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2014-10-17 | The fixed irreducible bridge ensemble for self-avoiding walks | We define a new ensemble for self-avoiding walks in the upper half-plane, the
fixed irredicible bridge ensemble, by considering self-avoiding walks in the
upper half-plane up to their $n$-th bridge height, $Y_n$, and scaling the walk
by $1/Y_n$ to obtain a curve in the unit strip, and then taking $n\to\infty$.
We then conjecture a relationship between this ensemble to $\SLE$ in the unit
strip from $0$ to a fixed point along the upper boundary of the strip,
integrated over the conjectured exit density of self-avoiding walk spanning a
strip in the scaling limit. We conjecture that there exists a positive constant
$\sigma$ such that $n^{-\sigma}Y_n$ converges in distribution to that of a
stable random variable as $n\to\infty$. Then the conjectured relationship
between the fixed irreducible bridge scaling limit and $\SLE$ can be described
as follows: If one takes a SAW considered up to $Y_n$ and scales by $1/Y_n$ and
then weights the walk by $Y_n$ to an appropriate power, then in the limit
$n\to\infty$, one should obtain a curve from the scaling limit of the
self-avoiding walk spanning the unit strip. In addition to a heuristic
derivation, we provide numerical evidence to support the conjecture and give
estimates for the boundary scaling exponent. | 1410.4796v1 |
2017-02-18 | Inf-sup stable finite-element methods for the Landau--Lifshitz--Gilbert and harmonic map heat flow equation | In this paper we propose and analyze a finite element method for both the
harmonic map heat and Landau--Lifshitz--Gilbert equation, the time variable
remaining continuous. Our starting point is to set out a unified saddle point
approach for both problems in order to impose the unit sphere constraint at the
nodes since the only polynomial function satisfying the unit sphere constraint
everywhere are constants. A proper inf-sup condition is proved for the Lagrange
multiplier leading to the well-posedness of the unified formulation. \emph{A
priori} energy estimates are shown for the proposed method.
When time integrations are combined with the saddle point finite element
approximation some extra elaborations are required in order to ensure both
\emph{a priori} energy estimates for the director or magnetization vector
depending on the model and an inf-sup condition for the Lagrange multiplier.
This is due to the fact that the unit length at the nodes is not satisfied in
general when a time integration is performed. We will carry out a linear Euler
time-stepping method and a non-linear Crank--Nicolson method. The latter is
solved by using the former as a non-linear solver. | 1702.05588v2 |
2010-04-07 | Concatenated quantum codes can attain the quantum Gilbert-Varshamov bound | A family of quantum codes of increasing block length with positive rate is
asymptotically good if the ratio of its distance to its block length approaches
a positive constant. The asymptotic quantum Gilbert-Varshamov (GV) bound states
that there exist $q$-ary quantum codes of sufficiently long block length $N$
having fixed rate $R$ with distance at least $N H^{-1}_{q^2}((1-R)/2)$, where
$H_{q^2}$ is the $q^2$-ary entropy function. For $q < 7$, only random quantum
codes are known to asymptotically attain the quantum GV bound. However, random
codes have little structure. In this paper, we generalize the classical result
of Thommesen to the quantum case, thereby demonstrating the existence of
concatenated quantum codes that can asymptotically attain the quantum GV bound.
The outer codes are quantum generalized Reed-Solomon codes, and the inner codes
are random independently chosen stabilizer codes, where the rates of the inner
and outer codes lie in a specified feasible region. | 1004.1127v6 |
2021-12-21 | Exponential decay of intersection volume with applications on list-decodability and Gilbert-Varshamov type bound | We give some natural sufficient conditions for balls in a metric space to
have small intersection. Roughly speaking, this happens when the metric space
is (i) expanding and (ii) well-spread, and (iii) a certain random variable on
the boundary of a ball has a small tail. As applications, we show that the
volume of intersection of balls in Hamming, Johnson spaces and symmetric groups
decay exponentially as their centers drift apart. To verify condition (iii), we
prove some large deviation inequalities `on a slice' for functions with
Lipschitz conditions.
We then use these estimates on intersection volumes to
$\bullet$ obtain a sharp lower bound on list-decodability of random $q$-ary
codes, confirming a conjecture of Li and Wootters; and
$\bullet$ improve the classical bound of Levenshtein from 1971 on constant
weight codes by a factor linear in dimension, resolving a problem raised by
Jiang and Vardy.
Our probabilistic point of view also offers a unified framework to obtain
improvements on other Gilbert--Varshamov type bounds, giving conceptually
simple and calculation-free proofs for $q$-ary codes, permutation codes, and
spherical codes. Another consequence is a counting result on the number of
codes, showing ampleness of large codes. | 2112.11274v2 |
2022-05-29 | Generalized Stochastic Matching | In this paper, we generalize the recently studied Stochastic Matching problem
to more accurately model a significant medical process, kidney exchange, and
several other applications. Up until now the Stochastic Matching problem that
has been studied was as follows: given a graph G = (V, E), each edge is
included in the realized sub-graph of G mutually independently with probability
p_e, and the goal is to find a degree-bounded sub-graph Q of G that has an
expected maximum matching that approximates the expected maximum matching of
the realized sub-graph. This model does not account for possibilities of vertex
dropouts, which can be found in several applications, e.g. in kidney exchange
when donors or patients opt out of the exchange process as well as in online
freelancing and online dating when online profiles are found to be faked. Thus,
we will study a more generalized model of Stochastic Matching in which vertices
and edges are both realized independently with some probabilities p_v, p_e,
respectively, which more accurately fits important applications than the
previously studied model.
We will discuss the first algorithms and analysis for this generalization of
the Stochastic Matching model and prove that they achieve good approximation
ratios. In particular, we show that the approximation factor of a natural
algorithm for this problem is at least $0.6568$ in unweighted graphs, and $1/2
+ \epsilon$ in weighted graphs for some constant $\epsilon > 0$. We further
improve our result for unweighted graphs to $2/3$ using edge degree constrained
subgraphs (EDCS). | 2205.14717v1 |
2009-10-20 | Bifurcation and chaos in spin-valve pillars in a periodic applied magnetic field | We study the bifurcation and chaos scenario of the macro-magnetization vector
in a homogeneous nanoscale-ferromagnetic thin film of the type used in
spin-valve pillars. The underlying dynamics is described by a generalized
Landau-Lifshitz-Gilbert (LLG) equation. The LLG equation has an especially
appealing form under a complex stereographic projection, wherein the
qualitative equivalence of an applied field and a spin-current induced torque
is transparent. Recently chaotic behavior of such a spin vector has been
identified by Zhang and Li using a spin polarized current passing through the
pillar of constant polarization direction and periodically varying magnitude,
owing to the spin-transfer torque effect. In this paper we show that the same
dynamical behavior can be achieved using a periodically varying applied
magnetic field, in the presence of a constant DC magnetic field and constant
spin current, which is technically much more feasible, and demonstrate
numerically the chaotic dynamics in the system for an infinitely thin film.
Further, it is noted that in the presence of a nonzero crystal anisotropy field
chaotic dynamics occurs at much lower magnitudes of the spin-current and DC
applied field. | 0910.3776v1 |
2021-11-21 | Explicit complex-valued solutions of the 2D eikonal equation | We present a method to obtain explicit solutions of the complex eikonal
equation in the plane. This equation arises in the approximation of Helmholtz
equation by the WKBJ or EWT methods. We obtain the complex-valued solutions
(called eikonals) as parameterizations in a complex variable. We consider both
the cases of constant and non-constant index of refraction. In both cases, the
relevant parameterizations depend on some holomorphic function. In the case of
non-constant index of refraction, the parametrization also depends on some
extra exponential complex-valued function and on a quasi-conformal
homeomorphism. This is due to the use of the theory of pseudo-analytic
functions and the related similarity principle. The parameterizations give
information about the formation of caustics and the light and shadow regions
for the relevant eikonals. | 2111.10852v1 |
1996-01-09 | Relaxation of Collective Excitations in LJ-13 Cluster | We have performed classical molecular dynamics simulation of $Ar_{13}$
cluster to study the behavior of collective excitations. In the solid ``phase''
of the cluster, the collective oscillation of the monopole mode can be well
fitted to a damped harmonic oscillator. The parameters of the equivalent damped
harmonic oscillator-- the damping coefficient, spring constant, time period of
oscillation and the mass of the oscillator -- all show a sharp change in
behavior at a kinetic temperature of about $7.0^oK$. This marks yet another
characteristic temperature of the system, a temperature $T_s$ below which
collective excitations are very stable, and at higher temperatures the single
particle excitations cause the damping of the collective oscillations. We argue
that so long as the cluster remains confined within the global potential energy
minimum the collective excitations do not decay; and once the cluster comes out
of this well, the local potential energy minima pockets act as single particle
excitation channels in destroying the collective motion. The effect is manifest
in almost all the physical observables of the cluster. | 9601026v2 |
2005-04-22 | Constraint damping in the Z4 formulation and harmonic gauge | We show that by adding suitable lower-order terms to the Z4 formulation of
the Einstein equations, all constraint violations except constant modes are
damped. This makes the Z4 formulation a particularly simple example of a
lambda-system as suggested by Brodbeck et al. We also show that the Einstein
equations in harmonic coordinates can be obtained from the Z4 formulation by a
change of variables that leaves the implied constraint evolution system
unchanged. Therefore the same method can be used to damp all constraints in the
Einstein equations in harmonic gauge. | 0504114v2 |
2008-10-21 | On Wigner functions and a damped star product in dissipative phase-space quantum mechanics | Dito and Turrubiates recently introduced an interesting model of the
dissipative quantum mechanics of a damped harmonic oscillator in phase space.
Its key ingredient is a non-Hermitian deformation of the Moyal star product
with the damping constant as deformation parameter. We compare the
Dito-Turrubiates scheme with phase-space quantum mechanics (or deformation
quantization) based on other star products, and extend it to incorporate Wigner
functions. The deformed (or damped) star product is related to a complex
Hamiltonian, and so necessitates a modified equation of motion involving
complex conjugation. We find that with this change the Wigner function
satisfies the classical equation of motion. This seems appropriate since
non-dissipative systems with quadratic Hamiltonians share this property. | 0810.3893v1 |
2017-09-12 | Temperature effects on MIPs in the BGO calorimeters of DAMPE | In this paper, we presented a study of temperature effects on BGO
calorimeters using proton MIP's collected in the first year operation of DAMPE.
By directly comparing MIP calibration constants used by DAMPE data production
pipe line, we found an experimental relation between temperature and signal
amplitudes of each BGO bar: a general deviation of -1.162%/$^{\circ}$C,and
-0.47%/$^{\circ}$C to -1.60%/$^{\circ}$C statistically for each detector
element. During 2016, DAMPE's temperature changed by about 7 degrees due to
solar elevation angle and the corresponding energy scale bias is about 8%. By
frequent MIP calibration operation, this kind of bias is eliminated to an
acceptable value. | 1709.03735v2 |
2015-02-01 | Nonlocal Damping of Helimagnets in One-Dimensional Interacting Electron Systems | We investigate the magnetization relaxation of a one-dimensional helimagnetic
system coupled to interacting itinerant electrons. The relaxation is assumed to
result from the emission of plasmons, the elementary excitations of the
one-dimensional interacting electron system, caused by slow changes of the
magnetization profile. This dissipation mechanism leads to a highly nonlocal
form of magnetization damping that is strongly dependent on the
electron-electron interaction. Forward scattering processes lead to a spatially
constant damping kernel, while backscattering processes produce a spatially
oscillating contribution. Due to the nonlocal damping, the thermal fluctuations
become spatially correlated over the entire system. We estimate the
characteristic magnetization relaxation times for magnetic quantum wires and
nuclear helimagnets. | 1502.00268v2 |
2017-07-08 | Nonlinear dynamics of damped DNA systems with long-range interactions | We investigate the nonlinear dynamics of a damped Peyrard-Bishop DNA model
taking into account long-range interactions with distance dependence |l|^-s on
the elastic coupling constant between different DNA base pairs. Considering
both Stokes and long-range hydrodynamical damping forces, we use the discrete
difference operator technique and show in the short wavelength modes that the
lattice equation can be governed by the complex Ginzburg-Landau equation. We
found analytically that the technique leads to the correct expression for the
breather soliton parameters. We found that the viscosity makes the amplitude of
the breather to damp out. We compare the approximate analytic results with
numerical simulations for the value s = 3 (dipole-dipole interactions). | 1707.02425v1 |
2019-07-10 | Determination of the damping co-efficient of electrons in optically transparent glasses at the true resonance frequency in the ultraviolet from an analysis of the Lorentz-Maxwell model of dispersion | The Lorentz-Maxwell model of dispersion of light has been analyzed in this
paper to determine the true resonance frequency in the ultraviolet for the
electrons in optically transparent glasses and the damping coefficient at this
frequency. For this we needed the refractive indices of glass in the optical
frequency range. We argue that the true resonance condition in the absorption
region prevails when the frequency at which the absorption coefficient is
maximum is the same as the frequency at which the average energy per cycle of
the electrons is also a maximum. We have simultaneously solved the two
equations obtained from the two maxima conditions numerically to arrive at a
unique solution for the true resonance frequency and the damping coefficient at
this frequency. Assuming the damping coefficient to be constant over a small
frequency range in the absorption region, we have determined the frequencies at
which the extinction coefficient and the reflectance are maxima. These
frequencies match very well with the published data for silica glasses
available from the literature. | 1907.04499v1 |
2019-07-21 | Critical Thresholds in One Dimensional Damped Euler-Poisson Systems | This paper is concerned with the critical threshold phenomenon for one
dimensional damped, pressureless Euler-Poisson equations with electric force
induced by a constant background, originally studied in [S. Engelberg and H.
Liu and E. Tadmor, Indiana Univ. Math. J., 50:109--157, 2001]. A simple
transformation is used to linearize the characteristic system of equations,
which allows us to study the geometrical structure of critical threshold curves
for three damping cases: overdamped, underdamped and borderline damped through
phase plane analysis. We also derive the explicit form of these critical
curves. These sharp results state that if the initial data is within the
threshold region, the solution will remain smooth for all time, otherwise it
will have a finite time breakdown. Finally, we apply these general results to
identify critical thresholds for a non-local system subjected to initial data
on the whole line. | 1907.09039v1 |
2022-06-17 | Resolvent estimates for the one-dimensional damped wave equation with unbounded damping | We study the generator $G$ of the one-dimensional damped wave equation with
unbounded damping. We show that the norm of the corresponding resolvent
operator, $\| (G - \lambda)^{-1} \|$, is approximately constant as $|\lambda|
\to +\infty$ on vertical strips of bounded width contained in the closure of
the left-hand side complex semi-plane, $\overline{\mathbb{C}}_{-} := \{\lambda
\in \mathbb{C}: \operatorname{Re} \lambda \le 0\}$. Our proof rests on a
precise asymptotic analysis of the norm of the inverse of $T(\lambda)$, the
quadratic operator associated with $G$. | 2206.08820v2 |
2023-12-14 | Smoluchowski-Kramers diffusion approximation for systems of stochastic damped wave equations with non-constant friction | We consider systems of damped wave equations with a state-dependent damping
coefficient and perturbed by a Gaussian multiplicative noise. Initially, we
investigate their well-posedness, under quite general conditions on the
friction. Subsequently, we study the validity of the so-called
Smoluchowski-Kramers diffusion approximation. We show that, under more
stringent conditions on the friction, in the small-mass limit the solution of
the system of stochastic damped wave equations converges to the solution of a
system of stochastic quasi-linear parabolic equations. In this convergence, an
additional drift emerges as a result of the interaction between the noise and
the state-dependent friction. The identification of this limit is achieved by
using a suitable generalization of the classical method of perturbed test
functions, tailored to the current infinite dimensional setting. | 2312.08925v1 |
2024-01-01 | Magnon Damping Minimum and Logarithmic Scaling in a Kondo-Heisenberg Model | Recently, an anomalous temperature evolution of spin wave excitations has
been observed in a van der Waals metallic ferromagnet Fe$_3$GeTe$_2$ (FGT) [S.
Bao, et al., Phys. Rev. X 12, 011022 (2022)], whose theoretical understanding
yet remains elusive. Here we study the spin dynamics of a ferromagnetic
Kondo-Heisenberg lattice model at finite temperature, and propose a mechanism
of magnon damping that explains the intriguing experimental results. In
particular, we find the magnon damping rate $\gamma(T)$ firstly decreases as
temperature lowers, due to the reduced magnon-magnon scatterings. It then
reaches a minimum at $T_{\rm d}^*$, and rises up again following a logarithmic
scaling $\gamma(T) \sim \ln{(T_0/T)}$ (with $T_0$ a constant) for $T < T_{\rm
d}^*$, which can be attributed to electron-magnon scatterings of spin-flip
type. Moreover, we obtain the phase diagram containing the ferromagnetic and
Kondo insulator phases by varying the Kondo coupling, which may be relevant for
experiments on pressured FGT. The presence of a magnon damping minimum and
logarithmic scaling at low temperature indicates the emergence of the Kondo
effect reflected in the collective excitations of local moments in a Kondo
lattice system. | 2401.00758v1 |
2024-01-19 | Upper bound of the lifespan of the solution to the nonlinear fractional wave equations with time-dependent damping | In this paper, we study the Cauchy problem of the nonlinear wave equation
with fractional Laplacian and time-dependent damping. Firstly, we derive the
weighted Sobolev estimate of the solution operators for the linear wave
equation with the damping of constant coefficient, and prove the local
existence and uniqueness in the weighted Sobolev space for the power-type
nonlinearity and $b(t)\in L^\infty$, by the contraction mapping principle.
Secondly, we consider the case of the source nonlinearity $f(u)\approx |u|^p$.
In the subcritical and critical cases $1<p\leq p_c=1+\frac \sigma N$, based on
the blow-up result on the ordinary differential inequality, we could prove the
blow-up of the solution and obtain the upper bound of the lifespan. And the
upper bound of the lifespan in the critical case is independent on the
coefficient of the time-dependent damping and is completely new even if the
classical case $b(t)=1$. | 2401.10552v1 |
2024-03-13 | Effects of wave damping and finite perpendicular scale on three-dimensional Alfvén wave parametric decay in low-beta plasmas | Shear Alfven wave parametric decay instability (PDI) provides a potential
path toward significant wave dissipation and plasma heating. However,
fundamental questions regarding how PDI is excited in a realistic
three-dimensional (3D) open system and how critically the finite perpendicular
wave scale -- as found in both the laboratory and space plasmas -- affects the
excitation remain poorly understood. Here, we present the first 3D,
open-boundary, hybrid kinetic-fluid simulations of kinetic Alfven wave PDI in
low-beta plasmas. Key findings are that the PDI excitation is strongly limited
by the wave damping present, including electron-ion collisional damping
(represented by a constant resistivity) and geometrical attenuation associated
with the finite-scale Alfven wave, and ion Landau damping of the child acoustic
wave. The perpendicular wave scale alone, however, plays no discernible role,
with different wave scales exhibiting similar instability growth. These
findings are corroborated by theoretical analysis and estimates. The new
understanding of 3D kinetic Alfven wave PDI physics is essential for laboratory
study of the basic plasma process and may also help evaluate the relevance/role
of PDI in low-beta space plasmas. | 2403.08179v1 |
2001-09-05 | Nuclear resonant scattering of Synchrotron radiation from nuclei in the Browninan motion | The time evolution of the coherent forward scattering of Synchrotron
radiation for resonant nuclei in Brownian motion is studied . Apart from target
thickness, the appearance of dynamical beats also depends on $\alpha$ which is
the ratio of harmonic force constant to the damping force constant of a
harmonic oscillator undergoing Brownian motion. | 0109074v2 |
2017-04-19 | Refractive index of dense materials | We show that applying the Lorentz-Lorenz transformation to the refractive
index of metals, semiconductors and insulators allows for a less empirical
modeling of this refractive index. | 1704.05718v1 |
2007-02-07 | Relativistic r-modes and shear viscosity | We derive the relativistic equations for stellar perturbations, including in
a consistent way shear viscosity in the stress-energy tensor, and we
numerically integrate our equations in the case of large viscosity. We consider
the slow rotation approximation, and we neglect the coupling between polar and
axial perturbations. In our approach, the frequency and damping time of the
emitted gravitational radiation are directly obtained. We find that,
approaching the inviscid limit from the finite viscosity case, the continuous
spectrum is regularized. Constant density stars, polytropic stars, and stars
with realistic equations of state are considered. In the case of constant
density stars and polytropic stars, our results for the viscous damping times
agree, within a factor two, with the usual estimates obtained by using the
eigenfunctions of the inviscid limit. For realistic neutron stars, our
numerical results give viscous damping times with the same dependence on mass
and radius as previously estimated, but systematically larger of about 60%. | 0702040v1 |
2009-08-19 | Nonlinear viscoelastic wave propagation: an extension of Nearly Constant Attenuation (NCQ) models | Hysteretic damping is often modeled by means of linear viscoelastic
approaches such as "nearly constant Attenuation (NCQ)" models. These models do
not take into account nonlinear effects either on the stiffness or on the
damping, which are well known features of soil dynamic behavior. The aim of
this paper is to propose a mechanical model involving nonlinear viscoelastic
behavior for isotropic materials. This model simultaneously takes into account
nonlinear elasticity and nonlinear damping. On the one hand, the shear modulus
is a function of the excitation level; on the other, the description of
viscosity is based on a generalized Maxwell body involving non-linearity. This
formulation is implemented into a 1D finite element approach for a dry soil.
The validation of the model shows its ability to retrieve low amplitude ground
motion response. For larger excitation levels, the analysis of seismic wave
propagation in a nonlinear soil layer over an elastic bedrock leads to results
which are physically satisfactory (lower amplitudes, larger time delays, higher
frequency content). | 0908.2715v2 |
2012-05-06 | Fractional wave equation and damped waves | In this paper, a fractional generalization of the wave equation that
describes propagation of damped waves is considered. In contrast to the
fractional diffusion-wave equation, the fractional wave equation contains
fractional derivatives of the same order $\alpha,\ 1\le \alpha \le 2$ both in
space and in time. We show that this feature is a decisive factor for
inheriting some crucial characteristics of the wave equation like a constant
propagation velocity of both the maximum of its fundamental solution and its
gravity and mass centers. Moreover, the first, the second, and the Smith
centrovelocities of the damped waves described by the fractional wave equation
are constant and depend just on the equation order $\alpha$. The fundamental
solution of the fractional wave equation is determined and shown to be a
spatial probability density function evolving in time that possesses finite
moments up to the order $\alpha$. To illustrate analytical findings, results of
numerical calculations and numerous plots are presented. | 1205.1199v2 |
2013-04-22 | Constant residual electrostatic electron plasma mode in Vlasov-Ampere system | In a collisionless Vlasov-Poisson (V-P) electron plasma system, two types of
modes for electric field perturbation exist: the exponentially Landau damped
electron plasma waves and the initial-value sensitive ballistic modes. Here,
the V-P system is modified slightly to a Vlasov-Ampere (V-A) system. A new
constant residual mode is revealed. Mathematically, this mode comes from the
Laplace transform of an initial electric field perturbation, and physically
represents that an initial perturbation (e.g., external electric field
perturbation) would not be damped away. Thus, this residual mode is more
difficult to be damped than the ballistic mode. [Physics of Plasmas 20, 112108
(2013); doi: 10.1063/1.4831761] | 1304.5883v2 |
2014-02-28 | A new way to evaluate x-ray Brillouin scattering data | Making use of the classical second moment sum rule, it is possible to convert
a series of constant-Q x-ray Brillouin scattering scans (Q momentum transfer)
into a series of constant frequency scans over the measured $Q$ range. The
method is applied to literature results for the phonon dispersion in liquid
vitreous silica and in glassy polybutadiene. It turns out that the constant
frequency scans are again well fitted by the damped harmonic oscillator
function, but now in terms of a Q-independent phonon damping depending
exclusively on the frequency. At low frequency, the sound velocity and the
damping of both evaluations agree, but at higher frequencies one gets
significant differences. The results in silica suggest a new interpretation of
x-ray Brillouin data in terms of a strong mixing of longitudinal and transverse
phonons toward higher frequencies. The results in polybutadiene enlighten the
crossover from Brillouin to Umklapp scattering. | 1402.7237v1 |
2014-08-27 | Quasi-particle Lifetime in a Mixture of Bose and Fermi Superfluids | In this letter, to reveal the effect of quasi-particle interactions in a
Bose-Fermi superfluid mixture, we consider the lifetime of quasi-particle of
Bose superfluid due to its interaction with quasi-particles in Fermi
superfluid. We find that this damping rate, i.e. inverse of the lifetime, has
quite different threshold behavior at the BCS and the BEC side of the Fermi
superfluid. The damping rate is a constant nearby the threshold momentum in the
BCS side, while it increases rapidly in the BEC side. This is because in the
BCS side the decay processe is restricted by constant density-of-state of
fermion quasi-particle nearby Fermi surface, while such a restriction does not
exist in the BEC side where the damping process is dominated by bosonic
quasi-particles of Fermi superfluid. Our results are related to collective mode
experiment in recently realized Bose-Fermi superfluid mixture. | 1408.6419v1 |
2017-05-10 | Negative mobility of a Brownian particle: strong damping regime | We study impact of inertia on directed transport of a Brownian particle under
non-equilibrium conditions: the particle moves in a one-dimensional periodic
and symmetric potential, is driven by both an unbiased time-periodic force and
a constant force, and is coupled to a thermostat of temperature T. Within
selected parameter regimes this system exhibits negative mobility, which means
that the particle moves in the direction opposite to the direction of the
constant force. It is known that in such a setup the inertial term is essential
for the emergence of negative mobility and it cannot be detected in the
limiting case of overdamped dynamics. We analyse inertial effects and show that
negative mobility can be observed even in the strong damping regime. We
determine the optimal dimensionless mass for the presence of negative mobility
and reveal three mechanisms standing behind this anomaly: deterministic
chaotic, thermal noise induced and deterministic non-chaotic. The last origin
has never been reported. It may provide guidance to the possibility of
observation of negative mobility for strongly damped dynamics which is of
fundamental importance from the point of view of biological systems, all of
which in situ operate in fluctuating environments. | 1705.03661v1 |
2018-04-09 | Damping and clustering into crowded environment of catalytic chemical oscillators | A system formed by a crowded environment of catalytic obstacles and complex
oscillatory chemical reactions is inquired. The obstacles are static spheres of
equal radius, which are placed in a random way. The chemical reactions are
carried out in a fluid following a multiparticle collision scheme where the
mass, energy and local momentum are conserved. Firstly, it is explored how the
presence of catalytic obstacles changes the oscillatory dynamics from a limit
cycle to a fix point reached after a damping. The damping is characterized by
the decay constant, which grows linearly with volume fraction for low values of
the mesoscale collision time and the catalytic reaction constant. Additionally,
it is shown that, although the distribution of obstacles is random, there are
regions in the system where the catalytic chemical reactions are favored. This
entails that in average the radius of gyrations of catalytic chemical reaction
does not match with the radius of gyration of obstacles, that is, clusters of
reactions emerge on the catalytic obstacles, even when the diffusion is
significant. | 1804.03174v1 |
2019-08-30 | Magnetization reversal, damping properties and magnetic anisotropy of L10-ordered FeNi thin films | L10 ordered magnetic alloys such as FePt, FePd, CoPt and FeNi are well known
for their large magnetocrystalline anisotropy. Among these, L10-FeNi alloy is
economically viable material for magnetic recording media because it does not
contain rare earth and noble elements. In this work, L10-FeNi films with three
different strengths of anisotropy were fabricated by varying the deposition
process in molecular beam epitaxy system. We have investigated the
magnetization reversal along with domain imaging via magneto optic Kerr effect
based microscope. It is found that in all three samples, the magnetization
reversal is happening via domain wall motion. Further ferromagnetic resonance
(FMR) spectroscopy was performed to evaluate the damping constant and magnetic
anisotropy. It was observed that the FeNi sample with moderate strength of
anisotropy exhibits low value of damping constant ~ 4.9X10^-3. In addition to
this, it was found that the films possess a mixture of cubic and uniaxial
anisotropies. | 1908.11761v1 |
2018-10-09 | The lifespan of solutions of semilinear wave equations with the scale-invariant damping in one space dimension | The critical constant of time-decaying damping in the scale-invariant case is
recently conjectured. It also has been expected that the lifespan estimate is
the same as for the associated semilinear heat equations if the constant is in
the \heat-like" domain. In this paper, we point out that this is not true if
the total integral of the sum of initial position and speed vanishes. In such a
case, we have a new type of the lifespan estimates which is closely related to
the non-damped case in shifted space dimensions. | 1810.03780v2 |
2015-02-24 | High Quality Yttrium Iron Garnet Grown by Room Temperature Pulsed Laser Deposition and Subsequent Annealing | We have investigated recrystallization of amorphous Yttrium Iron Garnet (YIG)
by annealing in oxygen atmosphere. Our findings show that well below the
melting temperature the material transforms into a fully epitaxial layer with
exceptional quality, both structural and magnetic.\\ In ferromagnetic resonance
(FMR) ultra low damping and extremely narrow linewidth can be observed. For a
56 nm thick layer a damping constant of
$\alpha$=(6.63$\pm$1.50)$\cdot$10$^{-5}$ is found and the linewidth at 9.6 GHz
is as small as 1.30$\pm$0.05 Oe which are the lowest values for PLD grown thin
films reported so far. Even for a 20 nm thick layer a damping constant of
$\alpha$=(7.51$\pm$1.40)$\cdot$10$^{-5}$ is found which is the lowest value for
ultrathin films published so far. The FMR linewidth in this case is
3.49$\pm$0.10 Oe at 9.6 GHz. Our results not only present a method of
depositing thin film YIG of unprecedented quality but also open up new options
for the fabrication of thin film complex oxides or even other crystalline
materials. | 1502.06724v2 |
2019-04-23 | Ultrafast depinning of domain wall in notched antiferromagnetic nanostructures | The pinning and depinning of antiferromagnetic (AFM) domain wall is certainly
the core issue of AFM spintronics. In this work, we study theoretically the
N\'eel-type domain wall pinning and depinning at a notch in an
antiferromagnetic (AFM) nano-ribbon. The depinning field depending on the notch
dimension and intrinsic physical parameters are deduced and also numerically
calculated. Contrary to conventional conception, it is revealed that the
depinning field is remarkably dependent of the damping constant and the
time-dependent oscillation of the domain wall position in the weakly damping
regime benefits to the wall depinning, resulting in a gradual increase of the
depinning field up to a saturation value with increasing damping constant. A
one-dimensional model accounting of the internal dynamics of domain wall is
used to explain perfectly the simulated results. It is demonstrated that the
depinning mechanism of an AFM domain wall differs from ferromagnetic domain
wall by exhibiting a depinning speed typically three orders of magnitude faster
than the latter, suggesting the ultrafast dynamics of an AFM system. | 1904.10197v2 |
2004-09-10 | Constraint on the Squeeze Parameter of Inflaton from Cosmological Constant | The inflaton is highly likely to settle in a squeezed vacuum state after
inflation. The relic inflaton after inflation and reheating undergoes a damped
oscillatory motion and contributes to the effective cosmological constant. We
interpret the renormalized energy density from the squeezed vacuum state as an
effective cosmological constant. Using the recent observational data on the
cosmological constant, we find the constraint on the squeeze parameter of the
inflaton in the early universe. | 0409044v1 |
2004-07-21 | A selfconsistent theory of current-induced switching of magnetization | A selfconsistent theory of the current-induced switching of magnetization
using nonequilibrium Keldysh formalism is developed for a junction of two
ferromagnets separated by a nonmagnetic spacer. It is shown that the
spin-transfer torques responsible for current-induced switching of
magnetization can be calculated from first principles in a steady state when
the magnetization of the switching magnet is stationary. The spin-transfer
torque is expressed in terms of one-electron surface Green functions for the
junction cut into two independent parts by a cleavage plane immediately to the
left and right of the switching magnet. The surface Green functions are
calculated using a tight-binding Hamiltonian with parameters determined from a
fit to an {\it ab initio} band structure.This treatment yields the spin
transfer torques taking into account rigorously contributions from all the
parts of the junction. To calculate the hysteresis loops of resistance versus
current, and hence to determine the critical current for switching, the
microscopically calculated spin-transfer torques are used as an input into the
phenomenological Landau-Lifshitz equation with Gilbert damping. The present
calculations for Co/Cu/Co(111) show that the critical current for switching is
$\approx 10^7A/cm^2$, which is in good agreement with experiment. | 0407562v2 |
2006-02-24 | Magnetization dynamics in dysprosium orthoferrites via inverse Faraday effect | The ultrafast non-thermal control of magnetization has recently become
feasible in canted antiferromagnets through photomagnetic instantaneous pulses
[A.V. Kimel {\it et al.}, Nature {\bf 435}, 655 (2005)]. In this experiment
circularly polarized femtosecond laser pulses set up a strong magnetic field
along the wave vector of the radiation through the inverse Faraday effect,
thereby exciting non-thermally the spin dynamics of dysprosium orthoferrites. A
theoretical study is performed by using a model for orthoferrites based on a
general form of free energy whose parameters are extracted from experimental
measurements. The magnetization dynamics is described by solving coupled
sublattice Landau-Lifshitz-Gilbert equations whose damping term is associated
with the scattering rate due to magnon-magnon interaction. Due to the inverse
Faraday effect and the non-thermal excitation, the effect of the laser is
simulated by magnetic field Gaussian pulses with temporal width of the order of
hundred femtoseconds. When the field is along the z-axis, a single resonance
mode of the magnetization is excited. The amplitude of the magnetization and
out-of-phase behavior of the oscillations for fields in z and -z directions are
in good agreement with the cited experiment. The analysis of the effect of the
temperature shows that magnon-magnon scattering mechanism affects the decay of
the oscillations on the picosecond scale. Finally, when the field pulse is
along the x-axis, another mode is excited, as observed in experiments. In this
case the comparison between theoretical and experimental results shows some
discrepancies whose origin is related to the role played by anisotropies in
orthoferrites. | 0602593v1 |
2006-04-19 | Stress - and Magneto-Impedance in Co71-xFexCr7Si8B14 (x = 0, 2) amorphous ribbons | Systematic measurements of stress impedance (SI) and magneto-impedance (MI)
have been carried out using Co-rich amorphous ribbons of nominal composition
Co71-xFexCr7Si8B14 (x = 0, 2) at various excitation frequencies and bias fields
and at room temperature. The impedance, Z, for both the samples was found to be
very sensitive functions of applied tensile stress (up to 100MPa) exhibiting a
maximum SI ratio as much as 80% at low frequency ~ 0.1MHz. The nature of
variation of impedance, Z, changes with the excitation frequency especially at
higher frequencies in MHz region where it exhibits a peak. Magnetization
measurements were also performed to observe the effects of applied stress and
magnetization decreases with the application of stress confirming the negative
magnetostriction co-efficient of both the samples. Both the samples exhibited
negative magneto-impedance when the variation of Z is observed with the applied
bias magnetic field, H. Maximum MI ratio as large as 99% has been observed for
both the samples at low fields ~ 27Oe. The impedance as functions of applied
magnetic field, Z(H), decreases with the application of stress thus making the
MI curves broader. Based on the electromagnetic screening and magnetization
dynamics and incorporating the Gilbert and the Bloch-Bloembergen damping and
stress dependent anisotropy, the SI has been calculated and is found to
describe well the stress and field dependence of impedance of the two samples. | 0604438v2 |
2011-06-22 | Effect of spin diffusion on current generated by spin motive force | Spin motive force is a spin-dependent force on conduction electrons induced
by magnetization dynamics. In order to examine its effects on magnetization
dynamics, it is indispensable to take into account spin accumulation, spin
diffusion, and spin-flip scattering since the spin motive force is in general
nonuniform. We examine the effects of all these on the way the spin motive
force generates the charge and spin currents in conventional situations, where
the conduction electron spin relaxation dynamics is much faster than the
magnetization dynamics. When the spin-dependent electric field is spatially
localized, which is common in experimental situations, we find that the
conservative part of the spin motive force is unable to generate the charge
current due to the cancelation effect by the diffusion current. We also find
that the spin current is a nonlocal function of the spin motive force and can
be effectively expressed in terms of nonlocal Gilbert damping tensor. It turns
out that any spin independent potential such as Coulomb potential does not
affect our principal results. At the last part of this paper, we apply our
theory to current-induced domain wall motion. | 1106.4389v2 |
2011-07-11 | Spin and charge transport induced by gauge fields in a ferromagnet | We present a microscopic theory of spin-dependent motive force ("spin motive
force") induced by magnetization dynamics in a conducting ferromagnet, by
taking account of spin relaxation of conduction electrons. The theory is
developed by calculating spin and charge transport driven by two kinds of gauge
fields; one is the ordinary electromagnetic field $A^{\rm em}_{\mu}$, and the
other is the effective gauge field $A^{z}_{\mu}$ induced by dynamical magnetic
texture. The latter acts in the spin channel and gives rise to a spin motive
force. It is found that the current induced as a linear response to
$A^{z}_{\mu}$ is not gauge-invariant in the presence of spin-flip processes.
This fact is intimately related to the non-conservation of spin via Onsager
reciprocity, so is robust, but indicates a theoretical inconsistency. This
problem is resolved by considering the time dependence of spin-relaxation
source terms in the "rotated frame", as in the previous study on Gilbert
damping [J. Phys. Soc. Jpn. {\bf 76}, 063710 (2007)]. This effect restores the
gauge invariance while keeping spin non-conservation. It also gives a
dissipative spin motive force expected as a reciprocal to the dissipative spin
torque ("$\beta$-term"). | 1107.2165v3 |
2013-03-14 | Spin-torque effects in thermally assisted magnetization reversal: Method of statistical moments | Thermal fluctuations of nanomagnets driven by spin-polarized currents are
treated via the Landau-Lifshitz-Gilbert equation generalized to include both
the random thermal noise field and the Slonczewski spin-transfer torque term.
By averaging this stochastic (Langevin) equation over its realizations, the
explicit infinite hierarchy of differential-recurrence relations for
statistical moments (averaged spherical harmonics) is derived for arbitrary
demagnetizing factors and magnetocrystalline anisotropy for the generic
nanopillar model of a spin-torque device comprising two ferromagnetic strata
representing the free and fixed layers and a nonmagnetic conducting spacer all
sandwiched between two ohmic contacts. The influence of thermal fluctuations
and spin-transfer torques on relevant switching characteristics, such as the
stationary magnetization, the magnetization reversal time, etc., is calculated
by solving the hierarchy for wide ranges of temperature, damping, external
magnetic field, and spin-polarized current indicating new spin-torque effects
in the thermally assisted magnetization reversal comprising several orders of
magnitude. In particular, a pronounced dependence of the switching
characteristics on the directions of the external magnetic field and the spin
polarization exists. | 1303.3476v4 |
2013-05-03 | Co2 FeAl thin films grown on MgO substrates: Correlation between static, dynamic and structural properties | Co2FeAl (CFA) thin films with thickness varying from 10 nm to 115 nm have
been deposited on MgO(001) substrates by magnetron sputtering and then capped
by Ta or Cr layer. X-rays diffraction (XRD) revealed that the cubic $[001]$ CFA
axis is normal to the substrate and that all the CFA films exhibit full
epitaxial growth. The chemical order varies from the $B2$ phase to the $A2$
phase when decreasing the thickness. Magneto-optical Kerr effect (MOKE) and
vibrating sample magnetometer measurements show that, depending on the field
orientation, one or two-step switchings occur. Moreover, the films present a
quadratic MOKE signal increasing with the CFA thickness, due to the increasing
chemical order. Ferromagnetic resonance, MOKE transverse bias initial inverse
susceptibility and torque (TBIIST) measurements reveal that the in-plane
anisotropy results from the superposition of a uniaxial and of a fourfold
symmetry term. The fourfold anisotropy is in accord with the crystal structure
of the samples and is correlated to the biaxial strain and to the chemical
order present in the films. In addition, a large negative perpendicular
uniaxial anisotropy is observed. Frequency and angular dependences of the FMR
linewidth show two magnon scattering and mosaicity contributions, which depend
on the CFA thickness. A Gilbert damping coefficient as low as 0.0011 is found. | 1305.0714v1 |
2013-06-19 | Asymmetric Ferromagnetic Resonance, Universal Walker Breakdown, and Counterflow Domain Wall Motion in the Presence of Multiple Spin-Orbit Torques | We study the motion of several types of domain wall profiles in spin-orbit
coupled magnetic nanowires and also the influence of spin-orbit interaction on
the ferromagnetic resonance of uniform magnetic films. We extend previous
studies by fully considering not only the field-like contribution from the
spin-orbit torque, but also the recently derived Slonczewski-like spin-orbit
torque. We show that the latter interaction affects both the domain wall
velocity and the Walker breakdown threshold non-trivially, which suggests that
it should be accounted in experimental data analysis. We find that the presence
of multiple spin-orbit torques may render the Walker breakdown to be universal
in the sense that the threshold is completely independent on the
material-dependent Gilbert damping, non-adiabaticity, and the chirality of the
domain wall. We also find that domain wall motion against the current injection
is sustained in the presence of multiple spin-orbit torques and that the wall
profile will determine the qualitative influence of these different types of
torques (e.g. field-like and Slonczewski-like). In addition, we consider a
uniform ferromagnetic layer under a current bias, and find that the resonance
frequency becomes asymmetric against the current direction in the presence of
Slonczewski-like spin-orbit coupling. This is in contrast with those cases
where such an interaction is absent, where the frequency is found to be
symmetric with respect to the current direction. This finding shows that
spin-orbit interactions may offer additional control over pumped and absorbed
energy in a ferromagnetic resonance setup by manipulating the injected current
direction. | 1306.4680v1 |
2014-03-03 | Observations and Implications of Large-Amplitude Longitudinal Oscillations in a Solar Filament | On 20 August 2010 an energetic disturbance triggered large-amplitude
longitudinal oscillations in a nearby filament. The triggering mechanism
appears to be episodic jets connecting the energetic event with the filament
threads. In the present work we analyze this periodic motion in a large
fraction of the filament to characterize the underlying physics of the
oscillation as well as the filament properties. The results support our
previous theoretical conclusions that the restoring force of large-amplitude
longitudinal oscillations is solar gravity, and the damping mechanism is the
ongoing accumulation of mass onto the oscillating threads. Based on our
previous work, we used the fitted parameters to determine the magnitude and
radius of curvature of the dipped magnetic field along the filament, as well as
the mass accretion rate onto the filament threads. These derived properties are
nearly uniform along the filament, indicating a remarkable degree of
cohesiveness throughout the filament channel. Moreover, the estimated mass
accretion rate implies that the footpoint heating responsible for the thread
formation, according to the thermal nonequilibrium model, agrees with previous
coronal heating estimates. We estimate the magnitude of the energy released in
the nearby event by studying the dynamic response of the filament threads, and
discuss the implications of our study for filament structure and heating. | 1403.0381v1 |
2015-01-16 | Direct measurement of the magnetic anisotropy field in Mn--Ga and Mn--Co--Ga Heusler films | The static and dynamic magnetic properties of tetragonally distorted Mn--Ga
based alloys were investigated. Static properties are determined in magnetic
fields up to 6.5~T using SQUID magnetometry. For the pure Mn$_{1.6}$Ga film,
the saturation magnetisation is 0.36~MA/m and the coercivity is 0.29~T. Partial
substitution of Mn by Co results in Mn$_{2.6}$Co$_{0.3}$Ga$_{1.1}$. The
saturation magnetisation of those films drops to 0.2~MA/m and the coercivity is
increased to 1~T.
Time-resolved magneto-optical Kerr effect (TR-MOKE) is used to probe the
high-frequency dynamics of Mn--Ga. The ferromagnetic resonance frequency
extrapolated to zero-field is found to be 125~GHz with a Gilbert damping,
$\alpha$, of 0.019. The anisotropy field is determined from both SQUID and
TR-MOKE to be 4.5~T, corresponding to an effective anisotropy density of
0.81~MJ/m$^3$.
Given the large anisotropy field of the Mn$_{2.6}$Co$_{0.3}$Ga$_{1.1}$ film,
pulsed magnetic fields up to 60~T are used to determine the field strength
required to saturate the film in the plane. For this, the extraordinary Hall
effect was employed as a probe of the local magnetisation. By integrating the
reconstructed in--plane magnetisation curve, the effective anisotropy energy
density for Mn$_{2.6}$Co$_{0.3}$Ga$_{1.1}$ is determined to be 1.23~MJ/m$^3$. | 1501.03973v1 |
2015-06-02 | Respective influence of in-plane and out-of-plane spin-transfer torques in magnetization switching of perpendicular magnetic tunnel junctions | The relative contributions of in-plane (damping-like) and out-of-plane
(field-like) spin-transfer-torques in the magnetization switching of
out-of-plane magnetized magnetic tunnel junctions (pMTJ) has been theoretically
analyzed using the transformed Landau-Lifshitz (LL) equation with the STT
terms. It is demonstrated that in a pMTJ structure obeying macrospin dynamics,
the out-of-plane torque influences the precession frequency but it does not
contribute significantly to the STT switching process (in particular to the
switching time and switching current density), which is mostly determined by
the in-plane STT contribution. This conclusion is confirmed by finite
temperature and finite writing pulse macrospin simulations of the current-field
switching diagrams. It contrasts with the case of STT-switching in in-plane
magnetized MTJ in which the field-like term also influences the switching
critical current. This theoretical analysis was successfully applied to the
interpretation of voltage-field STT switching diagrams experimentally measured
on perpendicular MTJ pillars 36 nm in diameter, which exhibit macrospin-like
behavior. The physical nonequivalence of Landau and Gilbert dissipation terms
in presence of STT-induced dynamics is also discussed. | 1506.00780v2 |
2015-08-28 | Control of magnetic relaxation by electric-field-induced ferroelectric phase transition and inhomogeneous domain switching | Electric-field modulation of magnetism in strain-mediated multiferroic
heterostructures is considered a promising scheme for enabling memory and
magnetic microwave devices with ultralow power consumption. However, it is not
well understood how electric-field-induced strain influences magnetic
relaxation, an important physical process for device applications. Here we
investigate resonant magnetization dynamics in ferromagnet/ferrolectric
multiferroic heterostructures, FeGaB/PMN-PT and NiFe/PMN-PT, in two distinct
strain states provided by electric-field-induced ferroelectric phase
transition. The strain not only modifies magnetic anisotropy but also magnetic
relaxation. In FeGaB/PMN-PT, we observe a nearly two-fold change in intrinsic
Gilbert damping by electric field, which is attributed to strain-induced tuning
of spin-orbit coupling. By contrast, a small but measurable change in extrinsic
linewidth broadening is attributed to inhomogeneous ferroelastic domain
switching during the phase transition of the PMN-PT substrate. | 1508.07290v2 |
2016-04-05 | Homodyne-detected ferromagnetic resonance of in-plane magnetized nano-contacts: composite spin wave resonances and their excitation mechanism | This work provides a detailed investigation of the measured in-plane
field-swept homodyne-detected ferromagnetic resonance (FMR) spectra of an
extended Co/Cu/NiFe pseudo spin valve stack using a nanocontact (NC) geometry.
The magnetodynamics are generated by a pulse-modulated microwave current and
the resulting rectified dc mixing voltage, which appears across the NC at
resonance, is detected using a lock-in amplifier. Most notably, we find that
the measured spectra of the NiFe layer are composite in nature and highly
asymmetric, consistent with the broadband excitation of multiple modes.
Additionally, the data must be fit with two Lorentzian functions in order to
extract a reasonable value for the Gilbert damping of the NiFe. Aided by
micromagnetic simulations, we conclude that (i) for in-plane fields the rf
Oersted field in the vicinity of the NC plays the dominant role in generating
the observed spectra, (ii) in addition to the FMR mode, exchange dominated spin
waves are also generated, and (iii) the NC diameter sets the mean wavevector of
the exchange dominated spin wave, in good agreement with the dispersion
relation. | 1604.01389v1 |
2017-01-10 | Motion of skyrmions in nanowires driven by magnonic momentum-transfer forces | We study the motion of magnetic skyrmions in a nanowire induced by a
spin-wave current $J$ flowing out of a driving layer close to the edge of the
wire. By applying micromagnetic simulation and an analysis of the effective
Thiele equation, we find that the skyrmion trajectory is governed by an
interplay of both forces due to the magnon current and the wire boundary. The
skyrmion is attracted to the driving layer and is accelerated by the repulsive
force due to the wire boundary. We consider both cases of a driving
longitudinal and transverse to the nanowire, but a steady-state motion of the
skyrmion is only obtained for a transverse magnon current. For the latter case,
we find in the limit of low current densities $J$ the velocity-current relation
$v \sim J/\alpha$ where $v$ is the skyrmion velocity and $\alpha$ is the
Gilbert damping. For large $J$ in case of strong driving, the skyrmion is
pushed into the driving layer resulting in a drop of the skyrmion velocity and,
eventually, the destruction of the skyrmion. | 1701.02430v2 |
2017-01-19 | Ultrafast Electron-Lattice Coupling Dynamics in VO2 and V2O3 Thin Films | Ultrafast optical pump - optical probe and optical pump - terahertz probe
spectroscopy were performed on vanadium dioxide (VO2) and vanadium sesquioxide
(V2O3) thin films over a wide temperature range. A comparison of the
experimental data from these two different techniques and two different
vanadium oxides, in particular a comparison of the electronic oscillations
generated by the photoinduced longitudinal acoustic modulation, reveals the
strong electron-phonon coupling that exists in the metallic state of both
materials. The low energy Drude response of V2O3 appears more susceptible than
VO2 to ultrafast strain control. Additionally, our results provide a
measurement of the temperature dependence of the sound velocity in both
systems, revealing a four- to fivefold increase in VO2 and a three- to fivefold
increase in V2O3 across the phase transition. Our data also confirm
observations of strong damping and phonon anharmonicity in the metallic phase
of VO2, and suggest that a similar phenomenon might be at play in the metallic
phase of V2O3. More generally, our simple table-top approach provides relevant
and detailed information about dynamical lattice properties of vanadium oxides,
opening the way to similar studies in other complex materials. | 1701.05531v1 |
2017-02-21 | All-optical Detection of Spin Hall Angle in W/CoFeB/SiO2 Heterostructures by Varying Tungsten Layer Thickness | The development of advanced spintronics devices hinges on the efficient
generation and utilization of pure spin current. In materials with large
spin-orbit coupling, the spin Hall effect may convert charge current to pure
spin current and a large conversion efficiency, which is quantified by spin
Hall angle (SHA), is desirable for the realization of miniaturized and energy
efficient spintronic devices. Here, we report a giant SHA in beta-tungsten
(\b{eta}-W) thin films in Sub/W(t)/Co20Fe60B20(3 nm)/SiO2(2 nm)
heterostructures with variable W thickness. We employed an all-optical
time-resolved magneto-optical Kerr effect microscope for an unambiguous
determination of SHA using the principle of modulation of Gilbert damping of
the adjacent ferromagnetic layer by the spin-orbit torque from the W layer. A
non-monotonic variation of SHA with W layer thickness (t) is observed with a
maximum of about 0.4 at about t = 3 nm, followed by a sudden reduction to a
very low value at t = 6 nm. This variation of SHA with W-thickness correlates
well with the thickness dependent structural phase transition and resistivity
variation of W above the spin diffusion length of W, while below this length
the interfacial electronic effect at W/CoFeB influences the estimation of SHA. | 1702.06258v1 |
2017-08-08 | Spin-orbit-torque driven magnetoimpedance in Pt-layer/magnetic-ribbon heterostructures | When a flow of electron passes through a paramagnetic layer with strong
spin-orbit-coupling such as platinum (Pt), a net spin current is produced via
spin Hall effect (SHE). This spin current can exert a torque on the
magnetization of an adjacent ferromagnetic layer which can be probed via
magnetization dynamic response, e.g. spin-torque ferromagnetic resonance
(ST-FMR). Nevertheless, that effect in lower frequency magnetization dynamic
regime (MHz) where skin effect occurs in high permeability ferromagnetic
conductors namely the magneto-impedance (MI) effect can be fundamentally
important which has not been studied so far. Here, by utilizing the MI effect
in magnetic-ribbon/Pt heterostructure with high magnetic permeability that
allows the ac current effectively confined at the skin depth of ~100 nm
thickness, the effect of spin-orbit-torque (SOT) induced by the SHE probed via
MI measurement is investigated. We observed a systematic MI frequency shift
that increases by increasing the applied current amplitude and thickness of the
Pt layer (varying from 0 nm to 20 nm). In addition, the role of Pt layer in
ribbon/Pt heterostructure is evaluated with ferromagnetic resonance (FMR)
effect representing standard Gilbert damping increase as the result of presence
of the SHE. Our results unveil the role of SOT in dynamic control of the
transverse magnetic permeability probed with impedance spectroscopy as useful
and valuable technique for detection of future SHE devices. | 1708.02402v2 |
2016-08-29 | Sub-micrometer yttrium iron garnet LPE films with low ferromagnetic resonance losses | Using liquid phase epitaxy (LPE) technique (111) yttrium iron garnet (YIG)
films with thicknesses of ~100 nm and surface roughnesses as low as 0.3 nm have
been grown as a basic material for spin-wave propagation experiments in
microstructured waveguides. The continuously strained films exhibit nearly
perfect crystallinity without significant mosaicity and with effective lattice
misfits of delta a(perpendicular)/a(substrate) ~10-4 and below. The
film/substrate interface is extremely sharp without broad interdiffusion layer
formation. All LPE films exhibit a nearly bulk-like saturation magnetization of
(1800+-20) Gs and an `easy cone' anisotropy type with extremely small in-plane
coercive fields <0.2 Oe. There is a rather weak in-plane magnetic anisotropy
with a pronounced six-fold symmetry observed for saturation field <1.5 Oe. No
significant out-of-plane anisotropy is observed, but a weak dependence of the
effective magnetization on the lattice misfit is detected. The narrowest
ferromagnetic resonance linewidth is determined to be 1.4 Oe @ 6.5 GHz which is
the lowest value reported so far for YIG films of 100 nm thicknesses and below.
The Gilbert damping coefficient for investigated LPE films is estimated to be
close to 1 x 10-4. | 1608.08043v1 |
2017-03-21 | Annealing stability of magnetic tunnel junctions based on dual MgO free layers and [Co/Ni] based thin synthetic antiferromagnet fixed system | We study the annealing stability of bottom-pinned perpendicularly magnetized
magnetic tunnel junctions based on dual MgO free layers and thin fixed systems
comprising a hard [Co/Ni] multilayer antiferromagnetically coupled to thin a Co
reference layer and a FeCoB polarizing layer. Using conventional magnetometry
and advanced broadband ferromagnetic resonance, we identify the properties of
each sub-unit of the magnetic tunnel junction and demonstrate that this
material option can ensure a satisfactory resilience to the 400$^\circ$C
thermal annealing needed in solid-state magnetic memory applications. The dual
MgO free layer possesses an anneal-robust 0.4 T effective anisotropy and
suffers only a minor increase of its Gilbert damping from 0.007 to 0.010 for
the toughest annealing conditions. Within the fixed system, the ferro-coupler
and texture-breaking TaFeCoB layer keeps an interlayer exchange above 0.8
mJ/m$^2$, while the Ru antiferrocoupler layer within the synthetic
antiferromagnet maintains a coupling above -0.5 mJ/m$^2$. These two strong
couplings maintain the overall functionality of the tunnel junction upon the
toughest annealing despite the gradual degradation of the thin Co layer
anisotropy that may reduce the operation margin in spin torque memory
applications. Based on these findings, we propose further optimization routes
for the next generation magnetic tunnel junctions. | 1703.07154v1 |
2019-09-12 | Spin Transport in Thick Insulating Antiferromagnetic Films | Spin transport of magnonic excitations in uniaxial insulating
antiferromagnets (AFs) is investigated. In linear response to spin biasing and
a temperature gradient, the spin transport properties of
normal-metal--insulating antiferromagnet--normal-metal heterostructures are
calculated. We focus on the thick-film regime, where the AF is thicker than the
magnon equilibration length. This regime allows the use of a drift-diffusion
approach, which is opposed to the thin-film limit considered by Bender {\it et
al.} 2017, where a stochastic approach is justified. We obtain the temperature-
and thickness-dependence of the structural spin Seebeck coefficient
$\mathcal{S}$ and magnon conductance $\mathcal{G}$. In their evaluation we
incorporate effects from field- and temperature-dependent spin conserving
inter-magnon scattering processes. Furthermore, the interfacial spin transport
is studied by evaluating the contact magnon conductances in a microscopic model
that accounts for the sub-lattice symmetry breaking at the interface. We find
that while inter-magnon scattering does slightly suppress the spin Seebeck
effect, transport is generally unaffected, with the relevant spin decay length
being determined by non-magnon-conserving processes such as Gilbert damping. In
addition, we find that while the structural spin conductance may be enhanced
near the spin flip transition, it does not diverge due to spin impedance at the
normal metal|magnet interfaces. | 1909.05881v2 |
2021-04-09 | Spin diffusion length associated to out-of-plane resistivity of Pt thin films in spin pumping experiments | We present a broadband ferromagnetic resonance study of the Gilbert damping
enhancement ($\Delta \alpha$) due to spin pumping in NiFe/Pt bilayers. The
bilayers, which have negligible interfacial spin memory loss, are studied as a
function of the Pt layer thickness ($t_{\text{Pt}}$) and temperature (100-293
K). Within the framework of diffusive spin pumping theory, we demonstrate that
Dyakonov-Perel (DP) or Elliot-Yaffet (EY) spin relaxation mechanisms acting
alone are incompatible with our observations. In contrast, if we consider that
the relation between spin relaxation characteristic time ($\tau_{\text{s}}$)
and momentum relaxation characteristic time ($\tau_{\text{p}}$) is determined
by a superposition of DP and EY mechanisms, the qualitative and quantitative
agreement with experimental results is excellent. Remarkably, we found that
$\tau_{\text{p}}$ must be determined by the out-of-plane electrical resistivity
($\rho$) of the Pt film and hence its spin diffusion length
($\lambda_{\text{Pt}}$) is independent of $t_{\text{Pt}}$. Our work settles the
controversy regarding the $t_{\text{Pt}}$ dependence of $\lambda_{\text{Pt}}$
by demonstrating its fundamental connection with $\rho$ considered along the
same direction of spin current flow. \end{abstract} | 2104.04426v1 |
2021-05-05 | Ni$_{80}$Fe$_{20}$ Nanotubes with Optimized Spintronic Functionalities Prepared by Atomic Layer Deposition | Permalloy Ni$_{80}$Fe$_{20}$ is one of the key magnetic materials in the
field of magnonics. Its potential would be further unveiled if it could be
deposited in three dimensional (3D) architectures of sizes down to the
nanometer. Atomic Layer Deposition, ALD, is the technique of choice for
covering arbitrary shapes with homogeneous thin films. Early successes with
ferromagnetic materials include nickel and cobalt. Still, challenges in
depositing ferromagnetic alloys reside in the synthesis via decomposing the
consituent elements at the same temperature and homogeneously. We report
plasma-enhanced ALD to prepare permalloy Ni$_{80}$Fe$_{20}$ thin films and
nanotubes using nickelocene and iron(III) tert-butoxide as metal precursors,
water as the oxidant agent and an in-cycle plasma enhanced reduction step with
hydrogen. We have optimized the ALD cycle in terms of Ni:Fe atomic ratio and
functional properties. We obtained a Gilbert damping of 0.013, a resistivity of
28 $\mu\Omega$cm and an anisotropic magnetoresistance effect of 5.6 $\%$ in the
planar thin film geometry. We demonstrate that the process also works for
covering GaAs nanowires, resulting in permalloy nanotubes with high aspect
ratios and diameters of about 150 nm. Individual nanotubes were investigated in
terms of crystal phase, composition and spin-dynamic response by microfocused
Brillouin Light Scattering. Our results enable NiFe-based 3D spintronics and
magnonic devices in curved and complex topology operated in the GHz frequency
regime. | 2105.01969v1 |
2013-11-29 | Magnon radiation by moving Abrikosov vortices in ferromagnetic superconductors and superconductor-ferromagnet multilayers | In systems combining type-II superconductivity and magnetism the
non-stationary magnetic field of moving Abrikosov vortices may excite spin
waves, or magnons. This effect leads to the appearance of an additional damping
force acting on the vortices. By solving the London and Landau-Lifshitz-Gilbert
equations we calculate the magnetic moment induced force acting on vortices in
ferromagnetic superconductors and superconductor/ferromagnet superlattices. If
the vortices are driven by a dc force, magnon generation due to the Cherenkov
resonance starts as the vortex velocity exceeds some threshold value. For an
ideal vortex lattice this leads to an anisotropic contribution to the
resistivity and to the appearance of resonance peaks on the current voltage
characteristics. For a disordered vortex array the current will exhibit a
step-like increase at some critical voltage. If the vortices are driven by an
ac force with a frequency \omega, the interaction with magnetic moments will
lead to a frequency-dependent magnetic contribution \eta_M to the vortex
viscosity. If \omega is below the ferromagnetic resonance frequency \omega_F,
vortices acquire additional inertia. For \omega > \omega_F dissipation is
enhanced due to magnon generation. The viscosity \eta_M can be extracted from
the surface impedance of the ferromagnetic superconductor. Estimates of the
magnetic force acting on vortices for the U-based ferromagnetic superconductors
and cuprate/manganite superlattices are given. | 1311.7620v1 |
2019-03-08 | Spin-transfer torques for domain walls in antiferromagnetically coupled ferrimagnets | Antiferromagnetic materials are outstanding candidates for next generation
spintronic applications, because their ultrafast spin dynamics makes it
possible to realize several orders of magnitude higher-speed devices than
conventional ferromagnetic materials1. Though spin-transfer torque (STT) is a
key for electrical control of spins as successfully demonstrated in
ferromagnetic spintronics, experimental understanding of STT in
antiferromagnets has been still lacking despite a number of pertinent
theoretical studies2-5. Here, we report experimental results on the effects of
STT on domain-wall (DW) motion in antiferromagnetically-coupled ferrimagnets.
We find that non-adiabatic STT acts like a staggered magnetic field and thus
can drive DWs effectively. Moreover, the non-adiabaticity parameter {\beta} of
STT is found to be significantly larger than the Gilbert damping parameter
{\alpha}, challenging our conventional understanding of the non-adiabatic STT
based on ferromagnets as well as leading to fast current-induced
antiferromagnetic DW motion. Our study will lead to further vigorous
exploration of STT for antiferromagnetic spin textures for fundamental physics
on spin-charge interaction as wells for efficient electrical control of
antiferromagnetic devices. | 1903.03251v1 |
2019-03-26 | Engineering of spin mixing conductance in Ru/FeCo/Ru interfaces: Effect of Re Doping | We have deposited polycrystalline Re doped $(Fe_{65}Co_{35})_{100-x}Re_{x}$
(0 $\leq$ x $\leq$ 12.6 at\%) thin films grown under identical conditions and
sandwiched between thin layers of Ru in order to study the phenomenon of spin
pumping as a function of Re concentration. In-plane and out-of-plane
ferromagnetic resonance spectroscopy results show an enhancement of the Gilbert
damping with an increase in Re doping. We found evidence of an increase in the
real part of effective spin mixing conductance
[Re($g^{\uparrow\downarrow}_{eff}$)] with the increase in Re doping of 6.6
at\%, while a decrease is evident at higher Re doping. The increase in
Re($g^{\uparrow\downarrow}_{eff}$) can be linked to the Re doping induced
change of the interface electronic structure in the non-magnetic Ru layer and
the effect interfacial spin-orbit coupling has on the effective spin-mixing
conductance. The lowest and highest values of
Re($g^{\uparrow\downarrow}_{eff}$) are found to be 9.883(02) $nm^{-2}$ and
19.697(02) $nm^{-2}$ for 0 at\% and 6.6 at\% Re doping, respectively. The
saturation magnetization decreases with increasing Re doping, from 2.362(13) T
for the undoped film to 1.740(03) T for 12.6 at\% Re doping. This study opens a
new direction of tuning the spin-mixing conductance in magnetic
heterostructures by doping of the ferromagnetic layerr, which is essential for
the realization of energy efficient operation of spintronic devices. | 1903.10966v2 |
2020-01-09 | Role of longitudinal fluctuations in L$1_0$ FePt | L$1_0$ FePt is a technologically important material for a range of novel data
storage applications. In the ordered FePt structure the normally non-magnetic
Pt ion acquires a magnetic moment, which depends on the local field originating
from the neighboring Fe atoms. In this work a model of FePt is constructed,
where the induced Pt moment is simulated by using combined longitudinal and
rotational spin dynamics. The model is parameterized to include a linear
variation of the moment with the exchange field, so that at the Pt site the
magnetic moment depends on the Fe ordering. The Curie temperature of FePt is
calculated and agrees well with similar models that incorporate the Pt dynamics
through an effective Fe-only Hamiltonian. By computing the dynamic correlation
function the anisotropy field and the Gilbert damping are extracted over a
range of temperatures. The anisotropy exhibits a power-law dependence with
temperature with exponent $n\approx2.1$. This agrees well with what observed
experimentally and it is obtained without including a two-ion anisotropy term
as in other approaches. Our work shows that incorporating longitudinal
fluctuations into spin dynamics calculations is crucial for understanding the
properties of materials with induced moments. | 2001.03074v1 |
2020-05-07 | Effect of interfacial oxidation layer in spin pumping experiments on Ni$_{80}$Fe$_{20}$/SrIrO$_3$ heterostructures | SrIrO$_3$ with its large spin-orbit coupling and low charge conductivity has
emerged as a potential candidate for efficient spin-orbit torque magnetization
control in spintronic devices. We here report on the influence of an
interfacial oxide layer on spin pumping experiments in Ni$_{80}$Fe$_{20}$
(NiFe)/SrIrO$_3$ bilayer heterostructures. To investigate this scenario we have
carried out broadband ferromagnetic resonance (BBFMR) measurements, which
indicate the presence of an interfacial antiferromagnetic oxide layer. We
performed in-plane BBFMR experiments at cryogenic temperatures, which allowed
us to simultaneously study dynamic spin pumping properties (Gilbert damping)
and static magnetic properties (such as the effective magnetization and
magnetic anisotropy). The results for NiFe/SrIrO$_3$ bilayer thin films were
analyzed and compared to those from a NiFe/NbN/SrIrO$_3$ trilayer reference
sample, where a spin-transparent, ultra-thin NbN layer was inserted to prevent
oxidation of NiFe. At low temperatures, we observe substantial differences in
the magnetization dynamics parameters of these samples, which can be explained
by an antiferromagnetic interfacial layer in the NiFe/SrIrO$_3$ bilayers. | 2005.03727v1 |
2020-05-28 | Hard antinodal gap revealed by quantum oscillations in the pseudogap regime of underdoped high-$T_{\rm c}$ superconductors | An understanding of the missing antinodal electronic excitations in the
pseudogap state is essential for uncovering the physics of the underdoped
cuprate high temperature superconductors. The majority of high temperature
experiments performed thus far, however, have been unable to discern whether
the antinodal states are rendered unobservable due to their damping, or whether
they vanish due to their gapping. Here we distinguish between these two
scenarios by using quantum oscillations to examine whether the small Fermi
surface pocket, found to occupy only 2% of the Brillouin zone in the underdoped
cuprates, exists in isolation against a majority of completely gapped density
of states spanning the antinodes, or whether it is thermodynamically coupled to
a background of ungapped antinodal states. We find that quantum oscillations
associated with the small Fermi surface pocket exhibit a signature sawtooth
waveform characteristic of an isolated two-dimensional Fermi surface pocket.
This finding reveals that the antinodal states are destroyed by a hard gap that
extends over the majority of the Brillouin zone, placing strong constraints on
a drastic underlying origin of quasiparticle disappearance over almost the
entire Brillouin zone in the pseudogap regime. | 2005.14123v1 |
2020-06-01 | Enhancement in Thermally Generated Spin Voltage at Pd/NiFe$_2$O$_4$ Interfaces by the Growth on Lattice-Matched Substrates | Efficient spin injection from epitaxial ferrimagnetic NiFe$_2$O$_4$ thin
films into a Pd layer is demonstrated via spin Seebeck effect measurements in
the longitudinal geometry. The NiFe$_2$O$_4$ films (60 nm to 1 $\mu$m) are
grown by pulsed laser deposition on isostructural spinel MgAl$_2$O$_4$,
MgGa$_2$O$_4$, and CoGa$_2$O$_4$ substrates with lattice mismatch varying
between 3.2% and 0.2%. For the thinner films ($\leq$ 330 nm), an increase in
the spin Seebeck voltage is observed with decreasing lattice mismatch, which
correlates well with a decrease in the Gilbert damping parameter as determined
from ferromagnetic resonance measurements. High resolution transmission
electron microscopy studies indicate substantial decrease of antiphase boundary
and interface defects that cause strain-relaxation, i.e., misfit dislocations,
in the films with decreasing lattice mismatch. This highlights the importance
of reducing structural defects in spinel ferrites for efficient spin injection.
It is further shown that angle-dependent spin Seebeck effect measurements
provide a qualitative method to probe for in-plane magnetic anisotropies
present in the films. | 2006.00777v1 |
2020-10-17 | Multiscale modelling of magnetostatic effects on magnetic nanoparticles with application to hyperthermia | We extend a renormalization group-based course-graining method for
micromagnetic simulations to include properly scaled magnetostatic
interactions. We apply the method in simulations of dynamic hysteresis loops at
clinically relevant sweep rates and at 310 K of iron oxide nanoparticles (NPs)
of the kind that have been used in preclinical studies of magnetic
hyperthermia. The coarse-graining method, along with a time scaling involving
sweep rate and Gilbert damping parameter, allow us to span length scales from
the unit cell to NPs approximately 50 nm in diameter with reasonable simulation
times. For both NPs and the nanorods composing them, we report effective
uniaxial anisotropy strengths and saturation magnetizations, which differ from
those of the bulk materials magnetite and maghemite of which they are made, on
account of the combined non-trivial effects of temperature, inter-rod exchange,
magnetostatic interactions and the degree of orientational order within the
nanorod composites. The effective parameters allow treating the NPs as single
macrospins, and we find for the test case of calculating loops for two aligned
NPs that using the dipole approximation is sufficient for distances beyond 1.5
times the NP diameter. We also present a study on relating integration time
step to micromagnetic cell size, finding that the optimal time step size scales
approximately linearly with cell volume. | 2010.08848v1 |
2021-02-09 | Unconventional quantum vortex matter state hosts quantum oscillations in the underdoped high-temperature cuprate superconductors | A central question in the underdoped cuprates pertains to the nature of the
pseudogap ground state. A conventional metallic ground state of the pseudogap
region has been argued to host quantum oscillations upon destruction of the
superconducting order parameter by modest magnetic fields. Here we use low
applied measurement currents and millikelvin temperatures on ultra-pure single
crystals of underdoped YBa$_2$Cu$_3$O$_{6+x}$ to unearth an unconventional
quantum vortex matter ground state characterized by vanishing electrical
resistivity, magnetic hysteresis, and non-ohmic electrical transport
characteristics beyond the highest laboratory accessible static fields. A new
model of the pseudogap ground state is now required to explain quantum
oscillations that are hosted by the bulk quantum vortex matter state without
experiencing sizeable additional damping in the presence of a large maximum
superconducting gap; possibilities include a pair density wave. | 2102.04927v2 |
2021-03-08 | Emerging magnetic nutation | Nutation has been recognized as of great significance for spintronics; but
justifying its presence has proven to be a hard problem. In this paper we show
that nutation can be understood as emerging from a systematic expansion of a
kernel that describes the history of the interaction of a magnetic moment with
a bath of colored noise. The parameter of the expansion is the ratio of the
colored noise timescale to the precession period. In the process we obtain the
Gilbert damping from the same expansion. We recover the known results, when the
coefficients of the two terms are proportional to one another, in the white
noise limit; and show how colored noise leads to situations where this simple
relation breaks down, but what replaces it can be understood by the appropriate
generalization of the fluctuation--dissipation theorem. Numerical simulations
of the stochastic equations support the analytic approach. In particular we
find that the equilibration time is about an order of magnitude longer than the
timescale set by the colored noise for a wide range of values of the latter and
we can identify the presence of nutation in the non-uniform way the
magnetization approaches equilibrium. | 2103.04787v3 |
2021-03-11 | Long-range spin transport on the surface of topological Dirac semimetal | We theoretically propose the long-range spin transport mediated by the
gapless surface states of topological Dirac semimetal (TDSM). Low-dissipation
spin current is a building block of next-generation spintronics devices. While
conduction electrons in metals and spin waves in ferromagnetic insulators
(FMIs) are the major carriers of spin current, their propagation length is
inevitably limited due to the Joule heating or the Gilbert damping. In order to
suppress dissipation and realize long-range spin transport, we here make use of
the spin-helical surface states of TDSMs, such as $\mathrm{Cd_3 As_2}$ and
$\mathrm{Na_3 Bi}$, which are robust against disorder. Based on a junction of
two FMIs connected by a TDSM, we demonstrate that the magnetization dynamics in
one FMI induces a spin current on the TDSM surface flowing to the other FMI. By
both the analytical transport theory on the surface and the numerical
simulation of real-time evolution in the bulk, we find that the induced spin
current takes a universal semi-quantized value that is insensitive to the
microscopic coupling structure between the FMI and the TDSM. We show that this
surface spin current is robust against disorder over a long range, which
indicates that the TDSM surface serves as a promising system for realizing
spintronics devices. | 2103.06519v1 |
2021-06-23 | Spin dynamics of itinerant electrons: local magnetic moment formation and Berry phase | The state-of-the-art theoretical description of magnetic materials relies on
solving effective Heisenberg spin problems or their generalizations to
relativistic or multi-spin-interaction cases that explicitly assume the
presence of local magnetic moments in the system. We start with a general
interacting fermionic model that is often obtained in ab initio electronic
structure calculations and show that the corresponding spin problem can be
introduced even in the paramagnetic regime, which is characterized by a zero
average value of the magnetization. Further, we derive a physical criterion for
the formation of the local magnetic moment and confirm that the latter exists
already at high temperatures well above the transition to the ordered magnetic
state. The use of path-integral techniques allows us to disentangle spin and
electronic degrees of freedom and to carefully separate rotational dynamics of
the local magnetic moment from Higgs fluctuations of its absolute value. It
also allows us to accurately derive the topological Berry phase and relate it
to a physical bosonic variable that describes dynamics of the spin degrees of
freedom. As the result, we demonstrate that the equation of motion in the case
of a large magnetic moment takes a conventional Landau-Lifshitz form that
explicitly accounts for the Gilbert damping due to itinerant nature of the
original electronic model. | 2106.12462v3 |
2021-11-20 | Skyrmionics in correlated oxides | While chiral magnets, metal-based magnetic multilayers, or Heusler compounds
have been considered as the material workhorses in the field of skyrmionics,
oxides are now emerging as promising alternatives, as they host special
correlations between the spin-orbital-charge-lattice degrees of freedom and/or
coupled ferroic order parameters. These interactions open new possibilities for
practically exploiting skyrmionics. In this article, we review the recent
advances in the observation and control of topological spin textures in various
oxide systems. We start with the discovery of skyrmions and related
quasiparticles in bulk and heterostructure ferromagnetic oxides. Next, we
emphasize the shortcomings of implementing ferromagnetic textures, which have
led to the recent explorations of ferrimagnetic and antiferromagnetic oxide
counterparts, with higher Curie temperatures, stray-field immunity, low Gilbert
damping, ultrafast magnetic dynamics, and/or absence of skyrmion deflection.
Then, we highlight the development of novel pathways to control the stability,
motion, and detection of topological textures using electric fields and
currents. Finally, we present the outstanding challenges that need to be
overcome to achieve all-electrical, nonvolatile, low-power oxide skyrmionic
devices. | 2111.10562v2 |
2021-12-01 | Unconditional well-posedness and IMEX improvement of a family of predictor-corrector methods in micromagnetics | Recently, Kim & Wilkening (Convergence of a mass-lumped finite element method
for the Landau-Lifshitz equation, Quart. Appl. Math., 76, 383-405, 2018)
proposed two novel predictor-corrector methods for the Landau-Lifshitz-Gilbert
equation (LLG) in micromagnetics, which models the dynamics of the
magnetization in ferromagnetic materials. Both integrators are based on the
so-called Landau-Lifshitz form of LLG, use mass-lumped variational formulations
discretized by first-order finite elements, and only require the solution of
linear systems, despite the nonlinearity of LLG. The first(-order in time)
method combines a linear update with an explicit projection of an intermediate
approximation onto the unit sphere in order to fulfill the LLG-inherent
unit-length constraint at the discrete level. In the second(-order in time)
integrator, the projection step is replaced by a linear constraint-preserving
variational formulation. In this paper, we extend the analysis of the
integrators by proving unconditional well-posedness and by establishing a close
connection of the methods with other approaches available in the literature.
Moreover, the new analysis also provides a well-posed integrator for the
Schr\"odinger map equation (which is the limit case of LLG for vanishing
damping). Finally, we design an implicit-explicit strategy for the treatment of
the lower-order field contributions, which significantly reduces the
computational cost of the schemes, while preserving their theoretical
properties. | 2112.00451v1 |
2022-01-27 | Magnon transport and thermoelectric effects in ultrathin Tm3Fe5O12/Pt nonlocal devices | The possibility of electrically exciting and detecting magnon currents in
magnetic insulators has opened exciting perspectives for transporting spin
information in electronic devices. However, the role of the magnetic field and
the nonlocal thermal gradients on the magnon transport remain unclear. Here, by
performing nonlocal harmonic voltage measurements, we investigate magnon
transport in perpendicularly magnetized ultrathin Tm3Fe5O12 (TmIG) films
coupled to Pt electrodes. We show that the first harmonic nonlocal voltage
captures spin-driven magnon transport in TmIG, as expected, and the second
harmonic is dominated by thermoelectric voltages driven by current-induced
thermal gradients at the detector. The magnon diffusion length in TmIG is found
to be on the order of 0.3 {\mu}m at 0.5 T and gradually decays to 0.2 {\mu}m at
0.8 T, which we attribute to the suppression of the magnon relaxation time due
to the increase of the Gilbert damping with field. By performing current,
magnetic field, and distance dependent nonlocal and local measurements we
demonstrate that the second harmonic nonlocal voltage exhibits five
thermoelectric contributions, which originate from the nonlocal spin Seebeck
effect and the ordinary, planar, spin, and anomalous Nernst effects. Our work
provides a guide on how to disentangle magnon signals from diverse
thermoelectric voltages of spin and magnetic origin in nonlocal magnon devices,
and establish the scaling laws of the thermoelectric voltages in
metal/insulator bilayers. | 2201.11353v1 |
2022-01-31 | Tuning spin-orbit torques across the phase transition in VO$_2$/NiFe heterostructure | The emergence of spin-orbit torques as a promising approach to
energy-efficient magnetic switching has generated large interest in material
systems with easily and fully tunable spin-orbit torques. Here, current-induced
spin-orbit torques in VO$_2$/NiFe heterostructures were investigated using
spin-torque ferromagnetic resonance, where the VO$_2$ layer undergoes a
prominent insulator-metal transition. A roughly two-fold increase in the
Gilbert damping parameter, $\alpha$, with temperature was attributed to the
change in the VO$_2$/NiFe interface spin absorption across the VO$_2$ phase
transition. More remarkably, a large modulation ($\pm$100%) and a sign change
of the current-induced spin-orbit torque across the VO$_2$ phase transition
suggest two competing spin-orbit torque generating mechanisms. The bulk spin
Hall effect in metallic VO$_2$, corroborated by our first-principles
calculation of spin Hall conductivity, $\sigma_{SH} \approx 10^4
\frac{\hbar}{e} \Omega^{-1} m^{-1}$, is verified as the main source of the
spin-orbit torque in the metallic phase. The self-induced/anomalous torque in
NiFe, of the opposite sign and a similar magnitude to the bulk spin Hall effect
in metallic VO$_2$, could be the other competing mechanism that dominates as
temperature decreases. For applications, the strong tunability of the torque
strength and direction opens a new route to tailor spin-orbit torques of
materials which undergo phase transitions for new device functionalities. | 2201.12984v1 |
2022-02-03 | Controlling spin pumping into superconducting Nb by proximity-induced spin-triplet Cooper pairs | Proximity-induced long-range spin-triplet supercurrents, important for the
field of superconducting spintronics, are generated in
superconducting/ferromagnetic heterostructures when interfacial magnetic
inhomogeneities responsible for spin mixing and spin flip scattering are
present. The multilayer stack Nb/Cr/Fe/Cr/Nb has been shown to support such
exotic currents when fabricated into Josephson junction devices. However,
creating pure spin currents controllably in superconductors outside of the
Josephson junction architecture is a bottleneck to progress. Recently,
ferromagnetic resonance was proposed as a possible direction, the signature of
pure supercurrent creation being an enhancement of the Gilbert damping below
the superconducting critical temperature, but the necessary conditions are
still poorly established. Consistent with theoretical prediction, we
demonstrate conclusively that pumping pure spin currents into a superconductor
is only possible when conditions supporting proximity-induced spin-triplet
effects are satisfied. Our study is an important step forward for
superconducting pure spin current creation and manipulation, considerably
advancing the field of superconducting spintronics. | 2202.01520v1 |
2022-06-17 | Multiscale Modelling of the Antiferromagnet Mn2Au: From ab-initio to Micromagnetics | Antiferromagnets (AFMs) are strong candidates for the future spintronic and
memory applications largely because of their inherently fast dynamics and lack
of stray fields, with Mn2Au being one of the most promising. For the numerical
modelling of magnetic material properties, it is common to use ab-initio
methods, atomistic models and micromagnetics. However, each method alone
describes the physics within certain limits. Multiscale methods bridging the
gap between these three approaches have been already proposed for ferromagnetic
materials. Here, we present a complete multiscale model of the AFM Mn2Au as an
exemplar material, starting with results from ab-initio methods going via
atomistic spin dynamics (ASD) to an AFM Landau-Lifshitz-Bloch (AFM-LLB) model.
Firstly, bulk is modelled using a classical spin Hamiltonian constructed based
on earlier first-principles calculations. Secondly, this spin model is used in
the stochastic Landau-Lifshitz-Gilbert (LLG) to calculate temperature-dependent
equilibrium properties, such as magnetization and magnetic susceptibilities.
Thirdly, the temperature dependent micromagnetic parameters are used in the
AFM-LLB. We validate our approach by comparing the ASD and AFM-LLB models for
three paradigmatic cases; (i) Damped magnetic oscillations, (ii) magnetization
dynamics following a heat pulse resembling pump-probe experiments, (iii)
magnetic domain wall motion under thermal gradients. | 2206.08625v1 |
2022-10-29 | Micromagnetic frequency-domain simulation methods for magnonic systems | We present efficient numerical methods for the simulation of small
magnetization oscillations in three-dimensional micromagnetic systems.
Magnetization dynamics is described by the Landau-Lifshitz-Gilbert (LLG)
equation, linearized in the frequency domain around a generic equilibrium
configuration, and formulated in a special operator form that allows leveraging
large-scale techniques commonly used to evaluate the effective field in
time-domain micromagnetic simulations. By using this formulation, we derive
numerical algorithms to compute the free magnetization oscillations (i.e., spin
wave eigenmodes) as well as magnetization oscillations driven by ac
radio-frequency fields for arbitrarily shaped nanomagnets. Moreover,
semi-analytical perturbation techniques based on the computation of a reduced
set of eigenmodes are provided for fast evaluation of magnetization frequency
response and absorption spectra as a function of damping and ac field. We
present both finite difference and finite element implementations and
demonstrate their effectiveness on a test case. These techniques open the
possibility to study generic magnonic systems discretized with several hundred
thousand (or even millions) of computational cells in a reasonably short time. | 2210.16564v3 |
2023-03-07 | Magnon currents excited by the spin Seebeck effect in ferromagnetic EuS thin films | A magnetic insulator is an ideal platform to propagate spin information by
exploiting magnon currents. However, until now, most studies have focused on
Y$_3$Fe$_5$O$_{12}$ (YIG) and a few other ferri- and antiferromagnetic
insulators, but not on pure ferromagnets. In this study, we demonstrate for the
first time that magnon currents can propagate in ferromagnetic insulating thin
films of EuS. By performing both local and non-local transport measurements in
18-nm-thick films of EuS using Pt electrodes, we detect magnon currents arising
from thermal generation by the spin Seebeck effect. By comparing the dependence
of the local and non-local signals with the temperature (< 30 K) and magnetic
field (< 9 T), we confirm the magnon transport origin of the non-local signal.
Finally, we extract the magnon diffusion length in the EuS film (~140 nm), a
short value in good correspondence with the large Gilbert damping measured in
the same film. | 2303.03833v2 |
2023-04-01 | A coupled magneto-structural continuum model for multiferroic $\mathrm{BiFeO}_3$ | A continuum approach to study magnetoelectric multiferroic $\mathrm{BiFeO}_3$
(BFO) is proposed. Our modeling effort marries the ferroelectric (FE) phase
field method and micromagnetic simulations in order to describe the entire
multiferroic order parameter sector (polarization, oxygen antiphase tilts,
strain, and magnetism) self-consistently on the same time and length scale. In
this paper, we discuss our choice of ferroelectric and magnetic energy terms
and demonstrate benchmarks against known behavior. We parameterize the lowest
order couplings of the structural distortions against previous predictions from
density functional theory calculations giving access to simulations of the FE
domain wall (DW) topology. This allows us to estimate the energetic hierarchy
and thicknesses of the numerous structural DWs. We then extend the model to the
canted antiferromagnetic order and demonstrate how the ferroelectric domain
boundaries influence the resulting magnetic DWs. We also highlight some
capabilities of this model by providing two examples relevant for applications.
We demonstrate spin wave transmission through the multiferroic domain
boundaries which identify rectification in qualitative agreement with recent
experimental observations. As a second example of application, we model
fully-dynamical magnetoelectric switching, where we find a sensitivity on the
Gilbert damping with respect to switching pathways. We envision that this
modeling effort will set the basis for further work on properties of arbitrary
3D nanostructures of BFO (and related multiferroics) at the mesoscale. | 2304.00270v1 |
2023-10-27 | Effect of interfacial Dzyaloshinskii-Moriya interaction in spin dynamics of an Antiferromagnet coupled Ferromagnetic double-barrier Magnetic Tunnel Junction | In this work, we have studied the spin dynamics of a synthethic
Antiferromagnet (SAFM)$|$Heavy Metal (HM)$|$Ferromagnet (FM) double barrier
magnetic tunnel junction (MTJ) in presence of Ruderman-Kittel-Kasuya-Yoside
interaction (RKKYI), interfacial Dzyaloshinskii-Moriya interaction (iDMI),
N\'eel field and Spin-Orbit Coupling (SOC) with different Spin Transfer Torque
(STT). We employ Landau-Lifshitz-Gilbert-Slonczewski (LLGS) equation to
investigate the AFM dynamics of the proposed system. We found that the system
exhibits a transition from regular to damped oscillations with the increase in
strength of STT for systems with weaker iDMI than RKKYI while display sustained
oscillatons for system having same order of iDMI and RKKYI. On the other hand
the iDMI dominating system exhibits self-similar but aperiodic patterns in
absence of N\'eel field. In the presence of N\'eel field, the RKKYI dominating
systems exhibit chaotic oscillations for low STT but display sustained
oscillation under moderate STT. Our results suggest that the decay time of
oscillations can be controlled via SOC. The system can works as an oscillator
for low SOC but display nonlinear characteristics with the rise in SOC for
systems having weaker iDMI than RKKYI while an opposite characteristic are
noticed for iDMI dominating systems. We found periodic oscillations under low
external magnetic field in RKKYI dominating systems while moderate field are
necessary for sustained oscillation in iDMI dominating systems. Moreover, the
system exhibits saddle-node bifurcation and chaos under moderate N\'eel field
and SOC with suitable iDMI and RKKYI. In addition, our results indicate that
the magnon lifetime can be enhanced by increasing the strength of iDMI for both
optical and acoustic modes. | 2310.18175v1 |
2024-03-01 | Spin current control of magnetism | Exploring novel strategies to manipulate the order parameter of magnetic
materials by electrical means is of great importance, not only for advancing
our understanding of fundamental magnetism, but also for unlocking potential
practical applications. A well-established concept to date uses gate voltages
to control magnetic properties, such as saturation magnetization, magnetic
anisotropies, coercive field, Curie temperature and Gilbert damping, by
modulating the charge carrier population within a capacitor structure. Note
that the induced carriers are non-spin-polarized, so the control via the
electric-field is independent of the direction of the magnetization. Here, we
show that the magnetocrystalline anisotropy (MCA) of ultrathin Fe films can be
reversibly modified by a spin current generated in Pt by the spin Hall effect.
The effect decreases with increasing Fe thickness, indicating that the origin
of the modification can be traced back to the interface. Uniquely, the change
in MCA due to the spin current depends not only on the polarity of the charge
current but also on the direction of magnetization, i.e. the change in MCA has
opposite sign when the direction of magnetization is reversed. The control of
magnetism by the spin current results from the modified exchange splitting of
majority- and minority-spin bands, and differs significantly from the
manipulation by gate voltages via a capacitor structure, providing a
functionality that was previously unavailable and could be useful in advanced
spintronic devices. | 2403.00709v1 |
2007-02-12 | The Ucsd/Keck Damped Lya Abundance Database: A Decade of High Resolution Spectroscopy | We publish the Keck/HIRES and Keck/ESI spectra that we have obtained during
the first 10 years of Keck observatory operations. Our full sample includes 42
HIRES spectra and 39 ESI spectra along 65 unique sightlines providing abundance
measurements on ~85 damped Lya systems. The normalized data can be downloaded
from the journal or from our supporting website:
http://www.ucolick.org/~xavier/DLA/. The database includes all of the
sightlines that have been included in our papers on the chemical abundances,
kinematics, and metallicities of the damped Lya systems. This data has also
been used to argue for variations in the fine-structure constant. We present
new chemical abundance measurements for 10 damped Lya systems and a summary
table of high-resolution metallicity measurements (including values from the
literature) for 153 damped Lya systems at z>1.6. We caution, however, that this
metallicity sample (and all previous ones) is biased to higher N(HI) values
than a random sample. | 0702325v1 |
1998-06-30 | Structure and Spin Dynamics of La$_{0.85}$Sr$_{0.15}$MnO$_3$ | Neutron scattering has been used to study the structure and spin dynamics of
La$_{0.85}$Sr$_{0.15}$MnO$_3$. The magnetic structure of this system is
ferromagnetic below T_C = 235 K. We see anomalies in the Bragg peak intensities
and new superlattice peaks consistent with the onset of a spin-canted phase
below T_{CA} = 205 K, which appears to be associated with a gap at q = (0, 0,
0.5) in the spin-wave spectrum. Anomalies in the lattice parameters indicate a
concomitant lattice distortion. The long-wavelength magnetic excitations are
found to be conventional spin waves, with a gapless (< 0.02 meV) isotropic
dispersion relation $E = Dq^2$. The spin stiffness constant D has a $T^{5/2}$
dependence at low T, and the damping at small q follows $q^4T^{2}$. An
anomalously strong quasielastic component, however, develops at small wave
vector above 200 K and dominates the fluctuation spectrum as T -> T_C. At
larger q, on the other hand, the magnetic excitations become heavily damped at
low temperatures, indicating that spin waves in this regime are not eigenstates
of the system, while raising the temperature dramatically increases the
damping. The strength of the spin-wave damping also depends strongly on the
symmetry direction in the crystal. These anomalous damping effects are likely
due to the itinerant character of the $e_g$ electrons. | 9806381v1 |
2008-02-11 | Eccentricity of masing disks in Active Galactic Nuclei | Observations of Keplerian disks of masers in NCG 4258 and other Seyfert
galaxies can be used to obtain geometric distance estimates and derive the
Hubble constant. The ultimate precision of such measurements could be limited
by uncertainties in the disk geometry. Using a time-dependent linear theory
model, we study the evolution of a thin initially eccentric disk under
conditions appropriate to sub-pc scales in Active Galactic Nuclei. The
evolution is controlled by a combination of differential precession driven by
the disk potential and propagating eccentricity waves that are damped by
viscosity. A simple estimate yields a circularization timescale of
approximately 10 Myr at 0.1 pc. Numerical solutions for the eccentricity
evolution confirm that damping commences on this timescale, but show that the
subsequent decay rate of the eccentricity depends upon the uncertain strength
of viscous damping of eccentricity. If eccentricity waves are important further
decay of the eccentricity can be slow, with full circularization requiring up
to 50 Myr for disks at radii of 0.1 pc to 0.2 pc. Observationally, this implies
that it is plausible that enough time has elapsed for the eccentricity of
masing disks to have been substantially damped, but that it may not be
justified to assume vanishing eccentricity. We predict that during the damping
phase the pericenter of the eccentric orbits describes a moderately tightly
wound spiral with radius. | 0802.1524v1 |
2013-09-26 | Non-Landau damping of magnetic excitations in systems with localized and itinerant electrons | We discuss the form of the damping of magnetic excitations in a metal near a
ferromagnetic instability. The paramagnon theory predicts that the damping term
should have the form $\Omega/\Gamma (q)$ with $\Gamma (q) \propto q$ (the
Landau damping). However, the experiments on uranium metallic compounds UGe$_2$
and UCoGe showed that $\Gamma (q)$ tends to a constant value at vanishing $q$.
A non-zero $\Gamma (0)$ is impossible in systems with one type of carriers
(either localized or itinerant) because it would violate the spin conservation.
It has been conjectured recently that a non-zero $\Gamma (q)$ in UGe$_2$ and
UCoGe may be due to the presence of both localized and itinerant electrons in
these materials, with ferromagnetism involving predominantly localized spins.
We present microscopic analysis of the damping of near-critical localized
excitations due to interaction with itinerant carriers. We show explicitly how
the presence of two types of electrons breaks the cancellation between the
contributions to $\Gamma (0)$ from self-energy and vertex correction insertions
into the spin polarization bubble and discuss the special role of the
Aslamazov-Larkin processes. We show that $\Gamma (0)$ increases with $T$ both
in the paramagnetic and ferromagnetic regions, but in-between it has a peak at
$T_c$. We compare our theory with the available experimental data. | 1309.7065v3 |
2016-04-20 | Nonlinear wave damping due to multi-plasmon resonances | For short wavelengths, it is well known that the linearized Wigner-Moyal
equation predicts wave damping due to wave-particle interaction, where the
resonant velocity shifted from the phase velocity by a velocity $v_q = \hbar
k/2m$. Here $\hbar$ is the reduced Planck constant, $k$ is the wavenumber and
$m$ is the electron mass. Going beyond linear theory, we find additional
resonances with velocity shifts $n v_q$, $n = 2, 3, \ldots$, giving rise to a
new wave-damping mechanism that we term \emph{multi-plasmon damping}, as it can
be seen as the simultaneous absorption (or emission) of multiple plasmon
quanta. Naturally this wave damping is not present in classical plasmas. For a
temperature well below the Fermi temperature, if the linear ($n = 1$) resonant
velocity is outside the Fermi sphere, the number of linearly resonant particles
is exponentially small, while the multi-plasmon resonances can be located in
the bulk of the distribution. We derive sets of evolution equations for the
case of two-plasmon and three-plasmon resonances for Langmuir waves in the
simplest case of a fully degenerate plasma. By solving these equations
numerically for a range of wave-numbers we find the corresponding damping
rates, and we compare them to results from linear theory to estimate the
applicability. Finally, we discuss the effects due to a finite temperature. | 1604.05983v2 |
2017-10-30 | Enhancement of intrinsic magnetic damping in defect-free epitaxial Fe3O4 thin films | We have investigated the magnetic damping of precessional spin dynamics in
defect-controlled epitaxial grown Fe$_3$O$_4$(111)/Yttria-stabilized Zirconia
(YSZ) nanoscale films by all-optical pump-probe measurements. The intrinsic
damping constant of the defect-free Fe$_3$O$_4$ film is found to be strikingly
larger than that of the as-grown Fe$_3$O$_4$ film with structural defects. We
demonstrate that the population of the first-order perpendicular standing spin
wave (PSSW) mode, which is exclusively observed in the defect-free film under
sufficiently high external magnetic fields, leads to the enhancement of the
magnetic damping of the uniform precession (Kittel) mode. We propose a physical
picture in which the PSSW mode acts as an additional channel for the extra
energy dissipation of the Kittel mode. The energy transfer from Kittel mode to
PSSW mode increases as in-plane magnetization precession becomes more uniform,
resulting in the unique intrinsic magnetic damping enhancement in the
defect-free Fe$_3$O$_4$ film. | 1710.10938v2 |
2022-06-08 | Motion control with optimal nonlinear damping: from theory to experiment | Optimal nonlinear damping control was recently introduced for the
second-order SISO systems, showing some advantages over a classical PD feedback
controller. This paper summarizes the main theoretical developments and
properties of the optimal nonlinear damping controller and demonstrates, for
the first time, its practical experimental evaluation. An extended analysis and
application to more realistic (than solely the double-integrator) motion
systems are also given in the theoretical part of the paper. As comparative
linear feedback controller, a PD one is taken, with the single tunable gain and
direct compensation of the plant time constant. The second, namely
experimental, part of the paper includes the voice-coil drive system with
relatively high level of the process and measurement noise, for which the
standard linear model is first identified in frequency domain. The linear
approximation by two-parameters model forms the basis for designing the PD
reference controller, which fixed feedback gain is the same as for the optimal
nonlinear damping control. A robust sliding-mode based differentiator is used
in both controllers for a reliable velocity estimation required for the
feedback. The reference PD and the proposed optimal nonlinear damping
controller, both with the same single design parameter, are compared
experimentally with respect to trajectory tracking and disturbance rejection. | 2206.03802v2 |
2023-07-12 | Exponential stability of damped Euler-Bernoulli beam controlled by boundary springs and dampers | In this paper, the vibration model of an elastic beam, governed by the damped
Euler-Bernoulli equation
$\rho(x)u_{tt}+\mu(x)u_{t}$$+\left(r(x)u_{xx}\right)_{xx}=0$, subject to the
clamped boundary conditions $u(0,t)=u_x(0,t)=0$ at $x=0$, and the boundary
conditions $\left(-r(x)u_{xx}\right)_{x=\ell}=k_r u_x(\ell,t)+k_a
u_{xt}(\ell,t)$, $\left(-\left(r(x)u_{xx}\right)_{x}\right )_{x=\ell}$$=- k_d
u(\ell,t)-k_v u_{t}(\ell,t)$ at $x=\ell$, is analyzed. The boundary conditions
at $x=\ell$ correspond to linear combinations of damping moments caused by
rotation and angular velocity and also, of forces caused by displacement and
velocity, respectively. The system stability analysis based on well-known
Lyapunov approach is developed. Under the natural assumptions guaranteeing the
existence of a regular weak solution, uniform exponential decay estimate for
the energy of the system is derived. The decay rate constant in this estimate
depends only on the physical and geometric parameters of the beam, including
the viscous external damping coefficient $\mu(x) \ge 0$, and the boundary
springs $k_r,k_d \ge 0$ and dampers $k_a,k_v \ge 0$. Some numerical examples
are given to illustrate the role of the damping coefficient and the boundary
dampers. | 2307.06170v2 |
2019-03-07 | Uniaxial anisotropy, intrinsic and extrinsic damping in Co$_{2}$FeSi Heusler alloy thin films | Ferromagnetic resonance (FMR) technique has been used to study the
magnetization relaxation processes and magnetic anisotropy in two different
series of the Co2FeSi (CFS) Heusler alloy thin films, deposited on the Si(111)
substrate by UHV sputtering. While the CFS films of fixed (50 nm) thickness,
deposited at different substrate temperatures (TS) ranging from room
temperature (RT) to 600^C, constitute the series-I, the CFS films with
thickness t varying from 12 nm to 100 nm and deposited at 550^C make up the
series-II. In series-I, the CFS films deposited at TS = RT and 200^C are
completely amorphous, the one at TS = 300^C is partially crystalline, and those
at TS equal 450^C, 550^C and 600^C are completely crystalline with B2 order. By
contrast, all the CFS films in series-II are in the fully-developed B2
crystalline state. Irrespective of the strength of disorder and film thickness,
angular variation of the resonance field in the film plane unambiguously
establishes the presence of global in-plane uniaxial anisotropy. Angular
variation of the linewidth in the film plane reveals that, in the CFS thin
films of varying thickness, a crossover from the in-plane local four-fold
symmetry (cubic anisotropy) to local two-fold symmetry (uniaxial anisotropy)
occurs as t exceeds 50 nm. Gilbert damping parameter {\alpha} decreases
monotonously from 0.047 to 0.0078 with decreasing disorder strength (increasing
TS) and jumps from 0.008 for the CFS film with t = 50 nm to 0.024 for the film
with t equal 75 nm. Such variations of {\alpha} with TS and t are understood in
terms of the changes in the total (spin-up and spin-down) density of states at
the Fermi level caused by the disorder and film thickness. | 1903.02976v1 |
2000-09-06 | The Cosmological Evolution of Quasar Damped Lyman-Alpha Systems | We present results from an efficient, non-traditional survey to discover
damped Lyman-alpha (DLA) absorption-line systems with neutral hydrogen column
densities N(HI)>2x10^{20} atoms cm^{-2} and redshifts z<1.65. Contrary to
previous studies at higher redshift that showed a decrease in the cosmological
mass density of neutral gas in DLA absorbers, Omega_{DLA}, with time, our
results indicate that Omega_{DLA} is consistent with remaining constant from
redshifts z \approx 4 to z \approx 0.5. There is no evidence that Omega_{DLA}
is approaching the value at z=0. Other interesting results from the survey are
also presented. | 0009098v1 |
2005-06-09 | Phantom damping of matter perturbations | Cosmological scaling solutions are particularly important in solving the
coincidence problem of dark energy. We derive the equations of sub-Hubble
linear matter perturbations for a general scalar-field Lagrangian--including
quintessence, tachyon, dilatonic ghost condensate and k-essence--and solve them
analytically for scaling solutions. We find that matter perturbations are
always damped if a phantom field is coupled to dark matter and identify the
cases in which the gravitational potential is constant. This provides an
interesting possibility to place stringent observational constraints on scaling
dark energy models. | 0506222v1 |
1995-02-10 | The influence of structure disorder on mean atomic momentum fluctuations and a spin-wave spectrum | The relation between atomic momenta fluctuations and density fluctuations is
obtained in frames of mean-field approximation. Using two-time temperature
Green functions within Tyablikov approximation the equations for spin
excitation energy and damping are obtained. The asymptotics of energy and
damping in the long-wave limit are investigated and the anomalous behaviour of
spin-wave stiffness constant is discussed. | 9502042v1 |
1999-01-19 | Damping of Growth Oscillations | Computer simulations and scaling theory are used to investigate the damping
of oscillations during epitaxial growth on high-symmetry surfaces. The
crossover from smooth to rough growth takes place after the deposition of
(D/F)^\delta monolayers, where D and F are the surface diffusion constant and
the deposition rate, respectively, and the exponent \delta=2/3 on a
two-dimensional surface. At the transition, layer-by-layer growth becomes
desynchronized on distances larger than a layer coherence length proportional
l^2, where l is a typical distance between two-dimensional islands in the
submonolayer region of growth. | 9901178v1 |
2000-03-27 | Effect of memory and dynamical chaos in long Josephson junctions | A long Josephson junction in a constant external magnetic field and in the
presence of a dc bias current is investigated. It is shown that the system,
simulated by the sine-Gorgon equation, "remembers" a rapidly damping initial
perturbation and final asymptotic states are determined exactly with this
perturbation. Numerical solving of the boundary sine-Gordon problem and
calculations of Lyapunov indices show that this system has a memory even when
it is in a state of dynamical chaos, i.e., dynamical chaos does not destroy
initial information having a character of rapidly damping perturbation. | 0003421v1 |
2003-09-24 | Landau Damping in a 2D Electron Gas with Imposed Quantum Grid | Dielectric properties of semiconductor substrate with imposed two dimensional
(2D) periodic grid of quantum wires or nanotubes (quantum crossbars, QCB) are
studied. It is shown that a capacitive contact between QCB and semiconductor
substrate does not destroy the Luttinger liquid character of the long wave QCB
excitations. However, the dielectric losses of a substrate surface are
drastically modified due to diffraction processes on the QCB superlattice.
QCB-substrate interaction results in additional Landau damping regions of the
substrate plasmons. Their existence, form and the density of losses are
strongly sensitive to the QCB lattice constant. | 0309546v2 |
2005-11-05 | Ratchet Effect in Magnetization Reversal of Stoner Particles | A new strategy is proposed aimed at substantially reducing the minimal
magnetization switching field for a Stoner particle. Unlike the normal method
of applying a static magnetic field which must be larger than the magnetic
anisotropy, a much weaker field, proportional to the damping constant in the
weak damping regime, can be used to switch the magnetization from one state to
another if the field is along the motion of the magnetization. The concept is
to constantly supply energy to the particle from the time-dependent magnetic
field to allow the particle to climb over the potential barrier between the
initial and the target states. | 0511135v1 |
1994-09-12 | Fermion damping rate in a hot medium | In principle every excitation acquires a finite lifetime in a hot system.
This nonzero spectral width is calculated self-consistently for massive
fermions coupled to massless scalar, vector and pseudoscalar bosons. It is
shown that the self-consistent summation of the corresponding Fock diagram for
fermions eliminates all infrared divergences although the bosons are not
screened at all. Our solutions for the fermion damping rate are analytical in
the coupling constant, but not analytical in the temperature parameter around
T=0. | 9409280v2 |
2004-02-06 | Critical Behavior of Damping Rate for Plasmon with Finite Momentum in φ^4 Theory | Applying thermal renormalization group (TRG) equations to $\phi^4$ theory
with spontaneous breaking symmetry, we investigate the critical behavior of the
damping rate for the plasmons with finite momentum at the symmetry-restoring
phase transition. From the TRG equation the IR cutoff provided by the external
momentum leads to that the momentum-dependent coupling constant stops running
in the critical region. As the result, the critical slowing down phenomenon
reflecting the inherently IR effect doesn't take place at the critical point
for the plasmon with finite external momentum. | 0402069v2 |
2006-11-26 | On the Lagrangian and Hamiltonian description of the damped linear harmonic oscillator | Using the modified Prelle- Singer approach, we point out that explicit time
independent first integrals can be identified for the damped linear harmonic
oscillator in different parameter regimes. Using these constants of motion, an
appropriate Lagrangian and Hamiltonian formalism is developed and the resultant
canonical equations are shown to lead to the standard dynamical description.
Suitable canonical transformations to standard Hamiltonian forms are also
obtained. It is also shown that a possible quantum mechanical description can
be developed either in the coordinate or momentum representations using the
Hamiltonian forms. | 0611048v1 |
2005-02-10 | Modulational instabilities in Josephson oscillations of elongated coupled condensates | We study the Josephson oscillations of two coupled elongated condensates.
Linearized calculations show that the oscillating mode uniform over the length
of the condensates (uniform Josephson mode) is unstable : modes of non zero
longitudinal momentum grow exponentially. In the limit of strong atom
interactions, we give scaling laws for the instability time constant and
unstable wave vectors. Beyond the linearized approach, numerical calculations
show a damped recurrence behavior : the energy in the Josephson mode presents
damped oscillations. Finally, we derive conditions on the confinement of the
condensates to prevent instabilities. | 0502050v3 |
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