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2007-12-19
|
Cosmic String Dynamics and Evolution in Warped Spacetime
|
We study the dynamics and evolution of Nambu-Goto strings in a warped
spacetime, where the warp factor is a function of the internal coordinates
giving rise to a `throat' region. The microscopic equations of motion for
strings in this background include potential and friction terms, which attract
the strings towards the bottom of the warping throat. However, by considering
the resulting macroscopic equations for the velocities of strings in the
vicinity of the throat, we note the absence of enough classical damping to
guarantee that the strings actually reach the warped minimum and stabilise
there. Instead, our classical analysis supports a picture in which the strings
experience mere deflections and bounces around the tip, rather than strongly
damped oscillations. Indeed, 4D Hubble friction is inefficient in the internal
dimensions and there is no other classical mechanism known, which could provide
efficient damping. These results have potentially important implications for
the intercommuting probabilities of cosmic superstrings.
|
0712.3224v2
|
2007-12-20
|
The Critical Exponent of the Fractional Langevin Equation is $α_c\approx 0.402$
|
We investigate the dynamical phase diagram of the fractional Langevin
equation and show that critical exponents mark dynamical transitions in the
behavior of the system. For a free and harmonically bound particle the critical
exponent $\alpha_c= 0.402\pm 0.002$ marks a transition to a non-monotonic
under-damped phase. The critical exponent $\alpha_{R}=0.441...$ marks a
transition to a resonance phase, when an external oscillating field drives the
system. Physically, we explain these behaviors using a cage effect, where the
medium induces an elastic type of friction. Phase diagrams describing the
under-damped, the over-damped and critical frequencies of the fractional
oscillator, recently used to model single protein experiments, show behaviors
vastly different from normal.
|
0712.3407v1
|
2008-01-24
|
Particle Acceleration by Fast Modes in Solar Flares
|
We address the problem of particle acceleration in solar flares by fast modes
which may be excited during the reconnection and undergo cascade and are
subjected to damping. We extend the calculations beyond quasilinear
approximation and compare the acceleration and scattering by transit time
damping and gyroresonance interactions. We find that the acceleration is
dominated by the so called transit time damping mechanism. We estimate the
total energy transferred into particles, and show that our approach provides
sufficiently accurate results We compare this rate with energy loss rate.
Scattering by fast modes appears to be sufficient to prevent the protons from
escaping the system during the acceleration. Confinement of electrons, on the
other hand, requires the existence of plasma waves. Electrons can be
accelerated to GeV energies through the process described here for solar flare
conditions.
|
0801.3786v2
|
2008-02-07
|
Analysis of squeal noise and mode coupling instabilities including damping and gyroscopic effects
|
This paper deals with an audible disturbance known as automotive clutch
squeal noise from the viewpoint of friction-induced mode coupling instability.
Firstly, an auto-coupling model is presented showing a non-conservative
circulatory effect originating from friction forces. Secondly, the stability of
an equilibrium is investigated by determining the eigenvalues of the system
linearized equations. The effects of the circulatory and gyroscopic actions are
examined analytically and numerically to determine their influence on the
stability region. Separate and combined effects are analysed with and without
structural damping and important information is obtained on the role of each
parameter and their interactions regarding overall stability. Not only is
structural damping shown to be of primary importance, as reported in many
previous works, this article also highlights a particular relationship with
gyroscopic effects. A method of optimizing both the stability range and its
robustness with respect to uncertainty on system parameters is discussed after
which practical design recommendations are given.
|
0802.0923v1
|
2008-02-12
|
Nonlinear Saturation of g-modes in Proto-Neutron Stars: Quieting the Acoustic Engine
|
According to Burrows et al.'s acoustic mechanism for core-collapse supernova
explosions, the primary, l=1, g-mode in the core of the proto-neutron star is
excited to an energy of ~ 10^{50} ergs and damps by the emission of sound
waves. Here we calculate the damping of the primary mode by the parametric
instability, i.e., by nonlinear, 3-mode coupling between the low-order primary
mode and pairs of high-order g-modes. We show that the primary mode is strongly
coupled to highly resonant, neutrino damped pairs with n>10; such short
wavelength interactions cannot be resolved in the simulations. We find that the
parametric instability saturates the primary mode energy at ~10^{48} ergs, well
below the energy needed to drive an explosion. We therefore conclude that
acoustic power is unlikely to be energetically significant in core-collapse
supernova explosions.
|
0802.1522v3
|
2008-02-21
|
Gas Damping Coefficient Research for MEMS Comb Linear Vibration Gyroscope
|
Silicon-MEMS gyroscope is an important part of MEMS (Micro Electrical
Mechanical System). There are some disturb ignored in traditional gyroscope
that must be evaluated newly because of its smaller size (reach the level of
micron). In these disturb, the air pressure largely influences the performance
of MEMS gyroscope. Different air pressure causes different gas damping
coefficient for the MEMS comb linear vibration gyroscope and different gas
damping coefficient influences the quality factor of the gyroscope directive.
The quality factor influences the dynamic working bandwidth of the MEMS comb
linear vibration gyroscope, so it is influences the output characteristic of
the MEMS comb linear vibration gyroscope. The paper shows the relationship
between the air pressure and the output amplified and phase of the detecting
axis through analyzing the air pressure influence on the MEMS comb linear
vibration gyroscope. It discusses the influence on the frequency distribute and
quality factor of the MEMS comb linear vibration gyroscope for different air
pressure.
|
0802.3048v1
|
2008-03-03
|
Damped harmonic oscillator interpretation of the soft-state power spectra of Cyg X-1
|
We develop a model of an accretion disc in which the variability induced at a
given radius is governed by a damped harmonic oscillator at the corresponding
epicyclic frequency. That variability induces both linear and non-linear
responses in the locally emitted radiation. The total observed variability of a
source is the sum of these contributions over the disc radius weighted by the
energy dissipation rate at each radius. It is shown that this simple model,
which effectively has only three parameters including the normalization, can
explain the range of the power spectra observed from Cyg X-1 in the soft state.
Although a degeneracy between the black hole mass and the strength of the
damping does not allow a unique determination of the mass, we can still
constrain it to <16--20 solar masses. We also show that our model preserves the
observed linear rms-flux relationship even in the presence of the non-linear
flux response.
|
0803.0238v2
|
2008-03-05
|
The Secular Evolution of a Close Ring-Satellite System: The Excitation of Spiral Density Waves at a Nearby Gap Edge
|
The Lagrange planetary equations are used to study to secular evolution of a
small, eccentric satellite that orbits within a narrow gap in a broad,
self-gravitating planetary ring. These equations show that the satellite's
secular perturbations of the ring will excite a very long-wavelength spiral
density wave that propagates away from the gap's outer edge. The amplitude of
these waves, as well as their dispersion relation, are derived here. That
dispersion relation reveals that a planetary ring can sustain two types of
density waves: long waves that, in Saturn's A ring, would have wavelengths of
order 100 km, and short waves that tend to be very nonlinear and are expected
to quickly damp. The excitation of these waves also transports angular momentum
from the ring to the satellite in a way that damps the satellite's eccentricity
e, which also tends to reduce the amplitude of subsequent waves. The rate of
eccentricity damping due to this wave action is then compared to the rates at
which the satellite's Lindblad and corotation resonances alter the satellite's
e. These results are then applied to the gap-embedded Saturnian satellites Pan
and Daphnis, and the long-term stability of their eccentricities is assessed.
|
0803.0576v1
|
2008-03-06
|
Hypersound damping in vitreous silica measured by picosecond acoustics
|
The attenuation of longitudinal acoustic phonons up to frequencies nearing
250 GHz is measured in vitreous silica with a picosecond optical technique.
Taking advantage of interferences on the probe beam, difficulties encountered
in early pioneering experiments are alleviated. Sound damping at 250 GHz and
room temperature is consistent with relaxation dominated by anharmonic
interactions with the thermal bath, extending optical Brillouin scattering
data. Our result is at variance with claims of a recent deep-UV experiment
which reported a rapid damping increase beyond 100 GHz. A comprehensive picture
of the frequency dependence of sound attenuation in $v$-SiO$_2$ can be
proposed.
|
0803.0832v1
|
2008-03-07
|
Resonance distribution in open quantum chaotic systems
|
In order to study the resonance spectra of chaotic cavities subject to some
damping (which can be due to absorption or partial reflection at the
boundaries), we use a model of damped quantum maps. In the high-frequency
limit, the distribution of (quantum) decay rates is shown to cluster near a
``typical'' value, which is larger than the classical decay rate of the
corresponding damped ray dynamics. The speed of this clustering may be quite
slow, which could explain why it has not been detected in previous numerical
data.
|
0803.1075v4
|
2008-04-03
|
Single flux quantum circuits with damping based on dissipative transmission lines
|
We propose and demonstrate the functioning of a special Rapid Single Flux
Quantum (RSFQ) circuit with frequency-dependent damping. This damping is
achieved by shunting individual Josephson junctions by pieces of open-ended RC
transmission lines. Our circuit includes a toggle flip-flop cell, Josephson
transmission lines transferring single flux quantum pulses to and from this
cell, as well as DC/SFQ and SFQ/DC converters. Due to the desired
frequency-dispersion in the RC line shunts which ensures sufficiently low noise
at low frequencies, such circuits are well-suited for integrating with the
flux/phase Josephson qubit and enable its efficient control.
|
0804.0442v1
|
2008-05-14
|
Reconciling results of LSND, MiniBooNE and other experiments with soft decoherence
|
We propose an explanation of the LSND signal via quantum-decoherence of the
mass states, which leads to damping of the interference terms in the
oscillation probabilities. The decoherence parameters as well as their energy
dependence are chosen in such a way that the damping affects only oscillations
with the large (atmospheric) $\Delta m^2$ and rapidly decreases with the
neutrino energy. This allows us to reconcile the positive LSND signal with
MiniBooNE and other null-result experiments. The standard explanations of
solar, atmospheric, KamLAND and MINOS data are not affected. No new particles,
and in particular, no sterile neutrinos are needed. The LSND signal is
controlled by the 1-3 mixing angle $\theta_{13}$ and, depending on the degree
of damping, yields $0.0014 < \sin^2\theta_{13} < 0.034$ at $3\sigma$. The
scenario can be tested at upcoming $\theta_{13}$ searches: while the comparison
of near and far detector measurements at reactors should lead to a null-result
a positive signal for $\theta_{13}$ is expected in long-baseline accelerator
experiments. The proposed decoherence may partially explain the results of
Gallium detector calibrations and it can strongly affect supernova neutrino
signals.
|
0805.2098v1
|
2008-06-02
|
Observations of Solar Doppler Shift Oscillations with the EUV Imaging Spectrometer on Hinode
|
Damped Doppler shift oscillations have been observed in emission lines from
ions formed at flare temperatures with the Solar Ultraviolet Measurements of
Emitted Radiation spectrometer on the Solar and Heliospheric Observatory and
with the Bragg Crystal Spectrometer on Yohkoh. This Letter reports the
detection of low-amplitude damped oscillations in coronal emission lines formed
at much lower temperatures observed with the EUV Imaging Spectrometer on the
Hinode satellite. The oscillations have an amplitude of about 2 km/s, and a
period of around 35 min. The decay times show some evidence for a temperature
dependence with the lowest temperature of formation emission line (Fe XII
195.12 Angstroms) exhibiting a decay time of about 43 min, while the highest
temperature of formation emission line (Fe XV 284.16 Angstroms) shows no
evidence for decay over more than two periods of the oscillation. The data
appear to be consistent with slow magnetoacoustic standing waves, but may be
inconsistent with conductive damping.
|
0806.0265v1
|
2008-07-07
|
Using squeezed field to preserve two-atom entanglement against spontaneous emissions
|
Tunable interaction between two atoms in a cavity is realized by interacting
the two atoms with an extra controllable single-mode squeezed field. Such a
controllable interaction can be further used to control entanglement between
the two atoms against amplitude damping decoherence caused by spontaneous
emissions. For the independent amplitude damping decoherence channel,
entanglement will be lost completely without controls, while it can be
partially preserved by the proposed strategy. For the collective amplitude
damping decoherence channel, our strategy can enhance the entanglement compared
with the uncontrolled case when the entanglement of the uncontrolled stationary
state is not too large.
|
0807.0965v2
|
2008-07-17
|
Connecting high-redshift galaxy populations through observations of local Damped Lyman Alpha dwarf galaxies
|
I report on observations of the z=0.01 dwarf galaxy SBS1543+593 which is
projected onto the background QSO HS1543+5921. As a star-forming galaxy first
noted in emission, this dwarf is playing a pivotal role in our understanding of
high-redshift galaxy populations, because it also gives rise to a Damped Lyman
Alpha system. This enabled us to analyze, for the first time, the chemical
abundance of $\alpha$ elements in a Damped Lyman Alpha galaxy using both,
emission and absorption diagnostics. We find that the abundances agree with one
another within the observational uncertainties. I discuss the implications of
this result for the interpretation of high-redshift galaxy observations. A
catalog of dwarf-galaxy--QSO projections culled from the Sloan Digital Sky
Survey is provided to stimulate future work.
|
0807.2853v1
|
2008-07-26
|
A Monte Carlo Method for Modeling Thermal Damping: Beyond the Brownian-Motion Master Equation
|
The "standard" Brownian motion master equation, used to describe thermal
damping, is not completely positive, and does not admit a Monte Carlo method,
important in numerical simulations. To eliminate both these problems one must
add a term that generates additional position diffusion. He we show that one
can obtain a completely positive simple quantum Brownian motion, efficiently
solvable, without any extra diffusion. This is achieved by using a stochastic
Schroedinger equation (SSE), closely analogous to Langevin's equation, that has
no equivalent Markovian master equation. Considering a specific example, we
show that this SSE is sensitive to nonlinearities in situations in which the
master equation is not, and may therefore be a better model of damping for
nonlinear systems.
|
0807.4211v3
|
2008-07-31
|
Finite-dimensional attractors for the quasi-linear strongly-damped wave equation
|
We present a new method of investigating the so-called quasi-linear strongly
damped wave equations $$ \partial_t^2u-\gamma\partial_t\Delta_x u-\Delta_x
u+f(u)= \nabla_x\cdot \phi'(\nabla_x u)+g $$ in bounded 3D domains. This method
allows us to establish the existence and uniqueness of energy solutions in the
case where the growth exponent of the non-linearity $\phi$ is less than 6 and
$f$ may have arbitrary polynomial growth rate. Moreover, the existence of a
finite-dimensional global and exponential attractors for the solution semigroup
associated with that equation and their additional regularity are also
established. In a particular case $\phi\equiv0$ which corresponds to the
so-called semi-linear strongly damped wave equation, our result allows to
remove the long-standing growth restriction $|f(u)|\leq C(1+ |u|^5)$.
|
0807.5078v1
|
2008-08-01
|
Field-Driven Domain-Wall Dynamics in GaMnAs Films with Perpendicular Anisotropy
|
We combine magneto-optical imaging and a magnetic field pulse technique to
study domain wall dynamics in a ferromagnetic (Ga,Mn)As layer with
perpendicular easy axis. Contrary to ultrathin metallic layers, the depinning
field is found to be smaller than the Walker field, thereby allowing for the
observation of the steady and precessional flow regimes. The domain wall width
and damping parameters are determined self-consistently. The damping, 30 times
larger than the one deduced from ferromagnetic resonance, is shown to
essentially originate from the non-conservation of the magnetization modulus.
An unpredicted damping resonance and a dissipation regime associated with the
existence of horizontal Bloch lines are also revealed.
|
0808.0119v1
|
2008-08-11
|
Effect of frequency and temperature on microwave-induced magnetoresistance oscillations in two-dimensional electron systems
|
Experimental results on microwave-induced magnetoresistance oscillation in
two-dimensional electron systems show a similar behavior of these systems
regarding temperature and microwave frequency. It is found that these
oscillations tend to quench when frequency or temperature increase, approaching
magnetoresistance to the response of the dark system. In this work we show that
this experimental behavior can be addressed on the same theoretical basis.
Microwave radiation forces the electron orbits to move back and forth being
damped by interaction with the lattice. We show that this damping depends
dramatically on microwave frequency and also on temperature. An increase in
frequency or temperature gives rise to an increase in the lattice damping
producing eventually a quenching effect in the magnetoresistance oscillations.
|
0808.1489v1
|
2008-09-26
|
Damping and magnetic anisotropy of ferromagnetic GaMnAs thin films
|
The magnetic properties of annealed, epitaxial Ga0.93Mn0.07As layers under
tensile and compressive stress have been investigated by X-band (9GHz) and
Q-band (35GHz) ferromagnetic resonance (FMR) spectroscopy. From the analysis of
the linewidths of the uniform mode spectra the FMR Gilbert damping factor
"alpha" has been determined. At T=4K we obtain a minimum damping factor of
"alpha" = 0.003 for the compressively stressed layer. Its value is not
isotropic. It has a minimum value for the easy axes orientations of the
magnetic field and increases with the measuring temperature. Its average value
is for both type of films of the order of 0.01 in spite of strong differences
in the inhomogeneous linewidth which vary between 20 Oe and 600 Oe for the
layers grown on GaAs and GaInAs substrates respectively.
|
0809.4644v2
|
2008-11-13
|
Higher order energy decay rates for damped wave equations with variable coefficients
|
Under appropriate assumptions the energy of wave equations with damping and
variable coefficients $c(x)u_{tt}-\hbox{div}(b(x)\nabla u)+a(x)u_t =h(x)$ has
been shown to decay. Determining the rate of decay for the higher order
energies involving the $k$th order spatial and time derivatives has been an
open problem with the exception of some sparse results obtained for $k=1,2,3$.
We establish estimates that optimally relate the higher order energies with the
first order energy by carefully analyzing the effects of linear damping. The
results concern weighted (in time) and also pointwise (in time) energy decay
estimates. We also obtain $L^\infty$ estimates for the solution $u$. As an
application we compute explicit decay rates for all energies which involve the
dimension $n$ and the bounds for the coefficients $a(x)$ and $b(x)$ in the case
$c (x)=1$ and $h(x)=0.$
|
0811.2159v1
|
2009-01-12
|
Nonlinear Wigner solid transport over superfluid helium under AC conditions
|
Nonlinear transport properties of the two-dimensional Wigner solid of surface
electrons on superfluid helium are studied for alternating current conditions.
For time-averaged quantities like Fourier coefficients, the field-velocity
characteristics are shown to be qualitatively different as compared to that
found in the DC theory. For a spatially uniform current we found a general
solution for the field-velocity relationship which appears to be strongly
dependent on the current frequency. If the current frequency is much lower than
the ripplon damping parameter, the Bragg-Cherenkov resonances which appear at
high enough drift velocities acquire a distinctive saw-tooth shape with long
right-side tails independent of small damping. For current frequencies which
are close or higher than the ripplon damping coefficient, the interference of
ripplons excited at different time intervals results in a new oscillatory (in
drift velocity) regime of Bragg-Cherenkov scattering.
|
0901.1508v1
|
2009-01-14
|
Brownian motion with respect to time-changing Riemannian metrics, applications to Ricci flow
|
We generalize Brownian motion on a Riemannian manifold to the case of a
family of metrics which depends on time. Such questions are natural for
equations like the heat equation with respect to time dependent Laplacians
(inhomogeneous diffusions). In this paper we are in particular interested in
the Ricci flow which provides an intrinsic family of time dependent metrics. We
give a notion of parallel transport along this Brownian motion, and establish a
generalization of the Dohrn-Guerra or damped parallel transport, Bismut
integration by part formulas, and gradient estimate formulas. One of our main
results is a characterization of the Ricci flow in terms of the damped parallel
transport. At the end of the paper we give an intrinsic definition of the
damped parallel transport in terms of stochastic flows, and derive an intrinsic
martingale which may provide information about singularities of the flow.
|
0901.1999v2
|
2009-01-26
|
Damping of sound waves in superfluid nucleon-hyperon matter of neutron stars
|
We consider sound waves in superfluid nucleon-hyperon matter of massive
neutron-star cores. We calculate and analyze the speeds of sound modes and
their damping times due to the shear viscosity and non-equilibrium weak
processes of particle transformations. For that, we employ the dissipative
relativistic hydrodynamics of a superfluid nucleon-hyperon mixture, formulated
recently [M.E. Gusakov and E.M. Kantor, Phys. Rev. D78, 083006 (2008)]. We
demonstrate that the damping times of sound modes calculated using this
hydrodynamics and the ordinary (nonsuperfluid) one, can differ from each other
by several orders of magnitude.
|
0901.4108v1
|
2009-03-02
|
Attenuation and damping of electromagnetic fields: Influence of inertia and displacement current
|
New results for attenuation and damping of electromagnetic fields in rigid
conducting media are derived under the conjugate influence of inertia due to
charge carriers and displacement current. Inertial effects are described by a
relaxation time for the current density in the realm of an extended Ohm's law.
The classical notions of poor and good conductors are rediscussed on the basis
of an effective electric conductivity, depending on both wave frequency and
relaxation time. It is found that the attenuation for good conductors at high
frequencies depends solely on the relaxation time. This means that the
penetration depth saturates to a minimum value at sufficiently high
frequencies. It is also shown that the actions of inertia and displacement
current on damping of magnetic fields are opposite to each other. That could
explain why the classical decay time of magnetic fields scales approximately as
the diffusion time. At very small length scales, the decay time could be given
either by the relaxation time or by a fraction of the diffusion time, depending
whether inertia or displacement current, respectively, would prevail on
magnetic diffusion.
|
0903.0210v1
|
2009-04-06
|
Scrutinizing single-qubit quantum channels: Theory and experiment with trapped ions
|
We report experimental implementation of various types of qubit channels
using an individual trapped ion. We analyzed experimental data and we performed
tomographic reconstruction of quantum channels based on these data.
Specifically, we studied phase damping channels, where the damping acts either
in the xy-plane of the Bloch sphere or in an arbitrary plane that includes the
origin of the Bloch sphere. We also experimentally realized and consequently
analyzed quantum channels that in addition to phase damping affect also a
polarization rotation. We used three reconstruction schemes for estimation of
quantum channels from experimental data: (1) a linear inverse method, (2) a
maximum likelihood estimation, and (3) a constrained maximum likelihood
estimation. We took into account realistic experimental conditions where
imperfect test-state preparations and biased measurements are incorporated into
the estimation schemes. As a result we found that imperfections present in the
process of preparation of test states and as well as in measurements of the
considered ion trap system do not limit the control of the implementation of
the desired channel. Even imperfect preparation of test state and subsequent
measurements still provide sufficient resources for the complete
quantum-channel tomography.
|
0904.0923v1
|
2009-05-13
|
Time-dependent barrier passage of Two-dimensional non-Ohmic damping system
|
The time-dependent barrier passage of an anomalous damping system is studied
via the generalized Langevin equation (GLE) with non-Ohmic memory damping
friction tensor and corresponding thermal colored noise tensor describing a
particle passing over the saddle point of a two-dimensional quadratic potential
energy surface. The time-dependent passing probability and transmission
coefficient are analytically obtained by using of the reactive flux method. The
long memory aspect of friction is revealed to originate a non-monotonic
$\delta$(power exponent of the friction) dependence of the passing probability,
the optimal incident angle of the particle and the steady anomalous
transmission coefficient. In the long time limit a bigger steady transmission
coefficient is obtained which means less barrier recrossing than the
one-dimensional case.
|
0905.2074v1
|
2009-06-04
|
Viscous cavity damping of a microlever in a simple fluid
|
We consider the problem of oscillation damping in air of a thermally actuated
microlever as it is gradually approached towards an infinite wall in parallel
geometry. As the gap is decreased from 20 nm down to 400 nm, we observe the
increasing damping of the lever Brownian motion in the fluid laminar regime.
This manifests itself as a linear decrease with distance of the lever quality
factor accompanied by a dramatic softening of its resonance, and eventually
leads to the freezing of the CL oscillation. We are able to quantitatively
explain this behavior by analytically solving the Navier-Stokes equation with
perfect slip boundary conditions. Our findings may have implications for
microfluidics and micro- nano-electromechanical applications.
|
0906.0782v1
|
2009-06-19
|
Wakefield damping for the CLIC crab cavity
|
A crab cavity is required in the CLIC to allow effective head-on collision of
bunches at the IP. A high operating frequency is preferred as the deflection
voltage required for a given rotation angle and the RF phase tolerance for a
crab cavity are inversely proportional to the operating frequency. The short
bunch spacing of the CLIC scheme and the high sensitivity of the crab cavity to
dipole kicks demand very high damping of the inter-bunch wakes, the major
contributor to the luminosity loss of colliding bunches. This paper
investigates the nature of the wakefields in the CLIC crab cavity and the
possibility of using various damping schemes to suppress them effectively.
|
0906.3593v1
|
2009-07-06
|
Non-Fermi liquid behavior due to U(1) gauge field in two dimensions
|
We study the damping rate of massless Dirac fermions due to the U(1) gauge
field in (2+1)-dimensional quantum electrodynamics. In the absence of a Maxwell
term for the gauge field, the fermion damping rate
$\mathrm{Im}\Sigma(\omega,T)$ is found to diverge in both perturbative and
self-consistent results. In the presence of a Maxwell term, there is still
divergence in the perturbative results for $\mathrm{Im}\Sigma(\omega,T)$. Once
the Maxwell term is included into the self-consistent equations for fermion
self-energy and vacuum polarization functions, the fermion damping rate is free
of divergence and exhibits non-Fermi liquid behavior:
$\mathrm{Im}\Sigma(\omega,T) \propto \mathrm{max}(\sqrt{\omega},\sqrt{T})$.
|
0907.1022v3
|
2009-07-30
|
Gas damping force noise on a macroscopic test body in an infinite gas reservoir
|
We present a simple analysis of the force noise associated with the
mechanical damping of the motion of a test body surrounded by a large volume of
rarefied gas. The calculation is performed considering the momentum imparted by
inelastic collisions against the sides of a cubic test mass, and for other
geometries for which the force noise could be an experimental limitation. In
addition to arriving at an accurated estimate, by two alternative methods, we
discuss the limits of the applicability of this analysis to realistic
experimental configurations in which a test body is surrounded by residual gas
inside an enclosure that is only slightly larger than the test body itself.
|
0907.5375v2
|
2009-08-26
|
Influence of an external magnetic field on forced turbulence in a swirling flow of liquid metal
|
We report an experimental investigation on the influence of an external
magnetic field on forced 3D turbulence of liquid gallium in a closed vessel. We
observe an exponential damping of the turbulent velocity fluctuations as a
function of the interaction parameter N (ratio of Lorentz force over inertial
terms of the Navier-Stokes equation). The flow structures develop some
anisotropy but do not become bidimensional. From a dynamical viewpoint, the
damping first occurs homogeneously over the whole spectrum of frequencies. For
larger values of N, a very strong additional damping occurs at the highest
frequencies. However, the injected mechanical power remains independent of the
applied magnetic field. The simultaneous measurement of induced magnetic field
and electrical potential differences shows a very weak correlation between
magnetic field and velocity fluctuations. The observed reduction of the
fluctuations is in agreement with a previously proposed mechanism for the
saturation of turbulent dynamos and with the order of magnitude of the Von
Karman Sodium dynamo magnetic field.
|
0908.3821v1
|
2009-09-30
|
Electronic damping of molecular motion at metal surfaces
|
A method for the calculation of the damping rate due to electron-hole pair
excitation for atomic and molecular motion at metal surfaces is presented. The
theoretical basis is provided by Time Dependent Density Functional Theory
(TDDFT) in the quasi-static limit and calculations are performed within a
standard plane-wave, pseudopotential framework. The artificial periodicity
introduced by using a super-cell geometry is removed to derive results for the
motion of an isolated atom or molecule, rather than for the coherent motion of
an ordered over-layer. The algorithm is implemented in parallel, distributed
across both ${\bf k}$ and ${\bf g}$ space, and in a form compatible with the
CASTEP code. Test results for the damping of the motion of hydrogen atoms above
the Cu(111) surface are presented.
|
0909.5495v1
|
2009-10-27
|
On the Interpretation of Magnetic Helicity Signatures in the Dissipation Range of Solar Wind Turbulence
|
Measurements of small-scale turbulent fluctuations in the solar wind find a
non-zero right-handed magnetic helicity. This has been interpreted as evidence
for ion cyclotron damping. However, theoretical and empirical evidence suggests
that the majority of the energy in solar wind turbulence resides in low
frequency anisotropic kinetic Alfven wave fluctuations that are not subject to
ion cyclotron damping. We demonstrate that a dissipation range comprised of
kinetic Alfven waves also produces a net right-handed fluctuating magnetic
helicity signature consistent with observations. Thus, the observed magnetic
helicity signature does not necessarily imply that ion cyclotron damping is
energetically important in the solar wind.
|
0910.5023v1
|
2009-12-08
|
Coupling Photosphere and Corona: Linear and Turbulent Regimes
|
In a recent work Grappin et al. [1] have shown that low- frequency movements
can be transmitted from one footpoint to the other along a magnetic loop, thus
mimicking a friction effect of the corona on the photosphere, and invalidating
the line-tying approximation. We consider here successively the effect of high
frequencies and turbulent damping on the process. We use a very simple
atmospheric model which allows to study analytically the laminar case, and to
study the turbulent case both using simple phenomenological arguments and a
more sophisticated turbulence model [2]. We find that, except when turbulent
damping is such that all turbulence is damped during loop traversal, coupling
still occurs between distant footpoints, and moreover the coronal field induced
by photospheric movements saturates at finite values.
|
0912.1497v1
|
2009-12-16
|
The role of $r$-mode damping in the thermal evolution of neutron stars
|
The thermal evolution of neutron stars (NSs) is investigated by coupling with
the evolution of $\textit{r}$-mode instability that is described by a second
order model.The heating effect due to shear viscous damping of the
$\textit{r}$-modes enables us to understand the high temperature of two young
pulsars (i.e., PSR B0531+21 and RX J0822-4300) in the framework of the simple
$npe$ NS model, without superfluidity or exotic particles.Moreover, the light
curves predicted by the model within an acceptable parameter regime may
probably cover all of the young and middle-aged pulsars in the $\lg
T_s^{\infty}-\lg t$ panel, and an artificially strong $p$ superfluidity invoked
in some early works is not needed here. Additionally, by considering the
radiative viscous damping of the $\textit{r}$-modes, a surprising extra cooling
effect is found, which can even exceed the heating effect sometimes although
plays an ignorable role in the thermal history.
|
0912.3052v1
|
2009-12-25
|
Noisy non-transitive quantum games
|
We study the effect of quantum noise in 3 by 3 entangled quantum games. By
considering different noisy quantum channels we analyze that how a two-player,
three-strategy Rock-Scissor-Paper game is influenced by the quantum noise. We
consider the winning non-transitive strategies R, S and P such as R beats S, S
beats P, and P beats R. The game behaves as a noiseless game for maximum value
of the quantum noise parameter. It is seen that Alice's payoff is heavily
influenced by the depolarizing noise as compared to the amplitude damping
noise. Depolarizing channel causes a monotonic decrease in players payoffs as
we increase the amount of of quantum noise. In case of amplitude damping
channel, the Alice's payoff function reaches its minimum for alpha=0.5 and is
symmetrical. This means that larger values of quantum noise influence the game
weakly. On the other hand, phase damping channel does not influence the game's
payoff. Furthermore, the game's Nash equilibrium and non-transitive character
of the game are not affected under the influence of quantum noise.
|
0912.4961v1
|
2010-01-26
|
Damping in high-frequency metallic nanomechanical resonators
|
We have studied damping in polycrystalline Al nanomechanical resonators by
measuring the temperature dependence of their resonance frequency and quality
factor over a temperature range of 0.1 - 4 K. Two regimes are clearly
distinguished with a crossover temperature of 1 K. Below 1 K we observe a
logarithmic temperature dependence of the frequency and linear dependence of
damping that cannot be explained by the existing standard models. We attribute
these phenomena to the effect of the two-level systems characterized by the
unexpectedly long (at least two orders of magnitude longer) relaxation times
and discuss possible microscopic models for such systems. We conclude that the
dynamics of the two-level systems is dominated by their interaction with
one-dimensional phonon modes of the resonators.
|
1001.4612v1
|
2010-04-28
|
Inviscid dynamical structures near Couette flow
|
Consider inviscid fluids in a channel {-1<y<1}. For the Couette flow
v_0=(y,0), the vertical velocity of solutions to the linearized Euler equation
at v_0 decays in time. At the nonlinear level, such inviscid damping has not
been proved. First, we show that in any (vorticity) H^{s}(s<(3/2)) neighborhood
of Couette flow, there exist non-parallel steady flows with arbitrary minimal
horizontal period. This implies that nonlinear inviscid damping is not true in
any (vorticity) H^{s}(s<(3/2)) neighborhood of Couette flow and for any
horizontal period. Indeed, the long time behavior in such neighborhoods are
very rich, including nontrivial steady flows, stable and unstable manifolds of
nearby unstable shears. Second, in the (vorticity) H^{s}(s>(3/2)) neighborhood
of Couette, we show that there exist no non-parallel steadily travelling flows
v(x-ct,y), and no unstable shears. This suggests that the long time dynamics in
H^{s}(s>(3/2)) neighborhoods of Couette might be much simpler. Such contrasting
dynamics in H^{s} spaces with the critical power s=(3/2) is a truly nonlinear
phenomena, since the linear inviscid damping near Couette is true for any
initial vorticity in L^2.
|
1004.5149v1
|
2010-06-14
|
Parallel electric field amplification by phase-mixing of Alfven waves
|
Previous numerical studies have identified "phase mixing" of low-frequency
Alfven waves as a mean of parallel electric field amplification and
acceleration of electrons in a collisionless plasma. Theoretical explanations
are given of how this produces an amplification of the parallel electric field,
and as a consequence, also leads to enhanced collisionless damping of the wave
by energy transfer to the electrons. Our results are based on the properties of
the Alfven waves in a warm plasma which are obtained from drift-kinetic theory,
in particular, the rate of their electron Landau damping. Phase mixing in a
collisionless low-$\beta$ plasma proceeds in a manner very similar to the
visco-resistive case, except for the fact that electron Landau damping is the
primary energy dissipation channel. The time and length scales involved are
evaluated. We also focus on the evolution of the parallel electric field and
calculate its maximum value in the course of its amplification.
|
1006.2729v1
|
2010-07-19
|
Anomalously large damping of long-wavelength quasiparticles caused by long-range interaction
|
We demonstrate that long-range interaction in a system can lead to a very
strong interaction between long-wavelength quasiparticles and make them heavily
damped. In particular, we discuss magnon spectrum using 1/S expansion in 3D
Heisenberg ferromagnet (FM) with arbitrary small dipolar forces at T<<T_C. We
obtain that a fraction of long-wavelength magnons with energies e_k<T has
anomalously large damping G_k (ratio G_k/e_k reaches 0.3 for certain k). This
effect is observed both in quantum and classical FMs. Remarkably, this result
contradicts expectation of the quasiparticle concept according which a weakly
excited state of a many-body system can be represented as a collection of
weakly interacting elementary excitations. Particular materials are pointed out
which are suitable for corresponding experiments.
|
1007.3081v2
|
2010-07-29
|
The Quasinormal Mode Spectrum of a Kerr Black Hole in the Eikonal Limit
|
It is well established that the response of a black hole to a generic
perturbation is characterized by a spectrum of damped resonances, called
quasinormal modes; and that, in the limit of large angular momentum ($l \gg
1$), the quasinormal mode frequency spectrum is related to the properties of
unstable null orbits. In this paper we develop an expansion method to explore
the link. We obtain new closed-form approximations for the lightly-damped part
of the spectrum in the large-$l$ regime. We confirm that, at leading order in
$l$, the resonance frequency is linked to the orbital frequency, and the
resonance damping to the Lyapunov exponent, of the relevant null orbit. We go
somewhat further than previous studies to establish (i) a spin-dependent
correction to the frequency at order $1 / l$ for equatorial ($m = \pm l$)
modes, and (ii) a new result for polar modes ($m = 0$). We validate the
approach by testing the closed-form approximations against frequencies obtained
numerically with Leaver's method.
|
1007.5097v1
|
2010-08-30
|
Caldeira-Leggett Model, Landau Damping, and the Vlasov-Poisson System
|
The Caldeira-Leggett Hamiltonian (Eq. (1) below) describes the interaction of
a discrete harmonic oscillator with a continuous bath of harmonic oscillators.
This system is a standard model of dissipation in macroscopic low temperature
physics, and has applications to superconductors, quantum computing, and
macroscopic quantum tunneling. The similarities between the Caldeira-Leggett
model and the linearized Vlasov-Poisson equation are analyzed, and it is shown
that the damping in the Caldeira-Leggett model is analogous to that of Landau
damping in plasmas [1]. An invertible linear transformation [2, 3] is presented
that converts solutions of the Caldeira-Leggett model into solutions of the
linearized Vlasov-Poisson system.
|
1008.5190v1
|
2010-09-09
|
A number-conserving linear response study of low-velocity ion stopping in a collisional magnetized classical plasma
|
The results of a theoretical investigation on the low-velocity stopping power
of the ions moving in a magnetized collisional plasma are presented. The
stopping power for an ion is calculated employing linear response theory using
the dielectric function approach. The collisions, which leads to a damping of
the excitations in the plasma, is taken into account through a
number-conserving relaxation time approximation in the linear response
function. In order to highlight the effects of collisions and magnetic field we
present a comparison of our analytical and numerical results obtained for a
nonzero damping or magnetic field with those for a vanishing damping or
magnetic field. It is shown that the collisions remove the anomalous friction
obtained previously [Nersisyan et al., Phys. Rev. E 61, 7022 (2000)] for the
collisionless magnetized plasmas at low ion velocities. One of major objectives
of this study is to compare and contrast our theoretical results with those
obtained through a novel diffusion formulation based on Dufty-Berkovsky
relation evaluated in magnetized one-component plasma models framed on target
ions and electrons.
|
1009.1700v1
|
2010-10-03
|
A High Phase Advance Damped and Detuned Structure for the Main Linacs of Clic
|
The main accelerating structures for the CLIC are designed to operate at an
average accelerating gradient of 100 MV/m. The accelerating frequency has been
optimised to 11.994 GHz with a phase advance of 2{\pi}/3 of the main
accelerating mode. The moderately damped and detuned structure (DDS) design is
being studied as an alternative to the strongly damped WDS design. Both these
designs are based on the nominal accelerating phase advance. Here we explore
high phase advance (HPA) structures in which the group velocity of the rf
fields is reduced compared to that of standard (2{\pi}/3) structures. The
electrical breakdown strongly depends on the fundamental mode group velocity.
Hence it is expected that electrical breakdown is less likely to occur in the
HPA structures. We report on a study of both the fundamental and dipole modes
in a CLIC_DDS_HPA structure, designed to operate at 5{\pi}/6 phase advance per
cell. Higher order dipole modes in both the standard and HPA structures are
also studied.
|
1010.0438v1
|
2010-10-23
|
Environmental influences on Quantum Monty Hall problem
|
We reformulate the quantum Monty Hall problem in the presence of decoherence.
The decoherence destroys the fairness of the game. A new Nash equilibrium for a
particular strategy profile in the presence of decoherence emerges. It is shown
that in the presence of decoherence under the action of amplitude damping
channel, Bob's winning probability is always higher than three-forth,
irrespective of Alice's strategy, if he does not switch to the other door and
always wins for a fully decohered case of the channel. Depolarizing channel
damps up Bob's winning probability and gets better off if he sticks to his
current selection. Phase damping channel leaves the winning probability
unaffected. Unlike the classical and the quantum forms of the game, Bob's
dominant strategy in the presence of decoherence is not switching.
|
1010.4875v2
|
2010-12-14
|
Controlling transfer of quantum correlations among bi-partitions of a composite quantum system by combining noisy environments
|
The correlation dynamics is investigated for various bi-partitions of a
composite system consisting of two qubits, and two independent and
non-identical noisy environments. The two qubits have no direct interaction
with each other and locally interact with their environments. Classical and
quantum correlations including entanglement are initially prepared only between
the two qubits. We find that, contrary to the identical noisy environment case,
the entanglement and quantum correlation transfer directions can be controlled
by combining different noisy environments. The amplitude damping environment
determines whether there exists entanglement transfer among the bi-partitions
of a composite system. When one qubit is coupled to an amplitude damping
environment but another one to a bit-flip one, we find a very interesting
result that all the quantum and classical correlations, and even the
entanglement, originally existing between the qubits, can be completely
transferred without any loss to the qubit coupled to the bit-flip environment
and the amplitude-damping environment. We also notice that it is possible to
distinguish the quantum correlation from the classical correlation and
entanglement by combining different noisy environments.
|
1012.3033v1
|
2010-12-22
|
Viscous damping of r-modes: Small amplitude instability
|
We study the viscous damping of r-modes of compact stars and analyze in
detail the regions where small amplitude modes are unstable to the emission of
gravitational radiation. We present general expressions for the viscous damping
times for arbitrary forms of interacting dense matter and derive general
semi-analytic results for the boundary of the instability region. These results
show that many aspects, like in particular the physically important minima of
the instability boundary, are surprisingly insensitive to detailed microscopic
properties of the considered form of matter. Our general expressions are
applied to the cases of hadronic stars, strange stars, and hybrid stars, and we
focus on equations of state that are compatible with the recent measurement of
a heavy compact star. We find that hybrid stars with a sufficiently small core
can "masquerade" as neutron stars and feature an instability region that is
indistinguishable from that of a neutron star, whereas neutron stars with a
core density high enough to allow direct Urca reactions feature a notch on the
right side of the instability region.
|
1012.4883v2
|
2010-12-25
|
Screw-pitch effect and velocity oscillation of domain-wall in ferromagnetic nanowire driven by spin-polarized current
|
We investigate the dynamics of domain wall in ferromagnetic nanowire with
spin-transfer torque. The critical current condition is obtained analytically.
Below the critical current, we get the static domain wall solution which shows
that the spin-polarized current can't drive domain wall moving continuously. In
this case, the spin-transfer torque plays both the anti-precession and
anti-damping roles, which counteracts not only the spin-precession driven by
the effective field but also Gilbert damping to the moment. Above the critical
value, the dynamics of domain wall exhibits the novel screw-pitch effect
characterized by the temporal oscillation of domain wall velocity and width,
respectively. Both the theoretical analysis and numerical simulation
demonstrate that this novel phenomenon arise from the conjunctive action of
Gilbert-damping and spin-transfer torque. We also find that the roles of
spin-transfer torque are entirely contrary for the cases of below and above the
critical current.
|
1012.5473v1
|
2010-12-27
|
Phenomenology of Current-Induced Dynamics in Antiferromagnets
|
We derive a phenomenological theory of current-induced staggered
magnetization dynamics in antiferromagnets. The theory captures the reactive
and dissipative current-induced torques and the conventional effects of
magnetic fields and damping. A Walker ansatz describes the dc current-induced
domain-wall motion when there is no dissipation. If magnetic damping and
dissipative torques are included, the Walker ansatz remains robust when the
domain-wall moves slowly. As in ferromagnets, the domain-wall velocity is
proportional to the ratio between the dissipative-torque and the magnetization
damping. In addition, a current-driven antiferromagnetic domain-wall acquires a
net magnetic moment.
|
1012.5655v2
|
2011-02-02
|
Harmonic Oscillator in Heat Bath: Exact simulation of time-lapse-recorded data, exact analytical benchmark statistics
|
The stochastic dynamics of the damped harmonic oscillator in a heat bath is
simulated with an algorithm that is exact for time steps of arbitrary size.
Exact analytical results are given for correlation functions and power spectra
in the form they acquire when computed from experimental time-lapse recordings.
Three applications are discussed: (i) Effects of finite sampling-rate and
-time, described exactly here, are similar for other stochastic dynamical
systems-e.g. motile micro-organisms and their time-lapse recorded trajectories.
(ii) The same statistics is satisfied by any experimental system to the extent
it is interpreted as a damped harmonic oscillator at finite temperature-such as
an AFM cantilever. (iii) Three other models of fundamental interest are
limiting cases of the damped harmonic oscillator at finite temperature; it
consequently bridges their differences and describes effects of finite sampling
rate and sampling time for these models as well. Finally, we give a brief
discussion of nondimensionalization.
|
1102.0524v1
|
2011-03-03
|
Determination of the pairing state in iron-based superconductors through neutron scattering
|
We calculate the spin susceptibility in the s_{+-} and s_{++} superconducting
states of the iron pnictides using the effective five orbital model and
considering the quasiparticle damping. For the experimentally evaluated
magnitude of the quasiparticle damping and the superconducting gap, the results
at the wave vector ~ (pi,0) show that the s_{+-} state is more consistent with
the neutron scattering experiments, while for larger quasiparticle damping and
the superconducting gap, the s_{++} state can be more consistent. To
distinguish between two cases that reproduce the experiments at the wave vector
~ (pi,0), we propose to investigate experimentally the wave vector ~ (pi,pi).
|
1103.0586v2
|
2011-03-03
|
Transmission of classical and quantum information through a quantum memory channel with damping
|
We consider the transfer of classical and quantum information through a
memory amplitude damping channel. Such a quantum channel is modeled as a damped
harmonic oscillator, the interaction between the information carriers - a train
of qubits - and the oscillator being of the Jaynes-Cummings kind. We prove that
this memory channel is forgetful, so that quantum coding theorems hold for its
capacities. We analyze entropic quantities relative to two uses of this
channel. We show that memory effects improve the channel aptitude to transmit
both classical and quantum information, and we investigate the mechanism by
which memory acts in changing the channel transmission properties.
|
1103.0747v2
|
2011-03-18
|
Time-periodic solitons in a damped-driven nonlinear Schrödinger equation
|
Time-periodic solitons of the parametrically driven damped nonlinear
Schr\"odinger equation are obtained as solutions of the boundary-value problem
on a two-dimensional spatiotemporal domain. We follow the transformation of the
periodic solitons as the strength of the driver is varied. The resulting
bifurcation diagrams provide a natural explanation for the overall form and
details of the attractor chart compiled previously via direct numerical
simulations. In particular, the diagrams confirm the occurrence of the
period-doubling transition to temporal chaos for small values of dissipation
and the absence of such transitions for larger dampings. This difference in the
soliton's response to the increasing driving strength can be traced to the
difference in the radiation frequencies in the two cases. Finally, we relate
the soliton's temporal chaos to the homoclinic bifurcation.
|
1103.3604v1
|
2011-03-28
|
Motion of position-dependent mass as a damping-antidamping process: Application to the Fermi gas and to the Morse potential
|
The object of this paper is to investigate, classically and quantum
mechanically, the relation existing between the position-dependent effective
mass and damping-antidamping dynamics. The quantization of the equations of
motion is carried out using the geometric interpretation of the motion, and we
compare it with the one based on the ordering ambiguity scheme. Furthermore, we
apply the obtained results to a Fermi gas of damped-antidamped particles, and
we solve the Schr\"odinger equation for an exponentially increasing
(decreasing) mass in the presence of the Morse potential.
|
1103.5440v3
|
2011-04-08
|
Magnetization Dissipation in Ferromagnets from Scattering Theory
|
The magnetization dynamics of ferromagnets are often formulated in terms of
the Landau-Lifshitz-Gilbert (LLG) equation. The reactive part of this equation
describes the response of the magnetization in terms of effective fields,
whereas the dissipative part is parameterized by the Gilbert damping tensor. We
formulate a scattering theory for the magnetization dynamics and map this
description on the linearized LLG equation by attaching electric contacts to
the ferromagnet. The reactive part can then be expressed in terms of the static
scattering matrix. The dissipative contribution to the low-frequency
magnetization dynamics can be described as an adiabatic energy pumping process
to the electronic subsystem by the time-dependent magnetization. The Gilbert
damping tensor depends on the time derivative of the scattering matrix as a
function of the magnetization direction. By the fluctuation-dissipation
theorem, the fluctuations of the effective fields can also be formulated in
terms of the quasistatic scattering matrix. The theory is formulated for
general magnetization textures and worked out for monodomain precessions and
domain wall motions. We prove that the Gilbert damping from scattering theory
is identical to the result obtained by the Kubo formalism.
|
1104.1625v1
|
2011-04-30
|
Resonantly Damped Propagating Kink Waves in Longitudinally Stratified Solar Waveguides
|
It has been shown that resonant absorption is a robust physical mechanism to
explain the observed damping of magnetohydrodynamic (MHD) kink waves in the
solar atmosphere due to naturally occurring plasma inhomogeneity in the
direction transverse to the direction of the magnetic field. Theoretical
studies of this damping mechanism were greatly inspired by the first
observations of post-flare standing kink modes in coronal loops using the
Transition Region And Coronal Explorer (TRACE). More recently, these studies
have been extended to explain the attenuation of propagating coronal kink waves
observed by the Coronal Multi-Channel Polarimeter (CoMP). In the present study,
for the first time we investigate the properties of propagating kink waves in
solar waveguides including the effects of both longitudinal and transverse
plasma inhomogeneity. Importantly, it is found that the wavelength is only
dependent on the longitudinal stratification and the amplitude is simply a
product of the two effects. In light of these results the advancement of solar
atmospheric magnetoseismology by exploiting high spatial/temporal resolution
observations of propagating kink waves in magnetic waveguides to determine the
length scales of the plasma inhomogeneity along and transverse to the direction
of the magnetic field is discussed.
|
1105.0067v1
|
2011-05-05
|
The effect of twisted magnetic field on the resonant absorption of MHD waves in coronal loops
|
The standing quasi modes in a cylindrical incompressible flux tube with
magnetic twist that undergoes a radial density structuring is considered in
ideal magnetohydrodynamics (MHD). The radial structuring is assumed to be a
linearly varying density profile. Using the relevant connection formulae, the
dispersion relation for the MHD waves is derived and solved numerically to
obtain both the frequencies and damping rates of the fundamental and
first-overtone modes of both the kink (m=1) and fluting (m=2,3) waves. It was
found that a magnetic twist will increase the frequencies, damping rates and
the ratio of the oscillation frequency to the damping rate of these modes. The
period ratio P_1/P_2 of the fundamental and its first-overtone surface waves
for kink (m=1) and fluting (m=2,3) modes is lower than 2 (the value for an
untwisted loop) in the presence of twisted magnetic field. For the kink modes,
particularly, the magnetic twists B_{\phi}/B_z=0.0065 and 0.0255 can achieve
deviations from 2 of the same order of magnitude as in the observations.
Furthermore, for the fundamental kink body waves, the frequency bandwidth
increases with increasing the magnetic twist.
|
1105.1120v1
|
2011-05-05
|
Interpreting Graph Cuts as a Max-Product Algorithm
|
The maximum a posteriori (MAP) configuration of binary variable models with
submodular graph-structured energy functions can be found efficiently and
exactly by graph cuts. Max-product belief propagation (MP) has been shown to be
suboptimal on this class of energy functions by a canonical counterexample
where MP converges to a suboptimal fixed point (Kulesza & Pereira, 2008).
In this work, we show that under a particular scheduling and damping scheme,
MP is equivalent to graph cuts, and thus optimal. We explain the apparent
contradiction by showing that with proper scheduling and damping, MP always
converges to an optimal fixed point. Thus, the canonical counterexample only
shows the suboptimality of MP with a particular suboptimal choice of schedule
and damping. With proper choices, MP is optimal.
|
1105.1178v1
|
2011-05-14
|
Crossovers in the non-Markovian dynamics of two-qubit entanglements
|
We study the entanglement dynamics of two non-interacting, spatially
separated qubits subject to local environment noises. Based on exactly solvable
models for non-Markovian amplitude damping and phase damping noises, we are
able to analyze the entanglement dynamics of the two qubits for different
coupling bandwidths and different detunings. We show that entanglement
oscillations can occur for both amplitude and phase damping noises. Moreover,
we demonstrate that changing the coupling bandwidth can lead to crossover
between dissipative and non-dissipative entanglement dynamics, while varying
the detuning controls the crossover between strong and weak coupling limits.
Our findings can help provide a synthesized picture for the entanglement
dynamics of two qubits subject to local environment noises.
|
1105.2859v2
|
2011-06-22
|
Tunable Magnonic Frequency and Damping in [Co/Pd]8 Multilayers with Variable Co Layer Thickness
|
We report the experimental observation of collective picosecond magnetization
dynamics in [Co/Pd]8 multilayers with perpendicular magnetic anisotropy. The
precession frequency shows large and systematic variation from about 5 GHz to
about 90 GHz with the decrease in the Co layer thickness from 1.0 nm to 0.22 nm
due to the linear increase in the perpendicular magnetic anisotropy. The
damping coefficient 'alpha' is found to be inversely proportional to the Co
layer thickness and a linear relation between the perpendicular magnetic
anisotropy and 'alpha' is established. We discuss the possible reasons behind
the enhanced damping as the d-d hybridization at the interface and spin
pumping. These observations are significant for the applications of these
materials in spintronics and magnonic crystals.
|
1106.4491v1
|
2011-07-04
|
An HI column density threshold for cold gas formation in the Galaxy
|
We report the discovery of a threshold in the HI column density of Galactic
gas clouds below which the formation of the cold phase of HI is inhibited. This
threshold is at $N_{HI} = 2 \times 10^{20}$ per cm$^{2}$; sightlines with lower
HI column densities have high spin temperatures (median $T_s \sim 1800$ K),
indicating low fractions of the cold neutral medium (CNM), while sightlines
with $N_{HI} \ge 2 \times 10^{20}$ per cm$^{2}$ have low spin temperatures
(median $T_s \sim 240$ K), implying high CNM fractions. The threshold for CNM
formation is likely to arise due to inefficient self-shielding against
ultraviolet photons at lower HI column densities. The threshold is similar to
the defining column density of a damped Lyman-$\alpha$ absorber; this indicates
a physical difference between damped and sub-damped Lyman-$\alpha$ systems,
with the latter class of absorbers containing predominantly warm gas.
|
1107.0744v2
|
2011-07-11
|
One-dimensional vertical dust strings in a glass box
|
The oscillation spectrum of a one-dimensional vertical dust string formed
inside a glass box on top of the lower electrode in a GEC reference cell was
studied. A mechanism for creating a single vertical dust string is described.
It is shown that the oscillation amplitudes, resonance frequencies, damping
coefficients, and oscillation phases of the dust particles separate into two
distinct groups. One group exhibits low damping coefficients, increasing
amplitudes and decreasing resonance frequencies for dust particles closer to
the lower electrode. The other group shows high damping coefficients but
anomalous resonance frequencies and amplitudes. At low oscillation frequencies,
the two groups are also separated by a {\pi}-phase difference. One possible
cause for the difference in behavior between the two groups is discussed.
|
1107.2074v1
|
2011-08-22
|
On conditions for asymptotic stability of dissipative infinite-dimensional systems with intermittent damping
|
We study the asymptotic stability of a dissipative evolution in a Hilbert
space subject to intermittent damping. We observe that, even if the
intermittence satisfies a persistent excitation condition, if the Hilbert space
is infinite-dimensional then the system needs not being asymptotically stable
(not even in the weak sense). Exponential stability is recovered under a
generalized observability inequality, allowing for time-domains that are not
intervals. Weak asymptotic stability is obtained under a similarly generalized
unique continuation principle. Finally, strong asymptotic stability is proved
for intermittences that do not necessarily satisfy some persistent excitation
condition, evaluating their total contribution to the decay of the trajectories
of the damped system. Our results are discussed using the example of the wave
equation, Schr\"odinger's equation and, for strong stability, also the special
case of finite-dimensional systems.
|
1108.4327v2
|
2011-08-26
|
Aligned Major Axes in a Planetary System without Tidal Evolution: The 61 Virginis example
|
Tidal damping of one of the orbits in a planetary system can lead to aligned
major-axes (the so-called "fixed-point" condition), but currently aligned major
axes do not necessarily imply such a history. An example is the nominal orbital
solution for the 61 Virginis system where two orbits librate about alignment,
but evaluation of the eigenmodes of the secular theory shows it could not be
the result of tidal damping but rather of initial conditions. Nevertheless, the
amplitudes of the eigenmodes suggest that this system may have undergone some
degree of tidal damping.
|
1108.5369v1
|
2011-09-09
|
Optimal linear optical implementation of a single-qubit damping channel
|
We experimentally demonstrate a single-qubit decohering quantum channel using
linear optics. We implement the channel, whose special cases include both the
amplitude-damping channel and the bit-flip channel, using a single, static
optical setup. Following a recent theoretical result [M. Piani et al., Phys.
Rev. A, 84, 032304 (2011)], we realize the channel in an optimal way,
maximizing the probability of success, i.e., the probability for the photonic
qubit to remain in its encoding. Using a two-photon entangled resource, we
characterize the channel using ancilla-assisted process tomography and find
average process fidelities of 0.9808 \pm 0.0002 and 0.9762 \pm 0.0002 for
amplitude-damping and the bit-flip case, respectively.
|
1109.2070v1
|
2011-11-20
|
Detection of picosecond magnetization dynamics of 50 nm magnetic dots down to the single dot regime
|
We report an all-optical time-domain detection of picosecond magnetization
dynamics of arrays of 50 nm Ni80Fe20 (permalloy) dots down to the single
nanodot regime. In the single nanodot regime the dynamics reveals one dominant
resonant mode corresponding to the edge mode of the 50 nm dot with slightly
higher damping than that of the unpatterned thin film. With the increase in
areal density of the array both the precession frequency and damping increases
significantly due to the increase in magnetostatic interactions between the
nanodots and a mode splitting and sudden jump in apparent damping are observed
at an edge-to-edge separation of 50 nm.
|
1111.4625v1
|
2012-01-09
|
Universal response of optimal granular damping devices
|
Granular damping devices constitute an emerging technology for the
attenuation of vibrations based on the dissipative nature of particle
collisions. We show that the performance of such devices is independent of the
material properties of the particles for working conditions where damping is
optimal. Even the suppression of a dissipation mode (collisional or frictional)
is unable to alter the response. We explain this phenomenon in terms of the
inelastic collapse of granular materials. These findings provide a crucial
standpoint for the design of such devices in order to achieve the desired low
maintenance feature that makes particle dampers particularly suitable to harsh
environments.
|
1201.1866v2
|
2012-01-09
|
Radiative energy loss reduction in an absorptive plasma
|
The influence of the damping of radiation on the radiative energy loss
spectrum of a relativistic charge in an infinite, absorptive plasma is studied.
We find increasing reduction of the spectrum with increasing damping. Our
studies, which represent an Abelian approximation for the colour charge
dynamics in the quark-gluon plasma, may influence the analysis of jet quenching
phenomena observed in high-energy nuclear collisions. Here, we focus on a
formal discussion of the limiting behaviour with increasing radiation
frequency. In an absorptive (and polarizable) medium, this is determined by the
behaviour of the exponential damping factor entering the spectrum and the
formation time of radiation.
|
1201.1890v1
|
2012-01-10
|
Nonequilibrium Damping of Collective Motion of Homogeneous Cold Fermi Condensates with Feshbach Resonances
|
Collisionless damping of a condensate of cold Fermi atoms, whose scattering
is controlled by a Feshbach resonance, is explored throughout the BCS and BEC
regimes when small perturbations on its phase and amplitude modes are turned on
to drive the system slightly out of equilibrium. Using a one-loop effective
action, we first recreate the known result that for a broad resonance the
amplitude of the condensate decays as $t^{-1/2}$ at late times in the BCS
regime whereas it decays as $t^{-3/2}$ in the BEC regime. We then examine the
case of an idealized narrow resonance, and find that this collective mode
decays as $t^{-3/2}$ throughout both the BCS and BEC regimes. Although this
seems to contradict earlier results that damping is identical for both broad
and narrow resonances, the breakdown of the narrow resonance limit restores
this universal behaviour. More measureably, the phase perturbation may give a
shift on the saturated value to which the collective amplitude mode decays,
which vanishes only in the deep BCS regime when the phase and amplitude modes
are decoupled.
|
1201.2019v1
|
2012-01-30
|
Modeling electricity spot prices using mean-reverting multifractal processes
|
We discuss stochastic modeling of volatility persistence and
anti-correlations in electricity spot prices, and for this purpose we present
two mean-reverting versions of the multifractal random walk (MRW). In the first
model the anti-correlations are modeled in the same way as in an
Ornstein-Uhlenbeck process, i.e. via a drift (damping) term, and in the second
model the anti-correlations are included by letting the innovations in the MRW
model be fractional Gaussian noise with H < 1/2. For both models we present
approximate maximum likelihood methods, and we apply these methods to estimate
the parameters for the spot prices in the Nordic electricity market. The
maximum likelihood estimates show that electricity spot prices are
characterized by scaling exponents that are significantly different from the
corresponding exponents in stock markets, confirming the exceptional nature of
the electricity market. In order to compare the damped MRW model with the
fractional MRW model we use ensemble simulations and wavelet-based variograms,
and we observe that certain features of the spot prices are better described by
the damped MRW model. The characteristic correlation time is estimated to
approximately half a year.
|
1201.6137v1
|
2012-05-14
|
Critical viscoelastic response in jammed solids
|
We determine the linear viscoelastic response of jammed packings of athermal
repulsive viscous spheres, a model for emulsions, wet foams, and soft colloidal
suspensions. We numerically measure the complex shear modulus, a fundamental
characterization of the response, and demonstrate that low frequency response
displays dynamic critical scaling near unjamming. Viscoelastic shear response
is governed by the relaxational eigenmodes of a packing. We use scaling
arguments to explain the distribution of eigenrates, which develops a
divergence at unjamming. We then derive the critical exponents characterizing
response, including a vanishing shear modulus, diverging viscosity, and
critical shear thinning regime. Finally, we demonstrate that macroscopic
rheology is sensitive to details of the local viscous force law. By varying the
ratio of normal and tangential damping coefficients, we identify and explain a
qualitative difference between systems with strong and weak damping of sliding
motion. When sliding is weakly damped there is no diverging time scale, no
diverging viscosity, and no critical shear thinning regime.
|
1205.2960v1
|
2012-06-11
|
Testing the 130 GeV gamma-ray line with high energy resolution detectors
|
Recently some hints of the existence of $\gamma$-ray line around 130 GeV are
reported according to the analysis of Fermi-LAT data. If confirmed it would be
the first direct evidence to show the existence of new physics beyond the
standard model. Here we suggest that using the forthcoming high energy
resolution $\gamma$-ray detectors, such as CALET and DAMPE, we may test whether
it is real line structure or just the background effect. For DAMPE like
detector with designed energy resolution $\sim1.5%$, a line significance will
reach $11\sigma$ for the same statistics as Fermi-LAT. For about 1.4 yr survey
observation, DAMPE may detect a $5\sigma$ signal of such a $\gamma$-ray line.
|
1206.2241v2
|
2012-06-14
|
Finite-temperature dynamics of matter-wave dark solitons in linear and periodic potentials: an example of an anti-damped Josephson junction
|
We study matter-wave dark solitons in atomic Bose-Einstein condensates at
finite temperatures, under the effect of linear and periodic potentials. Our
model, namely a dissipative Gross-Pitaevskii equation, is treated analytically
by means of dark soliton perturbation theory, which results in a Newtonian
equation of motion for the dark soliton center. This reduced model, which
incorporates an effective washboard potential and an anti-damping term,
constitutes an example of an anti-damped Josephson junction. We present a
qualitative (local and global) analysis of the equation of motion. For
sufficiently small wavenumbers of the periodic potential and weak linear
potentials, the results are found to be in good agreement with pertinent ones
obtained via a Bogoliubov-de Gennes analysis and direct numerical simulations.
|
1206.2993v1
|
2012-06-15
|
Damping of giant dipole resonance in hot rotating nuclei
|
The phonon damping model (PDM) is extended to include the effect of angular
momentum at finite temperature. The model is applied to the study of damping of
giant dipole resonance (GDR) in hot and noncollectively rotating spherical
nuclei. The numerical results obtained for Mo88 and Sn106 show that the GDR
width increases with both temperature T and angular momentum M. At T > 4 MeV
and M<= 60 hbar the increase in the GDR width slows down for Sn106, whereas at
M<= 80 hbar the GDR widths in both nuclei nearly saturate. By adopting the
nuclear shear viscosity extracted from fission data at T= 0, it is shown that
the maximal value of the angular momentum for Mo88 and Sn106 should be around
46 and 55 hbar, respectively, so that the universal conjecture for the lower
bound of the specific shear viscosity for all fluids is not violated up to T= 5
MeV.
|
1206.3361v1
|
2012-06-18
|
Sampled-data design for robust control of a single qubit
|
This paper presents a sampled-data approach for the robust control of a
single qubit (quantum bit). The required robustness is defined using a sliding
mode domain and the control law is designed offline and then utilized online
with a single qubit having bounded uncertainties. Two classes of uncertainties
are considered involving the system Hamiltonian and the coupling strength of
the system-environment interaction. Four cases are analyzed in detail including
without decoherence, with amplitude damping decoherence, phase damping
decoherence and depolarizing decoherence. Sampling periods are specifically
designed for these cases to guarantee the required robustness. Two sufficient
conditions are presented for guiding the design of unitary control for the
cases without decoherence and with amplitude damping decoherence. The proposed
approach has potential applications in quantum error-correction and in
constructing robust quantum gates.
|
1206.3897v2
|
2012-06-25
|
Trap anharmonicity and sloshing mode of a Fermi gas
|
For a gas trapped in a harmonic potential, the sloshing (or Kohn) mode is
undamped and its frequency coincides with the trap frequency, independently of
the statistics, interaction and temperature of the gas. However, experimental
trap potentials have usually Gaussian shape and anharmonicity effects appear as
the temperature and, in the case of Fermions, the filling of the trap are
increased. We study the sloshing mode of a degenerate Fermi gas in an
anharmonic trap within the Boltzmann equation, including in-medium effects in
both the transport and collision terms. The calculated frequency shifts and
damping rates of the sloshing mode due to the trap anharmonicity are in
satisfactory agreement with the available experimental data. We also discuss
higher-order dipole, octupole, and bending modes and show that the damping of
the sloshing mode is caused by its coupling to these modes.
|
1206.5688v2
|
2012-09-06
|
Radiative energy loss in the absorptive QGP: taming the long formation lengths in coherent emission
|
In an absorptive plasma, damping of radiation mechanisms can influence the
bremsstrahlung formation in case of large radiation formation lengths. We study
qualitatively the influence of this effect on the gluon bremsstrahlung spectrum
off heavy quarks in the quark-gluon plasma. Independent of the heavy-quark
mass, the spectrum is found to be strongly suppressed in an intermediate gluon
energy region which grows with increasing gluon damping rate and increasing
energy of the heavy quark. Thus, just as polarization effects in the plasma
render the bremsstrahlung spectra independent of the quark mass in the soft
gluon regime, damping effects tend to have a similar impact for larger gluon
energies.
|
1209.1149v1
|
2012-09-17
|
Power spectra in the eikonal approximation with adiabatic and non-adiabatic modes
|
We use the so-called eikonal approximation, recently introduced in the
context of cosmological perturbation theory, to compute power spectra for
multi-component fluids. We demonstrate that, at any given order in standard
perturbation theory, multipoint power spectra do not depend on the large-scale
adiabatic modes. Moreover, we employ perturbation theories to decipher how
nonadiabatic modes, such as a relative velocity between two different
components, damp the small-scale matter power spectrum, a mechanism recently
described in the literature. In particular, we do an explicit calculation at
1-loop order of this effect. While the 1-loop result eventually breaks down, we
show how the damping effect can be fully captured by the help of the eikonal
approximation. A relative velocity not only induces mode damping but also
creates large-scale anisotropic modulations of the matter power spectrum
amplitude. We illustrate this for the Local Group environment.
|
1209.3662v2
|
2012-09-26
|
Linear response theory for hydrodynamic and kinetic equations with long-range interactions
|
We apply the linear response theory to systems with long-range interactions
described by hydrodynamic equations such as the Euler, Smoluchowski, and damped
Euler equations. We analytically determine the response of the system submitted
to a pulse and to a step function. We compare these results with those obtained
for collisionless systems described by the Vlasov equation. We show that, in
the linear regime, the evolution of a collisionless system (Vlasov) with the
waterbag distribution is the same as the evolution of a collision-dominated gas
without dissipation (Euler). In this analogy, the maximum velocity of the
waterbag distribution plays the role of the velocity of sound in the
corresponding barotropic gas. When submitted to a step function, these systems
exhibit permanent oscillations. Other distributions exhibit Landau damping and
relax towards a steady state. We illustrate this behaviour with the Cauchy
distribution which can be studied analytically. We apply our results to the HMF
model and obtain a generalized Curie-Weiss law for the magnetic susceptibility.
Finally, we compare the linear response theory to the initial value problem for
the linearized Vlasov equation and report a case of algebraic damping of the
initial perturbation.
|
1209.5987v1
|
2012-09-15
|
Phase mixing of propagating Alfven waves in a stratified atmosphere: Solar spicules
|
Alfvenic waves are thought to play an important role in coronal heating and
solar wind acceleration. Recent observations by Hinode/SOT showed that the
spicules mostly exhibit upward propagating high frequency waves. Here we
investigate the dissipation of such waves due to phase mixing in stratified
environment of solar spicules. Since they are highly dynamic structures with
speeds at about significant fractions of the Alfven phase speed, we take into
account the effects of steady flows. Our numerical simulations show that in the
presence of stratification due to gravity, damping takes place in space than in
time. The exponential damping low, exp(-At^3), is valid under spicule
conditions, however the calculated damping time is much longer than the
reported spicule lifetimes from observations.
|
1210.0485v1
|
2012-10-03
|
Effect of temperature and velocity on superlubricity
|
We study the effects of temperature and sliding velocity on superlubricity in
numerical simulations of the Frenkel-Kontorova model. We show that resonant
excitations of the phonons in an incommensurate sliding body lead to an
effective friction and to thermal equilibrium with energy distributed over the
internal degrees of freedom. For finite temperature, the effective friction can
be described well by a viscous damping force, with a damping coefficient that
emerges naturally from the microscopic dynamics. This damping coefficient is a
non-monotonic function of the sliding velocity which peaks around resonant
velocities and increases with temperature. At low velocities, it remains finite
and nonzero, indicating the preservation of superlubricity in the zero-velocity
limit. Finally, we propose experimental systems in which our results could be
verified.
|
1210.1124v1
|
2012-10-04
|
Basic microscopic plasma physics unified and simplified by N-body classical mechanics
|
Debye shielding, collisional transport, Landau damping of Langmuir waves, and
spontaneous emission of these waves are introduced, in typical plasma physics
textbooks, in different chapters. This paper provides a compact unified
introduction to these phenomena without appealing to fluid or kinetic models,
but by using Newton's second law for a system of $N$ electrons in a periodic
box with a neutralizing ionic background. A rigorous equation is derived for
the electrostatic potential. Its linearization and a first smoothing reveal
this potential to be the sum of the shielded Coulomb potentials of the
individual particles. Smoothing this sum yields the classical Vlasovian
expression including initial conditions in Landau contour calculations of
Langmuir wave growth or damping. The theory is extended to accommodate a
correct description of trapping or chaos due to Langmuir waves. In the linear
regime, the amplitude of such a wave is found to be ruled by Landau growth or
damping and by spontaneous emission. Using the shielded potential, the
collisional diffusion coefficient is computed for the first time by a
convergent expression including the correct calculation of deflections for all
impact parameters. Shielding and collisional transport are found to be two
related aspects of the repulsive deflections of electrons.
|
1210.1546v2
|
2012-10-11
|
Measurement of the damping of nuclear shell effect in the doubly magic $^{208}$Pb region
|
The damping of the nuclear shell effect with excitation energy has been
measured through an analysis of the neutron spectra following the triton
transfer in the $^7$Li induced reaction on $^{205}$Tl. The measured neutron
spectra demonstrate the expected large shell correction energy for the nuclei
in the vicinity of doubly magic $^{208}$Pb and a small value for $^{184}$W. A
quantitative extraction of the allowed values of the damping parameter
$\gamma$, along with those for the asymptotic nuclear level density parameter
$\tilde{a}$, has been made for the first time.
|
1210.3213v2
|
2012-10-16
|
Optimal control of laser plasma instabilities using Spike Trains of Uneven Duration and Delay (STUD pulses) for ICF and IFE
|
An adaptive method of controlling parametric instabilities in laser produced
plasmas is proposed. It involves fast temporal modulation of a laser pulse on
the fastest instability's amplification time scale, adapting to changing and
unknown plasma conditions. These pulses are comprised of on and off sequences
having at least one or two orders of magnitude contrast between them. Such
laser illumination profiles are called STUD pulses for Spike Trains of Uneven
Duration and Delay. The STUD pulse program includes scrambling the speckle
patterns spatially in between the laser spikes. The off times allow damping of
driven waves. The scrambling of the hot spots allows tens of damping times to
elapse before hot spot locations experience recurring high intensity spikes.
Damping in the meantime will have healed the scars of past growth. Another
unique feature of STUD pulses on crossing beams is that their temporal profiles
can be interlaced or staggered, and their interactions thus controlled with an
on-off switch and a dimmer.
|
1210.4462v1
|
2012-10-28
|
Mass Dependence of Instabilities of an Oscillator with Multiplicative and Additive Noise
|
We study the instabilities of a harmonic oscillator subject to additive and
dichotomous multiplicative noise, focussing on the dependance of the
instability threshold on the mass. For multiplicative noise in the damping, the
instability threshold is crossed as the mass is decreased, as long as the
smaller damping is in fact negative. For multiplicative noise in the stiffness,
the situation is more complicated and in fact the transition is reentrant for
intermediate noise strength and damping. For multiplicative noise in the mass,
the results depend on the implementation of the noise. One can take the
velocity or the momentum to be conserved as the mass is changed. In these cases
increasing the mass destabilizes the system. Alternatively, if the change in
mass is caused by the accretion/loss of particles to the Brownian particle,
these processes are asymmetric with momentum conserved upon accretion and
velocity upon loss. In this case, there is no instability, as opposed to the
other two implementations. We also study the distribution of the energy,
finding a power-law cutoff at a value which increases with time.
|
1210.7433v1
|
2012-10-30
|
Extending the Concept of Analog Butterworth Filter for Fractional Order Systems
|
This paper proposes the design of Fractional Order (FO) Butterworth filter in
complex w-plane (w=sq; q being any real number) considering the presence of
under-damped, hyper-damped, ultra-damped poles. This is the first attempt to
design such fractional Butterworth filters in complex w-plane instead of
complex s-plane, as conventionally done for integer order filters. Firstly, the
concept of fractional derivatives and w-plane stability of linear fractional
order systems are discussed. Detailed mathematical formulation for the design
of fractional Butterworth-like filter (FBWF) in w-plane is then presented.
Simulation examples are given along with a practical example to design the FO
Butterworth filter with given specifications in frequency domain to show the
practicability of the proposed formulation.
|
1210.8194v3
|
2012-11-15
|
Spin transport and tunable Gilbert damping in a single-molecule magnet junction
|
We study time-dependent electronic and spin transport through an electronic
level connected to two leads and coupled with a single-molecule magnet via
exchange interaction. The molecular spin is treated as a classical variable and
precesses around an external magnetic field. We derive expressions for charge
and spin currents by means of the Keldysh non-equilibrium Green's functions
technique in linear order with respect to the time-dependent magnetic field
created by this precession. The coupling between the electronic spins and the
magnetization dynamics of the molecule creates inelastic tunneling processes
which contribute to the spin currents. The inelastic spin currents, in turn,
generate a spin-transfer torque acting on the molecular spin. This back-action
includes a contribution to the Gilbert damping and a modification of the
precession frequency. The Gilbert damping coefficient can be controlled by the
bias and gate voltages or via the external magnetic field and has a
non-monotonic dependence on the tunneling rates.
|
1211.3611v2
|
2012-11-24
|
Effects of Quantum Error Correction on Entanglement Sudden Death
|
We investigate the effects of error correction on non-local quantum coherence
as a function of time, extending the study by Sainz and Bj\"ork. We consider
error correction of amplitude damping, pure phase damping and combinations of
amplitude and phase damping as they affect both fidelity and quantum
entanglement. Initial two-qubit entanglement is encoded in arbitrary real
superpositions of both \Phi-type and \Psi-type Bell states. Our main focus is
on the possibility of delay or prevention of ESD (early stage decoherence, or
entanglement sudden death). We obtain the onset times for ESD as a function of
the state-superposition mixing angle. Error correction affects entanglement and
fidelity differently, and we exhibit initial entangled states for which error
correction increases fidelity but decreases entanglement, and vice versa.
|
1211.5654v2
|
2012-12-04
|
Polarization dependence of phonon influences in exciton-biexciton quantum dot systems
|
We report on a strong dependence of the phonon-induced damping of Rabi
dynamics in an optically driven exciton-biexciton quantum dot system on the
polarization of the exciting pulse. While for a fixed pulse intensity the
damping is maximal for linearly polarized excitation, it decreases with
increasing ellipticity of the polarization. This finding is most remarkable
considering that the carrier-phonon coupling is spin-independent. In addition
to simulations based on a numerically exact real-time path integral approach,
we present an analysis within a weak coupling theory that allows for analytical
expressions for the pertinent damping rates. We demonstrate that an efficient
coupling to the biexciton state is of central importance for the reported
polarization dependencies. Further, we discuss influences of various system
parameters and show that for finite biexciton binding energies Rabi scenarios
differ qualitatively from the widely studied two-level dynamics.
|
1212.0642v1
|
2012-12-10
|
Heat-induced damping modification in YIG/Pt hetero-structures
|
We experimentally demonstrate the manipulation of magnetization relaxation
utilizing a temperature difference across the thickness of an yttrium iron
garnet/platinum (YIG/Pt) hetero-structure: the damping is either increased or
decreased depending on the sign of the temperature gradient. This effect might
be explained by a thermally-induced spin torque on the magnetization
precession. The heat-induced variation of the damping is detected by microwave
techniques as well as by a DC voltage caused by spin pumping into the adjacent
Pt layer and the subsequent conversion into a charge current by the inverse
spin Hall effect.
|
1212.2073v1
|
2012-12-13
|
Surface plasmon polaritons in a semi-bounded degenerate plasma: role of spatial dispersion and collisions
|
Surface plasmon polaritons (SPPs) in a semi-bounded degenerate plasma (e.g.,
a metal) are studied using the quasiclassical mean-field kinetic model, taking
into account the spatial dispersion of the plasma (due to quantum degeneracy of
electrons) and electron-ion (electron-lattice, for metals) collisions. SPP
dispersion and damping are obtained in both retarded ($\omega/k_z\sim c$) and
non-retarded ($\omega/k_z\ll c$) regions, as well as in between. It is shown
that the plasma spatial dispersion significantly affects the properties of
SPPs, especially at short wavelengths (less than the collisionless skin depth,
$\lambda\lesssim c/\omega_{pe}$). Namely, the collisionless (Landau) damping of
SPPs (due to spatial dispersion) is comparable to the purely collisional
(Ohmic) damping (due to electron-lattice collisions) in a wide range of SPP
wavelengths, e.g., from $\lambda\sim20$ nm to $\lambda\sim0.8$ nm for SPP in
gold at T=293 K, and from $\lambda\sim400$ nm to $\lambda\sim0.7$ nm for SPPs
in gold at T=100 K. The spatial dispersion is also shown to affect, in a
qualitative way, the dispersion of SPPs at short wavelengths $\lambda\lesssim
c/\omega_{pe}$.
|
1212.3040v1
|
2012-12-13
|
Branching of quasinormal modes for nearly extremal Kerr black holes
|
We show that nearly extremal Kerr black holes have two distinct sets of
quasinormal modes, which we call zero-damping modes (ZDMs) and damped modes
(DMs). The ZDMs exist for all harmonic indices $l$ and $m \ge 0$, and their
frequencies cluster onto the real axis in the extremal limit. The DMs have
nonzero damping for all black hole spins; they exist for all counterrotating
modes ($m<0$) and for corotating modes with $0\leq \mu\lesssim \mu_c=0.74$ (in
the eikonal limit), where $\mu\equiv m/(l+1/2)$. When the two families coexist,
ZDMs and DMs merge to form a single set of quasinormal modes as the black hole
spin decreases. Using the effective potential for perturbations of the Kerr
spacetime, we give intuitive explanations for the absence of DMs in certain
areas of the spectrum and for the branching of the spectrum into ZDMs and DMs
at large spins.
|
1212.3271v1
|
2012-12-31
|
Effects of lateral device size and material properties on the ferromagnetic resonance response of spinwave eigen-modes in magnetic devices
|
We analyze the effects of lateral device size and magnetic material
parameters on the ferromagnetic resonance (FMR) response. Results presented are
directly relevant to widely used FMR experimental techniques for extracting
magnetic parameters from thin films, the results of which are often assumed to
carry over to corresponding nanometer-sized patterned devices. We show that
there can be significant variation in the FMR response with device size, and
that the extent of the variation depends on the magnetic material properties.
This explains, for example, why different experiments along these lines have
yielded different size-dependent trends from damping measurements. Observed
trends with increasing size and different material parameters are explained
through the evolution of three distinct eigen-modes, demonstrating the
respective roles of demagnetization and exchange. It is also shown that there
is a crossover of dominant eigen-modes in the response signal, accompanied by
conjugating edge-type modes, leading to evident effects in measured linewidth
and damping. Among the sizes considered, in higher saturation magnetization, we
observe as much as a 40% increase in apparent damping, due solely to device
size variation.
|
1212.6835v1
|
2013-01-16
|
Gain-tunable optomechanical cooling in a laser cavity
|
We study the optical cooling of the resonator mirror in a
cavity-optomechanical system that contains an optical gain medium. We find that
the optical damping rate is vanishingly small for an incoherently pumped laser
above threshold. In the presence of an external coherent drive however, the
optical damping rate can be enhanced substantially with respect to that of a
passive cavity. We show that the strength of the incoherent pump provides a
conduit to tune the damping rate and the minimum attainable phonon number with
the same radiation pressure force, and the latter can be lowered from that of a
passive cavity if the thermal contribution is nonnegligible. We also show that
the system can undergo a transition from the weak optomechanical coupling
regime to the strong optomechanical coupling regime as the incoherent pump
strength is varied.
|
1301.3762v2
|
2013-01-18
|
Adiabatic stability under semi-strong interactions: The weakly damped regime
|
We rigorously derive multi-pulse interaction laws for the semi-strong
interactions in a family of singularly-perturbed and weakly-damped
reaction-diffusion systems in one space dimension. Most significantly, we show
the existence of a manifold of quasi-steady N-pulse solutions and identify a
"normal-hyperbolicity" condition which balances the asymptotic weakness of the
linear damping against the algebraic evolution rate of the multi-pulses. Our
main result is the adiabatic stability of the manifolds subject to this normal
hyperbolicity condition. More specifically, the spectrum of the linearization
about a fixed N-pulse configuration contains essential spectrum that is
asymptotically close to the origin as well as semi-strong eigenvalues which
move at leading order as the pulse positions evolve. We characterize the
semi-strong eigenvalues in terms of the spectrum of an explicit N by N matrix,
and rigorously bound the error between the N-pulse manifold and the evolution
of the full system, in a polynomially weighted space, so long as the
semi-strong spectrum remains strictly in the left-half complex plane, and the
essential spectrum is not too close to the origin.
|
1301.4466v1
|
2013-01-24
|
Spin transport parameters in metallic multilayers determined by ferromagnetic resonance measurements of spin pumping
|
We measured spin transport in nonferromagnetic (NM) metallic multilayers from
the contribution to damping due to spin pumping from a ferromagnetic Co90Fe10
thin film. The multilayer stack consisted of NM1/NM2/Co90Fe10(2 nm)/NM2/NM3
with varying NM materials and thicknesses. Using conventional theory for one
dimensional diffusive spin transport in metals, we show that the effective
damping due to spin pumping can be strongly affected by the spin transport
properties of each NM in the multilayer, which permits the use of damping
measurements to accurately determine the spin transport properties of the
various NM layers in the full five-layer stack. We find that due to its high
electrical resistivity, amorphous Ta is a poor spin conductor, in spite of a
short spin-diffusion length of 1.0 nm, and that Pt is an excellent spin
conductor by virtue of its low electrical resistivity and a spin diffusion
length of only 0.5 nm. Spin Hall effect measurements may have underestimated
the spin Hall angle in Pt by assuming a much longer spin diffusion length.
|
1301.5861v1
|
2013-02-11
|
Low-damping epsilon-near-zero slabs: nonlinear and nonlocal optical properties
|
We investigate second harmonic generation, low-threshold multistability,
all-optical switching, and inherently nonlocal effects due to the free-electron
gas pressure in an epsilon-near-zero (ENZ) metamaterial slab made of
cylindrical, plasmonic nanoshells illuminated by TM-polarized light. Damping
compensation in the ENZ frequency region, achieved by using gain medium inside
the shells' dielectric cores, enhances the nonlinear properties. Reflection is
inhibited and the electric field component normal to the slab interface is
enhanced near the effective pseudo-Brewster angle, where the effective
\epsilon-near-zero condition triggers a non-resonant, impedance-matching
phenomenon. We show that the slab displays a strong effective, spatial
nonlocality associated with leaky modes that are mediated by the compensation
of damping. The presence of these leaky modes then induces further spectral and
angular conditions where the local fields are enhanced, thus opening new
windows of opportunity for the enhancement of nonlinear optical processes.
|
1302.2392v1
|
2013-02-14
|
A Matlab toolbox for fractional relaxation-oscillation equations
|
Stress relaxation and oscillation damping of complex viscoelastic media often
manifest history- and path-dependent physical behaviors and cannot accurately
be described by the classical models. Recent research found that fractional
derivative models can characterize such complex relaxation and damping.
However, to our best knowledge, easy-to-use numerical software is not available
for fractional relaxation-oscillation (FRO) equations. This paper is to
introduce an open source free Matlab toolbox which we developed in recent years
for numerical solution of the FRO equations. This FRO toolbox uses the
predictor-corrector approach for the discretization of time fractional
derivative, and non-expert users can accurately solve fractional
relaxation-oscillation equations via a friendly graphical user interface.
Compared with experimental data, our numerical experiments show that the FRO
toolbox is highly efficient and accurate to simulate viscoelastic stress
relaxation and damped vibration. This free toolbox will help promote the
research and practical use of fractional relaxation-oscillation equations.
|
1302.3384v1
|
2013-03-11
|
The Analysis of Long-Term Frequency and Damping Wandering in Buildings Using the Random Decrement Technique
|
The characterization and monitoring of buildings is an issue that has
attracted the interest of many sectors over the last two decades. With the
increasing use of permanent, continuous and real-time networks, ambient
vibrations can provide a simple tool for the identification of dynamic building
parameters. This study is focused on the long-term variation of frequency and
damping in several buildings, using the Random Decrement Technique (RDT). RDT
provides a fast, robust and accurate long-term analysis and improves the
reliability of frequency and damping measurements for structural health
monitoring. This reveals particularly useful information in finding out
precisely how far changes in modal parameters can be related to changes in
physical properties. This paper highlights the reversible changes of the
structure's dynamic parameters, correlated with external forces, such as
temperature and exposure to the sun. Contrasting behaviors are observed,
including correlation and anti-correlation with temperature variations.
|
1303.2642v1
|
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