19,157 research outputs found
Modelling of epitaxial film growth with a Ehrlich-Schwoebel barrier dependent on the step height
The formation of mounded surfaces in epitaxial growth is attributed to the
presence of barriers against interlayer diffusion in the terrace edges, known
as Ehrlich-Schwoebel (ES) barriers. We investigate a model for epitaxial growth
using a ES barrier explicitly dependent on the step height. Our model has an
intrinsic topological step barrier even in the absence of an explicit ES
barrier. We show that mounded morphologies can be obtained even for a small
barrier while a self-affine growth, consistent with the Villain-Lai-Das Sarma
equation, is observed in absence of an explicit step barrier. The mounded
surfaces are described by a super-roughness dynamical scaling characterized by
locally smooth (faceted) surfaces and a global roughness exponent .
The thin film limit is featured by surfaces with self-assembled
three-dimensional structures having an aspect ratio (height/width) that may
increase or decrease with temperature depending on the strength of step
barrier.Comment: To appear in J. Phys. Cond. Matter; 3 movies as supplementary
materia
Error threshold in finite populations
A simple analytical framework to study the molecular quasispecies evolution
of finite populations is proposed, in which the population is assumed to be a
random combination of the constiyuent molecules in each generation,i.e.,
linkage disequilibrium at the population level is neglected. In particular, for
the single-sharp-peak replication landscape we investigate the dependence of
the error threshold on the population size and find that the replication
accuracy at threshold increases linearly with the reciprocal of the population
size for sufficiently large populations. Furthermore, in the deterministic
limit our formulation yields the exact steady-state of the quasispecies model,
indicating then the population composition is a random combination of the
molecules.Comment: 14 pages and 4 figure
Determination of Gluten Peptides Associated with Celiac Disease by Mass Spectrometry
Gluten is a big protein network composed of monomeric fraction (prolamins) and polymeric fraction (glutelins), occurring in many cereal-based products, especially in those containing wheat. Gluten peptides can trigger food allergies and intolerances, including inflammatory reactions as the celiac disease, an autoimmune disorder of the small intestine characterized by mucosal degeneration and villous atrophy. The treatment is the permanent exclusion of gluten from diet. However, gluten analysis is a very difficult task, due to the high complexity of polypeptides and the lack of consensus on the most appropriate analytical method. Proteomics approaches, combining liquid chromatography and mass spectrometry in tandem (LC-MS/MS), have been pointed as the most promising non-immunological techniques for gluten detection. LC-MS analyses associated with bioinformatics and specific-prolamin database can solve methodological limitations since it is based on the accurate molecular mass of peptide biomarkers. One of the major contributions of proteomics has been the identification of epitopes of gluten peptides responsible for wheat-related diseases. Recent works have defined grain-specific gluten peptides and also the lowest concentration at which peptides could be confidently detected. Proteomic application for gluten quantification should support not only regulatory limits in processed foods, but also the safety of consumers about food labeled as gluten-free
Group selection models in prebiotic evolution
The evolution of enzyme production is studied analytically using ideas of the
group selection theory for the evolution of altruistic behavior. In particular,
we argue that the mathematical formulation of Wilson's structured deme model
({\it The Evolution of Populations and Communities}, Benjamin/Cumings, Menlo
Park, 1980) is a mean-field approach in which the actual environment that a
particular individual experiences is replaced by an {\it average} environment.
That formalism is further developed so as to avoid the mean-field approximation
and then applied to the problem of enzyme production in the prebiotic context,
where the enzyme producer molecules play the altruists role while the molecules
that benefit from the catalyst without paying its production cost play the
non-altruists role. The effects of synergism (i.e., division of labor) as well
as of mutations are also considered and the results of the equilibrium analysis
are summarized in phase diagrams showing the regions of the space of parameters
where the altruistic, non-altruistic and the coexistence regimes are stable. In
general, those regions are delimitated by discontinuous transition lines which
end at critical points.Comment: 22 pages, 10 figure
Methods for calculating nonconcave entropies
Five different methods which can be used to analytically calculate entropies
that are nonconcave as functions of the energy in the thermodynamic limit are
discussed and compared. The five methods are based on the following ideas and
techniques: i) microcanonical contraction, ii) metastable branches of the free
energy, iii) generalized canonical ensembles with specific illustrations
involving the so-called Gaussian and Betrag ensembles, iv) restricted canonical
ensemble, and v) inverse Laplace transform. A simple long-range spin model
having a nonconcave entropy is used to illustrate each method.Comment: v1: 22 pages, IOP style, 7 color figures, contribution for the JSTAT
special issue on Long-range interacting systems. v2: Open problem and
references added, minor typos corrected, close to published versio
Estudo da inibição de HMF e furfural na fermentação do hidrolisado de madeira de Eucalyptus urophylla.
Abundance trends in the inner and outer Galactic disk
Based on high-resolution spectra obtained with the MIKE spectrograph on the
Magellan telescopes we present detailed elemental abundances for 64 red giant
stars in the inner and outer Galactic disk. For the inner disk sample (4-7 kpc
from the Galactic centre) we find that stars with both thin and thick disk
abundance patterns are present while for Galactocentric distances beyond 10
kpc, we only find chemical patterns associated with the local thin disk, even
for stars far above the Galactic plane. Our results show that the relative
densities of the thick and thin disks are dramatically different from the solar
neighbourhood, and we therefore suggest that the radial scale length of the
thick disk is much shorter than that of the thin disk. A thick disk
scale-length of L_{thick}=2.0 kpc, and L_{thin}=3.8 kpc for the thin disk,
better match the data.Comment: Contributed talk at Galactic archeology, near-field cosmology and the
formation of the Milky Way, Shuzenji, Japan, 1-4 November 2011, to be
published in ASP Conference Serie
Twisted partial actions of Hopf algebras
In this work, the notion of a twisted partial Hopf action is introduced as a
unified approach for twisted partial group actions, partial Hopf actions and
twisted actions of Hopf algebras. The conditions on partial cocycles are
established in order to construct partial crossed products, which are also
related to partially cleft extensions of algebras. Examples are elaborated
using algebraic groups
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