Unequivocal determination of spin-triplet superconductivity using composite rings

Most superconductors are spin-singlet superconductors, such as Nb and cuprates, with Cooper pairs of spin 0. Spin-triplet superconductors with Cooper pairs of spin 1 are rare but essential for the realization of Majorana fermions and the noise-resilient topological quantum computing. The odd-parity of spin-triplet superconductivity, where the sign of gap function reverses upon the inversion of the momentum, or Δk = −Δ−k, can be distinguished from the even-parity of spin-singlet superconductivity via phase-sensitive experiments. The observation of half-integer quantum flux in a composite ring consisting of a single crystal of epitaxial triplet superconductor of interest connected by a singlet s-wave superconductor can unambiguously identify the otherwise elusive spin-triplet superconductor, but such composite rings are very challenging [1]. We have recently successfully fabricated μm-sized composite rings comprising of an epitaxial β-Bi₂Pd segment and Nb in various structures with Nb connecting to different crystalline facets of β-Bi₂Pd. The signature of half-integer quantum flux in the rings with Nb connecting to the opposite β-Bi₂Pd sides unequivocally determines the spin-triplet superconductivity in β-Bi₂Pd [2].

[1] V. B. Geshkenbein, A. I. Larkin, and A. Barone, Phys. Rev. B 36, 235 (1987).

[2] Xiaoying Xu, Yufan Li, C. L. Chien, arXiv:2210.08733 (2022).