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

We report the observation of a negative dynamic creep rate, i.e., increasin g irreversibility in the magnetic hysteresis loop for decreasing sweep rate of the magnetic field, in both YBa2Cu3O7-delta and 2H-NbSe2 single crystal s. This phenomenon is found to appear on the increasing branch of the peak effect that corresponds to a state which is intermediate between the disloc ation-free Bra,ag glass and a highly disordered vortex phase. The origin of this anomalous creep is shown to be connected to a negative differential r esistance resulting from the N-like shape of the current-voltage characteri stics.