Charge-transfer polarons and excitons

We show that polarons and excitons of charge-transfer origin arise quite naturally in the p-d model of high-temperature superconductors. We consider the limit in which on-site Coulomb repulsion on both Cu and O is taken to be infinite and treat the Cu-O repulsion ${\mathit{U}}_{\mathit{p}\mathit{d}}$ in the Hartree-Fock approximation. We find that the uniform Hartree-Fock solution is unstable (the compressibility is negative) for small enough doping. Using an unrestricted Hartree-Fock scheme we find in one dimension (1D) that one particle added to the half-filled case forms a self-trapped polaron state of charge-transfer origin. We also find that there is no phase separation. Within the same formalism we subsequently study exciton states. In 2D we obtain similar compressibility results, suggesting that the ground state is also polaronic. Our results are also relevant for one-dimensional charge-transfer systems.