3-DIMENSIONAL LOWEST-LANDAU-LEVEL THEORY APPLIED TO YBA2CU3O7-DELTA MAGNETIZATION AND SPECIFIC-HEAT DATA - IMPLICATIONS FOR THE CRITICAL-BEHAVIOR IN THE H-T PLANE

We study the applicability of magnetization and specific heat equation s derived from a lowest-landau-level (LLL) calculation, to the high-te mperature superconducting (HTSC) materials of the YBa2Cu3O7-delta (YBC O) 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 co rresponding two-dimensional functions are in describing data for the m ore anisotropic HTSC Bi- and Tl-based materials. The results discussed include scaling fits, evidence for the correspondence between the ons et of H-c2 (or LLL) fluctuations and the fur lattice melting transitio n, and reasons why three-dimensional (3D) XY scaling may have less sig nificance than previously believed. We also demonstrate how 3D XY scal ing does not describe the specific heat data of YBCO samples in the cr itical region. Throughout the paper, the importance of fitting the act ual functions to the data, in contradistinction to checking for scalin g behavior, is stressed.