Exotic Magnons and Excitons in Quantum Two-Dimensional Magnets Probed by Terahertz and Optical Spectroscopies

Highly unusual magnon and exciton dynamics in quantum two-dimensional magnets is reported based on terahertz and optical spectroscopies performed on representative materials such as CrI₃, NiI₂, and NiPS₃. By studying (anti)ferromagnetic resonance modes in the terahertz region, including their temperature and field dependences, we detected large magnetic anisotropies typical of quantum two-dimensional magnets. In contrast, a dramatic formation of Zhang-Rice excitons have been discovered in the near-infrared and visible regions in several materials. Such novel excitons are generated by a strong hybridization between the metal d- and the ligand p-orbitals. Our results also reveal an intricate connection between these excitons and the underlying magnetic order.