1,721,114 research outputs found
Structural, vibrational, and quasiparticle properties of the Peierls semiconductor BaBiO3: A hybrid functional and self-consistent GW plus vertex-corrections study
No full textBaBiO3 is characterized by a charge disproportionation with half of the Bi atoms possessing a valence 3+ and half a valence 5+. Because of self-interaction errors, local- and semilocal-density functionals fail to describe the charge disproportionation quantitatively, yielding a too small structural distortion and no band gap. Using hybrid functionals, we obtain a satisfactory description of the structural, electronic, optical, and vibrational properties of BaBiO3. The results obtained using GW (Green's function G and screened Coulomb potential W) based schemes on top of hybrid functionals, including fully self-consistent GW calculations with vertex corrections in the dielectric screening, qualitatively confirm the Heyd-Scuseria-Ernzerhof picture but a systematic overestimation of the band gap by about 0.4 eV is observed
Electron-phonon interactions using the projector augmented-wave method and Wannier functions
Full text linkWe present an ab initio density-functional-theory approach for calculating electron-phonon interactions within the projector augmented-wave (PAW) method. The required electron-phonon matrix elements are defined as the second derivative of the one-electron energies in the PAW method. As the PAW method leads to a generalized eigenvalue problem, the resulting electron-phonon matrix elements lack some symmetries that are usually present for simple eigenvalue problems and all-electron formulations. We discuss the relation between our definition of the electron-phonon matrix element and other formulations. To allow for efficient evaluation of physical properties, we introduce a Wannier-interpolation scheme, again adapted to generalized eigenvalue problems. To explore the method's numerical characteristics, the temperature-dependent band-gap renormalization of diamond is calculated and compared with previous publications. Furthermore, we apply the method to selected binary compounds and show that the obtained zero-point renormalizations agree well with other values found in literature and experiments
Assessing model-dielectric-dependent hybrid functionals on the antiferromagnetic transition-metal monoxides MnO, FeO, CoO, and NiO
Full text linkRecently, two nonempirical hybrid functionals, dielectric-dependent range-separated hybrid functional based on the Coulomb-attenuating method (DD-RSH-CAM) and doubly screened hybrid functional (DSH), have been suggested by Chen et al (2018 Phys. Rev. Mater. 2 073803) and Cui et al (2018 J. Phys. Chem. Lett. 9 2338), respectively. These two hybrid functionals are both based on a common model dielectric function approach, but differ in the way how to non-empirically obtain the range-separation parameter. By retaining the full short-range Fock exchange and a fraction of the long-range Fock exchange that equals the inverse of the dielectric constant, both DD-RSH-CAM and DSH turn out to perform very well in predicting the band gaps for a large variety of semiconductors and insulators. Here, we assess how these two hybrid functionals perform on challenging antiferromagnetic transition-metal monoxides MnO, FeO, CoO, and NiO by comparing them to other conventional hybrid functionals and the GW method. We find that single-shot DD0-RSH-CAM and DSH0 improve the band gaps towards experiments as compared to conventional hybrid functionals. The magnetic moments are slightly increased, but the predicted dielectric constants are decreased. The valence band density of states (DOS) predicted by DD0-RSH-CAM and DSH0 are as satisfactory as HSE03 in comparison to experimental spectra, however, the conduction band DOS are shifted to higher energies by about 2 eV compared to HSE03. Self-consistent DD-RSH-CAM and DSH deteriorate the results with a significant overestimation of band gaps
Structural, vibrational, and quasiparticle properties of the Peierls semiconductor BaBiO3: A hybrid functional and self-consistent GW+vertex-corrections study
No full textBaBiO3 is characterized by a charge disproportionation with half of the Bi atoms possessing a valence 3+ and half a valence 5+. Because of self-interaction errors, local- and semilocal-density functionals fail to describe the charge disproportionation quantitatively, yielding a too small structural distortion and no band gap. Using hybrid functionals, we obtain a satisfactory description of the structural, electronic, optical, and vibrational properties of BaBiO3. The results obtained using GW (Green's function G and screened Coulomb potential W) based schemes on top of hybrid functionals, including fully self-consistent GW calculations with vertex corrections in the dielectric screening, qualitatively confirm the Heyd-Scuseria-Ernzerhof picture but a systematic overestimation of the band gap by about 0.4 eV is observed. © 2010 The American Physical Society
Ground-state properties of multivalent manganese oxides: Density functional and hybrid density functional calculations
No full textWe present density functional theory (DFT) calculations for MnO, Mn3O4, alpha-Mn2O3, and beta-MnO2, using different gradient corrected functionals, such as Perdew-Burke-Ernzerhof (PBE), PBE+U, and the two hybrid density functional Hartree-Fock methods PBEO and Heyd-Scuseria-Ernzerhof (HSE). We investigate the structural, electronic, magnetic, and thermodynamical properties of the mentioned compounds. Despite the lack of sufficient experimental information allowing for a comprehensive comparison of our results, we find overall that hybrid functionals provide a more consistent picture than standard PBE. Although PBE+U is limited due to the uncertainty of choosing the parameter U, it nevertheless provides satisfactory results in terms of magnetic properties and energies of formation. This is in line with results of PBEO and HSE calculations, but the PBE+U approach tends to overestimate the equilibrium volumes, and also it favors a half-metallic state for the more reduced oxides Mn3O4, alpha-Mn2O3, and beta-MnO2, rather than an insulating character as derived from the hybrid functional approaches. The comparison of measured valence-band spectra with the HSE density of states offers a further assessment of the capability of hybrid approaches in overcoming the deficiencies of DFT in treating these kinds of materials
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
Ab Initio Calculations in Scintillator Research
No full textThe optical characteristics of the Ce3+ ion are largely determined by the geometry of the direct environment of the cerium-ion, and are often very sensitive to changes in this environment. These changes are for example periodic changes because of thermal movements of the atoms in the grid, but also because of static changes such as an optical transition on the cerium ion itself, so-called grid relaxation. Marsman looked for the most accurate method to calculate the geometry of the grid around the cerium-ion. He then combined this information with methods for calculating the optical absorption and emission energies. Calculations of these aspects require a detailed quantum mechanical description of the material. Marsman also tested a number of model systems, currently successfully used to determine the dynamic properties of semi-conductors and super-conductors, to determine if they can be used equally well on strongly iogenic materials. This study shows that that the accuracy of the determination of the dynamic properties of iogenic substances is substantially less than that of the determination of these properties in semi and super-conductors.Interfaculty Reactor Institut
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
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