First-Principles Many-Body Theory For Charged And Neutral Excitations: Trion Fine Structure Splitting In Transition Metal Dichalcogenides

A hybrid theory which combines configuration interaction and Green's function approaches is proposed here to treat charged and neutral excitations. The theory is parameter free and reduces to the well-known Bethe-Salpeter equation (BSE) in the case of excitons. However, unlike the BSE, the theory can be applied to calculate any excitation beyond the exciton. As this type of computation is generally time consuming, we show that in the case of Wannier-type excitations, the localization in reciprocal space can be used to reduce the required computation load. We apply our approach to excitons and trions in WS2 and MoS2 transition metal dichalcogenides monolayers and obtain optical spectra, binding energies, and dark-bright exciton splitting in good agreement with experimental measurements. Moreover, in the case of WS2, we have found that the negative trion peak shows a fine structure splitting (FSS) of 15 meV which corresponds to the FSS for an isolated monolayer.