arXiv subjects

Godfrey Gumbs

Publications and source records attributed to Godfrey Gumbs.

142 records · Page 8Linked to original sources

arXiv2010

Model of plasmon excitations in a bundle and two-dimensional array of nanotubes

We calculate the plasma excitations in a bundle as well as a two-dimensional (2D) periodic array of aligned parallel multishell nanotubes on a substrate. The carbon nanotubes are oriented perpendicular to the substrate. The model we use for the system is an electron gas confined to the surface of an infinitely long cylinder embedded in a background dielectric medium. Electron tunneling between individual tubules is neglected. We include the Coulomb interaction between electrons on the same tubule and on different tubules for the same nanotube and neighboring nanotubes. We present a self-consistent field theory for the dispersion equation for intrasubband and intersubband plasmon excitations. For both the bundle and 2D array of aligned parallel nanotubes, the dispersion relation of the collective modes is determined by a three-dimensional wave vector with components in the direction of the nanotube axes and in the transverse directions. The dispersion equation is solved numerically for a singlewall nanotube 2D array as well as a bundle, and the plasmon excitation energies are obtained as a function of wave vector. The intertube Coulomb interaction couples plasmons with different angular momenta in individual nanotubes, lifting the degeneracy of the single-nanotube modes. This effect is analyzed numerically as a function of the separation between the tubules. We show that the translational symmetry of the lattice is maintained in the plasmon spectrum for the periodic array, and the plasmon energies have a periodic dependence on the transverse wave vector. For the bundle, the Coulomb interaction between nanotubes gives rise to optical plasmon excitations.

cond-mat.mes-hall↗

arXiv2010

Tunable Band Structure Effects on Ballistic Transport in Graphene Nanoribbons

Graphene nanoribbons (GNR) in mutually perpendicular electric and magnetic fields are shown to exhibit dramatic changes in their band structure and electron transport properties. A strong electric field across the ribbon induces multiple chiral Dirac points, closing the semiconducting gap in armchair GNR's. A perpendicular magnetic field induces partially formed Landau levels as well as dispersive surface-bound states. Each of the applied fields on its own preserves the even symmetry $E_{k} = E_{-k}$ of the subband dispersion. When applied together, they reverse the dispersion parity to be odd and gives $E_{e,k} = -E_{h,-k}$ and mix the electron and hole subbands within the energy range corresponding to the change in potential across the ribbon. This leads to oscillations of the ballistic conductance within this energy range.

cond-mat.mtrl-sci↗

arXiv2010

Enhanced response of current-driven coupled quantum wells

We have investigated the conditions necessary to achieve stronger Cherenkov-like instability of plasma waves leading to emission in the terahertz (THz) regime for semiconductor quantum wells (QWs). The surface response function is calculated for a bilayer two-dimensional electron gas (2DEG) system in the presence of a periodic spatial modulation of the equilibrium electron density. The 2DEG layers are coupled to surface plasmons arising from excitations of free carriers in the bulk region between the layers. A current is passed through one of the layers and is characterized by a drift velocity for the driven electric charge. By means of a surface response function formalism, the plasmon dispersion equation is obtained as a function of angular frequency, the in-plane wave vector and reciprocal lattice vector of the density modulation. The dispersion equation,is solved numerically in the complex frequency plane for real wave vector. It is ascertained that the imaginary part of the angular frequency is enhanced with decreasing period of modulation, and with increasing the doping density of the free carriers in the bulk medium for fixed period of the spatial modulation.

cond-mat.mes-hall↗

arXiv2010

Unimpeded tunneling in graphene nanoribbons

We studied the Klein paradox in zigzag (ZNR) and anti-zigzag (AZNR) graphene nanoribbons. Due to the fact that ZNR (the number of lattice sites across the nanoribbon (N is even) and AZNR (N is odd) configurations are indistinguishable when treated by the Dirac equation, we supplemented the model with a pseudo-parity operator whose eigenvalues correctly depend on the sublattice wavefunctions for the number of carbon atoms across the ribbon, in agreement with the tight-binding model. We have shown that the Klein tunneling in zigzag nanoribbons is related to conservation of the pseudo-parity rather than pseudo-spin in infinite graphene. The perfect transmission in the case of head-on incidence is replaced by perfect transmission at the center of the ribbon and the chirality is interpreted as the projection of the pseudo-parity on momentum at different corners of the Brillouin zone.

cond-mat.mtrl-sci↗

arXiv2010

Comparison of inelastic and quasi-elastic scattering effects on nonlinear electron transport in quantum wires

When impurity and phonon scattering coexist, the Boltzmann equation has been solved accurately for nonlinear electron transport in a quantum wire. Based on the calculated non-equilibrium distribution of electrons in momentum space, the scattering effects on both the non-differential (for a fixed dc field) and differential (for a fixed temperature) mobilities of electrons as functions of temperature and dc field were demonstrated. The non-differential mobility of electrons is switched from a linearly increasing function of temperature to a parabolic-like temperature dependence as the quantum wire is tuned from an impurity-dominated system to a phonon-dominated one [see T. Fang, {\em et al.}, \prb {\bf 78}, 205403 (2008)]. In addition, a maximum has been obtained in the dc-field dependence of the differential mobility of electrons. The low-field differential mobility is dominated by the impurity scattering, whereas the high-field differential mobility is limited by the phonon scattering [see M. Hauser, {\em et al.}, Semicond. Sci. Technol. {\bf 9}, 951 (1994)]. Once a quantum wire is dominated by quasi-elastic scattering, the peak of the momentum-space distribution function becomes sharpened and both tails of the equilibrium electron distribution centered at the Fermi edges are raised by the dc field after a redistribution of the electrons is fulfilled in a symmetric way in the low-field regime. If a quantum wire is dominated by inelastic scattering, on the other hand, the peak of the momentum-space distribution function is unchanged while both shoulders centered at the Fermi edges shift leftward correspondingly with increasing dc field through an asymmetric redistribution of the electrons even in low-field regime [see C. Wirner, {\em et al.}, \prl {\bf 70}, 2609 (1993)].

cond-mat.str-el↗

arXiv2010

Collective Properties of Excitons in Presence of a Two-Dimensional Electron Gas

We have studied the collective properties of two-dimensional (2D) excitons immersed within a quantum well which contains 2D excitons and a two-dimensional electron gas (2DEG). We have also analyzed the excitations for a system of 2D dipole excitons with spatially separated electrons and holes in a pair of quantum wells (CQWs) when one of the wells contains a 2DEG. Calculations of the superfluid density and the Kosterlitz-Thouless (K-T) phase transition temperature for the 2DEG-exciton system in a quantum well have shown that the K-T transition temperature increases with increasing exciton density and that it might be possible to have fast long range transport of excitons. The superfluid density and the K-T transition temperature for dipole excitons in CQWs in the presence of a 2DEG in one of the wells increases with increasing inter-well separation.

cond-mat.mes-hall↗

arXiv2009

Quantum oscillations in the high frequency magnetoacoustic response of a quasi-two-dimensional metal

In this work we present the results of theoretical analysis of magnetic quantum oscillations of the velocity and attenuation of high frequency ultrasound waves traveling in quasi-two-dimensional conductors. We chose a geometry where the wave vector of the longitudinal sound wave and the external magnetic field are directed along the axis of symmetry of the Fermi surface. Assuming a moderately weak Fermi surface corrugation we showed that the oscillating correction to the sound velocity may include a special term besides an ordinary contribution originating from quantum oscillations of the charge carriers density of states at the Fermi surface. This additional term is generated by a "phase stability" resonance occurring when the charge carriers velocity in the direction of the wave propagation equals the sound velocity. The two oscillating contributions to the sound velocity are shown to differ in phase and shape, and they may have the same order of magnitude. The appearance of the extra term may bring significant changes in magnetic quantum oscillations of the velocity of sound in Q2D conductors, especially at low temperatures.

cond-mat.mtrl-sci↗

arXiv2009

Quasiparticles for quantum dot array in graphene and the associated Magnetoplasmons

We calculate the low-frequency magnetoplasmon excitation spectrum for a square array of quantum dots on a two-dimensional (2D) graphene layer. The confining potential is linear in the distance from the center of the quantum dot. The electron eigenstates in a magnetic field and confining potential are mapped onto a 2D plane of electron-hole pairs in an effective magnetic field without any confinement. The tight-binding model for the array of quantum dots leads to a wavefunction with inter-dot mixing of the quantum numbers associated with an isolated quantum dot. For chosen confinement, magnetic field, wave vector and frequency, we plot the dispersion equation as a function of the period $d$ of the lattice. We obtain those values of $d$ which yield collective plasma excitations. For the allowed transitions between the valence and conduction bands in our calculations, we obtain plasmons when $d \lesssim 100 Å$.

cond-mat.mes-hall↗

arXiv2008

Enhanced current quantization in high frequency electron pumps in a perpendicular magnetic field

We present experimental results of high frequency quantized charge pumping through a quantum dot formed by the electric field arising from applied voltages in a GaAs/AlGaAs system in the presence of a perpendicular magnetic field B. Clear changes are observed in the quantized current plateaus as a function of applied magnetic field. We report on the robustness in the length of the quantized plateaus and improvements in the quantization as a result of the applied B field.

cond-mat.mes-hall↗

arXiv2008

Magnetoplasmons in layered graphene structures

We calculate the dispersion equations for magnetoplasmons in a single layer, a pair of parallel layers, a graphite bilayer and a superlattice of graphene layers in a perpendicular magnetic field. We demonstrate the feasibility of a drift-induced instability of magnetoplasmons. The magnetoplasmon instability in a superlattice is enhanced compared to a single graphene layer. The energies of the unstable magnetoplasmons could be in the terahertz (THz) part of the electromagnetic spectrum. The enhanced instability makes superlattice graphene a potential source of THz radiation.

cond-mat.mes-hall↗

arXiv2008

On the low frequency electromagnetic waves in quasi-two-dimensional metals

Here, we theoretically analyze spectra of weakly attenuated electromagnetic waves which may appear in a Fermi-liquid of charge carriers in quasi-two-dimensional (Q2D) layered conductors when an external magnetic field is applied perpendicularly to the conducting layers. We study transverse modes propagating along the magnetic field. The frequencies of the modes are assumed to be lower than the cyclotron frequency of the charge carriers. It is shown that Fermi-liquid interaction of the charge carriers in Q2D conductors gives rise to a mode which cannot appear in a gase of charged quasiparticles, as well as it happens in conventional metals. Also, we show that the Fermi surface plofile may cause significant changes in the waves spectra and we analyze these changes.

cond-mat.mtrl-sci↗

arXiv2007

Bose-Einstein condensation and Superfluidity of magnetoexcitons in Graphene

We propose experiments to observe Bose-Einstein condensation (BEC) and superfluidity of quasi-two-dimensional (2D) spatially indirect magnetoexcitons in bilayer graphene. The magnetic field $B$ is assumed strong. The energy spectrum of collective excitations, the sound spectrum as well as the effective magnetic mass of magnetoexcitons are presented in the strong magnetic field regime. The superfluid density $n_S$ and the temperature of the Kosterlitz-Thouless phase transition $T_c$ are shown to be increasing functions of the excitonic density $n$ but decreasing functions of $B$ and the interlayer separation $D$. Numerical results are presented from these calculations.

cond-mat.str-el↗

arXiv2005

Local Geometry of the Fermi Surface and the Cyclotron Resonance in Metals in a Normal Magnetic Field

In this paper we present a detailed theoretical analysis of the cyclotron resonance in metals in the magnetic field directed along a normal to the surface of a sample. We show that this resonance occurs due to local geometry of the Fermi surface of a metal. When the Fermi surface (FS) includes segments where its curvature turns zero or diverges, this could give rise to resonance features in the frequency/magnetic field dependence of the surface impedance or its derivative with respect to the field. Otherwise the resonance is scarcely detectable unlike the well-known cyclotron resonance in a parallel magnetic field. The proposed theory agrees with experimantal results concerning both convenient and organic metals.

cond-mat.mtrl-sci↗

arXiv2005

Long-Range Electron Transfer and Electronic Transport Through Macromolecules

A theory of electrical transport through molecular wires is used to estimate the electronic factor in the intramolecular electron transfer (ET) in porphyrin-nitrobenzene supermolecules, and to analyze its structure. The chosen molecules have complex donor and acceptor configurations, and relatively simple structure of the bridge, which enables us to concentrate our studies on the donor/acceptor coupling to the bridge. We present analytical and numerical results concerning the effect of donor/acceptor coupling to the bridge on the ET process in molecules with complex donor/acceptor subsystems. PACS 05.60.Gg, 36.20.-r

cond-mat.soft↗

arXiv2005

Theory of the dc Magnetotransport in Laterally Modulated Quantum Hall Systems Near Filling $ν={1/2}$

A quasiclassical theory for dc magnetotransport in a modulated quantum Hall system near filling factor $ν={1/2}$ is presented. A weak one-dimensional electrostatic potential acts on the two-dimensional electron gas. Closed form analytic expressions are obtained for the resistivity $ρ_\perp$ corresponding to a current at right angles to the direction of the modulation lines as well as a smaller component $ρ_{||}$ for a current along the direction of the modulation lines. It is shown that both resistivity components are affected by the presence of the modulation. Numerical results are presented for $ρ_\perp$ and $ρ_{||}$ and show reasonable agreement with the results of recent experiments.

cond-mat.mes-hall↗

arXiv2005

The Effect of a Magnetic Field on the Acoustoelectric current in a Narrow Channel

The effect of a perpendicular magnetic field on the quantized current induced by a surface acoustic wave in a quasi-1D channel is studied. The channel has been produced experimentally in a GaAs heterostructure by shallow etching techniques and by the application of a negative gate voltage to Schottky split gates. Commensurability oscillations of the quantized current in this constriction have been observed in the interval of current between quantized plateaus. The results can be understood in terms of a moving quantum dot with the electron in the dot tunneling into the adjacent two-dimensional region. The goal is to explain qualitatively the mechanism for the steplike nature of the acoustoelectric current as a function of gate voltage and the oscillations when a magnetic field is applied. A transfer Hamiltonian formalism is employed.

cond-mat.mes-hall↗