Bilayer Graphene Research Articles

Mutant amplitude modulation behavior of MIS-like structure of few-layer graphene/SiO2/p-Si in 500–750 GHz band

Graphene has great application potential in the field of electromagnetic modulation field because of its excellent physical and electronic properties. Studies have demonstrated that the properties of graphene films with different layers are also different due to the difference in energy band structure. Nowadays, the modulation mechanism of monolayer graphene (MLG) and bilayer graphene (BLG) has been gradually discovered, but for graphene with more than three layers, the mechanism of whether it is tunable remains to be explored, especially on the proving from an experimental perspective. In this study, the CVD-prepared highly homogeneous few-layer graphene (FLG) film was combined with SiO2 nanolayers and P-doped Si substrate to form an MIS-like capacitor structure, a unique electromagnetic behavior of mutant amplitude modulation exhibited by FLG film was found, which was different from that of mono- and bi-layers of graphene. The results show that the structure exhibits obvious modulation behavior in the ultra-wideband frequency of 500–750 GHz and the bias of 0.9 V, up to 3.1 dB. This study makes a new supplement to a gap in the EM modulation system of graphene series material.

Fano resonances in gated phosphorene junctions

Fano resonances appear in plenty of physical phenomena due to the interference phenomena of a continuum spectrum and discrete states. In gated bilayer graphene junctions, the chiral matching at oblique incidence between the spectrum of electron states outside the electrostatic barrier and hole bound states inside it gives rise to an asymmetric line shape in the transmission as a function of the energy or Fano resonance. Here, we show that Fano resonances are also possible in gated phosphorene junctions along the zigzag direction. The special pseudospin texture of the charge carriers in the zigzag direction allows at oblique incidence the interference phenomena of the spectrum of electron states outside the electrostatic barrier with hole bound states inside it, giving rise to an asymmetric Fano line shape in the transmission. Due to the energy scale of the electrostatic barriers in phosphorene ultra thin barriers are required to observe the Fano resonance phenomenon. The preservation of the pseudospin texture with the closing of the phosphorene band gap opens the possibility to observe Fano resonances in smaller and wider electrostatic barriers. The asymmetric Fano line shape is susceptible to the transverse wave vector, the strength and width of the electrostatic barrier. Additionally, the conductance shows a characteristic mark in the position where the Fano resonances take place. The similarities and differences with respect to Fano resonances in bilayer graphene are also addressed.

Electronic and Optical Properties of 2D Heterostructure Bilayers of Graphene, Borophene and 2D Boron Carbides from First Principles.

In the present work the atomic, electronic and optical properties of two-dimensional graphene, borophene, and boron carbide heterojunction bilayer systems (Graphene-BC3, Graphene-Borophene and Graphene-B4C3) as well as their constituent monolayers are investigated on the basis of first-principles calculations using the HSE06 hybrid functional. Our calculations show that while borophene is metallic, both monolayer BC3 and B4C3 are indirect semiconductors, with band-gaps of 1.822 eV and 2.381 eV as obtained using HSE06. The Graphene-BC3 and Graphene-B4C3 bilayer heterojunction systems maintain the Dirac point-like character of graphene at the K-point with the opening of a very small gap (20-50 meV) and are essentially semi-metals, while Graphene-Borophene is metallic. All bilayer heterostructure systems possess absorbance in the visible region where the resonance frequency and resonance absorption peak intensity vary between structures. Remarkably, all heterojunctions support plasmons within the range 16.5-18.5 eV, while Graphene-B4C3 and Graphene-Borophene exhibit a π-type plasmon within the region 4-6 eV, with the latter possessing an additional plasmon at the lower energy of 1.5-3 eV. The dielectric tensor for Graphene-B4C3 exhibits complex off-diagonal elements due to the lower P3 space group symmetry indicating it has anisotropic dielectric properties and could exhibit optically active (chiral) effects. Our study shows that the two-dimensional heterostructures have desirable optical properties broadening the potential applications of the constituent monolayers.

Optical properties of gated bilayer graphene quantum dots with trigonal warping

Optical properties of gated bilayer graphene quantum dots with trigonal warping

Arrays of one-dimensional conducting channels in minimally twisted bilayer graphene

Arrays of one-dimensional conducting channels in minimally twisted bilayer graphene

Enantiomeric Adsorption of Alanine on Twisted Bilayer Graphene

Enantiomeric Adsorption of Alanine on Twisted Bilayer Graphene

Fine Structure of Flat Bands in a Chiral Model of Magic Angles

AbstractWe analyse symmetries of Bloch eigenfunctions at magic angles for the Tarnopolsky–Kruchkov–Vishwanath chiral model of the twisted bilayer graphene (TBG) following the framework introduced by Becker–Embree–Wittsten–Zworski. We show that vanishing of the first Bloch eigenvalue away from the Dirac points implies its vanishing at all momenta, that is, the existence of a flat band. We also show how the multiplicity of the flat band is related to the nodal set of the Bloch eigenfunctions. We conclude with two numerical observations about the structure of flat bands.

Magnon confinement in a nanomagnonic waveguide by a magnetic Moiré superlattice

The study of moiré superlattices has revealed intriguing phenomena in electronic systems, including unconventional superconductivity and ferromagnetism observed in magic-angle bilayer graphene. This approach has recently been adapted to the field of magnonics. In this Letter, we investigate the confinement of spin waves in a nanomagnonic waveguide integrated on top of a magnetic moiré superlattice. Our numerical analysis reveals a magnonic flatband at the center of the Brillouin zone, created by a 3.5° twist in the moiré superlattice. The flatband, characterized by a high magnon density of states and a zero group velocity, allows for the confinement of magnons within the AB stacking region. The flatband results from the mode anticrossing of several different magnon bands, covering a wavevector range of nearly 40 rad/μm and a 166 nm wide spatial distribution of the magnon trapping in the waveguide. Our results pave the way for nanomagnonic devices and circuits based on spin-wave trapping in magnon waveguides.

Ballistic Electron Source with Magnetically Controlled Valley Polarization in Bilayer Graphene.

The achievement of valley-polarized electron currents is a cornerstone for the realization of valleytronic devices. Here, we report on ballistic coherent transport experiments where two opposite quantum point contacts (QPCs) are defined by electrostatic gating in a bilayer graphene (BLG) channel. By steering the ballistic currents with an out-of-plane magnetic field we observe two current jets, a consequence of valley-dependent trigonal warping. Tuning the BLG carrier density and number of QPC modes (m) with a gate voltage we find that the two jets are present for m=1 and up to m=6, indicating the robustness of the effect. Semiclassical simulations confirm the origin of the signals by quantitatively reproducing the jet separations without fitting parameters. In addition, our model shows that the ballistic current jets have opposite valley polarization. As a consequence, by steering each jet toward the detector using a magnetic field, we achieve full control over the valley polarization of the collected currents, envisioning such devices as ballistic current sources with tunable valley polarization. We also show that collimation experiments are a sensitive probe to the trigonal warping of the Fermi surface.

Intrinsic anomalous, spin and valley Hall effects in ’ex-so-tic’ van-der-Waals structures

We consider the anomalous, spin, valley, and valley spin Hall effects in a pristine graphene-based van-der-Waals (vdW) heterostructure consisting of a bilayer graphene (BLG) sandwiched between a semiconducting van-der-Waals material with strong spin-orbit coupling (e.g., WS2\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\hbox {WS}_2$$\\end{document}) and a ferromagnetic insulating vdW material (e.g. Cr2\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\hbox {Cr}_2$$\\end{document}Ge2\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\hbox {Ge}_2$$\\end{document}Te6\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\hbox {Te}_6$$\\end{document}). Due to the exchange proximity effect from one side and spin-orbit proximity effect from the other side of graphene, such a structure is referred to as graphene based ’ex-so-tic’ structure. First, we derive an effective Hamiltonian describing the low-energy states of the structure. Then, using the Green’s function formalism, we obtain analytical results for the Hall conductivities as a function of the Fermi energy and gate voltage. For specific values of these parameters, we find a quantized valley Hall conductivity.

A deep equivariant neural network approach for efficient hybrid density functional calculations

Hybrid density functional calculations are essential for accurate description of electronic structure, yet their widespread use is restricted by the substantial computational cost. Here we develop DeepH-hybrid, a deep equivariant neural network method for learning the hybrid-functional Hamiltonian as a function of material structure, which circumvents the time-consuming self-consistent field iterations and enables the study of large-scale materials with hybrid-functional accuracy. Our extensive experiments demonstrate good reliability as well as effective transferability and efficiency of the method. As a notable application, DeepH-hybrid is applied to study large-supercell Moiré-twisted materials, offering the first case study on how the inclusion of exact exchange affects flat bands in magic-angle twisted bilayer graphene. The work generalizes deep-learning electronic structure methods to beyond conventional density functional theory, facilitating the development of deep-learning-based ab initio methods.

Designing Moiré Patterns by Shearing.

We analyze the elastic properties, structural effects, and low-energy physics of a sheared nanoribbon placed on top of graphene, which creates a gradually changing moiré pattern. By means of a classical elastic model we derive the strains in the ribbon and we obtain its electronic energy spectrum with a scaled tight-binding model. The size of the sheared region is determined by the balance between elastic and van der Waals energy, and different regimes are identified. Near the clamped edge, moderate strains and small twist angles lead to one-dimensional channels. Near the sheared edge, a long region behaves like magic angle twisted bilayer graphene (TBG), showing a sharp peak in the density of states, mostly isolated from the rest of the spectrum. We also calculate the band topology along the ribbon and we find that it is stable for large intervals of strains and twist angles. Together with the experimental observations, these results show that the sheared nanoribbon geometry is ideal for exploring superconductivity and correlated phases in TBG in the very sought-after regime of ultralow twist angle disorder.

Strongly correlated Hofstadter subbands in minimally twisted bilayer graphene

Strongly correlated Hofstadter subbands in minimally twisted bilayer graphene

Topological phases, van Hove singularities, and spin texture in magic-angle twisted bilayer graphene in the presence of proximity-induced spin-orbit couplings

Topological phases, van Hove singularities, and spin texture in magic-angle twisted bilayer graphene in the presence of proximity-induced spin-orbit couplings

Revisiting the magnetic responses of bilayer graphene from the perspective of quantum distance

Revisiting the magnetic responses of bilayer graphene from the perspective of quantum distance

Unraveling the multistage phase transformations in monolayer MoTe2−x

Monolayer MoTe2 exhibits a variety of derivative structural phases and associated intriguing electronic properties that enable a wealth of potential applications in future electronic and optoelectronic devices. However, a comprehensive study focusing on the complexities of the controllable phase evolution in this atomically thin film has yet to be performed. This work aims to address this issue by systematically investigating molecular beam epitaxial growth of monolayer Mo–Te compounds on bilayer graphene substrates. By utilizing scanning tunneling microscopy, we explored a series of thermally driven structural phase evolutions, including distinct T′-MoTe2, H-MoTe2, Mo6Te6 nanowires, and multistoichiometric MoTe2−x. Furthermore, we carefully investigated the critical effects of the growth parameters—annealing temperature and time and tellurium concentration—on the controllable and reversible phase transformation within monolayer MoTe2−x. The findings have significant implications for understanding the thin film synthesis and phase transformation engineering inherent to two-dimensional crystals, which can foster further development of high-performance devices.

Magnetic Response Properties of Twisted Bilayer Graphene

Magnetic Response Properties of Twisted Bilayer Graphene

Comparison between endogenous and exogenous nitrogen of nitrogen-doped carbon catalyst in the process of activating PMS

BackgroundAdvanced oxidation processes (AOPs) based on sulfate radicals (SO4•–) have proven to be highly effective in degrading organics in wastewater. Carbon-based materials have emerged as promising catalysts for activating persulfate, which generates environmentally friendly sulfate radicals (SO4•–), for remediation purposes. The nitrogen doping technique is an effective method for site-specific regulation and can significantly enhance the performance of carbon-based catalysts, which could promote the application of carbon-based catalysts in the future. Endogenous and exogenous nitrogen sources can provide nitrogen sources for N doping. However, there are few reports on the comparison of the structure and catalytic mechanism of these two types of N-doped biochar. It is also of great significance to reveal the mechanisms of constructing catalytic sites using endogenous and exogenous nitrogen. MethodsHerein, the preparation of endogenous nitrogen-doped biochar (BC) was achieved by using soybean as the precursor material, which is rich in natural nitrogen-containing components of proteins. Subsequently, the BC was doped by mixing it with urea and pyrolysis, resulting in the preparation of exogenous nitrogen-doped biochar (NBC). Significant findingsThe characterization of XRD and HRTEM showed that g-C3N4 formed in NBC. The results of catalytic degradation and quenching experiments demonstrate that the exogenous nitrogen-doped catalysts have a better performance than endogenous nitrogen-doped catalysts. The OFL removal rate in BC/PMS was higher than that in the BC/PMS system (71.68% vs. 61.83 %). The kobs in NBC/PMS are also higher than that in BC/PMS (0.00943 min−1vs. 0.01369 min−1). The NBC could be a promising catalyst for PMS activation in practical application. DFT results showed that the g-C3N4 generated from exogenous nitrogen can improve PMS activation performance in the g-C3N4/graphene bilayer structure. The influence on the charge distribution of surrounding carbon materials makes endogenous nitrogen doping a good choice for optimizing the local performance of the material in the absence of natural nitrogen components.

Graphene Bilayer as a Template for Manufacturing Novel Encapsulated 2D Materials.

Bilayer graphene (BLG) has recently been used as a tool to stabilize encapsulated single sheets of various layered materials and tune their properties. It was also discovered that the protecting action of graphene sheets makes it possible to synthesize completely new two-dimensional materials (2DMs) inside the BLG by intercalating various atoms and molecules. In comparison to the bulk graphite, BLG allows for easier intercalation and a much larger increase in the interlayer separation of the sheets. Moreover, it enables studying the atomic structure of the intercalated 2DM by using high-resolution transmission electron microscopy. In this review, we summarize the recent progress in this area, with a special focus on new materials created inside BLG. We compare the experimental findings with the theoretical predictions, pay special attention to the discrepancies, and outline the challenges in the field. Finally, we discuss unique opportunities offered by intercalation into 2DMs beyond graphene and their heterostructures.

Superconductivity and spin density wave in AA stacked bilayer graphene

This work theoretically analyzes electronic ordering in AA-stacked bilayer graphene and the role of the Coulomb interaction in these many-body phenomena. Using the random phase approximation to account for screening, we find intra-layer effective interactions to be much stronger than inter-layer interactions; under certain circumstances, the latter may also become attractive. At zero doping, the Coulomb repulsion stabilizes the spin-density wave state, with a Néel temperature in the tens of Kelvin. While dominant in the undoped system, the spin-density wave is destroyed by sufficiently strong doping and a superconducting phase emerges. We find that the effective Coulomb inter-layer interaction can give rise to superconductivity. However, the corresponding critical temperature is negligibly small, and phonon-mediated attraction must be introduced to observe it. Strong intra-layer repulsion suppresses order parameters that couple two intra-layer electrons. We point out a possible superconducting state with finite Cooper pair momentum.