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

Nature Physics volume 16, pages 1222–1226 (2020)Cite this article

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Abstract

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.

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Fig. 1: Colour maps of the dI/dV spectra of superconducting Pb–Pb junctions under high-frequency radiation at f = 40 GHz.
Fig. 2: Sketches of (resonant) Andreev processes and tunnelling spectra of YSR states in different transport regimes.
Fig. 3: Photon-assisted tunnelling into the YSR state under high-frequency irradiation.
Fig. 4: Photon-assisted resonant Andreev reflections with the YSR state.

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Data availability

All data included in the Supplementary Information are available at https://doi.org/10.17169/refubium-27301. Source data are provided with this paper.

Code availability

The code generated for the simulations is available at https://doi.org/10.17169/refubium-27301.

Change history

  • 04 November 2020

    The Supplementary Information file initially published online was corrupted and was replaced on 4th November.

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Acknowledgements

We thank K. Flensberg for fruitful discussions and C. Lotze for technical support. We gratefully acknowledge funding by the European Research Council under the Consolidator Grant ‘NanoSpin’, by Deutsche Forschungsgemeinschaft and Agence Nationale de la Recherche under grant ‘JOSPEC’ and by CRCs 183 and 910 of Deutsche Forschungsgemeinschaft.

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Authors and Affiliations

  1. Fachbereich Physik, Freie Universität Berlin, Berlin, Germany

    Olof Peters, Nils Bogdanoff, J. Rika Simon, Gaël Reecht & Katharina J. Franke

  2. Dahlem Center for Complex Quantum Systems and Fachbereich Physik, Freie Universität Berlin, Berlin, Germany

    Sergio Acero González, Larissa Melischek & Felix von Oppen

  3. Université Grenoble Alpes, CNRS, Institut Neél, Grenoble, France

    Clemens B. Winkelmann

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  1. Olof Peters
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Contributions

O.P. and N.B. designed and constructed the high-frequency experimental set-up and carried out the experiments with help from J.R.S. and G.R. O.P., S.A.G. and L.M. performed the simulations. O.P., S.A.G., F.v.O. and K.J.F. analysed the data. K.J.F. conceived the experiment, with the help of C.B.W. K.J.F. guided the experiment and F.v.O. guided the theory. O.P., F.v.O. and K.J.F. wrote the paper with input from all co-authors. All authors discussed the results.

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Correspondence to Katharina J. Franke.

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Peer review information Nature Physics thanks Milan Allan and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

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Supplementary information

Supplementary Information

Supplementary Figs. 1–11, Table 1, experimental details, theoretical considerations and comparison of experiment and theory.

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Peters, O., Bogdanoff, N., Acero González, S. et al. Resonant Andreev reflections probed by photon-assisted tunnelling at the atomic scale. Nat. Phys. 16, 1222–1226 (2020). https://doi.org/10.1038/s41567-020-0972-z

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