Second generation Dirac cones and inversion symmetry breaking induced gaps in graphene/hexagonal boron nitride

TL;DR

This study provides direct experimental evidence of second generation Dirac cones in graphene/h-BN heterostructures, revealing their locations, dispersion, and the effects of inversion symmetry breaking on gap opening, advancing understanding of band structure engineering.

Contribution

First direct experimental observation of SDCs in aligned graphene/h-BN, clarifying their dispersion, location, and the impact of inversion symmetry breaking.

Findings

SDCs located at superlattice Brillouin zone corners

Gaps of approximately 100 meV at SDCs and 160 meV at original Dirac cone

Strong inversion symmetry breaking influences band structure in graphene/h-BN

Abstract

Graphene/h-BN has emerged as a model van der Waals heterostructure, and the band structure engineering by the superlattice potential has led to various novel quantum phenomena including the self-similar Hofstadter butterfly states. Although newly generated second generation Dirac cones (SDCs) are believed to be crucial for understanding such intriguing phenomena, so far fundamental knowledge of SDCs in such heterostructure, e.g. locations and dispersion of SDCs, the effect of inversion symmetry breaking on the gap opening, still remains highly debated due to the lack of direct experimental results. Here we report first direct experimental results on the dispersion of SDCs in 0$^\circ$ aligned graphene/h-BN heterostructure using angle-resolved photoemission spectroscopy. Our data reveal unambiguously SDCs at the corners of the superlattice Brillouin zone, and at only one of the two superlattice valleys. Moreover, gaps of $\approx$ 100 meV and $\approx$ 160 meV are observed at the SDCs and the original graphene Dirac cone respectively. Our work highlights the important role of a strong inversion symmetry breaking perturbation potential in the physics of graphene/h-BN, and fills critical knowledge gaps in the band structure engineering of Dirac fermions by a superlattice potential.