Observation of steps in the microwave impedance of superconducting microbridges

Steps similar to those seen in the d.c. I—V characteristics have been noted in the onset of the microwave frequency current-driven superconducting-to-normal transition of tin and lead microbridges. These steps are well described by the theory of Skocpol, Beasley, and Tinkham, and are apparently caused by the generation of localized phase slip sites in the microbridges. The observed microwave dissipation and critical current hysteresis result from the diffusion of quasiparticles generated in the phase slip process. Heating effects were not found to limit the observation of these steps.