SKM 2021 – scientific programme
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TT: Fachverband Tiefe Temperaturen
TT 22: Poster Session: Disordered and Granular Superconductors: Fundamentals and Applications in Quantum Technology
TT 22.2: Poster
Thursday, September 30, 2021, 13:30–16:00, P
Collective excitations in weakly-coupled disordered superconductors — •Bo Fan1, Abhisek Samanta2, and Antonio Miguel Garcia-Garcia1 — 1Shanghai Center for Complex Physics, School of Physics and Astronomy, Shanghai Jiao Tong University, Shanghai 200240, China — 2Physics Department, Technion, Haifa 32000, Israel
Isolated islands in two-dimensional strongly-disordered and strongly-coupled superconductors become optically active inducing sub-gap collective excitations in the ac conductivity. Here, we investigate the fate of these excitations as a function of the disorder strength in the experimentally relevant case of weak electron-phonon coupling. An explicit calculation of the ac conductivity, that includes vertex corrections to restore gauge symmetry, reveals the existence of collective sub-gap excitations, related to phase fluctuations and therefore identified as the Goldstone modes, for intermediate to strong disorder. As disorder increases, the shape of the sub-gap excitation transits from peaked close to the spectral gap to a broader distribution reaching much smaller frequencies. Phase-coherence still holds in part of this disorder regime. The requirement to observe sub-gap excitations is not the existence of isolated islands acting as nano-antennas but rather the combination of a sufficiently inhomogeneous order parameter with a phase fluctuation correlation length smaller than the system size. Our results indicate that, by tuning disorder, the Goldstone mode may be observed experimentally in metallic superconductors based for instance on Al, Sn, Pb or Nb.