Nonlinear amplitude-phase interaction in charge-density-wave systems

A nonlinear amplitude-phase interaction Hamiltonian is derived for charge-density-wave (CDW) systems from the microscopic Fröhlich model. A perturbative formalism with the small parameter 1/√μ (μ is the dimensionless mass parameter of a CDW) is presented, and illustrated by calculating the amplitude-mode spectrum in one dimension. The calculation gives, without adjustable parameters, a quantitative explanation of the temperature dependence of the damping constant observed in KCP by Raman experiments. This result, together with the calculation of the metallic conductivity of TTF-TCNQ (S. Kurihara: J. Phys. Soc. Jpn. 44 (1978) 2011.), shows the importance of the nonlinear amplitude-phase interactions in CDW systems. It is suggested that some other dynamical degrees of freedom (most probably the motion of water molecules) should be taken into account to understand the behavior of the amplitude-mode frequency in KCP.