1,720,963 research outputs found
Estudio de las fluctuaciones de espín en hidruro de paladio superconductor
Full text link67 páginas. Maestría en Ciencias e Ingeniería de Materiales.Las fluctuaciones de espín o paramagnones, que deben su origen a electrones itinerantes que pasan de la banda de valencia a la de conducción, se han propuesto como uno de los mecanismos responsables de la superconductividad, ya sea para favorecerla o destruirla. Para dilucidar el papel que juegan estas fluctuaciones de espín en la termodinámica superconductora de los materiales, se estudia en este proyecto, el paladio (Pd) y el hidruro de paladio (PdH), en el marco de la Teoría del Funcional de la Densidad (DFT), implementada en los códigos WIEN2k y ELK, con la Aproximación del Gradiente Generalizado para la energía de intercambio correlación. Los cálculos realizados son completamente relativistas. Para la constante de acoplamiento electrón-fonón, electrón-paramagnón y temperatura crítica, se consideró la ecuación de Allen-Dynes modificada para fluctuaciones de espín por Mazin, dentro de la Teoría de Eliashberg. Encontramos que el hidrógeno modifica considerablemente las propiedades electrónicas, magnéticas y dinámicas de la red del Pd. Podemos concluir que el acoplamiento entre electrones y fonones es muy débil en el Pd, por lo cual, no presenta propiedades superconductoras en condiciones normales, en acuerdo con resultados experimentales. La constante de acoplamiento electrón-fonón del PdH (λef =2.12) es notablemente mayor con respecto al Pd (λef =0.25). El valor de la Tc encontrada para el PdH es 14.2 K, considerando únicamente la interacción electrón-fonón, incluyendo las fluctuaciones de espín, la Tc disminuye a un valor de 11 K, valor muy cercano al reportado en la literatura (9 K).Consejo Nacional de Ciencia y Tecnología (México)
Propiedades electrónicas y superconductoras del V y Nb bajo presión hidrostática
Full text link59 páginas. Maestría en Ciencias e Ingeniería de Materiales.En este trabajo estudiamos el papel que juegan los fonones en el efecto superconductor. Se investigaron las propiedades estructurales, electrónicas, de fonones y propiedades superconductoras para los metales de transición vanadio (V) y niobio (Nb) bajo presión hidrostática hasta 200 GPa. Analizamos las posibles transiciones fase estructural del V y Nb, asumiendo cinco estructuras cristalinas; la estructura cúbica centrada en el cuerpo (BCC), cúbica centrada en las caras (FCC), cúbica simple (SC), hexagonal compacta (HCP) y la estructura A15, para presiones hasta 200 GPa. Los cálculos se realizaron mediante el método de ondas planas aumentadas linealizadas con potencial completo (FLAPW), implementada en el código WIEN2k y ELK. Usamos la aproximación de gradiente generalizado (GGA) para la energía de intercambio-correlación. Los resultados de energía total fueron ajustados a la ecuación de estado de tercer orden de Birch-Murnaghan, mientras que para las propiedades superconductoras se utilizaron las ecuaciones de Eliashberg y para la Tc la ecuación de McMillan-Allen-Dynes. Los resultados indican que no ocurre ninguna transición de fase estructural, entre las estructuras propuestas y que en ambos casos la estructura de mínima energía corresponde a la estructura BCC, con un parámetro de red de α V = 3:00 _A para el V y aNb = 3:31 _A para el Nb. Encontramos que los fonones L, T1 y T2 en el V y Nb, aumentaron en energía con respecto a la presión, sin embargo, en el V la cantidad de fonones T1;2 aumento proporcionalmente con la presión, mientras, en el Nb no hubo cambios significativos. Adicionalmente, el V alcanzó una Tc = 17.58 K en 200 GPa, mientras, en el Nb la Tc se mantuvo disminuyendo en el rango de pequeños valores, con respecto a la presión
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
koamabayili/VECTRON-author-checklist: VECTRON author checklist
No full textWe have done our best to complete the author checklist relating to the use of animals in the hut study. Note that the objective for the hut study was to evaluate the IRS treatment applications for residual efficacy against Anopheles mosquitoes, including the local An. coluzzii mosquito population. Cows were only used to attract mosquitoes into the huts and no tests were carried out directly on the cows. The author checklist is intended for use with studies where experiments are carried out on animals, which is why we have had such difficulty in completing this for the hut study, as many of the questions do not relate to how the cows were used
Author-wise bibliometric analysis based on entropy.
No full textAuthor-wise bibliometric analysis based on entropy.</p
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