Nonlocal electron-phonon coupling of ionic charge-fluctuation type and phonon anomalies in high-temperature superconductors

The experimentally observed strong renormalization of the high-frequency copper-in-plane-oxygen bond-stretching vibrations in the metallic phase of YBa2Cu3O7 and La2CuO4 is investigated. In this context the effect of the a-b anisotropy in YBa2Cu3O7 on these anomalous phonon modes is discussed. It is demonstrated that the characteristic softening of the modes is a generic feature of the high-temperature superconductors. It is a consequence of a strong, nonlocal electron-phonon coupling of the ionic charge-fluctuation type and not related to Fermi-surface nesting. The phase space of this strong coupling effect is shown to be large. This reveals the importance of the phonons in the mechanism leading to high-temperature superconductivity.