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. 2012 Dec 28;13(5):054303. doi: 10.1088/1468-6996/13/5/054303

Physics and chemistry of layered chalcogenide superconductors

Keita Deguchi 1,2, Yoshihiko Takano 1,2, Yoshikazu Mizuguchi 1,3,✉
PMCID: PMC5099617  PMID: 27877516

Abstract

Structural and physical properties of layered chalcogenide superconductors are summarized. In particular, we review the remarkable properties of the Fe-chalcogenide superconductors, FeSe and FeTe-based materials. Furthermore, we introduce the recently discovered BiS2-based layered superconductors and discuss their prospects.

Keywords: Fe chalcogenide, FeSe, FeTe, superconductivity, magnetism, BiS2-based superconductor

Introduction to layered chalcogenide superconductors

Layered materials have provided us with many interesting fields in physics and chemistry. Owing to their two-dimensional crystal structure and electronic states, anomalous electronic and magnetic properties have often been observed. In particular, exotic superconductivity is likely to prefer such a layered crystal structure. For example, superconductivity at a high transition temperature (Tc) has been achieved in layered materials, such as cuprates [1–4], Fe-based [5–13] and MgB2 [14, 15] superconductors. We expect that a new high-Tc superconducting family will be found in layered materials.

Among the layered superconductors, the ‘chalcogenides’ are one of the notable groups, because of the variety of materials and the observation of exotic superconductivity. Sulfur (S), selenium (Se) and tellurium (Te) are categorized as chalcogens. Here we introduce the structural and physical properties of some layered chalcogenide superconductors. CdI2-type TiSe2 has a simple layered structure, composed of a stack of TiSe2 layers as depicted in figure 1(a), and exhibits a charge-density-wave (CDW) state. Intercalation of transition metals into the interlayer sites or application of external pressure suppresses the CDW state and induces superconductivity [16, 17]. A topological insulator Bi2Se3 also possesses a layered structure with a van der Waals gap as shown in figure 1(b). Similar to TiSe2, intercalation of Cu in the interlayer sites induces superconductivity in Bi2Se3 [18, 19]. One of the notable characteristics of chalcogenides is crystallization of a simple layered structure with van der Waals gaps. In such a structure, ions can be easily intercalated in the interlayer sites and dramatically change the physical properties of the chalcogenide layers.

Figure 1.

Figure 1

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Crystal structures of the typical layered chalcogenides (a) TiSe2, (b) BiSe2 and (c) FeSe.

The most remarkable layered chalcogenides are the Fe chalcogenides, FeSe and FeTe, which are the simplest Fe-based superconductors. The crystal structure of FeSe is shown in figure 1(c). FeSe exhibits a superconducting transition around 10 K, and shows a marked increase of Tc up to 37 K under high pressure. In contrast to FeSe, FeTe undergoes an antiferromagnetic transition at 70 K. However, a partial substitution of Te by S or Se suppresses the antiferromagnetic ordering and induces superconductivity. This family is very interesting because the physical properties markedly change upon the covalent substitutions of S, Se and Te. Recently, superconductivity above 40 K was observed in metal- or molecule-intercalated FeSe. To that respect, studies on not only fundamental physics but also applications of Fe-chalcogenide superconductors will be addressed. In the second section, we summarize the physical and structural properties of Fe-chalcogenide superconductors.

Very recently, we discovered a new superconducting family of the layered bismuth sulfides [20–22]. In this family, the BiS2 layers are the common superconducting structure essential for superconductivity as the CuO2 planes in the cuprates and the Fe2An2 (An: anions of P, As, S, Se or Te) layers in the Fe-based family. In section 3, we introduce the discovery of the novel BiS2-based layered superconducting family and the latest results.

Fe chalcogenides

In 2008, Kamihara et al. [5] reported superconductivity in the Fe-based compound LaFeAsO1−xFx. Although the parent compound LaFeAsO is an antiferromagnetic metal, the F substitution suppresses the magnetic ordering and induces superconductivity with a Tc as high as 26 K. As shown in figure 2(a), LaFeAsO has a layered structure with a stacking of the blocking La2O2 layers and the superconducting Fe2As2 layers. Many FeAs-based compounds analogous to LaFeAsO, for example, SmFeAsO1−xFx and Ba1−xKxFe2As2, were found to be superconducting with a maximum Tc as high as 55 K [7, 8]. All the Fe-based superconductors share a common layered structure based on the planar layer of an Fe square lattice. In Fe-pnictide superconductors, blocking layers containing alkali, alkali-earth or rare-earth and oxygen/fluorine elements are alternatively stacked with Fe–As conduction layers [5–9, 23–26].

Figure 2.

Figure 2

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Crystal structures of the typical Fe-based superconductors (a) LaFeAO, (b) FeSe and (c) Fe1+dTe. For Fe1+dTe, excess Fe occupies the interlayer sites with an occupancy of 7–25% as indicated with partially filled circles.

Superconductivity in layered Fe chalcogenides was initially found in FeSe by Hsu et al. [10] soon after the discovery of superconductivity in the LaFeAsO system. Contrary to the FeAs-based superconductors, FeSe is composed of only superconducting Fe2Se2 layers as shown in figure 2(b). Because of the lack of a blocking layer Fe-chalcogenides have the simplest crystal structure among the Fe-based superconductors. Furthermore, Fe1+dTe, which has a crystal structure analogous to FeSe as shown in figure 2(c), exhibits antiferromagnetic ordering as observed in the parent (non-doped) phases of FeAs-based superconductors. Thus, Fe-chalcogenides are the key materials for elucidating the mechanism of Fe-based superconductivity [10–12].

FeSe

PbO-type FeSe is well known as a commercial material, but the discovery of superconductivity in this compound was triggered by Fe-based superconductors reported in 2008 [10]. PbO-type FeSe is composed of only Fe2Se2 layers with a tetragonal structure (space group: P4/nmm). It has an iron-square planar sublattice equivalent to that of the iron pnictides and its crystal structure is the simplest among the Fe-based superconductors. The PbO structure is one of the stable phases of the Fe–Se binary compounds [27]. One can therefore obtain a polycrystalline sample of PbO-type FeSe using a conventional solid-state reaction method. However, FeSe synthesized by the solid-state reaction at high temperatures contains the NiAs-type (hexagonal) FeSe phase. Obtaining a single phase of tetragonal FeSe requires low-temperature annealing around 300–400 °C, which transforms the hexagonal phase to the tetragonal phase [28]. FeSe exhibits a structural transition from tetragonal to orthorhombic phase (Cmma) at 70–90 K. This structural transition is not accompanied by any magnetic transition, and superconductivity occurs in the orthorhombic phase below ∼12 K [10, 28–31]. Figure 3 shows the temperature dependence of resistivity with the insets presenting the resistivity in magnetic field and the estimated upper critical field (μ0Hc2) [10]. The resistivity becomes zero below 8 K, and a high μ0Hc2 is observed as the common feature of Fe-based superconductors. Although the Tc of FeSe at ambient pressure is relatively low as compared with other Fe-based superconductors, FeSe shows a marked enhancement of Tc under high pressure [32–35].

Figure 3.

Figure 3

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Temperature dependence of resistivity in FeSe. The left inset shows its variation with magnetic field, and the obtained field–temperature phase diagram is plotted in the right inset. Reproduced with permission from [10] ©2008 National Academy of Sciences.

Figures 4(a) and (b) show the temperature dependence of resistivity for FeSe under high pressure measured using a piston cylinder cell and an indenter cell, respectively [32, 34]. With increasing pressure, the pressure dependence of Tc shows an anomaly around 2 GPa and then a steep increase above 2 GPa. Above 4 GPa, Tconset reaches a maximum value of 37 K. The fact that the application of pressure markedly enhances Tc in FeSe indicates that the increase of Tc should be related to the change in the local crystal structure, because the carrier density does not significantly change with pressure. This enhancement of Tc was explained by investigating the structural parameter ‘anion height’ in the Fe layer [33, 36, 37].

Figure 4.

Figure 4

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Temperature dependence of resistivity in FeSe under high pressure measured using (a) a piston-cylinder cell and (b) an indenter cell.

As shown in figure 5, the pressure dependences of Tc and the anion height for FeSe show an obvious correlation, implying that Tc of FeSe can be optimized by tuning the anion height. Interestingly, the anion height dependence of Tc is applicable to all the Fe-based superconductors, not only FeSe [36–38]. Figure 6(a) shows the anion height dependence of Tc in typical Fe-based superconductors [36], and a schematic of anion height from the Fe layer is given in figure 6(b). The anion height dependence of Tc is a symmetric curve peaking at ∼1.38 Å, as indicated by the hand-drawn fitting curve. The data for FeSe fall on this fitting curve only for pressures above 2 GPa, at which the anomaly was observed in the pressure dependence of Tc as shown in figure 5. This observation indicates that intrinsic superconductivity in FeSe might be induced by the application of pressure above 2 GPa.

Figure 5.

Figure 5

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Pressure dependence of Tc and the Se height in FeSe.

Figure 6.

Figure 6

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(a) Anion height dependence of Tc in typical Fe-based superconductors. (b) Schematic of anion height.

High-Tc superconductivity in FeSe-related materials

As achieved by the application of the external pressure, the Tc of FeSe can be enhanced by decreasing the anion height upon intercalation of a metal element or a molecule into the interlayer sites. Guo et al. [13] reported superconductivity with Tc above 30 K in K-intercalated FeSe, K0.8Fe2Se2. Not only metal ions but also molecules such as Lix(NH2)y(NH3)1−y can be intercalated into the interlayer site of FeSe; Lix(NH2)y(NH3)1−yFe2Se2 shows a high Tc above 40 K [39]. To date, many FeSe-based superconductors with Tc above 30 K have been documented [13, 39–46]. Furthermore, Wang et al. [47] recently reported that single unit-cell FeSe films show signatures of superconducting transition with an onset temperature of 53 K. These facts indicate that the mechanisms of high-Tc superconductivity in Fe-based compounds are common in FeAs-based and Fe-chalcogenide-based superconductors.

FeTe

The crystal structure of FeTe is very similar to that of FeSe. However, FeTe, but not FeSe, exhibits a magnetic/structural transition at 70 K, and the physical properties of FeTe are different from those of FeSe [48, 49]. FeTe is not superconducting except in the special case of the tensile-stressed FeTe thin film [50], which showed superconductivity at 13 K. Neutron scattering studies indicate that the spin structure is different in FeTe and the Fe-pnictide parent compounds as shown figure 7 [48]. For FeSe and Fe-pnictide superconductors, the antiferromagnetic spin fluctuations with a wave vector Qs = (0.5, 0.5) were found to correlate with superconductivity [51–55]. On the other hand, FeTe shows magnetic wave vector Qd = (0.5, 0) [51, 52], which is not favorable for superconductivity [56–58]. The emergence of a Fermi surface nesting associated with Qd could be induced by excess Fe at the interlayer site as illustrated in figure 2(c). Excess Fe supplies a substantial amount of electrons and it has a magnetic moment. FeTe contains 7–25% of excess Fe in its crystal structure, which affects its physical properties [56, 58].

Figure 7.

Figure 7

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Schematic in-plane spin structure of FeTe and SrFe2As2. The solid arrows and hollow arrows represent two sublattices of spins. The shaded area indicates the magnetic unit cell. Reproduced with permission from [48] ©2009 American Physical Society.

FeTe-based superconductors

FeTe1−xSex.

As mentioned above, FeTe exhibits antiferromagnetic ordering associated with a lattice distortion at 70 K. Furthermore, FeTe possesses excess Fe (7–25%) at the interlayer sites. However, partial Se substitutions suppress the low-temperature structural/magnetic phase transition and reduce excess Fe, thereby inducing superconductivity [11, 59, 60]. Figure 8 shows the temperature dependence of magnetic susceptibility in FeTe1−xSex around TN (a) and around Tc (b). The long-range magnetic ordering is suppressed with increasing Se concentration and completely disappears at x = 0.15. The Tc is also gradually enhanced with increasing Se content and the optimal superconducting properties are obtained at a composition of FeTe0.5Se0.5 with Tc of 14 K, which is the highest Tc at ambient pressure among the FeTe-based superconductors. FeTe1−xSex shows a positive pressure effect on Tc as in the case of FeSe [61, 62]. As shown in figure 9, a dome-shaped Tc versus pressure curve arises with a maximum Tc of 23 K at 2 GPa [61]. Tensile-stress effects on Tc were also reported for thin films of FeTe1–xSex [63, 64]. The Tc of FeTe0.5Se0.5 films increases with decreasing lattice constant a reaching 21 K. The tensile stress can raise Tc similar to external pressure.

Figure 8.

Figure 8

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Temperature dependence of normalized magnetic susceptibility in FeTe1–xSex around TN (a) and around Tc (b). The arrows indicate the magnetic transition temperature. ZFC stands for zero-field cooling.

Figure 9.

Figure 9

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Tc as a function of applied pressure. Reproduced with permission from [61] ©2009 American Chemical Society.

FeTe1−xSx.

As the partial Se substitution for Te induces superconductivity in FeTe, the S substitution for Te also suppresses magnetism and induces superconductivity [12]. However, in the case of FeTe1−xSx, the optimal substitution cannot be achieved due to the low S/Te solubility limit as shown in figure 10 [65]. Although the antiferromagnetic ordering can be completely suppressed by 20% S substitution, bulk superconductivity is not observed. This can be understood as the appearance of bulk superconductivity in FeTe1−xSx is affected by the higher content of excess Fe resulting from low S concentration. In particular, the FeTe0.8S0.2 samples synthesized using a solid-state reaction method show a broad transition in the temperature dependence of resistivity, and diamagnetic signal corresponding to superconductivity is not observed. Ingenious attempts have been carried out to enhance this weak superconductivity and exotic annealing effects were discovered.

Figure 10.

Figure 10

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Nominal x dependence of xE determined by electron probe microanalysis. Reproduced with permission from [65] ©2011 IEEE.

Annealing effects of FeTe-based superconductors

Figure 11(a) shows the temperature dependence of resistivity for a FeTe0.8S0.2 sample that was kept in the air for up to 2 years. Exposure to air induced a distinct superconducting transition with zero resistivity, which was absent in the as-grown state [66, 67]. After 2 years, the Tczero reached 7.8 K. The superconducting signal of magnetic susceptibility was markedly enhanced with increasing air-exposure time as shown in figure 11(b).

Figure 11.

Figure 11

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Temperature dependence of (a) resistivity and (b) magnetic susceptibility in FeTe0.8S0.2 exposed to air.

The weak superconductivity was improved by annealing in oxygen [68, 69]. Figure 12 shows the temperature dependence of magnetic susceptibility for the samples annealed under various conditions. Only the oxygen-annealed sample shows a distinct superconducting behavior. Kawasaki et al. [70] reported that annealing in oxygen is effective for not only FeTe1–xSx but also FeTe1–xSex. Figure 13 shows the temperature dependence of magnetic susceptibility for O2-annealed samples with various Se concentrations around TN (a) and around Tc (b). Even a small (10%) substitution of Te by Se completely suppressed the magnetic ordering and induced bulk superconductivity. The difference in superconducting signals between as-grown and O2-annealed samples is obvious as shown in figures 8 and 13.

Figure 12.

Figure 12

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Temperature dependence of magnetic susceptibility in FeTe0.8S0.2 annealed in different atmospheres.

Figure 13.

Figure 13

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Temperature dependence of magnetic susceptibility in O2-annealed samples with various Se concentrations around TN (a) and around Tc (b). The arrows indicate the magnetic transition temperature.

Figures 14(a) and (b) show phase diagrams based on magnetic susceptibility measurement for the as-grown and O2-annealed samples. The as-grown samples exhibited a long-range AFM for x ⩽ 0.15 and weak superconductivity for 0.1 ⩽ x ⩽ 0.4 where ‘weak superconductivity’ means non-bulk (filamentary or partial) superconducting state interfered by excess Fe. Only the FeTe0.5Se0.5 sample was found to be a bulk superconductor. For O2-annealed samples, the coexistence of AFM ordering and weak superconductivity was observed only for x ⩽ 0.1. As the long-range AFM ordering was completely suppressed, the O2-annealed samples with x ≽ 0.1 became bulk superconductors. It is clear from figure 14 that the bulk superconductivity region spreads with O2 annealing.

Figure 14.

Figure 14

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Phase diagrams showing Tc and TN as a function of x for (a) as-grown FeTe1–xSex and (b) O2-annealed FeTe1–xSex.

Next, we discuss the relation between bulk superconductivity and antiferromagnetic fluctuations. As mentioned in section 2.1, Antiferromagnetic spin fluctuations with the wave Qs = (0.5, 0.5) were observed in FeSe and other FeAn compounds. In contrast, FeTe exhibits antiferromagnetic wave vector Qd = (0.5, 0). Neutron scattering measurements revealed that Qs and Qd are observed over a wide composition range where FeTe1−xSex exhibits weak superconductivity [57, 71]. For as-grown FeTe1−xSex samples, weak superconductivity was observed in the range 0.1 ⩽ x ⩽ 0.4, suggesting that the nesting vectors Qs and Qd coexist. It is expected that the nesting vector Qs becomes dominant for FeTe0.5Se0.5 where bulk superconductivity sets in. On the other hand, for the O2-annealed samples, the bulk superconductivity region extends down to x = 0.1, which implies that the nesting vector Qd is strongly suppressed by annealing in oxygen. It was concluded that the oxygen ions intercalated between superconducting layers compensate the over-doped electron carriers and suppress the magnetic wave vector Qd, which is responsible for the appearance of bulk superconductivity.

Soft chemical treatment for deintercalation of excess Fe

It was found that the alcoholic beverages induced superconductivity in FeTe0.8S0.2 [72]. FeTe0.8S0.2 samples were immersed in red wine, white wine, beer, Japanese sake (rice wine), shochu (distilled spirit) or whisky and then heated at 70 °C for 24 h. The obtained shielding volume fractions are summarized in figure 15 as a function of ethanol concentration; they vary between 6 and 9% for water–ethanol mixtures, but are much higher (21–63%) for alcoholic beverages, especially red wine. This effect was also confirmed for FeTe0.9Se0.1 (figure 16). It was explained by the deintercalation of excess Fe from the interlayer sites [73]. After analyzing the ingredients of alcoholic beverages we found that the solutions of malic acid, citric acid and β-alanine also induced the superconductivity in FeTe0.8S0.2 as shown in figures 17(a) and (b). Inductively coupled plasma spectroscopy results indicated that Fe ions were deintercalated from the sample to these solutions as shown in figure 17(c). Therefore, annealing in alcoholic beverages suppresses the magnetic moment of excess Fe simply by removing Fe, resulting in superconductivity. A similar enhancement of superconductivity by the deintercalation of excess Fe was reported in [74, 75]. This technique can be generally applied to the layered Fe-chalcogenide superconductors.

Figure 15.

Figure 15

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The shielding volume fraction of FeTe0.8S0.2 samples heated in various liquids as a function of ethanol concentration.

Figure 16.

Figure 16

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(a) Temperature dependence of magnetic susceptibility and (b) the shielding volume fraction of FeTe0.9Se0.1 samples heated in various liquids.

Figure 17.

Figure 17

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(a) Temperature dependence of magnetic susceptibility for FeTe0.8S0.2 annealed in various solutions. (b) The shielding volume fractions for the samples annealed in various liquids as a function of pH. (c) The pH dependence of Fe concentration in solutions after annealing.

BiS2-based layered superconductors

Discovery of a common superconducting layer is very important because many analogous superconductors can be designed by changing the spacer layers as in the high-Tc cuprates and Fe-based superconductors. Recently, we found that the BiS2 layer can be a basic superconducting layer in a new class of layered superconductors.

The first BiS2-based superconductor is Bi4O4S3 [20]. Its crystal structure (figure 18(a)) comprises a stack of rock-salt-type BiS2 layers and Bi4O4(SO4)1−x layers (blocks), where x indicates the lack of SO42− ions at the interlayer sites. The parent phase (x = 0) is Bi6O8S5, and Bi4O4S3 corresponds to x = 0.5. Such defects at the interlayer sites are common for layered materials.

Figure 18.

Figure 18

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Crystal structures of Bi4O4(SO4)1–xBi2S4 (a) and LaOBiS2 (b); x = 0.5 corresponds to Bi4O4S3.

Temperature dependence of resistivity is shown in figure 19. A gradual decrease of resistivity is observed below 8.6 K with the zero resistivity appearing at 4.5 K. Magnetic susceptibility measurements indicate that Bi4O4S3 is a bulk superconductor. To investigate the origin of superconductivity in Bi4O4S3, we performed band calculations. They indicate that Bi4O4S3 (x = 0.5) is a metal, whereas the parent phase of Bi6O8S5 (x = 0) is a band insulator containing Bi3+ ions. For Bi4O4S3, the Fermi level lies within the bands that mainly originate from the Bi 6p orbitals as shown in figure 20. In particular, the Fermi level coincides with the peak of the partial density of states of the Bi 6p orbital within the BiS2 layer.

Figure 19.

Figure 19

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Temperature dependence of resistivity of Bi4O4S3 in magnetic fields up to 5 T.

Figure 20.

Figure 20

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Left: the band structure of Bi4O4S3. The radius of the circles reflects the density of the Bi 6p orbitals within the BiS2 layer. Blue and red colors indicate the bands having mainly px and py characters, respectively. Right: the partial density of states of the Bi 6p orbital within the BiS2 layer.

The second BiS2-based system is REO1−xFxBiS2 (RE = rare earth). So far, superconductivity was observed in LaO1–xFxBiS2 [24], NdO1–xFxBiS2 [25], CeO1–xFxBiS2 [76] and PrO1–xFxBiS2 [77]. The crystal structure of LaO1–xFxBiS2 is shown in figure 18(b). These materials also possess the BiS2 layers as superconducting layers. The structure is simpler than that of Bi4O4S3; hence, we can consider this system as a prototype of BiS2-based superconductors. Temperature dependences of resistivity in magnetic fields for LaO1−xFxBiS2 and NdO1−xFxBiS2 are shown in figures 21(a) and (b), respectively. Superconducting transition occurs at 10.6 and 5.6 K for LaO1−xFxBiS2 and NdO1−xFxBiS2. We note that the blocking layer of La2O2 or Nd2O2 is analogous to those in FeAs-1111 system such as LaFeAsO. Owing to the similarities of crystal structure in the BiS2-based and Fe-based superconductors, we can easily design new BiS2-based superconductors by changing the blocking layers. We believe that Tc can be further enhanced by changing the blocking layers in the BiS2-based superconductor and that the discovery of new BiS2-based materials will open a new field in physics and chemistry of low-dimensional superconductors. Preliminary reports have indicated the possibility of unconventional superconductivity in the BiS2-based superconductors [78–81].

Figure 21.

Figure 21

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Temperature dependences of resistivity of LaO0.5F0.5BiS2 (a) and NdO0.7F0.3 BiS2 (b) for various magnetic fields.

Conclusions

We introduced the crystal structure and physical properties of layered chalcogenide superconductors. Chalcogenides tend to crystallize in a layered structure; hence, the intercalations/deintercalations of ions or molecules at the interlayer site markedly change the physical properties and induce exotic superconductivity. The most remarkable family is the Fe chalcogenides, which are the simplest Fe-based superconductors. In this series, the key factors to induce superconductivity are the suppression of antiferromagnetism of Fe planes and the reduction of magnetic moment of excess Fe at the interlayer site. The latter can be achieved by oxygen intercalation via annealing in an oxygen-containing atmosphere or by deintercalation of excess Fe via annealing in a liquid. Interestingly, red wine is most effective for this purpose. We also introduced the newly discovered BiS2-based superconducting family. The BiS2 layer is likely to play an important role in the superconductivity, as the CuO2 plane in cuprates and FeAn (FeAs, FeP, FeSe or FeTe) layers in Fe-based superconductors. New BiS2-based superconductors should be discovered by varying the blocking layers. We also believe in the existence of unidentified exotic chalcogenide superconductors other than the families introduced here.

Acknowledgments

The authors thank the group members of National Institute for Materials Science: Mr H Hara, Mr D Demura, Mr T Watanabe, Dr M Fujioka, Dr S I Denholme, Dr H Okazaki, Dr T Ozaki, Dr H Takeya and Dr T Yamaguchi for useful discussions and experimental help. The authors thank the group members of Tokyo Metropolitan University: Mr K Hamada, Mr H Izawa and Dr O Miura for fruitful discussions and experimental supports. This work was partly supported by a Grant-in-Aid for Scientific Research (KAKENHI) and a Strategic International Collaborative Research Program (SICORP), Japan Science and Technology Agency.

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