Sound propagation in charge- and spin-density waves

We study sound propagation in a quasi-one-dimensional charge- or spin-density waves (CDW or SDW) within mean-field theory. The quasiparticle damping is explicitly included in terms of randomly distributed impurities. In general the ionic potential is screened by both the quasiparticle and the phason. However, when the CDW or the SDW is pinned, the phason cannot participate in the screening. Therefore the depinning of the CDW or the SDW by an electric field always decreases the sound velocity. Further, due to the diffusion pole in correlation functions, the quasiparticle contribution depends crucially on ω/${\mathit{Dq}}^2$, where ω and q are the frequency and the wave vector of the sound wave and D is the diffusion constant. The present theory describes many features of sound propagation in the quasi-one-dimensional CDW systems.