Experimental signature of spin-triplet superconductors

Spin-triplet superconductors have attracted great attention for exploring Majorana Fermions and developing quantum computing. Superconductors with inversion crystal symmetry must be either spin-singlet with even gap symmetry (such as Nb) or spin-triplet with odd gap symmetry (such as β-Bi₂Pd). However, the broken crystal inversion symmetry in noncentrosymmetric superconductors compels an admixture of singlet and triplet pairing states. The unusual material BiPd can form both the centrosymmetric superconductor γ-BiPd (T_C = 3.3 K) and the noncentrosymmetric superconductor α-BiPd (T_C = 3.7 K), thus allowing BiPd to reveal the signatures of different pairing states. We report the discovery of multiple and single unconventional critical field-temperature phase diagrams in α-BiPd and β-Bi₂Pd, respectively, but only a single-component BCS-type phase diagram in γ-BiPd. We further perform a phase-sensitive experiment to confirm the singlet and triplet pairing states. We observe the quantization of half-magnetic and integer-magnetic fluxes in mesoscopic rings that provide conclusive evidence for triplet pairing in centrosymmetric β-Bi₂Pd, singlet pairing in centrosymmetric γ-BiPd, and the admixture of singlet-triplet pairing in α-BiPd. Our critical field and phase-sensitive measurements identify triplet pairing superconductors and the constraints of crystal symmetry on the pairing states, which is essential for the exploration of triplet superconductors and their quantum properties.