[Pw_forum] non-cubic dielectric tensor in a cubic crystal.
Paolo Giannozzi
p.giannozzi at gmail.com
Mon Jan 11 21:51:24 CET 2016
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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