CURRENT-VOLTAGE CHARACTERISTICS OF SEMICONDUCTOR-COUPLED SUPERCONDUCTING WEAK LINKS WITH LARGE ELECTRODE SEPARATIONS

We have studied the current-voltage characteristics of superconducting weak links in which the coupling medium is the two-dimensional electr on gas in InAs-based semiconductor quantum wells, with relatively larg e (typically 0.5 mu m) separations between niobium electrodes. The dev ices exhibit Josephson-like current-voltage characteristics; however, the falloff of the differential resistance with decreasing temperature is thermally activated, and is orders of magnitude slower than for mo re conventional weak links. Most unexpectedly, the thermal activation energies are found to be proportional to the width of the device, take n perpendicular to the current flow. This behavior falls outside the r ange of established theories; we propose that it is a fluctuation effe ct caused by giant shot noise associated with multiple Andreev reflect ions. The possibility of nonequilibrium effects is discussed. [S0163-1 829(98)05941-4].