NATURE OF VORTEX LATTICE DISORDERING AT THE ONSET OF THE PEAK EFFECT

We report a transport study of the onset of the peak effect in a 2H-Nb Se2 single crystal with very weak pinning. At low driving current, we find that at the onset of the peak effect, where the resistance starts to dip with increasing field, the resistance is hysteretic upon field cycling. The resistance is higher for upward field sweeps than for do wnward sweeps. The difference between the two has a double-peak featur e, indicative of two-stage disordering of a vortex lattice. The resist ance hysteresis and the two-stage feature disappear after cold-working the sample. The onset field of the peak effect is found to depend on driving current nonmonotonically, suggesting reentrant driven disorder -order and order-disorder transitions in the peak-effect regime.