  Inelastic neutron scattering investigations on optimally doped YBCO revealed
a very pronounced temperature dependence in the Cu-O in-plane bond-stretching
vibrations along the (010) direction: a shift of spectral weight by at least 10
meV has been observed in a narrow range of wave vectors halfway to the zone
boundary. The temperature evolution starts at about 200 K, well above the
superconducting transition temperature. The displacement pattern of the
anomalous phonons indicates a dynamic one-dimensional charge-density modulation
within the planes, but excludes a direct contribution from the Cu-O chains.
