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 > Link?ping University, Link?ping, Sweden > > > _______________________________________________ > Pw_forum mailing list > Pw_forum at pwscf.org > http://www.democritos.it/mailman/listinfo/pw_forum -- ---------------------------------------------------------------------- Prof Nicola Marzari, Chair of Theory and Simulation of Materials, EPFL
