Valley landau level crossings in weakly coupled bilayer WSe₂

Abstract WSe 2 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 WSe 2 flake are weakly coupled. The electric displacement field can easily break the layer degeneracy of the bilayer WSe 2 . Together with the strong spin–orbit coupling, it exhibits many novel quantum physical properties. In this work, we fabricate high quality dual-gated bilayer WSe 2 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 WSe 2 layers are filled independently and the bottom layer WSe 2 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 WSe 2 layer at Zeeman-to-cyclotron energy ratio <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:msub> <mml:mi>E</mml:mi> <mml:mi>Z</mml:mi> </mml:msub> <mml:mrow> <mml:mo>/</mml:mo> </mml:mrow> <mml:msub> <mml:mi>E</mml:mi> <mml:mi>N</mml:mi> </mml:msub> </mml:mrow> </mml:math> ≈ 4 and <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:msub> <mml:mi>E</mml:mi> <mml:mi>Z</mml:mi> </mml:msub> <mml:mrow> <mml:mo>/</mml:mo> </mml:mrow> <mml:msub> <mml:mi>E</mml:mi> <mml:mi>N</mml:mi> </mml:msub> </mml:mrow> </mml:math> ≈ 5. The interplay between two LLs from the two WSe 2 layers confirms the weak coupling between them. Our results show that the bilayer WSe 2 behave like two closely packed independent electronic systems under electric displacement fields.