Interactions between coupled thin-film microbridge Josephson junctions

The interactions between two coupled indium microbridge Josephson junctions separated by as little as 1.5μm of superconducting film and fabricated using electron beam lithography have been investigated. For convenience two classes of interactions are recognized, the general interaction and the voltage-locking interaction. The general interaction is best observed by monitoring the critical current, I/sub cl/, of one microbridge (designated bridge 1) as a function of the current, I2, and the voltage, V2, of the other microbridge. The effects of order parameter depression, heating, and quasiparticle diffusion are estimated and these three mechanisms are found to be consistent with the observed interactions. The latter mechanism, quasiparticle diffusion, depends upon the relative directions of current flow through the two bridges. Voltage locking occurs when the current flows in opposing directions through the two bridges. Both the general interaction and voltage locking are strongest for bridge separations of approximately less than 2 μm. Increasing the separation of the bridges sharply reduces all interactions. In addition the results are reported of measurements made on series arrays of microbridges. Observed synchronization of arrays to an applied microwave field have been observed. Results are interpreted in light of the interactions between two bridges described above. Finally, the status of weak-link theory is reviewed, and some of the experimental results for single microbridges are presented. Included are results on general properties, the variation of critical current with temperature, the AC Josephson effect, and heating effects