Abstract
The effects of electron-lattice coupling in a multiband Hubbard model are investigated by exact diagonalization. For a system with one doped hole, and with parameters relevant to cuprate superconductors, we find a continuous transition from a delocalized to a small polaron state at a critical value of the electron-phonon interaction. We characterize this state as a Zhang-Rice singlet with distortions in the CuO4 cluster. We find, in addition, a polaronic shift of the band near the Fermi level in the spectral function of the system. An enhancement of the transfer of the low-energy spectral weight is also observed. These results are in qualitative agreement with different spectroscopic measurements for high-Tc superconductors.