Optically generated excitons dictate the absorption and emission spectrum of doped semiconductor transition-metal dichalcogenide monolayers. We show that upon increasing the electron density, the elementary optical excitations develop a rotonlike dispersion, evidenced by a shift of the lowest-energy state to a finite momentum on the order of the Fermi momentum. This effect emerges due to Pauli exclusion between excitons and the electron Fermi sea, but the robustness of the roton minimum in these systems is a direct consequence of the long-range nature of the Coulomb interaction and the nonlocal dielectric screening characteristic of monolayers. Finally, we show that the emergence of rotons could be related to certain features of photoluminescence spectra in doped transition-metal dichalcogenide monolayers.

• Received 25 December 2018
• Revised 19 November 2019
• Accepted 21 April 2020

DOI:https://doi.org/10.1103/PhysRevB.101.205409