Subgap noise of a superconductor–normal-metal tunnel interface

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 $S(V,T)=4e\phantom{\rule{0.2em}{0ex}}\mathrm{coth}(eV∕T)I(V,T)$ holds quite generally even for nonlinear $I\text{\ensuremath{-}}V$ characteristics. Only when the normal metal is out of equilibrium do noise and current become independent. Their ratio, the Fano factor, depends then on the details of the layout.