CONTRIBUTION OF ANDREEV REFLECTION TO THE INCREASE IN THE RESISTANCE OF THE NORMAL-METAL IN A BIMETALLIC N-S STRUCTURE

We study the resistive properties of 3D normal-metal-superconductor sy stems in the pure mean-free-path limit l(N,S)much greater than xi(T) ( l(N,S) are the mean free paths in the metals, and xi is the coherence length) at liquid helium temperatures. In contrast to the situation wh ere l much less than xi, which is common in experiments involving eith er sandwiches or mesoscopic samples, here the N-S system exhibits unus ual temperature behavior that cannot be described by existing theories of boundary resistance. What is most remarkable is a rise in normal r esistance in regions that do not incorporate the N-S boundary as the t emperature decreases, with asymptotic behavior resembling that of the temperature curve of the gap of a superconductor in contact with a nor mal metal. We show that this effect, not observed earlier in 3D system s, is due to the nonequivalence of the cross sections of scattering by normal-metal impurities of electron and hole excitations in condition s of Andreev reflection. We also show that in standard measurements of the contribution of the N-S boundary lying between the test contacts, this effect is masked by accompanying effects, the proximity effect a nd the boundary resistance, whose estimate requires taking into accoun t the presence on the N-S boundary of an electrostatic barrier of the Schottky type, a barrier that redistributes the probabilities of ordin ary and Andreev reflections of quasiparticles in the nonequilibrium co nditions due to current how. (C) 1998 American Institute of Physics. [ S1063-7761(98)02303-8].