144,023 research outputs found
Effects of isotopic disorder on the Raman spectra of crystals: theory and ab initio calculations for diamond and germanium
No full textWe present a method to study the effects of isotopic composition on the Raman spectra of crystals, in which disorder is treated exactly without resorting to any kind of mean-field approximation. The Raman cross section is expressed in terms of a suitable diagonal element of the vibrational Green’s function, which is accurately and efficiently calculated using the recursion technique. This method can be used in conjunction with both semiempirical lattice-dynamical models and with first-principles interatomic force constants. We have applied our technique to diamond and germanium using the most accurate interatomic force constants presently available, obtained from density-functional perturbation theory. Our method correctly reproduces the light scattering in diamond—where isotopic effects dominates over the anharmonic ones—as well as in germanium, where anharmonic effects are larger
Electron energy loss and inelastic x-ray scattering cross sections from time-dependent density-functional perturbation theory
No full textThe Liouville-Lanczos approach to linear-response time-dependent density-functional theory is generalized so as to encompass electron energy loss and inelastic x-ray scattering spectroscopies in periodic solids. The computation of virtual orbitals and the manipulation of large matrices are avoided by adopting a representation of response orbitals borrowed from (time-independent) density functional perturbation theory and a suitable Lanczos recursion scheme. The latter allows the bulk of the numerical work to be performed at any given transferred momentum only once, for a whole extended frequency range. The numerical complexity of the method is thus greatly reduced, making the computation of the loss function over a wide frequency range at any given transferred momentum only slightly more expensive than a single standard ground-state calculation and opening the way to computations for systems of unprecedented size and complexity. Our method is validated on the paradigmatic examples of bulk silicon and aluminum, for which both experimental and theoretical results already exist in the literature
Atomic structure of icosahedral B4C boron carbide from a first principles analysis of NMR spectra
No full textDensity functional theory is demonstrated to reproduce the C-13 and B-11 NMR chemical shifts of icosahedral boron carbides with sufficient accuracy to extract previously unresolved structural information from experimental NMR spectra. B4C can be viewed as an arrangement of 3-atom linear chains and 12-atom icosahedra. According to our results, all the chains have a CBC structure. Most of the icosahedra have a B11C structure with the C atom placed in a polar site, and a few percent have a B-12 structure or a B10C2 structure with the two C atoms placed in two antipodal polar sites
Atomic structure and vibrational properties of icosahedral alpha-boron and B4C boron carbide
No full textThe Raman and infrared spectra of a-rhombohedral boron B-12 and of B4C boron carbide have been determined by accurate first-principles calculations based on density-functional perturbation theory. Our results account for all the features observed experimentally, including the characteristic Raman-active mode around 530 cm(-1), which is attributed to the libration of the icosahedra. A comparison of the calculated vibrational spectra with experimental data allows the first unambiguous determination of the atomic structure of B4C, Analysis of our data shows that the high bulk moduli of cc-rhombohedral boron and of B4C boron carbide - 220 and 240 GPa, respectively - are mainly determined by the stiff intramolecular bonding within each icosahedron. This finding is at variance with the current interpretation of recent neutron diffraction data on B4C in terms of a postulated larger stiffness of the intermolecular bonds in icosahedral solids (inverted molecular compressibility). Our results show that icosahedral boron-rich solids should be considered as members of a new class of covalently bonded materials. (C) 2000 Elsevier Science B.V. All rights reserved
A Multi-Language Comparison of Influences on Author Verification using Character N-Grams
No full textWe create a new multi-language corpus for author verification based on Wikipedia talkpages, and evaluate the influence that differences in topic and time have on character n-gram author profiles. Topic alignment between two texts is found to increase author verification precision, and an authors writing style is found to change over time, but not more significantly after 3 years than after 1 year.Information ArchitectureWISElectrical Engineering, Mathematics and Computer Scienc
Appropriate Similarity Measures for Author Cocitation Analysis
No full textWe 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
The vanishing author in computer-generated works: a critical analysis of recent Australian case law
No full textAbstract
The use of software is ubiquitous in the creation of many copyright works, yet the requirement in copyright law that every work have a human author who engages in independent intellectual effort means that its use may prevent copyright subsistence. Several recent Australian cases have refocused attention on authorship as an essential criterion of copyright subsistence, and these cases suggest that much computer-produced output may be authorless and thus lack copyright protection. This article, the first in a two-part series, analyses how each case deals with the question of authorship of computer-produced works and why the use of software diminishes copyright protection for a significant number of computer-generated works. The article critiques the application of conventional notions of human authorship developed in the pre-computer age to modern productions and suggests alternative approaches to authorship that satisfy both the major objectives of copyright policy and the need to adapt to the computer age. The article argues that, without a broader judicial approach to authorship of computer-generated works, Parliament must remedy the lacuna in protection for these ‘authorless’ works. Possible solutions for reform are suggested. In a forthcoming article, the author comprehensively examines those reform proposals
Vichy décentré. Le prisme de l’Aquitaine : Koscielniak (Jean-Pierre) et Souleau (Philippe) dir., Vichy en Aquitaine, Ivry-sur-Seine, Éditions de l’Atelier, 2011, 437 p
No full textVast Cécile. Vichy décentré. Le prisme de l’Aquitaine : Koscielniak (Jean-Pierre) et Souleau (Philippe) dir., Vichy en Aquitaine, Ivry-sur-Seine, Éditions de l’Atelier, 2011, 437 p. In: Annales du Midi : revue archéologique, historique et philologique de la France méridionale, Tome 124, N°278, 2012. La présence musulmane en narbonnaise et dans la vallée du Rhône. pp. 287-288
Improving Volunteering for people over 60.
No full textIn 2019 we worked with VAST to recruit members of their community researcher
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