4,831 research outputs found
Hexagonal spiral growth in the absence of a substrate
Experiments on the formation of spiraling hexagons (350 - 1000 nm in width)
from a solution of nanoparticles are presented. Transmission electron
microscopy images of the reaction products of chemically synthesized cadmium
nanocrystals indicate that the birth of the hexagons proceeds without
assistance from static screw or edge dislocatons, that is, they spiral without
constraints provided by an underlying substrate. Instead, the apparent growth
mechanism relies on what we believe is a dynamical dislocation identified as a
dense aggregate of small nanocrystals that straddles the spiraling hexagon at
the crystal surface. This nanocrystal bundle, which we term the "feeder", also
appears to release nanocrystals into the spiral during the growth process.Comment: 4 pages, 5 figure
Solvable Examples of Drift and Diffusion of Ions in Non-uniform Electric Fields
The drift and diffusion of a cloud of ions in a fluid are distorted by an
inhomogeneous electric field. If the electric field carries the center of the
distribution in a straight line and the field configuration is suitably
symmetric, the distortion can be calculated analytically. We examine the
specific examples of fields with cylindrical and spherical symmetry in detail
assuming the ion distributions to be of a generally Gaussian form. The effects
of differing diffusion coefficients in the transverse and longitudinal
directions are included
Azimuthal Correlation in Lepton-Hadron Scattering via Charged Weak-Current Processes
We consider the azimuthal correlation of the final-state particles in charged
weak-current processes. This correlation provides a test of perturbative
quantum chromodynamics. The azimuthal asymmetry is large in the semi-inclusive
processes in which we identify a final-state hadron, say, a charged pion
compared to that in the inclusive processes in which we do not identify
final-state particles and use only the calorimetric information. In
semi-inclusive processes the azimuthal asymmetry is more conspicuous when the
incident lepton is an antineutrino or a positron than when the incident lepton
is a neutrino or an electron. We analyze all the possible charged weak-current
processes and study the quantitative aspects of each process. We also compare
this result to the ep scattering with a photon exchange.Comment: 25 pages, 2 Postscript figures, uses RevTeX, fixes.st
Flavor changing interactions mediated by scalars at the weak scale
The quark and lepton mass matrices possess approximate flavor symmetries.
Several results follow if the interactions of new scalars possess these
approximate symmetries. Present experimental bounds allow these exotic scalars
to have a weak scale mass. The Glashow-Weinberg criterion is rendered
unnecessary. Finally, rare leptonic B meson decays provide powerful probes of
these scalars, especially if they are leptoquarks.Comment: 13 pages, report LBL-3234
Magnetocaloric Studies of the Peak Effect in Nb
We report a magnetocaloric study of the peak effect and Bragg glass
transition in a Nb single crystal. The thermomagnetic effects due to vortex
flow into and out of the sample are measured. The magnetocaloric signature of
the peak effect anomaly is identified. It is found that the peak effect
disappears in magnetocaloric measurements at fields significantly higher than
those reported in previous ac-susceptometry measurements. Investigation of the
superconducting to normal transition reveals that the disappearance of the bulk
peak effect is related to inhomogeneity broadening of the superconducting
transition. The emerging picture also explains the concurrent disappearance of
the peak effect and surface superconductivity, which was reported previously in
the sample under investigation. Based on our findings we discuss the
possibilities of multicriticality associated with the disappearance of the peak
effect.Comment: 30 pages, 10 figure
Planetary Nebula Abundances and Morphology: Probing the Chemical Evolution of the Milky Way
This paper presents a homogeneous study of abundances in a sample of 79
northern galactic planetary nebulae whose morphological classes have been
uniformly determined. Ionic abundances and plasma diagnostics were derived from
selected optical line strengths in the literature, and elemental abundances
were estimated with the Ionization Correction Factor developed by Kingsbourgh &
Barlow (1994). We compare the elemental abundances to the final yields obtained
from stellar evolution models of low-and intermediate-mass stars, and we
confirm that most Bipolar planetary nebulae have high nitrogen and helium
abundance, and are the likely progeny of stars with main-sequence mass larger
than 3 solar masses. We derive =0.27, and discuss the implication of such
a high ratio in connection with the solar neon abundance. We determine the
galactic gradients of oxygen and neon, and found Delta log (O/H)/Delta R=-0.01
dex/kpc$ and Delta log (Ne/H)/Delta R=-0.01 dex/kpc. These flat PN gradients do
not reconcile with galactic metallicity gradients flattening with time.Comment: The Astrophysical Journal, in pres
The physical parameters, excitation and chemistry of the rim, jets and knots of the planetary nebula NGC 7009
We present long-slit optical spectra along the major axis of the planetary
nebula NGC 7009. These data allow us to discuss the physical, excitation and
chemical properties of all the morphological components of the nebula,
including its remarkable systems of knots and jets. The main results of this
analysis are the following: i) the electron temperature throughout the nebula
is remarkably constant, T_e[OIII] = 10200K; ii) the bright inner rim and inner
pair of knots have similar densities of N_e = 6000cm^{-3}, whereas a much lower
density of N_e = 1500cm^{-3} is derived for the outer knots as well as for the
jets; iii) all the regions (rim, inner knots, jets and outer knots) are mainly
radiatively excited; and iv) there are no clear abundance changes across the
nebula for He, O, Ne, or S. There is a marginal evidence for an overabundance
of nitrogen in the outer knots (ansae), but the inner ones (caps) and the rim
have similar N/H values that are at variance with previous results. Our data
are compared to the predictions of theoretical models, from which we conclude
that the knots at the head of the jets are not matter accumulated during the
jet expansion through the circumstellar medium, neither can their origin be
explained by the proposed HD or MHD interacting-wind models for the formation
of jets/ansae, since the densities as well as the main excitation mechanisms of
the knots, disagree with model predictions.Comment: Figure 1 was changed because features were misidentified in the
previous version. 17 pages including 5 figures and 3 tables. ApJ in press.
Also available at http://www.iac.es/galeria/denise
Phase Segregation Dynamics in Particle Systems with Long Range Interactions I: Macroscopic Limits
We present and discuss the derivation of a nonlinear non-local
integro-differential equation for the macroscopic time evolution of the
conserved order parameter of a binary alloy undergoing phase segregation. Our
model is a d-dimensional lattice gas evolving via Kawasaki exchange dynamics,
i.e. a (Poisson) nearest-neighbor exchange process, reversible with respect to
the Gibbs measure for a Hamiltonian which includes both short range (local) and
long range (nonlocal) interactions. A rigorous derivation is presented in the
case in which there is no local interaction. In a subsequent paper (part II),
we discuss the phase segregation phenomena in the model. In particular we argue
that the phase boundary evolutions, arising as sharp interface limits of the
family of equations derived in this paper, are the same as the ones obtained
from the corresponding limits for the Cahn-Hilliard equation.Comment: amstex with macros (included in the file), tex twice, 20 page
Higgs signals and hard photons at the Next Linear Collider: the -fusion channel in the Standard Model
In this paper, we extend the analyses carried out in a previous article for
-fusion to the case of Higgs production via -fusion within the Standard
Model at the Next Linear Collider, in presence of electromagnetic radiation due
real photon emission. Calculations are carried out at tree-level and rates of
the leading order (LO) processes e^+e^-\rightarrow e^+e^- H \ar e^+e^- b\bar b
and e^+e^-\rightarrow e^+e^- H \ar e^+e^- WW \ar e^+e^- \mathrm{jjjj} are
compared to those of the next-to-leading order (NLO) reactions
e^+e^-\rightarrow e^+e^- H (\gamma)\ar e^+e^- b\bar b \gamma and
e^+e^-\rightarrow e^+e^- H (\gamma)\ar e^+e^- WW (\gamma) \ar e^+e^-
\mathrm{jjjj}\gamma, in the case of energetic and isolated photons.Comment: 12 pages, LaTeX, 5 PostScript figures embedded using epsfig and
bitmapped at 100dpi, complete paper including high definition figures
available at ftp://axpa.hep.phy.cam.ac.uk/stefano/cavendish_9611.ps or at
http://www.hep.phy.cam.ac.uk/theory/papers
A Far-UV Spectroscopic Analysis of the Central Star of the Planetary Nebula Longmore 1
We have performed a non-LTE spectroscopic analysis using far-UV and UV data
of the central star of the planetary nebula K1-26 (Longmore 1), and found Teff
= 120+/-10 kK, logg = 6.7 +0.3/-0.7, and y = 0.10. The temperature is
significantly hotter than previous results based on optical line analyses,
highlighting the importance of analyzing the spectra of such hot objects at
shorter wavelengths. The spectra show metal lines (from, e.g, carbon, oxygen,
sulfur, and iron). The signatures of most elements can be fit adequately using
solar abundances, confirming the classification of Longmore 1 as a high gravity
O(H) object. Adopting a distance of 800 pc, we derive R = 0.04 Rsun, L = 250
Lsun, and M = 0.6 Msun. This places the object on the white dwarf cooling
sequence of the evolutionary tracks with an age of ~= 65 kyr.Comment: 14 pages, 4 color figures. Accepted for publication in PAS
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