Evidence for Kosterlitz-Thouless and three-dimensional XY critical behavior in Bi2Sr2CaCu2O8+delta

We present reversible magnetization data of a high-quality Bi2Sr2CaCu2O8+delta single crystal and explore the occurrence of three-dimensional (3D)-xy critical behavior close to the bulk-transition temperature T-c and of Kosterlitz-Thouless (KT) behavior. Below and above the presumed Kosterlitz-Thouless transition temperature T-KT we observe the characteristic two-dimensional (2D)-xy behavior: a downward shift in the crossing point phenomenon toward T-KT as the field is decreased and sufficiently below T-KT the characteristic 2D-xy relationship between the magnetization and the in-plane magnetic penetration depth lambda(ab). In contrast, the measured temperature dependence of the superfluid density does not exhibit the characteristic KT behavior around the presumed T-KT. The absence of this feature is traced back to the 2D- to 3D-xy crossover setting in around and above T-KT. Invoking the Maxwell relation, the anomalous field dependence of the specific-heat peak is also traced back to the intermediate 2D-xy behavior. However, close to T-c we observe consistency with 3D-xy critical behavior in agreement with measurements of lambda(ab).