Subgap noise of a superconductor-normal-metal tunnel interface

TL;DR

This paper studies how impurity-induced interference affects low-frequency current noise in NIS tunnel junctions at low voltages and temperatures, revealing a universal relation in equilibrium and layout-dependent behavior out of equilibrium.

Contribution

It demonstrates that the relation between noise and current remains universal in equilibrium, while out of equilibrium the noise and current become independent with a Fano factor depending on device specifics.

Findings

In equilibrium, the noise-current relation $S(V,T) = 4e ext{coth}(eV/T) I(V,T)$ holds generally.

Out of equilibrium, noise and current are independent, with the Fano factor depending on device layout.

Impurity interference significantly influences subgap noise in NIS junctions.

Abstract

It is well established that the subgap conductivity through a normal-metal-insulator-superconductor (NIS) tunnel junction is strongly affected by interference of electron waves scattered by impurities. In this paper we investigate how the same phenomenon affects the low frequency current noise, $S$, for voltages $V$ and temperatures $T$ much smaller than the superconducting gap. If the normal metal is at equilibrium we find that the simple relation $S(V,T) = 4e {\rm coth} (eV/T) I(V,T) $ holds quite generally even for non-linear $I$-$V$ characteristics. Only when the normal metal is out of equilibrium, noise and current become independent. Their ratio, the Fano factor, depends then on the details of the layout.