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We have investigated the α–T3 model in the presence of a mass term which opens a gap in the energy dispersive spectrum, as well as under a uniform perpendicular quantizing magnetic field. The gap opening mass term plays the role of Zeeman splitting at low magnetic fields for this pseudospin-1 system, and, as a consequence, we are able to compare physical properties of the the α–T3 model at low and high magnetic fields. Specifically, we explore the magnetoplasmon dispersion relation in these two extreme limits. Central to the calculation of these collective modes is the dielectric function which is determined by the polarizability of the system. This latter function is generated by transition energies between subband states, as well as the overlap of their wave functions.

We investigate the localization of charged particles by the image potential of spherical shells, such as fullerene buckyballs. These spherical image states exist within surface potentials formed by the competition between the attractive image potential and the repulsive centripetal force arising from the angular motion. The image potential has a power law rather than a logarithmic behavior. This leads to fundamental differences in the nature of the effective potential for the two geometries. Our calculations have shown that the captured charge is more strongly localized closest to the surface for fullerenes than for cylindrical nanotube.

We have derived the energy loss spectrum for a beam of energetic electrons moving parallel to a set of two-dimensional (2D)-metasurfaces capable of carrying hyperbolic plasmons due to their anisotropic conductivity. The formalism is presented for naturally occurring metasurfaces such as transition metal dichalcogenides (TMDCs), as well as for an engineered array of graphene or phosphorene nanoribbons. Our formalism is applicable in the long-wavelength limit where one neglects (graphene) or emphasizes the cross-polarization of the nanoribbons (phosphorene). When the nanoribbons are aligned, the plasmon propagation along oblique directions is not permitted. This allows for plasmon probing at much lower electron energies in contrast with isotropic graphene and offers drastic modification of the energy loss probability. Additionally, the structure is akin to a thin uni-axial crystal supporting extraordinary refracted beams with negative group velocity.We suggest that electron energy loss spectroscopy (EELS) is capable of identifying two resonances separately once the beam and the direction of the nanoribbons are mismatched. For a twisted double layer configuration, one may observe a peculiar interference effect between ordinary and extraordinary plasmon modes. [ABSTRACT FROM AUTHOR]

We investigated the effect of perpendicular magnetic field on quantum wires when the 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 (Sd) are calculated as functions of the electron density, temperature and applied bias voltage. A low-temperature half-step feature in G that was observed experimentally by Quay et al. [Nat. Phys. 6, 336 (2010)], as well as a new peak in Sd are reproduced here in the low density region. These phenomena are related to the occurrence of Zeeman splitting and a 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 that are 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. [ABSTRACT FROM AUTHOR]

We calculated the effects of spin-orbit interaction on the energy bands, ballistic conductance (G), and the electron-diffusion thermoelectric power (Sd) 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 'polelike feature' in G which is obtained when a harmonic potential is employed for confinement. At low temperature, Sd increases linearly with T but deviates from the linear behavior for large values of T. [ABSTRACT FROM AUTHOR]

We have investigated the conditions necessary to achieve stronger Cherenkov-like instability of plasma waves leading to emission in the terahertz regime for semiconductor quantum wells. 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 vD for the driven electric charge. By means of a surface response function formalism, the plasmon dispersion equation is obtained as a function of frequency ω, in-plane wave vector q∥=(qx,qy), and reciprocal lattice vector nG, where n=0,±1,±2,... and G=2π/d, with d denoting the period of the density modulation. The dispersion equation, which yields the resonant frequencies, is solved numerically in the complex ω-plane for the real wave vector q∥. It is ascertained that the imaginary part of ω is enhanced with decreasing d and with increasing doping density of the free carriers in the bulk medium for a fixed period of the spatial modulation. [ABSTRACT FROM AUTHOR]

We perform a comprehensive analysis of the spectrum of graphene plasmons which arise when a pair of sheets are confined between conducting materials. The associated enhanced local fields may be employed in the manipulation of light on the nanoscale by adjusting the separation between the graphene layers, the energy band gap as well as the concentration of charge carriers in the conducting media surrounding the two‐dimensional (2D) layers. We present a theoretical formalism, based on the calculation of the surface response function, for determining the plasmon spectrum of an encapsulated pair of 2D layers and apply it to graphene. We solve the coupled equations involving the continuity of the electric potential and discontinuity of the electric field at the interfaces separating the constituents of the hybrid structure. We have compared the plasmon modes for encapsulated gapped and gapless graphene. The associated nonlocal graphene plasmon spectrum coupled to the “sandwich” system show a linear acoustic plasmon mode as well as a low‐frequency mode corresponding to in‐phase oscillations of the adjacent 2D charge densities. These calculations are relevant to the study of energy transfer via plasmon excitations when graphene is confined by a pair of thick conducting materials. [ABSTRACT FROM AUTHOR]

In the presence of an external vertical electric field and strain, it is evident that 1T ′ -MoS 2 exhibits tilted Dirac bands which are valley-spin-polarized. Additionally, this material experiences a topological phase change between a topological insulator and band insulator for a critical value of the electric field. Using linear response theory, we calculated the polarization function which is in turn employed to obtain the dielectric function. This latter quantity is subsequently utilized in calculations to determine the plasmons dispersion relation, their decay rate and impurity screening corresponding to various levels of doping, the critical applied vertical electric field strengths and the spin-orbit coupling gap in 1T ′ -MoS 2 with tilted Dirac bands. • Polarization function for tilted Dirac fermion materials. • Anisotropy of transverse magnetic modes. • Analytic expressions for the plasmon excitations. • Static shielding of impurities. • Landau damping of plasmons and the calculated decay rates. [ABSTRACT FROM AUTHOR]

A theory for the collective plasma excitations in a linear periodic array of spherical two-dimensional electron gases (S2DEGs) is presented. This is a simple model for an ultra thin and narrow microribbon of fullerenes or metallic shells. Coulomb coupling between electrons located on the same sphere and on different spheres is included in the random-phase approximation. Electron hopping between spheres is neglected in these calculations. The resulting plasmon-dispersion equation is solved numerically. Results are presented for a superlattice of single-wall S2DEGs as a function of the wave vector. The plasmon dispersions are obtained for different spherical separations. We show that the one-dimensional translational symmetry of the lattice is maintained in the plasmon spectrum. Additionally, we compare the plasmon dispersion when the superlatice direction is parallel or perpendicular to the axis of quantization. However, because of anisotropy in the Coulomb matrix elements, there is anticrossing in the plasmon dispersion only in the direction perpendicular to the quantization axis. The S2DEG may serve as a simple model for fullerenes, when their energy bands are far apart. [ABSTRACT FROM AUTHOR]

We present a formalism for calculating the image potential for a two-dimensional electron gas (2DEG) with Rashba spin-orbit interaction (SOI) as well as for a 2D topological insulator (TI). The formalism is further generalized for including the Coulomb coupled multiple layers. Roles of broken inversion symmetry near the surface and the dielectric environment are investigated by using a surface-response function. The insignificant role of SOI in 2DEG is dramatically enhanced in TI by selecting a small relative permittivity [ABSTRACT FROM AUTHOR]

We investigate the localization of charged particles by the image potential of spherical shells, such as fullerene buckyballs. These spherical image states exist within surface potentials formed by the competition between the attractive image potential and the repulsive centripetal force arising fromthe angular motion.The image potential has a power law rather than a logarithmic behavior. This leads to fundamental differences in the nature of the effective potential for the two geometries. Our calculations have shown that the captured charge is more strongly localized closest to the surface for fullerenes than for cylindrical nanotube. [ABSTRACT FROM AUTHOR]

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 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. [ABSTRACT FROM PUBLISHER]

Abstract: 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 Letter, 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. [Copyright &y& Elsevier]

We have investigated the α – T 3 model in the presence of a mass term which opens a gap in the energy dispersive spectrum, as well as under a uniform perpendicular quantizing magnetic field. The gap opening mass term plays the role of Zeeman splitting at low magnetic fields for this pseudospin-1 system, and, as a consequence, we are able to compare physical properties of the the α – T 3 model at low and high magnetic fields. Specifically, we explore the magnetoplasmon dispersion relation in these two extreme limits. Central to the calculation of these collective modes is the dielectric function which is determined by the polarizability of the system. This latter function is generated by transition energies between subband states, as well as the overlap of their wave functions. [ABSTRACT FROM AUTHOR]

We compare the two-dimensional (2D) plasmon dispersion relations for monolayer graphene when the sample is doped with carriers in the conduction band and the temperature T is zero with the case when the temperature is finite and there is no doping. Additionally, we have obtained the plasmon excitations when there is doping at finite temperature. The results were obtained in the random-phase approximation which employs energy electronic bands calculated using ab initio density functional theory. We found that in the undoped case the finite temperature results in appearance in the low-energy region of a 2D plasmon which is absent for the T=0 case. Its energy is gradually increased with increasing T. It is accompanied by expansion in the momentum range where this mode is observed as well. The 2D plasmon dispersion in the ΓM direction may differ in substantial ways from that along the ΓK direction at sufficiently high temperature and doping concentrations. Moreover, at temperatures exceeding ≈300 meV a second mode emerges along the ΓK direction at lower energies like it occurs at a doping level exceeding ≈300 meV. Once the temperature exceeds ≈0.75 eV this mode ceases to exist whereas the 2D plasmon exists as a well-defined collective excitation up to T=1.5eV, a maximal temperature investigated in this work. [ABSTRACT FROM AUTHOR]

Abstract: In this work we analyze the energy loss of a fast charged particle moving parallel to a two-dimensional (2D) graphene sheet. The response dynamics of the 2D graphene sheet are modeled using the random phase approximation in the degenerate limit (zero temperature). We determine the energy loss and stopping power for motion parallel to the graphene sheet as a function of the distance of the charged particle from the 2D sheet and of its velocity. [ABSTRACT FROM AUTHOR]

The presence of several kinds of carriers at the Fermi surface results in interesting complex dielectric properties of the bulk Pd in the low-energy excitation range. A most spectacular manifestation of this is the presence of a collective electronic excitation characterized by a soundlike dispersion, termed acoustic plasmon (AP). Due to the characteristic dispersion reaching zero energy in the long-wavelength limit, the question of the thermal stability of the excitation spectrum arises. In this work we explore this problem investigating the thermal effect on the electronic excitation spectrum in this material, tracing how the AP properties vary with the temperature increase. The main effect consists in the gradual destruction of AP in the energy range corresponding to the temperature. [ABSTRACT FROM AUTHOR]

1T-TiSe_{2} is one of the most studied charge density wave (CDW) systems, not only because of its peculiar properties related to the CDW transition, but also due to its status as a promising candidate of exciton insulator signaled by the proposed plasmon softening at the CDW wave vector. Using high-resolution electron energy loss spectroscopy, we report a systematic study of the temperature-dependent plasmon behaviors of 1T-TiSe_{2}. We unambiguously resolve the plasmon from phonon modes, revealing the existence of Landau damping to the plasmon at finite momentums, which does not support the plasmon softening picture for exciton condensation. Moreover, we discover that the plasmon lifetime at zero momentum responds dramatically to the band gap evolution associated with the CDW transition. The interband transitions near the Fermi energy in the normal phase are demonstrated to serve as a strong damping channel of plasmons, while such a channel in the CDW phase is suppressed due to the CDW gap opening, which results in the dramatic tunability of the plasmon in semimetals or small-gap semiconductors.

Using the α -T 3 model, we carried out analytical and numerical calculations for the static and dynamic polarization functions in the presence of a perpendicular magnetic field. The model involves a parameter α which is the ratio of the hopping strength from an atom at the center of a honeycomb lattice to one of the atoms on the hexagon to the hopping strength around its rim. Our results were employed to determine the longitudinal dielectric function and the magnetoplasmon dispersion relation. The magnetic field splits the continuous valence, conduction and flat energy subband into discrete Landau levels which present significant effects on the polarization function and magnetoplasmon dispersion. This includes the fact that the energies of the Landau levels are valley dependent which leads to different behaviors of the polarization function as the hopping parameter α (or ϕ = tan -1 ) is reduced continuously toward zero. This essential critical behavior of the polarization function leads to a softening of a magnetoplasmon mode. We present results for a doped layer in the integer quantum Hall regime for fixed hopping parameter α ) is reduced continuously toward zero. This essential critical behavior of the polarization function leads to a softening of a magnetoplasmon mode. We present results for a doped layer in the integer quantum Hall regime for fixed hopping parameter α and various magnetic fields as well as chosen magnetic field and different α in the random phase approximation., (© 2020 IOP Publishing Ltd.)