Equilibrium and driven vortex phases in the anomalous peak effect

We report experimental tests of the present models of the peak effect in we akly disordered type-II superconductors. Our results favour the scenario in which the peak effect arises from a crossover between the Larkin pinning l ength and a rapidly falling elastic length in a Vortex phase populated with thermally excited topological defects. A thickness dependence study of the onset of the peak effect at varying driving currents suggests that both sc rew and edge dislocations are involved in the vortex lattice disordering. T he driven dynamics in three-dimensional samples are drastically different f rom those in two-dimensional samples. We suggest that this may be a consequ ence of the absence of a Peierls potential for screw dislocations in a thre e-dimensional vortex line lattice.