51,202 research outputs found
On Retardation Effects in Space Charge Calculations Of High Current Electron Beams
Laser-plasma accelerators are expected to deliver electron bunches with high
space charge fields. Several recent publications have addressed the impact of
space charge effects on such bunches after the extraction into vacuum.
Artifacts due to the approximation of retardation effects are addressed, which
are typically either neglected or approximated. We discuss a much more
appropriate calculation for the case of laser wakefield acceleration with
negligible retardation artifacts due to the calculation performed in the mean
rest frame. This presented calculation approach also aims at a validation of
other simulation approaches
Compactifications of Heterotic Theory on Non-Kahler Complex Manifolds: I
We study new compactifications of the SO(32) heterotic string theory on
compact complex non-Kahler manifolds. These manifolds have many interesting
features like fewer moduli, torsional constraints, vanishing Euler character
and vanishing first Chern class, which make the four-dimensional theory
phenomenologically attractive. We take a particular compact example studied
earlier and determine various geometrical properties of it. In particular we
calculate the warp factor and study the sigma model description of strings
propagating on these backgrounds. The anomaly cancellation condition and
enhanced gauge symmetry are shown to arise naturally in this framework, if one
considers the effect of singularities carefully.
We then give a detailed mathematical analysis of these manifolds and
construct a large class of them. The existence of a holomorphic (3,0) form is
important for the construction. We clarify some of the topological properties
of these manifolds and evaluate the Betti numbers. We also determine the
superpotential and argue that the radial modulus of these manifolds can
actually be stabilized.Comment: 75 pages, Harvmac, no figures; v2: Some new results added, typos
corrected and references updated. Final version to appear in JHE
Compactification with Flux on K3 and Tori
We study compactifications of Type IIB string theory on a K3 \times T^2/Z_2
orientifold in the presence of RR and NS flux. We find the most general
supersymmetry preserving, Poincare invariant, vacua in this model. All the
complex structure moduli and some of the Kahler moduli are stabilised in these
vacua. We obtain in an explicit fashion the restrictions imposed by
supersymmetry on the flux, and the values of the fixed moduli. Some T-duals and
Heterotic duals are also discussed, these are non-Calabi-Yau spaces. A
superpotential is constructed describing these duals.Comment: Discussion of susy breaking vacua significantly altere
Antireflective nanotextures for monolithic perovskite silicon tandem solar cells
Recently, we studied the effect of hexagonal sinusoidal textures on the reflective properties of perovskite silicon tandem solar cells using the finite element method FEM . We saw that such nanotextures, applied to the perovskite top cell, can strongly increase the current density utilization from 91 for the optimized planar reference to 98 for the best nanotextured device period 500 nm and peak to valley height 500 nm , where 100 refers to the Tiedje Yablonovitch limit. [D. Chen et al., J. Photonics Energy 8, 022601, 2018 , doi 10.1117 1.JPE.8.022601] In this manuscript we elaborate on some numerical details of that work we validate an assumption based on the Tiedje Yablonovitch limit, we present a convergence study for simulations with the finite element method, and we compare different configurations for sinusoidal nanotexture
The X-ray nebula of the filled center supernova remnant 3C58 and its interaction with the environment
An \xmm observation of the plerionic supernova remnant 3C58 has allowed us to
study the X-ray nebula with unprecedented detail. A spatially resolved spectral
analysis with a resolution of 8\arcsec has yielded a precise determination of
the relation between the spectral index and the distance from the center. We do
not see any evidence for bright thermal emission from the central core. In
contrast with previous ASCA and {\em Einstein} results, we derive an upper
limit to the black-body 0.5-10 keV luminosity and emitting area of \ergsec and cm, respectively, ruling out
emission from the hot surface of the putative neutron star and also excluding
the "outer-gap" model for hot polar caps. We have performed for the first time
a spectral analysis of the outer regions of the X-ray nebula, where most of the
emission is still non-thermal, but where the addition of a soft (kT=0.2-0.3
keV) optically thin plasma component is required to fit the spectrum at
keV. This component provides 6% of the whole remnant observed flux in the
0.5-10.0 keV band. We show that a Sedov interpretation is incompatible with the
SN1181-3C58 association, unless there is a strong deviation from electron-ion
energy equipartition, and that an origin of this thermal emission in terms of
the expansion of the nebula into the ejecta core nicely fits all the radio and
X-ray observations.Comment: 10 pages, 7 figures, accepted for publication in A&
A Note on Flux Induced Superpotentials in String Theory
Non-vanishing fluxes in M-theory and string theory compactifications induce a
superpotential in the lower dimensional theory. Gukov has conjectured the
explicit form of this superpotential. We check this conjecture for the
heterotic string compactified on a Calabi-Yau three-fold as well as for warped
M-theory compactifications on Spin(7) holonomy manifolds, by performing a
Kaluza-Klein reduction.Comment: 19 pages, no figure
FGB1 and WSC3 are in planta-induced beta-glucan-binding fungal lectins with different functions
In the root endophyte Serendipita indica, several lectin-like members of the expanded multigene family of WSC proteins are transcriptionally induced in planta and are potentially involved in beta-glucan remodeling at the fungal cell wall. Using biochemical and cytological approaches we show that one of these lectins, SiWSC3 with three WSC domains, is an integral fungal cell wall component that binds to long-chain beta 1-3-glucan but has no affinity for shorter beta 1-3- or beta 1-6-linked glucose oligomers. Comparative analysis with the previously identified beta-glucan-binding lectin SiFGB1 demonstrated that whereas SiWSC3 does not require beta 1-6-linked glucose for efficient binding to branched beta 1-3-glucan, SiFGB1 does. In contrast to SiFGB1, the multivalent SiWSC3 lectin can efficiently agglutinate fungal cells and is additionally induced during fungus-fungus confrontation, suggesting different functions for these two beta-glucan-binding lectins. Our results highlight the importance of the beta-glucan cell wall component in plant-fungus interactions and the potential of beta-glucan-binding lectins as specific detection tools for fungi in vivo
X-ray observations of Algol
Observations of Algol using a solid state spectrometer onboard the Einstein Observatory are discussed. Two observations six months apart were made, both during a primary optical eclipse. No corresponding X-ray eclipses were seen. During the second observation the source was flaring and was on average a factor three brighter. The spectrum on both occasions was consistent with a two-component thermal equilibrium model with temperatures of approximately 7.5 and 40 million degrees. Attempts to insert a third component indicate the temperature distribution to be bimodal. Models for the X-ray emission are discussed and it is suggested that the emission most likely originates from an active corona surrounding the K star
The X-ray emitting galaxy Cen-A
OSO-8 X-ray observations of Cen-A in 1975 and 1976 are reported. The source spectrum is well fit in both years by a power law of number index 1.62 and absorption due to 1.3 x 10 to the 23rd power at/sq cm. The total flux varied by a factor 2 between 1975 and 1976. In 1976 there were approximately 40% flux variations on a time scale of days. The 6.4 keV Fe fluorescent line and the 7.1 keV absorption edge were measured implying Fe/H approximately equals .000016. Simultaneous radio measurements show variation in phase with X-ray variability. Models considering radio, milimeter, IR and X-ray data show that all the data can be accounted for by a model in which the X-rays are due to a synchrotron self-Compton source embedded in a cold H(2) cloud
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