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Daniel Erben1 Alexander Steinhoff1 Gunnar Schönhoff1 2 Tim Wehling1 2 3 Christopher Gies1 Frank Jahnke1 3

1, Institute for Theoretical Physics, University of Bremen, Bremen, , Germany
2, Bremen Center for Computational Materials Science, Bremen, , Germany
3, MAPEX Center for Materials and Processes, Bremen, , Germany

Monolayers of transition metal dichalcogenides (TMDCs) exhibit an exceptionally strong Coulomb interaction between charge carriers due to their two-dimensional nature and weak dielectrical screening. Modifying the interaction via changing the dielectric environment or exciting charge carriers offers a possibility to control the interactions and as a consequence also the macroscopic optical properties. More generally, due to their many-body interactions, excited charge carriers directly influence the electronic and optical properties of monolayer TMDCs. This includes scenarios of electrical and optical excitation as well as charge carrier doping.
In this talk the strong many-particle renormalizations caused by the Coulomb interaction of the excited carriers will be investigated for MX2 with M = (Mo, W) and X = (S, Se). To study the properties of these four TMDCs we solve the semiconductor Bloch equations on the full Brillouin zone using ab initio band structures and interaction matrix elements.
By increasing the density of excited carriers, we find a reduction of the exciton binding energy due to Coulomb screening and Pauli blocking. In addition to the excitation-induced band-gap shrinkage, these effects lead to redshifts of the excitonic resonances on the order of several hundred meV until the excitons dissociate into a fermionic electron-hole plasma. As excitons are charge-neutral bosonic particles, the degree of their ionization strongly influences many-body effects such as screening. We quantify the exciton ionization degree depending on excitation density and environmental screening.
The central finding of our investigation is a relative shift between the K- and the neighbouring Σ-valley in the conduction band induced by the renormalizations. All four TMDCs show a tendency to become more indirect as the Σ-valley shifts to energetically lower values than K. While monolayer TMDCs are usually celebrated for offering direct band gaps, we predict a transition from a direct to an indirect band gap for MoS2 and WS2. This transition should also lead to a drain of carriers from the K-valley and thus to a quenching of the photoluminescence, similar as in strained monolayers. Our findings are also relevant to transport and optical applications e.g. the emergence of superconductivity.

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