1,162 research outputs found
Magnetic structure and orbital ordering in BaCoO3 from first-principles calculations
Ab initio calculations using the APW+lo method as implemented in the WIEN2k
code have been used to describe the electronic structure of the
quasi-one-dimensional system BaCoO3. Both, GGA and LDA+U approximations were
employed to study different orbital and magnetic orderings. GGA predicts a
metallic ground state whereas LDA+U calculations yield an insulating and
ferromagnetic ground state (in a low-spin state) with an alternating orbital
ordering along the Co-Co chains, consistent with the available experimental
data.Comment: 8 pages, 9 figure
Calculation of the P-T phase diagram and tendency toward decomposition in equiatomic TiZr alloy
Electronic, structural and thermodynamic properties of the equiatomic alloy
TiZr are calculated within the electron density functional theory and the
Debye-Gruneisen model. The calculated values of the lattice parameters a and
c/a agree well with the experimental data for the alpha, omega and beta phases.
The omega phase is shown to be stable at atmospheric pressure and low
temperatures; it remains energetically preferable up to T=600K. The alpha phase
of the TiZr alloy becomes stable in the range 600K<T<900K, and the beta phase
at temperatures above 900K. The constructed phase diagram qualitatively agrees
with the experimental data available. The tendency toward decomposition in the
equiatomic alloy omega-TiZr is studied. It is shown that in the ground state
the omega phase of the ordered equiatomic alloy TiZr has a tendency toward
ordering, rather than decomposition.Comment: 6 pages, 8 figure
Is LaOFFeAs an electron-phonon superconductor ?
In this paper we calculate the electron-phonon coupling of the
newly-discovered superconductor LaOFFeAs from first-principles,
using Density Functional Perturbation Theory. For pure LaOFeAs, the calculated
electron-phonon coupling constant and logarithmic-averaged
frequency , give a maximum of 0.8 K, using the
standard Migdal-Eliashberg theory.
For the doped compounds, we predict even smaller coupling constants, due
to the strong suppression of the electronic Density of States at the Fermi
level. To reproduce the experimental , a 5-6 times larger coupling
constant would be needed.
Our results indicate that electron-phonon coupling is not sufficient to
explain superconductivity in the newly-discovered LaOFFeAs
superconductor, probably due to the importance of strong correlation effects
Extracorporeal Immunoglobulin Elimination for the Treatment of Severe Myasthenia Gravis
Myasthenia gravis (MG) is a neuromuscular disorder leading to fluctuating muscle weakness and fatigue. Rarely, long-term stabilization is not possible through the use of thymectomy or any known drug therapy. We present our experience with extracorporeal immunoglobulin (Ig) elimination by immunoadsorption (adsorbers with human Ig antibodies). Acetylcholine receptor antibodies (AChRAs) were measured during long-term monitoring (4.7 ± 2.9 years; range 1.1–8.0). A total of 474 samples (232 pairs) were analyzed, and a drop in AChRA levels was observed (P = .025). The clinical status of patients improved and stabilized. Roughly 6.8% of patients experienced clinically irrelevant side effects. The method of Ig elimination by extracorporeal immunoadsorption (IA) is a clinical application of the recent biotechnological advances. It offers an effective and safe therapy for severe MG even when the disease is resistant to standard therapy
Co-doped Ceria: Tendency towards ferromagnetism driven by oxygen vacancies
We perform an electronic structure study for cerium oxide homogeneously-doped
with cobalt impurities, focusing on the role played by oxygen vacancies and
structural relaxation. By means of full-potential ab-initio methods, we explore
the possibility of ferromagnetism as observed in recent experiments. Our
results indicate that oxygen vacancies seem to be crucial for the appearance of
a ferromagnetic alignment among Co impurities, obtaining an increasing tendency
towards ferromagnetism with growing vacancy concentration. The estimated
couplings cannot explain though, the experimentally observed room-temperature
ferromagnetism. In this systematic study, we draw relevant conclusions
regarding the location of the oxygen vacancies and the magnetic couplings
involved. In particular, we find that oxygen vacancies tend to nucleate in the
neighborhood of Co impurities and we get a remarkably strong ferromagnetic
coupling between Co atoms and the Ce^{3+} neighboring ions. The calculated
magnetic moments per cell depend on the degree of reduction which could explain
the wide spread in the magnetization values observed in the experiments
Influence of carbon and nitrogen on electronic structure and hyperfine interactions in fcc iron-based alloys
Carbon and nitrogen austenites, modeled by Fe8N and Fe8C superstructures are
studied by full-potential LAPW method. Structure parameters, electronic and
magnetic properties as well as hyperfine interaction parameters are obtained.
Calculations prove that Fe-C austenite can be successfully modeled by ordered
Fe8C superstructure. The results show that chemical Fe-C bond in Fe8C has
higher covalent part than in Fe8N. Detailed analysis of electric field gradient
formation for both systems is performed. The calculation of electric field
gradient allow us to carry out a good interpretation of Moessbauer spectra for
Fe-C and Fe-N systems.Comment: 8 pages, 3 figures, IOP-style LaTeX, submitted to J. Phys. Condens.
Matte
Evidence for magnetic clusters in BaCoO
Magnetic properties of the transition metal oxide BaCoO are analyzed on
the basis of the experimental and theoretical literature available via ab inito
calculations. These can be explained by assuming the material to be formed by
noninteracting ferromagnetic clusters of about 1.2 nm in diameter separated by
about 3 diameters. Above about 50 K, the so-called blocking temperature,
superparamagnetic behavior of the magnetic clusters occurs and, above 250 K,
paramagnetism sets in.Comment: 4 pages, 1 figur
Cleaving-temperature dependence of layered-oxide surfaces
The surfaces generated by cleaving non-polar, two-dimensional oxides are
often considered to be perfect or ideal. However, single particle
spectroscopies on Sr2RuO4, an archetypal non-polar two dimensional oxide, show
significant cleavage temperature dependence. We demonstrate that this is not a
consequence of the intrinsic characteristics of the surface: lattice parameters
and symmetries, step heights, atom positions, or density of states. Instead, we
find a marked increase in the density of defects at the mesoscopic scale with
increased cleave temperature. The potential generality of these defects to
oxide surfaces may have broad consequences to interfacial control and the
interpretation of surface sensitive measurements
The local magnetic moments and hyperfine magnetic fields in disordered metal-metalloid alloys
The local magnetic moments and hyperfine magnetic fields (HFF) in the ordered
alloys Fe_{15}Sn and Fe_{15}Si are calculated with the first-principles
full-potential linear augmented plane wave (FP LAPW) method. The results are
compared with the experimental data on Fe-M (M=Si, Sn) disordered alloys at
small metalloid concentration. The relaxation of the lattice around the
impurity and its influence on the quantities under consideration are studied.
The mechanism of the local magnetic moment formation is described. It is proved
that the main distinction between these alloys is connected with the different
lattice parameters. Three contributions to the HFF are discussed: the
contributions of the core and valence electron polarization to the
Fermi-contact part, and the contibution from the orbital magnetic moment.Comment: 3 pages, 3 figures, submitted to Phys. Rev.
Hubbard U and Hund's Exchange J in Transition Metal Oxides: Screening vs. Localization Trends from Constrained Random Phase Approximation
In this work, we address the question of calculating the local effective
Coulomb interaction matrix in materials with strong electronic Coulomb
interactions from first principles. To this purpose, we implement the
constrained random phase approximation (cRPA) into a density functional code
within the linearized augmented plane wave (LAPW) framework.
We apply our approach to the 3d and 4d early transition metal oxides SrMO3
(M=V, Cr, Mn) and (M=Nb, Mo, Tc) in their paramagnetic phases. For these
systems, we explicitly assess the differences between two physically motivated
low-energy Hamiltonians: The first is the three-orbital model comprising the
t2g states only, that is often used for early transition metal oxides. The
second choice is a model where both, metal d- and oxygen p-states are retained
in the construction of Wannier functions, but the Hubbard interactions are
applied to the d-states only ("d-dp Hamiltonian"). Interestingly, since -- for
a given compound -- both U and J depend on the choice of the model, so do their
trends within a family of these compounds. In the 3d perovskite series SrMO3
the effective Coulomb interactions in the t2g Hamiltonian decrease along the
series, due to the more efficient screening. The inverse -- generally expected
-- trend, increasing interactions with increasing atomic number, is however
recovered within the more localized "d-dp Hamiltonian". Similar conclusions are
established in the layered 4d perovskites series Sr2MO4 (M=Mo, Tc, Ru, Rh).
Compared to their isoelectronic and isostructural 3d analogues, the 4d 113
perovskite oxides SrMO3 (M=Nb, Mo, Tc) exhibit weaker screening effects.
Interestingly, this leads to an effectively larger U on 4d shells than on 3d
when a t2g model is constructed.Comment: 21 pages, 7 figure
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