Role of Anharmonicity

Figure shows the dependence of the frequencies of the anharmonic local modes on their amplitudes. In each case as the amplitude of the mode increases, its frequency move farther away from the top of the phonon band, Wm, and the mode becomes more localized.
In the region of small amplitude, the frequencies of the two local modes with different parities are almost identical and coincide with the frequency of the envelope soliton of the continuum approximation. A further increase in amplitude (or anharmonicity parameter) separates these modes from the envelope soliton region, and both odd and even parity anharmonic local modes become distinguishable elementary vibrations.

When the eigenvectors of the local modes are used as initial conditions for molecular dynamics (MD) simulations, there is excellent agreement between the simulated frequencies (circles) and the analytic results (solid curves) shown in Figure.

Last modified: August 12, 1997