[Pw_forum] Weird jump in phonon dispersion at Gamma point

[Eduardo Ariel Menendez Proupin]

Dear Yao,
The jumps probably are right. They appear in non-cubic systems. For very
very small wavevectors, when w_TO ~ cq (c the speed of light and q the
wavavector) , the phonon are coupled with photons and form the excitation
known as phonon-polariton (PP). When the PP equations are solved the jumps
dissapear. Quantum ESPRESSO only computes the phonons, and the LO-TO
splitting is what happens for wavevectors just larger than the region of
coupling. This topics is briefly explained in some Solid State Physics
books, and fully explained in this article
R. Loudon, Advances in Physics, 2001, Vol. 50, No. 7, 813-864

There is also  a short entry in Wikipedia
http://en.wikipedia.org/wiki/Polariton

Best regards,
Eduardo

Eduardo Menendez Proupin
Departamento de Química Fisica Aplicada
Facultad de Ciencias
Universidad Autónoma de Madrid
28049 Madrid, Spain
Phone: +34 91 497 6706

On leave from: Departamento de Fisica, Facultad de Ciencias, Universidad de
Chile URL: http://fisica.ciencias.uchile.cl/~emenendez





--------- Mensaje reenviado ----------
From: Yao Yao <yao.yao at unsw.edu.au>
To: <pw_forum at pwscf.org>
Cc:
Date: Fri, 9 Nov 2012 11:14:39 +1100
Subject: [Pw_forum] Weird jump in phonon dispersion at Gamma point
Dear All,

I calculated a orthorhombic cell with alternating AlSb/InAs mono {110} atom
layers with QE 4.3.2. I noticed that my phonon dispersion results were with
steps at Gamma point for some optical branches. Looks you will get different
frequency at Gamma when you approach Gamma from different direction
(X->Gamma,
Y->Gamma and Z->Gamma). One picture showing this is uploaded to
http://www.picvalley.net/v.php?p=u/2753/176658145760478725413524194617CoVRtxOq5iqZ20Wa3iK.PNG

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