139 research outputs found
Twin wall of cubic-tetragonal ferroelastics
We derive solutions for the twin wall linking two tetragonal variants of the
cubic-tetragonal ferroelastic transformation, including for the first time the
dilatational and shear energies and strains. Our solutions satisfy the
compatibility relations exactly and are obtained at all temperatures. They
require four non-vanishing strains except at the Barsch-Krumhansl temperature
TBK (where only the two deviatoric strains are needed). Between the critical
temperature and TBK, material in the wall region is dilated, while below TBK it
is compressed. In agreement with experiment and more general theory, the twin
wall lies in a cubic 110-type plane. We obtain the wall energy numerically as a
function of temperature and we derive a simple estimate which agrees well with
these values.Comment: 4 pages (revtex), 3 figure
Structure optimization effects on the electronic properties of BiSrCaCuO
We present detailed first-principles calculations for the normal state
electronic properties of the high T superconductor
BiSrCaCuO, by means of the linearized augmented plane wave
(LAPW) method within the framework of density functional theory (DFT). As a
first step, the body centered tetragonal (BCT) cell has been adopted, and
optimized regarding its volume, ratio and internal atomic positions by
total energy and force minimizations. The full optimization of the BCT cell
leads to small but visible changes in the topology of the Fermi surface,
rounding the shape of CuO barrels, and causing both the BiO bands,
responsible for the pockets near the \textit{\=M} 2D symmetry point, to dip
below the Fermi level. We have then studied the influence of the distortions in
the BiO plane observed in nature by means of a
orthorhombic cell (AD-ORTH) with space group. Contrary to what has been
observed for the Bi-2201 compound, we find that for Bi-2212 the distortion does
not sensibly shift the BiO bands which retain their metallic character. As a
severe test for the considered structures we present Raman-active phonon
frequencies () and eigenvectors calculated within the frozen-phonon
approximation. Focussing on the totally symmetric A modes, we observe
that for a reliable attribution of the peaks observed in Raman experiments,
both - and a-axis vibrations must be taken into account, the latter being
activated by the in-plane orthorhombic distortion.Comment: 22 pages, 4 figure
Statics and dynamics of domain patterns in hexagonal-orthorhombic ferroelastics
We study the statics and the dynamics of domain patterns in proper
hexagonal-orthorhombic ferroelastics; these patterns are of particular interest
because they provide a rare physical realization of disclinations in crystals.
Both our static and dynamical theories are based entirely on classical,
nonlinear elasticity theory; we use the minimal theory consistent with
stability, symmetry and ability to explain qualitatively the observed patterns.
After scaling, the only parameters of the static theory are a temperature
variable and a stiffness variable. For moderate to large stiffness, our static
results show nested stars, unnested stars, fans and other nodes, triangular and
trapezoidal regions of trapped hexagonal phase, etc observed in electron
microscopy of Ta4N and Mg-Cd alloys, and also in lead orthovanadate (which is
trigonal-monoclinic); we even find imperfections in some nodes, like those
observed. For small stiffness, we find patterns like those observed in the
mineral Mg-cordierite. Our dynamical studies of growth and relaxation show the
formation of these static patterns, and also transitory structures such as
12-armed bursts, streamers and striations which are also seen experimentally.
The major aspects of the growth-relaxation process are quite unlike those in
systems with conventional order parameters, for it is inherently nonlocal; for
example, the changes from one snapshot to the next are not predictable by
inspection.Comment: 9 pages, 3 figures (1 b&w, 2 colour); animations may be viewed at
http://huron.physics.utoronto.ca/~curnoe/sim.htm
Simulations of cubic-tetragonal ferroelastics
We study domain patterns in cubic-tetragonal ferroelastics by solving
numerically equations of motion derived from a Landau model of the phase
transition, including dissipative stresses. Our system sizes, of up to 256^3
points, are large enough to reveal many structures observed experimentally.
Most patterns found at late stages in the relaxation are multiply banded; all
three tetragonal variants appear, but inequivalently. Two of the variants form
broad primary bands; the third intrudes into the others to form narrow
secondary bands with the hosts. On colliding with walls between the primary
variants, the third either terminates or forms a chevron. The multipy banded
patterns, with the two domain sizes, the chevrons and the terminations, are
seen in the microscopy of zirconia and other cubic-tetragonal ferroelastics. We
examine also transient structures obtained much earlier in the relaxation;
these show the above features and others also observed in experiment.Comment: 7 pages, 6 colour figures not embedded in text. Major revisions in
conten
Genomic history of the seventh pandemic of cholera in Africa.
The seventh cholera pandemic has heavily affected Africa, although the origin and continental spread of the disease remain undefined. We used genomic data from 1070 Vibrio cholerae O1 isolates, across 45 African countries and over a 49-year period, to show that past epidemics were attributable to a single expanded lineage. This lineage was introduced at least 11 times since 1970, into two main regions, West Africa and East/Southern Africa, causing epidemics that lasted up to 28 years. The last five introductions into Africa, all from Asia, involved multidrug-resistant sublineages that replaced antibiotic-susceptible sublineages after 2000. This phylogenetic framework describes the periodicity of lineage introduction and the stable routes of cholera spread, which should inform the rational design of control measures for cholera in Africa
Genome-Wide Transcriptome Analyses of Silicon Metabolism in Phaeodactylum tricornutum Reveal the Multilevel Regulation of Silicic Acid Transporters
BACKGROUND:Diatoms are largely responsible for production of biogenic silica in the global ocean. However, in surface seawater, Si(OH)(4) can be a major limiting factor for diatom productivity. Analyzing at the global scale the genes networks involved in Si transport and metabolism is critical in order to elucidate Si biomineralization, and to understand diatoms contribution to biogeochemical cycles. METHODOLOGY/PRINCIPAL FINDINGS:Using whole genome expression analyses we evaluated the transcriptional response to Si availability for the model species Phaeodactylum tricornutum. Among the differentially regulated genes we found genes involved in glutamine-nitrogen pathways, encoding putative extracellular matrix components, or involved in iron regulation. Some of these compounds may be good candidates for intracellular intermediates involved in silicic acid storage and/or intracellular transport, which are very important processes that remain mysterious in diatoms. Expression analyses and localization studies gave the first picture of the spatial distribution of a silicic acid transporter in a diatom model species, and support the existence of transcriptional and post-transcriptional regulations. CONCLUSIONS/SIGNIFICANCE:Our global analyses revealed that about one fourth of the differentially expressed genes are organized in clusters, underlying a possible evolution of P. tricornutum genome, and perhaps other pennate diatoms, toward a better optimization of its response to variable environmental stimuli. High fitness and adaptation of diatoms to various Si levels in marine environments might arise in part by global regulations from gene (expression level) to genomic (organization in clusters, dosage compensation by gene duplication), and by post-transcriptional regulation and spatial distribution of SIT proteins
Réalisation de déflecteurs acousto-optiques composites à grande capacité
The use of a single transducer in acousto-optic deflectors does not allow us to use all the available bandwidth of the interaction material. In this paper, we present a new method of coupling cascaded and independent transducers of adjacent bandwidths. This new method increases the deflection capacity. When applied to quartz transducers vibrating in water, this method led us to a new XY deflector which, compared to a single transducer, had a much greater capacity and compacity. Moreover, the access time and the optical losses were substantially reduced.La constitution de déflecteurs acousto-optiques utilisant un seul transducteur ne permet pas de tirer parti de toute la bande acoustique disponible du matériau d'interaction. En vue d'augmenter la capacité de déflexion, nous proposons dans cet article une nouvelle méthode d'association de transducteurs. Ceux-ci sont disposés en cascade, étagés en fréquence et leurs bandes sont contiguës. En outre, chacun d'entre eux est orienté à l'incidence de Bragg pour sa fréquence centrale. Cette méthode que nous avons appliquée à un déflecteur XY quartz-eau, comparée à une réalisation antérieure ne faisant intervenir qu'un seul transducteur par direction, a permis en effet d'augmenter considérablement la capacité et la compacité du systÚme tout en diminuant substantiellement le temps d'accÚs et les pertes optiques
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