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 (Pr 1- x La x )Os 4 Sb 12 via Sb-NQR experiments. In the superconducting state for Pr-rich compounds, 1/ T 1 P r exponentially decreases down to T =0.7 K with no coherence peak below T c as well as in PrOs 4 Sb 12 . 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 PrOs 4 Sb 12 is inferred to be mediated by the local interaction between 4 f 2 -derived crystal-electric-field states with the electric quadrupole degree of freedom and conduction electrons. This coupling causes ...