Negative dynamic creep in the peak-effect regime in type-II superconductors

We report the observation of a negative dynamic creep rate, i.e., increasing irreversibility in the magnetic hysteresis loop for decreasing sweep rate of the magnetic field, in both YBa2Cu3O72d and 2H-NbSe2 single crystals. This phenomenon is found to appear on the increasing branch of the peak effect that corresponds to a state which is intermediate between the dislocation-free Bragg glass and a highly disordered vortex phase. The origin of this anomalous creep is shown to be connected to a negative differential resistance resulting from the N-like shape of the current-voltage characteristics.