Anomalous Dispersion of Longitudinal Optical Phonons in   $\mathrm{\mathbf{Nd_{1.86}Ce_{0.14}CuO_{4+\bm{\delta}}}}$ Determined by   Inelastic X-ray Scattering
  The phonon dispersions of $\mathrm{Nd_{1.86}Ce_{0.14}CuO_{4+\delta}}$ along the $[\xi,0,0]$ 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 around $\mathbf{q}=(0.2,0,0)$. These signatures provide evidence for strong electron-phonon coupling in this electron-doped high-temperature superconductor. 
M. d'Astuto, P. K. Mang, P. Giura, A. Shukla, P. Ghigna, A. Mirone, M.   Braden, M. Greven, M. Krisch, F. Sette