Sb-NQR probe for superconducting properties in the Pr-based filled-skutterudite compoundPrRu4Sb12

We report the electronic and superconducting properties in the Pr-based filled-skutterudite superconductor ${\mathrm{PrRu}}_4{\mathrm{Sb}}_{12}$ with $T_c=1.3\mathrm{}\mathrm{K}$ via the measurements of nuclear-quadrupole-resonance frequency ${\nu }_Q$ and nuclear-spin-lattice-relaxation time $T_1$ of Sb nuclei. The temperature dependence of ${\nu }_Q$ has revealed the energy scheme of ${\mathrm{Pr}}^{3+}$ crystal electric field that is consistent with an energy separation ${\Delta }_{\mathrm{CEF}}\sim 70\mathrm{K}$ between the ground state and the first-excited state. In the normal state, the Korringa relation of ${(1/T}_1{T)}_{\mathrm{Pr}}=\mathrm{const}$ is valid, with $[{(1/T}_1{T)}_{\mathrm{Pr}}{/(1/T}_1{T)}_{\mathrm{La}}{]}^{1/2}\sim 1.44,$ where ${(1/T}_1{T)}_{\mathrm{La}}$ is for ${\mathrm{LaRu}}_4{\mathrm{Sb}}_{12}.$ 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 $2\Delta {/k}_B{T}_c=\mathrm{3.1.}\mathrm{}$ These results demonstrate that ${\mathrm{PrRu}}_4{\mathrm{Sb}}_{12}$ is a typical weak-coupling s-wave superconductor, in strong contrast with the heavy-fermion superconductor ${\mathrm{PrOs}}_4{\mathrm{Sb}}_{12}$ that is in an unconventional strong coupling regime. The present study on ${\mathrm{PrRu}}_4{\mathrm{Sb}}_{12}$ highlights that the Pr-${4f}^2$-derived nonmagnetic doublet plays a key role in the unconventional electronic and superconducting properties in ${\mathrm{PrOs}}_4{\mathrm{Sb}}_{12}.$