Nonequilibrium transport in mesoscopic normal-metal-superconducting structures

We report measurements of the electrical transport properties of mesoscopic normal-metal–superconducting devices as a function of temperature, dc current, and magnetic field. In addition to properties associated with coherent Andreev reflection similar to those seen previously by other groups, these devices also exhibit sharp peaks and dips in the differential resistance as a function of dc current through the device. These features are occasionally metastable, and in doubly connected structures, their position and amplitude oscillate weakly with magnetic field. We believe these features arise from nonequilibrium transport phenomena induced by the dc current in the superconducting part of the samples.