Microscopic modelling of exciton-exciton interactions in atomically thin semiconductors

Sammanfattning: The successful isolation and characterization of the technologically promising material graphene in 2004 has spurred huge interest in other two-dimensional materials. Transition-metal dichalcogenides (TMDs), being one such class of materials, display intriguing properties from both a fundamental and techno- logical point of view. In particular, they constitute a platform for studying novel physical phenomena such as many-body correlations and exotic states of matter. The exceptionally strong Coulomb interaction in these materials gives rise to tightly bound excitons, i.e., electron-hole pairs, which are nei- ther of purely fermionic nor purely bosonic character. In the weak excitation regime, it is sufficient to treat excitons as pure bosons, but as the density of excitons increases their fermionic substructure becomes important, resulting in highly non-trivial scattering characteristics between excitons. In this work we have developed a microscopic theory of exciton-exciton in- teractions in TMD monolayers and van der Waals heterostructures. The latter are formed when stacking two monolayers on top of each other and this extends the rich exciton landscape to include also spatially separated in- terlayer excitons, consisting of electrons and holes located in different layers. We find that intralayer and interlayer excitons interact in fundamentally dif- ferent ways – while intralayer excitons interact primarily through quantum- mechanical exchange interactions, interlayer excitons can be viewed as re- pelling dipoles. We demonstrate the crucial role of these strong repulsive dipole-dipole interactions for exciton transport and diffusion. Moreover, we shed light on the particular importance of dark excitons in the context of exciton-exciton annihilation, which crucially governs the efficiency of TMD- based optoelectronic applications. The microscopic insights gained in this work can be used to study more exotic quantum phenomena such as strong correlations or exciton-exciton interactions between hybridised exciton states in moire materials.