Controlling the excitonic response and the electronic ground state in two-dimensional semiconductors

The vast amount of different two-dimensional (2D) materials and the possibility of combining them into arbitrary heterostructures provide an exciting playground for studying and exploiting novel physical phenomena. Semiconducting transition metal dichalcogenides (TMDs) are particularly interesting as they host strongly bound excitons, which dominate their optical response. Embedding TMDs in high-quality optoelectronic devices provides full control of the electrostatic environment and enables a large tunability of their excitonic response and their electronic ground state.
In bilayer TMD systems, interlayer excitons (IX) can form, where the electron and hole are spatially separated in the adjacent layers. A finite twist angle between the two layers leads to moiré and atomic reconstruction effects that dominate the excitonic properties. Interlayer excitons are studied in a type-II MoSe$_2$/WSe$_2$ heterobilayer. Their real space origin in the moiré potential and their momentum space origin are determined using photoluminescence spectroscopy. While these IX are widely tunable by electric fields, their coupling to light is considerably weak. Overcoming this deficit, hybridised interlayer excitons (IE) in naturally stacked homobilayer MoS$_2$ are discovered that combine a large tunability of their energy with a big oscillator strength. The large tunability is used to bring the IE into resonance with the intralayer excitons revealing two different types of exciton-exciton couplings. A classical model of two coupled optical dipoles is developed that shows a good agreement with the experimentally measured couplings. The model reveals that the measured optical susceptibility determines both the magnitude and the phase of the coupling constants.
The electronic ground state in monolayer MoS$_2$ is explored using photoluminescence spectroscopy as a local spin- and valley-sensitive probe. In a large external magnetic field, the electrons in MoS$_2$ form a ferromagnetic phase at low charge carrier densities. Evidence is presented that it is also possible to stabilise the ferromagnetic phase at zero magnetic field by using a circularly polarised excitation laser. On injecting electrons into the monolayer, a first-order phase transition from the ferromagnetic phase to a paramagnetic phase is observed at a certain critical carrier density.