms), and that the same atoms evidently participate in both collective phenomena, at least over a certain range of electron concentration. The problems of the relation between the electron structure, magnetism and superconductivity arise when considering the four known types of layered ironcontaining compounds [32]. In the 1111 system, there are three types of phase diagram for different rareearth components (see, e.g., the references in [6] and [29]), including those with overlapping regions of magnetic and superconducting order. We cannot, however, discuss these in detail here. We note only that doping with fluorine suppresses the spin ordering in the La1111 system. There is a sharp boundary in the phase diagram at х = 0.04, separating the magnetic ordering and superconductivity regions. It is impor tant to note that in the magnetic region, the tempera ture dependences of the average field determined from the Mössbauer spectra and the local field on the muon are proportional to each other [17, 18]. In the 111 and 011 systems, the Mössbauer spectra are quadrupolar doublets which are virtually identical above and below the superconducting transition tem perature. Whether the system 111 is superconducting only instoichiometric composition or not is not clear [33– 35]. According to the Mössbauer data in [32], a foreign FeAs phase of 11% was found in samples pre pared by the technique used in [34], favoring the importance of nonstoichiometry. The parameters of the main 111 phase were IS = 0.58 mm s–1 and QS = 0.59 mm s–1 at 95 K.
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Band theory (LDA) [36] predicts the existence of superconductivity in all iron chalcogenides. However, it has been observed experimentally only in selenide [37]. The superconducting transition temperature in the almost stoichiometric composition βFeSe rose to more than 36 K under a pressure of about 10 GPa [38]. In these studies, the extreme sensitivity of supercon ductivity to the stoichiometry of this system was revealed, but no traces of magnetism, sought by the magnetic splitting of the Mössbauer spectra, were recorded on the phase diagram. Magnetic contribu tions to the spectra were found only in samples con taminated by oxygen. The Mössbauer parameters of pure samples were IS = 0.44 mm s–1 and QS = 0.44 mm s–1. Attempts to observe the transition of tet ragonal FeTe to a superconducting state under pres sure failed [39]. At the same time, there was a report [40] on the growth of large single crystals (weight ~10 g) with variable composition Fe1 + yTexSe1 – x (0 < y < 0.15 and 0.5 < x < 1.0), in which iron can occupy additional positions in the Te/Se layers, and bulk superconductivity was observed for samples with х = 0.5 and y = 0.55. The absence of magnetic ordering in the LiFeAs and FeSe systems thus leads us to wonder just how general the scenario of superconductivity emergence in the new superconductors via the suppression of static magnetism upon electron or hole doping is, and how much the hypothesis of superconductivity due to antiferromagnetic fluctuations justifies. The discovery of superconductivity in doped cobalt parent systems (see [41] and the references therein), undermining the traditional ideas about the impossi bility of the Cooper pairing in the presence of the mag netic scattering centers, was unexpected as well. The temperature dependences of the resistance of the Sm1111 system, in which 10.or 15% of Fe is replaced by Co, show that the superconducting transition tem perature does not vary at rather high doping levels. This testifies to the stability of superconductivity to a high level of the disorder in the system, which is also usually considered incompatible with the phase coher ency of the superconducting electrons. Detailed stud ies of the substitution of iron by cobalt and other tran sition elements could provide important information for understanding the superconductivity mechanisms in ironcontaining superconductors. CONCLUSIONS It is clear that MS and emission MS in particular should be making contributions to these investiga tions. For a deep understanding of the results from such experiments, calculations of the electron struc ture that would allow direct comparison with the Mössbauer parameters are needed. These could be band or cluster calculations that take into account var ious mechanisms for the appearance of spin polariza
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