1,112 research outputs found
Random projections and the optimization of an algorithm for phase retrieval
Iterative phase retrieval algorithms typically employ projections onto
constraint subspaces to recover the unknown phases in the Fourier transform of
an image, or, in the case of x-ray crystallography, the electron density of a
molecule. For a general class of algorithms, where the basic iteration is
specified by the difference map, solutions are associated with fixed points of
the map, the attractive character of which determines the effectiveness of the
algorithm. The behavior of the difference map near fixed points is controlled
by the relative orientation of the tangent spaces of the two constraint
subspaces employed by the map. Since the dimensionalities involved are always
large in practical applications, it is appropriate to use random matrix theory
ideas to analyze the average-case convergence at fixed points. Optimal values
of the gamma parameters of the difference map are found which differ somewhat
from the values previously obtained on the assumption of orthogonal tangent
spaces.Comment: 15 page
Dense packing crystal structures of physical tetrahedra
We present a method for discovering dense packings of general convex hard
particles and apply it to study the dense packing behavior of a one-parameter
family of particles with tetrahedral symmetry representing a deformation of the
ideal mathematical tetrahedron into a less ideal, physical, tetrahedron and all
the way to the sphere. Thus, we also connect the two well studied problems of
sphere packing and tetrahedron packing on a single axis. Our numerical results
uncover a rich optimal-packing behavior, compared to that of other continuous
families of particles previously studied. We present four structures as
candidates for the optimal packing at different values of the parameter,
providing an atlas of crystal structures which might be observed in systems of
nano-particles with tetrahedral symmetry
Stability of the hard-sphere icosahedral quasilattice
The stability of the hard-sphere icosahedral quasilattice is analyzed using
the differential formulation of the generalized effective liquid approximation.
We find that the icosahedral quasilattice is metastable with respect to the
hard-sphere crystal structures. Our results agree with recent findings by
McCarley and Ashcroft [Phys. Rev. B {\bf 49}, 15600 (1994)] carried out using
the modified weighted density approximation.Comment: 15 pages, 2 figures available from authors upon request, (revtex),
submitted to Phys. Rev.
A method for dense packing discovery
The problem of packing a system of particles as densely as possible is
foundational in the field of discrete geometry and is a powerful model in the
material and biological sciences. As packing problems retreat from the reach of
solution by analytic constructions, the importance of an efficient numerical
method for conducting \textit{de novo} (from-scratch) searches for dense
packings becomes crucial. In this paper, we use the \textit{divide and concur}
framework to develop a general search method for the solution of periodic
constraint problems, and we apply it to the discovery of dense periodic
packings. An important feature of the method is the integration of the unit
cell parameters with the other packing variables in the definition of the
configuration space. The method we present led to improvements in the
densest-known tetrahedron packing which are reported in [arXiv:0910.5226].
Here, we use the method to reproduce the densest known lattice sphere packings
and the best known lattice kissing arrangements in up to 14 and 11 dimensions
respectively (the first such numerical evidence for their optimality in some of
these dimensions). For non-spherical particles, we report a new dense packing
of regular four-dimensional simplices with density
and with a similar structure to the densest known tetrahedron packing.Comment: 15 pages, 5 figure
Global convergence of a non-convex Douglas-Rachford iteration
We establish a region of convergence for the proto-typical non-convex
Douglas-Rachford iteration which finds a point on the intersection of a line
and a circle. Previous work on the non-convex iteration [2] was only able to
establish local convergence, and was ineffective in that no explicit region of
convergence could be given
International Federation of Clinical Chemistry (IFCC): Scientific Division, Committee on Enzymes. IFCC methods for the measurement of catalytic concentration of enzymes. Part 7. IFCC method for creatine kinase (ATP: creatine (N-phosphotransferase, EC 2.7.3.2). IFCC Recommendation
Proliferation of anomalous symmetries in colloidal monolayers subjected to quasiperiodic light fields
Quasicrystals provide a fascinating class of materials with intriguing
properties. Despite a strong potential for numerous technical applications, the
conditions under which quasicrystals form are still poorly understood.
Currently, it is not clear why most quasicrystals hold 5- or 10-fold symmetry
but no single example with 7 or 9-fold symmetry has ever been observed. Here we
report on geometrical constraints which impede the formation of quasicrystals
with certain symmetries in a colloidal model system. Experimentally, colloidal
quasicrystals are created by subjecting micron-sized particles to
two-dimensional quasiperiodic potential landscapes created by n=5 or seven
laser beams. Our results clearly demonstrate that quasicrystalline order is
much easier established for n = 5 compared to n = 7. With increasing laser
intensity we observe that the colloids first adopt quasiperiodic order at local
areas which then laterally grow until an extended quasicrystalline layer forms.
As nucleation sites where quasiperiodicity originates, we identify highly
symmetric motifs in the laser pattern. We find that their density strongly
varies with n and surprisingly is smallest exactly for those quasicrystalline
symmetries which have never been observed in atomic systems. Since such high
symmetry motifs also exist in atomic quasicrystals where they act as
preferential adsorption sites, this suggests that it is indeed the deficiency
of such motifs which accounts for the absence of materials with e.g. 7-fold
symmetry
Naturally-phasematched second harmonic generation in a whispering gallery mode resonator
We demonstrate for the first time natural phase matching for optical
frequency doubling in a high-Q whispering gallery mode resonator made of
Lithium Niobate. A conversion efficiency of 9% is achieved at 30 micro Watt
in-coupled continuous wave pump power. The observed saturation pump power of
3.2 mW is almost two orders of magnitude lower than the state-of-the-art. This
suggests an application of our frequency doubler as a source of non-classical
light requiring only a low-power pump, which easily can be quantum noise
limited. Our theoretical analysis of the three-wave mixing in a whispering
gallery mode resonator provides the relative conversion efficiencies for
frequency doubling in various modes
Chemical composition and radiative properties of nascent particulate matter emitted by an aircraft turbofan burning conventional and alternative fuels
Aircraft engines are a unique source of carbonaceous
aerosols in the upper troposphere. There, these particles can more
efficiently interact with solar radiation than at ground. Due to the lack of
measurement data, the radiative forcing from aircraft exhaust aerosol
remains uncertain. To better estimate the global radiative effects of
aircraft exhaust aerosol, its optical properties need to be comprehensively
characterized. In this work we present the link between the chemical
composition and the optical properties of the particulate matter (PM)
measured at the engine exit plane of a CFM56-7B turbofan. The measurements
covered a wide range of power settings (thrust), ranging from ground idle to
take-off, using four different fuel blends of conventional Jet A-1 and
hydro-processed ester and fatty acids (HEFA) biofuel. At the two measurement
wavelengths (532 and 870 nm) and for all tested fuels, the absorption and
scattering coefficients increased with thrust, as did the PM mass. The
analysis of elemental carbon (EC) and organic carbon (OC) revealed a
significant mass fraction of OC (up to 90 %) at low thrust levels, while
EC mass dominated at medium and high thrust. The use of HEFA blends induced
a significant decrease in the PM mass and the optical coefficients at all
thrust levels. The HEFA effect was highest at low thrust levels, where the
EC mass was reduced by up to 50 %–60 %. The variability in the chemical
composition of the particles was the main reason for the strong thrust
dependency of the single scattering albedo (SSA), which followed the same
trend as the fraction of OC to total carbon (TC). Mass absorption
coefficients (MACs) were determined from the correlations between aerosol
light absorption and EC mass concentration. The obtained MAC values
(MAC532=7.5±0.3 m2 g−1 and MAC870=5.2±0.9 m2 g−1) are in excellent agreement with previous
literature values of absorption cross section for freshly generated soot.
While the MAC values were found to be independent of the thrust level and
fuel type, the mass scattering coefficients (MSCs) significantly varied with
thrust. For cruise conditions we obtained MSC532=4.5±0.4 m2 g−1 and MSC870=0.54±0.04 m2 g−1,
which fall within the higher end of MSCs measured for fresh biomass smoke.
However, the latter comparison is limited by the strong dependency of MSC on
the particles' size, morphology and chemical composition. The use of the HEFA
fuel blends significantly decreased PM emissions, but no changes were
observed in terms of EC∕OC composition and radiative properties.</p
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