31,564 research outputs found
Polymeric foams from cross-linkable poly-N-ary lenebenzimidazoles
Foamed cross-linked poly-N-arylenebinzimidazoles are prepared by mixing an organic tetraamine and an ortho substituted aromatic dicarboxylic acid anhydride in the presence of a blowing agent, and then heating the prepolymer to a temperature sufficient to complete polymerization and foaming of the reactants. In another embodiment of the process, the reactants are heated to form a prepolymer. The prepolymer is then cured at higher temperatures to complete foaming and polymerization
Sub-Milliarcsecond Precision of Pulsar Motions: Using In-Beam Calibrators with the VLBA
We present Very Long Baseline Array phase-referenced measurements of the
parallax and proper motion of two pulsars, B0919+06 and B1857-26.
Sub-milliarcsecond positional accuracy was obtained by simultaneously observing
a weak calibrator source within the 40' field of view of the VLBA at 1.5 GHz.
We discuss the merits of using weak close calibrator sources for VLBI
observations at low frequencies, and outline a method of observation and data
reduction for these type of measurements. For the pulsar B1919+06 we measure a
parallax of 0.31 +/- 0.14 mas. The accuracy of the proper motions is
approximately 0.5 mas, an order of magnitude improvement over most previous
determinations.Comment: 11 pages plus 4 figures. In press, Astronomical Journa
Generation of Cosmological Seed Magnetic Fields from Inflation with Cutoff
Inflation has the potential to seed the galactic magnetic fields observed
today. However, there is an obstacle to the amplification of the quantum
fluctuations of the electromagnetic field during inflation: namely the
conformal invariance of electromagnetic theory on a conformally flat underlying
geometry. As the existence of a preferred minimal length breaks the conformal
invariance of the background geometry, it is plausible that this effect could
generate some electromagnetic field amplification. We show that this scenario
is equivalent to endowing the photon with a large negative mass during
inflation. This effective mass is negligibly small in a radiation and matter
dominated universe. Depending on the value of the free parameter of the theory,
we show that the seed required by the dynamo mechanism can be generated. We
also show that this mechanism can produce the requisite galactic magnetic field
without resorting to a dynamo mechanism.Comment: Latex, 16 pages, 2 figures, 4 references added, minor corrections;
v4: more references added, boundary term written in a covariant form,
discussion regarding other gauge fields added, submitted to PRD; v5: matched
with the published versio
Production Systems Involving Stocker Cattle and Soft Red Winter Wheat
A three year study at the Livestock and Forestry Research Station near Batesville, Arkansas evaluated production systems involving stocker cattle and soft red winter wheat. Grazing of soft red winter wheat forage from October through February followed by harvesting wheat grain or grazing through April with stocker cattle offers an alternative to conventional farming. Soft red winter wheat, when planted by September 15, produces an ample supply of high-quality forage that supports rapid growth of stocker cattle during October through April. Net income from stocker cattle averaged over 75,000,000 per year if 750,000 acres of wheat are grazed
Optical matrix elements in tight-binding models with overlap
We investigate the effect of orbital overlap on optical matrix elements in
empirical tight-binding models. Empirical tight-binding models assume an
orthogonal basis of (atomiclike) states and a diagonal coordinate operator
which neglects the intra-atomic part. It is shown that, starting with an atomic
basis which is not orthogonal, the orthogonalization process induces
intra-atomic matrix elements of the coordinate operator and extends the range
of the effective Hamiltonian. We analyze simple tight-binding models and show
that non-orthogonality plays an important role in optical matrix elements. In
addition, the procedure gives formal justification to the nearest-neighbor
spin-orbit interaction introduced by Boykin [Phys. Rev \textbf{B} 57, 1620
(1998)] in order to describe the Dresselahaus term which is neglected in
empirical tight-binding models.Comment: 16 pages 6 figures, to appear in Phys. Rev.
The development of structural adhesives systems suitable for use with liquid oxygen Annual summary report, 1 Jul. 1963 - 30 Jun. 1964
Fluorinated, chlorinated, and halogenated polymer adhesives prepared and tested for compatibility with liquid oxyge
Earnings distribution, corporate governance and CEO pay
We investigate the relationship between earnings differentials and the pay of CEOs of 190 British companies between 1970 and 1990. We find that (i) changes in the differential between the 90th and 50th weekly earnings percentiles for non-manual adult male workers [90:50] explain changes in the level of real CEO salary and bonus in our sample of companies; (ii) changes in this differential also account for changes in the elasticity of CEO pay to firm size; (iii) a broader measure of earnings inequality does far worse than 90:50 at explaining changes in both the level and the firm size elasticity of CEO pay; (iv) fitting the model on data for 1970-1983 and predicting pay levels for the period starting with the widespread adoption of executive share option schemes in 1984, we find a structural break in the relationship between lower management pay differentials and the pay of the CEO. We conclude first that top executive pay prior to 1984 was a stable function of both firm size and earnings differentials lower on the administrative ladder, consistent with a hypothesis advanced by Herbert Simon in 1957; and second that the use of share options from 1984 onward represents not simply a change in the mode of top executive compensation, but a de -linking of the pay of top executives and that of their subordinates
Quantum spin correlations in an organometallic alternating sign chain
High resolution inelastic neutron scattering is used to study excitations in
the organometallic magnet DMACuCl. The correct magnetic Hamiltonian
describing this material has been debated for many years. Combined with high
field bulk magnetization and susceptibility studies, the new results imply that
DMACuCl is a realization of the alternating
antiferromagnetic-ferromagnetic (AFM-FM) chain. Coupled-cluster calculations
are used to derive exchange parameters, showing that the AFM and FM
interactions have nearly the same strength. Analysis of the scattering
intensities shows clear evidence for inter-dimer spin correlations, in contrast
to existing results for conventional alternating chains. The results are
discussed in the context of recent ideas concerning quantum entanglement.Comment: 5 pages, 4 figures included in text. Submitted to APS Journal
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