698 research outputs found
Diagnosing space telescope misalignment and jitter using stellar images
Accurate knowledge of the telescope's point spread function (PSF) is
essential for the weak gravitational lensing measurements that hold great
promise for cosmological constraints. For space telescopes, the PSF may vary
with time due to thermal drifts in the telescope structure, and/or due to
jitter in the spacecraft pointing (ground-based telescopes have additional
sources of variation). We describe and simulate a procedure for using the
images of the stars in each exposure to determine the misalignment and jitter
parameters, and reconstruct the PSF at any point in that exposure's field of
view. The simulation uses the design of the SNAP (http://snap.lbl.gov)
telescope. Stellar-image data in a typical exposure determines secondary-mirror
positions as precisely as . The PSF ellipticities and size, which
are the quantities of interest for weak lensing are determined to and accuracies respectively in each exposure,
sufficient to meet weak-lensing requirements. We show that, for the case of a
space telescope, the PSF estimation errors scale inversely with the square root
of the total number of photons collected from all the usable stars in the
exposure.Comment: 20 pages, 6 figs, submitted to PAS
Fast diffusion of a Lennard-Jones cluster on a crystalline surface
We present a Molecular Dynamics study of large Lennard-Jones clusters
evolving on a crystalline surface. The static and the dynamic properties of the
cluster are described. We find that large clusters can diffuse rapidly, as
experimentally observed. The role of the mismatch between the lattice
parameters of the cluster and the substrate is emphasized to explain the
diffusion of the cluster. This diffusion can be described as a Brownian motion
induced by the vibrationnal coupling to the substrate, a mechanism that has not
been previously considered for cluster diffusion.Comment: latex, 5 pages with figure
Island diffusion on metal fcc(100) surfaces
We present Monte Carlo simulations for the size and temperature dependence of
the diffusion coefficient of adatom islands on the Cu(100) surface. We show
that the scaling exponent for the size dependence is not a constant but a
decreasing function of the island size and approaches unity for very large
islands. This is due to a crossover from periphery dominated mass transport to
a regime where vacancies diffuse inside the island. The effective scaling
exponents are in good agreement with theory and experiments.Comment: 13 pages, 2 figures, to be published in Phys. Rev. Let
Steady-State Properties of Single-File Systems with Conversion
We have used Monte-Carlo methods and analytical techniques to investigate the
influence of the characteristic parameters, such as pipe length, diffusion,
adsorption, desorption and reaction rate constants on the steady-state
properties of Single-File Systems with a reaction. We looked at cases when all
the sites are reactive and when only some of them are reactive. Comparisons
between Mean-Field predictions and Monte-Carlo simulations for the occupancy
profiles and reactivity are made. Substantial differences between Mean-Field
and the simulations are found when rates of diffusion are high. Mean-Field
results only include Single-File behavior by changing the diffusion rate
constant, but it effectively allows passing of particles. Reactivity converges
to a limit value if more reactive sites are added: sites in the middle of the
system have little or no effect on the kinetics. Occupancy profiles show
approximately exponential behavior from the ends to the middle of the system.Comment: 15 pages, 20 figure
Electromigration of Single-Layer Clusters
Single-layer atom or vacancy clusters in the presence of electromigration are
studied theoretically assuming an isotropic medium. A variety of distinctive
behaviors distinguish the response in the three standard limiting cases of
periphery diffusion (PD), terrace diffusion (TD), and evaporation-condensation
(EC). A general model provides power laws describing the size dependence of the
drift velocity in these limits, consistent with established results in the case
of PD. The validity of the widely used quasistatic limit is calculated. Atom
and vacancy clusters drift in opposite directions in the PD limit but in the
same direction otherwise. In absence of PD, linear stability analysis reveals a
new type of morphological instability, not leading to island break-down. For
strong electromigration, Monte Carlo simulations show that clusters then
destabilize into slits, in contrast to splitting in the PD limit.
Electromigration affects the diffusion coefficient of the cluster and
morphological fluctuations, the latter diverging at the instability threshold.
An instrinsic attachment-detachment bias displays the same scaling signature as
PD in the drift velocity.Comment: 11 pages, 4 figure
The bolometric focal plane array of the Polarbear CMB experiment
The Polarbear Cosmic Microwave Background (CMB) polarization experiment is
currently observing from the Atacama Desert in Northern Chile. It will
characterize the expected B-mode polarization due to gravitational lensing of
the CMB, and search for the possible B-mode signature of inflationary
gravitational waves. Its 250 mK focal plane detector array consists of 1,274
polarization-sensitive antenna-coupled bolometers, each with an associated
lithographed band-defining filter. Each detector's planar antenna structure is
coupled to the telescope's optical system through a contacting dielectric
lenslet, an architecture unique in current CMB experiments. We present the
initial characterization of this focal plane
Equilibrium Properties of A Monomer-Monomer Catalytic Reaction on A One-Dimensional Chain
We study the equilibrium properties of a lattice-gas model of an catalytic reaction on a one-dimensional chain in contact with a reservoir
for the particles. The particles of species and are in thermal contact
with their vapor phases acting as reservoirs, i.e., they may adsorb onto empty
lattice sites and may desorb from the lattice. If adsorbed and
particles appear at neighboring lattice sites they instantaneously react and
both desorb. For this model of a catalytic reaction in the
adsorption-controlled limit, we derive analytically the expression of the
pressure and present exact results for the mean densities of particles and for
the compressibilities of the adsorbate as function of the chemical potentials
of the two species.Comment: 19 pages, 5 figures, submitted to Phys. Rev.
A Nonzero Gap Two-Dimensional Carbon Allotrope from Porous Graphene
Graphene is considered one of the most promising materials for future
electronic. However, in its pristine form graphene is a gapless material, which
imposes limitations to its use in some electronic applications. In order to
solve this problem many approaches have been tried, such as, physical and
chemical functionalizations. These processes compromise some of the desirable
graphene properties. In this work, based on ab initio quantum molecular
dynamics, we showed that a two-dimensional carbon allotrope, named biphenylene
carbon (BPC) can be obtained from selective dehydrogenation of porous graphene.
BPC presents a nonzero bandgap and well-delocalized frontier orbitals.
Synthetic routes to BPC are also addressed.Comment: Published on J. Phys. Chem. C, 2012, 116 (23), pp 12810-1281
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