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2018-02-20
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Ultrafast magnetization dynamics in pure and doped Heusler and inverse Heusler alloys
|
By using a multiscale approach based on first-principles density functional
theory combined with atomistic spin dynamics, we investigate the electronic
structure and magnetization dynamics of an inverse Heusler and a Heusler
compound and their alloys, i. e. Mn$_{2-x}Z_x$CoAl and Mn$_{2-x}Z_x$VAl, where
$Z$ = Mo, W, Os and Ru, respectively. A signature of the ferrimagnetic ordering
of Mn$_{2}$CoAl and Mn$_{2}$VAl Heusler alloys is reflected in the calculated
Heisenberg exchange constants. They decay very rapidly with the interatomic
distance and have short range, which is a consequence of the existence of the
finite gap in the minority spin band. The calculated Gilbert damping parameter
of both Mn$_2$CoAl and Mn$_2$VAl is high compared to other half-metals, but
interestingly in the particular case of the inverse Mn$_{2}$CoAl alloys and due
to the spin-gapless semiconducting property, the damping parameters decrease
with the doping concentration in clear contradiction to the general trend.
Atomistic spin dynamics simulations predict ultrafast magnetisation switching
in Mn$_{2}$CoAl and Mn$_{2}$VAl under the influence of an external magnetic
field, starting from a threshold field of $2\text{T}$. Our overall finding
extends with Heusler and inverse Heusler alloys, the class of materials that
exhibits laser induced magnetic switching.
|
1802.07195v1
|
2019-01-24
|
Reaffirmation of Cosmological Oscillations in the Scale Factor from the Pantheon Compilation of 1048 Type Ia Supernovae
|
We observe damped temporal oscillations in the scale factor at a dominant
frequency of ~ 7 cycles/Hubble-time in the Pantheon Compilation of 1048 type Ia
supernovae (SNe). The residual oscillations observed in the Pantheon data
closely matches and reaffirms our initial observation of oscillations from
earlier SNe data (primarily SNLS3, 2011) at 2-sigma confidence. The nearly
identical shapes in amplitude, frequency, phase and damping constant makes it
highly likely the signal is real. Furthermore, 2/3 of the Pantheon SNe cover
different portions of the sky compared with SNLS3 strengthening this
conclusion. Our model describing the oscillation, presented in an earlier
paper, is a simple scalar field harmonic oscillator coupled to the LCDM
Friedmann eqn, but carried into the present epoch. The scalar field energy
density plays the role of the dark matter energy density in LCDM cosmology,
fits well as an average, and closely matches the present dark matter density
parameter, suggesting the oscillation play a role in the dark matter sector.
Temporal oscillations in the scale factor and its derivative, as described in
the present work, would also induce temporal oscillations of the Hubble
parameter.
|
1901.10311v3
|
2019-03-14
|
A new class of accelerated regularization methods, with application to bioluminescence tomography
|
In this paper we propose a new class of iterative regularization methods for
solving ill-posed linear operator equations. The prototype of these iterative
regularization methods is in the form of second order evolution equation with a
linear vanishing damping term, which can be viewed not only as an extension of
the asymptotical regularization, but also as a continuous analog of the
Nesterov's acceleration scheme. New iterative regularization methods are
derived from this continuous model in combination with damped symplectic
numerical schemes. The regularization property as well as convergence rates and
acceleration effects under the H\"older-type source conditions of both
continuous and discretized methods are proven.
The second part of this paper is concerned with the application of the newly
developed accelerated iterative regularization methods to the diffusion-based
bioluminescence tomography, which is modeled as an inverse source problem in
elliptic partial differential equations with both Dirichlet and Neumann
boundary data. A relaxed mathematical formulation is proposed so that the
discrepancy principle can be applied to the iterative scheme without the usage
of Sobolev embedding constants. Several numerical examples, as well as a
comparison with the state-of-the-art methods, are given to show the accuracy
and the acceleration effect of the new methods.
|
1903.05972v2
|
2019-05-15
|
Stop-and-Go Suppression in Two-Class Congested Traffic
|
This paper develops boundary feedback control laws in order to damp out
traffic oscillations in the congested regime of the linearized two-class
Aw-Rascle (AR) traffic model. The macroscopic second-order two-class AR traffic
model consists of four hyperbolic partial differential equations (PDEs)
describing the dynamics of densities and velocities on freeway. The concept of
area occupancy is used to express the traffic pressure and equilibrium speed
relationship yielding a coupling between the two classes of vehicles. Each
vehicle class is characterized by its own vehicle size and driver's behavior.
The considered equilibrium profiles of the model represent evenly distributed
traffic with constant densities and velocities of both classes along the
investigated track section. After linearizing the model equations around those
equilibrium profiles, it is observed that in the congested traffic one of the
four characteristic speeds is negative, whereas the remaining three are
positive. Backstepping control design is employed to stabilize the $4 \times 4$
heterodirectional hyperbolic PDEs. The control input actuates the traffic flow
at outlet of the investigated track section and is realized by a ramp metering.
A full-state feedback is designed to achieve finite time convergence of the
density and velocity perturbations to the equilibrium at zero. This result is
then combined with an anti-collocated observer design in order to construct an
output feedback control law that damps out stop-and-go waves in finite time by
measuring the velocities and densities of both vehicle classes at the inlet of
the investigated track section. The performance of the developed controllers is
verified by simulation.
|
1905.06476v1
|
2019-10-07
|
Chiral spin-wave velocities induced by all-garnet interfacial Dzyaloshinskii-Moriya interaction in ultrathin yttrium iron garnet films
|
Spin waves can probe the Dzyaloshinskii-Moriya interaction (DMI) which gives
rise to topological spin textures, such as skyrmions. However, the DMI has not
yet been reported in yttrium iron garnet (YIG) with arguably the lowest damping
for spin waves. In this work, we experimentally evidence the interfacial DMI in
a 7~nm-thick YIG film by measuring the nonreciprocal spin wave propagation in
terms of frequency, amplitude and most importantly group velocities using all
electrical spin-wave spectroscopy. The velocities of propagating spin waves
show chirality among three vectors, i.e. the film normal direction, applied
field and spin-wave wavevector. By measuring the asymmetric group velocities,
we extract a DMI constant of 16~$\mu$J/m$^{2}$ which we independently confirm
by Brillouin light scattering. Thickness-dependent measurements reveal that the
DMI originates from the oxide interface between the YIG and garnet substrate.
The interfacial DMI discovered in the ultrathin YIG films is of key importance
for functional chiral magnonics as ultra-low spin-wave damping can be achieved.
|
1910.02599v2
|
2019-11-20
|
Extreme events in a network of heterogeneous Josephson junctions
|
We report rare and recurrent large spiking events in a heterogeneous network
of superconducting Josephson junctions (JJ) connected through a resistive load
and driven by a radio-frequency (rf) current in addition to a constant bias.
The intermittent large spiking events show characteristic features of extreme
events (EE) since they are larger than a statistically defined significant
height. Under the influence of repulsive interactions and an impact of
heterogeneity of damping parameters, the network splits into three sub-groups
of junctions, one in incoherent rotational, another in coherent librational
motion and a third sub-group originating EE. We are able to scan the whole
population of junctions with their distinctive individual dynamical features
either in EE mode or non-EE mode in parameter space. EE migrates spatially from
one to another sub-group of junctions depending upon the repulsive strength and
the damping parameter. For a weak repulsive coupling, all the junctions
originate frequent large spiking events, in rotational motion when the average
inter-spike-interval (ISI) is small, but it increases exponentially with
repulsive interaction; it largely deviates from its exponential growth at a
break point where EE triggers in a sub-group of junctions. The probability
density of inter-event-intervals (IEI) in the subgroup exhibits a Poisson
distribution. EE originates via bubbling instability of in-phase
synchronization.
|
1911.10040v1
|
2019-12-05
|
Atypical Behavior of Collective Modes in Two-Dimensional Fermi Liquids
|
Using the Landau kinetic equation to study the non-equilibrium behavior of
interacting Fermi systems is one of the crowning achievements of Landau's Fermi
liquid theory. While thorough study of transport modes has been done for
standard three-dimensional Fermi liquids, an equally in-depth analysis for two
dimensional Fermi liquids is lacking. In applying the Landau kinetic equation
(LKE) to a two-dimensional Fermi liquid, we obtain unconventional behavior of
the zero sound mode $c_0$. As a function of the usual dimensionless parameter
$s=\omega/qv_F$, we find two peculiar results: First, for $|s|>1$ we see the
propagation of an undamped mode for weakly interacting systems. This differs
from the three dimensional case where an undamped mode only propagates for
repulsive interactions and the mode experiences Landau damping for any
arbitrary attractive interaction. Second, we find that regardless of
interaction strength, a propagating mode is forbidden for $|s|<1$. This is
profoundly different from the three-dimensional case where a mode can
propagate, albeit damped. In addition, we present a revised Pomeranchuk
instability condition for a two-dimensional Fermi liquid as well as equations
of motion for the fluid that follow directly from the LKE. In two dimensions,
we find a constant minimum for all Landau parameters for $\ell\geq 1$ which
differs from the three dimensional case. Finally we discuss the effect of a
Coulomb interaction on the system resulting in the plasmon frequency $\omega_p$
exhibiting a crossover to the zero sound mode.
|
1912.02699v2
|
2020-01-30
|
Ability of Markovian Master Equations to Model Quantum Computers and Other Systems Under Broadband Control
|
Most future quantum devices, including quantum computers, require control
that is broadband, meaning that the rate of change of the time-dependent
Hamiltonian is as fast or faster than the dynamics it generates. In many areas
of quantum physics, including quantum technology, one must include dissipation
and decoherence induced by the environment. While Markovian master equations
provide the only really efficient way to model these effects, these master
equations are derived for constant Hamiltonians (or those with a discrete set
of well-defined frequencies). In 2006, Alicky, Lidar, and Zanardi [Phys. Rev. A
73, 052311 (2006)] provided detailed qualitative arguments that Markovian
master equations could not describe systems under broadband control. Despite
apparently broad acceptance of these arguments, such master equations are
routinely used to model precisely these systems. This odd state of affairs is
likely due to a lack of quantitative results. Here we perform exact simulations
of two- and three-level systems coupled to an oscillator bath to obtain
quantitative results. Although we confirm that in general Markovian master
equations cannot predict the effects of damping under broadband control, we
find that there is a widely applicable regime in which they can. Master
equations are accurate for weak damping if both the Rabi frequencies and
bandwidth of the control are significantly smaller than the system's transition
frequencies. They also remain accurate if the bandwidth of control is as large
as the frequency of the driven transition so long as this bandwidth does not
overlap other transitions. Master equations are thus able to provide accurate
descriptions of many quantum information processing protocols for atomic
systems.
|
2001.11160v1
|
2020-02-06
|
Effects of transition-metal spacers on the spin-orbit torques, spin Hall magnetoresistance, and magnetic anisotropy of Pt/Co bilayers
|
We studied the effect of inserting 0.5 nm-thick spacer layers (Ti, V, Cr, Mo,
W) at the Pt/Co interface on the spin-orbit torques, Hall effect,
magnetoresistance, saturation magnetization, and magnetic anisotropy. We find
that the damping-like spin-orbit torque decreases substantially for all samples
with a spacer layer compared to the reference Pt/Co bilayer, consistently with
the opposite sign of the atomic spin-orbit coupling constant of the spacer
elements relative to Pt. The reduction of the damping-like torque is monotonic
with atomic number for the isoelectronic 3d, 4d, and 5d elements, with the
exception of V that has a stronger effect than Cr. The field-like spin-orbit
torque almost vanishes for all spacer layers irrespective of their composition,
suggesting that this torque predominantly originates at the Pt/Co interface.
The anomalous Hall effect, magnetoresistance, and saturation magnetization are
also all reduced substantially, whereas the sheet resistance is increased in
the presence of the spacer layer. Finally, we evidence a correlation between
the amplitude of the spin-orbit torques, the spin Hall-like magnetoresistance,
and the perpendicular magnetic anisotropy. These results highlight the
significant influence of ultrathin spacer layers on the magnetotransport
properties of heavy metal/ferromagnetic systems.
|
2002.02162v1
|
2020-04-30
|
Unifying femtosecond and picosecond single-pulse magnetic switching in GdFeCo
|
Many questions are still open regarding the physical mechanisms behind the
magnetic switching in GdFeCo alloys by single optical pulses. Phenomenological
models suggest a femtosecond scale exchange relaxation between sublattice
magnetization as the driving mechanism for switching. The recent observation of
thermally induced switching in GdFeCo by using both several picosecond optical
laser pulse as well as electric current pulses has questioned this previous
understanding. This has raised the question of whether or not the same
switching mechanics are acting at the femo- and picosecond scales. In this
work, we aim at filling this gap in the understanding of the switching
mechanisms behind thermal single-pulse switching. To that end, we have studied
experimentally thermal single-pulse switching in GdFeCo alloys, for a wide
range of system parameters, such as composition, laser power and pulse
duration. We provide a quantitative description of the switching dynamics using
atomistic spin dynamics methods with excellent agreement between the model and
our experiments across a wide range of parameters and timescales, ranging from
femtoseconds to picoseconds. Furthermore, we find distinct element-specific
damping parameters as a key ingredient for switching with long picosecond
pulses and argue, that switching with pulse durations as long as 15 picoseconds
is possible due to a low damping constant of Gd. Our findings can be easily
extended to speed up dynamics in other contexts where ferrimagnetic GdFeCo
alloys have been already demonstrated to show fast and energy-efficient
processes, e.g. domain-wall motion in a track and spin-orbit torque switching
in spintronics devices.
|
2004.14844v1
|
2020-06-12
|
Tidal Asteroseismology: Possible Evidence of Non-linear Mode Coupling in an Equilibrium State in Kepler Eclipsing Binary KIC 3230227
|
Previously, a series of tidally-excited oscillations were discovered in the
eccentric eclipsing binary KIC 3230227. The pulsation amplitudes and phases
suggest the observed oscillations are prograde quadruple modes. In this paper,
we refine the analysis and extract more oscillation frequencies. We also study
the temporal variations of amplitudes and phases and show that almost all modes
have stable phases and amplitudes. We then focus on the non-orbital-harmonic
oscillations. We consider two formation mechanisms: 1) nonlinear response of
the surface convective layer, and 2) nonlinear three/multi-mode coupling.
Although the former can explain some of the observed features, we find the
latter mechanism is more probable. Assuming that these are coupled modes, the
constant amplitude/phase over four years can be explained by either an
equilibrium state in the mode coupling or modes undergoing limit cycles with
very long periods. The observed frequency detuning and the calculated damping
rates of the daughter modes favor the equilibrium-state interpretation. This is
verified by integrating the amplitude equations of three-mode coupling. We find
that the steady-state relation derived in Weinberg et al., which relates the
observed frequency detuning, phase detuning, and mode damping rates, is
approximately satisfied for one mode triplet. We also try to identify the
azimuthal number of the modes based on the observed mode amplitude ratios and
the selection rules in nonlinear three-mode coupling. We discuss further
implications of these observations on nonlinear tidal asteroseismology.
|
2006.07417v1
|
2021-02-22
|
Anisotropic cosmological models in Horndeski gravity
|
It was found recently that the anisotropies in the homogeneous Bianchi I
cosmology considered within the context of a specific Horndeski theory are
damped near the initial singularity instead of being amplified. In this work we
extend the analysis of this phenomenon to cover the whole of the Horndeski
family. We find that the phenomenon is absent in the K-essence and/or Kinetic
Gravity Braiding theories, where the anisotropies grow as one approaches the
singularity. The anisotropies are damped at early times only in more general
Horndeski models whose Lagrangian includes terms quadratic and cubic in second
derivatives of the scalar field. Such theories are often considered as being
inconsistent with the observations because they predict a non-constant speed of
gravitational waves. However, the predicted value of the speed at present can
be close to the speed of light with any required precision, hence the theories
actually agree with the present time observations. We consider two different
examples of such theories, both characterized by a late self-acceleration and
an early inflation driven by the non-minimal coupling. Their anisotropies show
a maximum at intermediate times and approach zero at early and late times. The
early inflationary stage exhibits an instability with respect to inhomogeneous
perturbations, suggesting that the initial state of the universe should be
inhomogeneous. However, more general Horndeski models may probably be stable.
|
2102.10981v2
|
2021-03-18
|
Size limit of superparamagnetic inclusions in dust grains and difficulty of magnetic grain alignment in protoplanetary disks
|
Alignment of non-spherical grains with magnetic fields is an important
problem as it lays the foundation of probing magnetic fields with polarized
dust thermal emissions. In this paper, we investigate the feasibility of
magnetic alignment in protoplanetary disks (PPDs). We use an alignment
condition that Larmor precession should be fast compared with the damping
timescale. We first show that the Larmor precession timescale is some three
orders of magnitude longer than the damping time for millimeter-sized grains
under conditions typical of PPDs, making the magnetic alignment unlikely. The
precession time can be shortened by superparamagnetic inclusions (SPIs), but
the reduction factor strongly depends on the size of the SPI clusters, which we
find is limited by the so-called "N\'{e}el's relaxation process." In
particular, the size limit of SPIs is set by the so-called "anisotropic energy
constant" of the SPI material, which describes the energy barrier needed to
change the direction of the magnetic moment of an SPI. For the most common
iron-bearing materials, we find maximum SPI sizes corresponding to a reduction
factor of the Larmor precession timescale of order $10^3$. We also find that
reaching this maximum reduction factor requires fine-tuning on the SPI sizes.
Lastly, we illustrate the effects of the SPI size limits on magnetic alignment
of dust grains with a simple disk model, and we conclude that it is unlikely
for relatively large grains of order 100 $\mu$m or more to be aligned with
magnetic fields even with SPIs.
|
2103.10243v1
|
2021-05-19
|
Viscoelasticity and elastocapillarity effects in the impact of drops on a repellent surface
|
We investigate freely expanding viscoelastic sheets. The sheets are produced
by the impact of drops on a quartz plate covered with a thin layer of liquid
nitrogen that suppresses shear viscous dissipation as a result of the cold
Leidenfrost effect. The time evolution of the sheet is simultaneously recorded
from top and side views using high-speed cameras. The investigated viscoelastic
fluids are Maxwell fluids, which are characterized by low elastic moduli, and
relaxation times that vary over almost two orders of magnitude, thus giving
access to a large spectrum of viscoelastic and elastocapillary effects. For the
purposes of comparison, Newtonian fluids, with viscosity varying over three
orders of magnitude, are also investigated. In this study, $d_{\mathrm{max}}$,
the maximal expansion of the sheets, and $t_{\mathrm{max}}$ the time to reach
this maximal expansion from the time at impact, are measured as a function of
the impact velocity. By using a generalized damped harmonic oscillator model,
we rationalize the role of capillarity, bulk elasticity and viscous dissipation
in the expansion dynamics of all investigated samples. In the model, the spring
constant is a combination of the surface tension and the bulk dynamic elastic
modulus. The time-varying damping coefficient is associated to biaxial
extensional viscous dissipation and is proportional to the dynamic loss
modulus. For all samples, we find that the model reproduces accurately the
experimental data for $d_{\mathrm{max}}$ and $t_{\mathrm{max}}$.
|
2105.09244v1
|
2021-06-23
|
The dynamical exponent of a quantum critical itinerant ferromagnet: a Monte Carlo study
|
We consider the effect of the coupling between 2D quantum rotors near an XY
ferromagnetic quantum critical point and spins of itinerant fermions. We
analyze how this coupling affects the dynamics of rotors and the self-energy of
fermions.A common belief is that near a $q=0$ ferromagnetic transition,
fermions induce an $\Omega/q$ Landau damping of rotors (i.e., the dynamical
critical exponent is $z=3$) and Landau overdamped rotors give rise to non-Fermi
liquid fermionic self-energy $\Sigma\propto \omega^{2/3}$. This behavior has
been confirmed in previous quantum Monte Carlo (QMC) studies.Here we show that
for the XY case the behavior is different.We report the results of large scale
quantum Monte Carlo simulations,which show that at small frequencies $z=2$ and
$\Sigma\propto \omega^{1/2}$. We argue that the new behavior is associated with
the fact that a fermionic spin is by itself not a conserved quantity due to
spin-spin coupling to rotors, and a combination of self-energy and vertex
corrections replaces $1/q$ in the Landau damping by a constant. We discuss the
implication of these results to experiments.
|
2106.12601v3
|
2021-08-20
|
Cosmic-Ray Transport in Simulations of Star-forming Galactic Disks
|
Cosmic ray transport on galactic scales depends on the detailed properties of
the magnetized, multiphase interstellar medium (ISM). In this work, we
post-process a high-resolution TIGRESS magnetohydrodynamic simulation modeling
a local galactic disk patch with a two-moment fluid algorithm for cosmic ray
transport. We consider a variety of prescriptions for the cosmic rays, from a
simple purely diffusive formalism with constant scattering coefficient, to a
physically-motivated model in which the scattering coefficient is set by
critical balance between streaming-driven Alfv\'en wave excitation and damping
mediated by local gas properties. We separately focus on cosmic rays with
kinetic energies of $\sim 1$ GeV (high-energy) and $\sim 30$~MeV (low-energy),
respectively important for ISM dynamics and chemistry. We find that
simultaneously accounting for advection, streaming, and diffusion of cosmic
rays is crucial for properly modeling their transport. Advection dominates in
the high-velocity, low-density, hot phase, while diffusion and streaming are
more important in higher density, cooler phases. Our physically-motivated model
shows that there is no single diffusivity for cosmic-ray transport: the
scattering coefficient varies by four or more orders of magnitude, maximal at
density $n_\mathrm{H} \sim 0.01\, \mathrm{cm}^{-3}$. Ion-neutral damping of
Alfv\'en waves results in strong diffusion and nearly uniform cosmic ray
pressure within most of the mass of the ISM. However, cosmic rays are trapped
near the disk midplane by the higher scattering rate in the surrounding
lower-density, higher-ionization gas. The transport of high-energy cosmic rays
differs from that of low-energy cosmic rays, with less effective diffusion and
greater energy losses for the latter.
|
2108.09356v1
|
2021-09-13
|
Control of magnetization dynamics by substrate orientation in YIG thin films
|
Yttrium Iron Garnet (YIG) and bismuth (Bi) substituted YIG (Bi0.1Y2.9Fe5O12,
BYG) films are grown in-situ on single crystalline Gadolinium Gallium Garnet
(GGG) substrates [with (100) and (111) orientations] using pulsed laser
deposition (PLD) technique. As the orientation of the Bi-YIG film changes from
(100) to (111), the lattice constant is enhanced from 12.384 {\AA} to 12.401
{\AA} due to orientation dependent distribution of Bi3+ ions at dodecahedral
sites in the lattice cell. Atomic force microscopy (AFM) images show smooth
film surfaces with roughness 0.308 nm in Bi-YIG (111). The change in substrate
orientation leads to the modification of Gilbert damping which, in turn, gives
rise to the enhancement of ferromagnetic resonance (FMR) line width. The best
values of Gilbert damping are found to be (0.54)*10-4, for YIG (100) and
(6.27)*10-4, for Bi-YIG (111) oriented films. Angle variation measurements of
the Hr are also performed, that shows a four-fold symmetry for the resonance
field in the (100) grown film. In addition, the value of effective
magnetization (4{\pi}Meff) and extrinsic linewidth ({\Delta}H0) are observed to
be dependent on substrate orientation. Hence PLD growth can assist
single-crystalline YIG and BYG films with a perfect interface that can be used
for spintronics and related device applications.
|
2109.05901v1
|
2021-12-24
|
Excitation of ion-acoustic waves by non-linear finite-amplitude standing Alfvén waves
|
We investigate, using a multi-fluid approach, the main properties of standing
ion-acoustic modes driven by nonlinear standing Alfv\'en waves. The standing
character of the Alfv\'enic pump is because we study the superposition of two
identical circularly polarised counter-propagating waves. We consider parallel
propagation along the constant magnetic field and we find that left and
right-handed modes generate via ponderomotive forces the second harmonic of
standing ion-acoustic waves. We demonstrate that parametric instabilities are
not relevant in the present problem and the secondary ion-acoustic waves
attenuate by Landau damping in the absence of any other dissipative process.
Kinetic effects are included in our model where ions are considered as
particles and electrons as a massless fluid, and hybrid simulations are used to
complement the theoretical results. Analytical expressions are obtained for the
time evolution of the different physical variables in the absence of Landau
damping. From the hybrid simulations we find that the attenuation of the
generated ion-acoustic waves follows the theoretical predictions even under the
presence of a driver Alfv\'enic pump. Due to the nonlinear induced ion-acoustic
waves the system develops density cavities and an electric field parallel to
the magnetic field. Theoretical expressions for this density and electric field
fluctuations are derived. The implications of these results in the context of
standing slow mode oscillations in coronal loops is discussed.
|
2112.13048v1
|
2021-12-25
|
Internal modes and radiation damping for quadratic Klein-Gordon in 3D
|
We consider Klein-Gordon equations with an external potential $V$ and a
quadratic nonlinearity in $3+1$ space dimensions. We assume that $V$ is regular
and decaying and that the (massive) Schr\"odinger operator $H=-\Delta+V+m^2$
has a positive eigenvalue $\lambda^2<m^2$ with associated eigenfunction $\phi.$
This is a so-called internal mode and gives rise to time-periodic and spatially
localized solutions of the linear flow. We address the classical question of
whether such solutions persist under the full nonlinear flow, and describe the
behavior of all solutions in a suitable neighborhood of zero. Provided a
natural Fermi-Golden rule holds, our main result shows that a solution to the
nonlinear Klein-Gordon equation can be decomposed into a discrete component
$a(t)\phi$ where $a(t)$ decays over time, and a continuous component $v$ which
has some weak dispersive properties. We obtain precise asymptotic information
on these components such as the sharp rates of decay $\vert a(t) \vert \approx
t^{-1/2}$ and ${\| v(t) \|}_{L^\infty_x} \approx t^{-1}$, (where the implicit
constants are independent of the small size of the data) as well as the growth
of a natural weighted norm of the profile of $v.$ In particular, our result
extends the seminal work of Soffer-Weinstein for the cubic Klein-Gordon, and
shows that radiation damping also occurs in the quadratic case.
|
2112.13163v2
|
2022-01-17
|
Sizing of Energy Storage System for Virtual Inertia Emulation
|
The infusion of renewable energy sources into the conventional synchronous
generation system decreases the overall system inertia and negatively impacts
the stability of its primary frequency response. The lowered inertia is due to
the absence of inertia in some of the renewable energy-based systems. To
maintain the stability of the system, we need to keep the frequency in the
permissible limits and maintain low rotational inertia. Some authors in the
literature have used the virtual synchronous generators (VSG) as a solution to
this problem. Although the VSG based distributed recourses (DER) exhibits the
characteristics and behavior of synchronous generators (SG) such as inertia,
frequency droop functions and damping but it does not optimally solve the
question of frequency stability. This paper presents a solution for these
problems via an empirical model that sizes the Battery Energy Storage System
(BESS) required for the inertia emulation and damping control. The tested
system consists of a Photovoltaic (PV) based VSG that is connected to a 9-Bus
grid and the simulation experiments are carried out using EMTP software. The
VSG transient response is initiated by a symmetric fault on the grid side. Our
simulations show the battery energy sizing required to emulate the virtual
inertia corresponding to several design parameters, i.e., the droop gain,
K{\omega}, the droop coefficient, Kd, and the VSG time constant Ta.
|
2201.06566v2
|
2022-04-26
|
Galactic seismology: joint evolution of impact-triggered stellar and gaseous disc corrugations
|
Evidence for wave-like corrugations are well established in the Milky Way and
in nearby disc galaxies. These were originally detected as a displacement of
the interstellar medium about the midplane, either in terms of vertical
distance or vertical velocity. Over the past decade, similar patterns have
emerged in the Milky Way's stellar disc. We investigate how these vertical
waves are triggered by a passing satellite. Using high-resolution
N-body/hydrodynamical simulations, we systematically study how the corrugations
set up and evolve jointly in the stellar and gaseous discs. We find that the
gas corrugations follow the stellar corrugations, i.e. they are initially in
phase although, after a few rotation periods (500-700 Myr), the distinct waves
separate and thereafter evolve in different ways. The spatial and kinematic
amplitudes (and thus the energy) of the corrugations dampen with time, with the
gaseous corrugation settling at a faster rate (~800 Myr versus ~1 Gyr). In
contrast, the vertical energy of individual disc stars is fairly constant
throughout the galaxy's evolution. This difference arises because corrugations
are an emergent phenomenon supported by the collective, ordered motions of
co-spatial ensembles of stars. We show that the damping of the stellar
corrugations can be understood as a consequence of incomplete phase mixing,
while the damping of the gaseous corrugations is a natural consequence of the
dissipative nature of the gas. We suggest that - in the absence of further,
strong perturbations - the degree of correlation between the stellar and
gaseous waves may help to age-date the phenomenon.
|
2204.12096v2
|
2022-04-28
|
Low-frequency resolvent analysis of the laminar oblique shock wave / boundary layer interaction
|
Resolvent analysis is used to study the low-frequency behaviour of the
laminar oblique shock wave / boundary layer interaction (SWBLI). It is shown
that the computed optimal gain, which can be seen as a transfer function of the
system, follows a first-order low-pass filter equation, recovering the results
of Touber and Sandham (JFM, 2011). This behaviour is understood as proceeding
from the excitation of a single stable, steady global mode whose damping rate
sets the time scale of the filter. Different Mach and Reynolds numbers are
studied, covering different recirculation lengths $L$. This damping rate is
found to scale as $1/L$, leading to a constant Strouhal number $St_L$ as
observed in the literature. It is associated with a breathing motion of the
recirculation bubble. This analysis furthermore supports the idea that the
low-frequency dynamics of the SWBLI is a forced dynamics, in which background
perturbations continuously excite the flow. The investigation is then carried
out for 3D perturbations for which two regimes are identified. At low wave
numbers of the order of $L$, a modal mechanism similar to that of 2D
perturbations is found and exhibits larger values of the optimal gain. At
larger wave numbers of the order of the boundary layer thickness, the growth of
streaks, which results from a non-modal mechanism, is detected. No interaction
with the recirculation region is observed. Based on these results, the
potential prevalence of 3D effects in the low-frequency dynamics of the SWBLI
is discussed.
|
2204.13527v1
|
2022-09-01
|
Growth parameters of Bi0.1Y2.9Fe5O12 thin films for high frequency applications
|
The growth and characterization of Bismuth (Bi) substituted YIG (Bi-YIG,
Bi0.1Y2.9Fe5O12) thin films are reported. Pulsed laser deposited (PLD) films
with thicknesses ranging from 20 to 150 nm were grown on Gadolinium Gallium
Garnet substrates. Two substrate orientations of (100) and (111) were
considered. The enhanced distribution of Bi3+ ions at dodecahedral site along
(111) is observed to lead to an increment in lattice constant from 12.379
angstrom in (100) to 12.415 angstrom in (111) oriented films. Atomic force
microscopy images showed decreasing roughness with increasing film thickness.
Compared to (100) grown films, (111) oriented films showed an increase in
ferromagnetic resonance linewidth and consequent increase in Gilbert damping.
The lowest Gilbert damping values are found to be (1.06) * 10E-4 for (100) and
(2.30) * 10E-4 for (111) oriented films with thickness of 150 nm. The observed
values of extrinsic linewidth, effective magnetization, and anisotropic field
are related to thickness of the films and substrate orientation. In addition,
the in-plane angular variation established four-fold symmetry for the (100)
deposited films unlike the case of (111) deposited films. This study prescribes
growth conditions for PLD grown single-crystalline Bi-YIG films towards desired
high frequency and magneto-optical device applications.
|
2209.00558v1
|
2022-09-02
|
Identical damped harmonic oscillators described by coherent states
|
Some aspects of quantum damped harmonic oscillator (DHO) obeying a Markovian
master equation are considered in the absence of thermal noise. The continuity
equation is derived and Bohmian trajectories are constructed. As a solution of
the master equation, we take a single coherent state and compute analytically
the relative entropy of coherence, $C_r$, in the energy, position and momentum
bases. Although $C_r$ is constant in both the position and the momentum bases,
it is a decreasing function of time in the energy basis becoming zero at long
times, revealing its role as the preferred basis. Then, quantum coherence is
computed for a superposition of two coherent states, a cat state, and also a
superposition of two cat states in the energy basis as a function of
separation, in the complex plane, between the two superposed states. It is seen
that the quantum coherence increases with this separation. Furthermore, quantum
coherence of superposition is compared to that of decomposed states in the
superposition. Finally, considering a system of two non-interacting DHOs, the
effect of quantum statistics is studied on the coherence of reduced
single-particle states, the joint detection probability and the mean square
separation of particles. Our computations show that the single-particle
coherence for antisymmetric states is always less than that of symmetric ones.
Furthermore, boson anti-bunching and fermion bunching is seen in this open
system. This behavior of bosons is the matter-wave analogue of photon
anti-bunching seen in a modified Hanbury Brown-Twiss (HBT) interferometer.
|
2209.00914v2
|
2022-11-27
|
Dynamic surface tension of the pure liquid-vapor interface subjected to the cyclic loads
|
We demonstrate a methodology for computationally investigating the mechanical
response of a pure molten lead surface system to the lateral mechanical cyclic
loads and try to answer the question: how dose the dynamically driven liquid
surface system follow the classical physics of the elastic-driven oscillation?
The steady-state oscillation of the dynamic surface tension under cyclic load,
including the excitation of high frequency vibration mode at different driving
frequencies and amplitudes, was compared with the classical theory of
single-body driven damped oscillator. Under the highest studied frequency (50
GHz) and amplitude (5%) of the load, the increase of the (mean value) dynamic
surface tension could reach ~5%. The peak and trough values of the
instantaneous dynamic surface tension could reach (up to) 40% increase and (up
to) 20% decrease compared to the equilibrium surface tension, respectively. The
extracted generalized natural frequencies and the generalized damping constants
seem to be intimately related to the intrinsic timescales of the atomic
temporal-spatial correlation functions of the liquids both in the bulk region
and in the outermost surface layers. These insights uncovered could be helpful
for quantitative manipulation of the liquid surface tension using ultrafast
shockwaves or laser pulses.
|
2211.14766v1
|
2023-11-07
|
Strong electron-phonon coupling in Ba$_{1-x}$Sr$_x$Ni$_2$As$_2$
|
The charge density wave (CDW) or nematicity has been found to coexist with
superconductivity in many systems. It is thus interesting that the
superconducting transition temperature $T_c$ in the doped BaNi$_2$As$_2$ system
can be enhanced up to six times as the CDW or nematicity in the undoped
compound is suppressed. Here we show that the transverse acoustic phonons of
Ba$_{1-x}$Sr$_x$Ni$_2$As$_2$ are strongly damped in a wide doping range and
over the whole $Q$ range, which excludes its origin from either CDW or
nematicity. The damping of TA phonons can be understood as large
electron-phonon coupling and possible strong hybridization between acoustic and
optical phonons as shown by the first-principle calculations. The
superconductivity can be quantitatively reproduced by the change of
electron-phonon coupling constant calculated by the McMillan equation in the
BCS framework, which suggests that no quantum fluctuations of any order is
needed to promote the superconductivity. On the contrary, the change of $T_c$
in this system should be understood as the six-fold suppression of
superconductivity in undoped compounds.
|
2311.03649v2
|
2023-11-21
|
Numerical modelling of bulk viscosity in neutron stars
|
The early post-merger phase of a binary neutron-star coalescence is shaped by
characteristic rotational velocities as well as violent density oscillations
and offers the possibility to constrain the properties of neutron star matter
by observing the gravitational wave emission. One possibility to do so is the
investigation of gravitational wave damping through the bulk viscosity which
originates from violations of weak chemical equilibrium. Motivated by these
prospects, we present a comprehensive report about the implementation of the
self-consistent and second-order formulation of the equations of relativistic
hydrodynamics for dissipative fluids proposed by M\"uller, Israel and Stewart.
Furthermore, we report on the results of two test problems, namely the viscous
damping of linear density oscillations of isolated nonrotating neutron stars
and the viscous migration test, both of which confirm our implementation and
can be used for future code tests. Finally, we present fully
general-relativistic simulations of viscous binary neutron-star mergers. We
explore the structural and thermal properties of binary neutron-star mergers
with a constant bulk viscosity prescription and investigate the impact of bulk
viscosity on dynamical mass ejection. We find that inverse Reynolds numbers of
order $\sim 1\%$ can be achieved for the highest employed viscosity thereby
suppressing the dynamically ejected mass by a factor of $\sim 5$ compared to
the inviscid case.
|
2311.13027v1
|
2023-12-24
|
Transient growth of wavelet-based resolvent modes in the buffer layer of wall-bounded turbulence
|
In this work, we study the transient growth of the principal resolvent modes
in the minimal flow unit using a reformulation of resolvent analysis in a
time-localized wavelet basis. We target the most energetic spatial wavenumbers
for the minimal flow unit and obtain modes that are constant in the streamwise
direction and once-periodic in the spanwise direction. The forcing modes are in
the shape of streamwise rolls, though pulse-like in time, and the response
modes are in the form of transiently growing streaks. We inject the principal
transient forcing mode at different intensities into a simulation of the
minimal flow unit and compare the resulting nonlinear response to the linear
one. The peak energy amplification scales quadratically with the intensity of
the injected mode, and this peak occurs roughly at the same time for all
forcing intensities. However, the larger energy amplification intensifies the
magnitude of the nonlinear terms, which play an important role in damping the
energy growth and accelerating energy decay of the principal resolvent mode. We
also observe that the damping effect of the nonlinearities is less prominent
close to the wall. Finally, we find that the principal resolvent forcing mode
is more effective than other structures at amplifying the streak energy in the
turbulent minimal-flow unit. In addition to lending support to the claim that
linear mechanisms are important to near-wall turbulence, this work identifies
time scales for the nonlinear breakdown of linearly-generated streaks.
|
2312.15465v1
|
2024-02-27
|
Fourier analysis of near-field patterns generated by propagating polaritons
|
Scattering-type scanning near-field optical microscope (s-SNOM) has become an
essential tool to study polaritons - quasiparticles of light coupled to
collective charge oscillations - via direct probing of their near field with a
spatial resolution far beyond the diffraction limit. However, extraction of the
polariton complex propagation constant from the near-field images requires
subtle considerations that have not received necessary attention so far. In
this study, we discuss important yet overlooked aspects of the near-field
analysis. First, we experimentally demonstrate that the sample orientation
inside the s-SNOM may significantly affect the near-field interference pattern
of mid-infrared polaritons, leading to an error in momentum measurement up to
7.7% even for the modes with effective index of 12.5. Second, we establish a
methodology to correctly extract the polariton damping rate from the
interference fringes depending on their origin - the s-SNOM nano-tip or the
material edge. Overall, our work provides a unified framework for the accurate
extraction of the polariton momentum and damping from the near-field
interference fringes.
|
2402.17225v2
|
2000-09-11
|
Numerical Studies on Locally Damped Structures
|
In the JLC/NLC X-band linear collider, it is essential to reduce the
long-range dipole wakefields in the accelerator structure to prevent beam break
up (BBU) and emittance degradation. The two methods of reducing the long-range
wakefields are detuning and damping. Detuning reduces the wakefields rapidly as
the dipole modes de-cohere but, with a finite number of modes, the wakefield
will grow again as the modes re-cohere. In contrast, damping suppresses the
wakefields at a longer distance. There are two principal damping schemes:
synchronous damping using HOM manifolds such as that used in the RDDS1
structure and local damping similar to that used in the CLIC structure. In a
locally damped scheme, one can obtain almost any Q value, however, the damping
can have significant effects on the accelerating mode. In this paper, we
present a medium local-damping scheme where the wakefields are controlled to
meet the BBU requirement while minimizing the degradations of the fundamental
rf parameters. We will address the load design and pulse heating issues
associated with the medium damping scheme.
|
0009039v1
|
2015-03-13
|
A one-step optimal energy decay formula for indirectly nonlinearly damped hyperbolic systems coupled by velocities
|
In this paper, we consider the energy decay of a damped hyperbolic system of
wave-wave type which is coupled through the velocities. We are interested in
the asymptotic properties of the solutions of this system in the case of
indirect nonlinear damping, i.e. when only one equation is directly damped by a
nonlinear damping. We prove that the total energy of the whole system decays as
fast as the damped single equation. Moreover, we give a one-step general
explicit decay formula for arbitrary nonlinearity. Our results shows that the
damping properties are fully transferred from the damped equation to the
undamped one by the coupling in velocities, different from the case of
couplings through displacements as shown in \cite{AB01, ACK01, AB02, AL12} for
the linear damping case, and in \cite{AB07} for the nonlinear damping case. The
proofs of our results are based on multiplier techniques, weighted nonlinear
integral inequalities and the optimal-weight convexity method of \cite{AB05,
AB10}.
|
1503.04126v1
|
2015-08-21
|
Radiative damping in wave guide based FMR measured via analysis of perpendicular standing spin waves in sputtered Permalloy films
|
The damping $\alpha$ of the spinwave resonances in 75 nm, 120 nm, and 200nm
-thick Permalloy films is measured via vector-network-analyzer
ferromagnetic-resonance (VNA-FMR) in the out-of-plane geometry. Inductive
coupling between the sample and the waveguide leads to an additional radiative
damping term. The radiative contribution to the over-all damping is determined
by measuring perpendicular standing spin waves (PSSWs) in the Permalloy films,
and the results are compared to a simple analytical model. The damping of the
PSSWs can be fully explained by three contributions to the damping: The
intrinsic damping, the eddy-current damping, and the radiative damping. No
other contributions were observed. Furthermore, a method to determine the
radiative damping in FMR measurements with a single resonance is suggested.
|
1508.05265v1
|
2016-02-23
|
Experimental Investigation of Temperature-Dependent Gilbert Damping in Permalloy Thin Films
|
The Gilbert damping of ferromagnetic materials is arguably the most important
but least understood phenomenological parameter that dictates real-time
magnetization dynamics. Understanding the physical origin of the Gilbert
damping is highly relevant to developing future fast switching spintronics
devices such as magnetic sensors and magnetic random access memory. Here, we
report an experimental study of temperature-dependent Gilbert damping in
permalloy (Py) thin films of varying thicknesses by ferromagnetic resonance.
From the thickness dependence, two independent contributions to the Gilbert
damping are identified, namely bulk damping and surface damping. Of particular
interest, bulk damping decreases monotonically as the temperature decreases,
while surface damping shows an enhancement peak at the temperature of ~50 K.
These results provide an important insight to the physical origin of the
Gilbert damping in ultrathin magnetic films.
|
1602.07325v1
|
2017-09-29
|
Non-local Gilbert damping tensor within the torque-torque correlation model
|
An essential property of magnetic devices is the relaxation rate in magnetic
switching which depends strongly on the damping in the magnetisation dynamics.
It was recently measured that damping depends on the magnetic texture and,
consequently, is a non-local quantity. The damping enters the
Landau-Lifshitz-Gilbert equation as the phenomenological Gilbert damping
parameter $\alpha$, that does not, in a straight forward formulation, account
for non-locality. Efforts were spent recently to obtain Gilbert damping from
first principles for magnons of wave vector $\mathbf{q}$. However, to the best
of our knowledge, there is no report about real space non-local Gilbert damping
$\alpha_{ij}$. Here, a torque-torque correlation model based on a tight binding
approach is applied to the bulk elemental itinerant magnets and it predicts
significant off-site Gilbert damping contributions, that could be also
negative. Supported by atomistic magnetisation dynamics simulations we reveal
the importance of the non-local Gilbert damping in atomistic magnetisation
dynamics. This study gives a deeper understanding of the dynamics of the
magnetic moments and dissipation processes in real magnetic materials. Ways of
manipulating non-local damping are explored, either by temperature, material's
doping or strain.
|
1709.10365v1
|
2022-09-28
|
Tunable nonlinear damping in parametric regime
|
Nonlinear damping plays a significant role in several area of physics and it
is becoming increasingly important to understand its underlying mechanism.
However, microscopic origin of nonlinear damping is still a debatable topic.
Here, we probe and report nonlinear damping in a highly tunable MoS2 nano
mechanical drum resonator using electrical homodyne actuation and detection
technique. In our experiment, we achieve 2:1 internal resonance by tuning
resonance frequency and observe enhanced non-linear damping. We probe the
effect of non-linear damping by characterizing parametric gain. Geometry and
tunability of the device allow us to reduce the effect of other prominent
Duffing non-linearity to probe the non-linear damping effectively. The enhanced
non-linear damping in the vicinity of internal resonance is also observed in
direct drive, supporting possible origin of non-linear damping. Our experiment
demonstrates, a highly tunable 2D material based nanoresonator offers an
excellent platform to study the nonlinear physics and exploit nonlinear damping
in parametric regime.
|
2209.14120v1
|
2005-11-07
|
The Effects of Alfven Waves and Radiation Pressure in Dusty Winds of Late-Type Stars. II. Dust-Cyclotron Damping
|
There are in the literature several theories to explain the mass loss in
stellar winds. In particular, for late-type stars, some authors have proposed a
wind model driven by an outward-directed flux of damped Alfven waves. The winds
of these stars present great amounts of dust particles that, if charged, can
give rise to new wave modes or modify the pre-existing ones. In this work, we
study how the dust can affect the propagation of Alfven waves in these winds
taking into account a specific damping mechanism, dust-cyclotron damping. This
damping affects the Alfven wave propagation near the dust-cyclotron frequency.
Hence, if we assume a dust size distribution, the damping occurs over a broad
band of wave frequencies. In this work, we present a model of Alfven
wave-driven winds using the dust-cyclotron damping mechanism. On the basis of
coronal holes in the Sun, which present a superradial expansion, our model also
assumes a diverging geometry for the magnetic field. Thus, the mass, momentum,
and energy equations are obtained and then solved in a self-consistent
approach. Our results of wind velocity and temperature profiles for a typical
K5 supergiant star shows compatibility with observations. We also show that,
considering the presence of charged dust particles, the wave flux is less
damped due to the dust-cyclotron damping than it would be if we consider some
other damping mechanisms studied in the literature, such as nonlinear damping,
resonant surface damping, and turbulent damping.
|
0511192v2
|
2013-09-11
|
Initial versus tangent stiffness-based Rayleigh damping in inelastic time history seismic analyses
|
In the inelastic time history analyses of structures in seismic motion, part
of the seismic energy that is imparted to the structure is absorbed by the
inelastic structural model, and Rayleigh damping is commonly used in practice
as an additional energy dissipation source. It has been acknowledged that
Rayleigh damping models lack physical consistency and that, in turn, it must be
carefully used to avoid encountering unintended consequences as the appearance
of artificial damping. There are concerns raised by the mass proportional part
of Rayleigh damping, but they are not considered in this paper. As far as the
stiffness proportional part of Rayleigh damping is concerned, either the
initial structural stiffness or the updated tangent stiffness can be used. The
objective of this paper is to provide a comprehensive comparison of these two
types of Rayleigh damping models so that a practitioner (i) can objectively
choose the type of Rayleigh damping model that best fits her/his needs and (ii)
is provided with useful analytical tools to design Rayleigh damping model with
good control on the damping ratios throughout inelastic analysis. To that end,
a review of the literature dedicated to Rayleigh damping within these last two
decades is first presented; then, practical tools to control the modal damping
ratios throughout the time history analysis are developed; a simple example is
finally used to illustrate the differences resulting from the use of either
initial or tangent stiffness-based Rayleigh damping model.
|
1309.2741v1
|
2017-07-14
|
Damping of gravitational waves by matter
|
We develop a unified description, via the Boltzmann equation, of damping of
gravitational waves by matter, incorporating collisions. We identify two
physically distinct damping mechanisms -- collisional and Landau damping. We
first consider damping in flat spacetime, and then generalize the results to
allow for cosmological expansion. In the first regime, maximal collisional
damping of a gravitational wave, independent of the details of the collisions
in the matter is, as we show, significant only when its wavelength is
comparable to the size of the horizon. Thus damping by intergalactic or
interstellar matter for all but primordial gravitational radiation can be
neglected. Although collisions in matter lead to a shear viscosity, they also
act to erase anisotropic stresses, thus suppressing the damping of
gravitational waves. Damping of primordial gravitational waves remains
possible. We generalize Weinberg's calculation of gravitational wave damping,
now including collisions and particles of finite mass, and interpret the
collisionless limit in terms of Landau damping. While Landau damping of
gravitational waves cannot occur in flat spacetime, the expansion of the
universe allows such damping by spreading the frequency of a gravitational wave
of given wavevector.
|
1707.05192v2
|
2001-08-28
|
Galaxy number counts in the Hubble Deep Field as a strong constraint on a hierarchical galaxy formation model
|
Number counts of galaxies are re-analyzed using a semi-analytic model (SAM)
of galaxy formation based on the hierarchical clustering scenario. We have
determined the astrophysical parameters in the SAM that reproduce observations
of nearby galaxies, and used them to predict the number counts and redshifts of
faint galaxies for three cosmological models for (1) the standard cold dark
matter (CDM) universe, (2) a low-density flat universe with nonzero
cosmological constant, and (3) a low-density open universe with zero
cosmological constant. The novelty of our SAM analysis is the inclusion of
selection effects arising from the cosmological dimming of surface brightness
of high-redshift galaxies, and also from the absorption of visible light by
internal dust and intergalactic \ion{H}{1} clouds. Contrary to previous SAM
analyses which do not take into account such selection effects, we find, from
comparison with observed counts and redshifts of faint galaxies in the Hubble
Deep Field (HDF), that the standard CDM universe is {\it not} preferred, and a
low-density universe either with or without cosmological constant is favorable,
as suggested by other recent studies. Moreover, we find that a simple
prescription for the time scale of star formation (SF), being proportional to
the dynamical time scale of the formation of the galactic disk, is unable to
reproduce the observed number- redshift relation for HDF galaxies, and that the
SF time scale should be nearly independent of redshift, as suggested by other
SAM analyses for the formation of quasars and the evolution of damped
Ly-$\alpha$ systems.
|
0108433v1
|
2010-05-07
|
Quantum phase transitions of metals in two spatial dimensions: II. Spin density wave order
|
We present a field-theoretic renormalization group analysis of Abanov and
Chubukov's model of the spin density wave transition in two dimensional metals.
We identify the independent field scale and coupling constant renormalizations
in a local field theory, and argue that the damping constant of spin density
wave fluctuations tracks the renormalization of the local couplings. The
divergences at two-loop order overdetermine the renormalization constants, and
are shown to be consistent with our renormalization scheme. We describe the
physical consequences of our renormalization group equations, including the
breakdown of Fermi liquid behavior near the "hot spots" on the Fermi surface.
In particular, we find that the dynamical critical exponent z receives
corrections to its mean-field value z = 2. At higher orders in the loop
expansion, we find infrared singularities similar to those found by S.-S. Lee
for the problem of a Fermi surface coupled to a gauge field. A treatment of
these singularities implies that an expansion in 1/N, (where N is the number of
fermion flavors) fails for the present problem. We also discuss the
renormalization of the pairing vertex, and find an enhancement which scales as
logarithm-squared of the energy scale. A similar enhancement is also found for
a modulated bond order which is locally an Ising-nematic order.
|
1005.1288v2
|
2017-08-03
|
Evolution of the interfacial perpendicular magnetic anisotropy constant of the Co$_2$FeAl/MgO interface upon annealing
|
We investigate thickness series of films of the Heusler alloy Co$_2$FeAl in
order to study the effect of annealing on the interface with a MgO layer and on
the bulk magnetic properties. Our results reveal that while the perpendicular
interface anisotropy constant $K^{\perp}_{\rm S}$ is zero for the as-deposited
samples, its value increases with annealing up to a value of $1.14\, \pm
\,0.07$~mJ/m$^2$ for the series annealed at 320$^{\rm o}$C and of $2.07\, \pm
\,0.7$~mJ/m$^2$ for the 450$^{\rm o}$C annealed series owing to a strong
modification of the interface during the thermal treatment. This large value
ensures a stabilization of a perpendicular magnetization orientation for a
thickness below 1.7~nm. The data additionally shows that the in-plane biaxial
anisotropy constant has a different evolution with thickness in as-deposited
and annealed systems. The Gilbert damping parameter $\alpha$ shows minima for
all series for a thickness of 40~nm and an absolute minimum value of
$2.8\pm0.1\cdot10^{-3}$. The thickness dependence is explained in terms of an
inhomogenous magnetization state generated by the interplay between the
different anisotropies of the system and by crystalline disorder.
|
1708.01126v2
|
2023-09-10
|
Scalar fields around a loop quantum gravity black hole in de Sitter spacetime: Quasinormal modes, late-time tails and strong cosmic censorship
|
Loop quantum gravity, as one branch of quantum gravity, holds the potential
to explore the fundamental nature of black holes. Recently, according to the
quantum Oppenheimer-Snyder model in loop quantum cosmology, a novel loop
quantum corrected black hole in de Sitter spacetime has been discovered. Here,
we first investigate the corresponding quasinormal modes and late-time behavior
of massless neutral scalar field perturbations based on such a quantum-modified
black hole in de Sitter spacetime. The frequency and time domain analysis of
the lowest-lying quasinormal modes is derived by Prony method, Matrix method as
well as WKB approximation. The influences of loop quantum correction, the black
hole mass ratio, and the cosmological constant on the quasinormal frequencies
are studied in detail. The late-time behaviors of quantum-modified black holes
possess an exponential decay, which is mainly determined not only by the
multipole number but also by the cosmological constant. The impact of loop
quantum correction on the late-time tail is negligible, but it has a
significant impact on damping oscillation. To explore spacetime singularities,
we examine the validity of strong cosmic censorship for a near-extremal
quantum-modified black hole in de Sitter spacetime. As a result, it is found
that the strong cosmic censorship is destroyed as the black hole approaches the
near-extremal limit, but the violation becomes weaker as the cosmological
constant and the loop quantum correction increase.
|
2309.04962v2
|
2006-05-22
|
The entanglement of damped noon-state and its performance in phase measurement
|
The state evolution of the initial optical \textit{noon} state is
investigated. The residue entanglement of the state is calculated after it is
damped by amplitude and phase damping. The relative entropy of entanglement of
the damped state is exactly obtained. The performance of direct application of
the damped \textit{noon} state is compared with that of firstly distilling the
docoherence damped state then applying it in measurement.
|
0605184v1
|
2007-10-04
|
Channel-Adapted Quantum Error Correction for the Amplitude Damping Channel
|
We consider error correction procedures designed specifically for the
amplitude damping channel. We analyze amplitude damping errors in the
stabilizer formalism. This analysis allows a generalization of the [4,1]
`approximate' amplitude damping code of quant-ph/9704002. We present this
generalization as a class of [2(M+1),M] codes and present quantum circuits for
encoding and recovery operations. We also present a [7,3] amplitude damping
code based on the classical Hamming code. All of these are stabilizer codes
whose encoding and recovery operations can be completely described with
Clifford group operations. Finally, we describe optimization options in which
recovery operations may be further adapted according to the damping probability
gamma.
|
0710.1052v1
|
2011-11-30
|
Local phase damping of single qubits sets an upper bound on the phase damping rate of entangled states
|
I derive an inequality in which the phase damping rates of single qubits set
an upper bound for the phase damping rate of entangled states of many qubits.
The derivation is based on two assumptions: first, that the phase damping can
be described by a dissipator in Lindblad form and, second, that the phase
damping preserves the population of qubit states in a given basis.
|
1111.7152v2
|
2012-05-11
|
Quantum dynamics of the damped harmonic oscillator
|
The quantum theory of the damped harmonic oscillator has been a subject of
continual investigation since the 1930s. The obstacle to quantization created
by the dissipation of energy is usually dealt with by including a discrete set
of additional harmonic oscillators as a reservoir. But a discrete reservoir
cannot directly yield dynamics such as Ohmic damping (proportional to velocity)
of the oscillator of interest. By using a continuum of oscillators as a
reservoir, we canonically quantize the harmonic oscillator with Ohmic damping
and also with general damping behaviour. The dynamics of a damped oscillator is
determined by an arbitrary effective susceptibility that obeys Kramers-Kronig
relations. This approach offers an alternative description of nano-mechanical
oscillators and opto-mechanical systems.
|
1205.2545v1
|
2014-02-28
|
Escape rate for the power-law distribution in low-to-intermediate damping
|
Escape rate in the low-to-intermediate damping connecting the low damping
with the intermediate damping is established for the power-law distribution on
the basis of flux over population theory. We extend the escape rate in the low
damping to the low-to-intermediate damping, and get an expression for the
power-law distribution. Then we apply the escape rate for the power-law
distribution to the experimental study of the excited-state isomerization, and
show a good agreement with the experimental value. The extra current and the
improvement of the absorbing boundary condition are discussed.
|
1402.7194v2
|
2015-03-21
|
On damping created by heterogeneous yielding in the numerical analysis of nonlinear reinforced concrete frame elements
|
In the dynamic analysis of structural engineering systems, it is common
practice to introduce damping models to reproduce experimentally observed
features. These models, for instance Rayleigh damping, account for the damping
sources in the system altogether and often lack physical basis. We report on an
alternative path for reproducing damping coming from material nonlinear
response through the consideration of the heterogeneous character of material
mechanical properties. The parameterization of that heterogeneity is performed
through a stochastic model. It is shown that such a variability creates the
patterns in the concrete cyclic response that are classically regarded as
source of damping.
|
1503.07122v1
|
2016-01-20
|
Introduction to Landau Damping
|
The mechanism of Landau damping is observed in various systems from plasma
oscillations to accelerators. Despite its widespread use, some confusion has
been created, partly because of the different mechanisms producing the damping
but also due to the mathematical subtleties treating the effects. In this
article the origin of Landau damping is demonstrated for the damping of plasma
oscillations. In the second part it is applied to the damping of coherent
oscillations in particle accelerators. The physical origin, the mathematical
treatment leading to the concept of stability diagrams and the applications are
discussed.
|
1601.05227v1
|
2018-07-25
|
Regularity and asymptotic behaviour for a damped plate-membrane transmission problem
|
We consider a transmission problem where a structurally damped plate equation
is coupled with a damped or undamped wave equation by transmission conditions.
We show that exponential stability holds in the damped-damped situation and
polynomial stability (but no exponential stability) holds in the
damped-undamped case. Additionally, we show that the solutions first defined by
the weak formulation, in fact have higher Sobolev space regularity.
|
1807.09730v1
|
2021-08-04
|
Nonlinear fluid damping of elastically mounted pitching wings in quiescent water
|
We experimentally study the nonlinear fluid damping of a rigid but
elastically mounted pitching wing in the absence of a freestream flow. The
dynamics of the elastic mount are simulated using a cyber-physical system. We
perturb the wing and measure the fluid damping coefficient from damped
oscillations over a large range of pitching frequencies, pitching amplitudes,
pivot locations and sweep angles. A universal fluid damping scaling is proposed
to incorporate all these parameters. Flow fields obtained using particle image
velocimetry are analyzed to explain the nonlinear behaviors of the fluid
damping.
|
2108.02090v1
|
2019-06-18
|
Phase dynamics of effective drag and lift in vortex-induced vibration at low mass-damping
|
In this work, we investigate the dynamics of vortex-induced vibration of an
elastically mounted cylinder with very low values of mass and damping. We use
two methods to investigate this canonical problem: first we calculate the
instantaneous phase between the cylinder motion and the fluid forcing; second
we decompose the total hydrodynamic force into drag and lift components that
act along and normal to, respectively, the instantaneous effective angle of
attack. We focus on the phase dynamics in the large-amplitude-response range,
consisting of the initial, upper and lower branches of response. The
instantaneous phase between the transverse force and displacement shows
repeated phase slips separating periods of constant, or continuous-drifting,
phase in the second half of the upper branch. The phase between the lift
component and displacement shows strong phase locking throughout the
large-amplitude range - the average phase varies linearly with the primary
frequency - however the modulation of this phase is largest in the second half
of the upper branch. These observations suggest that the
large-amplitude-response dynamics is driven by two distinct limit cycles - one
that is stable over a very small range of reduced velocity at the beginning of
the upper branch, and another that consists of the lower branch. The chaotic
oscillation between them - the majority of the upper branch - occurs when
neither limit cycle is stable. The transition between the upper and lower
branches is marked by intermittent switching with epochs of time where
different states exist at a constant reduced velocity. These different states
are clearly apparent in the phase between the lift and displacement,
illustrating the utility of the force decomposition employed.
|
1906.07375v2
|
2002-11-03
|
Damping of coupled phonon--plasmon modes
|
The effect of free carriers on dispersion and damping of coupled
phonon-plasmon modes is considered in the long-wave approximation. The electron
and phonon scattering rate as well as Landau damping are taken into account.
|
0211040v1
|
2002-02-01
|
On "the authentic damping mechanism" of the phonon damping model
|
Some general features of the phonon damping model are presented. It is
concluded that the fits performed within this model have no physical content.
|
0202006v1
|
2010-12-20
|
Global attractors for the one dimensional wave equation with displacement dependent damping
|
We study the long-time behavior of solutions of the one dimensional wave
equation with nonlinear damping coefficient. We prove that if the damping
coefficient function is strictly positive near the origin then this equation
possesses a global attractor.
|
1012.4455v1
|
2018-01-28
|
Observations of excitation and damping of transversal oscillation in coronal loops by AIA/SDO
|
The excitation and damping of transversal coronal loop oscillations and
quantitative relation between damping time, damping quality (damping time per
period), oscillation amplitude, dissipation mechanism and the wake phenomena
are investigated. The observed time series data with the \textit{Atmospheric
Imaging Assembly} (AIA) telescope on NASA's \textit{Solar Dynamics Observatory}
(SDO) satellite on 2015 March 2, consisting of 400 consecutive images with 12
seconds cadence in the 171 $ \rm{{\AA}}$ pass band is analyzed for evidence of
transversal oscillations along the coronal loops by Lomb-Scargle periodgram. In
this analysis signatures of transversal coronal loop oscillations that are
damped rapidly were found with dominant oscillation periods in the range of
$\rm{P=12.25-15.80}$ minutes. Also, damping times and damping qualities of
transversal coronal loop oscillations at dominant oscillation periods are
estimated in the range of $ \rm{\tau_d=11.76-21.46}$ minutes and $
\rm{\tau_d/P=0.86-1.49}$, respectively. The observational results of this
analysis show that damping qualities decrease slowly with increasing the
amplitude of oscillation, but periods of oscillations are not sensitive
function of amplitude of oscillations. The order of magnitude of the damping
qualities and damping times are in good agreement with previous findings and
the theoretical prediction for damping of kink mode oscillations by dissipation
mechanism. Furthermore, oscillation of loop segments attenuate with time
roughly as $t^{-\alpha}$ that magnitude values of $\alpha$ for 30 different
segments change from 0.51 to 0.75.
|
1801.09217v1
|
1999-11-16
|
Probing supernovae ejecta by Halpha damping wings
|
It is predicted that H$\alpha$ emission line at the early nebular epoch of
type II-P supernovae may display robust observational effects of damping wings.
This is illustrated by Monte-Carlo simulations. The strength of damping wing
effects may be used to constrain parameters of the line-emitting zone. An
anomalous redshift, width and red wing of H$\alpha$ revealed by SN 1997D on day
150 are explained in terms of damping wing effects.
|
9911300v1
|
2009-01-23
|
Rheological Interpretation of Rayleigh Damping
|
Damping is defined through various terms such as energy loss per cycle (for
cyclic tests), logarithmic decrement (for vibration tests), complex modulus,
rise-time or spectrum ratio (for wave propagation analysis), etc. For numerical
modeling purposes, another type of damping is frequently used : it is called
Rayleigh damping. It is a very convenient way of accounting for damping in
numerical models, although the physical or rheological meaning of this approach
is not clear. A rheological model is proposed to be related to classical
Rayleigh damping : it is a generalized Maxwell model with three parameters. For
moderate damping (<25%), this model perfectly coincide with Rayleigh damping
approach since internal friction has the same expression in both cases and
dispersive phenomena are negligible. This is illustrated by finite element
(Rayleigh damping) and analytical (generalized Maxwell model) results in a
simple one-dimensional case.
|
0901.3717v1
|
2015-05-06
|
Remarks on the asymptotic behavior of the solution of an abstract damped wave equation
|
We study an abstract damped wave equation. We prove that the solution of the
damped wave equation becomes closer to the solution of a heat type equation as
time tend to infinity. As an application of our approach, we also study the
asymptotic behavior of the damped wave equation in Euclidean space under the
geometric control condition.
|
1505.01794v2
|
2017-01-18
|
Two types of spurious damping forces potentially modeled in numerical seismic nonlinear response history analysis
|
The purpose of this paper is to provide practitioners with further insight
into spurious damping forces that can be generated in nonlinear seismic
response history analyses (RHA). The term 'spurious' is used to refer to
damping forces that are not present in an elastic system and appear as
nonlinearities develop: such damping forces are not necessarily intended and
appear as a result of modifications in the structural properties as it yields
or damages due to the seismic action. In this paper, two types of spurious
damping forces are characterized. Each type has often been treated separately
in the literature, but each has been qualified as 'spurious', somehow blurring
their differences. Consequently, in an effort to clarify the consequences of
choosing a particular viscous damping model for nonlinear RHA, this paper shows
that damping models that avoid spurious damping forces of one type do not
necessarily avoid damping forces of the other type.
|
1701.05092v1
|
2017-02-02
|
Exponential stability for a coupled system of damped-undamped plate equations
|
We consider the transmission problem for a coupled system of undamped and
structurally damped plate equations in two sufficiently smooth and bounded
subdomains. It is shown that, independently of the size of the damped part, the
damping is strong enough to produce uniform exponential decay of the energy of
the coupled system.
|
1702.00637v1
|
2017-09-11
|
Comparison of damping mechanisms for transverse waves in solar coronal loops
|
We present a method to assess the plausibility of alternative mechanisms to
explain the damping of magnetohydrodynamic (MHD) transverse waves in solar
coronal loops. The considered mechanisms are resonant absorption of kink waves
in the Alfv\'en continuum, phase-mixing of Alfv\'en waves, and wave leakage.
Our methods make use of Bayesian inference and model comparison techniques. We
first infer the values for the physical parameters that control the wave
damping, under the assumption of a particular mechanism, for typically observed
damping time-scales. Then, the computation of marginal likelihoods and Bayes
factors enable us to quantify the relative plausibility between the alternative
mechanisms. We find that, in general, the evidence is not large enough to
support a single particular damping mechanism as the most plausible one.
Resonant absorption and wave leakage offer the most probable explanations in
strong damping regimes, while phase mixing is the best candidate for
weak/moderate damping. When applied to a selection of 89 observed transverse
loop oscillations, with their corresponding measurements of damping times
scales and taking into account data uncertainties, we find that only in a few
cases positive evidence for a given damping mechanism is available.
|
1709.03347v1
|
2019-03-25
|
Distributed Inter-Area Oscillation Damping Control for Power Systems by Using Wind Generators and Load Aggregators
|
This paper investigates the potential of wind turbine generators (WTGs) and
load aggregators (LAs) to provide supplementary damping control services for
low frequency inter-area oscillations (LFOs) through the additional distributed
damping control units (DCUs) proposed in their controllers. In order to provide
a scalable methodology for the increasing number of WTGs and LAs, a novel
distributed control framework is proposed to coordinate damping controllers.
Firstly, a distributed algorithm is designed to reconstruct the system Jacobian
matrix for each damping bus (buses with damping controllers). Thus, the
critical LFO can be identified locally at each damping bus by applying
eigen-analysis to the obtained system Jacobian matrix. Then, if the damping
ratio of the critical LFO is less than a preset threshold, the control
parameters of DCUs will be tuned in a distributed and coordinated manner to
improve the damping ratio and minimize the total control cost at the same time.
The proposed control framework is tested in a modified IEEE 39-bus test system.
The simulation results with and without the proposed control framework are
compared to demonstrate the effectiveness of the proposed framework.
|
1903.10135v1
|
2019-08-19
|
Spectral determinant for the damped wave equation on an interval
|
We evaluate the spectral determinant for the damped wave equation on an
interval of length $T$ with Dirichlet boundary conditions, proving that it does
not depend on the damping. This is achieved by analysing the square of the
damped wave operator using the general result by Burghelea, Friedlander, and
Kappeler on the determinant for a differential operator with matrix
coefficients.
|
1908.06862v1
|
2020-10-12
|
Decays rates for Kelvin-Voigt damped wave equations II: the geometric control condition
|
We study in this article decay rates for Kelvin-Voigt damped wave equations
under a geometric control condition. We prove that when the damping coefficient
is sufficiently smooth ($C^1$ vanishing nicely) we show that exponential decay
follows from geometric control conditions (see~\cite{BuCh, Te12} for similar
results under stronger assumptions on the damping function).
|
2010.05614v2
|
2020-12-05
|
On Periodical Damping Ratio of a Controlled Dynamical System with Parametric Resonances
|
This report provides an interpretation on the periodically varying damping
ratio of a dynamical system with direct control of oscillation or vibration
damping. The principal parametric resonance of the system and a new type of
parametric resonance, named "zero-th order" parametric resonance, are
investigated by using the method of multiple scales to find approximate,
analytical solutions of the system, which provide an interpretation on such
damping variations.
|
2012.02932v1
|
2021-06-09
|
Grammage of cosmic rays in the proximity of supernova remnants embedded in a partially ionized medium
|
We investigate the damping of Alfv\'en waves generated by the cosmic ray
resonant streaming instability in the context of the cosmic ray escape and
propagation in the proximity of supernova remnants. We consider ion-neutral
damping, turbulent damping and non linear Landau damping in the warm ionized
and warm neutral phases of the interstellar medium. For the ion-neutral
damping, up-to-date damping coefficients are used. We investigate in particular
whether the self-confinement of cosmic rays nearby sources can appreciably
affect the grammage. We show that the ion-neutral damping and the turbulent
damping effectively limit the residence time of cosmic rays in the source
proximity, so that the grammage accumulated near sources is found to be
negligible. Contrary to previous results, this also happens in the most extreme
scenario where ion-neutral damping is less effective, namely in a medium with
only neutral helium and fully ionized hydrogen. Therefore, the standard
picture, in which CR secondaries are produced during the whole time spent by
cosmic rays throughout the Galactic disk, need not to be deeply revisited.
|
2106.04948v1
|
2021-06-22
|
Sharp decay rate for the damped wave equation with convex-shaped damping
|
We revisit the damped wave equation on two-dimensional torus where the damped
region does not satisfy the geometric control condition. We show that if the
damping vanishes as a H\"older function $|x|^{\beta}$, and in addition, the
boundary of the damped region is strictly convex, the wave is stable at rate
$t^{-1+\frac{2}{2\beta+7}}$, which is better than the known optimal decay rate
$t^{-1+\frac{1}{\beta+3}}$ for strip-shaped dampings of the same H\"older
regularity. Moreover, we show by example that the decay rate is optimal. This
illustrates the fact that the energy decay rate depends not only on the order
of vanishing of the damping, but also on the shape of the damped region. The
main ingredient of the proof is the averaging method (normal form reduction)
developed by Hitrick and Sj\"ostrand (\cite{Hi1}\cite{Sj}).
|
2106.11782v3
|
2021-08-09
|
Effect of stepwise adjustment of Damping factor upon PageRank
|
The effect of adjusting damping factor {\alpha}, from a small initial value
{\alpha}0 to the final desired {\alpha}f value, upon then iterations needed for
PageRank computation is observed. Adjustment of the damping factor is done in
one or more steps. Results show no improvement in performance over a fixed
damping factor based PageRank.
|
2108.04150v1
|
2021-08-17
|
Asymptotic behaviour of the wave equation with nonlocal weak damping, anti-damping and critical nonlinearity
|
In this paper we prove the existence of the global attractor for the wave
equation with nonlocal weak damping, nonlocal anti-damping and critical
nonlinearity.
|
2108.07395v2
|
2023-02-23
|
Buckling Metamaterials for Extreme Vibration Damping
|
Damping mechanical resonances is a formidable challenge in an increasing
number of applications. Many of the passive damping methods rely on using low
stiffness dissipative elements, complex mechanical structures or electrical
systems, while active vibration damping systems typically add an additional
layer of complexity. However, in many cases, the reduced stiffness or
additional complexity and mass render these vibration damping methods
unfeasible. Here, we introduce a method for passive vibration damping by
allowing buckling of the primary load path, which sets an upper limit for
vibration transmission: the transmitted acceleration saturates at a maximum
value, no matter what the input acceleration is. This nonlinear mechanism leads
to an extreme damping coefficient tan delta ~0.23 in our metal
metamaterial|orders of magnitude larger than the linear damping of traditional
lightweight structural materials. We demonstrate this principle experimentally
and numerically in free-standing rubber and metal mechanical metamaterials over
a range of accelerations, and show that bi-directional buckling can further
improve its performance. Buckling metamaterials pave the way towards extreme
vibration damping without mass or stiffness penalty, and as such could be
applicable in a multitude of high-tech applications, including aerospace
structures, vehicles and sensitive instruments.
|
2302.11968v1
|
2005-08-26
|
Damping of MHD turbulence in Solar Flares
|
(Abridged) We describe the cascade of plasma waves or turbulence injected,
presumably by reconnection, at scales comparable to the size of a solar flare
loop to scales comparable to particle gyroradii, and evaluate their damping by
various mechanisms. We show that the classical viscous damping is unimportant
for magnetically dominated or low beta plasmas and the primary damping
mechanism is the collisionless damping by the background particles. We show
that the damping rate is proportional to the total random momentum density of
the particles. For solar flare conditions this means that in most flares,
except the very large ones, the damping is dominated by thermal background
electrons. For large flares one requires acceleration of essentially all
background electrons into a nonthermal distribution so that the accelerated
electrons can be important in the damping of the waves. In general, damping by
thermal or nonthermal protons is negligible compared to that of electrons
except for quasi-perpendicular propagating waves or for rare proton dominated
flares with strong nuclear gamma-ray line emission. Using the rate for damping
we determine the critical scale below which the damping becomes important and
the spectrum of the turbulence steepens. This critical scale, however, has
strong dependence on the angle of propagation with respect to the magnetic
field direction. The waves can cascade down to very small scales, such as the
gyroradii of the particles at small angles (quasi-parallel propagation) and
possibly near 90 degree (quasi-perpendicular propagation) giving rise to a
highly anisotropic spectral distribution.
|
0508567v1
|
2011-07-27
|
Constraint damping for the Z4c formulation of general relativity
|
One possibility for avoiding constraint violation in numerical relativity
simulations adopting free-evolution schemes is to modify the continuum
evolution equations so that constraint violations are damped away. Gundlach et.
al. demonstrated that such a scheme damps low amplitude, high frequency
constraint violating modes exponentially for the Z4 formulation of General
Relativity. Here we analyze the effect of the damping scheme in numerical
applications on a conformal decomposition of Z4. After reproducing the
theoretically predicted damping rates of constraint violations in the linear
regime, we explore numerical solutions not covered by the theoretical analysis.
In particular we examine the effect of the damping scheme on low-frequency and
on high-amplitude perturbations of flat spacetime as well and on the long-term
dynamics of puncture and compact star initial data in the context of spherical
symmetry. We find that the damping scheme is effective provided that the
constraint violation is resolved on the numerical grid. On grid noise the
combination of artificial dissipation and damping helps to suppress constraint
violations. We find that care must be taken in choosing the damping parameter
in simulations of puncture black holes. Otherwise the damping scheme can cause
undesirable growth of the constraints, and even qualitatively incorrect
evolutions. In the numerical evolution of a compact static star we find that
the choice of the damping parameter is even more delicate, but may lead to a
small decrease of constraint violation. For a large range of values it results
in unphysical behavior.
|
1107.5539v2
|
2023-12-14
|
Nonlocal damping of spin waves in a magnetic insulator induced by normal, heavy, or altermagnetic metallic overlayer: a Schwinger-Keldysh field theory approach
|
Understanding spin wave (SW) damping, and how to control it to the point of
being able to amplify SW-mediated signals, is one of the key requirements to
bring the envisaged magnonic technologies to fruition. Even widely used
magnetic insulators with low magnetization damping in their bulk, such as
yttrium iron garnet, exhibit 100-fold increase in SW damping due to inevitable
contact with metallic layers in magnonic circuits, as observed in very recent
experiments [I. Bertelli et al., Adv. Quantum Technol. 4, 2100094 (2021)]
mapping SW damping in spatially-resolved fashion. Here, we provide microscopic
and rigorous understanding of wavevector-dependent SW damping using extended
Landau-Lifshitz-Gilbert equation with nonlocal damping tensor, instead of
conventional local scalar Gilbert damping, as derived from Schwinger-Keldysh
nonequilibrium quantum field theory. In this picture, the origin of nonlocal
magnetization damping and thereby induced wavevector-dependent SW damping is
interaction of localized magnetic moments of magnetic insulator with conduction
electrons from the examined three different types of metallic overlayers --
normal, heavy, and altermagnetic. Due to spin-split energy-momentum dispersion
of conduction electrons in the latter two cases, the nonlocal damping is
anisotropic in spin and space, and it can be dramatically reduced by changing
the relative orientation of the two layers when compared to the usage of normal
metal overlayer.
|
2312.09140v1
|
1994-01-10
|
Radio Emitting Dust in the Free-Electron Layer of Spiral Galaxies: Testing the Disk/Halo Interface
|
We present a study of the radio emission from rotating, charged dust grains
immersed in the ionized gas constituting the thick, H$\alpha$-emitting disk of
many spiral galaxies. Using up-to-date optical constants, the charge on the
grains exposed to the diffuse galactic UV flux has been calculated. An
analytical approximation for the grain charge has been derived, which is then
used to obtain the grain rotation frequency. Grains are found to have
substantial radio emission peaked at a cutoff frequency in the range
10-100~GHz, depending on the grain size distribution and on the efficiency of
the radiative damping of the grain rotation. The dust radio emission is
compared to the free-free emission from the ionized gas component; some
constraints on the magnetic field strength in the observed dusty filaments are
also discussed. The model can be used to test the disk-halo interface
environment in spiral galaxies, to determine the amount and size distribution
of dust in their ionized component, and to investigate the rotation mechanisms
for the dust. Numerical estimates are given for experimental purposes.
|
9401010v1
|
1994-11-01
|
Toward Understanding CMB Anisotropies and Their Implications
|
Working toward a model independent understanding of cosmic microwave
background (CMB) anisotropies and their significance, we undertake a
comprehensive and self-contained study of scalar perturbation theory. Initial
conditions, evolution, thermal history, matter content, background dynamics,
and geometry all play a role in determining the anisotropy. By employing {\it
analytic} techniques to illuminate the numerical results, we are able to
separate and identify each contribution. We thus bring out the nature of the
{\it total} Sachs-Wolfe effect, acoustic oscillations, diffusion damping,
Doppler shifts, and reionization, as well as their particular manifestation in
a critical, curvature, or cosmological constant dominated universe. By studying
the full angular {\it and} spatial content of the resultant anisotropies, we
isolate the signature of these effects from the dependence on initial
conditions. Whereas structure in the Sachs-Wolfe anisotropy depends strongly on
the underlying power spectra, the acoustic oscillations provide features which
are nearly model independent. This may allow for future determination of the
matter content of the universe as well as the adiabatic and/or isocurvature
nature of the initial fluctuations.
|
9411008v1
|
1995-02-20
|
Constraints on Self-Interacting Dark Matter
|
We consider the growth of density perturbations in the presence of
self--interacting dark matter, SIDM, proposed by Carlson, Machacek and Hall
(1992). We determine the range of values for the coupling constant $\lambda$
and the particle mass $m^\prime$, for which the power spectrum lies in the
``allowed" range based on constraints from the IRAS galaxy survey and damped
Lyman--$\alpha $ systems. Our results show that no combination of parameters
can meet both limits. We consider constraints on the $\2-2$ scatterings which
keep the SIDM particles in pressure equilibrium, and we show that if such
interactions maintain pressure equilibrium down to the present, they will be
strong enough to disrupt galaxy mergers and may lead to stripping of galaxy
halos as galaxies move through the dark matter background of these particles.
Hence, we also investigate the evolution of large-scale structure in the SIDM
model when the particles drop out of pressure equilibrium at some higher
redshift. The resulting free-streaming leads to an additional suppression of
small-scale perturbations, but it does not significantly affect our results.
|
9502087v1
|
1996-12-16
|
Favored Variants of Cold Dark Matter Cosmologies
|
We discuss variants of Cold Dark Matter (CDM) dominated cosmological models
that give good agreement with a range of observations. We consider models with
hot dark matter, tilt, $\Omega < 1$, or a cosmological constant. We also
discuss the sensitivity of the results to other parameters, such as the Hubble
parameter and the baryon fraction. We obtain constraints by combining the COBE
data, cluster abundances, abundance of damped Lyman-$\alpha$ systems at
$z\sim3$, the small-angle Cosmic Microwave Background anisotropy, and the
small-scale non-linear power spectrum. We present non-linear power spectra from
a new suite of N-body simulations for the ``best-bet'' models from each
category.
|
9612156v1
|
1997-08-07
|
Gravitational Magnification of the Cosmic Microwave Background
|
Some aspects of gravitational lensing by large scale structure (LSS) are
investigated. We show that lensing causes the damping tail of the cosmic
microwave background (CMB) power spectrum to fall less rapidly with decreasing
angular scale than previously expected. This is due to a transfer of power from
larger to smaller angular scales which produces a fractional change in power
spectrum that increases rapidly beyond $\ell \sim 2000$. We also find that
lensing produces a nonzero mean magnification of structures on surfaces of
constant redshift if weighted by area on the sky. This is a result of the fact
that light-rays that are evenly distributed on the sky oversample overdense
regions. However this mean magnification has a negligible affect on the CMB
power spectrum. A new expression for the lensed power spectrum is derived and
it is found that future precision observations the high-$\ell$ tail of the
power spectrum will need to take into account lensing when determining
cosmological parameters.
|
9708059v1
|
1997-09-09
|
Thermochemical Instabilities in Optically Thin Reacting Plasmas
|
The linear stability analysis of an optically thin plasma where a general
reaction proceeds, including chemical relaxation time effects, is carried out .
A fifth order dispersion equation (instead of the fourth order one resulting
when such effects are neglected) is obtained. The new mode with the
corresponding instability criterion as well as the modifications of the
previous four modes and the corresponding instability criteria, are analyzed.
Generally, a further stabilizing effect on the unstable modes and an increasing
of the damping of stable modes appear because of the second viscosity generated
by the chemical reaction. The results are applied to: (1) a collisionally
ionized pure hydrogen plasma heated at a constant rate per unit mass and cooled
by free-free transitions, ionization, and e-H collisional excitations; (2) a
diffused gas with metallicity Z, photoionized and heated by a radiation field,
and cooled by excitation of hydrogen and heavy metal lines.
|
9709079v1
|
1998-11-10
|
Temperature Anisotropies and Distortions Induced by Hot Intracluster Gas on the Cosmic Microwave Background
|
The power spectrum of temperature anisotropies induced by hot intracluster
gas on the cosmic background radiation is calculated. For low multipoles it
remains constant while at multipoles above $l>2000$ it is exponentially damped.
The shape of the radiation power spectrum is almost independent of the average
intracluster gas density profile, gas evolution history or clusters virial
radii; but the amplitude depends strongly on those parameters and could be as
large as 20% that of intrinsic contribution. The exact value depends on the
global properties of the cluster population and the evolution of the
intracluster gas. The distortion on the Cosmic Microwave Background black body
spectra varies in a similar manner. The ratio of the temperature anisotropy to
the mean Comptonization parameters is shown to be almost independent of the
cluster model and, in first approximation, depends only on the number density
of clusters.
|
9811158v1
|
2001-12-13
|
Do the Fundamental Constants Vary in the Course of the Cosmological Evolution?
|
We estimate the cosmological variation of the proton-to-electron mass ratio
\mu=m_p/m_e by measuring the wavelengths of molecular hydrogen transitions in
the early universe. The analysis is performed using high spectral resolution
observations (FWHM ~ 7 km/s) of two damped Lyman-\alpha systems at
z_{abs}=2.3377 and 3.0249 observed along the lines of sight to the quasars Q
1232+082 and Q 0347-382 respectively. The most conservative result of the
analysis is a possible variation of \mu over the last ~ 10 Gyrs, with an
amplitude \Delta\mu/\mu = (5.7+-3.8)x10^{-5}. The result is significant at the
1.5\sigma level only and should be confirmed by further observations. This is
the most stringent estimate of a possible cosmological variation of \mu
obtained up to now.
|
0112323v2
|
2002-10-20
|
Non-Axisymmetric g-Mode and p-Mode Instability in a Hydrodynamic Thin Accretion Disk
|
It has been suggested that quasi-periodic oscillations of accreting X-ray
sources may relate to the modes named in the title. We consider
non-axisymmetric linear perturbations to an isentropic, isothermal,
unmagnetized thin accretion disk. The radial wave equation, in which the number
of vertical nodes (n) appears as a separation constant, admits a wave-action
current that is conserved except, in some cases, at corotation. Waves without
vertical nodes amplify when reflected by a barrier near corotation. Their
action is conserved. As was previously known, this amplification allows the n=0
modes to be unstable under appropriate boundary conditions. In contrast, we
find that waves with n >0 are strongly absorbed at corotation rather than
amplified; their action is not conserved. Therefore, non-axisymmetric p-modes
and g-modes with n>0 are damped and stable even in an inviscid disk. This
eliminates a promising explanation for quasi-periodic oscillations in
neutron-star and black-hole X-ray binaries.
|
0210455v3
|
2003-10-23
|
Atomic and Molecular Absorption at High Redshift
|
Strong constraints on possible variations in fundamental constants can be
derived from HI 21-cm and molecular rotational absorption lines observed
towards quasars. With the aim of forming a statistical sample of constraints we
have begun a program of systematic searches for such absorption systems. Here
we describe molecular rotational searches in 25 damped Lyman-alpha systems
where, in many cases, we set optical depth limits an order of magnitude better
than that required to detect the 4 known redshifted millimeter-wave absorbers.
We also discuss the contributory factors in the detectability of HI 21-cm
absorption, focusing on possible biases (e.g.low covering factors) in the
currently known sample of absorbers and non-detections.
|
0310672v2
|
2004-06-01
|
Constraints on Resonant Particle Production during Inflation from the Matter and CMB Power Spectra
|
We analyze the limits on resonant particle production during inflation based
upon the power spectrum of fluctuations in matter and the cosmic microwave
background. We show that such a model is consistent with features observed in
the matter power spectrum deduced from galaxy surveys and damped Lyman-alpha
systems at high redshift. It also provides an alternative explanation for the
excess power observed in the power spectrum of the cosmic microwave background
fluctuations in the range of 1000 < l < 3500. For our best-fit models, epochs
of resonant particle creation reenter the horizon at wave numbers ~ 0.4 and/or
0.2 (h/Mpc). The amplitude and location of these features correspond to the
creation of fermion species of mass ~ 1-2 Mpl during inflation with a coupling
constant between the inflaton field and the created fermion species of near
unity. Although the evidence is marginal, if this interpretation is correct,
this could be one of the first observational hints of new physics at the Planck
scale.
|
0406046v2
|
2005-11-28
|
Most precise single redshift bound to Delta alpha/alpha
|
Verification of theoretical predictions of an oscillating behavior of the
fine-structure constant alpha with cosmic time requires high precision Delta
alpha/alpha measurements at individual redshifts, while in earlier studies the
mean Delta alpha/alpha values averaged over wide redshift intervals were
usually reported. This requirement can be met via the single ion differential
alpha measurement (SIDAM) procedure proposed in Levshakov et al. (2005). We
apply the SIDAM to the FeII lines associated with the damped Ly-alpha system
observed at z=1.15 in the spectrum of HE0515-4414. The weighted mean <Delta
alpha/alpha> calculated on base of carefully selected 34 FeII pairs {1608,X} (X
= 2344, 2374, and 2586 A) is <Delta alpha/alpha> = (-0.07+/-0.84) 10^{-6}
(1sigma C.L.). The precision of this estimate improves by a factor 2 the
previous one reported for the same system by Quast et al. (2004). The obtained
result represents an absolute improvement with respect to what has been done in
the measurements of Delta alpha/alpha.
|
0511765v1
|
2006-06-08
|
Cosmological bounds on dark matter-neutrino interactions
|
We investigate the cosmological effects of a neutrino interaction with cold
dark matter. We postulate a neutrino that interacts with a ``neutrino
interacting dark matter'' (NIDM) particle with an elastic-scattering cross
section that either decreases with temperature as $T^2$ or remains constant
with temperature. The neutrino--dark-matter interaction results in a
neutrino--dark-matter fluid with pressure, and this pressure results in
diffusion-damped oscillations in the matter power spectrum, analogous to the
acoustic oscillations in the baryon-photon fluid. We discuss the bounds from
the Sloan Digital Sky Survey on the NIDM opacity (ratio of cross section to
NIDM-particle mass) and compare with the constraint from observation of
neutrinos from supernova 1987A. If only a fraction of the dark matter interacts
with neutrinos, then NIDM oscillations may affect current cosmological
constraints from measurements of galaxy clustering. We discuss how detection of
NIDM oscillations would suggest a particle-antiparticle asymmetry in the
dark-matter sector.
|
0606190v1
|
2006-07-26
|
Matter density perturbations in interacting quintessence models
|
Models with dark energy decaying into dark matter have been proposed to solve
the coincidence problem in cosmology. We study the effect of such coupling in
the matter power spectrum. Due to the interaction, the growth of matter density
perturbations during the radiation dominated regime is slower compared to
non-interacting models with the same ratio of dark matter to dark energy today.
This effect introduces a damping on the power spectrum at small scales
proportional to the strength of the interaction and similar to the effect
generated by ultrarelativistic neutrinos. The interaction also shifts
matter--radiation equality to larger scales. We compare the matter power
spectrum of interacting quintessence models with the measurments of 2dFGRS. We
particularize our study to models that during radiation domination have a
constant dark matter to dark energy ratio.
|
0607604v1
|
2006-11-27
|
High-Precision Measurements of Delta alpha/alpha from QSO Absorption Spectra
|
Precise radial velocity measurements (delta v/c ~ 10^{-7}) of FeII lines in
damped Ly-alpha systems from very high quality VLT/UVES spectra of quasars
HE0515-4414 and Q1101-264 are used to probe cosmological time dependence of the
fine structure constant, alpha. It is found that between two redshifts z1 =
1.15 and z2 = 1.84 the value of Delta alpha/alpha changes at the level of a few
ppm: (alpha_z2 - alpha_z1)/alpha_0 = 5.43 +/- 2.52 ppm. Variations of alpha can
be considered as one of the most reliable method to constrain the dark energy
equation of state and improvements on the accuracy of the wavelength
calibration of QSO spectra are of great importance.
|
0611803v1
|
2007-01-24
|
The N/O evolution on galaxies:the role played by the star formation history
|
We study the evolution of nitrogen resulting from a set of spiral and
irregular galaxy models computed for a large number of input mass radial
distributions and with various star formation efficiencies. We show that our
models produce a nitrogen abundance evolution in good agreement with the
observational data. In particular, low N/O values for high-redshift objects,
such as those obtained for Damped Lyman Alpha galaxies can be obtained with our
models simultaneously to higher and constant values of N/O as those observed
for irregular and dwarf galaxies, at the same low oxygen abundances $\rm
12+log(O/H) \sim 7$ dex. The differences in the star formation histories of the
regions and galaxies modeled are essential to reproduce the observational data
in the N/O-O/H plane.
|
0701691v1
|
1994-07-22
|
X-Ray Scattering Measurements of the Transient Structure of a Driven Charge-Density-Wave
|
We report time-resolved x-ray scattering measurements of the transient
structural response of the sliding {\bf Q}$_{1}$ charge-density-wave (CDW) in
NbSe$_{3}$ to a reversal of the driving electric field. The observed time scale
characterizing this response at 70K varies from $\sim$ 15 msec for driving
fields near threshold to $\sim$ 2 msec for fields well above threshold. The
position and time-dependent strain of the CDW is analyzed in terms of a
phenomenological equation of motion for the phase of the CDW order parameter.
The value of the damping constant, $\gamma = (3.2 \pm 0.7) \times 10^{-19}$ eV
$\cdot$ seconds $\cdot$ \AA$^{-3}$, is in excellent agreement with the value
determined from transport measurements. As the driving field approaches
threshold from above, the line shape becomes bimodal, suggesting that the CDW
does not depin throughout the entire sample at one well-defined voltage.
|
9407094v1
|
1995-07-03
|
Fundamental steps of group velocity for slow surface polariton under the quantum hall effect conditions
|
A new type of collective electromagnetic excitations, namely surface
polaritons (SP) --- in a 2D electronic layer in a high magnetic field under
Quantum Hall Effect (QHE) conditions is predicted. We have found the spectrum,
damping, and polarization of the SP in a wide range of frequencies $\omega$ and
wavevectors $\bf k$. It is shown that near the Cyclotron Resonance (CR)
($\omega\sim\Omega=\displaystyle eB/mc$) the phase velocity of the SP is
drastically slowed down and the group velocity undergoes fundamental steps
defined by the Fine Structure Constant $\alpha=e^2/\hbar c$. In the vicinity of
a CR subharmonic ($\omega\sim 2 \Omega$) the negative (anomalous) dispersion of
the SP occurs. The relaxation of electrons in the 2D layer gives rise to a new
dissipative collective threshold-type mode of the SP. We suggest a method for
calculating the kinetic coefficients for the 2D electronic layer under QHE
condition, using the Wigner distribution function formalism and determine their
spatial and frequency dispersion. Using this method we have calculated the
line-shape of the CR and the d.c. conductance under the QHE condition, which
are in good agreement with experimental data.
|
9507001v1
|
1995-08-18
|
On the normal phase of 2D Fermi liquid with weak attraction between particles
|
Proceeding from the simplest field theoretical model of 2D metal, the normal
phase Green functions of the weakly interacting fermions and the order
parameter fluctuations (responsible for the attraction between fermions) are
obtained. It is shown that taking into consideration the fluctuations mentioned
leads to a considerable reduction of the fermion wave function renormalization
constant (quasiparticle weight) as well as to a linear dependence of the
quasiparticle damping on the temperature. A general dependence of 2D Fermi
liquid properties on the fermion density is discussed. The relevance of the
proposed model to the marginal behavior of the Fermi liquid of high--$T_c$
superconductors, in particular, to their linear temperature dependence of the
resistivity is indicated.
|
9508076v1
|
1997-12-19
|
Spatiotemporal dynamics of discrete sine-Gordon lattices with sinusoidal couplings
|
The spatiotemporal dynamics of a damped sine-Gordon chain with sinusoidal
nearest-neighbor couplings driven by a constant uniform force are discussed.
The velocity characteristics of the chain versus the external force is shown.
Dynamics in the high- and low-velocity regimes are investigated. It is found
that in the high-velocity regime, the dynamics is dominated by rotating modes,
the velocity shows a branching bifurcation feature, while in the low-velocity
regime, the velocity exhibits step-like dynamical transitions, broken by the
destruction of strong resonances.
|
9712226v1
|
1998-11-06
|
Intermediate temperature dynamics of one-dimensional Heisenberg antiferromagnets
|
We present a general theory for the intermediate temperature (T) properties
of Heisenberg antiferromagnets of spin-S ions on p-leg ladders, valid for 2Sp
even or odd. Following an earlier proposal for 2Sp even (Damle and Sachdev,
cond-mat/9711014), we argue that an integrable, classical, continuum model of a
fixed-length, 3-vector applies over an intermediate temperature range; this
range becomes very wide for moderate and large values of 2Sp. The coupling
constants of the effective model are known exactly in terms of the energy gap
above the ground state (for 2Sp even) or a crossover scale (for 2Sp odd).
Analytic and numeric results for dynamic and transport properties are obtained,
including some exact results for the spin-wave damping. Numerous quantitative
predictions for neutron scattering and NMR experiments are made. A general
discussion on the nature of T>0 transport in integrable systems is also
presented: an exact solution of a toy model proves that diffusion can exist in
integrable systems, provided proper care is taken in approaching the
thermodynamic limit.
|
9811083v2
|
1999-02-25
|
Mobility of Bloch Walls via the Collective Coordinate Method
|
We have studied the problem of the dissipative motion of Bloch walls
considering a totally anisotropic one dimensional spin chain in the presence of
a magnetic field. Using the so-called "collective coordinate method" we
construct an effective Hamiltonian for the Bloch wall coupled to the magnetic
excitations of the system. It allows us to analyze the Brownian motion of the
wall in terms of the reflection coefficient of the effective potential felt by
the excitations due to the existence of the wall. We find that for finite
values of the external field the wall mobility is also finite. The spectrum of
the potential at large fields is investigated and the dependence of the damping
constant on temperature is evaluated. As a result we find the temperature and
magnetic field dependence of the wall mobility.
|
9902330v1
|
1999-04-06
|
Non-Fermi-liquid behavior in the Kondo lattices induced by peculiarities of magnetic ordering and spin dynamics
|
A scaling consideration of the Kondo lattices is performed with account of
singularities in the spin excitation spectral function. It is shown that a
non-Fermi-liquid (NFL) behavior between two critical values of the bare $s-f$
coupling constant occurs naturally for complicated magnetic structures with
several magnon branches. This may explain the fact that a NFL behavior takes
place often in the heavy-fermion systems with peculiar spin dynamics. Another
kind of a NFL-like state (with different critical exponents) can occur for
simple antiferromagnets with account of magnon damping, and for paramagnets,
especially with two-dimensional character of spin fluctuations. The mechanisms
proposed lead to some predictions about behavior of specific heat, resistivity,
magnetic susceptibility, and anisotropy parameter, which can be verified
experimentally.
|
9904072v3
|
1999-11-23
|
Collisionless dynamics of dilute Bose gases: Role of quantum and thermal fluctuations
|
We study the low-energy collective oscillations of a dilute Bose gas at
finite temperature in the collisionless regime. By using a time-dependent
mean-field scheme we derive for the dynamics of the condensate and
noncondensate components a set of coupled equations, which we solve
perturbatively to second order in the interaction coupling constant. This
approach is equivalent to the finite-temperature extension of the Beliaev
approximation and includes corrections to the Gross-Pitaevskii theory due both
to quantum and thermal fluctuations. For a homogeneous system we explicitly
calculate the temperature dependence of the velocity of propagation and damping
rate of zero sound. In the case of harmonically trapped systems in the
thermodynamic limit, we calculate, as a function of temperature, the frequency
shift of the low-energy compressional and surface modes.
|
9911377v1
|
2000-04-07
|
Coherent Atomic Oscillations and Resonances between Coupled Bose-Einstein Condensates with Time-Dependent Trapping Potential
|
We study the quantum coherent-tunneling between two Bose-Einstein condensates
separated through an oscillating trap potential. The cases of slowly and
rapidly varying in time trap potential are considered. In the case of a slowly
varying trap we study the nonlinear resonances and chaos in the oscillations of
the relative atomic population. Using the Melnikov function approach, we find
the conditions for chaotic macroscopic quantum-tunneling phenomena to exists.
Criteria for the onset of chaos are also given. We find the values of frequency
and modulation amplitude which lead to chaos on oscillations in the relative
population, for any given damping and the nonlinear atomic interaction. In the
case of a rapidly varying trap we use the multiscale expansion method in the
parameter epsilon = 1/Omega, where Omega is the frequency of modulations and we
derive the averaged system of equations for the modes. The analysis of this
system shows that new macroscopic quantum self trapping regions, in comparison
with the constant trap case, exist.
|
0004117v1
|
2000-05-02
|
Gravity-driven Dense Granular Flows
|
We report and analyze the results of numerical studies of dense granular
flows in two and three dimensions, using both linear damped springs and
Hertzian force laws between particles. Chute flow generically produces a
constant density profile that satisfies scaling relations suggestive of a
Bagnold grain inertia regime. The type of force law has little impact on the
behavior of the system. Bulk and surface flows differ in their failure criteria
and flow rheology, as evidenced by the change in principal stress directions
near the surface. Surface-only flows are not observed in this geometry.
|
0005051v1
|
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