First-order magnetic phase-transition of mobile electrons in monolayer MoS2_2

Evidence is presented for a first-order magnetic phase transition in a gated two-dimensional semiconductor, monolayer-MoS$_2$. The phase boundary separates a spin-polarised (ferromagnetic) phase at low electron density and a paramagnetic phase at high electron density. Abrupt changes in the optical response signal an abrupt change in the magnetism. The magnetic order is thereby controlled via the voltage applied to the gate electrode of the device. Accompanying the change in magnetism is a large change in the electron effective mass

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

Capacitively and Inductively Coupled Excitons in Bilayer MoS 2

Main text: 6 pages, 4 figures; Supplementary Information: 17 pages, 15 figuresInternational audienceThe interaction of intralayer and interlayer excitons is studied in a two-dimensional semiconductor, homobilayer MoS$_2$. It is shown that the measured optical susceptibility reveals both the magnitude and the sign of the coupling constants. The interlayer exciton interacts capacitively with the intralayer B-exciton (positive coupling constant) consistent with hole tunnelling from one monolayer to the other. Conversely, the interlayer exciton interacts inductively with the intralayer A-exciton (negative coupling constant). First-principles many-body calculations show that this coupling arises via an intravalley exchange-interaction of A- and B-excitons

Giant Stark splitting of an exciton in bilayer MoS2

Transition metal dichalcogenides (TMDs) constitute a versatile platform for atomically thin optoelectronics devices and spin-valley memory applications. In monolayers optical absorption is strong, but the transition energy is not tunable as the neutral exciton has essentially no out-of-plane electric dipole. In contrast, interlayer exciton transitions in heterobilayers are widely tunable in applied electric fields, but their coupling to light is considerably reduced. Here, we show tuning over 120 meV of interlayer excitons with high oscillator strength in bilayer MoS2. These shifts are due to the quantum confined Stark effect, here the electron is localised to one of the layers yet the hole is delocalised across the bilayer. We optically probe the interaction between intra- and interlayer excitons as they are energetically tuned into resonance. This allows studying their mixing supported by beyond standard density functional theory calculations including excitonic effects. In MoS2 trilayers our experiments uncover two types of interlayer excitons with and without in-built electric dipoles, respectively. Highly tunable excitonic transitions with large oscillator strength and in-built dipoles, that lead to considerable exciton-exciton interactions, hold great promise for non-linear optics with polaritons.Comment: final author versio