[Pw_forum] gap in the phonon DOS

[Eyvaz Isaev]

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