Most superconductors are spin-singlet superconductors (SCs), such as conventional SCs (Al, Nb, etc.) and high-TC cuprates, with Cooper pairs of spin 0. Spin-triplet SCs with Cooper pairs of spin 1 are rare but essential for the realization of Majorana fermions and the noise-resilient topological quantum computing. Experimentally identifying any triplet pairing state remains a challenge. The rare triplet pairing state differs from the common singlet pairing in two important ways. One is the spin configuration of the Cooper pair: a triplet pairing state allows nominally parallel spins and singlet allow only antiparallel alignment. The other is the parity symmetry of the gap function: odd parity for triplets, where the sign of gap function reverses upon the inversion of the momentum, or △k = - △-k ; and even parity for singlets, where△k = △-k . In order to unambiguously determine the pairing symmetry, we carried out a set of phase-sensitive experiments to probe the parity symmetry of pairing state. The triplet pairing state manifests half-integer flux quantization in polycrystalline superconducting rings [1,2], in stark contrast to the integer flux quantization for the conventional s-wave SCs. We have identified triplet pairing states in β-Bi2Pd and α-BiPd, both are candidate materials for topological SCs. Moreover, the odd-parity symmetry can be directly observed as the half-integer flux quantization in composite ring devices, where the triplet SC is connected by a conventional s-wave SC on the two opposite crystalline ends . I shall discuss the outlook of the phase-sensitive studies: how they may constitute a new paradigm for studying unconventional superconductivity, and how it may advise new designs of superconducting qubits , among other things. We shall also discuss our experiment of probing the topological properties in the superconducting phase by an anomalous transverse resistance .
Xiaoying Xu obtained her bachelor’s degree from University of Science and Technology Beijing and completed her Ph.D degree in physics at Fudan University, China. She is a postdoctoral fellow in the Department of Physics and Astronomy at the Johns Hopkins University, after having worked in the Oak Ridge National Laboratory and Kyushu University since graduation. She works in experimental condensed matter physics, exploring novel physical phenomena in materials with reduced dimensions, with her expertise in thin film material synthesis and device fabrication. Her current research interest, developed while working at the Johns Hopkins University, focuses on intrinsic topological superconductors, with particular emphases on experimental detection of the spin-triplet pairing state and potential applications in the quantum information sciences.
 Yufan Li, Xiaoying Xu, M. -H. Lee, M. -W. Chu, C. L. Chien, Science 366, 238 (2019)
 Xiaoying Xu, Yufan Li, C. L. Chien, Phys. Rev. Lett. 124, 167001 (2020)
 Xiaoying Xu, Yufan Li, C. L. Chien, arXiv:2210.08733 (2022)
 “Qubit devices comprising one or more polycrystalline or single crystalline spin-triplet superconductors” U.S. Patent Appl. No. 17610395.
 Xiaoying Xu, Yufan Li, C. L. Chien, Nat. Commun. 13, 5321 (2022)