Scaling of the Equilibrium Magnetization in the Mixed State of Type-II Superconductors

We discuss the analysis of mixed-state magnetization data of type-II superconductors using a recently developed scaling procedure. It is based on the fact that, if the Ginzburg-Landau parameter kappa does not depend on temperature, the magnetic susceptibility is a universal function of H/H_c2(T), leading to a simple relation between magnetizations at different temperatures. Although this scaling procedure does not provide absolute values of the upper critical fieldH_c2(T), its temperature variation can be established rather accurately. This provides an opportunity to validate theoretical models that are usually employed for the evaluation of H_c2(T) from equilibrium magnetization data. In the second part of the paper we apply this scaling procedure for a discussion of the notorious first order phase transition in the mixed state of high temperature superconductors. Our analysis, based on experimental magnetization data available in the literature, shows that the shift of the magnetization accross the transition may adopt either sign, depending on the particular chosen sample. We argue that this observation is inconsistent with the interpretation that this transition always represents the melting transition of the vortex lattice.Comment: 18 pages, 12 figure

Correlation of flux lines in single-crystal Bi

A detailed study has been made of the irreversibility line in Bi2Sr2CaCu2O8, both for unirradiated crystals and also for samples containing irradiation-induced columnar defects aligned at either 0°or 45°to the c axis. For the unirradiated reference sample the proposed scaling rules for the case of isotropic pinning provide an excellent description of the data. In contrast, for the case of the irradiated samples the breakdown of the scaling due to the presence of anisotropic pinning provides a powerful method with which to trace the development of anisotropic pinning due to correlated disorder. For fields below the matching field B the existence of strong anisotropic pinning is demonstrated, while for BB pinning is shown to be isotropic. A characteristic temperature T1 is identified, above which the effect of anisotropic pinning is strongly reduced due to increased thermally induced delocalization of the vortices away from the columnar defects.</p