Probing many-body interactions in monolayer transition-metal dichalcogenides

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.