 Re: optical in negative energy range ( No.1 ) |
- Date: 2024/12/30 05:31
- Name: YT Lee
- (1)
The energy difference (dE) of an optical transition between two states can be positive or negative.
According to the Kubo-Greenwood formula, regardless of whether dE is negative or positive, the energy range of the optical conductivity or dielectric function should not be negative.
(2)
In optical calculations using OpenMX, the scale of energy range doesn't correspond to eigenstates.
You do not need to check the Fermi level because optical transitions occur between two states.
Best regards,
YT Lee
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| Re: optical in negative energy range ( No.2 ) |
- Date: 2024/12/30 10:48
- Name: Debo Hao
- Dear YT Lee，
Thank you very much for your reply.
So, are there any other ways to get the optical properties near the Fermi level with OpenMX?
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| Re: optical in negative energy range ( No.3 ) |
- Date: 2025/01/02 14:36
- Name: YT Lee
- 1. Usually, I check the electronic band structure of a material first to confirm states at a k-point close to the Fermi level.
Based on the electronic band structure obtained, I can guess where the peaks should appear because optical transitions occur between two states.
(However, it is possible that an optical transition between two states is not allowed (forbidden), or not a sharp peak.)
2. After calculating the optical conductivity, I will try to connect the peaks in the optical conductivity to the electronic band structure of the material.
3. In the energy range of optical conductivity, it is the energy difference of an optical transition between two states.
For example, at the gamma point, state 1 is at -0.2 eV and state 2 is at 0.8 eV. (if Fermi level is set to be zero.)
There may be a weak (or sharp) peak at 1 eV (i.e. 0.8 - (-0.2) eV).
4. In your case, the energy range is set to be from 0 to 2 eV (i.e., -6 eV ~ -4 eV in your input).
You may check how many states exist from -2 eV to 2 eV at different k points (when Fermi level is set to be zero.)
5. Optical transitions usually take place near the Fermi level because of the Fermi-Dirac distribution in the Kubo-Greenwood formula ( listed in Physical Review B 102 (7), 075143 (2020), Physical Review B 98 (11), 115115 (2018), and relevant Journal papers ).
6. Absorption coefficients can be obtained when extinction coefficients (imaginary part of dielectric function) are calculated.
Best regards,
YT Lee
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| Re: optical in negative energy range ( No.4 ) |
- Date: 2025/01/03 14:46
- Name: Debo Hao
- Dear Prof. Lee,
Thank you very much for your patient and accurate reply.
Based on the literature and detailed explanations provided, I understand the relationship between band electron transitions and the position of light absorption peaks. However, I still have a few questions that require your assistance.
1. Is the calculation of the optical absorption coefficient in OpenMX as follows: optical conductivity -> dielectric constant -> extinction coefficient -> optical absorption coefficient?
2. A single optical absorption peak may correspond to many transition states. For example, a peak with a transition energy of 1 eV might correspond to transitions state from -0.2 eV to 0.8 eV at Gamma Point, or from -0.3 eV to 0.7 eV at M Point. How can I determine which electron transitions correspond to which peak clearly? And what are the percentage contributions of these transition states?
3. How does OpenMX consider intraband transitions?
4. Does OpenMX have the functionality for partial optical conductivity as mentioned in the article (Physical Review B 102 (7), 075143 (2020))?
Best regards,
Debo Hao
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| Re: optical in negative energy range ( No.5 ) |
- Date: 2025/01/06 19:51
- Name: Yung-Ting Lee
- 1. Please check
(1) https://www.openmx-square.org/openmx_man3.9/node198.html ,
(2) Some optical properties can be found in "Solid State Physics" by Ashcroft and Mermin,
(3) Optical conductivity in wiki : https://www.openmx-square.org/openmx_man3.9/node198.html ,
2.
(1) One may analyze electronic band structure of a material and corresponding electronic states first.
Then, one can modify codes to project certain optical transition(s) for analysis.
(2) It is hard to know a contribution of an optical transition in a peak before DFT calculations for a specific material.
3. Currently, the approach only includes optical transitions at the same k-point, not from k to k'.
I did not derive this Kubo-Greenwood formula for optical transitions from k to k'.
If one wishes to calculate optical transitions of a material from k to k', some programs have this functionality.
4. The functionality of partial optical conductivity is not included in OpenMX at present.
Best regards,
YT Lee
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| Re: optical in negative energy range ( No.6 ) |
- Date: 2025/01/07 16:24
- Name: Debo Hao
- Dear Prof. Lee,
Thank you for your reply.
I already know about these issues.
Best regards,
Debo Hao
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optical in negative energy range