1,721,016 research outputs found
Angular dependent time delay near correlation induced Cooper minima
Full text linkWe analyze an angular dependence of the Wigner time delay near the Cooper minimum (CM) of the sub-valent ns shell in argon, krypton and xenon. Such an angular dependence is a result of interplay between the relativistic and correlation effects. The correlation with the outermost np valence shell induces a CM in the sub-valent ns shell which is otherwise CM free. A phase difference between the two spin-orbit split ionization continua Ep 1/2 and Ep 3/2 makes the Wigner time delay angular dependent. Both these effects are accounted for within relativistic formulations of the random phase approximation and the time-dependent density functional theory. Comparison between these two approaches illustrates a very strong sensitivity of the observed effect to the computation detail, especially the account of the ground state correlation
Photoabsorption and photoionization dynamics study of silicon tetrafluoride in the framework of time-dependent density-functional theory
No full textPhotoionization cross sections and angular distributions of silicon tetrafluoride have been calculated in the
framework of the time-dependent density-functional theory approach. Both valence and inner shell ionizations
have been considered in an extended photon energy range. Calculations have been carried out by using two
different exchange-correlation xc potentials characterized by the correct asymptotic behavior. Theoretical
results obtained with both the van Leeuwen–Baerends and statistical average of orbital potentials SAOP xc
potentials are compared with photoabsorption, photoionization, and electron-scattering experiments as well as
with previous theoretical calculations. It is suggested that even if both xc potentials provide a reasonably good
description of the photoionization dynamics, correlation effects are phenomenologically better accounted for
by the SAOP xc potential. Overall, the good accuracy attained with the linear combination of atomic orbitalstime
dependent density-functional theory method in reproducing the experimental findings for SiF4 makes it a
promising and powerful method for the characterization of the photoionization dynamics from medium and
large-size molecules
Tribology at the atomic scale with density functional theory
Full text linkUnderstanding the quantum mechanical origins of friction forces has become increasingly important in the past decades with the advent of nanotechnology. At the nanometer scale, the universal Amontons-Coulomb laws cease to be valid and each interface requires individual scrutiny. Furthermore, measurements required to understand friction at the atomic scale are riddled with artificial factors such as the properties of the friction force microscope, effect of the environment, and the type of the substrate. It therefore proves difficult to isolate the actual behavior of interfaces from these effects. Electronic structure methods are an indispensable tool in understanding the details of interfaces, their interactions with lubricants, the environment and the support. In particular, density functional theory (DFT) has given large contributions to the field through accurate calculations of important properties such as the potential energy surfaces, shear strengths, adsorption of lubricant materials and the effect of the substrate. Although unable to tackle velocity- or temperature-dependent properties for which classical molecular dynamics is employed, DFT provides an affordable yet accurate means of understanding the quantum mechanical origins of the tribological behavior of interfaces in a parameter-free manner. This review attempts to give an overview of the ever-increasing literature on the use of DFT in the field of tribology. We start by summarizing the rich history of theoretical work on dry friction. We then identify the figures-of-merit which can be calculated using DFT. We follow by a summary of bulk interfaces and how to reduce friction via passivation and lubricants. The following section, namely friction involving two-dimensional materials is the focus of our review since these materials have gained increasing traction in the field thanks to the advanced manufacturing and manipulation techniques developed. Our review concludes with a brief touch on other interesting examples from DFT tribology literature such as rolling friction and the effect of photoexcitation in tribology
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
Combined effect of point defects and layer number on the adsorption of benzene and toluene on graphene
No full textUnderstanding the adsorption properties of organic molecules on graphene-based substrates is important for such applications as air and water filters. Pristine graphene is often the model substrate used in the theoretical investigations of this problem. While useful, pristine single-layer graphene is however an idealized model. In this work, we assess the effect of the presence of point defects (single vacancy, divacancy, and the Stone-Wales defect) in single-layer and bilayer graphene on the energetics of adsorption of benzene and toluene. Our calculations benchmark three different dispersion-corrected DFT schemes, namely PBE-D2, vdW-DF1, and vdW-DF2-C09. Whereas the presence of the single vacancy and the double vacancy does not appear to alter the adsorption energies of the aromatic molecules by an appreciable amount, the Stone-Wales defect and the addition of a second graphene layer stabilizes their interaction with the substrate by several tens of meV
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
Continuum Electronic States: The Tiresia Code
Full text linkA multicenter (LCAO) B-spline basis is described in detail, and its capabilities concerning affording convergent solutions for electronic continuum states and wavepacket propagation are presented. It forms the core of the Tiresia code, which implements static-DFT and TDDFT hamiltonians, as well as single channel Dyson-DFT and Dyson-TDDFT descriptions to include correlation in the bound states. Together they afford accurate and computationally efficient descriptions of photoionization properties of complex systems, both in the single photon and strong field environments. A number of examples are provided
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
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