11,904 research outputs found
Two Avenues to Self-Interaction Correction within Kohn-Sham Theory: Unitary Invariance is the Shortcut
The most widely-used density functionals for the exchange-correlation energy
are inexact for one-electron systems. Their self-interaction errors can be
severe in some applications. The problem is not only to correct the
self-interaction error, but to do so in a way that will not violate
size-consistency and will not go outside the standard Kohn-Sham density
functional theory. The solution via the optimized effective potential (OEP)
method will be discussed, first for the Perdew-Zunger self-interaction
correction (whose performance for molecules is briefly summarized) and then for
the more modern self-interaction corrections based upon unitarily-invariant
indicators of iso-orbital regions. For the latter approaches, the OEP
construction is greatly simplified. The kinetic-energy-based iso-orbital
indicator \tau^W_\sigma(\re)/\tau_\sigma(\re) will be discussed and plotted,
along with an alternative exchange-based indicator
The 5f localization/delocalization in square and hexagonal americium monolayers: A FP-LAPW electronic structure study
The electronic and geometrical properties of bulk americium and square and
hexagonal americium monolayers have been studied with the full-potential
linearized augmented plane wave (FP-LAPW) method. The effects of several common
approximations are examined: (1) non-spin polarization (NSP) vs. spin
polarization (SP); (2) scalar-relativity (no spin-orbit coupling (NSO)) vs.
full-relativity (i.e., with spin-orbit (SO) coupling included); (3)
local-density approximation (LDA) vs. generalized-gradient approximation (GGA).
Our results indicate that both spin polarization and spin orbit coupling play
important roles in determining the geometrical and electronic properties of
americium bulk and monolayers. A compression of both americium square and
hexagonal monolayers compared to the americium bulk is also observed. In
general, the LDA is found to underestimate the equilibrium lattice constant and
give a larger total energy compared to the GGA calculations. While spin orbit
coupling shows a similar effect on both square and hexagonal monolayer
calculations regardless of the model, GGA versus LDA, an unusual spin
polarization effect on both square and hexagonal monolayers is found in the LDA
results as compared with the GGA results. The 5f delocalization transition of
americium is employed to explain our observed unusual spin polarization effect.
In addition, our results at the LDA level of theory indicate a possible 5f
delocalization could happen in the americium surface within the same Am II (fcc
crystal structure) phase, unlike the usually reported americium 5f
delocalization which is associated with crystal structure change. The
similarities and dissimilarities between the properties of an Am monolayer and
a Pu monolayer are discussed in detail.Comment: 22 pages, 8 figure
Electronic Structure of Sodium Cobalt Oxide: Comparing Mono- and Bilayer-hydrate
To shed new light on the mechanism of superconductivity in sodium cobalt
oxide bilayer-hydrate (BLH), we perform a density functional calculation with
full structure optimization for BLH and its related nonsuperconducting phase,
monolayer hydrate (MLH). We find that these hydrates have similar band
structures, but a notable difference can be seen in the band around
the Fermi level. While its dispersion in the direction is negligibly small
for BLH, it is of the order of 0.1 eV for MLH. This result implies that the
three dimensional feature of the band may be the origin for the
absence of superconductivity in MLH.Comment: 5 pages, 7 figures, to be published in Phys. Rev.
Calculated elastic and electronic properties of MgB at high pressures
The effect of high pressure on structural and electronic properties of the
novel superconductor \MB has been calculated using the full-potential
linearized augmented-plane-wave method. Despite the layered crystal structure
of \MB nearly isotropic compression (bulk modulus GPa) is found
with only a 1.2% decrease of the ratio at 10 GPa. The effect of pressure
on the critical temperature has been estimated on the basis of BCS theory and
good agreement with experimental data is found. Our results suggest that it is
a combination of increasing phonon frequency and decreasing electronic density
of states at the Fermi level which leads to the observed decrease of the
critical temperature under pressure.Comment: 10 pages, 3 figures (EPS), Elsevier LaTeX. More detailed analysis of
the pressure dependence of Tc; results unchanged. Manuscript accepted for
publication in Solid State Commu
Describing static correlation in bond dissociation by Kohn-Sham density functional theory
We show that density functional theory within the RPA (random phase
approximation for the exchange-correlation energy) provides a correct
description of bond dissociation in H in a spin-restricted Kohn-Sham
formalism, i.e. without artificial symmetry breaking. We present accurate
adiabatic connection curves both at equilibrium and beyond the Coulson-Fisher
point. The strong curvature at large bond length implies important static
(left-right) correlation, justifying modern hybrid functional constructions but
also demonstrating their limitations. Although exact at infinite and accurate
around the equilibrium bond length, the RPA dissociation curve displays
unphysical repulsion at larger but finite bond lengths. Going beyond the RPA by
including the exact exchange kernel (RPA+X), we find a similar repulsion. We
argue that this deficiency is due to the absence of double excitations in
adiabatic linear response theory. Further analyzing the H dissociation
limit we show that the RPA+X is not size-consistent, in contrast to the RPA.Comment: 15 pages, 5 figure
{\it Ab initio} calculations of magnetic structure and lattice dynamics of Fe/Pt multilayers
The magnetization distribution, its energetic characterization by the
interlayer coupling constants and lattice dynamics of (001)-oriented Fe/Pt
multilayers are investigated using density functional theory combined with the
direct method to determine phonon frequencies. It is found that ferromagnetic
order between consecutive Fe layers is favoured, with the enhanced magnetic
moments at the interface. The bilinear and biquadratic coupling coefficients
between Fe layers are shown to saturate fast with increasing thickness of
nonmagnetic Pt layers which separate them. The phonon calculations demonstrate
a rather strong dependence of partial iron phonon densities of states on the
actual position of Fe monolayer in the multilayer structure.Comment: 7 pages, 8 figure
Electronic states and pairing symmetry in the two-dimensional 16 band d-p model for iron-based superconductor
The electronic states of the FeAs plane in iron-based superconductors are
investigated on the basis of the two-dimensional 16-band d-p model, where the
tight-binding parameters are determined so as to fit the band structure
obtained by the density functional calculation for LaFeAsO. The model includes
the Coulomb interaction on a Fe site: the intra- and inter-orbital direct terms
U and U', the exchange coupling J and the pair-transfer J'. Within the random
phase approximation (RPA), we discuss the pairing symmetry of possible
superconducting states including s-wave and d-wave pairing on the U'-J plane.Comment: 2 pages, 4 figures; Proceedings of the Int. Symposium on
Fe-Oxipnictide Superconductors (Tokyo, 28-29th June 2008
Intrinsic hole localization mechanism in magnetic semiconductors
The interplay between clustering and exchange coupling in magnetic
semiconductors for the prototype (Ga_{1-x},Mn_x)As with manganese
concentrations x of 1/16 and 1/32 in the interesting experimental range is
investigated. For x ~ 6 %, when all possible arrangements of two atoms within a
large supercell are considered, the clustering of Mn atoms at nearest-neighbour
Ga sites is energetically preferred. As shown by spin density analysis, this
minimum energy configuration localizes further one hole and reduces the
effective charge carrier concentration. Also the exchange coupling constant
increases to a value corresponding to lower Mn concentrations with decreasing
inter Mn distance.Comment: Accepted for publication in Journal of Physics: Condensed Matte
Physical signatures of discontinuities of the time-dependent exchange-correlation potential
The exact exchange-correlation (XC) potential in time-dependent
density-functional theory (TDDFT) is known to develop steps and discontinuities
upon change of the particle number in spatially confined regions or isolated
subsystems. We demonstrate that the self-interaction corrected adiabatic
local-density approximation for the XC potential has this property, using the
example of electron loss of a model quantum well system. We then study the
influence of the XC potential discontinuity in a real-time simulation of a
dissociation process of an asymmetric double quantum well system, and show that
it dramatically affects the population of the resulting isolated single quantum
wells. This indicates the importance of a proper account of the discontinuities
in TDDFT descriptions of ionization, dissociation or charge transfer processes.Comment: 17 pages, 6 figure
Spin Resolution of the Electron-Gas Correlation Energy: Positive same-spin contribution
The negative correlation energy per particle of a uniform electron gas of
density parameter and spin polarization is well known, but its
spin resolution into up-down, up-up, and down-down contributions is not.
Widely-used estimates are incorrect, and hamper the development of reliable
density functionals and pair distribution functions. For the spin resolution,
we present interpolations between high- and low-density limits that agree with
available Quantum Monte Carlo data. In the low-density limit for ,
we find that the same-spin correlation energy is unexpectedly positive, and we
explain why. We also estimate the up and down contributions to the kinetic
energy of correlation.Comment: new version, to appear in PRB Rapid Communicatio
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