1,721,009 research outputs found
A symmetrized-basis approach to excitons in carbon nanotubes
No full textWe calculate from first-principles the optical spectrum of a (4,2) single-wall carbon nanotube including quasi-particle corrections and excitonic effects. We expand every quantity on a special basis sets which completely exploits the symmetries of the nanotube, allowing calculations for tubes with a long unit cell. The results indicate that the excitonic effects are crucial and a strong peak in the absorption spectrum is predicted at 2.2 eV This value is compared with experimental results, with excellent agreement
MAURO CASADEI TURRONI MONTI, CESARINO RUINI (a cura di), Musica ed esperienza religiosa, Milano, Franco Angeli, 2017
No full textReview of Mauro Casadei Turroni Monti, Cesarino Ruini (a cura di), Musica ed esperienza religiosa, Milano, Franco Angeli, 2017, pp. 148
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
Publisher's Note: Optical properties of graphene nanoribbons: The role of many-body effects (Physical Review B - Condensed Matter and Materials Physics (2008) 77, (041404))
No full textGoing Beyond the GW Approximation Using the Time-Dependent Hartree-Fock Vertex
No full textThe time-dependent Hartree-Fock (TDHF) vertex of many-body perturbation theory (MBPT) makes it possible to extend TDHF theory to charged excitations. Here we assess its performance by applying it to spherical atoms in their neutral electronic configuration. On a theoretical level, we recast the TDHF vertex as a reducible vertex, highlighting the emergence of a self-energy expansion purely in orders of the bare Coulomb interaction; then, on a numerical level, we present results for polarizabilities, ionization energies (IEs), and photoemission satellites. We confirm the superiority of THDF over simpler methods such as the random phase approximation for the prediction of atomic polarizabilities. We then find that the TDHF vertex reliably provides better IEs than GW and low-order self-energies do in the light-atom, few-electron regime; its performance degrades in heavier, many-electron atoms instead, where an expansion in orders of an unscreened Coulomb interaction becomes less justified. New relevant features are introduced in the satellite spectrum by the TDHF vertex, but the experimental spectra are not fully reproduced due to a missing account of nonlinear effects connected to hole relaxation. We also explore various truncations of the self-energy given by the TDHF vertex, but do not find them to be more convenient than low-order approximations such as GW and second Born (2B), suggesting that vertex corrections should be carried out consistently both in the self-energy and in the polarizability
Theory of Resonance Raman Scattering in Carbon Nanotubes using Excitonic Intermediate States
No full textOptical transition energies in carbon nanotubes can be obtained with high precision from Raman excitation profiles (REPs) measured with continuously tunable laser sources. In the case of Raman scattering by the radial breathing mode (RBM), the measured frequency of this mode makes it possible to associate the optical transition energies with a particular tube diameter, a unique feature of the Raman approach. The theoretical interpretation of REPs has been so far limited to free-electron-hole pairs. However, large excitonic binding energies are expected in quasi one-dimensional systems, and this expectation has been recently confirmed by detailed first principles calculations. Here we present an extension of the ab initio approach to the computation of REPs in semiconducting tubes. A many-body method based on the GW approximation and the Bethe-Salpeter equation is developed to compute the REP for RBMs. This approach yields REPs that take into account the electron-hole interaction and excitonic effects. The method has been applied to the (4,2) tube. The calculated REP is compared with predictions for the case of free electron-hole pairs as well as with the theoretical optical absorption. Such a comparison makes it possible to identify the relative advantages of these two different optical techniques
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
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