Time-Reversal Symmetry Breaking in the Centrosymmetric Nonsymmorphic Superconductor RuB2

Centrosymmetric nonsymmorphic AB2 (A = Ru, Os) borides exhibit anomalous superconducting properties, but the microscopic nature of their superconducting ground state remains unresolved. Here, we investigate RuB2 using zero- and transverse-field muon spin relaxation (muSR), first-principles electronic-structure calculations, and Ginzburg-Landau symmetry analysis. Zero-field muSR reveals spontaneous internal magnetic fields below Tc = 1.6 K, providing direct evidence for time-reversal symmetry breaking (TRSB). Transverse-field measurements establish a fully gapped superconducting state, with a gap magnitude close to the BCS weak-coupling limit. Density-functional calculations show that RuB2 is a multiband Z2 topological metal with appreciable spin-orbit coupling. Guided by the experimental observations and the symmetry of the orthorhombic crystal structure, our Ginzburg-Landau analysis suggests that an internally antisymmetric, nonunitary spin-triplet state is a promising candidate for the superconducting order parameter. This candidate is consistent with both the observed TRSB and the fully gapped superconducting state. These results establish RuB2 as a promising platform for investigating unconventional superconductivity arising from the interplay of crystal symmetry, multiband electronic structure, and spin-orbit coupling, and motivate further studies of related nonsymmorphic borides. The two muSR measurements provide complementary constraints: zero-field data probe spontaneous magnetism associated with TRSB, while transverse-field data characterize superconducting field distribution and support a full gap. The topological electronic structure and spin-orbit coupling provide context for unconventional pairing. Although these results do not uniquely determine the order parameter, they narrow candidates and highlight that nonsymmorphic symmetry may shape superconductivity in RuB2.