Ladder lattice model of soft optical phonon Ladder lattice model of soft optical phonon

A linear lattice model for demonstrating a soft optical mode at a zone center is proposed. The unit cell consists of only two atoms. If the optical mode is sufficiently soft, anticrossing takes place between the optical and acoustic phonon branches. By using the generalized Langevin's equation method, the spectral function is calculated analytically. The intensity of the optical mode disappears at approximately the anticrossing wave number. In general, the spectral line shape of the optical mode is asymmetric because of the interaction between the two branches.