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Register a new userCharge-polarized interfacial superlattices in marginally twisted hexagonal boron nitride
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When two-dimensional crystals are brought into close proximity, their interaction results in strong reconstruction of electronic spectrum and local crystal structure. Such reconstruction strongly depends on the twist angle between the two crystals and has received growing attention due to new interesting electronic and optical properties that arise in graphene and transitional metal dichalcogenides. Similarly, novel and potentially useful properties are expected to appear in insulating crystals. Here we study two insulating crystals of hexagonal boron nitride (hBN) stacked at a small twist angle. Using electrostatic force microscopy, we observe ferroelectric-like domains arranged in triangular superlattices with a large surface potential that is independent on the size and orientation of the domains as well as the thickness of the twisted hBN crystals. The observation is attributed to interfacial elastic deformations that result in domains with a large density of out-of-plane polarized dipoles formed by pairs of boron and nitrogen atoms belonging to the opposite interfacial surfaces. This effectively creates a bilayer-thick ferroelectric with oppositely polarized (BN and NB) dipoles in neighbouring domains, in agreement with our modelling. The demonstrated electrostatic domains and their superlattices offer many new possibilities in designing novel van der Waals heterostructures.
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Twisted heterostructures of two-dimensional crystals offer almost unlimited scope for the design of novel metamaterials. Here we demonstrate a room-temperature ferroelectric semiconductor that is assembled using mono- or few- layer MoS2. These van der Waals heterostructures feature broken inversion symmetry, which, together with the asymmetry of atomic arrangement at the interface of two 2D crystals, enables ferroelectric domains with alternating out-of-plane polarisation arranged into a twist-controlled network. The latter can be moved by applying out-of-plane electrical fields, as visualized in situ using channelling contrast electron microscopy. The interfacial charge transfer for the observed ferroelectric domains is quantified using Kelvin probe force microscopy and agrees well with theoretical calculations. The movement of domain walls and their bending rigidity also agrees well with our modelling results. Furthermore, we demonstrate proof-of-principle field-effect transistors, where the channel resistance exhibits a pronounced hysteresis governed by pinning of ferroelectric domain walls. Our results show a potential venue towards room temperature electronic and optoelectronic semiconductor devices with built-in ferroelectric memory functions.
Moire superlattices (MSL) formed in angle-aligned bilayers of van der Waals materials have become a promising platform to realize novel two-dimensional electronic states. Angle-aligned trilayer structures can form two sets of MSLs which could potentially interfere with each other. In this work, we directly image the moire patterns in both monolayer graphene aligned on hBN and twisted bilayer graphene aligned on hBN, using combined scanning microwave impedance microscopy and conductive atomic force microscopy. Correlation of the two techniques reveals the contrast mechanism for the achieved ultrahigh spatial resolution (<2 nm). We observe two sets of MSLs with different periodicities in the trilayer stack. The smaller MSL breaks the 6-fold rotational symmetry and exhibits abrupt discontinuities at the boundaries of the larger MSL. Using a rigid atomic-stacking model, we demonstrate that the hBN layer considerably modifies the MSL of twisted bilayer graphene. We further analyze its effect on the reciprocal space spectrum of the dual-moire system.
Due to atomically thin structure, graphene/hexagonal boron nitride (G/hBN) heterostructures are intensively sensitive to the external mechanical forces and deformations being applied to their lattice structure. In particular, strain can lead to the modification of the electronic properties of G/hBN. Furthermore, moire structures driven by misalignment of graphene and hBN layers introduce new features to the electronic behavior of G/hBN. Utilizing {it ab initio} calculation, we study the strain-induced modification of the electronic properties of diverse stacking faults of G/hBN when applying in-plane strain on both layers, simultaneously. We observe that the interplay of few percent magnitude in-plane strain and moire pattern in the experimentally applicable systems leads to considerable valley drifts, band gap modulation and enhancement of the substrate-induced Fermi velocity renormalization. Furthermore, we find that regardless of the strain alignment, the zigzag direction becomes more efficient for electronic transport, when applying in-plane non-equibiaxial strains.
We study the stability and electronic structure of magic-angle twisted bilayer graphene on the hexagonal boron nitride (TBG/BN). Full relaxation has been performed for commensurate supercells of the heterostructures with different twist angles ($theta$) and stackings between TBG and BN. We find that the slightly misaligned configuration with $theta = 0.54^circ$ and the AA/AA stacking has the globally lowest total energy due to the constructive interference of the moir{e} interlayer potentials and thus the greatly enhanced relaxation in its $1 times 1$ commensurate supercell. Gaps are opened at the Fermi level ($E_F$) for small supercells with the stackings that enable strong breaking of the $C_2$ symmetry in the atomic structure of TBG. For large supercells with $theta$ close to those of the $1 times 1$ supercells, the broadened flat bands can still be resolved from the spectral functions. The $theta = 0.54^circ$ is also identified as a critical angle for the evolution of the electronic structure with $theta$, at which the energy range of the mini-bands around $E_F$ begins to become narrower with increasing $theta$ and their gaps from the dispersive bands become wider. The discovered stablest TBG/BN with a finite $theta$ of about $0.54^circ$ and its gapped flat bands agree with recent experimental observations.
We have previously reported ferromagnetism evinced by a large hysteretic anomalous Hall effect in twisted bilayer graphene (tBLG). Subsequent measurements of a quantized Hall resistance and small longitudinal resistance confirmed that this magnetic state is a Chern insulator. Here we report that, when tilting the sample in an external magnetic field, the ferromagnetism is highly anisotropic. Because spin-orbit coupling is negligible in graphene such anisotropy is unlikely to come from spin, but rather favors theories in which the ferromagnetism is orbital. We know of no other case in which ferromagnetism has a purely orbital origin. For an applied in-plane field larger than $5 mathrm{T}$, the out-of-plane magnetization is destroyed, suggesting a transition to a new phase.
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