Many Body Theory of Exciton and Trions in 2D Materials: Exciton-Trion Supersposition States and Exciton-Polarons*

The optical spectra of doped two-dimensional (2D) materials exhibit two sharp absorption peaks that are identified with excitons and trions. Many features of the measured spectra, such as the doping dependence of the energy separation and the spectral weights of these two peaks, cannot be explained by conventional models that treat excitons and trions as independent excitations. We show that in doped 2D materials, exciton and trions are strongly coupled as a result of Coulomb interactions and the resulting approximate eigenstates are exciton-trion supersposition states and correspond to the two peaks seen experimentally in the absorption spectra ^[1]. These eigenstates can be described by two coupled 2-body and 4-body Schrodinger equations. Solutions of these two coupled equations resemble variational exciton-polaron states ^[1] thereby establishing the relationship between our approach and Fermi polaron physics ^[2]. Our model can quantitatively explain all experimental results, including the observation of coherence in exciton-trion states and exciton-trion-polaritons in 2D materials. [1] Phys. Rev. B 102, 085304 (2020). [2] Nat. Phys. 13, 255 (2016).