Biexcitons fine structure and non-equilibrium effects in transition metal dichalcogenides monolayers from first principles

Transition metal dichalcogenides monolayers host strongly bounded Coulomb complexes such as exciton and trion due to charge confinement and screening reduction in two dimensions. Biexciton, a bound state of two electrons and two holes, has also been observed in these materials with a binding energy which is one order of magnitude larger than its counterpart in conventional semiconductors. Here, using first principles methods, we address the biexciton in WSe2 monolayer and unravel the important role of the electron-hole exchange interaction in dictating the valley character of biexciton states and their fine structure. In particular, the fundamental biexciton transition which is located between the exciton and trion peaks is shown to have a fine structure of 2.8 meV mainly due to the splitting of the dark exciton state under the intervalley electron-hole exchange interaction. Non equilibrium effects are also addressed and optical fingerprints of non-thermalized biexciton population are discussed.