Condensed Matter Seminar Series
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 . 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.
 M. Ruby, F. Pientka, Y. Peng, F. von Oppen, B. W. Heinrich, K. J. Franke, Phys. Rev. Lett., 115 087001 (2015).
 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).
 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