Influence of disorder on the irreversibility line and vortex transition intwin-free and twinned RBa2Cu3O7-delta (R = Y, Sm, Nd) single crystals

The irreversibility line, the vortex solid-liquid transition, and the peak effect of the current are studied as a function of disorder due to oxygen r eduction and Nd/Ba substitution in RBa2Cu3O7-delta (R=Y, Sm, Nd). In twinne d crystals the transition is obtained from the steplike dependence of the i rreversible magnetic moment vs field. The temperature dependence of this fi eld is in agreement with the melting line and the result does not depend on the electric field used for the measurement. In twinned and twin-free crys tals the upper critical point decreases similarly with increasing random di sorder but the evolution of the peak effect is completely different. A soli d-solid transition from a quasilattice below the current peak into a highly disordered vortex state above is in some qualitative agreement with result s from the twin-free crystal. [S0163-1829(99)10933-0].