In- and out-of-plane vortex correlations in YBa2Cu3O7

Transport measurements in the mixed state of YBa2Cu3O7 crystals (Hc) using the ''dc-flux-transformer'' configuration show that the glass-liquid phase transition at the irreversibility line, Ti(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 Tth(H), higher than Ti(H). However, the dependence of Tth(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 Ti(H).