[Sax-project-users] optical gap

[marsamos at democritos.it]

Dear user, for having the optical gap you first need to solve BSE. For 
this you'll need to run bse.x (see example3 for a run example). Once 
you run bse.x you should have as output a textual file named bse. 
Solving the BSE means to build a matrix (effective 2 particle 
Hamiltonian) and then diagonalization. The file is composed by 3 
vectors, that give the transitions taken into account, an other vector 
that begins with the tag <eigenvalues ... that contains the eigenvalues 
(the exciton energies) and a matrix that contains per columns the 
eigenvectors <line.1... <line.2... <line.3 ... (coefficient for 
excitons wave functions). The optical gap is usually the energy of the 
first electron-hole transition, i.e. the first eigenvalue (ATENTION 
energies are in RY.). Then there are two possibilies, bright or dark 
excitons. Dark excitons does not contribute to the absortion spectra 
(invisible to absorption experiements) , but they constitute "dark" 
path for de-excitations (usually the presence of dark excitons quench 
the luminescence efficiency). For knowing of you have dark or bright 
excitons you need to run (after bse.x), spectra.x that computes the 
optical absorption spectra (use a small lorentzian_broadening to have 
better energy resolution, and understand better which peaks of the 
optical absorption spectra,oas, correspond to a bse excitation). See 
example3 for a run example of spectra.x. If your first excitonns are 
dark then the experimental optical gap will be the energy of the fisrt 
bright excitation.

Best regards

SaX developers Team

PS optical gap is smaller than GW gap because in optical gap e-h 
attraction is included and this reduce the gap.

PS We gave a tutorial in Brazil last sommer, we will put, today, the 
.pdf files in the doc part of QE-forge.

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