We report results of inelastic neutron scattering measurements of phonon dispersions in optimally doped YBa2Cu3O6.95 and compare them with model calculations. The focus is on the in-plane oxygen bond-stretching phonon branches. The study of these modes is complicated by anticrossings with c-axis-polarized branches; such effects are interpreted through lattice-dynamical shell-model calculations. The in-plane anisotropy of the bond-stretching phonons was firmly ascertained from measurements on a detwinned sample. Studying the in-plane modes involving out-of-phase motion for the two Cu-O layers within a unit cell as well as those with in-phase motion was of great help for establishing a clear experimental picture. The measurements confirm that the in-plane oxygen bond-stretching phonon branches disperse steeply downwards from the zone center in both the a and the b directions indicating a strong electron-phonon coupling. For the b-axis-polarized bond-stretching phonons, there is an additional feature of considerable interest: a sharp local frequency minimum was found to develop on cooling from room temperature to T = 10 K at the wave vector q = 0.27 r.l.u..
@article{arxiv.cond-mat/0310183,
title = {Oxygen Phonon Branches in YBa2Cu3O7},
author = {L. Pintschovius and D. Reznik and W. Reichardt and Y. Endoh and H. Hiraka and J. M. Tranquada and H. Uchiyama and T. Masui and S. Tajima},
journal= {arXiv preprint arXiv:cond-mat/0310183},
year = {2009}
}