Yufan Li先生（香港中文大学）によるセミナーを開催いたしました

Yufan Li先生（香港中文大学）が中辻研究室を来訪され、Li先生が研究を進められている、トポロジカル超伝導体における磁束の量子化の観測についてセミナーをしていただきました。

◆Date ：2023/4/6 (Thu) 10:00-11:30 (10:00-11:00 talk & 11:00-11:30 Q&A)
◆Place：理学部4号館1320室
◆Presenter：Prof. Yufan Li （Depart. of Phys. the Chinese University of
Hong Kong）

Title: “Unconventional flux quantization of topological superconductors”
Prof. Yufan Li
Department of Physics, the Chinese University of Hong Kong

The most distinctive feature of superconductivity is the presence of the
complex-valued many-particle wave function that sustains phase coherence
over macroscopic distances. One of its experimental manifestations is
the flux quantization in a superconducting ring. The single-value nature
of the wave function dictates a universal phase change of 2π for any
closed path around the ring. As a result, the fluxoid can only take on
quantized values in integer steps of Φ_0=h/2e, where h is the Planck
constant and the denominator of 2e is the signature of electron pairing.
There are two hallmarks of the well established integer flux
quantization: the quantization is Φ_0 periodic, and energy minima are
achieved at Φ = n Φ_0. Our experiments in topological superconductors
found that both hallmarks of flux quantization may be violated. For
topological superconductors, the spin-triplet pairing state may give
rise to half-integer flux quantization Φ’ = (n+½) Φ_0, due to the odd
parity of the superconducting gap function, or ∆_k=-∆_(-k). The
half-quantum flux has been observed in polycrystalline rings of
superconducting β-Bi2Pd [1] and α-BiPd [2]. Using composite ring
devices, the odd parity can be observed directly [3]. Moreover, we have
observed multiple quantization periodicities of of 2Φ_0, 3Φ_0 and 4Φ_0,
in addition to the conventional Φ_0, in mesoscopic rings of epitaxial
β-Bi2Pd [4], which may reflect its topological properties. The highly
unconventional flux quantization are distinctive experimental features
for topological superconductors.
[1]    Science 366, 238 (2019).
[2]    Phys. Rev. Lett. 124, 167001 (2020).
[3]    ArXiv:2210.08733.
[4]    ArXiv:2003.00603.