Melting and dimensionality of the vortex lattice in ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{6.60}$

Muon spin rotation $\left(\mu \mathrm{SR}\right)$ measurements of the magnetic-field distribution $n\left(B\right)\mathrm{}$ 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 $n\left(B\right)\mathrm{}$ 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 ${\mathrm{Bi}}_{2+x}{\mathrm{Sr}}_{2-x}{\mathrm{CaCu}}_2{\mathrm{O}}_{8+y}.$