10,765 research outputs found
Theory of double-resonant Raman spectra in graphene: intensity and line shape of defect-induced and two-phonon bands
We calculate the double resonant (DR) Raman spectrum of graphene, and
determine the lines associated to both phonon-defect processes, and two-phonons
ones. Phonon and electronic dispersions reproduce calculations based on density
functional theory corrected with GW. Electron-light, -phonon, and -defect
scattering matrix elements and the electronic linewidth are explicitly
calculated. Defect-induced processes are simulated by considering different
kind of idealized defects. For an excitation energy of eV, the
agreement with measurements is very good and calculations reproduce: the
relative intensities among phonon-defect or among two-phonon lines; the
measured small widths of the D, , 2D and lines; the line shapes; the
presence of small intensity lines in the 1800, 2000 cm range. We
determine how the spectra depend on the excitation energy, on the light
polarization, on the electronic linewidth, on the kind of defects and on their
concentration. According to the present findings, the intensity ratio between
the and 2D lines can be used to determine experimentally the electronic
linewidth. The intensity ratio between the and lines depends on the
kind of model defect, suggesting that this ratio could possibly be used to
identify the kind of defects present in actual samples. Charged impurities
outside the graphene plane provide an almost undetectable contribution to the
Raman signal
Fine Structure of the Radial Breathing Mode in Double-Wall Carbon Nanotubes
The analysis of the Raman scattering cross section of the radial breathing
modes of double-wall carbon nanotubes allowed to determine the optical
transitions of the inner tubes. The Raman lines are found to cluster into
species with similar resonance behavior. The lowest components of the clusters
correspond well to SDS wrapped HiPco tubes. Each cluster represents one
particular inner tube inside different outer tubes and each member of the
clusters represents one well defined pair of inner and outer tubes. The number
of components in one cluster increases with decreasing of the inner tube
diameter and can be as high as 14.Comment: 5 pages, 3 figure
Large orbital magnetic moments in carbon nanotubes generated by resonant transport
The nonequilibrium Green's function method is used to study the ballistic
transport in metallic carbon nanotubes when a current is injected from the
electrodes with finite bias voltages. We reveal, both analytically and
numerically, that large loop currents circulating around the tube are induced,
which come from a quantum mechanical interference and are much larger than the
current along the tube axis when the injected electron is resonant with a
time-reversed pair of degenerate states, which are, in fact, inherent in the
zigzag and chiral nanotubes. This results in large orbital magnetic moments,
making the nanotube a molecular solenoid.Comment: 5 pages, 4 figures; typos correcte
Exciton binding energies in carbon nanotubes from two-photon photoluminescence
One- and two-photon luminescence excitation spectroscopy showed a series of
distinct excitonic states in single-walled carbon nanotubes. The energy
splitting between one- and two-photon-active exciton states of different
wavefunction symmetry is the fingerprint of excitonic interactions in carbon
nanotubes. We determine exciton binding energies of 0.3-0.4 eV for different
nanotubes with diameters between 0.7 and 0.9 nm. Our results, which are
supported by ab-initio calculations of the linear and non-linear optical
spectra, prove that the elementary optical excitations of carbon nanotubes are
strongly Coulomb-correlated, quasi-one dimensionally confined electron-hole
pairs, stable even at room temperature. This alters our microscopic
understanding of both the electronic structure and the Coulomb interactions in
carbon nanotubes, and has direct impact on the optical and transport properties
of novel nanotube devices.Comment: 5 pages, 4 figure
HI ``Tails'' from Cometary Globules in IC1396
IC 1396 is a relatively nearby (750 pc), large (>2 deg), HII region ionized
by a single O6.5V star and containing bright-rimmed cometary globules. We have
made the first arcmin resolution images of atomic hydrogen toward IC 1396, and
have found remarkable ``tail''-like structures associated with some of the
globules and extending up to 6.5 pc radially away from the central ionizing
star. These HI ``tails'' may be material which has been ablated from the
globule through ionization and/or photodissociation and then accelerated away
from the globule by the stellar wind, but which has since drifted into the
``shadow'' of the globules.
This report presents the first results of the Galactic Plane Survey Project
recently begun by the Dominion Radio Astrophysical Observatory.Comment: 11 pages, 5 postscript figures, uses aaspp4.sty macros, submitted in
uuencoded gzipped tar format, accepted for publication in Astrophysical
Journal Letters, colour figures available at
http://www.drao.nrc.ca/~schieven/news_sep95/ic1396.htm
G55.0+0.3: A Highly Evolved Supernova Remnant
Multi-frequency analysis has revealed the presence of a new supernova
remnant, G55.0+0.3, in the Galactic plane. A kinematic distance of 14 kpc has
been measured from HI spectral line data. The faint, clumpy half-shell is
non-thermal and has a physical radius of 70 pc. Using an evolutionary model,
the age of the remnant is estimated to be on the order of one million years,
which exceeds conventional limits by a factor of five. The remnant may be
associated with the nearby pulsar J1932+2020, which has a spin-down age of 1.1
million years. This work implies that the radiative lifetimes of remnants could
be much longer than previously suggested.Comment: 27 pages, 7 figures in 9 files (figures 1 and 2 require 2 files
each), Accepted for publication in The Astrophysical Journal (Jan. 20, 1998
volume
Radio and gamma-ray constraints on dark matter annihilation in the Galactic center
We determine upper limits on the dark matter (DM) self-annihilation cross
section for scenarios in which annihilation leads to the production of
electron--positron pairs. In the Galactic centre (GC), relativistic electrons
and positrons produce a radio flux via synchroton emission, and a gamma ray
flux via bremsstrahlung and inverse Compton scattering. On the basis of
archival, interferometric and single-dish radio data, we have determined the
radio spectrum of an elliptical region around the Galactic centre of extent 3
degrees semi-major axis (along the Galactic plane) and 1 degree semi-minor axis
and a second, rectangular region, also centered on the GC, of extent 1.6
degrees x 0.6 degrees. The radio spectra of both regions are non-thermal over
the range of frequencies for which we have data: 74 MHz -- 10 GHz. We also
consider gamma-ray data covering the same region from the EGRET instrument
(about GeV) and from HESS (around TeV). We show how the combination of these
data can be used to place robust constraints on DM annihilation scenarios, in a
way which is relatively insensitive to assumptions about the magnetic field
amplitude in this region. Our results are approximately an order of magnitude
more constraining than existing Galactic centre radio and gamma ray limits. For
a DM mass of m_\chi =10 GeV, and an NFW profile, we find that the
velocity-averaged cross-section must be less than a few times 10^-25 cm^3 s^-1.Comment: 14 pages, 9 figures. Version accepted for publication in PRD.
Reference section updated/extended
Spectrum of Electrons in Graphene as an Alternant Macromolecule and Its Specific Features in Quantum Conductance
An exact description of electrons based on the tight-binding model of
graphene as an alternant, plane macromolecule is presented. The model molecule
can contain an arbitrary number of benzene rings and has armchair- and
zigzag-shaped edges. This suggests an instructive alternative to the most
commonly used approach, where the reference is made to the honeycomb lattice
periodic in its A and B sublattices. Several advantages of the macromolecule
model are demonstrated. The newly derived analytical relations detail our
understanding of electron nature in achiral graphene ribbons and carbon
tubes and classify these structures as quantum wires.Comment: 13 pages 8 figures, revised in line with referee's comment
Theory of rigid-plane phonon modes in layered crystals
The lattice dynamics of low-frequency rigid-plane modes in metallic (graphene
multilayers, GML) and in insulating (hexagonal boron-nitride multilayers, BNML)
layered crystals is investigated. The frequencies of shearing and compression
(stretching) modes depend on the layer number {\EuScript N} and are presented
in the form of fan diagrams. The results for GML and BNML are very similar. In
both cases only the interactions (van der Waals and Coulomb) between
nearest-neighbor planes are effective, while the interactions between more
distant planes are screened. A comparison with recent Raman scattering results
on low-frequency shear modes in GML [Tan {\it et al.}, arXiv:1106.1146v1
(2011)] is made. Relations with the low-lying rigid-plane phonon dispersions in
the bulk materials are established. Master curves which connect the fan diagram
frequencies for any given {\EuScript N} are derived. Static and dynamic
thermal correlation functions for rigid-layer shear and compression modes are
calculated. The results might be of use for the interpretation of friction
force experiments on multilayer crystals
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