Freezing electronic correlations by polaronic instabilities in dopedLa2NiO4

In doped Mott-Hubbard insulators, the electron-phonon interaction and strong Coulomb repulsion can reinforce each other to stabilize small polarons, domain walls, and charge-density waves. We discuss how the electron-phonon coupling stabilizes domain-wall states in an antiferromagnetic spin background for small doping, and promotes coupled spin- and charge-density wave transitions at larger doping, and we explain why this coupling is more important in the nickelate than in the cuprate. We discuss our results in the light of recent experiments on ${\mathrm{La}}_{2\mathrm{-}\mathit{x}}$ ${\mathrm{Sr}}_{\mathit{x}}$ ${\mathrm{NiO}}_{4+\mathit{y}}$.