dc and ac Josephson Effect in Sputtered Nb–NbOx–Pb Junctions

Abstract

The magnetic field dependence of the maximum dc Josephson current and of the ac Josephson‐effect‐induced step structure have been measured for sputtered Nb–NbO_x–Pb Josephson junctions, and found to be in very good agreement with theoretical predictions; even for junctions made with ``dirty'' Nb thin films. From this agreement it is concluded that the observed anomalies in the I‐V characteristics, namely, excess currents for voltages Δ(Pb)<V<Δ(Pb) + Δ(Nb) and a ``knee'' in the I‐V curve at voltages just above Δ(Pb) + Δ(Nb), cannot be attributed to nonuniformities in the plane of the junction. It is further shown that these anomalies, especially the ``knee'', can be explained by a proximity‐effect model in which the surface of the Nb thin film is assumed to be gapless due to gaseous impurities. The junctions were found to be much more stable with respect to repeated‐thermal cycling and storage at room temperature than Pb–PbO_x–Pb junctions made under the same conditions.