Sb-NQR study on novel superconductivity in (Pr1−xLax)Os4Sb12

Abstract We report on superconducting (SC) properties in a series of filled-skutterudite compounds (Pr 1− x La x )Os 4 Sb 12 through the Sb nuclear-quadrupole-resonance (NQR). In the SC state, the nuclear spin-lattice relaxation rate 1/ T 1 Pr at Pr-cage decreases exponentially with no coherence peak below T C , consistent with the results for the pure PrOs 4 Sb 12 . In the Pr-rich compounds of x = 0.05 and 0.2, the residual density of states (RDOS) at the Fermi level are induced below T C due to the La substitution. It is concluded that the RDOS is not due to the impurity effect that used to be observed in unconventional superconductors with line-node gap. Rather, a part of the Fermi surface that contributes to 5.5% of the total is suggested to become gapless for x = 0.05 and 0.2, yielding the RDOS. For the La-rich compounds of x = 0.4 , 0.8 and 1, as the Pr-substitution for La increases, T C increases and a size of energy gap increases. The Pr-substitution for La makes the pairing interaction for forming the Cooper pairs strong and causes an anisotropy in its energy-gap structure.