Domain-Like Incommensurate Charge-Density-Wave States and Collective Modes

A theoretical study is presented for the incommensurate ( I ) state and the incommensurate-commensurate ( C ) transition with McMillan's single charge-density-wave model. The I state is determined precisely by taking account of a sufficiently large number of harmonics. A domain-like I state is obtained if the fundamental wave number is close to that of the C state. At the domain boundary the amplitude of the order parameter decreases together with the change of the phase of the order parameter. The I - C transition of the model is shown to be of second order. A detailed study is also made on the collective modes for the domain-like I state. Modes associated with domain boundaries are obtained.