30,155 research outputs found
The Milky Way Galaxy as a Strong Gravitational Lens
We study the gravitational lensing effects of spiral galaxies by taking a
model of the Milky Way and computing its lensing properties. The model is
composed of a spherical Hernquist bulge, a Miyamoto-Nagai disc and an
isothermal halo. As a strong lens, a spiral galaxy like the Milky Way can give
rise to four different imaging geometries. They are (i) three images on one
side of the galaxy centre (`disc triplets'), (ii) three images with one close
to the centre (`core triplets'), (iii) five images and (iv) seven images.
Neglecting magnification bias, we show that the core triplets, disc triplets
and fivefold imaging are roughly equally likely. Even though our models contain
edge-on discs, their image multiplicities are not dominated by disc triplets.
The halo has a small effect on the caustic structure, the time delays and
brightnesses of the images. The Milky Way model has a maximum disc (i.e., the
halo is not dynamically important in the inner parts). Strong lensing by nearly
edge-on disc galaxies breaks the degeneracy between the relative contribution
of the disc and halo to the overall rotation curve. If a spiral galaxy has a
sub-maximum disc, then the astroid caustic shrinks dramatically in size, whilst
the radial caustic shrinks more modestly. This causes changes in the relative
likelihood of the image geometries, specifically (i) core triplets are now 9/2
times more likely than disc triplets, (ii) the cross section for threefold
imaging is reduced by a factor of 2/3, whilst (iii) the cross section for
fivefold imaging is reduced by 1/2. Although multiple imaging is less likely
(the cross sections are smaller), the average total magnification is greater.Comment: MNRAS, in pres
Thermal And Mechanical Analysis of High-power Light-emitting Diodes with Ceramic Packages
In this paper we present the thermal and mechanical analysis of high-power
light-emitting diodes (LEDs) with ceramic packages. Transient thermal
measurements and thermo-mechanical simulation were performed to study the
thermal and mechanical characteristics of ceramic packages. Thermal resistance
from the junction to the ambient was decreased from 76.1 oC/W to 45.3 oC/W by
replacing plastic mould to ceramic mould for LED packages. Higher level of
thermo-mechanical stresses in the chip were found for LEDs with ceramic
packages despite of less mismatching coefficients of thermal expansion
comparing with plastic packages. The results suggest that the thermal
performance of LEDs can be improved by using ceramic packages, but the mounting
process of the high power LEDs with ceramic packages is critically important
and should be in charge of delaminating interface layers in the packages.Comment: Submitted on behalf of TIMA Editions
(http://irevues.inist.fr/tima-editions
Growth of single-crystal columns of CoSi2 embedded in epitaxial Si on Si(111) by molecular beam epitaxy
The codeposition of Si and Co on a heated Si(111) substrate is found to result in epitaxial columns of CoSi2 if the Si:Co ratio is greater than approximately 3:1. These columns are surrounded by a Si matrix which shows bulk-like crystalline quality based on transmission electron microscopy and ion channeling. This phenomenon has been studied as functions of substrate temperature and Si:Co ratio. Samples with columns ranging in average diameter from approximately 25 to 130 nm have been produced
Pairing without Superfluidity: The Ground State of an Imbalanced Fermi Mixture
Radio-frequency spectroscopy is used to study pairing in the normal and
superfluid phases of a strongly interacting Fermi gas with imbalanced spin
populations. At high spin imbalances the system does not become superfluid even
at zero temperature. In this normal phase full pairing of the minority atoms is
observed. This demonstrates that mismatched Fermi surfaces do not prevent
pairing but can quench the superfluid state, thus realizing a system of fermion
pairs that do not condense even at the lowest temperature
X-Ray Scanner for Atlas Barrel TRT Modules
X-ray scanners for gain mapping of ATLAS Barrel Transition Radiation Tracker
(TRT) modules were developed at Hampton University for quality assurance
purposes. Gas gain variations for each straw of the TRT modules were used to
decide whether wires should be removed or restrung, and to evaluate overall
module quality.Comment: Conference proceeding in the XXIV Physics in Collisions Conference
(PIC04), Boston, USA, June 2004, 3 pages, LaTeX, 6 eps figures. MONP0
Facilitation of polymer looping and giant polymer diffusivity in crowded solutions of active particles
We study the dynamics of polymer chains in a bath of self-propelled particles
(SPP) by extensive Langevin dynamics simulations in a two dimensional system.
Specifically, we analyse the polymer looping properties versus the SPP activity
and investigate how the presence of the active particles alters the chain
conformational statistics. We find that SPPs tend to extend flexible polymer
chains while they rather compactify stiffer semiflexible polymers, in agreement
with previous results. Here we show that larger activities of SPPs yield a
higher effective temperature of the bath and thus facilitate looping kinetics
of a passive polymer chain. We explicitly compute the looping probability and
looping time in a wide range of the model parameters. We also analyse the
motion of a monomeric tracer particle and the polymer's centre of mass in the
presence of the active particles in terms of the time averaged mean squared
displacement, revealing a giant diffusivity enhancement for the polymer chain
via SPP pooling. Our results are applicable to rationalising the dimensions and
looping kinetics of biopolymers at constantly fluctuating and often actively
driven conditions inside biological cells or suspensions of active colloidal
particles or bacteria cells.Comment: 15 pages, 9 figures, IOPLaTe
Fermionic Superfluidity with Imbalanced Spin Populations and the Quantum Phase Transition to the Normal State
Whether it occurs in superconductors, helium-3 or inside a neutron star,
fermionic superfluidity requires pairing of fermions, particles with
half-integer spin. For an equal mixture of two states of fermions ("spin up"
and "spin down"), pairing can be complete and the entire system will become
superfluid. When the two populations of fermions are unequal, not every
particle can find a partner. Will the system nevertheless stay superfluid? Here
we study this intriguing question in an unequal mixture of strongly interacting
ultracold fermionic atoms. The superfluid region vs population imbalance is
mapped out by employing two complementary indicators: The presence or absence
of vortices in a rotating mixture, as well as the fraction of condensed fermion
pairs in the gas. Due to the strong interactions near a Feshbach resonance, the
superfluid state is remarkably stable in response to population imbalance. The
final breakdown of superfluidity marks a new quantum phase transition, the
Pauli limit of superfluidity.Comment: 15 pages, 5 figure
Strong Analytic Controllability for Hydrogen Control Systems
The realization and representation of so(4,2) associated with the hydrogen
atom Hamiltonian are derived. By choosing operators from the realization of
so(4,2) as interacting Hamiltonians, a hydrogen atom control system is
constructed, and it is proved that this control system is strongly analytically
controllable based on a time-dependent strong analytic controllability theorem.Comment: 6 pages; corrected typo; added equations in section III for
representation states of so(4,2). accepted by CDC 200
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