[Pw_forum] Phonon LO-TO splitting in semiconductor with smearing

[Nicola Marzari]

Dear Hans,


for a semiconductor/insulator at finite temperature you have free
electrons in the conduction band that will screen the electric field
of the LO phonons - in the adiabatic limit of the PWSCF/phonon
calculations you have no macroscopic electric field, and no LO/TO 
splitting, but in the real world you might have non-adiabatic effects.

Depending on the material you'll have a wavevector/frequency
/temperature dependent dielectric function epsilon(q,omega,T) for your
electronic system that will screen more or less successfully the
electric field of the LO phonons - in the limit of very little screening
you have the full LO/TO phonon splitting of the zero-temperature 
semiconductor, and in the limit of very effective screening you'll have
no splitting.

Some related ab-initio work has been done in the group of Mauri:
http://prl.aps.org/pdf/PRL/v100/i22/e226401
http://prl.aps.org/pdf/PRL/v97/i26/e266407

It would be interesting to see experimental data for LO/TO phonons
in III-V as a function of temperature or doping, to see if the screening
is really effective or not. If you search and find them, please share!

			nicola


On 13/04/2012 11:26, Hans Lind wrote:
> Dear QE users
>
> I am trying to do phonon calculations on AlN at non-zero fermi
> temperature to get forces recalculated for higher temperatures. But
> since I need to use smearing for that the code won't do LO-TO splitting
> as is needed in a semiconductor. Normally to get LO-TO splitting handled
> correctly all I need to do is omit the smearing keyword, but since I
> can't do it now, is there anything that I can do to get the splitting
> even while using smearing?
> Grateful for any suggestions.
>
> Hans Lind
> PhD student
> Linköping University, Linköping, Sweden
>
>
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-- 

----------------------------------------------------------------------
Prof Nicola Marzari, Chair of Theory and Simulation of Materials, EPFL