Polaron formation in cuprates

We calculate the electron-phonon coupling for La2CuO4, and show that it is strong enough to lead to small polarons, as seen experimentally. The calculated line shapes are in reasonable agreement with experiment. Deriving sum rules, we show that for a weakly doped system, the Coulomb interaction strongly suppresses effects of the electron-phonon interaction on the phonon but not electron self-energy. Studying a Hubbard-Holstein model using a dynamical mean-field theory, we show that it is crucial to include antiferromagnetic correlations. The Coulomb interaction then only moderately suppresses the tendency to polaron formation, while the suppression of the phonon softening is very strong.