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

Abstract

We have studied superconductor/normal-metal/superconductor $(\mathrm{SNS})$ junctions consisting of short Au or Cu wires between Nb or Al banks. The Nb based junctions display inherent electron heating effects induced by the high thermal resistance of the $\mathrm{NS}$ 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 $\ensuremath{\propto}1/V$ remains, which suggests the presence of a long-range proximity effect in the noise.