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2008-08-03
|
Superradiant Instability of Five-Dimensional Rotating Charged AdS Black Holes
|
We study the instability of small AdS black holes with two independent
rotation parameters in minimal five-dimensional gauged supergravity to massless
scalar perturbations. We analytically solve the Klein-Gordon equation for
low-frequency perturbations in two regions of the spacetime of these black
holes: namely, in the region close to the horizon and in the far-region. By
matching the solutions in an intermediate region, we calculate the frequency
spectrum of quasinormal modes. We show that in the regime of superradiance only
the modes of even orbital quantum number undergo negative damping, resulting in
exponential growth of the amplitude. That is, the black holes become unstable
to these modes. Meanwhile, the modes of odd orbital quantum number do not
undergo any damping, oscillating with frequency-shifts. This is in contrast
with the case of four-dimensional small Kerr-AdS black holes which exhibit the
instability to all modes of scalar perturbations in the regime of
superradiance.
|
0808.0280v3
|
2008-08-15
|
Collective excitations in two-dimensional antiferromagnet in strong magnetic field
|
We discuss spin-$\frac12$ two-dimensional (2D) Heisenberg antiferromagnet
(AF) on a square lattice at T=0 in strong magnetic field H near its saturation
value $H_c$. A perturbation approach is proposed to obtain spectrum of magnons
with momenta not very close to AF vector in the leading order in small
parameter $(H_c-H)/H_c$. We find that magnons are well-defined quasi-particles
at $H>0.9H_c$ although the damping is quite large near the zone boundary. A
characteristic rotonlike local minimum in the spectrum is observed at ${\bf
k}=(\pi,0)$ accompanied by decrease of the damping near $(\pi,0)$. The
suggested approach can be used in discussion of short-wavelength excitations in
other 2D Bose gases of particles or quasi-particles.
|
0808.2127v3
|
2008-08-26
|
Nonlinear regularization techniques for seismic tomography
|
The effects of several nonlinear regularization techniques are discussed in
the framework of 3D seismic tomography. Traditional, linear, $\ell_2$ penalties
are compared to so-called sparsity promoting $\ell_1$ and $\ell_0$ penalties,
and a total variation penalty. Which of these algorithms is judged optimal
depends on the specific requirements of the scientific experiment. If the
correct reproduction of model amplitudes is important, classical damping
towards a smooth model using an $\ell_2$ norm works almost as well as
minimizing the total variation but is much more efficient. If gradients (edges
of anomalies) should be resolved with a minimum of distortion, we prefer
$\ell_1$ damping of Daubechies-4 wavelet coefficients. It has the additional
advantage of yielding a noiseless reconstruction, contrary to simple $\ell_2$
minimization (`Tikhonov regularization') which should be avoided. In some of
our examples, the $\ell_0$ method produced notable artifacts. In addition we
show how nonlinear $\ell_1$ methods for finding sparse models can be
competitive in speed with the widely used $\ell_2$ methods, certainly under
noisy conditions, so that there is no need to shun $\ell_1$ penalizations.
|
0808.3472v3
|
2008-09-09
|
Process tomography of field damping and measurement of Fock state lifetimes by quantum non-demolition photon counting in a cavity
|
The relaxation of a quantum field stored in a high-$Q$ superconducting cavity
is monitored by non-resonant Rydberg atoms. The field, subjected to repetitive
quantum non-demolition (QND) photon counting, undergoes jumps between photon
number states. We select ensembles of field realizations evolving from a given
Fock state and reconstruct the subsequent evolution of their photon number
distributions. We realize in this way a tomography of the photon number
relaxation process yielding all the jump rates between Fock states. The damping
rates of the $n$ photon states ($0\leq n \leq 7$) are found to increase
linearly with $n$. The results are in excellent agreement with theory including
a small thermal contribution.
|
0809.1511v1
|
2008-09-17
|
Spin-transfer torque induced reversal in magnetic domains
|
Using the complex stereographic variable representation for the macrospin,
from a study of the nonlinear dynamics underlying the generalized
Landau-Lifshitz(LL) equation with Gilbert damping, we show that the
spin-transfer torque is effectively equivalent to an applied magnetic field. We
study the macrospin switching on a Stoner particle due to spin-transfer torque
on application of a spin polarized current. We find that the switching due to
spin-transfer torque is a more effective alternative to switching by an applied
external field in the presence of damping. We demonstrate numerically that a
spin-polarized current in the form of a short pulse can be effectively employed
to achieve the desired macro-spin switching.
|
0809.2910v1
|
2008-10-01
|
Excitation of trapped oscillations in discs around black holes
|
High-frequency quasi-periodic oscillations detected in the light curves of
black hole candidates can, according to one model, be identified with
hydrodynamic oscillations of the accretion disc. We describe a non-linear
coupling mechanism, suggested by Kato, through which inertial waves trapped in
the inner regions of accretion discs around black holes are excited. Global
warping and/or eccentricity of the disc have a fundamental role in this
coupling: they combine with trapped modes, generating negative energy waves,
that are damped as they approach the inner edge of the disc or their corotation
resonance. As a result of this damping, inertial oscillations are amplified. We
calculate the resulting eigenfunctions and their growth rates.
|
0810.0116v1
|
2008-10-09
|
Atomistic spin dynamics of the CuMn spin glass alloy
|
We demonstrate the use of Langevin spin dynamics for studying dynamical
properties of an archetypical spin glass system. Simulations are performed on
CuMn (20% Mn) where we study the relaxation that follows a sudden quench of the
system to the low temperature phase. The system is modeled by a Heisenberg
Hamiltonian where the Heisenberg interaction parameters are calculated by means
of first-principles density functional theory. Simulations are performed by
numerically solving the Langevin equations of motion for the atomic spins. It
is shown that dynamics is governed, to a large degree, by the damping parameter
in the equations of motion and the system size. For large damping and large
system sizes we observe the typical aging regime.
|
0810.1645v1
|
2008-10-20
|
On the Existence of Exponentially Decreasing Solutions of the Nonlinear Landau Damping Problem
|
In this paper we prove the existence of a large class of periodic solutions
of the Vlasov-Poisson in one space dimension that decay exponentially as t goes
to infinity. The exponential decay is well known for the linearized version of
the Landau damping problem and it has been proved in [4] for a class of
solutions of the Vlasov-Poisson system that behaves asymptotically as free
streaming solutions and are sufficiently flat in the space of velocities. The
results in this paper enlarge the class of possible asymptotic limits,
replacing the fatness condition in [4] by a stability condition for the
linearized problem.
|
0810.3456v2
|
2008-10-22
|
Thermal (in)stability of type I collagen fibrils
|
We measured Young's modulus at temperatures ranging from 20 to 100 ^{\circ}$C
for a collagen fibril taken from rat's tendon. The hydration change under
heating and the damping decrement were measured as well. At physiological
temperatures $25-45^{\circ}$C Young's modulus decreases, which can be
interpreted as instability of collagen. For temperatures between
$45-80^{\circ}$C Young's modulus first stabilizes and then increases with
decreasing the temperature. The hydrated water content and the damping
decrement have strong maxima in the interval $70-80^{\circ}$C indicating on
complex inter-molecular structural changes in the fibril. All these effects
disappear after heat-denaturating the sample at $120^\circ$C. Our main result
is a five-stage mechanism by which the instability of a single collagen at
physiological temperatures is compensated by the interaction between collagen
molecules within the fibril.
|
0810.4172v1
|
2008-11-03
|
Ion thermal effects in oscillating multi-ion plasma sheath theory
|
The effects of ion temperature are discussed in a two-ion electron plasma and
for a model applicable to the oscillating sheath theory that has recently been
much in the focus of researchers. The differences between the fluid and kinetic
models have been pointed out, as well as the differences between the
approximative kinetic description (which involves the expansion of the plasma
dispersion function), and the exact kinetic description. It is shown that the
approximative kinetic description, first, can not describe the additional
acoustic mode which naturally exists in the plasma with an additional ion
population with a finite temperature, and, second, it yields an inaccurate
Landau damping of the bulk ion acoustic mode. The reasons for these two
failures are described. In addition to this, a fluid model is presented that is
capable of capturing both of these features that are missing in the
approximative kinetic description, i.e., two (fast and slow) ion acoustic
modes, and the corresponding Landau damping of both modes.
|
0811.0251v1
|
2008-12-01
|
Self-Diffusion in 2D Dusty Plasma Liquids: Numerical Simulation Results
|
We perform Brownian dynamics simulations for studying the self-diffusion in
two-dimensional (2D) dusty plasma liquids, in terms of both mean-square
displacement and velocity autocorrelation function (VAF). Super-diffusion of
charged dust particles has been observed to be most significant at infinitely
small damping rate $\gamma$ for intermediate coupling strength, where the
long-time asymptotic behavior of VAF is found to be the product of $t^{-1}$ and
$\exp{(-\gamma t)}$. The former represents the prediction of early theories in
2D simple liquids and the latter the VAF of a free Brownian particle. This
leads to a smooth transition from super-diffusion to normal diffusion, and then
to sub-diffusion with an increase of the damping rate. These results well
explain the seemingly contradictory scattered in recent classical molecular
dynamics simulations and experiments of dusty plasmas.
|
0812.0338v3
|
2008-12-11
|
Reduced nonlinear description of Farley-Buneman instability
|
In the study on nonlinear wave-wave processes in an ionosphere and a
magnetosphere usually the main attention is paid to investigation of plasma
turbulence at well developed stage, when the wide spectrum of plasma wave is
present. On the other side, it is well known that even if the number of
cooperating waves remains small due to a competition of processes of their
instability and attenuation, the turbulence appears in the result of their
stochastic behavior. The regimes of nonlinear dynamics of low frequency waves
excited due to Farley-Buneman instability in weakly ionized and inhomogeneous
ionospheric plasma in the presence of electric current perpendicular to ambient
magnetic field are considered. The problem is essentially three dimensional and
difficult for full numerical simulation, but the strong collisional damping of
waves allow to assume that in this case a perturbed state of plasma can be
described as finite set of interacting waves, some of which are unstable and
other strongly damping. The proposed nonlinear model allow to make full study
of nonlinear stabilization, conditions of stochasticity and to consider the
different regimes and properties of few mode plasma turbulence.
|
0812.2182v1
|
2008-12-16
|
Reconstructing Baryon Oscillations: A Lagrangian Theory Perspective
|
Recently Eisenstein and collaborators introduced a method to `reconstruct'
the linear power spectrum from a non-linearly evolved galaxy distribution in
order to improve precision in measurements of baryon acoustic oscillations. We
reformulate this method within the Lagrangian picture of structure formation,
to better understand what such a method does, and what the resulting power
spectra are. We show that reconstruction does not reproduce the linear density
field, at second order. We however show that it does reduce the damping of the
oscillations due to non-linear structure formation, explaining the improvements
seen in simulations. Our results suggest that the reconstructed power spectrum
is potentially better modeled as the sum of three different power spectra, each
dominating over different wavelength ranges and with different non-linear
damping terms. Finally, we also show that reconstruction reduces the
mode-coupling term in the power spectrum, explaining why mis-calibrations of
the acoustic scale are reduced when one considers the reconstructed power
spectrum.
|
0812.2905v3
|
2009-02-16
|
Plasmon excitations in homogeneous neutron star matter
|
We study the possible collective plasma modes which can affect neutron-star
thermodynamics and different elementary processes in the baryonic density range
between nuclear saturation ($\rho_0$) and $3\rho_0$. In this region, the
expected constituents of neutron-star matter are mainly neutrons, protons,
electrons and muons ($npe\mu$ matter), under the constraint of beta
equilibrium. The elementary plasma excitations of the $pe\mu$ three-fluid
medium are studied in the RPA framework. We emphasize the relevance of the
Coulomb interaction among the three species, in particular the interplay of the
electron and muon screening in suppressing the possible proton plasma mode,
which is converted into a sound-like mode. The Coulomb interaction alone is
able to produce a variety of excitation branches and the full spectral function
shows a rich structure at different energy. The genuine plasmon mode is pushed
at high energy and it contains mainly an electron component with a substantial
muon component, which increases with density. The plasmon is undamped for not
too large momentum and is expected to be hardly affected by the nuclear
interaction. All the other branches, which fall below the plasmon, are damped
or over-damped.
|
0902.2552v2
|
2009-02-27
|
Monogamy Inequality and Residual Entanglement of Three Qubits under Decoherence
|
Exploring an analytical expression for the convex roof of the pure state
squared concurrence for rank 2 mixed states the entanglement of a system of
three particles under decoherence is studied, using the monogamy inequality for
mixed states and the residual entanglement obtained from it. The monogamy
inequality is investigated both for the concurrence and the negativity in the
case of local independent phase damping channel acting on generalized GHZ
states of three particles and the local independent amplitude damping channel
acting on generalized W state of three particles. It is shown that the
bipartite entanglement between one qubit and the rest has a qualitative similar
behavior to the entanglement between individual qubits, and that the residual
entanglement in terms of the negativity cannot be a good entanglement measure
for mixed states, since it can increase under local decoherence.
|
0903.0019v2
|
2009-03-12
|
X-band crab cavities for the CLIC beam delivery system
|
The CLIC machine incorporates a 20 mrad crossing angle at the IP to aid the
extraction of spent beams. In order to recover the luminosity lost through the
crossing angle a crab cavity is proposed to rotate the bunches prior to
collision. The crab cavity is chosen to have the same frequency as the main
linac (11.9942 GHz) as a compromise between size, phase stability requirements
and beam loading. It is proposed to use a HE11 mode travelling wave structure
as the CLIC crab cavity in order to minimise beam loading and mode separation.
The position of the crab cavity close to the final focus enhances the effect of
transverse wake-fields so effective wake-field damping is required. A damped
detuned structure is proposed to suppress and de-cohere the wake-field hence
reducing their effect. Design considerations for the CLIC crab cavity will be
discussed as well as the proposed high power testing of these structures at
SLAC.
|
0903.2116v1
|
2009-03-16
|
Regularity of invariant sets in semilinear damped wave equations
|
Under fairly general assumptions, we prove that every compact invariant
subset $\mathcal I$ of the semiflow generated by the semilinear damped wave
equation \epsilon u_{tt}+u_t+\beta(x)u-\sum_{ij}(a_{ij}
(x)u_{x_j})_{x_i}&=f(x,u),&& (t,x)\in[0,+\infty[\times\Omega,
u&=0,&&(t,x)\in[0,+\infty[\times\partial\Omega in $H^1_0(\Omega)\times
L^2(\Omega)$ is in fact bounded in $D(\mathbf A)\times H^1_0(\Omega)$. Here
$\Omega$ is an arbitrary, possibly unbounded, domain in $\R^3$, $\mathbf A
u=\beta(x)u-\sum_{ij}(a_{ij}(x)u_{x_j})_{x_i}$ is a positive selfadjoint
elliptic operator 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 an attractor.
|
0903.2782v1
|
2009-03-20
|
Hawking-Unruh radiation as irreversible consequence of radiative action in dynamics
|
Hawking-Unruh thermal state of warm surrounding field encountered in
non-inertial frames is shown to be a real phenomenon, a marker of nonstationary
dynamic evolutions. In accelerated motion of a charged particle it is shown
that the recoiled damping effect of Larmor radiation relaxation leads to
distinctive thermal power, which is akin to that of Hawking-Unruh radiation
from warm surrounding field of the accelerated charge. The damping effect from
recoil-momentum of transverse electromagnetic field is worked out by
considering torque imparted to the inherently existing angular evolution of
spherically polarized vacuum field around the point-like charged particle in
acceleration. Hawking-Unruh effects is generally noted to be a universal marker
of decoherence in evolution in all scales of microscopic, macroscopic and
megascopic systems. Besides detailing the case of electrodynamics, the various
efficacies of H-U relaxation are considered in the nonstationary evolutions.
|
0903.3529v2
|
2009-03-25
|
The covering factor of high redshift damped Lyman-$α$ systems
|
We have used the Very Long Baseline Array to image 18 quasars with foreground
damped Lyman-$\alpha$ systems (DLAs) at 327, 610 or 1420 MHz, to measure the
covering factor $f$ of each DLA at or near its redshifted HI 21cm line
frequency. Including six systems from the literature, we find that none of 24
DLAs at $0.09 < z < 3.45$ has an exceptionally low covering factor, with $f
\sim 0.45 - 1$ for the 14 DLAs at $z > 1.5$, $f \sim 0.41 - 1$ for the 10
systems at $z < 1$, and consistent covering factor distributions in the two
sub-samples. The observed paucity of detections of HI 21cm absorption in
high-$z$ DLAs thus cannot be explained by low covering factors and is instead
likely to arise due to a larger fraction of warm HI in these absorbers.
|
0903.4483v1
|
2009-04-15
|
Size dependent Acoustic Phonon Dynamics of CdTe0.68Se0.32 Nanoparticles in Borosilicate glass
|
Low frequency acoustic vibration and phonon linewidth for CdTe0.68Se0.32
nanoparticle embedded in borosilicate glass are calculated using two different
approaches by considering the elastic continuum model and fixed boundary
condition. The presence of medium significantly affects the phonon peaks and
results into the broadening of the modes. The linewidth is found to depend
inversely on the size, similar to that reported experimentally. The damping
time and quality factor have also been calculated. The damping time that is of
the order of picoseconds decreases with the decrease in size. High value of
quality factor for l=2 normal mode suggests the less loss of energy for this
mode.
|
0904.2278v1
|
2009-04-19
|
Incorporating Human Body Mass in Standards of Helmet Impact Protection against Traumatic Brain Injury
|
Impact induced traumatic brain injury (ITBI) describes brain injury from head
impact not necessarily accompanied by skull fracture. For sufficiently abrupt
head impact decelerations, ITBI results from brain tissue stress incurred as
the brain crashes into the inside of the skull wall, displacing the surrounding
cerebral spinal fluid (CSF). Proper helmet cushioning can damp the impact force
and reduce ITBI. But force is mass times acceleration and commonly used helmet
blunt impact standards are based only on acceleration thresholds. Here I show
how this implies that present standards overestimate the minimum acceleration
onset for ITBI by implicitly assuming that the brain is mechanically decoupled
from the body. I quantify how an arbitrary orientation of the body with respect
to impact direction increases the effective mass that should be used in
calculating the required damping force and injury threshold accelerations. I
suggest a practical method to incorporate the body mass and impact angle into
ITBI helmet standards and provide direction for further work.
|
0904.2856v1
|
2009-04-21
|
Type II migration of planets on eccentric orbits
|
The observed extrasolar planets possess both large masses (with a median M
sin i of 1.65 MJ) and a wide range in orbital eccentricity (0 < e < 0.94). As
planets are thought to form in circumstellar disks, one important question in
planet formation is determining whether, and to what degree, a gaseous disk
affects an eccentric planet's orbit. Recent studies have probed the interaction
between a disk and a terrestrial planet on an eccentric orbit, and the
interaction between a disk and a gas giant on a nearly circular orbit, but
little is known about the interaction between a disk and an eccentric gas
giant. Such a scenario could arise due to scattering while the disk is still
present, or perhaps through planet formation via gravitational instability. We
fill this gap with simulations of eccentric, massive (gap-forming) planets in
disks using the hydrodynamical code FARGO. Although the long-term orbital
evolution of the planet depends on disk properties, including the boundary
conditions used, the disk always acts to damp eccentricity when the planet is
released into the disk. This eccentricity damping takes place on a timescale of
40 years, 15 times faster than the migration timescale.
|
0904.3336v1
|
2009-05-13
|
J-transform applied to the detection of Gravitational Waves: preliminary results
|
We propose to apply to the detection of Gravitational Waves a new method
developed for the spectral analysis of noisy time-series of damped oscillators.
From the Pad\'e Approximations of the time-series Z-transform, a Jacobi
Matrix (J-Matrix) is constructed. We show that the J-Matrix has bound states
with eigenvalues strictly inside the unit circle. Each bound state can be
identified with one precise damped oscillator. Beside these bound states, there
is an essential spectrum sitting on the unit circle which represents the noise.
In this picture, signal and noise are clearly separated and identified in the
complex plane. Furthermore, we show that the J-transform enjoys the exceptional
feature of lossless undersampling. We take advantage of the above properties of
the J-transform to develop a procedure for the search of Gravitational Wave
bursts in interferometric data series such as those of LIGO and VIRGO projects.
Successful application of our procedure to simulated data having a poor signal
to noise ratio, highlights the power of our method.
|
0905.2000v1
|
2009-05-25
|
Non-Markovian dynamics of a biased qubit coupled to a structured bath
|
A new analytical approach, beyond rotating wave approximation, based on
unitary transformations and the non-Markovian master equation for the density
operator, is applied to treat the biased spin boson model with a Lorentzian
structured bath for arbitrary detunings at zero temperature. Compared to zero
bias, we find that the dynamics demonstrates two more damping oscillation
frequencies and one additional relaxation frequency for non-zero bias, where
one of the damping oscillation frequencies is a new effect. Analytical
expressions for the non-Markovian dynamics and the corresponding spectrum, the
localized-delocalized transition point, the coherent-incoherent transition
point, the analytical ground energy, the renormalized tunneling factor and the
susceptibility are determined. The sum rule and the Shiba relation are checked
in the coherent regime.
|
0905.3965v3
|
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
|
2009-11-11
|
Ginzburg-Landau equation for dynamical four-wave mixing in gain nonlinear media with relaxation
|
We consider the dynamical degenerate four-wave mixing (FWM) model in a cubic
nonlinear medium including both the time relaxation of the induced nonlinearity
and the nonlocal coupling. The initial ten-dimensional FWM system can be
rewritten as a three-variable intrinsic system (namely the intensity pattern,
the amplitude of the nonlinearity and the total net gain) which is very close
to the pumped Maxwell-Bloch system. In the case of a purely nonlocal response
the initial system reduces to a real damped sine-Gordon (SG) equation. We
obtain a new solution of this equation in the form of a sech function with a
time-dependent coefficient. By applying the reductive perturbation method to
this damped SG equation, we obtain exactly the cubic complex Ginzburg Landau
equation (CGL3), but with a time dependence in the loss/gain coefficient. The
CGL3 describes the properties of the spatially localized interference pattern
formed by the FWM.
|
0911.2129v1
|
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
|
2009-12-30
|
Spin torque and critical currents for magnetic vortex nano-oscillator in nanopillars
|
We calculated the main dynamic parameters of the spin polarized current
induced magnetic vortex oscillations in nanopillars, such as the range of
current density, where a vortex steady oscillations exist, the oscillation
frequency and orbit radius. We accounted for both the non-linear vortex
frequency and non-linear vortex damping. To describe the vortex excitations by
the spin polarized current we used a generalized Thiele approach to motion of
the vortex core as a collective coordinate. All the calculation results are
represented via the free layer sizes, saturation magnetization, Gilbert damping
and the degree of the spin polarization of the fixed layer. Predictions of the
developed model can be checked experimentally.
|
0912.5521v1
|
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
|
2010-01-15
|
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
|
2010-02-03
|
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
|
2010-02-05
|
Damped-driven KdV and effective equation for long-time behaviour of its solutions
|
For the damped-driven KdV equation $$ \dot
u-\nu{u_{xx}}+u_{xxx}-6uu_x=\sqrt\nu \eta(t,x), x\in S^1, \int u dx\equiv
\int\eta dx\equiv0, $$ with $0<\nu\le1$ and smooth in $x$ white in $t$ random
force $\eta$, we study the limiting long-time behaviour of the KdV integrals of
motions $(I_1,I_2,...)$, evaluated along a solution $u^\nu(t,x)$, as $\nu\to0$.
We prove that %if $u=u^\nu(t,x)$ is a solution of the equation above, for
$0\le\tau:= \nu t \lesssim1$ the vector $
I^\nu(\tau)=(I_1(u^\nu(\tau,\cdot)),I_2(u^\nu(\tau,\cdot)),...), $ converges in
distribution to a limiting process $I^0(\tau)=(I^0_1,I^0_2,...)$. The $j$-th
component $I_j^0$ equals $\12(v_j(\tau)^2+v_{-j}(\tau)^2)$, where
$v(\tau)=(v_1(\tau), v_{-1}(\tau),v_2(\tau),...)$ is the vector of Fourier
coefficients of a solution of an {\it effective equation} for the
dam-ped-driven KdV. This new equation is a quasilinear stochastic heat equation
with a non-local nonlinearity, written in the Fourier coefficients. It is well
posed.
|
1002.1294v1
|
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
|
2010-03-31
|
Non-Markovian master equation for a damped oscillator with time-varying parameters
|
We derive an exact non-Markovian master equation that generalizes the
previous work [Hu, Paz and Zhang, Phys. Rev. D {\bf 45}, 2843 (1992)] to damped
harmonic oscillators with time-varying parameters. This is achieved by
exploiting the linearity of the system and operator solution in Heisenberg
picture. Our equation governs the non-Markovian quantum dynamics when the
system is modulated by external devices. As an application, we apply our
equation to parity kick decoupling problems. The time-dependent dissipative
coefficients in the master equation are shown to be modified drastically when
the system is driven by $\pi$ pulses. For coherence protection to be effective,
our numerical results indicate that kicking period should be shorter than
memory time of the bath. The effects of using soft pulses in an ohmic bath are
also discussed.
|
1003.5975v1
|
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
|
2010-06-12
|
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
|
2010-07-28
|
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
|
2010-10-01
|
Ferromagnetic resonance study of Co/Pd/Co/Ni multilayers with perpendicular anisotropy irradiated with Helium ions
|
We present a ferromagnetic resonance (FMR) study of the effect of Helium ion
irradiation on the magnetic anisotropy, the linewidth and the Gilbert damping
of a Co/Ni multilayer coupled to Co/Pd bilayers. The perpendicular magnetic
anisotropy decreases linearly with He ion fluence, leading to a transition to
in-plane magnetization at a critical fluence of 5x10^{14} ions/cm^2. We find
that the damping is nearly independent of fluence but the FMR linewidth at
fixed frequency has a maximum near the critical fluence, indicating that the
inhomogeneous broadening of the FMR line is a non-monotonic function of the He
ion fluence. Based on an analysis of the angular dependence of the FMR
linewidth, the inhomogeneous broadening is associated with spatial variations
in the magnitude of the perpendicular magnetic anisotropy. These results
demonstrate that ion irradiation may be used to systematically modify the
magnetic anisotropy and distribution of magnetic anisotropy parameters of
Co/Pd/Co/Ni multilayers for applications and basic physics studies.
|
1010.0268v2
|
2010-10-08
|
A unified first-principles study of Gilbert damping, spin-flip diffusion and resistivity in transition metal alloys
|
Using a formulation of first-principles scattering theory that includes
disorder and spin-orbit coupling on an equal footing, we calculate the
resistivity $\rho$, spin flip diffusion length $l_{sf}$ and the Gilbert damping
parameter $\alpha$ for Ni$_{1-x}$Fe$_x$ substitutional alloys as a function of
$x$. For the technologically important Ni$_{80}$Fe$_{20}$ alloy, permalloy, we
calculate values of $\rho = 3.5 \pm 0.15$ $\mu$Ohm-cm, $l_{sf}=5.5 \pm 0.3$ nm,
and $\alpha= 0.0046 \pm 0.0001$ compared to experimental low-temperature values
in the range $4.2-4.8$ $\mu$Ohm-cm for $\rho$, $5.0-6.0$ nm for $l_{sf}$, and
$0.004-0.013$ for $\alpha$ indicating that the theoretical formalism captures
the most important contributions to these parameters.
|
1010.1626v3
|
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
|
2010-10-20
|
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
|
2010-11-23
|
Ultra-fast magnetisation rates within the Landau-Lifshitz-Bloch model
|
The ultra-fast magnetisation relaxation rates during the laser-induced
magnetisation process are analyzed in terms of the Landau-Lifshitz-Bloch (LLB)
equation for different values of spin $S$. The LLB equation is equivalent in
the limit $S \rightarrow \infty$ to the atomistic Landau-Lifshitz-Gilbert (LLG)
Langevin dynamics and for $S=1/2$ to the M3TM model [B. Koopmans, {\em et al.}
Nature Mat. \textbf{9} (2010) 259]. Within the LLB model the ultra-fast
demagnetisation time ($\tau_{M}$) and the transverse damping ($\alpha_{\perp}$)
are parameterized by the intrinsic coupling-to-the-bath parameter $\lambda$,
defined by microscopic spin-flip rate. We show that for the phonon-mediated
Elliott-Yafet mechanism, $\lambda$ is proportional to the ratio between the
non-equilibrium phonon and electron temperatures.
We investigate the influence of the finite spin number and the scattering
rate parameter $\lambda$ on the magnetisation relaxation rates. The relation
between the fs demagnetisation rate and the LLG damping, provided by the LLB
theory, is checked basing on the available experimental data. A good agreement
is obtained for Ni, Co and Gd favoring the idea that the same intrinsic
scattering process is acting on the femtosecond and nanosecond timescale.
|
1011.5054v1
|
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
|
2011-02-22
|
Ab-initio calculation of the Gilbert damping parameter via linear response formalism
|
A Kubo-Greenwood-like equation for the Gilbert damping parameter $\alpha$ is
presented that is based on the linear response formalism. Its implementation
using the fully relativistic Korringa-Kohn-Rostoker (KKR) band structure method
in combination with Coherent Potential Approximation (CPA) alloy theory allows
it to be applied to a wide range of situations. This is demonstrated with
results obtained for the bcc alloy system Fe$_x$Co$_{1-x}$ as well as for a
series of alloys of permalloy with 5d transition metals.
To account for the thermal displacements of atoms as a scattering mechanism,
an alloy-analogy model is introduced. The corresponding calculations for Ni
correctly describe the rapid change of $\alpha$ when small amounts of
substitutional Cu are introduced.
|
1102.4551v1
|
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
|
2011-06-03
|
Shear viscous effects on the primordial power spectrum from warm inflation
|
We compute the primordial curvature spectrum generated during warm inflation,
including shear viscous effects. The primordial spectrum is dominated by the
thermal fluctuations of the radiation bath, sourced by the dissipative term of
the inflaton field. The dissipative coefficient \Upsilon, computed from first
principles in the close-to-equilibrium approximation, depends in general on the
temperature T, and this dependence renders the system of the linear
fluctuations coupled. Whenever the dissipative coefficient is larger than the
Hubble expansion rate H, there is a growing mode in the fluctuations before
horizon crossing. However, dissipation intrinsically means departures from
equilibrium, and therefore the presence of a shear viscous pressure in the
radiation fluid. This in turn acts as an extra friction term for the radiation
fluctuations that tends to damp the growth of the perturbations. Independently
of the T functional dependence of the dissipation and the shear viscosity, we
find that when the shear viscous coefficient \zeta_s is larger than 3 \rho_r/H
at horizon crossing, \rho_r being the radiation energy density, the shear
damping effect wins and there is no growing mode in the spectrum.
|
1106.0701v1
|
2011-06-06
|
Weakly nonlinear stochastic CGL equations
|
We consider the linear Schr\"odinger equation under periodic boundary
condition, driven by a random force and damped by a quasilinear damping: $$
\frac{d}{dt}u+i\big(-\Delta+V(x)\big) u=\nu \Big(\Delta u-\gr |u|^{2p}u-i\gi
|u|^{2q}u \Big) +\sqrt\nu\, \eta(t,x).\qquad (*) $$ The force $\eta$ is white
in time and smooth in $x$. We are concerned with the limiting, as $\nu\to0$,
behaviour of its solutions on long time-intervals $0\le t\le\nu^{-1}T$, and
with behaviour of these solutions under the double limit $t\to\infty$ and
$\nu\to0$. We show that these two limiting behaviours may be described in terms
of solutions for the {\it system of effective equations for $(*)$} which is a
well posed semilinear stochastic heat equation with a non-local nonlinearity
and a smooth additive noise, written in Fourier coefficients. The effective
equations do not depend on the Hamiltonian part of the perturbation
$-i\gi|u|^{2q}u$ (but depend on the dissipative part $-\gr|u|^{2p}u$). If $p$
is an integer, they may be written explicitly.
|
1106.1158v1
|
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
|
Increased Brownian force noise from molecular impacts in a constrained volume
|
We report on residual gas damping of the motion of a macroscopic test mass
enclosed in a nearby housing in the molecular flow regime. The damping
coefficient, and thus the associated thermal force noise, is found to increase
significantly when the distance between test mass and surrounding walls is
smaller than the test mass itself. The effect has been investigated with two
torsion pendulums of different geometry and has been modelled in a numerical
simulation whose predictions are in good agreement with the measurements.
Relevant to a wide variety of small-force experiments, the residual-gas force
noise power for the test masses in the LISA gravitational wave observatory is
roughly a factor 15 larger than in an infinite gas volume, though still
compatible with the target acceleration noise of 3 fm s^-2 Hz^-1/2 at the
foreseen pressure below 10^-6 Pa.
|
1107.2520v1
|
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
|
2011-07-20
|
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
|
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