Spin-triplet Superconductivity Evidenced by Half-Integer Quantum Flux in a Non-centrosymmetric Superconductor*

Spin-triplet superconductors, rare and challenging to identify, play central roles in Majorana fermions and quantum computing. It has been proposed that superconductors with broken inversion symmetry may host spin-triplet Cooper pairs. Unique features of half-quantum fluxoid in flux quantization is one of the few methods that can identify spin-triplet superconductors [1]. We fabricated thin films of non-centrosymmetric a-BiPd by sputtering and patterned them into sub-µm-sized rings. Measurements of the Little-Parks oscillations in magnetoresistance show half-quantum fluxoid (n + ½)Φ_o, a key signature of spin-triplet pairing, where flux quantum Φ_o = hc/2e and n is an integer. Our extensive measurements support mixing pairing states of spin-singlet and spin-triplet Cooper pairs in α-BiPd.