124,109 research outputs found
A Chandra ACIS view of the Thermal Composite Supernova Remnant 3C391
We present a 60 ks Chandra ACIS-S observation of the thermal composite
supernova remnant 3C391. The southeast-northwest elongated morphology is
similar to that previously found in radio and X-ray studies. This observation
unveils a highly clumpy structure of the remnant. Detailed spatially resolved
spectral analysis for the small-scale features reveals that the interior gas is
generally of normal metal abundance and has approached or basically reached
ionization equilibrium. The hydrogen column density increases from southeast to
northwest. Three mechanisms, radiative rim, thermal conduction, and cloudlet
evaporation, may all play roles in the X-ray appearance of 3C391 as a "thermal
composite" remnant, but there are difficulties with each of them in explaining
some physical properties. Comparatively, the cloudlet evaporation model is
favored by the main characteristics such as the highly clumpy structure and the
uniform temperature and density distribution over most of the remnant. The
directly measured postshock temperature also implies a young age, about 4 kyr,
for the remnant. The postshock gas pressure derived from the NE and SW rims,
which harbor maser spots, is consistent with the estimate for the maser
regions. An unresolved X-ray source is observed on the northwest border and its
spectrum is best fitted by a power-law.Comment: aastex, 27 pages (including 4 figures), to appear in the ApJ 1 Dec.
2004, v616 issu
Dynamical trapping and relaxation of scalar gravitational fields
We present a framework for nonlinearly coupled scalar-tensor theory of
gravity to address both inflation and core-collapse supernova problems. The
unified approach is based on a novel dynamical trapping and relaxation of
scalar gravity in highly energetic regimes. The new model provides a viable
alternative mechanism of inflation free from various issues known to affect
previous proposals. Furthermore, it could be related to observable violent
astronomical events, specifically by releasing a significant amount of
additional gravitational energy during core-collapse supernovae. A recent
experiment at CERN relevant for testing this new model is briefly outlined.Comment: 4 pages; version to appear in PL
n-CdSe/p-ZnTe based wide band-gap light emitters: Numerical simulation and design
The only II‐VI/II‐VI wide band‐gap heterojunction to provide both good lattice match and p‐ and n‐type dopability is CdSe/ZnTe. We have carried out numerical simulations of several light emitter designs incorporating CdSe, ZnTe, and Mg alloys. In the simulations, Poisson’s equation is solved in conjunction with the hole and electron current and continuity equations. Radiative and nonradiative recombination in bulk material and at interfaces are included in the model. Simulation results show that an n‐CdSe/p‐ZnTe heterostructure is unfavorable for efficient wide band‐gap light emission due to recombination in the CdSe and at the CdSe/ZnTe interface. An n‐CdSe/Mg_(x)Cd_(1−x)Se/p‐ZnTe heterostructure significantly reduces interfacial recombination and facilitates electron injection into the p‐ZnTe layer. The addition of a Mg_(y)Zn_(1−y)Te electron confining layer further improves the efficiency of light emission. Finally, an n‐CdSe/Mg_(x)Cd_(1−x)Se/Mg_(y)Zn_(1−y)Te/p‐ZnTe design allows tunability of the wavelength of light emission from green into the blue wavelength regime
A Feature-Based Comparison of Evolutionary Computing Techniques for Constrained Continuous Optimisation
Evolutionary algorithms have been frequently applied to constrained
continuous optimisation problems. We carry out feature based comparisons of
different types of evolutionary algorithms such as evolution strategies,
differential evolution and particle swarm optimisation for constrained
continuous optimisation. In our study, we examine how sets of constraints
influence the difficulty of obtaining close to optimal solutions. Using a
multi-objective approach, we evolve constrained continuous problems having a
set of linear and/or quadratic constraints where the different evolutionary
approaches show a significant difference in performance. Afterwards, we discuss
the features of the constraints that exhibit a difference in performance of the
different evolutionary approaches under consideration.Comment: 16 Pagesm 2 Figure
Models for the magnetic ac susceptibility of granular superferromagnetic CoFe/AlO
The magnetization and magnetic ac susceptibility, ,
of superferromagnetic systems are studied by numerical simulations. The
Cole-Cole plot, vs. , is used as a tool for classifying
magnetic systems by their dynamical behavior. The simulations of the
magnetization hysteresis and the ac susceptibility are performed with two
approaches for a driven domain wall in random media. The studies are motivated
by recent experimental results on the interacting nanoparticle system
CoFe/AlO showing superferromagnetic behavior. Its
Cole-Cole plot indicates domain wall motion dynamics similarly to a disordered
ferromagnet, including pinning and sliding motion. With our models we can
successfully reproduce the features found in the experimental Cole-Cole plots.Comment: 8 pages, 6 figure
X-ray photoelectron spectroscopy measurement of valence-band offsets for Mg-based semiconductor compounds
We have used x-ray photoelectron spectroscopy to measure the valence-band offsets for the lattice matched MgSe/Cd0.54Zn0.46Se and MgTe/Cd0.88Zn0.12Te heterojunctions grown by molecular beam epitaxy. By measuring core level to valence-band maxima and core level to core level binding energy separations, we obtain values of 0.56+/-0.07 eV and 0.43+/-0.11 eV for the valence-band offsets of MgSe/Cd0.54Zn0.46Se and MgTe/Cd0.88Zn0.12Te, respectively. Both of these values deviate from the common anion rule, as may be expected given the unoccupied cation d orbitals in Mg. Application of our results to the design of current II-VI wide band-gap light emitters is discussed
Two Approaches to Dislocation Nucleation in the Supported Heteroepitaxial Equilibrium Islanding Phenomenon
We study the dislocation formation in 2D nanoscopic islands with two methods,
the Molecular Static method and the Phase Field Crystal method. It is found
that both methods indicate the same qualitative stages of the nucleation
process. The dislocations nucleate at the film-substrate contact point and the
energy decreases monotonously when the dislocations are farther away from the
island-wetting film contact points than the distance of the highest energy
barrier.Comment: 4 page
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