We report on ac susceptibility measurements using a local Hall probe close to the melting point of the flux lattice in a clean, detwinned ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ (YBCO) single crystal. The critical current density, probed at a voltage level of ${10}^{\mathrm{-}11}$ 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 ${\mathrm{T}}_{\mathrm{irr}}$ where the critical current jumps coincides with the melting temperature ${\mathrm{T}}_{\mathrm{m}}$ 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.

DOI:https://doi.org/10.1103/PhysRevB.55.R14753