Doping states in a two-dimensional three-band Peierls-Hubbard model are investigated with inhomogeneous Hartree-Fock and random phase approximations. They are sensitive to small changes of electron-lattice and electron-electron interactions. For parameters relevant to the insulating copper oxides a small ferromagnetic polaron is found. Moderate intersite electron-lattice coupling triggers a nonlinear feedback mechanism resulting in a rapid crossover from a Zhang-Rice regime to a covalent molecular singlet state in which the local magnetic moment is quenched and local lattice distortion is large. Various states are characterized by distinct optical and infrared absorption spectra.

• Received 10 December 1991

DOI:https://doi.org/10.1103/PhysRevLett.69.965