Lattice effects in cuprates

We describe the fine details of the dependence of the dispersion of the Cu-O bond-stretching phonon in YBa2Cu3O6+x on temperature and composition, recently determined by inelastic neutron scattering. The results suggest the presence of a localized phonon mode above the pseudogap temperature, which softens at lower temperatures. Its implications and possible role in the mechanism of high-temperature superconductivity are discussed.