15,841 research outputs found
Accurate polarization within a unified Wannier function formalism
We present an alternative formalism for calculating the maximally localized
Wannier functions in crystalline solids, obtaining an expression which is
extremely simple and general. In particular, our scheme is exactly invariant
under Brillouin zone folding, and therefore it extends trivially to the
Gamma-point case. We study the convergence properties of the Wannier functions,
their quadratic spread and centers as obtained by our simplified technique. We
show how this convergence can be drastically improved by a simple and
inexpensive ``refinement'' step, which allows for very efficient and accurate
calculations of the polarization in zero external field.Comment: 9 pages, 4 figure
Ab-initio theory of metal-insulator interfaces in a finite electric field
We present a novel technique for calculating the dielectric response of
metal/insulator heterostructures. This scheme allows, for the first time, the
fully first-principles calculation of the microscopic properties of thin-film
capacitors at finite bias potential. The method can be readily applied to pure
insulators, where it provides an interesting alternative to conventional
finite-field techniques based on the Berry-phase formalism. We demonstrate the
effectiveness of our method by performing comprehensive numerical tests on a
model Ag/MgO/Ag heterostructure.Comment: 10 pages, 5 figures, major revisio
Vectorcardiographic changes during extended space flight
To assess the effects of space flight on cardiac electrical properties, vectorcardiograms were taken on the 9 Skylab astronauts during the flights of 28, 59, and 84 days. The Frank lead system was used and observations were made at rest; during 25%, 50% and 75% of maximum exercise; during a short pulse of exercise (150 watts, 2 minutes); and after exercise. Data from 131 in-flight tests were analyzed by computer and compared to preflight and postflight values. Statistically significant increase in QRS vector magnitude (six of nine crewmen); T vector magnitude (five of nine crewmen); and resting PR interval duration (six of nine crewmen) occurred. During exercise the PR interval did not differ from preflight. Exercise heart rates inflight were the same as preflight, but increased in the immediate postflight period. With the exception of the arrhythmias, no deleterious vectorcardiographic changes were observed during the Skylab missions
Influence of strain and oxygen vacancies on the magnetoelectric properties of multiferroic bismuth ferrite
The dependencies on strain and oxygen vacancies of the ferroelectric
polarization and the weak ferromagnetic magnetization in the multiferroic
material bismuth ferrite, BiFeO_3, are investigated using first principles
density functional theory calculations. The electric polarization is found to
be rather independent of strain, in striking contrast to most conventional
perovskite ferroelectrics. It is also not significantly affected by oxygen
vacancies, or by the combined presence of strain and oxygen vacancies. The
magnetization is also unaffected by strain, however the incorporation of oxygen
vacancies can alter the magnetization slightly, and also leads to the formation
of Fe^{2+}. These results are discussed in light of recent experiments on
epitaxial films of BiFeO_3 which reported a strong thickness dependence of both
magnetization and polarization.Comment: 9 pages, 3 figure
Systematic treatment of displacements, strains and electric fields in density-functional perturbation theory
The methods of density-functional perturbation theory may be used to
calculate various physical response properties of insulating crystals including
elastic, dielectric, Born charge, and piezoelectric tensors. These and other
important tensors may be defined as second derivatives of the total energy with
respect to atomic-displacement, electric-field, or strain perturbations, or as
mixed derivatives with respect to two of these perturbations. The resulting
tensor quantities tend to be coupled in complex ways in polar crystals, giving
rise to a variety of variant definitions. For example, it is generally
necessary to distinguish between elastic tensors defined under different
electrostatic boundary conditions, and between dielectric tensors defined under
different elastic boundary conditions. Here, we describe an approach for
computing all of these various response tensors in a unified and systematic
fashion. Applications are presented for two materials, wurtzite ZnO and
rhombohedral BaTiO3, at zero temperature.Comment: 14 pages. Uses REVTEX macros. Also available at
http://www.physics.rutgers.edu/~dhv/preprints/xfw_sys/index.htm
Nonlocality of Kohn-Sham exchange-correlation fields in dielectrics
The theory of the macroscopic field appearing in the Kohn-Sham
exchange-correlation potential for dielectric materials, as introduced by
Gonze, Ghosez and Godby, is reexamined. It is shown that this Kohn-Sham field
cannot be determined from a knowledge of the local state of the material (local
crystal potential, electric field, and polarization) alone. Instead, it has an
intrinsically nonlocal dependence on the global electrostatic configuration.
For example, it vanishes in simple transverse configurations of a polarized
dielectric, but not in longitudinal ones.Comment: 4 pages, two-column style with 2 postscript figures embedded. Uses
REVTEX and epsf macros. Also available at
http://www.physics.rutgers.edu/~dhv/preprints/index.html#dv_gg
Predicting polarization enhancement in multicomponent ferroelectric superlattices
Ab initio calculations are utilized as an input to develop a simple model of
polarization in epitaxial short-period CaTiO3/SrTiO3/BaTiO3 superlattices grown
on a SrTiO3 substrate. The model is then combined with a genetic algorithm
technique to optimize the arrangement of individual CaTiO3, SrTiO3 and BaTiO3
layers in a superlattice, predicting structures with the highest possible
polarization and a low in-plane lattice constant mismatch with the substrate.
This modelling procedure can be applied to a wide range of layered
perovskite-oxide nanostructures providing guidance for experimental development
of nanoelectromechanical devices with substantially improved polar properties.Comment: 4 pages, submitted to PR
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