[QE-users] Identifying LO & TO modes of phonon vibration

Sudip Kumar Mondal sudipkm1990 at gmail.com
Thu May 24 18:58:59 CEST 2018

Hi all,
I am having a problem with a phonon dispersion plot. In general for a
N-atom lattice there are 3N modes of vibration - 3 acoustic and 3N-3 are
optical.  If they are categorized according to their states of polarization
- among the 3 acoustic modes 1 is longitudinal acoustic (LA) and 2 are
transverse acoustic (TA) ; among the 3N-3 optical modes N-1 is longitudinal
optical (LO) and 2N-2 are transverse optical (TO).  It is simple to
identify the LA and TA modes on the plot  since the LA modes have higher
frequency than TA and both reside near the low frequency region having
frequencies  lower than optical modes. But for the optical mode it is
rather difficult to identify whether a branch is LO or TO.
So my questions are :

1. Is there any way to identify them from the plot or from the output
2. Like the acoustic modes , do the LO branches always reside above the TO

P.S.-  I am using QE-5.4.0.. The unit cell contains 24 atom and a few of
the optical modes are degenerate.

Happy computing.

Sudip Kumar Mondal
DST-INSPIRE Research Fellow
High Pressure Temperature Laboratory
Dept. Of Physics/Geological Sciences
Jadavpur University
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