Layer Graphene System Research Articles

Effect of impurities and carrier concentration on conductivity of bilayer graphene double layer system in different dielectric environments

The electronic transport properties of the bilayer graphene double layer (BLGDL) system at [Formula: see text][Formula: see text]K using the Boltzmann transport equation are reported here. The conductivity of the BLGDL system has been calculated with varying parameters as relative carrier concentration [Formula: see text], short-range (point defect) and long-range (Coulomb charge) impurity concentration, relative dielectric constant [Formula: see text] and interlayer distance [Formula: see text]. The maximum conductivity has been achieved in the absence of short-range impurity. The short-range impurity plays a non-trivial meaningful role in high dielectric and high carrier concentration regimes and limits the conductivity. For higher [Formula: see text] and higher [Formula: see text] values, the nature of the conductivity curve turns out to be of a sublinear nature, while in all other cases, it shows a superlinear trend. Suitable selection of relative dielectric constants, relative carrier concentration and interlayer distance helps to improve the conductivity of the BLGDL system. If the dielectric constant of the spacer is higher than the substrate material, the thickness of the spacer material helps remarkably in enhancing conductivity. Moreover, a comparison of BLGDL with our previously reported MLG–MLG double layer graphene system (DLGS) confirms that higher conductivity is achieved in the case of BLG–BLG system compared to DLGS.

Enhanced Heat Dissipation for Macroscopic Metals Achieved by a Single‐Layer Graphene

AbstractThe increasing demand for high‐performance devices on heat dissipation makes it approach the bottleneck even for metals with high thermal conductivities. The coating of only one layer of graphene, the heat dissipation performances of Cu, Ag, and Al can be further enhanced, e.g., with a maximum temperature reduction by ≈9% for a Cu foil is demonstrated. Molecular dynamics (MD) analysis of spectral phonon transmission reveals that low‐frequency phonons play a significant role in the thermal transport within the Cu/single‐layer graphene (SLG) system, and the high‐frequency phonons exhibit substantial mismatch. It suggests that the thermal anisotropy of graphene enables a rapid heat dispersion in the in‐plane direction and provides an effective thermal insulation in the out‐of‐plane direction. The thermal conductivity calculations demonstrate an enhanced participation of phonons in heat conduction by the graphene layer, indicating a novel heat conduction mechanism in the Cu/single‐layer graphene system. These findings highlight the positive impact of graphene on the heat conduction of metals, and they will hold crucial implications for the design and application of graphene‐based thermal devices is believed.

A consistent comparison of lattice thermal conductivities and phonon properties of single layer and bilayer graphene systems

Using nonequilibrium molecular dynamics (NEMD) based direct method and spectral energy density (SED) method, we calculate the size-dependent thermal conductivities (TCs) of single layer graphene (SLG), AB-stacked bilayer graphene (AB-BLG), and 21.78° twisted BLG (tBLG) in a robust and consistent manner. Our NEMD analysis reveals discrepancies in high TC reported for graphene systems in some of the earlier studies. Similarly, some of the previous SED based studies were done with unreliable SED Φ′ approach. We conduct size-dependent analysis of the graphene systems by the SED method for the first time and report that bulk TCs for SLG and tBLG systems are nearly the same when calculated by either the direct or the SED method. Contrary to studies that claim that phonon group velocities of AB-BLG and tBLG samples do not change, we find that although average group velocities in SLG and AB-BLG are almost the same, they are around 30% higher when compared to tBLG samples with different twist angles. On the other hand, average phonon lifetimes are almost similar for AB-BLG and 21.78° tBLG samples but around 43% lower than the average phonon lifetime of SLG. Together these trends suggest the reason behind the decreasing order of TCs across three systems. We also systematically study the basic phonon mode contributions to TCs and their properties and find that the high-symmetry modes contribute the most in all three systems.

Entanglement control in a laser driven single layer graphene system

Abstract In this letter, we have proposed a new model for quantum control of atom photon entanglement in a single layer graphene via von Neumann reduced entropy of entanglement. We consider the effect of terahertz laser field intensity on the degree of entanglement (DEM) in the resonance and off-resonance condition of the applied fields. We also investigate the spatially dependent of the DEM when two applied light becomes standing wave pattern in x and y directions. Our results show that in different parametric conditions, the population of the different states can be controlled and this leads to modifying the DEM of the system.

Coherent control of the incident light propagation in a defect one-dimensional photonic crystal

In this paper, we have discussed the transmission and reflection properties of the infrared laser pulse propagated through one-dimensional photonic crystal (1DPC) with single layer of graphene nanotrusses. The single layer graphene system interacts with a probe laser field and two coupling lights. We analytically solve the optical susceptibility of the single layer graphene system for adapting the dielectric function of the 1DPC. After that we discuss the transmission and reflection properties of the incident laser pulse on 1DPC by controlling the intensity for the coupling lights and relative phase between applied lights. We have also found that the absorption and population spectrums of the medium can be controlled by the relative phase between applied lights. Our results show that the gain without population inversion can be obtained for the simultaneous slow light propagation in the transmission and reflection pulses. Our results may have potential application in the future of the all-optical devices in quantum technologies.

Topological charge measurement in a four-level single layer graphene system

In this letter we have proposed a four-level graphene monolayer system for identifying the topological charge of Laguerre–Gaussian light. Here, we have shown that due to the four-wave mixing mechanism in the monolayer graphene system, a weak signal beam can be generated due to quantum coherence and interference effect. We have discussed the spatially dependent linear absorption spectrums of the weak probe and new generated signal beams via quantum mechanical density matrix formalism. We have found that by numbering the spot areas of the probe and signal beams, one can realize the topological charge of the Laguerre–Gaussian beam interacts by monolayer graphene system. Moreover, we have realized that for some topological charge the new generated signal beam can be amplified in the graphene system.

Influence of Long Range and Short Range Scattering Mechanisms on Electron Transport in Double Layer Graphene System

Theoretically investigated electronic transport properties of Double Layer Graphene System (DLGS) at absolute zero temperature using Boltzmann Transport equation are reported here with varying different parameters like relative carrier concentration (n(c)/n(CI)), relative dielectric constant (εr) and interlayer distance d. The contributions from two different scattering mechanisms namely, short range and long range scattering have been evaluated and found that the short range scatterers have influenced the scattering rate and the conductivity, long range scattering rate is more dominating in both high and low density regime. The conductivity curve is showing superlinear behavior in absence of short range impurities while the sublinear nature when some short range point defects are added. The role of screening effect between two layers and thus the dielectric constant on conductivity reaches to maximum very rapidly as the interlayer distance d is increased from 0.5 nm to 5 nm. The mismatch between dielectric constants of middle layer and substrate enhances the conductivity value.

Plasmonic tuning of near-field heat transfer between graphene monolayers

We discuss the non-radiative heat transfer in non-equilibrium double layer graphene system. We show that at the neutrality point the heat exchange is dominated by the inter-layer plasmon modes and derive analytic expressions for the heat current as a function of temperature and the interlayer separation. These results show that for a range of low temperatures the two graphene layers are much more efficient heat exchanger than conventional metals. The physical reason behind this phenomenon is the presence of inter-band excitations with a large energy, and a small momentum in graphene spectrum. Plasmonic mechanism of the heat transfer is sharply suppressed by electrostatic doping. This allows for tuning of the heat exchange by a small applied voltage between the two layers.

Effect of quasiparticle excitations and exchange-correlation in Coulomb drag in graphene

Coulomb drag in double layer graphene systems separated by an h-BN interlayer allows probing of the electron-electron interactions in the effective limit of zero layer separation. Although these interactions can be influenced by plasmons, phonons and exchange and correlation effects, these excitations have never been studied altogether, missing the effects of their coupling on the drag physics. Here we study theoretically the effects of these quasiparticles and their coupling, including also the effects of the electronic exchange and correlation, and demonstrate that the drag resistivity can attain a maximum value at room temperature and beyond, where hybridized plasmon-phonon modes contribute significantly. In particular, the hybridization of the plasmons with the hyperbolic phonons of h-BN, confined within the reststrahlen bands, enhance the drag resistivity. This study paves the way for the exploration of novel many-body physics phenomena in systems coupled through emerging 2D hyperbolic materials.

SPPs in a double layer graphene system with an anisotropic dielectric

We investigate theoretically surface plasmon polaritons (SPPs) dispersion relation for the configuration of an anisotropic dielectric medium sandwiched by two symmetric graphene layers and isotropic dielectrics. The dispersion splits into odd and even modes in the system. We show that the proper oriented optical axis of the dielectrics can enhance the confinement, propagation length and figure of merit (FOM) of two modes. And the proper orientation for propagation length of two modes and FOM for odd mode depends on the normalized separation of the graphene layers. The effect of the anisotropic dielectric between the two graphene layers is much greater than that outside the two graphene layers.

Curvature analysis of single layer graphene on the basis of extreme low-frequency Raman spectroscopy

Single layer graphene (SLG) sheets offer exciting optical and electronic properties, as well as excellent mechanical performance, which are desirable for countless potential applications in ultrathin optical, electronic, and mechanical devices. Typically, the mechanical properties of SLG are extrapolated from few layer graphene (FLG) systems in most existing experimental studies, despite the fact that the environmental mechanical response of SLG is quite different from FLG. Raman spectroscopy is one of the most versatile and nondestructive experimental techniques to probe graphene samples. Here, we provide direct experimental evidence for the vibrational behavior of SLG and its response to high pressure conditions (0–10 GPa) via Raman spectroscopy including the extreme low-frequency Raman region (5–250 cm–1). Artificial introduction of the curvature of the SLG sheets causes van Hove singularities within the range of Fermi energies (EF). The radius of curvature ρ can be predicted via a comparison of the shear mode and the breathing mode of SLG with the squash mode and the radial breathing mode of single wall carbon nanotubes (SWNTs). Furthermore, an additional polarization analysis further confirms similar low frequency modes of SLG and SWNTs under pressure. This direct investigation of SLG mechanical properties improves the quality of the available mechanical data, which is required for the design of new graphene-based nanocomposites and the development of electronic or mechatronic devices.

Kinked plasmon dispersion in borophene-borophene and borophene-graphene double layers

We investigate the collective plasmon modes in double layer of two dimensional materials where either one or both of the layers have tilted Dirac cone. Consistent with quite generic hydrodynamic treatment, similar to double layer graphene systems we find two branches of plasmons. The in-phase oscillations of the two layers disperses as $\sqrt q$, while the out-of-phase mode disperses as $q$. When even one of the layers hosts tilted Dirac cone spectrum, the plasmonic kink which is a salient feature of a monolayer of tilted Dirac cone is inherited by both of these branches. In double layers composed of graphene (nontilted) and borophene (tilted) where the two layers have two different Fermi velocities, the velocity scale of plasmonic modes is set by the greater of the two. The kink always takes place when each plasmon mode crosses an energy scale $\omega_{\rm kink}$. When the two layers have different chemical potentials, there will be two such scales, and each of in-phase and out-of-phase modes develop two kinks. Moreover we find that an additional linearly dispersing overdamped mode of monolayer tilted Dirac cone system survives in the double layer system.

Ballistic transport in graphene Y-junctions in transverse electric field

We investigate the prospects for current modulation in single layer graphene Y-junctions in the ballistic regime, under an external electric field. Overcoming the inability of inducing field effect in graphene nanoribbons by a stacked gate, the proposed in-plane electric field setup enables a controlled current transfer between the branches of the Y-junction. This behavior is further confirmed by changing the angular incidence of the electric field. The ballistic transmission functions are calculated for the three terminal system using the non-equilibrium Green’s function formalism, in the framework of density functional theory, under finite bias conditions. The edge currents dominating the transport in zigzag nanoribbons are strongly influenced by the induced dipole charge, facilitating the current modulation even for the metallic-like character of the Y-junctions. Spin polarization effects indicate the possibility of achieving spin filtering even in the absence of the external field provided the antiferromagnetic couplings between the edges are asymptotically set. Overall, our results indicate a robust behavior regarding the tunability of the charge current in the two outlet ports, showing the possibility of inducing field effect control in a single layer graphene system.

Quantum capacitance of double-layer graphene

We study the ground-state properties of a double layer graphene system with the Coulomb interlayer electron-electron interaction modeled within the random phase approximation. We first obtain an expression of the quantum capacitance of a two layer system. In addition, we calculate the many-body exchange-correlation energy and quantum capacitance of the hybrid double layer graphene system at zero-temperature. We show an enhancement of the majority density layer thermodynamic density-of-states owing to an increasing interlayer interaction between two layers near the Dirac point. The quantum capacitance near the neutrality point behaves like square root of the total density, $\alpha \sqrt{n}$, where the coefficient $\alpha$ decreases by increasing the charge density imbalance between two layers. Furthermore, we show that the quantum capacitance changes linearly by the gate voltage. Our results can be verified by current experiments.

Strong enhancement of third-harmonic generation in a double layer graphene system caused by electron-hole pairing

A manifestation of electron-hole pairing in nonlinear electromagnetic response of a double layer graphene system is studied. It is shown that the pairing causes the appearance of a number of peaks in the frequency dependence of the intensity of the third-harmonic generation (THG). The highest peak corresponds to , where ω is the incident wave frequency, and Δ is the order parameter of the electron-hole pairing. The absolute value of the THG intensity in the systems with electron-hole pairing is of several orders of magnitude greater than the THG intensity in the unpaired state. It is shown that huge enhancement of the THG intensity occurs both in the double monolayer and double bilayer graphene systems.

Polarized dependence of nonlinear susceptibility in a single layer graphene system in infrared region

In this study, the linear and nonlinear susceptibility of a single-layer graphene nanostructure driven by a weak probe light and an elliptical polarized coupling field is discussed theoretically. The Landau levels of graphene can be separated in infrared or terahertz regions under the strong magnetic field. Therefore, by using the density matrix formalism in quantum optic, the linear and nonlinear susceptibility of the medium can be derived. It is demonstrated that by adjusting the elliptical parameter, one can manipulate the linear and nonlinear absorption as well as Kerr nonlinearity of the medium. It is realized that the enhanced Kerr nonlinearity can be possible with zero linear absorption and nonlinear amplification at some values of elliptical parameter. Our results may be having potential applications in quantum information science based on Nano scales devices.

Electromagnetic properties of a double-layer graphene system with electron-hole pairing

We study electromagnetic properties of a double layer graphene system in which electrons from one layer are coupled with holes from the other layer. The gauge invariant linear response functions are obtained. The frequency dependences of the transmission, reflection and absorption coefficients are computed. We predict a peak in the reflection and absorption at the frequency equals to the gap in the quasiparticle spectrum. It is shown that the electron-hole pairing results in an essential modification of the spectrum of surface TM plasmons. We find that the optical TM mode splits into a low frequency undamped branch and a high frequency damped branch. At zero temperature the lower branch disappears. It is established that the pairing does not influence the acoustic TM mode. It is also shown that the pairing opens the frequency window in the subgap range for the surface TE wave.

The optical conductivity in double and three layer graphene systems

Abstract We investigate the longitudinal optical conductivity in few-layer monolayer graphene systems, which is different from the bilayer or trilayer graphene structures analytically and numerically. Here, few isolated parallel two-dimensional (2D) monolayer graphene are separated by a distance d with no interlayer tunneling, where the finite width thickness has to be taken into account. The carrier׳s energy structure and states for each layer are unaffected by the others. The carrier density in each layer is assumed to be tuned by the corresponding gate voltage. The optical conductivity depends on the electron density, the number of layer, and the broadening width at low temperature. However, analytical and numerical results show that the optical conductivity has little dependence on the distance between the adjacent layers. It is found that two intra- and inter-band transition channels for optical transition via absorption scattering in each layer can be observed. When the optical energy is larger than two times the kinetic energy at the Fermi energy, the optical conductivity is proportional to the layer numbers of monolayer graphene that can determine the number of the monolayer graphene layer. In sharp contrast to the bilayer or trilayer graphene systems, several turning points can be observed. Increasing the broadening width, the turning area becomes gradual. The main difference on the optical conductivity for double layer graphene and bilayer graphene is in the intermediate energy region where the threshold structure is observed.

Stacking dependence of carrier interactions in multilayer graphene systems

We identify qualitative trends in the stacking sequence dependence of carrier-carrier interaction phenomena in multilayer graphene. Our theory is based on a new approach which explicitly exhibits the important role in interaction phenomena of the momentum-direction dependent intersite phases determined by the stacking sequence. Using this method, we calculate and compare the self-energies, density--density response functions, collective modes, and ground-state energies of several different few layer graphene systems. The influence of electron--electron interactions on important electronic properties can be understood in terms of competition between intraband exchange, interband exchange and correlation contributions that vary systematically with stacking arrangement.

Direct Observation of a Gate Tunable Band Gap in Electrical Transport in ABC-Trilayer Graphene.

Few layer graphene systems such as Bernal stacked bilayer and rhombohedral (ABC-) stacked trilayer offer the unique possibility to open an electric field tunable energy gap. To date, this energy gap has been experimentally confirmed in optical spectroscopy. Here we report the first direct observation of the electric field tunable energy gap in electronic transport experiments on doubly gated suspended ABC-trilayer graphene. From a systematic study of the nonlinearities in current versus voltage characteristics and the temperature dependence of the conductivity, we demonstrate that thermally activated transport over the energy-gap dominates the electrical response of these transistors. The estimated values for energy gap from the temperature dependence and from the current voltage characteristics follow the theoretically expected electric field dependence with critical exponent 3/2. These experiments indicate that high quality few-layer graphene are suitable candidates for exploring novel tunable terahertz light sources and detectors.