Landau quantized excitonic absorption and photoluminescence in a monolayer valley semiconductor

We observe charge-density-dependent quantum oscillations in the excitonic absorption and luminescence of monolayer WSe₂ under magnetic fields up to B = 17.5 T. Valley-selective quantum oscillations occur for both the exciton and trions (or exciton-polarons) and reveal distinct intravalley and intervalley coupling between excitons and Landau levels (LLs). We observe spin- and valley-polarized LLs with filling factors n = +0, +1 in the lower conduction band and n = –0 to –6 in the valence band, including the Berry-curvature-induced n = ±0 LLs of massive Dirac fermions. The LL filling produces periodic plateaus in the exciton energy shift accompanied with sharp oscillations in the exciton absorption width and magnitude. This peculiar exciton behavior can be simulated by semi-empirical calculations. In addition, the experimentally deduced g-factors of the conduction band (g ~ 2.5) and valence band (g ~ 15) are much larger than the g-factors predicted in a single-particle model. Such g-factor enhancement implies strong many-body interactions at high charge density in monolayer WSe₂. The complex interplay between Landau quantization, excitonic effects, and many-body interactions, as demonstrated in our research, provides a new platform to explore novel correlated quantum phenomena.