# [Pw_forum] non-cubic dielectric tensor in a cubic crystal.

Eduardo Menendez eariel99 at gmail.com
Mon Jan 11 20:10:42 CET 2016

Hi,

I am computing the dielectric funciton of a cubic material (CdTe).
I am surprised that the to see a result like this the dielectric tensor
below:

# mode   [cm-1]    [THz]      IR
1      0.00    0.0000    0.0000
2      0.00    0.0000    0.0000
3      0.00    0.0000    0.0000
4    133.01    3.9875    2.3657
5    133.01    3.9875    2.3657
6    154.23    4.6238    2.3657

Electronic dielectric permittivity tensor (F/m units)
11.387818    0.000000   -0.000000
0.000000   11.387818   -0.000000
0.000000    0.000000   11.387818

... with zone-center polar mode contributions
14.306543    0.000000   -0.000000   (HERE IS ACKWARD)
0.000000   15.312431   -0.000000
-0.000000   -0.000000   15.312431

I (guess that) undertand the first tensor above as \epsilon_{\infty}, and
the second tensor as \epsilon_0. Why is the first component 14.3 different
from the others 15.31, shouldn't it be a diagonal tensor ? 15.31 is
consistent with epsilon_infty and the Lyddane-Sachs-Teller formula.

Well, I set q(1)=1, q(2)=0,q(3)=0, so I guess the component 11 is a
longitudinal dielectric constant. I see that changing the vector q also
change the tensor However, I think that for an LO phonon the electric
displacement is 0, so is null the longitudinal dielectric constant.

Sorry, I did never see this in textbooks. Finally, and practically,  if
14.3 is a longitudinal dielectric constant, is this the dielectric constant
that screens a static constant electric field ?

Thank you,

Eduardo Menendez Proupin
Departamento de Fisica, Facultad de Ciencias, Universidad de Chile
URL: http://www.gnm.cl/emenendez

“No cometerás actos impuros ni publicarás en revistas open-acces”
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