Long-wavelength dispersion of transverse acoustic phonons in untwinned YBa2Cu3O7-δ single crystals

In order to study the possibility of enhancing the electron-phonon coupling lambda in cuprates by soft transverse acoustic (TA) modes, using high-resolution (Delta E=1.3 meV) inelastic x-ray scattering, we measured the in-plane TA mode at low q less than or similar to 0.1 along the a(*) axis on optimally doped untwinned YBa2Cu3O7-delta single crystals. We find that the dispersion is linear with no softening down to q=0.02 and unchanged upon cooling from 300 to 98 K. The sound velocity is found to be 2700 +/- 70 m s(-1), in agreement with ultrasound data. Thus, no bending instability exists in the CuO2 layers and we conclude that larger lambda and T-c may be possible in cuprates by inducing this instability.