LOCALIZED (POLARONIC) CHARGE-TRANSFER EXCITATIONS IN CuO2 LAYERS

We show that the recently found polarons and excitons of charge-transfer origin in the 1D p-d model of high temperature superconductors can also occur in 2D. We use a 2D spinless model which is expected to qualitatively describe the charge degrees of freedom of the p-d model in the limit of infinite on-site Coulomb repulsion on both Cu and O. The Cu-O repulsion Upd is treated by an unrestricted Hartree-Fock scheme. We found that in addition to polarons and excitons, other nonlinear excitations like bipolarons and clustering of carriers (phase separation) arise as well. Since the energies of all these excitations are very close to each other at the mean field level, no definite statement can be made about the true ground state.