Valley landau level crossings in weakly coupled bilayer WSe₂

Abstract WSe₂ is a p-type 2D-semiconductor that can be mechanically exfoliated down to atomic layers. Unlike the Bernal stacked bilayer graphene, the two layers in a bilayer WSe₂ flake are weakly coupled. The electric displacement field can easily break the layer degeneracy of the bilayer WSe₂. Together with the strong spin–orbit coupling, it exhibits many novel quantum physical properties. In this work, we fabricate high quality dual-gated bilayer WSe₂ devices and observe twofold degenerate Landau levels(LLs) and density-dependent quantum Hall states which show transitions between even and odd filling. When introducing carriers into the system from the valence band edge by gating, two WSe₂ layers are filled independently and the bottom layer WSe₂ shows negative compressibility at the crossover point. Above 9 T, we observe the degeneracy of LLs is completely broken and there are two sets of LL crossings in the top WSe₂ layer at Zeeman-to-cyclotron energy ratio {E}_Z/{E}_N ≈ 4 and {E}_Z/{E}_N ≈ 5. The interplay between two LLs from the two WSe₂ layers confirms the weak coupling between them. Our results show that the bilayer WSe₂ behave like two closely packed independent electronic systems under electric displacement fields.