Chemical Pressure Effect on Superconductivity of BiS2-Based Ce1−xNdxO1−yFyBiS2 and Nd1−zSmzO1−yFyBiS2

Abstract

We have studied crystal structure and physical properties of REO1−yFyBiS2 (Ce1−xNdxO1−yFyBiS2 and Nd1−zSmzO1−yFyBiS2) with three different F concentration (y = 0.7, 0.5, and 0.3) to investigate relationship between the emergence of superconductivity and crystal structure in the REO1−yFyBiS2 series. REO1−yFyBiS2 is suitable for discussing chemical pressure effect on physical properties because the RE site at the blocking layer can be fully or partially substituted by various RE3+ ions, which could systematically tune the lattice volume. With increasing chemical pressure (with decreasing lattice volume), lattice constant of a-axis systematically decreases while lattice constant of c-axis does not show a remarkable change, indicating that the RE site substitution basically affect the lattice constant of a-axis. On the other hand, lattice constant of c-axis can be tuned by F concentration. On the basis of magnetic susceptibility measurements, we have obtained three kinds of superconductivity phase diagram with y = 0.7, 0.5, and 0.3 as a function of RE concentration (chemical pressure). For all the systems with y = 0.7, 0.5, and 0.3, chemical pressure basically increases superconducting transition temperature (Tc) with increasing chemical pressure. Having compared these three phase diagrams, we have suggested that there are at least two important structure parameters, (1) lattice contraction along a-axis, (2) optimal lattice contraction ratio (c/a), are essential for the emergence of superconductivity and increase in Tc in REO1−yFyBiS2.