-U-centers, percolation and metal-dielectric transition in HTSC

The mechanism of the -U-centers formation in the HTSC is considered. It is shown, that transition from the dielectric to metallic state by doping proceeds through a certain range of doping concentrations, whereby local transitions of singlet electron pairs from the oxygen ions on two neighbouring cations (-U-center) become possible. The conductivity in such a system occurs by the -U-centers concentration exceeding the percolation threshold for the singlet hole pairs orbitals. The phase diagram for the HTSC compounds of the Ln-214 class is plotted, which is in complete accordance with the experiment. The mechanisms of free hole carriers formation and relaxation are considered. It is shown, that the prevailing contribution of the electron-electron scattering into the charge carriers relaxation processes is a characteristic peculiarity of the HTSC normal state. The conclusion is made, that the HTSC constitute a special class of solid bodies, wherein the nonstandard superconductivity mechanism is realized