2,212 research outputs found
Multiscale Modeling of Grain Boundaries in ZrB2: Structure, Energetics, and Thermal Resistance
A combination of ab initio, atomistic and finite element methods (FEM) were used to investigate the structures, energetics and lattice thermal conductance of grain boundaries for the ultra high temperature ceramic ZrB2. Atomic models of idealized boundaries were relaxed using density functional theory. Information about bonding across the interfaces was determined from the electron localization function. The Kapitza conductance of larger scale versions of the boundary models were computed using non-equilibrium molecular dynamics. The interfacial thermal parameters together with single crystal thermal conductivities were used as parameters in microstructural computations. FEM meshes were constructed on top of microstructural images. From these computations, the effective thermal conductivity of the polycrystalline structure was determined
AF-algebras and topology of mapping tori
A covariant functor from the category of mapping tori to a category of
AF-algebras is constructed; the functor takes continuous maps between such
manifolds to stable homomorphisms between the corresponding AF-algebras. We use
this functor to develop an obstruction theory for the torus bundles of
dimension 2, 3 and 4.Comment: to appear Czechoslovak Math.
Setting Priorities for Space Research: Opportunities and Imperatives
This report represents the first phase of a study by a task group convened by the Space Studies Board to ascertain whether it should attempt to develop a methodology for recommending priorities among the various initiatives in space research (that is, scientific activities concerned with phenomena in space or utilizing observations from space). The report argues that such priority statements by the space research community are both necessary and desirable and would contribute to the formulation and implementation of public policy. The report advocates the establishment of priorities to enhance effective management of the nation's scientific research program in space. It argues that scientific objectives and purposes should determine how and under what circumstances scientific research should be done. The report does not take a position on the controversy between advocates of manned space exploration and those who favor the exclusive use of unmanned space vehicles. Nor does the report address questions about the value or appropriateness of Space Station Freedom or proposals to establish a permanent manned Moon base or to undertake a manned mission to Mars. These issues lie beyond the charge to the task group
Dirac-Hestenes spinor fields in Riemann-Cartan spacetime
In this paper we study Dirac-Hestenes spinor fields (DHSF) on a
four-dimensional Riemann-Cartan spacetime (RCST). We prove that these fields
must be defined as certain equivalence classes of even sections of the Clifford
bundle (over the RCST), thereby being certain particular sections of a new
bundle named Spin-Clifford bundle (SCB). The conditions for the existence of
the SCB are studied and are shown to be equivalent to the famous Geroch's
theorem concerning to the existence of spinor structures in a Lorentzian
spacetime. We introduce also the covariant and algebraic Dirac spinor fields
and compare these with DHSF, showing that all the three kinds of spinor fields
contain the same mathematical and physical information. We clarify also the
notion of (Crumeyrolle's) amorphous spinors (Dirac-K\"ahler spinor fields are
of this type), showing that they cannot be used to describe fermionic fields.
We develop a rigorous theory for the covariant derivatives of Clifford fields
(sections of the Clifford bundle (CB)) and of Dirac-Hestenes spinor fields. We
show how to generalize the original Dirac-Hestenes equation in Minkowski
spacetime for the case of a RCST. Our results are obtained from a variational
principle formulated through the multiform derivative approach to Lagrangian
field theory in the Clifford bundle.Comment: 45 pages, special macros kapproc.sty and makro822.te
Gauge Formalism for General Relativity and Fermionic Matter
A new formalism for spinors on curved spaces is developed in the framework of
variational calculus on fibre bundles. The theory has the same structure of a
gauge theory and describes the interaction between the gravitational field and
spinors. An appropriate gauge structure is also given to General Relativity,
replacing the metric field with spin frames. Finally, conserved quantities and
superpotentials are calculated under a general covariant form.Comment: 18 pages, Plain TEX, revision, explicit expression for superpotential
has been adde
Willmore Surfaces of Constant Moebius Curvature
We study Willmore surfaces of constant Moebius curvature in . It is
proved that such a surface in must be part of a minimal surface in
or the Clifford torus. Another result in this paper is that an isotropic
surface (hence also Willmore) in of constant could only be part of a
complex curve in or the Veronese 2-sphere in . It is
conjectured that they are the only examples possible. The main ingredients of
the proofs are over-determined systems and isoparametric functions.Comment: 16 pages. Mistakes occured in the proof to the main theorem (Thm 3.6)
has been correcte
A Fly in the Ointment: Evaluation of Traditional Use of Plants to Repel and Kill Blowfly Larvae in Fermented Fish
Introduction: In rural areas in Laos, fly larvae infestations are common in fermenting fish. Blowflies (Chrysomyamegacephala, Diptera: Calliphoridae) are attracted to oviposit (and/or larviposit) onto fermenting fish which results ininfestations with fly larvae. Knowledge of traditional use of plants to repel larvae during the production of fermented fish iscommon and widespread in Lao PDR. Research Questions: How effective are the most salient species in repelling, and killing fly larvae in fermenting fish? Material and Methods: The three plant species most frequently reported to repel fly larvae during an ethnobotanical surveythroughout Lao PDR were tested for repellence and larvicidal activity of fly larvae infesting fermented fish. The lethality andrepellence of Tadehagi triquetrum (L.) H. Ohashi (Fabaceae), Uraria crinita (L.) Desv. ex DC. (Fabaceae) and Bambusa multiplex(Lour.) Raeusch. ex Schult. & Schult. f. (Poaceae) were tested in an experimental design using fermenting fish in Vientiane,Lao PDR. Results: The repellent effect of fresh material of T. triquetrum and U. crinita, and the larvicidal effect of fresh B. multiplex, issignificantly more effective than that of dried material of the same species, and the total effect (repellence and larvicidaleffect combined) for each of the three species was significantly more effective for fresh than for dry material. Fresh materialof T. triquetrum, U. crinita, or B. multiplex added on top of the fermenting fish repelled 50%, 54%, 37%, and killed 22%, 28%,and 40% of fly larvae. The total effect was not significantly different per species at 72%, 82%, and 77%, respectively. Discussion and Conclusions: The three most salient species are effective in repelling and killing fly larvae in the productionof fermented fish, and may be essential to augment food safety during traditional fermentation in open jars
Coherent States of the q--Canonical Commutation Relations
For the -deformed canonical commutation relations for in some Hilbert
space we consider representations generated from a vector
satisfying , where .
We show that such a representation exists if and only if .
Moreover, for these representations are unitarily equivalent
to the Fock representation (obtained for ). On the other hand
representations obtained for different unit vectors are disjoint. We
show that the universal C*-algebra for the relations has a largest proper,
closed, two-sided ideal. The quotient by this ideal is a natural -analogue
of the Cuntz algebra (obtained for ). We discuss the Conjecture that, for
, this analogue should, in fact, be equal to the Cuntz algebra
itself. In the limiting cases we determine all irreducible
representations of the relations, and characterize those which can be obtained
via coherent states.Comment: 19 pages, Plain Te
Ovine pedomics : the first study of the ovine foot 16S rRNA-based microbiome
We report the first study of the bacterial microbiome of ovine interdigital skin based on 16S rRNA by pyrosequencing and conventional cloning with Sanger-sequencing. Three flocks were selected, one a flock with no signs of footrot or interdigital dermatitis, a second flock with interdigital dermatitis alone and a third flock with both interdigital dermatitis and footrot. The sheep were classified as having either healthy interdigital skin (H), interdigital dermatitis (ID) or virulent footrot (VFR). The ovine interdigital skin bacterial community varied significantly by flock and clinical condition. The diversity and richness of operational taxonomic units was greater in tissue from sheep with ID than H or VFR affected sheep. Actinobacteria, Bacteriodetes, Firmicutes and Proteobacteria were the most abundant phyla comprising 25 genera. Peptostreptococcus, Corynebacterium and Staphylococcus were associated with H, ID and VFR respectively. Sequences of Dichelobacter nodosus, the causal agent of ovine footrot, were not amplified due to mismatches in the 16S rRNA universal forward primer (27F). A specific real time PCR assay was used to demonstrate the presence of D. nodosus which was detected in all samples including the flock with no signs of ID or VFR. Sheep with ID had significantly higher numbers of D. nodosus (104-109 cells/g tissue) than those with H or VFR feet
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