Author Topic: calculating optical properties (permittivity) at very low frequencies (THz)  (Read 199 times)

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Online korand

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Hi dear friends

Earlier I have posted a question about the calculating optical properties at low frequencies. DFT does not give a reasonable answer, I found that this is possible by using Drude-lorentz model. In this model (attached figure), "wi" is natural frequency that will obtain from phonon band diagram or Raman. However, I dont know how to calculated  plasma frequencies in the numerator (here "fi*wi")!
I will appreciate it if you help me.

Best regard,
Korand

Offline Anders Blom

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Yep, that's how you do it.

Now, searching for "plasma frequency" on our excellent documentation site https://docs.quantumatk.com brings you to https://docs.quantumatk.com/manual/Types/OpticalSpectrum/OpticalSpectrum.html which has an example on precisely this (https://docs.quantumatk.com/manual/Types/OpticalSpectrum/OpticalSpectrum.html#intraband-contribution).

Note that this feature was added in a recent version only.
« Last Edit: May 17, 2021, 08:21 by Anders Blom »

Online korand

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Yep, that's how you do it.

Now, searching for "plasma frequency" on our excellent documentation site https://docs.quantumatk.com brings you to https://docs.quantumatk.com/manual/Types/OpticalSpectrum/ which has an example on precisely this (https://docs.quantumatk.com/manual/Types/OpticalSpectrum/OpticalSpectrum.html#intraband-contribution).

Note that this feature was added in a recent version only.


Dear Dr Blom

I want to thank you for taking the time to answer my question. I am sure that you are busy, and so I greatly appreciate your response.

I should clarify that I want to calculate plasma frequencies for every modes, in order to supply the Lorentz's formula summation (not Drude section). For instance, Bi2Se3 has 4 major phonon modes and each on them contributes in the permittivity. However, through the manner that described in documentation site that you mentioned, plasma freq for single resonance related to intraband in each direction will obtain.
Also, these phonon modes energy are far below the bandgap of Bi2Se3; therefore, my problem is not related to intraband contribution that appear in energies (freq) around energygap. My concern is belongs to interband counterpart.

I am looking forward to the hearing from you.

Best regard,
Korand

« Last Edit: May 14, 2021, 17:59 by korand »

Offline Anders Blom

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Sorry, I don't have a very good answer on that. I am not sure that you can compute the weights from DFT, a few articles I have seen rather treat these as fitting parameters when comparing to experiments. You can of course use QuantumATK to compute the phonon frequencies of active optical modes, and also study the Raman spectrum and identify Raman active modes this way. Besides that, I am not the best person to help, I fear...

Offline Tue Gunst

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Hi,
In the THz (infrared) regime one has to use the DielectricTensor object.
It takes in the OpticalSpectrum but then includes the low frequency coupling to vibrations.
See:
https://docs.quantumatk.com/manual/Types/DielectricTensor/DielectricTensor.html

In the section Notes you will see a functional form of the dielectric response from coupling with each phonon similar to the expression you refer to.
The numerator factor, termed mode oscillator strengths (not plasma frequencies), are calculated directly from the Born effective charges.

So in summary QuantumATK covers the full expression you show - plasma frequency can be calculated and used for interbands couplings in OpticalSpectrum. The DielectricTensor object can further calculate the THz coupling to vibrations.
Hope this answers your question,
Tue

Online korand

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Dear Dr Blom and dear Dr Tue

Thanks for your time and guidance.
What Dr Tue says will solve my problem.

Best regard,
Korand
« Last Edit: May 18, 2021, 15:18 by korand »