Commensurability effects and fluctuations of vortex flow in mesoscopic channels

We investigate vortex flow confined to mesoscopic, weak pinning channels in a NbN/Nb3Ge double layer. In the limit where the channels are sufficiently narrow, the flow behavior is profoundly affected by commensurability betwe en the channel width and the vortex spacing. We explore both static and dyn amic features of the resulting ordered and disordered vortex configurations by measuring the mean flow velocity as well as the velocity fluctuations. For fixed drive, we find distinct minima in the flow velocity, which we can associate with vortex lattices consisting of an integer number of parallel vortex rows inside a channel. These flow minima correspond to peaks in the depinning current, i.e., the static yield strength. To compare the dynamic response of the commensurate and the incommensurate configurations, we mea sure the low-frequency voltage noise resulting from slow rearrangements in the pinning landscape along the channel walls. (C) 2000 Elsevier Science B. V. All rights reserved.