24,129 research outputs found
The Social Costs of an MTBE Ban in California
Resource /Energy Economics and Policy,
Algebraic and combinatorial aspects of sandpile monoids on directed graphs
The sandpile group of a graph is a well-studied object that combines ideas
from algebraic graph theory, group theory, dynamical systems, and statistical
physics. A graph's sandpile group is part of a larger algebraic structure on
the graph, known as its sandpile monoid. Most of the work on sandpiles so far
has focused on the sandpile group rather than the sandpile monoid of a graph,
and has also assumed the underlying graph to be undirected. A notable exception
is the recent work of Babai and Toumpakari, which builds up the theory of
sandpile monoids on directed graphs from scratch and provides many connections
between the combinatorics of a graph and the algebraic aspects of its sandpile
monoid.
In this paper we primarily consider sandpile monoids on directed graphs, and
we extend the existing theory in four main ways. First, we give a combinatorial
classification of the maximal subgroups of a sandpile monoid on a directed
graph in terms of the sandpile groups of certain easily-identifiable subgraphs.
Second, we point out certain sandpile results for undirected graphs that are
really results for sandpile monoids on directed graphs that contain exactly two
idempotents. Third, we give a new algebraic constraint that sandpile monoids
must satisfy and exhibit two infinite families of monoids that cannot be
realized as sandpile monoids on any graph. Finally, we give an explicit
combinatorial description of the sandpile group identity for every graph in a
family of directed graphs which generalizes the family of (undirected)
distance-regular graphs. This family includes many other graphs of interest,
including iterated wheels, regular trees, and regular tournaments.Comment: v2: Cleaner presentation, new results in final section. Accepted for
publication in J. Combin. Theory Ser. A. 21 pages, 5 figure
Isotropic-medium three-dimensional cloaks for acoustic and electromagnetic waves
We propose a generalization of the two-dimensional eikonal-limit cloak
derived from a conformal transformation to three dimensions. The proposed cloak
is a spherical shell composed of only isotropic media; it operates in the
transmission mode and requires no mirror or ground plane. Unlike the well-known
omnidirectional spherical cloaks, it may reduce visibility of an arbitrary
object only for a very limited range of observation angles. In the
short-wavelength limit, this cloaking structure restores not only the
trajectories of incident rays, but also their phase, which is a necessary
ingredient to complete invisibility. Both scalar-wave (acoustic) and transverse
vector-wave (electromagnetic) versions are presented.Comment: 17 pages, 12 figure
Microstructure, magneto-transport and magnetic properties of Gd-doped magnetron-sputtered amorphous carbon
The magnetic rare earth element gadolinium (Gd) was doped into thin films of
amorphous carbon (hydrogenated \textit{a}-C:H, or hydrogen-free \textit{a}-C)
using magnetron co-sputtering. The Gd acted as a magnetic as well as an
electrical dopant, resulting in an enormous negative magnetoresistance below a
temperature (). Hydrogen was introduced to control the amorphous carbon
bonding structure. High-resolution electron microscopy, ion-beam analysis and
Raman spectroscopy were used to characterize the influence of Gd doping on the
\textit{a-}GdC(:H) film morphology, composition, density and
bonding. The films were largely amorphous and homogeneous up to =22.0 at.%.
As the Gd doping increased, the -bonded carbon atoms evolved from
carbon chains to 6-member graphitic rings. Incorporation of H opened up the
graphitic rings and stabilized a -rich carbon-chain random network. The
transport properties not only depended on Gd doping, but were also very
sensitive to the ordering. Magnetic properties, such as the spin-glass
freezing temperature and susceptibility, scaled with the Gd concentration.Comment: 9 figure
Recommended from our members
Identification of a Hemolysis Threshold That Increases Plasma and Serum Zinc Concentration.
Background: Plasma or serum zinc concentration (PZC or SZC) is the primary measure of zinc status, but accurate sampling requires controlling for hemolysis to prevent leakage of zinc from erythrocytes. It is not established how much hemolysis can occur without changing PZC/SZC concentrations.Objective: This study determines a guideline for the level of hemolysis that can significantly elevate PZC/SZC.Methods: The effect of hemolysis on PZC/SZC was estimated by using standard hematologic variables and mineral content. The calculated hemolysis threshold was then compared with results from an in vitro study and a population survey. Hemolysis was assessed by hemoglobin and iron concentrations, direct spectrophotometry, and visual assessment of the plasma or serum. Zinc and iron concentrations were determined by inductively coupled plasma spectrometry.Results: A 5% increase in PZC/SZC was calculated to result from the lysis of 1.15% of the erythrocytes in whole blood, corresponding to ∼1 g hemoglobin/L added into the plasma or serum. Similarly, the addition of simulated hemolysate to control plasma in vitro caused a 5% increase in PZC when hemoglobin concentrations reached 1.18 ± 0.10 g/L. In addition, serum samples from a population nutritional survey were scored for hemolysis and analyzed for changes in SZC; samples with hemolysis in the range of 1-2.5 g hemoglobin/L showed an estimated increase in SZC of 6% compared with nonhemolyzed samples. Each approach indicated that a 5% increase in PZC/SZC occurs at ∼1 g hemoglobin/L in plasma or serum. This concentration of hemoglobin can be readily identified directly by chemical hemoglobin assays or indirectly by direct spectrophotometry or matching to a color scale.Conclusions: A threshold of 1 g hemoglobin/L is recommended for PZC/SZC measurements to avoid increases in zinc caused by hemolysis. The use of this threshold may improve zinc assessment for monitoring zinc status and nutritional interventions
A 10 GHz Quasi-Optical Grid Amplifier Using Integrated HBT Differential Pairs
We report the fabrication and testing of a 10 GHz grid amplifier utilizing sixteen GaAs chips each
containing an HBT differential pair plus integral bias/feedback resistors. The overall amplifier consists of
a 4x4 array of unit cells on an RT Duroidâ„¢ board having a relative permittivity of 2.2. Each unit cell
consists of an emitter-coupled differential pair at the center, an input antenna which extends horizontally
in both directions from the two base leads, an output antenna which extends vertically in both directions
from the two collector leads, and high inductance bias lines. In operation, the active grid array is placed
between a pair of crossed polarizers. The horizontally polarized input wave passes through the input
polarizer and couples to the input leads. An amplified current then flows on the vertical leads, which
radiate a vertically polarized amplified signal through the output polarizer. The polarizers serve dual
functions, providing both input-output isolation as well as independent impedance matching for the input
and output ports. The grid thus functions essentially as a free-space beam amplifier. Calculations indicate
that output powers of several watts per square centimeter of grid area should be attainable with optimized
structures
Woe from stones: commemoration, identity politics and Estonia's 'War of Monuments'
No abstract available
Constraining the age of the NGC 4565 HI Disk Warp: Determining the Origin of Gas Warps
We have mapped the distribution of young and old stars in the gaseous HI warp
of NGC 4565. We find a clear correlation of young stars (<600 Myr) with the
warp, but no coincident old stars (>1 Gyr), which places an upper limit on the
age of the structure. The formation rate of the young stars, which increased
~300 Myr ago relative to the surrounding regions, is (6.3 +2.5/-1.5) x 10^-5
M_sol/yr/kpc^2. This implies a ~60+/-20 Gyr depletion time of the HI warp,
similar to the timescales calculated for the outer HI disks of nearby spiral
galaxies. While some stars associated with the warp fall into the asymptotic
giant branch (AGB) region of the color magnitude diagram, where stars could be
as old as 1 Gyr, further investigation suggests that they may be interlopers
rather than real AGB stars. We discuss the implications of these age
constraints for the formation of HI warps, and the gas fueling of disk
galaxies.Comment: 12 pages, 9 figures. Accepted for publication in Ap
Stabilizing the surface morphology of Si1–x–yGexCy/Si heterostructures grown by molecular beam epitaxy through the use of a silicon-carbide source
Si1–x–yGexCy/Si superlattices were grown by solid-source molecular beam epitaxy using silicon carbide as a source of C. Samples consisting of alternating layers of nominally 25 nm Si1–x–yGexCy and 35 nm Si for 10 periods were characterized by high-resolution x-ray diffraction, transmission electron microscopy (TEM), and Rutherford backscattering spectrometry to determine strain, thickness, and composition. C resonance backscattering and secondary ion mass spectrometries were used to measure the total C concentration in the Si1–x–yGexCy layers, allowing for an accurate determination of the substitutional C fraction to be made as a function of growth rate for fixed Ge and substitutional C compositions. For C concentrations close to 1%, high-quality layers were obtained without the use of Sb-surfactant mediation. These samples were found to be structurally perfect to a level consistent with cross-sectional TEM (< 10^7 defects/cm^2) and showed considerably improved homogeneity as compared with similar structures grown using graphite as the source for C. For higher Ge and C concentrations, Sb-surfactant mediation was found to be required to stabilize the surface morphology. The maximum value of substitutional C concentration, above which excessive generation of stacking fault defects caused polycrystalline and/or amorphous growth, was found to be approximately 2.4% in samples containing between 25 and 30% Ge. The fraction of substitutional C was found to decrease from roughly 60% by a factor of 0.86 as the Si1–x–yGexCy growth rate increased from 0.1 to 1.0 nm/s
- …