Multiple Andreev Reflection and Giant Excess Noise in Diffusive Superconductor/Normal-Metal/Superconductor Junctions

TL;DR

This study investigates superconductor/normal metal/superconductor junctions, revealing phenomena like multiple Andreev reflections, subharmonic gap structures, and giant excess noise, linked to supercurrent fluctuations and charge transfer mechanisms.

Contribution

It provides new insights into noise behavior and supercurrent fluctuations in SNS junctions with different superconducting materials, highlighting the role of electron heating and charge transfer.

Findings

Giant excess noise peak at low voltages in Al-based junctions.

Subharmonic gap structures observed in differential conductance.

Supercurrent fluctuations cause noise peaks at Josephson onset.

Abstract

We have studied superconductor/normal metal/superconductor (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 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 ~1/V remains, which may be interpreted as shot noise originating from large multiple charges.