ANOMALOUS ANDREEV CONDUCTANCE IN INAS-ALSB QUANTUM-WELL STRUCTURES WITH NB ELECTRODES

The differential conductance of InAs-AlSb quantum wells with superconducting Nb electrodes exhibits a sharp single peak at zero bias, followed by a drop below the normal conductance at voltages an order of magnitude higher than the gap voltage, before returning to the normal value at yet higher bias. The behavior is interpreted in terms of Andreev reflections, modified by multiple normal reflections of both electrons and Andreev holes between the Nb electrodes and the bottom barrier of the quantum well, in the presence of a nonuniform potential along the well.