Anomalous Dispersion of Longitudinal Optical Phonons in [Formula presented] Determined by Inelastic X-Ray Scattering

The phonon dispersions of [Formula presented] along the [Formula presented] direction have been determined by inelastic x-ray scattering. Compared to the undoped parent compound, the two highest longitudinal phonon branches, associated with the Cu-O bond stretching and out-of-plane oxygen vibration, are shifted to lower energies. Moreover, an anomalous softening of the bond-stretching band is observed at about [Formula presented]. These signatures provide evidence for strong electron-phonon coupling in this electron-doped high-temperature superconductor.