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Strong many-body interaction in two-dimensional transitional metal dichalcogenides provides a unique platform to study the interplay between different quasiparticles, such as prominent phonon replica emission and modified valley-selection rules. A large out-of-plane magnetic field is expected to modify the exciton-phonon interactions by quantizing excitons into discrete Landau levels, which is largely unexplored. Here, we observe the Landau levels originating from phonon-exciton complexes and directly probe exciton-phonon interaction under a quantizing magnetic field. Phonon-exciton interaction lifts the inter-Landau-level transition selection rules for dark trions, manifested by a distinctively different Landau fan pattern compared to bright trions. This allows us to experimentally extract the effective mass of both holes and electrons. The onset of Landau quantization coincides with a significant increase of the valley-Zeeman shift, suggesting strong many-body effects on the phonon-exciton interaction. Our work demonstrates monolayer WSe₂as an intriguing playground to study phonon-exciton interactions and their interplay with charge, spin, and valley.

 

We conduct a comprehensive study of three different magnetic semiconductors, CrI3, CrBr3, and CrCl3, by incorporating both few-layer and bilayer samples in van der Waals tunnel junctions. We find that the interlayer magnetic ordering, exchange gap, magnetic anisotropy, and magnon excitations evolve systematically with changing halogen atom. By fitting to a spin wave theory that accounts for nearest-neighbor exchange interactions, we are able to further determine a simple spin Hamiltonian describing all three systems. These results extend the 2D magnetism platform to Ising, Heisenberg, and XY spin classes in a single material family. Using magneto-optical measurements, we additionally demonstrate that ferromagnetism can be stabilized down to monolayer in more isotropic CrBr3, with transition temperature still close to that of the bulk. 

Engineering the properties of materials is of central importance in modern science and technology. In conventional bulk materials, the property changes are realized mainly through modifying the chemical bonds and crystalline structures, or introducing dopant atoms. Recent studies in van der Waals coupled 2D layered materials have demonstrated a different way of materials engineering through modifying interlayer interactions at the material interfaces, because the interlayer interactions in these materials can be easily controlled by changing interlayer stacking configurations and/or applying external fields. In this review, recent progresses in exploring the effects of van der Waals interlayer interactions on graphene, including the discovery of van Hove singularities, Fermi velocity renormalization, and Hofstadter's butterfly pattern, are discussed. Interlayer interactions at other 2D layered material interfaces, such as transition metal dichalcogenides and black phosphorus, are also discussed. Finally, the prospects of using the van der Waals coupled 2D layered materials for next‐generation electronics and optoelectronics are presented.