In-plane chemical pressure essential for superconductivity in BiCh 2 -based (Ch: S, Se) layered structure

BiCh 2 -based compounds (Ch: S, Se) are a new series of layered superconductors and the mechanisms for the emergence of superconductivity in these materials have not yet been elucidated. In this study, we investigate the relationship between crystal structure and superconducting properties of the BiCh 2 -based superconductor family, specifically, optimally doped Ce 1− x Nd x O 0.5 F 0.5 BiS 2 and LaO 0.5 F 0.5 Bi(S 1− y Se y ) 2 . We use powder synchrotron X-ray diffraction to determine the crystal structures. We show that the structure parameter essential for the emergence of bulk superconductivity in both systems is the in-plane chemical pressure, rather than Bi-Ch bond lengths or in-plane Ch-Bi-Ch bond angle. Furthermore, we show that the superconducting transition temperature for all REO 0.5 F 0.5 BiCh 2 superconductors can be determined from the in-plane chemical pressure.