Geometrical resonances in a high-injection-current nonequilibrium state of superconductor--normal-metal contacts

A hysteretic transition to a high-injection-current nonequilibrium state in metallic contacts between silver and high-purity single-crystal tantalum was recently observed by the author and H. Lerchner. Oscillations of conductance with bias in this state are interpreted as geometrical resonances of the excess current caused by a bias-driven variation of the superconductor--normal-metal phase-boundary position. Quantitative numerical results are obtained from an extension of the one-dimensional Blonder-Tinkham-Klapwijk microconstriction model and are successfully compared with part of the experimental results. Lacking any self-consistency features, the model is unable to describe nonlinearities and feedback effects which apparently dominate much of the experimental behavior, but supplies some indication that Andreev reflection might also play an essential role in these effects