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2009-05-28 | A black box method for solving the complex exponentials approximation problem | A common problem, arising in many different applied contexts, consists in
estimating the number of exponentially damped sinusoids whose weighted sum best
fits a finite set of noisy data and in estimating their parameters. Many
different methods exist to this purpose. The best of them are based on
approximate Maximum Likelihood estimators, assuming to know the number of
damped sinusoids, which can then be estimated by an order selection procedure.
As the problem can be severely ill posed, a stochastic perturbation method is
proposed which provides better results than Maximum Likelihood based methods
when the signal-to-noise ratio is low. The method depends on some
hyperparameters which turn out to be essentially independent of the
application. Therefore they can be fixed once and for all, giving rise to a
black box method. | 0905.4602v2 |
2009-06-10 | GALEX Discovery of a Damped Ly-alpha System at Redshift z = 1 | We report the first discovery of a QSO damped Ly-alpha (DLA) system by the
GALEX satellite. The system was initially identified as an MgII absorption-line
system (z_abs=1.028) in the spectrum of SDSS QSO J0203-0910 (z_em=1.58). The
presence of unusually strong absorption due to metal lines of ZnII, CrII, MnII,
and FeII clearly suggested that it might be a DLA system with N{HI} > 2 x 10^20
atoms cm^-2. Follow-up GALEX NUV grism spectroscopy confirms the system
exhibits a DLA absorption line, with a measured HI column density of N{HI} =
1.50+/-0.45 x 10^21 atoms cm^-2. By combining the GALEX N{HI} determination
with the SDSS spectrum measurements of unsaturated metal-line absorption due to
ZnII, which is generally not depleted onto grains, we find that the system's
neutral-gas-phase metal abundance is [Zn/H] = -0.69+/-0.22, or ~20% solar. By
way of comparison, although this system has one of the largest Zn^+ column
densities, its metal abundances are comparable to other DLAs at z~1.
Measurements of the abundances of Cr, Fe, and Mn help to further pin down the
evolutionary state of the absorber. | 0906.2018v1 |
2009-06-11 | Longitudinal Stability of Recycler Bunches; Part I: Thresholds for Loss of Landau Damping | We examine the stability of intense flat bunches in barrier buckets used in
the Recycler. We consider some common stationary distributions and show that
they would be unstable against rigid dipole oscillations. We then discuss an
analytical model for the line density that best describes measured bunch
profiles. We include space charge in this model to predict the bunch intensity
at which Landau damping would be lost. The dependence of this threshold on the
bunch length is studied and related to the results of an experimental study
with shorter bunch lengths. The threshold for the microwave instability is
estimated. These studies will be followed by more detailed numerical studies. | 0906.2188v1 |
2009-06-15 | Regge poles of the Schwarzschild black hole: a WKB approach | We provide simple and accurate analytical expressions for the Regge poles of
the Schwarzschild black hole. This is achieved by using third-order WKB
approximations to solve the radial wave equations for spins 0, 1 and 2. These
results permit us to obtain analytically the dispersion relation and the
damping of the "surface waves" lying on the photon sphere of the Schwarzschild
black hole and which generate the weakly damped quasinormal modes of its
spectrum. Our results could be helpful in order to simplify considerably the
description of wave scattering from the Schwarzschild black hole as well as the
analysis of the gravitational radiation created in many black hole processes.
Furthermore, the existence of dispersion relations for the photons propagating
close to the photon sphere could have also important consequences in the
context of gravitational lensing. | 0906.2601v3 |
2009-06-30 | Cooling a magnetic resonance force microscope via the dynamical back-action of nuclear spins | We analyze the back-action influence of nuclear spins on the motion of the
cantilever of a magnetic force resonance microscope. We calculate the
contribution of nuclear spins to the damping and frequency shift of the
cantilever. We show that, at the Rabi frequency, the energy exchange between
the cantilever and the spin system cools or heats the cantilever depending on
the sign of the high-frequency detuning. We also show that the spin noise leads
to a significant damping of the cantilever motion. | 0906.5420v2 |
2009-07-03 | Magnetic interference patterns in long disordered Josephson junctions | We study a diffusive superconductor - normal metal - superconductor (SNS)
junction in an external magnetic field. In the limit of a long junction, we
find that the form of the dependence of the Josephson current on the field and
on the length of the junction depends on the ratio between the junction width
and the length associated with the magnetic field. A certain critical ratio
between these two length scales separates two different regimes. In narrow
junctions, the critical current exhibits a pure decay as a function of the
junction length or of the magnetic field. In wide junctions, the critical
current exhibits damped oscillations as a function of the same parameters. This
damped oscillating behavior differs from the Fraunhofer pattern typical for
short or tunnel junctions. In wide and long junctions, superconducting pair
correlations and supercurrent are localized along the edges of the junction. | 0907.0632v3 |
2009-07-12 | Symmetries shape the current in ratchets induced by a bi-harmonic force | Equations describing the evolution of particles, solitons, or localized
structures, driven by a zero-average, periodic, external force, and invariant
under time reversal and a half-period time shift, exhibit a ratchet current
when the driving force breaks these symmetries. The bi-harmonic force
$f(t)=\epsilon_1\cos(q \omega t+\phi_1)+\epsilon_2\cos(p\omega t+\phi_2)$ does
it for almost any choice of $\phi_{1}$ and $\phi_{2}$, provided $p$ and $q$ are
two co-prime integers such that $p+q$ is odd. It has been widely observed, in
experiments in Josephson-junctions, photonic crystals, etc., as well as in
simulations, that the ratchet current induced by this force has the shape
$v\propto\epsilon_1^p\epsilon_2^q\cos(p \phi_{1} - q \phi_{2} + \theta_0)$ for
small amplitudes, where $\theta_0$ depends on the damping ($\theta_0=\pi/2$ if
there is no damping, and $\theta_0=0$ for overdamped systems). We rigorously
prove that this precise shape can be obtained solely from the broken symmetries
of the system and is independent of the details of the equation describing the
system. | 0907.2029v2 |
2009-07-21 | AFM Dissipation Topography of Soliton Superstructures in Adsorbed Overlayers | In the atomic force microscope, the nanoscale force topography of even
complex surface superstructures is extracted by the changing vibration
frequency of a scanning tip. An alternative dissipation topography with similar
or even better contrast has been demonstrated recently by mapping the
(x,y)-dependent tip damping but the detailed damping mechanism is still
unknown. Here we identify two different tip dissipation mechanisms: local
mechanical softness and hysteresis. Motivated by recent data, we describe both
of them in a onedimensional model of Moire' superstructures of incommensurate
overlayers. Local softness at "soliton" defects yields a dissipation contrast
that can be much larger than the corresponding density or corrugation contrast.
At realistically low vibration frequencies, however, a much stronger and more
effective dissipation is caused by the tip-induced nonlinear jumping of the
soliton, naturally developing bistability and hysteresis. Signatures of this
mechanism are proposed for experimental identification. | 0907.3585v4 |
2009-07-24 | Harmonic damped oscillators with feedback. A Langevin study | We consider a system in direct contact with a thermal reservoir and which, if
left unperturbed, is well described by a memory-less equilibrium Langevin
equation of the second order in the time coordinate. In such conditions, the
strength of the noise fluctuations is set by the damping factor, in accordance
with the Fluctuation and Dissipation theorem. We study the system when it is
subject to a feedback mechanism, by modifying the Langevin equation
accordingly. Memory terms now arise in the time evolution, which we study in a
non-equilibrium steady state. Two types of feedback schemes are considered, one
focusing on time shifts and one on phase shifts, and for both cases we evaluate
the power spectrum of the system's fluctuations. Our analysis finds application
in feedback cooled oscillators, such as the Gravitational Wave detector AURIGA. | 0907.4309v1 |
2009-08-19 | Quantum Energy Teleportation with Electromagnetic Field: Discrete vs. Continuous Variables | It is well known that usual quantum teleportation protocols cannot transport
energy. Recently, new protocols called quantum energy teleportation (QET) have
been proposed, which transport energy by local operations and classical
communication with the ground states of many-body quantum systems. In this
paper, we compare two different QET protocols for transporting energy with
electromagnetic field. In the first protocol, a 1/2 spin (a qubit) is coupled
with the quantum fluctuation in the vacuum state and measured in order to
obtain one-bit information about the fluctuation for the teleportation. In the
second protocol, a harmonic oscillator is coupled with the fluctuation and
measured in order to obtain continuous-variable information about the
fluctuation. In the spin protocol, the amount of teleported energy is
suppressed by an exponential damping factor when the amount of input energy
increases. This suppression factor becomes power damping in the case of the
harmonic oscillator protocol. Therefore, it is concluded that obtaining more
information about the quantum fluctuation leads to teleporting more energy.
This result suggests a profound relationship between energy and quantum
information. | 0908.2674v2 |
2009-08-25 | Designing materials for plasmonic systems | We use electronic structure calculations based upon density functional theory
to search for ideal plasmonic materials among the alkali noble intermetallics.
Importantly, we use density functional perturbation theory to calculate the
electron-phonon interaction and from there use a first order solution to the
Boltzmann equation to estimate the phenomenological damping frequency in the
Drude dielectric function. We discuss the necessary electronic features of a
plasmonic material and investigate the optical properties of the alkali-noble
intermetallics in terms of some generic plasmonic system quality factors. We
conclude that at low negative permittivities, KAu with a damping frequency of
0.0224 eV and a high optical gap to bare plasma frequency ratio, outperforms
gold and to some extent silver as a plasmonic material. Unfortunately, a low
plasma frequency (1.54 eV) reduces its utility in modern plasmonics
applications. We also discuss, briefly, the effect of local fields on the
optical properties of these materials. | 0908.3707v1 |
2009-09-15 | Quantum critical points of Helical Fermi Liquids | Following our previous work, we study the quantum phase transitions which
spontaneously develop ferromagnetic spin order in helical fermi liquids which
breaks continuous spin-space rotation symmetry, with application to the edge
states of 3d topological band insulators. With finite fermi surface, the
critical point has both z = 3 over-damped and z = 2 propagating quantum
critical modes, and the z = 3 mode will lead to non-fermi liquid behavior on
the entire fermi surface. In the ordered phase, the Goldstone mode is
over-damped unless it propagates along special directions, and quasiparticle is
ill defined on most parts of the fermi surface except for special points.
Generalizations of our results to other systems with spin-orbit couplings are
also discussed. | 0909.2647v3 |
2009-09-25 | Evidence for Landau's critical velocity in superfluid helium nanodroplets from wave packet dynamics of attached potassium dimers | Femtosecond pump-probe spectroscopy has been used to study vibrational
dynamics of potassium dimers attached to superfluid helium nanodroplets.
Comparing the measured data with theoretical results based on dissipative
quantum dynamics we propose that the most important effect of the helium
environment is a general damping of the vibrational dynamics as a result of the
interaction between dimer and collective degrees of freedom of the helium
droplet. The calculations allow us to explain crucial experimental findings
that are unobserved in gas-phase measurements. Remarkably, best agreement with
experiment is found for a model where we neglect damping once a wave packet
moves below a critical velocity. In this way the results provide first direct
evidence for the Landau critical velocity in superfluid nanodroplets. | 0909.4691v1 |
2009-10-23 | Fragmentation of the photoabsorption strength in neutral and charged metal microclusters | The line shape of the plasma resonance in both neutral and charged small
sodium clusters is calculated. The overall properties of the multipeak
structure observed in the photoabsorption cross section of spherical Na_8 and
Na_20 neutral clusters can be understood in terms of Landau damping. Quantal
configurations are shown to play an important role. In the case of charged
Na_9+ and Na_21^+ clusters a single peak is predicted that carries most of the
oscillator strength. | 0910.4576v1 |
2009-10-28 | Quantum Decoherence of Two Qubits | It is commonly stated that decoherence in open quantum systems is due to
growing entanglement with an environment. In practice, however, surprisingly
often decoherence may equally well be described by random unitary dynamics
without invoking a quantum environment at all. For a single qubit, for
instance, pure decoherence (or phase damping) is always of random unitary type.
Here, we construct a simple example of true quantum decoherence of two qubits:
we present a feasible phase damping channel of which we show that it cannot be
understood in terms of random unitary dynamics. We give a very intuitive
geometrical measure for the positive distance of our channel to the convex set
of random unitary channels and find remarkable agreement with the so-called
Birkhoff defect based on the norm of complete boundedness. | 0910.5364v1 |
2009-10-29 | System-reservoir dynamics of quantum and classical correlations | We address the system-reservoir dynamics of classical and quantum
correlations in the decoherence phenomenon, regarding a two qubit composite
system interacting with two independent environments. The most common noise
channels (amplitude damping, phase damping, bit flip, bit-phase flip, and phase
flip) was studied. By analytical and numerical analysis we found that, contrary
to what is usually stated in the literature, decoherence may occurs without
entanglement between the system and the environment. We also found that, in
some cases, the bipartite quantum correlation initially presented in the system
is completely evaporated, it is not transferred to the environments. | 0910.5711v3 |
2009-11-04 | Nonlinear damping in a micromechanical oscillator | Nonlinear elastic effects play an important role in the dynamics of
microelectromechanical systems (MEMS). Duffing oscillator is widely used as an
archetypical model of mechanical resonators with nonlinear elastic behavior. In
contrast, nonlinear dissipation effects in micromechanical oscillators are
often overlooked. In this work, we consider a doubly clamped micromechanical
beam oscillator, which exhibits nonlinearity in both elastic and dissipative
properties. The dynamics of the oscillator is measured in frequency domain and
time domain and compared to theoretical predictions based on Duffing-like model
with nonlinear dissipation. We especially focus on the behavior of the system
near bifurcation points. The results show that nonlinear dissipation can have a
significant impact on the dynamics of micromechanical systems. To account for
the results, we have developed a continuous model of a nonlinear viscoelastic
string with Voigt-Kelvin dissipation relation, which shows a relation between
linear and nonlinear damping. However, the experimental results suggest that
this model alone cannot fully account for all the experimentally observed
nonlinear dissipation, and that additional nonlinear dissipative processes
exist in our devices. | 0911.0833v2 |
2009-11-04 | Solar-like oscillations in massive main-sequence stars. I. Asteroseismic signatures of the driving and damping regions | Motivated by the recent detection of stochastically excited modes in the
massive star V1449 Aql (Belkacem et al., 2009b), already known to be a $\beta$
Cephei, we theoretically investigate the driving by turbulent convection. By
using a full non-adiabatic computation of the damping rates, together with a
computation of the energy injection rates, we provide an estimate of the
amplitudes of modes excited by both the convective region induced by the iron
opacity bump and the convective core. Despite uncertainties in the dynamical
properties of such convective regions, we demonstrate that both are able to
efficiently excite $p$ modes above the CoRoT observational threshold and the
solar amplitudes. In addition, we emphasise the potential asteroseismic
diagnostics provided by each convective region, which we hope will help to
identify the one responsible for solar-like oscillations, and to give
constraints on this convective zone. A forthcoming work will be dedicated to an
extended investigation of the likelihood of solar-like oscillations across the
Hertzsprung-Russell diagram. | 0911.0908v1 |
2009-12-10 | Bipartite quantum channels using multipartite cluster-type entangled coherent states | We propose a particular encoding for bipartite entangled states derived from
multipartite cluster-type entangled coherent states (CTECSs). We investigate
the effects of amplitude damping on the entanglement content of this bipartite
state, as well as its usefulness as a quantum channel for teleportation. We
find interesting relationships among the amplitude of the coherent states
constituting the CTECSs, the number of subsystems forming the logical qubits
(redundancy), and the extent to which amplitude damping affects the
entanglement of the channel. For instance, in the sense of sudden death of
entanglement, given a fixed value of the initial coherent state amplitude, the
entanglement life span is shortened if redundancy is increased. | 0912.1949v2 |
2009-12-18 | Oscillatory transient regime in the forced dynamics of a spin torque nano-oscillator | We demonstrate that the transient non-autonomous dynamics of a spin torque
nano-oscillator (STNO) under a radio-frequency (rf) driving signal is
qualitatively different from the dynamics described by the Adler model. If the
external rf current $I_{rf}$ is larger than a certain critical value $I_{cr}$
(determined by the STNO bias current and damping) strong oscillations of the
STNO power and phase develop in the transient regime. The frequency of these
oscillations increases with $I_{rf}$ as $\propto\sqrt{I_{rf} - I_{cr}}$ and can
reach several GHz, whereas the damping rate of the oscillations is almost
independent of $I_{rf}$. This oscillatory transient dynamics is caused by the
strong STNO nonlinearity and should be taken into account in most STNO rf
applications. | 0912.3650v1 |
2009-12-19 | Study of sdO models: mode trapping | We present the first description of mode trapping for sdO models. Mode
trapping of gravity modes caused by the He/H chemical transition is found for a
particular model, providing a selection effect for high radial order trapped
modes. Low- and intermediate-radial order {\em p}-modes (mixed modes with a
majority of nodes in the P-mode region) are found to be trapped by the C-O/He
transition, but with no significant effects on the driving. This region seems
to have also a subtle effect on the trapping of low radial order {\em g}-modes
(mixed modes with a majority of nodes in the G-mode region), but again with no
effect on the driving. We found that for mode trapping to have an influence on
the driving of sdO modes (1) the mode should be trapped in a way that the
amplitude of the eigenfunctions is lower in a damping region and (2) in this
damping region significant energy interchange has to be produced. | 0912.3911v1 |
2009-12-20 | Optimal Design of Fuzzy Based Power System Stabilizer Self Tuned by Robust Search Algorithm | In the interconnected power system network, instability problems are caused
mainly by the low frequency oscillations of 0.2 to 2.5 Hz. The supplementary
control signal in addition with AVR and high gain excitation systems are
provided by means of Power System Stabilizer (PSS). Conventional power system
stabilizers provide effective damping only on a particular operating point. But
fuzzy based PSS provides good damping for a wide range of operating points. The
bottlenecks faced in designing a fuzzy logic controller can be minimized by
using appropriate optimization techniques like Genetic Algorithm, Particle Swam
Optimization, Ant Colony Optimization etc.In this paper the membership
functions of FLC are optimized by the new breed optimization technique called
Genetic Algorithm. This design methodology is implemented on a Single Machine
Infinite Bus (SMIB) system. Simulation results on SMIB show the effectiveness
and robustness of the proposed PSS over a wide range of operating conditions
and system configurations. | 0912.3960v2 |
2009-12-23 | Decoherence and Entanglement Dynamics in Fluctuating Fields | We study pure phase damping of two qubits due to fluctuating fields. As
frequently employed, decoherence is thus described in terms of random unitary
(RU) dynamics, i.e., a convex mixture of unitary transformations. Based on a
separation of the dynamics into an average Hamiltonian and a noise channel, we
are able to analytically determine the evolution of both entanglement and
purity. This enables us to characterize the dynamics in a concurrence-purity
(CP) diagram: we find that RU phase damping dynamics sets constraints on
accessible regions in the CP plane. We show that initial state and dynamics
contribute to final entanglement independently. | 0912.4654v2 |
2010-01-02 | Distinguishing quantum channels via magic squares game | We study the effect of quantum memory in magic squares game when played in
quantum domain. We consider different noisy quantum channels and analyze their
influence on the magic squares quantum pseudo-telepathy game. We show that the
probability of success can be used to distinguish the quantum channels. It is
seen that the mean success probability decreases with increase of quantum
noise. Where as the mean success probability increases with increase of quantum
memory. It is also seen that the behaviour of amplitude damping and phase
damping channels is similar. On the other hand, the behaviour of depolarizing
channel is similar to the flipping channels. Therefore, the probability of
success of the game can be used to distinguish the quantum channels. | 1001.0295v1 |
2010-01-06 | Freezing of spin dynamics and omega/T scaling in underdoped cuprates | The memory function approach to spin dynamics in doped antiferromagnetic
insulator combined with the assumption of temperature independent static spin
correlations and constant collective mode damping leads to omega/T scaling in a
broad range. The theory involving a non universal scaling parameter is used to
analyze recent inelastic neutron scattering results for underdoped cuprates.
Adopting modified damping function also the emerging central peak in low-doped
cuprates at low temperatures can be explained within the same framework. | 1001.0837v1 |
2010-01-15 | Calculating Green Functions from Finite Systems | In calculating Green functions for interacting quantum systems numerically
one often has to resort to finite systems which introduces a finite size level
spacing. In order to describe the limit of system size going to infinity
correctly, one has to introduce an artificial broadening larger than the finite
size level discretization. In this work we compare various discretization
schemes for impurity problems, i.e. a small system coupled to leads. Starting
from a naive linear discretization we will then discuss the logarithmic
discretization of the Wilson NRG, compare it to damped boundary conditions and
arbitrary discretization in energy space. We then discuss the importance of
choosing the right single particle basis when calculating bulk spectral
functions. Finally we show the influence of damped boundary conditions on the
time evolution of wave packets leading to a NRG-tsunami. | 1001.2750v1 |
2010-02-02 | Inertial Oscillations of Pinned Dislocations | Dislocation pinning plays a vital role in the plastic behaviour of a
crystalline solid. Here we report the first observation of the damped
oscillations of a mobile dislocation after it gets pinned at an obstacle in the
presence of a constant static shear load. These oscillations are found to be
inertial, instead of forced as obtained in the studies of internal friction of
solid. The rate of damping enables us to determine the effective mass of the
dislocation. Nevertheless, the observed relation between the oscillation
frequency and the link length is found to be anomalous, when compared with the
theoretical results in the framework of Koehler's vibrating string model. We
assign this anomaly to the improper boundary conditions employed in the
treatment. Finally, we propose that the inertial oscillations may offer a
plausible explanation of the electromagnetic emissions during material
deformation and seismic activities. | 1002.0422v1 |
2010-02-03 | Nonlinear stability of viscous roll waves | Extending results of Oh--Zumbrun and Johnson--Zumbrun for parabolic
conservation laws, we show that spectral stability implies nonlinear stability
for spatially periodic viscous roll wave solutions of the one-dimensional St.
Venant equations for shallow water flow down an inclined ramp. The main new
issues to be overcome are incomplete parabolicity and the nonconservative form
of the equations, which leads to undifferentiated quadratic source terms that
cannot be handled using the estimates of the conservative case. The first is
resolved by treating the equations in the more favorable Lagrangian
coordinates, for which one can obtain large-amplitude nonlinear damping
estimates similar to those carried out by Mascia--Zumbrun in the related shock
wave case, assuming only symmetrizability of the hyperbolic part. The second is
resolved by the observation that, similarly as in the relaxation and detonation
cases, sources occurring in nonconservative components experience greater than
expected decay, comparable to that experienced by a differentiated source. | 1002.0788v2 |
2010-02-09 | Fate of non-Fermi liquid behavior in QED$_{3}$ at finite chemical potential | The damping rate of two-dimensional massless Dirac fermions exhibit non-Fermi
liquid behavior, $\propto \epsilon^{1/2}$, due to gauge field at zero
temperature and zero chemical potential. We study the fate of this behavior at
finite chemical potential. We fist calculate explicitly the temporal and
spatial components of vacuum polarization functions. The analytical expressions
imply that the temporal component of gauge field develops a static screening
length at finite chemical potential while the transverse component remains
long-ranged owing to gauge invariance. We then calculate the fermion damping
rate and show that the temporal gauge field leads to normal Fermi liquid
behavior but the transverse gauge field leads to non-Fermi liquid behavior
$\propto \epsilon^{2/3}$ at zero temperature. This energy-dependence is more
regular than $\propto \epsilon^{1/2}$ and does not change as chemical potential
varies. | 1002.1760v3 |
2010-02-18 | Direct Evidence for Two-Fluid Effects in Molecular Clouds | We present a combination of theoretical and simulation-based examinations of
the role of two-fluid ambipolar drift on molecular line widths. The dissipation
provided by ion-neutral interactions can produce a significant difference
between the widths of neutral molecules and the widths of ionic species,
comparable to the sound speed. We demonstrate that Alfven waves and certain
families of magnetosonic waves become strongly damped on scales comparable to
the ambipolar diffusion scale. Using the RIEMANN code, we simulate two-fluid
turbulence with ionization fractions ranging from 10^{-2} to 10^{-6}. We show
that the wave damping causes the power spectrum of the ion velocity to drop
below that of the neutral velocity when measured on a relative basis. Following
a set of motivational observations by Li & Houde (2008), we produce synthetic
line width-size relations that shows a difference between the ion and neutral
line widths, illustrating that two-fluid effects can have an observationally
detectable role in modifying the MHD turbulence in the clouds. | 1002.3443v1 |
2010-03-08 | Potential mechanical loss mechanisms in bulk materials for future gravitational wave detectors | Low mechanical loss materials are needed to further decrease thermal noise in
upcoming gravitational wave detectors. We present an analysis of the
contribution of Akhieser and thermoelastic damping on the experimental results
of resonant mechanical loss measurements. The combination of both processes
allows the fit of the experimental data of quartz in the low temperature region
(10 K to 25 K). A fully anisotropic numerical calculation over a wide
temperature range (10 K to 300 K) reveals, that thermoelastic damping is not a
dominant noise source in bulk silicon samples. The anisotropic numerical
calculation is sucessfully applied to the estimate of thermoelastic noise of an
advanced LIGO sized silicon test mass. | 1003.1613v1 |
2010-04-08 | Doppler cooling a microsphere | Doppler cooling the center-of-mass motion of an optically levitated
microsphere via the velocity dependent scattering force from narrow whispering
gallery mode (WGM) resonances is described. Light that is red detuned from the
WGM resonance can be used to damp the center-of-mass motion in a process
analogous to the Doppler cooling of atoms. Leakage of photons out of the
microsphere when the incident field is near resonant with the narrow WGM
resonance acts to damp the motion of the sphere. The scattering force is not
limited by saturation, but can be controlled by the incident power. Cooling
times on the order of seconds are calculated for a 20 micron diameter silica
microsphere trapped within optical tweezers, with a Doppler temperature limit
in the microKelvin regime. | 1004.1443v1 |
2010-05-17 | Concerning the statistics of cosmic magnetism | Magnetic fields appear to be a generic feature of the early universe and are
a natural source of secondary CMB non-Gaussianity. In recent years the
statistical nature of the stresses of a primordial magnetic field has been well
studied. In this paper we confirm and extend these studies at one- and
two-point level, and present analytical results for a wide range of power-law
spectra. We also consider two non-power law cases of interest: a blue spectrum
with an extended damping tail on small scales, which could be generated by the
non-linear mixing of density and vorticity; and a red spectrum with a damping
tail on large scales. We then briefly consider the CMB impacts that result from
such fields. While this paper focuses on the one- and two-point moments, the
techniques we employ are designed to ease the analysis of the full bispectra
induced by primordial magnetic fields. | 1005.2982v1 |
2010-05-20 | Line Solutions for the Euler and Euler-Poisson Equations with Multiple Gamma Law | In this paper, we study the Euler and Euler-Poisson equations in $R^{N}$,
with multiple $\gamma$-law for pressure function: \begin{equation}
P(\rho)=e^{s}\sum_{j=1}^{m}\rho^{\gamma_{j}}, \end{equation} where all
$\gamma_{i+1}>\gamma_{i}\geq1$, is the constants. The analytical line solutions
are constructed for the systems. It is novel to discover the analytical
solutions to handle the systems with mixed pressure function. And our solutions
can be extended to the systems with the generalized multiple damping and
pressure function. | 1005.3651v1 |
2010-06-12 | Mechanical filtering in forced-oscillation of two coupled pendulums | Forced oscillation of a system composed of two pendulums coupled by a spring
in the presence of damping is investigated. In the steady state and within the
small angle approximation we solve the system equations of motion and obtain
the amplitudes and phases of in terms of the frequency of the sinusoidal
driving force. The resonance frequencies are obtained and the amplitude ratio
is discussed in details. Contrary to a single oscillator, in this two-degree of
freedom system four resonant frequencies, which are close to mode frequencies,
appear. Within the pass-band interval the system is shown to exhibit a rich and
complicated behaviour. It is shown that damping crucially affects the system
properties. Under certain circumstances, the amplitude of the oscillator which
is directly connected to the driving force becomes smaller than the one far
from it. Particularly we show the existence of a driving frequency at which the
connected oscillator's amplitude goes zero. | 1006.2475v1 |
2010-07-12 | Ferromagnetic Excitations in La$_{0.82}$Sr$_{0.18}$CoO$_{3}$ Observed Using Neutron Inelastic Scattering | Polarized neutron inelastic scattering has been used to measure spin
excitations in ferromagnetic La$_{0.82}$Sr$_{0.18}$CoO$_{3}$. The magnon
spectrum of these spin excitations is well defined at low energies but becomes
heavily damped at higher energies, and can be modeled using a quadratic
dispersion. We determined a spin wave stiffness constant of $D=94\pm
3$\,meV\,\AA$^{2}$. Assuming a nearest-neighbor Heisenberg model we find
reasonable agreement between the exchange determined from D and the bulk Curie
temperature. Several possible mechanisms to account for the observed spin-wave
damping are discussed. | 1007.1919v1 |
2010-07-28 | Minimization of phonon-tunneling dissipation in mechanical resonators | Micro- and nanoscale mechanical resonators have recently emerged as
ubiquitous devices for use in advanced technological applications, for example
in mobile communications and inertial sensors, and as novel tools for
fundamental scientific endeavors. Their performance is in many cases limited by
the deleterious effects of mechanical damping. Here, we report a significant
advancement towards understanding and controlling support-induced losses in
generic mechanical resonators. We begin by introducing an efficient numerical
solver, based on the "phonon-tunneling" approach, capable of predicting the
design-limited damping of high-quality mechanical resonators. Further, through
careful device engineering, we isolate support-induced losses and perform the
first rigorous experimental test of the strong geometric dependence of this
loss mechanism. Our results are in excellent agreement with theory,
demonstrating the predictive power of our approach. In combination with recent
progress on complementary dissipation mechanisms, our phonon-tunneling solver
represents a major step towards accurate prediction of the mechanical quality
factor. | 1007.4948v1 |
2010-08-05 | Linear and Non-Linear Landau Resonance of Kinetic Alfvén Waves: Consequences for Electron Distribution and Wave Spectrum in the Solar Wind | Kinetic Alfven wave turbulence in solar wind is considered and it is shown
that non-Maxwellian electron distribution function has a significant effect on
the dynamics of the solar wind plasmas. Linear Landau damping leads to the
formation of a plateau in the parallel electron distribution function which
diminishes the Landau damping rate significantly. Nonlinear scattering of waves
by plasma particles is generalized to short wavelengths and it is found that
for the solar wind parameters this scattering is the dominant process as
compared to three wave decay and coalescence in the wave vector range .
Incorporation of these effects lead to the steepening of the wave spectrum
between the inertial and the dissipation ranges with a spectral index between 2
and 3. This region can be labeled as the scattering range. Such steepening has
been observed in the solar wind plasmas. | 1008.0993v1 |
2010-08-11 | Theory for a dissipative droplet soliton excited by a spin torque nanocontact | A novel type of solitary wave is predicted to form in spin torque oscillators
when the free layer has a sufficiently large perpendicular anisotropy. In this
structure, which is a dissipative version of the conservative droplet soliton
originally studied in 1977 by Ivanov and Kosevich, spin torque counteracts the
damping that would otherwise destroy the mode. Asymptotic methods are used to
derive conditions on perpendicular anisotropy strength and applied current
under which a dissipative droplet can be nucleated and sustained. Numerical
methods are used to confirm the stability of the droplet against various
perturbations that are likely in experiments, including tilting of the applied
field, non-zero spin torque asymmetry, and non-trivial Oersted fields. Under
certain conditions, the droplet experiences a drift instability in which it
propagates away from the nanocontact and is then destroyed by damping. | 1008.1898v1 |
2010-08-18 | Modulation stabilization of Bloch oscillations of two-component Bose-Einstein condensates in optical lattices | We study the Bloch oscillations (BOs) of two-component Bose-Einstein
condensates (BECs) trapped in spin-dependent optical lattices. Based on the
derived equations of motion of the wave packet in the basis of localized wave
functions of the lattice sites, the damping effect induced by the
intercomponent and intracomponent interactions to the BOs is explored
analytically and numerically. We also show that such damping of the BOs can be
suppressed entirely if all the atom-atom interactions are modulated
synchronously and harmonically in time with suitable frequency via the Feshbach
resonance. When the intercomponent and the intracomponent interactions have
inverse signs, we find that the long-living BOs and even the revival of the BOs
can be achieved via only statically modulating the configuration of optical
lattices. The results provide a valuable guidance for achieving long-living BOs
in the two-component BEC system by the Feshbach resonances and manipulating the
configuration of the optical lattices. | 1008.3004v1 |
2010-08-19 | Josephson Coupling and Fiske Dynamics in Ferromagnetic Tunnel Junctions | We report on the fabrication of Nb/AlO_x/Pd_{0.82}Ni_{0.18}/Nb
superconductor/insulator/ferromagnetic metal/superconductor (SIFS) Josephson
junctions with high critical current densities, large normal resistance times
area products, high quality factors, and very good spatial uniformity. For
these junctions a transition from 0- to \pi-coupling is observed for a
thickness d_F ~ 6 nm of the ferromagnetic Pd_{0.82}Ni_{0.18} interlayer. The
magnetic field dependence of the \pi-coupled junctions demonstrates good
spatial homogeneity of the tunneling barrier and ferromagnetic interlayer.
Magnetic characterization shows that the Pd_{0.82}Ni_{0.18} has an out-of-plane
anisotropy and large saturation magnetization, indicating negligible dead
layers at the interfaces. A careful analysis of Fiske modes provides
information on the junction quality factor and the relevant damping mechanisms
up to about 400 GHz. Whereas losses due to quasiparticle tunneling dominate at
low frequencies, the damping is dominated by the finite surface resistance of
the junction electrodes at high frequencies. High quality factors of up to 30
around 200 GHz have been achieved. Our analysis shows that the fabricated
junctions are promising for applications in superconducting quantum circuits or
quantum tunneling experiments. | 1008.3341v1 |
2010-09-03 | A Simple Numerical Absorbing Layer Method in Elastodynamics | The numerical analysis of elastic wave propagation in unbounded media may be
difficult to handle due to spurious waves reflected at the model artificial
boundaries. Several sophisticated techniques such as nonreflecting boundary
conditions, infinite elements or absorbing layers (e.g. Perfectly Matched
Layers) lead to an important reduction of such spurious reflections. In this
Note, a simple and efficient absorbing layer method is proposed in the
framework of the Finite Element Method. This method considers Rayleigh/Caughey
damping in the absorbing layer and its principle is presented first. The
efficiency of the method is then shown through 1D Finite Element simulations
considering homogeneous and heterogeneous damping in the absorbing layer. 2D
models are considered afterwards to assess the efficiency of the absorbing
layer method for various wave types (surface waves, body waves) and incidences
(normal to grazing). The method is shown to be efficient for different types of
elastic waves and may thus be used for various elastodynamic problems in
unbounded domains. | 1009.0592v1 |
2010-09-09 | Modulation of waves due to charge-exchange collisions in magnetized partially ionized space plasma | A nonlinear time dependent fluid simulation model is developed that describes
the evolution of magnetohydrodynamic waves in the presence of collisional and
charge exchange interactions of a partially ionized plasma. The partially
ionized plasma consists of electrons, ions and a significant number of neutral
atoms. In our model, the electrons and ions are described by a single fluid
compressible magnetohydrodynamic (MHD) model and are coupled self-consistently
to the neutral gas, described by the compressible hydrodynamic equations. Both
the plasma and neutral fluids are treated with different energy equations that
describe thermal energy exchange processes between them. Based on our
self-consistent model, we find that propagating Alfv\'enic and fast/slow modes
grow and damp alternately through a nonlinear modulation process. The
modulation appears to be robust and survives strong damping by the neutral
component. | 1009.1859v1 |
2010-09-15 | A discontinuous Galerkin method for the Vlasov-Poisson system | A discontinuous Galerkin method for approximating the Vlasov-Poisson system
of equations describing the time evolution of a collisionless plasma is
proposed. The method is mass conservative and, in the case that piecewise
constant functions are used as a basis, the method preserves the positivity of
the electron distribution function and weakly enforces continuity of the
electric field through mesh interfaces and boundary conditions. The performance
of the method is investigated by computing several examples and error estimates
associated system's approximation are stated. In particular, computed results
are benchmarked against established theoretical results for linear advection
and the phenomenon of linear Landau damping for both the Maxwell and Lorentz
distributions. Moreover, two nonlinear problems are considered: nonlinear
Landau damping and a version of the two-stream instability are computed. For
the latter, fine scale details of the resulting long-time BGK-like state are
presented. Conservation laws are examined and various comparisons to theory are
made. The results obtained demonstrate that the discontinuous Galerkin method
is a viable option for integrating the Vlasov-Poisson system. | 1009.3046v2 |
2010-10-03 | Measurement of damping and temperature: Precision bounds in Gaussian dissipative channels | We present a comprehensive analysis of the performance of different classes
of Gaussian states in the estimation of Gaussian phase-insensitive dissipative
channels. In particular, we investigate the optimal estimation of the damping
constant and reservoir temperature. We show that, for two-mode squeezed vacuum
probe states, the quantum-limited accuracy of both parameters can be achieved
simultaneously. Moreover, we show that for both parameters two-mode squeezed
vacuum states are more efficient than either coherent, thermal or single-mode
squeezed states. This suggests that at high energy regimes two-mode squeezed
vacuum states are optimal within the Gaussian setup. This optimality result
indicates a stronger form of compatibility for the estimation of the two
parameters. Indeed, not only the minimum variance can be achieved at fixed
probe states, but also the optimal state is common to both parameters.
Additionally, we explore numerically the performance of non-Gaussian states for
particular parameter values to find that maximally entangled states within
D-dimensional cutoff subspaces perform better than any randomly sampled states
with similar energy. However, we also find that states with very similar
performance and energy exist with much less entanglement than the maximally
entangled ones. | 1010.0442v1 |
2010-10-12 | Movers and shakers: Granular damping in microgravity | The response of an oscillating granular damper to an initial perturbation is
studied using experiments performed in microgravity and granular dynamics
mulations. High-speed video and image processing techniques are used to extract
experimental data. An inelastic hard sphere model is developed to perform
simulations and the results are in excellent agreement with the experiments.
The granular damper behaves like a frictional damper and a linear decay of the
amplitude is bserved. This is true even for the simulation model, where
friction forces are absent. A simple expression is developed which predicts the
optimal damping conditions for a given amplitude and is independent of the
oscillation frequency and particle inelasticities. | 1010.2343v1 |
2010-10-18 | K-shell photoionization of Na-like to Cl-like ions of Mg, Si, S, Ar, and Ca | We present $R$-matrix calculations of photoabsorption and photoionization
cross sections across the K-edge of Mg, Si, S, Ar, and Ca ions with more than
10 electrons. The calculations include the effects of radiative and Auger
damping by means of an optical potential. The wave functions are constructed
from single-electron orbital bases obtained using a Thomas--Fermi--Dirac
statistical model potential. Configuration interaction is considered among all
states up to $n=3$. The damping processes affect the resonances converging to
the K-thresholds causing them to display symmetric profiles of constant width
that smear the otherwise sharp edge at the photoionization threshold. These
data are important for modeling of features found in photoionized plasmas. | 1010.3734v1 |
2010-10-19 | A possible signature of cosmic neutrino decoupling in the nHz region of the spectrum of primordial gravitational waves | In this paper we study the effect of cosmic neutrino decoupling on the
spectrum of cosmological gravitational waves (GWs). At temperatures T>>1 MeV,
neutrinos constitute a perfect fluid and do not hinder GW propagation, while
for T<<1 MeV they free-stream and have an effective viscosity that damps
cosmological GWs by a constant amount. In the intermediate regime,
corresponding to neutrino decoupling, the damping is frequency-dependent. GWs
entering the horizon during neutrino decoupling have a frequency f ~ 1 nHz,
corresponding to a frequency region that will be probed by Pulsar Timing Arrays
(PTAs). In particular, we show how neutrino decoupling induces a spectral
feature in the spectrum of cosmological GWs just below 1 nHz. We briefly
discuss the conditions for a detection of this feature and conclude that it is
unlikely to be observed by PTAs. | 1010.3849v2 |
2010-10-20 | Modified Landau levels, damped harmonic oscillator and two-dimensional pseudo-bosons | In a series of recent papers one of us has analyzed in some details a class
of elementary excitations called {\em pseudo-bosons}. They arise from a special
deformation of the canonical commutation relation $[a,a^\dagger]=\1$, which is
replaced by $[a,b]=\1$, with $b$ not necessarily equal to $a^\dagger$. Here,
after a two-dimensional extension of the general framework, we apply the theory
to a generalized version of the two-dimensional Hamiltonian describing Landau
levels. Moreover, for this system, we discuss coherent states and we deduce a
resolution of the identity. We also consider a different class of examples
arising from a classical system, i.e. a damped harmonic oscillator. | 1010.4221v1 |
2010-11-16 | Forcibly driven coherent soft phonons in GeTe with intense THz-rate pump fields | We propose an experimental technique to generate large amplitude coherent
phonons with irradiation of THz-rate pump pulses and to study the dynamics of
phase transition in GeTe ferroelectrics. When a single pump pulse irradiates
the sample at various pump power densities, the frequency of the soft phonon
decreases sub-linearly and saturates at higher pump powers. By contrast, when
THz-rate pump pulse sequence irradiates the sample at matched time intervals to
forcibly drive the oscillation, a large red-shift of the phonon frequency is
observed without saturation effects. After excitation with a four pump pulse
sequence, the coherent soft phonon becomes strongly damped leading to a near
critical damping condition. This condition indicates that the lattice is driven
to a precursor state of the phase transition. | 1011.3624v2 |
2010-11-21 | Regular and chaotic transport of discrete solitons in asymmetric potentials | Ratchet dynamics of topological solitons of the forced and damped discrete
double sine-Gordon system are studied. Directed transport occurring both in
regular and in chaotic regions of the phase space and its dependence on
damping, amplitude and frequency of the driving, asymmetry parameter, coupling
constant, has been extensively investigated. We show that the passage from
ratchet phase-locked regime to chaotic ratchets occurs via a period doubling
route to chaos and that, quite surprisingly, pinned states can exist inside
phase-locking and chaotic transport regions for intermediate values of the
coupling constant. The possibility to control chaotic discrete soliton ratchets
by means of both small subharmonic signals and more general periodic drivings,
has also been investigated. | 1011.4707v1 |
2010-11-26 | Dependence of entanglement on initial states under amplitude damping channel in non-inertial frames | Under amplitude damping channel, the dependence of the entanglement on the
initial states $|\Theta>_{1}$ and $|\Theta>_{2}$, which reduce to four
orthogonal Bell states if we take the parameter of states $\alpha=\pm
1/\sqrt{2}$ are investigated. We find that the entanglements for different
initial states will decay along different curves even with the same
acceleration and parameter of the states. We note that, in an inertial frame,
the sudden death of the entanglement for $|\Theta>_{1}$ will occur if
$\alpha>1/\sqrt{2}$, while it will not take place for $|\Theta>_{2}$ for any
$\alpha$. We also show that the possible range of the sudden death of the
entanglement for $|\Theta>_{1}$ is larger than that for $|\Theta>_{2}$. There
exist two groups of Bell state here we can't distinguish only by concurrence. | 1011.5700v3 |
2010-12-21 | Characterization of Decoherence from an Environmental Perspective | For the case of phase damping (pure decoherence) we investigate the extent to
which environmental traits are imprinted on an open quantum system. The
dynamics is described using the quantum channel approach. We study what the
knowledge of the channel may reveal about the nature of its underlying dynamics
and, conversely, what the dynamics tells us about how to consistently model the
environment. We find that for a Markov phase-damping channel, that is, a
channel compatible with a time-continuous Markovian evolution, the environment
may adequately be represented by a mixture of only a few coherent states. For
arbitrary Hilbert space dimension $N\geq 4$ we refine the idea of {\it quantum
phase damping}, of which we show a means of identification. Symmetry
considerations are used to identify decoherence-free subspaces of the system. | 1012.4685v1 |
2010-12-28 | Quantum Leptogenesis I | Thermal leptogenesis explains the observed matter-antimatter asymmetry of the
universe in terms of neutrino masses, consistent with neutrino oscillation
experiments. We present a full quantum mechanical calculation of the generated
lepton asymmetry based on Kadanoff-Baym equations. Origin of the asymmetry is
the departure from equilibrium of the statistical propagator of the heavy
Majorana neutrino, together with CP violating couplings. The lepton asymmetry
is calculated directly in terms of Green's functions without referring to
"number densities". Compared to Boltzmann and quantum Boltzmann equations, the
crucial difference are memory effects, rapid oscillations much faster than the
heavy neutrino equilibration time. These oscillations strongly suppress the
generated lepton asymmetry, unless the standard model gauge interactions, which
cause thermal damping, are properly taken into account. We find that these
damping effects essentially compensate the enhancement due to quantum
statistical factors, so that finally the conventional Boltzmann equations again
provide rather accurate predictions for the lepton asymmetry. | 1012.5821v3 |
2011-01-06 | Chemical Enrichment in the Carbon-enhanced Damped Lyman $α$ System | We show that the recently observed elemental abundance pattern of the
carbon-rich metal-poor Damped Lyman $\alpha$ (DLA) system is in excellent
agreement with the nucleosynthesis yields of faint core-collapse supernovae of
primordial stars. The observed abundance pattern is not consistent with the
nucleosynthesis yields of pair-instability supernovae. The DLA abundance
pattern is very similar to that of carbon-rich extremely metal-poor (EMP)
stars, and the contributions from low-mass stars and/or binary effects should
be very small in DLAs. This suggests that chemical enrichment by the first
stars in the first galaxies is driven by core-collapse supernovae from $\sim
20-50 M_\odot$ stars, and also supports the supernova scenario as the
enrichment source of EMP stars in the Milky Way Galaxy. | 1101.1227v2 |
2011-02-08 | Quantization of Damping Particle Based On New Variational Principles | In this paper a new approach is proposed to quantize mechanical systems whose
equations of motion can not be put into Hamiltonian form. This approach is
based on a new type of variational principle, which is adopted to a describe a
relation: a damping particle may shares a common phase curve with a free
particle, whose Lagrangian in the new variational principle can be considered
as a Lagrangian density in phase space. According to Feynman's theory, the
least action principle is adopted to modify the Feynman's path integral
formula, where Lagrangian is replaced by Lagrangian density. In the case of
conservative systems, the modification reduces to standard Feynman's propagator
formula. As an example a particle with friction is analyzed in detail. | 1102.1573v2 |
2011-02-15 | Spin dynamics in the strong spin-orbit coupling regime | We study the spin dynamics in a high-mobility two dimensional electron gas
(2DEG) with generic spin-orbit interactions (SOIs). We derive a set of spin
dynamic equations which capture the purely exponential to the damped
oscillatory spin evolution modes observed in different regimes of SOI strength.
Hence we provide a full treatment of the D'yakonov-Perel's mechanism by using
the microscopic linear response theory from the weak to the strong SOI limit.
We show that the damped oscillatory modes appear when the electron scattering
time is larger than half of the spin precession time due to the SOI, in
agreement with recent observations. We propose a new way to measure the
scattering time and the relative strength of Rashba and linear Dresselhaus SOIs
based on these modes and optical grating experiments. We discuss the physical
interpretation of each of these modes in the context of Rabi oscillation. | 1102.3170v1 |
2011-03-03 | Collective modes and the speed of sound in the Fulde-Ferrell-Larkin-Ovchinnikov state | We consider the density response of a spin-imbalanced ultracold Fermi gas in
an optical lattice in the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state. We
calculate the collective mode spectrum of the system in the generalised random
phase approximation and find that though the collective modes are damped even
at zero tempererature, the damping is weak enough to have well-defined
collective modes. We calculate the speed of sound in the gas and show that it
is anisotropic due to the anisotropy of the FFLO pairing, which implies an
experimental signature for the FFLO state. | 1103.0696v2 |
2011-03-10 | Shocks in financial markets, price expectation, and damped harmonic oscillators | Using a modified damped harmonic oscillator model equivalent to a model of
market dynamics with price expectations, we analyze the reaction of financial
markets to shocks. In order to do this, we gather data from indices of a
variety of financial markets for the 1987 Black Monday, the Russian crisis of
1998, the crash after September 11th (2001), and the recent downturn of markets
due to the subprime mortgage crisis in the USA (2008). Analyzing those data we
were able to establish the amount by which each market felt the shocks, a
dampening factor which expresses the capacity of a market of absorving a shock,
and also a frequency related with volatility after the shock. The results gauge
the efficiency of different markets in recovering from such shocks, and measure
some level of dependence between them. We also show, using the correlation
matrices between the indices used, that financial markets are now much more
connected than they were two decades ago. | 1103.1992v2 |
2011-03-22 | Inductive determination of the optimum tunnel barrier thickness in magnetic tunnelling junction stacks for spin torque memory applications | We use pulsed inductive microwave magnetometry to study the precessional
magnetization dynamics of the free layer in CoFeB/MgO/CoFeB based magnetic
tunnelling junction stacks with varying MgO barrier thickness. From the field
dependence of the precession frequency we are able to derive the uniaxial
anisotropy energy and the exchange coupling between the free and the pinned
layer. Furthermore the field dependence of the effective damping parameter is
derived. Below a certain threshold barrier thickness we observe an increased
effective damping for antiparallel orientation of free and pinned layer which
would inhibit reversible low current density spin torque magnetization
reversal. Such inductive measurements, in combination with wafer probe station
based magneto transport experiments, allow a fast determination of the optimum
tunnel barrier thickness range for spin torque memory applications in a
lithography free process. | 1103.4248v1 |
2011-04-11 | Algebraic damping in the one-dimensional Vlasov equation | We investigate the asymptotic behavior of a perturbation around a spatially
non homogeneous stable stationary state of a one-dimensional Vlasov equation.
Under general hypotheses, after transient exponential Landau damping, a
perturbation evolving according to the linearized Vlasov equation decays
algebraically with the exponent -2 and a well defined frequency. The
theoretical results are successfully tested against numerical $N$-body
simulations, corresponding to the full Vlasov dynamics in the large $N$ limit,
in the case of the Hamiltonian mean-field model. For this purpose, we use a
weighted particles code, which allows us to reduce finite size fluctuations and
to observe the asymptotic decay in the $N$-body simulations. | 1104.1890v2 |
2011-05-06 | System-environment dynamics of X-type states in noninertial frames | The system-environment dynamics of noninertial systems is investigated. It is
shown that for the amplitude damping channel: (i) the biggest difference
between the decoherence effect and the Unruh radiation on the dynamics of the
entanglement is the former only leads to entanglement transfer in the whole
system, but the latter damages all types of entanglement; (ii) the
system-environment entanglement increases and then declines, while the
environment-environment entanglement always increases as the decay parameter
$p$ increases; and (iii) the thermal fields generated by the Unruh effect can
promote the sudden death of entanglement between the subsystems while postpone
the sudden birth of entanglement between the environments. It is also found
that there is no system-environment and environment-environment entanglements
when the system coupled with the phase damping environment. | 1105.1216v2 |
2011-05-10 | Spontaneous magnon decays in planar ferromagnet | We predict that spin-waves in an easy-plane ferromagnet have a finite
lifetime at zero temperature due to spontaneous decays. In zero field the
damping is determined by three-magnon decay processes, whereas decays in the
two-particle channel dominate in a transverse magnetic field. Explicit
calculations of the magnon damping are performed in the framework of the
spin-wave theory for the $XXZ$ square-lattice ferromagnet with an anisotropy
parameter $\lambda<1$. In zero magnetic field the decays occur for
$\lambda^*<\lambda<1$ with $\lambda^*\approx 1/7$. We also discuss possibility
of experimental observation of the predicted effect in a number of
ferromagnetic insulators. | 1105.1893v1 |
2011-05-19 | Scaling of the higher-order flow harmonics: implications for initial-eccentricity models and the "viscous horizon" | The scaling properties of the flow harmonics for charged hadrons $v_{n}$ and
their ratios $[ v_n/(v_2)^{n/2}]_{n\geq 3}$, are studied for a broad range of
transverse momenta ($p_T$) and centrality selections in Au+Au and Pb+Pb
collisions at $\sqrt{s_{NN}}=0.2 \text{and} 2.76$ TeV respectively. At
relatively low $p_T$, these scaling properties are found to be compatible with
the expected growth of viscous damping for sound propagation in the plasma
produced in these collisions. They also provide important constraints for
distinguishing between the two leading models of collision eccentricities, as
well as a route to constrain the relaxation time and make estimates for the
ratio of viscosity to entropy density $\eta/s$, and the "viscous horizon" or
length-scale which characterizes the highest harmonic which survives viscous
damping. | 1105.3782v2 |
2011-05-26 | Dynamics in the production of superheavy nuclei in low-energy heavy-ion collisions | We present a review of the recent progress of theoretical models on the
description of the formation of superheavy nuclei in collisions of heavy
systems. Two sorts of reactions that are the fusion-evaporation mechanism and
the massive damped collisions to produce superheavy nuclei are discussed.
Problems and further improvements of the capture of colliding partners, the
formation of compound nucleus and the de-excitation process are pointed out.
Possible combinations in the synthesis of the gap of the cold fusion and
$^{48}$Ca induced reactions are proposed by the calculations based on the
dinuclear system model and also compared with other approaches. The synthesis
of neutron-rich heavy isotopes near sub-shell closure N=162 via transfer
reactions in the damped collisions of two actinides and the influence of shell
closure on the production of heavy isotopes are investigated. Prospective
possibility to reach superheavy nuclei near N=184 via neutron-rich radioactive
beams of high intensity in the future is discussed. | 1105.5224v2 |
2011-06-01 | Effect of detuning on the phonon induced dephasing of optically driven InGaAs/GaAs quantum dots | Recently, longitudinal acoustic phonons have been identified as the main
source of the intensity damping observed in Rabi rotation measurements of the
ground-state exciton of a single InAs/GaAs quantum dot. Here we report
experiments of intensity damped Rabi rotations in the case of detuned laser
pulses, the results have implications for the coherent optical control of both
excitons and spins using detuned laser pulses. | 1106.0142v1 |
2011-06-09 | Investigating viscous damping using a webcam | We describe an experiment involving a mass oscillating in a viscous fluid and
analyze viscous damping of harmonic motion. The mechanical oscillator is
tracked using a simple webcam and an image processing algorithm records the
position of the geometrical center as a function of time. Interesting
information can be extracted from the displacement-time graphs, in particular
for the underdamped case. For example, we use these oscillations to determine
the viscosity of the fluid. Our mean value of 1.08 \pm 0.07 mPa s for distilled
water is in good agreement with the accepted value at 20\circC. This experiment
has been successfully employed in the freshman lab setting. | 1106.1823v1 |
2011-06-11 | Conformal and covariant formulation of the Z4 system with constraint-violation damping | We present a new formulation of the Einstein equations based on a conformal
and traceless decomposition of the covariant form of the Z4 system. This
formulation combines the advantages of a conformal decomposition, such as the
one used in the BSSNOK formulation (i.e. well-tested hyperbolic gauges, no need
for excision, robustness to imperfect boundary conditions) with the advantages
of a constraint-damped formulation, such as the generalized harmonic one (i.e.
exponential decay of constraint violations when these are produced). We
validate the new set of equations through standard tests and by evolving binary
black hole systems. Overall, the new conformal formulation leads to a better
behavior of the constraint equations and a rapid suppression of the violations
when they occur. The changes necessary to implement the new conformal
formulation in standard BSSNOK codes are very small as are the additional
computational costs. | 1106.2254v2 |
2011-06-14 | Oscillations of hot, young neutron stars: Gravitational wave frequencies and damping times | We study how the frequencies and damping times of oscillations of a newly
born, hot proto-neutron star depend on the physical quantities which
characterize the star quasi-stationary evolution which follows the bounce.
Stellar configurations are modeled using a microscopic equation of state
obtained within the Brueckner-Hartree-Fock, nuclear many-body approach,
extended to the finite-temperature regime. We discuss the mode frequency
behaviour as function of the lepton composition, and of the entropy gradients
which prevail in the interior of the star. We find that, in the very early
stages, gravitational wave emission efficiently competes with neutrino
processes in dissipating the star mechanical energy residual of the
gravitational collapse. | 1106.2736v1 |
2011-06-22 | Samll BGK waves and nonlinear Landau damping (higher dimensions) | Consider Vlasov-Poisson system with a fixed ion background and periodic
condition on the space variables, in any dimension d\geq2. First, we show that
for general homogeneous equilibrium and any periodic x-box, within any small
neighborhood in the Sobolev space W_{x,v}^{s,p} (p>1,s<1+(1/p)) of the steady
distribution function, there exist nontrivial travelling wave solutions (BGK
waves) with arbitrary traveling speed. This implies that nonlinear Landau
damping is not true in W^{s,p}(s<1+(1/p)) space for any homogeneous equilibria
and in any period box. The BGK waves constructed are one dimensional, that is,
depending only on one space variable. Higher dimensional BGK waves are shown to
not exist. Second, for homogeneous equilibria satisfying Penrose's linear
stability condition, we prove that there exist no nontrivial invariant
structures in the (1+|v|^{2})^{b}-weighted H_{x,v}^{s} (b>((d-1)/4), s>(3/2))
neighborhood. Since arbitrarilly small BGK waves can also be constructed near
any homogeneous equilibria in such weighted H_{x,v}^{s} (s<(3/2)) norm, this
shows that s=(3/2) is the critical regularity for the existence of nontrivial
invariant structures near stable homogeneous equilibria. These generalize our
previous results in the one dimensional case. | 1106.4368v1 |
2011-07-13 | Dimension of attractors and invariant sets of damped wave equations in unbounded domains | Under fairly general assumptions, we prove that every compact invariant set
$\mathcal I$ of the semiflow generated by the semilinear damped wave equation
u_{tt}+\alpha u_t+\beta(x)u-\Deltau = f(x,u), (t,x)\in[0,+\infty[\times\Omega,
u = 0, (t,x)\in[0,+\infty[\times\partial\Omega in $H^1_0(\Omega)\times
L^2(\Omega) has finite Hausdorff and fractal dimension. Here $\Omega$ is a
regular, possibly unbounded, domain in $\R^3$ and $f(x,u)$ is a nonlinearity of
critical growth. The nonlinearity $f(x,u)$ needs not to satisfy any
dissipativeness assumption and the invariant subset $\mathcal I$ needs not to
be an attractor. If $f(x,u)$ is dissipative and $\mathcal I$ is the global
attractor, we give an explicit bound on the Hausdorff and fractal dimension of
$\mathcal I$ in terms of the structure parameters of the equation. | 1107.2589v1 |
2011-07-15 | K-shell photoionization of Nickel ions using R-matrix | We present R-matrix calculations of photoabsorption and photoionization cross
sections across the K edge of the Li-like to Ca-like ions stages of Ni.
Level-resolved, Breit-Pauli calculations were performed for the Li-like to
Na-like stages. Term-resolved calculations, which include the mass-velocity and
Darwin relativistic corrections, were performed for the Mg-like to Ca-like ion
stages. This data set is extended up to Fe-like Ni using the distorted wave
approximation as implemented by AUTOSTRUCTURE. The R-matrix calculations
include the effects of radiative and Auger dampings by means of an optical
potential. The damping processes affect the absorption resonances converging to
the K thresholds causing them to display symmetric profiles of constant width
that smear the otherwise sharp edge at the K-shell photoionization threshold.
These data are important for the modeling of features found in photoionized
plasmas. | 1107.3146v1 |
2011-07-20 | Bayesian Magnetohydrodynamic Seismology of Coronal Loops | We perform a Bayesian parameter inference in the context of resonantly damped
transverse coronal loop oscillations. The forward problem is solved in terms of
parametric results for kink waves in one-dimensional flux tubes in the thin
tube and thin boundary approximations. For the inverse problem, we adopt a
Bayesian approach to infer the most probable values of the relevant parameters,
for given observed periods and damping times, and to extract their confidence
levels. The posterior probability distribution functions are obtained by means
of Markov Chain Monte Carlo simulations, incorporating observed uncertainties
in a consistent manner. We find well localized solutions in the posterior
probability distribution functions for two of the three parameters of interest,
namely the Alfven travel time and the transverse inhomogeneity length-scale.
The obtained estimates for the Alfven travel time are consistent with previous
inversion results, but the method enables us to additionally constrain the
transverse inhomogeneity length-scale and to estimate real error bars for each
parameter. When observational estimates for the density contrast are used, the
method enables us to fully constrain the three parameters of interest. These
results can serve to improve our current estimates of unknown physical
parameters in coronal loops and to test the assumed theoretical model. | 1107.3943v1 |
2011-07-31 | Evolution of cat states in a dissipative parametric amplifier: decoherence and entanglement | The evolution of the Schr\"{o}dinger-cat states in a dissipative parametric
amplifier is examined. The main tool in the analysis is the normally ordered
characteristic function. Squeezing, photon-number distribution and reduced
factorial moments are discussed for the single- and compound-mode cases. Also
the single-mode Wigner function is demonstrated. In addition to the decoherence
resulting from the interaction with the environment (damped case) there are two
sources which can cause such decoherence in the system even if it is completely
isolated: these are the decay of the pump and the relative phases of the
initial cat states. Furthermore, for the damped case there are two regimes,
which are underdamped and overdamped. In the first (second) regime the signal
mode or the idler mode "collapses" to a statistical mixture (thermal field). | 1108.0127v1 |
2011-07-31 | Second-Order, Dissipative Tâtonnement: Economic Interpretation and 2-Point Limit Cycles | This paper proposes an alternative to the classical price-adjustment
mechanism (called "t\^{a}tonnement" after Walras) that is second-order in time.
The proposed mechanism, an analogue to the damped harmonic oscillator, provides
a dynamic equilibration process that depends only on local information. We show
how such a process can result from simple behavioural rules. The discrete-time
form of the model can result in two-step limit cycles, but as the distance
covered by the cycle depends on the size of the damping, the proposed mechanism
can lead to both highly unstable and relatively stable behaviour, as observed
in real economies. | 1108.0188v3 |
2011-08-10 | Critical exponents of steady-state phase transitions in fermionic lattice models | We discuss reservoir induced phase transitions of lattice fermions in the
non-equilibrium steady state (NESS) of an open system with local reservoirs.
These systems may become critical in the sense of a diverging correlation
length upon changing the reservoir coupling. We here show that the transition
to a critical state is associated with a vanishing gap in the damping spectrum.
It is shown that although in linear systems there can be a transition to a
critical state there is no reservoir-induced quantum phase transition between
distinct phases with non-vanishing damping gap. We derive the static and
dynamical critical exponents corresponding to the transition to a critical
state and show that their possible values, defining universality classes of
reservoir-induced phase transitions are determined by the coupling range of the
independent local reservoirs. If a reservoir couples to N neighboring lattice
sites, the critical exponent can assume all fractions from 1 to 1/(N - 1). | 1108.2263v5 |
2011-09-09 | Observation of Spontaneous Brillouin Cooling | While radiation-pressure cooling is well known, the Brillouin scattering of
light from sound is considered an acousto-optical amplification-only process.
It was suggested that cooling could be possible in multi-resonance Brillouin
systems when phonons experience lower damping than light. However, this regime
was not accessible in traditional Brillouin systems since backscattering
enforces high acoustical frequencies associated with high mechanical damping.
Recently, forward Brillouin scattering in microcavities has allowed access to
low-frequency acoustical modes where mechanical dissipation is lower than
optical dissipation, in accordance with the requirements for cooling. Here we
experimentally demonstrate cooling via such a forward Brillouin process in a
microresonator. We show two regimes of operation for the Brillouin process:
acoustical amplification as is traditional, but also for the first time, a
Brillouin cooling regime. Cooling is mediated by an optical pump, and scattered
light, that beat and electrostrictively attenuate the Brownian motion of the
mechanical mode. | 1109.2084v2 |
2011-09-14 | Oscillations of simple networks | To describe the flow of a miscible quantity on a network, we introduce the
graph wave equation where the standard continuous Laplacian is replaced by the
graph Laplacian. This is a natural description of an array of inductances and
capacities, of fluid flow in a network of ducts and of a system of masses and
springs. The structure of the graph influences strongly the dynamics which is
naturally described using the basis of the eigenvectors. In particular, we show
that if two outer nodes are connected to a common third node with the same
coupling, then this coupling is an eigenvalue of the Laplacian. Assuming the
graph is forced and damped at specific nodes, we derive the amplitude
equations. These are analyzed for two simple non trivial networks: a tree and a
graph with a cycle. Forcing the network at a resonant frequency reveals that
damping can be ineffective if applied to the wrong node, leading to a
disastrous resonance and destruction of the network. These results could be
useful for complex physical networks and engineering networks like power grids. | 1109.3071v2 |
2011-09-21 | High-order explicit local time-stepping methods for damped wave equations | Locally refined meshes impose severe stability constraints on explicit
time-stepping methods for the numerical simulation of time dependent wave
phenomena. Local time-stepping methods overcome that bottleneck by using
smaller time-steps precisely where the smallest elements in the mesh are
located. Starting from classical Adams-Bashforth multi-step methods, local
time-stepping methods of arbitrarily high order of accuracy are derived for
damped wave equations. When combined with a finite element discretization in
space with an essentially diagonal mass matrix, the resulting time-marching
schemes are fully explicit and thus inherently parallel. Numerical experiments
with continuous and discontinuous Galerkin finite element discretizations
validate the theory and illustrate the usefulness of these local time-stepping
methods. | 1109.4480v2 |
2011-09-21 | Anisotropic critical magnetic fluctuations in the ferromagnetic superconductor UCoGe | We report neutron scattering measurements of critical magnetic excitations in
the weakly ferromagnetic superconductor UCoGe. The strong non-Landau damping of
the excitations we observe, although unusual has been found in another related
ferromagnet, UGe2 at zero pressure. However, we also find there is a
significant anisotropy of the magnetic correlation length in UCoGe that
contrasts with an almost isotropic length for UGe2. The values of the magnetic
correlation length and damping are found to be compatible with
superconductivity on small Fermi surface pockets. The anisotropy may be
important to explain why UCoGe is a superconductor at zero pressure while UGe2
is not. | 1109.4541v1 |
2011-09-30 | PT-symmetry, indefinite damping and dissipation-induced instabilities | With perfectly balanced gain and loss, dynamical systems with indefinite
damping can obey the exact PT-symmetry being marginally stable with a pure
imaginary spectrum. At an exceptional point where the symmetry is spontaneously
broken, the stability is lost via passing through a non-semisimple 1:1
resonance. In the parameter space of a general dissipative system, marginally
stable PT-symmetric ones occupy singularities on the boundary of the asymptotic
stability. To observe how the singular surface governs dissipation-induced
destabilization of the PT-symmetric system when gain and loss are not matched,
an extension of recent experiments with PT-symmetric LRC circuits is proposed. | 1110.0018v2 |
2011-10-01 | Electrical measurement of antivortex wall eigenfrequency | The dynamics of a ferromagnetic antivortex wall has been experimentally
studied in a magnetic nanostructure. Two different techniques have been used to
independently measure the eigenfrequency of an antivortex wall such as the
resonance excitation by sinusoidal microwave and the damped resonance
excitation induced by short voltage pulses. Direct observation of antivortex
wall nucleation has been measured in the frequency domain for the first time.
Electrical measurements of the antivortex dynamics in frequency domain reveal
the existence of multi-eigenmodes as well as nonlinear behaviors for large
excitation amplitudes. The time resolved measurements of the antivortex wall
show that the frequency of the damped gyration is similar to that of frequency
domain and coexistence of spin wave excitations. | 1110.0113v1 |
2011-10-06 | Comparative Wakefield Analysis of a First Prototype of a DDS Structure for CLIC Main Linac | A Damped Detuned Structure (DDS) for CLIC main linac has been proposed as an
alternative to the present baseline design which is based on heavy damping. A
first prototype, CLIC_DDS_A, for high power tests has been already designed and
is under construction. It is also foreseen to design a further prototype,
CLIC_DDS_B, to test both the wakefield suppression and high power performances.
Wakefield calculations for DDS are, in the early design stage, based on single
infinitely periodic cells. Though cell-to-cell interaction is taken into
account to calculate the wakefields, it is important to study full structure
properties using computational tools. In particular this is fundamental for
defining the input parameters for the HOM coupler that is crucial for the
performances of DDS. In the following a full analysis of wakefields and
impedances based on simulations conducted with finite difference based
electromagnetic computer code GdfidL will be presented. | 1110.1207v1 |
2007-08-24 | Enhancement of the Gilbert damping constant due to spin pumping in noncollinear ferromagnet/nonmagnet/ferromagnet trilayer systems | We analyzed the enhancement of the Gilbert damping constant due to spin
pumping in non-collinear ferromagnet / non-magnet / ferromagnet trilayer
systems. We show that the Gilbert damping constant depends both on the
precession angle of the magnetization of the free layer and on the direction of
the magntization of the fixed layer. We find the condition to be satisfied to
realize strong enhancement of the Gilbert damping constant. | 0708.3323v1 |
2006-12-01 | Gilbert damping and spin Coulomb drag in a magnetized electron liquid with spin-orbit interaction | We present a microscopic calculation of the Gilbert damping constant for the
magnetization of a two-dimensional spin-polarized electron liquid in the
presence of intrinsic spin-orbit interaction. First we show that the Gilbert
constant can be expressed in terms of the auto-correlation function of the
spin-orbit induced torque. Then we specialize to the case of the Rashba
spin-orbit interaction and we show that the Gilbert constant in this model is
related to the spin-channel conductivity. This allows us to study the Gilbert
damping constant in different physical regimes, characterized by different
orderings of the relevant energy scales -- spin-orbit coupling, Zeeman
coupling, momentum relaxation rate, spin-momentum relaxation rate, spin
precession frequency -- and to discuss its behavior in various limits.
Particular attention is paid to electron-electron interaction effects,which
enter the spin conductivity and hence the Gilbert damping constant via the spin
Coulomb drag coefficient. | 0612015v1 |
2023-09-20 | Evaluating Gilbert Damping in Magnetic Insulators from First Principles | Magnetic damping has a significant impact on the performance of various
magnetic and spintronic devices, making it a long-standing focus of research.
The strength of magnetic damping is usually quantified by the Gilbert damping
constant in the Landau-Lifshitz-Gilbert equation. Here we propose a
first-principles based approach to evaluate the Gilbert damping constant
contributed by spin-lattice coupling in magnetic insulators. The approach
involves effective Hamiltonian models and spin-lattice dynamics simulations. As
a case study, we applied our method to Y$_3$Fe$_5$O$_{12}$, MnFe$_2$O$_4$ and
Cr$_2$O$_3$. Their damping constants were calculated to be $0.8\times10^{-4}$,
$0.2\times10^{-4}$, $2.2\times 10^{-4}$, respectively at a low temperature. The
results for Y$_3$Fe$_5$O$_{12}$ and Cr$_2$O$_3$ are in good agreement with
experimental measurements, while the discrepancy in MnFe$_2$O$_4$ can be
attributed to the inhomogeneity and small band gap in real samples. The
stronger damping observed in Cr$_2$O$_3$, compared to Y$_3$Fe$_5$O$_{12}$,
essentially results from its stronger spin-lattice coupling. In addition, we
confirmed a proportional relationship between damping constants and the
temperature difference of subsystems, which had been reported in previous
studies. These successful applications suggest that our approach serves as a
promising candidate for estimating the Gilbert damping constant in magnetic
insulators. | 2309.11152v1 |
2011-05-20 | Magnetization Dissipation in the Ferromagnetic Semiconductor (Ga,Mn)As | We compute the Gilbert damping in (Ga,Mn)As based on the scattering theory of
magnetization relaxation. The disorder scattering is included
non-perturbatively. In the clean limit, the spin-pumping from the localized
d-electrons to the itinerant holes dominates the relaxation processes. In the
diffusive regime, the breathing Fermi-surface effect is balanced by the effects
of interband scattering, which cause the Gilbert damping constant to saturate
at around 0.005. In small samples, the system shape induces a large anisotropy
in the Gilbert damping. | 1105.4148v2 |
2019-11-07 | Quantum Oscillations of Gilbert Damping in Ferromagnetic/Graphene Bilayer Systems | We study the spin dynamics of a ferromagnetic insulator on which graphene is
placed. We show that the Gilbert damping is enhanced by the proximity exchange
coupling at the interface. The modulation of the Gilbert damping constant is
proportional to the product of the spin-up and spin-down densities of states of
graphene. Consequently, the Gilbert damping constant in a strong magnetic field
oscillates as a function of the external magnetic field that originates from
the Landau level structure of graphene. We find that a measurement of the
oscillation period enables the strength of the exchange coupling constant to be
determined. The results demonstrate in theory that the ferromagnetic resonance
measurements may be used to detect the spin resolved electronic structure of
the adjacent materials, which is critically important for future spin device
evaluations. | 1911.02775v2 |
2018-06-13 | Low magnetic damping of ferrimagnetic GdFeCo alloys | We investigate the Gilbert damping parameter for rare earth (RE)-transition
metal (TM) ferrimagnets over a wide temperature range. Extracted from the
field-driven magnetic domain-wall mobility, the Gilbert damping parameter was
as low as 0.0072 and was almost constant across the angular momentum
compensation temperature, starkly contrasting previous predictions that the
Gilbert damping parameter should diverge at the angular momentum compensation
temperature due to vanishing total angular momentum. Thus, magnetic damping of
RE-TM ferrimagnets is not related to the total angular momentum but is
dominated by electron scattering at the Fermi level where the TM has a dominant
damping role. | 1806.04881v1 |
2018-05-03 | Exact Intrinsic Localized Excitation of an Anisotropic Ferromagnetic Spin Chain in External Magnetic Field with Gilbert Damping, Spin Current and PT-Symmetry | We obtain the exact one-spin intrinsic localized excitation in an anisotropic
Heisenberg ferromagnetic spin chain in a constant/variable external magnetic
field with Gilbert damping included. We also point out how an appropriate
magnitude spin current term in a spin transfer nano-oscillator (STNO) can
stabilize the tendency towards damping. Further, we show how this excitation
can be sustained in a recently suggested PT-symmetric magnetic nanostructure.
We also briefly consider more general spin excitations. | 1805.01230v1 |
2019-10-24 | Spin waves in ferromagnetic thin films | A spin wave is the disturbance of intrinsic spin order in magnetic materials.
In this paper, a spin wave in the Landau-Lifshitz-Gilbert equation is obtained
based on the assumption that the spin wave maintains its shape while it
propagates at a constant velocity. Our main findings include: (1) in the
absence of Gilbert damping, the spin wave propagates at a constant velocity
with the increment proportional to the strength of the magnetic field; (2) in
the absence of magnetic field, at a given time the spin wave converges
exponentially fast to its initial profile as the damping parameter goes to zero
and in the long time the relaxation dynamics of the spin wave converges
exponentially fast to the easy-axis direction with the exponent proportional to
the damping parameter; (3) in the presence of both Gilbert damping and magnetic
field, the spin wave converges to the easy-axis direction exponentially fast at
a small timescale while propagates at a constant velocity beyond that. These
provides a comprehensive understanding of spin waves in ferromagnetic
materials. | 1910.11200v1 |
2018-07-20 | Another view on Gilbert damping in two-dimensional ferromagnets | A keen interest towards technological implications of spin-orbit driven
magnetization dynamics requests a proper theoretical description, especially in
the context of a microscopic framework, to be developed. Indeed, magnetization
dynamics is so far approached within Landau-Lifshitz-Gilbert equation which
characterizes torques on magnetization on purely phenomenological grounds.
Particularly, spin-orbit coupling does not respect spin conservation, leading
thus to angular momentum transfer to lattice and damping as a result. This
mechanism is accounted by the Gilbert damping torque which describes relaxation
of the magnetization to equilibrium. In this study we work out a microscopic
Kubo-St\v{r}eda formula for the components of the Gilbert damping tensor and
apply the elaborated formalism to a two-dimensional Rashba ferromagnet in the
weak disorder limit. We show that an exact analytical expression corresponding
to the Gilbert damping parameter manifests linear dependence on the scattering
rate and retains the constant value up to room temperature when no vibrational
degrees of freedom are present in the system. We argue that the methodology
developed in this paper can be safely applied to bilayers made of non- and
ferromagnetic metals, e.g., CoPt. | 1807.07897v2 |
2019-07-10 | The superior role of the Gilbert damping on the signal-to-noise ratio in heat-assisted magnetic recording | In magnetic recording the signal-to-noise ratio (SNR) is a good indicator for
the quality of written bits. However, a priori it is not clear which parameters
have the strongest influence on the SNR. In this work, we investigate the role
of the Gilbert damping on the SNR. Grains consisting of FePt like hard magnetic
material with two different grain sizes $d_1=5\,$nm and $d_2=7\,$nm are
considered and simulations of heat-assisted magnetic recording (HAMR) are
performed with the atomistic simulation program VAMPIRE. The simulations
display that the SNR saturates for damping constants larger or equal than 0.1.
Additionally, we can show that the Gilbert damping together with the bit length
have a major effect on the SNR whereas other write head and material parameters
only have a minor relevance on the SNR. | 1907.04577v2 |
2017-11-20 | Spin Pumping in Ion-beam Sputtered Co_{2}FeAl/Mo Bilayers:Interfacial Gilbert Damping | The spin pumping mechanism and associated interfacial Gilbert damping are
demonstrated in ion-beam sputtered Co2FeAl (CFA) /Mo bilayer thin films
employing ferromagnetic resonance spectroscopy. The dependence of the net spin
current transportation on Mo layer thickness, 0 to 10 nm, and the enhancement
of the net effective Gilbert damping are reported. The experimental data has
been analyzed using spin pumping theory in terms of spin current pumped through
the ferromagnet /nonmagnetic metal interface to deduce the effective spin
mixing conductance and the spin-diffusion length, which are estimated to be
1.16(0.19)x10^19 m^-2 and 3.50(0.35)nm, respectively. The damping constant is
found to be 8.4(0.3)x10^-3 in the Mo(3.5nm) capped CFA(8nm) sample
corresponding to a ~42% enhancement of the original Gilbert damping
(6.0(0.3)x10^-3) in the uncapped CFA layer. This is further confirmed by
inserting a Cu dusting layer which reduces the spin transport across the CFA
/Mo interface. The Mo layer thickness dependent net spin current density is
found to lie in the range of 1-3 MAm^-2, which also provides additional
quantitative evidence of spin pumping in this bilayer thin film system. | 1711.07455v1 |
2006-06-09 | Spin wave dynamics and the determination of intrinsic Gilbert damping in locally-excited Permalloy thin films | Time-resolved scanning Kerr effect microscopy has been used to study
magnetization dynamics in Permalloy thin films excited by transient magnetic
pulses generated by a micrometer-scale transmission line structure. The results
are consistent with magnetostatic spin wave theory and are supported by
micromagnetic simulations. Magnetostatic volume and surface spin waves are
measured for the same specimen using different bias field orientations and can
be accurately calculated by k-space integrations over all excited plane wave
components. A single damping constant of Gilbert form is sufficient to describe
both scenarios. The nonuniform pulsed field plays a key role in the spin wave
dynamics, with its Fourier transform serving as a weighting function for the
participating modes. The intrinsic Gilbert damping parameter $\alpha$ is most
conveniently measured when the spin waves are effectively stationary. | 0606235v3 |
2019-11-08 | Giant anisotropy of Gilbert damping in a Rashba honeycomb antiferromagnet | Giant Gilbert damping anisotropy is identified as a signature of strong
Rashba spin-orbit coupling in a two-dimensional antiferromagnet on a honeycomb
lattice. The phenomenon originates in spin-orbit induced splitting of
conduction electron subbands that strongly suppresses certain spin-flip
processes. As a result, the spin-orbit interaction is shown to support an
undamped non-equilibrium dynamical mode that corresponds to an ultrafast
in-plane N\'eel vector precession and a constant perpendicular-to-the-plane
magnetization. The phenomenon is illustrated on the basis of a two dimensional
$s$-$d$ like model. Spin-orbit torques and conductivity are also computed
microscopically for this model. Unlike Gilbert damping these quantities are
shown to reveal only a weak anisotropy that is limited to the semiconductor
regime corresponding to the Fermi energy staying in a close vicinity of
antiferromagnetic gap. | 1911.03408v1 |
2017-09-29 | Non-local Gilbert damping tensor within the torque-torque correlation model | An essential property of magnetic devices is the relaxation rate in magnetic
switching which depends strongly on the damping in the magnetisation dynamics.
It was recently measured that damping depends on the magnetic texture and,
consequently, is a non-local quantity. The damping enters the
Landau-Lifshitz-Gilbert equation as the phenomenological Gilbert damping
parameter $\alpha$, that does not, in a straight forward formulation, account
for non-locality. Efforts were spent recently to obtain Gilbert damping from
first principles for magnons of wave vector $\mathbf{q}$. However, to the best
of our knowledge, there is no report about real space non-local Gilbert damping
$\alpha_{ij}$. Here, a torque-torque correlation model based on a tight binding
approach is applied to the bulk elemental itinerant magnets and it predicts
significant off-site Gilbert damping contributions, that could be also
negative. Supported by atomistic magnetisation dynamics simulations we reveal
the importance of the non-local Gilbert damping in atomistic magnetisation
dynamics. This study gives a deeper understanding of the dynamics of the
magnetic moments and dissipation processes in real magnetic materials. Ways of
manipulating non-local damping are explored, either by temperature, material's
doping or strain. | 1709.10365v1 |
2021-05-08 | A second-order numerical method for Landau-Lifshitz-Gilbert equation with large damping parameters | A second order accurate numerical scheme is proposed and implemented for the
Landau-Lifshitz-Gilbert equation, which models magnetization dynamics in
ferromagnetic materials, with large damping parameters. The main advantages of
this method are associated with the following features: (1) It only solves
linear systems of equations with constant coefficients where fast solvers are
available, so that the numerical efficiency has been greatly improved, in
comparison with the existing Gauss-Seidel project method. (2) The second-order
accuracy in time is achieved, and it is unconditionally stable for large
damping parameters. Moreover, both the second-order accuracy and the great
efficiency improvement will be verified by several numerical examples in the 1D
and 3D simulations. In the presence of large damping parameters, it is observed
that this method is unconditionally stable and finds physically reasonable
structures while many existing methods have failed. For the domain wall
dynamics, the linear dependence of wall velocity with respect to the damping
parameter and the external magnetic field will be obtained through the reported
simulations. | 2105.03576v1 |
2016-02-23 | Experimental Investigation of Temperature-Dependent Gilbert Damping in Permalloy Thin Films | The Gilbert damping of ferromagnetic materials is arguably the most important
but least understood phenomenological parameter that dictates real-time
magnetization dynamics. Understanding the physical origin of the Gilbert
damping is highly relevant to developing future fast switching spintronics
devices such as magnetic sensors and magnetic random access memory. Here, we
report an experimental study of temperature-dependent Gilbert damping in
permalloy (Py) thin films of varying thicknesses by ferromagnetic resonance.
From the thickness dependence, two independent contributions to the Gilbert
damping are identified, namely bulk damping and surface damping. Of particular
interest, bulk damping decreases monotonically as the temperature decreases,
while surface damping shows an enhancement peak at the temperature of ~50 K.
These results provide an important insight to the physical origin of the
Gilbert damping in ultrathin magnetic films. | 1602.07325v1 |
2019-02-22 | Strongly Enhanced Gilbert Damping in 3d Transition Metal Ferromagnet Monolayers in Contact with Topological Insulator Bi2Se3 | Engineering Gilbert damping of ferromagnetic metal films is of great
importance to exploit and design spintronic devices that are operated with an
ultrahigh speed. Based on scattering theory of Gilbert damping, we extend the
torque method originally used in studies of magnetocrystalline anisotropy to
theoretically determine Gilbert dampings of ferromagnetic metals. This method
is utilized to investigate Gilbert dampings of 3d transition metal ferromagnet
iron, cobalt and nickel monolayers that are contacted by the prototypical
topological insulator Bi2Se3. Amazingly, we find that their Gilbert dampings
are strongly enhanced by about one order in magnitude, compared with dampings
of their bulks and free-standing monolayers, owing to the strong spin-orbit
coupling of Bi2Se3. Our work provides an attractive route to tailoring Gilbert
damping of ferromagnetic metallic films by putting them in contact with
topological insulators. | 1902.08700v1 |
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