Armchair Graphene Ribbons Research Articles

Transverse current response in armchair graphene ribbons

We demonstrate that the direction of transverse current in graphene nanoribbons under a magnetic field can be tuned with a gate voltage. It is shown that for armchair ribbons there exist extra energy regions where the direction of the Hall current can be switched between positive and negative values. The directional change of the Hall current coincides with the special points where the two lowest energy bands in the spectrum become degenerate (band crossing points). The number of such degenerate points depends on the width of the ribbons. The dependence of the sign reversal on the gate voltage provides a mechanism for tuning transverse response in graphene based devices.

Controlling the polarization fluctuation of excitons in armchair graphene ribbons by a laser field

We investigate the effect of the laser field on the polarization fluctuation of excitons in graphene ribbons. In order to calculate the fluctuation, we develop a bosonization method to deal with the electron-hole system. Our results show that the polarization fluctuation may be controlled by adjusting the strength and frequency of the laser field. The insulating armchair graphene ribbons may be divided into two types according to the width-dependences of the excitonic polarization fluctuation.

Andreev reflection and momentum filtering in quantum-wire/superconductive-graphene/quantum-wire junction

Transport property of superconductive armchair graphene ribbon (AGR) connected to quantum-wire (QW) contacts is investigated via Landauer formalism combined with transfer matrix method. The scattering at the AGR/QW interface induces an obvious asymmetry in conductance as gate voltage varies. The transmission peak is located at momentum ky=2π33a with a=0.142 nm. Andreev reflection (AR) enhances electronic transmission in the presence of hole reflection process. At lowest carrier density, the conductance of AGR in superconductive state becomes constant while the counterpart of semiconductive AGR in normal state decays exponentially with the length. The conductance increases with pair potential at low carrier density. The interplay between superconductivity and the scattering at the AGR/QW interface guides future application of superconductive graphene ribbon.

Electronic structure and transport of a carbon chain between graphene nanoribbonleads

The electronic structure and transport property of a carbon chain between two graphenenanoribbon leads are studied using an ab initio tight-binding (TB) model and Landauer’sformalism combined with a non-equilibrium Green’s function. The TB Hamiltonian andoverlap matrices are extracted from first-principles density functional calculations throughthe quasi-atomic minimal basis orbital scheme. The accuracy of the TB model isdemonstrated by comparing the electronic structure from the TB model with that fromfirst-principles density functional theory. The results of electronic transport on a carbonatomic chain connected to armchair and zigzag graphene ribbon leads, such asdifferent transport characters near the Fermi level and at most one quantizedconductance, reveal the effect of the electronic structure of the leads and the scatteringfrom the atomic chain. In addition, bond length alternation and an interestingtransmission resonance are observed in the atomic chain connected to zigzaggraphene ribbon leads. Our approach provides a promising route to quantitativeinvestigation of both the electronic structure and transport property of large systems.

Energy gap of strained graphene with tight-binding model

The tight-binding model is used to inquire the uniaxial strain effect on the electronic structure of 2D graphene sheet and quasi-1D armchair graphene nanoribbons. The dependence of Dirac points and gap on the uniaxial strain is investigated. The band edges deviate from the corners of the first Brillouin Zone (BZ) gradually with the strain applied. The gap can be opened only when graphene sheet is highly strained. The threshold of the gap induced by strain along armchair directions distinguishes from that by strain along zigzag direction. A controlled strain can be applied to achieve the semiconductor-metal transition in armchair graphene ribbons.

Low-energy electron transmission in a partially unzipped zigzag nanotube

Based on the nearest-neighbor tight-binding approximation, we present exact analytical expressions for electron transmission in nanotube/ribbon junctions, generated by incomplete unzipping of zigzag nanotubes. By assuming one-dimer-line difference in the widths of the leads, it is demonstrated that such a contact exhibits zero backscattering of low-energy electrons entering from the graphene side of the junction. We also show that a zigzag nanotube section sandwiched between two armchair graphene ribbons is completely transparent for incident low-energy electrons. Possible application of the results to nanosensor engineering is also included.

Ferromagnetism without flat bands in thin armchair nanoribbons

Describing by a Hubbard type of model a thin armchair graphene ribbon in the armchair hexagon chain limit, one shows in exact terms, that even if the system does not have flat bands at all, at low concentration a mesoscopic sample can have ferromagnetic ground state, being metallic in the same time. The mechanism is connected to a common effect of correlations and confinement.

Nanoengineering Structures on Graphene with Adsorbed Hydrogen “Lines”

It is shown that the lines of adsorbed hydrogen pair atoms divide a graphene sheet into electronically independent strips and form an electron waveguide or 2H-line graphene-based superlattice (2HG-SL). We investigated the electronic properties of such structures in detail. The electronic spectra of a “zigzag” (n,0)-2HG-SL are similar to those of armchair graphene ribbons and have similar oscillation of the band gap with the width between adjacent 2H-lines (number n). The induced strain with the direction perpendicular to the hydrogen pair “lines” significantly changes the electronic properties of the investigated structures. For example, in the case of the 2HG-SL (3n,0) (n > 2) we observed the semiconductor−metal transition. Superlattices of the (n,n) type with a “staircase” of adsorbed pairs of H atoms are semiconductors with nearly linear decreasing of the band gap with increasing n. We found that the configuration with the opposite spin (antiferromagnetic) orientation between ferromagnetically ordered edge states of the (n,n) 2HG-SL is energy favorable. We also suggested an experimental way of fabricating these superlattices. Finally, we discussed properties of possible hydrogen lined waveguide junctions.

Conduction suppression in graphene nanoribbons with a vacancy

The effect of a vacancy on the coherent transport properties of graphene nanoribbons is investigated numerically. For zigzag and metallic armchair graphene ribbons, a single vacancy within the interior can suppress the conductance at several Fermi energies in the first subband. This conductance suppression is related to the localization induced by the vacancy. A perpendicular magnetic field of sufficient strength can destroy the localization and reduce this conductance suppression.

Interband transmission in armchair graphene ribbons with a step‐like profile of potential energy: Relevance to Klein's tunneling

AbstractThree principal results concerning graphene‐based wires and their ambipolar behavior are presented. First, it is the exact expression of the transmission coefficient for armchair graphene wires described by the tight‐binding Hamiltonian with the step‐like change U of site energies. Second, the exact relation between the energy of incident electrons or holes and potential U at which there is no backscattering for the given mode of the transverse motion. Third, the range of relevance of Klein's formula describing the motion of relativistic particles in the same potential profile is established. Analysis of newly derived results shows that physics of interband transitions at constant energy in graphene wires is richer than it was believed.

Properties of Graphene Based Parametric Pump

The adiabatic parametric electron pump of the infinite zigzag graphene ribbons and the infinite armchair graphene ribbons is investigated by the tight binding method. The pumping signals are added by two gates around the ribbons. It is shown that the dc current can be pumped out by cyclically varying the two gate voltages and the pumped current strongly depends on the driving frequency, the pumping amplitude and the phase difference of the gate voltages. The pumped current is mediated by the graphene energy levels and its peaks occur around the energies where transmission coefficients and density of states are large. The pump current may give one peak or two opposite peaks corresponding to each transmission peak or transmission pair peaks. The height and width of the current peaks increase with the amplitude of the pumping driving voltages. The pumped current is antisymmetric about the phase difference φ = π and for small pumping amplitude the pumped current is a sinusoidal function of the phase difference. Some graphene ribbons, although with different widths, have very similar contours of the transmission coefficients and give the same pumped current figures.

Quantum transport in armchair graphene ribbons: analytical tight-binding solutions for propagation through step-like and barrier-like potentials

Based on the nearest-neighbor tight-binding approximation, we present exact analytical expressions for transmission coefficients through piecewise constant step-like and barrier-like electrostatic potentials. In the case of single mode propagation through semiconducting ribbon families our analytical solutions predict a new kind of resonances. Its features substantially change the behavior of the transmission coefficients in the range of moderate potentials, which become family-dependent. For semimetal ribbons our approach predicts no unit propagation. The non-zero backscattering is derived to be proportional as the square of the potential amplitude applied.

Quantum conductance of achiral graphene ribbons and carbon tubes

Explicit expressions of the band spectrum near the neutrality point are derived for armchair and zigzag graphene ribbons and carbon tubes. Several spectral features, which were previously observed only in numerical calculations, are given an adequate analytic description in terms of elementary functions. The obtained dispersion relations are used for a comparison of conductance ladders of graphene-based wires; these relations are also beneficial for many other applications.

Magnetic Properties of Mn Doped Armchair Graphene Nanoribbon

We investigate the electronic structure and magnetism of an armchair graphene ribbon doped with Mn by first-principles density functional calculations. Mn atoms are doped at the edge of the ribbon, with different densities. The local magnetic moment due to Mn atom is 5 μB/Mn for dilute doping and 4.5 μB/Mn for heavy doping. The structures of Mn doped at both edge of graphene ribbon were optimized in ferromagnetic and antiferromagnetic states to find the stable magnetic states of the two opposite ferromagnetic edges. Our calculations show that the ground state is ferromagnetic for heavy doping and does not have a preferable configuration for dilute doping because of the small energy difference. These structures have application in nanoelectronics and spintronics.

Electron-Electron and Spin-Orbit Interactions in Armchair Graphene Ribbons

The effects of intrinsic spin-orbit and Coulomb interactions on low-energy properties of finite width graphene armchair ribbons are studied by means of a Dirac Hamiltonian. It is shown that metallic states subsist in the presence of intrinsic spin-orbit interactions as spin-filtered edge states, in contrast with the insulating behavior predicted for graphene planes. A charge-gap opens due to Coulomb interactions in neutral ribbons, that vanishes as Delta approximately 1/W, with a gapless spin sector. Weak intrinsic spin-orbit interactions do not change the insulating behavior. Explicit expressions for the width-dependent gap and various correlation functions are presented.

Magnetic and quantum confinement effects on electronic and optical properties of grapheneribbons

Through the tight-binding calculation, we demonstrate that magnetic and quantumconfinements have a great influence on the low-energy band structures of one-dimensional(1D) armchair graphene ribbons. The magnetic field first changes 1D parabolic bands intothe Hall-edge states which originate in the Landau wavefunctions deformed by one or tworibbon edges. The quantum confinement dominates the characteristics of theHall-edge states only when the Landau wavefunctions touch two ribbon edges.Then, some of the Hall-edge states evolve as the Landau states when the fieldstrength grows. The partial flat bands (Landau levels), related to the Landaustates, appear. The magnetic field dramatically modifies the energy dispersionsand it changes the size of the bandgap, shifts the band-edge states, destroys thedegeneracy of the energy bands, induces the semiconductor–metal transition andgenerates the partial flat bands. The above-mentioned magneto-electronic propertiesare completely reflected in the low-frequency absorption spectra—the shift ofpeak position, the change of peak symmetry, the alteration of peak height, thegeneration of new peaks and the change of absorption edges. As a result, thereare magnetic-field-dependent absorption frequencies. The findings show that themagnetic field could be used to modulate the electronic properties and the absorptionspectra.