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 2 H-NbSe2 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 downward sweeps. The difference between the two has a double-peak feature, indicative of two-stage disordering of a vortex lattice. The resistance 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. @S0163-1829~98!51106-X# Pinning of Abrikosov vortex lines by disorder makes type-II superconductors useful in carrying electrical current with minimal loss in strong magnetic fields. Thermally activated vortex creep leads to reduction in critical current, 1 significant especially in high-Tc superconductors. 2 It was thus a surprise that the critical current in very clean high- T c superconducting YBa2Cu3O72d crystals was found to increase sharply with increasing temperature just before it vanishes. 3 It turns out that this phenomenon, known as the peak effect, occurs in many low-Tc type-II superconductors. 4,5 Although rarely discussed in the textbooks of superconductivity, the pursuit of its origin led to much improved understanding of the problems of random pinning, notably the collective pinning model of Larkin-Ovchinnikov. 6,7