The proximity effect and subgap conductivity in superconductor-barrier-normal metal contacts

Abstract

On the basis of microscopic equations for matrix Green's functions with boundary conditions at the interface, we calculate the conductance of S/N contacts of different types (a planar S/N junction and a one-dimensional S/N channel) as a function of bias voltage. It is shown that even at zero temperature the subgap conductance S( V) ( V < Δ) may be significant (of the order of the contact conductance in the normal state). The subgap conductance is caused by a term in the current which leads to the interference current at T < T c and to a contribution to the paraconductivity of the Maki-Thompson type at T > T c in SIS junctions. The form of S ( V) depends essentially on the relation between the barrier transmittance and the depairing rate in the N region.