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ABSTRACT: We study the energy band structure of magnetic graphene superlattices with delta-function magnetic barriers and zero average magnetic field. The dispersion relation obtained using the T-matrix approach shows the emergence of an infinite number of Dirac-like points at finite energies, while the original Dirac point is still located at the same place as that for pristine graphene. The carrier group velocity at the original Dirac point is isotropically renormalized, but at finite energy Dirac points it is generally anisotropic. An asymmetry in the width between the wells and the barriers of the periodic potential induces a shift of the original Dirac point in the zero-energy plane, keeping the velocity renormalization isotropic.
Journal of Physics Condensed Matter 07/2012; 24(34):345502. · 2.55 Impact Factor
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ABSTRACT: Using the nearest-neighbor tight-binding approach we study the electronic band structures of graphene nanoribbons with self-passivating edge reconstructions. For zigzag ribbons the edge reconstruction moves both the Fermi energy and the flat band down by several hundred meV, and the flat band is always found to be below the Fermi energy. The states featured by the flat band are shown to be mainly localized at the atoms belonging to several lattice lines closest to the edges. For armchair ribbons the edge reconstruction strongly modifies the band structure in the region close to the Fermi energy, leading to the appearance of a band gap even for ribbons which were predicted to be metallic in the model of standard armchair edges. The gap widths are, however, strongly different in magnitude and behave in different ways regarding the ribbon width.
Journal of Physics Condensed Matter 07/2011; 23(29):295503. · 2.55 Impact Factor
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ABSTRACT: Using the T-matrix approach, we study the effect of a Kronig-Penney periodic potential on the electronic states and the transport properties of graphene. The energy band structure and the group velocity of charge carriers are calculated and discussed in detail for potentials with varying amplitudes and barrier-to-well width ratios. The periodic potential is shown to cause a resonant structure and to enhance the magnitude of the conductivity.
Journal of Physics Condensed Matter 10/2010; 22(42):425501. · 2.55 Impact Factor
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ABSTRACT: Using the nonequilibrium Green’s function method, the electronic transport in a gate-induced barrier bilayer graphene structure is investigated. Strong resonant effects are shown to result in high amplitude oscillation of conductance as a function of Fermi energy and barrier height. Beyond a small effect of negative differential conductance (with peak to valley ratio less than 2), strong oscillations of transconductance are achieved. The amplitude of such oscillations between positive and negative values may exceed 5 mS /μ m . This effect might be helpful for further development of graphene-based nanoelectronics.
Applied Physics Letters 01/2010; · 3.84 Impact Factor
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ABSTRACT: Using the nonequilibrium Green’s functions formalism in a tight binding model, the spin-dependent transport in armchair graphene nanoribbons controlled by a ferromagnetic gate is investigated. Beyond the oscillatory behavior of conductance and spin polarization with respect to the barrier height, which can be tuned by the gate voltage, we especially analyze the effects of width-dependent band gap and of the nature of contacts. The oscillation of spin polarization in graphene nanoribbons with a large band gap is strong in comparison with that in infinite graphene sheets. Very high spin polarization (close to 100%) is observed in normal-conductor/graphene/normal-conductor junctions. Moreover, we find that the difference in electronic structure between normal conductor and graphene generates confined states which have a strong influence on the transport properties of the device. This study suggests that the device should be carefully designed to obtain a high controllability of spin-polarized current.
Journal of Applied Physics 10/2009; · 2.17 Impact Factor
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ABSTRACT: The standard T-matrix method can be effectively used for studying the dynamics of Dirac electrons under one-dimensional potentials in graphene. The transmission probability expressed in terms of T-matrices and the corresponding ballistic current are derived for any smooth one-dimensional potential, taking into account the chirality of Dirac massless carriers. Numerical calculations are illustrated for the potential approximately describing graphene n-p junctions.
Journal of Physics Condensed Matter 01/2009; 21(4):045305. · 2.55 Impact Factor
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ABSTRACT: We analyze the spin-dependent transport in single ferromagnetic gate structures based on armchair graphene nanoribbon (GNR) using the non-equilibrium Green's function method in a tight binding model. It is shown that the spin polarized current oscillates as a function of the gate-induced barrier height. For perfect GNRs, the larger the energy band gap, the stronger the oscillation of the spin polarization. However, though the edge roughness of the ribbons tends to enlarge the band gap, it also strongly reduces the conductance which finally degrades the spin polarized current.
J. Phys.: Conf. Ser. 01/2009; 19332.
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ABSTRACT: Using the nonequilibrium Green’s function formalism, the authors investigate the effect of the electron-phonon interaction on the current and shot noise in one dimensional resonant tunneling structures. Besides the well-known current behavior, they particularly show that the shot noise may be enhanced over the Poissonian value due to the phonon-assisted tunneling effect. The observed super-Poissonian noise is then interpreted as a result of the competition between the coherent and sequential current components.
Applied Physics Letters 08/2007; · 3.84 Impact Factor
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ABSTRACT: The non-equilibrium Green function (NEGF) technique is used to solve the quantum transport equation in resonant tunneling
diodes (RTDs). The charge interaction is treated self-consistently to include rigorously the space-charges effects. Reasonable
results for the potential profile, the transmission probability, and the current-voltage characteristics have been obtained.
The effect of temperature on the current-voltage (I-V) characteristics is investigated. Particularly, the current noise spectral
density has been extracted following both coherent and sequential tunneling approaches. Our conclusion of the dominance of
the coherent and sequential tunneling according to the transport regime is consistent with recent theoretical analyses and
experimental data.
Journal of Computational Electronics 01/2007; 6(1):125-128. · 1.21 Impact Factor
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ABSTRACT: The fully self-consistent nonequilibrium Green’s function approach to quantum transport is developed to investigate one-dimensional nanoscale devices. Numerical calculations performed for resonant tunneling diodes of different designs and at different temperatures show reasonable results for the potential and electron density profiles, as well as for the transmission coefficient and the current-voltage characteristics. The resonant behavior is discussed in detail with respect to the quantum-well width, the barrier thickness, and the temperature. It is also shown that the current noise spectral density can be straightforwardly calculated for both the coherent and the sequential tunneling models. In qualitative agreement with experiments, the obtained results highlight the role of charge interaction, which causes a fluctuation of the density of states in the well and therefore a noise enhancement in the negative differential conductance region.
Journal of Applied Physics 11/2006; 100(9):093705-093705-7. · 2.17 Impact Factor
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ABSTRACT: The shot noise of the current through a single electron transistor (SET), coupled capacitively with an electronic box, is calculated, using the master equation approach. We show that the noise may be sub-Poissonian or strongly super-Poissonian, depending mainly on the box parameters and the gate. The study also supports the idea that not negative differential conductance, but charge accumulation in the quantum dot, responds for the super-Poissonian noise observed. Comment: 4 Pages, 3 Figures
12/2005;
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ABSTRACT: The shot noise of current through a metallic double quantum dot structure exhibiting negative differential conductance is studied. We can exactly solve the master equation and derive an analytical expression of the spectral density of current fluctuations as a function of frequency in the first Coulomb staircase region. For a large range of bias voltage the noise is calculated by Monte Carlo simulation. We show that the noise is always sub-Poissonian though it is considerably enhanced in the negative differential conductance regime.
Applied Physics Letters 10/2005; · 3.84 Impact Factor
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ABSTRACT: We show that negative differential conductance (NDC) can be observed in metallic quantum dot structures. For a simple model at zero temperature we have derived an analytical expression for the current–voltage characteristics and a condition for observing NDC. For devices with gates at finite temperatures, using the Monte Carlo method we have suggested diagrams describing a correlation between the gate capacitance or temperature and the inter-dot coupling in producing (or removing) NDC.
Journal of Physics Condensed Matter 02/2005; 17(7):1157. · 2.55 Impact Factor
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ABSTRACT: We systematically analyze the stability diagrams and simulate the finite temperature current-voltage characteristics for metallic double-dot devices with cross couplings between dots and gates. The Coulomb blockade is described with respect to each device parameter. The negative differential conductance observed is essentially suppressed by increasing the temperature and/or introducing the offset charge and is very sensitive to the device parameters.
Journal of Applied Physics 09/2004; 96(6):3302-3306. · 2.17 Impact Factor
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ABSTRACT: The phonon-assisted resonant tunneling is studied for the double barrier structures in a longitudinal magnetic field. Using the scattering matrix approach with an appropriate one-particle Green's function we are able to calculate the current and the zero frequency shot noise power spectrum in a large range of the magnetic field and to any order of the electron–phonon interaction. Obtained results describe well the relevant experimental data and provide new suggestions for further examinations.
Physics Letters A. 372(29):4947-4952.
