Low-q dispersion of transverse acoustic phonons in YBa₂Cu₃O₇₋δ

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High-resolution (ΔE=1.3 meV) inelastic X-ray scattering (IXS) in optimally doped untwinned YBa₂Cu₃O₇₋δ 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.