4,651 research outputs found
Compartir l'esforç per millorar els serveis d'aigua i sanejament
La gestión comunitaria de los sistemas de suministro de agua será el enfoque primordial para conseguir los principales Objetivos de Desarrollo del Milenio en las zonas rurales. Discutimos tres aspectos específicos: el concepto de gestión comunitaria, los distintos papeles que pueden jugar los principales actores implicados en el desarrollo y la implementación de los servicios de agua y saneamiento; y la crucial necesidad de proveer capacitación utilizando técnicas participativas, de las cuales se describen algunas, apuntando a una toma de decisiones informada. Señalamos también que capacitar y estimular a los miembros de la comunidad es importante, pero aun lo es más, capacitar a los actores del nivel intermedio (gobierno local, ONG, sector privado, etc.) que son quienes respaldan y facilitan la implementación del proceso inicialmente, y luego el apoyo para asegurar la sostenibilidad del servicio proporcionado.La gestió comunitària dels sistemes de subministrament d'aigua serà l'enfocament primordial per a aconseguir els principals Objectius de Desenvolupament del Mil·lenni a les zones rurals.
Discutim tres aspectes específics: el concepte de gestió comunitària, els distints papers que poden jugar els principals actors implicats en el desenvolupament i la implementació dels serveis d'aigua i sanejament; i la crucial necessitat de proveir capacitació utilitzant tècniques participatives, de les quals es descriuen algunes, apuntant a una presa de decisions informada.
Assenyalem també que capacitar i estimular els membres de la comunitat és important, però fins i tot ho és més, capacitar als actors del nivell intermedi (govern local, ONG, sector privat, etc.) que són els qui donen suport i faciliten la implementació del procés inicialment, i després el suport per a assegurar la sostenibilitat del servei proporcionat.Peer Reviewe
Relativistic general-order coupled-cluster method for high-precision calculations: Application to Al+ atomic clock
We report the implementation of a general-order relativistic coupled-cluster
method for performing high-precision calculations of atomic and molecular
properties. As a first application, the static dipole polarizabilities of the
ground and first excited states of Al+ have been determined to precisely
estimate the uncertainty associated with the BBR shift of its clock frequency
measurement. The obtained relative BBR shift is -3.66+-0.44 for the 3s^2
^1S_0^0 --> 3s3p ^3P_0^0 transition in Al+ in contrast to the value obtained in
the latest clock frequency measurement, -9+-3 [Phys. Rev. Lett. 104, 070802
(2010)]. The method developed in the present work can be employed to study a
variety of subtle effects such as fundamental symmetry violations in atoms.Comment: 4 pages, 3 tables, submitte
Is Random Close Packing of Spheres Well Defined?
Despite its long history, there are many fundamental issues concerning random
packings of spheres that remain elusive, including a precise definition of
random close packing (RCP). We argue that the current picture of RCP cannot be
made mathematically precise and support this conclusion via a molecular
dynamics study of hard spheres using the Lubachevsky-Stillinger compression
algorithm. We suggest that this impasse can be broken by introducing the new
concept of a maximally random jammed state, which can be made precise.Comment: 6 pages total, 2 figure
Surface effects on nanowire transport: numerical investigation using the Boltzmann equation
A direct numerical solution of the steady-state Boltzmann equation in a
cylindrical geometry is reported. Finite-size effects are investigated in large
semiconducting nanowires using the relaxation-time approximation. A nanowire is
modelled as a combination of an interior with local transport parameters
identical to those in the bulk, and a finite surface region across whose width
the carrier density decays radially to zero. The roughness of the surface is
incorporated by using lower relaxation-times there than in the interior.
An argument supported by our numerical results challenges a commonly used
zero-width parametrization of the surface layer. In the non-degenerate limit,
appropriate for moderately doped semiconductors, a finite surface width model
does produce a positive longitudinal magneto-conductance, in agreement with
existing theory. However, the effect is seen to be quite small (a few per cent)
for realistic values of the wire parameters even at the highest practical
magnetic fields. Physical insights emerging from the results are discussed.Comment: 15 pages, 7 figure
Current measurement by real-time counting of single electrons
The fact that electrical current is carried by individual charges has been
known for over 100 years, yet this discreteness has not been directly observed
so far. Almost all current measurements involve measuring the voltage drop
across a resistor, using Ohm's law, in which the discrete nature of charge does
not come into play. However, by sending a direct current through a
microelectronic circuit with a chain of islands connected by small tunnel
junctions, the individual electrons can be observed one by one. The quantum
mechanical tunnelling of single charges in this one-dimensional array is time
correlated, and consequently the detected signal has the average frequency
f=I/e, where I is the current and e is the electron charge. Here we report a
direct observation of these time-correlated single-electron tunnelling
oscillations, and show electron counting in the range 5 fA-1 pA. This
represents a fundamentally new way to measure extremely small currents, without
offset or drift. Moreover, our current measurement, which is based on electron
counting, is self-calibrated, as the measured frequency is related to the
current only by a natural constant.Comment: 9 pages, 4 figures; v2: minor revisions, 2 refs added, words added to
title, typos correcte
Active eukaryotes in microbialites from Highborne Cay, Bahamas, and Hamelin Pool (Shark Bay), Australia
Author Posting. © The Author(s), 2013. This is the author's version of the work. It is posted here by permission of Nature Publishing Group for personal use, not for redistribution. The definitive version was published in The ISME Journal 8 (2014): 418–429, doi:10.1038/ismej.2013.130.Microbialites are organosedimentary structures that are formed through the interaction of
benthic microbial communities and sediments and include mineral precipitation. These
lithifying microbial mat structures include stromatolites and thrombolites. Exuma Sound
in the Bahamas, and Hamelin Pool in Shark Bay, Western Australia are two locations
where significant stands of modern microbialites exist. Although prokaryotic diversity in
these structures is reasonably well documented, little is known about the eukaryotic
component of these communities and their potential to influence sedimentary fabrics
through grazing, binding and burrowing activities. Accordingly, comparisons of
eukaryotic communities in modern stromatolitic and thrombolytic mats can potentially
provide insight into the coexistence of both laminated and clotted mat structures in close
proximity to one another. Here we examine this possibility by comparing eukaryotic
diversity based on Sanger and high-throughput pyrosequencing of small subunit
ribosomal RNA (18S rRNA) genes. Analyses were based on total RNA extracts as
template to minimize input from inactive or deceased organisms. Results identified
diverse eukaryotic communities particularly stramenopiles, Alveolata, Metazoa,
Amoebozoa, and Rhizaria within different mat types at both locations, as well as
abundant and diverse signatures of eukaryotes with <80% sequence similarity to
sequences in GenBank. This suggests presence of significant novel eukaryotic diversity,
particularly in hypersaline Hamelin Pool. There was evidence of vertical structuring of
protist populations and foraminiferal diversity was highest in bioturbated/clotted
thrombolite mats of Highborne Cay.This work was funded by grant
OCE-0926421 to JMB and VPE and OCE-0926372 to RES
Dynamical Screening and Superconducting State in Intercalated Layered Metallochloronitrides
An essential property of layered systems is the dynamical nature of the
screened Coulomb interaction. Low energy collective modes appear as a
consequence of the layering and provide for a superconducting-pairing channel
in addition to the electron-phonon induced attractive interaction. We show that
taking into account this feature allows to explain the high critical
temperatures (Tc~26K) observed in recently discovered intercalated
metallochloronitrides. The exchange of acoustic plasmons between carriers leads
to a significant enhancement of the superconducting critical temperature that
is in agreement with the experimental observations
Switching Distributions for Perpendicular Spin-Torque Devices within the Macrospin Approximation
We model "soft" error rates for writing (WSER) and for reading (RSER) for
perpendicular spin-torque memory devices by solving the Fokker-Planck equation
for the probability distribution of the angle that the free layer magnetization
makes with the normal to the plane of the film. We obtain: (1) an exact, closed
form, analytical expression for the zero-temperature switching time as a
function of initial angle; (2) an approximate analytical expression for the
exponential decay of the WSER as a function of the time the current is applied;
(3) comparison of the approximate analytical expression for the WSER to
numerical solutions of the Fokker-Planck equation; (4) an approximate
analytical expression for the linear increase in RSER with current applied for
reading; (5) comparison of the approximate analytical formula for the RSER to
the numerical solution of the Fokker-Planck equation; and (6) confirmation of
the accuracy of the Fokker-Planck solutions by comparison with results of
direct simulation using the single-macrospin Landau-Lifshitz-Gilbert (LLG)
equations with a random fluctuating field in the short-time regime for which
the latter is practical
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