IRREVERSIBILITY AND FLUX-LATTICE MELTING IN YBA2CU3O7-DELTA

We report on ac susceptibility measurements using a local Hall probe c lose to the melting point of the flux lattice in a clean, detwinned YB a2Cu3O7-delta (YBCO) single crystal. The critical current density, pro bed at a voltage level of 10(-11) V, abruptly jumps from a null value in the liquid state to a small but finite value in the solid state. Th e resulting irreversible magnetization is below the resolution level o f standard superconducting quantum interference device magnetometry. T he temperature T-irr where the critical current jumps coincides with t he melting temperature T-m as defined by de magnetization. The above m easurements establish a clear coincidence between the irreversibility line and the melting line in clean YBCO. The pinning force for the as- formed Bur solid is shown to increase with its density. A critical end point is observed above a threshold value of the pinning force.