ENHANCED CONDUCTANCE NEAR ZERO-VOLTAGE BIAS IN MESOSCOPIC SUPERCONDUCTOR-SEMICONDUCTOR JUNCTIONS

We have studied the conductance enhancement near zero voltage bias of double-barrier Nb-p(++) Si-E junctions, where we chose for the counter electrode E either Nb, bl, or W. The experiments show a large correcti on, Delta G approximate to 0.1 G(N), on the classical superconductor-i nsulator-normal-metal (SIN) conductance. We present measurements of th e temperature, magnetic-field, and voltage dependence, and we interpre t the observed results within the available theoretical models for coh erent Andreev reflection, as provided by several authors.