Melting and dimensionality of the vortex lattice inYBa2Cu3O6.60

Muon spin rotation $(\ensuremath{\mu}\mathrm{SR})$ measurements of the magnetic-field distribution $n(B)$ in the vortex state of the oxygen deficient high-${T}_{c}$ superconductor ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{6.60}$ reveal a vortex-lattice melting transition at much lower temperature than that in the fully oxygenated material. The transition is best described by a model in which adjacent layers of ``pancake'' vortices decouple in the liquid phase. In some samples we also observe changes in $n(B)$ which are consistent with a vortex lattice which becomes disordered along the field direction above a crossover field ${B}_{\mathrm{cr}}.$ A transition of this nature has previously been reported in the highly anisotropic superconductor ${\mathrm{Bi}}_{2+x}{\mathrm{Sr}}_{2\ensuremath{-}x}{\mathrm{CaCu}}_{2}{\mathrm{O}}_{8+y}.$