THE PEAK EFFECT AS A PHASE-BOUNDARY IN DETWINNED YBA2CU3O7-DELTA

We present magneto-resistance data from a high quality detwinned YBa2C u3O7-delta single crystal, taken using the conventional four-point met hod with the current applied along the ab plane and the field along th e c-axis. Measurements were taken in fields of up to 12T. At temperatu res just below the melting line, a sharp feature (a 'peak effect') app eared in the resistance when sufficiently high current densities were applied. We saw this feature as both a function of temperature, rho(T) , and field, rho(H). At higher fields, above the multi-critical point H=6T, the peak effect in rho(H) splits into two and spreads apart in field. This forms two branches in the H-T phase diagram, and their imp lications for the dynamics of the vortices are discussed.