Probing of superconducting tunnel junctions by photon-assisted tunneling with Nanonis Mimea

Researchers from FU Berlin have recently employed Nanonis Mimea to probe tunneling mechanisms between superconductors, including an impurity-induced Yu-Shiba-Rusinov (YSR) state by using photon-assisted tunneling under high frequency irradiation and in combination with the Tien-Gordon theoretical model for quantitative description. In superconducting junctions, tunneling occurs by different mechanisms where single electrons, Cooper pairs or larger numbers of electrons are transferred via Andreev reflection. Photon-assisted tunnelling between the tip and the bulk superconducting substrate is well described by the Tien–Gordon model, which also reveals the number of electrons involved in the elementary tunnelling process. Additionally,  the method may reveal a fingerprint as to which tunneling process is involved with the same number of electrons. Published results demonstrate the extension of this capability for YSR states and offer more insight into the contributions of resonant Andreev processes to subgap tunneling.
Measurements were performed with a superconducting Pb-coated tip on a Pb(111) surface at 1.3K in the presence of 40 GHz radiation. YSR states were induced by deposition of Mn atoms. First, single-electron tunneling above and within superconducting gap, as well as Cooper-pair tunneling and Andreev reflections at subgap voltages were investigated between the superconducting tip and the pristine superconducting substrate. Next, the procedure was repeated via tunneling through YSR states of individual magnetic adatoms. In the presence of subgap states, photon-assisted Andreev reflections showed distinct deviations from the standard Tien–Gordon theory. Therefore, scientists suggested an improved theoretical description which was in excellent agreement with their experimental data.
Presented results show the importance of photon-assisted tunnelling for analysis of tunnelling processes at the atomic scale which can be interesting for a better understanding of unconventional and topological superconductors. Subgap states in superconductors are promising grounds for quantum computing applications and a large amount of information can be obtained from tunnelling experiments through those states. In recent years there have been many attempts to distinguish Majorana from Andreev and YSR states.
Further information can be found here: doi.org/10.1038/s41567-020-0972-z