36,825 research outputs found
Transfer-matrix study of a hard-square lattice gas with two kinds of particles and density anomaly
Using transfer matrix and finite-size scaling methods, we study the
thermodynamic behavior of a lattice gas with two kinds of particles on the
square lattice. Only excluded volume interactions are considered, so that the
model is athermal. Large particles exclude the site they occupy and its four
first neighbors, while small particles exclude only their site. Two
thermodynamic phases are found: a disordered phase where large particles occupy
both sublattices with the same probability and an ordered phase where one of
the two sublattices is preferentially occupied by them. The transition between
these phases is continuous at small concentrations of the small particles and
discontinuous at larger concentrations, both transitions are separated by a
tricritical point. Estimates of the central charge suggest that the critical
line is in the Ising universality class, while the tricritical point has
tricritical Ising (Blume-Emery-Griffiths) exponents. The isobaric curves of the
total density as functions of the fugacity of small or large particles display
a minimum in the disordered phase.Comment: 9 pages, 7 figures and 4 table
Roughness exponents and grain shapes
In surfaces with grainy features, the local roughness shows a crossover
at a characteristic length , with roughness exponent changing from
to a smaller . The grain shape, the choice of
or height-height correlation function (HHCF) , and the procedure to
calculate root mean-square averages are shown to have remarkable effects on
. With grains of pyramidal shape, can be as low as 0.71,
which is much lower than the previous prediction 0.85 for rounded grains. The
same crossover is observed in the HHCF, but with initial exponent
for flat grains, while for some conical grains it may
increase to . The universality class of the growth process
determines the exponents after the crossover, but has no
effect on the initial exponents and , supporting the
geometric interpretation of their values. For all grain shapes and different
definitions of surface roughness or HHCF, we still observe that the crossover
length is an accurate estimate of the grain size. The exponents obtained
in several recent experimental works on different materials are explained by
those models, with some surface images qualitatively similar to our model
films.Comment: 7 pages, 6 figures and 2 table
Kinetic modelling of epitaxial film growth with up- and downward step barriers
The formation of three-dimensional structures during the epitaxial growth of
films is associated to the reflection of diffusing particles in descending
terraces due to the presence of the so-called Ehrlich-Schwoebel (ES) barrier.
We generalize this concept in a solid-on-solid growth model, in which a barrier
dependent on the particle coordination (number of lateral bonds) exists
whenever the particle performs an interlayer diffusion. The rules do not
distinguish explicitly if the particle is executing a descending or an
ascending interlayer diffusion. We show that the usual model, with a step
barrier in descending steps, produces spurious, columnar, and highly unstable
morphologies if the growth temperature is varied in a usual range of mound
formation experiments. Our model generates well-behaved mounded morphologies
for the same ES barriers that produce anomalous morphologies in the standard
model. Moreover, mounds are also obtained when the step barrier has an equal
value for all particles independently if they are free or bonded. Kinetic
roughening is observed at long times, when the surface roughness w and the
characteristic length scale as and where
and , independently of the growth
temperature.Comment: 15 pages, 7 figure
The use of genes for performance enhancement: doping or therapy?
Recent biotechnological advances have permitted the manipulation of genetic sequences to treat several diseases in a process called gene therapy. However, the advance of gene therapy has opened the door to the possibility of using genetic manipulation (GM) to enhance athletic performance. In such ‘gene doping’, exogenous genetic sequences are inserted into a specific tissue, altering cellular gene activity or leading to the expression of a protein product. The exogenous genes most likely to be utilized for gene doping include erythropoietin (EPO), vascular endothelial growth factor (VEGF), insulin-like growth factor type 1 (IGF-1), myostatin antagonists, and endorphin. However, many other genes could also be used, such as those involved in glucose metabolic pathways. Because gene doping would be very difficult to detect, it is inherently very attractive for those involved in sports who are prepared to cheat. Moreover, the field of gene therapy is constantly and rapidly progressing, and this is likely to generate many new possibilities for gene doping. Thus, as part of the general fight against all forms of doping, it will be necessary to develop and continually improve means of detecting exogenous gene sequences (or their products) in athletes. Nevertheless, some bioethicists have argued for a liberal approach to gene doping
Fitting isochrones to open cluster photometric data III. Estimating metallicities from UBV photometry
The metallicity is a critical parameter that affects the correct
determination fundamental characteristics stellar cluster and has important
implications in Galactic and Stellar evolution research. Fewer than 10 % of the
2174 currently catalog open clusters have their metallicity determined in the
literature. In this work we present a method for estimating the metallicity of
open clusters via non-subjective isochrone fitting using the cross-entropy
global optimization algorithm applied to UBV photometric data. The free
parameters distance, reddening, age, and metallicity simultaneously determined
by the fitting method. The fitting procedure uses weights for the observational
data based on the estimation of membership likelihood for each star, which
considers the observational magnitude limit, the density profile of stars as a
function of radius from the center of the cluster, and the density of stars in
multi-dimensional magnitude space. We present results of [Fe/H] for nine
well-studied open clusters based on 15 distinct UBV data sets. The [Fe/H]
values obtained in the ten cases for which spectroscopic determinations were
available in the literature agree, indicating that our method provides a good
alternative to determining [Fe/H] by using an objective isochrone fitting. Our
results show that the typical precision is about 0.1 dex
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