SYMMETRY-BREAKING, OFF-DIAGONAL SCATTERING, AND JOSEPHSON CURRENTS INMESOSCOPIC WEAK LINKS

In a unified description of superconductor-normal-metal-superconductor (SNS) and superconductor-semiconductor-superconductor (SSmS) junction s with mean free paths exceeding the Nor Sm-layer thickness it is show n how off-diagonal (Andreev) scattering from spatial variations of the pair potential in competition with diagonal scattering from interface potentials, mismatches of Fermi energies, and effective masses determ ines the quasiparticle states and Josephson current densities. The bou nd states with subgap energies provide the main channels for Cooper pa ir transfer via Andreev scattering. Scattering states only contribute to the net Cooper pair transfer in narrow ''resonance'' windows just a bove the gap at energies which are a continuation of the bound-states spectrum. The corresponding part of the Josephson current density flow s in or against the direction of the bias current, depending upon the phase difference and thickness of the nonsuperconducting layer. The cu rrent contributions from the bound and scattering states oscillate opp osite to each other as functions of this thickness. The weakening of p hase coherence by increases of temperature and interface potentials an d the corresponding approach to the sinusoidal current-phase relations hip are discussed. Perturbation theory and comparison with the exact c alculations show that the Bardeen-Johnson current density is exact in the limit of thin-film SNS junctions and a good approximation in junct ions with cross-sectional dimensions of the S-banks much larger than t he London penetration depth, if the N-layer thickness 2a exceeds consi derably the BCS coherence length xi0. The possibility of Josephson-Blo ch oscillations in SSmS junctions is discussed.