Doubts about the absorption of various anisotropic materials in different polarization directions

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Quxiao
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Doubts about the absorption of various anisotropic materials in different polarization directions

Post by Quxiao » Sat Apr 02, 2022 12:14 pm

Dear all:
When I try to study the effect of the polarization direction,I find something strange.
The first piscture is set LongDrXs 110,and the BlongDir is set the 1 1 0,means that include the px,py(polarization),note that the Y3 is dark exciton.However,the second picture is set LongDrXs 110,and the BlongDir is set the 100,means only consider the px(polarization),the Y3 in that picture is light exciton,I keep the same parameters(of course the kpoints),the Y3 is the 17th exciton,so I do not know how to explain that thing.It seems that the data processing is to average the BSE in the X and Y directions.
This seems unreasonable. The first picture considers two polarization directions, should contain all bright excitons, and then Y3 looks dark.
Thanks
Quxiao
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Daniele Varsano
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Re: Doubts about the absorption of various anisotropic materials in different polarization directions

Post by Daniele Varsano » Sat Apr 02, 2022 1:39 pm

Dear Quxiao ,

I suggest you look to the alpha and not to the epsilon. They are proportional here, but with the alpha you avoid to loot at values of the order 10^-7.
How do you understand that Y3 is dark in the first picture?
If you are looking at the intensity in the o-exc*sort file, please note that they are normalized to the maximum value so in the two cases you can have a different normalization factor. The normalization factor is reported in the output file as: "Maximum Residual Value"
Also from picture 1, Y3 does not seems completely dark, but it is possible that it is an excitation between two others having larger oscillator strengths (you can use a very small broadening to see if this is indeed the case). You can also look at the (100) and (010) separately.

Best,
Daniele
Dr. Daniele Varsano
S3-CNR Institute of Nanoscience and MaX Center, Italy
MaX - Materials design at the Exascale
http://www.nano.cnr.it
http://www.max-centre.eu/

Quxiao
Posts: 96
Joined: Fri Mar 26, 2021 11:27 am

Re: Doubts about the absorption of various anisotropic materials in different polarization directions

Post by Quxiao » Sat Apr 02, 2022 6:21 pm

Dear Daniele:
I have known that with your help,but there should not have a bright exciton,I would calculate it again.
The way to judge the bright exciton is to look at the o-Esort file and find the exciton state near the peak corresponding to the BSE spectrum. The energy difference between the two exciton states I calculated is 0.02ev. Near the same peak, I think it should be an exciton state, that is, Y2 and Y3 in the figure should basically be an exciton state.
Thanks!
Best wishes!
Quxiao
BIT
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Quxiao
Posts: 96
Joined: Fri Mar 26, 2021 11:27 am

Re: Doubts about the absorption of various anisotropic materials in different polarization directions

Post by Quxiao » Thu Apr 07, 2022 3:03 pm

Dear Daniele:
Thanks for your help,I have learn a lot with your help!
The last you suggest me use the alpha not the epsilon,I realize that you replied to me earlier that the dielectric function in 2D materials is not well defined,but the calculated image between the imaginary part of the dielectric function and the energy of the two-dimensional material is almost proportional to the image of the polarizability and energy.
I read the introduction of the previous article on Coulomb truncation. The real part of the derived dielectric constant is approximately 1, simulating a vacuum, and the corresponding dielectric function should be the corresponding dielectric function when the volume is infinite,like the formula which in the picture.The imaginary part of the dielectric function corresponds to the strength of absorption, or corresponds to the energy consumed to form an electric dipole. The imaginary part of the dielectric function calculated by using the Coulomb truncation technique is close to 0.
But the corresponding I am calculating a system, and the stress on it, one of the magnitude is 10^(-6), the other is 10^(-7), I think that although it is close to 0, it does lead to different levels of energy required to form dipoles, and it can be considered that the responses are different, but I am afraid that I do not understand the Coulomb truncation technology. The imaginary part of the said two-dimensional dielectric function is not well defined, which also makes me a little hesitant.Please give me some help to solve that.
Best wishes
Quxiao
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Daniele Varsano
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Re: Doubts about the absorption of various anisotropic materials in different polarization directions

Post by Daniele Varsano » Fri Apr 08, 2022 10:24 am

Dear Quxiao,

the formula you posted applies to 3D system where the coulomb potential is 1/q^2.
For a 2D system, you have a 1/q behaviour and this is the reason why you have very small numbers (at limit 0) in the Im(eps) for q-->0.

All the consideration you did applies to alpha. You can have a look at this paper Eq.16 and Eq.17 and related discussion on polarizability alpha 2D alpha 3D and dielectric function: Phys. Rev. B 84, 085406

Best,
Daniele
Dr. Daniele Varsano
S3-CNR Institute of Nanoscience and MaX Center, Italy
MaX - Materials design at the Exascale
http://www.nano.cnr.it
http://www.max-centre.eu/

Quxiao
Posts: 96
Joined: Fri Mar 26, 2021 11:27 am

Re: Doubts about the absorption of various anisotropic materials in different polarization directions

Post by Quxiao » Sun Apr 24, 2022 4:00 am

Dear Daniele:
Thanks for your help!I have known the dielectric function(2D) is defined by the effective potential,
however,when I try to use the formula in the picture below,and I found that multiplying the imaginary part of the polarizability by 2pi|q|, and taking |q| to take 10^ (-5), the order of magnitude obtained does not match the order of magnitude of the imaginary part of the dielectric function.
I want to know whether the calculation of the imaginary part of the dielectric function and the polarizability satisfies the conversion relationship of 2pi*|q|, or whether it is converted by setting q=1*10^(-5) in the program setting?
Please give me some help!
Best wishes!
Quxiao
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Daniele Varsano
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Re: Doubts about the absorption of various anisotropic materials in different polarization directions

Post by Daniele Varsano » Tue Apr 26, 2022 8:50 am

Dear Quxiao,

the polarizability is calculated as:
alpha=-X/qo^2*constant
where the constant depends on the dimension of the system: Vol/4*pi/periodic_length and qo=10^-5

Best,
Daniele
Dr. Daniele Varsano
S3-CNR Institute of Nanoscience and MaX Center, Italy
MaX - Materials design at the Exascale
http://www.nano.cnr.it
http://www.max-centre.eu/

Quxiao
Posts: 96
Joined: Fri Mar 26, 2021 11:27 am

Re: Doubts about the absorption of various anisotropic materials in different polarization directions

Post by Quxiao » Thu May 05, 2022 8:23 am

Dear Daniele:
Thanks for your help!I will continue to strengthen my theoretical study.
Best wishes
Quxiao
Quxiao in BIT,calculate the exciton

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