1,206 research outputs found
Transition state method and Wannier functions
We propose a computational scheme for materials where standard Local Density
Approximation (LDA) fails to produce a satisfactory description of excitation
energies. The method uses Slater's "transition state" approximation and Wannier
functions basis set. We define a correction to LDA functional in such a way
that its variation produces one-electron energies for Wannier functions equal
to the energies obtained in "transition state" constrained LDA calculations. In
the result eigenvalues of the proposed functional could be interpreted as
excitation energies of the system under consideration. The method was applied
to MgO, Si, NiO and BaBiO and gave an improved agreement with experimental
data of energy gap values comparing with LDA.Comment: 13 pages, 6 figures, 1 tabl
Geometric phases and Wannier functions of Bloch electrons in 1-dimension
We present a formal expression for Wannier functions of composite bands of
1-D Bloch electrons in terms of parallel-transported Bloch functions and their
non-Abelian geometric phases. Spatial decay properties of these Wannier
functions are studied in the case of simple bands of 1-D model insulator and
metal. Within first-principles density functional theory, we illustrate the
formalism through the construction of Wannier functions of polyethylene and
polyacetylene.Comment: 4 pages, 4 figure
Dust grain resonant capture: A statistical study
A statistical approach, based on a large number of simultaneous numerical integrations, is adopted to study the capture in external mean motion resonances with the Earth of micron size dust grains perturbed by solar radiation and wind forces. We explore the dependence of the resonant capture phenomenon on the initial eccentricity e(sub 0) and perihelion argument w(sub 0) of the dust particle orbit. The intensity of both the resonant and dissipative (Poynting-Robertson and wind drag) perturbations strongly depends on the eccentricity of the particle while the perihelion argument determines, for low inclination, the mutual geometrical configuration of the particle's orbit with respect to the Earth's orbit. We present results for three j:j+1 commensurabilities (2:3, 4:5 and 6:7) and also for particle sizes s = 15, 30 microns. This study extends our previous work on the long term orbital evolution of single dust particles trapped into resonances with the Earth
Hydrogen Dynamics in Superprotonic CsHSO4
We present a detailed study of proton dynamics in the hydrogen-bonded
superprotonic conductor CsHSO4 from first-principles molecular dynamics
simulations, isolating the subtle interplay between the dynamics of the O--H
chemical bonds, the O...H hydrogen bonds, and the SO4 tetrahedra in promoting
proton diffusion. We find that the Grotthus mechanism of proton transport is
primarily responsible for the dynamics of the chemical bonds, whereas the
reorganization of the hydrogen-bond network is dominated by rapid angular hops
in concert with small reorientations of the SO4 tetrahedra. Frequent proton
jumping across the O--H...O complex is countered by a high rate of jump
reversal, which we show is connected to the dynamics of the SO4 tetrahedra,
resulting in a diminished CsHSO4/CsDSO4 isotope effect. We also find evidence
of multiple timescales for SO4 reorientation events, leading to distinct
diffusion mechanisms along the different crystal lattice directions. Finally,
we employ graph-theoretic techniques to characterize the topology of the
hydrogen-bond network and demonstrate a clear relationship between certain
connectivity configurations and the likelihood for diffusive jump events.Comment: 12 pages, 10 figure
An anomalous alloy: Y_x Si_{1-x}
We study via density functional-based molecular dynamics the structural and
dynamical properties of the rare earth silicon amorphous alloy Y_xSi_{1-x} for
x=0.093 and x=0.156. The Si network forms cavities in which a Y^{3+} cation is
entrapped. Its electrons are transferred to the Si network and are located in
the dangling bonds of the Si atoms that line the Y cavities. This leads to the
presence of low coordinated Si atoms that can be described as monovalent or
divalent anions. For x=0.156, the cavities touch each other and share Si atoms
that have two dangling bonds. The vibrational spectrum is similar to that of
amorphous Si. However, doping induces a shoulder at 70 cm^{-1} and a pronounced
peak at 180 cm^{-1} due to low coordinated Si.Comment: 4 pages, 4 figure
Orbital densities functional
Local density approximation (LDA) to the density functional theory (DFT) has
continuous derivative of total energy as a number of electrons function and
continuous exchange-correlation potential, while in exact DFT both should be
discontinuous as number of electrons goes through an integer value. We propose
orbital densities functional (ODF) (with orbitals defined as Wannier functions)
that by construction obeys this discontinuity condition. By its variation
one-electron equations are obtained with potential in the form of projection
operator. The operator increases a separation between occupied and empty bands
thus curing LDA deficiency of energy gap value systematic underestimation.
Orbital densities functional minimization gives ground state orbital and total
electron densities. The ODF expression for the energy of orbital densities
fluctuations around the ground state values defines ODF fluctuation Hamiltonian
that allows to treat correlation effects. Dynamical mean-field theory (DMFT)
was used to solve this Hamiltonian with quantum Monte Carlo (QMC) method for
effective impurity problem. We have applied ODF method to the problem of
metal-insulator transition in lanthanum trihydride LaH_{3-x}. In LDA
calculations ground state of this material is metallic for all values of
hydrogen nonstoichiometry x while experimentally the system is insulating for x
< 0.3. ODF method gave paramagnetic insulator solution for LaH_3 and LaH_{2.75}
but metallic state for LaH_{2.5}.Comment: 35 pages, 5 figure
Calculation of electron density of periodic systems using non-orthogonal localised orbitals
Methods for calculating an electron density of a periodic crystal constructed
using non-orthogonal localised orbitals are discussed. We demonstrate that an
existing method based on the matrix expansion of the inverse of the overlap
matrix into a power series can only be used when the orbitals are highly
localised (e.g. ionic systems). In other cases including covalent crystals or
those with an intermediate type of chemical bonding this method may be either
numerically inefficient or fail altogether. Instead, we suggest an exact and
numerically efficient method which can be used for orbitals of practically
arbitrary localisation. Theory is illustrated by numerical calculations on a
model system.Comment: 12 pages, 4 figure
Alkali suppression within laser ion-source cavities and time structure of the laser ionized ion-bunches
The chemical selectivity of the target and ion-source production system is an asset for Radioactive Ion-Beam (RIB) facilities equipped with mass separators. Ionization via laser induced multiple resonant steps Ionization has such selectivity. However, the selectivity of the ISOLDE Resonant Ionization Laser Ion-Source (RILIS), where ionization takes place within high temperature refractory metal cavities, suffers from unwanted surface ionization of low ionization potential alkalis. In order to reduce this type of isobaric contaminant, surface ionization within the target vessel was used. On-line measurements of the efficiency of this method is reported, suppression factors of alkalis up to an order of magnitude were measured as a function of their ionization potential. The time distribution of the ion bunches produced with the RILIS was measured for a variety of elements and high temperature cavity materials. While all ions are produced within a few nanoseconds, the ion bunch sometimes spreads over more than 100 ms. This demonstrates that ions are confined within high temperature metallic cavities. It is the internal electrical field of these cavities that causes the ions to drifts to the extraction region and defines the dwell time of the ions in the cavity. Beam optics calculations were carried out to simulate the pulse shape of a RILIS ion bunch and are compared to the actual measurements
Full orbital calculation scheme for materials with strongly correlated electrons
We propose a computational scheme for the ab initio calculation of Wannier
functions (WFs) for correlated electronic materials. The full-orbital
Hamiltonian H is projected into the WF subspace defined by the physically most
relevant partially filled bands. The Hamiltonian H^{WF} obtained in this way,
with interaction parameters calculated by constrained LDA for the Wannier
orbitals, is used as an ab initio setup of the correlation problem, which can
then be solved by many-body techniques, e.g., dynamical mean-field theory
(DMFT). In such calculations the self-energy operator \Sigma(e) is defined in
WF basis which then can be converted back into the full-orbital Hilbert space
to compute the full-orbital interacting Green function G(r,r',e). Using
G(r,r',e) one can evaluate the charge density, modified by correlations,
together with a new set of WFs, thus defining a fully self-consistent scheme.
The Green function can also be used for the calculation of spectral, magnetic
and electronic properties of the system. Here we report the results obtained
with this method for SrVO3 and V2O3. Comparisons are made with previous results
obtained by the LDA+DMFT approach where the LDA DOS was used as input, and with
new bulk-sensitive experimental spectra.Comment: 36 pages, 14 figure
Maximally-localized Wannier Functions in Antiferromagnetic MnO within the FLAPW Formalism
We have calculated the maximally-localized Wannier functions of MnO in its
antiferromagnetic (AFM) rhombohedral unit cell, which contains two formula
units. Electron Bloch functions are obtained with the linearized augmented
plane-wave method within both the LSD and the LSD+U schemes. The thirteen
uppermost occupied spin-up bands correspond in a pure ionic scheme to the five
Mn 3d orbitals at the Mn_1 (spin-up) site, and the four O 2s/2p orbitals at
each of the O_1 and O_2 sites. Maximal localization identifies uniquely four
Wannier functions for each O, which are trigonally-distorted sp^3-like
orbitals. They display a weak covalent bonding between O 2s/2p states and
minority-spin d states of Mn_2, which is absent in a fully ionic picture. This
bonding is the fingerprint of the interaction responsible for the AFM ordering,
and its strength depends on the one-electron scheme being used. The five Mn
Wannier functions are centered on the Mn_1 site, and are atomic orbitals
modified by the crystal field. They are not uniquely defined by the criterion
of maximal localization and we choose them as the linear combinations which
diagonalize the r^2 operator, so that they display the D_3d symmetry of the
Mn_1 site.Comment: 11 pages, 6 PostScript figures. Uses Revtex4. Hi-res figures
available from the author
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