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 counterelectrode E either Nb, Al, or W. The experiments show a large correction, {Δ}G{}0.1GN, on the classical superconductor--insulator--normal-metal (SIN) conductance. We present measurements of the temperature, magnetic-field, and voltage dependence, and we interpret the observed results within the available theoretical models for coherent Andreev reflection, as provided by several authors.