1,890 research outputs found
Efficient calculation of local dose distribution for response modelling in proton and ion beams
We present an algorithm for fast and accurate computation of the local dose
distribution in MeV beams of protons, carbon ions or other heavy-charged
particles. It uses compound Poisson-process modelling of track interaction and
succesive convolutions for fast computation. It can handle mixed particle
fields over a wide range of fluences. Since the local dose distribution is the
essential part of several approaches to model detector efficiency or cellular
response it has potential use in ion-beam dosimetry and radiotherapy.Comment: 9 pages, 3 figure
Dynamic behavior of driven interfaces in models with two absorbing states
We study the dynamics of an interface (active domain) between different
absorbing regions in models with two absorbing states in one dimension;
probabilistic cellular automata models and interacting monomer-dimer models.
These models exhibit a continuous transition from an active phase into an
absorbing phase, which belongs to the directed Ising (DI) universality class.
In the active phase, the interface spreads ballistically into the absorbing
regions and the interface width diverges linearly in time. Approaching the
critical point, the spreading velocity of the interface vanishes algebraically
with a DI critical exponent. Introducing a symmetry-breaking field that
prefers one absorbing state over the other drives the interface to move
asymmetrically toward the unpreferred absorbing region. In Monte Carlo
simulations, we find that the spreading velocity of this driven interface shows
a discontinuous jump at criticality. We explain that this unusual behavior is
due to a finite relaxation time in the absorbing phase. The crossover behavior
from the symmetric case (DI class) to the asymmetric case (directed percolation
class) is also studied. We find the scaling dimension of the symmetry-breaking
field .Comment: 5 pages, 5 figures, Revte
Eigenvalue Separation in Some Random Matrix Models
The eigenvalue density for members of the Gaussian orthogonal and unitary
ensembles follows the Wigner semi-circle law. If the Gaussian entries are all
shifted by a constant amount c/Sqrt(2N), where N is the size of the matrix, in
the large N limit a single eigenvalue will separate from the support of the
Wigner semi-circle provided c > 1. In this study, using an asymptotic analysis
of the secular equation for the eigenvalue condition, we compare this effect to
analogous effects occurring in general variance Wishart matrices and matrices
from the shifted mean chiral ensemble. We undertake an analogous comparative
study of eigenvalue separation properties when the size of the matrices are
fixed and c goes to infinity, and higher rank analogues of this setting. This
is done using exact expressions for eigenvalue probability densities in terms
of generalized hypergeometric functions, and using the interpretation of the
latter as a Green function in the Dyson Brownian motion model. For the shifted
mean Gaussian unitary ensemble and its analogues an alternative approach is to
use exact expressions for the correlation functions in terms of classical
orthogonal polynomials and associated multiple generalizations. By using these
exact expressions to compute and plot the eigenvalue density, illustrations of
the various eigenvalue separation effects are obtained.Comment: 25 pages, 9 figures include
Optimal transport on wireless networks
We present a study of the application of a variant of a recently introduced
heuristic algorithm for the optimization of transport routes on complex
networks to the problem of finding the optimal routes of communication between
nodes on wireless networks. Our algorithm iteratively balances network traffic
by minimizing the maximum node betweenness on the network. The variant we
consider specifically accounts for the broadcast restrictions imposed by
wireless communication by using a different betweenness measure. We compare the
performance of our algorithm to two other known algorithms and find that our
algorithm achieves the highest transport capacity both for minimum node degree
geometric networks, which are directed geometric networks that model wireless
communication networks, and for configuration model networks that are
uncorrelated scale-free networks.Comment: 5 pages, 4 figure
The three species monomer-monomer model in the reaction-controlled limit
We study the one dimensional three species monomer-monomer reaction model in
the reaction controlled limit using mean-field theory and dynamic Monte Carlo
simulations. The phase diagram consists of a reactive steady state bordered by
three equivalent adsorbing phases where the surface is saturated with one
monomer species. The transitions from the reactive phase are all continuous,
while the transitions between adsorbing phases are first-order. Bicritical
points occur where the reactive phase simultaneously meets two adsorbing
phases. The transitions from the reactive to an adsorbing phase show directed
percolation critical behaviour, while the universal behaviour at the bicritical
points is in the even branching annihilating random walk class. The results are
contrasted and compared to previous results for the adsorption-controlled limit
of the same model.Comment: 12 pages using RevTeX, plus 4 postscript figures. Uses psfig.sty.
accepted to Journal of Physics
Interacting Monomer-Dimer Model with Infinitely Many Absorbing States
We study a modified version of the interacting monomer-dimer (IMD) model that
has infinitely many absorbing (IMA) states. Unlike all other previously studied
models with IMA states, the absorbing states can be divided into two equivalent
groups which are dynamically separated infinitely far apart. Monte Carlo
simulations show that this model belongs to the directed Ising universality
class like the ordinary IMD model with two equivalent absorbing states. This
model is the first model with IMA states which does not belong to the directed
percolation (DP) universality class. The DP universality class can be restored
in two ways, i.e., by connecting the two equivalent groups dynamically or by
introducing a symmetry-breaking field between the two groups.Comment: 5 pages, 5 figure
Mean-Field Analysis and Monte Carlo Study of an Interacting Two-Species Catalytic Surface Reaction Model
We study the phase diagram and critical behavior of an interacting one
dimensional two species monomer-monomer catalytic surface reaction model with a
reactive phase as well as two equivalent adsorbing phase where one of the
species saturates the system. A mean field analysis including correlations up
to triplets of sites fails to reproduce the phase diagram found by Monte Carlo
simulations. The three phases coexist at a bicritical point whose critical
behavior is described by the even branching annihilating random walk
universality class. This work confirms the hypothesis that the conservation
modulo 2 of the domain walls under the dynamics at the bicritical point is the
essential feature in producing critical behavior different from directed
percolation. The interfacial fluctuations show the same universal behavior seen
at the bicritical point in a three-species model, supporting the conjecture
that these fluctuations are a new universal characteristic of the model.Comment: 11 pages using RevTeX, plus 4 Postscript figures. Uses psfig.st
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