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TT: Fachverband Tiefe Temperaturen
TT 34: Superconductivity: Theory
TT 34.1: Talk
Wednesday, March 19, 2025, 15:00–15:15, H36
Eliashberg theory and band-off-diagonal superconductivity — •Bernhard Putzer1,2 and Mathias S. Scheurer1 — 1Institute for Theoretical Physics III, University of Stuttgart, 70550 Stuttgart, Germany — 2Institute for Theoretical Physics, University of Innsbruck, Innsbruck A-6020, Austria
In contrast to the mean field approximation of BCS theory, the Migdal-Eliashberg approach is a more sophisticated framework to describe phonon-mediated superconductivity. Allowing strong coupling between electronic and bosonic fields opens the door to investigate the effects of inter-band processes on the superconducting state in a controllable setting. We derive and solve the Eliashberg equations for a two-band model, inspired by twisted graphene systems, finding an entirely band-off-diagonal superconducting order parameter. By including full momentum and Matsubara fre- quency dependence, we uncover a mixing of even- and odd-frequency states induced by the band splitting. As a result, the superconductor exhibits very unconventional spectral properties for electron-phonon pairing; this includes a region with a nodal spectrum and a region with finite gap, which is, however, much smaller than the order parameter magnitude. Our findings have consequences for recent experiments on the superconducting state in twisted bilayer and trilayer graphene.
Keywords: Eliashberg Theory; band-off-diagonal superconductivity; strong coupling theroy