Berry curvature induced valley Hall effect in non-encapsulated hBN/Bilayer graphene heterostructure aligned with near-zero twist angle

TL;DR

This paper demonstrates the valley Hall effect in a non-encapsulated hBN/bilayer graphene heterostructure aligned at near-zero twist angle, showing that Berry curvature can induce valley polarization without complex stacking.

Contribution

It introduces a simplified, non-encapsulated heterostructure approach to observe the valley Hall effect, validated by experimental measurements and theoretical calculations.

Findings

Clear non-local resistance signature of valley Hall effect

Manipulation of resistance with displacement field

Validation through band structure and Berry curvature calculations

Abstract

Valley Hall effect has been 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 has been used in bilayer graphene to break the layer symmetry and thereby observe the valley Hall effect, the bilayer graphene was 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 ($R_{\rm{NL}}$) was observed. The observed non-local resistance could be manipulated by applying a displacement field across the heterostructure. Furthermore, the electronic band structure and Berry curvature calculations validate the experimental observations.