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

[Paolo Giannozzi]

Hi Eduardo

you used dynmat.x, didn't you? the \epsilon_0 tensor is computed is
subroutine polar_mode_permittivity of PHonon/PH/dynmat.f90. The header
mentions a nonexistent reference (the correct page number should be 184111):
  ! Algorithm from Fennie and Rabe, Phys. Rev. B 68, 18411 (2003)
My guess is that the algorithm assumes TO frequencies only, but LO
frequencies are used instead since you specified a direction for q=>0.

Paolo

On Mon, Jan 11, 2016 at 8:10 PM, Eduardo Menendez <eariel99 at gmail.com>
wrote:

> 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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-- 
Paolo Giannozzi, Dip. Scienze Matematiche Informatiche e Fisiche,
Univ. Udine, via delle Scienze 208, 33100 Udine, Italy
Phone +39-0432-558216, fax +39-0432-558222
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