Peak effect and vortex phase diagram in twin-free YBa2Cu3O7-delta single crystals

The peak effect of the current density j vs. magnetic field B and its relat ion to the vortex phase diagram is investigated in a twin-free YBa2Cu3O7-de lta single crystal. The fully oxidized crystal without any j(B)peak and wit h a first-order melting transition of the vortex lattice was subsequently o xygen-reduced, The solid/liquid transition, the upper critical point, the e volution of the peak, and the transition from the lattice below the peak in to the amorphous state above are investigated as a function of random disor der from oxygen reduction. The influence from disorder and thermal fluctuat ion on the lattice/glass transition in the solid vortex state is compared w ith the model of field-driven introduced dislocation into the vortex solid. (C) 2000 Elsevier Science B.V. All rights reserved.