277 research outputs found
Ferromagnetism of an all-carbon composite composed of a carbon nanowire nside a single-walled carbon nanotube
Using the first-principles spin density functional approach, we have studied
magnetism of a new type of all-carbon nanomaterials, i.e., the carbon nanowires
inserted into the single-walled carbon nanotubes. It is found that if the 1D
carbon nanowire density is not too higher, the ferromagnetic ground state will
be more stable than the antiferromagnetic one, which is caused by weak coupling
between the 1D carbon nanowire and the single-walled carbon nanotube. Also,
both dimerization of the carbon nanowire and carbon vacancy on the tube-wall
are found to enhance the magnetic moment of the composite.Comment: 3 figure
Structural transformations of double-walled carbon nanotube bundle under hydrostatic pressure
Three kinds of the response mechanisms to the external pressure have been
found for double-walled carbon nanotube (DWNT) bundle, depending strongly on
their average radius and symmetry. The small-diameter DWNT bundle undergoes a
small discontinuous volume change, and then deform continuously. The
intermediate-diameter DWNT bundle collapses completely after a structure phase
transition (SPT). Significantly, two SPTs exist for the larger-diameter DWNT
bundle if the outer tube has no or symmetry. It would be
interesting to search for signatures of these different structural
transformations by experimentally investigating mechanical, optical and thermal
response functions of DWNT bundle.Comment: 9 pages, 2 figure
Electronic Structure and Linear Optical Properties of SrCuOCl Studied from the First Principles Calculation
First-principles calculations with the full-potential linearized augmented
plane-wave (FP-LAPW) method have been performed to investigate detailed
electronic and linear optical properties of SrCuOCl, which is
a classical low-dimensional antiferromagnet (AFM) charge transfer ({\it CT})
insulator. Within the local-spin-density approximation (LSDA) plus the on-site
Coulomb interaction (LADA+) added on Cu 3d orbitals, our calculated band
gap and spin moments are well consistent with the experimental and other
theoretical values. The energy dispersion relation agrees well with the angle
resolved photoemission measurements. Its linear optical properties are
calculated within the electric-dipole approximation. The absorption spectrum is
found to agree well with the experimental result.Comment: 5 pages, 5 figure
Optical properties of 4 A single-walled carbon nanotubes inside the zeolite channels studied from first principles calculations
The structural, electronic, and optical properties of 4 A single-walled
carbon nanotubes (SWNTs) contained inside the zeolite channels have been
studied based upon the density-functional theory in the local-density
approximation (LDA). Our calculated results indicate that the relaxed
geometrical structures for the smallest SWNTs in the zeolite channels are much
different from those of the ideal isolated SWNTs, producing a great effect on
their physical properties. It is found that all three kinds of 4 A SWNTs can
possibly exist inside the Zeolite channels. Especially, as an example, we have
also studied the coupling effect between the ALPO_4-5 zeolite and the tube
(5,0) inside it, and found that the zeolite has real effects on the electronic
structure and optical properties of the inside (5,0) tube.Comment: 9 pages, 6figure
Document expansion for image retrieval
Successful information retrieval requires e�ective matching
between the user's search request and the contents of relevant
documents. Often the request entered by a user may
not use the same topic relevant terms as the authors' of the
documents. One potential approach to address problems
of query-document term mismatch is document expansion
to include additional topically relevant indexing terms in a
document which may encourage its retrieval when relevant
to queries which do not match its original contents well. We
propose and evaluate a new document expansion method
using external resources. While results of previous research
have been inconclusive in determining the impact of document
expansion on retrieval e�ectiveness, our method is
shown to work e�ectively for text-based image retrieval of
short image annotation documents. Our approach uses the
Okapi query expansion algorithm as a method for document
expansion. We further show improved performance can be
achieved by using a \document reduction" approach to include
only the signi�cant terms in a document in the expansion
process. Our experiments on the WikipediaMM task at
ImageCLEF 2008 show an increase of 16.5% in mean average
precision (MAP) compared to a variation of Okapi BM25 retrieval
model. To compare document expansion with query
expansion, we also test query expansion from an external resource
which leads an improvement by 9.84% in MAP over
our baseline. Our conclusion is that the document expansion
with document reduction and in combination with query expansion
produces the overall best retrieval results for shortlength
document retrieval. For this image retrieval task, we
also concluded that query expansion from external resource
does not outperform the document expansion method
Radial-breathing-like phonon modes of double-walled carbon nanotubes
The radial-breathing-like phonon modes (RBLMs) of the double-walled carbon
nanotubes are studied in a simple analytical model, in which the interaction
force constants (FCs) can be obtained analytically from the continuous model.
The RBLMs frequencies are obtained by solving the dynamical matrix, and their
relationship with the tube radii can be obtained analytically, offering a
powerful experimental tool for determining precisely the radii of the
multi-walled carbon nanotubes
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