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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 (RPA). 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.

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., 7 figures

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.

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., Comment: Updated corrected version of the original submission

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.

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 single-wall 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 $m$ in individual nanotubes, lifting the $\ifmmode\pm\else\textpm\fi{}m$ 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 ${\mathbf{q}}_{\ensuremath{\perp}}$. For the bundle, the Coulomb interaction between nanotubes gives rise to optical plasmon excitations.

We present ab initio time-dependent density-functional theory calculation results for low-energy collective electron excitations in ${\text{MgB}}_{2}$. The existence of a long-lived collective excitation corresponding to coherent charge density fluctuations between the boron $\ensuremath{\sigma}$ and $\ensuremath{\pi}$ bands ($\ensuremath{\sigma}\ensuremath{\pi}$ mode) is demonstrated. This mode has a sine-like oscillating dispersion for energies below 0.5 eV. At even lower energy we find another collective mode ($\ensuremath{\sigma}\ensuremath{\sigma}$ mode). We show the strong impact of local-field effects on dielectric functions in ${\text{MgB}}_{2}$. These effects account for the long $q$-range behavior of the modes. We discuss the physics that these collective excitations bring to the energy region typical for lattice vibrations.

We have formulated a theory to help us investigate the conditions which are needed to achieve stronger plasmon instability 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 metal grating placed on the surface which modulates the 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 2DEG layers and is characterized by a drift velocity υ D . With the use of the surface response function, the plasmon dispersion equation is obtained as a function of frequency ω, the in-plane wave vector q ll = ( q x , q y ) and reciprocal lattice vector nG where n = 0,±1,±2, ... with G = 2π/ d and d denoting the period of the grating. The dispersion equation, which yields the resonant frequencies, is solved in the complex ω-plane for real wave vector q ll . It is ascertained that the imaginary part of ω is enhanced with decreasing d , and with increasing the doping density of the free carriers in the bulk medium for fixed grating period.

An experimental and theoretical study of electronic excitations in MgB2 covering the domain of large energy and momentum transfers is reported. Energy-loss spectra for several values of momentum transfers were measured in a polycrystalline sample by means of inelastic X-ray scattering spectroscopy. Ab initio calculations of the dielectric function as well as the energy-loss function were performed in the frame of the time-dependent local density approximation with inclusion of crystal local-field effects. We obtained very good agreement between the experimental and the theoretical energy dispersion of the peak maximum of the loss function. We found that crystal local-field effects are responsible for this agreement at large momenta. Fine structure observed in the measured spectra was interpreted in terms of strong interband transitions predicted by the calculations in the and directions. The theoretical dispersion of these features is in good accordance with the experimental data. Further spectral features in the measured spectra due to Mg 2s and 2p core electron excitations are also discussed., Financial support from SeCyT (Universidad Nacional de Córdoba, Argentina) and FAPESP and CNPq (Brazil) is gratefully acknowledged. The San Sebastián team acknowledges the partial support from the University of the Basque Country (Grant No. GIC07IT36607), Departamento de Educación del Gobierno Vasco, and the Spanish Ministerio de Ciencia y Technología (MCyT) (Grant No. FIS200766711C0101). The work of V.M.S. is sponsored by the IKERBASQUE Foundation. The research was supported by LNLS-Brazilian Synchrotron Light Laboratory/MCT.

5 p., 4 figuras We present ab initio time-dependent density-functional theory calculation results for low-energy collective electron excitations in MgB2. The existence of a long-lived collective excitation corresponding to coherent charge density fluctuations between the boron sigma and pi bands (sigma pi mode) is demonstrated. This mode has a sine-like oscillating dispersion for energies below 0.5 eV. At even lower energy we find another collective mode (sigma sigma mode). We show the strong impact of local-field effects on dielectric functions in MgB2. These effects account for the long q-range behavior of the modes. We discuss the physics that these collective excitations bring to the energy region typical for lattice vibrations. Partial support from the University of the Basque Country (Grant No. GIC07IT36607), the Departamento de Educacion del Gobierno Vasco, and the Spanish Ministerio de Ciencia y Tecnologia (MCyT) (Grant No. FIS200766711C0101).

We report on the results of calculation of low-energy dielectric response and optical properties of MgB2. The calculations have been performed with full inclusion of the ab initio electron band structure making use of random-phase and time-dependent local-density approximations. The role of local-field and exchange-correlation effects in MgB2 dielectric function is thoroughly examined. Index of refraction, extinction coefficient, and normal-incidence reflectivity exhibiting strong anisotropy were calculated for electromagnetic waves polarized along the a∗ and c∗ axes by using the evaluated electron-density-response functions. Our results are in good agreement with recent x-ray and optical measurements performed on MgB2 single crystals., We acknowledge the partial support from the University of the Basque Country Grant No. GIC07IT36607, the Departamento de Educación del Gobierno Vasco, and the Spanish Ministerio de Ciencia y Tecnología MCyT Grant No.FIS200766711C0101. The work of V.M.S. is sponsored by the IKERBASQUE Foundation.

9 pages, 8 figures.-- PACS nrs.: 73.21.Fg, 73.20.At, 74.70.Ad, 73.50.Gr.-- Printed version published on Aug 2008., Recent advances in the theoretical description of electron and hole dynamics in several metallic systems are discussed. The role of initial and final states in the decay rates together with the dynamical screening is studied for the case of three different non-homogeneous systems, Na/Cu(111), MgB2, and Mg(0001)., We acknowledge partial support by the University of the Basque Country UPV/EHU (GIC07IT36607), and the Spanish MCyT (Grant No. FIS200766711C0101).

Carbon nanotubes have come under intense theoretical and experimental investigation focused on both their transport and photonic properties. Of recent interest is the observation that when a gate voltage is applied perpendicular to the axis of the nanotube, it can lead to spin-orbit interaction (SOI). This is of the same nature as the Rashba-Bychkov SOI at an asymmetric semiconductor heterojunction. We have calculated the "band states" for a cylindrical nanotube in the presence of SOI and then used the results to determine the collective plasma oscillations corresponding to electron transitions between the spin-split subbands for single-wall and multiwall nanotubes. These collective plasma modes determine the peak positions for the intersubband absorption coefficient as well as the energy loss spectrum. We show that the plasmon excitations for coaxial tubules have a region of instability which could lead to amplification in the energy transfer spectrum. The frequency regime where this instability occurs could be in the terahertz range. This source of radiation may be suitable for use in space-based detectors because of the durability and robustnes of carbon nanotubes.

We present a formalism for the rate of transfer of energy from a current of charged particles to multiwall and a linear array of nanotubes. The method is based on a self-consistent field theory involving Laplace's equation for the total screened potential and the density fluctuations on the nanotubes. It is demonstrated that one can identify the plasmon excitations from the spectrum of energy transfer. For a group of the excited plasmons, there is an instability associated with their decay. This occurrence is determined by the relationship between the phase velocity of the plasmon and the drift velocity of the current. A ``dip'' in the energy transfer spectrum corresponds to a plasmon mode instability. This is confirmed by solving the plasma dispersion equation in the complex frequency plane.

We calculate the image potential between the walls of a double-walled cylindrical nanotube which is assumed to be infinitely long. In this model, each tubule is infinitesimally thin and is embedded in a background dielectric medium. Interactions between electrons confined to move on the surface of each tubule are taken into account via the random-phase approximation. We demonstrate that the effective potential for an orbiting charge depends on its angular momentum as well as the separation between the tubules. We calculate the plasmon frequencies and their phase velocities as functions of the wave vector parallel to the axes of the tubules. The range for the drift velocity for current-driven plasma instabilities is discussed.

We develop a theory of collective plasma excitations in a pair of parallel nonoverlapping cylindrical nanotubes. A general dispersion equation is obtained as a function of the angular momentum and linear momentum transfer. We use our results to calculate the phase velocity of the plasmon excitations when current flows parallel to the axis of the nanotube in either the same or neighboring nanotube as the plasma excitations. We demonstrate that the less energetic undamped plasmon excitations may have a phase velocity which is smaller than the Fermi velocity. The plasmons with the largest frequencies have phase velocities which decrease with increasing wave vector. We discuss the instabilities which may arise when the drift velocity of the electrons lies within a range which is determined by the phase velocity for the plasmon modes of the pair of cylindrical nanotubes.