AA-stacked Bilayer Graphene Research Articles

Electric field-induced chiral d + id superconductivity in AA-stacked bilayer graphene: a quantum Monte Carlo study

Using the constrained-path quantum Monte Carlo method, we systematically study the half-filled Hubbard model on AA-stacked honeycomb lattice. Our simulations demonstrate that a dominant chiral d + id wave superconductivity can be induced by a perpendicular electric field. At a fixed electric field, the effective pairing interaction of chiral d + id superconductivity exhibits an increasing behavior with increasing the on-site Coulomb interaction. We attribute the electric field-induced d + id superconductivity to an increased density of states near the Fermi energy and robust antiferromagnetic spin correlation upon turning on electric field. Our results strongly suggest that the AA-stacked graphene system is a good candidate for chiral d + id superconductor.

Quantum dots in AA-stacked bilayer graphene

While electrostatic confinement in single-layer graphene and AA-stacked bilayer graphene is precluded by Klein tunneling and the gapless energy spectrum, we theoretically show that a circular domain wall that separates domains of single-layer graphene and AA-stacked bilayer graphene can provide bound states. Solving the Dirac-Weyl equation in the presence of a global mass potential and a local electrostatic potential, we obtain the energy spectrum of these states and the corresponding radial probability densities. Depending on the mass potential profile, regular bound states can exist inside the quantum dot and topological bound states at the domain wall. Controlling the electrostatic potential inside the quantum dot enables the simultaneous presence of both types of states. We find that the number of nodes of the radial wave function of the regular bound states inside the quantum dot is equal to the radial quantum number. The energy spectra of the bound states display anticrossings, reflecting coupling of electron- and holelike states.

Pressure and inversion symmetry breaking field-driven first-order phase transition and formation of Dirac circle in perovskites

Through model Hamiltonian studies and first-principle electronic structure calculations, we have examined the effect of inversion symmetry breaking (ISB) field and hydrostatic pressure on the band topology of halide perovskites by taking MAPbI$_3$ as a prototype. Our study shows that while hydrostatic pressure induces normal to topological insulator continuous phase transition, the ISB field makes it first order. The pressure smoothly reduces the normal bandgap, and without ISB, the system achieves a gapless state before it produces a non-trivial bandgap with inverted characters. The ISB field does not stabilize the gapless state, and therefore, the discontinuity in the bandgap with pressure gives rise to the first-order transition. Furthermore, in the non-trivial phase, the ISB field forms an invariant surface Dirac circle in the neighbourhood of TRIM, which is first of its kind. The circle is formed due to interpenetration of Dirac cones resembling the band topology of AA-stacked bilayer graphene.

Study the metal-insulator transitions of bilayer graphene: Abelian group schemes approach

Abstract Bilayer graphene (BLG), as a two-dimensional crystalline form of carbon, with a controllable band gap, has been proposed as an alternative to graphene for nanoscale electronics. Through modeling single-electron transport in BLG, using Abelian group schemes Z m × Z m as a novel approach, the present paper has numerically studied metal-insulator transition using random matrix theory in doped AA-stacking BLG. In this mathematical technique, initially, Abelian group schemes and the underlying graph of honeycomb periodic lattice of the single-layer graphene were constructed. Then, the tight-binding Hamiltonian matrix of this layer was represented by the adjacency matrices of the graph. Through using these matrices, it is possible to generate m-dimensional Bose-Mesner algebra. Finally, by applying the wreath product of these matrices, the Hamiltonian was calculated. In fact, instead of the hopping integral between individual layers, the mathematical combination of their underlying graphs was considered. This would be generalized to more than two layers. Numerical results indicated that insulator to metal transition occurred at the threshold doping value C = 0.5 % , which has already been achieved. After this threshold value, by increasing doping, a fast quantum chaotic transition from Poisson to Wigner energy-level statistic distribution was observed. The paper has suggested that a sufficient difference in the concentration of dopant atoms at individual layers would be an efficient way of controlling metal-insulator transitions.

Electronic Properties of Twisted Bilayer Graphene in the Presence of a Magnetic Field

In AA-stacked twisted bilayer graphene, the lower energy bands become completely flat when the twist angle passes through certain specific values: the so-called “magic angles”. The Dirac peak appears at zero energy due to the flattening of these bands when the twist angle is sufficiently small [1-3]. When a constant perpendicular magnetic field is applied, Landau levels start appearing as expected [5]. We used the Kernel Polynomial Method (KPM) [6] as implemented in KITE [7] to study the optical and electronic properties of these systems. The aim of this work is to analyze how the features of these quantities change with the twist angle in the presence of an uniform magnetic field.

Electronic structure and optical characteristics of AA stacked bilayer graphene: A first principles calculations

First-principle calculations based on full potential linearized augmented plane-wave method have been performed to investigate the electronic and optical properties of 1 × 1 and 2 × 2 supercells of non-Bernal AA stacked bilayer graphene. The exchange-correlation effects in the present work have been treated using the local density approximation which show good agreement of the calculated structural and electronic properties with previous reports. From the calculated electronic properties, semimetallic nature is found for 1 × 1 supercell of bilayer graphene, while a pseudogap is observed for the 2 × 2 supercell. Our results indicate that the pseudogap in 2 × 2 supercell of bilayer graphene originates from the increased number of valence and conduction band states contributing in the electronic structure of this configuration and suppressing the states in the vicinity of Fermi level. In order to understand the effects of non-Bernal AA stacking of two graphene layers on the optical response, we have computed the optical properties in terms of complex dielectric function for electromagnetic radiations having polarization vectors parallel and perpendicular to the xy-plane. Our calculated optical properties show a significant enhancement of optical absorption of electromagnetic radiations having polarization vector parallel to xy-plane, which is ascribed to the augmentation of 2D honeycomb lattice of carbon atoms in form of bilayer graphene. These results highlight tunable electronic and optical characteristics of graphene that might prove advantageous for its employment in electronic devices.

Reducible Fermi surfaces for non-symmetric bilayer quantum-graph operators

This work constructs a class of non-symmetric periodic Schrödinger operators on metric graphs (quantum graphs) whose Fermi, or Floquet, surface is reducible. The Floquet surface at an energy level is an algebraic set that describes all complex wave vectors admissible by the periodic operator at the given energy. The graphs in this study are obtained by coupling two identical copies of a periodic quantum graph by edges to form a bilayer graph. Reducibility of the Floquet surface for all energies ensues when the coupling edges have potentials belonging to the same asymmetry class. The notion of asymmetry class is defined in this article through the introduction of an entire spectral A-function a(\lambda) associated with a potential – two potentials belong to the same asymmetry class if their A-functions are identical. Symmetric potentials correspond to a(\lambda) \equiv 0 . If the potentials of the connecting edges belong to different asymmetry classes, then typically the Floquet surface is not reducible. An exception occurs when two copies of certain bipartite graphs are coupled; the Floquet surface in this case is always reducible. This includes AA-stacked bilayer graphene.

Electron collimation at van der Waals domain walls in bilayer graphene

We show that a domain wall separating single-layer graphene and AA-stacked bilayer graphene (AA-BLG) can be used to generate highly collimated electron beams which can be steered by a magnetic field. Two distinct configurations are studied, namely, locally delaminated AA-BLG and terminated AA-BLG whose terminal edge types are assumed to be either zigzag or armchair. We investigate the electron scattering using semiclassical dynamics and verify the results independently with wave-packet dynamics simulations. We find that the proposed system supports two distinct types of collimated beams that correspond to the lower and upper cones in AA-BLG. Our computational results also reveal that collimation is robust against the number of layers connected to AA-BLG and terminal edges.

Adsorption and ultrafast diffusion of lithium in bilayer graphene:Ab initioand kinetic Monte Carlo simulation study

In this paper, we adopt first-principles calculations based on density functional theory and kinetic Monte Carlo simulations to investigate the adsorption and diffusion of lithium in bilayer graphene (BLG) as anodes in lithium-ion batteries. Based on energy barriers directly obtained from first-principles calculations for single-Li and two-Li intercalated BLG, a new equation was deduced for predicting energy barriers considering ${\mathrm{Li}}^{{}^{\ensuremath{'}}}\mathrm{s}$ interactions for multi-Li intercalated BLG. Our calculated results indicate that Li energetically prefers to intercalate within rather than adsorb outside the BLG. Additionally, lithium exists in a cationic state in the BLG. More excitingly, ultrafast Li diffusion coefficient ($\ensuremath{\sim}{10}^{\ensuremath{-}5}\phantom{\rule{4pt}{0ex}}{\mathrm{cm}}^{2}\phantom{\rule{4pt}{0ex}}{\mathrm{s}}^{\ensuremath{-}1}$) within AB-stacked BLG near room temperature was obtained, which reproduces the ultrafast Li diffusion coefficient measured in recent experiment. However, ultrafast Li diffusion was not found within AA-stacked BLG near room temperature. The analyses of potential distribution indicate that the stacking structure of BLG greatly affects its height of potential well within BLG, which directly leads to the large difference in Li diffusion. Furthermore, it is found that both the interaction among Li ions and the stacking structure cause Li diffusion within AB-stacked BLG to exhibit directional preference. Finally, the temperature dependence of Li diffusion is described by the Arrhenius law. These findings suggest that the stacking structure of BLG has an important influence on Li diffusion within BLG, and changing the stacking structure of BLG is one possible way to greatly improve Li diffusion rate within BLG. At last, it is suggested that AB-stacked BLG can be regarded as an excellent candidate for anode material in lithium-ion batteries.

Ground state magnetic properties in AA-stacking bilayer graphene quantum dots using Lieb’s theorem

Abstract We have studied magnetic properties of ground state in perfect AA-stacking bilayer graphene quantum dots. Our main model is the single-orbital tight-binding Hamiltonian that is supplemented with a mean field Hubbard term. In addition, density functional method has been exploited to gain more confidence in our findings. The calculations are performed for some random and triangular shape of AA-stacking bilayer quantum dots and always yield an antiferromagnetic ordering with a total spin S = 0 ground state. Despite computational results of Hubbard model is admirably compatible with Lieb’s theorem, conventional crystallographic sublattices are not appropriate to interpret our computational results in terms of the theorem. Therefore, we have suggested a new sublattice decomposition to settle our numerical output to the consequences of Lieb’s theorem. Especially we demonstrate some characteristic including polarization of local magnetization, degeneracies in the energy spectrum, the ferromagnetic and antiferromagnetic coupling of vacancies in our system have decisive compatibility with new proposed sublattice decomposition.

Insights into the effect of the interlayer spacings of bilayer graphene on the desolvation of H+, Li+, Na+, and K+ ions with water as a solvent: a first-principles study.

The desolvation effect of ions plays an important role in adjusting the capacity of supercapacitors and has attracted considerable attention after its discovery. Here, first-principles calculations were conducted to calculate the reaction energies of ions, water, and hydrated ions in bilayer graphene (BG) with different interlayer spacings (d) and to explore the desolvation behaviors of H+, Li+, Na+, and K+ ions. The calculated results showed that H+ can only exist in the state of H3O+ in AA-stacking BG, and desolvation exists only in the case of AB-stacking BG. The complete desolvation size for H+ ions in the AB-stacking system reached 5.6 Å, which was the largest desolvation size of the four ions studied. The critical desolvation sizes of Li+, Na+, and K+ in the BG layers of AA- and AB-stacking increased sharply as a consequence of the increasing ionic radius. However, the complete desolvation sizes of all three ions were in the range of 4-5 Å and with the increase in ionic radius, the complete desolvation sizes showed a reverse tendency. The complete desolvation size of Na+ in AB-stacking BG was slightly larger than that in AA-stacking BG. Further analysis presented that the ionic radii of H+, Li+, Na+, and K+ ions make a dominant contribution to the critical size of desolvation. Our present results provide useful information for improving the capacity of supercapacitors by precisely matching the pore structure and electrolyte through the adjustment of the pore structure of carbon materials.

Tight-Binding Model Study of the Tunable Anti-ferromagnetism and Electron specific heat of AA-Staked bi-layer Graphene in a Transverse Electric Field

We communicate a microscopic theoretical model study of electronic and thermal properties of AA-stacked bi-layer graphene in a transverse electric field. In order to describe the system, we have considered a Hamiltonian consisting of nearest-neighbor electron hopping in both the layers in presence of a transverse electric field between the two layers created by a gate electrode. The inter-layer electron hopping from A1 atom of the first layer to A2 atom of the second layer is considered with a hopping energy εk,⊥=t⊥γ⊥(k). Due to strong on-site Coulomb correlation between the electrons of sub-lattices of both the layers, anti-ferromagnetic order is developed in the system. The electron Green's functions of both sub-lattices in the two layers are solved by Zubarev’s Green's function technique. The anti-ferromagnetic (AFM) gap equation is found from the electron correlations of the corresponding Green’s functions. It is assumed that the A-site magnetization dominates over the B-site magnetization giving rise to the anti-ferromagnetic order. The temperature dependent AFM gap exhibits high Neel’s temperature where electronic specific heat shows a sharp jump. Besides this, the specific heat exhibits another flat peak arising due to the onset of AFM order in A and B sub-lattices of the layer. The evolution of the gap equation and specific heat are reported by varying the model parameters like Coulomb energy, electric field, transverse electron hopping integral and chemical potential.

Strain impacts on commensurate bilayer graphene superlattices: Distorted trigonal warping, emergence of bandgap and direct-indirect bandgap transition

Due to low dimensionality, the controlled stacking of graphene films and their electronic properties are susceptible to environmental changes including strain. The strain-induced modification of the electronic properties such as the emergence and modulation of bandgaps crucially depends on the stacking of the graphene films. However, to date, only the impact of strain on electronic properties of Bernal and AA-stacked bilayer graphene has been extensively investigated in theoretical studies. Exploiting density functional theory and tight-binding calculation, we investigate the impacts of in-plane strain on two different classes of commensurate twisted bilayer graphene (TBG) which are even/odd under sublattice exchange (SE) parity. We find that the SE odd TBG remains gapless whereas the bandgap increases for the SE even TBG when applying equibiaxial tensile strain. Moreover, we observe that for extremely large mixed strains both investigated TBG superstructures demonstrate direct-indirect bandgap transition.

Electronic transport and Klein tunneling in gapped AA-stacked bilayer graphene

We theoretically investigate the electronic transport and Klein tunneling in AA-stacked bilayer graphene (AA-BLG) encapsulated by dielectric materials. Using the four-band continuum model, we evaluate the transmission and reflection probabilities along with the respective conductances. We find that the interlayer mass-term difference induced by the dielectric materials opens a gap in the energy spectrum and couples the upper and lower Dirac cones in AA-BLG. This cone coupling induces an inter-cone transport that is asymmetric with respect to the normal incidence in the presence of the asymmetric mass-term. The energy spectrum of the gapped AA-BLG exhibits electron-hole asymmetry that is reflected in the associated intra- and inter-cone transport channels. We also find that even though Klein tunneling exists in gated and biased AA-BLG, it is precluded by the interlayer mass-term difference and instead Fabry-Pérot resonances appear.

First-principles study of transition metal monatomic chains intercalated AA-stacked bilayer graphene nanoribbons

To explore the novel application of graphene nanomaterials, we investigate the structural, electronic and magnetic properties of transition metals (TM = V, Cr, Mn, Fe, Co and Ni) monatomic chains intercalated AA stacked bilayer graphene nanoribbons ([GTMG]w) using first-principles calculations. The combining processes of all the [GTMG]w compounds are exothermic except the [GCuG]w (w = 5, 6, 7, 8, 9) compounds, accompanying with larger formation energies and charges transfer. The formed TM–C ionic bonds are contributed to enhancing the stability of [GTMG]w compounds. The most stable intercalated structures, are the [GVG]w compounds for the different TM chain, and are the [GTMG]4 compounds for the different widths of bilayer graphene nanoribbons (w = 4, 5, 6, 7, 8, 9). The [GMnG]w compounds have the maximum magnetic moment, but the [GVG]w, [GNiG]w and [GCuG]w compounds have magnetic quenching phenomenon with zero magnetic moment. There are different magnetic properties for the even (w = 4, 6, 8) and odd (w = 5, 7, 9) widths of bilayer graphene nanoribbons in [GTMG] compounds. Therefore, the TM chains intercalated bilayer graphene nanoribbons exhibit tunable magnetic properties for potential spintronics applications.

Spin-dependent transport properties of AA-stacked bilayer graphene nanoribbon

In this paper, the spin-dependent transport of a spin-valve, based on bilayer graphene in which the AA-stacked bilayer zigzag graphene nanoribbon (BLG) connected to two ferromagnetic (FM) semi-infinite single-layer zigzag graphene nanoribbons (FM/BLG/FM system), is numerically studied. Using the non-equilibrium Green's function method and tight-binding model in the framework of Landauer-Büttiker formalism, the spin-dependent transmission function, current and tunnel magnetoresistance of the system are calculated. Results show that the transmission function is always nonzero near the zero energy for the parallel alignment of electrode magnetizations, while it is zero for the antiparallel alignment due to the band-selective rule. Consequently, there is a region for the tunnel magnetoresistance with the value of 100%. The magnitude of this region depends on the strength of magnetization in ferromagnetic electrodes. Also, the effects of bilayer/single-layer coupling geometry,inter-layer coupling strength and width of BLG on the spin-dependent transport of the system are investigated. These results can be useful for designing graphene based spintronic devices.

Substrate effects on transport properties of a biased AA-stacked bilayer graphene

The important experimental advances in graphene fabrication and its peculiar transport properties motivated researchers to utilize graphene as a potential basis for the next generation of fast and smart electronic devices. In this article, we investigate the influence of a potential substrate on the transport properties of a biased AA-stacked n-p-n bilayer graphene junction (AA-BLG). Using the Dirac Hamiltonian with the transfer matrix approach we obtain the transmission probabilities and thus the respective conductance. In the presence of the induced mass-term the energy spectrum and the intra-cone transmission drastically change while the inter-cone transmission remains zero. On the other hand, the bias slightly alters the energy spectrum but it significantly affects the transport properties due to its ability to switch on the inter-cone transmission. In addition, we find that Klein tunnelling is attenuated in the presence of the induced mass-term which can improve the carriers confinement in such configurations. Our findings provide possible experimental measurements to determine the interlayer coupling and the induced mass terms in graphene bilayer based on conductance and band structure measurements.

Ab initio calculation on the Optical Properties of AA- Stacked two dimensional Graphene, Silicene, Germanene, and Stanene

In this study, we have analyzed the electronic band structure and optical properties of AA-stacked bilayer graphene and its 2D analogues and compared the results with single layers. The calculations have been done using Density Functional Theory with Generalized Gradient Approximation as exchange correlation potential as in CASTEP. The study on electronic band structure shows the splitting of valence and conduction bands. A band gap of 0.342eV in graphene and an infinitesimally small gap in other 2D materials are generated. Similar to a single layer, AA-stacked bilayer materials also exhibit excellent optical properties throughout the optical region from infrared to ultraviolet. Optical properties are studied along both parallel (||) and perpendicular ( ) polarization directions. The complex dielectric function (ε) and the complex refractive index (N) are calculated. The calculated values of ε and N enable us to analyze optical absorption, reflectivity, conductivity, and the electron loss function. Inferences from the study of optical properties are presented. In general the optical properties are found to be enhanced compared to its corresponding single layer. The further study brings out greater inferences towards their direct application in the optical industry through a wide range of the optical spectrum.

Voltage tunable plasmon propagation in dual gated bilayer graphene

In this paper, we theoretically investigate plasmon propagation characteristics in AB and AA stacked bilayer graphene (BLG) in the presence of energy asymmetry due to an electrostatic field oriented perpendicularly to the plane of the graphene sheet. We first derive the optical conductivity of BLG using the Kubo formalism incorporating energy asymmetry and finite electron scattering. All results are obtained for room temperature (300 K) operation. By solving Maxwell's equations in a dual gate device setup, we obtain the wavevector of propagating plasmon modes in the transverse electric (TE) and transverse magnetic (TM) directions at terahertz frequencies. The plasmon wavevector allows us to compare the compression factor, propagation length, and the mode confinement of TE and TM plasmon modes in bilayer and monolayer graphene sheets and also to study the impact of material parameters on plasmon characteristics. Our results show that the energy asymmetry can be harnessed to increase the propagation length of TM plasmons in BLG. AA stacked BLG shows a larger increase in the propagation length than AB stacked BLG; conversely, it is very insensitive to the Fermi level variations. Additionally, the dual gate structure allows independent modulation of the energy asymmetry and the Fermi level in BLG, which is advantageous for reconfiguring plasmon characteristics post device fabrication.

Tight-Binding Model Study of Anti-ferromagnetic Order in AA-Stacked Bi-layer Graphene

We address here the anti-ferromagnetic order present in AA-stacked bi-layer graphene in a transversely applied electric field. The system is described by kinetic energy with nearest-neighbor electron hopping with same hopping integral t 1 for both the layers. Besides this, Coulomb interaction exists at A and B sub-lattices with same Coulomb correlation energy. The electron Green’s functions are calculated by Zubarev’s Green’s technique. The temperature-dependent anti-ferromagnetic magnetization is calculated from the Green’s function and is computed numerically and self-consistently. The strong on-site Coulomb interaction stabilizes the anti-ferromagnetic order in graphene. We assume that the electron spin at A site in the first layer is directed in the opposite direction to that of A site electron in the second layer. Similar spin order is observed for electrons in B site atom in reversed order. It is observed that anti-ferromagnetic (AFM) magnetization in the first layer nearly remains constant up to certain temperature and then increases with temperature, while the AFM magnetization in the second layer remains nearly constant and then rapidly decreases with temperature. The net AFM magnetization in bi-layer graphene remains constant and then rapidly increases with temperature. The evolution of the AFM magnetization is studied by varying transverse electric field, Coulomb energy, and temperature.