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

Abstract

We report on superconducting (SC) properties in a series of filled-skutterudite compounds (Pr1−xLax)Os4Sb12 through the Sb nuclear-quadrupole-resonance (NQR). In the SC state, the nuclear spin-lattice relaxation rate 1/T1Pr at Pr-cage decreases exponentially with no coherence peak below TC, consistent with the results for the pure PrOs4Sb12. 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 TC 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, TC 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.