Estimation of Gruneisen Parameter of Layered Superconductor LaO0.5F0.5BiS2-xSex (x 1/4 0.2, 0.4, 0.6, 0.8, 1.0)

The superconducting properties and structural parameters, including the Gruneisen parameter (gamma G), of the BiCh2-based (Ch: S, Se) layered superconductor LaO0.5F0.5BiS2-xSex were investigated. The superconducting transition temperature (Tc) increased with increasing Se concentration (x), and bulk superconductivity was induced by Se substitution. gamma G exhibited a larger value when x >= 0.4, where the superconducting properties were improved. These results suggest that the increase of lattice anharmonicity is preferable for superconductivity in LaO0.5F0.5BiS2-xSex. The discussion includes data for related superconductors REO0.5F0.5BiS2 (RE = Pr, Nd), where a positive correlation between gamma G and Tc is revealed for Se-poor samples. In addition, specific heat analyses suggest the presence of a low-energy optical phonon mode for LaO0.5F0.5BiS2-xSex.