Local pinning-force distribution in NbN films

The authors performed measurements of the flux-flow resistivity on a thin film of NbN. By fitting the resistivity, measured as a function of the current, to a theoretical expression, they were able to obtain the distribution function of local depinning forces in the film. It appears that the distribution is very broad and that only a minor fraction of flux lines is flowing at what is usually defined as J _c. These observations are in agreement with the results of the pinning-force measurements. The measurements reveal that at high fields pinning is limited by flux-line shear and that the effective channel width for shear is equal to the flux-line spacing. They suggest that variations of the local depinning force on various length scales always allow the flow of single rows of flux lines