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2008-07-11 | Superconductivity up to 29 K in SrFe2As2 and BaFe2As2 at high pressures | We report the discovery of superconductivity at high pressure in SrFe2As2 and
BaFe2As2. The superconducting transition temperatures are up to 27 K in
SrFe2As2 and 29 K in BaFe2As2, making these the highest pressure-induced
superconducting materials discovered thus far. | 0807.1896v2 |
2008-07-14 | An Adaptive Entanglement Distillation Scheme Using Quantum Low Density Parity Check Codes | Quantum low density parity check (QLDPC) codes are useful primitives for
quantum information processing because they can be encoded and decoded
efficiently. Besides, the error correcting capability of a few QLDPC codes
exceeds the quantum Gilbert-Varshamov bound. Here, we report a numerical
performance analysis of an adaptive entanglement distillation scheme using
QLDPC codes. In particular, we find that the expected yield of our adaptive
distillation scheme to combat depolarization errors exceed that of Leung and
Shor whenever the error probability is less than about 0.07 or greater than
about 0.28. This finding illustrates the effectiveness of using QLDPC codes in
entanglement distillation. | 0807.2122v1 |
2008-07-16 | Analytical solution of the equation of motion for a rigid domain wall in a magnetic material with perpendicular anisotropy | This paper reports the solution of the equation of motion for a domain wall
in a magnetic material which exhibits high magneto-crystalline anisotropy.
Starting from the Landau-Lifschitz-Gilbert equation for field-induced motion,
we solve the equation to give an analytical expression, which specifies the
domain wall position as a function of time. Taking parameters from a Co/Pt
multilayer system, we find good quantitative agreement between calculated and
experimentally determined wall velocities, and show that high field uniform
wall motion occurs when wall rigidity is assumed. | 0807.2604v3 |
2008-07-16 | A graphical extension for the Windows version of the Parallel Finite Element Micromagnetics Package (MagParExt) | In the current paper we present a graphical user interface useful for
settings input parameter of the Windows precompiled binaries for the Parallel
Finite Element Micromagnetics Package (MagPar). The Package is used for
magnetization dynamics analysis on a base of the Landau-Lifshitz-Gilbert (LLG)
equation. In an available version of the MagPar package there are several text
files which control simulations. Presented here graphical extension (MagParExt)
enables easy preparation of input and output data, stored in text files, and
additionally, direct and fast creation of figures obtained from dependencies
between simulated physical quantities. | 0807.2655v1 |
2008-08-17 | Attempt frequency of magnetization in nanomagnets with thin-film geometry | Solving the stochastic Landau-Lifshitz-Gilbert equation numerically, we
investigate the effect of the potential landscape on the attempt frequency of
magnetization in nanomagnets with the thin-film geometry. Numerical estimates
of the attempt frequency are analyzed in comparison with theoretical
predictions from the Fokker-Planck equation for the Neel-Brown model. It is
found that for a nanomagnet with the thin-film geometry, theoretically
predicted values for the universal case are in excellent agreement with
numerical estimates. | 0808.2281v1 |
2008-08-30 | Path integral study of the role of correlation in exchange coupling of spins in double quantum dots and optical lattices | We explore exchange coupling of a pair of spins in a double dot and in an
optical lattice. Our algorithm uses the frequency of exchanges in a bosonic
path integral, evaluated with Monte Carlo. This algorithm is simple enough to
be a "black box" calculator, yet gives insights into the role of correlation
through two-particle probability densities, visualization of instantons, and
pair correlation functions. We map the problem to Hubbard model and see that
exchange and correlation renormalize the effective parameters, dramatically
lowering U at larger separations. | 0809.0038v1 |
2008-12-09 | On densest packings of equal balls of $\rb^{n}$ and Marcinkiewicz spaces | We investigate, by "a la Marcinkiewicz" techniques applied to the
(asymptotic) density function, how dense systems of equal spheres of $\rb^{n},
n \geq 1,$ can be partitioned at infinity in order to allow the computation of
their density as a true limit and not a limsup. The density of a packing of
equal balls is the norm 1 of the characteristic function of the systems of
balls in the sense of Marcinkiewicz. Existence Theorems for densest sphere
packings and completely saturated sphere packings of maximal density are given
new direct proofs. | 0812.1720v1 |
2009-01-26 | Bioprotectant glassforming solutions confined in porous silicon nanocapillaries | Glycerol and trehalose-glycerol binary solutions are glass-forming liquids
with remarkable bioprotectant properties. In this paper, we address the effects
of confining of these solutions in straight channels of diameter D=8 nm formed
by porous silicon. Neutron diffraction and incoherent quasielastic neutron
scattering are used to reveal the different effects of nanoconfinement and
addition of trehalose on the intermolecular structure and molecular dynamics of
the liquid and glassy phases, on a nanosecond timescale. | 0901.3994v1 |
2009-02-12 | Computing equations for residually free groups | We show that there is no algorithm deciding whether the maximal residually
free quotient of a given finitely presented group is finitely presentable or
not.
Given a finitely generated subgroup G of a finite product of limit groups, we
discuss the possibility of finding an explicit set of defining equations (i.e.
of expressing G as the maximal residually free quotient of an explicit finitely
presented group). | 0902.2119v2 |
2009-03-06 | Simulation of current-induced microwave oscillation in geometrically confined domain wall | We studied magnetization dynamics of a geometrically confined domain wall
under dc current by solving simultaneously the Landau-Lifshitz-Gilbert equation
and diffusion equation for spin accumulation. We showed that the oscillation
motion of the domain wall is driven by the spin-transfer torque and the dc
current is converted to the ac voltage signal. The results means that the
geometrically confined domain wall is applicable as a source of microwave
oscillator. | 0903.1151v1 |
2009-06-25 | Nonlinear Schroedinger-Poisson Theory for Quantum-Dot Helium | We use a nonlinear Schroedinger-Poisson equation to describe two interacting
electrons with opposite spins confined in a parabolic potential, a quantum dot.
We propose an effective form of the Poisson equation taking into account the
dimensional mismatch of the two-dimensional electronic system and the
three-dimensional electrostatics. The results agree with earlier numerical
calculations performed in a large basis of two-body states and provide a simple
model for continuous quantum-classical transition with increasing nonlinearity.
Specific intriguing properties due to eigenstate non-orthogonality are
emphasized. | 0906.4650v1 |
2009-06-29 | Performance of Magnetic Quantum Cellular Automata and Limitations due to Thermal Noise | Operation parameters of magnetic quantum cellular automata are evaluated for
the purposes of reliable logic operation. The dynamics of the nanomagnets is
simulated via the Landau-Lifshitz-Gilbert equations with a stochastic magnetic
field corresponding to thermal fluctuations. It is found that in the macrospin
approximation the switching speed does not change under scaling of both size
and distances between nanomagnets. Thermal fluctuations put a limitation on the
size of nanomagnets, since the gate error rate becomes excessive for
nanomagnets smaller than 200nm at room temperature. | 0906.5172v1 |
2009-09-15 | Critical current density for spin transfer torque switching with composite free layer structure | Critical current density of composite free layer (CFL) in magnetic tunneling
junction is investigated. CFL consists of two exchange coupled ferromagnetic
layers, where the coupling is parallel or anti-parallel. Instability condition
of the CFL under the spin transfer torque, which is related with critical
current density, is obtained by analytic spin wave excitation model and
confirmed by macro-spin Landau-Lifshitz-Gilbert equation. The critical current
densities for the coupled two identical layers are investigated with various
coupling strengths, and spin transfer torque efficiencies. | 0909.2711v1 |
2009-09-25 | Tunable steady-state domain wall oscillator with perpendicular magnetic anisotropy | We theoretically study domain wall oscillations upon the injection of a dc
current through a geometrically constrained wire with perpendicular magnetic
anisotropy. The oscillation frequency spectrum can be tuned by the injected
current density, but additionally by the application of an external magnetic
field independent of the power. The results of analytical calculations are
supported by micromagnetic simulations based on the Landau-Lifshitz-Gilbert
equation. The simple concept of our localized steady-state oscillator might
prove useful as a nanoscale microwave generator with possible applications in
telecommunication or for rf-assisted writing in magnetic hard drives. | 0909.4733v1 |
2009-10-13 | Dipolar field effect on microwave oscillation in a domain wall spin-valve | We examined dipolar field effects on the microwave generation in the domain
wall spin-valve by solving simultaneously the Landau-Lifshitz-Gilbert and
Zhang-Levy-Fert diffusion equations. By numerically analyzing dipolar field
dependence, we showed that the microwave generation needs the dipole-dipole
interaction for a 180$^\circ$ domain wall and the amplitude of the microwave
voltage signal depends strongly on the exchange length. In order to design a
microwave generator using the domain wall spin-valve with high efficiency, we
propose that the materials with short exchange length are preferred. | 0910.2280v1 |
2009-10-30 | Dissipative dynamics of magnetic solitons in metals | Soliton dynamics in spin-textured metals generate electrical currents, which
produce backaction through spin torques. We modify the Landau-Lifshitz-Gilbert
equation and the corresponding solitonic equations of motion to include such
higher-order texture effects. We also find a quasistatic equation for the
induced electrochemical potential, which needs to be solved for
self-consistently, in the incompressible limit. As an example, we consider the
orbital motion of a vortex in a point-contact spin valve, and discuss
modifications of orbit radius, frequency, and dissipation power. | 0910.5912v2 |
2009-11-16 | JSJ decompositions: definitions, existence, uniqueness. I: The JSJ deformation space | This paper and its companion arXiv:1002.4564 have been replaced by
arXiv:1602.05139.
We give a general simple definition of JSJ decompositions by means of a
universal maximality property. The JSJ decomposition should not be viewed as a
tree (which is not uniquely defined) but as a canonical deformation space of
trees. We prove that JSJ decompositions of finitely presented groups always
exist, without any assumption on edge groups. Many examples are given. | 0911.3173v3 |
2010-02-15 | Domain wall motion in ferromagnetic nanowires driven by arbitrary time-dependent fields: An exact result | We address the dynamics of magnetic domain walls in ferromagnetic nanowires
under the influence of external time-dependent magnetic fields. We report a new
exact spatiotemporal solution of the Landau-Lifshitz-Gilbert equation for the
case of soft ferromagnetic wires and nanostructures with uniaxial anisotropy.
The solution holds for applied fields with arbitrary strength and time
dependence. We further extend this solution to applied fields slowly varying in
space and to multiple domain walls. | 1002.2913v1 |
2010-05-24 | SN2010U -- a Luminous Nova in NGC 4214 | The luminosity, light curve, post--maximum spectrum, and lack of a progenitor
on deep pre-outburst images suggest that SN 2010U was a luminous, fast nova.
Its outburst magnitude is consistent with that for a fast nova using the
Maximum Magnitude-Rate of Decline relationship for classical novae. | 1005.4356v1 |
2010-06-11 | Highly Parallel Sparse Matrix-Matrix Multiplication | Generalized sparse matrix-matrix multiplication is a key primitive for many
high performance graph algorithms as well as some linear solvers such as
multigrid. We present the first parallel algorithms that achieve increasing
speedups for an unbounded number of processors. Our algorithms are based on
two-dimensional block distribution of sparse matrices where serial sections use
a novel hypersparse kernel for scalability. We give a state-of-the-art MPI
implementation of one of our algorithms. Our experiments show scaling up to
thousands of processors on a variety of test scenarios. | 1006.2183v1 |
2010-06-24 | Boundary regularity for minimizers of the micromagnetic energy functional | Motivated by the construction of time-periodic solutions for the
three-dimensional Landau-Lifshitz-Gilbert equation in the case of soft and
small ferromagnetic particles, we investigate the regularity properties of
minimizers of the micromagnetic energy functional at the boundary. In
particular, we show that minimizers are regular provided the volume of the
particle is sufficiently small. The approach uses a reflection construction at
the boundary and an adaption of the well-known regularity theory for minimizing
harmonic maps into spheres. | 1006.4766v1 |
2010-06-24 | Time-periodic Néel wall motions | In thin ferromagnetic films, the predominance of the magnetic shape
anisotropy leads to in-plane magnetizations. The simplest domain wall in this
geometry is the one-dimensional Neel wall that connects two magnetizations of
opposite sign by a planar 180 degree rotation. In this paper, we perturb the
static Neel wall profile in order to construct time-periodic Neel wall motions
governed by to the Landau-Lifshitz-Gilbert equation. Our construction works
within a certain parameter regime and requires the restriction to external
magnetic fields with small amplitudes and suitable time averages. | 1006.4768v1 |
2010-06-30 | Supercurrent-Induced Magnetization Dynamics | We investigate supercurrent-induced magnetization dynamics in a Josephson
junction with two misaligned ferromagnetic layers, and demonstrate a variety of
effects by solving numerically the Landau-Lifshitz-Gilbert equation. In
particular, we demonstrate the possibility to obtain supercurrent-induced
magnetization switching for an experimentally feasible set of parameters, and
clarify the favorable condition for the realization of magnetization reversal.
These results constitute a superconducting analogue to conventional
current-induced magnetization dynamics and indicate how spin-triplet
supercurrents may be utilized for practical purposes in spintronics. | 1007.0004v1 |
2010-07-09 | Polarization and magnetization dynamics of a field-driven multiferroic structure | We consider a multiferroic chain with a linear magnetoelectric coupling
induced by the electrostatic screening at the ferroelectric/ferromagnet
interface. We study theoretically the dynamic ferroelectric and magnetic
response to external magnetic and electric fields by utilizing an approach
based on coupled Landau- Khalatnikov and finite-temperature
Landau-Lifshitz-Gilbert equations. Additionally, we compare with Monte Carlo
calculations. It is demonstrated that for material parameters corresponding to
BaTiO3/Fe the polarization and the magnetization are controllable by external
magnetic and electric fields respectively. | 1007.1543v1 |
2010-07-11 | Factorization of banded permutations | We consider the factorization of permutations into bandwidth 1 permutations,
which are products of mutually nonadjacent simple transpositions. We exhibit an
upper bound on the minimal number of such factors and thus prove a conjecture
of Gilbert Strang: a banded permutation of bandwidth $w$ can be represented as
the product of at most $2w-1$ permutations of bandwidth 1. An analogous result
holds also for infinite and cyclically banded permutations. | 1007.1760v2 |
2010-07-14 | Nonlinear interference in a mean-field quantum model | Using similar nonlinear stationary mean-field models for Bose-Einstein
Condensation of cold atoms and interacting electrons in a Quantum Dot, we
propose to describe the original many-particle ground state as a one-particle
statistical mixed state of the nonlinear eigenstates whose weights are provided
by the eigenstate non-orthogonality. We search for physical grounds in the
interpretation of our two main results, namely, quantum-classical nonlinear
transition and interference between nonlinear eigenstates. | 1007.2408v1 |
2011-02-04 | Current-induced dynamics of composite free layer with antiferromagnetic interlayer exchange coupling | Current-induced dynamics in spin valves including composite free layer with
antiferromagnetic interlayer exchange coupling is studied theoretically within
the diffusive transport regime. We show that current-induced dynamics of a
synthetic antiferromagnet is significantly different from dynamics of a
synthetic ferrimagnet. From macrospin simulations we obtain conditions for
switching the composite free layer, as well as for appearance of various
self-sustained dynamical modes. Numerical simulations are compared with simple
analytical models of critical current based on linearized
Landau-Lifshitz-Gilbert equation. | 1102.1028v2 |
2011-04-01 | Spin waves cause non-linear friction | Energy dissipation is studied for a hard magnetic tip that scans a soft
magnetic substrate. The dynamics of the atomic moments are simulated by solving
the Landau-Lifshitz-Gilbert (LLG) equation numerically. The local energy
currents are analysed for the case of a Heisenberg spin chain taken as
substrate. This leads to an explanation for the velocity dependence of the
friction force: The non-linear contribution for high velocities can be
attributed to a spin wave front pushed by the tip along the substrate. | 1104.0197v2 |
2011-05-31 | Upper and Lower Bounds on the Minimum Distance of Expander Codes | The minimum distance of expander codes over GF(q) is studied. A new upper
bound on the minimum distance of expander codes is derived. The bound is shown
to lie under the Varshamov-Gilbert (VG) bound while q >= 32. Lower bounds on
the minimum distance of some families of expander codes are obtained. A lower
bound on the minimum distance of low-density parity-check (LDPC) codes with a
Reed--Solomon constituent code over GF(q) is obtained. The bound is shown to be
very close to the VG bound and to lie above the upper bound for expander codes. | 1105.6224v1 |
2011-07-01 | Current induced magnetization reversal on the surface of a topological insulator | We study dynamics of the magnetization coupled to the surface Dirac fermions
of a three di- mensional topological insulator. By solving the
Landau-Lifshitz-Gilbert equation in the presence of charge current, we find
current induced magnetization dynamics and discuss the possibility of mag-
netization reversal. The torque from the current injection depends on the
transmission probability through the ferromagnet and shows nontrivial
dependence on the exchange coupling. The mag- netization dynamics is a direct
manifestation of the inverse spin-galvanic effect and hence another ferromagnet
is unnecessary to induce spin transfer torque in contrast to the conventional
setup. | 1107.0116v1 |
2011-08-05 | Thermoelectric detection of ferromagnetic resonance of a nanoscale ferromagnet | We present thermoelectric measurements of the heat dissipated due to
ferromagnetic resonance of a Permalloy strip. A microwave magnetic field,
produced by an on-chip coplanar strip waveguide, is used to drive the
magnetization precession. The generated heat is detected via Seebeck
measurements on a thermocouple connected to the ferromagnet. The observed
resonance peak shape is in agreement with the Landau-Lifshitz-Gilbert equation
and is compared with thermoelectric finite element modeling. Unlike other
methods, this technique is not restricted to electrically conductive media and
is therefore also applicable to for instance ferromagnetic insulators. | 1108.1286v1 |
2011-08-31 | Effects of Fermion Flavor on Exciton Condensation in Double Layer Systems | We use fermionic path integral quantum Monte Carlo to study the effects of
fermion flavor on the physical properties of dipolar exciton condensates in
double layer systems. We find that by including spin in the system weakens the
effective interlayer interaction strength, yet this has very little effect on
the Kosterlitz-Thouless transition temperature. We further find that, to obtain
the correct description of screening, it is necessary to account for
correlation in both the interlayer and intralayer interactions. We show that
while the excitonic binding cannot completely surpress screening by additional
fermion flavors, their screening effectiveness is reduced leading to a much
higher transition temperatures than predicted with large-N analysis. | 1108.6107v1 |
2011-09-30 | An ultrafast image recovery and recognition system implemented with nanomagnets possessing biaxial magnetocrystalline anisotropy | A circular magnetic disk with biaxial magnetocrystalline anisotropy has four
stable magnetization states which can be used to encode a pixel's shade in a
black/gray/white image. By solving the Landau-Lifshitz- Gilbert equation, we
show that if moderate noise deflects the magnetization slightly from a stable
state, it always returns to the original state, thereby automatically
de-noising the corrupted image. The same system can compare a noisy input image
with a stored image and make a matching decision using magneto-tunneling
junctions. These tasks are executed at ultrahigh speeds (~2 ns for a
512\times512 pixel image). | 1109.6932v1 |
2011-11-10 | Magnonic band structure of a two-dimensional magnetic superlattice | The frequencies and linewidths of spin waves in a two-dimensional periodic
superlattice of magnetic materials are found, using the Landau-Lifshitz-Gilbert
equations. The form of the exchange field from a surface-torque-free boundary
between magnetic materials is derived, and magnetic-material combinations are
identified which produce gaps in the magnonic spectrum across the entire
superlattice Brillouin zone for hexagonal and square-symmetry superlattices. | 1111.2506v1 |
2011-11-29 | Supercurrent induced domain wall motion | We study the dynamics of a magnetic domain wall, inserted in, or juxtaposed
to, a conventional superconductor, via the passage of a spin polarized current
through a FSF junction. Solving the Landau-Lifshitz-Gilbert equation of motion
for the magnetic moments we calculate the velocity of the domain wall and
compare it with the case of a FNF junction. We find that in several regimes the
domain wall velocity is larger when it is driven by a supercurrent. | 1111.6751v1 |
2011-12-02 | The Main Diagonal of a Permutation Matrix | By counting 1's in the "right half" of $2w$ consecutive rows, we locate the
main diagonal of any doubly infinite permutation matrix with bandwidth $w$.
Then the matrix can be correctly centered and factored into block-diagonal
permutation matrices. Part II of the paper discusses the same questions for the
much larger class of band-dominated matrices. The main diagonal is determined
by the Fredholm index of a singly infinite submatrix. Thus the main diagonal is
determined "at infinity" in general, but from only $2w$ rows for banded
permutations. | 1112.0582v2 |
2011-12-11 | Stability of the Greedy Algorithm on the Circle | We consider a single-server system with service stations in each point of the
circle. Customers arrive after exponential times at uniformly-distributed
locations. The server moves at finite speed and adopts a greedy routing
mechanism. It was conjectured by Coffman and Gilbert in~1987 that the service
rate exceeding the arrival rate is a sufficient condition for the system to be
positive recurrent, for any value of the speed. In this paper we show that the
conjecture holds true. | 1112.2389v3 |
2011-12-12 | Riffle shuffles with biased cuts | The well-known Gilbert-Shannon-Reeds model for riffle shuffles assumes that
the cards are initially cut 'about in half' and then riffled together. We
analyze a natural variant where the initial cut is biased. Extending results of
Fulman (1998), we show a sharp cutoff in separation and L-infinity distances.
This analysis is possible due to the close connection between shuffling and
quasisymmetric functions along with some complex analysis of a generating
function. | 1112.2650v1 |
2011-12-15 | Solutions of the Fully Compressible Semi-Geostrophic System | The fully compressible semi-geostrophic system is widely used in the
modelling of large-scale atmospheric flows. In this paper, we prove rigorously
the existence of weak Lagrangian solutions of this system, formulated in the
original physical coordinates. In addition, we provide an alternative proof of
the earlier result on the existence of weak solutions of this system expressed
in the so-called geostrophic, or dual, coordinates. The proofs are based on the
optimal transport formulation of the problem and on recent general results
concerning transport problems posed in the Wasserstein space of probability
measures. | 1112.3553v2 |
2012-01-23 | Micromagnetic Simulations for Spin Transfer Torque in Magnetic Multilayers | We investigate the spin transfer torque (STT) in the magnetic multilayer
structures with micromagnetic simulations. We implement the STT contribution
for the magnetic multilayer structures in addition to the
Landau-Lifshitz-Gilbert (LLG) micromagnetic simulators. Not only the Sloncewski
STT term, the zero, first, and second order field- like terms are also
considered, and the effects of the Oersted field by the current are addressed.
We determine the switching current densities of the free layer with the
exchange biased synthetic ferrimagnetic reference layers for various cases. | 1201.4707v1 |
2012-01-26 | Pseudospin Transfer Torques in Semiconductor Electron Bilayers | We use self-consistent quantum transport theory to investigate the influence
of electron-electron interactions on interlayer transport in semiconductor
electron bilayers in the absence of an external magnetic field. We conclude
that, even though spontaneous pseudospin order does not occur at zero field,
interaction-enhanced quasiparticle tunneling amplitudes and pseudospin transfer
torques do alter tunneling I-V characteristics, and can lead to time-dependent
response to a dc bias voltage. | 1201.5569v2 |
2012-03-03 | Skyrmion Generation by Current | Skyrmions, once a hypothesized field-theoretical object believed to describe
the nature of elementary particles, became common sightings in recent years
among several non-centrosymmetric metallic ferromagnets. For more practical
applications of Skyrmionic matter as carriers of information, thus realizing
the prospect of "Skyrmionics", it is necessary to have the means to create and
manipulate Skyrmions individually. We show through extensive simulation of the
Landau-Lifshitz-Gilbert equation that a circulating current imparted to the
metallic chiral ferromagnetic system can create isolated Skyrmionic spin
texture without the aid of external magnetic field. | 1203.0638v1 |
2012-04-25 | Numerical Study on Spin Torque Switching in Thermally Activated Region | We studied the spin torque switching of the single free layer in the
thermally activated region by numerically solving the Landau-Lifshitz-Gilbert
equation. We found that the temperature dependence of the switching time of the
in-plane magnetized system is nonlinear, which means $b \neq 1$. Here, $b$ is
the exponent of the current term in the switching rate formula and has been
widely assumed to be unity. This result enables us to evaluate the thermal
stability of spintronics devices. | 1204.5596v2 |
2012-07-13 | Theory of current-driven motion of Skyrmions and spirals in helical magnets | We study theoretically the dynamics of the spin textures, i.e., Skyrmion
crystal (SkX) and spiral structure (SS), in two-dimensional helical magnets
under external current. By numerically solving the Landau-Lifshitz-Gilbert
equation, it is found that (i) the critical current density of the motion is
much lower for SkX compared with SS in agreement with the recent experiment,
(ii) there is no intrinsic pinning effect for SkX and the deformation of the
internal structure of Skyrmion reduces the pinning effect dramatically, (iii)
the Bragg intensity of SkX shows strong time-dependence as can be observed by
neutron scattering experiment. | 1207.3224v1 |
2012-07-19 | Solar Orbiter: Exploring the Sun-heliosphere connection | The heliosphere represents a uniquely accessible domain of space, where
fundamental physical processes common to solar, astrophysical and laboratory
plasmas can be studied under conditions impossible to reproduce on Earth and
unfeasible to observe from astronomical distances. Solar Orbiter, the first
mission of ESA's Cosmic Vision 2015-2025 programme, will address the central
question of heliophysics: How does the Sun create and control the heliosphere?
In this paper, we present the scientific goals of the mission and provide an
overview of the mission implementation. | 1207.4579v2 |
2012-07-19 | Harnessing the modulation instability spectrum in optical fibers with a periodic dispersion landscape | We report the experimental demonstration of modulation instability process
assisted by a dispersion grating in an optical fiber. A simple analytical model
is developed to further analyze and explain the complex dynamics of this
process, showing that each of the multiple spectral components grows thanks to
a quasi phase-matching mechanism inherent to the periodicity of the waveguide
parameters. This model is confirmed by numerical simulations and it is
successfully used to tailor the multi-peak modulation instability spectrum
shape. These theoretical predictions are confirmed by experiments. | 1207.4667v1 |
2012-08-11 | Lexicodes over Rings | In this paper, we consider the construction of linear lexicodes over finite
chain rings by using a $B$-ordering over these rings and a selection criterion.
% and a greedy Algorithm. As examples we give lexicodes over $\mathbb{Z}_4$ and
$\mathbb{F}_2+u\mathbb{F}_2$. %First, greedy algorithms are presented to
construct %lexicodes using a multiplicative property. Then, greedy algorithms
%are given for the case when the selection criteria is not %multiplicative such
as the minimum distance constraint. It is shown that this construction produces
many optimal codes over rings and also good binary codes. Some of these codes
meet the Gilbert bound. We also obtain optimal self-dual codes, in particular
the octacode. | 1208.2361v2 |
2013-01-14 | magnum.fe: A micromagnetic finite-element simulation code based on FEniCS | We have developed a finite-element micromagnetic simulation code based on the
FEniCS package called magnum.fe. Here we describe the numerical methods that
are applied as well as their implementation with FEniCS. We apply a
transformation method for the solution of the demagnetization-field problem. A
semi-implicit weak formulation is used for the integration of the
Landau-Lifshitz-Gilbert equation. Numerical experiments show the validity of
simulation results. magnum.fe is open source and well documented. The broad
feature range of the FEniCS package makes magnum.fe a good choice for the
implementation of novel micromagnetic finite-element algorithms. | 1301.3061v2 |
2013-02-04 | Control of dissipation in superconducting films by magnetic stray fields | Hybrid superconducting/magnetic nanostructures on Si substrates have been
built with identical physical dimensions but different magnetic configurations.
By constructing arrays based on Co-dots with in-plane, out-of-plane, and vortex
state magnetic configurations, the stray fields are systematically tuned.
Dissipation in the mixed state of superconductors can be decreased (increased)
by several orders of magnitude by decreasing (increasing) the stray magnetic
fields. Furthermore, ordering of the stray fields over the entire array helps
to suppress dissipation and enhance commensurability effects increasing the
number of dissipation minima. | 1302.0680v1 |
2013-02-09 | Lower bounds on the minimum distance of long codes in the Lee metric | The Gilbert type bound for codes in the title is reviewed, both for small and
large alphabets. Constructive lower bounds better than these existential bounds
are derived from geometric codes, either over Fp or Fp2 ; or over even degree
extensions of Fp: In the latter case the approach is concatena- tion with a
good code for the Hamming metric as outer code and a short code for the Lee
metric as an inner code. In the former case lower bounds on the minimum Lee
distance are derived by algebraic geometric arguments inspired by results of
Wu, Kuijper, Udaya (2007). | 1302.2246v1 |
2013-03-04 | Mansuripur's Paradox | A recent article claims that the Lorentz force law is incompatible with
special relativity. We discuss the "paradox" on which this claim is based. The
resolution depends on whether one assumes a "Gilbert" model for the magnetic
dipole (separated monopoles) or the standard "Ampere" model (current loop). The
former was presented in these pages many years ago; the latter requires the
inclusion of "hidden momentum." | 1303.0732v2 |
2013-04-09 | Instability of Walker Propagating Domain Wall in Magnetic Nanowires | Stability of the well-known Walker propagating domain wall (DW) solution of
the Landau-Lifshitz-Gilbert equation is analytically investigated.
Surprisingly, the Walker's rigid body propagating DW mode is not stable against
the spin wave/wavepacket emission. In the low field region only stern spin
waves are emitted while both stern and bow waves are generated under high
fields. In a high enough field, but below the Walker breakdown field, the
Walker solution could be convective/absolute unstable if the transverse
magnetic anisotropy is larger than a critical value, corresponding to a
significant modification of the DW profile and DW propagating speed. | 1304.2439v1 |
2013-04-26 | Maximizing Spin Torque Diode Voltage by Optimizing Magnetization Alignment | The optimum condition of the magnetization alignment to maximize the spin
torque diode voltage is derived by solving the Landau-Lifshitz-Gilbert
equation. We show that the optimized diode voltage can be one order of
magnitude larger than that of the conventional alignment where the easy axes of
the free and the pinned layers are parallel. These analytical predictions are
confirmed by numerical simulations. | 1304.7079v1 |
2013-05-23 | Residual properties of automorphism groups of (relatively) hyperbolic groups | We show that Out(G) is residually finite if G is a one-ended group that is
hyperbolic relative to virtually polycyclic subgroups. More generally, if G is
one-ended and hyperbolic relative to proper residually finite subgroups, the
group of outer automorphisms preserving the peripheral structure is residually
finite. We also show that Out(G) is virtually p-residually finite for every
prime p if G is one-ended and toral relatively hyperbolic, or infinitely-ended
and virtually p-residually finite. | 1305.5403v3 |
2013-06-10 | Imperfect Construction of Microclusters | Microclusters are the basic building blocks used to construct cluster states
capable of supporting fault-tolerant quantum computation. In this paper, we
explore the consequences of errors on microcluster construction using two error
models. To quantify the effect of the errors we calculate the fidelity of the
constructed microclusters and the fidelity with which two such microclusters
can be fused together. Such simulations are vital for gauging the capability of
an experimental system to achieve fault tolerance. | 1306.2202v1 |
2013-06-19 | Weyl Phases in Point-Group Symmetric Superconductors | We study superconductivity in a Weyl semimetal with broken time-reversal
symmetry and stabilized by a point-group symmetry. The resulting
superconducting phase is characterized by topologically protected bulk nodes
and surface states with Fermi arcs. The topological invariant governing the
system is calculated using changes in eigenvalues of the point-group operator
along high-symmetry momentum lines. We show that this invariant is determined
by the Fermi surface topology of the Weyl semimetal. We discuss the effect of
surface orientation and $C_4$-breaking strain as possible experimental
consequences. | 1306.4696v1 |
2013-06-21 | The Polarization Signature of Local Bulk Flows | A large peculiar velocity of the intergalactic medium produces a Doppler
shift of the cosmic microwave background with a frequency-dependent quadrupole
term. This quadrupole will act as a source for polarization of the cosmic
microwave background, creating a large-scale polarization anisotropy if the
bulk flow is local and coherent on large scales. In the case where we are near
the center of the moving region, the polarization signal is a pure quadrupole.
We show that the signal is small, but detectable with future experiments for
bulk flows as large as some recent reports. | 1306.5248v3 |
2013-07-21 | Rigidity in the Positive Mass Theorem with Charge | In this paper we show how a natural coupling of the Dirac equation with the
generalized Jang equation, leads to a proof of the rigidity statement in the
positive mass theorem with charge, without the maximal slicing condition,
provided a solution to the coupled system exists. | 1307.5499v4 |
2014-02-05 | Spin-polarized transport in ferromagnetic multilayers: An unconditionally convergent FEM integrator | We propose and analyze a decoupled time-marching scheme for the coupling of
the Landau-Lifshitz-Gilbert equation with a quasilinear diffusion equation for
the spin accumulation. This model describes the interplay of magnetization and
electron spin accumulation in magnetic and non-magnetic multilayer structures.
Despite the strong nonlinearity of the overall PDE system, the proposed
integrator requires only the solution of two linear systems per time-step.
Unconditional convergence of the integrator towards weak solutions is proved. | 1402.0983v2 |
2014-02-12 | An efficient algorithm for the entropy rate of a hidden Markov model with unambiguous symbols | We demonstrate an efficient formula to compute the entropy rate $H(\mu)$ of a
hidden Markov process with $q$ output symbols where at least one symbol is
unambiguously received. Using an approximation to $H(\mu)$ to the first $N$
terms we give a $O(Nq^3$) algorithm to compute the entropy rate of the hidden
Markov model. We use the algorithm to estimate the entropy rate when the
parameters of the hidden Markov model are unknown.In the case of $q =2$ the
process is the output of the Z-channel and we use this fact to give bounds on
the capacity of the Gilbert channel. | 1402.2733v2 |
2014-03-24 | Skyrmionic spin Seebeck effect via dissipative thermomagnonic torques | We derive thermomagnonic torque and its "$\beta$-type" dissipative correction
from the stochastic Landau-Lifshitz-Gilbert equation. The "$\beta$-type"
dissipative correction describes viscous coupling between magnetic dynamics and
magnonic current and it stems from spin mistracking of the magnetic order. We
show that thermomagnonic torque is important for describing temperature
gradient induced motion of skyrmions in helical magnets while dissipative
correction plays an essential role in generating transverse Magnus force. We
propose to detect such skyrmionic motion by employing the transverse spin
Seebeck effect geometry. | 1403.6160v2 |
2014-08-11 | An optimal irrigation network with infinitely many branching points | The Gilbert-Steiner problem is a mass transportation problem, where the cost
of the transportation depends on the network used to move the mass and it is
proportional to a certain power of the "flow". In this paper, we introduce a
new formulation of the problem, which turns it into the minimization of a
convex functional in a class of currents with coefficients in a group. This
framework allows us to define calibrations, which can be used to prove the
optimality of concrete configurations. We apply this technique to prove the
optimality of a certain irrigation network, having the topological property
mentioned in the title. | 1408.2406v1 |
2014-09-04 | Integer Generalized Splines on Cycles | Let G be a graph whose edges are labeled by positive integers. Label each
vertex with an integer and suppose if two vertices are joined by an edge, the
vertex labels are congruent to each other modulo the edge label. The set of
vertex labels satisfying this condition is called a generalized spline.
Gilbert, Polster, and Tymoczko recently defined generalized splines based on
work on polynomial splines by Billera, Rose, Haas, Goresky-Kottwitz-Machperson,
and many others. We focus on generalized splines on $n$-cycles. We construct a
particularly nice basis for the module of splines on $n$-cycles. As an
application, we construct generalized splines on star graphs, wheel graphs, and
complete graphs. | 1409.1481v1 |
2014-09-10 | Proof of the Riemannian Penrose Inequality with Charge for Multiple Black Holes | We present a proof of the Riemannian Penrose inequality with charge in the
context of asymptotically flat initial data sets for the Einstein-Maxwell
equations, having possibly multiple black holes with no charged matter outside
the horizon, and satisfying the relevant dominant energy condition. The proof
is based on a generalization of Hubert Bray's conformal flow of metrics adapted
to this setting. | 1409.3271v2 |
2014-09-15 | Enhanced synchronization in an array of spin torque nano oscillators in the presence of oscillating external magnetic field | We demonstrate that the synchronization of an array of electrically coupled
spin torque nano-oscillators (STNO) modelled by
Landau-Lifshitz-Gilbert-Slonczewski (LLGS) equation can be enhanced appreciably
in the presence of a common external microwave magnetic field. The applied
microwave magnetic field stabilizes and enhances the regions of synchronization
in the parameter space of our analysis, where the oscillators are exhibiting
synchronized oscillations thereby emitting improved microwave power. To
characterize the synchronized oscillations we have calculated the locking range
in the domain of external source frequency. | 1409.4384v2 |
2014-10-19 | Extensions of the Charged Riemannian Penrose Inequality | In this paper we investigate the extension of the charged Riemannian Penrose
inequality to the case where charges are present outside the horizon. We prove
a positive result when the charge densities are compactly supported, and
present a counterexample when the charges extend to infinity. We also discuss
additional extensions to other matter models. | 1410.5027v2 |
2014-10-23 | Quantum spin dynamics | The classical Landau-Lifshitz equation has been derived from quantum
mechanics. Starting point is the assumption of a non-Hermitian Hamilton
operator to take the energy dissipation into account. The corresponding quantum
mechanical time dependent Schr\"odinger, Liouville and Heisenberg equation have
been described and the similarities and differences between classical and
quantum mechanical spin dynamics have been discussed. Furthermore, a time
dependent Schr\"odinger equation corresponding to the classical
Landau-Lifshitz-Gilbert equation and two ways to include temperature into the
quantum mechanical spin dynamics have been proposed. | 1410.6383v1 |
2014-12-04 | Role of Magnetic Field in Self-Oscillation of Nanomagnet Excited by Spin Torque | The critical current of the self-oscillation of spin torque oscillator (STO)
consisting of a perpendicularly magnetized free layer and an in-plane
magnetized pinned layer was studied by solving the Landau-Lifshitz-Gilbert
(LLG) equation. We found that the critical current diverged at certain field
directions, indicating that the self-oscillation does not occur at these
directions. It was also found that the sign of the critical current changed
depending on the applied field direction. | 1412.1532v1 |
2015-01-30 | Memory functions of magnetic skyrmions | Magnetic skyrmion, a swirling spin texture, in chiral magnets is
characterized by (i) nano-scale size ($\sim$1nm -- 100nm), (ii) topological
stability, and (iii) gyro-dynamics. These features are shown to be advantageous
for (a) high-density data-storage, (b) nonvolatile memory, and (c) ultra-low
current and energy cost manipulation, respectively. By the numerical
simulations of Landau-Lifshitz-Gilbert equation, the elementary functions of
skyrmions are demonstrated aiming at the design principles of skyrmionic memory
devices. | 1501.07650v1 |
2015-03-16 | Impact of Current on Static and Kinetic Depinning Fields of Domain Wall in Ferromagnetic Nanostrip | The impact of current on static and kinetic depinning fields of a domain wall
in an one dimensional ferromagnetic nanostrip is investigated by solving the
Landau-Lifshitz-Gilbert equation with adiabatic and non-adiabatic spin-transfer
torques analytically and numerically. The results show that in the absence of
current, the static depinning field is greater than the kinetic depinning field
and both the depinning fields decrease by the increase of current applied in a
direction opposite to the direction of the applied field. Both the depinning
fields can also be tuned by the current to make them equal. | 1503.04553v1 |
2015-04-17 | Critical analysis and remedy of switching failures in straintronic logic using Bennett clocking in the presence of thermal fluctuations | Straintronic logic is a promising platform for beyond Moore's law computing.
Using Bennett clocking mechanism, information can propagate through an array of
strain-mediated multiferroic nanomagnets exploiting the dipolar coupling
between the magnets without having to physically interconnect them. Here we
perform a critical analysis of switching failures, i.e., error in information
propagation due to thermal fluctuations through a chain of such straintronic
devices. We solved stochastic Landau-Lifshitz-Gilbert equation considering
room-temperature thermal perturbations and show that magnetization switching
may fail due to inherent magnetization dynamics accompanied by thermally
broadened switching delay distribution. Avenues available to circumvent such
issue are proposed. | 1504.04618v1 |
2015-06-18 | Landauer limit of energy dissipation in a magnetostrictive particle | According to Landauer's principle, a minimum amount of energy proportional to
temperature must be dissipated during the erasure of a classical bit of
information compensating the entropy loss, thereby linking the information and
thermodynamics. Here we show that the Landauer limit of energy dissipation is
achievable in a shape-anisotropic single-domain magnetostrictive nanomagnet
having two mutually anti-parallel degenerate magnetization states that store a
bit of information. We model the magnetization dynamics using stochastic
Landau-Lifshitz-Gilbert equation in the presence of thermal fluctuations and
show that on average the Landauer bound is satisfied, i.e., it accords to the
generalized Landauer's principle for small systems with stochastic
fluctuations. | 1506.07897v1 |
2015-06-29 | Pseudo-Spin Based Dynamical Model for Polarisation Switching in Ferroelectrics | A microscopic view of the response of the electric dipoles to a dynamic
external field in a ferroelectric (FE) chain has been studied by two spin
dynamics methods. One is the prominent micromagnetic approach, and the other is
the micromagnetic approach with a variable size of the pseudo-spin. The energy
stored in the ferroelectric chain is described by the transverse Ising model
(TIM) with electric pseudo-spins. The simulations are based on a modified
Landau-Lifshitz-Gilbert (LLG) equation which is precession free. The results
obtained are shown and compared with the result supplemented by
Landau-Devonshire (L-D) theory in the Appendix. | 1506.08500v2 |
2015-07-13 | Explicit Construction of AG Codes from Generalized Hermitian Curves | We present multi-point algebraic geometric codes overstepping the
Gilbert-Varshamov bound. The construction is based on the generalized Hermitian
curve introduced by A. Bassa, P. Beelen, A. Garcia, and H. Stichtenoth. These
codes are described in detail by constrcting a generator matrix. It turns out
that these codes have nice properties similar to those of Hermitian codes. It
is shown that the duals are also such codes and an explicit formula is given. | 1507.03418v1 |
2015-07-22 | Reflexive insensitive modal logics | We analyze a class of modal logics rendered insensitive to reflexivity by way
of a modification to the semantic definition of the modal operator. We explore
the extent to which these logics can be characterized, and prove a general
completeness theorem on the basis of a translation between normal modal logics
and their reflexive-insensitive counterparts. Lastly, we provide a sufficient
semantic condition describing when a similarly general soundness result is also
available. | 1507.06113v1 |
2015-08-24 | Spin Dynamics Simulation of the Magneto-Electric Effect in a Composite Multiferroic Chain | A composite multiferroic chain with an interfacial linear magneto-electric
coupling is used to study the magnetic and electric responses to an external
magnetic or electric field. The simulation uses continuous spin dynamics
through the Landau-Lifshitz-Gilbert equations of the magnetic spin and the
electric pseudo-spin. The results demonstrate an accurate description of the
distribution of the magnetisation and polarisation are induced by applied
electric and magnetic field, respectively. | 1508.05693v1 |
2015-10-16 | Reaction diffusion dynamics and the Schryer-Walker solution for domain walls of the Landau-Lifshitz-Gilbert equation | We study the dynamics of the equation obtained by Schryer and Walker for the
motion of domain walls. The reduced equation is a reaction diffusion equation
for the angle between the applied field and the magnetization vector. If the
hard axis anisotropy $K_d$ is much larger than the easy axis anisotropy $K_u$,
there is a range of applied fields where the dynamics does not select the
Schryer-Walker solution. We give analytic expressions for the speed of the
domain wall in this regime and the conditions for its existence. | 1510.04927v1 |
2015-11-06 | Dynamic Response in a Finite Size Composite Multiferroic Thin Film | Composite multiferroics, heterostructures of ferromagnetic (FM) and
ferroelectric (FE) materials, are characterized by a remarkable magnetoelectric
effect at the interface. Previous work has supported the ferromagnetic
structure with magnetic spins and the ferroelectric with pseudospins which act
as electric dipoles in a microscopic model, coupled with a magnetoelectric
interaction [J. Appl. Phys. 118, 124109 (2015)]. In this work, by solving the
stochastic Landau-Lifshitz-Gilbert equation, the electric-field-induced
magnetization switching in a twisted boundary condition has been studied, and a
behavior of domain wall in the ferromagnetic structure is discussed. | 1511.01982v2 |
2016-01-18 | On Simultaneous Percolation with Two Disk Types | In this paper we consider the simultaneous percolation of two Gilbert disk
models. The two models are connected through excluding disks, which prevent
elements of the second model to be in the vicinity of the first model. Under
these assumptions we characterize the region of densities in which the two
models both have a unique infinite connected component. The motivation for this
work is the co-existence of two cognitive radio networks. | 1601.04471v2 |
2016-01-22 | Toward a New Microscopic Framework for Kondo Lattice Materials | Understanding the emergence and subsequent behavior of heavy electrons in
Kondo lattice materials is one of the grand challenges in condensed matter
physics. In this perspective we review the progress that has been made during
the past decade and suggest some directions for future research. Our focus will
be on developing a new microscopic framework that incorporates the basic
concepts that emerge from a phenomenological description of the key
experimental findings. | 1601.06050v1 |
2016-02-01 | The Eddy Current-LLG Equations-Part I: FEM-BEM Coupling | We analyse a numerical method for the coupled system of the eddy current
equations in $\mathbb{R}^3$ with the Landau-Lifshitz-Gilbert equation in a
bounded domain. The unbounded domain is discretised by means of
finite-element/boundary-element coupling. Even though the considered problem is
strongly nonlinear, the numerical approach is constructed such that only two
linear systems per time step have to be solved. In this first part of the
paper, we prove unconditional weak convergence (of a subsequence) of the
finite-element solutions towards a weak solution. A priori error estimates will
be presented in the second part. | 1602.00744v1 |
2016-02-01 | The Eddy Current--LLG Equations: FEM-BEM Coupling and A Priori Error Estimates | We analyze a numerical method for the coupled system of the eddy current
equations in $\mathbb{R}^3$ with the Landau-Lifshitz-Gilbert equation in a
bounded domain. The unbounded domain is discretized by means of
finite-element/boundary-element coupling. Even though the considered problem is
strongly nonlinear, the numerical approach is constructed such that only two
linear systems per time step have to be solved. We prove unconditional weak
convergence (of a subsequence) of the finite-element solutions towards a weak
solution. We establish a priori error estimates if a sufficiently smooth strong
solution exists. Numerical experiments underlining the theoretical results are
presented. | 1602.00745v2 |
2016-02-24 | Partial Category Actions on Sets and Topological Spaces | We introduce (continuous) partial category actions on sets (topological
spaces) and show that each such action admits a universal globalization.
Thereby, we obtain a simultaneous generalization of corresponding results for
groups, by Kellendonk and Lawson, and for monoids, by Megrelishvili and
Schroder. We apply this result to the special case of partial groupoid actions
where we obtain a sharpening of a result by Gilbert, concerning ordered
groupoids, in the sense that mediating functions between universal
globalizations always are injective. | 1602.07541v4 |
2016-05-20 | Interlayer interaction in multilayer CoPt/Co structures | We report a study of interlayer exchange interaction in multilayer CoPt/Co
structures consisting of periodic CoPt multilayer film with an "easy axis"
anisotropy and thick Co layer with an "easy plane" anisotropy separated by Pt
spacer with variable thickness. The magnetooptical Kerr effect (MOKE) and
ferromagnetic resonance (FMR) measurements show up the essentially
non-collinear state of magnetic moments of the layers and strong exchange
coupling between CoPt and Co subsystems. The estimation of effective anisotropy
and exchange coupling in a simple model based on the Landau-Lifshitz-Gilbert
equation describing magnetization dynamics was performed. | 1605.06468v1 |
2016-07-12 | Tailoring the topological details of the magnetic skyrmion by the spin configuration at the edges | The magnetic skyrmion structure can be formed in the chiral magnets (CMs)
with strong Dzyaloshinskii-Moriya interactions. In this work, we propose a way
of artificially tailoring the topological details of the skyrmion such as its
radial and whirling symmetric patterns by external magnetic fields besieging
the CM slab. As long as the boundary magnetic fields are strong enough to fix
the boundary ferromagnetism, the attained skyrmion profile is stable over time.
The dynamics of spins is considered by numerically solving the non-equilibrium
Landau-Lifshitz-Gilbert equation. | 1607.03268v2 |
2016-09-07 | Entanglement manipulation by a magnetic pulse in Gd3N@C80 endohedral metallofullerenes on a Cu(001) surface | In this paper we present result of theoretical calculation of entanglement
within a spin structure of Gd3N@C80 under the in uence of rectangular impulses.
Research is conducted using general spin Hamiltonian within SSNQ (spin system
of N-qubits). Calculation of entanglement with variable impulse is performed
using the time-dependent Landau-Lifshitz-Gilbert equation with spin-spin
correlation function. We show that long rectangular impulse (t=850ps) can be
used for maintaining of entanglement value. This allows us to offer a new
algorithm which can be used to reduce the challenge of decoherence to logical
scheme optimization. | 1609.01959v1 |
2016-09-10 | Optical tomography on graphs | We present an algorithm for solving inverse problems on graphs analogous to
those arising in diffuse optical tomography for continuous media. In
particular, we formulate and analyze a discrete version of the inverse Born
series, proving estimates characterizing the domain of convergence,
approximation errors, and stability of our approach. We also present a
modification which allows additional information on the structure of the
potential to be incorporated, facilitating recovery for a broader class of
problems. | 1609.03041v1 |
2016-09-30 | Interaction Between a Domain Wall and Spin Supercurrent in Easy-cone Magnets | A domain wall and spin supercurrent can coexist in magnets with easy-cone
anisotropy owing to simultaneous spontaneous breaking of Z$_2$ and U(1)
symmetries. Their interaction is theoretically investigated in quasi
one-dimensional ferromagnets within the Landau-Lifshitz-Gilbert phenomenology.
Specifically, we show that spin supercurrent can exert the torque on a domain
wall and thereby drive it. We also show, as a reciprocal phenomenon, a
field-induced motion of a domain wall can generate spin supercurrent. | 1610.00034v1 |
2016-10-04 | A Local Inverse Formula and a Factorization | When a matrix has a banded inverse there is a remarkable formula that quickly
computes that inverse, using only local information in the original matrix.
This local inverse formula holds more generally, for matrices with sparsity
patterns that are examples of chordal graphs or perfect eliminators. The
formula has a long history going back at least as far as the completion problem
for covariance matrices with missing data. Maximum entropy estimates,
log-determinants, rank conditions, the Nullity Theorem and wavelets are all
closely related, and the formula has found wide applications in machine
learning and graphical models. We describe that local inverse and explain how
it can be understood as a matrix factorization. | 1610.01230v1 |
2016-10-10 | Parametric frequency mixing in the magneto-elastically driven FMR-oscillator | We demonstrate the nonlinear frequency conversion of ferromagnetic resonance
(FMR) frequency by optically excited elastic waves in a thin metallic film on
dielectric substrates. Time-resolved probing of the magnetization directly
witnesses magneto-elastically driven second harmonic generation, sum- and
difference frequency mixing from two distinct frequencies, as well as
parametric downconversion of each individual drive frequency. Starting from the
Landau-Lifshitz-Gilbert equations, we derive an analytical equation of an
elastically driven nonlinear parametric oscillator and show that frequency
mixing is dominated by the parametric modulation of FMR frequency. | 1610.02926v1 |
2016-11-05 | Magnonic analog of relativistic Zitterbewegung in an antiferromagnetic spin chain | We theoretically investigate the spin wave (magnon) excitations in a
classical antiferromagnetic spin chain with easy-axis anisotropy. We obtain a
Dirac-like equation by linearizing the Landau- Lifshitz-Gilbert equation in
this antiferromagnetic system, in contrast to the ferromagnetic system in which
a Schr\"{o}dinger equation is derived. The Hamiltonian operator in the
Dirac-like equation is a pseudo-Hermitian. We compute and demonstrate the
relativistic Zitterbewegung (trembling motion) in the antiferromagnetic spin
chain by measuring the expectation values of the wave packet position. | 1611.01512v2 |
2016-11-15 | The norm of the Fourier transform on compact or discrete abelian groups | We calculate the norm of the Fourier operator from $L^p(X)$ to $L^q(\hat{X})$
when $X$ is an infinite locally compact abelian group that is, furthermore,
compact or discrete. This subsumes the sharp Hausdorff-Young inequality on such
groups. In particular, we identify the region in $(p,q)$-space where the norm
is infinite, generalizing a result of Fournier, and setting up a contrast with
the case of finite abelian groups, where the norm was determined by Gilbert and
Rzeszotnik. As an application, uncertainty principles on such groups expressed
in terms of R\'enyi entropies are discussed. | 1611.04692v1 |
2017-02-02 | Magnon Condensation and Spin Superfluidity | We consider the phenomenon of Bose-Einstein condensation of quasi-equilibrium
magnons which leads to a spin superfluidity, the coherent quantum transfer of
magnetization in magnetic materials. These phenomena are beyond the classical
Landau-Lifshitz-Gilbert paradigm. The critical conditions for excited magnon
density for ferro- and antiferromagnets, bulk and thin films are estimated and
discussed. The BEC should occur in the antiferromagnetic hematite at much lower
excited magnon density compared to the ferromagnetic YIG. | 1702.00846v2 |
2017-02-09 | Transient spin dynamics in a single-molecule magnet | We explore the limitations and validity of semi-classically formulated spin
equations of motion. Using a single-molecule magnet as a test model, we employ
three qualitatively different approximation schemes. From a microscopic model,
we derive a generalized spin equation of motion in which the parameters have a
non-local time-dependence. This dynamical equation is simplified to the
Landau-Lifshitz-Gilbert equation with i) time-dependent, and ii)
time-independent parameters. We show that transient dynamics is essentially
non-existing in the latter approximation, while the former breaks down in the
regime of strong coupling between the spin and the itinerant electrons. | 1702.02820v2 |
2017-10-30 | Lattice calculation of hadronic tensor of the nucleon | We report an attempt to calculate the deep inelastic scattering structure
functions from the hadronic tensor calculated on the lattice. We used the
Backus-Gilbert reconstruction method to address the inverse Laplace
transformation for the analytic continuation from the Euclidean to the
Minkowski space. | 1710.11145v1 |
2017-10-31 | TF Boosted Trees: A scalable TensorFlow based framework for gradient boosting | TF Boosted Trees (TFBT) is a new open-sourced frame-work for the distributed
training of gradient boosted trees. It is based on TensorFlow, and its
distinguishing features include a novel architecture, automatic loss
differentiation, layer-by-layer boosting that results in smaller ensembles and
faster prediction, principled multi-class handling, and a number of
regularization techniques to prevent overfitting. | 1710.11555v1 |
2017-11-08 | Micromagnetic simulation study of a disordered model for one-dimensional granular perovskite manganite oxide nanostructures | Chemical techniques are an efficient method to synthesize one-dimensional
perovskite manganite oxide nanostructures with a granular morphology, that is,
formed by arrays of monodomain magnetic nanoparticles. Integrating the
stochastic Landau-Lifshitz-Gilbert equation, we simulate the dynamics of a
simple disordered model for such materials that only takes into account the
morphological characteristics of their nanograins. We show that it is possible
to describe reasonably well experimental hysteresis loops reported in the
literature for single La_0.67Ca_0.33MnO_3 nanotubes and powders of these
nanostructures, simulating small systems consisting of only 100 nanoparticles. | 1711.03159v2 |
2017-11-21 | Construction of asymptotically good locally repairable codes via automorphism groups of function fields | Locally repairable codes have been investigated extensively in recent years
due to practical application in distributed storage as well as theoretical
interest. However, not much work on asymptotical behavior of locally repairable
codes has been done until now. In particular, there is a little result on
constructive lower bound on asymptotical behavior of locally repairable codes.
In this paper, we extend the construction given in \cite{BTV17} via
automorphism groups of function field towers. The main advantage of our
construction is to allow more flexibility of locality. Furthermore, we show
that the Gilbert-Varshamov type bound on locally repairable codes can be
improved for all sufficiently large alphabet size $q$. | 1711.07703v1 |
2017-11-21 | Stability of axisymmetric chiral skyrmions | We examine topological solitons in a minimal variational model for a chiral
magnet, so-called chiral skyrmions. In the regime of large background fields,
we prove linear stability of axisymmetric chiral skyrmions under arbitrary
perturbations in the energy space, a long-standing open question in physics
literature. Moreover, we show strict local minimality of axisymmetric chiral
skyrmions and nearby existence of moving soliton solution for the
Landau-Lifshitz-Gilbert equation driven by a small spin transfer torque. | 1711.07717v1 |
2018-01-25 | Pharmacokinetics Simulations for Studying Correlates of Prevention Efficacy of Passive HIV-1 Antibody Prophylaxis in the Antibody Mediated Prevention (AMP) Study | A key objective in two phase 2b AMP clinical trials of VRC01 is to evaluate
whether drug concentration over time, as estimated by non-linear mixed effects
pharmacokinetics (PK) models, is associated with HIV infection rate. We
conducted a simulation study of marker sampling designs, and evaluated the
effect of study adherence and sub-cohort sample size on PK model estimates in
multiple-dose studies. With m=120, even under low adherence (about half of
study visits missing per participant), reasonably unbiased and consistent
estimates of most fixed and random effect terms were obtained. Coarsened marker
sampling schedules were also studied. | 1801.08626v1 |
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