Pr deficiency effect on the heavy-fermion superconductor PrxOs4Sb12

We have succeeded in growing single-crystalline samples of the filled-skutterudite heavy-fermion superconductor PrxOs4Sb12 having different Pr occupancy x by controlling the maximum temperature in Sb-flux sample growing process. From systematic measurements of the superconducting and normal state properties, it has been found that the superconducting transition temperature Tc and the energy splitting Δ between the singlet and triplet crystalline-electric-field (CEF) levels decrease significantly with decreasing x, much faster than the case for the La ion substitution in (PrxLa1-x)Os4Sb12. The observed correlation between Tc and Δ supports the idea that the CEF excitations of Pr 4f-electrons play a major role for the realization of the superconducting state in PrOs4Sb12.