Three-dimensional lowest-Landau-level theory applied to magnetization and specific heat data: Implications for the critical behavior in the plane

We study the applicability of magnetization and specific heat equations derived from a lowest-Landau-level (LLL) calculation, to the high-temperature superconducting (HTSC) materials of the (Formula presented) (YBCO) family. We find that significant information about these materials can be obtained from this analysis, even though the three-dimensional LLL functions are not quite as successful in describing them as the corresponding two-dimensional functions are in describing data for the more anisotropic HTSC Bi- and Tl-based materials. The results discussed include scaling fits, evidence for the correspondence between the onset of (Formula presented) (or LLL) fluctuations and the flux lattice melting transition, and reasons why three-dimensional (3D) (Formula presented) scaling may have less significance than previously believed. We also demonstrate how 3D (Formula presented) scaling does not describe the specific heat data of YBCO samples in the critical region. Throughout the paper, the importance of fitting the actual functions to the data, in contradistinction to checking for scaling behavior, is stressed.