Temperature- and field-dependence of critical currents in NbN microbridges

Abstract

We report measurements of the critical current of nano- and micrometer-sized NbN bridges as a function of temperature and magnetic eld. The bridges were fabricated from 4 to 10 nm thick, sputtered NbN lms using standard photo- and e-beam lithography. The detailed temperature- and eld-dependence is strongly influenced by the bridge width. We interpret our data taking into account geometrical edge barriers for vortex entry and conclude that submicrometer wide bridges remain free of vortices in zero-eld and measured critical currents are the depairing currents. The situation is much more complex inmicrometer-wide bridges. For certain conditions such bridges may even show non-monotonous dependence of the critical current on temperature. We develop a qualitative model that explains our main observations.