How can one analyze the phonon dispersion of 2D materials like graphene to discern specific curves corresponding to particular phonon modes, such as acoustic or optical, as well as longitudinal and transverse modes?


1 Answer 1


Do you know what longitudinal, transverse, acoustic, and optical mean in this context? Or you want a code that can tell you what they are?

If you know what they mean, you can plot the displacements using the *.yaml files output by phonopy here and just identify the modes manually. There are a lot of datasets already there that you can play with including the default one which is graphene.

For really large cells with lots of modes, it's a lot more tedious to plot the modes manually. I am a neutron scatterer, so I frequently simulate inelastic neutron scattering spectra from DFT phonon calculations. I use euphonic. The neutron scattering intensity depends on the phonon symmetry (~ acoustic vs optical) and polarization (~longitudinal vs optical), so if you are willing to invest the time to learn some of the underlying theory, this method is very powerful and can help you identify the "character" of phonons.

If you aren't sure what optical vs. acoustic and longitudinal vs. transverse means, that should be posted as another question or you can look in practically any solid-state physics text book. For complex unitcells in 3D, its really somewhat ambiguous to distinguish between acoustic and optical modes except near the zone-center where acoustic modes have vanishing frequency. Moreover, many phonons have mixed longitudinal/transverse character across the BZ. In my opinion, calculating the neutron scattering structure factor is one of the most direct ways to characterize the phonons since it has a real connection to experiment (there is really no other way to measure dispersions). It just requires a lot of "cost" up front to learn how to interpret the results.

I hope this helps!



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