IRREVERSIBILITY LINE IN DEOXYGENATED YBA2CU3O7-DELTA SINGLE-CRYSTAL

We have measured the AC susceptibility of a YBa<INF>2</INF>Cu<INF>3</I NF>O<INF>7-delta single crystal, and determined the irreversibility li ne through the dissipation peak in the imaginary component chi '' The sample investigated has a transition temperature T?c approximate to 62 .8 K, and transition width less than 1.0 K. At low fields we may fit t he results with a typical power law of the type (1-T/Tc</INF><SUP>n</S UP>, with n = 1.35, whereas at higher fields the data is better adjust ed with 1/T. We observed a clear change in behavior of the IL at H<INF >cr</INF> = 9.0 kOe, which characterizes two different regimes related to the dimensionally of the vortex fluctuations. This could be interp reted as a transition from three dimensional-vortex lines to two-dimen sional pancake-like vortices. The higher anisotropy of this system, as compared with a fully oxygenated crystal, favors the decoupling of th e vortices in adjacent CO<INF>2</INF> planes.