245 research outputs found
Optical absorption and energy-loss spectra of aligned carbon nanotubes
Optical-absorption cross-sections and energy-loss spectra of aligned
multishell carbon nanotubes are investigated, on the basis of photonic
band-structure calculations. A local graphite-like dielectric tensor is
assigned to every point of the tubules, and the effective transverse dielectric
function of the composite is computed by solving Maxwell's equations in media
with tensor-like dielectric functions. A Maxwell-Garnett-like approach
appropriate to the case of infinitely long anisotropic tubules is also
developed. Our full calculations indicate that the experimentally measured
macroscopic dielectric function of carbon nanotube materials is the result of a
strong electromagnetic coupling between the tubes. An analysis of the
electric-field pattern associated with this coupling is presented, showing that
in the close-packed regime the incident radiation excites a very localized
tangential surface plasmon.Comment: 7 pages, 12 figures, to appear in Eur. Phys. J.
Benchmark Quantum Monte Carlo calculations of the ground-state kinetic, interaction, and total energy of the three-dimensional electron gas
We report variational and diffusion Quantum Monte Carlo ground-state energies
of the three-dimensional electron gas using a model periodic Coulomb
interaction and backflow corrections for N=54, 102, 178, and 226 electrons. We
remove finite-size effects by extrapolation and we find lower energies than
previously reported. Using the Hellman-Feynman operator sampling method
introduced in Phys. Rev. Lett. 99, 126406 (2007), we compute accurately, within
the fixed-node pproximation, the separate kinetic and interaction contributions
to the total ground-state energy. The difference between the interaction
energies obtained from the original Slater-determinant nodes and the
backflow-displaced nodes is found to be considerably larger than the difference
between the corresponding kinetic energies
Ab initio calculations of the dynamical response of copper
The role of localized -bands in the dynamical response of Cu is
investigated, on the basis of {\em ab initio} pseudopotential calculations. The
density-response function is evaluated in both the random-phase approximation
(RPA) and a time-dependent local-density functional approximation (TDLDA). Our
results indicate that in addition to providing a polarizable background which
lowers the free-electron plasma frequency, d-electrons are responsible, at
higher energies and small momenta, for a double-peak structure in the dynamical
structure factor. These results are in agreement with the experimentally
determined optical response of copper. We also analyze the dependence of
dynamical scattering cross sections on the momentum transfer.Comment: 4 pages, 4 figures, to appear in Phys. Rev.
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