1,721,414 research outputs found
GEOMETRY AND CHARGE-DISTRIBUTION OF H-CENTERS IN THE FLUORITE STRUCTURE
No full textThe analysis of experimental optical and spin-resonance data for the H centre gives a consistent picture of the local geometry and one-electron wavefunctions. One of the two ions in the F2- molecular ion remains very close to the perfect lattice site the other is at a distance close to that found in other F2- centres. This analysis is confirmed by atomistic calculations using the HADES code. The results are used to give a preliminary analysis of the self-trapped exciton data
THE CALCULATION OF HUGONIOTS IN IONIC SOLIDS
No full textThe authors demonstrate the quantitative prediction of Hugoniots for an ionic crystal (NaCl) using the shell model with the best available potentials. The calculations show that these shock wave data can be predicted quite accurately by relatively simple use of established computer codes. The Hugoniot results, together with static phase-change data, also provide a very severe test of interatomic potentials. The two sets of empirical potentials ((i) Catlow, Diller and Norgett (1977), (ii) Sangster and Atwood (1978)) both perform much better than electron gas potentials. The same approach can be used to test potentials and to make predictions for the behaviour of solids for extreme conditions of temperature and pressure
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
THEORY OF THE STRUCTURE OF THE SELF-TRAPPED EXCITON IN QUARTZ
No full textQuartz is an insulator with an extremely wide band gap in the vacuum ultra-violet. However, under irradiation from high-energy electrons or X-rays, samples of high purity emit a luminescence band in the blue, corresponding to a Stokes shift of approximately 7 eV. This large Stokes shift has been ascribed to the self-trapping of an exciton in an otherwise perfect lattice owing to the distortion it induces; the authors review the evidence for this assignment, and describe electronic-structure calculations which reveal the structure of the distorted configuration and also explain various experimentally determined properties of the centre. The self-trapping process they postulate is a novel one as it is driven primarily by the electron component of the exciton
DEFECT ELECTRONIC STATES IN BETA-CAROTENE AND LOWER HOMOLOGS
No full textWe present semi-empirical calculations of the atomic geometries and electronic charge distributions of beta-carotene homologues of different chain lengths. We find defects in charged and photoexcited chains that are similar to the defects found in the degenerate polymer trans-polyacetylene, and we show how confinement affects these defects as the chains we shortened. Our results exhibit a generalized form of charge-conjugation symmetry in which the properties of a negatively charged defect are related to those of a positive one and vice versa
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
Modeling hydrocarbon oxidation mechanisms catalyzed by microporous materials
No full textIn this chapter we discuss the application of accurate electronic-structure computational methods based on hybrid-exchange density functional theory (DFT) to unravel the atomic-level details whereby a complex oxidation reaction of industrial relevance takes place. Detailed knowledge of the reaction mechanism represents crucial information in order to improve the catalytic properties, but is difficult to obtain from experimental techniques alone, which is especially true if complex mechanisms and short-lived intermediate species are involved, as is the case of radical-based autoxidations. The advances in quantum mechanical methods, especially the development of DFT, together with the huge increase in computational power experienced in the last decades, enable us to apply reliable computational methods to understand even very complex problems such as the radical-based aerobic oxidation of hydrocarbons catalyzed by transition-metal doped aluminophosphate frameworks discussed here
- …
