bright and dark exciton

[sunxl]

Dear all,

When I edit copy and submit posts, I always get blocked, so I present my questions in the form of images.
I appreciate any guidance and tutorial on that!
Best,
sunxl

q.pdf

[Daniele Varsano]

Dear Sunxl,

1) The weights iare defined as |Acvk|^2 where Acvk are the eigenvectors components of the exciton. They provide the weight of the single particle transitions participating in the excitons. \Psi_exc (r_e,r_h)=\Sum Acvk \psi_vk(r_h) \psi_ck(r_e)

If the exciton with the highest intensity has a weight of only 0.001, does such a small weight affect my analysis of exciton optical absorption spectra and similar analyses.

It is not clear what do you mean with "has a weight of only 0.001", given an exciton, there is a weight for each transition. The largest the weight, the more this transition is participating in the building up of the exciton. In some case you have few transitions with large weights, in other cases you can have an exciton formed by many transition with small weights.

2) These are not the exciton position, but the energy difference Ec-Ev (Kohn-Sham) of the transition participating in the exciton.

3) There is not a real threshold value. Properly, dark exciton (light forbidden excitation) have zero oscillator strength, but then you have the numerical precision to take into account. Exciton with very small oscillator strength can anyway be considered as dark.

Best,
Daniele

[abbas]

I hope you are doing well. I am using Yambo to study excitonic properties, and I am a bit confused about how to identify bright and dark excitons in my calculations.

Specifically, I would like to understand:

How to distinguish optically allowed (bright) and optically forbidden (dark) excitons in Yambo.

Which excitons follow optical selection rules and which ones are forbidden.

How this classification is affected by spin–orbit coupling (SOC), since my calculations include SOC.

How to analyze this at q = 0, where optical transitions are expected.

I could not find clear documentation or examples addressing the spin character of excitons and the bright/dark classification in SOC-based Yambo calculations, so I would really appreciate your guidance on this.

Abbas
Master Student
IIUI Isalamabad pakistan

[Daniele Varsano]

Dear Abbas,

bright and dark excitons can be distinguished looking at the oscillator strength.
Dark excitons have zero oscillator strength either because they are forbidden by symmetry (zero dipole), either because they are spin-forbidden. If SOC is included, you have a spinorial formulation and the way to distinguish bright/dark excitons is to look at the intensity.
Once you have solved the BS equation, you can inspect the excitation energy using ypp utility:

Code: Select all

ypp -e s -b 1

This will provide in output the excitations with their intensity normalized to the largest one for q=0 (iq=1) (optical limit).

Best,
Daniele