Coincidence of vortex-lattice melting and loss of vortex correlation along the c direction in untwinned YBa₂Cu₃O_7−δ Single Crystals

We report on transport measurements using the dc flux transformer configuration in clean and twinned single crystals of YBa₂Cu₃O_7−δ. 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.