Scaling behavior of the derivatives of the specific heat of YBa2Cu3O6.93 at the superconducting transition up to 16 tesla

The scaling properties of the specific heat of the high-T-c superconductor YBa2Cu3O6.93 (YBCO) are studied. We used a large crystal (0.29 g) at optimal doping. The measurements are performed with a high-resolution (similar to 10(-4)) quasi-adiabatic continuous heating type calorimeter in magnetic fields up to 16 T. Rather than scaling the electronic specific heat, we scale the derivatives versus field and versus temperature of the total specific heat. This new approach is less sensitive to arbitrary background subtraction and/or normalization. It is found unexpectedly that the low-field 3D-XY model is valid at high fields for H > 1 T, and that the lowest Landau level (3D-LLL) approximation is a poor description if H-c2(T) is taken to be linear in T. But if we take into account the experimentally observed curvature of H-c2(T) proportional to \T - T-c\(4/3), then 3D-LLL scaling is also satisfied for H > 1 T. With this modification, both descriptions are valid in the same range of fields. Measurements of the magnetization confirm this result. The shape of the scaling function agrees with that predicted in the modified 3D-LLL approximation.