Shot noise of large charge quanta in superconductor/semiconductor/superconductor junctions

TL;DR

This paper experimentally demonstrates enhanced shot noise in superconductor/semiconductor/superconductor junctions due to large effective charge quanta from multiple Andreev reflections, linking noise behavior to subharmonic gap structures.

Contribution

It provides experimental evidence for large charge quanta caused by multiple Andreev reflections, explaining noise enhancement in superconductor/semiconductor/superconductor junctions.

Findings

Noise exceeds thermal and shot noise limits in certain voltage regions.

Large effective charge quanta q* are observed, consistent with theoretical predictions.

Subharmonic gap structures correlate with multiple Andreev reflections.

Abstract

We have found experimentally that the noise of ballistic electron transport in a superconductor/semiconductor/superconductor junction is enhanced relative to the value given by the general relation, S_V=2eIR^2coth(eV/2kT), for two voltage regions in which this expression reduces to its thermal and shot noise limits. The noise enhancement is explained by the presence of large charge quanta, with effective charge q*=(1+2Delta/eV)e, that generate a noise spectrum S_V=2q*IR^2, as predicted in Phys. Rev. Lett. 76, 3814 (1996). These charge quanta result from multiple Andreev reflections at each junction interface, which are also responsible for the subharmonic gap structure observed in the voltage dependence of the junction's conductance.