Subgap noise of a superconductor-normal-metal tunnel interface

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 coth(eV/T)I(V,T) holds quite generally even for nonlinear I-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.