Topological Valley Currents in Bilayer Graphene/Hexagonal Boron Nitride Superlattices

TL;DR

This paper demonstrates the electrical detection of topological valley currents in bilayer graphene/hBN superlattices, revealing nonlocal resistance linked to Berry curvature effects without magnetic fields, advancing valleytronics research.

Contribution

It reports the first observation of nonlocal valley currents in bilayer graphene/hBN moiré superlattices using an all-electrical method, highlighting topological effects without magnetic fields.

Findings

Nonlocal resistance of about 1 kΩ observed.

Nonlocal resistance obeys a scaling relation.

Valley current linked to Berry curvature hot spots.

Abstract

Graphene superlattices have recently been attracting growing interest as an emergent class of quantum metamaterials. In this paper, we report the observation of nonlocal transport in bilayer graphene (BLG) superlattices encapsulated between two hexagonal boron nitride (hBN) layers, which formed hBN/BLG/hBN moir\'e superlattices. We then employed these superlattices to detect a long-range charge-neutral valley current using an all-electrical method. The moir\'e superlattice with broken inversion symmetry leads to a hot spot with Berry curvature accumulating at the charge neutral point (CNP), and it harbors satellites of the CNP. We observed nonlocal resistance on the order of 1 $\text{k}\Omega$, which obeys a scaling relation. This nonlocal resistance evolves from the quantum Hall effect but without magnetic field/time-reversal symmetry breaking, which is associated with a hot-spot-induced topological valley current. This study should pave the way to developing a Berry-phase-sensitive probe to detect hot spots in gapped Dirac materials with inversion-symmetry breaking.