239 research outputs found
Evaluation of an E-Learning Online Pecan Management Course
In February 2004, an online pecan management course was launched to educate pecan growers and assist them with decision-making. The interactive course was designed for both experienced pecan producers and first-time pecan producers. Since the inception of the course, only 24 persons have paid the registration fee. Several potential problems underlie the poor registration numbers, including low level of computer literacy, limited access to the Internet, download times, previous grower experience, cost, and awareness. Low registration numbers indicate that a more active approach to improve enrollment is needed to increase awareness
Two-dimensional wetting with binary disorder: a numerical study of the loop statistics
We numerically study the wetting (adsorption) transition of a polymer chain
on a disordered substrate in 1+1 dimension.Following the Poland-Scheraga model
of DNA denaturation, we use a Fixman-Freire scheme for the entropy of loops.
This allows us to consider chain lengths of order to ,
with disorder realizations. Our study is based on the statistics of
loops between two contacts with the substrate, from which we define Binder-like
parameters: their crossings for various sizes allow a precise determination
of the critical temperature, and their finite size properties yields a
crossover exponent .We then analyse at
criticality the distribution of loop length in both regimes
and , as well as the finite-size properties of the contact
density and energy. Our conclusion is that the critical exponents for the
thermodynamics are the same as those of the pure case, except for strong
logarithmic corrections to scaling. The presence of these logarithmic
corrections in the thermodynamics is related to a disorder-dependent
logarithmic singularity that appears in the critical loop distribution in the
rescaled variable as .Comment: 12 pages, 13 figure
Metastable lifetimes in a kinetic Ising model: Dependence on field and system size
The lifetimes of metastable states in kinetic Ising ferromagnets are studied
by droplet theory and Monte Carlo simulation, in order to determine their
dependences on applied field and system size. For a wide range of fields, the
dominant field dependence is universal for local dynamics and has the form of
an exponential in the inverse field, modified by universal and nonuniversal
power-law prefactors. Quantitative droplet-theory predictions are numerically
verified, and small deviations are shown to depend nonuniversally on the
details of the dynamics. We identify four distinct field intervals in which the
field dependence and statistical properties of the lifetimes are different. The
field marking the crossover between the weak-field regime, in which the decay
is dominated by a single droplet, and the intermediate-field regime, in which
it is dominated by a finite droplet density, vanishes logarithmically with
system size. As a consequence the slow decay characteristic of the former
regime may be observable in systems that are macroscopic as far as their
equilibrium properties are concerned.Comment: 18 pages single spaced. RevTex Version 3. FSU-SCRI-94-1
Test of the Kolmogorov-Johnson-Mehl-Avrami picture of metastable decay in a model with microscopic dynamics
The Kolmogorov-Johnson-Mehl-Avrami (KJMA) theory for the time evolution of
the order parameter in systems undergoing first-order phase transformations has
been extended by Sekimoto to the level of two-point correlation functions.
Here, this extended KJMA theory is applied to a kinetic Ising lattice-gas
model, in which the elementary kinetic processes act on microscopic length and
time scales. The theoretical framework is used to analyze data from extensive
Monte Carlo simulations. The theory is inherently a mesoscopic continuum
picture, and in principle it requires a large separation between the
microscopic scales and the mesoscopic scales characteristic of the evolving
two-phase structure. Nevertheless, we find excellent quantitative agreement
with the simulations in a large parameter regime, extending remarkably far
towards strong fields (large supersaturations) and correspondingly small
nucleation barriers. The original KJMA theory permits direct measurement of the
order parameter in the metastable phase, and using the extension to correlation
functions one can also perform separate measurements of the nucleation rate and
the average velocity of the convoluted interface between the metastable and
stable phase regions. The values obtained for all three quantities are verified
by other theoretical and computational methods. As these quantities are often
difficult to measure directly during a process of phase transformation, data
analysis using the extended KJMA theory may provide a useful experimental
alternative.Comment: RevTex, 21 pages including 14 ps figures. Submitted to Phys. Rev. B.
One misprint corrected in Eq.(C1
Impact of Load-Related Neural Processes on Feature Binding in Visuospatial Working Memory
BACKGROUND: The capacity of visual working memory (WM) is substantially limited and only a fraction of what we see is maintained as a temporary trace. The process of binding visual features has been proposed as an adaptive means of minimising information demands on WM. However the neural mechanisms underlying this process, and its modulation by task and load effects, are not well understood. OBJECTIVE: To investigate the neural correlates of feature binding and its modulation by WM load during the sequential phases of encoding, maintenance and retrieval. METHODS AND FINDINGS: 18 young healthy participants performed a visuospatial WM task with independent factors of load and feature conjunction (object identity and position) in an event-related functional MRI study. During stimulus encoding, load-invariant conjunction-related activity was observed in left prefrontal cortex and left hippocampus. During maintenance, greater activity for task demands of feature conjunction versus single features, and for increased load was observed in left-sided regions of the superior occipital cortex, precuneus and superior frontal cortex. Where these effects were expressed in overlapping cortical regions, their combined effect was additive. During retrieval, however, an interaction of load and feature conjunction was observed. This modulation of feature conjunction activity under increased load was expressed through greater deactivation in medial structures identified as part of the default mode network. CONCLUSIONS AND SIGNIFICANCE: The relationship between memory load and feature binding qualitatively differed through each phase of the WM task. Of particular interest was the interaction of these factors observed within regions of the default mode network during retrieval which we interpret as suggesting that at low loads, binding processes may be 'automatic' but at higher loads it becomes a resource-intensive process leading to disengagement of activity in this network. These findings provide new insights into how feature binding operates within the capacity-limited WM system
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