arXiv subjects

Godfrey Gumbs

Publications and source records attributed to Godfrey Gumbs.

At least 91 records · Page 5Linked to original sources

arXiv2017

Exploring the Optical States for Black Phosphorus: Anisotropy and Bandgap Tuning

The dressed states arising from the interaction between electrons and holes, and off-resonant electromagnetic radiation have been investigated for recently fabricated gapped and anisotropic black phosphorus. Our calculations were carried out for the low-energy electronic subbands near the $Γ$ point. States for both linear and circular polarizations of the incoming radiation have been computed. However, our principal emphasis is on linearly polarized light with arbitrary polarization since this case has not been given much attention for dressing fields imposed on anisotropic structures. We have considered various cases for one- and few-layer phosphorus, including massless Dirac fermions with tunable in-plane anisotropy. Initial Hamiltonian parameters are renormalized in a largely different way compared to those for previously reported for gapped Dirac structures and, most importantly, existing anisotropy which could be modified in every direction.

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arXiv2017

Controlling plasmon modes and damping in buckled two-dimensional material open systems

Full ranges of both hybrid plasmon-mode dispersions and their damping are studied systematically by our recently developed mean-field theory in open systems involving a conducting substrate and a two-dimensional (2D) material with a buckled honeycomb lattice, such as silicene, germanene, and a group \rom{4} dichalcogenide as well. In this hybrid system, the single plasmon mode for a free-standing 2D layer is split into one acoustic-like and one optical-like mode, leading to a dramatic change in the damping of plasmon modes. In comparison with gapped graphene, critical features associated with plasmon modes and damping in silicene and molybdenum disulfide are found with various spin-orbit and lattice asymmetry energy bandgaps, doping types and levels, and coupling strengths between 2D materials and the conducting substrate. The obtained damping dependence on both spin and valley degrees of freedom is expected to facilitate measuring the open-system dielectric property and the spin-orbit coupling strength of individual 2D materials. The unique linear dispersion of the acoustic-like plasmon mode introduces additional damping from the intraband particle-hole modes which is absent for a free-standing 2D material layer, and the use of molybdenum disulfide with a large bandgap simultaneously suppresses the strong damping from the interband particle-hole modes.

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arXiv2016

Tunneling of Hybridized Pairs of Electrons through a One-Dimensional Channel

Recently, the electron transport through a quasi-one dimensional (quasi-1D) electron gas was investigated experimentally as a function of the confining potential. We present a physical model for quantum ballistic transport of electrons through a short conduction channel, and investigate the role played by the Coulomb interaction in modifying the energy levels of two-electron states at low temperatures as the width of the channel is increased. In this regime, the effect of the Coulomb interaction on the two-electron states has been shown to lead to four split energy levels, including two anti-crossings and two crossing-level states. Due to the interplay between the anti-crossing and crossing of the energy levels, the ground state for the two-electron model switches from one anti-crossing state for strong confinement to a crossing state for intermediate confinement as the channel width is first increased, and then returned to its original anti-crossing state. This switching behavior is related to the triplet spin degeneracy as well as the Coulomb repulsion and reflected in the ballistic conductance. Here, many-body effects can still affect electron occupations in the calculation of quantum ballistic conductance although it cannot vary the center-of-mass velocity.

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arXiv2016

Field-created diverse quantizations in phosphorenes

Electronic properties of few-layer phosphorenes are investigated by the generalized tight-binding model. They are greatly diversified by the electric and magnetic fields ($E_z$ and $B_z$). The $E_z$-induced gap transition, Dirac cones, oscillatory bands and critical points are present in bilayer system, but absent in monolayer one. The diverse magnetic quantization phenomena cover the coexistent two subgroups of Landau levels, the uniform and non-uniform energy spacings, and the crossing and anti-crossing behaviors. Specifically, the wavefunctions exhibit the dramatic changes between the well-behaved and multi-mode oscillations. The feature-rich energy spectra are reveled in density of states as .many special structures which could be verified from scanning tunneling spectroscopy.

cond-mat.mtrl-sci↗

arXiv2016

Effect of Energy Band Gap in Graphene on Negative Refraction through the Veselago Lens and Electron Conductance

A remarkable property of intrinsic graphene is that upon doping, electrons and holes travel through the monolayer thick material with constant velocity which does not depend on energy up to about $0.3$ eV (Dirac fermions), as though the electrons and holes are massless particles and antiparticles which move at the Fermi velocity $v_F$. Consequently, there is Klein tunneling at a $p-n$ junction, in which there is no backscattering at normal incidence of massless Dirac fermions. However, this process yielding perfect transmission at normal incidence is expected to be affected when the group velocity of the charge carriers is energy dependent and there is non-zero effective mass for the target particle. We investigate how away from normal incidence the combined effect of incident electron energy $ε$ and band gap parameter $Δ$ can determine whether a $p-n$ junction would allow focusing of an electron beam by behaving like a Veselago lens with negative refractive index. We demonstrate that there is a specific region in $ε-Δ$ space where the index of refraction is negative, i.e., where monolayer graphene behaves as a metamaterial. Outside this region, the refractive index may be positive or there may be no refraction at all. We compute the ballistic conductance across a $p-n$ junction as a function of $Δ$ and $ε$ and compare our results with those for a single electrostatic potential barrier and multiple barriers.

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arXiv2016

Temperature-dependent collective effects for silicene, germanene and monolayer black phosphorus

We have calculated numerically electron exchange, correlation energies and dynamical polarization function for newly discovered silicene, germanene and black phosphorus (BP), consisting of puckered layers of elemental phosphorus atoms, broadening the range of two-dimensional (2D) materials at various temperatures. As a matter of fact, monolayer BP, produced by mechanical and liquid exfoliation techniques, has been predicted to be an insulator with a large energy splitting 1.6 eV for the quasiparticle band structure. We compare the dependence of these energies on the chemical potential, field-induced gap and the temperature and concluded that in many cases they behave qualitatively similarly, i.e., increasing with the doping, decreasing significantly at elevated temperatures, and displaying different dependence on the asymmetry gap at various temperatures. Furthermore, we used the dynamical polarizability to investigate the new "split" plasmon branches in the puckered lattices and predict a unique splitting, different from that in gapped graphene, for various energy gaps. Our results are crucial for stimulating electronic, transport and collective studies of these novel materials, as well as for enhancing silicene-based fabrication and technologies for photovoltaics and transistor devices.

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arXiv2016

Plasmon Excitations for Encapsulated Graphene

We have developed an analytical formulation to calculate the plasmon dispersion relation for a two-dimensional layer which is encapsulated within a narrow spatial gap between two bulk half-space plasmas. This is based on a solution of the inverse dielectric function integral equation within the random-phase approximation (RPA). We take into account the nonlocality of the plasmon dispersion relation for both gapped and gapless graphene as the sandwiched two-dimensional (2D) semiconductor plasma. The associated nonlocal graphene plasmon spectrum coupled to the "sandwich" system is exhibited in density plots, which show a linear mode and a pair of depolarization modes shifted from the bulk plasma frequency.

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arXiv2016

Thermal smearing of the magneto-Kohn anomaly for Dirac materials and comparison with the two-dimensional electron liquid

We compute and compare the effects due to a uniform perpendicular magnetic field as well as temperature on the static polarization functions for monolayer graphene (MLG), associated with the Dirac point, with that for the two-dimensional electron liquid (2DEL) with the use of comprehensive numerical calculations. Previous results for the 2DEL are discussed and, in particular, we point out a flaw in a reported analytic derivation which was carried out to exhibit the smearing of the Fermi surface for 2DEL. The relevance of our study to the Kohn anomaly in low-dimensional structures and the Friedel oscillations for the screening of the potential for a dilute distribution of impurities is reported. Our results show substantial differences due to screening for the 2DEL and MLG which have not been given adequate attention previously.

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arXiv2016

Velocity Autocorrelation Function of Magnetized Two-Dimensional One-Component Plasma

The velocity autocorrelation function (VAF) for a two-dimensional one-component plasma (OCP) is investigated by employing molecular dynamics techniques. The VAF exhibits well defined oscillations whose frequency is independent of the dimensionless coupling parameter Gamma. However, the presence of a uniform perpendicular external magnetic field increases the height of the first peak. Molecular dynamics computer simulation results are presented for a two-dimensional OCP in a perpendicular magnetic field as a function of the Plasma parameter. Our simulation results clearly indicate that at high temperature, the magnetic field affects the VAF in a substantial way.

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arXiv2015

Incident-Angle Dependence of Electromagnetic Wave Transmission through a Nano-hole in a Thin Plasmonic Semiconductor Layer

This work is focussed on the role of the angle of incidence of an incoming electromagnetic wave in its transmission through a subwavelength nano-hole in a thin semiconductor plasmonic layer. Fully detailed calculations and results are exhibited for $ p$- and $s$-polarizations of the incident wave for a variety of incident angles in the near, middle and far zones of the transmitted radiation. Our dyadic Green's function formulation includes both (1) the electromagnetic field transmitted directly through the $ 2D $ plasmonic layer superposed with (2) the radiation emanating from the nano-hole. Interference fringes due to this superposition are explicitly exhibited. Based on an integral equation formulation, this dyadic Green's function approach does not involve any appeal to metallic boundary conditions. It does incorporate the role of the $ 2D $ plasmon of the semiconductor layer, which is smeared due to its lateral wave number dependence. We find that the interference fringes, which are clustered near the nano-hole, flatten to a uniform level of transmission directly through the sheet alone at large distances from the nano-hole. Furthermore, as the incident angle increases, the axis of the relatively large central transmission maximum through the nano-hole follows it, accompanied by a spatial compression of interference fringe maxima forward of the large central transmission maximum, and a spatial thinning of the fringe maxima behind it.

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arXiv2015

Plasmon dissipation in gapped-graphene open systems at finite temperature

Numerical and closed-form analytic expressions for plasmon dispersion relations and rates of dissipation are first obtained at finite-temperatures for free-standing gapped graphene. These closed-system results are generalized to an open system with Coulomb coupling of graphene electrons to an external electron reservoir. New plasmon modes, as well as new plasmon dissipation channels, are found in this open system, including significant modifications arising from the combined effect of thermal excitation of electrons and an energy bandgap in gapped graphene. Moreover, the characteristics of the new plasmon mode and the additional plasmon dissipation may be fully controlled by adjusting the separation between the graphene layer from the surface of a thick conductor. Numerical results for the thermal shift of plasmon frequency in a doped gapped graphene layer, along with its sensitivity to the local environment, are demonstrated and analyzed. Such phenomenon associated with the frequency shift of plasmons may be applied to direct optical measurement of local electron temperature in transistors and nanoplasmonic structures.

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arXiv2015

Feature-rich electronic excitations in external fields of 2D silicene

Electronic Coulomb excitations in monolayer silicene are investigated by using the Lindhard dielectric function and a newly developed generalized tight-binding model (G-TBM). G-TBM simultaneously contains the atomic interactions, the spin-orbit coupling, the Coulomb interactions, and the various external fields at an arbitrary chemical potential. We exhibit the calculation results of the electrically tunable magnetoplasmons and the strong magnetic field modulation of plasmon behaviors. The two intriguing phenomena are well explained by determining the dominant transition channels in the dielectric function and through understanding the electron behavior under the multiple interactions (intrinsic and external). A further tunability of the plasmon features is demonstrated with the momentum transfer and the Fermi energy. The methodological strategy could be extended to several other 2D materials like germanene and stanene, and might open a pathway to search a better system in nanoplasmonic applications.

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arXiv2015

Environmental Effects on the Terahertz Surface Plasmons in Epitaxial Graphene

In this paper, we predict the existence of low-frequency nonlocal plasmon excitations at the vacuum-surface interface of a superlattice of $N$ graphene layers interacting with a thick conducting substrate. This is different from graphite which allows inter-layer hopping. A dispersion function is derived which incorporates the polarization function of the graphene monolayers (MLGs) and the dispersion function of a semi-infinite electron liquid at whose surface the electrons scatter specularly. We find that this surface plasmon-polariton is not damped by the particle-hole excitations (PHE's) or the bulk modes and separates below the continuum mini-band of bulk plasmon modes. For a conducting substrate with surface plasmon frequency $ω_s=ω_p/\sqrt{2}$, the surface plasmon frequency of the hybrid structure always lies below $ω_s$. The intensity of this mode depends on the distance of the graphene layers from the surface of the conductor, the energy band gap between the valence and conduction bands of MLG and, most importantly, on the number of two-dimensional (2D) layers. Furthermore, the hybrid structure has no surface plasmon for a sufficiently large number ($N\stackrel{>}{\sim} 7$) of layers. The existence of two plasmons with different dispersion relations indicates that quasiparticles with different group velocity may coexist for various ranges of wavelength which is determined by the number of layers in the superlattice.

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arXiv2015

The effect of perpendicular electric field on Temperature-induced plasmon excitations for intrinsic silicene

We use the tight-binding model and the random-phase approximation to investigate the intrinsic plasmon in silicene. At finite temperatures, an undamped plasmon is generated from the interplay between the intraband and the interband-gap transitions. The extent of the plasmon existence range in terms of momentum and temperature, which is dependent on the size of single-particle-excitation gap, is further tuned by applying a perpendicular electric field. The plasmon becomes damped in the interband-excitation region. A low damped zone is created by the field-induced spin split. The field-dependent plasmon spectrum shows a strong tunability in plasmon intensity and spectral bandwidth. This could make silicene a very suitable candidate for plasmonic applications.

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arXiv2015

Interplay of hybrid and unhybrid quantum transport of interacting electron pairs through a short conduction channel

For quantum ballistic transport of electrons through a short conduction channel, the role of Coulomb interaction may significantly modify the energy levels of an electron pair at low temperatures as the channel becomes wide. In this regime, the Coulomb effect on the orbital triplet and singlet electron-pair state is calculated and found to lead to four split energy levels, including two hybrid and two unhybrid states. Moreover, due to the interplay of hybrid and unhybrid Coulomb interactions between two electrons, our calculations reveal that the ground pair-state will switch from one hybrid orbit-triplet state (strong confinement) to the unhybrid orbit-singlet state (intermediate confinement) as the channel width gradually increases and then back to the original hybrid orbit-triplet state (weak confinement), due to larger total spin of the orbit-singlet state, as the channel width becomes larger than a threshold value. This switching behavior leaves a footprint in the conductance as well as in the diffusion thermoelectric power of electrons. Here, the predicted reoccurrence of the hybrid orbit-triplet state (spin-$0$ state) as a ground state is shown to relate to the higher spin degeneracy of the spin-$1$ state as well as to the strong Coulomb repulsion in the central region of the channel, which separates two electrons away and pushes them to different channel edges. The conductance reoccurrence region expands from the weak to the intermediate confinement regime with increasing linear electron density.

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arXiv2015

Temperature-dependent Plasmons and Their Damping Rates for Graphene with a Finite Energy Bandgap

We obtained numerical and closed-form analytic expressions for finite-temperature plasmon dispersion relations for intrinsic graphene in the presence of a finite energy gap in the energy spectrum. The calculations were carried out using the random-phase approximation. The analytic results have been derived in the high temperature regime and long-wavelength limit. We have found that the plasmon damping rate decreases in the presence of a band gap. Our method of calculation could also be applied to silicene and other buckled honeycomb lattice structures. The finite-temperature plasmon dispersion relations are presented when a single graphene layer is Coulomb coupled to a semi infinite conductor. Both cases of gapless and gapped monolayer graphene have been investigated when a thick substrate is in their proximity. Both the plasmon excitation frequency and damping rate are linear functions of the in-plane wave vector in the long wavelength limit when a monolayer interacts with a conducting substrate which is not the case for free-standing pristine or gapped graphene.

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arXiv2015

Coulomb Excitations for a Short Linear Chain of Metallic Shells

A self-consistent-field theory is given for the electronic collective modes of a chain containing a finite number, $N$, of Coulomb-coupled spherical two-dimensional electron gases (S2DE's) arranged with their centers along a straight line, simulating a narrow micro-ribbon of metallic shells. The separation between nearest-neighbor shells is arbitrary and because of the quantization of the electron energy levels due to their confinement to the spherical surface, all angular momenta $L$ of the Coulomb excitations and their projections $M$ on the quantization axis are coupled. However, for incoming light with a specific polarization, only one angular momentum quantum number is chosen. We show that when $N=3$ the next-nearest-neighbor Coulomb coupling is larger than its value if they are located at opposite ends of a right-angle triangle forming the triad. Additionally, the frequencies of the plasma excitations depend on the orientation of the line joining them with respect to the axis of quantization since the magnetic field generated from the induced oscillating electric dipole moment on one sphere can couple to the induced magnetic dipole moment on another.

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arXiv2014

Non-local Plasma Spectrum of Graphene Interacting with a Thick Conductor

Self-consistent field theory is used to obtain the non-local plasmon dispersion relation of monolayer graphene which is Coulomb-coupled to a thick conductor. We calculate numerically the undamped plasmon excitation spectrum for arbitrary wave number. For gapped graphene, both the low-frequency (acoustic) and high frequency (surface) plasmons may lie within an undamped opening in the particle-hole region. Furthermore, we obtain plasmon excitations in a region of frequency-wave vector space which do not exist for free-standing gapped graphene.

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