Superconductor-normal-superconductor microbridges: Fabrication, electrical behavior, and modeling

Abstract

Josephson-effect devices consisting of two superconducting thin films coupled by a short nonsuperconducting metallic bridge are investigated in this work. To fabricate these devices we have developed processing techniques which are compatible with refractory superconductors and show promise for the high-Tc A15 materials as well. We describe in detail the electrical behavior of Nb-Au-Nb and Nb-Cu-Nb bridges, which have been made with resistances up to 0.4 Ω and critical current-resistance (i.e., IcRn ) products as high as 0.5 mV. We have modified the usual theoretical model for the critical current in order to account for the actual geometry of our devices thereby obtaining semiquantitative agreement with experiment. A single parameter is used in this analysis to characterize the proximity effect and we find that values inferred from the critical currents of bridges agree well with those inferred from the behavior of the transition temperature of S/N bilayers. Typical current-voltage characteristics are in good agreement with the predictions of the time-dependent Ginzburg-Landau model with rigid boundary conditions. At high dissipations heating is observed and explained by a generalized version of the hotspot model. Finally, we have employed the models for superconductor-normal-superconductor microbridge behavior to evaluate the ultimate potential of this particular weak-link configuration for practical applications.