Long-wavelength dispersion of transverse acoustic phonons in untwinned YBa2Cu3O(7-delta) single crystals

In order to study the possibility of enhancing the electron-phonon coupling in cuprates by soft transverse acoustic TA modes, using high-resolution (dE=1.3 meV) inelastic x-ray scattering, we measured the inplane TA mode at low qd0.1 along the a* axis on optimally doped untwinned YBa2Cu3O(7-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, in agreement with ultrasound data. Thus, no bending instability exists in the CuO2 layers and we conclude that larger and Tc may be possible in cuprates by inducing this instability.