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2021-07-27 | Spin transport-induced damping of coherent THz spin dynamics in iron | We study the damping of perpendicular standing spin-waves (PSSWs) in
ultrathin Fe films at frequencies up to 2.4 THz. The PSSWs are excited by
optically generated ultrashort spin current pulses, and probed optically in the
time domain. Analyzing the wavenumber and thickness dependence of the damping,
we demonstrate that at sufficiently large wave vectors $k$ the damping is
dominated by spin transport effects scaling with k^4 and limiting the frequency
range of observable PSSWs. Although this contribution is known to originate in
the spin diffusion, we argue that at moderate and large k a more general
description is necessary and develop a model where the 'transverse spin mean
free path' is the a key parameter, and estimate it to be ~0.5 nm. | 2107.12812v2 |
2021-09-03 | Stabilization of the damped plate equation under general boundary conditions | We consider a damped plate equation on an open bounded subset of R^d, or a
smooth manifold, with boundary, along with general boundary operators
fulfilling the Lopatinskii-Sapiro condition. The damping term acts on a region
without imposing a geometrical condition. We derive a resolvent estimate for
the generator of the damped plate semigroup that yields a logarithmic decay of
the energy of the solution to the plate equation. The resolvent estimate is a
consequence of a Carleman inequality obtained for the bi-Laplace operator
involving a spectral parameter under the considered boundary conditions. The
derivation goes first though microlocal estimates, then local estimates, and
finally a global estimate. | 2109.01521v2 |
2021-09-07 | Fluid energy cascade rate and kinetic damping: new insight from 3D Landau-fluid simulations | Using an exact law for incompressible Hall magnetohydrodynamics (HMHD)
turbulence, the energy cascade rate is computed from three-dimensional HMHD-CGL
(bi-adiabatic ions and isothermal electrons) and Landau fluid (LF) numerical
simulations that feature different intensities of Landau damping over a broad
range of wavenumbers, typically $0.05\lesssim k_\perp d_i \lesssim100$. Using
three sets of cross-scale simulations where turbulence is initiated at large,
medium and small scales, the ability of the fluid energy cascade to "sense" the
kinetic Landau damping at different scales is tested. The cascade rate
estimated from the exact law and the dissipation calculated directly from the
simulation are shown to reflect the role of Landau damping in dissipating
energy at all scales, with an emphasis on the kinetic ones. This result
provides new prospects on using exact laws for simplified fluid models to
analyze dissipation in kinetic simulations and spacecraft observations, and new
insights into theoretical description of collisionless magnetized plasmas. | 2109.03123v2 |
2021-09-24 | Effect of nonlocal transformations on the linearizability and exact solvability of the nonlinear generalized modified Emden type equations | The nonlinear generalized modified Emden type equations (GMEE) are known to
be linearizable into simple harmonic oscillator (HO) or damped harmonic
oscillators (DHO) via some nonlocal transformations. Hereby, we show that the
structure of the nonlocal transformation and the linearizability into HO or DHO
determine the nature/structure of the dynamical forces involved (hence,
determine the structure of the dynamical equation). Yet, a reverse engineering
strategy is used so that the exact solutions of the emerging GMEE are
nonlocally transformed to find the exact solutions of the HO and DHO dynamical
equations. Consequently, whilst the exact solution for the HO remains a
textbook one, the exact solution for the DHO (never reported elsewhere, to the
best of our knowledge) turns out to be manifestly the most explicit and general
solution that offers consistency and comprehensive coverage for the associated
under-damping, critical-damping, and over-damping cases (i.e., no complex
settings for the coordinates and/or the velocities are eminent/feasible).
Moreover, for all emerging dynamical system, we report illustrative figures for
each solution as well as the corresponding phase-space trajectories as they
evolve in time. | 2109.12059v1 |
2022-01-12 | Implicit Bias of MSE Gradient Optimization in Underparameterized Neural Networks | We study the dynamics of a neural network in function space when optimizing
the mean squared error via gradient flow. We show that in the
underparameterized regime the network learns eigenfunctions of an integral
operator $T_{K^\infty}$ determined by the Neural Tangent Kernel (NTK) at rates
corresponding to their eigenvalues. For example, for uniformly distributed data
on the sphere $S^{d - 1}$ and rotation invariant weight distributions, the
eigenfunctions of $T_{K^\infty}$ are the spherical harmonics. Our results can
be understood as describing a spectral bias in the underparameterized regime.
The proofs use the concept of "Damped Deviations", where deviations of the NTK
matter less for eigendirections with large eigenvalues due to the occurence of
a damping factor. Aside from the underparameterized regime, the damped
deviations point-of-view can be used to track the dynamics of the empirical
risk in the overparameterized setting, allowing us to extend certain results in
the literature. We conclude that damped deviations offers a simple and unifying
perspective of the dynamics when optimizing the squared error. | 2201.04738v1 |
2022-01-19 | Variance-Reduced Stochastic Quasi-Newton Methods for Decentralized Learning: Part II | In Part I of this work, we have proposed a general framework of decentralized
stochastic quasi-Newton methods, which converge linearly to the optimal
solution under the assumption that the local Hessian inverse approximations
have bounded positive eigenvalues. In Part II, we specify two fully
decentralized stochastic quasi-Newton methods, damped regularized
limited-memory DFP (Davidon-Fletcher-Powell) and damped limited-memory BFGS
(Broyden-Fletcher-Goldfarb-Shanno), to locally construct such Hessian inverse
approximations without extra sampling or communication. Both of the methods use
a fixed moving window of $M$ past local gradient approximations and local
decision variables to adaptively construct positive definite Hessian inverse
approximations with bounded eigenvalues, satisfying the assumption in Part I
for the linear convergence. For the proposed damped regularized limited-memory
DFP, a regularization term is added to improve the performance. For the
proposed damped limited-memory BFGS, a two-loop recursion is applied, leading
to low storage and computation complexity. Numerical experiments demonstrate
that the proposed quasi-Newton methods are much faster than the existing
decentralized stochastic first-order algorithms. | 2201.07733v1 |
2022-01-19 | Active tuning of plasmon damping via light induced magnetism | Circularly polarized optical excitation of plasmonic nanostructures causes
coherent circulating motion of their electrons, which in turn, gives rise to
strong optically induced magnetization - a phenomenon known as the inverse
Faraday effect (IFE). In this study we report how the IFE also significantly
decreases plasmon damping. By modulating the optical polarization state
incident on achiral plasmonic nanostructures from linear to circular, we
observe reversible increases of reflectance by 78% as well as simultaneous
increases of optical field concentration by 35.7% under 10^9 W/m^2 continuous
wave (CW) optical excitation. These signatures of decreased plasmon damping
were also monitored in the presence of an externally applied magnetic field
(0.2 T). The combined interactions allow an estimate of the light-induced
magnetization, which corresponds to an effective magnetic field of ~1.3 T
during circularly polarized CW excitation (10^9 W/m^2). We rationalize the
observed decreases in plasmon damping in terms of the Lorentz forces acting on
the circulating electron trajectories. Our results outline strategies for
actively modulating intrinsic losses in the metal, and thereby, the optical
mode quality and field concentration via opto-magnetic effects encoded in the
polarization state of incident light. | 2201.07842v1 |
2022-03-02 | Simplified Stability Assessment of Power Systems with Variable-Delay Wide-Area Damping Control | Power electronic devices such as HVDC and FACTS can be used to improve the
damping of poorly damped inter-area modes in large power systems. This involves
the use of wide-area feedback signals, which are transmitted via communication
networks. The performance of the closed-loop system is strongly influenced by
the delay associated with wide-area signals. The random nature of this delay
introduces a switched linear system model. The stability assessment of such a
system requires linear matrix inequality based approaches. This makes the
stability analysis more complicated as the system size increases. To address
this challenge, this paper proposes a delay-processing strategy that simplifies
the modelling and analysis in discrete-domain. In contrast to the existing
stability assessment techniques, the proposed approach is advantageous because
the stability, as well as damping performance, can be accurately predicted by a
simplified analysis. The proposed methodology is verified with a case study on
the 2-area 4-machine power system with a series compensated tie-line. The
results are found to be in accordance with the predictions of the proposed
simplified analysis. | 2203.01362v1 |
2022-03-03 | Forward-modulated damping estimates and nonlocalized stability of periodic Lugiato-Lefever wave | In an interesting recent analysis, Haragus-Johnson-Perkins-de Rijk have shown
modulational stability under localized perturbations of steady periodic
solutions of the Lugiato-Lefever equation (LLE), in the process pointing out a
difficulty in obtaining standard "nonlinear damping estimates" on modulated
perturbation variables to control regularity of solutions. Here, we point out
that in place of standard "inverse-modulated" damping estimates, one can
alternatively carry out a damping estimate on the "forward-modulated"
perturbation, noting that norms of forward- and inverse-modulated variables are
equivalent modulo absorbable errors, thus recovering the classical argument
structure of Johnson-Noble-Rodrigues-Zumbrun for parabolic systems. This
observation seems of general use in situations of delicate regularity.
Applied in the context of (LLE) it gives the stronger result of stability and
asymptotic behavior with respect to nonlocalized perturbations. | 2203.01770v3 |
2022-03-31 | Observing Particle Energization above the Nyquist Frequency: An Application of the Field-Particle Correlation Technique | The field-particle correlation technique utilizes single-point measurements
to uncover signatures of various particle energization mechanisms in turbulent
space plasmas. The signature of Landau damping by electrons has been found in
both simulations and observations from Earth's magnetosheath using this
technique, but instrumental limitations of spacecraft sampling rates present a
challenge to discovering the full extent of the presence of Landau damping in
the solar wind. Theory predicts that field-particle correlations can recover
velocity-space energization signatures even from data that is undersampled with
respect to the characteristic frequencies at which the wave damping occurs. To
test this hypothesis, we perform a high-resoluation gyrokinetic simulation of
space plasma turbulence, confirm that it contains signatures of electron Landau
damping, and then systematically reduce the time resolution of the data to
identify the point at which the signatures become impossible to recover. We
find results in support of our theoretical prediction and look for a rule of
thumb that can be compared with the measurement capabilities of spacecraft
missions to inform the process of applying field-particle correlations to low
time resolution data. | 2204.00104v1 |
2022-04-06 | A Potential Based Quantization Procedure of the Damped Oscillator | Nowadays, two of the most prospering fields of physics are quantum computing
and spintronics. In both, the loss of information and dissipation plays a
crucial role. In the present work we formulate the quantization of the
dissipative oscillator, which aids understanding of the above mentioned, and
creates a theoretical frame to overcome these issues in the future. Based on
the Lagrangian framework of the damped spring system, the canonically
conjugated pairs and the Hamiltonian of the system are obtained, by which the
quantization procedure can be started and consistently applied. As a result,
the damping quantum wave equation of the dissipative oscillator is deduced, by
which an exact damping wave solution of this equation is obtained.
Consequently, we arrive at such an irreversible quantum theory by which the
quantum losses can be described. | 2204.02893v2 |
2022-04-19 | Role of shape anisotropy on thermal gradient-driven domain wall dynamics in magnetic nanowires | We investigate the magnetic domain wall (DW) dynamics in uniaxial/biaxial
nanowires under a thermal gradient (TG). The findings reveal that the DW
propagates toward the hotter region in both nanowires. The main physics of such
observations is the magnonic angular momentum transfer to the DW. The hard
(shape) anisotropy exists in biaxial nanowire, which contributes an additional
torque, hence DW speed is larger than that in uniaxial nanowire. With lower
damping, the DW velocity is smaller and DW velocity increases with damping
which is opposite to usual expectation. To explain this, it is predicted that
there is a probability to form the standing spin-waves (which do not carry net
energy/momentum) together with travelling spin-waves if the propagation length
of thermally-generated spin-waves is larger than the nanowire length. For
larger-damping, DW decreases with damping since the magnon propagation length
decreases. Therefore, the above findings might be useful in realizing the
spintronic (racetrack memory) devices. | 2204.09101v2 |
2022-04-25 | Energy decay estimates for the wave equation with supercritical nonlinear damping | We consider a damped wave equation in a bounded domain. The damping is
nonlinear and is homogeneous with degree p -- 1 with p > 2. First, we show that
the energy of the strong solution in the supercritical case decays as a
negative power of t; the rate of decay is the same as in the subcritical or
critical cases, provided that the space dimension does not exceed ten. Next,
relying on a new differential inequality, we show that if the initial
displacement is further required to lie in L p , then the energy of the
corresponding weak solution decays logarithmically in the supercritical case.
Those new results complement those in the literature and open an important
breach in the unknown land of super-critical damping mechanisms. | 2204.11494v1 |
2022-05-07 | Proposal for a Damping-Ring-Free Electron Injector for Future Linear Colliders | The current designs of future electron-positron linear colliders incorporate
large and complex damping rings to produce asymmetric beams for beamstrahlung
suppression. Here we present the design of an electron injector capable of
delivering flat electron beams with phase-space partition comparable to the
electron-beam parameters produced downstream of the damping ring in the
proposed international linear collider (ILC) design. Our design does not employ
a damping ring but is instead based on cross-plane phase-space-manipulation
techniques. The performance of the proposed configuration, its sensitivity to
jitter along with its impact on spin-polarization is investigated. The proposed
paradigm could be adapted to other linear collider concepts under consideration
and offers a path toward significant cost and complexity reduction. | 2205.03736v1 |
2022-06-02 | Optimal Control of the 3D Damped Navier-Stokes-Voigt Equations with Control Constraints | In this paper, we consider the 3D Navier-Stokes-Voigt (NSV) equations with
nonlinear damping $|u|^{r-1}u, r\in[1,\infty)$ in bounded and space-periodic
domains. We formulate an optimal control problem of minimizing the curl of the
velocity field in the energy norm subject to the flow velocity satisfying the
damped NSV equation with a distributed control force. The control also needs to
obey box-type constraints. For any $r\geq 1,$ the existence and uniqueness of a
weak solution is discussed when the domain $\Omega$ is periodic/bounded in
$\mathbb R^3$ while a unique strong solution is obtained in the case of
space-periodic boundary conditions. We prove the existence of an optimal pair
for the control problem. Using the classical adjoint problem approach, we show
that the optimal control satisfies a first-order necessary optimality condition
given by a variational inequality. Since the optimal control problem is
non-convex, we obtain a second-order sufficient optimality condition showing
that an admissible control is locally optimal. Further, we derive optimality
conditions in terms of adjoint state defined with respect to the growth of the
damping term for a global optimal control. | 2206.00988v2 |
2022-06-05 | Stationary measures for stochastic differential equations with degenerate damping | A variety of physical phenomena involve the nonlinear transfer of energy from
weakly damped modes subjected to external forcing to other modes which are more
heavily damped. In this work we explore this in (finite-dimensional) stochastic
differential equations in $\mathbb R^n$ with a quadratic, conservative
nonlinearity $B(x,x)$ and a linear damping term $-Ax$ which is degenerate in
the sense that $\mathrm{ker} A \neq \emptyset$. We investigate sufficient
conditions to deduce the existence of a stationary measure for the associated
Markov semigroups. Existence of such measures is straightforward if $A$ is full
rank, but otherwise, energy could potentially accumulate in $\mathrm{ker} A$
and lead to almost-surely unbounded trajectories, making the existence of
stationary measures impossible. We give a relatively simple and general
sufficient condition based on time-averaged coercivity estimates along
trajectories in neighborhoods of $\mathrm{ker} A$ and many examples where such
estimates can be made. | 2206.02240v1 |
2022-06-17 | Resolvent estimates for the one-dimensional damped wave equation with unbounded damping | We study the generator $G$ of the one-dimensional damped wave equation with
unbounded damping. We show that the norm of the corresponding resolvent
operator, $\| (G - \lambda)^{-1} \|$, is approximately constant as $|\lambda|
\to +\infty$ on vertical strips of bounded width contained in the closure of
the left-hand side complex semi-plane, $\overline{\mathbb{C}}_{-} := \{\lambda
\in \mathbb{C}: \operatorname{Re} \lambda \le 0\}$. Our proof rests on a
precise asymptotic analysis of the norm of the inverse of $T(\lambda)$, the
quadratic operator associated with $G$. | 2206.08820v2 |
2022-08-04 | Lp-asymptotic stability of 1D damped wave equations with localized and nonlinear damping | In this paper, we study the $L^p$-asymptotic stability with $p\in (1,\infty)$
of the one-dimensional nonlinear damped wave equation with a localized damping
and Dirichlet boundary conditions in a bounded domain $(0,1)$. We start by
addressing the well-posedness problem. We prove the existence and the
uniqueness of weak solutions for $p\in [2,\infty)$ and the existence and the
uniqueness of strong solutions for all $p\in [1,\infty)$. The proofs rely on
the well-posedness already proved in the $L^\infty$ framework by [4] combined
with a density argument. Then we prove that the energy of strong solutions
decays exponentially to zero. The proof relies on the multiplier method
combined with the work that has been done in the linear case in [8]. | 2208.02779v1 |
2022-08-07 | Damping of neutrino oscillations, decoherence and the lengths of neutrino wave packets | Spatial separation of the wave packets (WPs) of neutrino mass eigenstates
leads to decoherence and damping of neutrino oscillations. Damping can also be
caused by finite energy resolution of neutrino detectors or, in the case of
experiments with radioactive neutrino sources, by finite width of the emitted
neutrino line. We study in detail these two types of damping effects using
reactor neutrino experiments and experiments with radioactive $^{51}$Cr source
as examples. We demonstrate that the effects of decoherence by WP separation
can always be incorporated into a modification of the energy resolution
function of the detector and so are intimately entangled with it. We estimate
for the first time the lengths $\sigma_x$ of WPs of reactor neutrinos and
neutrinos from a radioactive $^{51}$Cr source. The obtained values, $\sigma_x =
(2\times 10^{-5} - 1.4\times 10^{-4})$ cm, are at least six orders of magnitude
larger than the currently available experimental lower bounds. We conclude that
effects of decoherence by WP separation cannot be probed in reactor and
radioactive source experiments. | 2208.03736v2 |
2022-08-23 | Fate of exceptional points in the presence of nonlinearities | The non-Hermitian dynamics of open systems deal with how intricate coherent
effects of a closed system intertwine with the impact of coupling to an
environment. The system-environment dynamics can then lead to so-called
exceptional points, which are the open-system marker of phase transitions,
i.e., the closing of spectral gaps in the complex spectrum. Even in the
ubiquitous example of the damped harmonic oscillator, the dissipative
environment can lead to an exceptional point, separating between under-damped
and over-damped dynamics at a point of critical damping. Here, we examine the
fate of this exceptional point in the presence of strong correlations, i.e.,
for a nonlinear oscillator. By employing a functional renormalization group
approach, we identify non-perturbative regimes of this model where the
nonlinearity makes the system more robust against the influence of dissipation
and can remove the exceptional point altogether. The melting of the exceptional
point occurs above a critical nonlinearity threshold. Interestingly, the
exceptional point melts faster with increasing temperatures, showing a
surprising flow to coherent dynamics when coupled to a warm environment. | 2208.11205v2 |
2022-09-10 | Data-driven, multi-moment fluid modeling of Landau damping | Deriving governing equations of complex physical systems based on first
principles can be quite challenging when there are certain unknown terms and
hidden physical mechanisms in the systems. In this work, we apply a deep
learning architecture to learn fluid partial differential equations (PDEs) of a
plasma system based on the data acquired from a fully kinetic model. The
learned multi-moment fluid PDEs are demonstrated to incorporate kinetic effects
such as Landau damping. Based on the learned fluid closure, the data-driven,
multi-moment fluid modeling can well reproduce all the physical quantities
derived from the fully kinetic model. The calculated damping rate of Landau
damping is consistent with both the fully kinetic simulation and the linear
theory. The data-driven fluid modeling of PDEs for complex physical systems may
be applied to improve fluid closure and reduce the computational cost of
multi-scale modeling of global systems. | 2209.04726v1 |
2022-09-25 | Formation of the cosmic-ray halo: The role of nonlinear Landau damping | We present a nonlinear model of self-consistent Galactic halo, where the
processes of cosmic ray (CR) propagation and excitation/damping of MHD waves
are included. The MHD-turbulence, which prevents CR escape from the Galaxy, is
entirely generated by the resonant streaming instability. The key mechanism
controlling the halo size is the nonlinear Landau (NL) damping, which
suppresses the amplitude of MHD fluctuations and, thus, makes the halo larger.
The equilibrium turbulence spectrum is determined by a balance of CR excitation
and NL damping, which sets the regions of diffusive and advective propagation
of CRs. The boundary $z_{cr}(E)$ between the two regions is the halo size,
which slowly increases with the energy. For the vertical magnetic field of
$\sim 1~\mu G$, we estimate $z_{cr} \sim 1$ kpc for GeV protons. The derived
proton spectrum is in a good agreement with observational data. | 2209.12302v1 |
2022-10-14 | Landau damping for gravitational waves in parity-violating theories | We discuss how tensor polarizations of gravitational waves can suffer Landau
damping in the presence of velocity birefringence, when parity symmetry is
explicitly broken. In particular, we analyze the role of the Nieh-Yan and
Chern-Simons terms in modified theories of gravity, showing how the
gravitational perturbation in collisionless media can be characterized by a
subluminal phase velocity, circumventing the well-known results of General
Relativity and allowing for the appearance of the kinematic damping. We
investigate in detail the connection between the thermodynamic properties of
the medium, such as temperature and mass of the particles interacting with the
gravitational wave, and the parameters ruling the parity violating terms of the
models. In this respect, we outline how the dispersion relations can give rise
in each model to different regions of the wavenumber space, where the phase
velocity is subluminal, superluminal or does not exist. Quantitative estimates
on the considered models indicate that the phenomenon of Landau damping is not
detectable given the sensitivity of present-day instruments. | 2210.07673v2 |
2022-10-25 | Formation of shifted shock for the 3D compressible Euler equations with damping | In this paper, we show the shock formation of the solutions to the
3-dimensional (3D) compressible isentropic and irrotational Euler equations
with damping for the initial short pulse data which was first introduced by
D.Christodoulou\cite{christodoulou2007}. Due to the damping effect, the
largeness of the initial data is necessary for the shock formation and we will
work on the class of large data (in energy sense). Similar to the undamped
case, the formation of shock is characterized by the collapse of the
characteristic hypersurfaces and the vanishing of the inverse foliation density
function $\mu$, at which the first derivatives of the velocity and the density
blow up. However, the damping effect changes the asymptotic behavior of the
inverse foliation density function $\mu$ and then shifts the time of shock
formation compared with the undamped case. The methods in the paper can also be
extended to a class of $3D$ quasilinear wave equations for the short pulse
initial data. | 2210.13796v1 |
2022-10-30 | Dynamics of a class of extensible beams with degenerate and non-degenerate nonlocal damping | This work is concerned with new results on long-time dynamics of a class of
hyperbolic evolution equations related to extensible beams with three
distinguished nonlocal nonlinear damping terms. In the first possibly
degenerate case, the results feature the existence of a family of compact
global attractors and a thickness estimate for their Kolmogorov's
$\varepsilon$-entropy. Then, in the non-degenerate context, the structure of
the helpful nonlocal damping leads to the existence of finite-dimensional
compact global and exponential attractors. Lastly, in a degenerate and critical
framework, it is proved the existence of a bounded closed global attractor but
not compact. To the proofs, we provide several new technical results by means
of refined estimates that open up perspectives for a new branch of nonlinearly
damped problems. | 2210.16851v1 |
2022-11-11 | Nonlinear fractional damped wave equation on compact Lie groups | In this paper, we deal with the initial value fractional damped wave equation
on $G$, a compact Lie group, with power-type nonlinearity. The aim of this
manuscript is twofold. First, using the Fourier analysis on compact Lie groups,
we prove a local in-time existence result in the energy space for the
fractional damped wave equation on $G$. Moreover, a finite time blow-up result
is established under certain conditions on the initial data. In the next part
of the paper, we consider fractional wave equation with lower order terms, that
is, damping and mass with the same power type nonlinearity on compact Lie
groups, and prove the global in-time existence of small data solutions in the
energy evolution space. | 2211.06155v1 |
2022-11-16 | Controlling the motional quality factor of a diamagnetically levitated graphite plate | Researchers seek methods to levitate matter for a wide variety of purposes,
ranging from exploring fundamental problems in science, through to developing
new sensors and mechanical actuators. Many levitation techniques require active
driving and most can only be applied to objects smaller than a few micrometers.
Diamagnetic levitation has the strong advantage of being the only form of
levitation which is passive, requiring no energy input, while also supporting
massive objects. Known diamagnetic materials which are electrical insulators
are only weakly diamagnetic, and require large magnetic field gradients to
levitate. Strong diamagnetic materials which are electrical conductors, such as
graphite, exhibit eddy damping, restricting motional freedom and reducing their
potential for sensing applications. In this work we describe a method to
engineer the eddy damping while retaining the force characteristics provided by
the diamagnetic material. We study, both experimentally and theoretically, the
motional damping of a magnetically levitated graphite plate in high vacuum and
demonstrate that one can control the eddy damping by patterning the plate with
through-slots which interrupt the eddy currents. We find we can control the
motional quality factor over a wide range with excellent agreement between the
experiment and numerical simulations. | 2211.08764v1 |
2022-12-03 | Strong On-Chip Microwave Photon-Magnon Coupling Using Ultra-low Damping Epitaxial Y3Fe5O12 Films at 2 Kelvin | Y3Fe5O12 is arguably the best magnetic material for magnonic quantum
information science (QIS) because of its extremely low damping. We report
ultralow damping at 2 K in epitaxial Y3Fe5O12 thin films grown on a diamagnetic
Y3Sc2Ga3O12 substrate that contains no rare-earth elements. Using these
ultralow damping YIG films, we demonstrate for the first time strong coupling
between magnons in patterned YIG thin films and microwave photons in a
superconducting Nb resonator. This result paves the road towards scalable
hybrid quantum systems that integrate superconducting microwave resonators, YIG
film magnon conduits, and superconducting qubits into on-chip QIS devices. | 2212.01708v1 |
2022-12-21 | Fractional damping effects on the transient dynamics of the Duffing oscillator | We consider the nonlinear Duffing oscillator in presence of fractional
damping which is characteristic in different physical situations. The system is
studied with a smaller and larger damping parameter value, that we call the
underdamped and overdamped regimes. In both we have studied the relation
between the fractional parameter, the amplitude of the oscillations and the
times to reach the asymptotic behavior, called asymptotic times. In the
overdamped regime, the study shows that, also here, there are oscillations for
fractional order derivatives and their amplitudes and asymptotic times can
suddenly change for small variations of the fractional parameter. In addition,
in this latter regime, a resonant-like behavior can take place for suitable
values of the parameters of the system. These results are corroborated by
calculating the corresponding Q-factor. We expect that these results can be
useful for a better understanding of fractional dynamics and its possible
applications as in modeling different kind of materials that normally need
complicated damping terms. | 2212.11023v1 |
2023-01-19 | Damped harmonic oscillator revisited: the fastest route to equilibrium | Theoretically, solutions of the damped harmonic oscillator asymptotically
approach equilibrium, i.e., the zero energy state, without ever reaching it
exactly, and the critically damped solution approaches equilibrium faster than
the underdamped or the overdamped solution. Experimentally, the systems
described with this model reach equilibrium when the system's energy has
dropped below some threshold corresponding to the energy resolution of the
measuring apparatus. We show that one can (almost) always find an optimal
underdamped solution that will reach this energy threshold sooner than all
other underdamped solutions, as well as the critically damped solution, no
matter how small this threshold is. We also comment on one exception to this
for a particular type of initial conditions, when a specific overdamped
solution reaches the equilibrium state sooner than all other solutions. We
confirm some of our findings experimentally. | 2301.08222v2 |
2023-01-22 | Boundary stabilization of a vibrating string with variable length | We study small vibrations of a string with time-dependent length $\ell(t)$
and boundary damping. The vibrations are described by a 1-d wave equation in an
interval with one moving endpoint at a speed $\ell'(t)$ slower than the speed
of propagation of the wave c=1. With no damping, the energy of the solution
decays if the interval is expanding and increases if the interval is shrinking.
The energy decays faster when the interval is expanding and a constant damping
is applied at the moving end. However, to ensure the energy decay in a
shrinking interval, the damping factor $\eta$ must be close enough to the
optimal value $\eta=1$, corresponding to the transparent condition. In all
cases, we establish lower and upper estimates for the energy with explicit
constants. | 2301.09086v1 |
2023-02-24 | Asymptotic behaviour of the semidiscrete FE approximations to weakly damped wave equations with minimal smoothness on initial data | Exponential decay estimates of a general linear weakly damped wave equation
are studied with decay rate lying in a range. Based on the $C^0$-conforming
finite element method to discretize spatial variables keeping temporal variable
continuous, a semidiscrete system is analysed, and uniform decay estimates are
derived with precisely the same decay rate as in the continuous case. Optimal
error estimates with minimal smoothness assumptions on the initial data are
established, which preserve exponential decay rate, and for a 2D problem, the
maximum error bound is also proved. The present analysis is then generalized to
include the problems with non-homogeneous forcing function, space-dependent
damping, and problems with compensator. It is observed that decay rates are
improved with large viscous damping and compensator. Finally, some numerical
experiments are performed to validate the theoretical results established in
this paper. | 2302.12476v1 |
2023-02-27 | Nonlinear acoustic imaging with damping | In this paper, we consider an inverse problem for a nonlinear wave equation
with a damping term and a general nonlinear term. This problem arises in
nonlinear acoustic imaging and has applications in medical imaging and other
fields. The propagation of ultrasound waves can be modeled by a quasilinear
wave equation with a damping term. We show the boundary measurements encoded in
the Dirichlet-to-Neumann map (DN map) determine the damping term and the
nonlinearity at the same time. In a more general setting, we consider a
quasilinear wave equation with a one-form (a first-order term) and a general
nonlinear term. We prove the one-form and the nonlinearity can be determined
from the DN map, up to a gauge transformation, under some assumptions. | 2302.14174v1 |
2023-04-11 | Sizable suppression of magnon Hall effect by magnon damping in Cr$_2$Ge$_2$Te$_6$ | Two-dimensional (2D) Heisenberg honeycomb ferromagnets are expected to have
interesting topological magnon effects as their magnon dispersion can have
Dirac points. The Dirac points are gapped with finite second nearest neighbor
Dzyaloshinskii-Moriya interaction, providing nontrivial Berry curvature with
finite magnon Hall effect. Yet, it is unknown how the topological properties
are affected by magnon damping. We report the thermal Hall effect in
Cr$_2$Ge$_2$Te$_6$, an insulating 2D honeycomb ferromagnet with a large Dirac
magnon gap and significant magnon damping. Interestingly, the thermal Hall
conductivity in Cr$_2$Ge$_2$Te$_6$ shows the coexisting phonon and magnon
contributions. Using an empirical two-component model, we successfully estimate
the magnon contribution separate from the phonon part, revealing that the
magnon Hall conductivity was 20 times smaller than the theoretical calculation.
Finally, we suggest that such considerable suppression in the magnon Hall
conductivity is due to the magnon damping effect in Cr$_2$Ge$_2$Te$_6$. | 2304.04922v1 |
2023-05-22 | Semi-active damping optimization of vibrational systems using the reduced basis method | In this article, we consider vibrational systems with semi-active damping
that are described by a second-order model. In order to minimize the influence
of external inputs to the system response, we are optimizing some damping
values. As minimization criterion, we evaluate the energy response, that is the
$\cH_2$-norm of the corresponding transfer function of the system. Computing
the energy response includes solving Lyapunov equations for different damping
parameters. Hence, the minimization process leads to high computational costs
if the system is of large dimension. We present two techniques that reduce the
optimization problem by applying the reduced basis method to the corresponding
parametric Lyapunov equations. In the first method, we determine a reduced
solution space on which the Lyapunov equations and hence the resulting energy
response values are computed approximately in a reasonable time. The second
method includes the reduced basis method in the minimization process. To
evaluate the quality of the approximations, we introduce error estimators that
evaluate the error in the controllability Gramians and the energy response.
Finally, we illustrate the advantages of our methods by applying them to two
different examples. | 2305.12946v1 |
2023-06-01 | A combined volume penalization / selective frequency damping approach for immersed boundary methods: application to moving geometries | This work extends, to moving geometries, the immersed boundary method based
on volume penalization and selective frequency damping approach [J. Kou, E.
Ferrer, A combined volume penalization/selective frequency damping approach for
immersed boundary methods applied to high-order schemes, Journal of
Computational Physics (2023)]. To do so, the numerical solution inside the
solid is decomposed into a predefined movement and an oscillatory part
(spurious waves), where the latter is damped by an SFD approach combined with
volume penalization. We challenge the method with two cases. First, a new
manufactured solution problem is proposed to show that the method can recover
high-order accuracy. Second, we validate the methodology by simulating the
laminar flow past a moving cylinder, where improved accuracy of the combined
method is reported. | 2306.00504v1 |
2023-06-09 | Damped nonlinear Schrödinger equation with Stark effect | We study the $L^2$-critical damped NLS with a Stark potential. We prove that
the threshold for global existence and finite time blowup of this equation is
given by $\|Q\|_2$, where $Q$ is the unique positive radial solution of $\Delta
Q + |Q|^{4/d} Q = Q$ in $H^1(\mathbb{R}^d)$. Moreover, in any small
neighborhood of $Q$, there exists an initial data $u_0$ above the ground state
such that the solution flow admits the log-log blowup speed. This verifies the
structural stability for the ``$\log$-$\log$ law'' associated to the NLS
mechanism under the perturbation by a damping term and a Stark potential. The
proof of our main theorem is based on the Avron-Herbst formula and the
analogous result for the unperturbed damped NLS. | 2306.05931v1 |
2023-06-19 | New Perspectives and Systematic Approaches for Analyzing Negative Damping-Induced Sustained Oscillation | Sustained oscillations (SOs) are commonly observed in systems dominated by
converters. Under specific conditions, even though the origin of SOs can be
identified through negative damping modes using conventional linear analysis,
utilizing the describing function to compute harmonic amplitude and frequency
remains incomplete. This is because a) it can not cover the cases where hard
limits are not triggered, and b) it can not provide a complete trajectory for
authentic linear analysis to confirm the presence of SO. Hence, two analytical
methods are proposed by returning to the essential principle of harmonic
balance. a) A dedicated approach is proposed to solving steady-state harmonics
via Newton-Raphson iteration with carefully chosen initial values. The method
encompasses all potential hard limit triggered cases. b) By employing extended
multiharmonic linearization theory and considering loop impedance, an authentic
linear analysis of SO is conducted. The analysis indicates that the initial
negative damping modes transform into multiple positive damping modes as SO
develops. Simulation validations are performed on a two-level voltage source
converter using both PSCAD and RT-LAB. Additionally, valuable insights into the
work are addressed considering the modularity and scalability of the proposed
methods. | 2306.10839v2 |
2023-06-24 | Numerical approximation of the invariant distribution for a class of stochastic damped wave equations | We study a class of stochastic semilinear damped wave equations driven by
additive Wiener noise. Owing to the damping term, under appropriate conditions
on the nonlinearity, the solution admits a unique invariant distribution. We
apply semi-discrete and fully-discrete methods in order to approximate this
invariant distribution, using a spectral Galerkin method and an exponential
Euler integrator for spatial and temporal discretization respectively. We prove
that the considered numerical schemes also admit unique invariant
distributions, and we prove error estimates between the approximate and exact
invariant distributions, with identification of the orders of convergence. To
the best of our knowledge this is the first result in the literature concerning
numerical approximation of invariant distributions for stochastic damped wave
equations. | 2306.13998v1 |
2023-07-31 | Estimation of Power in the Controlled Quantum Teleportation through the Witness Operator | Controlled quantum teleportation (CQT) can be considered as a variant of
quantum teleportation in which three parties are involved where one party acts
as the controller. The usability of the CQT scheme depends on two types of
fidelities viz. conditioned fidelity and non-conditioned fidelity. The
difference between these fidelities may be termed as power of the controller
and it plays a vital role in the CQT scheme. Thus, our aim is to estimate the
power of the controller in such a way so that its estimated value can be
obtained in an experiment. To achieve our goal, we have constructed a witness
operator and have shown that its expected value may be used in the estimation
of the lower bound of the power of the controller. Furthermore, we have shown
that it is possible to make the standard W state useful in the CQT scheme if
one of its qubits either passes through the amplitude damping channel or the
phase damping channel. We have also shown that the phase damping channel
performs better than the amplitude damping channel in the sense of generating
more power of the controller in the CQT scheme. | 2307.16574v1 |
2023-08-03 | Triple-Spherical Bessel Function Integrals with Exponential and Gaussian Damping: Towards an Analytic N-Point Correlation Function Covariance Model | Spherical Bessel functions appear commonly in many areas of physics wherein
there is both translation and rotation invariance, and often integrals over
products of several arise. Thus, analytic evaluation of such integrals with
different weighting functions (which appear as toy models of a given physical
observable, such as the galaxy power spectrum) is useful. Here we present a
generalization of a recursion-based method for evaluating such integrals. It
gives relatively simple closed-form results in terms of Legendre functions (for
the exponentially-damped case) and Gamma, incomplete Gamma functions, and
hypergeometric functions (for the Gaussian-damped case). We also present a new,
non-recursive method to evaluate integrals of products of spherical Bessel
functions with Gaussian damping in terms of incomplete Gamma functions and
hypergeometric functions. | 2308.01955v2 |
2023-08-28 | Quantized damped transversal single particle mechanical waves | In information transfer, the dissipation of a signal may have crucial
importance. The feasibility of reconstructing the distorted signal also depends
on this. That is why the study of quantized dissipative transversal single
particle mechanical waves may have an important role. It may be true,
particularly on the nanoscale in the case of signal distortion, loss, or
restoration. Based on the damped oscillator quantum description, we generalize
the canonical quantization procedure for the transversal waves. Furthermore, we
deduce the related damped wave equation and the state function. We point out
the two kinds of solutions of the wave equation. One involves the well-known
spreading solution superposed with the oscillation, in which the loss of
information is complete. The other is the Airy function solution, which is
non-spreading, so there is information loss only due to oscillation damping.
However, the structure of the wavefront remains unchanged. Thus, this result
allows signal reconstruction, which is important in restoring the lost
information. | 2308.14820v1 |
2023-11-15 | Integrated Local Energy Decay for Damped Magnetic Wave Equations on Stationary Space-Times | We establish local energy decay for damped magnetic wave equations on
stationary, asymptotically flat space-times subject to the geometric control
condition. More specifically, we allow for the addition of time-independent
magnetic and scalar potentials, which negatively affect energy coercivity and
may add in unwieldy spectral effects. By asserting the non-existence of
eigenvalues in the lower half-plane and resonances on the real line, we are
able to apply spectral theory from the work of Metcalfe, Sterbenz, and Tataru
and combine with a generalization of prior work by the present author to extend
the latter work and establish local energy decay, under one additional symmetry
hypothesis. Namely, we assume that either the imaginary part of the magnetic
potentials are uniformly small or, more interestingly and novelly, that the
damping term is the dominant principal term in the skew-adjoint part of the
damped wave operator within the region where the metric perturbation from that
of Minkowski space is permitted to be large. We also obtain an energy dichotomy
if we do not prohibit non-zero real resonances. In order to make the structure
of the argument more cohesive, we contextualize the present work within
requisite existing theory. | 2311.08628v1 |
2023-11-15 | Applications of $L^p-L^q$ estimates for solutions to semi-linear $σ$-evolution equations with general double damping | In this paper, we would like to study the linear Cauchy problems for
semi-linear $\sigma$-evolution models with mixing a parabolic like damping term
corresponding to $\sigma_1 \in [0,\sigma/2)$ and a $\sigma$-evolution like
damping corresponding to $\sigma_2 \in (\sigma/2,\sigma]$. The main goals are
on the one hand to conclude some estimates for solutions and their derivatives
in $L^q$ setting, with any $q\in [1,\infty]$, by developing the theory of
modified Bessel functions effectively to control oscillating integrals
appearing the solution representation formula in a competition between these
two kinds of damping. On the other hand, we are going to prove the global (in
time) existence of small data Sobolev solutions in the treatment of the
corresponding semi-linear equations by applying $(L^{m}\cap L^{q})- L^{q}$ and
$L^{q}- L^{q}$ estimates, with $q\in (1,\infty)$ and $m\in [1,q)$, from the
linear models. Finally, some further generalizations will be discussed in the
end of this paper. | 2311.09085v1 |
2023-12-07 | Generalized Damping Torque Analysis of Ultra-Low Frequency Oscillation in the Jerk Space | Ultra low frequency oscillation (ULFO) is significantly threatening the power
system stability. Its unstable mechanism is mostly studied via generalized
damping torque analysis method (GDTA). However, the analysis still adopts the
framework established for low frequency oscillation. Hence, this letter
proposes a GDTA approach in the jerk space for ULFO. A multi-information
variable is constructed to transform the system into a new state space, where
it is found that the jerk dynamics of the turbine-generator cascaded system is
a second-order differential equation. Benefiting from this characteristic, we
propose a new form for GDTA using jerk dynamics, which is established in the
frequency-frequency acceleration phase space. Then, analytical expressions of
all damping torque are provided. Finally, test results verified the proposed
theoretical results. The negative damping mechanism is revealed, and parameter
adjustment measures are concluded. | 2312.04148v1 |
2023-12-08 | Selective damping of plasmons in coupled two-dimensional systems by Coulomb drag | The Coulomb drag is a many-body effect observed in proximized low-dimensional
systems. It appears as emergence of voltage in one of them upon passage of bias
current in another. The magnitude of drag voltage can be strongly affected by
exchange of plasmonic excitations between the layers; however, the reverse
effect of Coulomb drag on properties of plasmons has not been studied. Here, we
study the plasmon spectra and damping in parallel two-dimensional systems in
the presence of Coulomb drag. We find that Coulomb drag leads to selective
damping of one of the two fundamental plasma modes of a coupled bilayer. For
identical electron doping of both layers, the drag suppresses the acoustic
plasma mode; while for symmetric electron-hole doping of the coupled pair, the
drag suppresses the optical plasma mode. The selective damping can be observed
both for propagating modes in extended bilayers and for localized plasmons in
bilayers confined by source and drain contacts. The discussed effect may
provide access to the strength of Coulomb interaction in 2d electron systems
from various optical and microwave scattering experiments. | 2312.05097v1 |
2023-12-13 | Geometrical Interpretation of Neutrino Oscillation with decay | The geometrical representation of two-flavor neutrino oscillation represents
the neutrino's flavor eigenstate as a magnetic moment-like vector that evolves
around a magnetic field-like vector that depicts the Hamiltonian of the system.
In the present work, we demonstrate the geometrical interpretation of neutrino
in a vacuum in the presence of decay, which transforms this circular trajectory
of neutrino into a helical track that effectively makes the neutrino system
mimic a classical damped driven oscillator. We show that in the absence of the
phase factor $\xi$ in the decay Hamiltonian, the neutrino exactly behaves like
the system of nuclear magnetic resonance(NMR); however, the inclusion of the
phase part introduces a $CP$ violation, which makes the system deviate from
NMR. Finally, we make a qualitative discussion on under-damped,
critically-damped, and over-damped scenarios geometrically by three different
diagrams. In the end, we make a comparative study of geometrical picturization
in vacuum, matter, and decay, which extrapolates the understanding of the
geometrical representation of neutrino oscillation in a more straightforward
way. | 2312.08178v1 |
2024-01-01 | Magnon Damping Minimum and Logarithmic Scaling in a Kondo-Heisenberg Model | Recently, an anomalous temperature evolution of spin wave excitations has
been observed in a van der Waals metallic ferromagnet Fe$_3$GeTe$_2$ (FGT) [S.
Bao, et al., Phys. Rev. X 12, 011022 (2022)], whose theoretical understanding
yet remains elusive. Here we study the spin dynamics of a ferromagnetic
Kondo-Heisenberg lattice model at finite temperature, and propose a mechanism
of magnon damping that explains the intriguing experimental results. In
particular, we find the magnon damping rate $\gamma(T)$ firstly decreases as
temperature lowers, due to the reduced magnon-magnon scatterings. It then
reaches a minimum at $T_{\rm d}^*$, and rises up again following a logarithmic
scaling $\gamma(T) \sim \ln{(T_0/T)}$ (with $T_0$ a constant) for $T < T_{\rm
d}^*$, which can be attributed to electron-magnon scatterings of spin-flip
type. Moreover, we obtain the phase diagram containing the ferromagnetic and
Kondo insulator phases by varying the Kondo coupling, which may be relevant for
experiments on pressured FGT. The presence of a magnon damping minimum and
logarithmic scaling at low temperature indicates the emergence of the Kondo
effect reflected in the collective excitations of local moments in a Kondo
lattice system. | 2401.00758v1 |
2024-01-04 | Simplified Information Geometry Approach for Massive MIMO-OFDM Channel Estimation -- Part II: Convergence Analysis | In Part II of this two-part paper, we prove the convergence of the simplified
information geometry approach (SIGA) proposed in Part I. For a general Bayesian
inference problem, we first show that the iteration of the common second-order
natural parameter (SONP) is separated from that of the common first-order
natural parameter (FONP). Hence, the convergence of the common SONP can be
checked independently. We show that with the initialization satisfying a
specific but large range, the common SONP is convergent regardless of the value
of the damping factor. For the common FONP, we establish a sufficient condition
of its convergence and prove that the convergence of the common FONP relies on
the spectral radius of a particular matrix related to the damping factor. We
give the range of the damping factor that guarantees the convergence in the
worst case. Further, we determine the range of the damping factor for massive
MIMO-OFDM channel estimation by using the specific properties of the
measurement matrices. Simulation results are provided to confirm the
theoretical results. | 2401.02037v1 |
2024-01-05 | Solving convex optimization problems via a second order dynamical system with implicit Hessian damping and Tikhonov regularization | This paper deals with a second order dynamical system with a Tikhonov
regularization term in connection to the minimization problem of a convex
Fr\'echet differentiable function. The fact that beside the asymptotically
vanishing damping we also consider an implicit Hessian driven damping in the
dynamical system under study allows us, via straightforward explicit
discretization, to obtain inertial algorithms of gradient type. We show that
the value of the objective function in a generated trajectory converges rapidly
to the global minimum of the objective function and depending the Tikhonov
regularization parameter the generated trajectory converges weakly to a
minimizer of the objective function or the generated trajectory converges
strongly to the element of minimal norm from the $\argmin$ set of the objective
function. We also obtain the fast convergence of the velocities towards zero
and some integral estimates. Our analysis reveals that the Tikhonov
regularization parameter and the damping parameters are strongly correlated,
there is a setting of the parameters that separates the cases when weak
convergence of the trajectories to a minimizer and strong convergence of the
trajectories to the minimal norm minimizer can be obtained. | 2401.02676v1 |
2024-01-16 | Influence of temperature, doping, and amorphization on the electronic structure and magnetic damping of iron | Hybrid magnonic quantum systems have drawn increased attention in recent
years for coherent quantum information processing, but too large magnetic
damping is a persistent concern when metallic magnets are used. Their intrinsic
damping is largely determined by electron-magnon scattering induced by
spin-orbit interactions. In the low scattering limit, damping is dominated by
intra-band electronic transitions, which has been theoretically shown to be
proportional to the electronic density of states at the Fermi level. In this
work, we focus on body-centered-cubic iron as a paradigmatic ferromagnetic
material. We comprehensively study its electronic structure using
first-principles density functional theory simulations and account for finite
lattice temperature, boron (B) doping, and structure amorphization. Our results
indicate that temperature induced atomic disorder and amorphous atomic
geometries only have a minor influence. Instead, boron doping noticeably
decreases the density of states near the Fermi level with an optimal doping
level of 6.25%. In addition, we show that this reduction varies significantly
for different atomic geometries and report that the highest reduction
correlates with a large magnetization of the material. This may suggest
materials growth under external magnetic fields as a route to explore in
experiment. | 2401.08076v1 |
2024-03-13 | Effects of wave damping and finite perpendicular scale on three-dimensional Alfvén wave parametric decay in low-beta plasmas | Shear Alfven wave parametric decay instability (PDI) provides a potential
path toward significant wave dissipation and plasma heating. However,
fundamental questions regarding how PDI is excited in a realistic
three-dimensional (3D) open system and how critically the finite perpendicular
wave scale -- as found in both the laboratory and space plasmas -- affects the
excitation remain poorly understood. Here, we present the first 3D,
open-boundary, hybrid kinetic-fluid simulations of kinetic Alfven wave PDI in
low-beta plasmas. Key findings are that the PDI excitation is strongly limited
by the wave damping present, including electron-ion collisional damping
(represented by a constant resistivity) and geometrical attenuation associated
with the finite-scale Alfven wave, and ion Landau damping of the child acoustic
wave. The perpendicular wave scale alone, however, plays no discernible role,
with different wave scales exhibiting similar instability growth. These
findings are corroborated by theoretical analysis and estimates. The new
understanding of 3D kinetic Alfven wave PDI physics is essential for laboratory
study of the basic plasma process and may also help evaluate the relevance/role
of PDI in low-beta space plasmas. | 2403.08179v1 |
2024-03-19 | Polarization Dynamics in Paramagnet of Charged Quark-Gluon Plasma | It is commonly understood that the strong magnetic field produced in heavy
ion collisions is short-lived. The electric conductivity of the quark-gluon
plasma is unable to significantly extend the life time of magnetic field. We
propose an alternative scenario to achieve this: with finite baryon density and
spin polarization by the initial magnetic field, the quark-gluon plasma behaves
as a paramagnet, which may continue to polarize quark after fading of initial
magnetic field. We confirm this picture by calculations in both quantum
electrodynamics and quantum chromodynamics. In the former case, we find a
splitting in the damping rates of probe fermion with opposite spin component
along the magnetic field with the splitting parametrically small than the
average damping rate. In the latter case, we find a similar splitting in the
damping rates of probe quark with opposite spin components along the magnetic
field. The splitting is parametrically comparable to the average damping rate,
providing an efficient way of polarizing strange quarks by the quark-gluon
plasma paramagnet consisting of light quarks. | 2403.12615v1 |
2006-04-14 | The UCSD Radio-Selected Quasar Survey for Damped Lyman alpha System | As large optical quasar surveys for damped Lya become a reality and the study
of star forming gas in the early Universe achieves statistical robustness, it
is now vital to identify and quantify the sources of systematic error. Because
the nature of optically-selected quasar surveys makes them vulnerable to dust
obscuration, we have undertaken a radio-selected quasar survey for damped Lya
systems to address this bias. We present the definition and results of this
survey. We then combine our sample with the CORALS dataset to investigate the
HI column density distribution function f(N) of damped Lya systems toward
radio-selected quasars. We find that f(N) is well fit by a power-law f(N) = k_1
N^alpha_1, with log k_1 = 22.90 and alpha_1 = -2.18. This power-law is in
excellent agreement with that of optically-selected samples at low N(HI), an
important yet expected result given that obscuration should have negligible
effect at these gas columns. However, because of the relatively small size of
the radio-selected sample, 26 damped Lya systems in 119 quasars, f(N) is not
well constrained at large N(HI) and the first moment of the HI distribution
function, Omega_g, is, strictly speaking, a lower limit. The power-law is steep
enough, however, that extrapolating it to higher column densities implies only
a modest, logarithmic increase in Omega_g. The radio-selected value of Omega_g
= 1.15 x 10^-3, agrees well with the results of optically-selected surveys.
While our results indicate that dust obscuration is likely not a major issue
for surveys of damped Lya systems, we estimate that a radio-selected sample of
approximately 100 damped Lya systems will be required to obtain the precision
necessary to absolutely confirm an absence of dust bias. | 0604334v1 |
2012-04-12 | Evidence of Gunn-Peterson damping wings in high-z quasar spectra: strengthening the case for incomplete reionization | The spectra of several high-redshift (z>6) quasars have shown evidence for a
Gunn-Peterson (GP) damping wing, indicating a substantial mean neutral hydrogen
fraction (x_HI > 0.03) in the z ~ 6 intergalactic medium (IGM). However,
previous analyses assumed that the IGM was uniformly ionized outside of the
quasar's HII region. Here we relax this assumption and model patchy
reionization scenarios for a range of IGM and quasar parameters. We quantify
the impact of these differences on the inferred x_HI, by fitting the spectra of
three quasars: SDSS J1148+5251 (z=6.419), J1030+0524 (z=6.308), and J1623+3112
(z=6.247). We find that the best-fit values of x_HI in the patchy models agree
well with the uniform case. More importantly, we confirm that the observed
spectra favor the presence of a GP damping wing, with peak likelihoods
decreasing by factors of > few - 10 when the spectra are modeled without a
damping wing. We also find that the Ly alpha absorption spectra, by themselves,
cannot distinguish the damping wing in a relatively neutral IGM from a damping
wing in a highly ionized IGM, caused either by an isolated neutral patch, or by
a damped Ly alpha absorber (DLA). However, neutral patches in a highly ionized
universe (x_HI < 0.01), and DLAs with the large required column densities (N_HI
> few x 10^{20} cm^{-2}) are both rare. As a result, when we include reasonable
prior probabilities for the line of sight (LOS) to intercept either a neutral
patch or a DLA at the required distance of ~ 40-60 comoving Mpc away from the
quasar, we find strong lower limits on the neutral fraction in the IGM, x_HI >
0.1 (at 95% confidence). This strengthens earlier claims that a substantial
global fraction of hydrogen in the z~6 IGM is in neutral form. | 1204.2838v2 |
2013-05-31 | Highly inclined and eccentric massive planets I: Planet-disc interactions | In the Solar System, planets have a small inclination with respect to the
equatorial plane of the Sun, but there is evidence that in extrasolar systems
the inclination can be very high. This spin-orbit misalignment is unexpected,
as planets form in a protoplanetary disc supposedly aligned with the stellar
spin. Planet-planet interactions are supposed to lead to a mutual inclination,
but the effects of the protoplanetary disc are still unknown. We investigate
therefore planet-disc interactions for planets above 1M_Jup. We check the
influence of the inclination i, eccentricity e, and mass M_p of the planet. We
perform 3D numerical simulations of protoplanetary discs with embedded
high-mass planets. We provide damping formulae for i and e as a function of i,
e, and M_p that fit the numerical data. For highly inclined massive planets,
the gap opening is reduced, and the damping of i occurs on time-scales of the
order of 10^-4 deg/yr M_disc/(0.01 M_star) with the damping of e on a smaller
time-scale. While the inclination of low planetary masses (<5M_Jup) is always
damped, large planetary masses with large i can undergo a Kozai-cycle with the
disc. These Kozai-cycles are damped in time. Eccentricity is generally damped,
except for very massive planets (M_p = 5M_Jup) where eccentricity can increase
for low inclinations. The dynamics tends to a final state: planets end up in
midplane and can then, over time, increase their eccentricity as a result of
interactions with the disc. The interactions with the disc lead to damping of i
and e after a scattering event of high-mass planets. If i is sufficiently
reduced, the eccentricity can be pumped up because of interactions with the
disc. If the planet is scattered to high inclination, it can undergo a
Kozai-cycle with the disc that makes it hard to predict the exact movement of
the planet and its orbital parameters at the dispersal of the disc. | 1305.7330v1 |
2014-10-20 | Frequency-dependent attenuation and elasticity in unconsolidated earth materials: effect of damping | We use the Discrete Element Method (DEM) to understand the underlying
attenuation mechanism in granular media, with special applicability to the
measurements of the so-called effective mass developed earlier. We consider
that the particles interact via Hertz-Mindlin elastic contact forces and that
the damping is describable as a force proportional to the velocity difference
of contacting grains. We determine the behavior of the complex-valued normal
mode frequencies using 1) DEM, 2) direct diagonalization of the relevant
matrix, and 3) a numerical search for the zeros of the relevant determinant.
All three methods are in strong agreement with each other. The real and the
imaginary parts of each normal mode frequency characterize the elastic and the
dissipative properties, respectively, of the granular medium. We demonstrate
that, as the interparticle damping, $\xi$, increases, the normal modes exhibit
nearly circular trajectories in the complex frequency plane and that for a
given value of $\xi$ they all lie on or near a circle of radius $R$ centered on
the point $-iR$ in the complex plane, where $R\propto 1/\xi$. We show that each
normal mode becomes critically damped at a value of the damping parameter $\xi
\approx 1/\omega_n^0$, where $\omega_n^0$ is the (real-valued) frequency when
there is no damping. The strong indication is that these conclusions carry over
to the properties of real granular media whose dissipation is dominated by the
relative motion of contacting grains. For example, compressional or shear waves
in unconsolidated dry sediments can be expected to become overdamped beyond a
critical frequency, depending upon the strength of the intergranular damping
constant. | 1410.5484v2 |
2020-08-05 | Fast optimization via inertial dynamics with closed-loop damping | In a Hilbert space $H$, in order to develop fast optimization methods, we
analyze the asymptotic behavior, as time $t$ tends to infinity, of inertial
continuous dynamics where the damping acts as a closed-loop control. The
function $f: H \to R$ to be minimized (not necessarily convex) enters the
dynamic through it gradient, which is assumed to be Lipschitz continuous on the
bounded subsets of $H$. This gives autonomous dynamical systems with nonlinear
damping and nonlinear driving force. We first consider the case where the
damping term $\partial \phi (\dot{x}(t))$ acts as a closed-loop control of the
velocity. The damping potential $\phi : H \to [0,+\infty)$ is a convex
continuous function which achieves its minimum at the origin. We show the
existence and uniqueness of a global solution to the associated Cauchy problem.
Then, we analyze the asymptotic convergence properties of the generated
trajectories generated. We use techniques from optimization, control theory,
and PDE's: Lyapunov analysis based on the decreasing property of an energy-like
function, quasi-gradient and Kurdyka-Lojasiewicz theory, monotone operator
theory for wave-like equations. Convergence rates are obtained based on the
geometric properties of the data $f$ and $\phi$. When $f$ is strongly convex,
we give general conditions which provide exponential convergence rates. Then,
we extend the results to the case where an additional Hessian-driven damping
enters the dynamic, which reduces the oscillations. Finally, we consider an
inertial system involving jointly the velocity $\dot{x}(t)$ and the gradient
$\nabla f(x(t))$. In addition to its original results, this work surveys the
numerous works devoted in recent years to the interaction between continuous
damped inertial dynamics and numerical algorithms for optimization, with the
emphasis on autonomous systems, closed-loop adaptive procedures, and
convergence rates. | 2008.02261v3 |
2023-01-10 | Cosmic Ray Drag and Damping of Compressive Turbulence | While it is well-known that cosmic rays (CRs) can gain energy from turbulence
via second order Fermi acceleration, how this energy transfer affects the
turbulent cascade remains largely unexplored. Here, we show that damping and
steepening of the compressive turbulent power spectrum are expected once the
damping time $t_{\rm damp} \sim \rho v^{2}/\dot{E}_{\rm CR} \propto E_{\rm
CR}^{-1}$ becomes comparable to the turbulent cascade time. Magnetohydrodynamic
(MHD) simulations of stirred compressive turbulence in a gas-CR fluid with
diffusive CR transport show clear imprints of CR-induced damping, saturating at
$\dot{E}_{\rm CR} \sim \tilde{\epsilon}$, where $\tilde{\epsilon}$ is the
turbulent energy input rate. In that case, almost all the energy in large scale
motions is absorbed by CRs and does not cascade down to grid scale. Through a
Hodge-Helmholtz decomposition, we confirm that purely compressive forcing can
generate significant solenoidal motions, and we find preferential CR damping of
the compressive component in simulations with diffusion and streaming,
rendering small-scale turbulence largely solenoidal, with implications for
thermal instability and proposed resonant scattering of $E > 300$ GeV CRs by
fast modes. When CR transport is streaming dominated, CRs also damp large scale
motions, with kinetic energy reduced by up to to an order of magnitude in
realistic $E_{\rm CR} \sim E_{\rm g}$ scenarios, but turbulence (with a reduced
amplitude) still cascades down to small scales with the same power spectrum.
Such large scale damping implies that turbulent velocities obtained from the
observed velocity dispersion may significantly underestimate turbulent forcing
rates, i.e. $\tilde{\epsilon} \gg \rho v^{3}/L$. | 2301.04156v2 |
2024-02-12 | Relaxation of weakly collisional plasma: continuous spectra, Landau eigenmodes, and transition from the collisionless to the fluid limit | The relaxation of a weakly collisional plasma is described by the
Boltzmann-Poisson equations with the Lenard-Bernstein collision operator. We
perform a perturbative analysis of these equations, and obtain, for the first
time, exact analytic solutions, enabling definitive resolutions to
long-standing controversies regarding the impact of weak collisions on
continuous spectra and Landau eigenmodes. Unlike some previous studies, we
retain both damping and diffusion terms in the collision operator. We find that
the linear response is a temporal convolution of a continuum that depends on
the continuous velocities of particles, and discrete normal modes that
encapsulate coherent oscillations. The normal modes are exponentially damped
over time due to collective effects (Landau damping) as well as collisional
dissipation. The continuum is also damped by collisions but somewhat
differently. Up to a collision time, which is the inverse of the collision
frequency $\nu_{\mathrm{c}}$, the continuum decay is driven by velocity
diffusion and occurs super-exponentially over a timescale $\sim
\nu^{-1/3}_{\mathrm{c}}$. After a collision time, however, the continuum decay
is driven by the collisional damping of particle velocities and diffusion of
their positions, and occurs exponentially over a timescale $\sim
\nu_{\mathrm{c}}$. This hitherto unknown, slow exponential decay causes
perturbations to damp the most on scales comparable to the mean free path, but
very slowly on larger scales, which establishes the local thermal equilibrium,
the essence of the fluid limit. The long-term decay of the response is driven
by the normal modes on scales smaller than the mean free path, but, on larger
scales, is governed by the slowly decaying continuum and the least damped
normal mode. Our analysis firmly establishes a long-sought connection between
the collisionless and fluid limits of weakly collisional plasmas. | 2402.07992v1 |
2008-06-23 | On multi F-nomial coefficients and Inversion formula for F-nomial coefficients | In response to [6], we discover the looked for inversion formula for F-nomial
coefficients. Before supplying its proof, we generalize F-nomial coefficients
to multi F-nomial coefficients and we give their combinatorial interpretation
in cobweb posets language, as the number of maximal-disjoint blocks of the form
sP_{k_1,k_2,...,k_s} of layer <Phi_1-->Phi_n>. Then we present inversion
formula for F-nomial coefficients using multi F-nomial coefficients for all
cobweb-admissible sequences. To this end we infer also some identities as
conclusions of that inversion formula for the case of binomial, Gaussian and
Fibonomial coefficients. | 0806.3626v2 |
2015-07-06 | On the Kendall Correlation Coefficient | In the present paper, we first discuss the Kendall rank correlation
coefficient. In continuous case, we define the Kendall rank correlation
coefficient in terms of the concomitants of order statistics, find the expected
value of the Kendall rank correlation coefficient and show that the later is
free of n. We also prove that in continuous case the Kendall correlation
coefficient converges in probability to its expected value. We then propose to
consider the expected value of the Kendall rank correlation coefficient as a
new theoretical correlation coefficient which can be an alternative to the
classical Pearson product-moment correlation coefficient. At the end of this
work we analyze illustrative examples. | 1507.01427v1 |
2017-10-12 | The co-Pieri rule for Kronecker coefficients | A fundamental problem in the representation theory of the symmetric group,
Sn, is to describe the coefficients in the decomposition of a tensor product of
two simple representations. These coefficients are known in the literature as
the Kronecker coefficients. The Littlewood--Richardson coefficients appear as
an important subfamily of the wider class of stable Kronecker coefficients.
This subfamily of coefficients can be calculated using a tableaux counting
algorithm known as the Littlewood--Richardson rule. This paper generalises one
half of this rule (the "co-Pieri" rule) to the the wider family of stable
Kronecker coefficients. | 1710.04523v3 |
2023-05-04 | All Kronecker coefficients are reduced Kronecker coefficients | We settle the question of where exactly the reduced Kronecker coefficients
lie on the spectrum between the Littlewood-Richardson and Kronecker
coefficients by showing that every Kronecker coefficient of the symmetric group
is equal to a reduced Kronecker coefficient by an explicit construction. This
implies the equivalence of a question by Stanley from 2000 and a question by
Kirillov from 2004 about combinatorial interpretations of these two families of
coefficients. Moreover, as a corollary, we deduce that deciding the positivity
of reduced Kronecker coefficients is $NP$-hard, and computing them is
$\#P$-hard under parsimonious many-one reductions. | 2305.03003v1 |
1995-09-21 | Damped Lyman-alpha and Lyman Limit Absorbers in the Cold Dark Matter Model | We study the formation of damped \lya and Lyman limit absorbers in a
hierarchical clustering scenario using a gas dynamical simulation of an $\Omega
= 1$, cold dark matter universe. In the simulation, these high column density
systems are associated with forming galaxies. Damped \lya absorption, $N_{HI}
\simgt 10^{20.2}\cm^{-2}$, arises along lines of sight that pass near the
centers of relatively massive, dense protogalaxies. Lyman limit absorption,
$10^{17}\cm^{-2} \simlt N_{HI} \simlt 10^{20.2}\cm^{-2}$, develops on lines of
sight that pass through the outer parts of such objects or near the centers of
smaller protogalaxies. The number of Lyman limit systems is less than observed,
while the number of damped \lya systems is quite close to the observed
abundance. Damped absorbers are typically $\sim 10$ kpc in radius, but the
population has a large total cross section because the systems are much more
numerous than present day $L_*$ galaxies. Our results demonstrate that high
column density systems like those observed arise naturally in a hierarchical
theory of galaxy formation and that it is now possible to study these absorbers
directly from numerical simulations. | 9509106v1 |
1995-09-21 | Nonlinear Damping of Oscillations in Tidal-Capture Binaries | We calculate the damping of quadrupole f and low order g modes (primary
modes) by nonlinear coupling to other modes of the star. This damping is orders
of magnitude more rapid than direct radiative damping when the primary
amplitude is large, as in tidal capture.
Primary modes destabilize high degree g-modes of half their frequency
(daughter modes) by 3-mode coupling in radiative zones. In sunlike stars, the
growth time $\equiv\eta^{-1}\approx 4 E_{0,42}^{-1/2}$ days, where $E_{0,42}$
is the initial energy of the primary mode in units of $10^{42}~$erg, and of
order $10^{10}E_{0,42}^{5/4}$ daughters are unstable. The growth rate is
approximately equal to the angular frequency of the primary mode times its
dimensionless radial amplitude, $\delta R/R_*\approx 0.002E_{0,42}^{1/2}$.
Although the daughter modes are limited by their own nonlinearities,
collectively they absorb most of the primary mode's energy after a time $\sim
10\eta^{-1}$ provided $E_{0}> 10^{40}~\mbox{erg}$. In fact nonlinear mode
interaction may be the dominant damping process if $E_0\gtrsim
10^{37}~\mbox{erg}$.
Our results have application to tidally captured main sequence globular
cluster stars of mass $\ge 0.5 M_{\sun}$; the tidal energy is dissipated in the
radiative core of the star in about a month, which is less than the initial
orbital period. | 9509112v1 |
1997-08-12 | Spectroscopy of PKS 0528-260: New Limits on CO Absorption and Emission | We have obtained a moderate resolution spectrum of the quasar PKS 0528-250
with the Red Channel Spectrograph on the Multiple Mirror Telescope (MMT) in
order to study a damped Lyman alpha absorption line system at z = 2.8115.
We obtain a new upper limit for the CO column density for the z = 2.8108
velocity component in the z = 2.8115 damped Lyman alpha system. The ionization
of different species in this component rules out a quasar spectral energy
distribution (SED) as the ionization field,and implies an ultraviolet radiation
field intensity a few times that of the Milky Way value. The estimated total
number density is n(H) about 20 cm^{-3}. The physical size for the z = 2.8108
component implied by these models is about 40 parsecs. The ionization of
different species also suggests a structure with a hot intercloud medium
associated with a H I cloud in this component, that is, most low ionized ions
are from the cold medium where photoionization and photodissociation dominates.
The highly ionized species may be from the intercloud medium where collisional
ionization dominates. We also present newly identified Ni II absorption lines
in the z = 2.1408 and z = 2.8115 damped Ly$\alpha$ systems. The derived
depletion of nickel by dust confirms previous results that the dust-to-gas
ratio in these two damped Lyman alpha systems is about 10% of the Milky Way
ratio. Millimeter wavelength observations obtained at the NRAO 12 meter
telescope provide new upper limits on CO (3-2) emission in the z = 2.8115
damped Lyman alpha system. | 9708104v1 |
1998-11-04 | GMRT Observations of Low z Damped Lyman-alpha Absorbers | We present Giant Metrewave Radio Telescope (GMRT) observations of redshifted
HI 21cm absorption in two low redshift (z=0.2212, z=0.0912) damped Lyman-alpha
systems seen towards the gigahertz peaked source OI 363 (z_em = 0.630). The
object at z=0.0912 is the lowest redshift damped Lyman-alpha system known to
date. Ground based imaging (Rao & Turnshek, 1998) shows that at neither
redshift is there a large spiral galaxy at low impact parameter to the line of
sight to OI 363, in contradiction with the suggestion that these systems are
large proto-disks.
Since OI 363 is a highly compact, core dominated source, the covering factor
of the HI gas is likely to be unity. Nonetheless, the spin temperatures derived
from the 21cm optical depth (and using the N_HI measured from HST spectra, Rao
& Turnshek, 1998) are high, viz. 1120 +/- 200 K and 825 +/- 110 K for the high
and low redshift systems respectively. These values are considerably higher
than typical values (100 - 200 K) measured in our Galaxy and Andromeda and are,
in fact, similar to those obtained in high redshift damped Lyman-alpha systems.
Our observations hence suggest that evolutionary effects may not be crucial in
understanding the difference in derived spin temperature values between local
spiral disks and high redshift damped Lyman-alpha systems. | 9811068v1 |
2002-01-25 | Galaxies Associated with z~4 Damped Lya Systems: I. Imaging and Photometric Selection | This paper describes the acquisition and analysis of imaging data for the
identification of galaxies associated with z~4 damped Lya systems. We present
deep BRI images of three fields known to contain four z~4 damped systems. We
discuss the reduction and calibration of the data, detail the color criteria
used to identify z~4 galaxies, and present a photometric redshift analysis to
complement the color selection. We have found no galaxy candidates closer to
the QSO than 7'' which could be responsible for the damped Lya systems.
Assuming that at least one of the galaxies is not directly beneath the QSO, we
set an upper limit on this damped Lya system of L < L*/4. Finally, we have
established a web site to release these imaging data to the public. | 0201417v2 |
2002-02-25 | Eccentricity Evolution for Planets in Gaseous Disks | We investigate the hypothesis that interactions between a giant planet and
the disk from which it forms promote eccentricity growth. These interactions
are concentrated at discrete Lindblad and corotation resonances. Interactions
at principal Lindblad resonances cause the planet's orbit to migrate and open a
gap in the disk if the planet is sufficiently massive. Those at first order
Lindblad and corotation resonances change the planet's orbital eccentricity.
Eccentricity is excited by interactions at external Lindblad resonances which
are located on the opposite side of corotation from the planet, and damped by
co-orbital Lindblad resonances which overlap the planet's orbit. If the planet
clears a gap in the disk, the rate of eccentricity damping by co-orbital
Lindblad resonances is reduced. Density gradients associated with the gap
activate eccentricity damping by corotation resonances at a rate which
initially marginally exceeds that of eccentricity excitation by external
Lindblad resonances. But the corotation torque drives a mass flux which reduces
the density gradient near the resonance. Sufficient partial saturation of
corotation resonances can tip the balance in favor of eccentricity excitation.
A minimal initial eccentricity of a few percent is required to overcome viscous
diffusion which acts to unsaturate corotation resonances by reestablishing the
large scale density gradient. Thus eccentricity growth is a finite amplitude
instability. Formally, interactions at the apsidal resonance, which is a
special kind of co-orbital Lindblad resonance, appears to damp eccentricity
faster than external Lindblad resonances can excite it. However, apsidal waves
have such long wavelengths that they do not propagate in protoplanetary disks.
This reduces eccentricity damping by the apsidal resonance to a modest level. | 0202462v1 |
2003-07-23 | Dusty Molecular Cloud Collapse in the Presence of Alfvén Waves | It has been shown that magnetic fields play an important role in the
stability of molecular clouds, mainly perpendicularly to the field direction.
However, in the parallel direction the stability is a serious problem still to
be explained. Interstellar turbulence may allow the generation of Alfv\'en
waves that propagate through the clouds in the magnetic field direction. These
regions also present great amounts of dust particles which can give rise to new
wave modes, or modify the pre-existing ones. The dust-cyclotron damping affects
the Alfv\'en wave propagation near the dust- cyclotron frequency. On the other
hand, the clouds present different grain sizes, which carry different charges.
In this sense, a dust particle distribution has several dust-cyclotron
frequencies and it will affect a broad band of wave frequencies. In this case,
the energy transfer to the gas is more efficient than in the case where the
ion-cyclotron damping is considered alone. This effect becomes more important
if a power law spectrum is considered for the wave energy flux, since the major
part of the energy is concentrated in low-frequency waves. In this work we
calculate the dust- cyclotron damping in a dusty and magnetized dwarf molecular
cloud, as well as determine the changes in the Alfv\'en wave flux. Then, we use
these results to study the gravitational stability of the cloud. We show that,
considering the presence of charged dust particles, the wave flux is rapidly
damped due to dust-cyclotron damping. Then the wave pressure acts in a small
length scale, and cannot explain the observable cloud sizes, but can explain
the existence of small and dense cores. | 0307411v1 |
2005-02-28 | Thermal Evolution of a Pulsating Neutron Star | We have derived a set of equations to describe the thermal evolution of a
neutron star which undergoes small-amplitude radial pulsations. We have taken
into account, in the frame of the General Theory of Relativity, the pulsation
damping due to the bulk and shear viscosity and the accompanying heating of the
star. The neutrino emission of a pulsating non-superfluid star and its heating
due to the bulk viscosity are calculated assuming that both processes are
determined by the non-equilibrium modified Urca process. Analytical and
numerical solutions to the set of equations of the stellar evolution are
obtained for linear and strongly non-linear deviations from beta-equilibrium.
It is shown that a pulsating star may be heated to very high temperatures,
while the pulsations damp very slowly with time (a power law damping for
100-1000 years), as long as the damping is determined by the bulk viscosity.
The contribution of the shear viscosity to the damping becomes important in a
rather cool star with a low pulsation energy. | 0502583v2 |
2005-05-02 | Collisionless Damping of Fast MHD Waves in Magneto-rotational Winds | We propose collisionless damping of fast MHD waves as an important mechanism
for the heating and acceleration of winds from rotating stars. Stellar rotation
causes magnetic field lines anchored at the surface to form a spiral pattern
and magneto-rotational winds can be driven. If the structure is a magnetically
dominated, fast MHD waves generated at the surface can propagate almost
radially outward and cross the field lines. The propagating waves undergo
collisionless damping owing to interactions with particles surfing on magnetic
mirrors that are formed by the waves themselves. The damping is especially
effective where the angle between the wave propagation and the field lines
becomes moderately large ($\sim 20$ to $80^{\circ}$). The angle tends naturally
to increase into this range because the field in magneto-rotational winds
develops an increasingly large azimuthal component. The dissipation of the wave
energy produces heating and acceleration of the outflow. We show using
specified wind structures that this damping process can be important in both
solar-type stars and massive stars that have moderately large rotation rates.
This mechanism can play a role in coronae of young solar-type stars which are
rapidly rotating and show X-ray luminosities much larger than the sun. The
mechanism could also be important for producing the extended X-ray emitting
regions inferred to exist in massive stars of spectral type middle B and later. | 0505013v5 |
2006-08-05 | The nature of damped Lyman alpha and sub-damped Lyman alpha absorbers | We present arguments based on the measured abundances in individual damped
Lyman alpha systems (DLAs) and sub-damped Lyman alpha systems (sub-DLAs), and
also the average abundances inferred in large samples of QSO absorption line
systems, to suggest that the amount of dust in intervening QSO absorbers is
small and is not responsible for missing many QSOs in magnitude limited QSO
surveys. While we can not totally rule out a bimodal dust distribution with a
population of very dusty, metal rich, absorbers which push the background QSOs
below the observational threshold of current optical spectroscopic studies,
based upon the current samples it appears that the metallicity in QSO absorbers
decreases with increase in H I column densities beyond 10^{19} cm^{-2}. Thus
the sub-DLA population is more metal rich than the DLAs, a trend which may
possibly extend to the non-damped Lyman limit systems (NDLLS). Based on the
recently discovered mass-metallicity relation for galaxies, we suggest that
most sub-DLAs and possibly NDLLS, are associated with massive spiral/elliptical
galaxies while most DLAs are associated with low mass galaxies. The sub-DLA
galaxies will then contribute a larger fraction of total mass (stellar and ISM)
and therefore metals, to the cosmic budget, specially at low redshifts, as
compared to the DLAs. | 0608127v2 |
2007-02-12 | The Ucsd/Keck Damped Lya Abundance Database: A Decade of High Resolution Spectroscopy | We publish the Keck/HIRES and Keck/ESI spectra that we have obtained during
the first 10 years of Keck observatory operations. Our full sample includes 42
HIRES spectra and 39 ESI spectra along 65 unique sightlines providing abundance
measurements on ~85 damped Lya systems. The normalized data can be downloaded
from the journal or from our supporting website:
http://www.ucolick.org/~xavier/DLA/. The database includes all of the
sightlines that have been included in our papers on the chemical abundances,
kinematics, and metallicities of the damped Lya systems. This data has also
been used to argue for variations in the fine-structure constant. We present
new chemical abundance measurements for 10 damped Lya systems and a summary
table of high-resolution metallicity measurements (including values from the
literature) for 153 damped Lya systems at z>1.6. We caution, however, that this
metallicity sample (and all previous ones) is biased to higher N(HI) values
than a random sample. | 0702325v1 |
1998-06-30 | Structure and Spin Dynamics of La$_{0.85}$Sr$_{0.15}$MnO$_3$ | Neutron scattering has been used to study the structure and spin dynamics of
La$_{0.85}$Sr$_{0.15}$MnO$_3$. The magnetic structure of this system is
ferromagnetic below T_C = 235 K. We see anomalies in the Bragg peak intensities
and new superlattice peaks consistent with the onset of a spin-canted phase
below T_{CA} = 205 K, which appears to be associated with a gap at q = (0, 0,
0.5) in the spin-wave spectrum. Anomalies in the lattice parameters indicate a
concomitant lattice distortion. The long-wavelength magnetic excitations are
found to be conventional spin waves, with a gapless (< 0.02 meV) isotropic
dispersion relation $E = Dq^2$. The spin stiffness constant D has a $T^{5/2}$
dependence at low T, and the damping at small q follows $q^4T^{2}$. An
anomalously strong quasielastic component, however, develops at small wave
vector above 200 K and dominates the fluctuation spectrum as T -> T_C. At
larger q, on the other hand, the magnetic excitations become heavily damped at
low temperatures, indicating that spin waves in this regime are not eigenstates
of the system, while raising the temperature dramatically increases the
damping. The strength of the spin-wave damping also depends strongly on the
symmetry direction in the crystal. These anomalous damping effects are likely
due to the itinerant character of the $e_g$ electrons. | 9806381v1 |
2002-08-29 | Some notes on ideology of waves in plasmas | Our last three papers provide an occasion to make some brief notes on
ideology of waves in plasmas and to rehabilitate Vlasov prescription to
calculate relevant logarithmically divergent integrals in the principal value
sense. In this approach asymptotical solutions of plasma oscillations are
selected according to self-consistent boundary physical conditions. Landau
damping is absent in this case by definition. Boundary electrical field
together with conditions of absence of unphysical backward and kinematical
waves define single-valued dependence of boundary distribution function on
electron velocity \vec{v} in the case of transversal waves and on the surface
break of the normal electrical field in the case of longitudinal oscillations.
We have proposed physically more justified modified iteration procedure of
collisional damping calculation and demonstrated some results of damping
decrements calculations in a low-collision electron-ion plasma. Dispersion
smearing of both longitudinal and transversal high-frequency waves, for which
the smearing decrement \delta_x is proportional to
\Delta\omega/(\omega\sqrt{\omega^2-\omega_L^2}), might be the main cause of
waves amplitude damping in collisionless plasmas imitating Landau damping. | 0208098v7 |
2007-04-12 | The effect of the solar corona on the attenuation of small-amplitude prominence oscillations. I. Longitudinal magnetic field | Context. One of the typical features shown by observations of solar
prominence oscillations is that they are damped in time and that the values of
the damping times are usually between one and three times the corresponding
oscillatory period. However, the mechanism responsible for the attenuation is
still not well-known. Aims. Thermal conduction, optically thin or thick
radiation and heating are taken into account in the energy equation, and their
role on the attenuation of prominence oscillations is evaluated. Methods. The
dispersion relation for linear non-adiabatic magnetoacoustic waves is derived
considering an equilibrium made of a prominence plasma slab embedded in an
unbounded corona. The magnetic field is orientated along the direction parallel
to the slab axis and has the same strength in all regions. By solving the
dispersion relation for a fixed wavenumber, a complex oscillatory frequency is
obtained, and the period and the damping time are computed. Results. The effect
of conduction and radiation losses is different for each magnetoacoustic mode
and depends on the wavenumber. In the observed range of wavelengths the
internal slow mode is attenuated by radiation from the prominence plasma, the
fast mode by the combination of prominence radiation and coronal conduction and
the external slow mode by coronal conduction. The consideration of the external
corona is of paramount importance in the case of the fast and external slow
modes, whereas it does not affect the internal slow modes at all. Conclusions.
Non-adiabatic effects are efficient damping mechanisms for magnetoacoustic
modes, and the values of the obtained damping times are compatible with those
observed. | 0704.1566v2 |
2007-10-01 | Lyman-alpha Damping Wing Constraints on Inhomogeneous Reionization | One well-known way to constrain the hydrogen neutral fraction, x_H, of the
high-redshift intergalactic medium (IGM) is through the shape of the red
damping wing of the Lya absorption line. We examine this method's effectiveness
in light of recent models showing that the IGM neutral fraction is highly
inhomogeneous on large scales during reionization. Using both analytic models
and "semi-numeric" simulations, we show that the "picket-fence" absorption
typical in reionization models introduces both scatter and a systematic bias to
the measurement of x_H. In particular, we show that simple fits to the damping
wing tend to overestimate the true neutral fraction in a partially ionized
universe, with a fractional error of ~ 30% near the middle of reionization.
This bias is generic to any inhomogeneous model. However, the bias is reduced
and can even underestimate x_H if the observational sample only probes a subset
of the entire halo population, such as quasars with large HII regions. We also
find that the damping wing absorption profile is generally steeper than one
would naively expect in a homogeneously ionized universe. The profile steepens
and the sightline-to-sightline scatter increases as reionization progresses. Of
course, the bias and scatter also depend on x_H and so can, at least in
principle, be used to constrain it. Damping wing constraints must therefore be
interpreted by comparison to theoretical models of inhomogeneous reionization. | 0710.0371v1 |
2008-02-11 | Eccentricity of masing disks in Active Galactic Nuclei | Observations of Keplerian disks of masers in NCG 4258 and other Seyfert
galaxies can be used to obtain geometric distance estimates and derive the
Hubble constant. The ultimate precision of such measurements could be limited
by uncertainties in the disk geometry. Using a time-dependent linear theory
model, we study the evolution of a thin initially eccentric disk under
conditions appropriate to sub-pc scales in Active Galactic Nuclei. The
evolution is controlled by a combination of differential precession driven by
the disk potential and propagating eccentricity waves that are damped by
viscosity. A simple estimate yields a circularization timescale of
approximately 10 Myr at 0.1 pc. Numerical solutions for the eccentricity
evolution confirm that damping commences on this timescale, but show that the
subsequent decay rate of the eccentricity depends upon the uncertain strength
of viscous damping of eccentricity. If eccentricity waves are important further
decay of the eccentricity can be slow, with full circularization requiring up
to 50 Myr for disks at radii of 0.1 pc to 0.2 pc. Observationally, this implies
that it is plausible that enough time has elapsed for the eccentricity of
masing disks to have been substantially damped, but that it may not be
justified to assume vanishing eccentricity. We predict that during the damping
phase the pericenter of the eccentric orbits describes a moderately tightly
wound spiral with radius. | 0802.1524v1 |
2008-02-20 | The Effect of Charon's Tidal Damping on the Orbits of Pluto's Three Moons | Pluto's recently discovered minor moons, Nix and Hydra, have almost circular
orbits, and are nearly coplanar with Charon, Pluto's major moon. This is
surprising because tidal interactions with Pluto are too weak to damp their
eccentricities. We consider an alternative possibility: that Nix and Hydra
circularize their orbits by exciting Charon's eccentricity via secular
interactions, and Charon in turn damps its own eccentricity by tidal
interaction with Pluto. The timescale for this process can be less than the age
of the Solar System, for plausible tidal parameters and moon masses. However,
as we show numerically and analytically, the effects of the 2:1 and 3:1
resonant forcing terms between Nix and Charon complicate this picture. In the
presence of Charon's tidal damping, the 2:1 term forces Nix to migrate outward
and the 3:1 term changes the eccentricity damping rate, sometimes leading to
eccentricity growth. We conclude that this mechanism probably does not explain
Nix and Hydra's current orbits. Instead, we suggest that they were formed
in-situ with low eccentricities.
We also show that an upper limit on Nix's migration speed sets a lower limit
on Pluto-Charon's tidal circularization timescale of >10^5 yrs. Moreover,
Hydra's observed proper eccentricity may be explained by the 3:2 forcing by
Nix. | 0802.2939v1 |
2008-03-18 | Non-adiabatic magnetohydrodynamic waves in a cylindrical prominence thread with mass flow | High-resolution observations show that oscillations and waves in prominence
threads are common and that they are attenuated in a few periods. In addition,
observers have also reported the presence of material flows in such prominence
fine-structures. Here we investigate the time damping of non-leaky oscillations
supported by a homogeneous cylindrical prominence thread embedded in an
unbounded corona and with a steady mass flow. Thermal conduction and radiative
losses are taken into account as damping mechanisms, and the effect of these
non-ideal effects and the steady flow on the attenuation of oscillations is
assessed. We solve the general dispersion relation for linear, non-adiabatic
magnetoacoustic and thermal waves supported by the model, and find that slow
and thermal modes are efficiently attenuated by non-adiabatic mechanisms. On
the contrary, fast kink modes are much less affected and their damping times
are much larger than those observed. The presence of flow has no effect on the
damping of slow and thermal waves, whereas fast kink waves are more (less)
attenuated when they propagate parallel (anti-parallel) to the flow direction.
Although the presence of steady mass flows improves the efficiency of
non-adiabatic mechanisms on the attenuation of transverse, kink oscillations
for parallel propagation to the flow, its effect is still not enough to obtain
damping times compatible with observations. | 0803.2600v2 |
2008-07-28 | Thermal fluctuations in moderately damped Josephson junctions: Multiple escape and retrapping, switching- and return-current distributions and hysteresis | A crossover at a temperature T* in the temperature dependence of the width s
of the distribution of switching currents of moderately damped Josephson
junctions has been reported in a number of recent publications, with positive
ds/dT and IV characteristics associated with underdamped behaviour for lower
temperatures T<T*, and negative ds/dT and IV characteristics resembling
overdamped behaviour for higher temperatures T>T*. We have investigated in
detail the behaviour of Josephson junctions around the temperature T* by using
Monte Carlo simulations including retrapping from the running state into the
supercurrent state as given by the model of Ben-Jacob et al. We develop
discussion of the important role of multiple escape and retrapping events in
the moderate-damping regime, in particular considering the behaviour in the
region close to T*. We show that the behaviour is more fully understood by
considering two crossover temperatures, and that the shape of the distribution
and s(T) around T*, as well as at lower T<T*, are largely determined by the
shape of the conventional thermally activated switching distribution. We show
that the characteristic temperatures T* are not unique for a particular
Josephson junction, but have some dependence on the ramp rate of the applied
bias current. We also consider hysteresis in moderately damped Josephson
junctions and discuss the less commonly measured distribution of return
currents for a decreasing current ramp. We find that some hysteresis should be
expected to persist above T* and we highlight the importance, even well below
T*, of accounting properly for thermal fluctuations when determining the
damping parameter Q. | 0807.4502v1 |
2009-02-26 | Viscous propagation of mass flow variability in accretion discs | We study mass flow rate through a disc resulting from a varying mass supply
rate. Variable mass supply rate occurs, e.g., during disc state transitions,
and in interacting eccentric binaries. It is, however, damped by the viscosity
of the disc. Here, we calculate this damping in detail. We derive an analytical
description of the propagation of the flow rate using the solution of
Lynden-Bell & Pringle, in which the disc is assumed to extend to infinity. In
particular, we derive the accretion-rate Green's function, and its Fourier
transform, which gives the fractional damping at a given variability frequency.
We then compare this model to that of a finite disc with the mass supply at its
outer edge. We find significant differences with respect to the infinite disc
solution, which we find to overestimate the viscous damping. In particular, the
asymptotic form of the Green's function is power-law for the infinite disc and
exponential for the finite one. We then find a simple fitting form for the
latter, and also calculate its Fourier transform. In general, the damping
becomes very strong when the viscous time at the outer edge of the disc becomes
longer than the modulation time scale. We apply our results to a number of
astrophysical systems. We find the effect is much stronger in low-mass X-ray
binaries, where the disc size is comparable to that of the Roche lobe, than in
high-mass binaries, where the wind-fed disc can have a much smaller size. | 0902.4530v2 |
2010-04-09 | Oscillations of weakly viscous conducting liquid drops in a strong magnetic field | We analyse small-amplitude oscillations of a weakly viscous electrically
conducting liquid drop in a strong uniform DC magnetic field. An asymptotic
solution is obtained showing that the magnetic field does not affect the shape
eigenmodes, which remain the spherical harmonics as in the non-magnetic case.
Strong magnetic field, however, constrains the liquid flow associated with the
oscillations and, thus, reduces the oscillation frequencies by increasing
effective inertia of the liquid. In such a field, liquid oscillates in a
two-dimensional (2D) way as solid columns aligned with the field. Two types of
oscillations are possible: longitudinal and transversal to the field. Such
oscillations are weakly damped by a strong magnetic field - the stronger the
field, the weaker the damping, except for the axisymmetric transversal and
inherently 2D modes. The former are overdamped because of being incompatible
with the incompressibility constraint, whereas the latter are not affected at
all because of being naturally invariant along the field. Since the magnetic
damping for all other modes decreases inversely with the square of the field
strength, viscous damping may become important in a sufficiently strong
magnetic field. The viscous damping is found analytically by a simple energy
dissipation approach which is shown for the longitudinal modes to be equivalent
to a much more complicated eigenvalue perturbation technique. This study
provides a theoretical basis for the development of new measurement methods of
surface tension, viscosity and the electrical conductivity of liquid metals
using the oscillating drop technique in a strong superimposed DC magnetic
field. | 1004.1548v2 |
2011-02-03 | Damping of Electron Density Structures and Implications for Interstellar Scintillation | The forms of electron density structures in kinetic Alfven wave turbulence
are studied in connection with scintillation. The focus is on small scales $L
\sim 10^8-10^{10}$ cm where the Kinetic Alfv\'en wave (KAW) regime is active in
the interstellar medium. MHD turbulence converts to a KAW cascade, starting at
10 times the ion gyroradius and continuing to smaller scales. These scales are
inferred to dominate scintillation in the theory of Boldyrev et al. From
numerical solutions of a decaying kinetic Alfv\'en wave turbulence model,
structure morphology reveals two types of localized structures, filaments and
sheets, and shows that they arise in different regimes of resistive and
diffusive damping. Minimal resistive damping yields localized current filaments
that form out of Gaussian-distributed initial conditions. When resistive
damping is large relative to diffusive damping, sheet-like structures form. In
the filamentary regime, each filament is associated with a non-localized
magnetic and density structure, circularly symmetric in cross section. Density
and magnetic fields have Gaussian statistics (as inferred from Gaussian-valued
kurtosis) while density gradients are strongly non-Gaussian, more so than
current. This enhancement of non-Gaussian statistics in a derivative field is
expected since gradient operations enhance small-scale fluctuations. The
enhancement of density gradient kurtosis over current kurtosis is not obvious,
yet it suggests that modest fluctuation levels in electron density may yield
large scintillation events during pulsar signal propagation in the interstellar
medium. In the sheet regime the same statistical observations hold, despite the
absence of localized filamentary structures. Probability density functions are
constructed from statistical ensembles in both regimes, showing clear formation
of long, highly non-Gaussian tails. | 1102.0810v2 |
2011-09-28 | Different dimensionality trends in the Landau damping of magnons in iron, cobalt and nickel: time dependent density functional study | We study the Landau damping of ferromagnetic magnons in Fe, Co, and Ni as the
dimensionality of the system is reduced from three to two. We resort to the
\textit{ab initio} linear response time dependent density functional theory in
the adiabatic local spin density approximation. The numerical scheme is based
on the Korringa-Kohn-Rostoker Green's function method. The key points of the
theoretical approach and the implementation are discussed. We investigate the
transition metals in three different forms: bulk phases, free-standing thin
films and thin films supported on a nonmagnetic substrate. We demonstrate that
the dimensionality trends in Fe and Ni are opposite: in Fe the transition from
bulk bcc crystal to Fe/Cu(100) film reduces the damping whereas in Ni/Cu(100)
film the attenuation increases compared to bulk fcc Ni. In Co, the strength of
the damping depends relatively weakly on the sample dimensionality. We explain
the difference in the trends on the basis of the underlying electronic
structure. The influence of the substrate on the spin-wave damping is analyzed
by employing Landau maps representing wave-vector resolved spectral density of
the Stoner excitations. | 1109.6217v2 |
2011-10-06 | Dissipative and conservative nonlinearity in carbon nanotube and graphene mechanical resonators | Graphene and carbon nanotubes represent the ultimate size limit of one and
two-dimensional nanoelectromechanical resonators. Because of their reduced
dimensionality, graphene and carbon nanotubes display unusual mechanical
behavior; in particular, their dynamics is highly nonlinear. Here, we review
several types of nonlinear behavior in resonators made from nanotubes and
graphene. We first discuss an unprecedented scenario where damping is described
by a nonlinear force. This scenario is supported by several experimental facts:
(i) the quality factor varies with the amplitude of the motion as a power law
whose exponent coincides with the value predicted by the nonlinear damping
model, (ii) hysteretic behavior (of the motional amplitude as a function of
driving frequency) is absent in some of our resonators even for large driving
forces, as expected when nonlinear damping forces are large, and (iii) when we
quantify the linear damping force (by performing parametric excitation
measurements) we find that it is significantly smaller than the nonlinear
damping force. We then review parametric excitation measurements, an
alternative actuation method which is based on nonlinear dynamics. Finally, we
discuss experiments where the mechanical motion is coupled to electron
transport through a nanotube. The coupling can be made so strong that the
associated force acting on the nanotube becomes highly nonlinear with
displacement and velocity. Overall, graphene and nanotube resonators hold
promise for future studies on classical and quantum nonlinear dynamics. | 1110.1234v1 |
2012-06-02 | Slow Mode Oscillations and Damping of Hot Solar Coronal Loops | The effect of temperature inhomogeneity on the periods, their ratios
(fundamental vs. first overtone), and the damping times of the standing slow
modes in gravitationally stratified solar coronal loops are studied. The
effects of optically thin radiation, compressive viscosity, and thermal
conduction are considered. The linearized one-dimensional magnetohydrodynamic
(MHD) equations (under low-$\beta$ condition) were reduced to a fourth--order
ordinary differential equation for the perturbed velocity. The numerical
results indicate that the periods of non-isothermal loops (i.e. temperature
increases from the loop base to apex) are smaller compared to those of
isothermal loops. In the presence of radiation, viscosity, and thermal
conduction, an increase in the temperature gradient is followed by a monotonic
decrease in the periods (compared with the isothermal case), while the period
ratio turns out to be a sensitive function of the gradient of the temperature
and the loop lengths. We verify that radiative dissipation is not a main
cooling mechanism of both isothermal and non-isothermal hot coronal loops and
has a small effect on the periods. Thermal conduction and compressive viscosity
are primary mechanisms in the damping of slow modes of the hot coronal loops.
The periods and damping times in the presence of compressive viscosity and/or
thermal conduction dissipation are consistent with the observed data in
specific cases. By tuning the dissipation parameters, the periods and the
damping times could be made consistent with the observations in more general
cases. | 1206.0366v1 |
2012-09-15 | Damped kink oscillations of flowing prominence threads | Transverse oscillations of thin threads in solar prominences are frequently
reported in high-resolution observations. Two typical features of the
observations are that the oscillations are damped in time and that simultaneous
mass flows along the threads are detected. Flows cause the dense threads to
move along the prominence magnetic structure while the threads are oscillating.
The oscillations have been interpreted in terms of standing magnetohydrodynamic
(MHD) kink waves of the magnetic flux tubes which support the threads. The
damping is most likely due to resonant absorption caused by plasma
inhomogeneity. The technique of seismology uses the observations combined with
MHD wave theory to estimate prominence physical parameters. This paper presents
a theoretical study of the joint effect of flow and resonant absorption on the
amplitude of standing kink waves in prominence threads. We find that flow and
resonant absorption can either be competing effects on the amplitude or both
can contribute to damp the oscillations depending on the instantaneous position
of the thread within the prominence magnetic structure. The amplitude profile
deviates from the classic exponential profile of resonantly damped kink waves
in static flux tubes. Flow also introduces a progressive shift of the
oscillation period compared to the static case, although this effect is in
general of minor importance. We test the robustness of seismological estimates
by using synthetic data aiming to mimic real observations. The effect of the
thread flow can significantly affect the estimation of the transverse
inhomogeneity length scale. The presence of random background noise adds
uncertainty to this estimation. Caution needs to be paid to the seismological
estimates that do not take the influence of flow into account. | 1209.3382v1 |
2013-04-13 | Parametric survey of longitudinal prominence oscillation simulations | It is found that both microflare-sized impulsive heating at one leg of the
loop and a suddenly imposed velocity perturbation can propel the prominence to
oscillate along the magnetic dip. An extensive parameter survey results in a
scaling law, showing that the period of the oscillation, which weakly depends
on the length and height of the prominence, and the amplitude of the
perturbations, scales with $\sqrt{R/g_\odot}$, where $R$ represents the
curvature radius of the dip, and $g_\odot$ is the gravitational acceleration of
the Sun. This is consistent with the linear theory of a pendulum, which implies
that the field-aligned component of gravity is the main restoring force for the
prominence longitudinal oscillations, as confirmed by the force analysis.
However, the gas pressure gradient becomes non-negligible for short
prominences. The oscillation damps with time in the presence of non-adiabatic
processes. Compared to heat conduction, the radiative cooling is the dominant
factor leading to the damping. A scaling law for the damping timescale is
derived, i.e., $\tau\sim l^{1.63} D^{0.66}w^{-1.21}v_{0}^{-0.30}$, showing
strong dependence on the prominence length $l$, the geometry of the magnetic
dip (characterized by the depth $D$ and the width $w$), and the velocity
perturbation amplitude $v_0$. The larger the amplitude, the faster the
oscillation damps. It is also found that mass drainage significantly reduces
the damping timescale when the perturbation is too strong. | 1304.3798v1 |
2013-06-08 | Observation of a Berry phase anti-damping spin-orbit torque | Recent observations of current-induced magnetization switching at
ferromagnet/normal-conductor interfaces have important consequences for future
magnetic memory technology. In one interpretation, the switching originates
from carriers with spin-dependent scattering giving rise to a relativistic
anti-damping spin-orbit torque (SOT) in structures with broken space-inversion
symmetry. The alternative interpretation combines the relativistic spin Hall
effect (SHE), making the normal-conductor an injector of a spin-current, with
the non-relativistic spin-transfer torque (STT) in the ferromagnet. Remarkably,
the SHE in these experiments originates from the Berry phase effect in the band
structure of a clean crystal and the anti-damping STT is also based on a
disorder-independent transfer of spin from carriers to magnetization. Here we
report the observation of an anti-damping SOT stemming from an analogous Berry
phase effect to the SHE. The SOT alone can therefore induce magnetization
dynamics based on a scattering-independent principle. The ferromagnetic
semiconductor (Ga,Mn)As we use has a broken space-inversion symmetry in the
crystal. This allows us to consider a bare ferromagnetic element which
eliminates by design any SHE related contribution to the spin torque. We
provide an intuitive picture of the Berry phase origin of the anti-damping SOT
and a microscopic modeling of measured data. | 1306.1893v1 |
2013-08-20 | Stringent constraints on the H I spin temperature in two z > 3 Damped Lyman-alpha systems from redshifted 21 cm absorption studies | Physical properties of Damped Lyman-alpha absorbers and their evolution are
closely related to galaxy formation and evolution theories, and have important
cosmological implications. H I 21 cm absorption study is one useful way of
measuring the temperature of these systems. In this work, very strong
constraints on the temperature of two Damped Lyman-alpha absorbers at z > 3 are
derived from low radio frequency observations. The H I spin temperature is
found to be greater than 2000 K for both the absorbers. The high spin
temperature of these high-redshift systems is in agreement with the trend found
in a compilation of temperatures for other Damped Lyman-alpha absorbers. We
also argue that the temperature - metallicity relation, reported earlier in the
literature, is unlikely to be a spurious line of sight effect, and that the
redshift evolution of the spin temperature does not arises due to a selection
effect. All of these are consistent with a redshift evolution of the warm gas
fraction in Damped Lyman-alpha systems. | 1308.4410v1 |
2013-09-26 | Non-Landau damping of magnetic excitations in systems with localized and itinerant electrons | We discuss the form of the damping of magnetic excitations in a metal near a
ferromagnetic instability. The paramagnon theory predicts that the damping term
should have the form $\Omega/\Gamma (q)$ with $\Gamma (q) \propto q$ (the
Landau damping). However, the experiments on uranium metallic compounds UGe$_2$
and UCoGe showed that $\Gamma (q)$ tends to a constant value at vanishing $q$.
A non-zero $\Gamma (0)$ is impossible in systems with one type of carriers
(either localized or itinerant) because it would violate the spin conservation.
It has been conjectured recently that a non-zero $\Gamma (q)$ in UGe$_2$ and
UCoGe may be due to the presence of both localized and itinerant electrons in
these materials, with ferromagnetism involving predominantly localized spins.
We present microscopic analysis of the damping of near-critical localized
excitations due to interaction with itinerant carriers. We show explicitly how
the presence of two types of electrons breaks the cancellation between the
contributions to $\Gamma (0)$ from self-energy and vertex correction insertions
into the spin polarization bubble and discuss the special role of the
Aslamazov-Larkin processes. We show that $\Gamma (0)$ increases with $T$ both
in the paramagnetic and ferromagnetic regions, but in-between it has a peak at
$T_c$. We compare our theory with the available experimental data. | 1309.7065v3 |
2014-06-16 | Design of the Readout Electronics for the Qualification Model of DAMPE BGO Calorimeter | The DAMPE (DArk Matter Particle Explorer) is a scientific satellite being
developed in China, aimed at cosmic ray study, gamma ray astronomy, and
searching for the clue of dark matter particles, with a planned mission period
of more than 3 years and an orbit altitude of about 500 km. The BGO
Calorimeter, which consists of 308 BGO (Bismuth Germanate Oxid) crystal bars,
616 PMTs (photomultiplier tubes) and 1848 dynode signals, has approximately 32
radiation lengths. It is a crucial sub-detector of the DAMPE payload, with the
functions of precisely measuring the energy of cosmic particles from 5 GeV to
10TeV, distinguishing positrons/electrons and gamma rays from hadron
background, and providing trigger information for the whole DAMPE payload. The
dynamic range for a single BGO crystal is about 2?105 and there are 1848
detector signals in total. To build such an instrument in space, the major
design challenges for the readout electronics come from the large dynamic
range, the high integrity inside the very compact structure, the strict power
supply budget and the long term reliability to survive the hush environment
during launch and in orbit. Currently the DAMPE mission is in the end of QM
(Qualification Model) stage. This paper presents a detailed description of the
readout electronics for the BGO calorimeter. | 1406.3886v1 |
2015-04-17 | Chiral damping of magnetic domain walls | Structural symmetry breaking in magnetic materials is responsible for a
variety of outstanding physical phenomena. Examples range from the existence of
multiferroics, to current induced spin orbit torques (SOT) and the formation of
topological magnetic structures. In this letter we bring into light a novel
effect of the structural inversion asymmetry (SIA): a chiral damping mechanism.
This phenomenon is evidenced by measuring the field driven domain wall (DW)
motion in perpendicularly magnetized asymmetric Pt/Co/Pt trilayers. The
difficulty in evidencing the chiral damping is that the ensuing DW dynamics
exhibit identical spatial symmetry to those expected from the
Dzyaloshinskii-Moriya interaction (DMI). Despite this fundamental resemblance,
the two scenarios are differentiated by their time reversal properties: while
DMI is a conservative effect that can be modeled by an effective field, the
chiral damping is purely dissipative and has no influence on the equilibrium
magnetic texture. When the DW motion is modulated by an in-plane magnetic
field, it reveals the structure of the internal fields experienced by the DWs,
allowing to distinguish the physical mechanism. The observation of the chiral
damping, not only enriches the spectrum of physical phenomena engendered by the
SIA, but since it can coexists with DMI it is essential for conceiving DW and
skyrmion devices. | 1504.04411v1 |
2015-07-28 | Spatial damping of propagating sausage waves in coronal cylinders | Sausage modes are important in coronal seismology. Spatially damped
propagating sausage waves were recently observed in the solar atmosphere. We
examine how wave leakage influences the spatial damping of sausage waves
propagating along coronal structures modeled by a cylindrical density
enhancement embedded in a uniform magnetic field. Working in the framework of
cold magnetohydrodynamics, we solve the dispersion relation (DR) governing
sausage waves for complex-valued longitudinal wavenumber $k$ at given real
angular frequencies $\omega$. For validation purposes, we also provide
analytical approximations to the DR in the low-frequency limit and in the
vicinity of $\omega_{\rm c}$, the critical angular frequency separating trapped
from leaky waves. In contrast to the standing case, propagating sausage waves
are allowed for $\omega$ much lower than $\omega_{\rm c}$. However, while able
to direct their energy upwards, these low-frequency waves are subject to
substantial spatial attenuation. The spatial damping length shows little
dependence on the density contrast between the cylinder and its surroundings,
and depends only weakly on frequency. This spatial damping length is of the
order of the cylinder radius for $\omega \lesssim 1.5 v_{\rm Ai}/a$, where $a$
and $v_{\rm Ai}$ are the cylinder radius and the Alfv\'en speed in the
cylinder, respectively. We conclude that if a coronal cylinder is perturbed by
symmetric boundary drivers (e.g., granular motions) with a broadband spectrum,
wave leakage efficiently filters out the low-frequency components. | 1507.07724v1 |
2015-10-19 | On the branching of the quasinormal resonances of near-extremal Kerr black holes | It has recently been shown by Yang. et. al. [Phys. Rev. D {\bf 87}, 041502(R)
(2013)] that rotating Kerr black holes are characterized by two distinct sets
of quasinormal resonances. These two families of quasinormal resonances display
qualitatively different asymptotic behaviors in the extremal ($a/M\to 1$)
black-hole limit: The zero-damping modes (ZDMs) are characterized by relaxation
times which tend to infinity in the extremal black-hole limit ($\Im\omega\to 0$
as $a/M\to 1$), whereas the damped modes (DMs) are characterized by non-zero
damping rates ($\Im\omega\to$ finite-values as $a/M\to 1$). In this paper we
refute the claim made by Yang et. al. that co-rotating DMs of near-extremal
black holes are restricted to the limited range $0\leq
\mu\lesssim\mu_{\text{c}}\approx 0.74$, where $\mu\equiv m/l$ is the
dimensionless ratio between the azimuthal harmonic index $m$ and the spheroidal
harmonic index $l$ of the perturbation mode. In particular, we use an
analytical formula originally derived by Detweiler in order to prove the
existence of DMs (damped quasinormal resonances which are characterized by
finite $\Im\omega$ values in the $a/M\to 1$ limit) of near-extremal black holes
in the $\mu>\mu_{\text{c}}$ regime, the regime which was claimed by Yang et.
al. not to contain damped modes. We show that these co-rotating DMs (in the
regime $\mu>\mu_{\text{c}}$) are expected to characterize the resonance spectra
of rapidly-rotating (near-extremal) black holes with $a/M\gtrsim 1-10^{-9}$. | 1510.05604v1 |
2016-02-16 | Damping and power spectra of quasi-periodic intensity disturbances above a solar polar coronal hole | We study intensity disturbances above a solar polar coronal hole seen in the
AIA 171 \AA\ and 193 \AA\ passbands, aiming to provide more insights into their
physical nature. The damping and power spectra of the intensity disturbances
with frequencies from 0.07 mHz to 10.5 mHz are investigated. The damping of the
intensity disturbances tends to be stronger at lower frequencies, and their
damping behavior below 980" (for comparison, the limb is at 945") is different
from what happens above. No significant difference is found between the damping
of the intensity disturbances in the AIA 171 \AA\ and that in the AIA 193 \AA.
The indices of the power spectra of the intensity disturbances are found to be
slightly smaller in the AIA 171 \AA\ than in the AIA 193 \AA, but the
difference is within one sigma deviation. An additional enhanced component is
present in the power spectra in a period range of 8--40 minutes at lower
heights. While the power spectra of spicule is highly correlated with its
associated intensity disturbance, it suggests that the power spectra of the
intensity disturbances might be a mixture of spicules and wave activities. We
suggest that each intensity disturbance in the polar coronal hole is possibly a
series of independent slow magnetoacoustic waves triggered by spicular
activities. | 1602.04883v1 |
2016-04-20 | Nonlinear wave damping due to multi-plasmon resonances | For short wavelengths, it is well known that the linearized Wigner-Moyal
equation predicts wave damping due to wave-particle interaction, where the
resonant velocity shifted from the phase velocity by a velocity $v_q = \hbar
k/2m$. Here $\hbar$ is the reduced Planck constant, $k$ is the wavenumber and
$m$ is the electron mass. Going beyond linear theory, we find additional
resonances with velocity shifts $n v_q$, $n = 2, 3, \ldots$, giving rise to a
new wave-damping mechanism that we term \emph{multi-plasmon damping}, as it can
be seen as the simultaneous absorption (or emission) of multiple plasmon
quanta. Naturally this wave damping is not present in classical plasmas. For a
temperature well below the Fermi temperature, if the linear ($n = 1$) resonant
velocity is outside the Fermi sphere, the number of linearly resonant particles
is exponentially small, while the multi-plasmon resonances can be located in
the bulk of the distribution. We derive sets of evolution equations for the
case of two-plasmon and three-plasmon resonances for Langmuir waves in the
simplest case of a fully degenerate plasma. By solving these equations
numerically for a range of wave-numbers we find the corresponding damping
rates, and we compare them to results from linear theory to estimate the
applicability. Finally, we discuss the effects due to a finite temperature. | 1604.05983v2 |
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