-
[show abstract]
[hide abstract]
ABSTRACT: The RKKY interaction between two magnetic impurities absorbed on the surface layer of half-filled AB-stacked multilayer graphene (ABSMLG) is theoretically studied based on the lattice Green's function technique. In comparison with the case of monolayer graphene, the RKKY interaction in such multilayer graphene presents distinct properties in some aspects. Firstly, from the numerical results, we find that the thickness of the ABSMLG influences the RKKY interaction in a complicated manner, depending on the odd/even parity of the number of layers and the sublattice attribution of the positions of the two magnetic impurities. Then, we derive the asymptotic expressions of the RKKY interactions in ABSMLG in the long-distance limit. For even-layered ABSMLG, we find that the RKKY interactions of the 1A-1A, 1B-1A and 1B-1B couplings fall off as 1/R(2), 1/R(4) and 1/R(6) (1A and 1B stand for, respectively, the sublattice points in the surface layer, which are positioned directly on the plaquette and on a lattice point of the layer underneath). On the other hand, in odd-layered ABSMLG, the decays of these interactions follow the 1/R(2), 1/R(3) and 1/R(3) power laws respectively. In addition, we also find that these analytical expressions are quantitatively valid to describe the RKKY interaction in ABSMLG when the distance between the two magnetic impurities is larger than the lattice constant of graphene by one order of magnitude.
Journal of Physics Condensed Matter 04/2012; 24(20):206003. · 2.55 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: The eigenenergy and the conductivity of a graphene sheet subject to a one-dimensional cosinusoidal potential and in the presence of a magnetic field are calculated. Such a graphene superlattice presents three distinct magnetic miniband structures as the magnetic field increases. They are, respectively; the triply degenerate Landau level spectrum, the nondegenerate minibands with finite dispersion and the same Landau level spectrum with the pristine graphene. The ratio of the magnetic length to the period of the potential function is the characteristic quantity to determine the electronic structure of the superlattice. Corresponding to these distinct electronic structures, the diagonal conductivity presents very strong anisotropy in the weak and moderate magnetic field cases, while the predominant magnetotransport orientation changes from the transverse to the longitudinal direction of the superlattice. More interestingly, in the weak magnetic field case, the superlattice exhibits half-integer quantum Hall effect, but with a large jump between the Hall plateaux. Thus, it is different from the one of the pristine graphene.
Journal of Applied Physics 04/2011; · 2.17 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: The electronic transmission spectrum of a graphene nanojunction formed by interconnecting two armchair-edged graphene nanoribbons is obtained by the first principle calculation. We find that in such a simple structure the electronic transmission is remarkably spin-polarized when its size is not very small. By calculating the local density of states and electron occupation numbers on some typical atoms at the scattering region, we conclude that the origin of the spin-polarization is the antiresonance effect, generated by the edge state localized at the zigzag-edged shoulder of the nanojunction.
Applied Physics Letters 04/2010; · 3.84 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: Based on the Kubo formula, we have studied the electron transport properties of a gapped graphene in the presence of a strong magnetic field. By solving the Dirac equation, we find that the Landau level spectra in two valleys differ from each other in that the n = 0 level in the K valley is located at top of the valence band, whereas it is at the bottom of the conduction band in the K' valley. Thus, in an individual valley, the symmetry between conduction and valence bands is broken by the presence of a magnetic field. By using the self-consistent Born approximation to treat the long range potential scattering, we formulate the diagonal and the Hall conductivities in terms of the Green function. To perform the numerical calculation, we find that a large bandgap can suppress the quantum Hall effect, owing to the enhancement of the bandgap squeezing the spacing between the low-lying Landau levels. On the other hand, if the bandgap is not very large, the odd integer quantum Hall effect experimentally, observed in the gapless graphene, remains in the gapped one. However, such a result does not indicate the half integer quantum Hall effect in an individual valley of the gapped graphene. This is because the heights of the Hall plateaux in either valley can be continuously tuned by the variation of the bandgap. More interestingly, we find that the height of the diagonal conductivity peak corresponding to the n = 0 Landau level is independent of the bandgap if the scattering is not very strong. In the weak scattering limit, we demonstrate analytically that such a peak takes a universal value e(2)/(hpi), regardless of the bandgap.
Nanotechnology 03/2010; 21(14):145703. · 3.98 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: Electron transport through a parallel double-dot structure with four terminals is theoretically studied. Introduction of a local Rashba spin-orbit interaction results in a distinct spin polarization of the electrons being transported through the structure. More interestingly, we find that the intradot electron interaction can influence the spin-polarized electron transport spectrum in a nontrivial way. It can enhance or reverse the electron spin polarization, depending on the strengths of the electron interaction. The underlying quantum interference that gives rise to such results is clarified by the analysis of the phase relation among the electron transmission paths.
Journal of Applied Physics 03/2009; · 2.17 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: The linear conductance spectrum of a metallic graphene junction formed by interconnecting two gapless graphene nanoribbons is calculated. A strong conductance suppression appears in the vicinity of the Dirac point. We found that such a conductance suppression arises from the antiresonance effect associated with an edge state localized at the zigzag-edged shoulder of the junction. The conductance valley due to the antiresonance is rather robust in the presence of the impurity and vacancy scattering. Also the center of the conductance valley can be readily tuned by an electric field exerted on the wider nanoribbon.
Journal of Applied Physics 02/2009; · 2.17 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: By applying a local Rashba spin-orbit interaction on an individual quantum dot of a four-terminal four-quantum-dot ring and introducing a finite bias between the longitudinal terminals, we theoretically investigate the charge and spin currents in the transverse terminals. It is found that when the quantum dot levels are separate from the chemical potentials of the transverse terminals, notable pure spin currents appear in the transverse terminals with the same amplitude and opposite polarization directions. Besides, the polarization directions of such pure spin currents can be inverted by altering structure parameters, i.e., the magnetic flux, the bias voltage, and the values of quantum dot levels with respect to the chemical potentials of the transverse terminals. Comment: 4 pages, 3 figures, Submitted to nanotechnology
01/2009;
-
[show abstract]
[hide abstract]
ABSTRACT: In the effective mass approximation, electronic property in graphene can be characterized by the relativistic Dirac equation. Within such a continuum model we investigate the electronic transport through graphene waveguides formed by connecting multiple segments of armchair-edged graphene nanoribbons of different widths. By using appropriate wavefunction connection conditions at the junction interfaces, we generalize the conventional transfer matrix approach to formulate the linear conductance of the graphene waveguide in terms of the structure parameters and the incident electron energy. In comparison with the tight-binding calculation, we find that the generalized transfer matrix method works well in calculating the conductance spectrum of a graphene waveguide even with a complicated structure and relatively large size. The calculated conductance spectrum indicates that the graphene waveguide exhibits a well-defined insulating band around the Dirac point, even though all the constituent ribbon segments are gapless. We attribute the occurrence of the insulating band to the antiresonance effect which is intimately associated with the edge states localized at the shoulder regions of the junctions. Furthermore, such an insulating band can be sensitively shifted by a gate voltage, which suggests a device application of the graphene waveguide as an electric nanoswitch.
12/2008;
-
[show abstract]
[hide abstract]
ABSTRACT: Electronic eigenstates of a square graphene quantum dot (GQD) terminated by both zigzag and armchair edges are derived in the theoretical framework of the Dirac equation. We find that the Dirac equation can determine the eigenenergy spectrum of a GQD with high accuracy even if its size is reduced to a few nanometers. More importantly, from the Dirac equation description we can readily work out the number and energy gap of the conjugate surface states, which are intimately associated with the magnetic properties of the GQD. By using the Hartree-Fock mean field approach, we study the size dependence of the magnetic ordering formation in this square GQD. We find that there exists a critical size of the width between the two zigzag edges to indicate the onset of the stable magnetic ordering. On the other hand, when such a width increases further, the magnetic ground state energy of a charge neutral GQD tends to a saturated value. These results coincide with the previous results obtained from the first-principles calculation. Then, based on the Dirac equation solution about the surface state, we establish a simple two-state model which can quantitatively explain the size dependence of the magnetic ordering in the square GQD.
Nanotechnology 10/2008; 19(43):435401. · 3.98 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: The contact conductance between graphene and two quantum wires which serve as the leads to connect graphene and electron reservoirs is theoretically studied. Our investigation indicates that the contact conductance depends sensitively on the graphene-lead coupling configuration. When each quantum wire couples solely to one carbon atom, the contact conductance vanishes at the Dirac point if the two carbon atoms coupling to the two leads belong to the same sublattice of graphene. We find that such a feature arises from the chirality of the Dirac electron in graphene. Such a chirality associated with conductance zero disappears when a quantum wire couples to multiple carbon atoms. The general result irrelevant to the coupling configuration is that the contact conductance decays rapidly with the increase of the distance between the two leads. In addition, in the weak graphene-lead coupling limit, when the distance between the two leads is much larger than the size of the graphene-lead contact areas and the incident electron energy is close to the Dirac point, the contact conductance is proportional to the square of the product of the two graphene-lead contact areas, and inversely proportional to the square of the distance between the two leads.
09/2008;
-
[show abstract]
[hide abstract]
ABSTRACT: Electron transport properties in a triple-quantum-dot ring with three terminals are theoretically studied. By introducing local Rashba spin-orbit interaction on an individual quantum dot, we calculate the charge and spin currents in one lead. We find that a pure spin current appears in the absence of a magnetic field. The polarization direction of the spin current can be inverted by altering the bias voltage. In addition, by tuning the magnetic field strength, the charge and spin currents reach their respective peaks alternately. Comment: 5 pages, 2 Figures
08/2008;
-
[show abstract]
[hide abstract]
ABSTRACT: The linear conductance spectrum of a metallic graphene junction formed by interconnecting two gapless graphene nanoribbons is calculated. A strong conductance suppression appears in the vicinity of the Dirac point. We found that such a conductance suppression arises from the antiresonance effect associated with the edge state localized at the zigzag-edged shoulder of the junction. The conductance valley due to the antiresonance is rather robust in the presence of the impurity and vacancy scattering. And the center of the conductance valley can be readily tuned by an electric field exerted on the wider nanoribbon. Comment: 14 pages, 4 figures
08/2008;
-
[show abstract]
[hide abstract]
ABSTRACT: The effect of electron–phonon interaction on the Fano lineshape in the linear conductance spectrum in the parallel double quantum dot structure is studied. The local and non-local phonon modes are considered and the corresponding electron transport formulae are derived on an equal footing. It is found that the inelastic conductance vanishes in the zero bias and zero temperature case. The local and non-local phonon modes influence the Fano lineshape distinctly. The non-local phonon can change the Fano lineshape effectively, while the local phonon mainly shifts the initial Fano lineshape. These properties are still distinct for the finite temperature case when the inelastic scattering process is more complex. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
physica status solidi (b) 04/2008; 245(6):1175 - 1180. · 1.32 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: The multiple level Anderson model and the exact many-body Hamiltionian are employed to describe the interacting electrons in the two-dimensional quantum dots, respectively. By diagonalizing the Hamiltonians of the two models, the numerical results of the conductance through a quantum dot with relatively large size in the weak coupling limit are obtained. The remarkable difference of the calculated conductances between the two models shows that the multiple level Anderson model is not applicable to such a quantum dot. By comparing the two-electron ground state energies calculated via the two models, the validity of the Anderson model to the quantum dots with different sizes is examined. It is found that only for very small quantum dot in which the confinement energy is far larger than electron interaction energy the Anderson model can give the accurate results.
Physica E Low-dimensional Systems and Nanostructures 5(3):137-141. · 1.53 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: Using the continuum phonon model and effective-mass approximation, we have calculated the scattering rate via electron–acoustic phonon interaction in a quantum wire. Both taking and not taking into account electron effective-mass mismatch (EEMM), the scattering rates decrease with increasing electron energy. This is attributed to rapid decrease of the state density of the quantum wire. The rate for the case with EEMM is lower than that without EEMM, since EEMM can make the barrier drop along the growth direction. The scattering rate increases with increasing Al concentration, which is attributed to lower well height and smaller wave vector along the growth direction. The relative deviation of the scattering rates between the without and with EEMM cases decreases with increasing Al concentration because of the lowering of the effective-barrier height caused by EEMM. With increasing well width along the growth direction the scattering rate decreases, which is ascribed to the decrease of wave vector α along the z-axis, that is, weakening of the bounding for electron in the quantum well.
Materials Science and Engineering: B. 75:130-133.
-
[show abstract]
[hide abstract]
ABSTRACT: We investigate the electronic transport through the metallic armchair T-shaped graphene nanoribbon. We found that the conductance spectrum exhibits a well-defined insulating band around the Dirac point. Our calculation also indicates that the defect can destroy the edge state localized at the zigzag-edged shoulder of the junction. When the concentration of the vacancies of each edge is about , a notable feature is the conductance insulating band can be completely eliminated. And such a conclusion is independent of the size of the shoulder.
Physica B Condensed Matter 405(16):3316-3319. · 1.06 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: Optical conductivity of a graphene sheet subject to a one-dimensional cosinusoidal potential is studied. We find that the optical conductivity can be tuned by the strength of the superlattice potential. When this strength is within a certain range the terahertz conductivity can be enhanced by two orders of magnitude.
Physics Letters A.
-
[show abstract]
[hide abstract]
ABSTRACT: The I–V spectrum of electronic transport through a quantum dot chain is calculated by means of the nonequilibrium Green function technique. In such a system, two arbitrary quantum dots are connected with two electron reservoirs through leads. When the dot-lead coupling is very weak, a series of discrete resonant peaks in electron transmission function cause staircase-like I–V characteristic. On the contrary, in the relatively strong dot-lead coupling regime, stairs in the I–V spectrum due to resonance vanish. However, when there are some dangling quantum dots in the chain outside two leads, the antiresonance which corresponds to the zero points of electron transmission function brings about novel staircase characteristic in the I–V spectrum. Moreover, two features in the I–V spectrum arising from the antiresonance are pointed out, which are significant for possible device applications. One is the multiple negative differential conductance regions, and another is regarding to create a highly spin-polarized current through the quantum dot chain by the interplay of the resonance and antiresonance. Finally, we focus on the role that the many-body effect plays on the antiresonance. Our result is that the antiresonance remains when the electron interaction is considered to the second order approximation.
Physics Letters A.
-
[show abstract]
[hide abstract]
ABSTRACT: Electron transport properties in a parallel double-quantum-dot structure with three terminals are theoretically studied. By introducing a local Rashba spin–orbit coupling, we find that an incident electron from one terminal can select a specific terminal to depart from the quantum dots according to its spin state. As a result, spin polarization and spin separation can be simultaneously realized in this structure. And spin polarizations in different terminals can be inverted by tuning the structure parameters. The underlying quantum interference that gives rise to such a result is analyzed in the language of Feynman paths for the electron transmission.
Physica E: Low-dimensional Systems and Nanostructures.
