116 research outputs found
Smectic order, pinning, and phase transition in a smectic liquid crystal cell with a random substrate
We study smectic-liquid-crystal order in a cell with a heterogeneous
substrate imposing surface random positional and orientational pinnings.
Proposing a minimal random elastic model, we demonstrate that, for a thick
cell, the smectic state without a rubbed substrate is always unstable at long
scales and, for weak random pinning, is replaced by a smectic glass state. We
compute the statistics of the associated substrate-driven distortions and the
characteristic smectic domain size on the heterogeneous substrate and in the
bulk. We find that for weak disorder, the system exhibits a three-dimensional
temperature-controlled phase transition between a weakly and strongly pinned
smectic glass states akin to the Cardy-Ostlund phase transition. We explore
experimental implications of the predicted phenomenology and suggest that it
provides a plausible explanation for the experimental observations on polarized
light microscopy and x-ray scattering.Comment: 30 pages, 11 figures, Published in PRE, with minor typos correcte
Nonequilibrium steady states in a vibrated-rod monolayer: tetratic, nematic and smectic correlations
We study experimentally the nonequilibrium phase behaviour of a horizontal
monolayer of macroscopic rods. The motion of the rods in two dimensions is
driven by vibrations in the vertical direction. Aside from the control
variables of packing fraction and aspect ratio that are typically explored in
molecular liquid crystalline systems, due to the macroscopic size of the
particles we are also able to investigate the effect of the precise shape of
the particle on the steady states of this driven system. We find that the shape
plays an important role in determining the nature of the orientational ordering
at high packing fraction. Cylindrical particles show substantial tetratic
correlations over a range of aspect ratios where spherocylinders have
previously been shown by Bates et al (JCP 112, 10034 (2000)) to undergo
transitions between isotropic and nematic phases. Particles that are thinner at
the ends (rolling pins or bails) show nematic ordering over the same range of
aspect ratios, with a well-established nematic phase at large aspect ratio and
a defect-ridden nematic state with large-scale swirling motion at small aspect
ratios. Finally, long-grain, basmati rice, whose geometry is intermediate
between the two shapes above, shows phases with strong indications of smectic
order.Comment: 18 pages and 13 eps figures, references adde
Fluctuations and phase transitions in Larkin-Ovchinnikov liquid crystal states of population-imbalanced resonant Fermi gas
Motivated by a realization of imbalanced Feshbach-resonant atomic Fermi
gases, we formulate a low-energy theory of the Fulde-Ferrell and the
Larkin-Ovchinnikov (LO) states and use it to analyze fluctuations, stability,
and phase transitions in these enigmatic finite momentum-paired superfluids.
Focusing on the unidirectional LO pair-density wave state, that spontaneously
breaks the continuous rotational and translational symmetries, we show that it
is characterized by two Goldstone modes, corresponding to a superfluid phase
and a smectic phonon. Because of the liquid-crystalline "softness" of the
latter, at finite temperature the 3d state is characterized by a vanishing LO
order parameter, quasi-Bragg peaks in the structure and momentum distribution
functions, and a "charge"-4, paired Cooper-pairs, off-diagonal-long-range
order, with a superfluid-stiffness anisotropy that diverges near a transition
into a nonsuperfluid state. In addition to conventional integer vortices and
dislocations the LO superfluid smectic exhibits composite half-integer
vortex-dislocation defects. A proliferation of defects leads to a rich variety
of descendant states, such as the "charge"-4 superfluid and Fermi-liquid
nematics and topologically ordered nonsuperfluid states, that generically
intervene between the LO state and the conventional superfluid and the
polarized Fermi-liquid at low and high imbalance, respectively. The fermionic
sector of the LO gapless superconductor is also quite unique, exhibiting a
Fermi surface of Bogoliubov quasiparticles associated with the Andreev band of
states, localized on the array of the LO domain-walls.Comment: 56 pages, 21 figure
Histogram Monte Carlo study of multicritical behavior in the hexagonal easy-axis Heisenberg antiferromagnet
The results of a detailed histogram Monte-Carlo study of critical-fluctuation
effects on the magnetic-field temperature phase diagram associated with the
hexagonal Heisenberg antiferromagnet with weak axial anisotropy are reported.
The multiphase point where three lines of continuous transitions merge at the
spin-flop boundary exhibits a structure consistent with scaling theory but
without the usual umbilicus as found in the case of a bicritical point.Comment: 7 pages (RevTex 3.0), 1 figure available upon request, CRPS-93-1
Attitudes Toward Organizational Change among Public Middle Managers
Positive attitudes toward change (PATC) are an important current issue in public
organizations facing profound financial and managerial reforms. This study aims to
identify social and organizational antecedents of PATC. The investigated population
is composed of middle managers working in Swiss public hospitals (N = 720), which
are currently being confronted by major reforms. Partial mediation effects of
organizational commitment (OC) in the relationships between independent variables
and PATC are also controlled. The findings show that perceived social support (work
relationships with colleagues and supervisors) as well as perceived organizational
support (employee voice and participation, information and communication, work-life
balance) are positively and significantly related to PATC. Stress perception is shown
to have a negative impact on PATC. This article provides valuable contributions with
respect to antecedents of attitudes toward change in a population of public middle
managers
“Interactive Technology Assessment” and Beyond: the Field Trial of Genetically Modified Grapevines at INRA-Colmar
International audienc
Producing the nation through philanthropy: legitimizing co-ethnic and pro-refugee civic action in Hungary
Dopamine in the dorsal hippocampus impairs the late-consolidation of cocaine-associated memory
Cocaine is thought to be addictive because it elevates dopamine levels in the striatum, reinforcing drug-seeking habits. Cocaine also elevates dopamine levels in the hippocampus, a structure involved in contextual conditioning as well as in reward function. Hippocampal dopamine promotes the late phase of consolidation of an aversive step-down avoidance memory. Here, we examined the role of hippocampal dopamine function in the persistence of the conditioned increase in preference for a cocaine-associated compartment. Blocking dorsal hippocampal D1-type receptors (D1Rs) but not D2-type receptors (D2Rs) 12 h after a single training trial extended persistence of the normally short-lived memory; conversely, a general and a specific phospholipase C-coupled D1R agonist (but not a D2R or adenylyl cyclase-coupled D1R agonist) decreased the persistence of the normally long-lived memory established by three-trial training. These effects of D1 agents were opposite to those previously established in a step-down avoidance task, and were here also found to be opposite to those in a lithium chloride-conditioned avoidance task. After returning to normal following cocaine injection, dopamine levels in the dorsal hippocampus were found elevated again at the time when dopamine antagonists and agonists were effective: between 13 and 17 h after cocaine injection. These findings confirm that, long after the making of a cocaine-place association, hippocampal activity modulates memory consolidation for that association via a dopamine-dependent mechanism. They suggest a dynamic role for dorsal hippocampal dopamine in this late-phase memory consolidation and, unexpectedly, differential roles for late consolidation of memories for places that induce approach or withdrawal because of a drug association.Fil: Kramar, Cecilia Paula. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Houssay. Instituto de Biología Celular y Neurociencia "Prof. Eduardo de Robertis". Universidad de Buenos Aires. Facultad de Medicina. Instituto de Biología Celular y Neurociencia; ArgentinaFil: Chefer, Vladimir I.. National Institutes of Health; Estados UnidosFil: Wise, Roy A.. National Institutes of Health; Estados UnidosFil: Medina, Jorge Horacio. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Houssay. Instituto de Biología Celular y Neurociencia "Prof. Eduardo de Robertis". Universidad de Buenos Aires. Facultad de Medicina. Instituto de Biología Celular y Neurociencia; Argentina. Universidad de Buenos Aires. Facultad de Medicina. Departamento de Ciencias Fisiológicas; ArgentinaFil: Barbano, María Flavia. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Houssay. Instituto de Biología Celular y Neurociencia "Prof. Eduardo de Robertis". Universidad de Buenos Aires. Facultad de Medicina. Instituto de Biología Celular y Neurociencia; Argentin
Increased Secreted Amyloid Precursor Protein-α (sAPPα) in Severe Autism: Proposal of a Specific, Anabolic Pathway and Putative Biomarker
Autism is a neurodevelopmental disorder characterized by deficits in verbal communication, social interactions, and the presence of repetitive, stereotyped and compulsive behaviors. Excessive early brain growth is found commonly in some patients and may contribute to disease phenotype. Reports of increased levels of brain-derived neurotrophic factor (BDNF) and other neurotrophic-like factors in autistic neonates suggest that enhanced anabolic activity in CNS mediates this overgrowth effect. We have shown previously that in a subset of patients with severe autism and aggression, plasma levels of the secreted amyloid-β (Aβ) precursor protein-alpha form (sAPPα) were significantly elevated relative to controls and patients with mild-to-moderate autism. Here we further tested the hypothesis that levels of sAPPα and sAPPβ (proteolytic cleavage products of APP by α- and β-secretase, respectively) are deranged in autism and may contribute to an anabolic environment leading to brain overgrowth. We measured plasma levels of sAPPα, sAPPβ, Aβ peptides and BDNF by corresponding ELISA in a well characterized set of subjects. We included for analysis 18 control, 6 mild-to-moderate, and 15 severely autistic patient plasma samples. We have observed that sAPPα levels are increased and BDNF levels decreased in the plasma of patients with severe autism as compared to controls. Further, we show that Aβ1-40, Aβ1-42, and sAPPβ levels are significantly decreased in the plasma of patients with severe autism. These findings do not extend to patients with mild-to-moderate autism, providing a biochemical correlate of phenotypic severity. Taken together, this study provides evidence that sAPPα levels are generally elevated in severe autism and suggests that these patients may have aberrant non-amyloidogenic processing of APP
Impaired Adult Neurogenesis in the Dentate Gyrus of a Triple Transgenic Mouse Model of Alzheimer's Disease
It has become generally accepted that new neurones are added and integrated mainly in two areas of the mammalian CNS, the subventricular zone and the subgranular zone (SGZ) of the dentate gyrus (DG) of the hippocampus, which is of central importance in learning and memory. The newly generated cells display neuronal morphology, are able to generate action potentials and receive functional synaptic inputs, i.e. their properties are similar to those found in mature neurones. Alzheimer's disease (AD) is the primary and widespread cause of dementia and is an age-related, progressive and irreversible neurodegenerative disease that deteriorates cognitive functions. Here, we have used male and female triple transgenic mice (3xTg-AD) harbouring three mutant genes (β-amyloid precursor protein, presenilin-1 and tau) and their respective non-transgenic (non-Tg) controls at 2, 3, 4, 6, 9 and 12 months of age to establish the link between AD and neurogenesis. Using immunohistochemistry we determined the area density of proliferating cells within the SGZ of the DG, measured by the presence of phosphorylated Histone H3 (HH3), and their possible co-localisation with GFAP to exclude a glial phenotype. Less than 1% of the HH3 labeled cells co-localised with GFAP. Both non-Tg and 3xTg-AD showed an age-dependent decrease in neurogenesis. However, male 3xTg-AD mice demonstrated a further reduction in the production of new neurones from 9 months of age (73% decrease) and a complete depletion at 12 months, when compared to controls. In addition, female 3xTg-AD mice showed an earlier but equivalent decrease in neurogenesis at 4 months (reduction of 63%) with an almost inexistent rate at 12 months (88% decrease) compared to controls. This reduction in neurogenesis was directly associated with the presence of β-amyloid plaques and an increase in the number of β-amyloid containing neurones in the hippocampus; which in the case of 3xgTg females was directly correlated. These results suggest that 3xTg-AD mice have an impaired ability to generate new neurones in the DG of the hippocampus, the severity of which increases with age and might be directly associated with the known cognitive impairment observed from 6 months of age onwards . The earlier reduction of neurogenesis in females, from 4 months, is in agreement with the higher prevalence of AD in women than in men. Thus it is conceivable to speculate that a recovery in neurogenesis rates in AD could help to rescue cognitive impairment
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