Proximity effect in Nb/Al, AlOoxide, Al/Nb Josephson tunnel junctions

Regions with reduced energy gap induced by the proximity effect give rise to quasi-particle loss in Josephson-junction X-ray detectors, but may also be used advantageously for quasi-particle collection. The influence of the thickness of the Al proximity layers in Nb/Al/sub 1/, AlO/sub x/, Al/sub 2//Nb Josephson tunnel junctions on the electrical characteristics has been investigated theoretically and experimentally. Theoretically it is found that the strength of the proximity effect is mainly determined by the proximity effect is mainly determined by the proximity parameters gamma /sub M1/ ( gamma /sub M2/) of the electrodes. Good fits of the measured I-V curves with theory were obtained for junctions with thicknesses d/sub A11/ ranging from 4 to 25 nm and d/sub A12/=3 nm, with gamma /sub M2/ approximately=0.12 and gamma /sub M1// gamma /sub M2/=d/sub A11//d/sub A12/. For all junctions the proximity knee remains more pronounced than predicted.<
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