Condensed Matter Seminar Series

Katharina Franke

Freie Universität Berlin

Photon-assisted tunneling at the atomic scale: Probing resonant Andreev reflections from Yu-Shiba-Rusinov states

Exchange coupling of magnetic adsorbates to a superconducting substrate leads to Yu-Shiba-Rusinov (YSR) states within the superconducting energy gap. These can be probed by scanning tunneling spectroscopy as a pair of resonances at positive and negative bias voltage and over a wide range of tunnel conductances. At low tunneling rates, the current is carried by single-electron processes, where each excitation is sufficiently quickly followed by a relaxation into the energetic continuum. Upon increasing the junction conductance, the relaxation rates suppress single-electron tunneling and resonant Andreev processes start to dominate the transport process. The cross-over of these processes is expressed in the variation of the ratio of YSR peak height at positive and negative bias voltage [1]. Here, we investigate these transport processes by photon-assisted tunneling. While applying high-frequency radiation to the tunneling junction, we record the differential conductance spectra in the low and high-conductance regime. At low conductance, the YSR states exhibit symmetrically spaced sidebands with their spacing directly evidencing single-electron tunneling. Surprisingly, at large junction conductance, the spacing remains the same while the patterns become asymmetric. We show that this asymmetry is direct evidence of a resonant Andreev reflection with tunneling threshold conditions imposed on its electron and hole component [2,3]. We suggest that photon-assisted tunneling can be a powerful tool for the determination of the nature of the charge carriers in a single tunneling event.
[1] M. Ruby, F. Pientka, Y. Peng, F. von Oppen, B. W. Heinrich, K. J. Franke, Phys. Rev. Lett., 115 087001 (2015).
[2] O. Peters, N. Bogdanoff, S. Acero Gonzalez, L. Melischek, R. Simon, G. Reecht, C. B. Winkelmann, F. von Oppen, K. J. Franke, Nature Phys. 16, 128 (2020).
[3] S. Acero Gonzalez, L. Melischek, O. Peters, K. Flensberg, K. J. Franke, F. von Oppen, Phys. Rev. B 102, 045413 (2020).

In collaboration with: Olof Peters, Nils Bogdanoff, Sergio Acero Gonzalez, Larissa Melischek, J. Rika Simon, Gaël Reecht, Clemens B. Winkelmann, Felix von Oppen


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