Biased Bilayer Graphene Research Articles

Photoinduced polarization enhancement on biased bilayer graphene in the Landau level regime

We investigate the charge carrier dynamics in bilayer graphene subject to monochromatic laser irradiation within the Landau level quantization regime. Even though the radiation field does not lift the energy degeneracy of the lowest Landau levels (LLs), it nevertheless has a strong effect on the photoinduced pseudospin polarization response for higher LLs (). Our results show that the photoinduced bandgaps lead to a finite response of the averaged pseudospin polarization with nontrivial oscillating behavior. It is shown that the contribution from these higher LL transitions turns out to be crucial to achieve an enhanced photoinduced polarization in radiated bilayer graphene. The experimental feasibility of our findings is also discussed.

Effect of magnetic field on specific heat and magnetic susceptibility of biased bilayer graphene: A full band approach

We have addressed the behaviors density of states and thermodynamic properties of doped biased bilayer graphene for AA stacking in the context of tight binding model hamiltonian. Specially we have studied the temperature dependence of spin susceptibility and specific heat of simple bilayer graphene in the presence of external magnetic field and bias voltage. The effects of electron doping and magnetic field on specific heat have been investigated. Green’s function approach has been implemented to find the behavior of electronic density of states and thermodynamic properties of bilayer graphene. We have found the temperature dependence of specific heat and spin susceptibility for different values of magnetic field and bias voltage in the presence of doping effects. Our results for density of states of bilayer graphene in the presence of bias voltage show the increase of magnetic field reduces the band gap width. Also there is a peak in temperature dependence of specific heat for all values of magnetic field and bias voltage. Moreover the effects of magnetic field and chemical potential on the behaviors of Pauli paramagnetic spin susceptibility have been addressed in details.

Electrical conductivity of undoped bilayer Graphene: Beyond nearest neighbor approximation

We have addressed density of states and electrical conductivity of undoped biased bilayer graphene for both AA and AB stacking in the context of tight binding model hamiltonian. The effects of next nearest neighbor hopping amplitude for electrons and gap parameter on electronic density of states and electrical conductivity have been investigated. Green’s function approach has been implemented to find the behavior of electrical conductivity of bilayer graphene within linear response theory. We have found the temperature dependence of electrical conductivity for different values of gap parameter and bias voltage in the presence of neat nearest neighbor hopping integral. Our results for density of states of bilayer graphene in the presence of gap parameter show the increase of bias voltage reduces the band gap width. Also a peak in the dependence of thermal conductivity on temperature has been observed for all physical parameters. An anisotropic behavior for electrical conductivity of unbiased simple bilayer graphene in the presence of gap parameter has been resulted.

Excitons without effective mass: Biased bilayer graphene

Understanding the dynamics of excitons in two dimensional semiconductors requires a theory that incorporates the essential physics distinct from their three-dimensional counterparts. In addition to the modified dielectric environment, single-particle states with strongly non-parabolic dispersion appear in many two-dimensional band structures, so that "effective mass" is ill-defined. Focusing on electrostatically-biased bilayer graphene as an example where quartic (and higher) dispersion terms are necessary, we present a semi-analytic theory used to investigate the properties of ground and excited excitonic states. This includes determination of relative oscillator strengths and magnetic moments (g-factors) which can be directly compared to recent experimental measurements.

Electrically Tunable Gauge Fields in Tiny-Angle Twisted Bilayer Graphene.

Twisted bilayer graphene has recently attracted a lot of attention for its rich electronic properties and tunability. Here we show that for very small twist angles, α≪1°, the application of a perpendicular electric field is mathematically equivalent to a new kind of artificial gauge field. This identification opens the door for the generation and detection of pseudo-Landau levels in graphene platforms within robust setups, which do not depend on strain engineering and therefore can be realistically harvested for technological applications. Furthermore, this new artificial gauge field leads to the development of highly localized modes associated with flat bands close to charge neutrality, which form an emergent kagome lattice in real space. Our findings indicate that for tiny angles biased twisted bilayer graphene is a promising platform that can realize frustrated lattices of highly localized states, opening a new direction for the investigation of strongly correlated phases of matter.

Effect of high-frequency electromagnetic radiation on the plasmon dispersion in biased graphene bilayer

The Floquet spectrum of electron in biased graphene bilayer subjected to the high-frequency electromagnetic radiation has been derived. The magnitude of gap in electron spectrum renormalized by radiation has been calculated. The quasienergy gap has been shown to increase with electromagnetic wave intensity. The possibility of controlling of curvature of dispersion curve for plasmon by changing of both bias voltage and high-frequency field intensity has been shown. The dependence of this curvature on both bias voltage and electromagnetic field amplitude has been predicted to have the nonmonotonous character. Intensity of electromagnetic radiation causing the suppression of the plasma oscillations has been found.

Valley-controlled nonlocal transistor based on irradiated and biased bilayer graphene

We theoretically study the nonlocal Andreev transport through a normal/superconducting/normal junction formed on bilayer graphene, in which an off-resonant circularly polarized light and interlayer biases are applied to the two normal regions. The effects of the light field, bias voltage, and superconductor length on the crossed Andreev reflection are calculated and analyzed. We show that the pure crossed Andreev reflection can be obtained by modulating the completely opposite valley polarizations in the two terminals under the optical and electrical coaction, and a valley-switch effect can be realized between the pure crossed Andreev reflection and the pure valley polarized elastic cotunneling by reversing the interlayer bias of one terminal. In addition, a very small subthreshold swing can be acquired when the pure nonlocal conductance changes with variation of the Fermi energy. This observation suggests that our device can be operated as a fast nonlocal switching transistor.

Thermal properties of biased bilayer graphene and boron nitride nanoribbons

The electronic structure, temperature-dependent thermal conductivity κ(T) and heat capacity CV(T) of bilayer graphene nanoribbons (G/G), boron nitride nanoribbons (BN/BN) and graphene/boron nitride nanoribbons (G/BN) with AA and AB stacking are investigated using the tight binding model and Green’s function method. The results show that the bilayer G/G with AA stacking is metallic independent of the bias while other structures with AA and AB stacking are semiconductor in absence of bias and their band gap reduce in the presence of bias. The κ(T) for all structures increases with temperature to its maximum value and then decreases by further temperature increasing. The κ(T) of bilayer BN/BN is smaller than that of bilayer G/G and G/BN. The CV(T) of all structures increases up to a maximum value at T = TM (Schottky anomaly) and the corresponding TM value depends on the nanoribbons types and stacking as TM(G/G)<TM(G/BN)<TM(BN/BN). The Cv(T) of bilayer G/G is larger than G/BN and BN/BN.

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.

Longitudinal Magnetic Field Effects on Thermal Transport of Doped Bilayer Graphene

We address thermal conductivity of doped biased bilayer graphene for AA-stacking in the context of tight binding model hamiltonian. The effect of magnetic field as its direction is along perpendicular to graphene plane on thermal conductivity has been investigated. Green’s function approach has been implemented to find the behavior of thermal conductivity of bilayer graphene within linear response theory. We have found the chemical potential dependence of thermal conductivity for different values of magnetic field and bias voltage. Also a peak in the dependence of thermal conductivity on magnetic field has been observed. Moreover, in the absence of magnetic field, we find a monotonic decreasing behavior for chemical dependence of thermal conductivity. However, a peak appears in chemical potential dependence of thermal conductivity at nonzero magnetic field. Finally, the behavior of Seebeck coefficient in terms of magnetic field and chemical potential has been studied in details.

Phonon-induced renormalization of the electron spectrum of biased bilayer graphene

The effect of the electron-phonon interaction on the electron subsystem of the bilayer graphene has been investigated in the case when there is a potential bias between the graphene layers. The electron-phonon interaction has been shown to lead to increasing of the curvature of the lower dispersion branch of the conduction band of the bigraphene in the vicinity of the Dirac point. The latter corresponds to the decreasing of the absolute value of the electron effective mass. The corresponding correction to the effective mass has been calculated. Dependence of this correction on the bias has been investigated. Influence of such effect on the bigraphene conductivity is discussed.

Topological confinement effects of electron-electron interactions in biased zigzag-edge bilayer graphene nanoribbons

We have investigated topological confinement effects of electron-electron interactions on gapless edge states in zigzag-edge bilayer graphene nanoribbons under a voltage bias between the layers by using a tight-binding model with on-site Coulomb interactions. Spin-dependent potentials at the four edge sites resulting from the electron-electron interactions not only determine the spin configurations but also can result in topological confinement effects. In the magnetic phases with interlayer antiferromagnetic spin configurations, the edge potentials do not produce gapless edge states, and only gapless edge states can exist due to the edge shape. In the magnetic phases with interlayer ferromagnetic spin configurations, various gapless edge states due to the edge potential and edge shape are produced. Gapless edge states corresponding to the quantum spin-valley Hall phase and the layered antiferromagnetic phase can be described by topological confinement effects alone. The half-metallic quantum valley Hall phase can be described by topological confinement effects at inequivalent edge sites.

Valley surface-wave photonic crystal and its bulk/edge transport

The valley Hall effect and topological valley edge states are two fundamental properties in gapped valleytronic materials, such as MoS${}_{2}$ and biased bilayer graphene. Such properties have paved the way for applications in valleytronics. Here, the authors experimentally demonstrate a valley surface-wave photonic crystal on a single metal surface, as the photonic analog of the valley-Hall topological insulator phase. The photonic valley-Hall effect with valley-chirality locked beam splitting, and topological valley-polarized edge states, are demonstrated for the first time on a photonic platform.

Valley filtering due to orbital magnetic moment in bilayer graphene

We investigate the effect of valley-dependent orbital magnetic moment on the transmission of quasiparticles through biased bilayer graphene npn and pnp junctions in the presence of out-of-plane magnetic field. It is shown that the valley-polarized Zeeman-like energy splitting, due to the interaction of orbital magnetic moment with magnetic field, can suppress the transmission of quasiparticles of one valley while transmitting those of the other valley. This valley-selective transmission property can be exploited for valley filtering. We demonstrate that the npn and pnp junction, respectively, filters off the K′-valley and K-valley particles, with nearly perfect degree of filtration.

Nonlinear response of biased bilayer graphene at terahertz frequencies

A density-matrix formalism within the length gauge is developed to calculate the nonlinear response of both doped and undoped biased bilayer graphene at terahertz frequencies. Employing a tight-binding model, we derive an effective two-band Hamiltonian with which we calculate the conduction and valence band dispersion, as well as their respective Bloch states. We then solve for the dynamic equations of the density-matrix elements, allowing for the calculation of the intraband and interband current densities and the transmitted and reflected terahertz fields. We find that the third harmonic amplitude generated for undoped biased bilayer graphene with a gap size of 4 meV is larger than that for monolayer graphene or unbiased bilayer graphene for an incident 1 THz single-cycle pulse with a field amplitude of 2.0 kV/cm. We also find for doped biased bilayer graphene that, although the dispersion becomes highly nonparabolic as a bias is applied, the third harmonic is a maximum when there is no bias and diminishes with an increase in bias.

Numerical simulation of vertical tunneling transistor with bilayer graphene as source and drain regions

In this paper, the electrical characteristics of vertical tunneling bilayer graphene field effect transistor (VTBGFET) are theoretically investigated. We evaluate the device behavior using nonequilibrium Green's function (NEGF) formalism combined with an atomistic tight binding model. By using this method, we extract the most significant figures of merit such as ON/OFF current ratio, subthreshold swing, and intrinsic gate‐delay time.The results indicate that using a bilayer graphene instead of a monolayer graphene as the base material for the source and drain regions leads to a larger ON/OFF current ratio due to the presence of an energy bandgap in biased bilayer graphene. Also, the subthreshold swing of VTBGFET can be much lower than that of vertical tunneling monolayer graphene field‐effect transistor (VTMGFET). We find that the increase of the number of hBN layers enhances the ON/OFF current ratio but degrades the intrinsic gate‐delay time.

Quantum pumping of layer pseudospin current in biased bilayer graphene

In bilayer graphene, the top layer and the bottom layer can be regarded as a kind of ‘pseudospin’. By applying a voltage bias between the two layers, the pseudospin is polarized along the direction perpendicular to the graphene plane. Considering a biased–unbiased–biased bilayer graphene structure, we show that it is possible to use quantum pumping to generate a layer pseudospin polarized current which is highly localized in the layer with a higher potential. By reversing the bias direction, the pumped current is tuned to flow in the opposite layer. The proposed structure which generates layer pseudospin current will have applications in the development of layer pseudospin devices.

Half-metallic quantum valley Hall effect in biased zigzag-edge bilayer graphene nanoribbons

We have investigated electron-electron interaction effects on gapless edge states in the antiferromagnetic phase of zigzag-edge bilayer graphene nanoribbons under a voltage bias between the layers by using a tight-binding model with on-site Coulomb interactions. We found that a zigzag-edge bilayer graphene nanoribbon can have gapless edge states corresponding to peculiar topologically nontrivial insulator phases, such as a half-metallic quantum valley Hall phase. The half-metallicity was found to be due to the on-site Coulomb interactions through which excess charges produced by the voltage bias between the layers give rise to the potential difference between the opposite edges. A topological phase diagram in the antiferromagnetic phase of a zigzag-edge bilayer graphene nanoribbon was determined as a function of the bias voltage and the ribbon width. The quantum confinement effect, which was found to be graphene-like for narrow ribbons and to be bilayer graphene-like for wide ribbons, was also confirmed to play an important role in determining the topologically nontrivial insulator phases.

Band tunneling through double barrier in biased graphene bilayer

We calculate the transport properties of charge carriers in graphene bilayers across symmetric and asymmetric double potential barrier considering energies exceeding the inter-layer coupling where two transport modes exist. Evaluating the transmission and reflection probabilities and corresponding conductances, we show that the transport is sensitive to the distance between the two barriers. Moreover, we explain the characteristic features observed in the numerical calculations, such as resonance tunneling at normal incidence, based on the Febry–Pèrot oscillations and ballistic transmission carried out by the evanescent waves. Finally, we compute the conductance of each mode separately and investigate contributions from inter-mode scattering and show that some geometric potential parameters can be used to control the total conductance of the system.

Electrons at the monkey saddle: A multicritical Lifshitz point

We consider 2D interacting electrons at a monkey saddle with dispersion $\propto p_x^3-3p_xp_y^2$. Such a dispersion naturally arises at the multicritical Lifshitz point when three van Hove saddles merge in an elliptical umbilic elementary catastrophe, which we show can be realized in biased bilayer graphene. A multicritical Lifshitz point of this kind can be identified by its signature Landau level behavior $E_m\propto (Bm)^{3/2}$ and related oscillations in thermodynamic and transport properties, such as de Haas-van Alphen and Shubnikov-de Haas oscillations, whose period triples as the system crosses the singularity. We show, in the case of a single monkey saddle, that the non-interacting electron fixed point is unstable to interactions under the renormalization group flow, developing either a superconducting instability or non-Fermi liquid features. Biased bilayer graphene, where there are two non-nested monkey saddles at the $K$ and $K^\prime$ points, exhibits an interplay of competing many-body instabilities, namely s-wave superconductivity, ferromagnetism, and spin- and charge-density wave.