Strongly localized image states of spherical graphitic particles.

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.

Plasmon Damping Rates in Coulomb-Coupled 2D Layers in a Heterostructure.

The Coulomb excitations of charge density oscillation are calculated for a double-layer heterostructure. Specifically, we consider two-dimensional (2D) layers of silicene and graphene on a substrate. From the obtained surface response function, we calculated the plasmon dispersion relations, which demonstrate how the Coulomb interaction renormalizes the plasmon frequencies. Most importantly, we have conducted a thorough investigation of how the decay rates of the plasmons in these heterostructures are affected by the Coulomb coupling between different types of two-dimensional materials whose separations could be varied. A novel effect of nullification of the silicene band gap is noticed when graphene is introduced into the system. To utilize these effects for experimental and industrial purposes, graphical results for the different parameters are presented.

Floquet engineering of tilted and gapped Dirac bandstructure in 1T[Formula: see text]-MoS[Formula: see text].

We have developed a rigorous theoretical formalism for Floquet engineering, investigating, and subsequently tailoring most crucial electronic properties of 1T[Formula: see text]-MoS[Formula: see text] by applying an external high-frequency dressing field within the off-resonance regime. It was recently demonstrated that monolayer semiconducting 1T[Formula: see text]-MoS[Formula: see text] exhibits tunable and gapped spin- and valley-polarized tilted Dirac bands. The electron-photon dressed states depend strongly on the polarization of the applied irradiation and reflect a full complexity of the low-energy Hamiltonian for non-irradiated material. We have calculated and analyzed the properties of the electron dressed states corresponding to linear and circular polarization of a dressing field by focusing on their symmetry, anisotropy, tilting, direct and indirect band gaps. Circularly polarized dressing field is known for transition into a new electronic state with broken time-reversal symmetry and a non-zero Chern number, and therefore, the combination of these topologically non-trivial phases and transitions between them could reveal some truly unique and previously unknown phenomena and applications. We have also computed and discussed the density of states for various types of 1T[Formula: see text]-MoS[Formula: see text] materials and its modification in the presence of a dressing field.

Coherent-scatterer enhancement and Klein-tunneling suppression by potential barriers in gapped graphene with chirality-time-reversal symmetry.

We have utilized the finite-difference approach to explore electron-tunneling properties in gapped graphene through various electrostatic-potential barriers ranging from Gaussian to a triangular envelope function in comparison with a square potential barrier. The transmission coefficient is calculated numerically for each case and applied to the corresponding tunneling conductance. It is well known that Klein tunneling in graphene will be greatly reduced in gapped graphene. Our results further demonstrate that such a decrease of transmission can be significantly enhanced for spatially-modulated potential barriers. Moreover, we investigate the effect from a bias field applied to those barrier profiles, from which we show that it enables the control of electron flow under normal incidence. Meanwhile, the suppression of Klein tunneling is found more severe for a non-square barrier and exhibits a strong dependence on bias-field polarity for all kinds of barriers. Finally, roles of a point impurity on electron transmission and conductance are analyzed with a sharp peak appearing in electron conductance as the impurity atom is placed in the middle of a square barrier. For narrow triangular and Gaussian barriers, however, the conductance peaks become significantly broadened, associated with an enhancement in tunneling conductance.

Defect capturing and charging dynamics and their effects on magneto-transport of electrons in quantum wells.

The calculated defect corrections to the polarization and dielectric functions for Bloch electrons in quantum wells are presented. These results were employed to derive the first two moment equations from the Boltzmann transport theory and then applied to explore the role played by defects on the magneto-transport of Bloch electrons. Additionally, we have derived analytically the inverse momentum-relaxation time and mobility tensor for Bloch electrons by making use of the screened defect-corrected polarization function. Based on quantum-statistical theory, we have investigated the defect capture and charging dynamics by employing a parameterized physics-based model for defects to obtain defect wave functions. Both capture and relaxation rates, as well as the density for captured Bloch electrons, were calculated self-consistently as functions of temperature, doping density and chosen defect parameters. By applying the energy-balance equation, the number of occupied energy levels and the chemical potential of defects were determined, with which the transition rate for defect capturing was obtained. By applying these results, the defect energy-relaxation, capture and escape rates, and Bloch-electron chemical potential were calculated self-consistently for a non-canonical subsystem of Bloch electrons. At the same time, the energy- and momentum-relaxation rates of Bloch electrons, as well as the current suppression factor, were also investigated quantitatively. By combining all these results, the temperature dependence of the Hall and longitudinal mobilities was presented for Bloch electrons in either single- or multi-quantum wells.

Tailoring plasmon excitations in [Formula: see text] armchair nanoribbons.

We have calculated and investigated the electronic states, dynamical polarization function and the plasmon excitations for [Formula: see text] nanoribbons with armchair-edge termination. The obtained plasmon dispersions are found to depend significantly on the number of atomic rows across the ribbon and the energy gap which is also determined by the nanoribbon geometry. The bandgap appears to have the strongest effect on both the plasmon dispersions and their Landau damping. We have determined the conditions when relative hopping parameter [Formula: see text] of an [Formula: see text] lattice has a strong effect on the plasmons which makes our material distinguished from graphene nanoribbons. Our results for the electronic and collective properties of [Formula: see text] nanoribbons are expected to find numerous applications in the development of the next-generation electronic, nano-optical and plasmonic devices.

Many-body effects and optical properties of single and double layer α-[Formula: see text] lattices.

An extensive analytical and numerical investigation has been carried out to examine the role played by many-body effects on various α-[Formula: see text] materials under an off-resonance optical dressing field. Additionally, we explore its dependence on the hopping parameter α as well as the electron-light coupling strength λ 0. The obtained dressed states due to mutual interaction between Dirac electrons and incident light are shown to demonstrate rather different electronic and optical properties in comparison with those in the absence of incident light. Specifically, various collective transport and optical properties of these electron dressed states are discussed in detail and compared for both single- and double layer α-[Formula: see text] lattices. All of these novel properties are due to the presence of a middle flat band and the interband transitions between it and an upper conduction band. Also, coupled plasmon dispersions for interacting double layer α-[Formula: see text] lattices are calculated, revealing a lower acoustic-like plasmon branch with tunable group velocity determined by both the layer separation and Fermi energy of each layer. Finally, a many-body theory is presented within the random-phase approximation for calculating the optical absorbance of doped multi-layered α-[Formula: see text] lattices in a linearly-polarized light field. We anticipate that the discoveries reported here could impact the design of the next-generation nano-optical and nano-plasmonic devices.

Magnetoplasmons for the α-T3 model with filled Landau levels.

Using the $\alpha-T_3$ 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 $\alpha$ 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 at 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 $\alpha$ (or $\phi = tan^{-1}\alpha$) 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 $\alpha$ and various magnetic fields as well as chosen magnetic field and different $\alpha$ in the random phase approximation.

Effects of site asymmetry and valley mixing on Hofstadter-type spectra of bilayer graphene in a square-scatter array potential.

Under a magnetic field perpendicular to an monolayer graphene, the existence of a two-dimensional periodic scatter array can not only mix Landau levels of the same valley for displaying split electron-hole Hofstadter-type energy spectra, but also couple two sets of Landau subbands from different valleys in a bilayer graphene. Such a valley mixing effect with a strong scattering strength has been found observable and studied thoroughly in this paper by using a Bloch-wave expansion approach and a projected [Formula: see text] effective Hamiltonian including interlayer effective mass, interlayer coupling and asymmetrical on-site energies due to a vertically-applied electric field. For bilayer graphene, we find two important characteristics, i.e. mixing and interference of intervalley scatterings in the presence of a scatter array, as well as a perpendicular-field induced site-energy asymmetry which deforms severely or even destroys completely the Hofstadter-type band structures due to the dependence of Bloch-wave expansion coefficients on the applied electric field.

Role of electron back action on photons in hybridizing double-layer graphene plasmons with localized photons.

In this paper, we deal with the electromagnetic coupling between an incident surface-plasmon-polariton wave and relativistic electrons in two graphene layers. Our previous investigation was limited to single-layer graphene (Iurov et al 2017 Phys. Rev. B 96 081408). However, the present work, is both an expanded and extended version of this previous Phys. Rev. B paper after having included very detailed theoretical formalisms and extensive comparisons of results from either one or two graphene layers embedded in a dielectric medium. The additional retarded Coulomb interaction between two graphene layers will compete with the coupling between the single graphene layer and the surface of a conductor. Consequently, some distinctive features, such as triply-hybridized absorption peaks and a new acoustic-like graphene plasmon mode within the anticrossing region, have been found for the double-layer graphene system. Physically, our theory is self-consistent, in comparison with a commonly adopted perturbative theory, for studying hybrid light-plasmon modes and the electron back action on photons. Instead of usual radiation or grating-deflection field coupling, a surface-plasmon-polariton localized field coupling is introduced with completely different dispersion relations for radiative (small wave numbers) and evanescent (large wave numbers) field modes. Technically, the exactly calculated effective scattering matrix for this theory can be employed to construct an effective-medium theory in order to improve the accuracy of the well-known finite-difference time-domain method for solving Maxwell's equations numerically. Practically, the predicted triply-hybridized absorption peaks can excite polaritons only, giving rise to a possible polariton-condensation based laser.

Temperature-dependent collective effects for silicene and germanene.

We have numerically calculated electron exchange and correlation energies and dynamical polarization functions for recently discovered silicene, germanene and other buckled honeycomb lattices at various temperatures. We have compared the dependence of these energies on the chemical potential, field-induced gap and temperature and we have concluded that in many cases they behave qualitatively in a similar way, i.e. increasing with the doping, decreasing significantly at elevated temperatures, and displaying different dependences on the asymmetry gap at various temperatures. Furthermore, we have used the dynamical polarizability to study the 'split' plasmon branches in the buckled lattices and predicted 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 silicene and germanene, as well as for enhancing silicene-based fabrication and technologies for photovoltaics and transistor devices.

Plasmon Excitations of Multi-layer Graphene on a Conducting Substrate.

We predict the existence of low-frequency nonlocal plasmons at the vacuum-surface interface of a superlattice of N graphene layers interacting with conducting substrate. We derive a dispersion function that incorporates the polarization function of both the graphene monolayers and the semi-infinite electron liquid at whose surface the electrons scatter specularly. We find a surface plasmon-polariton that is not damped by particle-hole excitations or the bulk modes and which separates below the continuum mini-band of bulk plasmon modes. The surface plasmon frequency of the hybrid structure always lies below ωs = ωp/√2, the surface plasmon frequency of the conducting substrate. The intensity of this mode depends on the distance of the graphene layers from the conductor's surface, the energy band gap between valence and conduction bands of graphene monolayer and, most importantly, on the number of two-dimensional layers. For a sufficiently large number of layers (N ≥ 7) the hybrid structure has no surface plasmon. The existence of plasmons with different dispersion relations indicates that quasiparticles with different group velocity may coexist for various ranges of wavelengths determined by the number of layers in the superlattice.

Anisotropic plasmon-coupling dimerization of a pair of spherical electron gases.

We have discovered a novel feature in the plasmon excitations for a pair of Coulomb-coupled non-concentric spherical two-dimensional electron gases (S2DEGs). Our results show that the plasmon excitations for such pairs depend on the orientation with respect to the external electromagnetic probe field. The origin of this anisotropy of the inter-sphere Coulomb interaction is due to the directional asymmetry of the electrostatic coupling of electrons in excited states which depend on both the angular momentum quantum number L and its projection M on the axis of quantization taken as the probe E-field direction. We demonstrate the anisotropic inter-sphere Coulomb coupling in space and present semi-analytic results in the random-phase approximation both perpendicular and parallel to the axis of quantization. For the incidence of light with a finite orbital or spin angular momentum, the magnetic field generated from an induced oscillating electric dipole on one sphere can couple to an induced magnetic dipole on another sphere in a way that is dependent on whether the direction is parallel or perpendicular to the probe E field. Such an effect from the plasmon spatial correlation is expected to be experimentally observable by employing circularly polarized light or a helical light beam for incidence. The S2DEG serves as a simple model for fullerenes as well as metallic dimers, when the energy bands are far apart.

Photon dressed electronic states in topological insulators: tunneling and conductance.

We have obtained analytic results for the surface states of three-dimensional topological insulators in the presence of circularly polarized light. This electron-photon interaction results in an energy gap as well as a novel energy dispersion of the dressed electron-photon states, different from both graphene and the standard two-dimensional electron gas (2DEG). Additionally, we made calculations of the ballistic conductance and Klein tunneling in both two- and three-dimensional topological insulators as well as investigating how these phenomena are affected in the presence of circularly polarized light. We have found a critical energy for an incoming particle, separating two substantially different types of tunneling.

Anomalous photon-assisted tunneling in graphene.

We investigated the transmission of Dirac electrons through a potential barrier in the presence of circularly polarized light. An anomalous photon-assisted enhanced transmission is predicted and explained. It is demonstrated that the perfect transmission for nearly head-on collision in infinite graphene is suppressed in gapped dressed states of electrons, which is further accompanied by a shift of peaks as a function of the incident angle away from head-on collision. In addition, the perfect transmission 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 transmission is reduced at the same time.