Irreversibility and flux-lattice melting in YBa2Cu3O7-delta

We report on ac susceptibility measurements using a local Hall probe close to the melting point of the flux lattice in a clean, detwinned YBa2Cu3O7-delta (YBCO) single crystal. The critical current density, probed 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. The resulting irreversible magnetization is below the resolution level of standard superconducting quantum interference device magnetometry. The temperature T-irr where the critical current jumps coincides with the melting temperature T-m as defined by de magnetization. The above measurements 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.