Theory of Coulomb Complexes with Negative Reduced Mass in Two-Dimensional Semiconductors

Monolayers of transition metal dichalcogenides are characterized by the appearance of correlated Coulomb complexes: excitons, trions, biexcitons, near the K+ and K– points of the Brillouin zone, in which a direct band gap is realized. In recent experiments, Coulomb complexes have been discovered in which one of the electrons is located not in the ground band (the lowest energy conduction band), but in the excited conduction band. A distinctive feature of the dispersion of the excited conduction band is the negative effective mass. In this work, the binding energy of excitons and trions with the electron in the high-lying band is calculated using the variational method and scattering theory, and the condition for the existence of trions with a negative reduced mass in the parabolic dispersion approximation is studied.