Microscopic superconducting state of RuB2

Determine the microscopic nature of the superconducting ground state of centrosymmetric nonsymmorphic RuB2, including the pairing structure responsible for its time-reversal symmetry breaking and fully gapped excitation spectrum.

Background

RuB2 is reported to exhibit spontaneous internal magnetic fields below its superconducting transition temperature, indicating time-reversal symmetry breaking, while transverse-field muon-spin relaxation measurements indicate a fully gapped superconducting state. First-principles calculations identify RuB2 as a multiband Z2 topological metal with appreciable spin–orbit coupling.

The paper uses Ginzburg–Landau symmetry analysis to suggest an internally antisymmetric nonunitary spin-triplet state as a promising candidate, but the microscopic superconducting state is not established. The unresolved problem is therefore to determine the actual pairing mechanism and order-parameter structure that account simultaneously for time-reversal symmetry breaking, a full gap, multiband electronic structure, and spin–orbit coupling.