Lateral proximity effect and long-range energy-gap gradients in Ta/Al and Nb/Al superconducting tunnel junctions - art. no. 214507

We present the results of current-voltage characterizations of Nb/Al and Ta /Al superconducting tunnel junctions, which reveal a dependence of the meas ured energy gap on the size of the junction. This implies a geometrical dep endence of the energy gap, which suggests that the effective local energy g ap has a lateral spatial variation on a scale of several mum. An extended v ersion of the theory of the proximity effect could explain this phenomenon when lateral coherence lengths are introduced, which are of the order of th e bulk coherence length in Al. One of the consequences of this theory is th at the coherence length in a thin film cannot be taken to be isotropic. Ano ther is that the effective lateral coherence length, which features in the lateral Usadel equation, actually depends on the quasiparticle energy.