Tunneling Study of the Superconducting Proximity Effect in the Thin-Normal Limit

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Abstract
We report the tunneling characteristics of Pb-Zn-oxide-Pb junctions in which the average Zn thickness is approximately one atomic layer. This thin layer is produced either by indirect evaporation of Zn or by dissociation of ZnS upon evaporation. The tunneling characteristics show an added current shoulder just above the gap-edge voltage similar to that frequently reported for Nb junctions and several other systems, but unexplained until recently. We discuss the McMillan theory of the proximity effect in relation to earlier experiments using thicker normal films. We then use the theory to demonstrate that the observed current shoulder results from the proximity effect, involving a much thinner normal layer than in any previous experiments. A similar effect can explain the observations with Nb and other materials.