Spin-Triplet Pairing State Evidenced by Half-Quantum Flux in a Noncentrosymmetric Superconductor

A prime category of superconducting materials in which to look for spin-triplet pairing and topological superconductivity are superconductors without inversion symmetry. It is predicted that the broken parity symmetry gives rise to an admixture of spin-triplet and spin-singlet pairing states. However, experimental confirmation of pairing mixing in any material remains elusive. In this work, we perform a phase-sensitive experiment to examine the pairing state of noncentrosymmetric superconductor α-BiPd. The Little-Parks effect observed in mesoscopic polycrystalline α-BiPd rings reveals the presence of half-integer magnetic flux quantization, which provides a decisive evidence for the spin-triplet pairing state. We find both half-quantum fluxes and integer-quantum fluxes of different proportions, consistent with the scenario of an admixture of singlet-triplet pairing.