666 research outputs found
First principles modelling of magnesium titanium hydrides
Mixing Mg with Ti leads to a hydride Mg(x)Ti(1-x)H2 with markedly improved
(de)hydrogenation properties for x < 0.8, as compared to MgH2. Optically, thin
films of Mg(x)Ti(1-x)H2 have a black appearance, which is remarkable for a
hydride material. In this paper we study the structure and stability of
Mg(x)Ti(1-x)H2, x= 0-1 by first-principles calculations at the level of density
functional theory. We give evidence for a fluorite to rutile phase transition
at a critical composition x(c)= 0.8-0.9, which correlates with the
experimentally observed sharp decrease in (de)hydrogenation rates at this
composition. The densities of states of Mg(x)Ti(1-x)H2 have a peak at the Fermi
level, composed of Ti d states. Disorder in the positions of the Ti atoms
easily destroys the metallic plasma, however, which suppresses the optical
reflection. Interband transitions result in a featureless optical absorption
over a large energy range, causing the black appearance of Mg(x)Ti(1-x)H2.Comment: 22 pages, 9 figures, 4 table
Spin-Wave Spectrum in `Single-Domain' Magnetic Ground State of Triangular Lattice Antiferromagnet CuFeO2
By means of neutron scattering measurements, we have investigated spin-wave
excitation in a collinear four-sublattice (4SL) magnetic ground state of a
triangular lattice antiferromagnet CuFeO2, which has been of recent interest as
a strongly frustrated magnet, a spin-lattice coupled system and a multiferroic.
To avoid mixing of spin-wave spectrum from magnetic domains having three
different orientations reflecting trigonal symmetry of the crystal structure,
we have applied uniaxial pressure on [1-10] direction of a single crystal
CuFeO2. By elastic neutron scattering measurements, we have found that only 10
MPa of the uniaxial pressure results in almost 'single domain' state in the 4SL
phase. We have thus performed inelastic neutron scattering measurements using
the single domain sample, and have identified two distinct spin- wave branches.
The dispersion relation of the upper spin-wave branch cannot be explained by
the previous theoretical model [R. S. Fishman: J. Appl. Phys. 103 (2008)
07B109]. This implies the importance of the lattice degree of freedom in the
spin-wave excitation in this system, because the previous calculation neglected
the effect of the spin-driven lattice distortion in the 4SL phase. We have also
discussed relationship between the present results and the recently discovered
"electromagnon" excitation.Comment: 5 pages, 3 figures, accepted for publication in J. Phys. Soc. Jp
Superconductivity in novel BiS2-based layered superconductor LaO1-xFxBiS2
Layered superconductors have provided some interesting fields in condensed
matter physics owing to the low dimensionality of their electronic states. For
example, the high-Tc (high transition temperature) cuprates and the Fe-based
superconductors possess a layered crystal structure composed of a stacking of
spacer (blocking) layers and conduction (superconducting) layers, CuO2 planes
or Fe-Anion layers. The spacer layers provide carriers to the conduction layers
and induce exotic superconductivity. Recently, we have reported
superconductivity in the novel BiS2-based layered compound Bi4O4S3. It was
found that superconductivity of Bi4O4S3 originates from the BiS2 layers. The
crystal structure is composed of a stacking of BiS2 superconducting layers and
the spacer layers, which resembles those of high-Tc cuprate and the Fe-based
superconductors. Here we report a discovery of a new type of BiS2-based layered
superconductor LaO1-xFxBiS2, with a Tc as high as 10.6 K.Comment: 23 pages, 5 figures, 1 table (table caption has been revised), to
appear in J. Phys. Soc. Jp
Crystal structure of Cu-Sn-In alloys around the {\eta} phase field studied by neutron diffraction
The study of the Cu-Sn-In ternary system has become of great importance in
recent years, due to new environmental regulations forcing to eliminate the use
of Pb in bonding technologies for electronic devices. A key relevant issue
concerns the intermetallic phases which grow in the bonding zone and are
determining in their quality and performance. In this work, we focus in the
{\eta}-phase (Cu2In or Cu6Sn5) that exists in both end binaries and as a
ternary phase. We present a neutron diffraction study of the constitution and
crystallography of a series of alloys around the 60 at.% Cu composition, and
with In contents ranging from 0 to 25 at.%, quenched from 300\degreeC. The
alloys were characterized by scanning electron microscopy, probe microanalysis
and high-resolution neutron diffraction. The Rietveld refinement of neutron
diffraction data allowed to improve the currently available model for site
occupancies in the hexagonal {\eta}-phase in the binary Cu-Sn as well as in
ternary alloys. For the first time, structural data is reported in the ternary
Cu-Sn-In {\eta}-phase as a function of composition, information that is of
fundamental technological importance as well as valuable input data for ongoing
modelisations of the ternary phase diagram.Comment: 8 pages, 10 figure
On Quantum Markov Chains on Cayley tree II: Phase transitions for the associated chain with XY-model on the Cayley tree of order three
In the present paper we study forward Quantum Markov Chains (QMC) defined on
a Cayley tree. Using the tree structure of graphs, we give a construction of
quantum Markov chains on a Cayley tree. By means of such constructions we prove
the existence of a phase transition for the XY-model on a Cayley tree of order
three in QMC scheme. By the phase transition we mean the existence of two now
quasi equivalent QMC for the given family of interaction operators
.Comment: 34 pages, 1 figur
Effects of antenatal betamethasone on preterm human and mouse ductus arteriosus: comparison with baboon data.
BackgroundAlthough studies involving preterm infants ≤34 weeks gestation report a decreased incidence of patent ductus arteriosus after antenatal betamethasone, studies involving younger gestation infants report conflicting results.MethodsWe used preterm baboons, mice, and humans (≤276/7 weeks gestation) to examine betamethasone's effects on ductus gene expression and constriction both in vitro and in vivo.ResultsIn mice, betamethasone increased the sensitivity of the premature ductus to the contractile effects of oxygen without altering the effects of other contractile or vasodilatory stimuli. Betamethasone's effects on oxygen sensitivity could be eliminated by inhibiting endogenous prostaglandin/nitric oxide signaling. In mice and baboons, betamethasone increased the expression of several developmentally regulated genes that mediate oxygen-induced constriction (K+ channels) and inhibit vasodilator signaling (phosphodiesterases). In human infants, betamethasone increased the rate of ductus constriction at all gestational ages. However, in infants born ≤256/7 weeks gestation, betamethasone's contractile effects were only apparent when prostaglandin signaling was inhibited, whereas at 26-27 weeks gestation, betamethasone's contractile effects were apparent even in the absence of prostaglandin inhibitors.ConclusionsWe speculate that betamethasone's contractile effects may be mediated through genes that are developmentally regulated. This could explain why betamethasone's effects vary according to the infant's developmental age at birth
Rietveld refinement of Sr5(AsO4)3Cl from high-resolution synchrotron data
The apatite-type compound, pentastrontium tris[arsenate(V)] chloride, Sr5(AsO4)3Cl, has been synthesized by ion exchange at high temperature from a synthetic sample of mimetite [Pb5(AsO4)3Cl] with SrCO3 as a by-product. The results of the Rietveld refinement, based on high resolution synchrotron X-ray powder diffraction data, show that the title compound crystallizes in the same structure as other halogenoapatites with general formula A
5(YO4)3
X (A = divalent cation, Y = pentavalent cation, and X = F, Cl or Br) in the space group P63/m. The structure consists of isolated tetrahedral AsO4
3− anions (the As atom and two O atoms have m symmetry), separated by two crystallographically independent Sr2+ cations that are located on mirror planes and threefold rotation axes, respectively. One Sr atom is coordinated by nine O atoms and the other by six. The chloride anions (site symmetry ) are at the 2a sites and are located in the channels of the structure
Argentina, Brasil e Paraguai: educação superior e seus marcos normativos
O presente artigo é parte de relatório da pesquisa interinstitucional e internacional intitulada “Dilemas de nuevas culturas de producción de conocimiento. Los postgrados en Argentina, Brasil y Paraguay en el contexto de la evaluación de la calidad de la educación superior” e tem como objetivo compartilhar resultados referentes à etapa de descrição, a partir do desenho metodológico da pesquisa. O texto está dividido em duas partes: na primeira, apresentamos a metodologia da pesquisa, baseada em literatura da educação comparada; na segunda, há uma divisão entre a dimensão dos aspectos nacionais, com foco na definição de marco normativo em contraposição ao marco regulatório e no contexto macro legal da Educação Superior no Brasil, e a dimensão dos aspectos MERCOSUL, em que destacamos o contexto atual da pós-graduação e apresentamos informações acerca da origem e do desenvolvimento deste nível de ensino nos países envolvidos no projeto. Ao compartilhar os resultados, pretendemos contribuir com a discussão do tema não apenas em estudos nacionais, mas, em especial, para estudos de educação comparada
A classification model to predict synergism/antagonism of cytotoxic mixtures using protein-drug docking scores
- …