Observation of Time-Reversal Symmetry Breaking in the Type-I Superconductor YbSb_{2}.

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 muon spin relaxation and rotation (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 YbSb_{2} as a 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 suggest that this INT state may host gapless Majorana surface modes, pointing to the possibility of topological superconductivity in YbSb_{2}. Our results highlight YbSb_{2} as a unique material platform where type-I superconductivity coexists with triplet pairing and nontrivial topology.