Dear all,
I have been trying to study electronic and optical absorption properties of a material (1D) through GW+BSE simulation. In the GW level the system has a direct band gap of 3.05 eV at the gamma point and 4.85 eV at Z point of the Brillouin zone. Afterwards, I did the BSE study to obtain the optical absoption spectra.The light is polarised along the Z direction. But, I am getting the first optical gap at 3.26 eV, which is more than the direct gap at gamma poin. Is there any explanation for this?
Usually the optical gap is lower than the minimum direct band gap.
Yes, there could be several reasons why the first optical gap obtained from the BSE simulation differs from the direct band gap at the gamma point obtained from the GW calculation.
1. The Bethe-Salpeter Equation (BSE) takes into account excitonic effects, which are electron-hole interactions that can lead to the formation of bound electron-hole pairs (excitons). These excitonic effects can modify the energy of the optical transitions compared to the single-particle picture obtained from the GW calculation.
2. The optical transition at 3.26 eV may involve different symmetry properties compared to the direct band gap at the gamma point. The optical transition may be allowed by selection rules, whereas the direct band gap at the gamma point may have different selection rules or may be forbidden due to symmetry constraints.
3. If your system is finite in size (e.g., a nanowire or nanotube), quantum confinement effects can influence the electronic and optical properties. The finite size can lead to a shift in the energy levels and optical transitions compared to the bulk material.
4. It's also important to ensure that the Brillouin zone sampling used in both the GW and BSE calculations is sufficient to accurately capture the electronic structure and optical transitions. Insufficient k-point sampling can lead to inaccuracies in the calculated band structures and optical spectra.
In summary, the discrepancy between the first optical gap obtained from the BSE simulation and the direct band gap at the gamma point from the GW calculation can be attributed to excitonic effects, symmetry considerations, finite size effects, k-point sampling, and approximations in the calculations. Further analysis and potentially refinement of the calculation parameters may be necessary to reconcile the results.
- Badges
- Science topic
- Similar topics
- Electrochemistry
- Battery