Composite super-moiré lattices in double-aligned graphene heterostructures

Abstract

When two-dimensional atomic crystals are brought into close proximity to form a van der Waals heterostructure, neighbouring crystals can start influencing each other's electronic properties. Of particular interest is the situation when the periodicity of the two crystals closely match and a moiré pattern forms, which results in specific electron scattering, reconstruction of electronic and excitonic spectra, crystal reconstruction, and many other effects. Thus, formation of moiré patterns is a viable tool of controlling the electronic properties of 2D materials. At the same time, the difference in the interatomic distances for the two crystals combined, determines the range in which the electronic spectrum is reconstructed, and thus is a barrier to the low energy regime. Here we present a way which allows spectrum reconstruction at all energies. By using graphene which is aligned simultaneously to two hexagonal boron nitride layers, one can make electrons scatter in the differential moiré pattern, which can have arbitrarily small wavevector and, thus results in spectrum reconstruction at arbitrarily low energies. We demonstrate that the strength of such a potential relies crucially on the atomic reconstruction of graphene within the differential moiré super-cell. Such structures offer further opportunity in tuning the electronic spectra of two-dimensional materials.Introduction: Van der Waals heterostructures allow combining different two-dimensional (2D) materials into functional stacks(1, 2), which has already produced a range of interesting electronic(3, 4) and optoelectronic(5-8) devices and resulted in observation of exciting physical phenomena. The large variety of the heterostructures is mainly due to the large selection of 2D materials. However, the assembly of van der Waals heterostructures allow one extra degree of freedom: apart from the selection of the sequence of the 2D crystalsthe individual crystals can be differently oriented with respect to each other. Previously such control over the rotational alignment between crystals resulted in the observation of the resonant tunnelling(9-11), renormalisation of exciton binding energy(12) insulating(13) and superconducting(4) states. 01129a). J.Y. and A.M. acknowledge the support of EPSRC Early Career Fellowship EP/N007131/1.

Ultrahigh-resolution scanning microwave impedance microscopy of moiré lattices and superstructures

“…In addition to the conventional moiré lattices, our ultrahigh-sensitivity uMIM allows for imaging of moiré superstructures from three underlying lattices with different lattice vectors. Although heterostructures with multiple coexisting moiré have been imaged before with AFM ( 25 ), the resulting moiré superstucture has not been observed in ambient conditions previously. Figure 3 (A to C) shows a twisted double bilayer graphene on hBN (BG/BG/hBN) heterostructure where the two BGs are slightly twisted and the bottom BG is nearly aligned with the underlying hBN flake.…”

“…Traditionally, the structure of moiré lattices is imaged with transmission electron microscopy (TEM) (13,14) and scanning tunneling microscopy (STM) (15)(16)(17)(18)(19)(20), but these methods have low throughput and require specialized sample preparation that is largely unsuitable for functional devices. Different imaging modalities based on atomic force microscopy (AFM) have also been explored to study moiré lattices (21)(22)(23)(24)(25)(26), but imaging moiré lattices and superstructures with sufficient sensitivity and resolution is challenging.…”

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.…”

Charge-polarized interfacial superlattices in marginally twisted hexagonal boron nitride

“…Like in the case of graphene, atomically thin crystals of hBN can be obtained through exfoliating the bulk material 11,12 . A wide-band insulator with strong polar covalent bonding between boron and nitrogen, hBN has proven itself indispensable for making high-quality vdW heterostructures 13,14 and lateral superlattices, especially in combination with graphene 12,[15][16][17][18][19][20] . Very recently, ferroelectric-like charge polarization has been observed on bilayer-graphene/hBN superlattices 21 .…”