Abstract
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 not only the in-plane modes involving
in-phase motion for the two Cu-O layers within a unit cell but also
those with opposite-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=10K at wave vector q≈0.27 r.l.u.
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