POLARONS IN THE 3-BAND PEIERLS-HUBBARD MODEL - AN EXACT DIAGONALIZATION STUDY

We have studied the three-band Peierls-Hubbard model describing the Cu -O layers in high-T(c) superconductors by using Lanczos diagonalizatio n and assuming infinite mass for the ions. When the system is doped wi th one hole, and for lambda (the electron-lattice coupling strength) g reater than a critical value, we found that the oxygens around one Cu contract and the hole self-traps forming a lattice and electronic smal l polaron. The self-trapped hole forms a local singlet analogous to th e Zhang-Rice singlet in the undeformed lattice. We also studied the si ngle-particle spectral function and the optical conductivity. We have found that the spectral weight, in general, is similar to that found i n previous studies where the coupling with the lattice was absent. The re is an anomalous transfer of spectral weight but, contrary to those studies, the spectral weight goes to these localized polaronic states. However, this polaronic shift does not seem enough by itself to expla in pinning of the chemical potential observed in real materials. The p eaks in the optical conductivity are also shifted, according to the po laronic shift, in the single-particle spectral functions. We compare o ur results to those obtained in inhomogeneous Hartree-Fock calculation s and we discuss their relation with experiments.