1,042 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
Microscopic Coexistence of Ferromagnetism and Superconductivity in Single-Crystal UCoGe
Unambiguous evidence for the microscopic coexistence of ferromagnetism and
superconductivity in UCoGe ( K and
0.6 K) is reported from Co nuclear quadrupole resonance (NQR). The
Co-NQR signal below 1 K indicates ferromagnetism throughout the sample
volume, while nuclear spin-lattice relaxation rate in the ferromagnetic
(FM) phase decreases below due to the opening of the
superconducting(SC) gap. The SC state was found to be inhomogeneous, suggestive
of a self-induced vortex state, potentially realizable in a FM superconductor.
In addition, the Co-NQR spectrum around show that the FM
transition in UCoGe possesses a first-order character, which is consistent with
the theoretical prediction that the low-temperature FM transition in itinerant
magnets is generically of first-order.Comment: 5 pages, 5 figure
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
Achirality in the low temperature structure and lattice modes of tris(acetylacetonate)iron(iii)
Tris(acetylacteonate) iron(III) is a relatively ubiquitous mononuclear inorganic coordination complex. The bidentate nature of the three acetylacteonate ligands coordinating around a single centre inevitably leads to structural isomeric forms, however whether or not this relates to chirality in the solid state has been questioned in the literature. Variable temperature neutron diffraction data down to T = 3 K, highlights the dynamic nature of the ligand environment, including the motions of the hydrogen atoms. The Fourier transform of the molecular dynamics simulation based on the experimentally determined structure was shown to closely reproduce the low temperature vibrational density of states obtained using inelastic neutron scattering
Quasi-molecular and atomic phases of dense solid hydrogen
The high-pressure phases of solid hydrogen are of fundamental interest and
relevant to the interior of giant planets; however, knowledge of these phases
is far from complete. Particle swarm optimization (PSO) techniques were applied
to a structural search, yielding hitherto unexpected high-pressure phases of
solid hydrogen at pressures up to 5 TPa. An exotic quasi-molecular mC24
structure (space group C2/c, stable at 0.47-0.59 TPa) with two types of
intramolecular bonds was predicted, providing a deeper understanding of
molecular dissociation in solid hydrogen, which has been a mystery for decades.
We further predicted the existence of two atomic phases: (i) the oC12 structure
(space group Cmcm, stable at > 2.1 TPa), consisting of planar H3 clusters, and
(ii) the cI16 structure, previously observed in lithium and sodium, stable
above 3.5 TPa upon consideration of the zero-point energy. This work clearly
revised the known zero-temperature and high-pressure (>0.47 TPa) phase diagram
for solid hydrogen and has implications for the constituent structures of giant
planets.Comment: accepted in The Journal of Physical Chemistr
3,12-Diaza-6,9-diazonia-2,13-dioxotetradecane bis(perchlorate)
The crystal structure of the title diprotonated diacetyltriethylenetetramine (DAT) perchorate salt, C10H24N4O2
2+·2ClO4
−, can be described as a three-dimensional assembly of alternating layers consisting of diprotonated diacetyltriethylenetetramine (H2DAT)2+ strands along [100] and the anionic species ClO4
−. The (H2DAT)2+ cations in the strands are connected via N—H⋯O hydrogen bonding between the acetyl groups and the amine groups of neighbouring (H2DAT)2+ cations. Layers of (H2DAT)2+ strands and perchlorate anions are connected by a network of hydrogen bonds between the NH and NH2 groups and the O atoms of the perchlorate anion. The asymmetric unit consits of one perchlorate anion in a general position, as well as of one cation that is located on a center of inversion
Heavy-Mass Behavior of Ordered Perovskites ACu3Ru4O12 (A = Na, Ca, La)
We synthesized ACu3Ru4O12 (A = Na, Na0.5Ca0.5, Ca, Ca0.5La0.5, La) and
measured their DC magnetization, AC susceptibility, specific heat, and
resistivity, in order to investigate the effects of the hetero-valent
substitution. A broad peak in the DC magnetization around 200 K was observed
only in CaCu3Ru4O12, suggesting the Kondo effect due to localized Cu2+ ions.
However, the electronic specific heat coefficients exhibit large values not
only for CaCu3Ru4O12 but also for all the other samples. Moreover, the Wilson
ratio and the Kadowaki-Woods ratio of our samples are all similar to the values
of other heavy-fermion compounds. These results question the Kondo effect as
the dominant origin of the mass enhancement, and rather indicate the importance
of correlations among itinerant Ru electrons.Comment: 6 pages, 6 figures, to be published in J. Phys. Soc. Jp
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
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
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
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