3,092 research outputs found
Relativistic Compact Objects in Isotropic Coordinates
We present a matrix method for obtaining new classes of exact solutions for
Einstein's equations representing static perfect fluid spheres. By means of a
matrix transformation, we reduce Einstein's equations to two independent
Riccati type differential equations for which three classes of solutions are
obtained. One class of the solutions corresponding to the linear barotropic
type fluid with an equation of state is discussed in detail.Comment: 9 pages, no figures, accepted for publication in Pramana-Journal of
Physic
IL-33 ameliorates Alzheimer’s disease-like pathology and cognitive decline
Alzheimer’s disease (AD) is a devastating condition with no known effective treatment. AD is characterized by memory loss as well as impaired locomotor ability, reasoning, and judgment. Emerging evidence suggests that the innate immune response plays a major role in the pathogenesis of AD. In AD, the accumulation of β-amyloid (Aβ) in the brain perturbs physiological functions of the brain, including synaptic and neuronal dysfunction, microglial activation, and neuronal loss. Serum levels of soluble ST2 (sST2), a decoy receptor for interleukin (IL)-33, increase in patients with mild cognitive impairment, suggesting that impaired IL-33/ST2 signaling may contribute to the pathogenesis of AD. Therefore, we investigated the potential therapeutic role of IL-33 in AD, using transgenic mouse models. Here we report that IL-33 administration reverses synaptic plasticity impairment and memory deficits in APP/PS1 mice. IL-33 administration reduces soluble Aβ levels and amyloid plaque deposition by promoting the recruitment and Aβ phagocytic activity of microglia; this is mediated by ST2/p38 signaling activation. Furthermore, IL-33 injection modulates the innate immune response by polarizing microglia/macrophages toward an antiinflammatory phenotype and reducing the expression of proinflammatory genes, including IL-1β, IL-6, and NLRP3, in the cortices of APP/PS1 mice. Collectively, our results demonstrate a potential therapeutic role for IL-33 in AD
Absolute Stability Limit for Relativistic Charged Spheres
We find an exact solution for the stability limit of relativistic charged
spheres for the case of constant gravitational mass density and constant charge
density. We argue that this provides an absolute stability limit for any
relativistic charged sphere in which the gravitational mass density decreases
with radius and the charge density increases with radius. We then provide a
cruder absolute stability limit that applies to any charged sphere with a
spherically symmetric mass and charge distribution. We give numerical results
for all cases. In addition, we discuss the example of a neutral sphere
surrounded by a thin, charged shell.Comment: 25 pages, 1 figure 1 June 07: Replaced with added citations to prior
work along same line
Graphene field-effect-transistors with high on/off current ratio and large transport band gap at room temperature
Graphene is considered to be a promising candidate for future
nano-electronics due to its exceptional electronic properties. Unfortunately,
the graphene field-effect-transistors (FETs) cannot be turned off effectively
due to the absence of a bandgap, leading to an on/off current ratio typically
around 5 in top-gated graphene FETs. On the other hand, theoretical
investigations and optical measurements suggest that a bandgap up to a few
hundred meV can be created by the perpendicular E-field in bi-layer graphenes.
Although previous carrier transport measurements in bi-layer graphene
transistors did indicate a gate-induced insulating state at temperature below 1
Kelvin, the electrical (or transport) bandgap was estimated to be a few meV,
and the room temperature on/off current ratio in bi-layer graphene FETs remains
similar to those in single-layer graphene FETs. Here, for the first time, we
report an on/off current ratio of around 100 and 2000 at room temperature and
20 K, respectively in our dual-gate bi-layer graphene FETs. We also measured an
electrical bandgap of >130 and 80 meV at average electric displacements of 2.2
and 1.3 V/nm, respectively. This demonstration reveals the great potential of
bi-layer graphene in applications such as digital electronics,
pseudospintronics, terahertz technology, and infrared nanophotonics.Comment: 3 Figure
Stabilization of test particles in Induced Matter Kaluza-Klein theory
The stability conditions for the motion of classical test particles in an -dimensional Induced Matter Kaluza-Klein theory is studied. We show that
stabilization requires a variance of the strong energy condition for the
induced matter to hold and that it is related to the hierarchy problem.
Stabilization of test particles in a FRW universe is also discussed.Comment: 15 pages, 1 figure, to appear in Class. Quantum Gra
Crossover from Fermi liquid to Wigner molecule behavior in quantum dots
The crossover from weak to strong correlations in parabolic quantum dots at
zero magnetic field is studied by numerically exact path-integral Monte Carlo
simulations for up to eight electrons. By the use of a multilevel blocking
algorithm, the simulations are carried out free of the fermion sign problem. We
obtain a universal crossover only governed by the density parameter . For
, the data are consistent with a Wigner molecule description, while
for , Fermi liquid behavior is recovered. The crossover value is surprisingly small.Comment: 4 pages RevTeX, 3 figures, corrected Tabl
Thermodynamic phase diagram and phase competition in BaFe2(As1-xPx)2 studied by thermal expansion
High-resolution thermal-expansion and specific-heat measurements were
performed on single crystalline BaFe2(As1-xPx)2 (0 < x < 0.33, x = 1). The
observation of clear anomalies allows to establish the thermodynamic phase
diagram which features a small coexistence region of SDW and superconductivity
with a steep rise of Tc on the underdoped side. Samples that undergo the
tetragonal-orthorhombic structural transition are detwinned in situ, and the
response of the sample length to the magneto-structural and superconducting
transitions is studied for all three crystallographic directions. It is shown
that a reduction of the magnetic order by superconductivity is reflected in all
lattice parameters. On the overdoped side, superconductivity affects the
lattice parameters in much the same way as the SDW on the underdoped side,
suggesting an intimate relation between the two types of order. Moreover, the
uniaxial pressure derivatives of Tc are calculated using the Ehrenfest relation
and are found to be large and anisotropic. A correspondence between
substitution and uniaxial pressure is established, i.e., uniaxial pressure
along the b-axis (c-axis) corresponds to a decrease (increase) of the P
content. By studying the electronic contribution to the thermal expansion we
find evidence for a maximum of the electronic density of states at optimal
doping
Excitonic Effects on Optical Absorption Spectra of Doped Graphene
We have performed first-principles calculations to study optical absorption
spectra of doped graphene with many-electron effects included. Both self-energy
corrections and electron-hole interactions are reduced due to the enhanced
screening in doped graphene. However, self-energy corrections and excitonic
effects nearly cancel each other, making the prominent optical absorption peak
fixed around 4.5 eV under different doping conditions. On the other hand, an
unexpected increase of the optical absorbance is observed within the infrared
and visible-light frequency regime (1 ~ 3 eV). Our analysis shows that a
combining effect from the band filling and electron-hole interactions results
in such an enhanced excitonic effect on the optical absorption. These unique
variations of the optical absorption of doped graphene are of importance to
understand relevant experiments and design optoelectronic applications.Comment: 15 pages, 5 figures; Nano Lett., Article ASAP (2011
Electrical Control of Plasmon Resonance with Graphene
Surface plasmon, with its unique capability to concentrate light into
sub-wavelength volume, has enabled great advances in photon science, ranging
from nano-antenna and single-molecule Raman scattering to plasmonic waveguide
and metamaterials. In many applications it is desirable to control the surface
plasmon resonance in situ with electric field. Graphene, with its unique
tunable optical properties, provides an ideal material to integrate with
nanometallic structures for realizing such control. Here we demonstrate
effective modulation of the plasmon resonance in a model system composed of
hybrid graphene-gold nanorod structure. Upon electrical gating the strong
optical transitions in graphene can be switched on and off, which leads to
significant modulation of both the resonance frequency and quality factor of
plasmon resonance in gold nanorods. Hybrid graphene-nanometallic structures, as
exemplified by this combination of graphene and gold nanorod, provide a general
and powerful way for electrical control of plasmon resonances. It holds promise
for novel active optical devices and plasmonic circuits at the deep
subwavelength scale
High On/Off Ratios in Bilayer Graphene Field Effect Transistors Realized by Surface Dopants
The unique property of bilayer graphene to show a band gap tunable by
external electrical fields enables a variety of different device concepts with
novel functionalities for electronic, optoelectronic and sensor applications.
So far the operation of bilayer graphene based field effect transistors
requires two individual gates to vary the channel's conductance and to create a
band gap. In this paper we report on a method to increase the on/off ratio in
single gated bilayer graphene field effect transistors by adsorbate doping. The
adsorbate dopants on the upper side of the graphene establish a displacement
field perpendicular to the graphene surface breaking the inversion symmetry of
the two graphene layers. Low temperature measurements indicate, that the
increased on/off ratio is caused by the opening of a mobility gap. Beside field
effect transistors the presented approach can also be employed for other
bilayer graphene based devices like photodetectors for THz to infrared
radiation, chemical sensors and in more sophisticated structures such as
antidot- or superlattices where an artificial potential landscape has to be
created.Comment: 4 pages, 4 figure
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