Irreversibility and flux-lattice melting

Abstract

We report on ac susceptibility measurements using a local Hall probe close to the melting point of the flux lattice in a clean, detwinned (Formula presented)(Formula presented)(Formula presented) (YBCO) single crystal. The critical current density, probed at a voltage level of (Formula presented) 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 (Formula presented) where the critical current jumps coincides with the melting temperature (Formula presented) as defined by dc 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 flux solid is shown to increase with its density. A critical end point is observed above a threshold value of the pinning force. © 1997 The American Physical Society.