Evidence for low-field crossover in the vortex critical velocity of type-II superconducting thin films

We measure current-voltage characteristics as function of magnetic field and temperature in Nb strips of different thickness and width. The instability voltage of the flux-flow state related to the vortex critical velocity v* is studied and compared with the Larkin-Ovchinnikov theory. Beside the usual power-law dependence v* approximate to B-1/2, in the low-field range a new crossover field, B-cr1, is observed below which v* decreases by further lowering the external magnetic field B. We ascribe this unexpected crossover to vortex channeling due to a fanlike penetration of the applied magnetic field as confirmed by magneto-optic imaging. The observation of B-cr1 becomes a direct evidence of a general feature in type-II superconducting films at low fields that is a channel-like vortex motion induced by the inhomogeneous magnetic state caused by the relatively strong pinning.