Fluctuations in single-crystalYBa2Cu3O6.5:Evidence for crossover from two-dimensional to three-dimensional behavior

Abstract

Magnetization measurements as a function of temperature are reported for ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{6.5}$ crystal ${(T}_{c}=45.2 \mathrm{K})$ for fields between 0.2 and 3.5 T. All isochamps for $H>1$ T intersect at ${T}_{2D}^{\ensuremath{\star}}\ensuremath{\simeq}42.8$ 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 ${T}_{3D}^{\ensuremath{\star}}\ensuremath{\simeq}43.4$ 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 $2\stackrel{\ensuremath{\rightarrow}}{D}3\mathrm{D}$ crossover are consistent with the experimental observation.