359 research outputs found
In Memoriam: Dr. W. John Weilgart
Dr. W. John Weilgart, professor emeritus of psychology at Luther College died at age 67, Monday, January 26. He suffered from leukemia. Weilgart served on the Luther faculty from 1964-78. He previously taught at the University of Portland, Mills College, Xavier University, Notre Dame, and California Lutheran College
Simulation and analysis of in vitro DNA evolution
We study theoretically the in vitro evolution of a DNA sequence by binding to
a transcription factor. Using a simple model of protein-DNA binding and
available binding constants for the Mnt protein, we perform large-scale,
realistic simulations of evolution starting from a single DNA sequence. We
identify different parameter regimes characterized by distinct evolutionary
behaviors. For each regime we find analytical estimates which agree well with
simulation results. For small population sizes, the DNA evolutional path is a
random walk on a smooth landscape. While for large population sizes, the
evolution dynamics can be well described by a mean-field theory. We also study
how the details of the DNA-protein interaction affect the evolution.Comment: 11 pages, 11 figures. Submitted to PNA
Tropical climate–vegetation–fire relationships: multivariate evaluation of the land surface model JSBACH
The interactions between climate, vegetation and fire can strongly influence
the future trajectories of vegetation in Earth system models. We evaluate the
relationships between tropical climate, vegetation and fire in the global
vegetation model JSBACH, using a simple fire scheme and the complex fire
model SPITFIRE with the aim to identify potential for model improvement. We
use two remote-sensing products (based on MODIS and Landsat) in different
resolutions to assess the robustness of the obtained observed relationships.
We evaluate the model using a multivariate comparison that allows us to focus
on the interactions between climate, vegetation and fire and test the
influence of land use change on the modelled patterns.
Climate–vegetation–fire relationships are known to differ between
continents; we therefore perform the analysis for each continent separately.The observed relationships are
similar in the two satellite data sets, but maximum tree cover is reached at
higher precipitation values for coarser resolution. This shows that the
spatial scale of models and data needs to be consistent for meaningful
comparisons. The model captures the broad spatial patterns with regional
differences, which are partly due to the climate forcing derived from an
Earth system model. Compared to the simple fire scheme, SPITFIRE strongly
improves the spatial pattern of burned area and the distribution of burned
area along increasing precipitation. The correlation between precipitation
and tree cover is higher in the observations than in the largely climate-driven vegetation model, with both fire models. The multivariate comparison
identifies excessive tree cover in low-precipitation areas and a too-strong relationship between high fire occurrence and low tree cover for the
complex fire model. We therefore suggest that drought effects on tree cover
and the impact of burned area on tree cover or the adaptation of trees to
fire can be improved.The observed variation in the relationship between precipitation and maximum
tree cover between continents is higher than the simulated one. Land use
contributes to the intercontinental differences in fire regimes with SPITFIRE
and strongly overprints the modelled multimodality of tree cover with
SPITFIRE.The multivariate model–data comparison used here has several
advantages: it improves the attribution of model–data mismatches to model
processes, it reduces the impact of biases in the meteorological forcing on
the evaluation and it allows us to evaluate not only a specific target variable
but also the interactions.</p
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Mechanistic Study Of Plasma Damage Of Low k Dielectric Surfaces
Plasma damage to low k dielectric materials was investigated from a mechanistic point of view. Low k dielectric films were treated by plasma Ar, O-2, N-2/H-2, N-2 and H-2 in a standard RIE chamber and the damage was characterized by Angle Resolved X-ray Photoelectron Spectroscopy (ARXPS), X-Ray Reflectivity (XRR), Fourier Transform Infrared Spectroscopy (FTIR) and Contact Angle measurements. Both carbon depletion and surface densification were observed on the top surface of damaged low k materials while the bulk remained largely unaffected. Plasma damage was found to be a complicated phenomenon involving both chemical and physical effects, depending on chemical reactivity and the energy and mass of the plasma species. A downstream hybrid plasma source with separate ions and atomic radicals was employed to study their respective roles in the plasma damage process. Ions were found to play a more important role in the plasma damage process. The dielectric constant of low k materials can increase up to 20% due to plasma damage and we attributed this to the removal of the methyl group making the low k surface hydrophilic. Annealing was generally effective in mitigating moisture uptake to restore the k value but the recovery was less complete for higher energy plasmas. Quantum chemistry calculation confirmed that physisorbed water in low k materials induces the largest increase of dipole moments in comparison with changes of surface bonding configurations, and is primarily responsible for the dielectric constant increase.Microelectronics Research Cente
Dime como educas y te diré como es tu ambiente
La crisis ambiental es un problema de conocimiento, por ello involucra a la educación, siendo el desafío de ella el de pasar de una cultura economicista, que refuerza y que es reforzada por la globalización, a una cultura de pertenencia, de compromiso, de resistencia, de solidaridad. Debemos focalizar los esfuerzos por construir desde la complejidad ambiental una ciencia para la sustentabilidad que hunda sus raíces en la justicia ambiental y que destierre las ideas simplificadoras y reduccionistas de la sociedad y comience a fundar el nuevo paradigma basado en la interdisciplinaridad y la transdisciplinariedad. Este proyecto didáctico se elabora para aplicar en dos cursos "paralelos" de diferentes escuelas de la ciudad de Rafaela, provincia de Santa Fe. Se formarán equipos de trabajo que interactuarán en esta propuesta: docentes (biología, historia, derecho, lengua, geografía, economía), directivos y los alumnos, con las adaptaciones a sus problemáticas, su universo y territorialidad, implementando ILS-indicadores locales de sustentabilidad.Trabajos del área Ciencias NaturalesDepartamento de Ciencias Exactas y Naturale
Scaling in the time-dependent failure of a fiber bundle with local load sharing
We study the scaling behaviors of a time-dependent fiber-bundle model with
local load sharing. Upon approaching the complete failure of the bundle, the
breaking rate of fibers diverges according to ,
where is the lifetime of the bundle, and is a quite
universal scaling exponent. The average lifetime of the bundle scales
with the system size as , where depends on the
distribution of individual fiber as well as the breakdown rule.Comment: 5 pages, 4 eps figures; to appear in Phys. Rev.
Burst avalanches in solvable models of fibrous materials
We review limiting models for fracture in bundles of fibers, with
statistically distributed thresholds for breakdown of individual fibers. During
the breakdown process, avalanches consisting of simultaneous rupture of several
fibers occur, and the distribution of the magnitude of
such avalanches is the central characteristics in our analysis. For a bundle of
parallel fibers two limiting models of load sharing are studied and contrasted:
the global model in which the load carried by a bursting fiber is equally
distributed among the surviving members, and the local model in which the
nearest surviving neighbors take up the load. For the global model we
investigate in particular the conditions on the threshold distribution which
would lead to anomalous behavior, i.e. deviations from the asymptotics
, known to be the generic behavior. For the local
model no universal power-law asymptotics exists, but we show for a particular
threshold distribution how the avalanche distribution can nevertheless be
explicitly calculated in the large-bundle limit.Comment: 28 pages, RevTeX, 3 Postscript figure
Gravastar Solutions with Continuous Pressures and Equation of State
We study the gravitational vacuum star (gravastar) configuration as proposed
by other authors in a model where the interior de Sitter spacetime segment is
continuously extended to the exterior Schwarzschild spacetime. The multilayered
structure in previous papers is replaced by a continuous stress-energy tensor
at the price of introducing anisotropy in the (fluid) model of the gravastar.
Either with an ansatz for the equation of state connecting the radial and
tangential pressure or with a calculated equation of state with
non-homogeneous energy/fluid density, solutions are obtained which in all
aspects satisfy the conditions expected for an anisotropic gravastar. Certain
energy conditions have been shown to be obeyed and a polytropic equation of
state has been derived. Stability of the solution with respect to possible
axial perturbation is shown to hold.Comment: 19 pages, 9 figures. Latest version contains new and updated
references along with some clarifying remarks in the stability analysi
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