Critical Josephson current in the dynamical Coulomb blockade regime

Superconductivity is commonly described as a macroscopic quantum phenomenon. However, it arises from microscopic mechanisms occurring at the nanometer scale as illustrated, for example, by the non-trivial pairing in unconventional superconductors. More recently, also local interactions with superconductors in the context of Majorana fermions became of interest. A very direct way to study the atomic scale properties of superconductors is given by the combination of the Josephson effect with scanning tunneling microscopy (STM), also referred to as JSTM. Here, the critical Josephson current serves as a direct local probe of the superconducting ground state and may reveal valuable information that is often inaccessible when studying quasi-particle excitation spectra. We show that we can extract local values of the critical Josephson current from JSTM measurements in the dynamical Coulomb blockade regime. Furthermore, we experimentally determine the regime of sequential Cooper pair tunneling, which is in accordance to theoretical predictions. Our study presents new insights on the tunneling mechanisms in Josephson junctions and lays the basis for the implementation of JSTM as a versatile probe for superconductivity.