Effect of inhomogeneities and measuring current on the current-voltage characteristics of superconducting nonsymmetric tunnel junctions

It is shown experimentally that the shape of the current-voltage characteristics of superconducting nonsymmetric thin-film junctions depends on both the inhomogeneity of the films and the tunnel resistivity which determines the measurement current. Reproducible results consistent with the BCS theory were obtained only for low-resistance junctions in which the shape of the I-V curve is close to the theoretical shape. In the case of slight deviations from the theoretical I-V curve the smearing of the current jump at V≃ (Δ1 + Δ2)/e can be attributed to the superconductor with the higher Tc. Comparison of experiment with theory for highly inhomogeneous Al and Zn thin films revealed that the criteria for reckoning both the energy gap and Tc are important. Measurements on low-resistance aluminum-insulator-thallium and zinc-insulator-aluminum junctions yielded the following values for 2Δ (0)/kTc: 3.55±0.04 and 3.53±0.04 for Al films, 3.54±0.04 for thallium films, and 3.53±0.06 for zinc films.