Maximum dc Josephson Current inPb−PbOx−PbJunctions

Abstract

We have measured the magnetic field dependence of the maximum dc Josephson current for square $\mathrm{P}\mathrm{b}-\mathrm{P}\mathrm{b}{\mathrm{O}}_x-\mathrm{P}\mathrm{b}$ Josephson junctions, with the ratio of junction width $w$ to Josephson penetration depth ${\lambda }_J$ as a parameter that was systematically varied. For wide Josephson junctions, the maximum dc Josephson current in zero external magnetic field $I_0$ is limited by the current-induced self-field, while for very narrow Josephson junctions, $I_0$ is limited by thermal noise. However, we have shown that for square Pb-Pb Josephson junctions with $1.00>\mathrm{}\frac{w}{{\lambda }_J}>0.\mathrm{}22$, the self-field as well as thermal noise is negligible. For these junctions, our experimental results are in consistent agreement with the theoretical prediction of Fulton and McCumber, who have shown that strong-coupling effects will reduce $I_0$ to 78.8% of the weak-coupling prediction for Pb-Pb junctions. We present evidence for the uniformity of our $\mathrm{Pb}{\mathrm{O}}_x$ tunneling barrier layers, and a model for the formation of our $\mathrm{Pb}{\mathrm{O}}_x$ layers.