publicationDate
stringlengths 1
2.79k
| title
stringlengths 1
36.5k
⌀ | abstract
stringlengths 1
37.3k
⌀ | id
stringlengths 9
47
|
|---|---|---|---|
2019-09-14
|
Measurement-Based Wide-Area Damping of Inter-Area Oscillations based on MIMO Identification
|
Interconnected power grid exhibits oscillatory response after a disturbance
in the system. One such type of oscillations, the inter-area oscillations has
the oscillation frequency in the range of 0.1 to 1 Hz. The damping of
inter-area oscillations is difficult with local controllers, but it can be
achieved using a Wide Area Damping Controller (WADC). For effective control,
the input to the WADC should be the most observable signal and the WADC output
should be sent to the most controllable generator. This paper presents a
measurement-based novel algorithm for multi-input-multi-output (MIMO) transfer
function identification of the power system based on optimization to estimate
such oscillation frequencies. Based on the MIMO transfer function the optimal
control loop for WADC is estimated. The WADC design is based on the discrete
linear quadratic regulator (DLQR) and Kalman filtering for damping of
inter-area oscillations. Since the MIMO identification is based on actual
measurements, the proposed method can accurately monitor changes in the power
grid whereas the conventional methods are based on small-signal analysis of a
linearized model which does not consider changing operating conditions. The
overall algorithm is implemented and validated on a RTDS/RSCAD and MATLAB
real-time co-simulation platform using two-area and IEEE 39 bus power system
models.
|
1909.06687v1
|
2020-02-07
|
Engineering Co$_2$MnAl$_x$Si$_{1-x}$ Heusler compounds as a model system to correlate spin polarization, intrinsic Gilbert damping and ultrafast demagnetization
|
Engineering of magnetic materials for developing better spintronic
applications relies on the control of two key parameters: the spin polarization
and the Gilbert damping responsible for the spin angular momentum dissipation.
Both of them are expected to affect the ultrafast magnetization dynamics
occurring on the femtosecond time scale. Here, we use engineered Co2MnAlxSi1-x
Heusler compounds to adjust the degree of spin polarization P from 60 to 100%
and investigate how it correlates with the damping. We demonstrate
experimentally that the damping decreases when increasing the spin polarization
from 1.1 10-3 for Co2MnAl with 63% spin polarization to an ultra-low value of
4.10-4 for the half-metal magnet Co2MnSi. This allows us investigating the
relation between these two parameters and the ultrafast demagnetization time
characterizing the loss of magnetization occurring after femtosecond laser
pulse excitation. The demagnetization time is observed to be inversely
proportional to 1-P and as a consequence to the magnetic damping, which can be
attributed to the similarity of the spin angular momentum dissipation processes
responsible for these two effects. Altogether, our high quality Heusler
compounds allow controlling the band structure and therefore the channel for
spin angular momentum dissipation.
|
2002.02686v1
|
2020-03-16
|
Spin-orbit torques originating from bulk and interface in Pt-based structures
|
We investigated spin-orbit torques in prototypical Pt-based spintronic
devices. We found that, in Pt/Ni and Pt/Fe bilayers, the damping-like torque
efficiency depends on the thickness of the Pt layer. We also found that the
damping-like torque efficiency is almost identical in the Pt/Ni and Pt/Fe
bilayers despite the stronger spin memory loss at the Pt/Fe interface. These
results suggest that although the dominant source of the damping-like torque is
the bulk spin Hall effect in the Pt layer, a sizable damping-like torque is
generated by the interface in the Pt/Fe bilayer due to the stronger interfacial
spin-orbit coupling. In contrast to the damping-like torque, whose magnitude
and sign are almost identical in the Pt/Ni and Pt/Fe bilayers, the field-like
torque strongly depends on the choice of the ferromagnetic layer. The sign of
the field-like torque originating from the bulk spin Hall effect in the Pt
layer is opposite between the Pt/Ni and Pt/Fe bilayers, which can be attributed
to the opposite sign of the imaginary part of the spin-mixing conductance.
These results demonstrate that the spin-orbit torques are quite sensitive to
the electronic structure of the FM layer.
|
2003.07271v2
|
2020-03-23
|
Heat-like and wave-like lifespan estimates for solutions of semilinear damped wave equations via a Kato's type lemma
|
In this paper we study several semilinear damped wave equations with
"subcritical" nonlinearities, focusing on demonstrating lifespan estimates for
energy solutions. Our main concern is on equations with scale-invariant damping
and mass. Under different assumptions imposed on the initial data, lifespan
estimates from above are clearly showed. The key fact is that we find
"transition surfaces", which distinguish lifespan estimates between "wave-like"
and "heat-like" behaviours. Moreover we conjecture that the lifespan estimates
on the "transition surfaces" can be logarithmically improved. As direct
consequences, we reorganize the blow-up results and lifespan estimates for the
massless case in which the "transition surfaces" degenerate to "transition
curves". Furthermore, we obtain improved lifespan estimates in one space
dimension, comparing to the known results. We also study semilinear wave
equations with the scattering damping and negative mass term, and find that if
the decay rate of the mass term equals to 2, the lifespan estimate is the same
as one special case of the equations with the scale-invariant damping and
positive mass. The main strategy of the proof consists of a Kato's type lemma
in integral form, which is established by iteration argument.
|
2003.10578v1
|
2020-09-03
|
Dynamics of magnetic collective modes in the square and triangular lattice Mott insulators at finite temperature
|
We study the equilibrium dynamics of magnetic moments in the Mott insulating
phase of the Hubbard model on the square and triangular lattice. We rewrite the
Hubbard interaction in terms of an auxiliary vector field and use a recently
developed Langevin scheme to study its dynamics. A thermal `noise', derivable
approximately from the Keldysh formalism, allows us to study the effect of
finite temperature. At strong coupling, $U \gg t$, where $U$ is the local
repulsion and $t$ the nearest neighbour hopping, our results reproduce the well
known dynamics of the nearest neighbour Heisenberg model with exchange $J \sim
{\cal O}(t^2/U)$. These include crossover from weakly damped dispersive modes
at temperature $T \ll J$ to strong damping at $T \sim {\cal O}(J)$, and
diffusive dynamics at $T \gg J$. The crossover temperatures are naturally
proportional to $J$. To highlight the progressive deviation from Heisenberg
physics as $U/t$ reduces we compute an effective exchange scale $J_{eff}(U)$
from the low temperature spin wave velocity. We discover two features in the
dynamical behaviour with decreasing $U/t$: (i)~the low temperature dispersion
deviates from the Heisenberg result, as expected, due to longer range and
multispin interactions, and (ii)~the crossovers between weak damping, strong
damping, and diffusion take place at noticeably lower values of $T/J_{eff}$. We
relate this to enhanced mode coupling, in particular to thermal amplitude
fluctuations, at weaker $U/t$. A comparison of the square and triangular
lattice reveals the additional effect of geometric frustration on damping.
|
2009.01833v2
|
2020-09-15
|
Classification of the mechanisms of wave energy dissipation in the nonlinear oscillations of coated and uncoated bubbles
|
Acoustic waves are dissipated when they pass through bubbly media.
Dissipation by bubbles takes place through thermal damping (Td), radiation
damping (Rd) and damping due to the friction of the liquid (Ld) and friction of
the coating (Cd). Knowledge of the contributions of the Td, Rd, Ld and Cd
during nonlinear bubble oscillations will help in optimizing bubble and
ultrasound exposure parameters for the relevant applications by maximizing a
desirable parameter. In this work we investigate the mechanisms of dissipation
in bubble oscillations and their contribution to the total damping (Wtotal) in
various nonlinear regimes. By using bifurcation analysis, we have classified
nonlinear dynamics of bubbles that are sonicated with their 3rd superharmonic
(SuH) and 2nd SuH resonance frequency (fr), pressure dependent resonance
frequency (PDfr), fr, subharmonic (SH) resonance (fsh=2fr), pressure dependent
SH resonance (PDfsh) and 1/3 order SH resonance. The corresponding Td, Rd, Ld,
Cd, Wtotal, scattering to dissipation ratio (STDR), maximum wall velocity and
maximum back-scattered pressure from non-destructive oscillations of bubbles
were calculated and analyzed using the bifurcation diagrams. We classified
different regimes of dissipation and provided parameter regions in which a
particular parameter of interest (e.g. Rd) can be enhanced. Afterwards enhanced
bubble activity is linked to some relevant applications in ultrasound. This
paper represents the first comprehensive analysis of the nonlinear oscillations
regimes and the corresponding damping mechanisms.
|
2009.07380v1
|
2020-11-18
|
The effect of redshift degeneracy and the damping effect of viscous medium on the information extracted from gravitational wave signals
|
Considering the cosmological redshift $z_c$ , the mass of GW source extracted
from GW signal is $1+z_c$ times larger than its intrinsic value, and distance
between detector and GW source should be regarded as luminosity distance.
However, besides cosmological redshift, there are other kinds of redshifts
should be considered, which is actually ignored, in the analysis of GW data,
such as Doppler redshift and gravitational redshift, so the parameters
extracted from GW may deviate from their intrinsic values. Another factor that
may affect GW is the viscous medium in propagation path of GW, which may damp
the GW with a damping rate of $16{\pi}G{\eta}$. Some studies indicate dark
matter may interact with each other, thus dark matter may be the origin of
viscosity of cosmic medium. Then the GW may be rapidly damped by the viscous
medium that is made of dark matter, such as dark matter "mini-spike" around
intermediate mass black hole. In this article, we mainly discuss how Doppler
and gravitational redshift, together with the damping effect of viscous medium,
affect the information, such as the mass and redshift of GW source, extracted
from GW signals.
|
2011.09169v2
|
2020-12-28
|
On the Efficacy of Friction Damping in the Presence of Nonlinear Modal Interactions
|
This work addresses friction-induced modal interactions in jointed
structures, and their effects on the passive mitigation of vibrations by means
of friction damping. Under the condition of (nearly) commensurable natural
frequencies, the nonlinear character of friction can cause so-called nonlinear
modal interactions. If harmonic forcing near the natural frequency of a
specific mode is applied, for instance, another mode may be excited due to
nonlinear energy transfer and thus contribute considerably to the vibration
response. We investigate how this phenomenon affects the performance of
friction damping. To this end, we study the steady-state, periodic forced
vibrations of a system of two beams connected via a local mechanical friction
joint. The system can be tuned to continuously adjust the ratio between the
first two natural frequencies in the range around the $1:3$ internal resonance,
in order to trigger or suppress the emergence of modal interactions. Due to the
re-distribution of the vibration energy, the vibration level can in fact be
reduced in certain situations. However, in other situations, the multi-harmonic
character of the vibration has detrimental effects on the effective damping
provided by the friction joint. The resulting response level can be
significantly larger than in the absence of modal interactions. Moreover, it is
shown that the vibration behavior is highly sensitive in the neighborhood of
internal resonances. It is thus concluded that the condition of internal
resonance should be avoided in the design of friction-damped systems.
|
2101.03232v1
|
2021-06-30
|
Origin of Nonlinear Damping due to Mode Coupling in Auto-Oscillatory Modes Strongly Driven by Spin-Orbit Torque
|
We investigate the physical origin of nonlinear damping due to mode coupling
between several auto-oscillatory modes driven by spin-orbit torque in
constricted Py/Pt heterostructures by examining the dependence of
auto-oscillation on temperature and applied field orientation. We observe a
transition in the nonlinear damping of the auto-oscillation modes extracted
from the total oscillation power as a function of drive current, which
coincides with the onset of power redistribution amongst several modes and the
crossover from linewidth narrowing to linewidth broadening in all individual
modes. This indicates the activation of another relaxation process by nonlinear
magnon-magnon scattering within the modes. We also find that both nonlinear
damping and threshold current in the mode-interaction damping regime at high
drive current after transition are temperature independent, suggesting that the
mode coupling occurs dominantly through a non-thermal magnon scattering process
via a dipole or exchange interaction rather than thermally excited
magnon-mediated scattering. This finding presents a promising pathway to
overcome the current limitations of efficiently controlling the interaction
between two highly nonlinear magnetic oscillators to prevent mode crosstalk or
inter-mode energy transfer and deepens understanding of complex nonlinear spin
dynamics in multimode spin wave systems.
|
2107.00150v2
|
2021-07-15
|
On the long-time behavior for a damped Navier-Stokes-Bardina model
|
In this paper, we consider a damped Navier-Stokes-Bardina model posed on the
whole three-dimensional. These equations have an important physical motivation
and they arise from some oceanic model. From the mathematical point of view,
they write down as the well-know Navier-Stokes equations with an additional
nonlocal operator in their nonlinear transport term, and moreover, with an
additional damping term depending of a parameter $\beta>0$. We study first the
existence and uniqueness of global in time weak solutions in the energy space.
Thereafter, our main objective is to describe the long time behavior of these
solutions. For this, we use some tools in the theory of dynamical systems to
prove the existence of a global attractor, which is a compact subset in the
energy space attracting all the weak solutions when the time goes to infinity.
Moreover, we derive an upper bound for the fractal dimension of the global
attractor associated to these equations.
Finally, we find a range of values for the damping parameter $\beta>0$, where
we are able to give an acutely description of the internal structure of the
global attractor. More precisely, we prove that the global attractor only
contains the stationary (time-independing) solution of the damped
Navier-Stokes-Bardina equations.
|
2107.07070v2
|
2021-07-17
|
Plasmon-Exciton Coupling Effect on Plasmon Damping
|
Plasmon decay via the surface or interface is a critical process for
practical energy conversion and plasmonic catalysis. However, the relationship
between plasmon damping and the coupling between the plasmon and 2D materials
is still unclear. The spectral splitting due to plasmon-exciton interaction
impedes the conventional single-particle method to evaluate the plasmon damping
rate by the spectral linewidth directly. Here, we investigated the interaction
between a single gold nanorod (GNR) and 2D materials using the single-particle
spectroscopy method assisted with in situ nanomanipulation technique by
comparing scattering intensity and linewidth together. Our approach allows us
to indisputably identify that the plasmon-exciton coupling in the GNR-WSe2
hybrid would induce plasmon damping. We can also isolate the contribution
between the charge transfer channel and resonant energy transfer channel for
the plasmon decay in the GNR-graphene hybrid by comparing that with thin hBN
layers as an intermediate medium to block the charge transfer. We find out that
the contact layer between the GNR and 2D materials contributes most of the
interfacial plasmon damping. These findings contribute to a deep understanding
of interfacial excitonic effects on the plasmon and 2D materials hybrid.
|
2107.08230v1
|
2021-10-12
|
Outflows in the presence of cosmic rays and waves with cooling
|
Plasma outflow from a gravitational potential well with cosmic rays and
self-excited Alfv\'en waves with cooling and wave damping is studied in the
hydrodynamics regime. We study outflows in the presence of cosmic ray and
Alfv\'en waves including the effect of cooling and wave damping. We seek
physically allowable steady-state subsonic-supersonic transonic solutions. We
adopted a multi-fluid hydrodynamical model for the cosmic ray plasma system.
Thermal plasma, cosmic rays, and self-excited Alfv\'en waves are treated as
fluids. Interactions such as cosmic-ray streaming instability, cooling, and
wave damping were fully taken into account. We considered one-dimensional
geometry and explored steady-state solutions. The model is reduced to a set of
ordinary differential equations, which we solved for subsonic-supersonic
transonic solutions with given boundary conditions at the base of the
gravitational potential well. We find that physically allowable
subsonic-supersonic transonic solutions exist for a wide range of parameters.
We studied the three-fluid system (considering only forward-propagating
Alfv\'en waves) in detail. We examined the cases with and without cosmic ray
diffusion separately. Comparisons of solutions with and without cooling and
with and without wave damping for the same set of boundary conditions (on
density, pressures of thermal gas, cosmic rays and waves) are presented. We
also present the interesting case of a four-fluid system (both forward- and
backward-propagating Alfv\'en waves are included), highlighting the intriguing
relation between different components.
|
2110.06170v1
|
2021-11-19
|
Finite time extinction for a class of damped Schr{ö}dinger equations with a singular saturated nonlinearity
|
We present some sharper finite extinction time results for solutions of a
class of damped nonlinear Schr{\"o}dinger equations when the nonlinear damping
term corresponds to the limit cases of some ``saturating non-Kerr law''
$F(|u|^2)u=\frac{a}{\varepsilon+(|u|^2)^\alpha}u,$ with $a\in\mathbb{C},$
$\varepsilon\geqslant0,$ $2\alpha=(1-m)$ and $m\in[0,1).$ To carry out the
improvement of previous results in the literature we present in this paper a
careful revision of the existence and regularity of weak solutions under very
general assumptions on the data. We prove that the problem can be solved in the
very general framework of the maximal monotone operators theory, even under a
lack of regularity of the damping term. This allows us to consider, among other
things, the singular case $m=0.$ We replace the above approximation of the
damping term by a different one which keeps the monotonicity for any
$\varepsilon\geqslant0$. We prove that, when $m=0,$ the finite extinction time
of the solution arises for merely bounded right hand side data $f(t,x).$ This
is specially useful in the applications in which the Schr{\"o}dinger equation
is coupled with some other functions satisfying some additional equations.
|
2111.10136v2
|
2022-01-26
|
Effect of Chiral Damping on the dynamics of chiral domain walls and skyrmions
|
Friction plays an essential role in most physical processes that we
experience in our everyday life. Examples range from our ability to walk or
swim, to setting boundaries of speed and fuel efficiency of moving vehicles. In
magnetic systems, the displacement of chiral domain walls (DW) and skyrmions
(SK) by Spin Orbit Torques (SOT), is also prone to friction. Chiral damping,
the dissipative counterpart of the Dzyaloshinskii Moriya Interaction (DMI),
plays a central role in these dynamics. Despite experimental observation, and
numerous theoretical studies confirming its existence, the influence of chiral
damping on DW and SK dynamics has remained elusive due to the difficulty of
discriminating from DMI. Here we unveil the effect that chiral damping has on
the flow motion of DWs and SKs driven by current and magnetic field. We use a
static in-plane field to lift the chiral degeneracy. As the in-plane field is
increased, the chiral asymmetry changes sign. When considered separately,
neither DMI nor chiral damping can explain the sign reversal of the asymmetry,
which we prove to be the result of their competing effects. Finally, numerical
modelling unveils the non-linear nature of chiral dissipation and its critical
role for the stabilization of moving SKs.
|
2201.10742v1
|
2022-01-27
|
A Study on Monte Carlo simulation of the radiation environment above GeV at the DAMPE orbit
|
The Dark Matter Particle Explorer (DAMPE) has been undergoing a stable
on-orbit operation for more than 6 years and acquired observation of over 11
billion events. And a better understanding of the overall radiation environment
on the DAMPE orbit is crucial for both simulation data production and flight
data analysis. In this work, we study the radiation environment at the low
Earth orbit and develop a simulation software package using the framework of
ATMNC3, in which state-of-the-art full 3D models of the Earth's atmospheric and
magnetic-field configurations is integrated. We consider in our Monte Carlo
procedure event-by-event propagation of the cosmic rays in the geomagnetic
field and their interaction with the Earth's atmosphere, focusing on the
particles above GeV that are able to trigger the DAMPE data acquisition system.
We compare the simulation results with the cosmic-ray electrons and positrons
(CREs) flux measurements made by DAMPE. The overall agreement on both the
spectral and angular distribution of the CREs flux demonstrates that our
simulation is well established. Our software package could be of more general
usage for the simulation of the radiation environment at the low Earth orbit of
various altitudes.
|
2201.11364v1
|
2022-05-10
|
Nonlinear damping quantification from phase-resonant tests under base excitation
|
The present work addresses the experimental identification of
amplitude-dependent modal parameters (modal frequency, damping ratio, Fourier
coefficients of periodic modal oscillation). Phase-resonant testing has emerged
as an important method for this task, as it substantially reduces the amount of
data required for the identification compared to conventional
frequency-response testing at different excitation/response levels. In the case
of shaker-stinger excitation, the applied excitation force is commonly measured
in order to quantify the amplitude-dependent modal damping ratio from the
phase-resonant test data. In the case of base excitation, however, the applied
excitation force is challenging or impossible to measure. In this work we
develop an original method for damping quantification from phase-resonant
tests. It relies solely on response measurement; it avoids the need to resort
to force measurement. The key idea is to estimate the power provided by the
distributed inertia force imposed by the base motion. We develop both a
model-free and a model-based variant of the method. We validate the developed
method first in virtual experiments of a friction-damped and a geometrically
nonlinear system, and then in a physical experiment involving a thin beam
clamped at both ends via bolted joints. We conclude that the method is highly
robust and provides high accuracy already for a reasonable number of sensors.
|
2205.04735v1
|
2022-09-22
|
Neutrino Fast Flavor Pendulum. Part 2: Collisional Damping
|
In compact astrophysical objects, the neutrino density can be so high that
neutrino-neutrino refraction can lead to fast flavor conversion of the kind
$\nu_e \bar\nu_e \leftrightarrow \nu_x \bar\nu_x$ with $x=\mu,\tau$, depending
on the neutrino angle distribution. Previously, we have shown that in a
homogeneous, axisymmetric two-flavor system, these collective solutions evolve
in analogy to a gyroscopic pendulum. In flavor space, its deviation from the
weak-interaction direction is quantified by a variable $\cos\vartheta$ that
moves between $+1$ and $\cos\vartheta_{\rm min}$, the latter following from a
linear mode analysis. As a next step, we include collisional damping of flavor
coherence, assuming a common damping rate $\Gamma$ for all modes. Empirically
we find that the damped pendular motion reaches an asymptotic level of pair
conversion $f=A+(1-A)\cos\vartheta_{\rm min}$ (numerically $A\simeq 0.370$)
that does not depend on details of the angular distribution (except for fixing
$\cos\vartheta_{\rm min}$), the initial seed, nor $\Gamma$. On the other hand,
even a small asymmetry between the neutrino and antineutrino damping rates
strongly changes this picture and can even enable flavor instabilities in
otherwise stable systems.
|
2209.11235v3
|
2022-10-12
|
Second order two-species systems with nonlocal interactions: existence and large damping limits
|
We study the mathematical theory of second order systems with two species,
arising in the dynamics of interacting particles subject to linear damping, to
nonlocal forces and to external ones, and resulting into a nonlocal version of
the compressible Euler system with linear damping. Our results are limited to
the $1$ space dimensional case but allow for initial data taken in a
Wasserstein space of probability measures. We first consider the case of smooth
nonlocal interaction potentials, not subject to any symmetry condition, and
prove existence and uniqueness. The concept of solutions relies on a stickiness
condition in case of collisions, in the spirit of previous works in the
literature. The result uses concepts from classical Hilbert space theory of
gradient flows (cf. Brezis [7]) and a trick used in [4]. We then consider a
large-time and large-damping scaled version of our system and prove convergence
to solutions to the corresponding first order system. Finally, we consider the
case of Newtonian potentials -- subject to symmetry of the cross-interaction
potentials -- and external convex potentials. After showing existence in the
sticky particles framework in the spirit of [4], we prove convergence for large
times towards Dirac delta solutions for the two densities. All the results
share a common technical framework in that solutions are considered in a
Lagrangian framework, which allows to estimate the behavior of solutions via
$L^2$ estimates of the pseudo-inverse variables corresponding to the two
densities. In particular, due to this technique, the large-damping result holds
under a rather weak condition on the initial data, which does not require
well-prepared initial velocities. We complement the results with numerical
simulations.
|
2210.06162v1
|
2022-10-12
|
Stability of the Néel quantum critical point in the presence of Dirac fermions
|
We investigate the stability of the N\'eel quantum critical point of
two-dimensional quantum antiferromagnets, described by a non-linear $\sigma$
model (NL$\sigma$M), in the presence of a Kondo coupling to $N_f$ flavours of
two-component Dirac fermion fields. The long-wavelength order parameter
fluctuations are subject to Landau damping by electronic particle-hole
fluctuations. Using momentum-shell RG, we demonstrate that the Landau damping
is weakly irrelevant at the N\'eel quantum critical point, despite the fact
that the corresponding self-energy correction dominates over the quadratic
gradient terms in the IR limit. In the ordered phase, the Landau damping
increases under the RG, indicative of damped spin-wave excitations. Although
the Kondo coupling is weakly relevant, sufficiently strong Landau damping
renders the N\'eel quantum critical point quasi-stable for $N_f\ge 4$ and
thermodynamically stable for $N_f<4$. In the latter case, we identify a new
multi-critical point which describes the transition between the N\'eel critical
and Kondo run-away regimes. The symmetry breaking at this fixed point results
in the opening of a gap in the Dirac fermion spectrum. Approaching the
multi-critical point from the disordered phase, the fermionic quasiparticle
residue vanishes, giving rise to non-Fermi-liquid behavior.
|
2210.06577v3
|
2022-11-13
|
Damping analysis of Floating Offshore Wind Turbine (FOWT): a new control strategy reducing the platform vibrations
|
In this paper, the coupled dynamics of the floating platform and the WTG
rotor is analysed. In particular, the damping is explicitly derived from the
coupled equations of rotor and floating platform. The analysis of the damping
leads to the study of the instability phenomena and it derives the explicit
conditions that lead to the Non Minimum Phase Zero (NMPZ). Two NMPZs, one
related to the rotor dynamics and the other one to the platform pitch dynamics,
are analysed. The latter is a novelty and it is analysed in this work,
providing the community of an explicit condition for its verification. The
domain of the instability of the platform is explicitly derived from the
coupled system of equations. In the second part of the paper, from the analysis
of the damping of the floating platform, a new strategy for the control of
FOWTs is proposed. This strategy allows one to impose to the controller an
explicit level of damping in the platform pitch motion without changing the
period of platform pitching. Finally the new strategy is compared to the one
without compensation by performing aero-hydro-servo-elastic numerical
simulations of the UMaine IEA15MW FOWT. Generated power, movements, blade pitch
and tower base fatigue are compared showing that the new control strategy can
reduce fatigue in the structure without affecting the power production.
|
2211.10362v1
|
2022-11-22
|
Universal Dynamics of Damped-Driven Systems: The Logistic Map as a Normal Form for Energy Balance
|
Damped-driven systems are ubiquitous in engineering and science. Despite the
diversity of physical processes observed in a broad range of applications, the
underlying instabilities observed in practice have a universal characterization
which is determined by the overall gain and loss curves of a given system. The
universal behavior of damped-driven systems can be understood from a
geometrical description of the energy balance with a minimal number of
assumptions. The assumptions on the energy dynamics are as follows: the energy
increases monotonically as a function of increasing gain, and the losses become
increasingly larger with increasing energy, i.e. there are many routes for
dissipation in the system for large input energy. The intersection of the gain
and loss curves define an energy balanced solution. By constructing an
iterative map between the loss and gain curves, the dynamics can be shown to be
homeomorphic to the logistic map, which exhibits a period doubling cascade to
chaos. Indeed, the loss and gain curves allow for a geometrical description of
the dynamics through a simple Verhulst diagram (cobweb plot). Thus irrespective
of the physics and its complexities, this simple geometrical description
dictates the universal set of logistic map instabilities that arise in complex
damped-driven systems. More broadly, damped-driven systems are a class of
non-equilibrium pattern forming systems which have a canonical set of
instabilities that are manifest in practice.
|
2211.11748v1
|
2023-01-23
|
Optimal Inter-area Oscillation Damping Control: A Transfer Deep Reinforcement Learning Approach with Switching Control Strategy
|
Wide-area damping control for inter-area oscillation (IAO) is critical to
modern power systems. The recent breakthroughs in deep learning and the broad
deployment of phasor measurement units (PMU) promote the development of
datadriven IAO damping controllers. In this paper, the damping control of IAOs
is modeled as a Markov Decision Process (MDP) and solved by the proposed Deep
Deterministic Policy Gradient (DDPG) based deep reinforcement learning (DRL)
approach. The proposed approach optimizes the eigenvalue distribution of the
system, which determines the IAO modes in nature. The eigenvalues are evaluated
by the data-driven method called dynamic mode decomposition. For a given power
system, only a subset of generators selected by participation factors needs to
be controlled, alleviating the control and computing burdens. A Switching
Control Strategy (SCS) is introduced to improve the transient response of IAOs.
Numerical simulations of the IEEE-39 New England power grid model validate the
effectiveness and advanced performance of the proposed approach as well as its
robustness against communication delays. In addition, we demonstrate the
transfer ability of the DRL model trained on the linearized power grid model to
provide effective IAO damping control in the non-linear power grid model
environment.
|
2301.09321v1
|
2023-03-15
|
Blow-up and decay for a class of variable coefficient wave equation with nonlinear damping and logarithmic source
|
In this paper, we consider the long time behavior for the solution of a class
of variable coefficient wave equation with nonlinear damping and logarithmic
source. The existence and uniqueness of local weak solution can be obtained by
using the Galerkin method and contraction mapping principle. However, the long
time behavior of the solution is usually complicated and it depends on the
balance mechanism between the damping and source terms. When the damping
exponent $(p+1)$ (see assumption (H3)) is greater than the source term exponent
$(q-1)$ (see equation (1.1)), namely, $p+2>q$, we obtain the global existence
and accurate decay rates of the energy for the weak solutions with any initial
data. Moreover, whether the weak solution exists globally or blows up in finite
time, it is closely related to the initial data. In the framework of modified
potential well theory, we construct the stable and unstable sets (see (2.8))
for the initial data. For the initial data belonging to the stable set, we
prove that the weak solution exists globally and has similar decay rates as the
previous results. For $p+2<q$ and the initial data belonging to the unstable
set, we prove that the weak solution blows up in finite time for a little
special damping $g(u_{t})=|u_{t}|^{p}u_{t}$.
|
2303.08629v1
|
2023-04-13
|
Centralised Multimode Power Oscillation Damping Controller for Photovoltaic Plants with Communication Delay Compensation
|
Low-frequency oscillations are an inherent phenomena in transmission networks
and renewable energy plants should be configured to damp them. Commonly, a
centralised controller is used in PV plants to coordinate PV generators via
communication channels. However, the communication systems of PV plants
introduce delays of a stochastic nature that degrade the performance of
centralised control algorithms. Therefore, controllers for oscillation damping
may not operate correctly unless the communication channel characteristics are
not considered and compensated. In this paper, a centralised controller is
proposed for the oscillation damping that uses a PV plant with all the
realistic effects of communication channels taken into consideration. The
communication channels are modelled based on measurements taken in a laboratory
environment. The controller is designed to damp several modes of oscillation by
using the open-loop phase shift compensation. Theoretical developments were
validated in a laboratory using four converters acting as two PV inverters, a
battery and a STATCOM. A real-time processing platform was used to implement
the centralised controller and to deploy the communication infrastructure.
Experimental results show the communication channels impose severe restrictions
on the performance of centralised POD controllers, highlighting the importance
of their accurate modelling and consideration during the controller design
stage.
|
2304.06415v1
|
2023-05-09
|
Glassy heat capacity from overdamped phasons and a hypothetical phason-induced superconductivity in incommensurate structures
|
Phasons are collective low-energy modes that appear in disparate condensed
matter systems such as quasicrystals, incommensurate structures, fluctuating
charge density waves, and Moir\'e superlattices. They share several
similarities with acoustic phonon modes, but they are not protected by any
exact translational symmetry. As a consequence, they are subject to a
wavevector independent damping, and they develop a finite pinning frequency,
which destroy their acoustic linearly propagating dispersion. Under a few and
simple well-motivated assumptions, we compute the phason density of states, and
we derive the phason heat capacity as a function of the temperature. Finally,
imagining a hypothetical s-wave pairing channel with electrons, we compute the
critical temperature $T_c$ of the corresponding superconducting state as a
function of phason damping using the Eliashberg formalism. We find that for
large phason damping, the heat capacity is linear in temperature, showing a
distinctive glass-like behavior. Additionally, we observe that the phason
damping can strongly enhance the effective Eliashberg coupling, and we reveal a
sharp non-monotonic dependence of the superconducting temperature $T_c$ on the
phason damping, with a maximum located at the underdamped to overdamped
crossover scale. Our simple computations confirm the potential role of
overdamped modes in explaining the glassy properties of incommensurate
structures, but also in possibly inducing strongly-coupled superconductivity
therein, and enhancing the corresponding $T_c$.
|
2305.05407v2
|
2023-08-03
|
Flavor-wave theory with quasiparticle damping at finite temperatures: Application to chiral edge modes in the Kitaev model
|
We propose a theoretical framework to investigate elementary excitations at
finite temperatures within a localized electron model that describes the
interactions between multiple degrees of freedom, such as quantum spin models
and Kugel-Khomskii models. Thus far, their excitation structures have been
mainly examined using the linear flavor-wave theory, an SU($N$) generalization
of the linear spin-wave theory. These techniques introduce noninteracting
bosonic quasiparticles as elementary excitations from the ground state, thereby
elucidating numerous physical phenomena, including excitation spectra and
transport properties characterized by topologically nontrivial band structures.
Nevertheless, the interactions between quasiparticles cannot be ignored in
systems exemplified by $S=1/2$ quantum spin models, where strong quantum
fluctuations are present. Recent studies have investigated the effects of
quasiparticle damping at zero temperature in such models. In our study,
extending this approach to the flavor-wave theory for general localized
electron models, we construct a comprehensive method to calculate excitation
spectra with the quasiparticle damping at finite temperatures. We apply our
method to the Kitaev model under magnetic fields, a typical example of models
with topologically nontrivial magnon bands. Our calculations reveal that chiral
edge modes undergo significant damping in weak magnetic fields, amplifying the
damping rate by the temperature increase. This effect is caused by collisions
with thermally excited quasiparticles. Since our approach starts from a general
Hamiltonian, it will be widely applicable to other localized systems, such as
spin-orbital coupled systems derived from multi-orbital Hubbard models in the
strong correlation limit.
|
2308.01711v1
|
2024-02-13
|
Investigating the Effect of Noise on the Training Performance of Hybrid Quantum Neural Networks
|
In this paper, we conduct a comprehensively analyze the influence of
different quantum noise gates, including Phase Flip, Bit Flip, Phase Damping,
Amplitude Damping, and the Depolarizing Channel, on the performance of HyQNNs.
Our results reveal distinct and significant effects on HyQNNs training and
validation accuracies across different probabilities of noise. For instance,
the Phase Flip gate introduces phase errors, and we observe that HyQNNs exhibit
resilience at higher probability (p = 1.0), adapting effectively to consistent
noise patterns, whereas at intermediate probabilities, the performance
declines. Bit Flip errors, represented by the PauliX gate, impact HyQNNs in a
similar way to that Phase Flip error gate. The HyQNNs, can adapt such kind of
errors at maximum probability (p = 1.0). Unlike Phase and Bit Flip error gates,
Phase Damping and Amplitude Damping gates disrupt quantum information, with
HyQNNs demonstrating resilience at lower probabilities but facing challenges at
higher probabilities. Amplitude Damping error gate, in particular, poses
efficiency and accuracy issues at higher probabilities however with lowest
probability (p = 0.1),it has the least effect and the HyQNNs, however not very
effectively, but still tends to learn. The Depolarizing Channel proves most
detrimental to HyQNNs performance, with limited or no training improvements.
There was no training potential observed regardless of the probability of this
noise gate. These findings underscore the critical need for advanced quantum
error mitigation and resilience strategies in the design and training of
HyQNNs, especially in environments prone to depolarizing noise. This paper
quantitatively investigate that understanding the impact of quantum noise gates
is essential for harnessing the full potential of quantum computing in
practical applications.
|
2402.08523v1
|
2024-03-03
|
Magnonic $\varphi$ Josephson Junctions and Synchronized Precession
|
There has been a growing interest in non-Hermitian physics. One of its main
goals is to engineer dissipation and to explore ensuing functionality. In
magnonics, the effect of dissipation due to local damping on magnon transport
has been explored. However, the effects of non-local damping on the magnonic
analog of the Josephson effect remain missing, despite that non-local damping
is inevitable and has been playing a central role in magnonics. Here, we
uncover theoretically that a surprisingly rich dynamics can emerge in magnetic
junctions due to intrinsic non-local damping, using analytical and numerical
methods. In particular, under microwave pumping, we show that coherent spin
precession in the right and left insulating ferromagnet (FM) of the junction
becomes synchronized by non-local damping and thereby a magnonic analog of the
$\varphi$ Josephson junction emerges, where $\varphi$ stands here for the
relative precession phase of right and left FM in the stationary limit.
Remarkably, $\varphi$ decreases monotonically from $ \pi$ to $\pi/2$ as the
magnon-magnon interaction, arising from spin anisotropies, increases. Moreover,
we also find a magnonic diode effect giving rise to rectification of magnon
currents. Our predictions are readily testable with current device and
measurement technologies at room temperatures.
|
2403.01625v1
|
2024-04-09
|
A Frequency-Domain Beamforming Procedure for Extracting Rayleigh Wave Attenuation Coefficients and Small-Strain Damping Ratio from 2D Ambient Noise Array Measurements
|
The small-strain damping ratio plays a crucial role in assessing the response
of soil deposits to earthquake-induced ground motions and general dynamic
loading. The damping ratio can theoretically be inverted for after extracting
frequency-dependent Rayleigh wave attenuation coefficients from wavefields
collected during surface wave testing. However, determining reliable estimates
of in-situ attenuation coefficients is much more challenging than achieving
robust phase velocity dispersion data, which are commonly measured using both
active-source and ambient-wavefield surface wave methods. This paper introduces
a new methodology for estimating frequency-dependent attenuation coefficients
through the analysis of ambient noise wavefield data recorded by
two-dimensional (2D) arrays of surface seismic sensors for the subsequent
evaluation of the small-strain damping ratio. The approach relies on the
application of an attenuation-specific wavefield conversion and
frequency-domain beamforming. Numerical simulations are employed to verify the
proposed approach and inform best practices for its application. Finally, the
practical efficacy of the proposed approach is showcased through its
application to field data collected at a deep, soft soil site in Logan, Utah,
USA, where phase velocity and attenuation coefficients are extracted from
surface wave data and then simultaneously inverted to develop deep shear wave
velocity and damping ratio profiles.
|
2404.06650v1
|
1997-01-17
|
Evidence for Rotation in the Galaxy at z=3.15 Responsible for a Damped Lyman-alpha Absorption System in the Spectrum of Q2233+1310
|
Proof of the existence of a significant population of normal disk galaxies at
redshift z>2 would have profound implications for theories of structure
formation and evolution. We present evidence based on Keck HIRES observations
that the damped Lyman-alpha absorber at z=3.15 toward the quasar Q2233+1310 may
well be such an example. Djorgovski et al have recently detected the
Lyman-alpha emission from the absorber, which we assume is at the systemic
redshift of the absorbing galaxy. By examining the profiles of the metal
absorption lines arising from the absorbing galaxy in relation to its systemic
redshift, we find strong kinematical evidence for rotation. Therefore the
absorber is likely to be a disk galaxy. The inferred circular velocity for the
galaxy is >200 km/s. With a separation of ~17 kpc between the galaxy and the
quasar sightline, the implied dynamic mass for the galaxy is >1.6x10(11) solar
mass. The metallicity of the galaxy is found to be [Fe/H]=-1.4, typical of
damped Lyman-alpha galaxies at such redshifts. However, in another damped
galactic rotation is evident. In the latter case, the damped Lyman-alpha
absorber occurs near the background quasar in redshift so its properties may be
influenced by the background quasar. These represent the only two cases at
present for which the technique used here may be applied. Future applications
of the same technique to a large sample of damped Lyman-alpha galaxies may
allow us to determine if a significant population of disk galaxies already
existed only a few billion years after the Big Bang.
|
9701116v2
|
1997-04-11
|
The Metallicity of High Redshift Galaxies: The Abundance of Zinc in 34 Damped Lyman Alpha Systems from z = 0.7 to 3.4
|
We report new observations of ZnII and CrII absorption lines in 10 damped
\lya systems (DLAs), mostly at redshift $z_{abs} \simgt 2.5$ . By combining
these results with those from our earlier survey (Pettini et al. 1994) and
other recent data, we construct a sample of 34 measurements (or upper limits)
of the Zn abundance relative to hydrogen [Zn/H]; the sample includes more than
one third of the total number of DLAs known.
The plot of the abundance of Zn as a function of redshift reinforces the two
main findings of our previous study. (1) Damped \lya systems are mostly
metal-poor, at all redshifts sampled; the column density weighted mean for the
whole data set is [Zn/H] $= -1.13 \pm 0.38$ (on a logarithmic scale), or
approximately 1/13 of solar. (2) There is a large spread, by up to two orders
of magnitude, in the metallicities we measure at essentially the same
redshifts. We propose that damped \lya systems are drawn from a varied
population of galaxies of different morphological types and at different stages
of chemical evolution, supporting the idea of a protracted epoch of galaxy
formation.
At redshifts $z \simgt 2$ the typical metallicity of the damped \lya systems
is in agreement with expectations based on the consumption of HI gas implied by
the recent measurements of $\Omega_{DLA}$ by Storrie-Lombardi et al. (1996a),
and with the metal ejection rates in the universe at these epochs deduced by
Madau (1996) from the ultraviolet luminosities of high redshift galaxies
revealed by deep imaging surveys. There are indications in our data for an
increase in the mean metallicity of the damped \lya systems from $z > 3$ to
$\approx 2$, consistent with the rise in the comoving star formation rate
indicated by the relative numbers of $U$ and $B$ drop-outs in the Hubble Deep
Field. Although such comparisons are still tentative, it appears that these
different avenues for exploring the early evolution of galaxies give a broadly
consistent picture.
|
9704102v1
|
1997-04-17
|
On the Kinematics of the Damped Lyman Alpha Protogalaxies
|
We present the first results of an ongoing program to investigate the
kinematic characteristics of high redshift damped lya systems. Because damped
lya systems are widely believed to be the progenitors of current massive
galaxies, an analysis of their kinematic history allows a direct test of galaxy
formation scenarios.
We have collected a kinematically unbiased sample of 17 high S/N ratio, high
resolution damped lya spectra taken with HIRES on the 10m W.M. Keck Telescope.
Our study focuses on the unsaturated, low-ion transitions of these systems
which reveal their kinematic traits. The profiles exhibit a nearly uniform
distribution of velocity widths ranging from 20 - 200 km/s and a relatively
high degree of asymmetry. In an attempt to explain these characteristics, we
introduce several physical models, which have previously been attributed to
damped lya systems, including rapidly rotating cold disks, slowly rotating hot
disks, massive isothermal halos, and a hydrodynamic spherical accretion model.
Using standard Monte Carlo techniques, we run sightlines through these model
systems to derive simulated low-ion profiles. Comparing statistical measures of
the simulated profiles with the observed profiles, we determine that the
rapidly rotating cold disk model is the only tested model consistent with the
data at high confidence levels. A Relative Likelihood Test of the rapidly
rotating cold disk model indicates the disks must have large rotation speeds; v
> 180 km/s at the 99% c.l. In turn, we demonstrate that the Cold Dark Matter
Model, as developed by Kauffmann (1996), is inconsistent with the damped lya
data at very high c.l. This is because the CDM Model does not predict a large
enough fraction of rapidly rotating disks at z approx 2.5.
|
9704169v2
|
2000-11-20
|
H-alpha Imaging with HST+NICMOS of An Elusive Damped Ly-alpha Cloud at z=0.6
|
Despite previous intensive ground-based imaging and spectroscopic campaigns
and wide-band HST imaging of the z=0.927 QSO 3C336 field, the galaxy that hosts
the damped Ly-alpha system along this line-of-sight has eluded detection. We
present a deep narrow-band H-alpha image of the field of this z=0.656 damped
Ly-alpha absorber, obtained through the F108N filter of NICMOS 1 onboard the
Hubble Space Telescope. The goal of this project was to detect any H-alpha
emission 10 times closer than previous studies to unveil the damped absorber.
We do not detect H-alpha emission between 0.05'' and 6'' (0.24 and 30 $h^{-1}$
kpc) from the QSO, with a 3-sigma flux limit of $3.70 \times 10^{-17} h^{-2}$
erg/s/cm^2 for an unresolved source, corresponding to a star formation rate
(SFR) of $0.3 h^{-2}$ M_sun/yr. This leads to a 3-sigma upper limit of 0.15
M_sun/yr/kpc^2 on the SFR density, or a maximum SFR of 1.87 M_sun/yr assuming a
disk of 4 kpc in diameter. This result adds to the number of low redshift
damped Ly-alpha absorbers that are not associated with the central regions of
Milky-Way-like disks. Damped Ly-alpha absorption can arise from high density
concentrations in a variety of galactic environments including some that,
despite their high local HI densities, are not conducive to widespread star
formation.
|
0011374v2
|
2005-08-17
|
The SDSS Damped Lya Survey: Data Release 3
|
We present the results from a damped Lya survey of the Sloan Digital Sky
Survey, Data Release 3 based on over 500 new damped Lya systems at z>2.2. We
measure the HI column density distribution f(N) and its zeroth and first
moments (the incidence l(X) and gas mass-density O_dla of damped Lya systems,
respectively) as a function of redshift. The key results include: (1) the f(N)
distribution is well fit by a Gamma-function with `break' column density log
N_g=10^21.5 and `faint-end' slope alpha=-1.8; (2) the shape of the f(N)
distributions do not show evolution with redshift; (3) l(X) and O_dla decrease
by 35% and 50% during ~1Gyr between redshift z=[3.,3.5] to z=[2.2,2.5]; and (4)
l(X) and O_dla in the lowest SDSS redshift bin (z=2.2) are consistent with the
current values. We investigate systematic errors in damped Lya analysis and
identify only one important effect: we measure 40 +/- 20% higher O_dla values
toward a subset of brighter quasars than toward a faint subset. This effect
runs contrary to the bias associated with dust obscuration and suggests that
gravitational lensing may be important. Comparing the results against models of
galaxy formation, we find all of the models significantly underpredict l(X) at
z=3 and only SPH models with significant feedback may reproduce O_dla at high
redshift. We argue that the Lyman limit systems contribute ~1/3 of the
universe's HI atoms at all redshifts z=2 to 5 and that the f(N) distribution
for N(HI)<10^20 has an inflection with slope >-1. We advocate a new mass
density definition -- the mass density of predominantly neutral gas O_neut --
to be contrasted with the mass density of gas associated with HI atoms. We
contend the damped Lya systems contribute >80% of O_neut at all redshifts and
therefore are the main reservoirs for star formation. [abridged]
|
0508361v1
|
2010-03-11
|
Damping of MHD turbulence in partially ionized gas and the observed difference of velocities of neutrals and ions
|
Theoretical and observational studies on the turbulence of the interstellar
medium developed fast in the past decades. The theory of supersonic magnetized
turbulence, as well as the understanding of projection effects of observed
quantities, are still in progress. In this work we explore the characterization
of the turbulent cascade and its damping from observational spectral line
profiles. We address the difference of ion and neutral velocities by clarifying
the nature of the turbulence damping in the partially ionized. We provide
theoretical arguments in favor of the explanation of the larger Doppler
broadening of lines arising from neutral species compared to ions as arising
from the turbulence damping of ions at larger scales. Also, we compute a number
of MHD numerical simulations for different turbulent regimes and explicit
turbulent damping, and compare both the 3-dimensional distributions of velocity
and the synthetic line profile distributions. From the numerical simulations,
we place constraints on the precision with which one can measure the 3D
dispersion depending on the turbulence sonic Mach number. We show that no
universal correspondence between the 3D velocity dispersions measured in the
turbulent volume and minima of the 2D velocity dispersions available through
observations exist. For instance, for subsonic turbulence the correspondence is
poor at scales much smaller than the turbulence injection scale, while for
supersonic turbulence the correspondence is poor for the scales comparable with
the injection scale. We provide a physical explanation of the existence of such
a 2D-3D correspondence and discuss the uncertainties in evaluating the damping
scale of ions that can be obtained from observations. However, we show that the
statistics of velocity dispersion from observed line profiles can provide the
spectral index and the energy transfer rate of turbulence. Also, comparing two
similar simulations with different viscous coefficients it was possible to
constrain the turbulent cut-off scale. This may especially prove useful since
it is believed that ambipolar diffusion may be one of the dominant dissipative
mechanism in star-forming regions. In this case, the determination of the
ambipolar diffusion scale may be used as a complementary method for the
determination of magnetic field intensity in collapsing cores. We discuss the
implications of our findings in terms of a new approach to magnetic field
measurement proposed by Li & Houde (2008).
|
1003.2346v1
|
2011-09-07
|
Weakly collisional Landau damping and three-dimensional Bernstein-Greene-Kruskal modes: New results on old problems
|
Landau damping and Bernstein-Greene-Kruskal (BGK) modes are among the most
fundamental concepts in plasma physics. While the former describes the
surprising damping of linear plasma waves in a collisionless plasma, the latter
describes exact undamped nonlinear solutions of the Vlasov equation. There does
exist a relationship between the two: Landau damping can be described as the
phase-mixing of undamped eigenmodes, the so-called Case-Van Kampen modes, which
can be viewed as BGK modes in the linear limit. While these concepts have been
around for a long time, unexpected new results are still being discovered. For
Landau damping, we show that the textbook picture of phase-mixing is altered
profoundly in the presence of collision. In particular, the continuous spectrum
of Case-Van Kampen modes is eliminated and replaced by a discrete spectrum,
even in the limit of zero collision. Furthermore, we show that these discrete
eigenmodes form a complete set of solutions. Landau-damped solutions are then
recovered as true eigenmodes (which they are not in the collisionless theory).
For BGK modes, our interest is motivated by recent discoveries of electrostatic
solitary waves in magnetospheric plasmas. While one-dimensional BGK theory is
quite mature, there appear to be no exact three-dimensional solutions in the
literature (except for the limiting case when the magnetic field is
sufficiently strong so that one can apply the guiding-center approximation). We
show, in fact, that two- and three-dimensional solutions that depend only on
energy do not exist. However, if solutions depend on both energy and angular
momentum, we can construct exact three-dimensional solutions for the
unmagnetized case, and two-dimensional solutions for the case with a finite
magnetic field. The latter are shown to be exact, fully electromagnetic
solutions of the steady-state Vlasov-Poisson-Amp\`ere system.
|
1109.1353v1
|
2012-11-06
|
Torsional Alfvén waves in solar partially ionized plasma: effects of neutral helium and stratification
|
Ion-neutral collisions may lead to the damping of Alfven waves in
chromospheric and prominence plasmas. Neutral helium atoms enhance the damping
in certain temperature interval, where the ratio of neutral helium and neutral
hydrogen atoms is increased. Therefore, the height-dependence of ionization
degrees of hydrogen and helium may influence the damping rate of Alfven waves.
We aim to study the effect of neutral helium in the damping of Alfven waves in
stratified partially ionized plasma of the solar chromosphere. We consider a
magnetic flux tube, which is expanded up to 1000 km height and then becomes
vertical due to merging with neighboring tubes, and study the dynamics of
linear torsional Alfven waves in the presence of neutral hydrogen and neutral
helium atoms. We start with three-fluid description of plasma and consequently
derive single-fluid magnetohydrodynamic (MHD) equations for torsional Alfven
waves. Thin flux tube approximation allows to obtain the dispersion relation of
the waves in the lower part of tubes, while the spatial dependence of
steady-state Alfven waves is governed by Bessel type equation in the upper part
of tubes. Consecutive derivation of single-fluid MHD equations results in a new
Cowling diffusion coefficient in the presence of neutral helium which is
different from previously used one. We found that shorter-period (< 5 s)
torsional Alfven waves damp quickly in the chromospheric network due to
ion-neutral collision. On the other hand, longer-period (> 5 s) waves do not
reach the transition region as they become evanescent at lower heights in the
network cores. Propagation of torsional Alfven waves through the chromosphere
into the solar corona should be considered with caution: low-frequency waves
are evanescent due to the stratification, while high-frequency waves are damped
due to ion neutral collisions.
|
1211.1348v2
|
2013-05-16
|
Application of vibration-transit theory to distinct dynamic response for a monatomic liquid
|
We examine the distinct part of the density autocorrelation function Fd(q,t),
also called the intermediate scattering function, from the point of view of the
vibration-transit (V-T) theory of monatomic liquid dynamics. A similar study
has been reported for the self part, and we study the self and distinct parts
separately because their damping processes are not simply related. We begin
with the perfect vibrational system, which provides precise definitions of the
liquid correlations, and provides the vibrational approximation Fdvib(q,t) at
all q and t. Two independent liquid correlations are defined, motional and
structural, and these are decorrelated sequentially, with a crossover time
tc(q). This is done by two independent decorrelation processes: the first,
vibrational dephasing, is naturally present in Fdvib(q,t) and operates to damp
the motional correlation; the second, transit-induced decorrelation, is invoked
to enhance the damping of motional correlation, and then to damp the structural
correlation. A microscopic model is made for the "transit drift", the averaged
transit motion that damps motional correlation on 0 < t < tc(q). Following the
previously developed self-decorrelation theory, a microscopic model is also
made for the "transit random walk," which damps the structural correlation on t
> tc(q). The complete model incorporates a property common to both self and
distinct decorrelation: simple exponential decay following a delay period,
where the delay is tc(q, the time required for the random walk to emerge from
the drift. Our final result is an accurate expression for Fd(q,t) for all q
through the first peak in Sd(q). The theory is calibrated and tested using
molecular dynamics (MD) calculations for liquid Na at 395K; however, the theory
itself does not depend on MD, and we consider other means for calibrating it.
|
1305.3954v2
|
2013-09-16
|
Two-atom system as a nano-antenna for mode switching and light routing
|
We determine how a system composed of two nonidentical two-level atoms with
different resonance frequencies and different damping rates could work as a
nano-antenna for controlled mode switching and light routing. We calculate the
angular distribution of the emitted field detected in a far-field zone of the
system including the direct interatomic interactions and arbitrary linear
dimensions of the system. The calculation is carried out in terms of the
symmetric and antisymmetric modes of the two atom system. We find that as long
as the atoms are identical, the emission cannot be switched between the
symmetric and antisymmetric modes. The switching may occur when the atoms are
non-identical and the emission can then be routed to different modes by
changing the relative ratio of the atomic frequencies, or damping rates or by a
proper tuning of the laser frequency to the atomic resonance frequencies. It is
shown that in the case of atoms of different resonance frequencies but equal
damping rates, the light routing is independent of the frequency of the driving
laser field. It depends only on the sign of the detuning between the atomic
resonance frequencies. In the case of atoms of different damping rates, the
emission can be switched between different modes by changing the laser
frequency from the blue to red detuned from the atomic resonance. The effect of
the interatomic interactions is also considered and it is found that in the
case of unequal resonance frequencies of the atoms, the interactions slightly
modify the visibility of the intensity pattern. The case of unequal damping
rates of the atoms is affected rather more drastically, the light routing
becoming asymmetric under the dipole-dipole interaction with the enhanced
intensities of the modes turned towards the atom of smaller damping rate.
|
1309.3924v1
|
2015-04-01
|
Landau damping of Gardner solitons in a dusty bi-ion plasma
|
The effects of linear Landau damping on the nonlinear propagation of
dust-acoustic solitary waves (DASWs) are studied in a collisionless
unmagnetized dusty plasma with two species of positive ions. The extremely
massive, micron-seized, cold and negatively charged dust particles are
described by fluid equations, whereas the two species of positive ions, namely
the cold (heavy) and hot (light) ions are described by the kinetic Vlasov
equations. Following Ott and Sudan [Phys. Fluids {\bf 12}, 2388 (1969)], and by
considering lower and higher-order perturbations, the evolution of DASWs with
Landau damping is shown to be governed by Korteweg-de Vries (KdV), modified KdV
(mKdV) or Gardner (KdV-mKdV)-like equations. The properties of the phase
velocity and the Landau damping rate of DASWs are studied for different values
of the ratios of the temperatures $(\sigma)$ and the number densities $(\mu)$
of hot and cold ions as well the cold to hot ion mass ratio $m$. The
distinctive features of the decay rates of the amplitudes of the KdV, mKdV and
Gardner solitons with a small effect of Landau damping are also studied in
different parameter regimes. It is found that the Gardner soliton points to
lower wave amplitudes than the KdV and mKdV solitons. The results may be useful
for understanding the localization of solitary pulses and associated wave
damping (collisionless) in laboratory and space plasmas (e.g., the F-ring of
Saturn) in which the number density of free electrons is much smaller than that
of ions and the heavy, micron seized dust grains are highly charged.
|
1504.00089v2
|
2017-03-09
|
Material developments and domain wall based nanosecond-scale switching process in perpendicularly magnetized STT-MRAM cells
|
We investigate the Gilbert damping and the magnetization switching of
perpendicularly magnetized FeCoB-based free layers embedded in tunnel junctions
adequate for spin-torque operated memories. We study the influence of the boron
content in MgO / FeCoB /Ta systems alloys on their Gilbert damping after
crystallization annealing. Increasing the boron content from 20 to 30\%
increases the crystallization temperature, thereby postponing the onset of
elemental diffusion within the free layer. This reduction of the interdiffusion
of the Ta atoms helps maintaining the Gilbert damping at a low level of 0.009
without any penalty on the anisotropy and the magneto-transport properties up
to the 400$^\circ$C annealing required in CMOS back-end of line processing. In
addition, we show that dual MgO free layers of composition
MgO/FeCoB/Ta/FeCoB/MgO have a substantially lower damping than their
MgO/FeCoB/Ta counterparts, reaching damping parameters as low as 0.0039 for a 3
\r{A} thick Tantalum spacer. This confirms that the dominant channel of damping
is the presence of Ta impurities within the FeCoB alloy. On optimized tunnel
junctions, we then study the duration of the switching events induced by
spin-transfer-torque. We focus on the sub-threshold thermally activated
switching in optimal applied field conditions. From the electrical signatures
of the switching, we infer that once the nucleation has occurred, the reversal
proceeds by a domain wall sweeping though the device at a few 10 m/s. The
smaller the device, the faster its switching. We present an analytical model to
account for our findings. The domain wall velocity is predicted to scale
linearly with the current for devices much larger than the wall width. The wall
velocity depends on the Bloch domain wall width, such that the devices with the
lowest exchange stiffness will be the ones that host the domain walls with the
slowest mobilities.
|
1703.03198v3
|
2017-07-18
|
Explanations of the DAMPE high energy electron/positron spectrum in the dark matter annihilation and pulsar scenarios
|
Many studies have shown that either the nearby astrophysical source or dark
matter (DM) annihilation/decay is required to explain the origin of high energy
cosmic ray (CR) $e^\pm$, which are measured by many experiments, such as PAMELA
and AMS-02. Recently, the Dark Matter Particle Explorer (DAMPE) collaboration
has reported its first result of the total CR $e^\pm$ spectrum from $25
\,\mathrm{GeV}$ to $4.6 \,\mathrm{TeV}$ with high precision. In this work, we
study the DM annihilation and pulsar interpretations of the DAMPE high energy
$e^\pm$ spectrum. In the DM scenario, the leptonic annihilation channels to
$\tau^+\tau^-$, $4\mu$, $4\tau$, and mixed charged lepton final states can well
fit the DAMPE result, while the $\mu^+\mu^-$ channel has been excluded. In
addition, we find that the mixed charged leptons channel would lead to a sharp
drop at $\sim$ $\mathrm{TeV}$. However, these DM explanations are almost
excluded by the observations of gamma-ray and CMB, unless some complicated DM
models are introduced. In the pulsar scenario, we analyze 21 nearby known
pulsars and assume that one of them is the primary source of high energy CR
$e^\pm$.Considering the constraint from the Fermi-LAT observation of the
$e^\pm$ anisotropy, we find that two pulsars are possible to explain the DAMPE
data. Our results show that it is difficult to distinguish between the DM
annihilation and single pulsar explanations of high energy $e^\pm$ with the
current DAMPE result.
|
1707.05664v2
|
2018-06-27
|
In-flight performance of the DAMPE silicon tracker
|
DAMPE (DArk Matter Particle Explorer) is a spaceborne high-energy cosmic ray
and gamma-ray detector, successfully launched in December 2015. It is designed
to probe astroparticle physics in the broad energy range from few GeV to 100
TeV. The scientific goals of DAMPE include the identification of possible
signatures of Dark Matter annihilation or decay, the study of the origin and
propagation mechanisms of cosmic-ray particles, and gamma-ray astronomy. DAMPE
consists of four sub-detectors: a plastic scintillator strip detector, a
Silicon-Tungsten tracKer-converter (STK), a BGO calorimeter and a neutron
detector. The STK is composed of six double layers of single-sided silicon
micro-strip detectors interleaved with three layers of tungsten for photon
conversions into electron-positron pairs. The STK is a crucial component of
DAMPE, allowing to determine the direction of incoming photons, to reconstruct
tracks of cosmic rays and to estimate their absolute charge (Z). We present the
in-flight performance of the STK based on two years of in-flight DAMPE data,
which includes the noise behavior, signal response, thermal and mechanical
stability, alignment and position resolution.
|
1806.10355v1
|
2018-10-30
|
Effect of Landau damping on ion acoustic solitary waves in a multi-species collisionless unmagnetized plasma consisting of nonthermal and isothermal electrons
|
A Korteweg-de Vries (KdV) equation including the effect of Landau damping is
derived to study the propagation of weakly nonlinear and weakly dispersive ion
acoustic waves in a collisionless unmagnetized plasma consisting of warm
adiabatic ions and two different species of electrons at different
temperatures. The hotter energetic electron species follows the nonthermal
velocity distribution of Cairns et al. [Geophys. Res. Lett. 22, 2709 (1995)]
whereas the cooler electron species obeys the Boltzmann distribution. It is
found that the coefficient of the nonlinear term of this KdV like evolution
equation vanishes along different family of curves in different parameter
planes. In this context, a modified KdV (MKdV) equation including the effect of
Landau damping effectively describes the nonlinear behaviour of ion acoustic
waves. It has also been observed that the coefficients of the nonlinear terms
of the KdV and MKdV like evolution equations including the effect of Landau
damping, are simultaneously equal to zero along a family of curves in the
parameter plane. In this situation, we have derived a further modified KdV
(FMKdV) equation including the effect of Landau damping to describe the
nonlinear behaviour of ion acoustic waves. In fact, different modified KdV like
evolution equations including the effect of Landau damping have been derived to
describe the nonlinear behaviour of ion acoustic waves in different region of
parameter space. The method of Ott & Sudan [Phys. Fluids 12, 2388 (1969)] has
been applied to obtain the solitary wave solution of the evolution equation
having the nonlinear term $(\phi^{(1)})^{r}\frac{\partial \phi^{(1)}}{\partial
\xi}$, where $\phi^{(1)}$ is the first order perturbed electrostatic potential
and $r =1,2,3$. We have found that the amplitude of the solitary wave solution
decreases with time for all $r =1,2,3$.
|
1810.12739v1
|
2019-03-28
|
Improving convergence of volume penalised fluid-solid interactions
|
We analyse and improve the volume-penalty method, a simple and versatile way
to model objects in fluid flows. The volume-penalty method is a kind of
fictitious-domain method that approximates no-slip boundary conditions with
rapid linear damping inside the object. The method can then simulate complex,
moving objects in general numerical solvers without specialised algorithms or
boundary-conforming grids. Volume penalisation pays for this simplicity by
introducing an equation-level error, the $\textit{model error}$, that is
related to the damping time $\eta \ll 1$. While the model error has been proven
to vanish as the damping time tends to zero, previous work suggests convergence
at a slow rate of $\mathcal{O}(\eta^{1/2})$. The stiffness of the damping
implies conventional volume penalisation only achieves first order numerical
accuracy. We analyse the volume-penalty method using multiple-scales
matched-asymptotics with a signed-distance coordinate system valid for
arbitrary smooth geometries. We show the dominant model error stems from a
displacement length that is proportional to a Reynolds number $\text{Re}$
dependent boundary layer of size $\mathcal{O}(\eta^{1/2}\text{Re}^{-1/2})$. The
relative size of the displacement length and damping time leads to multiple
error regimes. Our key finding derives a simple smoothing prescription for the
damping that eliminates the displacement length and reduces the model error to
$\mathcal{O}(\eta)$ in all regimes. This translates to second order numerical
accuracy. We validate our findings in several comprehensive benchmark problems
and finally combine Richardson extrapolation of the model error with our
correction to further improve convergence to $\mathcal{O}(\eta^{2})$.
|
1903.11914v4
|
2019-06-12
|
Study of Alfven Eigenmodes stability in plasma with multiple NBI driven energetic particle specie
|
The aim of this study is to analyze the destabilization of Alfven Eigenmodes
(AE) by multiple energetic particles (EP) species in DIII-D and LHD discharges.
We use the reduced MHD equations to describe the linear evolution of the
poloidal flux and the toroidal component of the vorticity in a full 3D system,
coupled with equations of density and parallel velocity moments for the
energetic particles species, including the effect of the acoustic modes,
diamagnetic currents and helical couplings. We add the Landau damping and
resonant destabilization effects using a closure relation. The simulations with
multiple NBI lines show three different regimes: the non damped regime where
the multi beam AEs growth rate is larger compared to the growth rate of the AEs
destabilized by the individual NBI lines, the interaction regime where the
multi beam AEs growth rate is smaller than the single NBI AEs and the damped
regime where the AEs are suppressed. Operations in the damped regime requires
EP species with different density profile flatness or gradient locations. In
addition, the AEs growth rate in the interaction regime is further reduced if
the combined NBI lines have similar beam temperatures and the beta of the NBI
line with flatter EP density profile increases. Then, optimization trends are
identified in DIII-D high poloidal beta and LHD low density / magnetic field
discharges with multiple NBI lines as well as the configuration requirements to
operate in the damped and interaction regimes. DIII-D simulations show a
decrease of the n=2 to 6 AEs growth rate and n=1 AE are stabilized in the LHD
case. The helical coupling effects in LHD simulations lead to a transition from
the interaction to the damped regime of the n=2,-8,12 helical family.
|
1906.05701v1
|
2020-06-08
|
Stochastic re-acceleration and magnetic-field damping in Tycho's supernova remnant
|
A number of studies suggest that shock acceleration with particle feedback
and very efficient magnetic-field amplification combined with Alfv\'{e}nic
drift are needed to explain the rather soft radio spectrum and the narrow rims
observed for Tycho's SNR. We show that the broadband spectrum of Tycho's SNR
can alternatively be well explained when accounting for stochastic acceleration
as a secondary process. The re-acceleration of particles in the turbulent
region immediately downstream of the shock should be efficient enough to impact
particle spectra over several decades in energy. The so-called Alfv\'{e}nic
drift and particle feedback on the shock structure are not required in this
scenario. Additionally, we investigate whether synchrotron losses or
magnetic-field damping play a more profound role in the formation of the
non-thermal filaments. We solve the full particle transport equation in
test-particle mode using hydrodynamic simulations of the SNR plasma flow. The
background magnetic field is either computed from the induction equation or
follows analytic profiles, depending on the model considered. Fast-mode waves
in the downstream region provide the diffusion of particles in momentum space.
We show that the broadband spectrum of Tycho can be well explained if
magnetic-field damping and stochastic re-acceleration of particles are taken
into account. Although not as efficient as standard DSA, stochastic
acceleration leaves its imprint on the particle spectra, which is especially
notable in the emission at radio wavelengths. We find a lower limit for the
post-shock magnetic-field strength $\sim330\,\mathrm{\mu G}$, implying
efficient amplification even for the magnetic-field damping scenario. For the
formation of the filaments in the radio range magnetic-field damping is
necessary, while the X-ray filaments are shaped by both the synchrotron losses
and magnetic-field damping.
|
2006.04832v1
|
2021-02-23
|
Influence of Ion-Neutral Damping on the Cosmic-Ray Streaming Instability: Magnetohydrodynamic Particle-in-cell Simulations
|
We explore the physics of the gyro-resonant cosmic ray streaming instability
(CRSI) including the effects of ion-neutral (IN) damping. This is the main
damping mechanism in (partially-ionized) atomic and molecular gas, which are
the primary components of the interstellar medium (ISM) by mass. Limitation of
CRSI by IN damping is important in setting the amplitude of Alfv\'en waves that
scatter cosmic rays and control galactic-scale transport. Our study employs the
MHD-PIC hybrid fluid-kinetic numerical technique to follow linear growth as
well as post-linear and saturation phases. During the linear phase of the
instability -- where simulations and analytical theory are in good agreement --
IN damping prevents wave growth at small and large wavelengths, with the
unstable bandwidth lower for higher ion-neutral collision rate $\nu_{\rm in}$.
Purely MHD effects during the post-linear phase extend the wave spectrum
towards larger $k$. In the saturated state, the cosmic ray distribution evolves
toward greater isotropy (lower streaming velocity) by scattering off of Alv\'en
waves excited by the instability. In the absence of low-$k$ waves, CRs with
sufficiently high momentum are not isotropized. The maximum wave amplitude and
rate of isotropization of the distribution function decreases at higher
$\nu_{\rm in}$. When the IN damping rate approaches the maximum growth rate of
CSRI, wave growth and isotropization is suppressed. Implications of our results
for CR transport in partially ionized ISM phases are discussed.
|
2102.11878v3
|
2022-06-17
|
Quantum Dynamics of Magnetic Skyrmions: Consistent Path Integral Formulation
|
We present a path integral formalism for the intrinsic quantum dynamics of
magnetic skyrmions coupled to a thermal background of magnetic fluctuations.
Upon promoting the skyrmion's collective coordinate $\boldsymbol{R}$ to a
dynamic variable and integrating out the magnonic heat bath, we derive the
generalized equation of motion for $\boldsymbol{R}$ with a non-local damping
term that describes a steady-state skyrmion dynamics at finite temperatures.
Being essentially temperature dependent, the intrinsic damping is shown to
originate from the coupling of thermally activated magnon modes to the
adiabatic potential driven by a rigid skyrmion motion, which can be regarded as
another manifestation of emergent electrodynamics inherent to topological
magnetic textures. We further argue that the diagonal components of the damping
term act as the source of dissipation and inertia, while its off-diagonal
components modify the gyrotropic motion of a magnetic skyrmion. By means of
numerical calculations for the lattice spin model of chiral ferromagnets, we
study the temperature behavior of the intrinsic damping as a function of
magnetic field in periodic and confined geometries. The intrinsic damping is
demonstrated to be highly non-local, revealing its quantum-mechanical nature,
that becomes more pronounced with increasing temperature. At high temperatures
when the magnon occupation factors are large, the intrinsic damping is shown to
yield a modified Thiele's equation with the additional non-local dissipative
and mass terms that exhibit an almost linear temperature behavior. Our results
provide a microscopic background for semiclassical magnetization dynamics and
establish a framework for understanding spin caloritronics effects in
topological magnetic textures.
|
2206.08532v2
|
2024-02-05
|
Revisiting the role of cosmic-ray driven Alfvén waves in pre-existing magnetohydrodynamic turbulence. I. Turbulent damping rates and feedback on background fluctuations
|
Alfv\'en waves (AWs) excited by the cosmic-ray (CR) streaming instability
(CRSI) are a fundamental ingredient for CR confinement. The effectiveness of
self-confinement relies on a balance between CRSI growth rate and damping
mechanisms acting on quasi-parallel AWs excited by CRs. One relevant mechanism
is the so-called turbulent damping, in which an AW packet injected in
pre-existing turbulence undergoes a cascade process due to its nonlinear
interaction with fluctuations of the background. The turbulent damping of an AW
packet in pre-existing magnetohydrodynamic turbulence is re-examined, revised,
and extended to include most-recent theories of MHD turbulence that account for
dynamic alignment and reconnection-mediated regime. The case in which the role
of feedback of CR-driven AWs on pre-existing turbulence is important will also
be discussed. Particular attention is given to the nonlinearity parameter
$\chi^w$ that estimates the strength of nonlinear interaction between CR-driven
AWs and background fluctuations. We point out the difference between $\chi^w$
and $\chi^z$ that instead describes the strength of nonlinear interactions
between pre-existing fluctuations. When $\chi^w$ is properly taken into
account, one finds that (i) the turbulent damping rate of quasi-parallel AWs in
anisotropic turbulence depends on the background-fluctuations' amplitude to the
third power, hence is strongly suppressed, and (ii) the dependence on the AW's
wavelength (and thus on the CR gyro-radius from which it is excited) is
different from what has been previously obtained. Finally, (iii) when dynamic
alignment of cascading fluctuations and the possibility of a
reconnection-mediated range is included in the picture, the turbulent damping
rate exhibits novel regimes and breaks. Finally, a criterion for CR-feedback is
derived and simple phenomenological models of CR-modified turbulent scaling are
provided.
|
2402.02901v1
|
2006-10-24
|
Logical contradictions of Landau damping
|
Landau damping/growing at boundary condition of excitation of a harmonic wave
in collisionless ion-electron-neutrals plasma contradicts to the law of energy
conservation of a wave damping/growing in space. There is also no criterion of
a choice either damping or growing solution in difference from always
non-damping in the direction of propagation Vlasov waves. Variety of other
incongruities as consequence of Landau damping is specified also. Absence of
explicit positivity and finiteness of wave solutions for electron distribution
function near singularity point leads to need of imposing additional cutting
off constraints with resulting positivity and finiteness of the electron
distribution function at the singularity points and finiteness of the complex
dispersion integral. Landau damping as a real physical phenomenon of
collisionless damping does not exist. A relation is established for the real
dispersion equation with real waves (see Appendices 2,4) between the averaged
over period wave damping decrement and the collisional energy-exchange term of
kinetic equation. Collisionless Vlasov-Landau damping is explained finally by
the usual wrong use of nonlinearly complex wave functions leading to complex
dispersion equation. All used solution of the complex dispersion equation for
the simultaneously existing collisionless both exponentially damping and
growing nonlinear complex waves is entirely, quantitatively and in its logical
sense, different from the solution of initially real dispersion equation for
real either damping or growing waves and should be discarded (see Appendices
2,4,5,6). Collisionless damping is caused by unreasonable use of wave functions
with complex frequency or complex wave number leading to complex dispersion
relation with unphysical binomial virtual complex roots. Thus finding roots of
the complex dispersion equation has only abstract mathematical interest.
|
0610220v67
|
2000-05-31
|
The afterglow of the short/intermediate-duration gamma-ray burst GRB 000301C: A jet at z=2.04
|
We present Ulysses and NEAR data from the detection of the short or
intermediate duration (2 s) gamma-ray burst GRB000301C (2000 March 1.41 UT).
The gamma-ray burst (GRB) was localised by the Inter Planetary Network (IPN)
and RXTE to an area of 50 arcmin^2. A fading optical counterpart was
subsequently discovered with the Nordic Optical Telescope (NOT) about 42h after
the burst. The GRB lies at the border between the long-soft and the short-hard
classes of GRBs. If GRB000301C belongs to the latter class, this would be the
first detection of an afterglow to a short-hard burst. We present UBRI and JHK
photometry from the time of the discovery until 11 days after the burst.
Finally, we present spectroscopic observations of the optical afterglow
obtained with the ESO VLT Antu telescope 4 and 5 days after the burst. The
optical light curve is consistent with being achromatic from 2 to 11 days after
the burst and exhibits a break. A broken power-law fit yields a shallow
pre-break decay power-law slope of a_1=-0.72+-0.06, a break time of
t_b=4.39+-0.26 days after the burst, and a post-break slope of a_2=-2.29+-0.17,
which is best explained by a sideways expanding jet in an ambient medium of
constant mean density. In the optical spectrum we find absorption features that
are consistent with FeII, CIV, CII, SiII and Ly-a at a redshift of
2.0404+-0.0008. We find evidence for a curved shape of the spectral energy
distribution of the observed afterglow. It is best fitted with a power-law
spectral distribution with index b ~ -0.7 reddened by an SMC-like extinction
law with A_V~0.1 mag. Based on the Ly-a absorption line we estimate the HI
column density to be log(N(HI))=21.2+-0.5. This is the first direct indication
of a connection between GRB host galaxies and Damped Ly-a Absorbers.
|
0005609v2
|
2011-05-16
|
A Measurement of the Damping Tail of the Cosmic Microwave Background Power Spectrum with the South Pole Telescope
|
We present a measurement of the angular power spectrum of the cosmic
microwave background (CMB) using data from the South Pole Telescope (SPT). The
data consist of 790 square degrees of sky observed at 150 GHz during 2008 and
2009. Here we present the power spectrum over the multipole range 650 < ell <
3000, where it is dominated by primary CMB anisotropy. We combine this power
spectrum with the power spectra from the seven-year Wilkinson Microwave
Anisotropy Probe (WMAP) data release to constrain cosmological models. We find
that the SPT and WMAP data are consistent with each other and, when combined,
are well fit by a spatially flat, LCDM cosmological model. The SPT+WMAP
constraint on the spectral index of scalar fluctuations is ns = 0.9663 +/-
0.0112. We detect, at ~5-sigma significance, the effect of gravitational
lensing on the CMB power spectrum, and find its amplitude to be consistent with
the LCDM cosmological model. We explore a number of extensions beyond the LCDM
model. Each extension is tested independently, although there are degeneracies
between some of the extension parameters. We constrain the tensor-to-scalar
ratio to be r < 0.21 (95% CL) and constrain the running of the scalar spectral
index to be dns/dlnk = -0.024 +/- 0.013. We strongly detect the effects of
primordial helium and neutrinos on the CMB; a model without helium is rejected
at 7.7-sigma, while a model without neutrinos is rejected at 7.5-sigma. The
primordial helium abundance is measured to be Yp = 0.296 +/- 0.030, and the
effective number of relativistic species is measured to be Neff = 3.85 +/-
0.62. The constraints on these models are strengthened when the CMB data are
combined with measurements of the Hubble constant and the baryon acoustic
oscillation feature. Notable improvements include ns = 0.9668 +/- 0.0093, r <
0.17 (95% CL), and Neff = 3.86 +/- 0.42. The SPT+WMAP data show...
|
1105.3182v2
|
2016-10-07
|
The Atacama Cosmology Telescope: Two-Season ACTPol Spectra and Parameters
|
We present the temperature and polarization angular power spectra measured by
the Atacama Cosmology Telescope Polarimeter (ACTPol). We analyze night-time
data collected during 2013-14 using two detector arrays at 149 GHz, from 548
deg$^2$ of sky on the celestial equator. We use these spectra, and the spectra
measured with the MBAC camera on ACT from 2008-10, in combination with Planck
and WMAP data to estimate cosmological parameters from the temperature,
polarization, and temperature-polarization cross-correlations. We find the new
ACTPol data to be consistent with the LCDM model. The ACTPol
temperature-polarization cross-spectrum now provides stronger constraints on
multiple parameters than the ACTPol temperature spectrum, including the baryon
density, the acoustic peak angular scale, and the derived Hubble constant.
Adding the new data to planck temperature data tightens the limits on damping
tail parameters, for example reducing the joint uncertainty on the number of
neutrino species and the primordial helium fraction by 20%.
|
1610.02360v1
|
1995-02-16
|
Lyman alpha Emission from High-Redshift Galaxies
|
We summarise the results of a deep search for Lyman alpha emission from
star-forming regions associated with damped Lyman alpha absorption systems and
conclude that the Lyman alpha luminosity of high redshift galaxies is generally
less than 10^(42) erg/s . We also present a newly discovered case, in the field
of the QSO Q2059-360, where the emission is unusually strong, possibly because
the damped system is close in redshift to the QSO.
|
9502076v1
|
1995-10-12
|
Limits on diffusive shock acceleration in dense and incompletely ionised media
|
The limits imposed on diffusive shock acceleration by upstream ion-neutral
Alfven wave damping, and by ionisation and Coulomb losses of low energy
particles, are calculated. Analytic solutions are given for the steady upstream
wave excitation problem with ion-neutral damping and the resulting escaping
upstream flux calculated. The time dependent problem is discussed and numerical
solutions presented. Finally the significance of these results for possible
observational tests of shock acceleration in supernova remnants is discussed.
|
9510066v2
|
1995-11-28
|
Damping of GRR instability by direct URCA reactions
|
The role of direct URCA reactions in damping of the gravitational radiation
driven instability is discussed. The temperature at which bulk viscosity
suppresses completely this instability is calculated.
The results are obtained analytically using recent calculations performed in
the case of bulk viscosity due to the modified URCA processes (Lindblom 1995;
Yoshida & Eriguchi 1995).
The bulk viscosity caused by direct URCA reactions is found to reduce
significantly the region of temperatures and rotation frequencies where a
neutron star is subject to GRR instability.
|
9511136v1
|
1997-10-31
|
Abundances in Damped Lyman-alpha Systems and Chemical Evolution of High Redshift Galaxies
|
Recent abundance measurements in damped Lyman-alpha galaxies, supplemented
with unpublished Keck observations, are discussed. The metallicity distribution
with cosmic time is examined for clues about the degree of enrichment, the
onset of initial star formation, and the nature of the galxies. The relative
abundances of the elements are compared with the abundnce pattern in Galactic
halo stars and in the Sun, taking into account of the effects of dust
depletion, in order to gain insight into the stellar processes and the time
scales by which the enrichment occurred.
|
9710370v1
|
1998-05-08
|
Exploring the Damped Lyman-alpha Clouds with AXAF
|
The High Energy Transmission Grating (HETG) Spectrometer on the Advanced
X-ray Astrophysics Facility (AXAF) (scheduled for launch in August, 1998) will
provide a new tool for the study of absorption in the X-ray spectra of high
redshift quasars due to the material along the line of sight. In this paper we
try to explore the possibility of using AXAF HETG to detect resonance
absorption lines from the Damped Lyman-alpha (DLA) clouds.
|
9805110v1
|
1998-05-28
|
Photon Damping of Waves in Accretion Disks
|
MHD turbulence is generally believed to have two important functions in
accretion disks: it transports angular momentum outward, and the energy in its
shortest wavelength modes is dissipated into the heat that the disks radiate.
In this paper we examine a pair of mechanisms which may play an important role
in regulating the amplitude and spectrum of this turbulence: photon diffusion
and viscosity. We demonstrate that in radiation pressure-dominated disks,
photon damping of compressive MHD waves is so rapid that it likely dominates
all other dissipation mechanisms.
|
9805358v1
|
1998-06-11
|
Damping of differential rotation in neutron stars
|
We derive the transport relaxation times for quasiparticle-vortex scattering
processes via nuclear force, relevant for the damping of differential rotation
of superfluids in the quantum liquid core of a neutron star. The proton
scattering off the neutron vortices provides the dominant resistive force on
the vortex lattice at all relevant temperatures in the phase where neutrons
only are in the paired state. If protons are superconducting, a small fraction
of hyperons and resonances in the normal state would be the dominant source of
friction on neutron and proton vortex lattices at the core temperatures $T\ge
10^{7}$ K.
|
9806156v1
|
1999-03-10
|
Elemental abundances at early times: the nature of Damped Lyman-alpha systems
|
The distribution of element abundances with redshift in Damped Ly-alpha (DLA)
systems can be adequately reproduced by the same model reproducing the halo and
disk components of the Milky Way Galaxy at different galactocentric distances:
DLA systems are well represented by normal spiral galaxies in their early
evolutionary stages.
|
9903150v1
|
1999-07-26
|
Are Damped Ly-alpha Systems Large, Galactic Disks ?
|
The hypothesis that the Damped Ly-alpha systems (DLAs) are large, galactic
disks (Milky Way sized) is tested by confronting predictions of models of the
formation and evolution of (large) disk galaxies with observations, in
particular the Zinc abundance distribution with neutral hydrogen column density
found for DLAs. A pronounced mismatch is found strongly hinting that the
majority of DLAs may not be large, galactic disks.
|
9907349v1
|
1999-11-25
|
Probing Solar Convection
|
In the solar convection zone acoustic waves are scattered by turbulent sound
speed fluctuations. In this paper the scattering of waves by convective cells
is treated using Rytov's technique. Particular care is taken to include
diffraction effects which are important especially for high-degree modes that
are confined to the surface layers of the Sun. The scattering leads to damping
of the waves and causes a phase shift. Damping manifests itself in the width of
the spectral peak of p-mode eigenfrequencies. The contribution of scattering to
the line widths is estimated and the sensitivity of the results on the assumed
spectrum of the turbulence is studied. Finally the theoretical predictions are
compared with recently measured line widths of high-degree modes.
|
9911469v1
|
1999-12-14
|
The Gas Reservoir for present day Galaxies : Damped Ly-alpha Absorption Systems
|
We present results from an ongoing search for galaxy counterparts of a
subgroup of Quasar Absorption Line Systems called Damped Ly-alpha Absorbers
(DLAs). DLAs have several characteristics that make them essential in the
process of understanding how galaxies formed in the early universe and evolved
to the galaxies we see today in the local universe.
Finally we compare DLAs with recent findings of a population of starforming
galaxies at high redshifts, so called Lyman-break galaxies.
|
9912268v1
|
2000-06-22
|
Nuclear Reaction Rates in a Plasma: The Effect of Highly Damped Modes
|
The fluctuation-dissipation theorem is used to evaluate the screening factor
of nuclear reactions due to the electromagnetic fluctuations in a plasma. We
show that the commonly used Saltpeter factor is obtained if only fluctuations
near the plasma eigenfrequency are assumed to be important (\omega \sim
\omega_{pe}\ll T (\hbar=k_{B}=1)). By taking into account all the fluctuations,
the highly damped ones, with \omega >\omega_{pe}, as well as those with
\omega\leq\omega_{pe}, we find that nuclear reaction rates are higher than
those obtained using the Saltpeter factor, for many interesting plasmas.
|
0006326v1
|
2001-01-13
|
Measuring Feedback in Damped Lyman Alpha Systems
|
We measure feedback (heating rates) in damped Lyman alpha systems from the
cooling rate of the neutral gas. Since cooling occurs through [C II] 158 micron
emission, we infer cooling from C II^{*} 1335.7 absorption lines detected with
HIRES on the Keck I telescope. The inferred heating rates are about 30 times
lower than for the Galaxy ISM. At z = 2.8, the implied star formation rate per
unit area is 10^{-2.4+-0.3} solar masses per kpc^{2} per year, and the the star
formation rate per unit comoving volume is 10^{-0.8+-0.2} solar masses per
Mpc^{3} per year. This is the first measurement of star formation rates in
objects likely to be the progenitors of current galaxies.
|
0101218v1
|
2001-04-18
|
The First Detection of Cobalt in a Damped Lyman Alpha System
|
We present the first ever detection of Cobalt in a Damped Lyman Alpha system
(DLA) at z = 1.92. In addition to providing important clues to the star
formation history of these high redshift galaxies, we discuss how studying the
Co abundance in DLAs may also help to constrain models of stellar
nucleosynthesis in a regime not probed by Galactic stars.
|
0104301v1
|
2001-05-09
|
Nuclear reaction rates and energy in stellar plasmas : The effect of highly damped modes
|
The effects of the highly damped modes in the energy and reaction rates in a
plasma are discussed. These modes, with wavenumbers $k \gg k_{D}$, even being
only weakly excited, with less than $k_{B}T$ per mode, make a significant
contribution to the energy and screening in a plasma. When the de Broglie
wavelength is much less than the distance of closest approach of thermal
electrons, a classical analysis of the plasma can
|
0105153v1
|
2001-07-03
|
The HI Content and Extent of Low Surface Brightness Galaxies - Could LSB Galaxies be Responsible for Damped Ly-alpha Absorption?
|
Low surface brightness galaxies, those galaxies with a central surface
brightness at least one magnitude fainter than the night sky, are often not
included in discussions of extragalactic gas at z < 0.1. In this paper we
review many of the properties of low surface brightness galaxies, including
recent studies which indicate low surface brightness systems may contribute far
more to the local HI luminosity function than previously thought. Additionally,
we use the known (HI) gas properties of low surface brightness galaxies to
consider their possible contribution to nearby damped Lyman-alpha absorbers.
|
0107064v1
|
2001-09-10
|
H_2 molecules in damped systems
|
Damped Lyman alpha systems seen in the spectra of high-z QSOs arise in
high-density neutral gas in which molecular hydrogen (H_2) should be
conspicuous. Systematic searches to detect the H_2 lines redshifted into the
Lyman alpha forest at <3400\AA are now possible thanks to the unique
capabilities of UVES on the VLT. Here we summarise the present status of our on
going programme to search for H_2 in DLAs, discuss the physical conditions in
the systems where H_2 is detected and the implications of non-detections.
|
0109155v1
|
2001-10-23
|
A scaling law of interstellar depletions as a tool for abundance studies of Damped Ly alpha systems
|
An analytical expression is presented that allows dust depletions to be
estimated in different types of interstellar environments, including Damped Ly
alpha systems. The expression is a scaling law of a reference depletion pattern
and takes into account the possibility that the dust chemical composition may
vary as a function of the dust-to-metals ratio and of the intrinsic abundances
of the medium. Preliminary tests and applications of the proposed scaling law
are briefly reported.
|
0110499v1
|
2002-09-23
|
Outflows in Galaxies and Damped Ly-alpha System
|
Although quasar absorbers, and in particular Damped Lyman-alpha systems
(DLAs) have proven a valuable tool to study the early Universe, their exact
nature is so far poorly constrained. It has been suggested that outflows in
galaxies might account for at least part of the DLA population. Observational
evidences and models in support of this hypothesis are reviewed, including
recent observations of Lyman Break Galaxies (LBGs). Observational
counter-arguments and theoretical limitations are also given. Finally,
implications of such a model for the environment of galaxies at high-redshifts
are discussed.
|
0209463v1
|
2004-03-15
|
The Damping Wing of the Gunn-Peterson Absorption and Lyman-Alpha Emitters in the Pre-Reionization Era
|
We use a numerical simulation of cosmological reionization to estimate the
likelihood of detecting Lyman-alpha emitting galaxies during the
pre-reionization era. We show that it is possible to find galaxies even at z~9
that are barely affected by the dumping wing of the Gunn-Peterson absorption
from the neutral IGM outside of their HII regions. The damping wing becomes
rapidly more significant at z>9, but even at z>10 is it not inconceivable
(although quite hard) to see a Lyman-alpha emission line from a star-forming
galaxy.
|
0403345v1
|
2005-05-28
|
Cosmic ray transport in MHD turbulence
|
Recent advances in understanding of magnetohydrodynamic (MHD) turbulence call
for revisions in the picture of cosmic ray transport. In this paper we use
recently obtained scaling laws for MHD modes to obtain the scattering frequency
for cosmic rays. We account for the turbulence cutoff arising from both
collisional and collisionless damping. We obtain the scattering rate and show
that fast modes provide the dominant contribution to cosmic ray scattering for
the typical interstellar conditions in spite of the fact that fast modes are
subjected to damping. We determine how the efficiency of the scattering depends
on the characteristics of ionized media, e.g. plasma $\beta$. We show that
streaming instability is suppressed by the ambient MHD turbulence.
|
0505575v1
|
2005-06-22
|
A Damped Ly-alpha Absorption-line System in an Apparent Void at Redshift 2.38
|
We study the contents of an apparent void in the distribution of Ly-alpha
emitting galaxies at redshift 2.38. We show that this void is not empty, but
contains a damped Ly-alpha absorption-line system, seen in absorption against
background QSO 2138-4427. Imaging does not reveal any galaxy associated with
this absorption-line system, but it contains metals (Fe/H ~ -1.3), and its
large velocity range (~ 180 km/s) implies a significant mass.
|
0506525v1
|
2005-08-08
|
Fluorescence in damp air and comments on the radiative life time
|
Photon yields in damp air excited by an electron using a Sr90 $\beta$ source
are compared withthose in dry air. Water vapors considerably reduce the yields,
however, a further study is needed to evaluate the effects on the energy
estimation of ultrahigh-energy cosmic rays. The relation of fluorescence
efficiency to the life time of de-excitation by radiation is discussed.
|
0508183v1
|
2006-08-17
|
Electron thermal conductivity owing to collisions between degenerate electrons
|
We calculate the thermal conductivity of electrons produced by
electron-electron Coulomb scattering in a strongly degenerate electron gas
taking into account the Landau damping of transverse plasmons. The Landau
damping strongly reduces this conductivity in the domain of ultrarelativistic
electrons at temperatures below the electron plasma temperature. In the inner
crust of a neutron star at temperatures T < 1e7 K this thermal conductivity
completely dominates over the electron conductivity due to electron-ion
(electron-phonon) scattering and becomes competitive with the the electron
conductivity due to scattering of electrons by impurity ions.
|
0608371v1
|
2006-09-19
|
Dust, Metals and Diffuse Interstellar Bands in Damped Lyman Alpha Systems
|
Although damped Lyman alpha (DLA) systems are usually considered metal-poor,
it has been suggested that this could be due to observational bias against
metal-enriched absorbers. I review recent surveys to quantify the particular
issue of dust obscuration bias and demonstrate that there is currently no
compelling observational evidence to support a widespread effect due to
extinction. On the other hand, a small sub-set of DLAs may be metal-rich and I
review some recent observations of these metal-rich absorbers and the detection
of diffuse interstellar bands in one DLA at z ~ 0.5.
|
0609530v1
|
2006-11-08
|
Comments on Viscous Damping of Non-Adiabatic MHD Waves in an Unbounded Solar Coronal Plasma by Kumar and Kumar
|
Considering thermal conduction, compressive viscosity and optically thin
radiation as damping mechanisms for MHD waves, we derive a six-order general
dispersion relation. We point out a fundamental flaw in the derivation of
five-order dispersion relation by Kumar and Kumar (2006) who adopt as a basis
vector. The correct definition of the motion in the x-z plane (2-D vector
space) stems from the two independent variables, namely .
|
0611252v2
|
2007-01-10
|
Non-gaussianity in fluctuations from warm inflation
|
The scalar mode density perturbations in a the warm inflationary scenario are
analysed with a view to predicting the amount of non-gaussianity produced by
this scenario. The analysis assumes that the inflaton evolution is strongly
damped by the radiation, with damping terms that are temperature independent.
Entropy fluctuations during warm inflation play a crucial role in generating
non-gaussianity and result in a distinctive signal which should be observable
by the Planck satellite.
|
0701302v2
|
1998-05-22
|
WKB for a damped spin
|
The master equation for a damped spin well known from the theory of
superradiance, is written as a finite-difference equation and solved by a
WKB-like method. The propagator thus obtained looks like the van Vleck
propagator of a certain classical Hamiltonian system with one degree of
freedom. A new interpretation is provided of the temporal broadening of
initially sharp probability distributions as the analogue of the spreading of
the quantum mechanical wave packet.
|
9805018v1
|
1998-11-04
|
Cascades of energy and helicity in the GOY shell model of turbulence
|
The effect of extreme hyperviscous damping, $\nu k_n^p, p=\infty$ is studied
numerically in the GOY shell model of turbulence. It has resently been
demonstrated [Leveque and She, Phys. Rev. Lett, 75,2690 (1995)] that the
inertial range scaling in the GOY model is non-universal and depending on the
viscous damping. The present study shows that the deviation from Kolmogorov
scaling is due to the cascade of the second inviscid invariant. This invariant
is non-positive definite and in this sense analogous to the helicity of 3D
turbulent flow.
|
9811009v1
|
1994-02-04
|
Effects of Disorder in a Dilute Bose Gas
|
We discuss the effects of a weak random external potential on the properties
of the dilute Bose gas at zero temperature. The results recently obtained by
Huang and Meng for the depletion of the condensate and of the superfluid
density are recovered. Results for the shift of the velocity of sound as well
as for its damping due to collisions with the external field are presented. The
damping of phonons is calculated also for dense superfluids. (submitted to
Phys.Rev.B)
|
9402015v1
|
1997-02-13
|
Comment on "Collective Excitations of a Bose-Einstein Condensate in a Magnetic Trap"
|
We calculate the damping rate of collective excitations for a nearly pure
Bose-Einstein condensate regarding the recent experiments in MIT [M.-O. Mews et
al, Phys. Rev. Lett. 77, 988 (1996)]. The decay time of collective excitations
obtained in our theoretical calculations agrees well with their experimental
result. We argue that the damping of collective excitations is due to thermal
contributions rather than interactions between collective modes.
|
9702122v1
|
1997-08-14
|
Landau damping in dilute Bose gases
|
Landau damping in weakly interacting Bose gases is investigated by means of
perturbation theory. Our approach points out the crucial role played by
Bose-Einstein condensation and yields an explicit expression for the decay rate
of elementary excitations in both uniform and non uniform gases. Systematic
results are derived for the phonon width in homogeneous gases interacting with
repulsive forces. Special attention is given to the low and high temperature
regimes.
|
9708104v1
|
1997-11-07
|
Coulomb suppression of NMR coherence peak in fullerene superconductors
|
The suppressed NMR coherence peak in the fullerene superconductors is
explained in terms of the dampings in the superconducting state induced by the
Coulomb interaction between conduction electrons. The Coulomb interaction,
modelled in terms of the onsite Hubbard repulsion, is incorporated into the
Eliashberg theory of superconductivity with its frequency dependence considered
self-consistently at all temperatures. The vertex correction is also included
via the method of Nambu. The frequency dependent Coulomb interaction induces
the substantial dampings in the superconducting state and, consequently,
suppresses the anticipated NMR coherence peak of fullerene superconductors as
found experimentally.
|
9711060v2
|
1997-12-09
|
The Sound of Sonoluminescence
|
We consider an air bubble in water under conditions of single bubble
sonoluminescence (SBSL) and evaluate the emitted sound field nonperturbatively
for subsonic gas-liquid interface motion. Sound emission being the dominant
damping mechanism, we also implement the nonperturbative sound damping in the
Rayleigh-Plesset equation for the interface motion. We evaluate numerically the
sound pulse emitted during bubble collapse and compare the nonperturbative and
perturbative results, showing that the usual perturbative description leads to
an overestimate of the maximal surface velocity and maximal sound pressure. The
radius vs. time relation for a full SBSL cycle remains deceptively unaffected.
|
9712097v1
|
1998-12-02
|
Vortex lattice melting and the damping of the dHvA oscillations in the mixed state
|
Phase fluctuations in the superconducting order parameter, which are
responsible for the melting of the Abrikosov vortex lattice below the mean
field $H_{c2}$, are shown to dramatically enhance the scattering of
quasi-particles by the fluctuating pair potential, thus leading to enhanced
damping of the dHvA oscillations in the liquid mixed state. This effect is
shown to quantitatively account for the detailed field dependence of the dHvA
amplitude observed recently in the mixed state of a Quasi 2D organic SC.
|
9812040v1
|
1999-06-15
|
Temperature-induced resonances and Landau damping of collective modes in Bose-Einstein condensed gases in spherical traps
|
Interaction between collective monopole oscillations of a trapped
Bose-Einstein condensate and thermal excitations is investigated by means of
perturbation theory. We assume spherical symmetry to calculate the matrix
elements by solving the linearized Gross-Pitaevskii equations. We use them to
study the resonances of the condensate induced by temperature when an external
perturbation of the trapping frequency is applied and to calculate the Landau
damping of the oscillations.
|
9906214v1
|
1999-08-03
|
Kinetic Theory of Collective Modes in Atomic Clouds above the Bose-Einstein Transition Temperature
|
We calculate frequencies and damping rates of the lowest collective modes of
a dilute Bose gas confined in an anisotropic trapping potential above the
Bose-Einstein transition temperature. From the Boltzmann equation with a
simplified collision integral we derive a general dispersion relation that
interpolates between the collisionless and hydrodynamic regimes. In the case of
axially symmetric traps we obtain explicit expressions for the frequencies and
damping rates of the lowest modes in terms of a phenomenological collision
time. Our results are compared with microscopic calculations and experiments.
|
9908043v1
|
1999-09-01
|
Normal Fermi Liquid Behavior of Quasiholes in the Spin-Polaron Model for Copper Oxides
|
Based on the t-J model and the self-consistent Born approximation, the
damping of quasiparticle hole states near the Fermi surface is calculated in a
low doping regime. Renormalization of spin-wave excitations due to hole doping
is taken into account. The damping is shown to be described by a familiar form
$\text{Im}\Sigma({\bf k}^{\prime},\epsilon)\propto (\epsilon^{2}/
\epsilon_{F})\ln(\epsilon/ \epsilon_{F})$ characteristic of the 2-dimensional
Fermi liquid, in contrast with the earlier statement reported by Li and Gong
[Phys. Rev. B {\bf 51}, 6343 (1995)] on the marginal Fermi liquid behavior of
quasiholes.
|
9909020v1
|
1999-12-01
|
Impurity relaxation mechanism for dynamic magnetization reversal in a single domain grain
|
The interaction of coherent magnetization rotation with a system of two-level
impurities is studied. Two different, but not contradictory mechanisms, the
`slow-relaxing ion' and the `fast-relaxing ion' are utilized to derive a system
of integro-differential equations for the magnetization. In the case that the
impurity relaxation rate is much greater than the magnetization precession
frequency, these equations can be written in the form of the Landau-Lifshitz
equation with damping. Thus the damping parameter can be directly calculated
from these microscopic impurity relaxation processes.
|
9912014v1
|
2000-02-16
|
Dissipative dynamics of Bose condensates in optical cavities
|
We study the zero temperature dynamics of Bose-Einstein condensates in driven
high-quality optical cavities in the limit of large atom-field detuning. We
calculate the stationary ground state and the spectrum of coupled atom and
field mode excitations for standing wave cavities as well as for travelling
wave cavities. Finite cavity response times lead to damping or controlled
amplification of these excitations. Analytic solutions in the Lamb-Dicke
expansion are in good agreement with numerical results for the full problem and
show that oscillation frequencies and the corresponding damping rates are
qualitatively different for the two cases.
|
0002247v1
|
2000-09-13
|
Oscillations of the superconducting order parameter in a ferromagnet
|
Planar tunneling spectroscopy reveals damped oscillations of the
superconducting order parameter induced into a ferromagnetic thin film by the
proximity effect. The oscillations are due to the finite momentum transfer
provided to Cooper pairs by the splitting of the spin up and down bands in the
ferromagnet. As a consequence, for negative values of the superconducting order
parameter the tunneling spectra are capsized ("$\pi$-state"). The oscillations'
damping and period are set by the same length scale, which depends on the spin
polarization.
|
0009192v1
|
2000-09-29
|
Damping and revivals of collective oscillations in a finite-temperature model of trapped Bose-Einstein condensation
|
We utilize a two-gas model to simulate collective oscillations of a
Bose-Einstein condensate at finite temperatures. The condensate is described
using a generalized Gross-Pitaevskii equation, which is coupled to a thermal
cloud modelled by a Monte Carlo algorithm. This allows us to include the
collective dynamics of both the condensed and non-condensed components
self-consistently. We simulate quadrupolar excitations, and measure the damping
rate and frequency as a function of temperature. We also observe revivals in
condensate oscillations at high temperatures, and in the thermal cloud at low
temperature. Extensions of the model to include non-equilibrium effects and
describe more complex phenomena are discussed.
|
0009468v1
|
2001-04-18
|
Effective rate equations for the over-damped motion in fluctuating potentials
|
We discuss physical and mathematical aspects of the over-damped motion of a
Brownian particle in fluctuating potentials. It is shown that such a system can
be described quantitatively by fluctuating rates if the potential fluctuations
are slow compared to relaxation within the minima of the potential, and if the
position of the minima does not fluctuate. Effective rates can be calculated;
they describe the long-time dynamics of the system. Furthermore, we show the
existence of a stationary solution of the Fokker-Planck equation that describes
the motion within the fluctuating potential under some general conditions. We
also show that a stationary solution of the rate equations with fluctuating
rates exists.
|
0104330v1
|
2001-09-05
|
Spin Excitations in a Fermi Gas of Atoms
|
We have experimentally investigated a spin excitation in a quantum degenerate
Fermi gas of atoms. In the hydrodynamic regime the damping time of the
collective excitation is used to probe the quantum behavior of the gas. At
temperatures below the Fermi temperature we measure up to a factor of 2
reduction in the excitation damping time. In addition we observe a strong
excitation energy dependence for this quantum statistical effect.
|
0109098v2
|
2001-10-09
|
Freezing of a Stripe Liquid
|
The existence of a stripe-liquid phase in a layered nickelate,
La(1.725)Sr(0.275)NiO(4), is demonstrated through neutron scattering
measurements. We show that incommensurate magnetic fluctuations evolve
continuously through the charge-ordering temperature, although an abrupt
decrease in the effective damping energy is observed on cooling through the
transition. The energy and momentum dependence of the magnetic scattering are
parametrized with a damped-harmonic-oscillator model describing overdamped
spin-waves in the antiferromagnetic domains defined instantaneously by charge
stripes.
|
0110191v2
|
2001-12-13
|
Magnon softening and damping in the ferromagnetic manganites due to orbital correlations
|
We present a theory for spin excitations in ferromagnetic metallic manganites
and demonstrate that orbital fluctuations have strong effects on the magnon
dynamics in the case these compounds are close to a transition to an orbital
ordered state. In particular we show that the scattering of the spin
excitations by low-lying orbital modes with cubic symmetry causes both the
magnon softening and damping observed experimentally.
|
0112252v2
|
Subsets and Splits
No community queries yet
The top public SQL queries from the community will appear here once available.