A self-consistent scheme for the calculations of the interacting groundstate and the near bandgap optical spectra of mono-and multilayer transition-metal-dichalcogenide systems is presented. The approach combines a dielectric model for the Coulomb interaction potential in a multilayer environment, gap equations for the renormalized groundstate, and the Dirac-Wannier-equation to determine the excitonic properties. To account for the extension of the individual monolayers perpendicular to their basic plane, an effective thickness parameter in the Coulomb interaction potential is introduced. Numerical evaluations for the example of MoS2 show that the resulting finite size effects lead to significant modifications in the optical spectra, reproducing the experimentally observed non hydrogenic features of the excitonic resonance series. Applying the theory for multi-layer configurations, a consistent description of the near bandgap optical properties is obtained all the way from monolayer to bulk. In addition to the well-known in-plane excitons, also interlayer excitons occur in multilayer systems suggesting a reinterpretation of experimental results obtained for bulk material.

Optical absorption and gain spectra in MoS2 monolayers with thermal carrier distributions are calculated from the combined gap and Dirac-Bloch equations. It is shown that the excited carriers lead to a bandgap renormalization as large as 800 meV for a suspended monolayer MoS2. Above the critical density, optical gain is obtained over an approx. 400 meV broad spectral range above the gap. Whereas the absorption spectra in the low density regime are very sensitive to the dielectric environment, the spectra become purely intrinsic at elevated carrier densities.

The dynamics of band-gap renormalization and gain build-up in monolayer MoTe2-H is investigated by evaluating the non-equilibrium Dirac-Bloch equations with the incoherent carrier-carrier and carrier-phonon scattering treated via quantum-Boltzmann type scattering equations. For the case where an approximately 300 fs-long high intensity optical pulse generates charge-carrier densities in the gain regime, the strong Coulomb coupling leads to a relaxation of excited carriers on a few fs time scale. The pump-pulse generation of excited carriers induces a large band-gap renormalization during the time scale of the pulse. Efficient phonon coupling leads to a subsequent carrier thermalization within a few ps, which defines the time scale for the optical gain build-up energetically close to the low-density exciton resonance. arXiv:1903.08553v2 [cond-mat.mes-hall]

Valley-selective optical selection rules and a spin-valley locking in transition-metal dichalcogenide (TMDC) monolayers are at the heart of "valleytronic physics", which exploits the valley degree of freedom and has been a major research topic in recent years. In contrast, valleytronic properties of TMDC bilayers have not been in the focus so much by now. Here, we report on the valleytronic properties and optical characterization of bilayers of WS 2 as a representative TMDC material. In particular, we study the influence of the relative layer alignment in TMDC homo-bilayer samples on their polarization-dependent optical properties. Therefore, CVD-grown WS 2 bilayer samples have been prepared that favor either the inversion symmetric AA' stacking or AB stacking without inversion symmetry during synthesis. Subsequently, a detailed analysis of reflection contrast and photoluminescence spectra under different polarization conditions has been performed. We observe circular and linear dichroism of the photoluminescence that is more pronounced for the AB stacking configuration. Our experimental findings are supported by theoretical calculations showing that the observed dichroism can be linked to optical selection rules, that maintain the spin-valley locking in the AB-stacked WS 2 bilayer, whereas a spin-layer-locking is present the inversion symmetric AA' bilayer instead. Furthermore, our theoretical calculations predict a small relative shift of the excitonic resonances in both stacking configurations, which is also experimentally observed.The ability to obtain van-der-Waals (vdW) materials as monolayers has rendered them an emerging novel material class. Transition metal dichalcogenides (TMDCs) as two-dimensional semiconductors have attracted considerable attention, because of their extraordinary strong light-matter interaction and excitonic effects, but also because of their potential application in valleytronic devices.Common to TMDCs and other layered vdW materials is the honeycomb geometry in real space as well as in reciprocal space with direct band gaps occuring at the corners of the Brillouin zone. Opposite corners are related by the parity or time-reversal symmetry and are referred to as K and K' valleys. The broken spatial inversion symmetry in a TMDC monolayer allows addressing opposite valleys separately with circular polarized light. This opens up the possibility of using the valley index -or pseudo-spin -for information storage and processing, to name but a few applications.Many optical investigations on TMDC monolayers have focused on the aspect of valley coherence and found a pronounced optical helicity [1-7] as well as a linear-polarization anisotropy for emitted light after

The quasi-two dimensional Coulomb interaction potential in transition metal dichalcogenides is determined using the Kohn–Sham wave functions obtained from ab initio calculations. An effective form factor is derived that accounts for the finite extension of the wave functions in the direction perpendicular to the material layer. The resulting Coulomb matrix elements are used in microscopic calculations based on the Dirac Bloch equations yielding an efficient method to calculate the band gap and the opto-electronic material properties in different environments and under various excitation conditions.