Observation of a new non-equilibrium state in superconductors caused by sequential tunnelling

A previously unrecognised non-equilibrium phenomenon in superconducting tunnel junctions is identified. The situation may arise in low-loss, low-gap, multi-tunnelling devices in which quasiparticles retain accumulated energy through several cycles of sequential forward and back tunnelling. As a consequence, a stable non-equilibrium distribution is established, in which many quasiparticles have energies exceeding the 3Delta threshold for breaking Cooper pairs, resulting in a large excess current. By solving the system of coupled kinetic equations for quasiparticles and phonons, we have modelled the quasiparticle distribution and excess current, and have confirmed the predictions through experiments on an appropriate superconducting tunnel junction.