  Recent studies have shown a number of surprising vortex dynamics phenomena
both in low and high temperature superconductors, which include: low frequency
noise, slow voltage oscillations, history dependent dynamic response, memory of
the direction, amplitude, duration, and frequency of the previously applied
current, suppression of a large ac vortex response by a very small dc bias, and
a strong frequency dependence. Taken together, these phenomena are incompatible
with the current understanding of bulk vortex dynamics. We propose a generic
mechanism to account for these observations in terms of the competition between
the injection of a disordered vortex phase through the surface barriers at the
sample edges, and the annealing of this metastable disorder by the transport
current. The model is confirmed by investigating the current distribution
across NbSe2 single crystals using arrays of Hall sensors. For an ac current
only narrow regions near the edges are in the pinned disordered phase resulting
in a large response. In the presence of a dc bias a wide region of the sample
is filled by the disordered phase preventing vortex motion. The resulting
spatial variation of the disorder across the sample acts as an active memory of
the previously applied current sequence.
