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. In the first approach, we complement the capabilities of a scanning tunneling microscope by introducing high-frequencies (HF) up to 40 GHz 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.
Here we have used 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. Our model is in excellent agreement with our data.
In a second spectroscopic approach, we investigate Cooper pair tunneling in current-biased Josephson junctions. We show that the critical current is strongly reduced by magnetic impurities, which reflects a reduced superconducting order parameter in the vicinity of the magnetic adatom.
Our results of photon-assisted tunneling and Josephson spectroscopy show that we have established two powerful methods for the investigation of superconducting tunneling processes at the atomic scale. These methods could be particularly informative for the investigation of unconventional and topological superconductors.