Sb-NQR probe for superconducting properties in the Pr-based filled-skutterudite compound PrRu4Sb12

We report the electronic and superconducting properties in the Pr-based filled-skutterudite superconductor PrRu 4 Sb 1 2 with T c =1.3 K via the measurements of nuclear-quadrupole-resonance frequency ν Q and nuclear-spin-lattice-relaxation time T 1 of Sb nuclei. The temperature dependence of v Q has revealed the energy scheme of Pr 3 + crystal electric field that is consistent with an energy separation Δ C E F ∼70 K between the ground state and the first-excited state. In the normal state, the Korringa relation of (1/T 1 T) P r =const is valid, with [( 1/T 1 T) p r /(1/T 1 T) L a ] 1 / 2 ∼1.44, where (1/T 1 T) L a is for LaRu 4 Sb 1 2 . These results are understood in terms of a conventional Fermi-liquid picture in which the Pr-4f 2 state derives neither magnetic nor quadrupolar degrees of freedom at low temperatures. In the superconducting state, 1/T 1 shows a distinct coherence peak just below T c , followed by an exponential decrease with a value of 2Δ/k B T c =3.1. These results demonstrate that PrRu 4 Sb 1 2 is a typical weak-coupling s-wave superconductor, in strong contrast with the heavy-fermion superconductor PrOs 4 Sb 1 2 that is in an unconventional strong coupling regime. The present study on PrRu 4 Sb 1 2 highlights that the Pr-4f 2 -derived nonmagnetic doublet plays a key role in the unconventional electronic and superconducting properties in PrOs 4 Sb 1 2 .