Low-q dispersion of transverse acoustic phonons in YBa2Cu3O7

High-resolution (�E=1.3 meV) inelastic X-ray scattering (IXS) in optimally doped untwinned YBa2Cu3O7−� single crystals show that the dispersion of transverse acoustic phonons in the a- direction remains linear with no softening due to bending modes in the long wavelength limit, down to q=0.02. The sound velocity is found to be 2700 ± 70 m s−1, in excellent agreement with ultrasound measurements, and unchanged upon cooling from room temperature to 98 K. These results rule out a strong coupling between bending dynamics of the CuO2 planes and charge carriers in cuprates.