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-${\mathit{p}}^{++}$Si-E junctions, where we chose for the counterelectrode E either Nb, Al, or W. The experiments show a large correction, ΔG≊0.1${\mathit{G}}_{\mathit{N}}$, 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.