The Josephson effect in scanning tunneling microscopy: Exploring the quantum nature of the tunneling effect

The Josephson effect describes the tunneling of Cooper pairs in a weak link. The Josephson energy, describing the coupling between the tunneling contacts, is typically very small (10-100µeV), so that temperatures in the mK-range are necessary to operate in the low temperature limit. In a typical scanning tunneling microscopy (STM) setup, where the charge is better defined than the phase, the dominant energy scale, however, is the charging energy (about 100µeV) due to the capacitance of the tunnel junction. This is the dynamical Coulomb blockade regime, where sequential charge tunneling dominates the Josephson effect as opposed to correlated phase tunneling. In this regime, the tunneling probability for a Cooper pair is directly proportional to the probability for emitting its kinetic energy into the electromagnetic environment. This process can be modeled within the so-called P(E)-theory and is a direct manifestation of the quantum nature of the tunneling effect. In my presentation, I will discuss the Josephson effect in the dynamical Coulomb blockade regime, how we can understand the interaction with the electromagnetic environment, and what information of the Josephson effect can be extracted in this regime. In this way, we can not only use STM to learn about the Josephson effect, but also use it as a probe for superconductivity.