568 research outputs found
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
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
Inhibited Al diffusion and growth roughening on Ga-coated Al (100)
Ab initio calculations indicate that the ground state for Ga adsorption on Al
(100) is on-surface with local unit coverage. On Ga-coated Al (100), the bridge
diffusion barrier for Al is large, but the AlGa {\it exchange
barrier is zero}: the ensuing incorporation of randomly deposited Al's into the
Ga overlayer realizes a percolation network, efficiently recoated by Ga atoms.
Based on calculated energetics, we predict rough surface growth at all
temperatures; modeling the growth by a random deposition model with partial
relaxation, we find a power-law divergent roughness .Comment: 4 pages RevTeX-twocolumn, no figures. to appear in Phys. Rev. Lett.,
July 199
Phonon Properties of Knbo3 and Ktao3 from First-Principles Calculations
The frequencies of transverse-optical phonons in KNbO and
KTaO are calculated in the frozen-phonon scheme making use of the
full-potential linearized muffin-tin orbital method. The calculated frequencies
in the cubic phase of KNbO and in the tetragonal ferroelectric phase are in
good agreement with experimental data. For KTaO, the effect of lattice
volume was found to be substantial on the frequency of the soft mode, but
rather small on the relative displacement patterns of atoms in all three modes
of the symmetry. The TO frequencies in KTaO are found to be of the
order of, but somehow higher than, the corresponding frequencies in cubic
KNbO.Comment: 8 pages + 1 LaTeX figure, Revtex 3.0, SISSA-CM-94-00
Electronic structure and total energy of interstitial hydrogen in iron: Tight binding models
An application of the tight binding approximation is presented for the
description of electronic structure and interatomic force in magnetic iron,
both pure and containing hydrogen impurities. We assess the simple canonical
d-band description in comparison to a non orthogonal model including s and d
bands. The transferability of our models is tested against known properties
including the segregation energies of hydrogen to vacancies and to surfaces of
iron. In many cases agreement is remarkably good, opening up the way to quantum
mechanical atomistic simulation of the effects of hydrogen on mechanical
properties
Gamma phonons and microscopic structure of orthorhombic KNbO3 from first-principles calculations
{}From a series of total energy calculations by the full-potential linear
muffin-tin orbital method, the total energy hypersurface as function of atomic
displacements from equilibrium positions has been fitted for different Gamma
phonon modes in orthorhombic KNbO3. Frequencies and eigenvectors of all TO
Gamma phonons have been calculated in the harmonic approximation, and in the
quantum oscillator scheme -- for A2 and B2 modes. The microscopic structure of
the orthorhombic phase has been analyzed in a series of supercell calculations
for different patterns of Nb displacements, providing indications in favour of
the chain structure, with oppositely directed neighboring chains.Comment: 10 pages, including 3 LaTeX figure
Systematic Control of Carrier Doping without Disorder at Interface of Oxide Heterostructures
We propose a method to systematically control carrier densities at the
interface of transition-metal oxide heterostructures without introducing
disorders. By inserting non-polar layers sandwiched by polar layers, continuous
carrier doping into the interface can be realized. This method enables us to
control the total carrier densities per unit cell systematically up to high
values of the order unity.Comment: 8 pages, 9 figure
Nature of bonding and electronic structure in MgB2, a boron intercalation superconductor
Chemical bonding and electronic structure of MgB2, a boron-based newly
discovered superconductor, is studied using self-consistent band structure
techniques. Analysis of the transformation of the band structure for the
hypothetical series of graphite - primitive graphite - primitive graphite-like
boron - intercalated boron, shows that the band structure of MgB2 is
graphite-like, with pi-bands falling deeper than in ordinary graphite. These
bands possess a typically delocalized and metallic, as opposed to covalent,
character. The in-plane sigma-bands retain their 2D covalent character, but
exhibit a metallic hole-type conductivity. The coexistence of 2D covalent
in-plane and 3D metallic-type interlayer conducting bands is a peculiar feature
of MgB2. We analyze the 2D and 3D features of the band structure of MgB2 and
related compounds, and their contributions to conductivity.Comment: 4 pages in revtex, 3 figures in 4 separate EPS file
An analysis of photoemission and inverse photoemission spectra of Si(111) and sulphur-passivated InP(001) surfaces
Photoemission (PES) and inverse-photoemission spectra (IPES) for the
sulphur-passivated InP(001) surface are compared with theoretical predictions
based on density-functional calculations. As a test case for our methods, we
also present a corresponding study of the better known Si(111) surface. The
reported spectra for InP(001)-S agree well with the calculated ones if the
surface is assumed to consist of a mixture of two phases, namely, the fully
S-covered -reconstructed structure, which contains four S atoms in
the surface unit-cell, and a structure containing two S and two P
atoms per unit cell. The latter has recently been identified in total-energy
calculations as well as in core-level spectra of S-passivated
Si(111)- is in excellent agreement with the calculations. The
comparison of the experimental-PES with our calculations provides additional
considerations regarding the nature of the sample surface. It is also found
that the commonly-used density-of-states approximation to the photo- and
inverse- photoemission spectra is not valid for these systems.Comment: Submitted to Phys. Rev. B; 6 postscript formatted pages; 7 figures in
gif format; postscript figures available upon reques
Structural and chemical embrittlement of grain boundaries by impurities: a general theory and first principles calculations for copper
First principles calculations of the Sigma 5 (310)[001] symmetric tilt grain
boundary in Cu with Bi, Na, and Ag substitutional impurities provide evidence
that in the phenomenon of Bi embrittlement of Cu grain boundaries electronic
effects do not play a major role; on the contrary, the embrittlement is mostly
a structural or "size" effect. Na is predicted to be nearly as good an
embrittler as Bi, whereas Ag does not embrittle the boundary in agreement with
experiment. While we reject the prevailing view that "electronic" effects
(i.e., charge transfer) are responsible for embrittlement, we do not exclude
the role of chemistry. However numerical results show a striking equivalence
between the alkali metal Na and the semi metal Bi, small differences being
accounted for by their contrasting "size" and "softness" (defined here). In
order to separate structural and chemical effects unambiguously if not
uniquely, we model the embrittlement process by taking the system of grain
boundary and free surfaces through a sequence of precisely defined gedanken
processes; each of these representing a putative mechanism. We thereby identify
three mechanisms of embrittlement by substitutional impurities, two of which
survive in the case of embrittlement or cohesion enhancement by interstitials.
Two of the three are purely structural and the third contains both structural
and chemical elements that by their very nature cannot be further unravelled.
We are able to take the systems we study through each of these stages by
explicit computer simulations and assess the contribution of each to the nett
reduction in intergranular cohesion. The conclusion we reach is that
embrittlement by both Bi and Na is almost exclusively structural in origin;
that is, the embrittlement is a size effect.Comment: 13 pages, 5 figures; Accepted in Phys. Rev.
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