Phase diagrams of La2-xSrxCuO4 and YBa2Cu3O{6+/delta} as the keys to   understanding the nature of high-Tc superconductors
  The model that explains many of the details of superconducting and magnetic phase diagrams of YBa2Cu3O{6+/delta} and La2-xSrxCuO4 is presented. The model is based on the assumption of rigid localization of doped charges in the close vicinity of doped ion. This localization results in local variation of electronic structure of the parent charge-transfer insulator that depends on the local mutual arrangement of the doped charges. It is shown that in such system the negative-U centers (NUCs) may form under certain conditions on the pairs of neighboring Cu cations in CuO2 plane. We consider the mechanism of hole generation. The calculated dependences of hole concentration in YBa2Cu3O{6+/delta} on doping and temperature are found to be in a perfect quantitative agreement with experimental data. In the framework of the model the phase diagrams of YBa2Cu3O{6+/delta} is considered and the interpretation of pseudogap and 60 K-phases in YBa2Cu3O{6+/delta} is offered. The pseudogap has superconducting nature and arises at temperature T*>Tc/inf>Tc in small clusters uniting a number of NUCs due to large fluctuations of NUC occupation. (Tc/inf and Tc are the temperatures of superconducting transition for infinite and finite clusters, accordingly). The calculated T*(/delta) and Tc(/delta) dependences are in accordance with experiment. The range between T*(/delta) and Tc(/delta) corresponds to the range of fluctuations where small clusters fluctuate between superconducting and normal states owing to fluctuations of NUC occupation. The phase diagram of La2-xSrxCuO4 is calculated. It is shown the features of the superconducting phase diagram and the characteristics of stripe textures of La2-xSrxCuO4 only reflect the geometrical relations existing in a square lattice and the competition of different types of dopant ordering. 
K.V. Mitsen, O.M. Ivanenko