[Wien] Some Queries about Polarization and Strain

[Jameela Fatheema]

Thank you Sir, It is really helpful.



On Saturday, July 29, 2017, 7:29:54 AM GMT+5, Rubel, Oleg <rubelo at mcmaster.ca> wrote:

Definition of the spontaneous polarization is not always straightforward. It is determined with respect to a reference structure. The choice of the reference structure is a tricky part and depends on how the polarization manifests itself in an experimental situation. I would suggest checking those two tutorials to begin with:https://github.com/spichardo/BerryPI/wiki/Tutorial-1:-Spontaneous-Polarization-in-BaTiO3https://github.com/spichardo/BerryPI/wiki/Tutorial-4:-Polarization-in-GaN
Calculation of the response to strain (a piezoelectric coefficient) requires relaxation of atomic positions to be realistic. It also becomes not a trivial analysis, since the polarization changes due to two effects: the volume change as well as atomic displacements. It seems that the "raw" Berry phase is more suitable for this purpose. Please check the section "Calculation of piezoelectric coefficients" in http://olegrubel.mcmaster.ca/publications/2017/Rubel_SR_2017.pdf
I hope this will help Oleg
--
Oleg Rubel (PhD, PEng)
Department of Materials Science and Engineering
McMaster University
JHE 359, 1280 Main Street West, Hamilton, Ontario L8S 4L8, Canada
Email: rubelo at mcmaster.ca
Tel: +1-905-525-9140, ext. 24094
Web: http://olegrubel.mcmaster.ca
On Jul 28, 2017, at 2:39 AM, Jameela Fatheema <j.fatheema96 at yahoo.com> wrote:


Dear all,
I had a few questions regarding the calculation of spontaneous polarization.
First, for sponteous polarization, if the displacement are larger or smaller, the spontaneous polarization will remain same or it will change? if it changes how do we say that anyone of those is the spontaneous polarization of that specific material?

Second, for calculating the effect of strain i.e. we change any one lattice parameter by x%, would the atoms still need to be displaced as is done in non-centro-symmentric for spontaneous polarization or only the lattice parameter will be changed?
Third, if the atoms are to be displaced in non-centro-symmetric case, should it be along the axis in which strain has been introduced?
It would be very helpful to have answer to any of these questions.
Regards
Jameela Fatheema

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