C. L. Chien, Department of Physics and Astronomy, Johns Hopkins University
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Abstract: The overwhelming majority of superconductors (SCs) are spin singlet SCs, mostly s-wave (e.g., Nb, MgB2, Fe pnictides) and some d-wave (high TC cuprates). Spin triplet SCs, crucial for Majorana physics and fault tolerant quantum computing, are rare and difficult to identify. In the past, large upper critical field (HC2) and NMR Knight shift have been used but not successful in comfirming triplet SCs, with Sr2RuO4 being a well-known disappointment. We instead use phase-sensitive method of half-quantum flux (HQF) to identify triplet SCs, which have odd-parity gaps as opposed to the even-parity gaps in singlet SCs. We have observed HQF of (n + ½)o, where o = hc/2e the flux quantum and n an integer in patterned sub-µm rings of spin triplet β-Bi2Pd thin films [1], instead of the integer flux no universally observed in singlet SC rings. Noncentrosymmetric SCs with an admixture of spin triplet pairing also exhibit HQF [2]. Very recently, we have succeeded in fabricating Geshkenbein-Larkin-Barone (GLB) composite rings consisting of two SCs that can unequivocally identity singlet or triplet pairing in any SC. The attributes of triplet SCs may be exploited in flux qubits.
[1] Yufan Li, Xiaoying Xu, M. H. Lee, M. W. Chu, and C. L. Chien, Science, 366, 238 (2019).
[2] Xiaoying Xu, Yufan Li, and C. L. Chien, Phys. Rev. Lett. 124, 167001 (2020).
[3] Xiaoying Xu, Yufan Li, and C. L. Chien (submitted), arXiv:2210.08733 (2022).
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