Coexisting Massive and Massless Dirac Fermions in Moire'-Reconstructed Bilayer Graphene

TL;DR

This study demonstrates the coexistence of massive and massless Dirac fermions in moiré-reconstructed bilayer graphene aligned with hBN, revealing topological band reconstruction and tunable band topology through moiré engineering.

Contribution

It uncovers the emergence of massless Dirac fermions in moiré bands of bilayer graphene/hBN, showing topological transition from massive to massless carriers.

Findings

Massless Dirac fermions observed at moiré-induced secondary bands.

Primary bilayer graphene retains parabolic dispersion with Berry phase 2π.

Transition driven by topological band reconstruction from moiré potential.

Abstract

We report the emergence of massless Dirac fermions in moir\'{e}-reconstructed bands of bilayer graphene (BLG) aligned with hexagonal boron nitride (hBN). Magnetotransport measurements reveal that while the primary BLG band retains a parabolic dispersion with a Berry phase of $2\pi$, the moir\'{e}-induced secondary bands at $n/n_0 = \pm 4$ host chiral massless quasiparticles with a Berry phase $\pi$ and a Fermi velocity $v_m \approx 3.6 \times 10^5 \mathrm{m s^{-1}}$. This transition from massive to massless carriers arises from topological band reconstruction driven by the hBN moir\'{e} potential. Our results demonstrate that moir\'{e} engineering in BLG/hBN offers a powerful route to tune band topology and realize coexisting Dirac and massive fermions within a single crystalline platform.