ELECTROMECHANICAL EFFECT IN CHARGE-DENSITY AND SPIN-DENSITY WAVES

We study tile sound propagation in charge- and spin-density wave syste ms 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 th e phason (i.e. no electromechanical effect) independent of the directi on of the polarization vector u, while the transverse wave couples to the phason only if u is parallel to the chain direction. This describe s at least qualitatively a peculiar asymmetry found recently in sound propagation in (TaSe4)(2)I.