643 research outputs found
Temperature dependence of the band gap shrinkage due to electron-phonon interaction in undoped n-type GaN
The photoluminescence spectra of band-edge transitions in GaN is studied as a
function of temperature. The parameters that describe the temperature
dependence red-shift of the band-edge transition energy and the broadening of
emission line are evaluated using different models. We find that the
semi-empirical relation based on phonon-dispersion related spectral function
leads to excellent fit to the experimental data. The exciton-phonon coupling
constants are determined from the analysis of linewidth broadening
Fatigue-Life Prediction Methodology Using Small-Crack Theory
This paper reviews the capabilities of a plasticity-induced crack-closure model to predict fatigue lives of metallic materials using 'small-crack theory' for various materials and loading conditions. Crack-tip constraint factors, to account for three-dimensional state-of-stress effects, were selected to correlate large-crack growth rate data as a function of the effective-stress-intensity factor range (delta K(eff)) under constant-amplitude loading. Some modifications to the delta k(eff)-rate relations were needed in the near-threshold regime to fit measured small-crack growth rate behavior and fatigue endurance limits. The model was then used to calculate small- and large-crack growth rates, and to predict total fatigue lives, for notched and un-notched specimens made of two aluminum alloys and a steel under constant-amplitude and spectrum loading. Fatigue lives were calculated using the crack-growth relations and microstructural features like those that initiated cracks for the aluminum alloys and steel for edge-notched specimens. An equivalent-initial-flaw-size concept was used to calculate fatigue lives in other cases. Results from the tests and analyses agreed well
Continuous Percolation Phase Transitions of Two-dimensional Lattice Networks under a Generalized Achlioptas Process
The percolation phase transitions of two-dimensional lattice networks under a
generalized Achlioptas process (GAP) are investigated. During the GAP, two
edges are chosen randomly from the lattice and the edge with minimum product of
the two connecting cluster sizes is taken as the next occupied bond with a
probability . At , the GAP becomes the random growth model and leads
to the minority product rule at . Using the finite-size scaling analysis,
we find that the percolation phase transitions of these systems with are always continuous and their critical exponents depend on .
Therefore, the universality class of the critical phenomena in two-dimensional
lattice networks under the GAP is related to the probability parameter in
addition.Comment: 7 pages, 14 figures, accepted for publication in Eur. Phys. J.
Pedagogies of critique : struggling with what and how to think
No abstract available
Lyapunov exponents and transport in the Zhang model of Self-Organized Criticality
We discuss the role played by the Lyapunov exponents in the dynamics of
Zhang's model of Self-Organized Criticality. We show that a large part of the
spectrum (slowest modes) is associated with the energy transpor in the lattice.
In particular, we give bounds on the first negative Lyapunov exponent in terms
of the energy flux dissipated at the boundaries per unit of time. We then
establish an explicit formula for the transport modes that appear as diffusion
modes in a landscape where the metric is given by the density of active sites.
We use a finite size scaling ansatz for the Lyapunov spectrum and relate the
scaling exponent to the scaling of quantities like avalanche size, duration,
density of active sites, etc ...Comment: 33 pages, 6 figures, 1 table (to appear
Teachers as leaders in a knowledge society: encouraging signs of a new professionalism
[Abstract]: Challenges confronting schools worldwide are greater than ever,and, likewise, many teachers possess capabilities, talents, and formal credentials more sophisticated than ever. However, the responsibility and authority accorded
to teachers have not grown significantly, nor has the image of teaching as a profession advanced significantly. The question becomes, what are the implications for the image and status of the teaching profession as the concept of knowledge society takes a firm hold in the industrialized world? This article addresses the philosophical underpinnings of teacher leadership manifested in case studies where schools sought to achieve the generation of new knowledge as part of a process of whole-school revitalization. Specifically, this article reports on Australian research that has illuminated the work of teacher leaders engaged in the IDEAS project, a joint school revitalization initiative of the University
of Southern Queensland and the Queensland Department of Education and the Arts
Properties of a random attachment growing network
In this study we introduce and analyze the statistical structural properties
of a model of growing networks which may be relevant to social networks. At
each step a new node is added which selects 'k' possible partners from the
existing network and joins them with probability delta by undirected edges. The
'activity' of the node ends here; it will get new partners only if it is
selected by a newcomer. The model produces an infinite-order phase transition
when a giant component appears at a specific value of delta, which depends on
k. The average component size is discontinuous at the transition. In contrast,
the network behaves significantly different for k=1. There is no giant
component formed for any delta and thus in this sense there is no phase
transition. However, the average component size diverges for delta greater or
equal than one half.Comment: LaTeX, 19 pages, 6 figures. Discussion section, comments, a new
figure and a new reference are added. Equations simplifie
System size resonance in coupled noisy systems and in the Ising model
We consider an ensemble of coupled nonlinear noisy oscillators demonstrating
in the thermodynamic limit an Ising-type transition. In the ordered phase and
for finite ensembles stochastic flips of the mean field are observed with the
rate depending on the ensemble size. When a small periodic force acts on the
ensemble, the linear response of the system has a maximum at a certain system
size, similar to the stochastic resonance phenomenon. We demonstrate this
effect of system size resonance for different types of noisy oscillators and
for different ensembles -- lattices with nearest neighbors coupling and
globally coupled populations. The Ising model is also shown to demonstrate the
system size resonance.Comment: 4 page
Time series irreversibility: a visibility graph approach
We propose a method to measure real-valued time series irreversibility which
combines two differ- ent tools: the horizontal visibility algorithm and the
Kullback-Leibler divergence. This method maps a time series to a directed
network according to a geometric criterion. The degree of irreversibility of
the series is then estimated by the Kullback-Leibler divergence (i.e. the
distinguishability) between the in and out degree distributions of the
associated graph. The method is computationally effi- cient, does not require
any ad hoc symbolization process, and naturally takes into account multiple
scales. We find that the method correctly distinguishes between reversible and
irreversible station- ary time series, including analytical and numerical
studies of its performance for: (i) reversible stochastic processes
(uncorrelated and Gaussian linearly correlated), (ii) irreversible stochastic
pro- cesses (a discrete flashing ratchet in an asymmetric potential), (iii)
reversible (conservative) and irreversible (dissipative) chaotic maps, and (iv)
dissipative chaotic maps in the presence of noise. Two alternative graph
functionals, the degree and the degree-degree distributions, can be used as the
Kullback-Leibler divergence argument. The former is simpler and more intuitive
and can be used as a benchmark, but in the case of an irreversible process with
null net current, the degree-degree distribution has to be considered to
identifiy the irreversible nature of the series.Comment: submitted for publicatio
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