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 Qu?mica Fisica Aplicada Facultad de Ciencias Universidad Aut?noma 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 <[email protected]> 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 -- -------------- next part -------------- An HTML attachment was scrubbed... URL: http://pwscf.org/pipermail/pw_forum/attachments/20121109/5f03dbd7/attachment.html
