Broken time-reversal symmetry in cubic skutterudite-like superconductor Y$_3$Ru$_4$Ge$_{13}$

TL;DR

This study uses muon spin relaxation to demonstrate broken time-reversal symmetry in the superconducting state of Y$_3$Ru$_4$Ge$_{13}$, a cubic skutterudite-like material, revealing unconventional superconductivity.

Contribution

It provides the first evidence of spontaneous internal magnetic fields indicating broken time-reversal symmetry in Y$_3$Ru$_4$Ge$_{13}$, highlighting electronic correlations beyond electron-phonon interactions.

Findings

Spontaneous internal magnetic field of ~0.18 mT below Tc

Superconductivity exhibits a fully developed gap

Electronic correlations may drive unconventional superconductivity

Abstract

The microscopic properties of superconducting cubic skutterudite-like material Y$_3$Ru$_4$Ge$_{13}$ are investigated using muon spin relaxation and rotation ($\mu$SR) measurements. Zero-field $\mu$SR measurements reveal the presence of a spontaneous internal field with a magnitude of $\approx$ 0.18~mT below the superconducting transition temperature, indicating broken time-reversal symmetry in the ground state. In line with previous experiments, transverse-field $\mu$SR measurements are consistent with a fully developed superconductivity gap in Y$_3$Ru$_4$Ge$_{13}$. Our observations point towards the relevance of electronic correlations beyond electron-phonon coupling as origin and indicate that spin-orbit coupling is likely not the key driving force behind the spontaneous breaking of time-reversal symmetry in this system.