CRITICAL-BEHAVIOR AND ANISOTROPY OF HIGH-TEMPERATURE SUPERCONDUCTORS

The weak pinning and relatively large flux creep in extended regions of the mixed state and even at low temperatures are intimately related to the vortex lattice anisotropy. This vortex lattice anisotropy itself is a result of the layered structure of the oxidic cuprates and is very large for the Bi and Tl compounds. In this contribution we discuss some of the characteristics of the field-temperature phase diagram as appropriate for Bi2Sr2CaCu2O8. Collective flux pinning and flux creep in case oxygen vacancies are the dominant pinning centers, are briefly reviewed. Finally, magnetization measurements versus temperature or field in the reversible vortex liquid regime about T(c) are presented. They show clear evidence for strong two-dimensional diamagnetic fluctuations and behaviour deviating from the well-known mean-field (Abrikosov) characteristics.