275 research outputs found
Charge screening and magnetic anisotropy in metallic rare-earth systems
The calculation of magnetic anisotropy constants is performed beyond the
point charge model for a continuous charge density distribution of screening
conduction electrons. An important role of the non-uniform electron density, in
particular, of the Friedel oscillations, in the formation of crystal field is
demonstrated. Such effects can modify strongly the effective ion (impurity)
charge and even change its sign. This enables one to justify the anion model,
which is often used at discussing experimental data on hydrogen-containing
systems. Possible applications to the pure rare-earth metals and RCo5 compounds
are discussed. The deformation of magnetic structure near the interstitial
positive muon owing to the strong local anisotropy, and the corresponding
contribution to the dipole field at the muon are considered.Comment: 10 pages, RevTeX, 3 figure
First principles study of local electronic and magnetic properties in pure and electron-doped NdCuO
The local electronic structure of Nd2CuO4 is determined from ab-initio
cluster calculations in the framework of density functional theory.
Spin-polarized calculations with different multiplicities enable a detailed
study of the charge and spin density distributions, using clusters that
comprise up to 13 copper atoms in the CuO2plane. Electron doping is simulated
by two different approaches and the resulting changes in the local charge
distribution are studied in detail and compared to the corresponding changes in
hole doped La2CuO4. The electric field gradient (EFG) at the copper nucleus is
investigated in detail and good agreement is found with experimental values. In
particular the drastic reduction of the main component of the EFG in the
electron-doped material with respect to LaCuO4 is explained by a reduction of
the occupancy of the 3d3z^2-r^2 atomic orbital. Furthermore, the chemical
shieldings at the copper nucleus are determined and are compared to results
obtained from NMR measurements. The magnetic hyperfine coupling constants are
determined from the spin density distribution
Deformation quantization of cosmological models
The Weyl-Wigner-Groenewold-Moyal formalism of deformation quantization is
applied to cosmological models in the minisuperspace. The quantization
procedure is performed explicitly for quantum cosmology in a flat
minisuperspace. The de Sitter cosmological model is worked out in detail and
the computation of the Wigner functions for the Hartle-Hawking, Vilenkin and
Linde wave functions are done numerically. The Wigner function is analytically
calculated for the Kantowski-Sachs model in (non)commutative quantum cosmology
and for string cosmology with dilaton exponential potential. Finally, baby
universes solutions are described in this context and the Wigner function is
obtained.Comment: 37 pages, 16 figure
Linear-response theory and lattice dynamics: a muffin-tin orbital approach
A detailed description of a method for calculating static linear-response
functions in the problem of lattice dynamics is presented. The method is based
on density functional theory and it uses linear muffin-tin orbitals as a basis
for representing first-order corrections to the one-electron wave functions. As
an application we calculate phonon dispersions in Si and NbC and find good
agreement with experiments.Comment: 18 pages, Revtex, 2 ps figures, uuencoded, gzip'ed, tar'ed fil
Noncommutativity from spectral flow
We investigate the transition from second to first order systems. This
transforms configuration space into phase space and hence introduces
noncommutativity in the former. Quantum mechanically, the transition may be
described in terms of spectral flow. Gaps in the energy or mass spectrum may
become large which effectively truncates the available state space. Using both
operator and path integral languages we explicitly discuss examples in quantum
mechanics, (light-front) quantum field theory and string theory.Comment: 31 pages, one Postscript figur
The problem of a metal impurity in an oxide: ab-initio study of electronic and structural properties of Cd in Rutile TiO2
In this work we undertake the problem of a transition metal impurity in an
oxide. We present an ab-initio study of the relaxations introduced in TiO2 when
a Cd impurity replaces substitutionally a Ti atom. Using the Full-Potential
Linearized-Augmented-Plane-Wave method we obtain relaxed structures for
different charge states of the impurity and computed the electric-field
gradients (EFGs) at the Cd site. We find that EFGs, and also relaxations, are
dependent on the charge state of the impurity. This dependence is very
remarkable in the case of the EFG and is explained analyzing the electronic
structure of the studied system. We predict fairly anisotropic relaxations for
the nearest oxygen neighbors of the Cd impurity. The experimental confirmation
of this prediction and a brief report of these calculations have recently been
presented [P.R.L. 89, 55503 (2002)]. Our results for relaxations and EFGs are
in clear contradiction with previous studies of this system that assumed
isotropic relaxations and point out that no simple model is viable to describe
relaxations and the EFG at Cd in TiO2 even approximately.Comment: 11 pages, 8 figures, Revtex 4, published in Physical Review
Core reconstruction in pseudopotential calculations
A new method is presented for obtaining all-electron results from a
pseudopotential calculation. This is achieved by carrying out a localised
calculation in the region of an atomic nucleus using the embedding potential
method of Inglesfield [J.Phys. C {\bf 14}, 3795 (1981)]. In this method the
core region is \emph{reconstructed}, and none of the simplifying approximations
(such as spherical symmetry of the charge density/potential or frozen core
electrons) that previous solutions to this problem have required are made. The
embedding method requires an accurate real space Green function, and an
analysis of the errors introduced in constructing this from a set of numerical
eigenstates is given. Results are presented for an all-electron reconstruction
of bulk aluminium, for both the charge density and the density of states.Comment: 14 pages, 5 figure
First-Principles Calculations of Hyperfine Interactions in La_2CuO_4
We present the results of first-principles cluster calculations of the
electronic structure of La_2CuO_4. Several clusters containing up to nine
copper atoms embedded in a background potential were investigated.
Spin-polarized calculations were performed both at the Hartree-Fock level and
with density functional methods with generalized gradient corrections to the
local density approximation. The distinct results for the electronic structure
obtained with these two methods are discussed. The dependence of the
electric-field gradients at the Cu and the O sites on the cluster size is
studied and the results are compared to experiments. The magnetic hyperfine
coupling parameters are carefully examined. Special attention is given to a
quantitative determination of on-site and transferred hyperfine fields. We
provide a detailed analysis that compares the hyperfine fields obtained for
various cluster sizes with results from additional calculations of spin states
with different multiplicities. From this we conclude that hyperfine couplings
are mainly transferred from nearest neighbor Cu^{2+} ions and that
contributions from further distant neighbors are marginal. The mechanisms
giving rise to transfer of spin density are worked out. Assuming conventional
values for the spin-orbit coupling, the total calculated hyperfine interaction
parameters are compared to informations from experiments.Comment: 23 pages, 9 figure
On the constitution of sodium at higher densities
Using density functional theory the atomic and electronic structure of sodium
are predicted to depart substantially from those expected of simple metals for
GPa). Newly-predicted phases include those with low
structural symmetry, semi-metallic electronic properties (including zero-gap
semiconducting limiting behavior), unconventional valence charge density
distributions, and even those that raise the possibility of superconductivity,
all at currently achievable pressures. Important differences emerge between
sodium and lithium at high densities, and these are attributable to
corresponding differences in their respective cores.Comment: 13 pages; 3 figure
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