The singularities of quasiparticle current in superconducting multiple tunnelling junctions

We have developed a theory of the anomalous sub-gap currents in small gap superconducting multiple tunnelling junctions. The sub-gap current at low bath temperatures is predicted to follow a sequence of steps at 2Δ/n, where n is an integer. These steps occur because in small gap multiple tunnelling structures a quasiparticle has the chance to increase its energy successively after each tunnelling event, until eventually its energy exceeds 3Δ. At this point, it relaxes down the edge with the emission of 2Δ phonon which triggers a Cooper pair break resulting in excess sub-gap current.