12,239 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.
Observation of magnetism in Au thin films
Direct magnetization measurements of thin gold films are presented. These
measurements integrate the signal from the thin film under study and the
magnetic contribution of the film's interface with the substrate. The
diamagnetic contribution to the signal from the bulk substrate is of the same
order as the noise level. we find that thin gold films can exhibit positive
magnetization. The character of their magnetic behavior is strongly substrate
dependent.Comment: 9 pages, 4 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
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 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
Convergence of generic infinite products of affine operators
We establish several results concerning the asymptotic behavior of random infinite products of generic sequences of affine uniformly continuous operators on bounded closed convex subsets of a Banach space. In addition to weak ergodic theorems we also obtain convergence to a unique common fixed point and more generally, to an affine retraction
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
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