907 research outputs found

### Connection between charge transfer and alloying core-level shifts based on density-functional calculations

The measurement of alloying core-level binding energy (CLBE) shifts has been
used to give a precise meaning to the fundamental concept of charge transfer.
Here, ab-initio density-functional calculations for the intermetallic compound
MgAu are used to investigate models which try to make a connection between the
core levels shifts and charge transfer. The calculated CLBE shifts agree well
with experiment, and permit an unambiguous separation into initial-state and
screening contributions. Interestingly, the screening contribution is large and
cannot be neglected in any reasonable description. Comparison of the calculated
results with the predictions of simple models show that these models are not
adequate to describe the realistic situation. On the positive side, the
accuracy of the density-functional calculations indicates that the combination
of experiments with such calculations is a powerful tool to investigate unknown
systems.Comment: RevTeX 10 pages incl 8 figure

### Reconstruction Mechanism of FCC Transition-Metal (001) Surfaces

The reconstruction mechanism of (001) fcc transition metal surfaces is
investigated using a full-potential all-electron electronic structure method
within density-functional theory. Total-energy supercell calculations confirm
the experimental finding that a close-packed quasi-hexagonal overlayer
reconstruction is possible for the late 5$d$-metals Ir, Pt, and Au, while it is
disfavoured in the isovalent 4$d$ metals (Rh, Pd, Ag). The reconstructive
behaviour is driven by the tensile surface stress of the unreconstructed
surfaces; the stress is significantly larger in the 5$d$ metals than in 4$d$
ones, and only in the former case it overcomes the substrate resistance to the
required geometric rearrangement. It is shown that the surface stress for these
systems is due to $d$ charge depletion from the surface layer, and that the
cause of the 4th-to-5th row stress difference is the importance of relativistic
effects in the 5$d$ series.Comment: RevTeX 3.0, 12 pages, 1 PostScript figure available upon request] 23
May 199

### Many-body effects in iron pnictides and chalcogenides -- non-local vs dynamic origin of effective masses

We apply the quasi-particle self-consistent GW (QSGW) approximation to some
of the iron pnictide and chalcogenide superconductors. We compute Fermi
surfaces and density of states, and find excellent agreement with experiment,
substantially improving over standard band-structure methods. Analyzing the
QSGW self-energy we discuss non-local and dynamic contributions to effective
masses. We present evidence that the two contributions are mostly separable,
since the quasi-particle weight is found to be essentially independent of
momentum. The main effect of non locality is captured by the static but
non-local QSGW effective potential. Moreover, these non-local self-energy
corrections, absent in e.g. dynamical mean field theory (DMFT), can be
relatively large. We show, on the other hand, that QSGW only partially accounts
for dynamic renormalizations at low energies. These findings suggest that QSGW
combined with DMFT will capture most of the many-body physics in the iron
pnictides and chalcogenides.Comment: 4+ pages, 3 figure

### Theory of Adsorption and Surfactant Effect of Sb on Ag (111)

We present first-principles studies of the adsorption of Sb and Ag on clean
and Sb-covered Ag (111). For Sb, the {\it substitutional} adsorption site is
found to be greatly favored with respect to on-surface fcc sites and to
subsurface sites, so that a segregating surface alloy layer is formed. Adsorbed
silver adatoms are more strongly bound on clean Ag(111) than on Sb-covered Ag.
We propose that the experimentally reported surfactant effect of Sb is due to
Sb adsorbates reducing the Ag adatom mobility. This gives rise to a high
density of Ag islands which coalesce into regular layers.Comment: RevTeX 3.0, 11 pages, 0 figures] 13 July 199

### Extracting convergent surface energies from slab calculations

The formation energy of a solid surface can be extracted from slab
calculations if the bulk energy per atom is known. It has been pointed out
previously that the resulting surface energy will diverge with slab thickness
if the bulk energy is in error, in the context of calculations which used
different methods to study the bulk and slab systems. We show here that this
result is equally relevant for state-of-the-art computational methods which
carefully treat bulk and slab systems in the same way. Here we compare
different approaches, and present a solution to the problem that eliminates the
divergence and leads to rapidly convergent and accurate surface energies.Comment: 3 revtex pages, 1 figure, in print on J. Phys. Cond. Mat

### GW correlation effects on plutonium quasiparticle energies: changes in crystal-field splitting

We present results for the electronic structure of plutonium by using a
recently developed quasiparticle self-consistent $GW$ method (\qsgw). We
consider a paramagnetic solution without spin-orbit interaction as a function
of volume for the face-centered cubic (fcc) unit cell. We span unit-cell
volumes ranging from 10% greater than the equilibrium volume of the $\delta$
phase to 90 % of the equivalent for the $\alpha$ phase of Pu. The
self-consistent $GW$ quasiparticle energies are compared to those obtained
within the Local Density Approximation (LDA). The goal of the calculations is
to understand systematic trends in the effects of electronic correlations on
the quasiparticle energy bands of Pu as a function of the localization of the
$f$ orbitals. We show that correlation effects narrow the $f$ bands in two
significantly different ways. Besides the expected narrowing of individual $f$
bands (flatter dispersion), we find that an even more significant effect on the
$f$ bands is a decrease in the crystal-field splitting of the different bands.Comment: 9 pages, 7 figures, 3 table

### Ab-initio procedure for effective models of correlated materials with entangled band structure

We present a first-principles method for deriving effective low-energy models
of electrons in solids having entangled band structure. The procedure starts
with dividing the Hilbert space into two subspaces, the low-energy part ("$d$
space'') and the rest of the space ("$r$ space''). The low-energy model is
constructed for the $d$ space by eliminating the degrees of freedom of the $r$
space. The thus derived model contains the strength of electron correlation
expressed by a partially screened Coulomb interaction, calculated in the
constrained random-phase-approximation (cRPA) where screening channels within
the $d$ space, $P_d$, are subtracted. One conceptual problem of this
established downfolding method is that for entangled bands it is not clear how
to cut out the $d$ space and how to distinguish $P_d$ from the total
polarization. Here, we propose a simple procedure to overcome this difficulty.
In our scheme, the $d$ subspace is cut out from the Hilbert space of the Kohn
Sham eigenfunctions with the help of a procedure to construct a localized
Wannier basis. The $r$ subspace is constructed as the complementary space
orthogonal to the $d$ subspace. After this disentanglement, $P_d$ becomes well
defined. Using the disentangled bands, the effective parameters are uniquely
determined in the cRPA. The method is successfully applied to 3$d$ transition
metals.Comment: 14 pages, 4 figure

### Charge redistribution at Pd surfaces: ab initio grounds for tight-binding interatomic potentials

A simplified tight-binding description of the electronic structure is often
necessary for complex studies of surfaces of transition metal compounds. This
requires a self-consistent parametrization of the charge redistribution, which
is not obvious for late transition series elements (such as Pd, Cu, Au), for
which not only d but also s-p electrons have to be taken into account. We show
here, with the help of an ab initio FP-LMTO approach, that for these elements
the electronic charge is unchanged from bulk to the surface, not only per site
but also per orbital. This implies different level shifts for each orbital in
order to achieve this orbital neutrality rule. Our results invalidate any
neutrality rule which would allow charge redistribution between orbitals to
ensure a common rigid shift for all of them. Moreover, in the case of Pd, the
power law which governs the variation of band energy with respect to
coordination number, is found to differ significantly from the usual
tight-binding square root.Comment: 6 pages, 2 figures, Latex; Phys.Rev. B 56 (1997

### Screened Coulomb interaction in the maximally localized Wannier basis

We discuss a maximally localized Wannier function approach for constructing
lattice models from first-principles electronic structure calculations, where
the effective Coulomb interactions are calculated in the constrained
random-phase-approximation. The method is applied to the 3d transition metals
and a perovskite (SrVO_3). We also optimize the Wannier functions by unitary
transformation so that U is maximized. Such Wannier functions unexpectedly
turned out to be very close to the maximally localized ones.Comment: 22 pages, 6 figure

### Realistic many-body models for Manganese Monoxide under pressure

In materials like transition metals oxides where electronic Coulomb
correlations impede a description in terms of standard band-theories, the
application of genuine many-body techniques is inevitable. Interfacing the
realism of density-functional based methods with the virtues of Hubbard-like
Hamiltonians, requires the joint ab initio construction of transfer integrals
and interaction matrix elements (like the Hubbard U) in a localized basis set.
In this work, we employ the scheme of maximally localized Wannier functions and
the constrained random phase approximation to create effective low-energy
models for Manganese monoxide, and track their evolution under external
pressure. We find that in the low pressure antiferromagnetic phase, the
compression results in an increase of the bare Coulomb interaction for specific
orbitals. As we rationalized in recent model considerations [Phys. Rev. B 79,
235133 (2009)], this seemingly counter-intuitive behavior is a consequence of
the delocalization of the respective Wannier functions. The change of screening
processes does not alter this tendency, and thus, the screened on-site
component of the interaction - the Hubbard U of the effective low-energy system
- increases with pressure as well. The orbital anisotropy of the effects
originates from the orientation of the orbitals vis-a-vis the deformation of
the unit-cell. Within the high pressure paramagnetic phase, on the other hand,
we find the significant increase of the Hubbard U is insensitive to the orbital
orientation and almost exclusively owing to a substantial weakening of
screening channels upon compression.Comment: 13 pages, 6 figure

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