Exciton fission in monolayer transition metal dichalcogenide semiconductors

When electron-hole pairs are excited in a semiconductor, it is a priori not clear if subsequent relaxation leads to a gas of bound excitons or to an interacting plasma of unbound electrons and holes. Usually, the exciton phase is associated with low temperatures. In atomically thin transition metal dichalcogenide semiconductors, excitons are particularly important even at room temperature due to strong Coulomb interaction and a large exciton density of states.
Using state-of-the-art many-body theory, we show that the thermodynamic fission-fusion balance of excitons and electron-hole plasma can be efficiently tuned via the dielectric environment as well as charge carrier doping. We observe entropy ionization of excitons at low excitation densities and a Mott transition to a fully ionized plasma at high densities between 3x10¹² cm^-2 and 1x10¹³ cm^-2 depending on experimental parameters.
We propose the observation of these effects by studying exciton satellites in photoemission and tunneling spectroscopy, which are sensitive to the single-particle spectral functions, thus containing information about the degree of exciton fission and the extent of exciton wave functions in reciprocal space.