Fluctuations in single-crystal YBa₂Cu₃O_6.5: Evidence for crossover from two-dimensional to three-dimensional behavior

Magnetization measurements as a function of temperature are reported for YBa₂Cu₃O_6.5 crystal (T_c = 45.2 K) for fields between 0.2 and 3.5 T. All isochamps for H > 1 T intersect at T*_2D ≃43.4 K, implying a fluctuation contribution to the magnetization. These curves collapse into a single curve when magnetization and temperature are scaled according to the predicted “two-dimensional (2D) scaling” in the fluctuation regime. Surprisingly, the low-field curves also intersect, at T*_3D≃43.4 K, and they obey a 3D scaling. We provide a theoretical picture of the magnetization in the fluctuation regime based on the Lawrence-Doniach model. Within this model we calculate the field and temperature dependence of the magnetization. The two intersection points and the 2D^→3D crossover are consistent with the experimental observation.