Revealing the exciton masses and dielectric properties of monolayer semiconductors with high magnetic fields

In semiconductor physics, many essential optoelectronic material parameters can be experimentally revealed via optical spectroscopy in sufficiently large magnetic fields. For monolayer transition-metal dichalcogenide semiconductors, this field scale is substantial -- of order 100 tesla! -- due to heavy carrier masses and huge exciton binding energies. Here we report absorption spectroscopy of monolayer MoS₂, MoSe₂, MoTe₂, and WS₂ in very high magnetic fields to 91 T. We follow the diamagnetic shifts and valley Zeeman splittings of not only the exciton’s 1s ground state but also its excited 2s, 3s, … ns Rydberg states. This provides a direct experimental measure of the effective (reduced) exciton masses and dielectric properties. Exciton binding energies, exciton radii, and free-particle bandgaps are also determined. Unexpectedly, the measured exciton masses are significantly heavier than theoretically predicted, especially for the Mo-based monolayers. These results provide essential and quantitative parameters for the rational design of optoelectronic van der Waals heterostructures incorporating 2D semiconductors. [1] Goryca et al., Nature Comm. 10, 4172 (2019).