Quantum theory of moiré excitons in atomically thin semiconductors

Doktorsavhandling, 2024

In recent years, atomically thin nanomaterials have garnered significant attention for their intriguing physics and potential applications in novel technological devices. Transition metal dichalcogenides (TMDs) stand out due to the reduced screening resulting in a remarkably strong Coulomb interaction and the formation of excitons, Coulomb-bound electron-hole pairs, which dominate the material's optical properties even at room temperature. By vertically stacking two TMD layers, long-lived interlayer excitons emerge, with electrons and holes spatially separated in different layers. The introduction of another layer introduces interlayer tunneling and layer polarization. These can be finely tuned by adjusting the relative twist angle between layers, forming a moiré pattern capable of spatially trapping excitons.

This thesis aims to explore the exciton energy landscape in vertically stacked TMDs using a material-specific, quantum mechanical approach. The study delves into the various coupling mechanisms and their impact on the exciton band structure, subsequently influencing the material's optical properties. Additionally, the thesis investigates electric fields and twist angle engineering as externally accessible tuning knobs. The role of interlayer tunneling in bilayer systems is emphasized, revealing strongly layer-hybridized excitons that often dominate the material's optical spectra. Atomic reconstruction in the small twist angle regime is also examined, demonstrating its significant impact on the exciton energy landscape and exciton wave function. Finally, the twist-angle-dependent in-plane charge-separation is investigated and its relation to the inter-site hopping of the moiré exciton. In summary, this research sheds light on the physics governing moiré excitons in this promising class of atomically thin nanomaterials.