430 research outputs found
Silicon nanoparticles and interstellar extinction
To examine a recently proposed hypothesis that silicon nanoparticles are the
source of extended red emission (ERE) in the interstellar medium, we performed
a detailed modeling of the mean Galactic extinction in the presence of silicon
nanoparticles. For this goal we used the appropriate optical constants of
nanosized Si, essentially different from those of bulk Si due to quantum
confinement. It was found that a dust mixture of silicon nanoparticles, bare
graphite grains, silicate core-organic refractory mantle grains and three-layer
silicate-water ice-organic refractory grains works well in explaining the
extinction and, in addition, results in the acceptable fractions of UV/visible
photons absorbed by silicon nanoparticles: 0.071-0.081. Since these fractions
barely agree with the fraction of UV/visible photons needed to excite the
observed ERE, we conclude that the intrinsic photon conversion efficiency of
the photoluminescence by silicon nanoparticles must be near 100%, if they are
the source of the ERE.Comment: Latex2e, uses emulateapj.sty (included), multicol.sty, epsf.sty, 6
pages, 3 figures (8 Postscript files), accepted for publication in ApJ
Letters, complete Postscript file is also available at
http://physics.technion.ac.il/~zubko/eb.html#SNP
New Interstellar Dust Models Consistent with Extinction, Emission, and Abundance Constraints
We present new interstellar dust models which have been derived by
simultaneously fitting the far-ultraviolet to near-infrared extinction, the
diffuse infrared (IR) emission and, unlike previous models, the elemental
abundance constraints on the dust for different interstellar medium abundances,
including solar, F and G star, and B star abundances. The fitting problem is a
typical ill-posed inversion problem, in which the grain size distribution is
the unknown, which we solve by using the method of regularization. The dust
model contains various components: PAHs, bare silicate, graphite, and amorphous
carbon particles, as well as composite particles containing silicate, organic
refractory material, water ice, and voids. The optical properties of these
components were calculated using physical optical constants. As a special case,
we reproduce the Li & Draine (2001) results, however their model requires an
excessive amount of silicon, magnesium, and iron to be locked up in dust: about
50 ppm (atoms per million of H atoms), significantly more than the upper limit
imposed by solar abundances of these elements, about 34, 35, and 28 ppm,
respectively. A major conclusion of this paper is that there is no unique
interstellar dust model that simultaneously fits the observed extinction,
diffuse IR emission, and abundances constraints.Comment: 70 pages, 23 figures, accepted for publication in the Astrophysical
Journal Supplemen
Effects of electromagnetic fields emitted by GSM phones on working memory: a meta-analysis
Background and Objective: Current treatments for Alzheimer's Disease (AD) do not affect the course of the illness and brain stimulation techniques are increasingly promoted as potential therapeutic inter-ventions for AD. This study reviews the effects of electromagnetic field (EMF) exposure versus sham exposure on working memory (WM) performance of healthy human participants.
Method: Online literature databases and previous systematic reviews were searched for studies of EMF and WM in participants without reported memory problems. Two thousand eight hundred and fifty seven studies were identified, and 10 studies met the inclusion criteria. An assessment of study quality was completed, and separate, random effects meta-analyses were conducted for each of the three WM tasks included: n-back, substitution and digit span forward.
Results: No differences were found between participants exposed to active EMF versus sham conditions in any of the three working memory tasks examined.
Conclusion: Results indicate that EMF does not affect WM during the n-back, substitution and digit-span tasks. Future studies should focus on the possible effects of chronic exposure to EMF in older adults with AD using a battery of comparable WM and attention tasks, before EMF can be seriously considered as a potential modulator of WM in AD
Comet 9P/Tempel 1: Interpretation with the Deep Impact Results
According to our common understandings, the original surface of a
short-period comet nucleus has been lost by sublimation processes during its
close approaches to the Sun. Sublimation results in the formation of a dust
mantle on the retreated surface and in chemical differentiation of ices over
tens or hundreds of meters below the mantle. In the course of NASA's Deep
Impact mission, optical and infrared imaging observations of the ejecta plume
were conducted by several researchers, but their interpretations of the data
came as a big surprise: (1) The nucleus of comet 9P/Tempel 1 is free of a dust
mantle, but maintains its pristine crust of submicron-sized carbonaceous
grains; (2) Primordial materials are accessible already at a depth of several
tens of cm with abundant silicate grains of submicrometer sizes. In this study,
we demonstrate that a standard model of cometary nuclei explains well available
observational data: (1) A dust mantle with a thickness of ~1-2 m builds up on
the surface, where compact aggregates larger than tens of micrometers dominate;
(2) Large fluffy aggregates are embedded in chemically differentiated layers as
well as in the deepest part of the nucleus with primordial materials. We
conclude that the Deep Impact results do not need any peculiar view of a comet
nucleus.Comment: 11 pages, 1 figure, 1 table. ApJ letters, 673, L199-20
Crystal structure of the high temperature phase of strontium barium niobate
Pure and undoped strontium-barium niobate Sr0:40Ba0:60Nb2O6 (SBN40) single crystals grown by the
Czochralski method were investigated by single crystal X-ray diffraction methods. The study below TC (429 K for
SBN40) confirmed the structure with P4bm space group. Above this temperature the structure transforms into the
paraelectric, centrosymmetric one with P4=mbm space group. Analysis of the recorded diffraction patterns allowed
to observe several signs of crystal structure modulation. On the registered diffraction images satellite reflections
were found. A modulation vector q = ( ; ; ), where = 0:3075(6) (at room temperature) was found and it was
similar to that occurring in the SBN61. In addition, above the phase transition temperature on the (hk) planes
with l integer a weak diffuse scattering was observed
Probing of local ferroelectricity in BiFeO3 thin films and (BiFeO3)m(SrTiO3)m superlattices
Ferroelectric BiFeO3 thin films and artificial superlattices of
(BiFeO3)m(SrTiO3)m (m~ 1 to 10 unit cells) were fabricated on (001)-oriented
SrTiO3 substrates by pulsed laser ablation. The variation of leakage current
and macroscopic polarization with periodicity was studied. Piezo force
microscopy studies revealed the presence of large ferroelectric domains in the
case of BiFeO3 thin films while a size reduction in ferroelectric domains was
observed in the case of superlattice structures. The results show that the
modification of ferroelectric domains through superlattice, could provide an
additional control on engineering the domain wall mediated functional
properties.Comment: 14 pages, To be published in J. Mag. Mag Mater. proceedings of EMRS
200
Strain gradient induced polarization in SrTiO3 single crystals
Piezoelectricity is inherent only in noncentrosymmetric materials, but a
piezoelectric response can also be obtained in centrosymmetric crystals if
subjected to inhomogeneous deformation. This phenomenon, known as
flexoelectricity, affects the functional properties of insulators, particularly
thin films of high permittivity materials. We have measured
strain-gradient-induced polarization in single crystals of paraelectric
SrTiO as a function of temperature and orientation down to and below the
105 K phase transition. Estimates were obtained for all the components of the
flexoelectric tensor, and calculations based on these indicate that local
polarization around defects in SrTiO may exceed the largest ferroelectric
polarizations. A sign reversal of the flexoelectric response detected below the
phase transition suggests that the ferroelastic domain walls of SrTiO may
be polar.Comment: 4 pages, 3 figures, 1 tabl
Diodes with Breakdown Voltages Enhanced by the Metal-Insulator Transition of LaAlO-SrTiO Interfaces
Using the metal-insulator transition that takes place as a function of
carrier density at the LaAlO-SrTiO interface, oxide diodes have been
fabricated with room-temperature breakdown voltages of up to 200 V. With
applied voltage, the capacitance of the diodes changes by a factor of 150. The
diodes are robust and operate at temperatures up to 270 C
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