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 files? 2. Like the acoustic modes , do the LO branches always reside above the TO branches? 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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