ABSTRACT
The key question about high-temperature superconductivity is the variation of the critical temperature TC with the material characteristics. In the recent years, several models have been proposed in order to explain the high value of TC. Even though these models are not unanimously agreed upon within the scientific community, it is not less true that they partly predict the variations of certain parameters. The most realistic theories seem to be those which refine or modify the already existing classical theories (London model, Ginzburg-Landau theory, BCS theory). Of the many surprising facts which have appeared with these new materials, some are1: an unusual irreversibility line in the H-T plane with a possible fusion of the vortices lattice above this line, an enormous non exponential relaxation time of the magnetic properties and its ageing effects, a disagreement in the estimation of the critical current densities, an almost exponential decrease of the critical current density as a function of the temperature and the field, an anomaly in the profile of the variation of the susceptibility as a function of the field, an unforeseen widening of the resistive transition under field and many others. These properties, which are the starting point of the present and future interpretations, require a full and detailed knowledge of the structure of these materials. Indeed, although the fundamental interactions which make superconductors of these compounds remain to this day little known, their structural characteristics seem to play a crucial role in the appearance of this phenomenon because almost all of these new materials contain copper oxide planes alternated by insulating or weakly metallic layers. Below the superconducting transition temperature, these planes turn into ideal propagation ways for the charge carriers: the two-dimensionality of the crystalline structure and the mixed valence of the copper appear to be at the origin of the superconducting character of these materials.
