11,103 research outputs found
Chirality distribution and transition energies of carbon nanotubes
From resonant Raman scattering on isolated nanotubes we obtained the optical
transition energies, the radial breathing mode frequency and Raman intensity of
both metallic and semiconducting tubes. We unambiguously assigned the chiral
index (n_1,n_2) of approximately 50 nanotubes based solely on a third-neighbor
tight-binding Kataura plot and find omega_RBM=214.4cm^-1nm/d+18.7cm^-1. In
contrast to luminescence experiments we observe all chiralities including
zig-zag tubes. The Raman intensities have a systematic chiral-angle dependence
confirming recent ab-initio calculations.Comment: 4 pages, to be published in Phys. Rev. Let
The phonon dispersion of graphite by inelastic x-ray scattering
We present the full in-plane phonon dispersion of graphite obtained from
inelastic x-ray scattering, including the optical and acoustic branches, as
well as the mid-frequency range between the and points in the Brillouin
zone, where experimental data have been unavailable so far. The existence of a
Kohn anomaly at the point is further supported. We fit a fifth-nearest
neighbour force-constants model to the experimental data, making improved
force-constants calculations of the phonon dispersion in both graphite and
carbon nanotubes available.Comment: 7 pages; submitted to Phys. Rev.
The strength of the radial-breathing mode in single-walled carbon nanotubes
We show by ab initio calculations that the electron-phonon coupling matrix
element M of the radial breathing mode in single-walled carbon nanotubes
depends strongly on tube chirality. For nanotubes of the same diameter the
coupling strength |M|^2 is up to one order of magnitude stronger for zig-zag
than for armchair tubes. For (n,m) tubes M depends on the value of (n-m) mod 3,
which allows to discriminate semiconducting nano tubes with similar diameter by
their Raman scattering intensity. We show measured resonance Raman profiles of
the radial breathing mode which support our theoretical predictions
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
Tuning the electron-phonon coupling in multilayer graphene with magnetic fields
Magneto Raman scattering study of the E optical phonons in multi-layer
epitaxial graphene grown on a carbon face of SiC are presented. At 4.2K in
magnetic field up to 33 T, we observe a series of well pronounced avoided
crossings each time the optically active inter Landau level transition is tuned
in resonance with the E phonon excitation (at 196 meV). The width of the
phonon Raman scattering response also shows pronounced variations and is
enhanced in conditions of resonance. The experimental results are well
reproduced by a model that gives directly the strength of the electron-phonon
interaction.Comment: 4 pages, 3 figure
Phase diagram and spin Hamiltonian of weakly-coupled anisotropic S=1/2 chains in CuCl2*2((CD3)2SO)
Field-dependent specific heat and neutron scattering measurements were used
to explore the antiferromagnetic S=1/2 chain compound CuCl2 * 2((CD3)2SO). At
zero field the system acquires magnetic long-range order below TN=0.93K with an
ordered moment of 0.44muB. An external field along the b-axis strengthens the
zero-field magnetic order, while fields along the a- and c-axes lead to a
collapse of the exchange stabilized order at mu0 Hc=6T and mu0 Hc=3.5T,
respectively (for T=0.65K) and the formation of an energy gap in the excitation
spectrum. We relate the field-induced gap to the presence of a staggered
g-tensor and Dzyaloshinskii-Moriya interactions, which lead to effective
staggered fields for magnetic fields applied along the a- and c-axes.
Competition between anisotropy, inter-chain interactions and staggered fields
leads to a succession of three phases as a function of field applied along the
c-axis. For fields greater than mu0 Hc, we find a magnetic structure that
reflects the symmetry of the staggered fields. The critical exponent, beta, of
the temperature driven phase transitions are indistinguishable from those of
the three-dimensional Heisenberg magnet, while measurements for transitions
driven by quantum fluctuations produce larger values of beta.Comment: revtex 12 pages, 11 figure
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
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
Gamma-ray Flares and VLBI Outbursts of Blazars
A model is developed for the time dependent electromagnetic - radio to
gamma-ray - emission of active galactic nuclei, specifically, the blazars,
based on the acceleration and creation of leptons at a propagating
discontinuity or {\it front} of a Poynting flux jet. The front corresponds to a
discrete relativistic jet component as observed with
very-long-baseline-interferometry (VLBI). Equations are derived for the number,
momentum, and energy of particles in the front taking into account synchrotron,
synchrotron-self-Compton (SSC), and inverse-Compton processes as well as
photon-photon pair production. The apparent synchrotron, SSC, and
inverse-Compton luminosities as functions of time are determined. Predictions
of the model are compared with observations in the gamma, optical and radio
bands. The delay between the high-energy gamma-ray flare and the onset of the
radio is explained by self-absorption and/or free-free absorption by external
plasma. Two types of gamma-ray flares are predicted depending on pair creation
in the front.Comment: 11 pages, submitted to ApJ. 10 figures can be obtained from R.
Lovelace by sending postal address to [email protected]
- …