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TT: Fachverband Tiefe Temperaturen

TT 66: Superconductivity: Yu-Shiba-Rusinov and Andreev Physics

TT 66.10: Talk

Thursday, March 21, 2024, 12:00–12:15, H 3005

Impact of evanescent scattering modes in short junctions — •Daniel Kruti and Roman-Pascal Riwar — Jülich Research Centre and Cologne University

Superconducting junctions are essential building blocks for quantum hardware, and their fundamental behaviour is still a highly active research field. For generic junctions, the emergence of subgap Andreev bound states is conveniently described by Beenakker's formula, representing an important starting point for both analytic and numeric studies. In this work, we critically reassess two common assumptions: that in the short junction limit scattering is approximately energy independent and dominated by planar channels. We argue that cross-channel scattering can lead to a strong energy-dependence which is particularly pronounced when the system is tuned close to a change of the channel number -- even if energy is small compared to the Thouless energy. We provide a general recipe to include evanescent modes, and apply it to the example of a ballistic junction with nontrivial geometry. We show that the interplay between evanescent modes and energy-dependant scattering has a significant influence on the Andreev spectrum, especially close to the superconducting gap. In particular, the energy-dependence of the scattering matrix breaks the symmetry which allows for the Andreev spectrum to touch the gap. Evanescent modes effectively restore this symmetry. We expect our work to be relevant for contemporary experimental and theoretical studies of ultra-small junctions and their (dissipative) transport properties.

Keywords: Andreev bound states; Tunnel junction; Scattering; Superconducting qubit; Weak link