Exciton dispersion interpolation

[sitangshu]

Dear all,

I am using Version 5.1.0 Revision 21422 Hash (prev commit) fde6e2a07 and doing some excitonic dispersion. I found that the code do not compute whether the q/=0 exciton is bright or dark. Intensity here is not reported. Is this true?

Regards,
Sitangshu

[Daniele Varsano]

Dear Sitangshu,

let me first stress that strictly speaking finite-q exciton are not bright or dark as they cannot be accessed with an optic experiments.
Having said that, you can have a generalized strength of the excitation as seen in an EELS experiment, and this is reported when you sort your excitons using ypp.

Code: Select all

ypp -e s -b #qindex

Best,
Daniele

[sitangshu]

Many Thanks Danielle,

I have an interpolated exciton dispersion on which I want to superimpose the exciton energy data from the file "r-output_excitons_interpolate". However, I could not find the way to do since the interpolated one shows the corresponding |q| while the r-output_excitons_interpolate file does not.
How can I convert the usual 19 regular symmetry k points found in 12 12 1 sampled values to their respective |q|?

Regards,
Sitangshu

[sabrine]

Dear Yambo developers and users,

Recently, I start to do GW-BSE calculation by using yambo 5.1.0

After that, we have calculated the exciton oscillator strength using the yambo post-processing ypp. We have obtained the exciton oscillation strength and the amplitude.

Now, we aim to identify the symmetries of the excitonic state ( 1s,2s,2p...). I mean I want to be able to index every exciton states
with the quasi-hydrogenoide (mixing state) series, please see the attached figure (h), In fact we want to do index the exciton state using BSE method
as is done in this figure ( see h in the figure ) and compare to hydrogenoide system.

From this data file o-BSE.exc_qpt1_E_sorted, can we have an idea about the exciton symmetries (1s, 2s,2p ) or it will just give us an idea only about dark and bright exciton. Actually, plotting the density of exciton state, doesn't give us a clear view about the symmetries of the different state since the exitonic wave function in this case take into account also the orbital symmetries (Bloch function) not just the symmetries of the envelope function.

Thanks

[palful]

Dear Sitangshu,

I have an interpolated exciton dispersion on which I want to superimpose the exciton energy data from the file "r-output_excitons_interpolate". However, I could not find the way to do since the interpolated one shows the corresponding |q| while the r-output_excitons_interpolate file does not.
How can I convert the usual 19 regular symmetry k points found in 12 12 1 sampled values to their respective |q|?

Unfortunately there is no automatic way to do this operation at the moment. I plan to introduce this possibility in yambopy but so far I have only sparse python scripts.
I think you may do your own script following these steps.

1) Read the q-coordinates of both the interpolated points (o-*) and the original ones (r-*). These are written by yambo in reduced units.
2) Convert them to cartesian units (you need the reciprocal lattice vectors for this).
3) The original q-points are written in a grid over the irreducible BZ. Find the subset of points lying along the high-symmetry directions used in the interpolation (e.g., the GM direction in a hexagonal system would be either all the (0,q_y,0) points or all the (q_x,0,0) points).
4) Obtain a |q|-axis to plot these points based on their norms: this should be consistent with the |q|-path provided by the interpolated ones.
5) Now you can plot the interpolated and calculated exciton energies together.

Hope this works! If not, I will try to speed up the introduction of this option in yambopy!

Cheers,
Fulvio

[palful]

Dear Sabrine,

Dear Yambo developers and users,

Recently, I start to do GW-BSE calculation by using yambo 5.1.0

After that, we have calculated the exciton oscillator strength using the yambo post-processing ypp. We have obtained the exciton oscillation strength and the amplitude.

Now, we aim to identify the symmetries of the excitonic state ( 1s,2s,2p...). I mean I want to be able to index every exciton states
with the quasi-hydrogenoide (mixing state) series, please see the attached figure (h), In fact we want to do index the exciton state using BSE method
as is done in this figure ( see h in the figure ) and compare to hydrogenoide system.

From this data file o-BSE.exc_qpt1_E_sorted, can we have an idea about the exciton symmetries (1s, 2s,2p ) or it will just give us an idea only about dark and bright exciton. Actually, plotting the density of exciton state, doesn't give us a clear view about the symmetries of the different state since the exitonic wave function in this case take into account also the orbital symmetries (Bloch function) not just the symmetries of the envelope function.

Thanks

First of all a suggestion: maybe this question should have been put in a different thread and not in Sitangshu's thread which is about a different topic.

At the moment, there is no way to automatically extract symmetry information from a yambo BSE calculation in an automatic way. We have plans to add this capability to yambopy-ypp but it will take time. So far, the symmetry analysis must be done manually from the knowledge of the point group of the system and indirect evidence.

You can look at this paper for an example of symmetry analysis in the case of monolayer hexagonal boron nitride.

For example, the brightness or darkness of an exciton already tells you some indirect information about its symmetry, since bright states must transform as the dipole. Then, knowing the symmetry point group of your system, you can vary the dipole direction (BLongDir) in the BSE input and check which states become bright. Another possibility to assign "hydrogenic" labels to the excitonic states is to plot the phase of the excitonic wave function in reciprocal space instead of the modulus. A uniform phase around the transition area, for example, might indicate an s-type state. 120-degree patterns may be linked to p-states, and so on. However I'm not sure if it is always possible to find an "hydrogenic" series for excitons in complex materials. In my experience, support from simple tight-binding models, when viable, is very helpful.

Cheers,
Fulvio

[sabrine]

Dear Fulvio,

thank you very much for your reply and the suggested solution.

I hope, that this identification of exciton state will be implemented in yambopy in the near future.

Best regards.

[Davide Sangalli]

Dear Sabrine,

as Fulvio explained there is no automatic tool. Moreover any automatic tool would, at best, detect the exciton symmetry w.r.t. the point group.

The mapping to the hydrogen atom series would be only approximated. Look for example to this table in the book of Dresselhaus "Group Theory, application to the physics of condensed matter" for the Oh point group.

Symmetries_table.png

I also add another reference which might be useful: https://arxiv.org/abs/2211.12241

Best,
D

[sabrine]

Dear Davide
i understand you , thanks a lot for the proposed paper .
Best Regards.