arXiv subjects

Godfrey Gumbs

Publications and source records attributed to Godfrey Gumbs.

At least 127 records · Page 7Linked to original sources

arXiv2012

Effects of localized trap-states and corrugation on charge transport in graphene nanoribbons

We investigate the role played by electron traps on adiabatic charge transport for graphene nanoribbons in the presence of an acoustically induced longitudinal surface acoustic wave (SAW) potential. Due to the weak longitudinal SAW-induced potential as well as the strong transverse confinement by a nanoribbon, minibandsof sliding tunnel-coupled quantum dots are formed so that by varying the chemical potential to pass through the minigaps, quantized adiabatic charge transport may be obtained. We analyze the way that the minigaps may be closed, thereby destroying the likelihood of current quantization in a nanoribbon. We present numerical calculations showing the effects due to electron traps which lead to localized-trap energy levels within the minigaps. Additionally, for comparison, we present results for the minibands of a corrugated nanoribbon in the absence of a SAW.

cond-mat.mes-hall↗

arXiv2012

Effects of nonlocal plasmons in gapped graphene micro-ribbon array and 2DEG on near-field electromagnetic response in the deep-subwavelength regime

A self-consistent theory involving Maxwell equations and a density-matrix linear-response theory is solved for an electromagnetically-coupled doped graphene micro-ribbon array and a quantum-well electron gas sitting at an interface between a half-space of air and another half-space of a doped semiconductor substrate which supports a surface-plasmon mode in our system. The coupling between a spatially-modulated total electromagnetic field and the electron dynamics in a Dirac-cone of a graphene ribbon, as well as the coupling of the far-field specular and near-field higher-order diffraction modes, are included in the derived electron optical-response function. Full analytical expressions are obtained with non-locality for the optical-response functions of a two-dimensional electron gas and a graphene layer with an induced bandgap, and are employed in our numerical calculations beyond the long-wavelength limit (Drude model). Both the near-field transmissivity and reflectivity spectra, as well as their dependence on different configurations of our system and on the array period, ribbon width, graphene chemical potential of quantum-well electron gas and bandgap in graphene, are studied. Moreover, the transmitted E-field intensity distribution is calculated to demonstrate its connection to the mixing of specular and diffraction modes of the total electromagnetic field. An externally-tunable electromagnetic coupling among the surface, conventional electron-gas and massless graphene intraband plasmon excitations is discovered and explained. Furthermore, a comparison is made between the dependence of the graphene-plasmon energy on the ribbon width and chemical potential in this paper and the recent experimental observation given by Ju, et al., [Nature Nanotechnology, 6, 630 (2011)] for a graphene micro-ribbon array in the terahertz-frequency range.

cond-mat.mtrl-sci↗

arXiv2012

Gap-modulated doping effects on indirect exchange interaction between magnetic impurities in graphene

A dilute distribution of magnetic impurities is assumed to be present in doped graphene. We calculate the interaction energy between two magnetic impurities which are coupled via the indirect-exchange or Ruderman-Kittel-Kasuva-Yosida (RKKY) interaction by the doped conduction electrons. The current model is a half-filled $AB$-lattice structure. Our calculations are based on the retarded lattice Green's function formalism in momentum-energy space which is employed in linear response theory to determine the magnetic susceptibility in coordinate space. Analytic results are obtained for gapped graphene when the magnetic impurities are placed on the $A$ and $B$ sublattice sites of the structure. This interaction, which is important in determining spin ordering, has been found to be significantly different for $AA$ and $BB$ exchange energies in doped graphene due to the existence of an energy gap, and is attributed to a consequence of the local fields not being equal on the $A$ and $B$ sublattices. For doped graphene, the oscillations of all three RKKY interactions from ferromagnetic to antiferromagnetic with increasing Fermi energy is significantly modified by the energy gap both in magnitude and phase. Additionally, the $AB$ exchange energy may be modified by the presence of a gap for undoped graphene but not for doped graphene due to the dominance of doped conduction electrons.

cond-mat.mes-hall↗

arXiv2012

Dipole-dipole interaction between a quantum dot and graphene nanodisk

We study theoretically the dipole-dipole interaction and energy transfer in a hybrid system consisting of a quantum dot and graphene nanodisk embedded in a nonlinear photonic crystal. In our model a probe laser field is applied to measure the energy transfer between the quantum dot and graphene nanodisk while a control field manipulates the energy transfer process. These fields create excitons in the quantum dot and surface plasmon polaritons in the graphene nanodisk which interact via the dipole-dipole interaction. Here the nonlinear photonic crystal acts as a tunable photonic reservoir for the quantum dot, and is used to control the energy transfer. We have found that the spectrum of power absorption in the quantum dot has two peaks due to the creation of two dressed excitons in the presence of the dipole-dipole interaction. The energy transfer rate spectrum of the graphene nanodisk also has two peaks due to the absorption of these two dressed excitons. Additionally, energy transfer between the quantum dot and the graphene nanodisk can be switched on and off by applying a pump laser to the photonic crystal or by adjusting the strength of the dipole-dipole interaction. We show that the intensity and frequencies of the peaks in the energy transfer rate spectra can be modified by changing the number of graphene monolayers in the nanodisk or the separation between the quantum dot and graphene. Our results agree with existing experiments on a qualitative basis. The principle of our system can be employed to fabricate nano-biosensors, optical nano-switches, and energy transfer devices.

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arXiv2012

Anisotropy of π-plasmon Dispersion Relation of AA-stacked Graphite

The dispersion relation of the high energy optical π-plasmons of simple hexagonal intrinsic graphite was calculated within the self-consistent-field approximation. The plasmon frequency ω_p is determined as functions of the transferred momentum $q_{\parallel}$ along the hexagonal plane in the Brillouin zone and its perpendicular component $q_z$. These plasmons are isotropic within the plane in the long wavelength limit. As the in-plane transferred momentum is increased, the plasmon frequency strongly depends on its magnitude and direction (ϕ). With increasing angle, the dispersion relation within the hexagonal plane is gradually changed from quadratic to nearly linear form. There are many significant differences for the π-plasmon dispersion relations between 2D graphene and 3D AA-stacked graphite. They include $q_\parallel$- and ϕ-dependence and π-plasmon bandwidth. This result reveals that interlayer interaction could enhance anisotropy of in-plane π-plasmons. For chosen $\textbf{q}_\parallel$, we also obtain the plasmon frequency as a function of $q_z$ and show that there is an upper bound on $q_z$ for plasmons to exist in graphite. Additionally, the group velocity for plasmon propagation along the perpendicular direction may be positive or negative depending on the choice of $\textbf{q}_\parallel$. Consequently, the forward and backward propagation of π-plasmons in AA-stacked graphite in which the energy flow is respectively parallel or antiparallel to the transferred momentum, can be realized. The backward flowing resonance is an intrinsic property of AA-stacked graphite, arising from the energy band structure and the interlayer coupling.

cond-mat.mtrl-sci↗

arXiv2012

Plasmons in single- and double-component helical liquids: Application to two-dimensional topological insulators

The plasmon excitations in proposed single- and double-component helical liquid (HL) models are investigated within the random-phase approximation, by calculating the density-density, spin-density and spin-spin waves. The effect due to broken time-reversal symmetry on intraband-plasmon dispersion relation in the single-component HL system is analyzed and compared to those of well-known cases, such as conventional quasi-one-dimensional electron gases and armchair graphene nanoribbons. The equivalence between the density-density wave in the single-component HL to the coupled spin-density and density-density waves in the double-component HL is shown here and explained, in addition to the difference between intraband and interband-plasmon excitations in these two systems. Since the two-component HL can physically be thought of as a Kramers pair in two-dimensional topological insulators, our proposed single-component HL model with broken time-reversal symmetry, which is an artificial construct, can be viewed as an "effective" model in this sense and its prediction may be verified in realistic systems in future experiments.

cond-mat.mtrl-sci↗

arXiv2012

Photon dressed electronic states in topological insulators: Tunneling and conductance

The surface bound electronic states of three-dimensional topological insulators, as well as the edge states in two-dimensional topological insulators, are investigated in the presence of a circularly polarized light. The strong coupling between electrons and photons is found t o give rise to an energy gap as well as a unique energy dispersion of the dressed states, different from both graphene and conventional two-dimensional electron gas (2DEG). The effects of electron-photon interaction, barrier height and width on the electron tunneling through a p-n junction and on the ballistic conductance in topological insulators are demonstrated by numerical calculations. A critical energy for an incident electron to tunnel perfectly through a barrier is predicted, where electrons behave as either massless Dirac-like or massive Schrodinger-like particles above or below this threshold value. Additionally, these effects are compared with those in zigzag graphene nanoribbons and a 2DEG. Both the similarities and the differences are demonstrated and explained.

cond-mat.mes-hall↗

arXiv2011

A half-step in quantized conductance for low-density electrons in a quantum wire

We investigated the effect due to perpendicular magnetic field on quantum wires where spin-orbit interaction (SOI) of electrons is not neglected. Based on the calculated energy dispersion, the nonlinear ballistic conductance ($G$) and electron-diffusion thermoelectric power ($S_d$) are calculated as functions of electron density, temperature and applied bias voltage. A low-temperature half-step feature in $G$, which was observed experimentally by Quay et al. [see Nature Physics {\bf 6}, 336 (2010)], as well as a new peak in $S_d$ are reproduced here in the low density regime. These phenomena are related to the occurrence of the Zeeman splitting and SOI induced saddle point in the band structure, where the channel chemical potential lies within an anticrossing gap between the saddle point of the lower subband and the bottom of the upper subband. Additionally, side peaks in $G$ far away from the zero bias for the nonlinear transport, as well as a quadratic bias-voltage dependence of $G$ near zero voltage, are predicted and discussed.

cond-mat.mes-hall↗

arXiv2011

Anomalous Photon-Assisted Tunneling in Graphene

We investigated the Dirac electrons transmission through a potential barrier in the presence of circularly polarized light. An anomalous photon-assisted enhanced transmission is predicted and explained in a comparison with the well-known Klein paradox. It is demonstrated that the perfect transmission for nearly-head-on collision in an infinite graphene is suppressed in gapped dressed states of electrons, which is further accompanied by shift of peaks as a function of the incident angle away from the head-on collision. In addition, the perfect transmission in the absence of potential barrier is partially suppressed by a photon-induced gap in illuminated graphene. After the effect of rough edges of the potential barrier or impurity scattering is included, the perfect transmission with no potential barrier becomes completely suppressed and the energy range for the photon-assisted perfect transmission is reduced at the same time.

cond-mat.mtrl-sci↗

arXiv2011

Spectroscopic Characterization of Gapped Graphene in the Presence of Circularly Polarized Light

We present a description of the energy loss of a charged particle moving parallel to a graphene layer and graphene double layers. Specifically, we compare the stopping power of the plasma oscillations for these two configurations in the absence as well as the presence of circularly polarized light whose frequency and intensity can be varied to yield an energy gap of several hundred $\texttt{meV}$ between the valence and conduction bands. The dressed states of the Dirac electrons by the photons yield collective plasma excitations whose characteristics are qualitatively and quantitatively different from those produced by Dirac fermions in gapless graphene, due in part to the finite effective mass of the dressed electrons. For example, the range of wave numbers for undamped self-sustaining plasmons is increased as the gap is increased, thereby increasing the stopping power of graphene for some range of charged particle velocity when graphene is radiated by circularly polarized light.

cond-mat.mtrl-sci↗

arXiv2011

Optical Modulation Effects on Nonlinear Electron Transport in Graphene in Terahertz Frequency Range

We describe very fast electron dynamics for a graphene nanoribbon driven by a control electromagnetic field in the terahertz regime. The mobility as a function of bias possesses a large threshold value when entering a nonlinear transport regime. This value depends on the temperature, electron density, impurity scattering strength, nanoribbon width and correlation length for the line-edge roughness. An enhanced electron mobility beyond this threshold has been observed, which is related to the initially-heated electrons in high energy states with a larger group velocity. However, this mobility enhancement quickly reaches a maximum governed by the Fermi velocity in graphene and the dramatically increased phonon scattering. Super-linear and sub-linear temperature dependences of the mobility are seen in the linear and nonlinear transport regimes, which is attributed separately to the results of sweeping electrons from the right Fermi edge to the left one through elastic scattering and moving electrons from low-energy states to high-energy ones through field-induced electron heating. The threshold field is pushed up by a decreased correlation length in the high field regime, and is further accompanied by a reduced magnitude in the mobility enhancement. This implies an anomalous high-field increase of the line-edge roughness scattering with decreasing correlation length due to the occupation of high-energy states by field-induced electron heating. Additionally, a self-consistent device modeling has been proposed for graphene transistors under an optical modulation on its gate, which employs Boltzmann moment equations up to the third order for describing fast carrier dynamics and full wave electromagnetics coupled to the Boltzmann equation for describing spatial-temporal dependence of the total field.

cond-mat.mes-hall↗

arXiv2011

Plasma excitations of dressed Dirac electrons in graphene layers

The dispersion relation for the collective plasma excitations of optically dressed Dirac electrons in single and double graphene layers is calculated in the random-phase approximation. The presence of circularly polarized light gives rise to an energy gap $E_g$ between the conduction and valence energy bands. The value of $E_g$ may be adjusted by varying the frequency and intensity of the light and could be much larger compared to that which is generated by spin-orbit coupling, and may reach values of the gap reported for epitaxially grown graphene. We report plasmon dispersion relations for various energy gaps and separation between graphene layers. For a single graphene sheet, we find that plasmon modes may be excited for larger wave vector and frequency when subjected to light. For double layers, we obtained an optical and phonon-like mode and found that the optical mode is not as sensitive as the phonon-like mode in the long wavelength limit when the layer separation is varied, for a chosen $E_g$. Contrary to the notion that the effective mass acquired by Dirac electrons provides a crossover to two-dimensional electron gas (2DEG) behavior, we found that the response of dressed Dirac electrons to an external perturbation is governed by the Dirac cone $ω= v_F q$, where $_F$ is the Fermi velocity and $q$ is the wave vector. Consequently, the dressed electron plasma although massive still has Dirac origin giving rise to differences in the properties of the plasmon modes compared with those for the 2DEG.

cond-mat.mtrl-sci↗

arXiv2011

Field enhanced electron mobility by nonlinear phonon scattering of Dirac electrons in semiconducting graphene nanoribbons

The calculated electron mobility for a graphene nanoribbon as a function of applied electric field has been found to have a large threshold field for entering a nonlinear transport regime. This field depends on the lattice temperature, electron density, impurity scattering strength, nanoribbon width and correlation length for the line-edge roughness. An enhanced electron mobility beyond this threshold has been observed, which is related to the initially-heated electrons in high energy states with a larger group velocity. However, this mobility enhancement quickly reaches a maximum due to the Fermi velocity in graphene and the dramatically increased phonon scattering. Super-linear and sub-linear temperature dependence of mobility seen in the linear and nonlinear transport regimes. By analyzing the calculated non-equilibrium electron distribution function, this difference is attributed separately to the results of sweeping electrons from the right Fermi edge to the left one through the elastic scattering and moving electrons from low-energy states to high-energy ones through field-induced electron heating. The threshold field is pushed up by a decreased correlation length in the high field regime, and is further accompanied by a reduced magnitude in the mobility enhancement. This implies an anomalous high-field increase of the line-edge roughness scattering with decreasing correlation length due to the occupation of high-energy states by field-induced electron heating.

cond-mat.mes-hall↗

arXiv2010

Plasma Excitations in Graphene: Their Spectral Intensity and Temperature Dependence in Magnetic Field

In this paper, we calculated the dielectric function, the loss function, the magnetoplasmon dispersion relation and the temperature-induced transitions for graphene in a uniform perpendicular magnetic field B. The calculations were performed using the Peierls tight-binding model to obtain the energy band structure and the random-phase approximation to determine the collective plasma excitation spectrum. The single-particle and collective excitations have been precisely identified based on the resonant peaks in the loss function. The critical wave vector at which plasmon damping takes place is clearly established. This critical wave vector depends on the magnetic field strength as well as the levels between which the transition takes place. The temperature effects were also investigated. At finite temperature, there are plasma resonances induced by the Fermi distribution function. Whether such plasmons exist is mainly determined by the field strength, temperature, and momentum. The inelastic light scattering spectroscopies could be used to verify the magnetic field and temperature induced plasmons.

cond-mat.mes-hall↗

arXiv2010

Energy bands, conductance and thermoelectric power for ballistic electrons in a nanowire with spin-orbit interaction

We calculated the effects of spin-orbit interaction (SOI) on the energy bands, ballistic conductance and the electron-diffusion thermoelectric power of a nanowire by varying the temperature, electron density and width of the wire. The potential barriers at the edges of the wire are assumed to be very high. A consequence of the boundary conditions used in this model is determined by the energy band structure, resulting in wider plateaus when the electron density is increased due to larger energy-level separation as the higher subbands are occupied by electrons. The nonlinear dependence of the transverse confinement on position with respect to the well center excludes the "pole-like feature" in the conductance which is obtained when a harmonic potential is employed for confinement. At low temperature, the electron diffusion thermoelectric power increases linearly with T but deviates from the linear behavior for large values of T.

cond-mat.mes-hall↗

arXiv2010

Spin-dependent Scattering by a Potential Barrier on a Nanotube

The electron spin effects on the surface of a nanotube have been considered through the spin-orbit interaction (SOI), arising from the electron confinement on the surface of the nanotube. This is of the same nature as the Rashba-Bychkov SOI at a semiconductor heterojunction. We estimate the effect of disorder within a potential barrier on the transmission probability. Using a continuum model, we obtained analytic expressions for the spin-split energy bands for electrons on the surface of nanotubes in the presence of SOI. First we calculate analytically the scattering amplitudes from a potential barrier located around the axis of the nanotube into spin-dependent states. The effect of disorder on the scattering process is included phenomenologically and induces a reduction in the transition probability. We analyzed the relative role of SOI and disorder on the transmission probability which depends on the angular and linear momentum of the incoming particle, and its spin orientation. We demonstrated that in the presence of disorder perfect transmission may not be achieved for finite barrier heights.

cond-mat.mes-hall↗

arXiv2010

Single- and few-electron dynamic quantum dots in a perpendicular magnetic field

We present experimental studies of the current pumped through a dynamic quantum dot over a wide range of magnetic fields. At low fields we observe repeatable structure indicating increased confinement of the electrons in the dynamic dot. At higher fields (B>3T), we observe structure which changes markedly from device to device suggesting that in this regime the transport is sensitive to local disorder. The results are significant for the development of dynamic quantum dot pumps as quantum standards of electrical current.

cond-mat.mes-hall↗

arXiv2010

Formula for the Absorption Coefficient for Multi-Wall Nanotubes

We present a formalism for calculating the absorption coefficient of a pair of coaxial tubules. A spatially nonlocal, dynamical self-consistent field theory is obtained by calculating the electrostatic potential produced by the charge density fluctuations as well as the external electric field. There are peaks in the absorption spectrum arising from plasma excitations corresponding either to plasmon or particle-hole modes. In this paper, we numerically calculate the plasmon contribution to the absorption spectrum when an external electric field is applied. The number of peaks depends on the radius of the inner as well as outer tubule. The height of each peak is determined by the plasmon wavelength and energy. For a chosen wave number, the most energetic plasmon has the highest peak corresponding to the largest oscillator strength of the excited modes. Some of the low-frequency plasmon modes have such weak coupling to an external electric field that they are not seen on the same scale as the modes with larger energy of excitation. We plot the peak positions of the plasmon excitations for a pair of coaxial tubules. The coupled modes on the two tubules are split by the Coulomb interaction. The energies of the two highest plasmon branches increase with the radius of the outer tubule. On the contrary, the lowest modes decrease in energy as this radius is increased. No effects due to inter-tubule hopping are included in these calculations.

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