Resonant Andreev reflections probed by photon-assisted tunnelling at the atomic scale

Tunnelling across superconducting junctions proceeds by a rich variety of processes, which transfer single electrons, Cooper pairs or even larger numbers of electrons by multiple Andreev reflections. Photon-assisted tunnelling combined with the venerable Tien–Gordon model has long been a powerful to...

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Bibliographic Details
Published in:Nature physics 2020-12, Vol.16 (12), p.1222-1226
Main Authors: Peters, Olof, Bogdanoff, Nils, Acero González, Sergio, Melischek, Larissa, Simon, J. Rika, Reecht, Gaël, Winkelmann, Clemens B., von Oppen, Felix, Franke, Katharina J.
Format: Article
Language:English
Subjects:
639/766/119/1003
639/766/119/544
Atomic
Classical and Continuum Physics
Complex Systems
Condensed Matter Physics
Cooper pairs
Electrons
General Physics
Impurities
Mathematical and Computational Physics
Microwave radiation
Microwaves
Molecular
Optical and Plasma Physics
Photons
Physics
Physics and Astronomy
Scanning tunneling microscopy
Single electrons
Substrates
Superconductivity
Theoretical
Topological superconductors
Tunnel junctions
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Summary:Tunnelling across superconducting junctions proceeds by a rich variety of processes, which transfer single electrons, Cooper pairs or even larger numbers of electrons by multiple Andreev reflections. Photon-assisted tunnelling combined with the venerable Tien–Gordon model has long been a powerful tool to identify tunnelling processes between superconductors. Here, we probe superconducting tunnel junctions including an impurity-induced Yu–Shiba–Rusinov (YSR) state by exposing a scanning tunnelling microscope with a superconducting tip to microwave radiation. We find that a simple Tien–Gordon description describes tunnelling of single electrons and Cooper pairs into the bare substrate, but breaks down for tunnelling via YSR states by resonant Andreev reflections. We develop an improved theoretical description that is in excellent agreement with the data. Our results establish photon-assisted tunnelling as a powerful tool to analyse tunnelling processes at the atomic scale, which should be particularly informative for unconventional and topological superconductors. Scanning tunnelling microscopy is enhanced by microwave radiation that allows photon-assisted tunnelling processes. This technique is demonstrated on impurity states in a superconductor.
ISSN:1745-2473
1745-2481
1476-4636
DOI:10.1038/s41567-020-0972-z