181 research outputs found
Motional narrowing effect in certain random binary lattices
We present a model for a class of random binary lattices by introducing a
one-dimensional system where impurities are placed in one sublattice while host
atoms lie on the other sublattice. The source of disorder is the stochastic
fluctuation of the impurity energy from site to site. We study the optical
absorption spectra and the peculiarities of the motional narrowing effect at
the band edges for perturbative and nonperturbative degrees of disorder.
Analytical results agree well with numerical simulations.Comment: 4 pages, REVTeX, 2 Postscript figures included. To appear in Physics
Letters
Digital homotopy with obstacles
AbstractIn (Ayala et al. (Discrete Appl. Math. 125 (1) (2003) 3) it was introduced the notion of a digital fundamental group π1d(O/S;σ) for a set of pixels O in relation to another set S which plays the role of an “obstacle”. This notion intends to be a generalization of the digital fundamental groups of both digital objects and their complements in a digital space. However, the suitability of this group was only checked for digital objects in that paper. As a sequel, we extend here the results in Ayala et al. (2003) for complements of objects. More precisely, we prove that for arbitrary digital spaces the group π1d(O/S;σ) maps onto the usual fundamental group of the difference of continuous analogues |AO∪S|−|AS|. Moreover, this epimorphism turns to be an isomorphism for a large class of digital spaces including most of the examples in digital topology
Chaotic transients in the switching of roto-breathers
By integrating a set of model equations for Josephson ladder subjected to a
uniform transverse bias current we have found almost all of the kinds of
breathers described in recent experiments, and closely reproduced their
voltage-current characteristics and switching behaviour. Our main result is
that a chaotic transient occurs in the switching process. The growth of tiny
perturbations during the chaotic transient causes the new breather
configuration to be extremely sensitive to the precise history of the initial
breather and can also cause the new breather to have a new centre of symmetry.Comment: 6 pages, 4 figure
One-dimensional Dirac oscillator in a thermal bath
We analyze the one-dimensional Dirac oscillator in a thermal bath. We found
that the heat capacity is two times greater than the heat capacity of the
one-dimensional harmonic oscillator for higher temperatures.Comment: 4 pages, 3 figures, to appear in Physics Letters
Distributed Algorithms for Consensus and Coordination in the Presence of Packet-Dropping Communication Links - Part II: Coefficients of Ergodicity Analysis Approach
In this two-part paper, we consider multicomponent systems in which each
component can iteratively exchange information with other components in its
neighborhood in order to compute, in a distributed fashion, the average of the
components' initial values or some other quantity of interest (i.e., some
function of these initial values). In particular, we study an iterative
algorithm for computing the average of the initial values of the nodes. In this
algorithm, each component maintains two sets of variables that are updated via
two identical linear iterations. The average of the initial values of the nodes
can be asymptotically computed by each node as the ratio of two of the
variables it maintains. In the first part of this paper, we show how the update
rules for the two sets of variables can be enhanced so that the algorithm
becomes tolerant to communication links that may drop packets, independently
among them and independently between different transmission times. In this
second part, by rewriting the collective dynamics of both iterations, we show
that the resulting system is mathematically equivalent to a finite inhomogenous
Markov chain whose transition matrix takes one of finitely many values at each
step. Then, by using e a coefficients of ergodicity approach, a method commonly
used for convergence analysis of Markov chains, we prove convergence of the
robustified consensus scheme. The analysis suggests that similar convergence
should hold under more general conditions as well.Comment: University of Illinois at Urbana-Champaign. Coordinated Sciences
Laboratory technical repor
Gamma-Rays as Probes for the Multi-Dimensionality of Type Ia Supernovae
We present -ray spectra for a set of Type Ia supernovae models. Our
study is based on a detailed Monte Carlo transport scheme for both spherical
and full 3-D geometries. Classical and new challenges of the ray
astronomy are addressed. We find that -rays are very suitable to
reveal the structure of the envelope and, thus, they allow to probe properties
of the nuclear burning front and the progenitor, namely its central density and
global asphericities. The potential problems are discussed for the quantitative
comparison between theoretical and observed line fluxes during the first few
months after the explosion.Comment: in Astronomy with Radioactivities,ed.R.Diehl,SpaceScienceRev.,in
pres
Effective potential for the massless KPZ equation
In previous work we have developed a general method for casting a classical
field theory subject to Gaussian noise (that is, a stochastic partial
differential equation--SPDE) into a functional integral formalism that exhibits
many of the properties more commonly associated with quantum field theories
(QFTs). In particular, we demonstrated how to derive the one-loop effective
potential. In this paper we apply the formalism to a specific field theory of
considerable interest, the massless KPZ equation (massless noisy vorticity-free
Burgers equation), and analyze its behaviour in the ultraviolet
(short-distance) regime. When this field theory is subject to white noise we
can calculate the one-loop effective potential and show that it is one-loop
ultraviolet renormalizable in 1, 2, and 3 space dimensions, and fails to be
ultraviolet renormalizable in higher dimensions. We show that the one-loop
effective potential for the massless KPZ equation is closely related to that
for lambda phi^4 QFT. In particular we prove that the massless KPZ equation
exhibits one-loop dynamical symmetry breaking (via an analog of the
Coleman-Weinberg mechanism) in 1 and 2 space dimensions, and that this
behaviour does not persist in 3 space dimensions.Comment: 13 pages, LaTeX 209, ReV_TeX 3.2, three *.eps figures, epsf.st
Weak lighting functions and strong 26-surfaces
AbstractThe goal of this paper is to introduce the notion of weak lighting function in order to replicate the “continuous perception” associated with strong 26-surfaces. As a consequence, the continuous analogue defined ad hoc by Malgouyres and Bertrand only for these surfaces is extended for arbitrary objects, and the local characterization of finite strong 26-surfaces given in (Malgouyres and Bertrand, Int. J. Pattern Recognition Art. Intell. 13(4) (1999) 465–484) is generalized to possibly infinite surfaces. Moreover, weak lighting functions also replicate the “continuous perception” associated with (α,β)-surfaces, (α,β)≠(6,6), since they are generalizing the lighting functions previously defined by the authors
The direct boundary element method: 2D site effects assessment on laterally varying layered media (methodology)
The Direct Boundary Element Method (DBEM) is presented to solve the elastodynamic field equations in 2D, and a complete comprehensive implementation is given. The DBEM is a useful approach to obtain reliable numerical estimates of site effects on seismic ground motion due to irregular geological configurations, both of layering and topography. The method is based on the discretization of the classical Somigliana's elastodynamic representation equation which stems from the reciprocity theorem. This equation is given in terms of the Green's function which is the full-space harmonic steady-state fundamental solution. The formulation permits the treatment of viscoelastic media, therefore site models with intrinsic attenuation can be examined. By means of this approach, the calculation of 2D scattering of seismic waves, due to the incidence of P and SV waves on irregular topographical profiles is performed. Sites such as, canyons, mountains and valleys in irregular multilayered media are computed to test the technique. The obtained transfer functions show excellent agreement with already published results
Plastic deformation at high temperatures of pure and Mn-doped GaSb
In this work the plastic behavior of GaSb and Mn-doped GaSb at high temperature has been analyzed. Several experiments at different constant load and temperatures around 500 °C were carried out. The parameters used in the Haasen model have been obtained experimentally and compared with the ones obtained from simulations
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