77 research outputs found
Percolation model for structural phase transitions in LiHIO mixed crystals
A percolation model is proposed to explain the structural phase transitions
found in LiHIO mixed crystals as a function of the
concentration parameter . The percolation thresholds are obtained from Monte
Carlo simulations on the specific lattices occupied by lithium atoms and
hydrogen bonds. The theoretical results strongly suggest that percolating
lithium vacancies and hydrogen bonds are indeed responsible for the solid
solution observed in the experimental range .Comment: 4 pages, 2 figure
The Value of Information for Populations in Varying Environments
The notion of information pervades informal descriptions of biological
systems, but formal treatments face the problem of defining a quantitative
measure of information rooted in a concept of fitness, which is itself an
elusive notion. Here, we present a model of population dynamics where this
problem is amenable to a mathematical analysis. In the limit where any
information about future environmental variations is common to the members of
the population, our model is equivalent to known models of financial
investment. In this case, the population can be interpreted as a portfolio of
financial assets and previous analyses have shown that a key quantity of
Shannon's communication theory, the mutual information, sets a fundamental
limit on the value of information. We show that this bound can be violated when
accounting for features that are irrelevant in finance but inherent to
biological systems, such as the stochasticity present at the individual level.
This leads us to generalize the measures of uncertainty and information usually
encountered in information theory
Fraction of uninfected walkers in the one-dimensional Potts model
The dynamics of the one-dimensional q-state Potts model, in the zero
temperature limit, can be formulated through the motion of random walkers which
either annihilate (A + A -> 0) or coalesce (A + A -> A) with a q-dependent
probability. We consider all of the walkers in this model to be mutually
infectious. Whenever two walkers meet, they experience mutual contamination.
Walkers which avoid an encounter with another random walker up to time t remain
uninfected. The fraction of uninfected walkers is investigated numerically and
found to decay algebraically, U(t) \sim t^{-\phi(q)}, with a nontrivial
exponent \phi(q). Our study is extended to include the coupled
diffusion-limited reaction A+A -> B, B+B -> A in one dimension with equal
initial densities of A and B particles. We find that the density of walkers
decays in this model as \rho(t) \sim t^{-1/2}. The fraction of sites unvisited
by either an A or a B particle is found to obey a power law, P(t) \sim
t^{-\theta} with \theta \simeq 1.33. We discuss these exponents within the
context of the q-state Potts model and present numerical evidence that the
fraction of walkers which remain uninfected decays as U(t) \sim t^{-\phi},
where \phi \simeq 1.13 when infection occurs between like particles only, and
\phi \simeq 1.93 when we also include cross-species contamination.Comment: Expanded introduction with more discussion of related wor
Fitting the integrated Spectral Energy Distributions of Galaxies
Fitting the spectral energy distributions (SEDs) of galaxies is an almost
universally used technique that has matured significantly in the last decade.
Model predictions and fitting procedures have improved significantly over this
time, attempting to keep up with the vastly increased volume and quality of
available data. We review here the field of SED fitting, describing the
modelling of ultraviolet to infrared galaxy SEDs, the creation of
multiwavelength data sets, and the methods used to fit model SEDs to observed
galaxy data sets. We touch upon the achievements and challenges in the major
ingredients of SED fitting, with a special emphasis on describing the interplay
between the quality of the available data, the quality of the available models,
and the best fitting technique to use in order to obtain a realistic
measurement as well as realistic uncertainties. We conclude that SED fitting
can be used effectively to derive a range of physical properties of galaxies,
such as redshift, stellar masses, star formation rates, dust masses, and
metallicities, with care taken not to over-interpret the available data. Yet
there still exist many issues such as estimating the age of the oldest stars in
a galaxy, finer details ofdust properties and dust-star geometry, and the
influences of poorly understood, luminous stellar types and phases. The
challenge for the coming years will be to improve both the models and the
observational data sets to resolve these uncertainties. The present review will
be made available on an interactive, moderated web page (sedfitting.org), where
the community can access and change the text. The intention is to expand the
text and keep it up to date over the coming years.Comment: 54 pages, 26 figures, Accepted for publication in Astrophysics &
Space Scienc
Preparation, structural, dielectric and magnetic properties of LaFeO3-PbTiO3 solid solutions
Solid solutions of (1-x)LaFeO3-(x)PbTiO3 (0<x<1) have been prepared by
conventional solid-state reaction. These complex perovskites have been studied
by means of X-ray (XRPD) and neutron powder (NPD) diffraction, complemented
with dielectric, magnetic, heat capacity and M\"ossbauer measurements. Complete
solubility in the perovskite series was demonstrated. The NPD and XRPD patterns
were successfully refined as orthorhombic (x \leq 0.7) and tetragonal (x \geq
0.8). A composition-driven phase transformation occurs within the interval
0.7<x<0.8. The samples with x<0.5 showed evidence of long-range magnetic
ordering with an G-type antiferromagnetic arrangement of the magnetic moments
of the Fe3+ cations in the B-site with propagation vector k = (0,0,0). Based on
the obtained experimental data, a combined structural and magnetic phase
diagram has been constructed. The factors governing the structural, dielectric
and magnetic properties of (1-x) LaFeO3 - (x)PbTiO3 solid solutions are
discussed, as well as their possible multiferroicity.Comment: 33 pages, 15 figure
Checking Individual Agent Behaviours in Markov Population Models by Fluid Approximation
In this chapter, we will describe, in a tutorial style, recent work on the use of fluid approximation techniques in the context of stochastic model checking. We will discuss the theoretical background and the algorithms working out an example.
This approach is designed for population models, in which a (large) number of individual agents interact, which give rise to continuous time Markov chain (CTMC) models with a very large state space. We then focus on properties of individual agents in the system, specified by Continuous Stochastic Logic (CSL) formulae, and use fluid approximation techniques (specifically, the so called fast simulation) to check those properties. We will show that verification of such CSL formulae reduces to the computation of reachability probabilities in a special kind of time-inhomogeneous CTMC with a small state space, in which both the rates and the structure of the CTMC can change (discontinuously) with time. In this tutorial, we will discuss only briefly the theoretical issues behind the approach, like the decidability of the method and the consistency of the approximation scheme
CW blue-green light emission from GaN and SiC by sum-frequency generation and second harmonic generation
Optical birefringence studies of thiourea complex crystals using Cauchy’s two term model by the channeled spectrum method
Powder second harmonic generation efficiencies of saccharide materials
Saccharide materials are potential candidates for frequency conversion applications. In addition to being chiral, which ensures crystallization in a space group necessary for three-wave mixing processes, they generally possess useful physical and optical properties. We have examined the powder second harmonic generation efficiencies of both saturated saccharides and sugars with simple polar π-functionalities. Powder efficiencies of up to 5 times that of sucrose were observed for simple saturated sugars, whereas values of 18 times sucrose (or 0.45 x urea) were observed for unsaturated saccharide derivatives. We have noted that for both classes of material, there is a tendency for more efficient nonlinear compounds to reside in a monoclinic rather than an orthorhombic space group. We have also noted that there appears to be a correlation between the phase-matching potential and the crystal symmetry. In addition, two promising saccharide materials have been identified for frequency conversion applications, based on their powder second-harmonic generation efficiencies, their phase-matching capabilities, and their UV transparency. © 1993 American Chemical Society
- …