Fluctuations in single-crystal YBa2Cu3O6.5: Evidence for crossover from two-dimensional to three-dimensional behavior

Abstract

Magnetization measurements as a function of temperature are reported for YBa2Cu3O6.5 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(star) similar or equal to 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 (20) scaling" in the fluctuation regime. Surprisingly, the low-field curves also intersect, at T-3D(star) similar or equal to 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.