681 research outputs found
Brownian yet non-Gaussian diffusion in heterogeneous media: from superstatistics to homogenization
We discuss the situations under which Brownian yet non-Gaussian (BnG)
diffusion can be observed in the model of a particle's motion in a random
landscape of diffusion coefficients slowly varying in space. Our conclusion is
that such behavior is extremely unlikely in the situations when the particles,
introduced into the system at random at , are observed from the
preparation of the system on. However, it indeed may arise in the case when the
diffusion (as described in Ito interpretation) is observed under equilibrated
conditions. This paradigmatic situation can be translated into the model of the
diffusion coefficient fluctuating in time along a trajectory, i.e. into a kind
of the "diffusing diffusivity" model.Comment: 12 pages; 10 figure
Density functional simulation of small Fe nanoparticles
We calculate from first principles the electronic structure, relaxation and
magnetic moments in small Fe particles, applying the numerical local orbitals
method in combination with norm-conserving pseudopotentials. The accuracy of
the method in describing elastic properties and magnetic phase diagrams is
tested by comparing benchmark results for different phases of crystalline iron
to those obtained by an all-electron method. Our calculations for the
bipyramidal Fe_5 cluster qualitatively and quantitatively confirm previous
plane-wave results that predicted a non-collinear magnetic structure. For
larger bcc-related (Fe_35) and fcc-related (Fe_38, Fe_43, Fe_62) particles, a
larger inward relaxation of outer shells has been found in all cases,
accompanied by an increase of local magnetic moments on the surface to beyond 3
mu_B.Comment: 15 pages with 6 embedded postscript figures, updated version,
submitted to Eur.Phys.J.
Linearized force constants method for lattice dynamics in mixed semiconductors
A simple and accurate method of calculating phonon spectra in mixed
semiconductors alloys, on the basis of preliminarily (from first principles)
relaxed atomic structure, is proposed and tested for (Zn,Be)Se and (Ga,In)As
solid solutions. The method uses an observation that the interatomic force
constants, calculated ab initio for a number of microscopic configurations in
the systems cited, show a clear linear variation of the main (diagonal) values
of the interatomic force constants with the corresponding bond length. We
formulate simple rules about how to recover the individual 3x3 subblocks of the
force constants matrix in their local (bonds-related) coordinate systems and
how to transform them into a global (crystal cell-related) coordinate system.
Test calculations done for 64-atom supercells representing different
concentrations of (Zn,Be)Se and (Ga,In)As show that the phonon frequencies and
compositions of eigenvectors are faithfully reproduced in a linearized force
constants calculation, as compared to true ab initio calculations.Comment: to appear in the proceedings of the Phonons2007 conference (Paris,
July 2007
Parametric Analysis of Cherenkov Light LDF from EAS for High Energy Gamma Rays and Nuclei: Ways of Practical Application
In this paper we propose a 'knee-like' approximation of the lateral
distribution of the Cherenkov light from extensive air showers in the energy
range 30-3000 TeV and study a possibility of its practical application in high
energy ground-based gamma-ray astronomy experiments (in particular, in
TAIGA-HiSCORE). The approximation has a very good accuracy for individual
showers and can be easily simplified for practical application in the HiSCORE
wide angle timing array in the condition of a limited number of triggered
stations.Comment: 4 pages, 5 figures, proceedings of ISVHECRI 2016 (19th International
Symposium on Very High Energy Cosmic Ray Interactions
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