558 research outputs found
Nonlinear dispersion of stationary waves in collisionless plasmas
A nonlinear dispersion of a general stationary wave in collisionless plasma
is obtained in a non-differential form from a single-particle
oscillation-center Hamiltonian. For electrostatic oscillations in nonmagnetized
plasma, considered as a paradigmatic example, the linear dielectric function is
generalized, and the trapped particle contribution to the wave frequency shift
is found analytically as a function of the wave amplitude .
Smooth distributions yield , as usual. However,
beam-like distributions of trapped electrons result in different power laws, or
even a logarithmic nonlinearity, which are derived as asymptotic limits of the
same dispersion relation
Unoccupied Band Structure of NbSe2 by Very-Low-Energy Electron Diffraction: Experiment and Theory
A combined experimental and theoretical study of very-low-energy electron
diffraction at the (0001) surface of 2H-NbSe2 is presented. Electron
transmission spectra have been measured for energies up to 50 eV above the
Fermi level with k|| varying along the GammaK line of the Brillouin zone. Ab
initio calculations of the spectra have been performed with the extended linear
augmented plane wave k-p method. The experimental spectra are interpreted in
terms of three-dimensional one-electron band structure. Special attention is
paid to the quasi-particle lifetimes: by comparing the broadening of the
spectral structures in the experimental and calculated spectra the energy
dependence of the optical potential Vi is determined. A sharp increase of Vi at
20 eV is detected, which is associated with a plasmon peak in the
Im(-1/epsilon) function. Furthermore, the electron energy loss spectrum and the
reflectivity of NbSe2 are calculated ab initio and compared with optical
experiments. The obtained information on the dispersions and lifetimes of the
unoccupied states is important for photoemission studies of the 3D band
structure of the valence band.Comment: 17 pages, 11 Postscript figures, submitted to Phys. Rev.
Adiabatic nonlinear waves with trapped particles: II. Wave dispersion
A general nonlinear dispersion relation is derived in a nondifferential form
for an adiabatic sinusoidal Langmuir wave in collisionless plasma, allowing for
an arbitrary distribution of trapped electrons. The linear dielectric function
is generalized, and the nonlinear kinetic frequency shift is
found analytically as a function of the wave amplitude . Smooth
distributions yield , as usual. However,
beam-like distributions of trapped electrons result in different power laws, or
even a logarithmic nonlinearity, which are derived as asymptotic limits of the
same dispersion relation. Such beams are formed whenever the phase velocity
changes, because the trapped distribution is in autoresonance and thus evolves
differently from the passing distribution. Hence, even adiabatic is generally nonlocal.Comment: submitted together with Papers I and II
Band Mapping in One-Step Photoemission Theory: Multi-Bloch-Wave Structure of Final States and Interference Effects
A novel Bloch-waves based one-step theory of photoemission is developed
within the augmented plane wave formalism. Implications of multi-Bloch-wave
structure of photoelectron final states for band mapping are established.
Interference between Bloch components of initial and final states leads to
prominent spectral features with characteristic frequency dispersion
experimentally observed in VSe_2 and TiTe_2. Interference effects together with
a non-free-electron nature of final states strongly limit the applicability of
the common direct transitions band mapping approach, making the tool of
one-step analysis indispensable.Comment: 4 jpg figure
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