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–NbOx–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–PbOx–Pb junctions made under the same conditions.

A study of VVOxPb Josephson tunnel junctions

“…The relation between the effective A, and the penetration depth in the case of finite thickness of the electrode is given by [23] A, = A t m h ($-) with d as the thickness of the sitperconducting electrode. I n the case of 3, > d the effective penetration depth is limited by the value Ae = d / 2 .…”

Anomalous Inner-Gap Structure in Transport Characteristics of Superconducting Junctions with Degraded Interfaces

“…We mean the so-called knee, a sudden current decrease in low-temperature I - V characteristics of Nb-based S-I-S junctions at voltages slightly just above a strong increase of the current that was expected to occur at the sum of V Δ values for the two superconducting electrodes [ 4 ] (see also [ 5 ]). Moreover, the current values at very low temperatures and at voltages lower than the sum of V Δ were considerably larger than theoretically anticipated for the BCS densities of states [ 6 ]. Later, it was shown experimentally that this anomaly may be caused by the presence of a normal conducting layer on the superconducting niobium film [ 6 , 7 ].…”

“…Moreover, the current values at very low temperatures and at voltages lower than the sum of V Δ were considerably larger than theoretically anticipated for the BCS densities of states [ 6 ]. Later, it was shown experimentally that this anomaly may be caused by the presence of a normal conducting layer on the superconducting niobium film [ 6 , 7 ]. This statement was confirmed theoretically [ 8 ] in the framework of the proximity-effect model which treats incoherent single-particle scatterings from a normal layer to a superconducting one within a tunnel-barrier approximation [ 9 ].…”

A simple technique for fabrication of tunnel junctions using Nb wire

“…Secondly there is a negative resistance region in the vicinity of LI Nb + d Pb. Both anomalies are commonly seen in Nb junctions qualitative explanations involving proximity effect and gaplessness having been offered [10], [11 ]. 4 and, as can be seen it has a higher value of Y2 and a leakage at least as small as the best film devices.…”

“…The Josephson effect. -Although the quasiparticle tunneling characteristics of Nb based tunnel junctions are not ideal, Schwidtal [10] has shown that nearly ideal Josephson tunneling behavior is possible. As is evident from figure 2, our wire junctions exhibit TABLE 1 large critical currents which are quite repeatable from junction to junction.…”