Multiple Andreev reflection and giant excess noise in diffusive superconductor/normal-metal/superconductor junctions

Abstract

We have studied superconductor/normal-metal/superconductor (SNS) junctions consisting of shea Au or Cu wires between Nh or Al banks. The Nb based junctions display inherent electron heating effects induced by the high thermal resistance of the NS boundaries. The Al based junctions show in addition subharmonic gap structures in the differential conductance dI/dV and a pronounced peak in the excess noise at very low voltages V. We suggest that the noise peak is caused by fluctuations of the supercurrent at the onset of Josephson coupling between the superconducting banks. At intermediate temperatures where the supercurrent is suppressed a noise contribution proportional to 1/V remains, which suggests the presence of a long-range proximity effect in the noise.