235 research outputs found
An Optimized and Scalable Eigensolver for Sequences of Eigenvalue Problems
In many scientific applications the solution of non-linear differential
equations are obtained through the set-up and solution of a number of
successive eigenproblems. These eigenproblems can be regarded as a sequence
whenever the solution of one problem fosters the initialization of the next. In
addition, in some eigenproblem sequences there is a connection between the
solutions of adjacent eigenproblems. Whenever it is possible to unravel the
existence of such a connection, the eigenproblem sequence is said to be
correlated. When facing with a sequence of correlated eigenproblems the current
strategy amounts to solving each eigenproblem in isolation. We propose a
alternative approach which exploits such correlation through the use of an
eigensolver based on subspace iteration and accelerated with Chebyshev
polynomials (ChFSI). The resulting eigensolver is optimized by minimizing the
number of matrix-vector multiplications and parallelized using the Elemental
library framework. Numerical results show that ChFSI achieves excellent
scalability and is competitive with current dense linear algebra parallel
eigensolvers.Comment: 23 Pages, 6 figures. First revision of an invited submission to
special issue of Concurrency and Computation: Practice and Experienc
Spin-caloric transport properties of cobalt nanostructures: spin disorder effects from first principles
The fundamental aspects of spin-dependent transport processes and their
interplay with temperature gradients, as given by the spin Seebeck coefficient,
are still largely unexplored and a multitude of contributing factors must be
considered. We used density functional theory together with a Monte-Carlo-based
statistical method to simulate simple nanostructures, such as Co nanowires and
films embedded in a Cu host or in vacuum, and investigated the influence of
spin-disorder scattering on electron transport at elevated temperatures. While
we show that the spin-dependent scattering of electrons due to temperature
induced disorder of the local magnetic moments contributes significantly to the
resistance, thermoelectric and spin-caloric transport coefficients, we also
conclude that the actual magnitude of these effects cannot be predicted,
quantitatively or qualitatively, without such detailed calculations.Comment: 10 pages, 6 figure
Functionalized Bismuth Films: Giant Gap Quantum Spin Hall and Valley-Polarized Quantum Anomalous Hall States
The search for new large band gap quantum spin Hall (QSH) and quantum
anomalous Hall (QAH) insulators is critical for their realistic applications at
room temperature. Here we predict, based on first principles calculations, that
the band gap of QSH and QAH states can be as large as 1.01 eV and 0.35 eV in an
H-decorated Bi(111) film. The origin of this giant band gap lies both in the
large spin-orbit interaction of Bi and the H-mediated exceptional electronic
and structural properties. Moreover, we find that the QAH state also possesses
the properties of quantum valley Hall state, thus intrinsically realising the
so-called valley-polarized QAH effect. We further investigate the realization
of large gap QSH and QAH states in an H-decorated Bi(\={1}10) film and
X-decorated (X=F, Cl, Br, and I) Bi(111) films.Comment: submitted to Physical Reveiw Letters on 25th of September 201
Quantum well states and amplified spin-dependent Friedel oscillations in thin films
Electrons mediate many of the interactions between atoms in a solid. Their
propagation in a material determines its thermal, electrical, optical, magnetic
and transport properties. Therefore, the constant energy contours
characterizing the electrons, in particular the Fermi surface, have a prime
impact on the behavior of materials. If anisotropic, the contours induce strong
directional dependence at the nanoscale in the Friedel oscillations surrounding
impurities. Here we report on giant anisotropic charge density oscillations
focused along specific directions with strong spin-filtering after scattering
at an oxygen impurity embedded in the surface of a ferromagnetic thin film of
Fe grown on W(001). Utilizing density functional theory, we demonstrate that by
changing the thickness of the Fe films, we control quantum well states confined
to two dimensions that manifest as multiple flat energy contours, impinging and
tuning the strength of the induced charge oscillations which allow to detect
the oxygen impurity at large distances ( 50nm).Comment: This paper has an explanatory supplemen
Solution to the modified Helmholtz equation for arbitrary periodic charge densities
We present a general method for solving the modified Helmholtz equation
without shape approximation for an arbitrary periodic charge distribution,
whose solution is known as the Yukawa potential or the screened Coulomb
potential. The method is an extension of Weinert's pseudo-charge method [M.
Weinert, J. Math. Phys. 22, 2433 (1981)] for solving the Poisson equation for
the same class of charge density distributions. The inherent differences
between the Poisson and the modified Helmholtz equation are in their respective
radial solutions. These are polynomial functions, for the Poisson equation, and
modified spherical Bessel functions, for the modified Helmholtz equation. This
leads to a definition of a modified pseudo-charge density and modified
multipole moments. We have shown that Weinert's convergence analysis of an
absolutely and uniformly convergent Fourier series of the pseudo-charge density
is transferred to the modified pseudo-charge density. We conclude by
illustrating the algorithmic changes necessary to turn an available
implementation of the Poisson solver into a solver for the modified Helmholtz
equation.Comment: submitted to the Journal of Mathematical Physic
Mixed topological semimetals driven by orbital complexity in two-dimensional ferromagnets
The concepts of Weyl fermions and topological semimetals emerging in
three-dimensional momentum space are extensively explored owing to the vast
variety of exotic properties that they give rise to. On the other hand, very
little is known about semimetallic states emerging in two-dimensional magnetic
materials, which present the foundation for both present and future information
technology. Here, we demonstrate that including the magnetization direction
into the topological analysis allows for a natural classification of
topological semimetallic states that manifest in two-dimensional ferromagnets
as a result of the interplay between spin-orbit and exchange interactions. We
explore the emergence and stability of such mixed topological semimetals in
realistic materials, and point out the perspectives of mixed topological states
for current-induced orbital magnetism and current-induced domain wall motion.
Our findings pave the way to understanding, engineering and utilizing
topological semimetallic states in two-dimensional spin-orbit ferromagnets
Phonons from Density-Functional Perturbation Theory using the All-Electron Full-Potential Linearized Augmented Plane-Wave Method FLEUR
Phonons are quantized vibrations of a crystal lattice that play a crucial
role in understanding many properties of solids. Density functional theory
(DFT) provides a state-of-the-art computational approach to lattice vibrations
from first-principles. We present a successful software implementation for
calculating phonons in the harmonic approximation, employing density-functional
perturbation theory (DFPT) within the framework of the full-potential
linearized augmented plane-wave (FLAPW) method as implemented in the electronic
structure package FLEUR. The implementation, which involves the Sternheimer
equation for the linear response of the wave function, charge density, and
potential with respect to infinitesimal atomic displacements, as well as the
setup of the dynamical matrix, is presented and the specifics due to the
muffin-tin sphere centered LAPW basis-set and the all-electron nature are
discussed. As a test, we calculate the phonon dispersion of several solids
including an insulator, a semiconductor as well as several metals. The latter
are comprised of magnetic, simple, and transition metals. The results are
validated on the basis of phonon dispersions calculated using the finite
displacement approach in conjunction with the FLEUR code and the phonopy
package, as well as by some experimental results. An excellent agreement is
obtained.Comment: 44 pages, 6 figure
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