352 research outputs found
A tetragonal-to-monoclinic phase transition in a ferroelectric perovskite: the structure of PbZr(0.52)Ti(0.48)O3
The perovskite-like ferroelectric system PbZr(1-x)Ti(x)O3 (PZT) has a nearly
vertical morphotropic phase boundary (MPB) around x=0.45-0.50. Recent
synchrotron x-ray powder diffraction measurements by Noheda et al. [Appl. Phys.
Lett. 74, 2059 (1999)] have revealed a new monoclinic phase between the
previously-established tetragonal and rhombohedral regions. In the present work
we describe a Rietveld analysis of the detailed structure of the tetragonal and
monoclinic PZT phases on a sample with x= 0.48 for which the lattice parameters
are respectively: at= 4.044 A, ct= 4.138 A, at 325 K, and am= 5.721 A, bm=
5.708 A, cm= 4.138 A, beta= 90.496 deg., at 20K. In the tetragonal phase the
shifts of the atoms along the polar [001] direction are similar to those in
PbTiO3 but the refinement indicates that there are, in addition, local
disordered shifts of the Pb atoms of ~0.2 A perpendicular to the polar axis..
The monoclinic structure can be viewed as a condensation along one of the
directions of the local displacements present in the tetragonal phase. It
equally well corresponds to a freezing-out of the local displacements along one
of the directions recently reported by Corker et al.[J. Phys. Condens.
Matter 10, 6251 (1998)] for rhombohedral PZT. The monoclinic structure
therefore provides a microscopic picture of the MPB region in which one of the
"locally" monoclinic phases in the "average" rhombohedral or tetragonal
structures freezes out, and thus represents a bridge between these two phases.Comment: REVTeX, 7 figures. Modifications after referee's suggestion: new
figure (figure 5), comments in 2nd para. (Sect.III) and in 2nd & 3rd para.
(Sect. IV-a), in the abstract: "...of ~0.2 A perpendicular to the polar
axis.
Stability of the monoclinic phase in the ferroelectric perovskite PbZr(1-x)TixO3
Recent structural studies of ferroelectric PbZr(1-x)TixO3 (PZT) with x= 0.48,
have revealed a new monoclinic phase in the vicinity of the morphotropic phase
boundary (MPB), previously regarded as the the boundary separating the
rhombohedral and tetragonal regions of the PZT phase diagram. In the present
paper, the stability region of all three phases has been established from high
resolution synchrotron x-ray powder diffraction measurements on a series of
highly homogeneous samples with 0.42 <=x<= 0.52. At 20K the monoclinic phase is
stable in the range 0.46 <=x<= 0.51, and this range narrows as the temperature
is increased. A first-order phase transition from tetragonal to rhombohedral
symmetry is observed only for x= 0.45. The MPB, therefore, corresponds not to
the tetragonal-rhombohedral phase boundary, but instead to the boundary between
the tetragonal and monoclinic phases for 0.46 <=x<= 0.51. This result provides
important insight into the close relationship between the monoclinic phase and
the striking piezoelectric properties of PZT; in particular, investigations of
poled samples have shown that the monoclinic distortion is the origin of the
unusually high piezoelectric response of PZT.Comment: REVTeX file, 7 figures embedde
Hybrid exchange-correlation functional for accurate prediction of the electronic and structural properties of ferroelectric oxides
Using a linear combination of atomic orbitals approach, we report a
systematic comparison of various Density Functional Theory (DFT) and hybrid
exchange-correlation functionals for the prediction of the electronic and
structural properties of prototypical ferroelectric oxides. It is found that
none of the available functionals is able to provide, at the same time,
accurate electronic and structural properties of the cubic and tetragonal
phases of BaTiO and PbTiO. Some, although not all, usual DFT
functionals predict the structure with acceptable accuracy, but always
underestimate the electronic band gaps. Conversely, common hybrid functionals
yield an improved description of the band gaps, but overestimate the volume and
atomic distortions associated to ferroelectricity, giving rise to an
unacceptably large ratio for the tetragonal phases of both compounds.
This super-tetragonality is found to be induced mainly by the exchange energy
corresponding to the Generalized Gradient Approximation (GGA) and, to a lesser
extent, by the exact exchange term of the hybrid functional. We thus propose an
alternative functional that mixes exact exchange with the recently proposed GGA
of Wu and Cohen [Phys. Rev. B 73, 235116 (2006)] which, for solids, improves
over the treatment of exchange of the most usual GGA's. The new functional
renders an accurate description of both the structural and electronic
properties of typical ferroelectric oxides.Comment: 13 pages, 4 figures, 7 table
An investigation into aripiprazole's partial D(2) agonist effects within the dorsolateral prefrontal cortex during working memory in healthy volunteers
Rationale:
Working memory impairments in schizophrenia have been attributed to dysfunction of the dorsolateral prefrontal cortex (DLPFC) which in turn may be due to low DLPFC dopamine innervation. Conventional antipsychotic drugs block DLPFC D2 receptors, and this may lead to further dysfunction and working memory impairments. Aripiprazole is a D2 receptor partial agonist hypothesised to enhance PFC dopamine functioning, possibly improving working memory.
Objectives:
We probed the implications of the partial D2 receptor agonist actions of aripiprazole within the DLPFC during working memory. Investigations were carried out in healthy volunteers to eliminate confounds of illness or medication status. Aripiprazoleβs prefrontal actions were compared with the D2/5-HT2A blocker risperidone to separate aripiprazoleβs unique prefrontal D2 agonist actions from its serotinergic and striatal D2 actions that it shares with risperidone.
Method:
A double-blind, placebo-controlled, parallel design was implemented. Participants received a single dose of either 5 mg aripiprazole, 1 mg risperidone or placebo before performing the n-back task whilst undergoing fMRI scanning.
Results:
Compared with placebo, the aripiprazole group demonstrated enhanced DLPFC activation associated with a trend for improved discriminability (dβ) and speeded reaction times. In contrast to aripiprazoleβs neural effects, the risperidone group demonstrated a trend for reduced DLPFC recruitment. Unexpectedly, the risperidone group demonstrated similar effects to aripiprazole on dβ and additionally had reduced errors of commission compared with placebo.
Conclusion:
Aripiprazole has unique DLPFC actions attributed to its prefrontal D2 agonist action. Risperidoneβs serotinergic action that results in prefrontal dopamine release may have protected against any impairing effects of its prefrontal D2 blockade
Active Carboxylic Acid-Terminated Alkanethiol Self-Assembled Monolayers on Gold Bead Electrodes for Immobilization of Cytochromes c
Reduced conditioned fear response in mice that lack Dlx1 and show subtype-specific loss of interneurons
The inhibitory GABAergic system has been implicated in multiple neuropsychiatric diseases such as schizophrenia and autism. The Dlx homeobox transcription factor family is essential for development and function of GABAergic interneurons. Mice lacking the Dlx1 gene have postnatal subtype-specific loss of interneurons and reduced IPSCs in their cortex and hippocampus. To ascertain consequences of these changes in the GABAergic system, we performed a battery of behavioral assays on the Dlx1 mutant mice, including zero maze, open field, locomotor activity, food intake, rotarod, tail suspension, fear conditioning assays (context and trace), prepulse inhibition, and working memory related tasks (spontaneous alteration task and spatial working memory task). Dlx1 mutant mice displayed elevated activity levels in open field, locomotor activity, and tail suspension tests. These mice also showed deficits in contextual and trace fear conditioning, and possibly in prepulse inhibition. Their learning deficits were not global, as the mutant mice did not differ from the wild-type controls in tests of working memory. Our findings demonstrate a critical role for the Dlx1 gene, and likely the subclasses of interneurons that are affected by the lack of this gene, in behavioral inhibition and associative fear learning. These observations support the involvement of particular components of the GABAergic system in specific behavioral phenotypes related to complex neuropsychiatric diseases
Deficits in visuo-spatial working memory, inhibition and oculomotor control in boys with ADHD and their non-affected brothers.
Few studies have assessed visuo-spatial working memory and inhibition in attention-deficit/hyperactivity disorder (ADHD) by recording saccades and consequently little additional knowledge has been gathered on oculomotor functioning in ADHD. Moreover, this is the first study to report the performance of non-affected siblings of children with ADHD, which may shed light on the familiality of deficits. A total of 14 boys with ADHD, 18 non-affected brothers, and 15 control boys aged 7-14 years, were administered a memory-guided saccade task with delays of three and seven seconds. Familial deficits were found in accuracy of visuo-spatial working memory, percentage of anticipatory saccades, and tendency to overshoot saccades relative to controls. These findings suggest memory-guided saccade deficits may relate to a familial predisposition for ADHD
Towards Loop Quantum Supergravity (LQSG) I. Rarita-Schwinger Sector
In our companion papers, we managed to derive a connection formulation of
Lorentzian General Relativity in D+1 dimensions with compact gauge group
SO(D+1) such that the connection is Poisson commuting, which implies that Loop
Quantum Gravity quantisation methods apply. We also provided the coupling to
standard matter. In this paper, we extend our methods to derive a connection
formulation of a large class of Lorentzian signature Supergravity theories, in
particular 11d SUGRA and 4d, N = 8 SUGRA, which was in fact the motivation to
consider higher dimensions. Starting from a Hamiltonian formulation in the time
gauge which yields a Spin(D) theory, a major challenge is to extend the
internal gauge group to Spin(D+1) in presence of the Rarita-Schwinger field.
This is non trivial because SUSY typically requires the Rarita-Schwinger field
to be a Majorana fermion for the Lorentzian Clifford algebra and Majorana
representations of the Clifford algebra are not available in the same spacetime
dimension for both Lorentzian and Euclidean signature. We resolve the arising
tension and provide a background independent representation of the non trivial
Dirac antibracket *-algebra for the Majorana field which significantly differs
from the analogous construction for Dirac fields already available in the
literature.Comment: 43 pages. v2: Journal version. Some nonessential sign errors in
sections 2 and 3 corrected. Minor clarifications and correction
Gene expression markers of tendon fibroblasts in normal and diseased tissue compared to monolayer and three dimensional culture systems
<p>Abstract</p> <p>Background</p> <p>There is a paucity of data regarding molecular markers that identify the phenotype of the tendon cell. This study aims to quantify gene expression markers that distinguish between tendon fibroblasts and other mesenchymal cells which may be used to investigate tenogenesis.</p> <p>Methods</p> <p>Expression levels for 12 genes representative of musculoskeletal tissues, including the proposed tendon progenitor marker scleraxis, relative to validated reference genes, were evaluated in matched samples of equine tendon (harvested from the superficial digital flexor tendon), cartilage and bone using quantitative PCR (qPCR). Expression levels of genes associated with tendon phenotype were then evaluated in healthy, including developmental, and diseased equine tendon tissue and in tendon fibroblasts maintained in both monolayer culture and in three dimensional (3D) collagen gels.</p> <p>Results</p> <p>Significantly increased expression of scleraxis was found in tendon compared with bone (P = 0.002) but not compared to cartilage. High levels of COL1A2 and scleraxis and low levels of tenascin-C were found to be most representative of adult tensional tendon phenotype. While, relative expression of scleraxis in developing mid-gestational tendon or in acute or chronically diseased tendon did not differ significantly from normal adult tendon, tenascin-C message was significantly upregulated in acutely injured equine tendon (P = 0.001). Relative scleraxis gene expression levels in tendon cell monolayer and 3D cultures were significantly lower than in normal adult tendon (P = 0.002, P = 0.02 respectively).</p> <p>Conclusion</p> <p>The findings of this study indicate that high expression of both COL1A2 and scleraxis, and low expression of tenascin-C is representative of a tensional tendon phenotype. The <it>in vitro </it>culture methods used in these experiments however, may not recapitulate the phenotype of normal tensional tendon fibroblasts in tissues as evidenced by gene expression.</p
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