Gaps induced by inversion symmetry breaking and second-generation Dirac cones in graphene/hexagonal boron nitride

Abstract

Graphene/hexagonal boron nitride (h-BN) has emerged as a model van der Waals heterostructure 1 as the superlattice potential, which is induced by lattice mismatch and crystal orientation, gives rise to various novel quantum phenomena, such as the self-similar Hofstadter butterfly states 2-5 . Although the newly generated second-generation Dirac cones (SDCs) are believed to be crucial for understanding such intriguing phenomena, fundamental knowledge of SDCs, such as locations and dispersion, and the e ect of inversion symmetry breaking on the gap opening, still remains highly debated due to the lack of direct experimental results. Here we report direct experimental results on the dispersion of SDCs in 0 • -aligned graphene/h-BN heterostructures using angle-resolved photoemission spectroscopy. Our data unambiguously reveal SDCs at the corners of the superlattice Brillouin zone, and at only one of the two superlattice valleys. Moreover, gaps of approximately 100 meV and approximately 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.Hexagonal boron nitride (h-BN) shares a similar honeycomb lattice structure to graphene, yet its lattice is stretched by 1.8%. Moreover, the breaking of the inversion symmetry by distinct boron and nitrogen sublattices leads to a large bandgap (5.97 eV) in the π band, which is in sharp contrast to the gapless Dirac cones in graphene. By stacking graphene atop h-BN to form a van der Waals heterostructure 1 , graphene/h-BN not only exhibits greatly improved properties for device applications, such as reduced ripples, suppressed charge inhomogeneities and higher mobility 6,7 , but also provides unique opportunities for band structure engineering of Dirac fermions by a periodic potential 8,9 . The superlattice potential induced by the lattice mismatch and crystal orientation can significantly modify the electronic properties of graphene and lead to various novel quantum phenomena, for example, the emergence of second-generation Dirac cones (SDCs), which are crucial for the realization of Hofstadter butterfly states under an applied magnetic field 2-5 , renormalization of the Fermi velocity 8,[10][11][12] , gap opening at the Dirac point 4,13-16 , topological currents 15 and gate-dependent pseudospin mixing 17 . Hence, understanding the effects of the superlattice potential on the band structure of graphene is crucial for advancing its device applications, and for gaining new knowledge about the fundamental physics of Dirac fermions in a periodic potential.Previously, the existence of SDCs has been deduced from scanning tunnelling spectroscopy, resistivity and capacitance measurements 2,5,18,19 . However, such measurements are not capable of mapping out the electronic dispersion with momentum-resolved informatio...

Giant spin-splitting and gap renormalization driven by trions in single-layer WS2/h-BN heterostructures

“…The clear electronic states and lack of band hybridization reveal a weak interlayer interaction between the two materials. Similar to work 22 on graphene/h-BN, we expect these data to represent the intrinsic electronic structure of single-layer WS 2 with negligible substrate influence.…”

“…This should enable direct investigation of the intrinsic electronic structure and many-body effects of the adjacent TMD. Hexagonal BN is often used as a substrate for graphene heterostructures 8,20 with high device performance 21 and new exotic electronic states such as quantized Dirac cones 22 . Unfortunately, the lateral size of mechanically assembled heterostructures is usually of the order of 10 μ m, much smaller than the beam spot of typical ARPES setups (≳ 100 μ m).…”

Moiré band model and band gaps of graphene on hexagonal boron nitride

“…Collectively, these results imply that G/BN should not have a band gap. However, the experimental observation of sizeable band gaps [19,[21][22][23] has led to theories where the nonzero average mass generation introduced by the partial commensuration of the G/BN layers [24,25], and electron-electron interaction effects [26,27] play a relevant role. Other manifestations of the moiré pattern effects in G/BN include the Hofstadter butterfly [28,29], topological valley current [30,31], tunable Van Hove singularities in the low-energy regime [32], and the emergence of secondary Dirac cones (sDC) at the edge of the moiré Brilluoin Zone (mBZ) [23,29].…”

Real-space mapping of topological invariants using artificial neural networks

“…The sublattice imbalance could arise for a graphene monolayer deposited on boron nitride in a commensurate fashion [11,60]. The Rashba and exchange fields naturally arise for a graphene monolayer deposited over a ferromagnetic insulator, such as YIG [61,62], EuO [63], or CrI 3 [64,65].…”

“…This situation might lead to protected modes between different regions of the system, dramatically changing the low-energy properties of the whole material. This is the natural scenario in van der Waals heterostructures, where Moire patterns [9][10][11] could coexist with any topological state [12,13]. A more controlled situation is the proposals for topological superconductivity involving nanowires, where the topological state is controlled locally by electric gates [14,15].…”