Broken time-reversal symmetry in the cubic skutterudite-like superconductor Y3Ru4Ge13

Abstract

The microscopic properties of the superconducting cubic skutterudite-like material Y3Ru4Ge13 are investigated using muon spin relaxation and rotation (μSR) measurements. Zero-field μSR measurements reveal the presence of a spontaneous internal field with a magnitude of ≃0.18 mT below the superconducting transition temperature, indicating broken time-reversal symmetry in the ground state. In line with previous experiments, transverse-field μSR measurements are consistent with a fully developed superconductivity gap in Y3Ru4Ge13. Our observations point towards the relevance of electronic correlations beyond electron-phonon coupling as the superconducting 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.