Observation of Time-Reversal Symmetry Breaking in the Type-I Superconductor YbSb2
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 YbSb2. Zero-field μSR measurements reveal spontaneous internal magnetic fields emerging just below the superconducting transition, while transverse-field μSR confirms a fully gapped type-I superconducting state. Our first-principles calculations identify YbSb2 as a Z2 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 YbSb2 as a topological superconductor. Our results highlight YbSb2 as a unique material platform where type-I superconductivity coexists with triplet-pairing and nontrivial topology.
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