Electromechanical effect in charge- and spin-density waves

We study the sound propagation in charge- and spin-density wave systems in the hydrodynamic limit. For the longitudinal sound wave the long-range Coulomb interaction plays a crucial role in suppressing charge fluctuations. We find that the longitudinal wave does not couple to the phason (i.e. no electromechanical effect) independent of the direction of the polarization vector u, while the transverse wave couples to the phason only if u is parallel to the chain direction. This describes at least qualitatively a peculiar asymmetry found recently in sound propagation in (TaSe4)2I.