425 research outputs found
The Probability Distribution Function of Column Density in Molecular Clouds
(Abridged) We discuss the probability distribution function (PDF) of column
density resulting from density fields with lognormal PDFs, applicable to
isothermal gas (e.g., probably molecular clouds). We suggest that a
``decorrelation length'' can be defined as the distance over which the density
auto-correlation function has decayed to, for example, 10% of its zero-lag
value, so that the density ``events'' along a line of sight can be assumed to
be independent over distances larger than this, and the Central Limit Theorem
should be applicable. However, using random realizations of lognormal fields,
we show that the convergence to a Gaussian is extremely slow in the high-
density tail. Thus, the column density PDF is not expected to exhibit a unique
functional shape, but to transit instead from a lognormal to a Gaussian form as
the ratio of the column length to the decorrelation length increases.
Simultaneously, the PDF's variance decreases. For intermediate values of
, the column density PDF assumes a nearly exponential decay. We then
discuss the density power spectrum and the expected value of in actual
molecular clouds. Observationally, our results suggest that may be
inferred from the shape and width of the column density PDF in
optically-thin-line or extinction studies. Our results should also hold for gas
with finite-extent power-law underlying density PDFs, which should be
characteristic of the diffuse, non-isothermal neutral medium (temperatures
ranging from a few hundred to a few thousand degrees). Finally, we note that
for , the dynamic range in column density is small
( a factor of 10), but this is only an averaging effect, with no
implication on the dynamic range of the underlying density distribution.Comment: 13 pages, 7 figures (10 postscript files). Accepted in ApJ.
Eliminated implication that ratio of column length to correlation length
necessarily increases with resolution, and thus that 3D simulations are
unresolved. Added discussion of dependence of autocorrelation function with
parameters of the turbulenc
Simulation studies of a phenomenological model for elongated virus capsid formation
We study a phenomenological model in which the simulated packing of hard,
attractive spheres on a prolate spheroid surface with convexity constraints
produces structures identical to those of prolate virus capsid structures. Our
simulation approach combines the traditional Monte Carlo method with a modified
method of random sampling on an ellipsoidal surface and a convex hull searching
algorithm. Using this approach we identify the minimum physical requirements
for non-icosahedral, elongated virus capsids, such as two aberrant flock house
virus (FHV) particles and the prolate prohead of bacteriophage , and
discuss the implication of our simulation results in the context of recent
experimental findings. Our predicted structures may also be experimentally
realized by evaporation-driven assembly of colloidal spheres
Geographical Indication as a vector of regional development and the possibility of its application in the artisanal crystal sector of Vale do Itajaí
In a context marked by the deepening of globalization, geographic dispersion of industrial production and international division of labor, the sector of crystal craft of the Vale of Itajaí in the State of Santa Catarina/ Brazil, has been facing different challenges and looking for new perspectives of competitiveness. The aim of the paper is to address, more specifically, the question of Geographic Indication as a key factor in the strategy of regional development, linking the productive organization and performance in the industrial sector to the socioeconomic situation of the region and its development. To this end, the paper discussed the feasibility of obtaining Geographic Indication and its impacts on restructuring of territories, reorganization of production thru clusters and regional development. Finally, the study pointed out to which extent geographical proximity and territorial economic activity may affect the strategy of competitiveness and international integration of the firms connected with the crystal industry in the regio
Absence of correlation between built-in electric dipole moment and quantum Stark effect in InAs/GaAs self-assembled quantum dots
We report significant deviations from the usual quadratic dependence of the
ground state interband transition energy on applied electric fields in
InAs/GaAs self-assembled quantum dots. In particular, we show that conventional
second-order perturbation theory fails to correctly describe the Stark shift
for electric field below kV/cm in high dots. Eight-band calculations demonstrate this effect is predominantly due to
the three-dimensional strain field distribution which for various dot shapes
and stoichiometric compositions drastically affects the hole ground state. Our
conclusions are supported by two independent experiments.Comment: 4 pages, 4 figure
Anomalous quantum confined Stark effects in stacked InAs/GaAs self-assembled quantum dots
Vertically stacked and coupled InAs/GaAs self-assembled quantum dots (SADs)
are predicted to exhibit a strong non-parabolic dependence of the interband
transition energy on the electric field, which is not encountered in single SAD
structures nor in other types of quantum structures. Our study based on an
eight-band strain-dependent Hamiltonian indicates that
this anomalous quantum confined Stark effect is caused by the three-dimensional
strain field distribution which influences drastically the hole states in the
stacked SAD structures.Comment: 4 pages, 4 figure
Heterogeneous aging in spin glasses
We introduce a set of theoretical ideas that form the basis for an analytical
framework capable of describing nonequilibrium dynamics in glassy systems. We
test the resulting scenario by comparing its predictions with numerical
simulations of short-range spin glasses. Local fluctuations and responses are
shown to be connected by a generalized local out-of-equilibrium
fluctuation-dissipation relation. Scaling relationships are uncovered for the
slow evolution of heterogeneities at all time scales.Comment: Substantially reorganized to improve clarity of exposition. Accepted
for publication in Physical Review Letters. 5 pages, 4 figure
Thermal noise properties of two aging materials
In this lecture we review several aspects of the thermal noise properties in
two aging materials: a polymer and a colloidal glass.
The measurements have been performed after a quench for the polymer and
during the transition from a fluid-like to a solid-like state for the gel. Two
kind of noise has been measured: the electrical noise and the mechanical noise.
For both materials we have observed that the electric noise is characterized
by a strong intermittency, which induces a large violation of the Fluctuation
Dissipation Theorem (FDT) during the aging time, and may persist for several
hours at low frequency. The statistics of these intermittent signals and their
dependance on the quench speed for the polymer or on sample concentration for
the gel are studied. The results are in a qualitative agreement with recent
models of aging, that predict an intermittent dynamics. For the mechanical
noise the results are unclear. In the polymer the mechanical thermal noise is
still intermittent whereas for the gel the violation of FDT, if it exists, is
extremely small.Comment: to be published in the Proceedings of the XIX Sitges Conference on
''Jammming, Yielding and Irreversible Deformation in Condensed Matter'',
M.-C.Miguel and M. Rubi eds.,Springer Verlag, Berli
Micellization of Sliding Polymer Surfactants
Following up a recent paper on grafted sliding polymer layers (Macromolecules
2005, 38, 1434-1441), we investigated the influence of the sliding degree of
freedom on the self-assembly of sliding polymeric surfactants that can be
obtained by complexation of polymers with cyclodextrins. In contrast to the
micelles of quenched block copolymer surfactants, the free energy of micelles
of sliding surfactants can have two minima: the first corresponding to small
micelles with symmetric arm lengths, and the second corresponding to large
micelles with asymmetric arm lengths. The relative sizes and concentrations of
small and large micelles in the solution depend on the molecular parameters of
the system. The appearance of small micelles drastically reduces the kinetic
barrier signifying the fast formation of equilibrium micelles.Comment: Submitted to Macromolecule
Sources of UHECRs in view of the TUS and JEM-EUSO experiments
The origin of ultra-high-energy cosmic rays (UHECRs) is one of the most
intriguing problems of modern cosmic ray physics. We briefly review the main
astrophysical models of their origin and the forthcoming orbital experiments
TUS and JEM-EUSO, and discuss how the new data can help one solve the
long-standing puzzle.Comment: 4 pages; prepared for ECRS-2012 (http://ecrs2012.sinp.msu.ru/); v2: a
reference adde
Properties of electrons scattered on a strong plane electromagnetic wave with a linear polarization: classical treatment
The relations among the components of the exit momenta of ultrarelativistic
electrons scattered on a strong electromagnetic wave of a low (optical)
frequency and linear polarization are established using the exact solutions to
the equations of motion with radiation reaction included (the Landau-Lifshitz
equation). It is found that the momentum components of the electrons traversed
the electromagnetic wave depend weakly on the initial values of the momenta.
These electrons are mostly scattered at the small angles to the direction of
propagation of the electromagnetic wave. The maximum Lorentz factor of the
electrons crossed the electromagnetic wave is proportional to the work done by
the electromagnetic field and is independent of the initial momenta. The
momentum component parallel to the electric field strength vector of the
electromagnetic wave is determined only by the diameter of the laser beam
measured in the units of the classical electron radius. As for the reflected
electrons, they for the most part lose the energy, but remain relativistic.
There is a reflection law for these electrons that relates the incident and the
reflection angles and is independent of any parameters.Comment: 12 pp, 3 fig
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