Sb-NQR probe for multipole degree of freedom in the first Pr-based heavy-fermion superconductor PrOs$_{4}$Sb$_{12}$

TL;DR

This study uses Sb-NQR measurements to explore the multipole degrees of freedom in the heavy-fermion superconductor PrOs$_{4}$Sb$_{12}$, revealing unusual temperature-dependent electric field gradients linked to multipole coupling.

Contribution

It introduces Sb-NQR as a probe for multipole interactions in PrOs$_{4}$Sb$_{12}$, highlighting the coupling between Sb quadrupole moments and Pr multipole moments, especially hexadecapole.

Findings

EFG exhibits unusual temperature dependence below 30 K.

Coupling between Sb quadrupole moments and Pr multipole moments is significant.

The temperature dependence aligns with a CEF quasi-quartet model.

Abstract

We report $^{121,123}$Sb nuclear quadrupole resonance (NQR) measurements in the filled-skutterudite superconductor PrOs$_4$Sb$_{12}$ in the temperature range of 0.05-30 K. The electric field gradients (EFG), $V_{zz}$ and $V_{xx}-V_{yy}$, at the Sb site exhibit unusual temperature dependence below 30 K. To explain these features, we discuss the coupling between the Sb nuclear quadrupole moment and Pr $4f^2$-derived multipole moments. The observed $T$ dependence of EFG is well explained by the CEF quasi-quartet consisted of $\Gamma_1$ singlet and $\Gamma_4^{(2)}$ triplet states in the cubic point group $T_h$. These results, in turn, are indicative of the importance of the coupling between the $^{121,123}$Sb quadrupole moments and the hexadecapole moment caused by the quasi-quartet.