13,392 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
Structure and formation energy of carbon nanotube caps
We present a detailed study of the geometry, structure and energetics of
carbon nanotube caps. We show that the structure of a cap uniquely determines
the chirality of the nanotube that can be attached to it. The structure of the
cap is specified in a geometrical way by defining the position of six pentagons
on a hexagonal lattice. Moving one (or more) pentagons systematically creates
caps for other nanotube chiralities. For the example of the (10,0) tube we
study the formation energy of different nanotube caps using ab-initio
calculations. The caps with isolated pentagons have an average formation energy
0.29+/-0.01eV/atom. A pair of adjacent pentagons requires a much larger
formation energy of 1.5eV. We show that the formation energy of adjacent
pentagon pairs explains the diameter distribution in small-diameter nanotube
samples grown by chemical vapor deposition.Comment: 8 pages, 8 figures (gray scale only due to space); 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
The existence of a 2Po excited state for the e+Ca system
The Configuration Interaction method is used to demonstrate that there is an
electronically stable state of positronic calcium with an orbital angular
momentum of L=1. This prediction relies on the use of an asymptotic series to
estimate the variational limit of the energy. The best estimate of the binding
energy is 37 meV. A discussion of the structure of the system is also
presented.Comment: 4 pages, 2 figures, in press PR
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.
Fluctuating Elastic Rings: Statics and Dynamics
We study the effects of thermal fluctuations on elastic rings. Analytical
expressions are derived for correlation functions of Euler angles, mean square
distance between points on the ring contour, radius of gyration, and
probability distribution of writhe fluctuations. Since fluctuation amplitudes
diverge in the limit of vanishing twist rigidity, twist elasticity is essential
for the description of fluctuating rings. We find a crossover from a small
scale regime in which the filament behaves as a straight rod, to a large scale
regime in which spontaneous curvature is important and twist rigidity affects
the spatial configurations of the ring. The fluctuation-dissipation relation
between correlation functions of Euler angles and response functions, is used
to study the deformation of the ring by external forces. The effects of inertia
and dissipation on the relaxation of temporal correlations of writhe
fluctuations, are analyzed using Langevin dynamics.Comment: 43 pages, 9 Figure
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