4,553 research outputs found
Forces and conductances in a single-molecule bipyridine junction
Inspired by recent measurements of forces and conductances of bipyridine
nano-junctions, we have performed density functional theory calculations of
structure and electron transport in a bipyridine molecule attached between gold
electrodes for seven different contact geometries. The calculations show that
both the bonding force and the conductance are sensitive to the surface
structure, and that both properties are in good agreement with experiment for
contact geometries characterized by intermediate coordination of the metal
atoms corresponding to a stepped surface. The conductance is mediated by the
lowest unoccupied molecular orbital, which can be illustrated by a quantitative
comparison with a one-level model. Implications for the interpretation of the
experimentally determined force and conductance distributions are discussed
Phylogenetic analysis accounting for age-dependent death and sampling with applications to epidemics
The reconstruction of phylogenetic trees based on viral genetic sequence data
sequentially sampled from an epidemic provides estimates of the past
transmission dynamics, by fitting epidemiological models to these trees. To our
knowledge, none of the epidemiological models currently used in phylogenetics
can account for recovery rates and sampling rates dependent on the time elapsed
since transmission.
Here we introduce an epidemiological model where infectives leave the
epidemic, either by recovery or sampling, after some random time which may
follow an arbitrary distribution.
We derive an expression for the likelihood of the phylogenetic tree of
sampled infectives under our general epidemiological model. The analytic
concept developed in this paper will facilitate inference of past
epidemiological dynamics and provide an analytical framework for performing
very efficient simulations of phylogenetic trees under our model. The main idea
of our analytic study is that the non-Markovian epidemiological model giving
rise to phylogenetic trees growing vertically as time goes by, can be
represented by a Markovian "coalescent point process" growing horizontally by
the sequential addition of pairs of coalescence and sampling times.
As examples, we discuss two special cases of our general model, namely an
application to influenza and an application to HIV. Though phrased in
epidemiological terms, our framework can also be used for instance to fit
macroevolutionary models to phylogenies of extant and extinct species,
accounting for general species lifetime distributions.Comment: 30 pages, 2 figure
Towards Grower-friendly Apple Crop Thinning by Tree Shading
Light management with shading nets, which reduce sunlight by 74%, might be an
alternative to chemicals commonly used for thinning on apple trees. To study the effect of
shading on crop load and fruit quality, trials were conducted in field experiments with the
cultivars Golden Delicious and Elstar in 2006. Trees were either covered 25 days after full
bloom (DAFB) with a net during three days, or until the peak of fruit fall, observed after
seven days shading. Ideal time length for optimal crop yield was seven days shading for
Elstar and three days shading for Golden Delicious. Alternate bearing could be decreased
as flower initiation counts the following year showed. In both experiments, inner quality of
fruit such as sugar and firmness showed good values at optimal shading duration
compared with chemical + hand thinning. In 2007, a second field trial was conducted with
cultivars Golden Delicious and Topaz to study the time period for shading in further detail.
Shading was done for three days at 19, 26 and 33 DAFB using two net types (three- and
two-meter-net width, covering the trees entirely or only down to 50 cm above ground). For
Golden Delicious, shading after 19 and 26 days reduced fruits per 100 flower cluster to the
same extent as with chemical + hand thinning. There was no difference between the two
net types. For Topaz, shading after 19 days showed the best results. Regarding inner
quality of both cultivars, only sugar content for Golden Delicious could be significantly
improved after 19 and 26 days shading. Further analyses are still under way (e.g. for
acidity).
This study is part of an effort for increasing European consumption with fruit from
sustainable production systems, the ISAFRUIT-EU-project
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