Magneto-optics of layered two-dimensional semiconductors and heterostructures: Progress and prospects

Beginning with the "conventional" two-dimensional (2D) quantum wells based on III-V and II-VI semiconductors in the 1970s, to the recent atomically thin sheets of van der Waals materials such as 2D semiconducting transition metal dichalcogenides (TMDCs) and 2D magnets, the research in 2D materials is continuously evolving and providing new challenges. Magneto-optical spectroscopy has played a significant role in this area of research, both from fundamental physics and technological perspectives. A major challenge in 2D semiconductors such as TMDCs is to understand their spin-valley-resolved physics and their implications in quantum computation and information research. Since the discovery of valley Zeeman effects, deep insights into the spin-valley physics of TMDCs and their heterostructures have emerged through magneto-optical spectroscopy. In this Perspective, we highlight the role of magneto-optics in many milestones such as the discovery of interlayer excitons, phase control between coherently excited valleys, determination of exciton-reduced masses, Bohr radii and binding energies, physics of the optically bright and dark excitons, trions, and other many-body species such as biexcitons and their phonon replicas in TMDC monolayers. The discussion accompanies open questions, challenges, and future prospects in the field including comments on the magneto-optics of van der Waals heterostructures involving TMDCs and 2D magnets.