Fluctuations in single-crystal YBa2Cu3O6.5: Evidence for crossover from two-dimensional to three-dimensional behavior
Magnetization measurements as a function of temperature are reported for YBa2Cu3O6.5 crystal (Tc=45.2 K) for fields between 0.2 and 3.5 T. All isochamps for H>1 T intersect at T*2D~=42.8 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.