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

Abstract

We report the emergence of massless Dirac fermions in moiré-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 moiré-induced secondary bands at $n/n_0=\pm 4$ host chiral massless quasiparticles with a Berry phase $\pi$ and a Fermi velocity $v_m\approx 3.6\times 10^5\mathrm{m}{\mathrm{s}}^{-1}$. This transition from massive to massless carriers arises from topological band reconstruction driven by the hBN moiré potential. Our results demonstrate that moiré engineering in BLG/hBN offers a powerful route to tune band topology and realize coexisting Dirac and massive fermions within a single crystalline platform.

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