Subscribe to the gold package and get unlimited access to Shamra Academy
Register a new userMisalignment instability in magic-angle twisted bilayer graphene on hexagonal boron nitride
84
0
0.0
(
0
)
Ask ChatGPT about the research
No Arabic abstract
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.
rate research
Read More
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.
The effect of an hexagonal boron nitride (hBN) layer close aligned with twisted bilayer graphene (TBG) is studied. At sufficiently low angles between twisted bilayer graphene and hBN, $theta_{hBN} lesssim 2^circ$, the graphene electronic structure is strongly disturbed. The width of the low energy peak in the density of states changes from $W sim 5 - 10$ meV for a decoupled system to $sim 20 - 30$ meV. Spikes in the density of states due to van Hove singularities are smoothed out. We find that for a realistic combination of the twist angle in the TBG and the twist angle between the hBN and the graphene layer the system can be described using a single moire unit cell.
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.
When a crystal is subjected to a periodic potential, under certain circumstances (such as when the period of the potential is close to the crystal periodicity; the potential is strong enough, etc.) it might adjust itself to follow the periodicity of the potential, resulting in a, so called, commensurate state. Such commensurate-incommensurate transitions are ubiquitous phenomena in many areas of condensed matter physics: from magnetism and dislocations in crystals, to vortices in superconductors, and atomic layers adsorbed on a crystalline surface. Of particular interest might be the properties of topological defects between the two commensurate phases: solitons, domain walls, and dislocation walls. Here we report a commensurate-incommensurate transition for graphene on top of hexagonal boron nitride (hBN). Depending on the rotational angle between the two hexagonal lattices, graphene can either stretch to adjust to a slightly different hBN periodicity (the commensurate state found for small rotational angles) or exhibit little adjustment (the incommensurate state). In the commensurate state, areas with matching lattice constants are separated by domain walls that accumulate the resulting strain. Such soliton-like objects present significant fundamental interest, and their presence might explain recent observations when the electronic, optical, Raman and other properties of graphene-hBN heterostructures have been notably altered.
Graphene, a one-layer honeycomb lattice of carbon atoms, exhibits unconventional phenomena and attracts much interest since its discovery. Recently, an unexpected Mott-like insulator state induced by moire pattern and a superconducting state are observed in magic-angle-twisted bilayer graphene, especially, without correlations between electrons, which gives more hints for the understanding and investigation of strongly correlated phenomena. The photon as boson, behaving differently with fermion, can also retrieve the unconventional phenomena of graphene, such as the bearded edge state which is even never been observed in graphene due to the unstability. Here, we present a direct observation of magic angle and wall state in twisted bilayer photonic graphene. We successfully observe the strong localization and rapid diffusion of photon at the regions with AA and AB stacking order around the magic angle, respectively. Most importantly, we find a wall state showing the photon distribution distinctly separate at the regions with AA and AB/BA stacking order in the lowest-energy band. The mechanism underlying the wall states may help to understand the existence of both Mott-like insulating state and superconducting state in magic-angle twisted bilayer graphene. The accessibility of magic angle in twisted bilayer photonic graphene adds the boson behavior into graphene superlattice and the observation of wall state will also deep the understanding of matter.
Log in to be able to interact and post comments
comments
Fetching comments
Sign in to be able to follow your search criteria
