  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.
