Sb-NQR Probe for Multiband Superconductivity in Filled-Skutterudite Compounds (Pr1-xLax)Os4Sb12

We report on the systematic evolution of normal-state properties and superconducting characteristics in filled-skutterudite compounds (Pr1-xLax)Os4Sb12 via Sb-NQR experiments. In the superconducting state for Pr-rich compounds, 1/T-1(Pr) exponentially decreases down to T = 0.7 K with no coherence peak below T-c as well as in PrOs4Sb12. A remarkable finding is that the residual density of states (RDOS) at the Fermi level below T-c is induced by La substitution for Pr. However, the RDOS does not increase and T-c does not decrease with increasing La content up to x = 0.2. These results are proposed to be understood in terms of a multiband-superconductivity model that assumes a full gap for part of the FS and an anisotropic gap for a small FS. The novel strong-coupling superconductivity in PrOs4Sb12 is inferred to be mediated by the local interaction between 4f(2)-derived crystal-electric-field states with the electric quadrupole degree of freedom and conduction electrons. This coupling causes a mass enhancement of quasi-particles for a part of FS and induces a small FS. Note that the small FS does not play any primary role for the strong-coupling superconductivity in PrOs4Sb12.