Many-body interactions in monolayer transition-metal dichalcogenides are strongly affected by their unique band structure. We study these interactions by measuring the energy shift of neutral excitons (bound electron-hole pairs) in gated ${\mathrm{WSe}}_2$ and ${\mathrm{MoSe}}_2$. Surprisingly, while the blueshift of the neutral exciton $X^0$ in electron-doped samples can be more than 10 meV, the blueshift in hole-doped samples is nearly absent. Taking into account dynamical screening and local-field effects, we present a transparent and analytical model that elucidates the crucial role played by intervalley plasmons in electron-doped conditions. The energy shift of $X^0$ as a function of charge density is computed showing agreement with experiment, where the renormalization of $X^0$ by intervalley plasmons yields a stronger blueshift in ${\mathrm{MoSe}}_2$ than in ${\mathrm{WSe}}_2$ due to differences in their band ordering.

• Received 27 June 2016
• Revised 16 January 2019

DOI:https://doi.org/10.1103/PhysRevB.99.085301