Superconductivity in La2O2M4S6 -Type Bi-based Compounds: A Review on Element Substitution Effects

Abstract

Since 2012, layered compounds containing Bi-Ch (Ch: S and Se) layers have been extensively studied in the field of superconductivity. The most-studied system is BiS2-based superconductors with two-layer-type conducting layers. Recently, superconductivity was observed in La2O2M2S6 (M = metals), which contains four-layer-type conducting layers. The four-layer-type Bi-based superconductors are new systems in the family of Bi-based superconductors; we can expect further development of Bi-based layered superconductors. In this review article, we summarize the progress of synthesis, structural analysis, investigations on superconducting properties, and material design of the four-layer-type Bi-based superconductors. In-plane chemical pressure is the factor essential for the emergence of bulk superconductivity in the system. The highest Tc of 4.1 K was observed in Rare Earth elements (RE) substituted La2-xRExO2Bi3Ag0.6Sn0.4S6.

High-pressure effects on superconducting properties and crystal structure of Bi-based layered superconductor La2O2Bi3Ag0.6Sn0.4S6

“…Figure 6(a) shows the temperature dependences of the susceptibility for La 2 O 2 Bi 3 Ag 0.6 Sn 0.4 S 6 under pressures. As reported in reference [22], the superconducting states at ambient pressure is not bulk in nature, and bulk superconductivity was induced by chemical pressure effects via a partial substitution of S by Se (or a partial substitution of La by smaller RE) [22,[27][28][29]. With increasing pressure, T c of La 2 O 2 Bi 3 Ag 0.6 Sn 0.4 S 6 reached 4 K, and bulk superconductivity was induced at pressures above 0.48 GPa, which was evaluated through the emergence of large shielding volume fraction of magnetic susceptibility.…”

“…The low Tc in La2O2Bi3AgS6 was found to be related to the charge-density-wave-like (CDW-like) transition, and the partial substitution of Sn for the Ag site was effective to suppress the transition and improve superconducting properties in La2O2Bi3Ag0.6Sn0.4S6, in which Tc reaches 2.5 K [22]. Furthermore, application of chemical pressures in La2O2Bi3Ag0.6Sn0.4S6 further improved Tc up to 4 K in Eusubstituted La1.6Eu0.4O2Bi3Ag0.6Sn0.4S6 and similar phases with various RE [27][28][29]. On the basis of the knowledge on the dramatic external pressure effects in two-layer-type BiS2-based systems, we expected further increases in Tc by external pressures in La2O2Bi3Ag0.6Sn0.4S6.…”

From Ta2S5 Wires to Ta2O5 and Ta2O5–xSx

“…25 Since the first reported bulk bismuth oxysulfide superconductor, Bi 4 O 4 S 3 26 [T c = 4.4 K], only bismuth-based, BiS-layered oxysulfide compounds have been reported. 27 These include 28 LaOBiS 2 [T c = 3.6 K] and 27 La 1.7 Eu 0.3 O 2 Bi 3 Ag 0.6 Sn 0.4 S 6 [T c = 3.64 K], which have between four and seven elements. Therefore, Bi 4 O 4 S 3 and Ta 2 O 5−x S x can be considered "simple oxysulfides," as they are composed of only three elements each.…”

“…Superconductivity in oxysulfides is exceedingly rare . Since the first reported bulk bismuth oxysulfide superconductor, Bi 4 O 4 S 3 [ T c = 4.4 K], only bismuth-based, BiS-layered oxysulfide compounds have been reported . These include LaOBiS 2 [ T c = 3.6 K] and La 1.7 Eu 0.3 O 2 Bi 3 Ag 0.6 Sn 0.4 S 6 [ T c = 3.64 K], which have between four and seven elements.…”

Experimental overview on pairing mechanisms of BiCh2-based (Ch: S, Se) layered superconductors

“…SrFBiS 2 has a structure composed of BiS 2 layers and SrF blocking layers; the crystal structure is same as the REOBiS 2 -based system. Recently, new types of BiCh 2 -based compounds whose conducting layers are four-layer-type (Bi 2 M 2 Ch 6 layer with M = Ag, Sn, Pb, Bi) and one-layer-type (BiClS 2 layer) [32,33], whereas that for typical BiCh 2 -based systems is two-layer-type (BiCh 2 bilayer = Bi 2 Ch 4 layer). As a common feature in BiS 2 -based systems, Bi 4 O 4 S 3 and RE(O,F)BiS 2 , four bands mainly originating from Bi 6p x /6p y orbitals in the BiS 2 bilayer cross the Fermi level [1,21,34].…”