Two-Particle Tunneling in Superconducting PbIn|Oxide|Pb Junctions
Tunnel junctions have been fabricated and studied which reproducibly show structure in their I-V characteristics which we believe can be understood in terms of two-particle tunneling. The tunnel junctions were made by depositing a Pb—8 wt In film, oxidizing it, and depositing a second film of pure Pb. The measured T c of the PbIn does not differ by more than 0.1°K from that of the Pb film, and the value of the junction gap 2∆ is the same as for Pb|oxide|Pb junctions. The structure which we ascribe to two-particle tunneling has three salient features: a current step at V = ∆, a rapid rise in current for ∆ < V < 2∆, and a negative resistance region superimposed upon the usual single-particle features at V= 2∆. In Fig. 1 the solid curve is a measured I-V characteristic at T = 1.4°K in the range ∆ < V < 2∆, showing the step at V= A. We cannot easily show the negative resistance region in the same plot because of the difference in scale. Hence we show this feature in the insert. The step at A is somewhat rounded but not more than is the step at 2∆. The sharpness of these steps varies somewhat from sample to sample and is due, we believe, to some compositional inhomogeneity in the PbIn film. We should also point out that these features that we attribute to two-particle tunneling are not observed in similar Pb|oxide|Pb junctions prepared in the same manner.
KeywordsTunnel Junction Current Step Thin Region Barrier Thickness Excess Current
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