Negative U centers, percolation, and the insulator-metal transition in high-T c superconductors

The mechanism of formation of ?U centers in high-T _c superconductors (HTSCs) is considered. It is shown that the transition from the insulator to the metallic state on doping passes through a certain range of dopant concentrations in which it becomes possible for local transitions of singlet electron pairs to occur from oxygen ions to two neighboring cations (a ?U center), while single-electron transitions are still forbidden. Conduction arises in such systems at a concentration of ?U centers exceeding the percolation threshold for the orbitals of singlet hole pairs. A phase diagram constructed on the basis of the proposed model for the HTSC compounds of the Ln-214 class is in complete agreement with experiment. The mechanisms of formation and relaxation of free hole carriers are considered. It is shown that a distinctive feature of the normal state of HTSCs is the dominant contribution of electron-electron scattering to the charge carrier relaxation processes. It is concluded from the analysis presented that HTSCs comprise a special class of solids in which a nonstandard mechanism of superconductivity, different from the BCS mechanism, is realized.