Large-scale superconductivity-induced conductance suppression in mesoscopic normal-superconducting structures

Abstract Experiments on hybrid superconducting normal-metal structures have revealed that even in the absence of tunnel junctions the onset of superconductivity can lead to a decrease in the electrical conductance by an amount many orders of magnitude greater than e 2 / h . In this paper, we provide a theory of this phenomenon which shows that it originates from an instability in the four-probe conductance which is absent from two-probe measurements. We compare the zero-bias, zero-temperature four-probe conductances G N and G S of a normal diffusive metal in contact with a superconductor in both the normal (N) and superconducting (S) states, respectively. In the absence of tunnel barriers, the ensemble average of the difference δ G = G S − G N vanishes, in agreement with quasiclassical theory. However, we also predict that there exist macroscopic sample specific fluctuations in δ G , which lie beyond quasiclassical theory and allow large negative values of δ G to occur.