Superconductivity in HEA-type compounds

Since the discovery of superconductivity in a high-entropy alloy (HEA) Ti-Zr-Nb-Hf-Ta in 2014, the community of superconductor science has explored new HEA superconductors to find the merit of the HEA states on superconducting properties. Since 2018, we have developed HEA-type compounds as superconductors or thermoelectric materials. As well known, compounds like intermetallic compounds or layered compounds are composed of multi crystallographic sites. In a HEA-type compounds, one or more sites are alloyed and total mixing entropy satisfies with the criterion of HEA. Herein, we summarize the synthesis methods, the crystal structural variation and superconducting properties of the HEA-type compounds, which include NaCl-type metal tellurides, CuAl2-type transition metal zirconides, high-Tc cuprates, and BiS2-based layered superconductors. The effects of the introduction of a HEA site in various kinds of complicated compounds are discussed from the structural-dimensionality viewpoint.

Crystal structure, site selectivity, and electronic structure of layered chalcogenide LaOBiPbS3

We have investigated the crystal structure of LaOBiPbS3 using neutron diffraction and synchrotron X-ray diffraction. From structural refinements, we found that the two metal sites, occupied by Bi and Pb, were differently surrounded by the sulfur atoms. Calculated bond valence sum suggested that one metal site was nearly trivalent and the other was nearly divalent. Neutron diffraction also revealed site selectivity of Bi and Pb in the LaOBiPbS3 structure. These results suggested that the crystal structure of LaOBiPbS3 can be regarded as alternate stacks of the rock-salt-type Pb-rich sulfide layers and the LaOBiS2-type Bi-rich layers. From band calculations for an ideal (LaOBiS2)(PbS) system, we found that the S bands of the PbS layer were hybridized with the Bi bands of the BiS plane at around the Fermi energy, which resulted in the electronic characteristics different from that of LaOBiS2. Stacking the rock-salt type sulfide (chalcogenide) layers and the BiS2-based layered structure could be a new strategy to exploration of new BiS2-based layered compounds, exotic two-dimensional electronic states, or novel functionality.

Compositional and temperature evolution of crystal structure of new thermoelectric compound LaOBiS2-xSex

We examined the crystal structure of the new thermoelectric material LaOBiS2-xSex, whose thermoelectric performance is enhanced by Se substitution, by using powder synchrotron X-ray diffraction and Rietveld refinement. The emergence of metallic conductivity and enhancement of the thermoelectric power factor of LaOBiS2-xSex can be explained with the higher in-plane chemical pressure caused by the increase of Se concentration at the in-plane Ch1 site (Ch = S, Se). High-temperature X-ray diffraction measurements for optimally substituted LaOBiSSe revealed anomalously large atomic displacement parameters (Uiso) for Bi and Ch atoms in the BiCh2 conduction layers. The anisotropic analysis of the atomic displacement parameters (U11 and U33) for the in-plane Bi and Ch1 sites suggested that Bi atoms exhibit large atomic displacement along the c-axis direction above 300 K, which could be the origin of the low thermal conductivity in LaOBiSSe. The large Bi vibration along the c-axis direction could be related to in-plane rattling, which is a new strategy for attaining low thermal conductivity and phonon-glass-electron-crystal states.

Evolution of superconductivity in LaO1-xFxBiS2 prepared by high pressure technique

Novel BiS2-based superconductors LaO1-xFxBiS2 prepared by the high pressure synthesis technique were systematically studied. It was found that the high pressure annealing strongly the lattice as compared to the LaO1-xFxBiS2 samples prepared by conventional solid state reaction at ambient pressure. Bulk superconductivity was observed within a wide F-concentration range of x = 0.2 ~ 0.7. On the basis of those results, we have established a phase diagram of LaO1-xFxBiS2.

Substitution Effects on FeSe Superconductor

We have investigated the effect of atomic substitutions in the FeSe system, which exhibits the simplest crystal structure among the iron-based superconductors. An enhancement of the superconducting transition temperature Tc was observed with the substitution of S or Te for Se; the Tc increased with S substitution by up to 20 %, and also increased with Te substitution up to 75 %. In contrast, Co or Ni substitutions for the Fe site significantly suppressed superconductivity. In this work we present a detailed description of the substitution technique employed to determine Tc in the FeSe system.

Superconductivity in S-substituted FeTe

We have successfully synthesized a new superconducting phase of FeTe1-xSx with a PbO-type structure. It has the simplest crystal structure in iron-based superconductors. Superconducting transition temperature is about 10 K at x = 0.2. The upper critical field Hc2 was estimated to be ~70 T. The coherent length was calculated to be ~2.2 nm. Because FeTe1-xSx is composed of nontoxic elements, this material is a candidate for applications and will activate more and more research on iron-based superconductor.

FeTe as a candidate material for new iron-based superconductor

Tetragonal FeSe is a superconductor with a transition temperature Tc of 8 K and shows a huge enhancement of Tc with applying pressure. Tetragonal FeTe has a structure very analogous to superconducting FeSe, but does not show superconducting transition. We investigated the pressure effect of resistivity on FeTe. The resistivity at room temperature decreased with increasing pressure. An anomaly in resistivity around 80 K shifted towards a lower temperature with increasing pressure.