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

Experimental investigations of the proximity effects in symmetric Nb/Al-AlOx-Al/Nb superconducting tunneling junctions with a fixed Nb thickiness of 200 nm, and Al layer thicknesses ranging from 4 to 120 nm, are reported. Both de Josephson-tunneling and quasiparticle-tunneling current voltage characteristics were measured, and analyzed using the proximity effect model proposed by Golubov and Kupriyanov. The numerical computations were performed without the approximation used by Golubov et nl. In particular, the order parameter 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 gap energies are satisfactorily reproduced by the model calculations provided 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 transmission at the Nb/Al interface which is smaller than unity. [S0163-1829(99)01013-9].