254 research outputs found
Observational study of type of surgical training and outcome of definitive surgery for primary malignant melanoma
No abstract available
Quantifying nanoparticle dispersion: application of the Delaunay network for objective analysis of sample micrographs
Measuring quantitatively the nanoparticle dispersion of a composite material requires more than choosing a particular parameter and determining its correspondence to good and bad dispersion. It additionally requires anticipation of the measure’s behaviour towards imperfect experimental data, such as that which can be obtained from a limited number of samples. It should be recognised that different samples from a common parent population can give statistically different responses due to sample variation alone and a measure of the likelihood of this occurring allows a decision on the dispersion to be made. It is also important to factor into the analysis the quality of the data in the micrograph with it: (a) being incomplete because some of the particles present in the micrograph are indistinguishable or go unseen; (b) including additional responses which are false. With the use of our preferred method, this article investigates the effects on the measured dispersion quality of nanoparticles of the micrograph’s magnification settings, the role of the fraction of nanoparticles visible and the number of micrographs used. It is demonstrated that the best choice of magnification, which gives the clearest indication of dispersion type, is dependent on the type of nanoparticle structure present. Furthermore, it is found that the measured dispersion can be modified by particle loss, through the limitations of micrograph construction, and material/microscope imperfections such as cut marks and optical aberrations which could lead to the wrong conclusions being drawn. The article finishes by showing the versatility of the dispersion measure by characterising various different spatial features. <br/
A Singular Perturbation Analysis for \\Unstable Systems with Convective Nonlinearity
We use a singular perturbation method to study the interface dynamics of a
non-conserved order parameter (NCOP) system, of the reaction-diffusion type,
for the case where an external bias field or convection is present. We find
that this method, developed by Kawasaki, Yalabik and Gunton for the
time-dependant Ginzburg-Landau equation and used successfully on other NCOP
systems, breaks down for our system when the strength of bias/convection gets
large enough.Comment: 5 pages, PostScript forma
Interface Motion and Pinning in Small World Networks
We show that the nonequilibrium dynamics of systems with many interacting
elements located on a small-world network can be much slower than on regular
networks. As an example, we study the phase ordering dynamics of the Ising
model on a Watts-Strogatz network, after a quench in the ferromagnetic phase at
zero temperature. In one and two dimensions, small-world features produce
dynamically frozen configurations, disordered at large length scales, analogous
of random field models. This picture differs from the common knowledge
(supported by equilibrium results) that ferromagnetic short-cuts connections
favor order and uniformity. We briefly discuss some implications of these
results regarding the dynamics of social changes.Comment: 4 pages, 5 figures with minor corrections. To appear in Phys. Rev.
Smeared phase transition in a three-dimensional Ising model with planar defects: Monte-Carlo simulations
We present results of large-scale Monte Carlo simulations for a
three-dimensional Ising model with short range interactions and planar defects,
i.e., disorder perfectly correlated in two dimensions. We show that the phase
transition in this system is smeared, i.e., there is no single critical
temperature, but different parts of the system order at different temperatures.
This is caused by effects similar to but stronger than Griffiths phenomena. In
an infinite-size sample there is an exponentially small but finite probability
to find an arbitrary large region devoid of impurities. Such a rare region can
develop true long-range order while the bulk system is still in the disordered
phase. We compute the thermodynamic magnetization and its finite-size effects,
the local magnetization, and the probability distribution of the ordering
temperatures for different samples. Our Monte-Carlo results are in good
agreement with a recent theory based on extremal statistics.Comment: 9 pages, 6 eps figures, final version as publishe
Solution of voter model dynamics on annealed small-world networks
An analytical study of the behavior of the voter model on the small-world
topology is performed. In order to solve the equations for the dynamics, we
consider an annealed version of the Watts-Strogatz (WS) network, where
long-range connections are randomly chosen at each time step. The resulting
dynamics is as rich as on the original WS network. A temporal scale
separates a quasi-stationary disordered state with coexisting domains from a
fully ordered frozen configuration. is proportional to the number of
nodes in the network, so that the system remains asymptotically disordered in
the thermodynamic limit.Comment: 11 pages, 4 figures, published version. Added section with extension
to generic number of nearest neighbor
Ground-state energy and entropy of the two-dimensional Edwards-Anderson spin-glass model with different bond distributions
We study the two-dimensional Edwards-Anderson spin-glass model using a
parallel tempering Monte Carlo algorithm. The ground-state energy and entropy
are calculated for different bond distributions. In particular, the entropy is
obtained by using a thermodynamic integration technique and an appropriate
reference state, which is determined with the method of high-temperature
expansion. This strategy provide accurate values of this quantity for
finite-size lattices. By extrapolating to the thermodynamic limit, the
ground-state energy and entropy of the different versions of the spin-glass
model are determined.Comment: 18 pages, 5 figure
Metastable States in Spin Glasses and Disordered Ferromagnets
We study analytically M-spin-flip stable states in disordered short-ranged
Ising models (spin glasses and ferromagnets) in all dimensions and for all M.
Our approach is primarily dynamical and is based on the convergence of a
zero-temperature dynamical process with flips of lattice animals up to size M
and starting from a deep quench, to a metastable limit. The results (rigorous
and nonrigorous, in infinite and finite volumes) concern many aspects of
metastable states: their numbers, basins of attraction, energy densities,
overlaps, remanent magnetizations and relations to thermodynamic states. For
example, we show that their overlap distribution is a delta-function at zero.
We also define a dynamics for M=infinity, which provides a potential tool for
investigating ground state structure.Comment: 34 pages (LaTeX); to appear in Physical Review
Scaling in Late Stage Spinodal Decomposition with Quenched Disorder
We study the late stages of spinodal decomposition in a Ginzburg-Landau mean
field model with quenched disorder. Random spatial dependence in the coupling
constants is introduced to model the quenched disorder. The effect of the
disorder on the scaling of the structure factor and on the domain growth is
investigated in both the zero temperature limit and at finite temperature. In
particular, we find that at zero temperature the domain size, , scales
with the amplitude, , of the quenched disorder as with and in two
dimensions. We show that , where is the
Lifshitz-Slyosov exponent. At finite temperature, this simple scaling is not
observed and we suggest that the scaling also depends on temperature and .
We discuss these results in the context of Monte Carlo and cell dynamical
models for phase separation in systems with quenched disorder, and propose that
in a Monte Carlo simulation the concentration of impurities, , is related to
by .Comment: RevTex manuscript 5 pages and 5 figures (obtained upon request via
email [email protected]
Relaxation Properties of Small-World Networks
Recently, Watts and Strogatz introduced the so-called small-world networks in
order to describe systems which combine simultaneously properties of regular
and of random lattices. In this work we study diffusion processes defined on
such structures by considering explicitly the probability for a random walker
to be present at the origin. The results are intermediate between the
corresponding ones for fractals and for Cayley trees.Comment: 16 pages, 6 figure
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