Charge localization and phonon spectra in hole-doped La2NiO4

The in-plane oxygen vibrations in La2NiO4 are investigated for several hole -doping concentrations both theoretically and experimentally via inelastic neutron scattering. Using an inhomogeneous Hartree-Fock plus random-phase a pproximation numerical method in a two-dimensional Peierls-Hubbard model, i t is found that the doping induces stripe ordering of localized charges, an d that the strong electron-lattice coupling causes the in-plane oxygen mode s to split into two subbands. This result agrees with the phonon band split ting observed by inelastic neutron scattering in La2-xSrxNiO4. Predictions of strong electron-lattice coupling in La2NiO4, the proximity of both oxyge n-centred and nickel-centred charge ordering, and the relation between char ged stripe ordering and the splitting of the in-plane phonon band upon dopi ng are emphasized.