6,565 research outputs found
Meeting the millennium development goal in education : a cost-effectiveness analysis for Ecuador
Ecuador;education;development strategy;input output analysis;primary education;secondary education
Towards predictive many-body calculations of phonon-limited carrier mobilities in semiconductors
We probe the accuracy limit of {\it ab initio} calculations of carrier
mobilities in semiconductors, within the framework of the Boltzmann transport
equation. By focusing on the paradigmatic case of silicon, we show that fully
predictive calculations of electron and hole mobilities require many-body
quasiparticle corrections to band structures and electron-phonon matrix
elements, the inclusion of spin-orbit coupling, and an extremely fine sampling
of inelastic scattering processes in momentum space. By considering all these
factors we obtain excellent agreement with experiment, and we identify the band
effective masses as the most critical parameters to achieve predictive
accuracy. Our findings set a blueprint for future calculations of carrier
mobilities, and pave the way to engineering transport properties in
semiconductors by design.Comment: 11 pages and 8 figure
A framework for closed-loop supply chains of reusable articles
Reuse practices contribute to the environmental and economical sustainability of production and distribution systems. Surprisingly, reuse closed-loop supply chains (CLSC) have not been widely researched for the moment. In this paper, we explore the scientific literature on reuse and we propose a framework for reusable articles. This conceptual structure includes a typology integrating under the reusable articles term different categories of articles (transportation items, packaging materials, tools) and addresses the management issues that arise in reuse CLSC. We ground our results in a set of case studies developed in real industrial settings, which have also been contrasted with cases available in existing literature.reverse logistics;case studies;closed-loop supply chains;returns managment
Stellar models with Schwarzschild and non-Schwarzschild vacuum exteriors
A striking characteristic of non-Schwarzschild vacuum exteriors is that they
contain not only the total gravitational mass of the source, but also an {\it
arbitrary} constant. In this work, we show that the constants appearing in the
"temporal Schwarzschild", "spatial Schwarzschild" and
"Reissner-Nordstr{\"o}m-like" exteriors are not arbitrary but are completely
determined by star's parameters, like the equation of state and the
gravitational potential. Consequently, in the braneworld scenario the
gravitational field outside of a star is no longer determined by the total mass
alone, but also depends on the details of the internal structure of the source.
We show that the general relativistic upper bound on the gravitational
potential , for perfect fluid stars, is significantly increased in
these exteriors. Namely, , and for the
temporal Schwarzschild, spatial Schwarzschild and Reissner-Nordstr{\"o}m-like
exteriors, respectively. Regarding the surface gravitational redshift, we find
that the general relativistic Schwarzschild exterior as well as the braneworld
spatial Schwarzschild exterior lead to the same upper bound, viz., .
However, when the external spacetime is the temporal Schwarzschild metric or
the Reissner-Nordstr{\"o}m-like exterior there is no such constraint: . This infinite difference in the limiting value of is because for
these exteriors the effective pressure at the surface is negative. The results
of our work are potentially observable and can be used to test the theory.Comment: 19 pages, 3 figures and caption
Consistent discretizations: the Gowdy spacetimes
We apply the consistent discretization scheme to general relativity
particularized to the Gowdy space-times. This is the first time the framework
has been applied in detail in a non-linear generally-covariant gravitational
situation with local degrees of freedom. We show that the scheme can be
correctly used to numerically evolve the space-times. We show that the
resulting numerical schemes are convergent and preserve approximately the
constraints as expected.Comment: 10 pages, 8 figure
MHD Remote Numerical Simulations: Evolution of Coronal Mass Ejections
Coronal mass ejections (CMEs) are solar eruptions into interplanetary space
of as much as a few billion tons of plasma, with embedded magnetic fields from
the Sun's corona. These perturbations play a very important role in
solar--terrestrial relations, in particular in the spaceweather. In this work
we present some preliminary results of the software development at the
Universidad Nacional Autonoma de Mexico to perform Remote MHD Numerical
Simulations. This is done to study the evolution of the CMEs in the
interplanetary medium through a Web-based interface and the results are store
into a database. The new astrophysical computational tool is called the Mexican
Virtual Solar Observatory (MVSO) and is aimed to create theoretical models that
may be helpful in the interpretation of observational solar data.Comment: 2 pages, 1 color figure. To appear in Proceedings IAU Symposium No.
259. Cosmic Magnetic Fields: From Planets, to Stars and Galaxies. In pres
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