|The frequency of an intrinsic gap mode as a function of normalized amplitude, a/d. The left set of data are for the zinc blende and the right set for the fcc lattice. The numerical solutions of the time-independent nonlinear equations are represented by the solid lines for the  and by dashed lines for the  crystal direction. The result of MD simulations are given by the open diamonds for the  direction and by open circles for the  direction.|
In both systems the elastic distortion associated with the IGM has lower symmetry than the corresponding point group symmetry of the crystal. To recover the full crystal point group symmetry for this perfect crystal it must be possible with a different set of initial conditions to rotate the IGM about the equilibrium lattice site. To test this idea we have examined IGM excitations along the three different crystal directions. Although a stable IGM mode does not appear for initial excitation along the  direction, the dashed lines and open circles in the figure demonstrate that for both point group symmetries a  directed mode can occur. These sets of data support the idea of an interchange of the IGM vibration direction as would be expected, for example, with hindered rotational motion of the excitation about the lattice site. This concomitant low frequency component of the IGM may provide new experimental ways to excite and identify these nonlinear excitations.