Malte Rösner

Bio: Malte Rösner is an academic researcher from Radboud University Nijmegen. The author has contributed to research in topics: Electronic band structure & Exciton. The author has an hindex of 23, co-authored 59 publications receiving 1751 citations. Previous affiliations of Malte Rösner include University of Bremen & University of Southern California.

Influence of Excited Carriers on the Optical and Electronic Properties of MoS2

Abstract: We study the ground-state and finite-density optical response of molybdenum disulfide by solving the semiconductor Bloch equations, using ab initio band structures and Coulomb interaction matrix elements. Spectra for excited carrier densities up to 1013 cm–2 reveal a redshift of the excitonic ground-state absorption, whereas higher excitonic lines are found to disappear successively due to Coulomb-induced band gap shrinkage of more than 500 meV and binding-energy reduction. Strain-induced band variations lead to a redshift of the lowest exciton line by ∼110 meV/% and change the direct transition to indirect while maintaining the magnitude of the optical response.

Optimal Hubbard models for materials with nonlocal Coulomb interactions: graphene, silicene, and benzene.

Abstract: To understand how nonlocal Coulomb interactions affect the phase diagram of correlated electron materials, we report on a method to approximate a correlated lattice model with nonlocal interactions by an effective Hubbard model with on-site interactions ${U}^{*}$ only. The effective model is defined by the Peierls-Feynman-Bogoliubov variational principle. We find that the local part of the interaction $U$ is reduced according to ${U}^{*}=U\ensuremath{-}\overline{V}$, where $\overline{V}$ is a weighted average of nonlocal interactions. For graphene, silicene, and benzene we show that the nonlocal Coulomb interaction can decrease the effective local interaction by more than a factor of 2 in a wide doping range.

Efficient Excitonic Photoluminescence in Direct and Indirect Band Gap Monolayer MoS2

Abstract: We discuss the photoluminescence (PL) of semiconducting transition metal dichalcogenides on the basis of experiments and a microscopic theory The latter connects ab initio calculations of the single-particle states and Coulomb matrix elements with a many-body description of optical emission spectra For monolayer MoS2, we study the PL efficiency at the excitonic A and B transitions in terms of carrier populations in the band structure and provide a quantitative comparison to an (In)GaAs quantum well-structure Suppression and enhancement of PL under biaxial strain is quantified in terms of changes in the local extrema of the conduction and valence bands The large exciton binding energy in MoS2 enables two distinctly different excitation methods: above-band gap excitation and quasi-resonant excitation of excitonic resonances below the single-particle band gap The latter case creates a nonequilibrium distribution of carriers predominantly in the K-valleys, which leads to strong emission from the A-exciton transition and a visible B-peak even if the band gap is indirect For above-band gap excitation, we predict a strongly reduced emission intensity at comparable carrier densities and the absence of B-exciton emission The results agree well with PL measurements performed on monolayer MoS2 at excitation wavelengths of 405 nm (above) and 532 nm (below the band gap)

Exciton fission in monolayer transition metal dichalcogenide semiconductors

Abstract: When electron-hole pairs are excited in a semiconductor, it is a priori not clear if they form a plasma of unbound fermionic particles or a gas of composite bosons called excitons. Usually, the exciton phase is associated with low temperatures. In atomically thin transition metal dichalcogenide semiconductors, excitons are particularly important even at room temperature due to strong Coulomb interaction and a large exciton density of states. Using state-of-the-art many-body theory, we show that the thermodynamic fission–fusion balance of excitons and electron-hole plasma can be efficiently tuned via the dielectric environment as well as charge carrier doping. We propose the observation of these effects by studying exciton satellites in photoemission and tunneling spectroscopy, which present direct solid-state counterparts of high-energy collider experiments on the induced fission of composite particles. Owing to their atomically thin nature, 2D transition metal dichalcogenides host room temperature, strongly bound excitons. Here, the authors show that the thermodynamical balance between fission and fusion of excitons can be tuned by the dielectric environment and charge carrier doping and observed by photoemission spectroscopy.

Observation of Exciton Redshift–Blueshift Crossover in Monolayer WS2

Abstract: We report a rare atom-like interaction between excitons in monolayer WS2, measured using ultrafast absorption spectroscopy. At increasing excitation density, the exciton resonance energy exhibits a pronounced redshift followed by an anomalous blueshift. Using both material-realistic computation and phenomenological modeling, we attribute this observation to plasma effects and an attraction–repulsion crossover of the exciton–exciton interaction that mimics the Lennard-Jones potential between atoms. Our experiment demonstrates a strong analogy between excitons and atoms with respect to interparticle interaction, which holds promise to pursue the predicted liquid and crystalline phases of excitons in two-dimensional materials.

Valleytronics in 2D materials

Abstract: Semiconductor technology is currently based on the manipulation of electronic charge; however, electrons have additional degrees of freedom, such as spin and valley, that can be used to encode and process information. Over the past several decades, there has been significant progress in manipulating electron spin for semiconductor spintronic devices, motivated by potential spin-based information processing and storage applications. However, experimental progress towards manipulating the valley degree of freedom for potential valleytronic devices has been limited until very recently. We review the latest advances in valleytronics, which have largely been enabled by the isolation of 2D materials (such as graphene and semiconducting transition metal dichalcogenides) that host an easily accessible electronic valley degree of freedom, allowing for dynamic control. The energy extrema of an electronic band are referred to as valleys. In 2D materials, two distinguishable valleys can be used to encode information and explore other valleytronic applications.

Colloquium : Excitons in atomically thin transition metal dichalcogenides

Abstract: Atomically thin materials such as graphene and monolayer transition metal dichalcogenides (TMDs) exhibit remarkable physical properties resulting from their reduced dimensionality and crystal symmetry. The family of semiconducting transition metal dichalcogenides is an especially promising platform for fundamental studies of two-dimensional (2D) systems, with potential applications in optoelectronics and valleytronics due to their direct band gap in the monolayer limit and highly efficient light-matter coupling. A crystal lattice with broken inversion symmetry combined with strong spin-orbit interactions leads to a unique combination of the spin and valley degrees of freedom. In addition, the 2D character of the monolayers and weak dielectric screening from the environment yield a significant enhancement of the Coulomb interaction. The resulting formation of bound electron-hole pairs, or excitons, dominates the optical and spin properties of the material. Here recent progress in understanding of the excitonic properties in monolayer TMDs is reviewed and future challenges are laid out. Discussed are the consequences of the strong direct and exchange Coulomb interaction, exciton light-matter coupling, and influence of finite carrier and electron-hole pair densities on the exciton properties in TMDs. Finally, the impact on valley polarization is described and the tuning of the energies and polarization observed in applied electric and magnetic fields is summarized.