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 PrRu₄Sb₁₂ with T_c = 1.3 K via the measurements of nuclear-quadrupole-resonance frequency ν_Q and nuclear-spin-lattice-relaxation time T₁ of Sb nuclei. The temperature dependence of ν_Q has revealed the energy scheme of Pr³⁺ crystal electric field that is consistent with an energy separation Δ_CEF 70 K between the ground state and the first-excited state. In the normal state, the Korringa relation of (1/T₁T)_Pr = const is valid, with [(1/T₁T)_Pr/(1/T₁T)_La]^1/2 1.44, where (1/T₁T)_La is for LaRu₄Sb₁₂. These results are understood in terms of a conventional Fermi-liquid picture in which the Pr-4f² state derives neither magnetic nor quadrupolar degrees of freedom at low temperatures. In the superconducting state, 1/T₁ shows a distinct coherence peak just below T_c, followed by an exponential decrease with a value of 2Δ/k_BT_c = 3.1. These results demonstrate that PrRu₄Sb₁₂ is a typical weak-coupling s-wave superconductor, in strong contrast with the heavy-fermion superconductor PrOs₄Sb₁₂ that is in an unconventional strong coupling regime. The present study on PrRu₄Sb₁₂ highlights that the Pr-4f²-derived nonmagnetic doublet plays a key role in the unconventional electronic and superconducting properties in PrOs₄Sb₁₂.