784 research outputs found
Direct generation of charge carriers in c-Si solar cells due to embedded nanoparticles
It is known that silicon is an indirect band gap material, reducing its
efficiency in photovoltaic applications. Using surface plasmons in metallic
nanoparticles embedded in a solar cell has recently been proposed as a way to
increase the efficiency of thin film silicon solar cells. The dipole mode that
dominates the plasmons in small particles produces an electric field having
Fourier components with all wave numbers. In this work, we show that such a
field creates electron-hole-pairs without phonon assistance, and discuss the
importance of this effect compared to radiation from the particle and losses
due to heating.Comment: 1 figur
Epigenetic modifications affect the rate of spontaneous mutations in a pathogenic fungus
Mutations are the source of genetic variation and the substrate for evolution. Genome-widemutation rates appear to be affected by selection and are probably adaptive. Mutation ratesare also known to vary along genomes, possibly in response to epigenetic modifications, butcausality is only assumed. In this study we determine the direct impact of epigenetic mod-ifications and temperature stress on mitotic mutation rates in a fungal pathogen using amutation accumulation approach. Deletion mutants lacking epigenetic modifications confirmthat histone mark H3K27me3 increases whereas H3K9me3 decreases the mutation rate.Furthermore, cytosine methylation in transposable elements (TE) increases the mutation rate15-fold resulting in significantly less TE mobilization. Also accessory chromosomes havesignificantly higher mutation rates. Finally, wefind that temperature stress substantiallyelevates the mutation rate. Taken together, wefind that epigenetic modifications andenvironmental conditions modify the rate and the location of spontaneous mutations in thegenome and alter its evolutionary trajectory
Derivation of the Lorentz Force Law, the Magnetic Field Concept and the Faraday-Lenz Law using an Invariant Formulation of the Lorentz Transformation
It is demonstrated how the right hand sides of the Lorentz Transformation
equations may be written, in a Lorentz invariant manner, as 4--vector scalar
products. This implies the existence of invariant length intervals analogous to
invariant proper time intervals. This formalism, making essential use of the
4-vector electromagnetic potential concept, provides a short derivation of the
Lorentz force law of classical electrodynamics, the conventional definition of
the magnetic field, in terms of spatial derivatives of the 4--vector potential
and the Faraday-Lenz Law. An important distinction between the physical
meanings of the space-time and energy-momentum 4--vectors is pointed out.Comment: 15 pages, no tables 1 figure. Revised and extended version of
physics/0307133 Some typos removed and minor text improvements in this
versio
Dynamics of transposable elements in recently diverged fungal pathogens: lineage-specific transposable element content and efficiency of genome defences
Transposable elements (TEs) impact genome plasticity, architecture and evolution in fungal plant pathogens. The wide range of TE content observed in fungal genomes reflects diverse efficacy of host-genome defence mechanisms that can counter-balance TE expansion and spread. Closely related species can harbour drastically different TE repertoires, suggesting variation in the efficacy of genome defences. The evolution of fungal effectors, which are crucial determinants of pathogenicity, has been linked to the activity of TEs in pathogen genomes. Here we describe how TEs have shaped genome evolution of the fungal wheat pathogen Zymoseptoria tritici and four closely related species. We compared de novo TE annotations and Repeat-Induced Point mutation signatures in thirteen genomes from the Zymoseptoria species-complex. Then, we assessed the relative insertion ages of TEs using a comparative genomics approach. Finally, we explored the impact of TE insertions on genome architecture and plasticity. The thirteen genomes of Zymoseptoria species reflect different TE dynamics with a majority of recent insertions. TEs associate with distinct genome compartments in all Zymoseptoria species, including chromosomal rearrangements, genes showing presence/absence variation and effectors. European Z. tritici isolates have reduced signatures of Repeat-Induced Point mutations compared to Iranian isolates and closely related species. Our study supports the hypothesis that ongoing but moderate TE mobility in Zymoseptoria species shapes pathogen genome evolution.Competing Interest StatementThe authors have declared no competing interest
Link Prediction Based on Local Random Walk
The problem of missing link prediction in complex networks has attracted much
attention recently. Two difficulties in link prediction are the sparsity and
huge size of the target networks. Therefore, the design of an efficient and
effective method is of both theoretical interests and practical significance.
In this Letter, we proposed a method based on local random walk, which can give
competitively good prediction or even better prediction than other
random-walk-based methods while has a lower computational complexity.Comment: 6 pages, 2 figure
A Precision Angle Sensor using an Optical Lever inside a Sagnac Interferometer
We built an ultra low noise angle sensor by combining a folded optical lever
and a Sagnac interferometer. The instrument has a measured noise floor of 1.3
prad / Hz^(1/2) at 2.4 kHz. We achieve this record angle sensitivity using a
proof-of-concept apparatus with a conservative N=11 bounces in the optical
lever. This technique could be extended to reach sub-picoradian / Hz^(1/2)
sensitivities with an optimized design.Comment: 3 pages, 4 figure
Responses of two scleractinian corals to cobalt pollution and ocean acidification
The effects of ocean acidification alone or in combination with warming on coral metabolism have been extensively investigated, whereas none of these studies consider that most coral reefs near shore are already impacted by other natural anthropogenic inputs such as metal pollution. It is likely that projected ocean acidification levels will aggravate coral reef health. We first investigated how ocean acidification interacts with one near shore locally abundant metal on the physiology of two major reef-building corals: Stylophora pistillata and Acropora muricata. Two pH levels (pH(T) 8.02; pCO(2) 366 mu atm and pH(T) 7.75; pCO(2) 1140 mu atm) and two cobalt concentrations (natural, 0.03 mu g L-1 and polluted, 0.2 mu g L-1) were tested during five weeks in aquaria. We found that, for both species, cobalt input decreased significantly their growth rates by 28% while it stimulated their photosystem II, with higher values of rETR(max) (relative Electron Transport Rate). Elevated pCO(2) levels acted differently on the coral rETR(max) values and did not affect their growth rates. No consistent interaction was found between pCO(2) levels and cobalt concentrations. We also measured in situ the effect of higher cobalt concentrations (1.06 +/- 0.16 mu g L-1) on A. muricata using benthic chamber experiments. At this elevated concentration, cobalt decreased simultaneously coral growth and photosynthetic rates, indicating that the toxic threshold for this pollutant has been reached for both host cells and zooxanthellae. Our results from both aquaria and in situ experiments, suggest that these coral species are not particularly sensitive to high pCO(2) conditions but they are to ecologically relevant cobalt concentrations. Our study reveals that some reefs may be yet subjected to deleterious pollution levels, and even if no interaction between pCO(2) levels and cobalt concentration has been found, it is likely that coral metabolism will be weakened if they are subjected to additional threats such as temperature increase, other heavy metals, and eutrophication
Vertex similarity in networks
We consider methods for quantifying the similarity of vertices in networks.
We propose a measure of similarity based on the concept that two vertices are
similar if their immediate neighbors in the network are themselves similar.
This leads to a self-consistent matrix formulation of similarity that can be
evaluated iteratively using only a knowledge of the adjacency matrix of the
network. We test our similarity measure on computer-generated networks for
which the expected results are known, and on a number of real-world networks
Electromagnetic force density in dissipative isotropic media
We derive an expression for the macroscopic force density that a narrow-band
electromagnetic field imposes on a dissipative isotropic medium. The result is
obtained by averaging the microscopic form for Lorentz force density. The
derived expression allows us to calculate realistic electromagnetic forces in a
wide range of materials that are described by complex-valued electric
permittivity and magnetic permeability. The three-dimensional energy-momentum
tensor in our expression reduces for lossless media to the so-called Helmholtz
tensor that has not been contradicted in any experiment so far. The momentum
density of the field does not coincide with any well-known expression, but for
non-magnetic materials it matches the Abraham expression
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