[Pw_forum] LO-TO splitting

[Stefano de Gironcoli]

LO-TO splitting is a general feature  of polar materials.
It means that the frequency of the (3N-3) optical modes at gamma  
depends  on  the direction you are approaching from.
for any given direction the number of optical modes is always 3N-3 but 
they are in general different in different directions.
In the particular case of elemental semiconductors with cubic symmetry 
the 3 optical modes are split in 2 TO and 1 LO modes whose eigenvalues 
do not depends on the direction BUT their eigenvectors DO depend on the 
direction ...
In general both the eigenvalues and the eigenvectors depend on the direction

stefano

Reza Ebraahimi wrote:
> Hello everyone
> i have a question about the LO-TO splitting. Imagine an insulator or semiconducting bulk material that has no degenerate phonon mode at Gamma point (no "E" or "T" species after diagonalizing the Dynamical matrix). Should this bulk exhibit LO-TO splitting? Cause in this case, the # of the optical modes becomes more than 3N-3, right?
> Thank You in Advance
>
> R. Ebraahimi
> Graduate Student- Tehran University
>
>
>
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