Melting and dimensionality of the vortex lattice in YBa2Cu3O6.60

Muon spin rotation (mu SR) measurements of the magnetic-field distribution n(B) in the vortex state of the oxygen deficient high-T-c superconductor YB a2Cu3O6.60 reveal a vortex-lattice melting transition at much lower tempera ture than that in the fully oxygenated material. The transition is best des cribed by a model in which adjacent layers of "pancake" vortices decouple i n the Liquid phase. In some samples we also observe changes in n(B) which a re consistent with a vortex lattice which becomes disordered along the fiel d direction above a crossover held B-cr. A transition of this nature has pr eviously been reported in the highly anisotropic superconductor Bi2+xSr2-xC aCu2O8+y.