Absence of T_c-pinning phenomenon under high pressure in high-entropy REO_0.5F_0.5BiS₂ layered superconductor

Recently, robustness of superconductivity (transition temperature, T_c) under high pressures has been observed in high-entropy alloy (HEA), bcc-type Ti-Zr-Hf-Nb-Ta, and HEA-type compounds (Ag, In, Sn, Pb, Bi)Te with a NaCl-type structure. Since those materials have three-dimensional crystal structure, investigation on the pressure dependence of T_c of low-dimensional materials is needed to understand the phenomena. Here, we investigated the superconducting properties and the crystal structure of BiS₂-based layered system REO_0.5F_0.5BiS₂. Although the robustness of T_c was induced in MTe with increasing M-site configurational entropy, the increase in RE-site configurational entropy does not induce robustness of T_c under high pressures in REO_0.5F_0.5BiS₂. The crystal structure of HEA-type REO_0.5F_0.5BiS₂ was confirmed as monoclinic P2₁/m, which is the same space group as the zero-entropy counterpart LaO_0.5F_0.5BiS₂. The results suggest that an increase in configurational entropy at blocking layers do not affect crystal structure and superconducting properties under high pressures in the BiS₂-based layered system. (author)