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

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 (T-c) 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 T-c in REO1-yFyBiS2.