Enhancing superconductivity of Y5Rh6Sn18 by atomic disorder

We investigate the effect of enhancement of superconducting transition temperature Tc by nonmagnetic atom disorder in the filled skutterudite-related tetragonal Y5Rh6Sn18 compound doped with Ca. We documented experimentally that Y5Rh6Sn18 and its Ca-doped alloys are electronically inhomogeneous at the nanoscale, when Ca content is smaller than ∼1.5 in the Y5−xCaxRh6Sn18 system, while for x≥1.5 much stronger chemical phase inhomogeneity is observed with an about 20% volume fraction of the second 3:4:13 cubic phase, which we interpret as a fluctuation of stoichiometry within the bulk sample. Then the enhancement of Tc vs x could be modeled by a mechanism proposed in recent theoretical reports for increasing the mean-field transition temperature Tc in the presence of nonmagnetic disorder. The increase in disorder with doping increases the sample inhomogeneity and causes a systematic increase in Tc, while for two-phase x≥1.5 samples the critical temperature Tc★≈2×Tc rapidly increases. Based on band structure calculations performed under pressure, we demonstrate how the change in density of states would affect Tc of Y5Rh6Sn18. We obtained the Grüneisen parameter γG larger for the inhomogeneous phase with respect to γG of the bulk Tc phase and attribute the enhancement of Tc to larger stiffening of the high-temperature Tc★ phase.