Valley-spin polarized Landau levels in a monolayer semiconductor

Electrons in monolayer transition metal dichalcogenides (TMDs) possess both the valley and spin degree of freedom. These internal quantum degrees of freedom have provided an ideal laboratory for exploring both new physical phenomena and electronics and photonics applications. Valley- and spin-dependent optical and electrical properties, originated from Berry curvature effects, have been recently demonstrated. Such Berry curvature effects, together with strong spin-orbit interactions, can further generate unconventional Landau levels (LLs) under a perpendicular magnetic field that support valley- and spinpolarized chiral edge states in the quantum Hall regime. The unique LL structure, however, has not been demonstrated in TMDs. Here we report the observation of fully valley- and spin-polarized LLs in high-quality WSe2 monolayers achieved using a van der Waals heterostructure device platform. Handedness-resolved optical reflection spectroscopy has been applied to probe the inter-LL transitions at individual valleys. The LL structure has been constructed. A doping-induced mass renormalization driven by the strong Coulomb interactions has also been observed. Our results open the door for studies of the unconventional LL physics and quantum Hall effect in monolayer TMDs.