Berry Curvature Induced Valley Hall Effect in Non-Encapsulated hBN/Bilayer Graphene Heterostructure Aligned with Near-Zero Twist Angle

Valley Hall effect is observed in asymmetric single-layer and bilayer graphene systems. In single-layer graphene systems, asymmetry is introduced by aligning graphene with hexagonal boron nitride (hBN) with a near-zero twist angle, breaking the sub-lattice symmetry. Although a similar approach is used in bilayer graphene to break the layer symmetry and thereby observe the valley Hall effect, the bilayer graphene is sandwiched with hBN on both sides in those studies. This study looks at a much simpler, non-encapsulated structure where hBN is present only at the top of graphene. The crystallographic axes of both hBN and bilayer graphene are aligned. A clear signature of the valley Hall effect through non-local resistance measurement (RNL) is observed. The observed non-local resistance can be manipulated by applying a displacement field across the heterostructure. Furthermore, the electronic band structure and Berry curvature calculations validate the experimental observations. This study details the observation of the valley Hall effect in non-encapsulated hBN/bilayer graphene heterostructure (hBN/bilayer graphene/SiO2) where the hBN and bilayer graphene are aligned with near zero-twist angle. Aligning bilayer graphene with hBN gives rise to a global bandgap and a finite Berry curvature, leading to the valley Hall effect. image