The electronic structure of a graphene superlattice composed by two periodic regions with different Fermi velocity, energy gap and electrostatic potential is investigated by using an effective Dirac-like Hamiltonian. It must be expected that the change of the Fermi velocity in one region of the graphene superlattice is equivalent to changing the width of this region keeping the Fermi velocity unchanged, provided that the time taken to charge carriers cross the region is the same. However, it is shown here that these two systems are not equivalent. We found extra Dirac points induced by the periodic potential and their location in the \textbf{k} space. It is shown that the Fermi velocity modulation breaks the symmetry between the electron and hole minibands and that it is possible to control the behavior of the extra Dirac points. The results obtained here can be used in the fabrication of graphene-based electronic devices.
We report the Raman spectroscopy of 12C/13C graphene isotope superlattices synthesized by chemical vapour deposition. At large periods the Raman spectra corresponds to the sum of the bulk 12C and 13C contributions. However, at small periods we observe the formation of mixed 12C/13C modes for Raman processes that involve two phonons, which results in the tripling of the 2D and 2D' Raman peaks. This tripling can be well understood in the framework of real space Raman spectroscopy, where the two emitted phonons stem from different regions of the superlattice. The intensity of the mixed peak incre…
Armchair graphene nanoribbons, when forming a superlattice, can be classified in different topological phases, with or without edge states. By means of tight-binding and classical molecular dynamics (MD) simulations, we studied the electronic and mechanical properties of some of these superlattices. MD shows that fracture in modulated superlattices is brittle, as for unmodulated ribbons, and that occurs at the thinner regions, with staggered superlattices achieving a larger fracture strain. We found a general mechanism to induce a topological transition with strain, related to the electronic p…
Most studies on moiré superlattices formed from a stack of h-BN and graphene have focused on single layer graphene; graphene with multiple layers is less understood. Here, we show that a moiré superlattice of multilayer graphene shows new features arising from the anisotropic Fermi surface affected by the superlattice structure. The moiré superlattice of a h-BN/AB-stacked tetralayer graphene heterostructure exhibited resistivity peaks showing a complicated dependence on the perpendicular electric field. The peaks were not due to secondary Dirac cones forming, but rather opening of the energy g…
Symmetry-induced gap opening in graphene superlattices
我们研究了石墨烯中缺陷的nxn蜂窝超晶格。所考虑的缺陷是缺失的p_z轨道，可以通过在给定位置引入C原子空位或化学结合简单原子物种来实现。利用对称性论证，我们证明了当n= 3 m +1，3 m +2（m的整数）时如何可能打开一个能隙，并估计其值与缺陷浓度x=1/n^2的平方根近似相关。紧束缚计算证实了这些发现，并表明诱导能隙可以相当大，例如，x~10^{-3}时约为100 meV。对吸附在石墨烯上的H原子所形成的超晶格进行了修正的密度泛函理论计算，结果与紧束缚的结果吻合得很好，从而表明所提出的结构可以在实践中用于打开石墨烯中的间隙。
In two-dimensional (2D) twisted bilayers, the van der Waals (vdW) interlayer interaction introduces atomic-scale reconstruction at interface by locally rotating lattice to form strain-field vortex networks in their moiré superlattice. However, direct imaging the tiny local lattice rotation of the strain-field vortex requires extremely high spatial resolution and is an outstanding challenge in experiment. Here, a topmost small-period graphene moiré pattern is introduced to magnify sub-Angstrom distortions of the lattice and tiny local lattice rotation in underlying twisted bilayer graphene (TBG…
We provide a critical discussion concerning the claim of topological valley currents, driven by a global Berry curvature and valley Hall effect proposed in recent literature. After pointing out a major inconsistency of the theoretical scenario proposed to interpret giant nonlocal resistance, we discuss various possible alternative explanations and open directions of research to solve the mystery of nonlocal transport in graphene superlattices.
Monte Carlo Simulation of Sinusoidally Modulated Superlattice Growth
利用蒙特卡罗模拟研究了在非均匀磁通分布下旋转衬底代替连续关闭和打开源光闸生长ZnSe/ZnTe超晶格的过程。据发现，每种化合物的浓度是正弦调制沿生长方向，所造成的不均匀到达的Se和Te原子在样品的一个给定的点，并通过在该点的Te/Se比的变化，由于旋转的衬底。以这种方式，我们获得了ZnSe $_{1-x}$ Te $_x$合金，其中组分$x$沿生长方向正弦变化。调制的周期由衬底旋转的速率直接控制。成分调制的幅度对于衬底旋转的小角速度是单调的，但是其本身对于大的角速度是调制的。调制图案的平均振幅随着衬底旋转的角速度增加和测量位置接近旋转中心而减小。模拟结果与以前发表的实验测量结果吻合得很好，以这种方式制造的超晶格。
Moiré superlattices in van der Waals heterostructures are gaining increasing attention because they offer new opportunities to tailor and explore unique electronic phenomena when stacking 2D materials with small twist angles. Here, we reveal local surface potentials associated with stacking domains in twisted double bilayer graphene (TDBG) moiré superlattices. Using a combination of both lateral Piezoresponse Force Microscopy (LPFM) and Scanning Kelvin Probe Microscopy (SKPM), we distinguish between Bernal (ABAB) and rhombohedral (ABCA) stacked graphene and directly correlate these stacking co…
