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

We report measurements of the critical current of nano- and micrometer-sized NbN bridges as a function of temperature and magnetic field. The bridges were fabricated from 4 to 10 nm thick, sputtered NbN films using standard photo- and e-beam lithography. The detailed temperature- and field-dependence is strongly influenced by the bridge width. We interpret our data taking into account geometrical edge barriers for vortex entry and conclude that sub-micrometer wide bridges remain free of vortices in zero-field and measured critical currents are the depairing currents. The situation is much more complex in micrometer-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