Shift in the longitudinal sound velocity due to sliding charge density waves.

The nonlinear conductivity observed for moderate electric fields in NbSe3, TaS3, (TaS4)2I, and K(0.3)MoO(3) below the charge density wave (CDW) transition is believed to be due to the sliding of the CDWs. The sliding motion leads to a Doppler shift of the x-ray diffraction peaks, but this effect has not yet been resolved. We show here that besides the Doppler shift, a sliding incommensurate CDW causes a change in the longitudinal sound velocity of the crystal that is linear in the CDW velocity.