Tunable crystal symmetry in graphene–boron nitride heterostructures with coexisting moiré superlattices

Abstract

In heterostructures consisting of atomically thin crystals layered on top of one another, lattice mismatch or rotation between the layers results in long-wavelength moiré superlattices. These moiré patterns can drive significant band structure reconstruction of the composite material, leading to a wide range of emergent phenomena including superconductivity [1][2][3], magnetism [4], fractional Chern insulating states [5], and moiré excitons [6][7][8][9]. Here, we investigate monolayer graphene encapsulated between two crystals of boron nitride (BN), where the rotational alignment between all three components can be varied. We find that band gaps in the graphene arising from perfect rotational alignment with both BN layers can be modified substantially depending on whether the relative orientation of the two BN layers is 0 or 60 degrees, suggesting a tunable transition between the absence or presence of inversion symmetry in the heterostructure. Small deviations (< 1 • ) from perfect alignment of all three layers leads to coexisting longwavelength moiré potentials, resulting in a highly reconstructed graphene band structure featuring multiple secondary Dirac points. Our results demonstrate that the interplay between multiple moiré patterns can be utilized to controllably modify the electronic properties of the composite heterostructure.The ability to combine diverse vdW materials into a heterostructure enables engineering of new properties not observed in the constituent materials alone. A unique degree of freedom within these vdW heterostructures is the twist angle between layers, and changing this angle can strongly modify the material properties owing to the formation of moiré patterns. In graphene-BN heterostructures, the moiré pattern introduces a spatiallyperiodic effective potential that modifies the graphene band structure, giving rise to emergent secondary Dirac points (SDPs) at finite energy [10][11][12][13] and band gaps at the charge neutrality point and valence band SDP [13][14][15][16][17][18][19][20][21][22][23][24][25][26]. In twisted bilayer graphene (tBLG), correlated insulating states and superconductivity emerge at a twist angle of ∼1.1 • where the lowest energy moiré bands be-come exceptionally flat [1,2,4,27,28]. However, typical vdW heterostructures comprising many flakes possess numerous crystal interfaces, and in principle multiple long-wavelength moiré patterns may coexist within a single heterostructure, likely with profound consequences on moiré-driven physics. For example, topological bands have been shown to potentially arise in tBLG aligned to BN [4]. So far, little has been done to controllably tune the alignment of multiple pairs of crystals within a single device, and it is not well understood how multiple moiré patterns interact to influence the properties of the vdW heterostructure.In a heterostructure where graphene is encapsulated on both sides by BN, there are a number of qualitatively distinct stacking orders that can be realized by independently controlling the twist angle ...

Long-range ballistic transport of Brown-Zak fermions in graphene superlattices

“…The alignment was verified by Raman spectroscopy 32 prior to encapsulation with the second hBN crystal. The latter was intentionally misaligned to avoid competing moiré patterns [33][34][35] . The assembled stacks were placed on an oxidized Si wafer, which allowed us to apply the back-gate voltage V g to control the carrier density n. We studied six devices that were shaped into the multiterminal Hall bar geometry and had the main channel widths W ranging from 2 to 17 μm (see Fig.…”

Giant orbital magnetoelectric effect and current-induced magnetization switching in twisted bilayer graphene

“…The detailed source of symmetry breaking will not affect our conclusion that currents can induce magnetization in TBGs. Moreover, we expect that other materials with low crystal symmetries such as twisted bilayer-bilayer graphene [56][57][58], twisted boron nitridegraphene heterostructure [59,60], and twisted transition metal dichacolgenides [61] will exhibit similar magnetoelectric effects, though the magnitude of the magnetoelectric response will depend on the details of the materials.…”

Graphene Plasmonics in Sensor Applications: A Review

“…The combination of 2D materials such as hBN [ 153 ], transition metal dichalcogenides [ 162 , 163 ] and black phosphorous [ 164 ] with various polaritons, including SPPs, exciton polaritons and phonon polaritons, is also expected to allow the development of more advanced graphene-based plasmonic devices [ 162 , 165 ]. Heterostructures involving such materials together with bilayer graphene are also promising, because these have the potential to exhibit novel physical properties, such as valleytronics [ 166 , 167 , 168 ], magic angle effects [ 169 ], twisted graphene plasmons [ 170 , 171 ] and Moiré superlattices [ 172 , 173 ]. Devices based on direct electrical current control of graphene SPPs are promising for mid-IR and THz light sources for sensor systems [ 174 , 175 , 176 ].…”