Titel der Dissertation: Transport Processes through Superconducting Junctions at the Atomic Scale
Abstract: Tunneling across superconducting junctions is associated with a variety of different processes that transfer single electrons, Cooper pairs, or even larger amounts of electrons by multiple Andreev reflections. Resonances inside the superconducting energy gap, like e.g. induced by magnetic adatoms, add resonant Andreev reflections to the variety of tunneling processes. We have successfully established two spectroscopic methods to study the nature of tunneling processes in superconductors at the atomic scale – Photon-assisted tunneling and Josephson spectroscopy. Both could be particularly informative for the investigation of unconventional and topological superconductors. The disputation will focus on photon-assisted tunneling. For this approach, we complement the capabilities of a scanning tunneling microscope by introducing high-frequencies (HF) up to 40GHz into the tunnel junction. The charge carriers involved in the tunneling process can exchange energy with the radiated HF field which leads to photon-assisted tunneling. Based on the theory of Tien und Gordon it is predicted that the sideband spacing in the bias voltage is a direct fingerprint of the number of electrons transferred in a single tunneling event. We have applied photon-assisted tunneling to study superconducting tunnel junctions that exhibit Yu-Shiba-Rusinov states (YSR) induced by magnetic Mn adatoms on Pb(111). By exploiting the tunability of the junction conductivity we could specifically obtain insights into the contributions of single-electron tunneling and resonant Andreev processes to the YSR states. While the simple Tien-Gordon description is sufficient to describe single-electron tunneling and Cooper pair tunneling into the pure substrate, we show that the description breaks down for resonant Andreev reflections. We developed an improved theoretical model based on rate equations and the ac modulation of the bias voltage, which is in excellent agreement with our data.
Zeit & Ort
10.12.2020 | 16:00