2,963 research outputs found
Inflationary universe in loop quantum cosmology
Loop quantum cosmology provides a nice solution of avoiding the big bang
singularity through a big bounce mechanism in the high energy region. In loop
quantum cosmology an inflationary universe is emergent after the big bounce, no
matter what matter component is filled in the universe. A super-inflation phase
without phantom matter will appear in a certain way in the initial stage after
the bounce; then the universe will undergo a normal inflation stage. We discuss
the condition of inflation in detail in this framework. Also, for slow-roll
inflation, we expect the imprint from the effects of the loop quantum cosmology
should be left in the primordial perturbation power spectrum. However, we show
that this imprint is too weak to be observed.Comment: 21 pages, 4 figures; accepted for publication in JCA
Bouncing Universe with Quintom Matter
The bouncing universe provides a possible solution to the Big Bang
singularity problem. In this paper we study the bouncing solution in the
universe dominated by the Quintom matter with an equation of state (EoS)
crossing the cosmological constant boundary. We will show explicitly the
analytical and numerical bouncing solutions in three types of models for the
Quintom matter with an phenomenological EoS, the two scalar fields and a scalar
field with a modified Born-Infeld action.Comment: 7 pages, 5 figure
Cyclic cosmology from Lagrange-multiplier modified gravity
We investigate cyclic and singularity-free evolutions in a universe governed
by Lagrange-multiplier modified gravity, either in scalar-field cosmology, as
well as in one. In the scalar case, cyclicity can be induced by a
suitably reconstructed simple potential, and the matter content of the universe
can be successfully incorporated. In the case of -gravity, cyclicity can
be induced by a suitable reconstructed second function of a very
simple form, however the matter evolution cannot be analytically handled.
Furthermore, we study the evolution of cosmological perturbations for the two
scenarios. For the scalar case the system possesses no wavelike modes due to a
dust-like sound speed, while for the case there exist an oscillation
mode of perturbations which indicates a dynamical degree of freedom. Both
scenarios allow for stable parameter spaces of cosmological perturbations
through the bouncing point.Comment: 8 pages, 3 figures, references added, accepted for publicatio
Reversible room-temperature ferromagnetism in Nb-doped SrTiO3 single crystals
The search for oxide-based room-temperature ferromagnetism has been one of
the holy grails in condensed matter physics. Room-temperature ferromagnetism
observed in Nb-doped SrTiO3 single crystals is reported in this Rapid
Communication. The ferromagnetism can be eliminated by air annealing (making
the samples predominantly diamagnetic) and can be recovered by subsequent
vacuum annealing. The temperature dependence of magnetic moment resembles the
temperature dependence of carrier density, indicating that the magnetism is
closely related to the free carriers. Our results suggest that the
ferromagnetism is induced by oxygen vacancies. In addition, hysteretic
magnetoresistance was observed for magnetic field parallel to current,
indicating that the magnetic moments are in the plane of the samples. The x-ray
photoemission spectroscopy, the static time-of-flight and the dynamic secondary
ion mass spectroscopy and proton induced x-ray emission measurements were
performed to examine magnetic impurities, showing that the observed
ferromagnetism is unlikely due to any magnetic contaminant.Comment: 6 pages, 6 figure
Bounce and cyclic cosmology in extended nonlinear massive gravity
We investigate non-singular bounce and cyclic cosmological evolutions in a
universe governed by the extended nonlinear massive gravity, in which the
graviton mass is promoted to a scalar-field potential. The extra freedom of the
theory can lead to certain energy conditions violations and drive cyclicity
with two different mechanisms: either with a suitably chosen scalar-field
potential under a given Stuckelberg-scalar function, or with a suitably chosen
Stuckelberg-scalar function under a given scalar-field potential. Our analysis
shows that extended nonlinear massive gravity can alter significantly the
evolution of the universe at both early and late times.Comment: 20 pages, 5 figures, version published at JCA
Novel role for the innate immune receptor toll-like receptor 4 (TLR4) in the regulation of the wnt signaling pathway and photoreceptor apoptosis
Recent evidence has implicated innate immunity in regulating neuronal survival in the brain during stroke and other neurodegenerations. Photoreceptors are specialized light-detecting neurons in the retina that are essential for vision. In this study, we investigated the role of the innate immunity receptor TLR4 in photoreceptors. TLR4 activation by lipopolysaccharide (LPS) significantly reduced the survival of cultured mouse photoreceptors exposed to oxidative stress. With respect to mechanism, TLR4 suppressed Wnt signaling, decreased phosphorylation and activation of the Wnt receptor LRP6, and blocked the protective effect of the Wnt3a ligand. Paradoxically, TLR4 activation prior to oxidative injury protected photoreceptors, in a phenomenon known as preconditioning. Expression of TNFα and its receptors TNFR1 and TNFR2 decreased during preconditioning, and preconditioning was mimicked by TNFα antagonists, but was independent of Wnt signaling. Therefore, TLR4 is a novel regulator of photoreceptor survival that acts through the Wnt and TNFα pathways. © 2012 Yi et al
Water‐assisted programmable assembly of flexible and self‐standing Janus membranes
Janus membranes with asymmetric wettability have been considered cutting-edge for energy/environmental-sustainable applications like water/fog harvester, breathable skin, and smart sensor; however, technical challenges in fabrication and accurate regulation of asymmetric wettability limit their development. Herein, by using water-assisted hydrogen-bonded (H-bonded) assembly of small molecules at water/oil interface, a facile strategy is proposed for one-step fabrication of membranes with well-regulable asymmetric wettability. Asymmetric orderly patterns, beneficial for mass transport based on abundant high-permeability sites and large surface area, are constructed on opposite membrane surfaces. Upon tuning water-assisted H-bonding via H-sites/configuration design and temperature/pH modulation, double-hydrophobic, double-hydrophilic, and hydrophobic-hydrophilic membranes are facilely fabricated. The Janus membranes show smart vapor-responsive curling and unidirectional water transport with promising flux of 1158±25 L m−2 h−1 under natural gravity and 31500±670 L·(m−2 h−1 bar−1) at negative pressure. This bottom-up approach offers a feasible-to-scalable avenue to precise-manipulation of Janus membranes for advanced applications, providing an effective pathway for developing tailor-made self-assembled nanomaterials
Association of LEP G2548A and LEPR Q223R Polymorphisms with Cancer Susceptibility: Evidence from a Meta-Analysis
__Background:__ Numerous epidemiological studies have examined associations of genetic variations in LEP (G2548A, -2548 nucleotide upstream of the ATG start site) and LEPR (Q223R, nonsynonymous SNP in exon 6) with cancer susceptibility; however, the findings are inconsistent. Therefore, we performed a meta-analysis to comprehensively evaluate such associations.
__Methods:__ We searched published literature from MEDLINE, EMBASE, Web of Science and CBM for eligible publications. We also assessed genotype-based mRNA expression data from HapMap for rs7799039 (G2548A) and rs1137101 (Q223R) in normal cell lines derived from 270 subjects with different ethnicities.
__Results:__ The final analysis included 16 published studies of 6569 cases and 8405 controls for the LEP G2548A and 19 studies of 7504 cases and 9581 controls for the LEPR Q223R. Overall, LEP G2548A was statistically significantly associated with an increased risk of overall cancer (AA vs. GG: OR=1.27, 95% CI=1.05-1.54; recessive model: OR=1.19, 9
p-type delta-doped layers in silicon: structural and electronic properties
We report on the properties of p-type delta-doped layers prepared in molecular beam epitaxy-Si by growth interruption and evaporation of elemental B. Secondary-ion mass spectrometry measurements at several primary ion energies have been used to show that the full width at half maximum is ~2 nm. Hall measurements confirm that the layers are completely activated at 300 K with a mobility of 30±5 cm2/V s for a carrier density of (9±2)×1012 cm−2. At temperatures below 70 K nonmetallic behavior is observed which we have attributed to conduction between impurity states. It is concluded that the critical acceptor separation for the Mott metal-insulator transition in this system is significantly less than the value found in uniformly doped Si:B
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