Nonequilibrium and relaxation effects in tunnel superconducting junctions

A specific property of a planar tunnel junction with thin-film diffusive plates and long enough leads, typical for most of practical situations, is essential enhancement of its transmission coefficient compared to the bare transparency of the tunnel barrier [1, 2]. In voltage-biased junctions, this creates favorable conditions for strong nonequilibrium of quasiparticles in the junction plates and leads, produced by multiparticle tunneling. We study theoretically the interplay between the nonequilibrium and relaxation processes in such junctions and found that nonequilibrium in the leads noticeably modifies the current-voltage characteristic at eV > 2∆, especially the excess current, whereas strong diffusive relaxation restores the result of the classical tunnel model. At eV ≤ 2∆, diffusive relaxation decreases the peaks of the multiparticle currents. Inelastic relaxation in the junction plates essentially suppresses the n-particle currents (n > 2) by the factor n for odd and n/2 for even n. The results may be important for the problem of decoherence in Josephsonjunction based superconducting qubits. PACS numbers: 74.45.+c, 72.15.Lh, 74.40.Gh, 74.50.+r.