Observation of Time-Reversal Symmetry Breaking in the Type-I Superconductor YbSb$_2$

TL;DR

This study provides evidence of time-reversal symmetry breaking in the type-I superconductor YbSb$_2$, revealing its unconventional topological superconducting nature with triplet pairing and Majorana surface modes.

Contribution

It is the first demonstration of time-reversal symmetry breaking in a type-I superconductor and identifies YbSb$_2$ as a topological superconductor with unique properties.

Findings

Spontaneous internal magnetic fields below $T_c$

YbSb$_2$ hosts a fully gapped type-I superconducting state

Presence of gapless Majorana surface modes

Abstract

The spontaneous breaking of time-reversal symmetry is a hallmark of unconventional superconductivity, typically observed in type-II superconductors. Here, we report evidence of time-reversal symmetry breaking in the type-I superconductor YbSb$_2$. Zero-field $\mu$SR measurements reveal spontaneous internal magnetic fields emerging just below the superconducting transition, while transverse-field $\mu$SR confirms a fully gapped type-I superconducting state. Our first-principles calculations identify YbSb$_2$ as a ${\mathbb Z}_2$ topological metal hosting a Dirac nodal line near the Fermi level. Symmetry analysis within the Ginzburg Landau framework indicates an internally antisymmetric nonunitary triplet (INT) state as the most probable superconducting ground state. Calculations based on an effective low-energy model further demonstrate that this INT state hosts gapless Majorana surface modes, establishing YbSb$_2$ as a topological superconductor. Our results highlight YbSb$_2$ as a unique material platform where type-I superconductivity coexists with triplet-pairing and nontrivial topology.