Yambo Community Forum

Hi Yambo developers

I am trying to calculate the exciton binding energy, according to the QP gap and optical gap from this tutorial (http://www.yambo-code.org/tutorials/GaSb/absorption.php)

The QP gap is about 0.54 eV (the experimental gap is about 0.73 eV), and the optical gap (the lowest few excitation energy) is more than 1 eV.

Therefore, the exciton binding energy is negative for GaSb???

Is this reasonable?

thanks

I'm not expert with this materials,
anyway excitons can be bound or unbound.

Best,
Daniele

Hi Daniele

The exciton binding energy in bulk GaSb is about 1.5 meV from this paper (http://journals.aps.org/prb/pdf/10.1103 ... B.59.15395), I am not clear what's wrong with my results.

The experimental gap is just 0.73 eV, and the lowest optical gap is more than 1 eV. So the exciton is definitely negative.

Maybe I could try to find out more papers on the discussion of exciton bing energy in GaSb

thanks

Dear Eric,
if you took the input from the tutorial, please bear in mind that they are meant for school tutorial and it is needed the run should not be too time consuming for
obvious reason. So they are useful to learn how to carry on the calculations and the meaning of main variables, but the results are usually out-of-convergence.
Convergence tests are mandatory.
Best,
Daniele

Hi Daniele

So even without the usage of yambo, we can still conclude the exciton binding energy is negative or not based on the available experimental data.

For example, the experimental gap is 0.73 eV, and the lowest absorption spectrum is more than 1 eV. So the binding energy is negative, right?

thanks

Dear Eric

First, you should not compare the experimental gap with the calculated position of the exciton.
You should compare the fundamental gap in your calculation with the position of the exciton in the same calculation.
Second as Daniele said before any conclusion one needs to converge the calculation carefully: both the GW and the BSE.
By comparing the QP and QP+BSE spectra (columns 1:4 and 1:2 respectively) you can see whether you have bound or unbound excitons.