Abstract:
Multipole electron modes beyond the Mie plasmon in atomic clusters are investigated within the time-dependent local density approximation theory (TDLDA). We consider the origin of the modes, their connection with basic cluster properties and possible routes of experimental observation. Particular attention is paid to infrared magnetic orbital modes, scissors and twist, and electric quadrupole mode. The scissors and twist modes determine orbital magnetism of clusters while the electric quadrupole mode provides direct access to the single electron spectra of the cluster. We examine two-photon processes (Raman scattering, stimulated emission pumping and stimulated adiabatic Raman passage) as the most promising tools for experimental investigation of the modes.

Abstract:
We present here a modified hydrodynamical model for the collective electronic excitations of spherical atomic clusters. The model is applied to the giant-dipole plasmon mode of the free fullerene molecule.

Abstract:
The existence and nature of a new mode of electronic collective excitations (quadrupole plasmons) in confined one-dimensional electronic systems have been predicted by an eigen-equation method. The eigen-equation based on the time-dependent density-functional theory is presented for calculating the collective excitations in confined systems. With this method, all modes of collective excitations in the 1D systems may be found out. These modes include dipole plasmons and quadrupole plasmons. The dipole plasmon mode corresponds to the antisymmetric oscillation of induced charge, and can be shown as a resonance of the dipole response. In the quadrupole plasmon modes, the induced charge distribution is symmetric, and the dipole response vanishes. The motion of the electrons in the quadrupole modes is similar to the vibration of atoms in the breathing mode of phonons. This type of plasmons can be shown as a resonance of the quadrupole response, and has to be excited by al non-uniform field.

Abstract:
A metal film supports the continuum of propagating surface plasmon waves. The interaction of these waves with a dipole (nanoparticle) positioned some distance from the surface of the film can produce well defined localized plasmon modes whose frequency nonetheless resides inside the continuum. This leads to the resonant enhancement of scattering of surface plasmons off the dipole. The maximum of scattering is found to occur when the distance from the dipole to the surface of the film is equal to one half of the film thickness. The possibility of controllable plasmon scattering could be advantageous for the field of nanoplasmonics.

Abstract:
The recent experimental discovery of ${\rm Cd_3 As_2}$ and ${\rm Na_3 Bi}$ Dirac semimetals enables the study of the properties of chiral quasi-particles in three spatial dimensions. As demonstrated by photoemission, Dirac semimetals are characterized by a linear dispersion relation for fermion quasi-particles, and thus represent three dimensional analogs of graphene. While the distinctive behavior of chiral fermions (e.g. Klein tunneling) is already evident in two dimensional graphene, the physics of chirality in three dimensions opens a number of new possibilities. In this paper we investigate the properties of the collective plasmon excitations in Dirac semimetals by using the methods of relativistic field theory. We find a strong and narrow plasmon excitation whose frequency is in the terahertz (THz) range which may be important for practical applications. The properties of the plasmon appear universal for all Dirac semimetals, due to the large degeneracy of the quasi-particles and the small Fermi velocity, $v_F \ll c$. This universality is closely analogous to the phenomenon of "dimensional transmutation", that is responsible for the emergence of dimensionful scales in relativistic field theories such as Quantum Chromodynamics, the modern theory of nuclear physics.

Abstract:
Some properties of small and medium sodium clusters are described within the RPA approach using a projected spherical single particle basis. The oscillator strengths calculated with a Schiff-like dipole transition operator and folded with Lorentzian functions are used to calculate the photoabsorbtion cross section spectra. The results are further employed to establish the dependence of the plasmon frequency on the number of cluster components. Static electric polarizabilities of the clusters excited in a RPA dipole state are also calculated. Comparison of our results with the corresponding experimental data show an overall good agreement.

Abstract:
The role of anharmonic effects on the excitation of the double giant dipole resonance is investigated in a simple macroscopic model.Perturbation theory is used to find energies and wave functions of the anharmonic ascillator.The cross sections for the electromagnetic excitation of the one- and two-phonon giant dipole resonances in energetic heavy-ion collisions are then evaluated through a semiclassical coupled-channel calculation.It is argued that the variations of the strength of the anharmonic potential should be combined with appropriate changes in the oscillator frequency,in order to keep the giant dipole resonance energy consistent with the experimental value.When this is taken into account,the effects of anharmonicities on the double giant dipole resonance excitation probabilities are small and cannot account for the well-known discrepancy between theory and experiment.

Abstract:
We study the possible collective plasma modes which can affect neutron-star thermodynamics and different elementary processes in the baryonic density range between nuclear saturation ($\rho_0$) and $3\rho_0$. In this region, the expected constituents of neutron-star matter are mainly neutrons, protons, electrons and muons ($npe\mu$ matter), under the constraint of beta equilibrium. The elementary plasma excitations of the $pe\mu$ three-fluid medium are studied in the RPA framework. We emphasize the relevance of the Coulomb interaction among the three species, in particular the interplay of the electron and muon screening in suppressing the possible proton plasma mode, which is converted into a sound-like mode. The Coulomb interaction alone is able to produce a variety of excitation branches and the full spectral function shows a rich structure at different energy. The genuine plasmon mode is pushed at high energy and it contains mainly an electron component with a substantial muon component, which increases with density. The plasmon is undamped for not too large momentum and is expected to be hardly affected by the nuclear interaction. All the other branches, which fall below the plasmon, are damped or over-damped.

Abstract:
The collective electronic excitation in planar sodium clusters is studied by time-dependent density functional theory calculations. The formation and development of the resonances in photoabsorption spectra are investigated in terms of the shape and size of the 2-dimensional (2-D) systems. The nature of these resonances is revealed by the frequency-resolved induced charge densities present on a real-space grid. For long double chains, the excitation is similar to that in long single atomic chains, showing longitudinal modes, end and central transverse modes. However, for 2-D planes consisting of ($n \times n$) atoms with $n$ up to 16, new 2-D characteristic modes emerge regardless of the symmetries considered. For a kick parallel to the plane, besides the equivalent end mode, mixed modes with contrary polarity occur, while for an impulse perpendicular to the plane there will be corner, side center, bulk center, and circuit modes. Our calculation reveals the importance of dimensionality for plasmon excitation and how it evolves from 1-D to 2-D.

Abstract:
The plasmon oscillations of a metallic triaxial ellipsoid nanoparticle have been studied within the framework of the quasistatic approximation. A general method has been proposed for finding the analytical expressions describing the potential and frequencies of the plasmon oscillations of an arbitrary multipolarity order. The analytical expressions have been derived for an electric potential and plasmon oscillation frequencies of the first 24 modes. Other higher orders plasmon modes are investigated numerically.