[Pw_forum] zone center phonons of Si lattice

[Miguel Martínez Canales]

Hi Ezad,

There's an example in Ashcroft's and Mermin's "Solid State Physics" (Ch. 
22, p 433 in my copy) in which they take the parametres of a linear chain 
with a basis so that it's equivalent to solving the monoatomic linear 
chain. What you see in this case is similar to "folding" the phonon branch.

Doing something similar for the bandstructure would also result in folding, 
since there is no bragg reflection to split the bands near the "zone boundary".

I suppose one sees the same behaviour on "SC Si", but one should have to be 
very careful to select the same (or equivalent) q points (due to the 
different basis you're using).

I hope this gives some hints. Regards,

Miguel

Ezad Shojaee escribió:
> Hi
> It seems that if we look to Si lattice as an SC with 8 atoms per cell, 
> the symmetries connected to the fractional translations will not be 
> accepted, so if we use this scheme to obtain the zone-center phonons of 
> Si, we will get frequencies rather different from the main one(FCC 
> scheme), with different symmetries and degenerecies.
> How is it possible to obtain the right ones (zone-center phonons) of Si, 
> with the right symmetries and degenerecies, by the SC scheme?
> Thanx 
> 
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-- 
----------------------------------------
Miguel Martínez Canales
    Dto. Física de la Materia Condensada
    UPV/EHU
    Facultad de Ciencia y Tecnología
    Apdo. 644
    48080 Bilbao (Spain)
Fax:  +34 94 601 3500
Tlf:  +34 94 601 5326
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