PHONON-DISPERSION CURVES OF DIAMOND AND SILICON WITHIN THE QUASI-ION MODEL

The Phonon dispersion curves for diamond and silicon in the quasi-ion model are calculated and additionally the effect of anisotropic breath ing is studied. Within the rigid quasi-ion model (RIM) the influence o n the phonon dispersion of the localization of the expansion set (Gaus sians) for the quasi-ions is discussed. Furthermore the phonon dispers ion curves are investigated taking anisotropic breathing into account. Such an extension allows to influence special modes in the spectrum, in particular the LA-mode at the L point which is poorly represented i n the RIM. Also calculated are phonon-induced changes of the electroni c density in this model for some modes. Anisotropic breathing is assoc iated with a quadrupole contribution to the displacement-induced chang e in density in contrast to the RIM where the quadrupole moment vanish es for symmetry reasons.