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Monolayers of transition metal dichalcogenides (TMDCs) exhibit anexceptionally strong Coulomb interaction between charge carriers due to thetwo-dimensional carrier confinement in connection with weak dielectricscreening. High densities of excited charge carriers in the variousband-structure valleys cause strong many-body renormalizations that influenceboth the electronic properties and the optical response of the material. Weinvestigate electronic and optical properties of the typical monolayer TMDCsMoS$_2$, MoSe$_2$, WS$_2$ and WSe$_2$ in the presence of excited carriers bysolving semiconductor Bloch equations on the full Brillouin zone. Withincreasing carrier density, we systematically find a reduction of the excitonbinding energies due to Coulomb screening and Pauli blocking. Together withexcitation-induced band-gap shrinkage this leads to redshifts of excitonicresonances up to the dissociation of excitons. As a central result, we predictfor all investigated monolayer TMDCs that the $\Sigma$-valley shifts strongerthan the K-valley. Two of the materials undergo a transition from direct toindirect band gaps under carrier excitation similar to well-knownstrain-induced effects. Our findings have strong implications for the fillingof conduction-band valleys with excited carriers and are relevant to transportand optical applications as well as the emergence of phonon-drivensuperconductivity.

Excitation-induced transition to indirect band gaps in atomically thin transition metal dichalcogenide semiconductors