401 research outputs found
Application of the Exact Muffin-Tin Orbitals Theory: the Spherical Cell Approximation
We present a self-consistent electronic structure calculation method based on
the {\it Exact Muffin-Tin Orbitals} (EMTO) Theory developed by O. K. Andersen,
O. Jepsen and G. Krier (in {\it Lectures on Methods of Electronic Structure
Calculations}, Ed. by V. Kumar, O.K. Andersen, A. Mookerjee, Word Scientific,
1994 pp. 63-124) and O. K. Andersen, C. Arcangeli, R. W. Tank, T.
Saha-Dasgupta, G. Krier, O. Jepsen, and I. Dasgupta, (in {\it Mat. Res. Soc.
Symp. Proc.} {\bf 491}, 1998 pp. 3-34). The EMTO Theory can be considered as an
{\it improved screened} KKR (Korringa-Kohn-Rostoker) method which is able to
treat large overlapping potential spheres. Within the present implementation of
the EMTO Theory the one electron equations are solved exactly using the Green's
function formalism, and the Poisson's equation is solved within the {\it
Spherical Cell Approximation} (SCA). To demonstrate the accuracy of the
SCA-EMTO method test calculations have been carried out.Comment: 20 pages, 10 figure
Robust half-metallic antiferromagnets LaVOsO and LaMoO ( = Ca, Sr, Ba; = Re, Tc) from first-principles calculations
We have theoretically designed three families of the half-metallic (HM)
antiferromagnets (AFM), namely, LaVOsO, LaMoTcO and
LaMoReO ( = Ca, Sr, Ba), based on a systematic {\it ab initio} study
of the ordered double perovskites LaO with the possible and
pairs from all the 3, 4 and 5 transtion metal elements being
considered. Electronic structure calculations based on first-principles
density-functional theory with generalized gradient approximation (GGA) for
more than sixty double perovskites LaCaO have been performed using the
all-electron full-potential linearized augmented-plane-wave method. The found
HM-AFM state in these materials survives the full {\it ab initio} lattice
constant and atomic position optimizations which were carried out using
frozen-core full potential projector augmented wave method. It is found that
the HM-AFM properties predicted previously in some of the double perovskites
would disappear after the full structural optimizations. The AFM is attributed
to both the superexchange mechanism and the generalized double exchange
mechanism via the () - O (2) - () coupling
and the latter is also believed to be the origin of the HM. Finally, in our
search for the HM-AFMs, we find LaCrTcO and LaCrReO to be AFM
insulators of an unconventional type in the sense that the two
antiferromagnetic coupled ions consist of two different elements and that the
two spin-resolved densities of states are no longer the same.Comment: To appear in Phys. Rev.
Construction of transferable spherically-averaged electron potentials
A new scheme for constructing approximate effective electron potentials
within density-functional theory is proposed. The scheme consists of
calculating the effective potential for a series of reference systems, and then
using these potentials to construct the potential of a general system. To make
contact to the reference system the neutral-sphere radius of each atom is used.
The scheme can simplify calculations with partial wave methods in the
atomic-sphere or muffin-tin approximation, since potential parameters can be
precalculated and then for a general system obtained through simple
interpolation formulas. We have applied the scheme to construct electron
potentials of phonons, surfaces, and different crystal structures of silicon
and aluminum atoms, and found excellent agreement with the self-consistent
effective potential. By using an approximate total electron density obtained
from a superposition of atom-based densities, the energy zero of the
corresponding effective potential can be found and the energy shifts in the
mean potential between inequivalent atoms can therefore be directly estimated.
This approach is shown to work well for surfaces and phonons of silicon.Comment: 8 pages (3 uuencoded Postscript figures appended), LaTeX,
CAMP-090594-
A first-principles comparison of the electronic properties of MgC_{y}Ni_{3} and ZnC_{y}Ni_{3} alloys
First-principles, density-functional-based electronic structure calculations
are employed to study the changes in the electronic properties of ZnC_{y}Ni_{3}
and MgC_{y}Ni_{3} using the Korringa-Kohn-Rostoker coherent-potential
approximation method in the atomic sphere approximation (KKR-ASA CPA). As a
function of decreasing C at%, we find a steady decrease in the lattice constant
and bulk modulus in either alloys. However, the pressure derivative of the bulk
modulus displays an opposite trend. Following the Debye model, which relates
the pressure derivative of the bulk modulus with the average phonon frequency
of the crystal, it can thus be argued that ZnCNi_{3} and its disordered alloys
posses a different phonon spectra in comparison to its MgCNi_{3} counterparts.
This is further justified by the marked similarity we find in the electronic
structure properties such as the variation in the density of states and the
Hopfield parameters calculated for these alloys. The effects on the equation of
state parameters and the density of states at the Fermi energy, for partial
replacement of Mg by Zn are also discussed.Comment: 19 pages, 15 figure
Towards a first-principles theory of surface thermodynamics and kinetics
Understanding of the complex behavior of particles at surfaces requires
detailed knowledge of both macroscopic and microscopic processes that take
place; also certain processes depend critically on temperature and gas
pressure. To link these processes we combine state-of-the-art microscopic, and
macroscopic phenomenological, theories. We apply our theory to the O/Ru(0001)
system and calculate thermal desorption spectra, heat of adsorption, and the
surface phase diagram. The agreement with experiment provides validity for our
approach which thus identifies the way for a predictive simulation of surface
thermodynamics and kinetics.Comment: 4 pages including 3 figures. Related publications can be found at
http://www.fhi-berlin.mpg.de/th/paper.htm
Pareto-optimal alloys
Large databases that can be used in the search for new materials with
specific properties remain an elusive goal in materials science. The search
problem is complicated by the fact that the optimal material for a given
application is usually a compromise between a number of materials properties
and the price. In this letter we present a database consisting of the lattice
parameters, bulk moduli, and heats of formation for over 64,000 ordered
metallic alloys, which has been established by direct first-principles
density-functional-theory calculations. Furthermore, we use a concept from
economic theory, the Pareto-optimal set, to determine optimal alloy solutions
for the compromise between low compressibility, high stability and price.Comment: 5 pages, 2 figures, To be published in Appl. Phys. Let
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