Low-lying collective modes in the charge density wave state inn-type inversion layers of semiconductors

Acoustic-phonon like low-energy long-wave length collective excitations are shown to exist in the charge density wave (CDW) ground state of a two-valley model forn-type inversion layers. The dispersion of the collective modes are obtained by calculating the two-particle Green's function within the random phase approximation, and the sound velocity is investigated as a function of interaction parameters and temperature. Physically the modes represent the phase oscillation of the CDW and the fluctuation of the population difference between two valleys; these two modes are coupled to each other.On leave of absence from the Department of Physics, University of Tokyo, Tokyo, Japan