609 research outputs found
Learning about knowledge: A complex network approach
This article describes an approach to modeling knowledge acquisition in terms
of walks along complex networks. Each subset of knowledge is represented as a
node, and relations between such knowledge are expressed as edges. Two types of
edges are considered, corresponding to free and conditional transitions. The
latter case implies that a node can only be reached after visiting previously a
set of nodes (the required conditions). The process of knowledge acquisition
can then be simulated by considering the number of nodes visited as a single
agent moves along the network, starting from its lowest layer. It is shown that
hierarchical networks, i.e. networks composed of successive interconnected
layers, arise naturally as a consequence of compositions of the prerequisite
relationships between the nodes. In order to avoid deadlocks, i.e. unreachable
nodes, the subnetwork in each layer is assumed to be a connected component.
Several configurations of such hierarchical knowledge networks are simulated
and the performance of the moving agent quantified in terms of the percentage
of visited nodes after each movement. The Barab\'asi-Albert and random models
are considered for the layer and interconnecting subnetworks. Although all
subnetworks in each realization have the same number of nodes, several
interconnectivities, defined by the average node degree of the interconnection
networks, have been considered. Two visiting strategies are investigated:
random choice among the existing edges and preferential choice to so far
untracked edges. A series of interesting results are obtained, including the
identification of a series of plateaux of knowledge stagnation in the case of
the preferential movements strategy in presence of conditional edges.Comment: 18 pages, 19 figure
Hyperon Nonleptonic Weak Decays Revisited
We first review the current algebra - PCAC approach to nonleptonic octet
baryon 14 weak decay B (\to) (B^{\prime})(\pi) amplitudes. The needed four
parameters are independently determined by (\Omega \to \Xi \pi),(\Lambda K) and
(\Xi ^{-}\to \Sigma ^{-}\gamma) weak decays in dispersion theory tree order. We
also summarize the recent chiral perturbation theory (ChPT) version of the
eight independent B (\to) (B^{\prime}\pi) weak (\Delta I) = 1/2 amplitudes
containing considerably more than eight low-energy weak constants in one-loop
order.Comment: 10 pages, RevTe
Evidence for a Peierls phase-transition in a three-dimensional multiple charge-density waves solid
The effect of dimensionality on materials properties has become strikingly
evident with the recent discovery of graphene. Charge ordering phenomena can be
induced in one dimension by periodic distortions of a material's crystal
structure, termed Peierls ordering transition. Charge-density waves can also be
induced in solids by strong Coulomb repulsion between carriers, and at the
extreme limit, Wigner predicted that crystallization itself can be induced in
an electrons gas in free space close to the absolute zero of temperature.
Similar phenomena are observed also in higher dimensions, but the microscopic
description of the corresponding phase transition is often controversial, and
remains an open field of research for fundamental physics. Here, we photoinduce
the melting of the charge ordering in a complex three-dimensional solid and
monitor the consequent charge redistribution by probing the optical response
over a broad spectral range with ultrashort laser pulses. Although the
photoinduced electronic temperature far exceeds the critical value, the
charge-density wave is preserved until the lattice is sufficiently distorted to
induce the phase transition. Combining this result with it ab initio}
electronic structure calculations, we identified the Peierls origin of multiple
charge-density waves in a three-dimensional system for the first time.Comment: Accepted for publication in Proc. Natl. Acad. Sci. US
Roughness of Sandpile Surfaces
We study the surface roughness of prototype models displaying self-organized
criticality (SOC) and their noncritical variants in one dimension. For SOC
systems, we find that two seemingly equivalent definitions of surface roughness
yields different asymptotic scaling exponents. Using approximate analytical
arguments and extensive numerical studies we conclude that this ambiguity is
due to the special scaling properties of the nonlinear steady state surface. We
also find that there is no such ambiguity for non-SOC models, although there
may be intermediate crossovers to different roughness values. Such crossovers
need to be distinguished from the true asymptotic behaviour, as in the case of
a noncritical disordered sandpile model studied in [10].Comment: 5 pages, 4 figures. Accepted for publication in Phys. Rev.
Message Transfer in a Communication Network
We study message transfer in a communication network of regular nodes
and randomly distributed hubs. We study both single message transfer and
multiple message transfer on the lattice. The average travel time for single
messages travelling between source and target pairs of fixed separations shows
exponential behaviour as a function of hub density with a characteristic
power-law tail, indicating a rapid drop in the average travel time as a
function of hub density. This power-law tail arises as a consequence of the
log-normal distribution of travel times seen at high hub densities. When many
messages travel on the lattice, a congestion-decongestion transition can be
seen. The waiting times of messages in the congested phase show a Gaussian
distribution, whereas the decongested phase shows a log-normal distribution.
Thus, the congested or decongested behaviour is encrypted in the behaviour of
the waiting time distributions.Comment: 7 Pages, 6 figure, to appear in the Proceeding of the conference
Perspectives in Nonlinear Dynamics 2007, a special issue of the Journal
Praman
Mirror matter admixtures and isospin breaking in the \Delta I=1/2 rule in \Omega^- two body non-leptonic decays
We discuss a description of \Omega^- two body non-leptonic decays based on
possible, albeit tiny, admixtures of mirror matter in ordinary hadrons. The
\Delta I=1/2 rule enhancement is obtained as a result of isospin symmetry and,
more importantly, the rather large observed deviations from this rule result
from small isospin breaking. This analysis lends support to the possibility
that the enhancement phenomenon observed in low energy weak interactions may be
systematically described by mirror matter admixtures in ordinary hadrons.Comment: Changed conten
An Effective Field Theory Calculation of the Parity Violating Asymmetry in n+p -> d+gamma
Weak interactions are expected to induce a parity violating pion-nucleon
coupling, h_{\pi NN}^{(1)}. This coupling should be measurable in a proposed
experiment to study the parity violating asymmetry A_\gamma in the process \vec
n + p \to d+\gamma. We compute the leading dependence of A_\gamma on the
coupling h_{\pi NN}^{(1)} using recently developed effective field theory
techniques and find an asymmetry of A_\gamma = +0.17 h_{\pi NN}^{(1)} at
leading order. This asymmetry has the opposite sign to that given by
Desplanques, Donoghue and Holstein.Comment: 7 pages, 2 figures from 3 eps files, late
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