7,459 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
Brane classical and quantum cosmology from an effective action
Motivated by the Randall-Sundrum brane-world scenario, we discuss the
classical and quantum dynamics of a (d+1)-dimensional boundary wall between a
pair of (d+2)-dimensional topological Schwarzschild-AdS black holes. We assume
there are quite general -- but not completely arbitrary -- matter fields living
on the boundary ``brane universe'' and its geometry is that of an
Friedmann-Lemaitre-Robertson-Walker (FLRW) model. The effective action
governing the model in the mini-superspace approximation is derived. We find
that the presence of black hole horizons in the bulk gives rise to a complex
action for certain classically allowed brane configurations, but that the
imaginary contribution plays no role in the equations of motion. Classical and
instanton brane trajectories are examined in general and for special cases, and
we find a subset of configuration space that is not allowed at the classical or
semi-classical level; these correspond to spacelike branes carrying tachyonic
matter. The Hamiltonization and Dirac quantization of the model is then
performed for the general case; the latter involves the manipulation of the
Hamiltonian constraint before it is transformed into an operator that
annihilates physical state vectors. The ensuing covariant Wheeler-DeWitt
equation is examined at the semi-classical level, and we consider the possible
localization of the brane universe's wavefunction away from the cosmological
singularity. This is easier to achieve for branes with low density and/or
spherical spatial sections.Comment: Shortened to match version accepted by Phys. Rev. D (unabridged text
found in version 2), 42 pages, 9 figures, Rextex
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
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