Explanation of the phase diagram of high-temperature superconductors in terms of the model of negative-U centres superconductivity

This paper demonstrates qualitatively how a unified explanation of the wide variety of different regions in a typical phase diagram of high-temperature superconductors (HTSC) can be given in terms of the model of negative-U centres superconductivity. The existence of four regions (pseudogap, superconducting, non-Fermi liquid and Fermi liquid) in the phase diagram and all the transitions between these regions can be explained by assuming that negative-U centres exist in HTSC and qualitatively taking into account their thermodynamic and direct quantum-mechanical interaction with ordinary electrons from the valence band. It is found that the principal parameter determining the properties of HTSC in the negative-U centres model is the relative concentration of electrons belonging to the negative-U centres. The negative-U centres naturally determine both the superconducting properties of HTSC systems and the entire phase diagram in the normal state