Graphene Dirac Research Articles

Evolution of Graphene Dirac Fermions in Electric Double‐Layer Transistors with a Soft Barrier (Adv. Funct. Mater. 34/2024)

Evolution of Graphene Dirac Fermions in Electric Double‐Layer Transistors with a Soft Barrier (Adv. Funct. Mater. 34/2024)

Designable exciton mixing through layer alignment in WS2-graphene heterostructures

Optical properties of heterostructures composed of layered 2D materials, such as transition metal dichalcogenides (TMDs) and graphene, are broadly explored. Of particular interest are light-induced energy transfer mechanisms in these materials and their structural roots. Here, we use state-of-the-art first-principles calculations to study the excitonic composition and the absorption properties of WS2–graphene heterostructures as a function of interlayer alignment and the local strain resulting from it. We find that Brillouin zone mismatch and the associated energy level alignment between the graphene Dirac cone and the TMD bands dictate an interplay between interlayer and intralayer excitons, mixing together in the many-body representation upon the strain-induced symmetry breaking in the interacting layers. Examining the representative cases of the 0° and 30° interlayer twist angles, we find that this exciton mixing strongly varies as a function of the relative alignment. We quantify the effect of these structural modifications on exciton charge separation between the layers and the associated graphene-induced homogeneous broadening of the absorption resonances. Our findings provide guidelines for controllable optical excitations upon interface design and shed light on the importance of many-body effects in the understanding of optical phenomena in complex heterostructures.

Investigation of transport properties of graphene Dirac fluid by holographic two-current axion coupling model

Recently, there has been great interest in the phenomenon of severe violation of the Wiedemann–Franz law in graphene Dirac fluids around 75 K, due to the strong coupling relativistic plasma near the neutral point, where traditional perturbation theory fails. To explain this phenomenon, this article proposes a holographic dual two-current axion coupling model, describing the interaction between electrons and holes in graphene near the charge neutrality point (CNP) and revealing the related physical mechanism. The study shows that the holographic two-current model aligns with experimental results at 100 μ\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\upmu $$\\end{document}m-2\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$^{-2}$$\\end{document}, and correctly predicts conductivity as temperature increases. Additionally, the article explores the behavior of β+γ\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\beta +\\gamma $$\\end{document} and its impact on conductivity and thermal conductivity. The results suggest a frictional effect between electrons and holes. Consequently, this study provides us with a clearer understanding of the properties of graphene Dirac fluids and further confirms the reliability of the holographic duality method.

Evolution of Graphene Dirac Fermions in Electric Double‐Layer Transistors with a Soft Barrier

AbstractThe interface and dielectric environment of graphene transistors are of great importance to commercial circuit integrations. The tangling bond in oxide‐based dielectric severely lagged the carrier mobility while the 2D dielectric layer (for instance, hexagonal boron nitride) unavoidably hastened complicated condensed physics even at room temperature. Herein, multilayer black phosphorus (BP) a versatile and widely‐tunable dielectric candidate for manifesting graphene fermions is demonstrated. Because of hetero‐interfacial charge redistributions, a vertical electric double‐layer between the bottom BP layer and top graphene spontaneously forms with the central BP layer as a soft barrier. Under dual‐gate modulation, abnormal step‐like evolution of Dirac fermions and charge‐transfer quantum Hall effect arises while the intrinsic Dirac behavior of graphene is preserved, ascribing to the gate‐tunable charge redistributions of dielectric BP layer. Moreover, the electric double‐layer transistors apply equally well to bilayer graphene with similar Dirac behavior but an enhanced interfacial charge interference. The findings not only create a new avenue to manipulate the fermions by assembling graphene with narrow‐gapped 2D layered materials but also promote electric double‐layer transistors as a new build block to design multifunctional devices.

Effectuating tunable valley selection via multiterminal monolayer graphene devices

Valleytronics using two-dimensional materials opens unprecedented opportunities for information processing using a valley polarizer as a basic building block. Various methodologies, such as strain engineering, the inclusion of line defects, and the application of static magnetic fields, have been widely explored for creating valley polarization. However, these methods suffer from low transmission or lack of polarization directionality. To overcome the above limitations, we propose an all-electrical valley polarizer using zigzag edge graphene nanoribbons in a multiterminal device geometry. The proposed device can be gate-tuned to operate along two independent regimes: (i) a terminal-specific valley filter that utilizes band-structure engineering, and (ii) a parity-specific valley filter that exploits the parity selection rule in zigzag edge graphene. We show that the device exhibits intriguing physics in the multimode regime of operation affecting the valley polarization; we investigate various factors influencing polarization in wide-device geometries, such as optical analogs of graphene Dirac fermions, angle-selective transmission via $p\ensuremath{-}n$ junctions, and the localization of edge states. Furthermore, we evaluate the performance of the proposed device structures in the presence of Anderson short-range disorder at the edges and the bulk, and we find it to be resilient to edge disorder even for a higher disorder strength. The device geometry is optimized to achieve maximum valley polarization, thereby paving the way for a physics-based tunable valleytronic device.

Twist-angle dependent proximity induced spin-orbit coupling in graphene/topological insulator heterostructures

The proximity-induced spin-orbit coupling (SOC) in heterostructures of twisted graphene and topological insulators (TIs) Bi$_2$Se$_3$ and Bi$_2$Te$_3$ is investigated from first principles. To build commensurate supercells, we strain graphene and correct thus resulting band offsets by applying a transverse electric field. We then fit the low-energy electronic spectrum to an effective Hamiltonian that comprises orbital and spin-orbit terms. For twist angles 0$^\circ\leq\Theta \lessapprox 20^\circ$, we find the dominant spin-orbit couplings to be of the valley-Zeeman and Rashba types, both a few meV strong. We also observe a sign change in the induced valley-Zeeman SOC at $\Theta\approx 10^\circ$. Additionally, the in-plane spin structure resulting from the Rashba SOC acquires a non-zero radial component, except at $0^\circ$ or $30^\circ$. At $30^\circ$ the graphene Dirac cone interacts directly with the TI surface state. We therefore explore this twist angle in more detail, studying the effects of gating, TI thicknesses, and lateral shifts on the SOC parameters. We find, in agreement with previous results, the emergence of the proximitized Kane-Mele SOC, with a change in sign possible by electrically tuning the Dirac cone within the TI bulk band gap.

Tuning structural and electronic properties of β12-Borophene/Graphene heterostructure

Heterostructures, owing to their synergistic effects between the layers, have prompted interest as they supply the essential physical properties for extensive potential applications. In this letter, the electronic and structural properties of the newly synthesized vertical [Formula: see text]12-borophene/graphene ([Formula: see text]12-B/Gr) heterostructure are investigated by the density functional theory. This study reveals proper stability for the [Formula: see text]12-B/Gr heterostructure from the binding energies, which facilitate the instability issue of the borophene monolayer. The binding energy shows this heterostructure is a van der Waals (vdW) heterostructure, and the layers are stacked collectively with the aid of the vdW type bond. Additionally, the electronic properties of this novel heterostructure mostly preserve the electronic properties of monolayers. Furthermore, there is a bit of shift in graphene Dirac to the negative energy references to the Fermi level, due to which the graphene n-doped and the transferred charge along with the charge carrier concentration are calculated. The band structure of the [Formula: see text]12-B/Gr heterostructure is a plain sum total of the band structures of monolayers. Owing to this reality, this heterostructure exhibits great conductivity. This property makes the [Formula: see text]12-B/Gr heterostructure to be a good applicant as an anode for ion batteries, lithium, and hydrogen storage applications.

Determining the Proximity Effect-Induced Magnetic Moment in Graphene by Polarized Neutron Reflectivity and X-ray Magnetic Circular Dichroism.

We report the magnitude of the induced magnetic moment in CVD-grown epitaxial and rotated-domain graphene in proximity with a ferromagnetic Ni film, using polarized neutron reflectivity (PNR) and X-ray magnetic circular dichroism (XMCD). The XMCD spectra at the C K-edge confirm the presence of a magnetic signal in the graphene layer, and the sum rules give a magnetic moment of up to ∼0.47 μB/C atom induced in the graphene layer. For a more precise estimation, we conducted PNR measurements. The PNR results indicate an induced magnetic moment of ∼0.41 μB/C atom at 10 K for epitaxial and rotated-domain graphene. Additional PNR measurements on graphene grown on a nonmagnetic Ni9Mo1 substrate, where no magnetic moment in graphene is measured, suggest that the origin of the induced magnetic moment is due to the opening of the graphene's Dirac cone as a result of the strong C pz-Ni 3d hybridization.

Photoemission study of twisted monolayers and bilayers of WSe2 on graphite substrates

Using microfocused angle-resolved photoemission spectroscopy we investigated microstructures containing regions of single-layer (SL) and bilayer (BL) ${\mathrm{WSe}}_{2}$ on graphite substrates at different twist angles between SL ${\mathrm{WSe}}_{2}$ and graphite and within the BL ${\mathrm{WSe}}_{2}$. Fermi level electrons emitted from the graphite are sharply focused near their $K{}_{gr}$ points in the Brillouin zone, and, when passing through the ${\mathrm{WSe}}_{2}$, get diffracted to form band replicas readily observed in experimental Fermi surface maps from twisted SL ${\mathrm{WSe}}_{2}$/graphite. We investigated two twisted BL ${\mathrm{WSe}}_{2}$ at twist angles $\ensuremath{\sim}{28}^{\ensuremath{\circ}}$ and $\ensuremath{\sim}{10}^{\ensuremath{\circ}}$ and found no evidence of hybridization gaps at the interlayer band-crossing points, that could be precursors of the flat bands at smaller twist angles. Similarly, no such gaps were found for SL ${\mathrm{WSe}}_{2}$/graphite. Experimental results are complemented by theoretical density functional theory calculations, which suggest that a formation of hybridization gaps in the ${\mathrm{WSe}}_{2}$/graphene (which approximates the experimental ${\mathrm{WSe}}_{2}$/graphite system) sensitively depends on the ${\mathrm{WSe}}_{2}$ band character at the crossing point with the graphene Dirac band.

Giant orbital magnetic moments and paramagnetic shift in artificial relativistic atoms and molecules.

Materials such as graphene and topological insulators host massless Dirac fermions that enable the study of relativistic quantum phenomena. Single quantum dots and coupled quantum dots formed with massless Dirac fermions can be viewed as artificial relativistic atoms and molecules, respectively. Such structures offer a unique testbed to study atomic and molecular physics in the ultrarelativistic regime (particle speed close to the speed of light). Here we use a scanning tunnelling microscope to create and probe single and coupled electrostatically defined graphene quantum dots to unravel the magnetic-field responses of artificial relativistic nanostructures. We observe a giant orbital Zeeman splitting and orbital magnetic moment up to ~70 meV T-1 and ~600μB (μB, Bohr magneton) in single graphene quantum dots. For coupled graphene quantum dots, Aharonov-Bohm oscillations and a strong Van Vleck paramagnetic shift of ~20 meV T-2 are observed. Our findings provide fundamental insights into relativistic quantum dot states, which can be potentially leveraged for use in quantum information science.

Gate-enhanced broadband photodetection based on Cd3As2/graphene Dirac heterojunctions

Dirac semimetals are promising materials for broadband and fast photodetection due to their gapless nature. Dirac heterostructures consisting of 2D Dirac semimetal graphene and its 3D analogue Cd3As2 should take the ascendency of high carrier mobility in both materials, while overcome the limitation of weak optical absorption in graphene-based devices and suppress the dark current occurring in pure Cd3As2 photodetectors. Herein, we report high-performance photodetectors based on a 3D Dirac semimetal Cd3As2/monolayer graphene heterostructure, which show broadband photoresponse from visible (488 nm) to mid-infrared (10 μm) wavelength region at room temperature without an external bias. The photodetectors are with a maximum responsivity of 0.34 mA/W at 488 nm and a fast response speed of ∼13 μs. In addition, the photoresponse can be enhanced by a gate voltage even in a long wavelength region. Our work suggests that the combination of the graphene and 3D Dirac semimetal is promising for high-performance photodetectors with broadband detection, high sensitivity, and rapid response.

Band gap opening in graphene by hybridization with Au (001) reconstructed surfaces

Au(001) surfaces exhibit a complex reconstructed structure $[\mathrm{Hex}\ensuremath{-}\mathrm{Au}(001)]$ comprising a hexagonal surface and square bulk lattices, yielding a quasi-one-dimensional corrugated surface. When graphene was grown on this surface, the periodicity of the corrugated surface was predicted to change the electronic structure of graphene, forming bandgaps and new Dirac points. Furthermore, the graphene--Au interface is promising for bandgap generation and spin injection due to band hybridization. Here, we report the angle-resolved photoemission spectroscopy and density functional calculation of graphene on a $\mathrm{Hex}\ensuremath{-}\mathrm{Au}(001)$ surface. The crossing point of the original and replica graphene \ensuremath{\pi} bands showed no bandgap, suggesting that the one-dimensional potential was too small to modify the electronic structure. A bandgap of 0.2 eV was observed at the crossing point of the graphene \ensuremath{\pi} and Au $6sp$ bands, indicating that the bandgap is generated using hybridization of the graphene \ensuremath{\pi} and Au $6sp$ bands. We discussed the hybridization mechanism and concluded that the R30 configuration between graphene and Au and an isolated electronic structure of Au are essential for effective hybridization between graphene and Au. We anticipate that hybridization between graphene \ensuremath{\pi} and Au $6sp$ would result in spin injection into graphene.

Adsorption of Na Monolayer on Graphene Covered Pt(111) Substrate

Modification of graphene electronic properties via contact with atoms of different kind allows for designing a number of functional post-silicon electronic devices. Specifically, 2D metallic layer formation over graphene is a promising approach to improving the electronic properties of graphene-based systems. In this work we analyse the electronic and spin structure of graphene synthesized on Pt(111) after sodium monolayer adsorption by means of angle-resolved photoemission spectroscopy and ab initio calculations. Here, we show that sodium layer formation leads to a shift of the graphene π states towards higher binding energies, but the most intriguing property of the studied system is the appearance of a partially spin-polarized Kanji symbol-like feature resembling the graphene Dirac cone in the electronic structure of adsorbed sodium. Our findings reveal that this structure is caused by a strong interaction between Na orbitals and Pt 5d spin-polarized states, where the graphene monolayer between them serves as a mediator of such interaction.

Graphene Dirac fermions in symmetric electric and magnetic fields: the case of an electric square well

In this paper, a simple method is proposed to get analytical solutions (or with the help of a few numerical calculations) of the Dirac-Weyl equation for low energy electrons in graphene in the presence of certain electric and magnetic fields. In order to decouple the Dirac-Weyl equation we have assumed a displacement symmetry of the system along a direction and some conditions on the magnetic and electric fields. The resulting equations have the natural form to apply the technique of supersymmetric quantum mechanics. The example of an electric well with square profile is worked out in detail to illustrate some of the most interesting features of this procedure.

Passivation of Bi2Te3 Topological Insulator by Transferred CVD‐Graphene: Toward Intermixing‐Free Interfaces

AbstractThe investigation, and ultimate application, of topological insulators, typically involve exposure to ambient conditions or their integration with metals, which lead to surface oxidation or material intermixing. X‐ray photoelectron spectroscopy (XPS) measurements that demonstrate passivated and intermixing‐free interfaces in the topological insulator Bi2Te3 by means of dry‐transferred CVD graphene are reported. After air exposure, no traces of Bi2Te3 oxidation are found. Furthermore, it is demonstrated that graphene acts as a very efficient metal and chalcogen diffusion barrier in Bi2Te3/graphene/permalloy (Py) heterostructures, which are relevant for spintronics. Such results are in stark contrast with the significant surface degradation observed in bare Bi2Te3 under ambient conditions and the deep BiTe bonding disruption that occurs in Bi2Te3/Py heterostructures. These findings provide a new approach to control and engineer topological insulator interfaces for spintronic applications and a new platform to investigate the combined use of graphene and topological insulator Dirac states.

Tailoring Dirac Fermions by In-Situ Tunable High-Order Moiré Pattern in Graphene-Monolayer Xenon Heterostructure

We report an experimental study of a high-order moiré pattern formed in graphene-monolayer xenon heterostructure. The moiré period is insitu tuned from few nanometers to +∞, by adjusting the lattice constant of the xenon monolayer through annealing. Using angle-resolved photoemission spectroscopy, we observe that Dirac node replicas move closer and finally overlap with a gap opening, as the moiré pattern expands to +∞ and evolves into a Kekulé distortion. A moiré Hamiltonian coupling Dirac fermions from different valleys explains experimental results and indicates narrow moiré band. Our Letter demonstrates a platform to study continuous evolution of the moiré pattern, and provides an unprecedented approach for tailoring Dirac fermions with tunable intervalley coupling.

Multi-control plasmon-induced transparency via graphene and bulk Dirac semimetal

Based on graphene and bulk Dirac semimetal (BDS), we proposed a terahertz metamaterial which can realize plasmon-induced transparency (PIT) effect. By independently or jointly tuning the Fermi energy of graphene and BDS, the frequency selection, single frequency optical switch and the double frequency synchronous optical switch can be achieved. The physical mechanism of single PIT effect is explained via electric field distribution and coupled mode theory. In addition, we improve the proposed metamaterial by etching grooves on the BDS gratings and changing the structural parameters of graphene and BDS gratings, which results in a dual-PIT effect. When independently or jointly tuning the Fermi energy of graphene and BDS, the frequency of the dual-PIT peaks can also be tuned, and the quatary frequency asynchronous and triple frequency synchronous optical switch can be achieved. By analyzing the electric field distribution, we found that the double PIT effect is caused by different mode coupling between graphene and BDS gratings. This work provides a new direction for the future design of multi-band filters and optical switch in terahertz band.

Explaining Steep-Slope Switching in Carbon Nanotube Dirac-Source Field-Effect Transistors

Dirac-source field-effect transistors (DS-FETs) have been proposed as steep-slope transistors for low-power switching. The steep-slope switching of a DS-FET originates from the “low-pass” energy filtering effect of the graphene Dirac source, which requires the number of modes in the graphene source to be lower than the ones in the channel. However, despite the fact that this requirement is not satisfied in a DS-FET with a carbon nanotube (CNT) channel, steep-slope switching has been experimentally demonstrated in such devices. In this article, we propose a mechanism of switching in CNT DS-FETs that is consistent with the small number of modes in a CNT. We argue that the CNT acts as a transverse momentum selector, which effectively introduces a bandgap in the graphene source, and thus, the Klein tunneling in the graphene n-p junction becomes band-to-band tunneling (BTBT). This makes the CNT DS-FET essentially a new type of tunneling field-effect transistor (TFET), which we call momentum-selector TFET (MS-TFET). Moreover, we study the impact of misorientation between graphene and CNT on the performance of a CNT MS-TFET. Finally, we show that scattering may actually be beneficial in such a device and could potentially enhance the on-current of a CNT MS-TFET.

Erratum: Pseudogauge field driven acoustoelectric current in two-dimensional hexagonal Dirac materials [Phys. Rev. B 105 , 125407 (2022)

Using a diagrammatic scheme, we study the acoustoelectric effects in two-dimensional (2D) hexagonal Dirac materials due to the sound-induced pseudo-gauge field. We analyze both uniform and {\em spatially dispersive} currents in response to copropagating and counterpropagating sound waves, respectively. In addition to the longitudinal acoustoelectric current, we obtain an exotic {\em transverse} charge current flowing perpendicular to the sound propagation direction owing to the interplay of transverse and longitudinal gauge field components $j_T\propto A_L A^\ast_T$. In contrast to the almost isotropic directional profile of the longitudinal uniform current, a highly anisotropic transverse component $j_T\sim\sin(6\theta)$ is achieved that stems from the inherited three-fold symmetry of the hexagonal lattice. However, both longitudinal and transverse parts of the dispersive current are predicted to be strongly anisotropic $\sim\sin^2(3\theta)$ or $\cos^2(3\theta)$. We quantitatively estimate the pseudogauge field contribution to the acoustoelectric current that can be probed in future experiments in graphene and other 2D hexagonal Dirac materials.

Hunting Quantum Gravity with Analogs: The Case of Graphene

Analogs of fundamental physical phenomena can be used in two ways. One way consists in reproducing specific aspects of the classical or quantum gravity of quantum fields in curved space or of other high-energy scenarios on lower-energy corresponding systems. The “reverse way” consists in building fundamental physical theories, for instance, quantum gravity models, inspired by the lower-energy corresponding systems. Here, we present the case of graphene and other Dirac materials.