Lead alloy Josephson junctions with Pb-Bi counterelectrodes

An experimental investigation has indicated that the ε-phase Pb-Bi alloy is an attractive alternative to Pb-1.7 wt.% Au for use as a counterelectrode for Josephson tunneling devices containing Pb-12 wt.% In-4 wt.% Au alloy films as base electrodes and control lines. Use of this Pb-Bi alloy as a counterelectrode results in a significant improvement in junction quality; the normalized subgap single-particle tunneling current is ∼2× lower at 2 mV. In addition, devices with the Pb-Bi counterelectrodes exhibit ∼3× fewer failures after repeated thermal cycling between 300 and 4.2°K. Out ot a total of 72 such devices having large area (6.2×10-6 cm2) junctions, 94% survived 1000 thermal cycles with negligible change.