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

Experiments on hybrid superconducting normal-metal structures have revealed that even in the absence of tunnel junctions the onset of superconductivit y can lead to a decrease in the electrical conductance by an amount many or ders of magnitude greater than e(2)/h. In this paper, we provide a theory o f this phenomenon which shows that it originates from an instability in the four-probe conductance which is absent from two-probe measurements. We com pare the zero-bias, zero-temperature four-probe conductances GN and Gs 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 delta G = G(S) - G(N) van ishes, in agreement with quasiclassical theory. However, we also predict th at there exist macroscopic sample specific fluctuations in delta G, which l ie beyond quasiclassical theory and allow large negative values of 6G to oc cur. (C) 1999 Academic Press.