45,597 research outputs found
Normal modes of carbon nanotubes: similarities and differences with their continuum counterpart
Carbon nanotubes (CNTs) possess a range of unusually interesting and useful physicochemical properties. In this paper, the mechanical properties of single wall CNTs are investigated via free vibration normal modes using molecular mechanics models. The forcefield used is empirical and the usual assumptions of potential energy contributions coming from bondstretching, bond angle bending, and bond twisting for two, three, and four atom interactions respectively, are made. The validity of continuum behaviour is examined by comparing the modal spacing obtained from the molecular mechanics models and that obtained from classical continuum elastodynamics. The breakdown of continuum behaviour is systematically characterised for various combinations of length to diameter ratio as well as for the number of atoms per circumference
Investigations in space-related molecular biology
Improved instrumentation and preparation techniques for high resolution, high voltage cryo-electron microscopic and diffraction studies on terrestrial and extraterrestrial specimens are reported. Computer correlated ultrastructural and biochemical work on hydrated and dried cell membranes and related biological systems provided information on membrane organization, ice crystal formation and ordered water, RNA virus linked to cancer, lunar rock samples, and organometallic superconducting compounds. Apollo 11, 12, 14, and 15 specimens were analyze
Mission: Impossible (Escape from the Lyman Limit)
We investigate the intrinsic opacity of high-redshift galaxies to outgoing
ionising photons using high-quality photometry of a sample of 27
spectroscopically-identified galaxies of redshift 1.9<z<3.5 in the Hubble Deep
Field. Our measurement is based on maximum-likelihood fitting of model galaxy
spectral energy distributions-including the effects of intrinsic Lyman-limit
absorption and random realizations of intervening Lyman-series and Lyman-limit
absorption-to photometry of galaxies from space- and ground-based broad-band
images. Our method provides several important advantages over the methods used
by previous groups, including most importantly that two-dimensional sky
subtraction of faint-galaxy images is more robust than one-dimensional sky
subtraction of faint-galaxy spectra. We find at the 3sigma statistical
confidence level that on average no more than 4% of the ionising photons escape
galaxies of redshift 1.9<z<3.5. This result is consistent with observations of
low- and moderate-redshift galaxies but is in direct contradiction to a recent
result based on medium-resolution spectroscopy of high-redshift (z~3) galaxies.
Dividing our sample in subsamples according to luminosity, intrinsic
ultraviolet colour, and redshift, we find no evidence for selection effects
that could explain such discrepancy. Even when all systematic effects are
included, the data could not realistically accomodate any escape fraction value
larger than ~15%.Comment: Accepted for publication in Monthly Notices of the Royal Astronomical
Society. 8 pages, 4 b/w figures, MNRAS styl
Antiferromagnetic O(N) models in four dimensions
We study the antiferromagnetic O(N) model in the F_4 lattice. Monte Carlo
simulations are applied for investigating the behavior of the transition for
N=2,3. The numerical results show a first order nature but with a large
correlation length. The limit is also considered with analytical
methods.Comment: 14 pages, 3 postscript figure
Critical behavior of long straight rigid rods on two-dimensional lattices: Theory and Monte Carlo simulations
The critical behavior of long straight rigid rods of length (-mers) on
square and triangular lattices at intermediate density has been studied. A
nematic phase, characterized by a big domain of parallel -mers, was found.
This ordered phase is separated from the isotropic state by a continuous
transition occurring at a intermediate density . Two analytical
techniques were combined with Monte Carlo simulations to predict the dependence
of on , being . The first involves
simple geometrical arguments, while the second is based on entropy
considerations. Our analysis allowed us also to determine the minimum value of
(), which allows the formation of a nematic phase on a
triangular lattice.Comment: 23 pages, 5 figures, to appear in The Journal of Chemical Physic
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