In- and out-of-plane vortex correlations in ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$

Transport measurements in the mixed state of ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_7$ crystals (H∥c) using the ‘‘dc-flux-transformer’’ configuration show that the glass-liquid phase transition at the irreversibility line, ${\mathit{T}}_{\mathit{i}}$(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 ${\mathit{T}}_{\mathrm{th}}$(H), higher than ${\mathit{T}}_{\mathit{i}}$(H). However, the dependence of ${\mathit{T}}_{\mathrm{th}}$(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 ${\mathit{T}}_{\mathit{i}}$(H).