In-plane chemical pressure essential for superconductivity in BiCh2-based (Ch: S, Se) layered structure

Abstract

BiCh2-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 BiCh2-based superconductor family, specifically, optimally doped Ce1−xNdxO0.5F0.5BiS2 and LaO0.5F0.5Bi(S1−ySey)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 REO0.5F0.5BiCh2 superconductors can be determined from the in-plane chemical pressure.

Intrinsic Phase Diagram of Superconductivity in the BiCh2-Based System Without In-Plane Disorder

“…Similar chemical pressure effect can be produced by Se substitution for the S site, as demonstrated in LaO0.5F0.5BiS2-ySey and Eu0.5La0.5FBiS2-ySey [23,24]. Although Bi-Ch plane expands by the substitution of in-plane S 2-by larger Se 2- [25], the packing density of Bi-Ch plane is enhanced due to the almost fixed structure (volume) of the LaO blocking layer [21]. Then, bulk superconductivity is induced by in-plane chemical pressure effect in LaO0.5F0.5BiS2-ySey and Eu0.5La0.5FBiS2-ySey, as well.…”

“…In this study, we have clarified that the suppression of the in-plane local disorder, which is a typical characteristic of BiCh2-based compounds, is the most essential for the emergence of superconductivity. The in-plane disorder in the BiCh2-based system has been detected using extended X-ray absorption fine structure (EXAFS), X-ray diffraction, and neutron diffraction [21,[26][27][28][29]. In addition, we showed that the in-plane disorder could be reduced by the chemical pressure effects [27,29]; the details on the suppression of in-plane disorder in LaO0.5F0.5BiS2-ySey and LaO1-xFxBiSSe will be discussed later with Fig.…”

“…Namely, both electron carrier doping and crystal structure optimization are required for the emergence of bulk superconductivity in the BiCh2-based system. Recently, we proposed that the enhancement of in-plane chemical pressure is the factor essential for the BiCh2-based superconductivity [21]. The in-plane chemical pressure is related to the orbital overlaps between Bi and Ch and can be tuned by element substitutions at the blocking layer or the conducting layer.…”

“…In Ref. 21, we proposed that the in-plane chemical pressure was a good indicator of the emergence of bulk superconductivity in BiCh2-based compounds and Tc. In addition, the Se substitution for the S site of the BiS2 conducting layer is one of the most effective method to enhance in-plane chemical pressure, as revealed in LaO0.5F0.5BiS2-ySey and Eu0.5La0.5FBiS2-ySey.…”

“…To further clarify the essence of the in-plane chemical pressure effect in the REOBiCh2 system, we have performed Rietveld refinements for LaO0.5F0.5BiS2-ySey [21,23] and LaO1-xFxBiSSe using anisotropic displacement parameters of in-plane Bi and Ch1. See Supplemental Materials (Table S1) for all displacement parameters of LaO1-xFxBiSSe.…”