Vortex dynamics below the flux-lattice melting transition in ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$

We report measurements of the critical current, differential resistance, and voltage noise below the vortex-lattice melting transition in clean, untwinned ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ crystals. The differential resistance at dc current bias much greater than ${\mathit{J}}_{\mathit{c}}$ shows a sharp feature at the melting transition accompanied by a rapid drop in the voltage noise, demonstrating that the melting transition is insensitive to the static pinning potential. In the solid phase, the measurements all show a marked change from the low-field, pinned lattice to the high-field vortex-glass contiguous with the tricritical point in this sample.