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

Abstract

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

4 page with 3 figures embedded in the text. Submitted in Phys. Rev. B