[Pw_forum] transverse acoustic, longitudinal acoustic, and out-of-plane transverse acoustic

[Uri Argaman]

> 1- Does the dimension of Supercell for phonon calculation determine

> by nq1, nq2,and nq3 in the phonon input file?

The parameters nq1, nq2,and nq3 is determine the Monkhorst-Pack grid for
the DFPT calculation (phonon calculation) that done in q-space (you
calculate the dynamical matrices explicitly and not the force constants).
In DFPT there is no supercell but the convergence with respect to q-points
is similar to the convergence with respect to the size of the supercell in
supercell approaches.

> 2- we know that the lowest three dispersion in the phonon spectra

> show the three acoustic branches, but how can I find which one is

> for TA, LA, and ZA?

Phonons do not have just transverse or longitudinal polarization. There are
phonons with polarization that is neither transverse nor longitudinal
(although transverse and longitudinal polarizations are off-course
possible). The most general polarization is elliptical. In general, the
polarization vector is complex and you can know it from the calculation
(after diagonalization of the dynamical matrices). You have this
information in the dynamical matrices files which is output of the phonon
calculation and also you have it in the output of matdyn.x in the file
matdyn.modes.

We recently publish a work in which we analyze the polarization of a
complex acoustic phonon in the corner of a hexagonal Brillouin zone:

http://journals.aps.org/prb/abstract/10.1103/PhysRevB.94.174305

You can maybe find it interesting.



Uri Argaman

Ben-Gurion University

Israel
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