658 research outputs found
Maximally localized Wannier functions in LaMnO3 within PBE+U, hybrid functionals, and partially self-consistent GW: an efficient route to construct ab-initio tight-binding parameters for e_g perovskites
Using the newly developed VASP2WANNIER90 interface we have constructed
maximally localized Wannier functions (MLWFs) for the e_g states of the
prototypical Jahn-Teller magnetic perovskite LaMnO3 at different levels of
approximation for the exchange-correlation kernel. These include conventional
density functional theory (DFT) with and without additional on-site Hubbard U
term, hybrid-DFT, and partially self-consistent GW. By suitably mapping the
MLWFs onto an effective e_g tight-binding (TB) Hamiltonian we have computed a
complete set of TB parameters which should serve as guidance for more elaborate
treatments of correlation effects in effective Hamiltonian-based approaches.
The method-dependent changes of the calculated TB parameters and their
interplay with the electron-electron (el-el) interaction term are discussed and
interpreted. We discuss two alternative model parameterizations: one in which
the effects of the el-el interaction are implicitly incorporated in the
otherwise "noninteracting" TB parameters, and a second where we include an
explicit mean-field el-el interaction term in the TB Hamiltonian. Both models
yield a set of tabulated TB parameters which provide the band dispersion in
excellent agreement with the underlying ab initio and MLWF bands.Comment: 30 pages, 7 figure
On the dynamics of Extrasolar Planetary Systems under dissipation. Migration of planets
We study the dynamics of planetary systems with two planets moving in the
same plane, when frictional forces act on the two planets, in addition to the
gravitational forces. The model of the general three-body problem is used.
Different laws of friction are considered. The topology of the phase space is
essential in understanding the evolution of the system. The topology is
determined by the families of stable and unstable periodic orbits, both
symmetric and non symmetric. It is along the stable families, or close to them,
that the planets migrate when dissipative forces act. At the critical points
where the stability along the family changes, there is a bifurcation of a new
family of stable periodic orbits and the migration process changes route and
follows the new stable family up to large eccentricities or to a chaotic
region. We consider both resonant and non resonant planetary systems. The 2/1,
3/1 and 3/2 resonances are studied. The migration to larger or smaller
eccentricities depends on the particular law of friction. Also, in some cases
the semimajor axes increase and in other cases they are stabilized. For
particular laws of friction and for special values of the parameters of the
frictional forces, it is possible to have partially stationary solutions, where
the eccentricities and the semimajor axes are fixed.Comment: Accepted in Celestial Mechanics and Dynamical Astronom
Ab initio Derivation of Low-energy Model for Iron-Based Superconductors LaFeAsO and LaFePO
Effective Hamiltonians for LaFeAsO and LaFePO are derived from the
downfolding scheme based on first-principles calculations and provide insights
for newly discovered superconductivity in the family of LnFeAsOF,
Ln = La, Ce, Pr, Nd, Sm, and Gd. Extended Hubbard Hamiltonians for five
maximally localized Wannier orbitals per Fe are constructed dominantly from
five-fold degenerate iron-3 bands. They contain parameters for effective
Coulomb and exchange interactions screened by the polarization of other
electrons away from the Fermi level. The onsite Coulomb interaction estimated
as 2.2-3.3 eV is compared with the transfer integrals between the
nearest-neighbor Fe-3 Wannier orbitals, 0.2-0.3 eV, indicating moderately
strong electron correlation. The Hund's rule coupling is found to be 0.3-0.6
eV. The derived model offers a firm basis for further studies on physics of
this family of materials. The effective models for As and P compounds turn out
to have very similar screened interactions with slightly narrower bandwidth for
the As compound.Comment: 5 pages, 3 figures, 1 table; to appear in J. Phys. Soc. Jpn. Vol. 77
No.9: Revised version contains corrected table values and discussions of
quantitative accuracy of constrained random-phase approximatio
The GAPS Programme with HARPS-N at TNG. X. Differential abundances in the XO-2 planet hosting binary
Binary stars hosting exoplanets are a unique laboratory where chemical
tagging can be performed to measure with high accuracy the elemental abundances
of both stellar components, with the aim to investigate the formation of
planets and their subsequent evolution. Here, we present a high-precision
differential abundance analysis of the XO-2 wide stellar binary based on high
resolution HARPS-N@TNG spectra. Both components are very similar K-dwarfs and
host planets. Since they formed presumably within the same molecular cloud, we
expect they should possess the same initial elemental abundances. We
investigate if the presence of planets can cause some chemical imprints in the
stellar atmospheric abundances. We measure abundances of 25 elements for both
stars with a range of condensation temperature K, achieving
typical precisions of dex. The North component shows abundances in
all elements higher by dex on average, with a mean
difference of +0.078 dex for elements with K. The
significance of the XO-2N abundance difference relative to XO-2S is at the
level for almost all elements. We discuss the possibility that this
result could be interpreted as the signature of the ingestion of material by
XO-2N or depletion in XO-2S due to locking of heavy elements by the planetary
companions. We estimate a mass of several tens of in heavy
elements. The difference in abundances between XO-2N and XO-2S shows a positive
correlation with the condensation temperatures of the elements, with a slope of
dex K, which could mean that both
components have not formed terrestrial planets, but that first experienced the
accretion of rocky core interior to the subsequent giant planets.Comment: 10 pages, 5 figures, accepted by Astronomy & Astrophysics. Numbering
of the series change
Non-linear optical susceptibilities, Raman efficiencies and electrooptic tensors from first-principles density functional perturbation theory
The non-linear response of infinite periodic solids to homogenous electric
fields and collective atomic displacements is discussed in the framework of
density functional perturbation theory. The approach is based on the 2n + 1
theorem applied to an electric-field-dependent energy functional. We report the
expressions for the calculation of the non-linear optical susceptibilities,
Raman scattering efficiencies and electrooptic coefficients. Different
formulations of third-order energy derivatives are examined and their
convergence with respect to the k-point sampling is discussed. We apply our
method to a few simple cases and compare our results to those obtained with
distinct techniques. Finally, we discuss the effect of a scissors correction on
the EO coefficients and non-linear optical susceptibilities
Charging Induced Emission of Neutral Atoms from NaCl Nanocube Corners
Detachment of neutral cations/anions from solid alkali halides can in
principle be provoked by donating/subtracting electrons to the surface of
alkali halide crystals, but generally constitutes a very endothermic process.
However, the amount of energy required for emission is smaller for atoms
located in less favorable positions, such as surface steps and kinks. For a
corner ion in an alkali halide cube the binding is the weakest, so it should be
easier to remove that atom, once it is neutralized. We carried out first
principles density functional calculations and simulations of neutral and
charged NaCl nanocubes, to establish the energetics of extraction of
neutralized corner ions. Following hole donation (electron removal) we find
that detachment of neutral Cl corner atoms will require a limited energy of
about 0.8 eV. Conversely, following the donation of an excess electron to the
cube, a neutral Na atom is extractable from the corner at the lower cost of
about 0.6 eV. Since the cube electron affinity level (close to that a NaCl(100)
surface state, which we also determine) is estimated to lie about 1.8 eV below
vacuum, the overall energy balance upon donation to the nanocube of a zero
energy electron from vacuum will be exothermic. The atomic and electronic
structure of the NaCl(100) surface, and of the nanocube Na and Cl corner
vacancies are obtained and analyzed as a byproduct.Comment: 16 pages, 2 table, 7 figure
Low thrust propulsion in a coplanar circular restricted four body problem
This paper formulates a circular restricted four body problem (CRFBP), where the three primaries are set in the stable Lagrangian equilateral triangle configuration and the fourth body is massless. The analysis of this autonomous coplanar CRFBP is undertaken, which identies eight natural equilibria; four of which are close to the smaller body, two stable and two unstable, when considering the primaries to be the Sun and two smaller bodies of the solar system. Following this, the model incorporates `near term' low-thrust propulsion capabilities to generate surfaces of articial equilibrium points close to the smaller primary, both in and out of the plane containing the celestial bodies. A stability analysis of these points is carried out and a stable subset of them is identied. Throughout the analysis the Sun-Jupiter-Asteroid-Spacecraft system is used, for conceivable masses of a hypothetical asteroid set at the libration point L4. It is shown that eight bounded orbits exist, which can be maintained with a constant thrust less than 1:5 10􀀀4N for a 1000kg spacecraft. This illustrates that, by exploiting low-thrust technologies, it would be possible to maintain an observation point more than 66% closer to the asteroid than that of a stable natural equilibrium point. The analysis then focusses on a major Jupiter Trojan: the 624-Hektor asteroid. The thrust required to enable close asteroid observation is determined in the simplied CRFBP model. Finally, a numerical simulation of the real Sun-Jupiter-624 Hektor-Spacecraft is undertaken, which tests the validity of the stability analysis of the simplied model
Combined approach of density functional theory and quantum Monte Carlo method to electron correlation in dilute magnetic semiconductors
We present a realistic study for electronic and magnetic properties in dilute
magnetic semiconductor (Ga,Mn)As. A multi-orbital Haldane-Anderson model
parameterized by density-functional calculations is presented and solved with
the Hirsch-Fye quantum Monte Carlo algorithm. Results well reproduce
experimental results in the dilute limit. When the chemical potential is
located between the top of the valence band and an impurity bound state, a
long-range ferromagnetic correlations between the impurities, mediated by
antiferromagnetic impurity-host couplings, are drastically developed. We
observe an anisotropic character in local density of states at the
impurity-bound-state energy, which is consistent with the STM measurements. The
presented combined approach thus offers a firm starting point for realistic
calculations of the various family of dilute magnetic semiconductors.Comment: 5 pages, 4 figure
Coulomb Parameter U and Correlation Strength in LaFeAsO
First principles constrained density functional theory scheme in Wannier
functions formalism has been used to calculate Coulomb repulsion U and Hund's
exchange J parameters for iron 3d electrons in LaFeAsO. Results strongly depend
on the basis set used in calculations: when O-2p, As-4p, and Fe-3d orbitals and
corresponding bands are included, computation results in U=3-4 eV, however,
with the basis set restricted to Fe-3d orbitals and bands only, computation
gives parameters corresponding to F^0=0.8 eV, J=0.5 eV. LDA+DMFT (the Local
Density Approximation combined with the Dynamical Mean-Field Theory)
calculation with this parameters results in weakly correlated electronic
structure that is in agreement with X-ray experimental spectra
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