Shortwave Coulomb excitations and local-field effects in monolayer transition-metal dichalcogenides.*

Many-body interactions in monolayer transition-metal dichalcogenides are strongly affected by local-field effects and their spin-split band structure. The former is caused by a strong contribution of umklapp processes to Coulomb excitations between the time-reversed valleys, where the effect is stronger for conduction-band electrons because of the nature of their atomic orbital. As a result, the blueshift of the neutral exciton, X₀, in electron-doped samples can be larger than 10 meV when the electron density increases from 0 to 5x10¹² cm^-2, while the blueshift in hole-doped samples is nearly absent. We develop an analytical theoretical model that elucidates the important role played by intervalley plasmons and local-field effects. We compute the energy shift of X₀ as a function of charge density and show that similar to experiment, the blueshift is evident only in electron-doped conditions, and that it is stronger in MoSe₂ than in WSe₂ due to differences in their band ordering and direct vs indirect exciton energies. In addition, the theory elucidates the observed emergence of exciton-plasmon peak in electron-doped WSe₂.