Hi,
--- On Sun, 10/25/09, Reza Ebraahimi <rezaebraahimi at yahoo.com> wrote:
> Thanks for your reply
Welcome!
> What i meant to say was, does not the ionicity (Born charges and ..) of > the
> crystal, which splits the TO-LO phonons, show itself as a gap in
> high-energy region of the phonon DOS?
Sorry, I am not sure I understood this sentence properly.
And I suppose you mixup two quanities: gap and splitting.
1. A gap for high-energy optical modes is related to the mass difference.
It usually occures along high symmetry directions. In fact, in some cases it
might be related to specific atomic vibrations (see below).
2. LO-TO splitting occurs ONLY!!! at the Gamma point. It is always connected to
dipols in the crystal (and respectively, to Born effective charges and
macroscopic dielectric constant)
For both cases, again, Gamma-Boron is a good example. Because it has three
optical modes which are separaed from others, though, all B atoms have the same
mass. Another example (for mass difference case) is MeH.
Besides, LO-TO splitting takes place for B28 at low frequency range (but
optical modes again).
For MgO LO-TO takes place, but there is no gap in the phonon dispersion
relations.
> If not, what is the sign of this splitting?in the phonon dos diagram?
This one is quite difficult to predict apriori, as it might be different
for different systems. In fact, there will be some re-distribution of phonon
DOS. For a system of interest just follow Nicola's advice.
Bests,
Eyvaz.
-------------------------------------------------------------------
Prof. Eyvaz Isaev,
Theoretical Physics Department, Moscow State Institute of Steel & Alloys,
Russia,
Department of Physics, Chemistry, and Biology (IFM), Linkoping University,
Sweden
Condensed Matter Theory Group, Uppsala University, Sweden
Eyvaz.Isaev at fysik.uu.se, isaev at ifm.liu.se, eyvaz_isaev at yahoo.com
