1,721,183 research outputs found
Are Hydrogen-Bonded Charge Transfer Crystals Room Temperature Ferroelectrics?
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Theoretical investigation of the electronic, vibrational and transport properties of layered transition metal chalcogenides and their stacked heterostructures
No full textChalcogenides exhibit a wide range of physical properties making them very at-
tractive for future electronic and thermoelectric applications. Their layered nature
allows them to be easily isolated in layers just few atoms thick. The new exciting
properties arising when reducing the dimensionality of materials have motivated
the scientific community to investigate these materials. In this thesis, we theoret-
ically investigate the structural, electronic and vibrational properties of two fam-
ilies of chalcogenides : mono-chalcogenides and Transition Metal Dichalcogenides
(TMDs). This theoretical investigation was conducted with the powerful predictive
capacities of Density Functional Theory and the Temperature Dependent Effec-
tive Potential method. We investigate the excellent thermoelectric properties of
SnSe, the nature of the phase transition occurring with temperature and highlight
the link between the two. We study the structural transformation in few-layer
SnSe and SnS, and its effect on vibrational properties. The finite temperature
behavior of the vibrational properties of a selection of TMDs in their bulk form
is presented, and their thermal conductivity is compared. We study the effects of
heterostructuring by alternatively stacking different TMDs. We show a contrac-
tion of the Van der Waals gap when TMDs with different chalcogens are combined.
We also explain the suppression of the charge density wave instability in certain
heterostructures. Finally we investigate the transfer of charge and the electronic
properties of heterostructures of mono and di-chalcogenides called ferecrystals
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
Lattice thermal conductivity and anharmonic phonon properties in the mode coupling approach
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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
Thermal properties of phonons and electrons in materials from ab initio
No full textThe properties of thermal transport in solids are very important in research and industry. Different kinds of materials own different thermal transport process. For instance, the free electrons carry the most part of thermal transport in metals, meanwhile, the phonon will dominate the thermal transport process in most of semiconductors and insulators. Although there were lots of theoretical and experimental works on the topic of thermal properties of electrons and phonon in condensed matter materials, some problems and mysteries are left, especially, the evaluation of electronic thermal conductivity in metals and the high-order anharmonicity of phonon are still open and challenging questions. In the thesis, the conventional theory of electronic thermal conductivity and the phonon theory are briefly reviewed. Based on the first principle calculations, firstly, we provided a new methodology for determining the electronic thermal conductivity of metals via non-equilibrium ab initio molecular dynamics. We present a new physical picture to describe the thermal transport process by electrons in metals based on the traditional free Fermi gas model theory. With this method, five representative pure metals at room conditions were studied and the thermal conductivity of iron at earth’s core conditions was also estimated. Secondly, we provided a new metric for indicating the strong fourth-order phonon anharmonicity in solids. We applied this method to several of materials, i.e. UO2, CeO2, HgTe, Si, Ge, and some carbon allotropes. Thirdly, in the study of the carbon allotropes, we also explored other related properties of the atomic bonds of these allotropes with the density functional theory, i.e., the atomic orbital hybridization, effective spring constants of atomic bonds, etc. Finally, in this thesis, we also studied the phonon properties of the organic-inorganic perovskite solar cells which is treated as unprecedented photovoltaic technology materials
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
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