Abstract
The origin of the frictional force between pinning sites and moving fluxons is investigated. It is shown that the elastic distortion introduced by the pinning sites gives rise to a frictional force which in general depends on the velocity of the fluxons. If the current is appreciably greater than the critical current, however, the frictional force is essentially independent of velocity (in agreement with experiment) unless the current is extremely large, in which case the frictional force vanishes. The experimental fact that the frictional force is of the same order of magnitude as phi oJc implies that the critical current Jc depends on the elastic constants of the fluxons as well as on the pinning forces.