1,672 research outputs found
Asymmetric core combustion in neutron stars and a potential mechanism for gamma ray bursts
We study the transition of nuclear matter to strange quark matter (SQM)
inside neutron stars (NSs). It is shown that the influence of the magnetic
field expected to be present in NS interiors has a dramatic effect on the
propagation of a laminar deflagration (widely studied so far), generating a
strong acceleration of the flame in the polar direction. This results in a
strong asymmetry in the geometry of the just formed core of hot SQM which
resembles a cylinder orientated in the direction of the magnetic poles of the
NS. This geometrical asymmetry gives rise to a bipolar emission of the thermal
neutrino-antineutrino pairs produced in the process of SQM formation. The
neutrino-antineutrino pairs annihilate into electron-positron pairs just above
the polar caps of the NS giving rise to a relativistic fireball, thus providing
a suitable form of energy transport and conversion to gamma emission that may
be associated to short gamma ray bursts (GRBs).Comment: 2 figure
Enhancement of piezoelectricity in a mixed ferroelectric
We use first-principles density-functional total energy and polarization
calculations to calculate the piezoelectric tensor at zero temperature for both
cubic and simple tetragonal ordered supercells of Pb_3GeTe_4. The largest
piezoelectric coefficient for the tetragonal configuration is enhanced by a
factor of about three with respect to that of the cubic configuration. This can
be attributed to both the larger strain-induced motion of cations relative to
anions and higher Born effective charges in the tetragonal case. A normal mode
decomposition shows that both cation ordering and local relaxation weaken the
ferroelectric instability, enhancing piezoelectricity.Comment: 5 pages, revtex, 2 eps figure
Lattice instabilities of PbZrO3/PbTiO3 [1:1] superlattices from first principles
Ab initio phonon calculations for the nonpolar reference structures of the
(001), (110), and (111) PbZrO_3/PbTiO_3 [1:1] superlattices are presented. The
unstable polar modes in the tetragonal (001) and (110) structures are confined
in either the Ti- or the Zr-centered layers and display two-mode behavior,
while in the cubic (111) case one-mode behavior is observed. Instabilities with
pure oxygen character are observed in all three structures. The implications
for the ferroelectric behavior and related properties are discussed.Comment: 12 pages, 2 figures, 7 tables, submitted to PR
Thermal and Bulk Comptonization in Accretion-Powered X-Ray Pulsars
We develop a new theoretical model for the spectral formation process in
accretion-powered X-ray pulsars based on a detailed treatment of the bulk and
thermal Comptonization occurring in the accreting, shocked gas. A rigorous
eigenfunction expansion method is employed to obtain the analytical solution
for the Green's function describing the scattering of radiation injected into
the column from a monochromatic source located at an arbitrary height above the
stellar surface. The emergent spectrum is calculated by convolving the Green's
function with source terms corresponding to bremsstrahlung, cyclotron, and
blackbody emission. The energization of the photons in the shock, combined with
cyclotron absorption, naturally produces an X-ray spectrum with a relatively
flat continuum shape and a high-energy quasi-exponential cutoff. We demonstrate
that the new theory successfully reproduces the phase-averaged spectra of the
bright pulsars Her X-1, LMC X-4, and Cen X-3. In these luminous sources, it is
shown that the emergent spectra are dominated by Comptonized bremsstrahlung
emission.Comment: accepted for publication in Ap
Electrospun amplified fiber optics
A lot of research is focused on all-optical signal processing, aiming to
obtain effective alternatives to existing data transmission platforms.
Amplification of light in fiber optics, such as in Erbium-doped fiber
amplifiers, is especially important for an efficient signal transmission.
However, the complex fabrication methods, involving high-temperature processes
performed in highly pure environment, slow down the fabrication and make
amplified components expensive with respect to an ideal, high-throughput and
room temperature production. Here, we report on near infrared polymer fiber
amplifiers, working over a band of about 20 nm. The fibers are cheap, spun with
a process entirely carried out at room temperature, and show amplified
spontaneous emission with good gain coefficients as well as low optical losses
(a few cm^-1). The amplification process is favoured by the high fiber quality
and low self-absorption. The found performance metrics promise to be suitable
for short-distance operation, and the large variety of commercially-available
doping dyes might allow for effective multi-wavelength operation by electrospun
amplified fiber optics.Comment: 27 pages, 8 figure
Global compactness for a class of quasi-linear elliptic problems
We prove a global compactness result for Palais-Smale sequences associated
with a class of quasi-linear elliptic equations on exterior domains.Comment: 19 page
Phonons and related properties of extended systems from density-functional perturbation theory
This article reviews the current status of lattice-dynamical calculations in
crystals, using density-functional perturbation theory, with emphasis on the
plane-wave pseudo-potential method. Several specialized topics are treated,
including the implementation for metals, the calculation of the response to
macroscopic electric fields and their relevance to long wave-length vibrations
in polar materials, the response to strain deformations, and higher-order
responses. The success of this methodology is demonstrated with a number of
applications existing in the literature.Comment: 52 pages, 14 figures, submitted to Review of Modern Physic
Expression and Differential Responsiveness of Central Nervous System Glial Cell Populations to the Acute Phase Protein Serum Amyloid A
Acute-phase response is a systemic reaction to environmental/inflammatory insults and involves hepatic production of acute-phase proteins, including serum amyloid A (SAA). Extrahepatically, SAA immunoreactivity is found in axonal myelin sheaths of cortex in Alzheimer's disease and multiple sclerosis (MS), although its cellular origin is unclear. We examined the responses of cultured rat cortical astrocytes, microglia and oligodendrocyte precursor cells (OPCs) to master pro-inflammatory cytokine tumour necrosis factor (TNF)-\u3b1 and lipopolysaccaride (LPS). TNF-\u3b1 time-dependently increased Saa1 (but not Saa3) mRNA expression in purified microglia, enriched astrocytes, and OPCs (as did LPS for microglia and astrocytes). Astrocytes depleted of microglia were markedly less responsive to TNF-\u3b1 and LPS, even after re-addition of microglia. Microglia and enriched astrocytes showed complementary Saa1 expression profiles following TNF-\u3b1 or LPS challenge, being higher in microglia with TNF-\u3b1 and higher in astrocytes with LPS. Recombinant human apo-SAA stimulated production of both inflammatory mediators and its own mRNA in microglia and enriched, but not microglia-depleted astrocytes. Co-ultramicronized palmitoylethanolamide/luteolin, an established anti-inflammatory/neuroprotective agent, reduced Saa1 expression in OPCs subjected to TNF-\u3b1 treatment. These last data, together with past findings suggest that co-ultramicronized palmitoylethanolamide/luteolin may be a novel approach in the treatment of inflammatory demyelinating disorders like MS
Phonon-assisted radiofrequency absorption by gold nanoparticles resulting in hyperthermia
It is suggested that in gold nanoparticles (GNPs) of about 5 nm sizes used in
the radiofrequency (RF) hyperthermia, an absorption of the RF photon by the
Fermi electron occurs with involvement of the longitudinal acoustic vibrational
mode (LAVM), the dominating one in the distribution of vibrational density of
states (VDOS). This physical mechanism helps to explain two observed phenomena:
the size dependence of the heating rate (HR) in GNPs and reduced heat
production in aggregated GNPs. The argumentation proceeds within the
one-electron approximation, taking into account the discretenesses of energies
and momenta of both electrons and LAVMs. The heating of GNPs is thought to
consist of two consecutive processes: first, the Fermi electron absorbs
simultaneously the RF photon and the LAVM available in the GNP; hereafter the
excited electron gets relaxed within the GNP's boundary, exciting a LAVM with
the energy higher than that of the previously absorbed LAVM. GNPs containing
the Ta and/or Fe impurities are proposed for the RF hyperthermia as promising
heaters with enhanced HRs, and GNPs with rare-earth impurity atoms are also
brought into consideration. It is shown why the maximum HR values should be
expected in GNPs with about 5-7 nm size.Comment: proceedings at the NATO Advanced Research workshop FANEM-2015 (Minsk,
May 25-27, 2015). To be published in the final form in: "Fundamental and
Applied NanoElectroMagnetics" (Springer Science + Business Media B.V.
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