6,845 research outputs found
Charge order in Magnetite. An LDA+ study
The electronic structure of the monoclinic structure of FeO is
studied using both the local density approximation (LDA) and the LDA+. The
LDA gives only a small charge disproportionation, thus excluding that the
structural distortion should be sufficient to give a charge order. The LDA+
results in a charge disproportion along the c-axis in good agreement with the
experiment. We also show how the effective can be calculated within the
augmented plane wave methods
Electronic structure of the ferromagnetic superconductor UCoGe from first principles
The superconductor UCoGe is analyzed with electronic structure calculations
using Linearized Augmented Plane Wave method based on Density Functional
Theory. Ferromagnetic and antiferromagnetic calculations with and without
correlations (via LDA+U) were done. In this compound the Fermi level is
situated in a region where the main contribution to DOS comes from the U-5f
orbital. The magnetic moment is mainly due to the Co-3d orbital with a small
contribution from the U-5f orbital. The possibility of fully non-collinear
magnetism in this compound seems to be ruled out. These results are compared
with the isostructural compound URhGe, in this case the magnetism comes mostly
from the U-5f orbital
C and S induces changes in the electronic and geometric structure of Pd(533) and Pd(320)
We have performed ab initio electronic structure calculations of C and S
adsorption on two vicinal surfaces of Pd with different terrace geometry and
width. We find both adsorbates to induce a significant perturbation of the
surface electronic and geometric structure of Pd(533) and Pd(320). In
particular C adsorbed at the bridge site at the edge of a Pd chain in Pd(320)
is found to penetrate the surface to form a sub-surface structure. The
adsorption energies show almost linear dependence on the number of
adsorbate-metal bonds, and lie in the ranges of 5.31eV to 8.58eV for C and
2.89eV to 5.40eV for S. A strong hybridization between adsorbate and surface
electronic states causes a large splitting of the bands leading to a drastic
decrease in the local densities of electronic states at the Fermi-level for Pd
surface atoms neighboring the adsorbate which may poison catalytic activity of
the surface. Comparison of the results for Pd(533) with those obtained earlier
for Pd(211) suggests the local character of the impact of the adsorbate on the
geometric and electronic structures of Pd surfaces.Comment: 14 pages 9 figs, Accepted J. Phys: Conden
Einstein-Weyl structures and Bianchi metrics
We analyse in a systematic way the (non-)compact four dimensional
Einstein-Weyl spaces equipped with a Bianchi metric. We show that Einstein-Weyl
structures with a Class A Bianchi metric have a conformal scalar curvature of
constant sign on the manifold. Moreover, we prove that most of them are
conformally Einstein or conformally K\"ahler ; in the non-exact Einstein-Weyl
case with a Bianchi metric of the type or , we show that the
distance may be taken in a diagonal form and we obtain its explicit
4-parameters expression. This extends our previous analysis, limited to the
diagonal, K\"ahler Bianchi case.Comment: Latex file, 12 pages, a minor modification, accepted for publication
in Class. Quant. Gra
Compact Einstein-Weyl four-dimensional manifolds
We look for four dimensional Einstein-Weyl spaces equipped with a regular
Bianchi metric. Using the explicit 4-parameters expression of the distance
obtained in a previous work for non-conformally-Einstein Einstein-Weyl
structures, we show that only four 1-parameter families of regular metrics
exist on orientable manifolds : they are all of Bianchi type and
conformally K\"ahler ; moreover, in agreement with general results, they have a
positive definite conformal scalar curvature. In a Gauduchon's gauge, they are
compact and we obtain their topological invariants. Finally, we compare our
results to the general analyses of Madsen, Pedersen, Poon and Swann : our
simpler parametrisation allows us to correct some of their assertions.Comment: Latex file, 13 pages, an important reference added and a critical
discussion of its claims offered, others minor modification
Solving the m-mixing problem for the three-dimensional time-dependent Schr\"{o}dinger equation by rotations: application to strong-field ionization of H2+
We present a very efficient technique for solving the three-dimensional
time-dependent Schrodinger equation. Our method is applicable to a wide range
of problems where a fullly three-dimensional solution is required, i.e., to
cases where no symmetries exist that reduce the dimensionally of the problem.
Examples include arbitrarily oriented molecules in external fields and atoms
interacting with elliptically polarized light. We demonstrate that even in such
cases, the three-dimensional problem can be decomposed exactly into two
two-dimensional problems at the cost of introducing a trivial rotation
transformation. We supplement the theoretical framework with numerical results
on strong-field ionization of arbitrarily oriented H2+ molecules.Comment: 5 pages, 4 figure
Physics and Astrophysics of Strange Quark Matter
3-flavor quark matter (strange quark matter; SQM) can be stable or metastable
for a wide range of strong interaction parameters. If so, SQM can play an
important role in cosmology, neutron stars, cosmic ray physics, and
relativistic heavy-ion collisions. As an example of the intimate connections
between astrophysics and heavy-ion collision physics, this Chapter gives an
overview of the physical properties of SQM in bulk and of small-baryon number
strangelets; discusses the possible formation, destruction, and implications of
lumps of SQM (quark nuggets) in the early Universe; and describes the structure
and signature of strange stars, as well as formation and detection of
strangelets in cosmic rays. It is concluded, that astrophysical and laboratory
searches are complementary in many respects, and that both should be pursued to
test the intriguing possibility of a strange ground state for hadronic matter,
and (more generally) to improve our knowledge of the strong interactions.Comment: 45 pages incl. figures. To appear in "Hadrons in Dense Matter and
Hadrosynthesis", Lecture Notes in Physics, Springer Verlag (ed. J.Cleymans
Neural-Network Force Field Backed Nested Sampling: Study of the Silicon p-T Phase Diagram
Nested sampling is a promising method for calculating phase diagrams of
materials, however, the computational cost limits its applicability if
ab-initio accuracy is required. In the present work, we report on the efficient
use of a neural-network force field in conjunction with the nested-sampling
algorithm. We train our force fields on a recently reported database of silicon
structures and demonstrate our approach on the low-pressure region of the
silicon pressure-temperature phase diagram between 0 and \SI{16}{GPa}. The
simulated phase diagram shows a good agreement with experimental results,
closely reproducing the melting line. Furthermore, all of the experimentally
stable structures within the investigated pressure range are also observed in
our simulations. We point out the importance of the choice of
exchange-correlation functional for the training data and show how the meta-GGA
r2SCAN plays a pivotal role in achieving accurate thermodynamic behaviour using
nested-sampling. We furthermore perform a detailed analysis of the exploration
of the potential energy surface and highlight the critical role of a diverse
training data set
The loss of anisotropy in MgB2 with Sc substitution and its relationship with the critical temperature
The electrical conductivity anisotropy of the sigma-bands is calculated for
the (Mg,Sc)B2 system using a virtual crystal model. Our results reveal that
anisotropy drops with relatively little scandium content (< 30%); this
behaviour coincides with the lowering of Tc and the reduction of the Kohn
anomaly. This anisotropy loss is also found in the Al and C doped systems. In
this work it is argued that the anisotropy, or 2D character, of the sigma-bands
is an important parameter for the understanding of the high Tc found in MgB2
Cancellation of probe effects in measurements of spin polarized momentum density by electron positron annihilation
Measurements of the two dimensional angular correlation of the
electron-positron annihilation radiation have been done in the past to detect
the momentum spin density and the Fermi surface. We point out that the momentum
spin density and the Fermi Surface of ferromagnetic metals can be revealed
within great detail owing to the large cancellation of the electron-positron
matrix elements which in paramagnetic multiatomic systems plague the
interpretation of the experiments. We prove our conjecture by calculating the
momentum spin density and the Fermi surface of the half metal CrO2, who has
received large attention due to its possible applications as spintronics
material
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