Quantum Theory of Exciton-Exciton Interactions in Atomically Thin Semiconductors

Sammanfattning: Atomically thin materials such as transition-metal dichalcogenides (TMDs) have emerged as an unprecedented platform for engineering future optoelectronic devices and studying exotic quantum phases of matter. In particular, the strong Coulomb interaction in these materials enables the formation of tightly-bound excitons, electron-hole pairs with neither purely fermionic nor bosonic character. These intriguing quasiparticles dominate the optical response, dynamics and transport properties of TMDs. The aim of this thesis is to investigate the impact of exciton-exciton interactions on optics, relaxation dynamics and diffusion in 2D materials. Based on the density-matrix formalism, we develop equations of motion which are used to study intriguing and technologically relevant phenomena in the high-excitation regime where interexcitonic interactions dominate. We shed light on the fundamentally different interactions between intra- and interlayer excitons in TMD monolayers and bilayers, respectively. Exciton-exciton interactions in monolayers are governed by quantum-mechanical exchange interactions and interlayer excitons in heterobilayers interact predominantly via dipole-dipole repulsion. Furthermore, we demonstrate the crucial importance of optically-dark recombination channels in exciton-exciton annihilation and reveal the dominant role of dipole-dipole repulsion in interlayer exciton transport. Finally, we show that interactions between layer-hybridized excitons are electrically tunable and that electric fields can be used to boost and control the exciton propagation in van der Waals heterostructures. Our material-specific and predictive theory has allowed us to predict experimental observations, such as density-dependent exciton line-shifts and exciton-exciton annihilation rates, that have been verified through joint theory-experiment collaborations. Overall, this thesis provides microscopic insights into exciton-exciton interactions, which are expected to play a central role in the optimal operation of optoelectronic devices and the realization of strongly correlated electron-hole systems.