Dynamic instabilities and memory effects in vortex matter

The magnetic flux line lattice in type II superconductors serves as a usefu l system in which to study condensed matter flow, as its dynamic properties are tunable. Recent studies have shown a number of puzzling phenomena asso ciated with vortex motion, including: low-frequency noise(1-5) and slow vol tage oscillations(3,6); a history-dependent dynamic response(7-12), and mem ory of the direction, amplitude duration and frequency of the previously ap plied current(13,14); high vortex mobility for alternating current, but no apparent vortex motion for direct currents(13,15,16); and strong suppressio n of an a.c. response by small d.c. bias(13). Taken together, these phenome na are incompatible with current understanding of vortex dynamics. Here we report a generic mechanism that accounts for these observations. Our model, which is derived from investigations of the current distribution across si ngle crystals of NbSe2, is based on a competition between the injection of a disordered vortex phase at the sample edges, and the dynamic annealing of this metastable disorder by the transport current. For an alternating curr ent, only narrow regions near the edges are in the disordered phase, while for d.c. bias, most of the sample is in the disordered phase-preventing vor tex motion because of more efficient pinning. The resulting spatial depende nce of the disordered vortex system serves as an active memory of the previ ous history.