Coincidence of Vortex-Lattice Melting and Loss of Vortex Correlation along the $\mathit{c}$ Direction in Untwinned ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{O}_{7-\delta }$ Single Crystals

We report on transport measurements using the dc flux transformer configuration in clean and twinned single crystals of ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7-\delta }$. We show that correlated disorder changes not only the order of the solid-liquid phase transition but also the nature of the vortices in the liquid state. In clean crystals, melting is shown to take place between a solid of vortex lines and a liquid with no correlation in the direction of the magnetic field.