1,721,040 research outputs found
Chemical-pressure-induced modifications on the magnetic and electronic properties of Ba1-xSrxVS3
No full textBy means of ab initio calculations based on the local spin density approximation within the density functional theory, we study the effect of the isovalent Sr substitution at the Ba site on the structural, electronic and magnetic properties of Ba1-xSrxVS3 for different Sr substitution levels, x = 0.25, 0.5, 0.75 and 1. We find that the Sr-induced chemical pressure causes an anisotropic shrinking of the unit cell that stabilizes the metallic state and induces a magnetic antiferro- (AF) to ferro-magnetic (FM) transition for sufficiently high x values. Our results shed light on the complex experimental scenario towards a deeper understanding of the intriguing properties of this system
Role of electronic correlations on the ground-state properties and on the pressure-induced metal-insulator transition in BaVS3
No full textWe investigated the structural, magnetic and electronic properties of the hexagonal perovskite BaVS3 by means of first-principles calculations within the density functional theory in the local spin density approximation (LSDA) that includes the Hubbard repulsion term U (LSDA+U) to take into account electronic correlations. We find that the LSDA+U scheme greatly improves on the LSDA results previously reported, and quantitatively accounts for all ground state properties found experimentally. First, the LSDA+U predicts an orthorhombic structure and a quasi-metallic ground state with a long-range antiferromagnetic (AFM) order in the quasi-hexagonal ab-plane, ferromagnetically (FM) coupled along the c-axis. Second, we studied the stability of competing crystal structures and competing magnetic orderings in terms of exchange integrals. The results account well for the experimental pressure-dependence of the metal-insulator transition and for the chemical-pressure induced AFM-FM transition reported recently in Sr-substituted samples. In particular, at the experimental value of the volume, V, we obtain an energy gap Delta=47 meV, which falls in the range of experimental values (43-59 meV), while, at smaller V values, we find an AFM-FM transition, in agreement with the above effects of Sr-induced chemical pressure. Finally, in the metallic phase, we find a nearly isotropic electrical conductivity, in agreement with experiments, despite the presence of quasi-one-dimensional chains of VS6 along the c-axis. We account for such three-dimensional conductivity in terms of the strong interaction between V and S orbitals within each VS3 chain and between adjacent chains
Structural and electronic properties of Hg1-yMoyBa2CuO4+delta
No full textWe discuss the effect of experimentally relevant defects on the structural and electronic properties of HgBa2CuO4+delta, using the full-potential linearized-augmented-plane-waves method, within the local-density approximation to density-functional theory. In particular, we study interstitial O and F atoms (delta = 0.5), and the 50% substitution of Hg atoms with Mo. We perform structural optimization, and obtain a good agreement with the experimental results and with their trends in terms of doping. Mo forms strong bonds with nearby apical oxygen atoms, bringing about large changes in both structural and electronic properties. The filling of the relevant Cu-O bands is increased (decrease of doping), consistently with the high T-c approximate to 40 K experimentally found with delta approximate to 0.5. O and F atoms are found to provide a remarkably similar band structure near E-F, as far as the shape and the filling of the relevant Cu-O bands are concerned. This confirms an anomalous contribution of interstitial O to the doping
Disorder-induced localisation and suppression of superconductivity in YSr2Cu3O6+x
No full textBy means of ab initio calculations within the local density approximation to density functional theory, we investigate the electronic structure of the 60 K superconductor YSr2Cu3O6+x (YSCO). We focus on the effects of the Sr/Ba substitution and on the main structural modifications induced by this substitution experimentally found in the Sr compound, namely the tetragonal symmetry and the oxygen disorder in the basal plane. In the calculations, this disorder is simulated by using a supercell approach. Due to band structure effects, we find a larger stabilisation free energy of the orthorhombic structure in YBa2Cu3O6+x (YBCO). In YSCO, the tetragonal disordered phase is found to be stabilized by oxygen overdoping (x > 1) and by sufficiently large mass-enhancement factors, γ > 5. The analysis of the atomic site projected density of states suggests that oxygen disorder in the CuO basal planes of YSCO induces hole localisation, which accounts for the large 30 K reduction of Tc with respect to YBCO
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
Structural, transport, and electronic properties of a layered dichalcogenide AuVS2 with semimetallic properties
No full textWe report on preparation, crystal structure, transport properties, and ab initio band-structure calculations of the layered compound AuVS2. Single crystals of submillimeter size are obtained by reacting powders of BaVS3 with metallic gold at 700 degreesC under an hydrostatic pressure of 3 GPa. According to single-crystal x-ray-diffraction data, the crystal structure is hexagonal with unit-cell parameters a=b=3.224+/-0.001 Angstrom, c=15.039+/-0.002 Angstrom, and space-group symmetry P6(3)/mmc. Temperature-dependent ac resistivity measurements yield a value of room-temperature in-plane resistivity rho(ab)approximate to400 muOmega cm and show a slightly metallic temperature dependence above 250 K. Below this temperature the dependence becomes weakly insulating but the resistivity remains less than or similar to800 muOmega cm at 4.2 K. This behavior is in agreement with the above band-structure calculations which indicate that AuVS2 is a semimetal with an almost vanishing density of states at the Fermi level. Both electron and hole doping would strongly enhance the metallic properties and change the very small Fermi surfaces of the undoped compound into larger pockets with mostly V 3d character
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