Experimental evidence for a three-dimensional-two-dimensional crossover for magnetic fields parallel to the ab plane in oxygen-deficient single crystals of YBa2Cu3O7-δ

Resistive measurements on oxygen-deficient YBa2Cu3O7-delta single crystals of varying anisotropy gamma (22 less than or equal to gamma less than or equal to 36) have been used for investigations of the transition into a glassy vortex state for magnetic fields 0 less than or equal to B less than or equal to 12 T parallel to the ab plane. When B is accurately aligned parallel to the nb plane there is a remarkable change in behavior to a field-independent vortex solid-to-liquid transition at high magnetic fields. A possible explanation is suggested for the vertical transition involving a complete decoupling of the superconducting planes that leads to a two-dimensional system where the resistivity is given by nucleation of vortex-antivortex pairs.