Impact of Coulomb exchange interaction on exciton diffusion in 2D materials

Abstract

Transition metal dichalcogenides (TMDs) are a promising class of materials for future technologies due to their remarkable optical and electronic properties. In contrast to their bulk form, monolayer TMDs are direct bandgap semiconductors, which makes them promising candidates for optoelectronic devices. Furthermore, their atomically thin structure implies a reduced dielectric screening, resulting in a pronounced Coulomb interaction. This creates strongly bound electron-hole pairs, called excitons. These excitons can interact in many ways, which accounts for the rich exciton physics observed in monolayer TMDs. One example of possible interaction is the Coulomb exchange coupling. In this process, an exciton in the K valley is exchanged with an exciton in the K’ valley and vice versa. It has been shown that the exchange coupling results in the entangling of K and K’ excitons, creating a novel excitonic band geometry. In this thesis we use the framework of density matrix theory to investigate the impact of the exchange coupling on exciton-phonon interaction and exciton diffusion in TMDs and in particular MoSe2. Moreover, we generalise the band geometry to include a detuning, e.g. induced by a magnetic field, and study how this changes the impact of Coulomb exchange coupling. We find that the exciton-phonon coupling becomes dependent on excitonic phase and exciton momentum. Furthermore, we reveal that the momentum-resolved exciton-phonon scattering rates show new scattering channels arising from the modified band geometry. Finally, the knowledge of the excitonic band structure and exciton-phonon scattering rates is used to calculate excitonic diffusion. We find that the diffusion coefficient can be controlled with a magnetic field-induced detuning of K and K’ exciton states.