9,335 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.
Tensor network states and algorithms in the presence of a global SU(2) symmetry
The benefits of exploiting the presence of symmetries in tensor network
algorithms have been extensively demonstrated in the context of matrix product
states (MPSs). These include the ability to select a specific symmetry sector
(e.g. with a given particle number or spin), to ensure the exact preservation
of total charge, and to significantly reduce computational costs. Compared to
the case of a generic tensor network, the practical implementation of
symmetries in the MPS is simplified by the fact that tensors only have three
indices (they are trivalent, just as the Clebsch-Gordan coefficients of the
symmetry group) and are organized as a one-dimensional array of tensors,
without closed loops. Instead, a more complex tensor network, one where tensors
have a larger number of indices and/or a more elaborate network structure,
requires a more general treatment. In two recent papers, namely (i) [Phys. Rev.
A 82, 050301 (2010)] and (ii) [Phys. Rev. B 83, 115125 (2011)], we described
how to incorporate a global internal symmetry into a generic tensor network
algorithm based on decomposing and manipulating tensors that are invariant
under the symmetry. In (i) we considered a generic symmetry group G that is
compact, completely reducible and multiplicity free, acting as a global
internal symmetry. Then in (ii) we described the practical implementation of
Abelian group symmetries. In this paper we describe the implementation of
non-Abelian group symmetries in great detail and for concreteness consider an
SU(2) symmetry. Our formalism can be readily extended to more exotic symmetries
associated with conservation of total fermionic or anyonic charge. As a
practical demonstration, we describe the SU(2)-invariant version of the
multi-scale entanglement renormalization ansatz and apply it to study the low
energy spectrum of a quantum spin chain with a global SU(2) symmetry.Comment: 32 pages, 37 figure
Single-atom control of the optoelectronic response in sub-nanometric cavities
By means of ab-initio time dependent density functional theory calculations
carried out on an prototypical hybrid plasmonic device (two metallic
nanoparticles bridged by a one-atom junction), we demonstrate the strong
interplay between photoinduced excitation of localized surface plasmons and
electron transport through the single atom. Such an interplay is remarkably
sensitive to the atomic orbitals of the junction. Therefore, we show the
possibility of a twofold tuning (plasmonic response and photoinduced current
across the juntion) just by changing a single atom in the device.Comment: 5 pages, 5 figure
Wavelength de-multiplexing properties of a single aperture flanked by periodic arrays of indentations
In this paper we explore the transmission properties of single subwavelength
apertures perforated in thin metallic films flanked by asymmetric
configurations of periodic arrays of indentations. It is shown how the
corrugation in the input side can be used to transmit selectively only two
different wavelengths. Also, by tuning the geometrical parameters defining the
corrugation of the output side, these two chosen wavelengths can emerge from
the structure as two very narrow beams propagating at well-defined directions.
This new ability of structured metals can be used as a base to build
micron-sized wavelength de-multiplexers.Comment: Accepted for publication in Photonics and Nanostructure
Edge and waveguide THz surface plasmon modes in graphene micro-ribbons
Surface plasmon modes supported by graphene ribbon waveguides are studied and
classified. The properties of both modes with the field concentration within
the ribbon area (waveguiding modes) and on the edges (edge modes) are
discussed. The waveguide and edge modes are shown to be separated from each
other by a gap in wavenumbers. The even-parity hybridized edge mode results to
be the fundamental electromagnetic mode of the ribbon, possessing also the
lowest losses. All the plasmonic modes in the ribbons have an optimum
frequency, at which the absorption losses are minimum, due to competition
between the plasmon confinement and the frequency dependence of absorption in
graphene.Comment: 4 pages, 4 figure
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