INPLANE AND OUT-OF-PLANE VORTEX CORRELATIONS IN YBA2CU3O7

Transport measurements in the mixed state of YBa2Cu3O7 crystals (Hparallelc) using the ''dc-flux-transformer'' configuration show that the glass-liquid phase transition at the irreversibility line, T(i)(H), behaves as a three-dimensional vortex phase transition. These results together with resistance measurements in the c direction show that the vortex correlation across the sample is destroyed at a temperature T(th)(H), higher than T(i)(H). However, the dependence of T(th)(H) on sample thickness suggests that, in real thermodynamic phase transitions (infinite sample), the long-range correlation in the ab plane, as well as in the c direction, is lost at the single temperature T(i)(H).