Proximity effects in Nb/Al-AlOx-Al/Nb superconducting tunneling junctions

Experimental investigations of the proximity effects in symmetric Nb/Al-AlO x-Al/Nb superconducting tunneling junctions with a fixed Nb thickiness of 2 00 nm, and Al layer thicknesses ranging from 4 to 120 nm, are reported. Bot h de Josephson-tunneling and quasiparticle-tunneling current voltage charac teristics were measured, and analyzed using the proximity effect model prop osed by Golubov and Kupriyanov. The numerical computations were performed w ithout the approximation used by Golubov et nl. In particular, the order pa rameter was allowed to spatially vary in Nb and Al and the Usadel functions were not linearized. The overall shape of the experimental I-V curves, as well as the temperature dependence of the de Josephson current and of the g ap energies are satisfactorily reproduced by the model calculations provide d the parameter gamma(BN) describing the resistance at the Al-Nb, interface is taken into account. From ys, we extract a value for the quasiparticle t ransmission at the Nb/Al interface which is smaller than unity. [S0163-1829 (99)01013-9].