Signature of chiral superconducting order parameter evidenced in mesoscopic superconductors

Chiral superconductivity is an unconventional superconducting state in which the Cooper pair wavefunction breaks time-reversal symmetry and carries finite orbital angular momentum. In the momentum space, this manifests itself as a multi-component complex superconducting gap function, with the phase winding clockwise or counterclockwise around the Fermi surface. Confirming chiral superconductivity remains challenging because experimental probes that detect time-reversal-symmetry breaking (μSR, polar Kerr, etc) often yield controversial results and can be flawed by extrinsic factors.

We demonstrate a different strategy that focuses on the finite angular momentum of the Cooper pairs  [1]. A straightforward Ginzburg–Landau analysis shows that, when the Cooper pair carries orbital angular momentum, its coupling to an external field produces a linear‑in‑field shift of the critical temperature. The apex of the temperature-magnetic field (T−H) phase boundary is therefore displaced from zero to finite field—an unambiguous fingerprint of chiral order. We demonstrate experimentally that the Little-Parks effect observed in mesoscopic rings of β-Bi₂Pd exhibits the predicted linear-in-field modulation of the T−H phase boundary. The dimensional dependence of this effect aligns precisely with theoretical analysis. Together with the previously observed half‑integer fluxoid quantization in the same material [2,3], our result points convincingly to a chiral p‑wave order parameter in β-Bi₂Pd.

 

[1]         X. Xu, W. Qin, Y. Shen, Z. Huang, Z. Zhou, Z. Wang, and Y. Li, arXiv:2509.18781.

[2]         Y. Li, X. Xu, M.-H. Lee, M.-W. Chu, and C. L. Chien, Science 366, 238 (2019).

[3]         X. Xu, Y. Li, and C. L. Chien, Phys. Rev. Lett. 132, 056001 (2024).