1,068 research outputs found
Structural stability, magnetic and electronic properties of Co2MnSi(001)/MgO heterostructures: A density functional theory study
A computational study of the epitaxial Co2MnSi(001)/MgO(001) interface
relevant to tunneling magnetoresistive (TMR) devices is presented. Employing ab
initio atomistic thermodynamics, we show that the Co- or MnSi-planes of
bulk-terminated Co2MnSi form stable interfaces, while pure Si or pure Mn
termination requires non-equilibrium conditions. Except for the pure Mn
interface, the half-metallic property of bulk Co2MnSi is disrupted by interface
bands. Even so, at homogeneous Mn or Co interfaces these bands contribute
little to the minority-spin conductance through an MgO barrier, and hence such
terminations could perform strongly in TMR devices.Comment: 4 pages, 3 fig
First-principles studies of kinetics in epitaxial growth of III-V semiconductors
We demonstrate how first-principles calculations using density-functional
theory (DFT) can be applied to gain insight into the molecular processes that
rule the physics of materials processing. Specifically, we study the molecular
beam epitaxy (MBE) of arsenic compound semiconductors. For homoepitaxy of GaAs
on GaAs(001), a growth model is presented that builds on results of DFT
calculations for molecular processes on the beta2-reconstructed GaAs(001)
surface, including adsorption, desorption, surface diffusion and nucleation.
Kinetic Monte Carlo simulations on the basis of the calculated energetics
enable us to model MBE growth of GaAs from beams of Ga and As_2 in atomistic
detail. The simulations show that island nucleation is controlled by the
reaction of As_2 molecules with Ga adatoms on the surface. The analysis reveals
that the scaling laws of standard nucleation theory for the island density as a
function of growth temperature are not applicable to GaAs epitaxy. We also
discuss heteroepitaxy of InAs on GaAs(001), and report first-principles DFT
calculations for In diffusion on the strained GaAs substrate. In particular we
address the effect of heteroepitaxial strain on the growth kinetics of
coherently strained InAs islands. The strain field around an island is found to
cause a slowing-down of material transport from the substrate towards the
island and thus helps to achieve more homogeneous island sizes.Comment: 12 pages, 7 figures, REVTeX, Final version to appear in Appl. Phys. A
(2002). Other related publications can be found at
http://www.fhi-berlin.mpg.de/th/paper.htm
Effect of the cluster size in modeling the H_2 desorption and dissociative adsorption on Si(001)
Three different clusters, Si_9H_12, Si_15H_16, and Si_21H_20, are used in
density-functional theory calculations in conjunction with ab initio
pseudopotentials to study how the energetics of H_2 dissociativ e adsorption on
and associative desorption from Si(001) depends on the cluster size. The
results are compared to five-layer slab calculations using the same
pseudopotentials and high qu ality plane-wave basis set. Several
exchange-correlation functionals are employed. Our analysis suggests that the
smaller clusters generally overestimate the activation barriers and reaction
energy. The Si_21H_20 cluster, however, is found to predict reaction
energetics, with E_{a}^{des}=56 +- 3 kcal/mol (2.4 +- 0.1 eV), reasonably close
(though still different) to that obtained from the slab calculations.
Differences in the calculated activation energies are discussed in relation to
the efficiency of clusters to describe the properties of the clean Si(001)-2x1
surface.Comment: 10 pages, 6 figures, submitted to J. Chem. Phy
Electron-hole spectra created by adsorption on metals from density-functional theory
Non-adiabaticity in adsorption on metal surfaces gives rise to a number of
measurable effects, such as chemicurrents and exo-electron emission. Here we
present a quantitative theory of chemicurrents on the basis of ground-state
density-functional theory (DFT) calculations of the effective electronic
potential and the Kohn-Sham band structure. Excitation probabilities are
calculated both for electron-hole pairs and for electrons and holes separately
from first-order time-dependent perturbation theory. This is accomplished by
evaluating the matrix elements (between Kohn-Sham states) of the rate of change
of the effective electronic potential between subsequent (static) DFT
calculations. Our approach is related to the theory of electronic friction, but
allows for direct access to the excitation spectra. The method is applied to
adsorption of atomic hydrogen isotopes on the Al(111) surface. The results are
compatible with the available experimental data (for noble metal surfaces); in
particular, the observed isotope effect in H versus D adsorption is described
by the present theory. Moreover, the results are in qualitative agreement with
computationally elaborate calculations of the full dynamics within
time-dependent density-functional theory, with the notable exception of effects
due to the spin dynamics. Being a perturbational approach, the method proposed
here is simple enough to be applied to a wide class of adsorbates and surfaces,
while at the same time allowing us to extract system-specific information.Comment: 23 pages, 9 figures, accepted for publication in Phys. Rev. B,
http://prb.aps.org/, v2: some major improvements, plus correction of minor
error
Magnetic monolayer LiN: Density Functional Theory Calculations
Density functional theory (DFT) calculations are used to investigate the
electronic and magnetic structures of a two-dimensional (2D) monolayer
LiN. It is shown that bulk LiN is a non-magnetic semiconductor. The
non-spinpolarized DFT calculations show that electrons of N in 2D LiN
form a narrow band at the Fermi energy due to a low coordination
number, and the density of states at the Fermi energy ()) is
increased as compared with bulk LiN. The large ) shows
instability towards magnetism in Stoner's mean field model. The spin-polarized
calculations reveal that 2D LiN is magnetic without intrinsic or impurity
defects. The magnetic moment of 1.0\, in 2D LiN is mainly
contributed by the electrons of N, and the band structure shows
half-metallic behavior. {Dynamic instability in planar LiN monolayer is
observed, but a buckled LiN monolayer is found to be dynamically stable.}
The ferromagnetic (FM) and antiferromagnetic (AFM) coupling between the N atoms
is also investigated to access the exchange field strength. {We found that
planar (buckled) 2D LiN is a ferromagnetic material with Curie
temperature of 161 (572) K.}Comment: Euro Phys. Lett. 2017 (Accepted
Density-functional theory study of half-metallic heterostructures: interstitial Mn in Si
Using density-functional theory within the generalized gradient
approximation, we show that Si-based heterostructures with 1/4 layer
-doping of {\em interstitial} Mn (Mn) are
half-metallic. For Mn concentrations of 1/2 or 1 layer, the
states induced in the band gap of -doped heterostructures still display
high spin polarization, about 85% and 60%, respectively. The proposed
heterostructures are more stable than previously assumed -layers of
{\em substitutional} Mn. Contrary to wide-spread belief, the present study
demonstrates that {\em interstitial} Mn can be utilized to tune the magnetic
properties of Si, and thus provides a new clue for Si-based spintronics
materials.Comment: 5 pages, 4 figures, PRL accepte
Atomic processes in molecular beam epitaxy on strained InAs(137): A density-functional theory study
The atomic processes in molecular beam epitaxy of InAs on the InAs(137) surface are investigated by means of first-principles total-energy calculations. We consider layer-by-layer growth on InAs(137) facets as a typical process during the evolution of shallow InAs islands in the Stranski-Krastanov growth mode of InAs on GaAs that is exploited for the self-assembly of heteroepitaxial quantum dots. From the calculated energetics we conclude that a growth scenario where an As2 molecule adsorbs on a single In adatom, followed by capture of another In adatom, is most likely. Moreover, our calculations of the potential-energy surface for In adatoms on the InAs(137) surface show that In adatoms are highly mobile. Surface diffusion on InAs(137) is found to be almost isotropic with energy barriers 2 molecule is destabilized by compressive strain in excess of −5%. This finding leads us to the conclusion that layer growth on InAs(137) facets ceases in highly strained regions of InAs islands on GaAs, in line with the observed shape evolution of such islands
Three-dimensional visualization of relational databases
Published ArticleIn recent years, databases have reached unprecedented complexity and volume. A database structure comprising tens of thousands of tables with a staggering number of inter-table relationships to match can hardly be understood and managed by the human mind. This holds in particular, if two-dimensional visualizations of such a structure are used. This paper describes a project to enhance human comprehension by another dimension enabling database users to move among database tables in three dimensions. The project applies techniques of virtual reality enabling users to control the individual aspect and the level of detail while gaining valuable insights into the structure, the contents, and the retrieval strategies within their databases
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