Fluctuations in $YB{a}_{2}C{u}_3{O}_{6.5}$ single crystal: an evidence for 2D->3D crossover

Magnetization measurements as a function of temperature are reported for $YB{a}_{2}C{u}_3{O}_{6.5}$ crystal (T_{c}=45.2 K) for fields between 0.2 to $3.5
Tesla$\mathrm{.}AllisochampsforH>1Teslaintersectat$T_{2D}^{\star}\simeq 42.8
K$, implying fluctuations contribution to the magnetization. These curves collapse into a single curve when magnetization and temperature are scaled according to the predicted ‘‘2D scaling’’ in the fluctuation regime. Surprisingly, the low field curves also intersect, at $T_{3D}^{\star}\simeq 43.4 K$, 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 $2D\to 3D$ crossover are consistent with the experimental observation.