1,720,966 research outputs found
Many-body effects in the electronic and optical properties of materials for UV applications: the case of BaF2
No full textMany-body effects in the electronic and optical properties of materials for UV applications: the case of BaF
Electronic and Optical properties of cadmium fluoride: the role of many-body effects
No full textElectronic and Optical properties of cadmium fluoride: the role of many-body effect
Calculation of optical properties for systems with large supercells
No full textSi and Ge nanocrystals embedded in a wide-gap semiconductor like -SiC are potentially interesting systems for possible electroluminescence applications. A theoretical understanding of these systems is yet to be achieved, both for the optical properties under inclusion of the relevant many-body effects and for the structural and electronic properties. We present first attempts to describe these properties using parameter-free electronic structure calculations and large supercells. A plane-wave-pseudopotential code (VASP ) is used to calculate the electronic structure within density-functional theory (DFT) in local-density approximation (LDA). Ultrasoft non-normconserving pseudopotentials allow the {\it ab-initio} treatment of supercells with up to 512 atoms, even in the case of first-row elements. Each supercell contains one cluster, the remaining space is filled with matrix material. The maximum dot diameters are about 1 nm. Examples are mainly structures made of group-IV materials. In the present talk we focus our attention on three main problems.
The problem of wave-function augmentation for non-normconserving pseudopotentials is solved by constructing all-electron wave functions using Bl\"ochl's projector-augmented wave (PAW) method [1]. As an advantage nonlocal contributions to the optical matrix elements do not occur. Spectra are compared with those obtained using normconserving pseudopotentials and the FLAPW method.
The huge supercell size drastically restricts the number of -points. Nevertheless we prefer to use the tetrahedron method for the optical calculations. Unfortunately the increase of the supercell size gives rise to many band crossings which prevent the identification of the same band at the tetrahedron vertices. We present a highly efficient and robust extrapolative Brillouin-zone integration scheme based on second-order perturbation theory (cf. [2]).
We use the simplified treatment of the GW self-energy developed by Cappellini et al. [3] in the beginning of the 90's to calculate the quasiparticle corrections for supercells with several hundreds of atoms . The dielectric constant and the electron density determining the RPA screening are obtained within DFT-LDA. Despite the complications with the augmentation of the Bloch integrals we show agreement with calculations using two-atom cells. Computations for embedded clusters are in progress
Classical versus ab initio structural relaxation: electronic excitations and optical properties of Ge nanocrystals embedded in a SiC matrix
No full textWe discuss and test a combined method to efficiently perform ground- and excited-state calculations for relaxed structures using both a quantum first-principles approach and a classical molecular-dynamics scheme. We apply this method to calculate the ground state, the optical properties, and the electronic excitations of Ge nanoparticles embedded in a cubic SiC matrix. Classical molecular dynamics is used to relax the large-supercell system. First-principles quantum techniques are then used to calculate the electronic structure and, in turn, the electronic excitation and optical properties. The proposed procedure is tested with data resulting from a full first-principles scheme. The agreement is quantitatively discussed between the results after the two computational paths with respect to the structure, the optical properties, and the electronic excitations. The combined method is shown to be applicable to embedded nanocrystals in large simulation cells for which the first-principle treatment of the ionic relaxation is presently out of reach, whereas the electronic, optical and excitation properties can already be obtained ab initio. The errors incurred from the relaxed structure are found to be non-negligible but controllable
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
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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