Electron correlations and bond-length fluctuations in layered copper oxides: electron versus hole doping

We investigate the nature of the electronic ground state and electron-lattice couplings for doped chains of CuO_4 plaquettes or CuO_6 octahedra. The undoped configuration implies here Cu 3d^9 and O 2p^6 formal valence states. The results of multiconfiguration calculations on 4-plaquette (or 4-octahedra) linear clusters indicate strong electron-lattice interactions and polaronic behavior of the doped particles, for both electron and hole doping. For certain phases of the oxygen-ion half-breathing distortions a multi-well energy landscape is predicted. Since each well is associated to carriers localized at different sites, the half-breathing displacements induce charge transfer along the chain. In the case of hole-doping, the trends found by ab initio multiconfiguration calculations on 4-octahedra clusters are confirmed by density-matrix renormalization-group calculations for a p-d, extended Hubbard model with chains of few tens of CuO_4 plaquettes. Under the assumption of charge separation and the formation of 1/3-doped stripes, our results seem to support the vibronic mechanism and the traveling charge-density wave scenario proposed in some recent contributions for superconductivity in copper oxides.