Probing of energy gap distribution in superconducting tunnel junctions by low temperature scanning electron microscopy

A two-dimensional voltage image of the energy gap distribution of a superconducting tunnel junction was obtained by scanning the current biased junction with an electron beam and detecting the voltage change δV. The value of the energy gap at the point of irradiation was determined quantitatively from the δV σ(V) curves, where σ(V) is the electric conductance of the junction. Further the quasiparticle diffusion length was found by measuring the length of the transition between a high- and low-gap region generated by a double tunnel junction configuration. The theoretical predictions could be verified by investigating a double tunnel junction configuration, where the energy gap could be changed deliberately by quasiparticle injection.