1,721,070 research outputs found
Thickness dependent structural and electronic properties of CuO grown on SrTiO3(100): a hybrid density functional theory study
No full textWe discuss the structural and electronic properties of tetragonal CuO grown on SrTiO3(100) by means of hybrid density functional theory. Our analysis explains the anomalously large Cu-O vertical distance observed in the experiments (approximate to 2.7 angstrom) in terms of a peculiar frustration between two competing local Cu-O environments characterized by different in-plane and out-of-plane bond lengths and Cu electronic populations. The proper inclusion of substrate effects is crucial to understanding the tetragonal expansion and to reproduce correctly the measured valence band spectrum for a CuO thickness of 3-3.5 unit cells, in agreement with the experimentally estimated thickness
Structural transitions and transport-half-metallic ferromagnetism in LaMnO3 at elevated pressure
No full textBy means of hybrid density functional theory we investigate the evolution of the structural, electronic, and magnetic properties of the colossal magnetoresistance (CMR) parent compound LaMnO3 under pressure. We predict a transition from a low-pressure antiferromagnetic (AFM) insulator to a high-pressure ferromagnetic (FM) transport half metal (tHM), characterized by a large spin polarization (approximate to 80-90%). The FM-tHM transition is associated with a progressive quenching of the cooperative Jahn-Teller (JT) distortions which transform the Pnma orthorhombic phase into a perfect cubic one (through a mixed phase in which JT-distorted and regular MnO6 octahedra coexist), and with a high-spin (S = 2, m(Mn) = 3.7 mu(B)) to low-spin (S = 1, m(Mn) = 1.7 mu(B)) magnetic moment collapse. These results interpret the progression of the experimentally observed non-Mott metalization process and open up the possibility of realizing CMR behaviors in a stoichiometric manganite
Hardness of T-carbon: Density functional theory calculations
No full textWe reconsider and interpret the mechanical properties of the recently proposed allotrope of carbon, T-carbon [Sheng et al., Phys. Rev. Lett. 106, 155703 (2011)], using density functional theory in combination with different empirical hardness models. In contrast with the early estimation based on Gao et al.'s model, which attributes to T-carbon a high Vickers hardness of 61 GPa comparable to that of superhard cubic boron nitride (c-BN), we find that T-carbon is not a superhard material, since its Vickers hardness does not exceed 10 GPa. Besides providing clear evidence for the absence of superhardness in T-carbon, we discuss the physical reasons behind the failure of Gao et al.'s and Simunek and Vackar's (SV) models in predicting the hardness of T-carbon, residing in their improper treatment of the highly anisotropic distribution of quasi-sp(3)-like C-C hybrids. A possible remedy for the Gao et al. and SV models based on the concept of the superatom is suggested, which indeed yields a Vickers hardness of about 8 GPa
Polymeric forms of carbon in dense lithium carbide
No full textThe immense interest in carbon nanomaterials continues to stimulate intense research activities aimed at realizing carbon nanowires, since linear chains of carbon atoms are expected to display novel and technologically relevant optical, electrical and mechanical properties. Although various allotropes of carbon (e. g., diamond, nanotubes, graphene, etc) are among the best-known materials, it remains challenging to stabilize carbon in the one-dimensional form because of the difficulty of suitably saturating the dangling bonds of carbon. Here, we show through first-principles calculations that ordered polymeric carbon chains can be stabilized in solid Li(2)C(2) under moderate pressure. This pressure-induced phase (above 5 GPa) consists of parallel arrays of twofold zigzag carbon chains embedded in lithium cages, which display a metallic character due to the formation of partially occupied carbon lone-pair states in sp(2)-like hybrids. It is found that this phase remains the most favorable one in a wide range of pressures. At extreme pressure (larger than 215 GPa) a structural and electronic phase transition towards an insulating single-bonded threefold-coordinated carbon network is predicted
Hybrid density-functional calculation of the electronic and magnetic structures of tetragonal CuO
No full textThe electronic and magnetic properties of recently synthesized tetragonal CuO with c/a>1 is calculated by means of hybrid density-functional theory. We predict that this tetragonal phase orders antiferromagnetically and has an exceptionally high Neeacutel temperature T(N)approximate to 800 K, which makes it an ideal candidate for doping experiments and a potential parent of superconductors. The electronic structure is characterized by a charge-transfer gap of 2.7 eV whereas the magnetic properties are dominated by the antiferromagnetic Cu-O-Cu interactions along the nearest-neighbor [100] direction. In addition, we predict the second tetragonal CuO phase with a c/a ratio < 1 with a different antiferromagnetic ordering and a similar high T(N). We suggest that this phase could be synthesized by epitaxial growth
Electronic, optical, and mechanical properties of superhard cold-compressed phases of carbon
No full textBy means of standard and hybrid density functional theory, we analyzed the electronic, optical, and mechanical properties of the two discovered superhard orthorhombic (W) and monoclinic (M) phases of carbon, synthesized by cold compression. We demonstrated that both phases exhibit a transparent insulating behaviour with indirect band gaps of about 5.4 eV (W) and 4.5 eV (M), and highly isotropic optical spectra, substantially different to those of the related body centered tetragonal C-4 phase. The analysis of the elastic constants and Vickers hardness confirmed that these phases are as hard as the second hardest material c-BC2N (C) 201/ American Institute of Physics. [doi:10.1063/1.3610996
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
Electron and hole doping in the relativistic Mott insulator Sr2IrO4: a first-principles study using band unfolding technique
No full textWe study the effects of dilute La and Rh substitutional doping on the electronic structure of the relativistic Mott
insulator Sr 2 IrO 4 using fully relativistic and magnetically noncollinear density functional theory with the inclusion
of an on-site Hubbard U . To model doping effects, we have adopted the supercell approach, that allows for a
realistic treatment of structural relaxations and electronic effects beyond a purely rigid band approach. By means of
the band unfolding technique we have computed the spectral function and constructed the effective band structure
and Fermi surface (FS) in the primitive cell, which are readily comparable with available experimental data. Our
calculations clearly indicate that La and Rh doping can be interpreted as effective electron and (fractional) hole
doping, respectively. We found that both electron and hole doping induce an insulating-to-metal transition (IMT)
but with different characteristics. In Sr 2−x La x IrO 4 the IMT is accompanied by a moderate renormalization of the
electronic correlation substantiated by a reduction of the effective on-site Coulomb repulsion U − J from 1.6 eV
(x = 0) to 1.4 eV (metallic regime of x = 12.5%). The progressive closing of the relativistic Mott gap leads to
the emergence of connected elliptical electron pockets at (π/2,π/2) and less intense features at X on the Fermi
surface. The average ordered magnetic moment is slightly reduced upon doping, but the canted antiferromagnetic
state is perturbed on the Ir-O planes located near the La atoms. The substitution of Ir with the nominally isovalent
Rh is accompanied by a substantial hole transfer from the Rh site to the nearest-neighbor Ir sites. This shifts
down the chemical potential, creates almost circular disconnected hole pockets in the FS, and establishes the
emergence of a two-dimensional metallic state formed by conducting Rh planes intercalated by insulating Ir
planes. Finally, our data indicate that hole doping causes a flipping of the in-plane net ferromagnetic moment on
the Rh plane and induces a magnetic transition from the antiferromagnetic (AF)-I to the AF-II ordering
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
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