The lateral proximity effect and long-range energy-gap gradients in Ta/Al and Nb/Al Josephson junctions

We present two independent experiments, each of which suggests that the loc al energy gap in Ta (and Nb) has a lateral spatial variation on a scale of several mu m. The first experiment is a series of current-voltage character izations of Nb/AI/AlOx and Ta/Al/AlOx Josephson junctions, which reveals a dependence of the measured energy gag on the size of the junction. This imp lies a geometrical dependence of the energy gap, An extended version of the current: theory of the proximity effect could explain this phenomenon when a lateral coherence length is introduced, which is of the order of the bul k coherence length of the materials. The second experiment is a series of c oincidence measurements of photon absorption events in a Ta absorber betwee n two Ta/Al junctions. There is a clear distinction in the pulse-height cha racteristics between events detected in the absorber and the junctions. Int erestingly, there are also events indicating the presence of a transition r egion between the absorber and the junction. Event statistics imply that th is region has a size of similar to 6 mu m, independent of photon energy, wh ich is quite a bit larger than even the bulk coherence length in Ta. It is argued that an additional effect due to 'smearing' by the relaxed quasi-par ticle cloud must also be present. These effects are interesting and intrigu ing; not only from a theoretical viewpoint, but also for energy-gap enginee ring of superconducting materials for practical applications, e.g. in a var iety of photon and particle detectors. (C) 2000 Elsevier Science B.V. All r ights reserved.