Multiple Andreev reflections and enhanced shot noise in diffusive SNS junctions

We study the dc conductance and current fluctuations in diffusive voltage biased SNS junctions with a tunnel barrier inside the mesoscopic normal region. We find that at subgap voltages, eV < 2∆ /n , the current associated with the chain of n Andreev reflections is mapped onto the quasiparticle flow through a structure of n + 1 voltage biased barriers connected by diffusive conductors. As a result, the current-voltage characteristic of a long SNINS structure obeys Ohm’s law, in spite of the complex multiparticle transport process. At the same time, nonequilibrium heating of subgap electrons produces giant shot noise with pronounced subharmonic gap structure which corresponds to stepwise growth of the effective transferred charge. At eV → 0, the shot noise approaches the magnitude of the Johnson-Nyquist noise with the effective temperature T ⋆ = ∆ / 3, and the effective charge increases as ( e/ 3)(1+2∆ /eV ), with the universal “one third suppression” factor. We analyse the role of inelastic scattering and present a criterion of strong nonequilibrium.