A Comparative Study of High-Field Diamagnetic Fluctuations in Deoxygenated YBa2Cu3O7-x and Polycrystalline (Bi–Pb)2Sr2Ca2Cu3O10

Abstract

We studied three single crystals of YBa2Cu3O7-x, (Y123) with superconducting transition temperature, Tc=62.5, 52, and 41 K, and a highly textured polycrystalline specimen of (Bi–Pb)2Sr2Ca2Cu3O10 (Bi2223), with Tc=108K. Isofield magnetization data were obtained as a function of temperature, with the magnetic field applied parallel to the c axis of each sample. The reversible magnetization data for all samples exhibited a rounded transition as magnetization tended toward zero. The reversible data were interpreted in terms of two-dimensional diamagnetic lowest-Landau-level (LLL) fluctuation theory. The LLL scaling analysis yielded consistent values of the superconducting transition temperatures Tc(H) for the various samples. The resulting scaling data were fit well by the two-dimensional LLL expression for magnetization obtained by Tesanovic and colaborators, producing reasonable values of κ but the fitting parameter ∂Hc2/∂T produced values that were larger than the experimentally determined ones. We performed simultaneous scaling of Y123 data and Bi2223, obtaining a single collapsed curve. The single curve was obtained after multiplying the x and y axis of each scaling curve by appropriate sample-dependent scaling factors. An expression for the two-dimensional x-axis LLL scaling was extracted from theory, allowing comparison of theoretical values of the x-axis scaling factors with the experimental values. The comparison between the values of the x-axis produced a deviation of 40% which suggests that the hypothesis of universality of the two-dimensional LLL fluctuations is not supported by the studied samples. We also observe that Y123 magnetization data for temperatures above Tc obbey a universal scaling obtained for the diamagnetic fluctuation magnetization from a theory considering non-local field effects. The same scaling was not obbeyed by the corresponding magnetization calculated from the two-dimensional LLL theory.