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2016-11-29
|
Dissipative self-gravitating Bose-Einstein condensates with arbitrary nonlinearity as a model of dark matter halos
|
We develop a general formalism applying to Newtonian self-gravitating
Bose-Einstein condensates. This formalism may find application in the context
of dark matter halos. We introduce a generalized Gross-Pitaevskii equation
including a source of dissipation (damping) and an arbitrary nonlinearity.
Using the Madelung transformation, we derive the hydrodynamic representation of
this generalized Gross-Pitaevskii equation and obtain a damped quantum Euler
equation involving a friction force proportional and opposite to the velocity
and a pressure force associated with an equation of state determined by the
nonlinearity present in the generalized Gross-Pitaevskii equation. In the
strong friction limit, we obtain a quantum Smoluchowski equation. These
equations satisfy an $H$-theorem for a free energy functional constructed with
a generalized entropy. We specifically consider the Boltzmann and Tsallis
entropies associated with isothermal and polytropic equations of state. We also
consider the entropy associated with the logotropic equation of state. We
derive the virial theorem corresponding to the generalized Gross-Pitaevskii
equation, damped quantum Euler equation, and quantum Smoluchowski equation.
Using a Gaussian ansatz, we obtain a simple equation governing the dynamical
evolution of the size of the condensate. We highlight a specific model of dark
matter halos corresponding to a generalized Gross-Pitaevskii equation with a
logarithmic nonlinearity and a cubic nonlinearity. It leads to dark matter
halos with an equation of state $P=\rho k_B T_{\rm eff}/m+2\pi
a_s\hbar^2\rho^{2}/m^3$ presenting a condensed core (BEC/soliton) and an
isothermal halo with an effective temperature $T_{\rm eff}$. We propose that
this model provides an effective coarse-grained parametrization of dark matter
halos experiencing gravitational cooling.
|
1611.09610v1
|
2016-12-06
|
Breakdown of Fermi liquid theory in topological multi-Weyl semimetals
|
Fermi liquid theory works very well in most normal metals, but is found
violated in many strongly correlated electron systems, such as cuprate and
heavy-fermion superconductors. A widely accepted criterion is that, the Fermi
liquid theory is valid when the interaction-induced fermion damping rate
approaches zero more rapidly than the energy. Otherwise, it is invalid. Here,
we demonstrate that this criterion breaks down in topological double-and
triple-Weyl semimetals. Renormalization group analysis reveals that, although
the damping rate of double- and triple-Weyl fermions induced by the Coulomb
interaction approaches zero more rapidly than the energy, the quasiparticle
residue vanishes and the Fermi liquid theory is invalid. This behavior
indicates a weaker-than-marginal violation of the Fermi liquid theory. Such an
unconventional non-Fermi liquid state originates from the special dispersion of
double- and triple-Weyl fermions, and is qualitatively different from all the
other Fermi-liquid and non-Fermi-liquid states. The predicted properties of the
fermion damping rate and the spectral function can be probed by the
angle-resolved photoemission spectroscopy. The density of states, specific
heat, and conductivities are also calculated and analyzed after incorporating
the corrections induced by the Coulomb interaction.
|
1612.01729v2
|
2016-12-08
|
Quantifying acoustic damping using flame chemiluminescence
|
Thermoacoustic instabilities in gas turbines and aeroengine combustors falls
within the category of complex systems. They can be described
phenomenologically using nonlinear stochastic differential equations, which
constitute the grounds for output-only model-based system identification. It
has been shown recently that one can extract the governing parameters of the
instabilities, namely the linear growth rate and the nonlinear component of the
thermoacoustic feedback, using dynamic pressure time series only. This is
highly relevant for practical systems, which cannot be actively controlled due
to a lack of cost-effective actuators. The thermoacoustic stability is given by
the linear growth rate, which results from the combination of the acoustic
damping and the coherent feedback from the flame. In this paper, it is shown
that it is possible to quantify the acoustic damping of the system, and thus to
separate its contribution to the linear growth rate from the one of the flame.
This is achieved by post-processing in a simple way simultaneously acquired
chemiluminescence and acoustic pressure data. It provides an additional
approach to further unravel from observed time series the key mechanisms
governing the system dynamics. This straightforward method is illustrated here
using experimental data from a combustion chamber operated at several linearly
stable and unstable operating conditions.
|
1612.02609v1
|
2016-12-29
|
A quasi-mode theory of chiral phonons
|
The coherence properties of mechanical resonators are often limited by
multiple unavoidable forms of loss -- including phonon-phonon and phonon-defect
scattering -- which result in the scattering of sound into other resonant modes
and into the phonon bath. Dynamic suppression of this scattering loss can lift
constraints on device structure and can improve tolerance to defects in the
material, even after fabrication. Inspired by recent experiments, here we
introduce a model of phonon losses resulting from disorder in a whispering
gallery mode resonator with acousto-optical coupling between optical and
mechanical modes. We show that a typical elastic scattering mechanism of high
quality factor (Q) mechanical modes flips the direction of phonon propagation
via high-angle scattering, leading to damping into modes with the opposite
parity. When the optical mode overlaps co-propagating high-Q and bulk
mechanical modes, the addition of laser cooling via sideband-resolved damping
of the mechanical mode of a chosen parity also damps and modifies the response
of the bulk modes of the same parity. This, in turn, simultaneously improves
the quality factor and reduces the thermal load of the counter-propagating
high-Q modes, leading to the dynamical creation of a cold phononic shield. We
compare our theoretical results to the recent experiments of Kim et al., and
find quantitative agreement with our theory.
|
1612.09240v1
|
2017-01-03
|
A Model for Dissipation of Solar Wind Magnetic Turbulence by Kinetic Alfvén Waves at Electron Scales: Comparison with Observations
|
In hydrodynamic turbulence, it is well established that the length of the
dissipation scale depends on the energy cascade rate, i.e., the larger the
energy input rate per unit mass, the more the turbulent fluctuations need to be
driven to increasingly smaller scales to dissipate the larger energy flux.
Observations of magnetic spectral energy densities indicate that this intuitive
picture is not valid in solar wind turbulence. Dissipation seems to set in at
the same length scale for different solar wind conditions independently of the
energy flux. To investigate this difference in more detail, we present an
analytic dissipation model for solar wind turbulence at electron scales, which
we compare with observed spectral densities. Our model combines the energy
transport from large to small scales and collisionless damping, which removes
energy from the magnetic fluctuations in the kinetic regime. We assume
wave-particle interactions of kinetic Alfv\'{e}n waves (KAW) to be the main
damping process. Wave frequencies and damping rates of KAW are obtained from
the hot plasma dispersion relation. Our model assumes a critically balanced
turbulence, where larger energy cascade rates excite larger parallel
wavenumbers for a certain perpendicular wavenumber. If the dissipation is
additionally wave driven such that the dissipation rate is proportional to the
parallel wavenumber - as with KAW - then an increase of the energy cascade rate
is counter-balanced by an increased dissipation rate for the same perpendicular
wavenumber leading to a dissipation length independent of the energy cascade
rate.
|
1701.00680v1
|
2017-02-07
|
Constraining color flavor locked strange stars in the gravitational wave era
|
We perform a detailed analysis of the fundamental mode of non-radial
pulsations of color flavor locked strange stars. Solving the general
relativistic equations for non-radial pulsations for an equation of state
derived within the MIT bag model, we calculate the frequency and the
gravitational damping time of the fundamental mode for all the parametrizations
of the equation of state that lead to self-bound matter.
Our results show that color flavor locked strange stars can emit
gravitational radiation in the optimal range for present gravitational wave
detectors and that it is possible to constrain the equation of state's
parameters if the fundamental oscillation mode is observed and the stellar mass
is determined.
We also show that the $f$-mode frequency can be fitted as a function of the
square root of the average stellar density $\sqrt{M/R^3}$ by a single linear
relation that fits quite accurately the results for all parametrizations of the
equation of state. All results for the damping time can also be fitted as a
function of the compactness $M/R$ by a single empirical relation. Therefore, if
a given compact object is identified as a color flavor locked strange star
these two relations could be used to determine the mass and the radius from the
knowledge of the frequency and the damping time of gravitational waves from the
$f$ mode.
|
1702.02081v1
|
2017-02-16
|
Designing the Optimal Bit: Balancing Energetic Cost, Speed and Reliability
|
We consider the technologically relevant costs of operating a reliable bit
that can be erased rapidly. We find that both erasing and reliability times are
non-monotonic in the underlying friction, leading to a trade-off between
erasing speed and bit reliability. Fast erasure is possible at the expense of
low reliability at moderate friction, and high reliability comes at the expense
of slow erasure in the underdamped and overdamped limits. Within a given class
of bit parameters and control strategies, we define "optimal" designs of bits
that meet the desired reliability and erasing time requirements with the lowest
operational work cost. We find that optimal designs always saturate the bound
on the erasing time requirement, but can exceed the required reliability time
if critically damped. The non-trivial geometry of the reliability and erasing
time-scales allows us to exclude large regions of parameter space as
sub-optimal. We find that optimal designs are either critically damped or close
to critical damping under the erasing procedure.
|
1702.04950v2
|
2017-03-07
|
Higgs Modes in the Pair Density Wave Superconducting State
|
The pair density wave (PDW) superconducting state has been proposed to
explain the layer- decoupling effect observed in the compound
La$_{2-x}$Ba$_x$CuO$_4$ at $x=1/8$ (Phys. Rev. Lett. 99, 127003). In this state
the superconducting order parameter is spatially modulated, in contrast with
the usual superconducting (SC) state where the order parameter is uniform. In
this work, we study the properties of the amplitude (Higgs) modes in a
unidirectional PDW state. To this end we consider a phenomenological model of
PDW type states coupled to a Fermi surface of fermionic quasiparticles. In
contrast to conventional superconductors that have a single Higgs mode,
unidirectional PDW superconductors have two Higgs modes. While in the PDW state
the Fermi surface largely remains gapless, we find that the damping of the PDW
Higgs modes into fermionic quasiparticles requires exceeding an energy
threshold. We show that this suppression of damping in the PDW state is due to
kinematics. As a result, only one of the two Higgs modes is significantly
damped. In addition, motivated by the experimental phase diagram, we discuss
the mixing of Higgs modes in the coexistence regime of the PDW and uniform SC
states. These results should be observable directly in a Raman spectroscopy, in
momentum resolved electron energy loss spectroscopy, and in resonant inelastic
X-ray scattering, thus providing evidence of the PDW states.
|
1703.02541v2
|
2017-04-29
|
Low-frequency wide band-gap elastic/acoustic meta-materials using the K-damping concept
|
The terms "acoustic/elastic meta-materials" describe a class of periodic
structures with unit cells exhibiting local resonance. This localized resonant
structure has been shown to result in negative effective stiffness and/or mass
at frequency ranges close to these local resonances. As a result, these
structures present unusual wave propagation properties at wavelengths well
below the regime corresponding to band-gap generation based on spatial
periodicity, (i.e. "Bragg scattering"). Therefore, acoustic/elastic
meta-materials can lead to applications, especially suitable in the
low-frequency range. However, low frequency range applications of such
meta-materials require very heavy internal moving masses, as well as additional
constraints at the amplitudes of the internally oscillating locally resonating
structures, which may prohibit their practical implementation. In order to
resolve this disadvantage, the K-Damping concept will be analyzed. According to
this concept, the acoustic/elastic meta-materials are designed to include
negative stiffness elements instead or in addition to the internally resonating
added masses. This concept removes the need for the heavy locally added heavy
masses, while it simultaneously exploits the negative stiffness damping
phenomenon. Application of both Bloch's theory and the classical modal analysis
at the one-dimensional mass-in-mass lattice is analyzed and corresponding
dispersion relations are derived. The results indicate significant advantages
over the conventional mass-in-a mass lattice, such as broader band-gaps and
increased damping ratio and reveal significant potential in the proposed
solution. Preliminary feasibility analysis for seismic meta-structures and low
frequency acoustic isolation-damping confirm the strong potential and
applicability of this concept.
|
1705.00226v2
|
2017-05-07
|
Precision cosmology with redshift-space bispectrum: a perturbation theory based model at one-loop order
|
The large-scale matter distribution in the late-time Universe exhibits
gravity-induced non-Gaussianity, and the bispectrum, three-point cumulant is
expected to contain significant cosmological information. In particular, the
measurement of the bispectrum helps to tighten the constraints on dark energy
and modified gravity through the redshift-space distortions (RSD). In this
paper, extending the work by Taruya, Nishimichi & Saito (2010, Phys.Rev.D 82,
063522), we present a perturbation theory (PT) based model of redshift-space
matter bispectrum that can keep the non-perturbative damping effect under
control. Characterizing this non-perturbative damping by a univariate function
with single free parameter, the PT model of the redshift-space bispectrum is
tested against a large set of cosmological $N$-body simulations, finding that
the predicted monopole and quadrupole moments are in a good agreement with
simulations at the scales of baryon acoustic oscillations (well beyond the
range of agreement of standard PT). The validity of the univariate ansatz of
the damping effect is also examined, and with the PT calculation at
next-to-leading order, the fitted values of the free parameter is shown to
consistently match those obtained from the PT model of power spectrum by
Taruya, Nishimichi & Saito (2010).
|
1705.02574v1
|
2017-05-13
|
Large-amplitude longitudinal oscillations in a solar filament
|
In this paper, we report our multiwavelength observations of the
large-amplitude longitudinal oscillations of a filament on 2015 May 3. Located
next to active region 12335, the sigmoidal filament was observed by the
ground-based H$\alpha$ telescopes from GONG and by AIA aboard SDO. The filament
oscillations were most probably triggered by the magnetic reconnection in the
filament channel. The directions of oscillations have angles of
4$^\circ$-36$^\circ$ with respect to the filament axis. The whole filament did
not oscillate in phase as a rigid body. Meanwhile, the periods (3100$-$4400 s)
of oscillations have a spatial dependence. The values of $R$ are estimated to
be 69.4$-$133.9 Mm, and the minimum transverse magnetic field of the dips is
estimated to be 15 G. The amplitudes of S5-S8 grew with time, while the
amplitudes of S9-S14 damped with time. The amplitudes of oscillations range
from a few to ten Mm, and the maximal velocity can reach 30 km s$^{-1}$.
Interestingly, the filament experienced mass drainage southwards at a speed of
$\sim$27 km s$^{-1}$. The oscillations continued after the mass drainage and
lasted for more than 11 hr. After the mass drainage, the phases of oscillations
did not change a lot. The periods of S5-S8 decreased, while the periods of
S9-S14 increased. The amplitudes of S5$-$S8 damped with time, while the
amplitudes of S9-S14 grew. Most of the damping (growing) ratios are between -9
and 14. We propose a schematic cartoon to explain the complex behaviors of
oscillations by introducing thread-thread interaction.
|
1705.04820v1
|
2017-05-14
|
Inter-Area Oscillation Damping With Non-Synchronized Wide-Area Power System Stabilizer
|
One of the major issues in an interconnected power system is the low damping
of inter-area oscillations which significantly reduces the power transfer
capability. Advances in Wide-Area Measurement System (WAMS) makes it possible
to use the information from geographical distant location to improve power
system dynamics and performances. A speed deviation based Wide-Area Power
System Stabilizer (WAPSS) is known to be effective in damping inter-area modes.
However, the involvement of wide-area signals gives rise to the problem of
time-delay, which may degrade the system performance. In general, time-stamped
synchronized signals from Phasor Data Concentrator (PDC) are used for WAPSS, in
which delays are introduced in both local and remote signals. One can opt for a
feedback of remote signal only from PDC and uses the local signal as it is
available, without time synchronization. This paper utilizes configurations of
time-matched synchronized and nonsychronized feedback and provides the
guidelines to design the controller. The controllers are synthesized using
$H_\infty$ control with regional pole placement for ensuring adequate dynamic
performance. To show the effectiveness of the proposed approach, two power
system models have been used for the simulations. It is shown that the
controllers designed based on the nonsynchronized signals are more robust to
time time delay variations than the controllers using synchronized signal.
|
1705.04953v2
|
2017-05-19
|
Analytical Prediction of Reflection Coefficients for Wave Absorbing Layers in Flow Simulations of Regular Free-Surface Waves
|
Undesired wave reflections, which occur at domain boundaries in flow
simulations with free-surface waves, can be minimized by applying source terms
in the vicinity of the boundary to damp the waves. Examples of such approaches
are absorbing layers, damping zones, forcing zones, relaxation zones and sponge
layers. A problem with these approaches is that the effectivity of the wave
damping depends on the parameters in the source term functions, which are
case-dependent and must be adjusted to the wave. The present paper presents a
theory which analytically predicts the reflection coefficients and which can be
used to optimally select the source term parameters before running the
simulation. The theory is given in a general form so that it is applicable to
many existing implementations. It is validated against results from
finite-volume-based flow simulations of regular free-surface waves and found to
be of satisfactory accuracy for practical purposes.
|
1705.06940v2
|
2017-06-16
|
Challenges testing the no-hair theorem with gravitational waves
|
General relativity's no-hair theorem states that isolated astrophysical black
holes are described by only two numbers: mass and spin. As a consequence, there
are strict relationships between the frequency and damping time of the
different modes of a perturbed Kerr black hole. Testing the no-hair theorem has
been a longstanding goal of gravitational-wave astronomy. The recent detection
of gravitational waves from black hole mergers would seem to make such tests
imminent. We investigate how constraints on black hole ringdown parameters
scale with the loudness of the ringdown signal---subject to the constraint that
the post-merger remnant must be allowed to settle into a perturbative,
Kerr-like state. In particular, we require that---for a given detector---the
gravitational waveform predicted by numerical relativity is indistinguishable
from an exponentially damped sine after time $t^\text{cut}$. By requiring the
post-merger remnant to settle into such a perturbative state, we find that
confidence intervals for ringdown parameters do not necessarily shrink with
louder signals. In at least some cases, more sensitive measurements probe later
times without necessarily providing tighter constraints on ringdown frequencies
and damping times. Preliminary investigations are unable to explain this result
in terms of a numerical relativity artifact.
|
1706.05152v2
|
2017-06-26
|
Simulating the effect of high column density absorbers on the one-dimensional Lyman-alpha forest flux power spectrum
|
We measure the effect of high column density absorbing systems of neutral
hydrogen (HI) on the one-dimensional (1D) Lyman-alpha forest flux power
spectrum using cosmological hydrodynamical simulations from the Illustris
project. High column density absorbers (which we define to be those with HI
column densities $N(\mathrm{HI}) > 1.6 \times
10^{17}\,\mathrm{atoms}\,\mathrm{cm}^{-2}$) cause broadened absorption lines
with characteristic damping wings. These damping wings bias the 1D Lyman-alpha
forest flux power spectrum by causing absorption in quasar spectra away from
the location of the absorber itself. We investigate the effect of high column
density absorbers on the Lyman-alpha forest using hydrodynamical simulations
for the first time. We provide templates as a function of column density and
redshift, allowing the flexibility to accurately model residual contamination,
i.e., if an analysis selectively clips out the largest damping wings. This
flexibility will improve cosmological parameter estimation, e.g., allowing more
accurate measurement of the shape of the power spectrum, with implications for
cosmological models containing massive neutrinos or a running of the spectral
index. We provide fitting functions to reproduce these results so that they can
be incorporated straightforwardly into a data analysis pipeline.
|
1706.08532v2
|
2017-07-19
|
Engineering elliptical spin-excitations by complex anisotropy fields in Fe adatoms and dimers on Cu(111)
|
We investigate the dynamics of Fe adatoms and dimers deposited on the Cu(111)
metallic surface in the presence of spin-orbit coupling, within time-dependent
density functional theory. The \textit{ab initio} results provide
material-dependent parameters that can be used in semiclassical approaches,
which are used for insightful interpretations of the excitation modes. By
manipulating the surroundings of the magnetic elements, we show that elliptical
precessional motion may be induced through the modification of the magnetic
anisotropy energy. We also demonstrate how different kinds of spin precession
are realized, considering the symmetry of the magnetic anisotropy energy, the
ferro- or antiferromagnetic nature of the exchange coupling between the
impurities, and the strength of the magnetic damping. In particular, the normal
modes of a dimer depend on the initial magnetic configuration, changing
drastically by going from a ferromagnetic metastable state to the
antiferromagnetic ground state. By taking into account the effect of the
damping into their resonant frequencies, we reveal that an important
contribution arises for strongly biaxial systems and specially for the
antiferromagnetic dimers with large exchange couplings. Counter intuitively,
our results indicate that the magnetic damping influences the quantum
fluctuations by decreasing the zero-point energy of the system.
|
1707.06087v2
|
2017-10-02
|
The gas and stellar mass of low-redshift damped Lyman-$α$ absorbers
|
We report Hubble Space Telescope Cosmic Origins Spectrograph far-ultraviolet
and Arecibo Telescope H{\sc i} 21cm spectroscopic studies of six damped and
sub-damped Lyman-$\alpha$ absorbers (DLAs and sub-DLAs, respectively) at $z
\lesssim 0.1$, that have yielded estimates of their H{\sc i} column density,
metallicity and atomic gas mass. This significantly increases the number of
DLAs with gas mass estimates, allowing the first comparison between the gas
masses of DLAs and local galaxies. Including three absorbers from the
literature, we obtain H{\sc i} masses $\approx (0.24 - 5.2) \times 10^9 \: {\rm
M}_\odot$, lower than the knee of the local H{\sc i} mass function. This
implies that massive galaxies do not dominate the absorption cross-section for
low-$z$ DLAs. We use Sloan Digital Sky Survey photometry and spectroscopy to
identify the likely hosts of four absorbers, obtaining low stellar masses,
$\approx 10^7-10^{8.7} M_\odot$, in all cases, consistent with the hosts being
dwarf galaxies. We obtain high H{\sc i} 21\,cm or CO emission line widths,
$\Delta V_{20} \approx 100-290$~km~s$^{-1}$, and high gas fractions, $f_{\rm
HI} \approx 5-100$, suggesting that the absorber hosts are gas-rich galaxies
with low star formation efficiencies. However, the H{\sc i} 21\,cm velocity
spreads ($\gtrsim 100$~km~s$^{-1}$) appear systematically larger than the
velocity spreads in typical dwarf galaxies.
|
1710.00710v1
|
2017-10-25
|
Two-Level System Damping in a Quasi-One-Dimensional Optomechanical Resonator
|
Nanomechanical resonators have demonstrated great potential for use as
versatile tools in a number of emerging quantum technologies. For such
applications, the performance of these systems is restricted by the decoherence
of their fragile quantum states, necessitating a thorough understanding of
their dissipative coupling to the surrounding environment. In bulk amorphous
solids, these dissipation channels are dominated at low temperatures by
parasitic coupling to intrinsic two-level system (TLS) defects, however, there
remains a disconnect between theory and experiment on how this damping
manifests in dimensionally-reduced nanomechanical resonators. Here, we present
an optomechanically-mediated thermal ringdown technique, which we use to
perform simultaneous measurements of the dissipation in four mechanical modes
of a cryogenically-cooled silicon nanoresonator, with resonant frequencies
ranging from 3 - 19 MHz. Analyzing the device's mechanical damping rate at
fridge temperatures between 10 mK - 10 K, we demonstrate quantitative agreement
with the standard tunneling model for TLS ensembles confined to one dimension.
From these fits, we extract the defect density of states ($P_0 \sim$ 1 - 4
$\times$ 10$^{44}$ J$^{-1}$ m$^{-3}$) and deformation potentials ($\gamma \sim$
1 - 2 eV), showing that each mechanical mode couples on average to less than a
single thermally-active defect at 10 mK.
|
1710.09439v3
|
2017-11-10
|
Vortex axisymmetrization, inviscid damping, and vorticity depletion in the linearized 2D Euler equations
|
Coherent vortices are often observed to persist for long times in turbulent
2D flows even at very high Reynolds numbers and are observed in experiments and
computer simulations to potentially be asymptotically stable in a weak sense
for the 2D Euler equations. We consider the incompressible 2D Euler equations
linearized around a radially symmetric, strictly monotone decreasing vorticity
distribution. For sufficiently regular data, we prove the inviscid damping of
the $\theta$-dependent radial and angular velocity fields with the optimal
rates $\|u^r(t)\| \lesssim \langle t \rangle^{-1}$ and $\|u^\theta(t)\|
\lesssim \langle t \rangle^{-2}$ in the appropriate radially weighted $L^2$
spaces. We moreover prove that the vorticity weakly converges back to radial
symmetry as $t \rightarrow \infty$, a phenomenon known as vortex
axisymmetrization in the physics literature, and characterize the dynamics in
higher Sobolev spaces. Furthermore, we prove that the $\theta$-dependent
angular Fourier modes in the vorticity are ejected from the origin as $t \to
\infty$, resulting in faster inviscid damping rates than those possible with
passive scalar evolution. This non-local effect is called vorticity depletion.
Our work appears to be the first to find vorticity depletion relevant for the
dynamics of vortices.
|
1711.03668v1
|
2017-11-15
|
Anomalous spin-orbit torque switching due to field-like torque-assisted domain wall reflection
|
Spin-orbit torques (SOT) allow the electrical control of magnetic states.
Current-induced SOT switching of the perpendicular magnetization is of
particular technological importance. The SOT consists of damping-like and
field-like torques so that the efficient SOT switching requires to understand
combined effects of the two torque-components. Previous quasi-static
measurements have reported an increased switching probability with the width of
current pulses, as predicted with considering the damping-like torque only.
Here we report a decreased switching probability at longer pulse-widths, based
on time-resolved measurements. Micromagnetic analysis reveals that this
anomalous SOT switching results from domain wall reflections at sample edges.
The domain wall reflection is found to strongly depend on the field-like torque
and its relative sign to the damping-like torque. Our result demonstrates a key
role of the field-like torque in the deterministic SOT switching and notifies
the importance of sign correlation of the two torque-components, which may shed
light on the SOT switching mechanism.
|
1711.05367v1
|
2017-11-24
|
Influence of surfactants on the electrohydrodynamic stretching of water drops in oil
|
In this paper we present experimental and numerical studies of the
electrohydrodynamic stretching of a sub-millimetre-sized salt water drop,
immersed in oil with added non-ionic surfactant, and subjected to a suddenly
applied electric field of magnitude approaching 1 kV/mm. By varying the drop
size, electric field strength and surfactant concentration we cover the whole
range of electric capillary numbers ($Ca_E$) from 0 up to the limit of drop
disintegration. The results are compared with the analytical result by Taylor
(1964) which predicts the asymptotic deformation as a function of $Ca_E$. We
find that the addition of surfactant damps the transient oscillations and that
the drops may be stretched slightly beyond the stability limit found by Taylor.
We proceed to study the damping of the oscillations, and show that increasing
the surfactant concentration has a dual effect of first increasing the damping
at low concentrations, and then increasing the asymptotic deformation at higher
concentrations. We explain this by comparing the Marangoni forces and the
interfacial tension as the drops deform. Finally, we have observed in the
experiments a significant hysteresis effect when drops in oil with large
concentration of surfactant are subjected to repeated deformations with
increasing electric field strengths. This effect is not attributable to the
flow nor the interfacial surfactant transport.
|
1711.08969v2
|
2017-11-30
|
Model-independent analysis of the DAMPE excess
|
The Dark Matter Particle Explorer (DAMPE) recently released measurements of
the electron spectrum with a hint of a narrow peak at about 1.4 TeV. We
investigate dark matter (DM) models that could produce such a signal by
annihilation in a nearby subhalo whilst simultaneously satisfying constraints
from DM searches. In our model-independent approach, we consider all
renormalizable interactions via a spin 0 or 1 mediator between spin 0 or 1/2 DM
particles and the Standard Model leptons. We find that of the 20 combinations,
10 are ruled out by velocity or helicity suppression of the annihilation cross
section to fermions. The remaining 10 models, though, evade constraints from
the relic density, collider and direct detection searches, and include models
of spin 0 and 1/2 DM coupling to a spin 0 or 1 mediator. We delineate the
regions of mediator mass and couplings that could explain the DAMPE excess. In
all cases the mediator is required to be heaver than about 2 TeV by LEP limits.
|
1711.11376v3
|
2017-12-07
|
Flavor Structure of the Cosmic-Ray Electron/Positron Excesses at DAMPE
|
The Dark Matter Particle Explorer (DAMPE) satellite detector announced its
first result for measuring the cosmic-ray electron/positron (CRE) energy
spectrum up to 4.6TeV, including a tentative peak-like event excess at
(1.3-1.5)TeV. In this work, we uncover a significant hidden excess in the DAMPE
CRE spectrum over the energy range (0.6-1.1)TeV, which has a non-peak-like
structure. We propose a new mechanism to explain this excess by a set of 1.5TeV
$\mu^\pm$ events with subsequent decays into $e^\pm$ plus neutrinos. For
explaining this new excess together with the peak excess around 1.4TeV, we
demonstrate that the {\it flavor structure} of the original lepton final-state
produced by dark matter (DM) annihilations (or other mechanism) should have a
composition ratio $N_e : (N_\mu +\frac{1}{6}N_\tau) = 1 : y$, with $y \simeq
2.6-10.8$. For lepton portal DM models, this puts important constraint on the
lepton-DM-mediator couplings $\lambda_e : (\lambda_\mu^4 +
\frac{1}{6}\lambda_\tau^4)^{\frac{1}{4}} = 1 : y^{\frac{1}{4}}$ with a narrow
range $y^{\frac{1}{4}} \simeq 1.3-1.8$.
|
1712.02744v3
|
2017-12-20
|
Unifying ultrafast demagnetization and intrinsic Gilbert damping in Co/Ni bilayers with electronic relaxation near the Fermi surface
|
The ability to controllably manipulate the laser-induced ultrafast magnetic
dynamics is a prerequisite for future high speed spintronic devices. The
optimization of devices requires the controllability of the ultrafast
demagnetization time, , and intrinsic Gilbert damping, . In previous attempts
to establish the relationship between and , the rare-earth doping of a
permalloy film with two different demagnetization mechanism is not a suitable
candidate. Here, we choose Co/Ni bilayers to investigate the relations between
and by means of time-resolved magneto-optical Kerr effect (TRMOKE) via
adjusting the thickness of the Ni layers, and obtain an approximately
proportional relation between these two parameters. The remarkable agreement
between TRMOKE experiment and the prediction of breathing Fermi-surface model
confirms that a large Elliott-Yafet spin-mixing parameter is relevant to the
strong spin-orbital coupling at the Co/Ni interface. More importantly, a
proportional relation between and in such metallic films or heterostructures
with electronic relaxation near Fermi surface suggests the local spin-flip
scattering domains the mechanism of ultrafast demagnetization, otherwise the
spin-current mechanism domains. It is an effective method to distinguish the
dominant contributions to ultrafast magnetic quenching in metallic
heterostructures by investigating both the ultrafast demagnetization time and
Gilbert damping simultaneously. Our work can open a novel avenue to manipulate
the magnitude and efficiency of Terahertz emission in metallic heterostructures
such as the perpendicular magnetic anisotropic Ta/Pt/Co/Ni/Pt/Ta multilayers,
and then it has an immediate implication of the design of high frequency
spintronic devices.
|
1712.07323v1
|
2017-12-22
|
Low-momentum dynamic structure factor of a strongly interacting Fermi gas at finite temperature: A two-fluid hydrodynamic description
|
We provide a description of the dynamic structure factor of a homogeneous
unitary Fermi gas at low momentum and low frequency, based on the dissipative
two-fluid hydrodynamic theory. The viscous relaxation time is estimated and is
used to determine the regime where the hydrodynamic theory is applicable and to
understand the nature of sound waves in the density response near the
superfluid phase transition. By collecting the best knowledge on the shear
viscosity and thermal conductivity known so far, we calculate the various
diffusion coefficients and obtain the damping width of the (first and second)
sounds. We find that the damping width of the first sound is greatly enhanced
across the superfluid transition and very close to the transition the second
sound might be resolved in the density response for the transferred momentum up
to the half of Fermi momentum. Our work is motivated by the recent measurement
of the local dynamic structure factor at low momentum at Swinburne University
of Technology and the on-going experiment on sound attenuation of a homogeneous
unitary Fermi gas at Massachusetts Institute of Technology. We discuss how the
measurement of the velocity and damping width of the sound modes in
low-momentum dynamic structure factor may lead to an improved determination of
the universal superfluid density, shear viscosity and thermal conductivity of a
unitary Fermi gas.
|
1712.08320v1
|
2018-01-15
|
Amplitude- and gas pressure-dependent nonlinear damping of high-Q oscillatory MEMS micro mirrors
|
Silicon-based micro-electromechanical systems (MEMS) can be fabricated using
bulk and surface micromachining technology. A micro mirror designed as an
oscillatory MEMS constitutes a prominent example. Typically, in order to
minimize energy consumption, the micro mirror is designed to have high quality
factors. In addition, a phase-locked loop guarantees resonant actuation despite
the occurrence of frequency shifts. In these cases, the oscillation amplitude
of the micro mirror is expected to scale linearly with the actuation input
power. Here, however, we report on an experimental observation which clearly
shows an amplitude depletion that is not in accordance with any linear
behaviour. As a consequence, the actuation forces needed to reach the desired
oscillation amplitude are by multiples higher than expected. We are able to
explain the experimental observations accurately by introducing a single
degree-of-freedom model including an amplitude-dependent nonlinear damping
term. Remarkably, we find that the nonlinear damping shows a clear gas pressure
dependency. We investigate the concepts and compare our findings on two
different micro mirror design layouts.
|
1801.04758v2
|
2018-01-30
|
Model Based Active Slosh Damping Experiment
|
This paper presents a model based experimental investigation to demonstrate
the usefulness of an active damping strategy to manage fluid sloshing motion in
spacecraft tanks. The active damping strategy is designed to reduce the
degrading impact on maneuvering and pointing performance via a force feedback
strategy. Many problems have been encountered until now, such as instability of
the closed loop system, excessive consumption in the attitude propellant or
problems for engine re-ignition in upper stages. Mostly, they have been
addressed in a passive way via the design of baffles and membranes, which on
their own have mass and constructive impacts. Active management of propellant
motion in launchers and satellites has the potential to increase performance on
various levels. This paper demonstrates active slosh management using force
feedback for the compensation of the slosh resonances. Force sensors between
tank and the carrying structure provide information of the fluid motion via the
reaction force. The control system is designed to generate an appropriate
acceleration profile that leads to desired attenuation profiles in amplitude,
frequency and time. Two robust control design methods, one based on $\mu$
design and the other on parametric structured design based on non-smooth
optimization of the worst-case $H_{\infty}$ norm, are applied. The controller
is first tested with a computational fluid dynamics simulation in the loop.
Finally a water tank mounted on a Hexapod with up to $1100$ liter is used to
evaluate the control performance. The paper illustrates that is possible to
actively influence sloshing via closed loop.
|
1801.10017v1
|
2018-03-22
|
Propagative and diffusive regimes of acoustic damping in bulk amorphous material
|
In amorphous solids, a non-negligible part of thermal conductivity results
from phonon scattering on the structural disorder. The conversion of acoustic
energy into thermal energy is often measured by the Dynamical Structure Factor
(DSF) thanks to inelastic neutron or X-Ray scattering. The DSF is used to
quantify the dispersion relation of phonons, together with their damping.
However, the connection of the dynamical structure factor with dynamical
attenuation of wave packets in glasses is still a matter of debate. We focus
here on the analysis of wave packets propagation in numerical models of
amorphous silicon. We show that the DHO fits (Damped Harmonic Oscillator model)
of the dynamical structure factors give a good estimate of the wave packets
mean-free path, only below the Ioffe-Regel limit. Above the Ioffe-Regel limit
and below the mobility edge, a pure diffusive regime without a definite mean
free path is observed. The high-frequency mobility edge is characteristic of a
transition to localized vibrations. Below the Ioffe-Regel criterion, a mixed
regime is evidenced at intermediate frequencies, with a coexistence of
propagative and diffusive wave fronts. The transition between these different
regimes is analyzed in details and reveals a complex dynamics for energy
transportation, thus raising the question of the correct modeling of thermal
transport in amorphous materials.
|
1803.08594v1
|
2018-04-11
|
Axial quasi-normal modes of neutron stars in $R^2$ gravity
|
In the present paper the axial quasi-normal modes of neutron stars in $f(R)$
gravity are examined using a large set of equations of state. The numerical
calculations are made using two different approaches -- performing time
evolution of the perturbation equations and solving the time-independent
representation of the equations as a boundary value problem. According to the
results the mode frequencies and the damping times decrease with the increase
of the free parameter of the theory in comparison to the pure general
relativistic case. While the frequencies deviate significantly from Einstein's
theory for all realistic neutron star masses (say above $1M_\odot$), the
damping times reach non-negligible differences only for the more massive
models. We have constructed as well universal (equation of state independent)
gravitational wave asteroseismology relations involving the frequencies and the
damping times. It turns out that the equation of state independence is
preserved using the same normalization as in pure general relativity and the
qualitative differences of the phenomenological relations with respect to
Einstein's theory of gravity can be large for large values of the free
parameter in $f(R)$ gravity.
|
1804.04060v1
|
2018-05-10
|
Dust modification of the plasma conductivity in the mesosphere
|
Relative transverse drift (with respect to the ambient magnetic field)
between the weakly magnetized electrons and the unmagnetized ions at the lower
altitude (80 km) and between the weakly magnetized ions and unmagnetized dust
at the higher altitude (90 km) gives rise to the finite Hall conductivity in
the Earth's mesosphere. If, on the other hand, the number of free electrons is
sparse in the mesosphere and most of the negative charge resides on the weakly
magnetized, fine, nanometre sized dust powder and positive charge on the more
massive, micron sized, unmagnetized dust, the sign of the Hall conductivity due
to their relative transverse drift will be opposite to the previous case. Thus
the sign of the Hall effect not only depends on the direction of the local
magnetic field but also on the nature of the charge carrier in the partially
ionized dusty medium.
As the Hall and the Ohm diffusion are comparable below 80 km, the low
frequency long wavelength waves will be damped at this altitude with the
damping rate typically of the order of few minutes. Therefore, the ultra--low
frequency magnetohydrodynamic waves can not originate below 80 km in the
mesosphere. However, above 80 km since Hall effect dominates Ohm diffusion the
mesosphere can host the ultra--low frequency waves which can propagate across
the ionosphere with little or, no damping.
|
1805.03799v1
|
2018-05-19
|
Migration of Planets Into and Out of Mean Motion Resonances in Protoplanetary Discs: Overstability of Capture and Nonlinear Eccentricity Damping
|
A number of multiplanet systems are observed to contain planets very close to
mean motion resonances, although there is no significant pileup of precise
resonance pairs. We present theoretical and numerical studies on the outcome of
capture into first-order mean motion resonances (MMRs) using a parametrized
planet migration model that takes into account nonlinear eccentricity damping
due to planet-disk interaction. This parametrization is based on numerical
hydrodynamical simulations and is more realistic than the simple linear
parametrization widely used in previous analytic studies. We find that
nonlinear eccentricity damping can significantly influence the stability and
outcome of resonance capture. In particular, the equilibrium eccentricity of
the planet captured into MMRs become larger, and the captured MMR state tends
to be more stable compared to the prediction based on the simple migration
model. In addition, when the migration is sufficiently fast or/and the planet
mass ratio is sufficiently small, we observe a novel phenomenon of eccentricity
overshoot, where the planet's eccentricity becomes very large before settling
down to the lower equilibrium value. This can lead to the ejection of the
smaller planet if its eccentricity approaches unity during the overshoot. This
may help explain the lack of low-mass planet companion of hot Jupiters when
compared to warm Jupiters.
|
1805.07501v1
|
2018-06-04
|
Density Waves and the Viscous Overstability in Saturn's Rings
|
This paper addresses resonantly forced spiral density waves in a dense
planetary ring which is close to the threshold for viscous overstability. We
solve numerically the hydrodynamical equations for a dense, axisymmetric thin
disk in the vicinity of an inner Lindblad resonance with a perturbing
satellite. The spiral shape of a density wave is taken into account through a
suitable approximation of the advective terms arising from the fluid orbital
motion. This paper is a first attempt to model the co-existence of resonantly
forced density waves and short-scale axisymmetric overstable wavetrains in
Saturn's rings by conducting large-scale hydrodynamical integrations. These
integrations reveal that the two wave types undergo complex interactions, not
taken into account in existing models for the damping of density waves. In
particular it is found that, depending on the relative magnitude of both wave
types, the presence of viscous overstability can lead to a damping of an
unstable density wave and vice versa. The damping of viscous overstability by a
density wave is investigated further by employing a simplified model of an
axisymmetric ring perturbed by a nearby Lindblad resonance. A linear
hydrodynamic stability analysis as well as local N-body simulations of this
model system are performed and support the results of our large-scale
hydrodynamical integrations.
|
1806.01211v3
|
2018-07-02
|
Thermoplasmonic behavior of semiconductor nanoparticles: A comparison with metals
|
A number of applications in nanoplasmonics utilize noble metals, gold (Au)
and silver (Ag), as the materials of choice. However, these materials suffer
from problems of poor thermal and chemical stability accompanied by significant
dissipative losses under high-temperature conditions. In this regard,
semiconductor nanoparticles have attracted attention with their promising
characteristics of highly tunable plasmonic resonances, low ohmic losses and
greater thermochemical stability. Here, we investigate the size-dependent
thermoplasmonic properties of semiconducting silicon and gallium arsenide
nanoparticles to compare them with metallic Au nanoparticles using Mie theory.
To this end, we employ experimentally estimated models of dielectric
permittivity in our computations. Among the various permittivity models for Au,
we further compare the Drude-Lorentz (DL) and the Drude and critical points
(DCP) models. Results show a redshift in the scattering and absorption
resonances for the DL model while the DCP model presents a blueshift. The
dissipative damping in the semiconductor nanoparticles is strongest for the
sharp electric octupole resonances followed by the quadrupole and dipole modes.
However, a reverse order with strongest values for the broad dipole resonance
is observed for the Au nanoparticles. A massive Drude broadening contributes
strongly to the damping of resonances in Au nanoparticles at elevated
temperatures. In contrast, the semiconductor nanoparticles do not exhibit any
significant deterioration in their scattering and absorption resonances at high
temperatures. In combination with low dissipative damping, this makes the
semiconductor nanoparticles better suited for high-temperature applications in
nanoplasmonics wherein the noble metals suffer from excessive heating.
|
1807.00881v1
|
2018-07-26
|
Aspherical deformations of the Choptuik spacetime
|
We perform dynamical and nonlinear numerical simulations to study critical
phenomena in the gravitational collapse of massless scalar fields in the
absence of spherical symmetry. We evolve axisymmetric sets of initial data and
examine the effects of deviation from spherical symmetry. For small deviations
we find values for the critical exponent and echoing period of the discretely
self-similar critical solution that agree well with established values;
moreover we find that such small deformations behave like damped oscillations
whose damping coefficient and oscillation frequencies are consistent with those
predicted in the linear perturbation calculations of Martin-Garcia and
Gundlach. However, we also find that the critical exponent and echoing period
appear to decrease with increasing departure from sphericity, and that, for
sufficiently large departures from spherical symmetry, the deviations become
unstable and grow, confirming earlier results by Choptuik et.al.. We find some
evidence that these growing modes lead to a bifurcation, similar to those
reported by Choptuik et.al., with two centers of collapse forming on the
symmetry axis above and below the origin. These findings suggest that nonlinear
perturbations of the critical solution lead to changes in the effective values
of the critical exponent, echoing period and damping coefficient, and may even
change the sign of the latter, so that perturbations that are stable in the
linear regime can become unstable in the nonlinear regime.
|
1807.10342v2
|
2018-08-03
|
Witnessing galaxy assembly at the edge of the reionization epoch
|
We report the discovery of Serenity-18, a galaxy at z=5.939 for which we
could measure the content of molecular gas, M(H_2)~ 5 x10^9 M_sun, traced by
the CO(6-5) emission, together with the metal-poor ([Fe/H]=-3.08 +- 0.12,
[Si/H]=-2.86 +- 0.14) gas clump/filament which is possibly feeding its growth.
The galaxy has an estimated star formation rate of ~100 M_sun yr^{-1}, implying
that it is a typical main sequence galaxy at these redshifts. The metal-poor
gas is detected through a damped Lyman-alpha absorber (DLA) observed at a
spatial separation of 40 kpc and at the same redshift of Serenity-18, along the
line of sight to the quasar SDSS J2310+1855 (z_em = 6.0025). The chemical
abundances measured for the damped Lyman-alpha system are in very good
agreement with those measured for other DLAs discovered at similar redshifts,
indicating an enrichment due to massive PopII stars. The galaxy/Damped system
we discovered is a direct observational evidence of the assembly of a galaxy at
the edge of the reionization epoch.
|
1808.01146v2
|
2018-08-13
|
Fluidization of collisionless plasma turbulence
|
In a collisionless, magnetized plasma, particles may stream freely along
magnetic-field lines, leading to phase "mixing" of their distribution function
and consequently to smoothing out of any "compressive" fluctuations (of
density, pressure, etc.,). This rapid mixing underlies Landau damping of these
fluctuations in a quiescent plasma-one of the most fundamental physical
phenomena that make plasma different from a conventional fluid. Nevertheless,
broad power-law spectra of compressive fluctuations are observed in turbulent
astrophysical plasmas (most vividly, in the solar wind) under conditions
conducive to strong Landau damping. Elsewhere in nature, such spectra are
normally associated with fluid turbulence, where energy cannot be dissipated in
the inertial scale range and is therefore cascaded from large scales to small.
By direct numerical simulations and theoretical arguments, it is shown here
that turbulence of compressive fluctuations in collisionless plasmas strongly
resembles one in a collisional fluid and does have broad power-law spectra.
This "fluidization" of collisionless plasmas occurs because phase mixing is
strongly suppressed on average by "stochastic echoes", arising due to nonlinear
advection of the particle distribution by turbulent motions. Besides resolving
the long-standing puzzle of observed compressive fluctuations in the solar
wind, our results suggest a conceptual shift for understanding kinetic plasma
turbulence generally: rather than being a system where Landau damping plays the
role of dissipation, a collisionless plasma is effectively dissipationless
except at very small scales. The universality of "fluid" turbulence physics is
thus reaffirmed even for a kinetic, collisionless system.
|
1808.04284v1
|
2018-08-15
|
Neural Material: Learning Elastic Constitutive Material and Damping Models from Sparse Data
|
The accuracy and fidelity of deformation simulations are highly dependent
upon the underlying constitutive material model. Commonly used linear or
nonlinear constitutive material models only cover a tiny part of possible
material behavior. In this work we propose a unified framework for modeling
deformable material. The key idea is to use a neural network to correct a
nominal model of the elastic and damping properties of the object. The neural
network encapsulates a complex function that is hard to explicitly model. It
injects force corrections that help the forward simulation to more accurately
predict the true behavior of a given soft object, which includes non-linear
elastic forces and damping. Attempting to satisfy the requirement from real
material interference and animation design scenarios, we learn material models
from examples of dynamic behavior of a deformable object's surface. The
challenge is that such data is sparse as it is consistently given only on part
of the surface. Sparse reduced space-time optimization is employed to gradually
generate increasingly accurate training data, which further refines and
enhances the neural network. We evaluate our choice of network architecture and
show evidence that the modest amount of training data we use is suitable for
the problem tackled. Our method is demonstrated with a set of synthetic
examples.
|
1808.04931v1
|
2018-09-06
|
Forming Gliese 876 Through Smooth Disk Migration
|
We run a suite of dissipative N-body simulations to determine which regions
of phase space for smooth disk migration are consistent with the GJ876 system,
an M-dwarf hosting three planets orbiting in a chaotic 4:2:1 Laplace resonance.
We adopt adaptive mesh refinement (AMR) methods which are commonly used in
hydrodynamical simulations to efficiently explore the parameter space defined
by the semi-major axis and eccentricity damping timescales. We find that there
is a large region of phase space which produces systems in the chaotic Laplace
resonance and a smaller region consistent with the observed eccentricities and
libration amplitudes for the resonant angles. Under the assumptions of Type I
migration for the outer planet, we translate these damping timescales into
constraints on the protoplanetary disk surface density and thickness. When we
strongly (weakly) damp the eccentricities of the inner two Laplace planets,
these timescales correspond to disk surface densities around ten thousand (a
few hundred) grams per square centimeter and disk aspect ratios between 1-10%.
Additionally, smooth migration produces systems with a range of chaotic
timescales, from decades and centuries to upwards of thousands of years. In
agreement with previous studies, the less chaotic regions of phase space
coincide with the system being in a low energy double apsidal corotation
resonance. Our detailed modeling of multi-planetary systems coupled with our
AMR exploration method enhances our ability to map out the parameter space of
planet formation models, and is well suited to study other resonant chain
systems such as Trappist-1, Kepler-60, and others.
|
1809.02200v2
|
2018-09-15
|
New closures for more precise modeling of Landau damping in the fluid framework
|
Incorporation of kinetic effects such as Landau damping into a fluid
framework was pioneered by Hammett and Perkins PRL 1990, by obtaining closures
of the fluid hierarchy, where the gyrotropic heat flux fluctuations or the
deviation of the 4th-order gyrotropic fluid moment, are expressed through
lower-order fluid moments. To obtain a closure of a fluid model expanded around
a bi-Maxwellian distribution function, the usual plasma dispersion function
$Z(\zeta)$ that appears in kinetic theory or the associated plasma response
function $R(\zeta)=1 + \zeta Z(\zeta)$, have to be approximated with a suitable
Pad\'e approximant in such a way, that the closure is valid for all $\zeta$
values. Such closures are rare, and the original closures of Hammett and
Perkins are often employed. Here we present a complete mapping of all plausible
Landau fluid closures that can be constructed at the level of 4th-order moments
in the gyrotropic limit and we identify the most precise closures. Furthermore,
by considering 1D closures at higher-order moments, we show that it is possible
to reproduce linear Landau damping in the fluid framework to any desired
precision, thus showing convergence of the fluid and collisionless kinetic
descriptions.
|
1809.05718v1
|
2018-10-04
|
Sub-photospheric turbulence as a heating mechanism in gamma-ray bursts
|
We examine the possible role of turbulence in feeding the emission of
gamma-ray bursts (GRBs). Turbulence may develop in a GRB jet as the result of
hydrodynamic or current-driven instabilities. The jet carries dense radiation
and the turbulence cascade can be damped by Compton drag, passing kinetic fluid
energy to photons through scattering. We identify two regimes of turbulence
dissipation: (1) "Viscous" - the turbulence cascade is Compton damped on a
scale $\ell_{\rm damp}$ greater than the photon mean free path $\ell_\star$.
Then turbulence energy is passed to photons via bulk Comptonization by smooth
shear flows on scale $\ell_\star<\ell_{\rm damp}$. (2) "Collisionless" - the
cascade avoids Compton damping and extends to microscopic plasma scales much
smaller than $\ell_\star$. The collisionless dissipation energizes plasma
particles, which radiate the received energy; how the dissipated power is
partitioned between particles needs further investigation with kinetic
simulations. We show that the dissipation regime switches from viscous to
collisionless during the jet expansion, at a critical value of the jet optical
depth which depends on the amplitude of turbulence. Turbulent GRB jets are
expected to emit nonthermal photospheric radiation. Our analysis also suggests
revisions of turbulent Comptonization in black hole accretion disks discussed
in previous works.
|
1810.02228v1
|
2018-10-15
|
Zombie Vortex Instability. III. Persistence with Nonuniform Stratification and Radiative Damping
|
The Zombie Vortex Instability (ZVI) occurs in the dead zones of
protoplanetary disks (PPDs) where perturbations excite baroclinic critical
layers, generating "zombie" vortices and turbulence. In this work, we
investigate ZVI with nonuniform vertical stratification; while ZVI is triggered
in the stratified regions away from the midplane, the subsequent turbulence
propagates into and fills the midplane. ZVI turbulence alters the background
Keplerian shear flow, creating a steady-state zonal flow. Intermittency is
observed, where the flow cycles through near-laminar phases of zonal flow
punctuated by chaotic bursts of new vortices. ZVI persists in the presence of
radiative damping, as long as the thermal relaxation timescale is more than a
few orbital periods. We refute the premature claim by Lesur & Latter (2016)
that radiative damping inhibits ZVI for disk radii r>0.3 au. Their conclusions
were based on unrealistically short cooling times using opacities with
virtually no grain growth. We explore different grain growth and vertical
settling scenarios, and find that the gas and dust in off-midplane regions are
not necessarily in local thermodynamic equilibrium (LTE) with each other. In
such cases, thermal relaxation timescales can be orders of magnitude longer
than the optically thin cooling times assuming LTE because of the finite time
for energy to be exchanged between gas and dust grains via collisions. With
minimal amounts of grain growth and dust settling, the off-midplane regions of
disks are susceptible to ZVI and much of the planet-forming regions can be
filled with zombie vortices and turbulence.
|
1810.06588v1
|
2018-09-21
|
High performance passive vibration isolation system for optical tables using six-degree-of-freedom viscous damping combined with steel springs
|
Mechanical vibrations in buildings are ubiquitous. Such vibrations limit the
performance of sensitive instruments used, for example, for high-precision
manufacturing, nanofabrication, metrology, medical systems, or microscopy. For
improved precision, instruments and optical tables need to be isolated from
mechanical vibrations. However, common active or passive vibration isolation
systems often perform poorly when low-frequency vibration isolation is required
or are expensive. Furthermore, a simple solution such as suspension from common
bungee cords may require high ceilings. Here we developed a vibration isolation
system that uses steel springs to suspend an optical table from a common-height
ceiling. The system was designed for a fundamental resonance frequency of 0.5
Hz. Resonances and vibrations were efficiently damped in all translational and
rotational degrees of freedom of the optical table by spheres, which were
mounted underneath the table and immersed in a highly viscous silicone oil. Our
low-cost, passive system outperformed several state-of-the-art passive and
active systems in particular in the frequency range between 1-10 Hz. We
attribute this performance to a minimal coupling between the degrees of freedom
and the truly three dimensional viscous damping combined with a nonlinear
hydrodynamic finite-size effect. Furthermore, the system can be adapted to
different loads, resonance frequencies, and dimensions. In the long term, the
excellent performance of the system will allow high-precision measurements for
many different instruments.
|
1810.06641v4
|
2018-10-17
|
Resonance-broadened transit time damping of particles in MHD turbulence
|
As a fundamental astrophysical process, the scattering of particles by
turbulent magnetic fields has its physical foundation laid by the
magnetohydrodynamic (MHD) turbulence theory. In the framework of the modern
theory of MHD turbulence, we derive a generalized broadened resonance function
by taking into account both the magnetic fluctuations and nonlinear
decorrelation of turbulent magnetic fields arising in MHD turbulence, and we
specify the energy range of particles for the dominance of different broadening
mechanisms. The broadened resonance allows for scattering of particles beyond
the energy threshold of the linear resonance. By analytically determining the
pitch-angle diffusion coefficients for transit time damping (TTD) with slow and
fast modes, we demonstrate that the turbulence anisotropy of slow modes
suppresses their scattering efficiency. Furthermore, we quantify the dependence
of the relative importance between slow and fast modes in TTD scattering on (i)
particle energy, (ii) plasma $\beta$ (the ratio of gas pressure to magnetic
pressure), and (iii) damping of MHD turbulence, and we also provide the
parameter space for the dominance of slow modes. To exemplify its applications,
we find that among typical partially ionized interstellar phases, in the warm
neutral medium slow and fast modes have comparable efficiencies in TTD
scattering of cosmic rays. For low-energy particles, e.g., sub-Alfv\'{e}nic
charged grains, we show that slow modes always dominate TTD scattering.
|
1810.07726v1
|
2018-10-23
|
Calibration of the DAMPE Plastic Scintillator Detector and its on-orbit performance
|
DArk Matter Particle Explorer (DAMPE) is a space-borne apparatus for
detecting the high-energy cosmic-rays like electrons, $\gamma$-rays, protons
and heavy-ions. Plastic Scintillator Detector (PSD) is the top-most
sub-detector of the DAMPE. The PSD is designed to measure the charge of
incident high-energy particles and it also serves as a veto detector for
discriminating $\gamma$-rays from charged particles. In this paper, PSD
on-orbit calibration procedure is described, which includes five steps of
pedestal, dynode correlation, response to minimum-ionizing particles (MIPs),
light attenuation function and energy reconstruction. A method for
reconstructing the charge of incident high energy cosmic-ray particles is
introduced. The detection efficiency of each PSD strip is verified to be above
99.5%, the total efficiency of the PSD for charged particles is above 99.99%.
|
1810.09901v1
|
2018-10-25
|
Charge Measurement of Cosmic Ray Nuclei with the Plastic Scintillator Detector of DAMPE
|
One of the main purposes of the DArk Matter Particle Explorer (DAMPE) is to
measure the cosmic ray nuclei up to several tens of TeV or beyond, whose origin
and propagation remains a hot topic in astrophysics. The Plastic Scintillator
Detector (PSD) on top of DAMPE is designed to measure the charges of cosmic ray
nuclei from H to Fe and serves as a veto detector for discriminating gamma-rays
from charged particles. We propose in this paper a charge reconstruction
procedure to optimize the PSD performance in charge measurement. Essentials of
our approach, including track finding, alignment of PSD, light attenuation
correction, quenching and equalization correction are described detailedly in
this paper after a brief description of the structure and operational principle
of the PSD. Our results show that the PSD works very well and almost all the
elements in cosmic rays from H to Fe are clearly identified in the charge
spectrum.
|
1810.10784v1
|
2018-11-14
|
Anderson-Bogoliubov and Carlson-Goldman modes in counterflow superconductors: Case study of a double monolayer graphene
|
The impact of electron-hole pairing on the spectrum of plasma excitations in
double layer systems is investigated. The theory is developed with reference to
a double monolayer graphene. Taking into account the coupling of scalar
potential oscillations with oscillations of the order parameter $\Delta$, we
show that the spectrum of antisymmetric (acoustic) plasma excitations contains
two modes: a weakly damped mode below the gap $2\Delta$ and a strongly damped
mode above the gap. The lower mode can be interpreted as an analog of the
Carlson-Goldman mode. This mode has an acoustic dispersion relation at small
wave vectors and it saturates at the level $2\Delta$ at large wave vectors. Its
velocity is larger than the velocity of the Anderson-Bogoliubov mode
$v_{AB}=v_F$/$\sqrt{2}$, and it can be smaller than the Fermi velocity $v_F$.
The damping rate of this mode strongly increases under increase of temperature.
Out-of-phase oscillations of two order parameters in two spin subsystems are
also considered. This part of the spectrum contains two more modes. One of them
is interpreted as an analog of the Anderson-Bogoliubov (phase) mode and the
other, as an analog of the Schmid (amplitude) mode. With minor modifications
the theory can be extended to describe collective modes in a double bilayer
graphene as well.
|
1811.05899v3
|
2018-12-07
|
Magnetic Braking and Damping of Differential Rotation in Massive Stars
|
Fragmentation of highly differentially rotating massive stars that undergo
collapse has been suggested as a possible channel for binary black hole
formation. Such a scenario could explain the formation of the new population of
massive black holes detected by the LIGO/VIRGO gravitational wave laser
interferometers. We probe that scenario by performing general relativistic
magnetohydrodynamic simulations of differentially rotating massive stars
supported by thermal radiation pressure plus a gas pressure perturbation. The
stars are initially threaded by a dynamically weak, poloidal magnetic field
confined to the stellar interior. We find that magnetic braking and turbulent
viscous damping via magnetic winding and the magnetorotational instability in
the bulk of the star redistribute angular momentum, damp differential rotation
and induce the formation of a massive and nearly uniformly rotating inner core
surrounded by a Keplerian envelope. The core + disk configuration evolves on a
secular timescale and remains in quasi-stationary equilibrium until the
termination of our simulations. Our results suggest that the high degree of
differential rotation required for $m=2$ seed density perturbations to trigger
gas fragmentation and binary black hole formation is likely to be suppressed
during the normal lifetime of the star prior to evolving to the point of
dynamical instability to collapse. Other cataclysmic events, such as stellar
mergers leading to collapse, may therefore be necessary to reestablish
sufficient differential rotation and density perturbations to drive
nonaxisymmetric modes leading to binary black hole formation.
|
1812.03176v3
|
2018-12-18
|
Inferring physical parameters in solar prominence threads
|
High resolution observations have permitted to resolve the solar
prominences/filaments as sets of threads/fibrils. However, the values of the
physical parameters of these threads and their structuring remain poorly
constrained. We use prominence seismology techniques to analyse transverse
oscillations in threads through the comparison between magnetohydrodynamic
(MHD) models and observations. We apply Bayesian methods to obtain two
different types of information. We first infer the marginal posterior
distribution of physical parameters, such as the magnetic field strength or the
length of the thread, when a totally filled tube, a partially filled tube, and
three damping models (resonant absorption in the Alfv\'en continuum, resonant
absorption in the slow continuum, and Cowling's diffusion) are considered as
certain. Then, we compare the relative plausibility between alternative MHD
models by computing the Bayes factors. Well constrained probability density
distributions can be obtained for the magnetic field strength, the length of
the thread, the density contrast, and parameters associated to damping models.
When comparing the damping models of resonant absorption in the Alfv\'en
continuum, resonant absorption in the slow continuum and Cowling's diffusion
due to partial ionisation of prominence plasma, the resonant absorption in the
Alfv\'en continuum is the most plausible mechanism in explaining the existing
observations. Relations between periods of fundamental and first overtone kink
modes with values around 1 are better explained by expressions of the period
ratio in the long thread approximation, while the rest of the values are more
probable in the short thread limit for the period ratio. Our results show that
Bayesian analysis offers valuable methods for performing parameter inference
and model comparison in the context of prominence seismology.
|
1812.07262v1
|
2019-01-07
|
Abnormal anti-crossing effect in photon-magnon coupling
|
We report the experimental demonstration of an abnormal, opposite
anti-crossing effect in a photon-magnon-coupled system that consists of an
Yttrium Iron Garnet film and an inverted pattern of split-ring resonator
structure (noted as ISRR) in a planar geometry. It is found that the normal
shape of anti-crossing dispersion typically observed in photon-magnon coupling
is changed to its opposite anti-crossing shape just by changing the
position/orientation of the ISRR's split gap with respect to the microstrip
line axis along which ac microwave currents are applied. Characteristic
features of the opposite anti-crossing dispersion and its linewidth evolution
are analyzed with the help of analytical derivations based on electromagnetic
interactions. The observed opposite anti-crossing dispersion is ascribed to the
compensation of both intrinsic damping and coupling-induced damping in the
magnon modes. This compensation is achievable by controlling the relative
strength and phase of oscillating magnetic fields generated from the ISRR's
split gap and the microstrip feeding line. The position/orientation of an
ISRR's split gap provides a robust means of controlling the dispersion shape of
anti-crossing and its damping in a photon-magnon coupling, thereby offering
more opportunity for advanced designs of microwave devices.
|
1901.01729v2
|
2019-01-24
|
A compact actively damped vibration isolation platform for optical experiments in ultra-high vacuum
|
We present a tabletop six-axis vibration isolation system, compatible with
Ultra-High Vacuum (UHV), which is actively damped and provides 25 dB of
isolation at 10 Hz and 65 dB at 100 Hz. While this isolation platform has been
primarily designed to support optics in the Laser Interferometer
Gravitational-Wave Observatory (LIGO) detectors, it is suitable for a variety
of applications. The system has been engineered to facilitate the construction
and assembly process, while minimizing cost. The platform provides passive
isolation for six degrees of freedom using a combination of vertical springs
and horizontal pendula. It is instrumented with voice-coil actuators and
optical shadow sensors to damp the resonances. All materials are compatible
with stringent vacuum requirements. Thanks to its architecture, the system's
footprint can be adapted to meet spatial requirements, while maximizing the
dimensions of the optical table. Three units are currently operating for LIGO.
We present the design of the system, controls principle, and experimental
results.
|
1901.09666v2
|
2019-01-28
|
Strong damping-like spin-orbit torque and tunable Dzyaloshinskii-Moriya interaction generated by low-resistivity Pd$_{1-x}$Pt$_x$ alloys
|
Despite their great promise for providing a pathway for very efficient and
fast manipulation of magnetization at the nanoscale, spin-orbit torque (SOT)
operations are currently energy inefficient due to a low damping-like SOT
efficiency per unit current bias, and/or the very high resistivity of the spin
Hall materials. Here, we report an advantageous spin Hall material, Pd1-xPtx,
which combines a low resistivity with a giant spin Hall effect as evidenced
through the use of three independent SOT ferromagnetic detectors. The optimal
Pd0.25Pt0.75 alloy has a giant internal spin Hall ratio of >0.47 (damping-like
SOT efficiency of ~ 0.26 for all three ferromagnets) and a low resistivity of
~57.5 {\mu}{\Omega} cm at 4 nm thickness. Moreover, we find the
Dzyaloshinskii-Moriya interaction (DMI), the key ingredient for the
manipulation of chiral spin arrangements (e.g. magnetic skyrmions and chiral
domain walls), is considerably strong at the Pd1-xPtx/Fe0.6Co0.2B0.2 interface
when compared to that at Ta/Fe0.6Co0.2B0.2 or W/Fe0.6Co0.2B0.2 interfaces and
can be tuned by a factor of 5 through control of the interfacial spin-orbital
coupling via the heavy metal composition. This work establishes a very
effective spin current generator that combines a notably high energy efficiency
with a very strong and tunable DMI for advanced chiral spintronics and spin
torque applications.
|
1901.09954v1
|
2019-02-13
|
Two-mediator dark matter models and cosmic electron excess
|
The cosmic electron energy spectrum recently observed by the DAMPE experiment
exhibits two interesting features, including a break around 0.9 TeV and a sharp
resonance near 1.4 TeV. In this analysis, we propose a dark matter explanation
to both exotic features seen by DAMPE. In our model, dark matter annihilates in
the galaxy via two different channels that lead to both a narrow resonance
spectrum near 1.4 TeV and electron excess events over an extended energy range
thus generating the break structure around TeV. The two annihilation channels
are mediated by two gauge bosons that interact both with dark matter and with
the standard model fermions. Dark matter annihilations through the s-channel
process mediated by the heavier boson produce monoenergetic electron-positron
pairs leading to the resonance excess. The lighter boson has a mass smaller
than the dark matter such that they can be on-shell produced in dark matter
annihilations in the galaxy; the lighter bosons in the final state subsequently
decay to generate the extended excess events due to the smeared electron energy
spectrum in this process. We further analyze constraints from various
experiments, including HESS, Fermi, AMS, and LHC, to the parameter space of the
model where both excess events can be accounted for. In order to interpret the
two new features in the DAMPE data, dark matter annihilation cross sections in
the current galaxy are typically much larger than the canonical thermal cross
section needed for the correct dark matter relic abundance. This discrepancy,
however, is remedied by the nonperturbative Sommerfeld enhancement because of
the existence of a lighter mediator in the model.
|
1902.04916v1
|
2019-02-18
|
Coherent control of magnon radiative damping with local photon states
|
The collective excitation of ordered spins, known as spin waves or magnons,
can in principle radiate by emitting travelling photons to an open system when
decaying to the ground state. However, in contrast to the electric dipoles,
magnetic dipoles contributed by magnons are more isolated from electromagnetic
environment with negligible radiation in the vacuum, limiting their application
in coherent communication by photons. Recently, strong interaction between
cavity standing-wave photons and magnons has been reported, indicating the
possible manipulation of magnon radiation via tailoring photon states. Here,
with loading an yttrium iron garnet sphere in a one-dimensional circular
waveguide cavity in the presence of both travelling and standing photon modes,
we demonstrate an efficient photon emissions from magnon and a significant
magnon radiative damping with radiation rate found to be proportional to the
local density of states (LDOS) of photon. By modulating the LDOS including its
magnitude and/or polarization, we can flexibly tune the photon emission and
magnon radiative damping on demand. Our findings provide a general way in
manipulating photon emission from magnon radiation for harnessing energy and
angular momentum generation, transfer and storage modulated by magnon in the
cavity and waveguide electrodynamics.
|
1902.06795v2
|
2019-03-04
|
Quantum speed limit time for the damped Jaynes-Cummings and Ohmic-like dephasing models in Schwarzschild spacetime
|
Quantum theory sets the bound on the minimal evolution time between initial
and final states of the quantum system. This minimal evolution time can be used
to specify the maximal speed of the evolution in open and closed quantum
systems. Quantum speed limit is one of the interesting issue in the theory of
open quantum systems. One may investigate the influence of the relativistic
effect on the quantum speed limit time. When several observers are placed in
different inertial or non-inertial frames, or in Schwarzschild space-time, the
relativistic effect should be taken into account. In this work, the quantum
speed limit time in Schwarzschild space-time will be studied for two various
model consist of damped Jaynes-Cummings and Ohmic-like dephasing. First, it
will be observed that how quantum coherence is affected by Hawking radiation.
According to the dependence of quantum speed limit time on quantum coherence
and the dependence of quantum coherence on relative distance of quantum system
to event horizon $R_{0}$, it will be represented that the quantum speed limit
time in Schwarzschild space-time is decreased by increasing $R_{0}$ for damped
Jaynes-Cummings model and conversely, It is increased by increasing $R_{0}$ for
Ohmic-like dephasing model .
|
1903.01230v2
|
2019-03-07
|
Non-linear diffusion of cosmic rays escaping from supernova remnants - II. Hot ionized media
|
We study the problem of the escape and transport of Cosmic-Rays (CR) from a
source embedded in a fully ionised, hot phase of the interstellar medium (HIM).
In particular, we model the CR escape and their propagation in the source
vicinity taking into account excitation of Alfv\'enic turbulence by CR
streaming and mechanisms damping the self-excited turbulence itself. Our
estimates of escape radii and times result in large values (100 pc,
$2\times10^5$ yr) for particle energies $\lesssim20$ GeV and smaller values for
particles with increasing energies (35 pc and 14 kyr at 1 TeV). These escape
times and radii, when used as initial conditions for the CR propagation outside
the source, result in relevant suppression of the diffusion coefficient (by a
factor 5-10) on time-scales comparable with their (energy dependent) escape
time-scale. The damping mechanisms are fast enough that even on shorter time
scales the Alfv\'enic turbulence is efficiently damped, and the ratio between
random and ordered component of the magnetic field is $\delta B/B_0\ll 1$,
justifying the use of quasi-linear theory. In spite of the suppressed diffusion
coefficient, and then the increased residence time in the vicinity (<200 pc) of
their source, the grammage accumulated by CRs after their escape is found to be
negligible (at all energies) as compared to the one accumulated while diffusing
in the whole Galaxy, due to the low density of the HIM.
|
1903.03193v1
|
2019-03-25
|
Stabilised Asynchronous Fast Adaptive Composite Multigrid using Additive Damping
|
Multigrid solvers face multiple challenges on parallel computers. Two
fundamental ones read as follows: Multiplicative solvers issue coarse grid
solves which exhibit low concurrency and many multigrid implementations suffer
from an expensive coarse grid identification phase plus adaptive mesh
refinement overhead. We propose a new additive multigrid variant for
spacetrees, i.e. meshes as they are constructed from octrees and quadtrees: It
is an additive scheme, i.e. all multigrid resolution levels are updated
concurrently. This ensures a high concurrency level, while the transfer
operators between the mesh levels can still be constructed algebraically. The
novel flavour of the additive scheme is an augmentation of the solver with an
additive, auxiliary damping parameter per grid level per vertex that is in turn
constructed through the next coarser level---an idea which utilises smoothed
aggregation principles or the motivation behind AFACx: Per level, we solve an
additional equation whose purpose is to damp too aggressive solution updates
per vertex which would otherwise, in combination with all the other levels,
yield an overcorrection and, eventually, oscillations. This additional equation
is constructed additively as well, i.e. is once more solved concurrently to all
other equations. This yields improved stability, closer to what is seen with
multiplicative schemes, while pipelining techniques help us to write down the
additive solver with single-touch semantics for dynamically adaptive meshes.
|
1903.10367v3
|
2019-04-04
|
The DAMPE excess and gamma-ray constraints
|
The direct measurements of the cosmic electron-positron spectrum around 1 TeV
made by DAMPE have induced many theoretical speculations about possible
excesses in the data above the standard astrophysical predictions that might
have the dark matter (DM) origin. These attempts mainly fall into two
categories: i) DM annihilation (or decay) in the Galactic halo producing the
broad spectrum excess; ii) DM annihilation in the nearby compact subhalo
producing the sharp peak at 1.4 TeV. We investigate the gamma-ray emission
accompanying $e^+e^-$ production in DM annihilation, as well as various
theoretical means to suppress the prompt radiation, such as specific
interaction vertices or multi-cascade modes, and conclude that these attempts
are in tension with various gamma-ray observations. We show that the DM
explanations of the broad spectrum excess tend to contradict the diffuse
isotropic gamma-ray background (IGRB), measured by Fermi-LAT, while the nearby
subhalo scenario is constrained by nonobservation in the surveys, performed by
Fermi-LAT, MAGIC and HESS. We also briefly review other types of gamma-ray
constraints, which seem to rule out the DM interpretations of the DAMPE broad
spectrum excess as well.
|
1904.02456v2
|
2019-04-10
|
Stochastic nonlinear wave dynamics on compact surfaces
|
We study the Cauchy problem for the nonlinear wave equations (NLW) with
random data and/or stochastic forcing on a two-dimensional compact Riemannian
manifold without boundary. (i) We first study the defocusing stochastic damped
NLW driven by additive space-time white-noise, and with initial data
distributed according to the Gibbs measure. By introducing a suitable
space-dependent renormalization, we prove local well-posedness of the
renormalized equation. Bourgain's invariant measure argument then allows us to
establish almost sure global well-posedness and invariance of the Gibbs measure
for the renormalized stochastic damped NLW. (ii) Similarly, we study the random
data defocusing NLW (without stochastic forcing), and establish the same
results as in the previous setting. (iii) Lastly, we study the stochastic NLW
without damping. By introducing a space-time dependent renormalization, we
prove its local well-posedness with deterministic initial data in all
subcritical spaces.
These results extend the corresponding recent results on the two-dimensional
torus obtained by (i) Gubinelli-Koch-Oh-Tolomeo (2018), (ii) Oh-Thomann (2017),
and (iii) Gubinelli-Koch-Oh (2018), to a general class of compact manifolds.
The main ingredient is the Green's function estimate for the Laplace-Beltrami
operator in this setting to study regularity properties of stochastic terms
appearing in each of the problems.
|
1904.05277v3
|
2019-04-30
|
Damping rates and frequency corrections of Kepler LEGACY stars
|
Linear damping rates and modal frequency corrections of radial oscillation
modes in selected LEGACY main-sequence stars are estimated by means of a
nonadiabatic stability analysis. The selected stellar sample covers stars
observed by Kepler with a large range of surface temperatures and surface
gravities. A nonlocal, time-dependent convection model is perturbed to assess
stability against pulsation modes. The mixing-length parameter is calibrated to
the surface-convection-zone depth of a stellar model obtained from fitting
adiabatic frequencies to the LEGACY observations, and two of the nonlocal
convection parameters are calibrated to the corresponding LEGACY linewidth
measurements. The remaining nonlocal convection parameters in the 1D
calculations are calibrated so as to reproduce profiles of turbulent pressure
and of the anisotropy of the turbulent velocity field of corresponding 3D
hydrodynamical simulations. The atmospheric structure in the 1D stability
analysis adopts a temperature-optical-depth relation derived from 3D
hydrodynamical simulations. Despite the small number of parameters to adjust,
we find good agreement with detailed shapes of both turbulent pressure profiles
and anisotropy profiles with depth, and with damping rates as a function of
frequency. Furthermore, we find the absolute modal frequency corrections,
relative to a standard adiabatic pulsation calculation, to increase with
surface temperature and surface gravity.
|
1904.13170v1
|
2019-05-09
|
An excess of excesses examined via dark matter radio emissions from galaxies
|
Cosmic-ray and gamma-ray observations have yielded several notable excesses
that often lend themselves to explanation by various dark matter
annihilation/decay models. In particular, the AMS-02 anti-proton and positron
excesses have continued to grow more robust with the collection of more data.
This is supplemented by gamma-ray excesses in the Galactic Centre and a
high-energy break in spectrum of electron/positron cosmic rays seen by DAMPE.
In this work we carefully model the magnetic field environments of M31 and M33
and use this to estimate expected synchrotron emissions from electrons produced
via dark matter annihilation. By comparing this to available radio data we
review simplifying assumptions used previously for dark matter hunting in these
environments and produce novel constraints that are capable of fully ruling out
dark matter models proposed to accommodate all the aforementioned excesses
barring that of DAMPE. However, we do show that significant constraints can be
placed upon the DAMPE parameter space with M31 data. In addition to this we
project SKA non-observation constraints for the Reticulum II and Triangulum II
dwarf galaxies and find these have potential to rule out cosmic-ray and
gamma-ray excess-producing models of dark matter, even when the most
conservative assumptions are employed.
|
1905.05599v5
|
2019-05-17
|
Statics and Dynamics of Polymeric Droplets on Chemically Homogeneous and Heterogeneous Substrates
|
We present a molecular dynamics study of the motion of cylindrical polymer
droplets on striped surfaces. We first consider the equilibrium properties of
droplets on different surfaces, we show that for small stripes the
Cassie-Baxter equation gives a good approximation of the equilibrium contact
angle. As the stripe width becomes non-negligible compared to the dimension of
the droplets, the droplet has to deform significantly to minimize its free
energy, this results in a smaller value of the contact angle than the continuum
model predicts. We then evaluate the slip length, and thus the damping
coefficient as a function of the stripe width. For very small stripes, the
heterogeneous surface behaves as an effective surface, with the same damping as
an homogeneous surface with the same contact angle. However, as the stripe
width increases, damping at the surface increases until reaching a plateau.
Afterwards, we study the dynamics of droplets under a bulk force. We show that
if the stripes are large enough the droplets are pinned until a critical
acceleration. The critical acceleration increases linearly with stripe width.
For large enough accelerations, the average velocity increases linearly with
the acceleration, we show that it can then be predicted by a model depending
only the size of droplet, viscosity and slip length. We show that the velocity
of the droplet varies sinusoidally as a function of its position on the
substrate. On the other hand, for accelerations just above the depinning
acceleration we observe a characteristic stick-slip motion, with successive
pinnings and depinnings.
|
1905.07214v1
|
2019-05-20
|
Exploring the damping of Alfvén waves along a long off-limb coronal loop, up to 1.4 R$_\odot$
|
The Alfv\'en wave energy flux in the corona can be explored using the
electron density and velocity amplitude of the waves. The velocity amplitude of
Alfv\'en waves can be obtained from the non-thermal velocity of the spectral
line profiles. Previous calculations of the Alfv\'en wave energy flux with
height in active regions and polar coronal holes have provided evidence for the
damping of Alfv\'en waves with height. We present off-limb Hinode EUV imaging
spectrometer (EIS) observations of a long coronal loop up to 1.4~R$_\odot$. We
have obtained the electron density along the loop and found the loop to be
almost in hydrostatic equilibrium. We obtained the temperature using the
EM-loci method and found the loop to be isothermal across, as well as along,
the loop with a temperature of about 1.37 MK. We significantly improve the
estimate of non-thermal velocities over previous studies by using the estimated
ion (equal to electron) temperature. Estimates of electron densities are
improved using the significant updates of the CHIANTI v.8 atomic data. More
accurate measurements of propagating Alfv\'en wave energy along the coronal
loop and its damping are presented up to distances of 1.4 R$_\odot$, further
than have been previously explored. The Alfv\'en wave energy flux obtained
could contribute to a significant part of the coronal losses due to radiation
along the loop.
|
1905.08194v2
|
2019-06-14
|
Influence of External Magnetic Field on Dust$-$Acoustic Waves in a Capacitive RF Discharge
|
This paper reports experiments on self$-$excited dust acoustic waves (DAWs)
and its propagation characteristics in a magnetized rf discharge plasma. The
DAWs are spontaneously excited in dusty plasma after adding more particles in
the confining potential well and found to propagate in the direction of
streaming ions. The spontaneous excitation of such low-frequency modes is
possible due to the instabilities associated with streaming ions through the
dust grain medium. The background E-field and neutral pressure determine the
stability of excited DAWs. The characteristics of DAWs strongly depend on the
strength of external magnetic field. The magnetic field of strength B $<$ 0.05
T only modifies the characteristics of propagating waves in dusty plasma at
moderate power and pressure, P = 3.5 W and p = 27 Pa respectively. It is found
that DAWs start to be damped with increasing the magnetic field beyond B $>$
0.05 T and get completely damped at higher magnetic field B $\sim$ 0.13 T.
After lowering the power and pressure to 3 W and 23 Pa respectively, the
excited DAWs in the absence of B are slightly unstable. In this case, the
magnetic field only stabilizes and modifies the propagation characteristics of
DAWs while the strength of B is increased up to 0.1 T or even higher. The
modification of the sheath electric field where particles are confined in the
presence of the external magnetic field is the main cause of the modification
and damping of the DAWs in a magnetized rf discharge plasma.
|
1906.06255v2
|
2019-06-18
|
A Dynamic Robotic Actuator with Variable Physical Stiffness and Damping
|
This study is part of research aiming at increasing the range of dynamic
tasks for teleoperated field robotics in order to allow operators to use the
full range of human motions without being limited by the dynamics of the
robotic manipulator. A new variable impedance actuator (VIA) was designed,
capable of reproducing motions through teleoperation from precise positioning
tasks to highly dynamic tasks. The design requirements based on previous human
user studies were a stiffness changing time of 50 ms, a peak output velocity of
20 rad/s and variable damping allowing to suppress undesired oscillations. This
is a unique combination of features that was not met by other VIAs. The new
design has three motors in parallel configuration: two responsible for changing
the VIA's neutral position and effective stiffness through a sliding pivot
point lever mechanism, and the third acting as variable damper. A prototype was
built and its performance measured with an effective stiffness changing time of
50 to 120 ms for small to large stiffness steps, nominal output velocity of 16
rad/s and a variable damper with a damping torque from 0 to 3 Nm. Its effective
stiffness range is 0.2 to 313 Nm/rad. This concludes that the new actuator is
particularly suitable for highly dynamic tasks. At the same time, the new
actuator is also very versatile, making it especially interesting for
teleoperation and human-robot collaboration.
|
1906.07669v2
|
2019-06-27
|
Frequency Fluctuations in Tunable and Nonlinear Microwave Cavities
|
We present a model for how frequency fluctuations comparable to the total
cavity linewidth may arise in tunable and nonlinear microwave cavities, and how
these fluctuations affect the measurement of scattering matrix elements.
Applying this model to the specific case of a two-sided cavity, we obtain
closed-form expressions for the average scattering matrix elements in several
important cases. A key signature of our model is the subtle deformation of the
trajectories swept out by scattering matrix elements in the complex plane.
Despite this signature, the fluctuating and non-fluctuating models are
qualitatively similar enough to be mistaken for one another. In the case of
tunable cavities we show that if one fails to account for these fluctuations
then one will find damping rates that appear to depend on the tuning parameter,
which is a common observation in such systems. In the case of a Kerr cavity, we
show that there exists a fundamental lower bound to the scale of these
frequency fluctuations in the steady state, imposed by quantum mechanical
uncertainty, which can appreciably affect the apparent damping rates of the
cavity as the strength of the nonlinearity approaches the single-photon level.
By using the model we present as a fitting function for experimental data,
however, one can extract both the true damping rates of the cavity and the
effective scale of these frequency fluctuations over the scattering
measurement's bandwidth. Lastly, we compare this new method for observing
frequency fluctuations to other methods, one of which we extend beyond the
regime of small fluctuations.
|
1906.11989v3
|
2019-08-08
|
Anisotropic damping of the spin fluctuations in doped La2-xSrxCuO4 studied by resonant inelastic x-ray scattering
|
We report high-resolution resonant inelastic x-ray scattering (RIXS)
measurements of the collective spin fluctuations in three compositions of the
superconducting cuprate system La2-xSrxCuO4. We have mapped out the excitations
throughout much of the 2-D (h,k) Brillouin zone. The spin fluctuations in
La2-xSrxCuO4 are found to be fairly well-described by a damped harmonic
oscillator model, thus our data allows us to determine the full wavevector
dependence of the damping parameter. This parameter increases with doping and
is largest along the (h, h) line, where it is peaked near (0.2,0.2). We have
used a new procedure to determine the absolute wavevector-dependent
susceptibility for the doped compositions La2-xSrxCuO4 (x=0.12,0.16) by
normalising our data to La2CuO4 measurements made with inelastic neutron
scattering (INS). We find that the evolution with doping of the intensity of
high-energy excitations measured by RIXS and INS is consistent. For the doped
compositions, the wavevector-dependent susceptibility is much larger at
(1/4,1/4) than at (1/2,0). It increases rapidly along the (h,h) line towards
the antiferromagnetic wavevector of the parent compound (1/2,1/2). Thus, the
strongest magnetic excitations, and those predicted to favour superconductive
pairing, occur towards the (1/2,1/2) position as observed by INS.
|
1908.03086v2
|
2019-08-14
|
Two-fluid simulations of waves in the solar chromosphere II. Propagation and damping of fast magneto-acoustic waves and shocks
|
Waves and shocks traveling through the solar chromospheric plasma are
influenced by its partial ionization and weak collisional coupling, and may
become susceptible to multi-fluid effects, similar to interstellar shock waves.
In this study, we consider fast magneto-acoustic shock wave formation and
propagation in a stratified medium, that is permeated by a horizontal magnetic
field, with properties similar to that of the solar chromosphere. The evolution
of plasma and neutrals is modeled using a two-fluid code that evolves a set of
coupled equations for two separate fluids. We observed that waves in neutrals
and plasma, initially coupled at the upper photosphere, become uncoupled at
higher heights in the chromosphere. This decoupling can be a consequence of
either the characteristic spatial scale at the shock front, that becomes
similar to the collisional scale, or the change in the relation between the
wave frequency, ion cyclotron frequency, and the collisional frequency with
height. The decoupling height is a sensitive function of the wave frequency,
wave amplitude, and the magnetic field strength. We observed that decoupling
causes damping of waves and an increase in the background temperature due to
the frictional heating. The comparison between analytical and numerical results
allows us to separate the role of the nonlinear effects from the linear ones on
the decoupling and damping of waves.
|
1908.05262v1
|
2019-09-16
|
Inviscid damping and enhanced dissipation of the boundary layer for 2D Navier-Stokes linearized around Couette flow in a channel
|
We study the 2D Navier-Stokes equations linearized around the Couette flow
$(y,0)^t$ in the periodic channel $\mathbb T \times [-1,1]$ with no-slip
boundary conditions in the vanishing viscosity $\nu \to 0$ limit. We split the
vorticity evolution into the free evolution (without a boundary) and a boundary
corrector that is exponentially localized to at most an $O(\nu^{1/3})$ boundary
layer. If the initial vorticity perturbation is supported away from the
boundary, we show inviscid damping of both the velocity and the vorticity
associated to the boundary layer. For example, our $L^2_t L^1_y$ estimate of
the boundary layer vorticity is independent of $\nu$, provided the initial data
is $H^1$. For $L^2$ data, the loss is only logarithmic in $\nu$. Note both such
estimates are false for the vorticity in the interior. To the authors'
knowledge, this inviscid decay of the boundary layer vorticity seems to be a
new observation not previously isolated in the literature. Both velocity and
vorticity satisfy the expected $O(\exp(-\delta\nu^{1/3}\alpha^{2/3}t))$
enhanced dissipation in addition to the inviscid damping. Similar, but slightly
weaker, results are obtained also for $H^1$ data that is against the boundary
initially. For $L^2$ data against the boundary, we at least obtain the boundary
layer localization and enhanced dissipation.
|
1909.07230v1
|
2019-10-19
|
Anomalies in the switching dynamics of C-type antiferromagnets and antiferromagnetic nanowires
|
Antiferromagnets (AFMs) are widely believed to be superior than ferromagnets
in spintronics because of their high stability due to the vanishingly small
stray field. It is thus expected that the order parameter of AFM should always
align along the easy-axis of the crystalline anisotropy. In contrast to this
conventional wisdom, we find that the AFM order parameter switches away from
the easy-axis below a critical anisotropy strength when an AFM is properly
tailored into a nano-structure. The switching time first decreases and then
increases with the damping. Above the critical anisotropy, the AFM order
parameter is stable and precesses under a microwave excitation. However, the
absorption peak is not at resonance frequency even for magnetic damping as low
as 0.01. To resolve these anomalies, we first ascertain the hidden role of
dipolar interaction that reconstructs the energy landscape of the nano-system
and propose a model of damped non-linear pendulum to explain the switching
behavior. In this framework, the second anomaly appears when an AFM is close to
the boundary between underdamped and overdamped phases, where the observed
absorption lineshape has small quality factor and thus is not reliable any
longer. Our results should be significant to extract the magnetic parameters
through resonance techniques.
|
1910.08668v1
|
2019-11-01
|
Importance of Giant Impact Ejecta for Orbits of Planets Formed during the Giant Impact Era
|
Terrestrial planets are believed to be formed via giant impacts of Mars-sized
protoplanets. Planets formed via giant impacts have highly eccentric orbits. A
swarm of planetesimals around the planets may lead to eccentricity damping for
the planets via the equipartition of random energies (dynamical friction).
However, dynamical friction increases eccentricities of planetesimals,
resulting in high velocity collisions between planetesimals. The collisional
cascade grinds planetesimals to dust until dust grains are blown out due to
radiation pressure. Therefore, the total mass of planetesimals decreases due to
collisional fragmentation, which weakens dynamical friction. We investigate the
orbital evolution of protoplanets in a planetesimal disk, taking into account
collisional fragmentation of planetesimals. For 100 km-sized or smaller
planetesimals, dynamical friction is insignificant for eccentricity damping of
planets because of collisional fragmentation. On the other hand, giant impacts
eject collisional fragments. Although the total mass of giant impact ejecta is
0.1-0.3 Earth masses, the largest impact ejecta are ~ 1,000 km in size. We also
investigate the orbital evolution of single planets with initial eccentricities
0.1 in a swarm of such giant impact ejecta. Although the total mass of giant
impact ejecta decreases by a factor of 3 in 30 Myrs, eccentricities of planets
are damped down to the Earth level (~0.01) due to interaction with giant impact
ejecta. Therefore, giant impact ejecta play an important role for determination
of terrestrial planet orbits.
|
1911.00278v3
|
2019-11-06
|
Damping in Ru/Co-based multilayer films with large Dzyaloshinskii-Moriya interaction
|
Recent development of the magnetic material engineering led to achievement of
the systems with a high interfacial Dzyaloshinskii-Moriya interaction (DMI). As
a result, the formation of non-collinear magnetic soliton states or
nonreciprocal spin wave dynamics is achievable. Typically used materials are
based on bi-layers Heavy Metal/Ferromagnet, e.g., Pt/Co. These layers are
characterized not only by a strong DMI, but also by the spin pumping effect and
the resulting relatively large damping. Here, we show that the considerable
interfacial DMI can be also present in bi-layers based on Ru/Co, characterized
with low spin pumping effect and low damping. It is therefore a good candidate
for the dynamical studies and implementations of chiral DMI. It is demonstrated
by theoretical calculations that the value of DMI can be strongly affected and
controlled by the strain of the lattice. We show a systematic experimental and
theoretical comparison of magnetic material parameters between Pt/Co and Ru/Co
bi-layers as a deserving candidate for spintronic and spin-orbitronic
applications.
|
1911.02467v1
|
2019-11-14
|
Studies of the beam-ion instability and its mitigation with feedback system
|
The beam-ion interaction is a potential limitation of beam performance in
electron accelerators, especially where the beam emittance is of a great
concern in future ultra-low emittance light source. "Conventionally", the beam
instability due to beam-ion interaction is attributed to two types of effects:
ion trapping effect and fast ion effect, which emphasize the beam-ion dynamics
in different time scales. Whereas, in accelerators, the beam suffers from a
mixture of ion trapping effect and fast ion effect, leading to a more
complicated process and requiring a self-consistent treatment. To evaluate the
beam characteristics, as emittance growth under the influence from beam-ion
effect, a new numerical simulation code based on the "quasi-strong-strong"
model has been developed, including modules of ionization, beam-ion
interaction, synchrotron radiation damping, quantum excitation, bunch-by-bunch
feedback, etc. In the study, we do not regularly distinguish the ion trapping
effect and the fast ion effect, but treat beam-ion interaction more generally
and consistently. The lattice of High Energy Photon Source, a diffraction limit
ring under construction in Beijing, is used as an example to show the beam-ion
effect. It is found that in this low emittance ring, the beam-ion instability
is not a dominant mechanism in operation mode with a high beam current, but
seriously occurs in a lower beam current region. When the beam-ion instability
were significantly driven and can not be damped by the synchrotron radiation
damping, the simulations show the bunch-by-bunch feedback system based on the
Finite Impulse Response filter technique can be adopted to mitigate it
effectively.
|
1911.05958v1
|
2019-12-05
|
Steering magnonic dynamics and permeability at exceptional points in a parity-time symmetric waveguide
|
Tuning the low-energy magnetic dynamics is a key element in designing novel
magnetic metamaterials, spintronic devices and magnonic logic circuits. This
study uncovers a new, highly effective way of controlling the magnetic
permeability via shaping the magnonic properties in coupled magnetic waveguides
separated by current carrying spacer with strong spin-orbit coupling. The
spin-orbit torques exerted on the waveguides leads to an externally tunable
enhancement of magnetic damping in one waveguide and a decreased damping in the
other, constituting so a magnetic parity-time (PT) symmetric system with
emergent magnetic properties at the verge of the exceptional point where
magnetic gains/losses are balanced. In addition to controlling the magnetic
permeability, phenomena inherent to PT-symmetric systems are identified,
including the control on magnon power oscillations, nonreciprocal magnon
propagation, magnon trapping and enhancement as well as the increased
sensitivity to magnetic perturbation and abrupt spin reversal. These
predictions are demonstrated analytically and confirmed by full numerical
simulations under experimentally feasible conditions. The position of the
exceptional points and the strength of the spontaneous PT symmetry breaking can
be tuned by external electric and/or magnetic fields. The roles of the
intrinsic magnetic damping, and the possibility of an electric control via
Dzyaloshinskii-Moriya interaction are exposed and utilized for mode dispersion
shaping and magnon amplification and trapping. The results point to a new route
to designing optomagnonic waveguides, traps, sensors, and circuits.
|
1912.02500v1
|
2020-01-23
|
Skyrmion Dynamics and Topological Sorting on Periodic Obstacle Arrays
|
We examine skyrmions under a dc drive interacting with a square array of
obstacles for varied obstacle size and damping. When the drive is applied in a
fixed direction, we find that the skyrmions are initially guided in the drive
direction but also move transverse to the drive due to the Magnus force. The
skyrmion Hall angle, which indicates the difference between the skyrmion
direction of motion and the drive direction, increases with drive in a series
of quantized steps as a result of the locking of the skyrmion motion to
specific symmetry directions of the obstacle array. On these steps, the
skyrmions collide with an integer number of obstacles to create a periodic
motion. The transitions between the different locking steps are associated with
jumps or dips in the velocity-force curves. In some regimes, the skyrmion Hall
angle is actually higher than the intrinsic skyrmion Hall angle that would
appear in the absence of obstacles. In the limit of zero damping, the skyrmion
Hall angle is 90$^\circ$, and we find that it decreases as the damping
increases. For multiple interacting skyrmion species in the collective regime,
we find jammed behavior at low drives where the different skyrmion species are
strongly coupled and move in the same direction. As the drive increases, the
species decouple and each can lock to a different symmetry direction of the
obstacle lattice, making it possible to perform topological sorting in analogy
to the particle sorting methods used to fractionate different species of
colloidal particles moving over two-dimensional obstacle arrays.
|
2001.08835v1
|
2020-03-06
|
Lattice dynamics and polarization-dependent phonon damping in $α$-phase FeSi$_{2}$ nanoislands
|
We determined the lattice dynamics of metastable, surface-stabilized
$\alpha$-phase FeSi$_2$ nanoislands epitaxially grown on the Si(111) surface
with average heights and widths ranging from 1.5 to 20 nm and 18 to 72 nm,
respectively. The crystallographic orientation, surface morphology and local
crystal structure of the nanoislands were investigated by reflection
high-energy electron diffraction, atomic force microscopy and X-ray absorption
spectroscopy. The Fe-partial phonon density of states (PDOS), obtained by
nuclear inelastic scattering, exhibits a pronounced damping and broadening of
the spectral features with decreasing average island height. First-principles
calculations of the polarization-projected Si- and Fe-partial phonon
dispersions and PDOS enable the disentanglement of the contribution of the
$xy$- and $z$-polarized phonons to the experimental PDOS. Modeling of the
experimental data with the theoretical results unveils an enhanced damping of
the $z$-polarized phonons for islands with average sizes below 10 nm. This
phenomenon is attributed to the fact that the low-energy $z$-polarized phonons
couple to the low-energy surface/interface vibrational modes. The thermodynamic
and elastic properties obtained from the experimental data show a pronounced
size-dependent behavior.
|
2003.02969v1
|
2020-03-20
|
The resonant drag instability of dust streaming in turbulent protoplanetary disc
|
Damping of the previously discovered resonant drag instability (RDI) of dust
streaming in protoplanetary disc is studied using the local approach to
dynamics of gas-dust perturbations in the limit of the small dust fraction.
Turbulence in a disc is represented by the effective viscosity and diffusivity
in equations of motion for gas and dust, respectively. In the standard case of
the Schmidt number (ratio of the effective viscosity to diffusivity) Sc = 1,
the reduced description of RDI in terms of the inertial wave (IW) and the
streaming dust wave (SDW) falling in resonance with each other reveals that
damping solution differs from the inviscid solution simply by adding the
characteristic damping frequency to its growth rate. RDI is fully suppressed at
the threshold viscosity, which is estimated analytically, first, for radial
drift, next, for vertical settling of dust, and at last, in the case of
settling combined with radial drift of the dust. In the last case, RDI survives
up to the highest threshold viscosity, with a greater excess for smaller
solids. Once Sc \neq 1, a new instability specific for dissipative
perturbations on the dust settling background emerges. This instability of the
quasi-resonant nature is referred to as settling viscous instability (SVI). The
mode akin to SDW (IW) becomes growing in a region of long waves provided that
Sc > 1 (Sc < 1). SVI leads to an additional increase of the threshold
viscosity.
|
2003.09212v1
|
2020-05-22
|
Quasinormal modes, shadow and greybody factors of 5D electrically charged Bardeen black holes
|
We study quasinormal modes (QNMs) in 5D electrically charged Bardeen black
holes spacetime by considering the scalar and electromagnetic field
perturbations. The black holes spacetime is an exact solution of Einstein
gravity coupled to nonlinear electrodynamics in five dimensions, which has
nonsingular behavior. To calculate QNMs, we use the WKB approximation method up
to sixth order. Due to the presence of electric charge $q_e > 0$, both the
scalar and electromagnetic field perturbations decay more slowly when compared
to the Schwarzschild-Tangherlini black holes. We discover that the scalar field
perturbations oscillate more rapidly when compared to the electromagnetic field
perturbations. In terms of damping, the scalar field perturbations damp more
quickly. Graphically we show that the transmission (reflection) coefficients
decrease (increase) with an increase in the magnitude of the electric charge
$q_e$. The emission of gravitational waves allows spacetime to undergo damped
oscillations due to the nonzero value of the imaginary part, which is always
negative. The imaginary part of the QNMs frequencies is continuously decreasing
with an increase in the magnitude of the electric charge $q_e$ for a given mode
($l,n$). A connection between the QNMs frequencies and the black hole shadow,
as well as the geometric cross-section in the eikonal limit, is also described.
|
2005.11080v2
|
2020-05-28
|
Spintronics meets nonadiabatic molecular dynamics: Geometric spin torque and damping on noncollinear classical magnetism due to electronic open quantum system
|
We analyze a quantum-classical hybrid system of steadily precessing slow
classical localized magnetic moments, forming a head-to-head domain wall,
embedded into an open quantum system of fast nonequilibrium electrons. The
electrons reside within a metallic wire connected to macroscopic reservoirs.
The model captures the essence of dynamical noncollinear and noncoplanar
magnetic textures in spintronics, while making it possible to obtain the exact
time-dependent nonequilibrium density matrix of electronic system and split it
into four contributions. The Fermi surface contribution generates dissipative
(or damping-like in spintronics terminology) spin torque on the moments, and
one of the two Fermi sea contributions generates geometric torque dominating in
the adiabatic regime. When the coupling to the reservoirs is reduced, the
geometric torque is the only nonzero contribution. Locally it has both
nondissipative (or field-like in spintronics terminology) and damping-like
components, but with the sum of latter being zero, which act as the
counterparts of geometric magnetism force and electronic friction in
nonadiabatic molecular dynamics. Such current-independent geometric torque is
absent from widely used micromagnetics or atomistic spin dynamics modeling of
magnetization dynamics based on the Landau-Lifshitz-Gilbert equation, where
previous analysis of Fermi surface-type torque has severely underestimated its
magnitude.
|
2005.14153v2
|
2020-07-15
|
On the Extension of Linear Damping to Quantum Mechanics through Fractionary Momentum Operators Pt. I
|
The use of fractional momentum operators and fractionary kinetic energy used
to model linear damping in dissipative systems such as resistive circuits and a
spring-mass ensambles was extended to a quantum mechanical formalism. Three
important associated 1 dimensional problems were solved: the free particle
case, the infinite potential well, and the harmonic potential. The wave
equations generated reproduced the same type of 2-order ODE observed in
classical dissipative systems, and produced quantized energy levels. In the
infinite potential well, a zero-point energy emerges, which can be fitted to
the rest energy of the particle described by special relativity, given by
relationship $E_r=mc^2$. In the harmonic potential, new fractional creation and
destruction operators were introduced to solve the problem in the energy basis.
The energy eigenvalues found are different to the ones reported by earlier
approaches to the quantum damped oscillator problem reported by other authors.
In this case, a direct relationship between the relativistic rest energy of the
particle and the expected value of the fractionary kinetic energy in the base
state was obtained. We conclude that there exists a relationship between
fractional kinetic energy and special relativity energies, that remains unclear
and needs further exploration, but also conclude that the current form of
transforming fractionary momentum operators to the position basis will yield
non-observable imaginary momentum quantities, and thus a correction to the way
of transforming them needs to be explored further.
|
2007.07434v3
|
2020-07-18
|
Results from the Alfvén Eigenmode Active Diagnostic during the 2019-2020 JET deuterium campaign
|
This paper presents results of extensive analysis of mode excitation observed
during the operation of the Alfv\'{e}n Eigenmode Active Diagnostic (AEAD) in
the JET tokamak during the 2019-2020 deuterium campaign. Six of eight
toroidally spaced antennas, each with independent power and phasing, were
successful in actively exciting stable MHD modes in 479 plasmas. In total, 4768
magnetic resonances were detected with up to fourteen fast magnetic probes. In
this work, we present the calculations of resonant frequencies $f_0$, damping
rates $\gamma < 0$, and toroidal mode numbers $n$, spanning the parameter range
$f_0 \approx$ 30 - 250 kHz, $-\gamma \approx$ 0 - 13 kHz, and $\vert n \vert
\leq 30$. In general, good agreement is seen between the resonant and the
calculated toroidal Alfv\'{e}n Eigenmode frequencies, and between the toroidal
mode numbers applied by the AEAD and estimated of the excited resonances. We
note several trends in the database: the probability of resonance detection
decreases with plasma current and external heating power; the normalized
damping rate increases with edge safety factor but decreases with external
heating. These results provide key information to prepare future experimental
campaigns and to better understand the physics of excitation and damping of
Alfv\'{e}n Eigenmodes in the presence of alpha particles during the upcoming DT
campaign, thereby extrapolating with confidence to future tokamaks.
|
2007.09412v1
|
2020-08-18
|
A Quasi-Linear Diffusion Model for Resonant Wave-Particle Instability in Homogeneous Plasma
|
In this paper, we develop a model to describe the generalized wave-particle
instability in a quasi-neutral plasma. We analyze the quasi-linear diffusion
equation for particles by expressing an arbitrary unstable and resonant wave
mode as a Gaussian wave packet, allowing for an arbitrary direction of
propagation with respect to the background magnetic field. We show that the
localized energy density of the Gaussian wave packet determines the
velocity-space range in which the dominant wave-particle instability and
counter-acting damping contributions are effective. Moreover, we derive a
relation describing the diffusive trajectories of resonant particles in
velocity space under the action of such an interplay between the wave-particle
instability and damping. For the numerical computation of our theoretical
model, we develop a mathematical approach based on the Crank-Nicolson scheme to
solve the full quasi-linear diffusion equation. Our numerical analysis solves
the time evolution of the velocity distribution function under the action of a
dominant wave-particle instability and counteracting damping and shows a good
agreement with our theoretical description. As an application, we use our model
to study the oblique fast-magnetosonic/whistler instability, which is proposed
as a scattering mechanism for strahl electrons in the solar wind. In addition,
we numerically solve the full Fokker-Planck equation to compute the time
evolution of the electron-strahl distribution function under the action of
Coulomb collisions with core electrons and protons after the collisionless
action of the oblique fast-magnetosonic/whistler instability.
|
2008.08169v2
|
2020-09-10
|
Spin waves in alloys at finite temperatures: application for FeCo magnonic crystal
|
We study theoretically the influence of the temperature and disorder on the
spin wave spectrum of the magnonic crystal Fe$_{1-c}$Co$_{c}$. Our formalism is
based on the analysis of a Heisenberg Hamiltonian by means of the wave vector
and frequency dependent transverse magnetic susceptibility. The exchange
integrals entering the model are obtained from the \emph{ab initio} magnetic
force theorem. The coherent potential approximation is employed to treat the
disorder and random phase approximation in order to account for the softening
of the magnon spectrum at finite temperatures. The alloy turns out to exhibit
many advantageous properties for spintronic applications. Apart from high Curie
temperature, its magnonic bandgap remains stable at elevated temperatures and
is largely unaffected by the disorder. We pay particular attention to the
attenuation of magnons introduced by the alloying. The damping turns out to be
a non-monotonic function of the impurity concentration due to the non-trivial
evolution of the value of exchange integrals with the Co concentration. The
disorder induced damping of magnons is estimated to be much smaller than their
Landau damping.
|
2009.04712v6
|
2020-09-14
|
On the response of a star cluster to a tidal perturbation
|
We study the response of star clusters to individual tidal perturbations
using controlled $N$-body simulations. We consider perturbations by a moving
point mass and by a disc, and vary the duration of the perturbation as well as
the cluster density profile. For fast perturbations (i.e. `shocks'), the
cluster gains energy in agreement with theoretical predictions in the impulsive
limit. For slow disc perturbations, the energy gain is lower, and this has
previously been attributed to adiabatic damping. However, the energy gain due
to slow perturbations by a point-mass is similar to, or larger than that due to
fast shocks, which is not expected because adiabatic damping should be almost
independent of the nature of the tides. We show that the geometric distortion
of the cluster during slow perturbations is of comparable importance for the
energy gain as adiabatic damping, and that the combined effect can
qualitatively explain the results. The half-mass radius of the bound stars
after a shock increases up to $\sim$7\% for low-concentration clusters, and
decreases $\sim$3\% for the most concentrated ones. The fractional mass loss is
a non-linear function of the energy gain, and depends on the nature of the
tides and most strongly on the cluster density profile, making semi-analytic
model predictions for cluster lifetimes extremely sensitive to the adopted
density profile.
|
2009.06643v2
|
2020-09-18
|
African Easterly Waves in an Idealized General Circulation Model: Instability and Wavepacket Diagnostics
|
We examine the group dynamic of African easterly waves (AEW) generated in a
realistic, spatially non-homogeneous African easterly jet (AEJ) using an
idealized general circulation model. Our objective is to investigate whether
the limited zonal extent of the AEJ is an impediment to AEW development. We
construct a series of basic states using global reanalysis fields and
initialize waves via transient heating over West Africa. The dominant response
is a localized wavepacket that disperses upstream and downstream. The inclusion
of a crude representation of boundary layer damping stabilizes the waves in
most cases. In some basic states, however, exponential growth occurs even in
the presence of damping. This shows that AEWs can occasionally emerge
spontaneously. The key result is that the wavepacket in almost all cases
remains within the AEJ instead of being swept away. Drawing from other studies,
this also suggests that even the damped waves can grow if coupled with
additional sources of energy such as moist convection and dust radiative
feedback. The wavepacket in the localized AEJ appears to satisfy a condition
for absolute instability, a form of spatial hydrodynamic instability. However,
this needs to be verified more rigorously. Our results also suggest that the
intermittent nature of AEWs is mediated, not by transitions between convective
and absolute instability, but likely by external sources such as propagating
equatorial wave modes
|
2009.08604v1
|
2020-09-25
|
Polaronic Contributions to Friction in a Manganite Thin Film
|
Despite the huge importance of friction in regulating movement in all natural
and technological processes, the mechanisms underlying dissipation at a sliding
contact are still a matter of debate. Attempts to explain the dependence of
measured frictional losses at nanoscale contacts on the electronic degrees of
freedom of the surrounding materials have so far been controversial. Here, it
is proposed that friction can be explained by considering damping of stick-slip
pulses in a sliding contact. Based on friction force microscopy studies of
La$_{(1-x)}$Sr$_x$MnO$_3$ films at the ferromagnetic-metallic to
paramagnetic-polaronic conductor phase transition, it is confirmed that the
sliding contact generates thermally-activated slip pulses in the nanoscale
contact, and argued that these are damped by direct coupling into phonon bath.
Electron-phonon coupling leads to the formation of Jahn-Teller polarons and a
clear increase in friction in the high temperature phase. There is no evidence
for direct electronic drag on the atomic force microscope tip nor any
indication of contributions from electrostatic forces. This intuitive scenario,
that friction is governed by the damping of surface vibrational excitations,
provides a basis for reconciling controversies in literature studies as well as
suggesting possible tactics for controlling friction.
|
2009.12137v1
|
2020-09-25
|
Direct computation of nonlinear mapping via normal form for reduced-order models of finite element nonlinear structures
|
The direct computation of the third-order normal form for a geometrically
nonlinear structure discretised with the finite element (FE) method, is
detailed. The procedure allows to define a nonlinear mapping in order to derive
accurate reduced-order models (ROM) relying on invariant manifold theory. The
proposed reduction strategy is direct and simulation free, in the sense that it
allows to pass from physical coordinates (FE nodes) to normal coordinates,
describing the dynamics in an invariant-based span of the phase space. The
number of master modes for the ROM is not a priori limited since a complete
change of coordinate is proposed. The underlying theory ensures the quality of
the predictions thanks to the invariance property of the reduced subspace,
together with their curvatures in phase space that accounts for the nonresonant
nonlinear couplings. The method is applied to a beam discretised with 3D
elements and shows its ability in recovering internal resonance at high energy.
Then a fan blade model is investigated and the correct prediction given by the
ROMs are assessed and discussed. A method is proposed to approximate an
aggregate value for the damping, that takes into account the damping
coefficients of all the slave modes, and also using the Rayleigh damping model
as input. Frequency-response curves for the beam and the blades are then
exhibited, showing the accuracy of the proposed method.
|
2009.12145v1
|
2020-09-29
|
Structural Phase Dependent Giant Interfacial Spin Transparency in W/CoFeB Thin Film Heterostructure
|
Pure spin current has transfigured the energy-efficient spintronic devices
and it has the salient characteristic of transport of the spin angular
momentum. Spin pumping is a potent method to generate pure spin current and for
its increased efficiency high effective spin-mixing conductance (Geff) and
interfacial spin transparency (T) are essential. Here, a giant T is reported in
Sub/W(t)/Co20Fe60B20(d)/SiO2(2 nm) heterostructures in \beta-tungsten (\beta-W)
phase by employing all-optical time-resolved magneto-optical Kerr effect
technique. From the variation of Gilbert damping with W and CoFeB thicknesses,
the spin diffusion length of W and spin-mixing conductances are extracted.
Subsequently, T is derived as 0.81 \pm 0.03 for the \beta-W/CoFeB interface. A
sharp variation of Geff and T with W thickness is observed in consonance with
the thickness-dependent structural phase transition and resistivity of W. The
spin memory loss and two-magnon scattering effects are found to have negligible
contributions to damping modulation as opposed to spin pumping effect which is
reconfirmed from the invariance of damping with Cu spacer layer thickness
inserted between W and CoFeB. The observation of giant interfacial spin
transparency and its strong dependence on crystal structures of W will be
important for pure spin current based spin-orbitronic devices.
|
2009.14143v1
|
2020-10-08
|
Modeling of the ECCD injection effect on the Heliotron J and LHD plasma stability
|
The aim of the study is to analyze the stability of the Energetic Particle
Modes (EPM) and Alfven Eigenmodes (AE) in Helitron J and LHD plasma if the
electron cyclotron current drive (ECCD) is applied. The analysis is performed
using the code FAR3d that solves the reduced MHD equations describing the
linear evolution of the poloidal flux and the toroidal component of the
vorticity in a full 3D system, coupled with equations of density and parallel
velocity moments for the energetic particle (EP) species, including the effect
of the acoustic modes. The Landau damping and resonant destabilization effects
are added via the closure relation. The simulation results show that the n=1
EPM and n=2 Global AE (GAE) in Heliotron J plasma can be stabilized if the
magnetic shear is enhanced at the plasma periphery by an increase (co-ECCD
injection) or decrease (ctr-ECCD injection) of the rotational transform at the
magnetic axis iota0. In the ctr-ECCD simulations, the EPM/AE growth rate
decreases only below a given iota0, similar to the ECCD intensity threshold
observed in the experiments. In addition, ctr-ECCD simulations show an
enhancement of the continuum damping. The simulations of the LHD discharges
with ctr-ECCD injection indicate the stabilization of the n=1 EPM, n=2 Toroidal
AE (TAE) and n=3 TAE, caused by an enhancement of the continuum damping in the
inner plasma leading to a higher EP beta threshold with respect to the co- and
no-ECCD simulations.
|
2010.03892v1
|
2020-10-08
|
A blow-up result for the wave equation with localized initial data: the scale-invariant damping and mass term with combined nonlinearities
|
We are interested in this article in studying the damped wave equation with
localized initial data, in the \textit{scale-invariant case} with mass term and
two combined nonlinearities. More precisely, we consider the following
equation: $$ (E) {1cm} u_{tt}-\Delta
u+\frac{\mu}{1+t}u_t+\frac{\nu^2}{(1+t)^2}u=|u_t|^p+|u|^q, \quad \mbox{in}\
\mathbb{R}^N\times[0,\infty), $$ with small initial data. Under some
assumptions on the mass and damping coefficients, $\nu$ and $\mu>0$,
respectively, we show that blow-up region and the lifespan bound of the
solution of $(E)$ remain the same as the ones obtained in \cite{Our2} in the
case of a mass-free wave equation, it i.e. $(E)$ with $\nu=0$.
Furthermore, using in part the computations done for $(E)$, we enhance the
result in \cite{Palmieri} on the Glassey conjecture for the solution of $(E)$
with omitting the nonlinear term $|u|^q$. Indeed, the blow-up region is
extended from $p \in (1, p_G(N+\sigma)]$, where $\sigma$ is given by (1.12)
below, to $p \in (1, p_G(N+\mu)]$ yielding, hence, a better estimate of the
lifespan when $(\mu-1)^2-4\nu^2<1$. Otherwise, the two results coincide.
Finally, we may conclude that the mass term {\it has no influence} on the
dynamics of $(E)$ (resp. $(E)$ without the nonlinear term $|u|^q$), and the
conjecture we made in \cite{Our2} on the threshold between the blow-up and the
global existence regions obtained holds true here.
|
2010.05455v1
|
2020-10-14
|
Kink Oscillations in Solar Coronal Loops with Elliptical Cross-Sections. I. the linear regime
|
The cross sections of solar coronal loops are suggested to be rarely
circular. We examine linear kink oscillations in straight, density-enhanced,
magnetic cylinders with elliptical cross-sections by solving the
three-dimensional magnetohydrodynamic equations from an initial-value-problem
perspective. Motivated by relevant eigen-mode analyses, we distinguish between
two independent polarizations, one along the major axis (the M-modes) and the
other along the minor one (the m-modes). We find that, as happens for coronal
loops with circular cross-sections, the apparent damping of the transverse
displacement of the loop axis is accompanied by the accumulation of transverse
Alfv\'enic motions and the consequent development of small-scales therein,
suggesting the robustness of the concepts of resonant absorption and
phase-mixing. In addition, two stages can in general be told apart in the
temporal evolution of the loop displacement; a Gaussian time dependence
precedes an exponential one. For the two examined density ratios between loops
and their surroundings, the periods of the M-modes (m-modes) tend to increase
(decrease) with the major-to-minor-half-axis ratio, and the damping times in
the exponential stage for the M-modes tend to exceed their m-mode counterparts.
This is true for the two transverse profiles we examine. However, the relative
magnitudes of the damping times in the exponential stage for different
polarizations depend on the specification of the transverse profile and/or the
density contrast. The applications of our numerical findings are discussed in
the context of coronal seismology.
|
2010.06991v1
|
2020-11-04
|
The impact of astrophysical dust grains on the confinement of cosmic rays
|
We argue that charged dust grains could significantly impact the confinement
and transport of galactic cosmic rays. For sub-GeV to ~1000GeV cosmic rays,
small-scale parallel Alfv\'en waves, which isotropize cosmic rays through
gyro-resonant interactions, are also gyro-resonant with charged grains. If the
dust is nearly stationary, as in the bulk of the interstellar medium, Alfv\'en
waves are damped by dust. This will reduce the amplitude of Alfv\'en waves
produced by the cosmic rays through the streaming instability, thus enhancing
cosmic-ray transport. In well-ionized regions, the dust damping rate is larger
by a factor of ~10 than other mechanisms that damp parallel Alfv\'en waves at
the scales relevant for ~GeV cosmic rays, suggesting that dust could play a key
role in regulating cosmic-ray transport. In astrophysical situations in which
the dust moves through the gas with super-Alfv\'enic velocities, Alfv\'en waves
are rendered unstable, which could directly scatter cosmic rays. This
interaction has the potential to create a strong feedback mechanism where dust,
driven through the gas by radiation pressure, then strongly enhances the
confinement of cosmic rays, increasing their capacity to drive outflows. This
mechanism may act in the circumgalactic medium around star-forming galaxies and
active galactic nuclei.
|
2011.02497v2
|
2020-11-17
|
A Phase Resonance Approach for Modal Testing of Structures with Nonlinear Dissipation
|
The concept of nonlinear modes is useful for the dynamical characterization
of nonlinear mechanical systems. While efficient and broadly applicable methods
are now available for the computation of nonlinear modes, nonlinear modal
testing is still in its infancy. The purpose of this work is to overcome its
present limitation to conservative nonlinearities. Our approach relies on the
recently extended periodic motion concept, according to which nonlinear modes
of damped systems are defined as family of periodic motions induced by an
appropriate artificial excitation that compensates the natural dissipation. The
particularly simple experimental implementation with only a single-point,
single-frequency, phase resonant forcing is analyzed in detail. The method
permits the experimental extraction of natural frequencies, modal damping
ratios and deflection shapes (including harmonics), for each mode of interest,
as function of the vibration level. The accuracy, robustness and current
limitations of the method are first demonstrated numerically. The method is
then verified experimentally for a friction-damped system. Moreover, a
self-contained measure for estimating the quality of the extracted modal
properties is investigated. The primary advantages over alternative vibration
testing methods are noise robustness, broad applicability and short measurement
duration. The central limitation of the identified modal quantities is that
they only characterize the system in the regime near isolated resonances.
|
2011.08500v1
|
2020-12-08
|
Meta Learning-based MIMO Detectors: Design, Simulation, and Experimental Test
|
Deep neural networks (NNs) have exhibited considerable potential for
efficiently balancing the performance and complexity of multiple-input and
multiple-output (MIMO) detectors. We propose a receiver framework that enables
efficient online training by leveraging the following simple observation:
although NN parameters should adapt to channels, not all of them are
channel-sensitive. In particular, we use a deep unfolded NN structure that
represents iterative algorithms in signal detection and channel decoding
modules as multi layer deep feed forward networks. An expectation propagation
(EP) module, called EPNet, is established for signal detection by unfolding the
EP algorithm and rendering the damping factors trainable. An unfolded turbo
decoding module, called TurboNet, is used for channel decoding. This component
decodes the turbo code, where trainable NN units are integrated into the
traditional max-log-maximum a posteriori decoding procedure. We demonstrate
that TurboNet is robust for channels and requires only one off-line training.
Therefore, only a few damping factors in EPNet must be re-optimized online. An
online training mechanism based on meta learning is then developed. Here, the
optimizer, which is implemented by long short-term memory NNs, is trained to
update damping factors efficiently by using a small training set such that they
can quickly adapt to new environments. Simulation results indicate that the
proposed receiver significantly outperforms traditional receivers and that the
online learning mechanism can quickly adapt to new environments. Furthermore,
an over-the-air platform is presented to demonstrate the significant robustness
of the proposed receiver in practical deployment.
|
2012.04379v1
|
2020-12-31
|
Adaptive Surgical Robotic Training Using Real-Time Stylistic Behavior Feedback Through Haptic Cues
|
Surgical skill directly affects surgical procedure outcomes; thus, effective
training is needed to ensure satisfactory results. Many objective assessment
metrics have been developed and some are widely used in surgical training
simulators. These objective metrics provide the trainee with descriptive
feedback about their performance however, often lack feedback on how to proceed
to improve performance. The most effective training method is one that is
intuitive, easy to understand, personalized to the user and provided in a
timely manner. We propose a framework to enable user-adaptive training using
near-real-time detection of performance, based on intuitive styles of surgical
movements (e.g., fluidity, smoothness, crispness, etc.), and propose a haptic
feedback framework to assist with correcting styles of movement. We evaluate
the ability of three types of force feedback (spring, damping, and spring plus
damping feedback), computed based on prior user positions, to improve different
stylistic behaviors of the user during kinematically constrained reaching
movement tasks. The results indicate that four out of the six styles studied
here were statistically significantly improved (p<0.05) using spring guidance
force feedback and a significant reduction in task time was also found using
spring feedback. The path straightness and targeting error in the task were
other task performance metrics studied which were improved significantly using
the spring-damping feedback. This study presents a groundwork for adaptive
training in robotic surgery based on near-real-time human-centric models of
surgical behavior.
|
2101.00097v3
|
2021-01-08
|
Damped dust-ion-acoustic solitons in collisional magnetized nonthermal plasmas
|
A multi-species magnetized collisional nonthermal plasma system containing
inertial ion species, non-inertial electron species following nonthermal
$\kappa-$ distribution, and immobile dust particles, is considered to examine
the characteristics of the dissipative dust-ion-acoustic (DIA) soliton modes,
\textbf{theoretically and parametrically}. The electrostatic solitary modes are
found to be associated with the low frequency dissipative dust-ion-acoustic
solitary waves (DIASWs). The ion-neutral collision is taken into account, and
the influence of ion-neutral collisional effects on the dynamics of dissipative
DIASWs is investigated. It is reported that most of the plasma mediums in space
and laboratory are far from thermal equilibrium, and the particles in such
plasma systems are well fitted via the $\kappa-$nonthermal distribution than
via the thermal Maxwellian distribution. The reductive perturbation approach is
adopted to derive the damped KdV (dKdV) equation, and the solitary wave
solution of the dKdV equation is derived via the tangent hyperbolic method to
analyze the basic features (amplitude, width, speed, time evolution, etc.) of
dissipative DIASWs. The propagation nature and also the basic features of
dissipative DIASWs are seen to influence significantly due to the variation of
the plasma configuration parameters and also due to the variation of the
supethermality index $\kappa$ in the considered plasma system. The implication
of the results of this study could be useful for better understanding the
electrostatic localized disturbances, in the ion length and time scale, in
space and experimental dusty plasmas, where the presence of excess energetic
electrons and ion-neutral collisional damping are accountable.
|
2101.03183v2
|
2020-12-28
|
Global complexity effects due to local damping in a nonlinear system in 1:3 internal resonance
|
It is well-known that nonlinearity may lead to localization effects and
coupling of internally resonant modes. However, research focused primarily on
conservative systems commonly assumes that the near-resonant forced response
closely follows the autonomous dynamics. Our results for even a simple system
of two coupled oscillators with a cubic spring clearly contradict this common
belief. We demonstrate analytically and numerically global effects of a weak
local damping source in a harmonically forced nonlinear system under condition
of 1:3 internal resonance: The global motion becomes asynchronous, i.e., mode
complexity is introduced with a non-trivial phase difference between the modal
oscillations. In particular, we show that a maximum mode complexity with a
phase difference of $90^\circ$ is attained in a multi-harmonic sense. This
corresponds to a transition from generalized standing to traveling waves in the
system's modal space. We further demonstrate that the localization is crucially
affected by the system's damping. Finally, we propose an extension of the
definition of mode complexity and mode localization to nonlinear quasi-periodic
motions, and illustrate their application to a quasi-periodic regime in the
forced response.
|
2101.03233v1
|
2021-01-27
|
New estimations of the added mass and damping of two cylinders vibrating in a viscous fluid, from theoretical and numerical approaches
|
This paper deals with the small oscillations of two circular cylinders
immersed in a viscous stagnant fluid. A new theoretical approach based on an
Helmholtz expansion and a bipolar coordinate system is presented to estimate
the fluid forces acting on the two bodies. We show that these forces are linear
combinations of the {\textcolor{black}{cylinder accelerations}} and velocities,
through viscous fluid added coefficients. {\textcolor{black}{To assess the
validity of this theory, we consider the case of two equal size cylinders, one
of them being stationary while the other one is forced sinusoidally}}. The
self-added mass and damping coefficients are shown to decrease with both the
Stokes number and the separation distance. The cross-added mass and damping
coefficients tend to increase with the Stokes number and the separation
distance. Compared to the inviscid results, the effect of viscosity is to add a
correction term which scales as $Sk^{-1/2}$. When the separation distance is
sufficiently large, the two cylinders behave as if they were independent and
the Stokes predictions for an isolated cylinder are recovered. Compared to
previous works, the present theory offers a simple and flexible alternative for
an easy determination of the fluid forces and related added coefficients. To
our knowledge, this is also the first time that a numerical approach based on a
penalization method is presented in the context of fluid-structure interactions
for relatively small Stokes numbers, and successfully compared to theoretical
predictions.
|
2101.11346v1
|
2021-03-08
|
A Self-Consistent, Time-Dependent Treatment of Dynamical Friction: New Insights regarding Core Stalling and Dynamical Buoyancy
|
Dynamical friction is typically regarded a secular process, in which the
subject ('perturber') evolves very slowly (secular approximation), and has been
introduced to the host over a long time (adiabatic approximation). These
assumptions imply that dynamical friction arises from the LBK torque with
non-zero contribution only from pure resonance orbits. However, dynamical
friction is only of astrophysical interest if its timescale is shorter than the
age of the Universe. In this paper we therefore relax the adiabatic and secular
approximations. We first derive a generalized LBK torque, which reduces to the
LBK torque in the adiabatic limit, and show that it gives rise to transient
oscillations due to non-resonant orbits that slowly damp out, giving way to the
LBK torque. This is analogous to how a forced, damped oscillator undergoes
transients before settling to a steady state, except that here the damping is
due to phase mixing rather than dissipation. Next, we present a self-consistent
treatment, that properly accounts for time-dependence of the perturber
potential and circular frequency (memory effect), which we use to examine
orbital decay in a cored galaxy. We find that the memory effect results in a
phase of accelerated, super-Chandrasekhar friction before the perturber stalls
at a critical radius, $R_{\mathrm{crit}}$, in the core (core-stalling). Inside
of $R_{\mathrm{crit}}$ the torque flips sign, giving rise to dynamical
buoyancy, which counteracts friction and causes the perturber to stall. This
phenomenology is consistent with $N$-body simulations, but has thus far eluded
proper explanation.
|
2103.05004v1
|
2021-04-10
|
Non-Markovian open quantum system approach to the early universe: I. Damping of gravitational waves by matter
|
By revising the application of the open quantum system approach to the early
universe and extending it to the conditions beyond the Markovian approximation,
we obtain a new non-Markovian quantum Boltzmann equation. Throughout the paper,
we also develop an extension of the quantum Boltzmann equation to describe the
processes that are irreversible at the macroscopic level. This new kinetic
equation is, in principle, applicable to a wide variety of processes in the
early universe. For instance, using this equation one can accurately study the
microscopic influence of a cosmic environment on a system of cosmic background
photons or stochastic gravitational waves. In this paper, we apply the
non-Markovian quantum Boltzmann equation to study the damping of gravitational
waves propagating in a medium consisting of decoupled ultra-relativistic
neutrinos. For such a system, we study the time evolution of the intensity and
the polarization of the gravitational waves. It is shown that, in contrast to
intensity and linear polarization which are damped, the circular polarization
(V-mode) of the gravitational wave (if present) is amplified by propagating
through such a medium.
|
2104.04836v2
|
2021-04-19
|
Giant spin-orbit torque efficiency in all-epitaxial heterostructures
|
A large anti-damping spin-obit torque (SOT) efficiency in magnetic
heterostructures is a prerequisite to realize energy efficient spin torque
based magnetic memories and logic devices. The efficiency can be characterized
in terms of the spin-orbit fields generated by anti-damping torques when an
electric current is passed through the non-magnetic layer. We report a giant
spin-orbit field of 48.96 (27.50) mT at an applied current density of 1 MAcm-2
in beta-W interfaced Co60Fe40 (Ni81Fe19)/TiN epitaxial structures due to an
anti-damping like torque, which results in a magnetization auto-oscillation
current density as low as 1.68(3.27) MAcm-2. The spin-orbit field value
increases with decrease of beta-W layer thickness, which affirms that epitaxial
surface states are responsible for the extraordinary large efficiency. SOT
induced energy efficient in-plane magnetization switching in large 20x100 um2
structures has been demonstrated by Kerr microscopy and the findings are
supported by results from micromagnetic simulations. The observed giant SOT
efficiencies in the studied all-epitaxial heterostructures are comparable to
values reported for topological insulators. These results confirm that by
utilizing epitaxial material combinations an extraordinary large SOT efficiency
can be achieved using semiconducting industry compatible 5d heavy metals, which
provides immediate solutions for the realization of energy efficient spin-logic
devices.
|
2104.09168v1
|
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