Imaging of spatial structures of fiske-, shapiro-, and photon-assisted tunnelling step states in Josephson tunnel junctions

Using Low Temperature Scanning Electron Microscopy (LTSEM) the Fiske-Steps generated in a Josephson tunnel junction in the presence of a magnetic field have been studied. Spatial structures of the beam-induced change of the maximum step currents and also of the voltage signals of current biased step states have been observed with a spatial resolution of a few μm. In combination with a microwave source LTSEM offers the possibility to study the spatial distribution of rf-induced constant-voltage steps (Shapiro-steps) and of photon-assisted tunnelling steps. In the former case the spatial distribution of the maximum step current of various Shapiro-steps has been imaged. In the latter case we obtained evidence for the nucleation of an inhomogeneous energy gap state in the junction electrodes due to microwave irradiation.