Phason, Sound Wave and their Coupling in the Quasi Two Dimensional Charge Density Wave

Abstract

Mixed modes of a phason and a sound wave are studied for the incommensurate charge density wave state. Our model Hamiltonian has two types of electrons. One type satisfies the nesting condition and the other one does not satisfy this condition and its single excitations do not have an energy gap even in the ordered state. The band structure of electrons takes the form of a two dimensional system. Both types of electrons can travel along layers, but can not transit between layers. The energy dispersion of these mixed modes retain a sound wave character in contrast with that of a linear chain model in which every single excitations has a gap. Near T_c and near T=0 the velocity of one of the mixed modes is equivalent to that of the sound wave of the normal state.