155 research outputs found
Exchange Interactions in Paramagnetic Amorphous and Disordered Crystalline CrN-based Systems
We present a first principles supercell methodology for the calculation of
exchange interactions of magnetic materials with arbitrary degrees of
structural and chemical disorder in their high temperature paramagnetic state.
It is based on a projection of the total magnetic energy of the system onto
local pair clusters, allowing the interactions to vary independently as a
response to their local environments. We demonstrate our method by deriving the
distance dependent exchange interactions in vibrating crystalline CrN, a
TiCrN solid solution as well as in amorphous CrN. Our method
reveals strong local environment effects in all three systems. In the amorphous
case we use the full set of exchange interactions in a search for the
non-collinear magnetic ground state.Comment: 5 pages, 3 figure
An \emph{ab initio} method for locating characteristic potential energy minima of liquids
It is possible in principle to probe the many--atom potential surface using
density functional theory (DFT). This will allow us to apply DFT to the
Hamiltonian formulation of atomic motion in monatomic liquids [\textit{Phys.
Rev. E} {\bf 56}, 4179 (1997)]. For a monatomic system, analysis of the
potential surface is facilitated by the random and symmetric classification of
potential energy valleys. Since the random valleys are numerically dominant and
uniform in their macroscopic potential properties, only a few quenches are
necessary to establish these properties. Here we describe an efficient
technique for doing this. Quenches are done from easily generated "stochastic"
configurations, in which the nuclei are distributed uniformly within a
constraint limiting the closeness of approach. For metallic Na with atomic pair
potential interactions, it is shown that quenches from stochastic
configurations and quenches from equilibrium liquid Molecular Dynamics (MD)
configurations produce statistically identical distributions of the structural
potential energy. Again for metallic Na, it is shown that DFT quenches from
stochastic configurations provide the parameters which calibrate the
Hamiltonian. A statistical mechanical analysis shows how the underlying
potential properties can be extracted from the distributions found in quenches
from stochastic configurations
Disorder-induced cavities, resonances, and lasing in randomly-layered media
We study, theoretically and experimentally, disorder-induced resonances in
randomly-layered samples,and develop an algorithm for the detection and
characterization of the effective cavities that give rise to these resonances.
This algorithm enables us to find the eigen-frequencies and pinpoint the
locations of the resonant cavities that appear in individual realizations of
random samples, for arbitrary distributions of the widths and refractive
indices of the layers. Each cavity is formed in a region whose size is a few
localization lengths. Its eigen-frequency is independent of the location inside
the sample, and does not change if the total length of the sample is increased
by, for example, adding more scatterers on the sides. We show that the total
number of cavities, , and resonances, , per
unit frequency interval is uniquely determined by the size of the disordered
system and is independent of the strength of the disorder. In an active,
amplifying medium, part of the cavities may host lasing modes whose number is
less than . The ensemble of lasing cavities behaves as
distributed feedback lasers, provided that the gain of the medium exceeds the
lasing threshold, which is specific for each cavity. We present the results of
experiments carried out with single-mode optical fibers with gain and
randomly-located resonant Bragg reflectors (periodic gratings). When the fiber
was illuminated by a pumping laser with an intensity high enough to overcome
the lasing threshold, the resonances revealed themselves by peaks in the
emission spectrum. Our experimental results are in a good agreement with the
theory presented here.Comment: minor correction
Liquid state properties from first principles DFT calculations: Static properties
In order to test the Vibration-Transit (V-T) theory of liquid dynamics, ab
initio density functional theory (DFT) calculations of thermodynamic properties
of Na and Cu are performed and compared with experimental data. The
calculations are done for the crystal at T = 0 and T_m, and for the liquid at
T_m. The key theoretical quantities for crystal and liquid are the structural
potential and the dynamical matrix, both as function of volume. The theoretical
equations are presented, as well as details of the DFT computations. The
properties compared with experiment are the equilibrium volume, the isothermal
bulk modulus, the internal energy and the entropy. The agreement of theory with
experiment is uniformly good. Our primary conclusion is that the application of
DFT to V-T theory is feasible, and the resulting liquid calculations achieve
the same level of accuracy as does ab initio lattice dynamics for crystals.
Moreover, given the well established reliability of DFT, the present results
provide a significant confirmation of V-T theory itself.Comment: 9 pages, 3 figures, 5 tables, edited to more closely match published
versio
Producción y cálida de forraje de dos variedades de ballico (lolium multiflorum) en el sur de sonora
EI presente trabajo se IIevó a cabo con objeto de evaluar el efecto de diferentes fechas de siembra sobre la producción, contenido de proteína cruda y digestibilidad de la materia seca de ballico italiano y tetraploide en la zona sur del estado de Sonora
Resonance optimization of polychromatic light in disordered structures
Disorder offers rich possibilities for manipulating the phase and intensity of light and designing photonic devices for various applications including random lasers, light storage, and speckle-free imaging. Disorder-based optical systems can be implemented in one-dimensional structures based on random or pseudo-random alternating layers with different refractive indices. Such structures can be treated as sequences of scatterers, in which spatial light localization is characterized by random sets of spectral transmission resonances, each accompanied by a relatively high-intensity concentration. The control and manipulation of resonances is the key element in designing disorder-based photonic systems. In this work, we introduce a method of controlling disorder-induced resonances by using the established non-trivial interconnection between the symmetry of bi-directional light propagation properties and the features of the resonant transmissions. Considering a fiber with resonant Bragg gratings as an example, the mechanism of enhancing or suppressing the resonant transmission of polychromatic light and the effectiveness of the method have been demonstrated both theoretically and experimentally. The proposed algorithm of controlling disorder-induced resonances is general and applicable to classical waves and quantum particles, for disordered systems both with and without gain
Post-Construction Support and Sustainability in Community-Managed Rural Water Supply
Executive Summary This volume reports the main findings from a multi-country research project that was designed to develop a better understanding of how rural water supply systems are performing in developing countries. We began the research in 2004 to investigate how the provision of support to communities after the construction of a rural water supply project affected project performance in the medium term. We collected information from households, village water committees, focus groups of village residents, system operators, and key informants in 400 rural communities in Bolivia, Ghana, and Peru; in total, we discussed community water supply issues with approximately 10,000 individuals in these communities. To our surprise, we found the great majority of the village water systems were performing well. Our findings on the factors influencing their sustainability will, we hope, be of use to policy makers, investors, and managers in rural water supply
Reflexiones universitarias. San Pedro Valencia: renovación urbana, saneamiento ambiental y emprendimientos turísticos. Otoño 2015
Los trabajos que aquí se presentan se elaboraron por las y los estudiantes como parte de las actividades del Proyecto de Aplicación Profesional (PAP) “San Pedro Valencia: renovación urbana, saneamiento ambiental y emprendimientos turísticos”, durante el periodo de Otoño de 2015. A lo largo del periodo los autores compartieron sus reflexiones en torno a su percepción sobre el contexto en el que actúa el PAP; sobre las alternativas posibles a la problemática detectada y lo que significa pensar una alternativa; sobre los sujetos con los que se ha interactuado a lo largo de la experiencia de trabajo y sobre el papel del profesionista y del ciudadano en un mundo como el que nos tocó vivir. La obra está compuesta por reflexiones personales de las y los estudiantes que, explorando estas temáticas, comparten sus aprendizajes y observaciones de forma vívida.ITESO, A.C
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