Dissipative Processes in Superconducting Diffusive Tunnel Junctions

Doktorsavhandling, 2006

A central subject of this thesis is the dissipative current transport in superconducting tunnel junctions with diffusive electrodes. Tunnel current transport in superconducting junctions is a classical topic of interest and research. Our investigation is focused on the most interesting from the physical viewpoint region of small applied voltages, where the charge is carried by multiparticle tunneling processes, and strong nonequilibrium quasiparticle distribution is established. Such a study required development of a theoretical formalism going beyond the tunnel model approximation and based on a general nonequilibrium Keldysh-Green function technique for diffusive superconductors. Recently the problem attracted new attention motivated by development of transmissive tunnel structures, in particular, with high temperature superconductors, and by the problem of decoherence in Josephson junction based superconducting qubits. The first part of the thesis (paper 1) is devoted to nonequilibrium effects at low applied voltage and finite temperatures. Due to a small value of the Josephson frequency, the quasiparticle spectrum adiabatically follows the time evolution of the superconducting phase difference, which results in the formation of oscillating bound states in the vicinity of the tunnel junction (Andreev band). The quasiparticles trapped by the Andreev band generate higher even harmonics of the Josephson ac current, and also, in the presence of inelastic scattering, a nonequilibrium dc current, which may considerably exceed the dc quasiparticle current given by the tunnel model. The distribution of travelling quasiparticles also deviates from the equilibrium due to the spectrum oscillations, which results in an additional contribution to the dc current, proportional to the square root of applied voltage. In the second part of the thesis (papers 2 and 3), we examine the subharmonic gap structure (SGS) in the tunnel current focusing on the effect of diffusive electrodes and junction geometry. The main result is that the SGS features scale with an effective tunneling transparency which may exceed the junction transparency by up to two orders of magnitude depending on the junction geometry, and the ratio between the coherence length and the elastic scattering length. Our result provides an alternative explanation of anomalously high values of the subgap current in tunnelling experiments often ascribed to imperfectness of the insulating layer.