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

TL;DR

This study uses Sb-NQR measurements to investigate the electronic and superconducting properties of PrRu4Sb12, revealing it as a conventional weak-coupling s-wave superconductor with a Fermi liquid ground state.

Contribution

First detailed NQR analysis of PrRu4Sb12 showing its conventional superconductivity and clarifying the role of Pr 4f electrons compared to the unconventional PrOs4Sb12.

Findings

PrRu4Sb12 exhibits a coherence peak in 1/T1 below Tc.

The superconducting gap ratio 2Δ/kBTc is 3.1, indicating weak-coupling s-wave pairing.

Normal state follows Fermi liquid behavior with Pr 4f^2 states lacking magnetic or quadrupolar degrees of freedom.

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 $(1/T_1T)_{Pr}$=const. is valid, with [$(1/T_{1}T$)$_{Pr}$/$(1/T_{1}T$)$_{La}$]$^{1/2}$ $\sim$ 1.44 where $(1/T_1T)_{La}$ 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$_{12}$.