Unconventional superconductivity in Y$_5$Rh$_6$Sn$_{18}$ probed by muon spin relaxation

TL;DR

This study investigates the unconventional superconductivity in Y$_5$Rh$_6$Sn$_{18}$, revealing broken time-reversal symmetry and an anisotropic gap with point nodes, contrasting with conventional s-wave behavior.

Contribution

It provides experimental evidence of broken time-reversal symmetry in Y$_5$Rh$_6$Sn$_{18}$ using muon spin relaxation, highlighting its unconventional superconducting state.

Findings

Spontaneous internal magnetic fields appear below T_c.

Heat capacity indicates an anisotropic gap with point nodes.

Muon spin relaxation confirms broken time-reversal symmetry.

Abstract

Conventional superconductors are robust diamagnets that expels magnetic fields through the Meissner effect. It would therefore be unexpected if a superconducting ground state would support spontaneous magnetics fields. Such broken time-reversal symmetry states have been suggested for the high$-$temperature superconductors, but their identification remains experimentally controversial. We present magnetization, heat capacity, zero field and transverse field muon spin relaxation experiments on the recently discovered caged type superconductor \yrhsn\ ($T_{\bf c}$ = 3.0 K). The electronic heat capacity of \yrhsn\ shows a $T^3$ dependence below $T_{\bf c}$ indicating an anisotropic superconducting gap with a point node. This result is in sharp contrast to that observed in the isostructural Lu$_5$Rh$_6$Sn$_{18}$ which is a strong coupling $s-$wave superconductor. The temperature dependence of the deduced superfluid density \yrhsn\ is consistent with a BCS $s-$wave gap function, while the zero-field muon spin relaxation measurements strongly evidences unconventional superconductivity through a spontaneous appearance of an internal magnetic field below the superconducting transition temperature, signifying that the superconducting state is categorized by the broken time-reversal symmetry.