280 research outputs found
Ballistic Spin Injection from Fe into ZnSe and GaAs with a (001), (111), and (110) orientation
We present first-principles calculations of ballistic spin injection in
Fe/GaAs and Fe/ZnSe junctions with orientation (001), (111), and (110). We find
that the symmetry mismatch of the Fe minority-spin states with the
semiconductor conduction states can lead to extremely high spin polarization of
the current through the (001) interface for hot and thermal injection
processes. Such a symmetry mismatch does not exist for the (111) and (110)
interfaces, where smaller spin injection efficiencies are found. The presence
of interface states is found to lower the current spin polarization, both with
and without a Schottky barrier. Finally, a higher bias can also affect the spin
injection efficiency.Comment: 12 pages, 18 figure
Cd-vacancy and Cd-interstitial complexes in Si and Ge
The electrical field gradient (EFG), measured e.g. in perturbed angular
correlation (PAC) experiments, gives particularly useful information about the
interaction of probe atoms like 111In / 111Cd with other defects. The
interpretation of the EFG is, however, a difficult task. This paper aims at
understanding the interaction of Cd impurities with vacancies and interstitials
in Si and Ge, which represents a controversial issue. We apply two
complementary ab initio methods in the framework of density functional theory
(DFT), (i) the all electron Korringa-Kohn-Rostoker (KKR) Greenfunction method
and (ii) the Pseudopotential-Plane-Wave (PPW) method, to search for the correct
local geometry. Surprisingly we find that both in Si and Ge the substitutional
Cd-vacancy complex is unstable and relaxes to a split-vacancy complex with the
Cd on the bond-center site. This complex has a very small EFG, allowing a
unique assignment of the small measured EFGs of 54MHz in Ge and 28MHz in Si.
Also, for the Cd-selfinterstitial complex we obtain a highly symmetrical split
configuration with large EFGs, being in reasonable agreement with experiments
Vacancy complexes with oversized impurities in Si and Ge
In this paper we examine the electronic and geometrical structure of
impurity-vacancy complexes in Si and Ge. Already Watkins suggested that in Si
the pairing of Sn with the vacancy produces a complex with the Sn-atom at the
bond center and the vacancy split into two half vacancies on the neighboring
sites. Within the framework of density-functional theory we use two
complementary ab initio methods, the pseudopotential plane wave (PPW) method
and the all-electron Kohn-Korringa-Rostoker (KKR) method, to investigate the
structure of vacancy complexes with 11 different sp-impurities. For the case of
Sn in Si, we confirm the split configuration and obtain good agreement with EPR
data of Watkins. In general we find that all impurities of the 5sp and 6sp
series in Si and Ge prefer the split-vacancy configuration, with an energy gain
of 0.5 to 1 eV compared to the substitutional complex. On the other hand,
impurities of the 3sp and 4sp series form a (slightly distorted) substitutional
complex. Al impurities show an exception from this rule, forming a split
complex in Si and a strongly distorted substitutional complex in Ge. We find a
strong correlation of these data with the size of the isolated impurities,
being defined via the lattice relaxations of the nearest neighbors.Comment: 8 pages, 4 bw figure
A polarizable interatomic force field for TiO parameterized using density functional theory
We report a classical interatomic force field for TiO, which has been
parameterized using density functional theory forces, energies, and stresses in
the rutile crystal structure. The reliability of this new classical potential
is tested by evaluating the structural properties, equation of state, phonon
properties, thermal expansion, and some thermodynamic quantities such as
entropy, free energy, and specific heat under constant volume. The good
agreement of our results with {\em ab initio} calculations and with
experimental data, indicates that our force-field describes the atomic
interactions of TiO in the rutile structure very well. The force field can
also describe the structures of the brookite and anatase crystals with good
accuracy.Comment: Accepted for publication in Phys. Rev. B; Changes from v1 include
multiple minor revisions and a re-write of the description of the force field
in Section II
Dynamics of mesoscopic precipitate lattices in phase separating alloys under external load
We investigate, via three-dimensional atomistic computer simulations, phase
separation in an alloy under external load. A regular two-dimensional array of
cylindrical precipitates, forming a mesoscopic precipitate lattice, evolves in
the case of applied tensile stress by the movement of mesoscopic lattice
defects. A striking similarity to ordinary crystals is found in the movement of
"meso-dislocations", but new mechanisms are also observed. Point defects such
as "meso-vacancies" or "meso-interstitials" are created or annihilated locally
by merging and splitting of precipitates. When the system is subjected to
compressive stress, we observe stacking faults in the mesoscopic
one-dimensional array of plate-like precipitates.Comment: 4 pages, 4 figures, REVTE
Coulomb parameters and photoemission for the molecular metal TTF-TCNQ
We employ density-functional theory to calculate realistic parameters for an
extended Hubbard model of the molecular metal TTF-TCNQ. Considering both intra-
and intermolecular screening in the crystal, we find significant longer-range
Coulomb interactions along the molecular stacks, as well as inter-stack
coupling. We show that the long-range Coulomb term of the extended Hubbard
model leads to a broadening of the spectral density, likely resolving the
problems with the interpretation of photoemission experiments using a simple
Hubbard model only.Comment: 4 pages, 2 figure
Density Functional Application to Strongly Correlated Electron Systems
The LSDA+U approach to density functional theory is carefully reanalyzed. Its
possible link to single-particle Green's function theory is occasionally
discussed. A simple and elegant derivation of the important sum rules for the
on-site interaction matrix elements linking them to the values of U and J is
presented. All necessary expressions for an implementation of LSDA+U into a
non-orthogonal basis solver for the Kohn-Sham equations are given, and
implementation into the FPLO solver is made. Results of application to several
planar cuprate structures are reported in detail and conclusions on the
interpretation of the physics of the electronic structure of the cuprates are
drawn.Comment: invited paper in Journal of Solid State Chemistr
Evidence of anisotropic magnetic polarons in laSrMnO by neutron scattering and comparison with Ca-doped manganites
Elastic and inelastic neutron scattering experiments have been performed in a
LaSrMnO untwinned crystal, which exhibits an
antiferromagnetic canted magnetic structure with ferromagnetic layers.
The elastic small q scattering exhibits a modulation with an anisotropic
q-dependence. It can be pictured by ferromagnetic inhomogeneities or polarons
with a platelike shape, the largest size () and largest
inter-polaron distance ( 38) being within the ferromagnetic
layers. Comparison with observations performed on Ca-doped samples, which show
the growth of the magnetic polarons with doping, suggests that this growth is
faster for the Sr than for the Ca substitution. Below the gap of the spin wave
branch typical of the AF layered magnetic structure, an additional spin wave
branch reveals a ferromagnetic and isotropic coupling, already found in
Ca-doped samples. Its q-dependent intensity, very anisotropic, closely reflects
the ferromagnetic correlations found for the static clusters. All these results
agree with a two-phase electronic segregation occurring on a very small scale,
although some characteristics of a canted state are also observed suggesting a
weakly inhomogeneous state.Comment: 11 pages, 11 figure
- …