1,720,954 research outputs found
Simulation of charge carrier diffusion in algan epitaxial layers.
No full textDiffusion in disordered systems, such as amorphous or disordered semiconductors, does not follow classical laws which describe particle transport in ordered crystalline media, and this leads to anomalous subdiffusive transport properties. While in classical diffusion the mean square displacement of a particle is proportional to time to the first power, in subdiffusion the mean square displacement is proportional to time to some real power α < 1. Subdiffusion is described by fractional diffusion equation whose solution gives the evolution of particle concentration. Subdiffusive process can be simulated in computer assuming that particles move in energetically irregular lattice. Two one-dimensional lattice models are presented, each with different hopping mechanisms form one node to another. Statistically generated distributions of particle concentration are compared to the solutions of fractional diffusion equation. After analysing characteristics of subdiffusion in one dimension, the lattice model is upgraded to three dimensions and is used for simulating charge carrier dynamics in real materials. Particularly, in this work the lattice model is used for trying to explain the decrease of charge carrier lifetimes at lower energies in AlGaN epitaxial layers which was observed by using photoluminescence spectroscopy. We conclude that in order for a system to undergo subdiffusive process described by fractional diffusion equation, the energies of localized states (which in our models are the energies of the nodes of the lattice) have to be distributed according to the exponential law. Since both models produce same results, we also conclude that a particular mechanism of random walk has no influence to overall evolution of particle concentration. By simulating charge carrier dynamics and recombination in the upgraded three-dimensional lattice we can achieve the desired decrease of charge carrier lifetimes at lower energies with a particular set of parameters. However, the model cannot explain all experimentally observed characteristics and we conclude that there are more complicated mechanisms of charge carrier transport and recombination in AlGaN epitaxial layers
Calculation of atomic photoionization cross section considering the influence of coordination environment on photoelectron states.
No full textPhotoelectron spectroscopy of zinc metallic compounds revealed that the emission intensity ratio between Zn 3d level spin-orbit split components, known as the branching ratio, deviates from its statistical value of 3/2 and oscillates as a function of photon energy. Similar oscillatory fine structure, which is called XAFS, is measured in X-ray absorption spectroscopy where it is interpreted as arising due to alternation of the photoelectron’s state by the interference between the outgoing photoelectron wave with the one backscattered by atoms in coordination environment. In the present work we hypothesize that the same process is responsible for the observed branching ratio oscillations. In order to prove it or disprove it, a theoretical analysis of photoionization in coordination environment is needed. For that matter we choose not use existing XAFS analysis software which work in a black box manner. Instead, we develop our own simple method for calculating photoionization cross section and its fine structure due to photoelectron scattering. This approach lets us calculate the main features of the fine structure and get a better understanding of the physical processes behind it. Firstly, our analysis has shown that the branching ratio deviations from its statistical value should be present even without the coordination environment. In the presence of nearby atoms, the branching ratio should oscillate, oscillations should be proportional to the derivative of the fine structure and the energy difference between split levels. The calculated fine structure in its shape is similar to the one measured in XAFS spectroscopy. The specific discrepancies are mostly the result of the inaccurate zero-range potential model which has been used to describe the potentials of neighboring atoms. However, the branching ratio calculations in crystalline zinc do not agree with the experimental measurements, in which the deviations from statistical value are much larger. Except for some short-period small-amplitude oscillations seen in the data, we conclude that the overall shape of the observed photon energy–branching ratio dependence cannot be attributed solely to the backscattering effect. Furthermore, the long period oscillation of the branching ratio seen in ZnMgY and other quasicrystals cannot be attributed to this effect, too, because otherwise this would imply three times shorter distances between adjacent atoms than it is measured experimentally. We conclude that there are other processes involved in determining the specific branching ratio deviations. One of them could be the influence coordination environment has on electrons in atomic 3d orbitals. Interaction with nearby atoms could modify electron spin-orbit split orbitals increasing the difference between them, which is the main factor behind nonstatistical branching ratio. This modification is supported by the fact that the energy gap between the two split states was measured to be nearly two times greater than it is calculated for isolated atoms, and the same calculation for deeper levels differs from the measurements only by few percent. It is possible that the bigger difference between the bound electron states amplifies the fine structure effect caused by photoelectron scattering
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe 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
Get PDF“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
Get PDFWe 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
Get PDFWe 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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