Proximity effects in ${\mathrm{N}\mathrm{b}/\mathrm{A}\mathrm{l}-\mathrm{A}\mathrm{l}\mathrm{O}}_x-\mathrm{A}\mathrm{l}/\mathrm{N}\mathrm{b}$ superconducting tunneling junctions

Experimental investigations of the proximity effects in symmetric ${\mathrm{N}\mathrm{b}/\mathrm{A}\mathrm{l}-\mathrm{A}\mathrm{l}\mathrm{O}}_x-\mathrm{A}\mathrm{l}/\mathrm{N}\mathrm{b}$ superconducting tunneling junctions with a fixed Nb thickness of 200 nm, and Al layer thicknesses ranging from 4 to 120 nm, are reported. Both dc 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 al. 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 dc Josephson current and of the gap energies are satisfactorily reproduced by the model calculations provided the parameter ${\gamma }_{\mathrm{BN}}$ describing the resistance at the Al-Nb interface is taken into account. From ${\gamma }_{\mathrm{BN}}$ we extract a value for the quasiparticle transmission at the Nb/Al interface which is smaller than unity.