publicationDate
stringlengths 1
2.79k
| title
stringlengths 1
36.5k
⌀ | abstract
stringlengths 1
37.3k
⌀ | id
stringlengths 9
47
|
|---|---|---|---|
2024-02-02
|
Controllable frequency tunability and parabolic-like threshold current behavior in spin Hall nano-oscillators
|
We investigate the individual impacts of critical magnetodynamical
parameters-effective magnetization and magnetic damping-on the auto-oscillation
characteristics of nano-constriction-based Spin Hall Nano-Oscillators (SHNOs).
Our micromagnetic simulations unveil a distinctive non-monotonic relationship
between current and auto-oscillation frequency in out-of-plane magnetic fields.
The influence of effective magnetization on frequency tunability varies with
out-of-plane field strengths. At large out-of-plane fields, the frequency
tunability is predominantly governed by effective magnetization, achieving a
current tunability of 1 GHz/mA-four times larger than that observed at the
lowest effective magnetization. Conversely, at low out-of-plane fields,
although a remarkably high-frequency tunability of 4 GHz/mA is observed, the
effective magnetization alters the onset of the transition from a linear-like
mode to a spin-wave bullet mode. Magnetic damping primarily affects the
threshold current with negligible impact on auto-oscillation frequency
tunability. The threshold current scales linearly with increased magnetic
damping at a constant out-of-plane field but exhibits a parabolic behavior with
variations in out-of-plane fields. This behavior is attributed to the
qualitatively distinct evolution of the auto-oscillation mode across different
out-of-plane field values. Our study not only extends the versatility of SHNOs
for oscillator-based neuromorphic computing with controllable frequency
tunability but also unveils the intricate auto-oscillation dynamics in
out-of-plane fields.
|
2402.01570v1
|
2024-03-18
|
Radiative loss and ion-neutral collisional effects in astrophysical plasmas
|
In this paper we study the role of radiative cooling in a two-fluid model
consisting of coupled neutrals and charged particles. We first analyze the
linearized two-fluid equations where we include radiative losses in the energy
equation for the charged particles. In a 1D geometry for parallel propagation
and in the limiting cases of weak and strong coupling, it can be shown
analytically that the instability conditions for the thermal mode and the sound
waves, the isobaric and isentropic criteria, respectively, remain unchanged
with respect to one-fluid radiative plasmas. For the parameters considered in
this paper, representative for the solar corona, the radiative cooling produces
growth of the thermal mode and damping of the sound waves. When neutrals are
included and are sufficiently coupled to the charges, the thermal mode growth
rate and the wave damping both reduce by the same factor, which depends on the
ionization fraction only. For a heating function which is constant in time, we
find that the growth of the thermal mode and the damping of the sound waves are
slightly larger. The numerical calculation of the eigenvalues of the general
system of equations in a 3D geometry confirm the analytic results. We then run
2D fully nonlinear simulations which give consistent results: a higher
ionization fraction or lower coupling will increase the growth rate. The
magnetic field contribution is negligible in the linear phase.
Ionization-recombination effects might play an important role because the
radiative cooling produces a large range of temperatures in the system. In the
numerical simulation, after the first condensation phase, when the minimum
temperature is reached, the fraction of neutrals increases four orders of
magnitude because of the recombination.
|
2403.11900v1
|
2001-01-12
|
Spatial gradients in the cosmological constant
|
It is possible that there may be differences in the fundamental physical
parameters from one side of the observed universe to the other. I show that the
cosmological constant is likely to be the most sensitive of the physical
parameters to possible spatial variation, because a small variation in any of
the other parameters produces a huge variation of the cosmological constant. It
therefore provides a very powerful {\em indirect} evidence against spatial
gradients or temporal variation in the other fundamental physical parameters,
at least 40 orders of magnitude more powerful than direct experimental
constraints. Moreover, a gradient may potentially appear in theories where the
variability of the cosmological constant is connected to an anthropic selection
mechanism, invoked to explain the smallness of this parameter. In the Hubble
damping mechanism for anthropic selection, I calculate the possible gradient.
While this mechanism demonstrates the existence of this effect, it is too small
to be seen experimentally, except possibly if inflation happens around the
Planck scale.
|
0101130v1
|
1999-01-22
|
Longitudinal wavevector- and frequency-dependent dielectric constant of the TIP4P water model
|
A computer adapted theory for self-consistent calculations of the wavevector-
and frequency-dependent dielectric constant for interaction site models of
polar systems is proposed. A longitudinal component of the dielectric constant
is evaluated for the TIP4P water model in a very wide scale of wavenumbers and
frequencies using molecular dynamics simulations. It is shown that values for
the dielectric permittivity, calculated within the exact interaction site
description, differ in a characteristic way from those obtained by the point
dipole approximation which is usually used in computer experiment. It is also
shown that the libration oscillations, existing in the shape of longitudinal
time-dependent polarization fluctuations at small and intermediate wavevector
values, vanish however for bigger wavenumbers. A comparison between the
wavevector and frequency behaviour of the dielectric constant for the TIP4P
water and the Stockmayer model is made. The static screening of external
charges and damping of longitudinal electric excitations in water are
considered as well. A special investigation is devoted to the time dependence
of dielectric quantities in the free motion regime.
|
9901036v1
|
2010-07-05
|
On the Karman constant
|
Numerous studies in the past 40 years have established that turbulent flow
fields are populated by transient coherent structures that represent patches of
fluids moving cohesively for significant distances before they are worn out by
momentum exchange with the surrounding fluid. Two particular well-documented
structures are the hairpin vortices that move longitudinally above the wall and
ejections inclined with respect to the wall that bring the fluid from the
transient viscous layers underneath these vortices into the outer region of the
boundary layer.
It is proposed that the Karman universal constant in the logarithmic law the
sine of the angle between the transient ejections and the direction normal to
the wall. The edge of the buffer layer is represented by a combination of the
Karman constant and the damping function in the wall layer.
Computation of this angle from experimental data of velocity distributions in
turbulent shear flows matches published traces of fronts of turbulence obtained
from the time shifts in the peak of the correlation function of the velocity.
Key works: Turbulence, coherent structures, Karman constant, mixing-length,
shear layers
|
1007.0605v1
|
2020-01-08
|
Assessing different approaches to ab initio calculations of spin wave stiffness
|
Ab initio calculations of the spin wave stiffness constant $D$ for elemental
Fe and Ni performed by different groups in the past have led to values with a
considerable spread of 50-100 %. We present results for the stiffness constant
$D$ of Fe, Ni, and permalloy Fe$_{0.19}$Ni$_{0.81}$ obtained by three different
approaches: (i) by finding the quadratic term coefficient of the power
expansion of the spin wave energy dispersion, (ii) by a damped real-space
summation of weighted exchange coupling constants, and (iii) by integrating the
appropriate expression in reciprocal space. All approaches are implemented by
means of the same Korringa-Kohn-Rostoker (KKR) Green function formalism. We
demonstrate that if properly converged, all procedures yield comparable values,
with uncertainties of 5-10 % remaining. By a careful analysis of the influence
of various technical parameters we estimate the margin of errors for the
stiffness constants evaluated by different approaches and suggest procedures to
minimize the risk of getting incorrect results.
|
2001.02558v2
|
2021-05-19
|
Sound attenuation derived from quenched disorder in solids
|
In scattering experiments, the dynamical structure factor (DSF) characterizes
inter-particle correlations and their time evolution. We analytically evaluated
the DSF of disordered solids with disorder in the spring constant, by averaging
over quenched disorder in the values of lattice bond strength, along the
acoustic branch. The width of the resulting acoustic excitation peak is treated
as the effective damping constant $\Gamma(q)$, which we found to grow linearly
with exchanged momentum $q$. This is verified by numerically calculating a
model system consisting of harmonic linear chains with disorder in spring
constant. We also found that the quenched averaging of the vibrational density
of states produces a characteristic peak at a frequency related to the average
acoustic resonance. Such a peak (the excess over Debye law) may be related to
the "boson peak" frequently discussed in disordered solids, in our case
explicitly arising from the quenched disorder in the distribution of spring
constants.
|
2105.09393v1
|
2022-05-17
|
Acoustic gravitational interaction revised
|
In this paper, we deduce the expression of the gravito-acoustic force between
two oscillating bubbles using the hypothesis that this type of force is a force
of scattering-absorption of the energy of excitatory waves. The expression of
the gravito-acoustic force at resonance highlights the dependence of this force
on the product of the virtual masses of the two bubbles and on an acoustic
gravitational constant. The acoustic gravitational constant depends on the
absorption damping coefficient. We may say also that the expression of the
acoustic gravitational constant is analogous to the expression of the
gravitational constant in the electromagnetic world, that one obtained in the
Einstein-Sciama model and the Dirac-Eddington large numbers hypothesis. The
results obtained for this type of phenomenon in the acoustic world support the
similarity between the acoustic world and the electromagnetic world.
|
2206.00435v1
|
2022-07-27
|
Determination of Thickness-dependent Damping Constant and Plasma Frequency for Ultrathin Ag and Au Films: Nanoscale Dielectric Function
|
There is an ever increasing interest in the development of plasmonic 2D
nanomaterials, with widespread applications in optoelectronics, high resolution
microscopy, imaging and sensing, among others. With the current ability of
ultrathin noble metal film deposition down to a few monolayers in thickness,
there is a need for an analytical expression of the thickness dependent complex
dielectric function for predicting optical properties for arbitrary
thicknesses. The free and bound electron contributions to the dielectric
function are dealt with independently, since their influences affect separate
wavelengths ranges. The former is dealt within the Drude model framework for
large wavelengths with appropriately addressed damping constant and plasma
frequency parameters to account for thickness dependence. Applying our
previously developed method, we determine these parameters for specific film
thicknesses, based on refractive index experimental values for Ag and Au thin
films. Fitting separately each one of these parameters allowed us to find an
analytical expression for their dependence on arbitrary film thickness and
consequently for the free electron contribution. Concerning bound electrons, it
is seen that its contribution for small wavelengths is the same for all
analyzed thicknesses and may be set equal to the bulk bound contribution.
Taking all these facts into account, the complex dielectric function can be
rewritten analytically, in terms of the bulk dielectric function plus
corrective film thickness dependent terms. In particular, the fitting process
for the damping constant allows us to determine that the electron scattering at
the film boundary is mainly diffusive (inelastic) for both silver and gold thin
films. It is also shown that, in accordance with theoretical studies, plasma
frequency shows a red shift as the film thickness decreases.
|
2207.13580v1
|
1999-11-03
|
Tensor Microwave Anisotropies from a Stochastic Magnetic Field
|
We derive an expression for the angular power spectrum of cosmic microwave
background anisotropies due to gravity waves generated by a stochastic magnetic
field and compare the result with current observations; we take into account
the non-linear nature of the stress energy tensor of the magnetic field.
For almost scale invariant spectra, the amplitude of the magnetic field at
galactic scales is constrained to be of order 10^{-9} Gauss. If we assume that
the magnetic field is damped below the Alfven damping scale, we find that its
amplitude at
0.1 h^{-1}Mpc, B_\lambda, is constrained to be B_\lambda<7.9 x10^{-6} e^{3n}
Gauss, for n<-3/2, and B_\lambda<9.5x10^{-8} e^{0.37n} Gauss, for n>-3/2, where
n is the spectral index of the magnetic field and H_0=100h km s^{-1}Mpc^{-1} is
the Hubble constant today.
|
9911040v1
|
2002-07-15
|
On the Structure of the Iron K-Edge
|
It is shown that the commonly held view of a sharp Fe K edge must be modified
if the decay pathways of the series of resonances converging to the K
thresholds are adequately taken into account. These resonances display damped
Lorentzian profiles of nearly constant widths that are smeared to impose
continuity across the threshold. By modeling the effects of K damping on
opacities, it is found that the broadening of the K edge grows with the
ionization level of the plasma and that the appearance at high ionization of a
localized absorption feature at 7.2 keV is identified as the K-beta unresolved
transition array.
|
0207324v2
|
2006-12-15
|
Damp Mergers: Recent Gaseous Mergers without Significant Globular Cluster Formation?
|
Here we test the idea that new globular clusters (GCs) are formed in the same
gaseous ("wet") mergers or interactions that give rise to the young stellar
populations seen in the central regions of many early-type galaxies. We compare
mean GC colors with the age of the central galaxy starburst. The red GC
subpopulation reveals remarkably constant mean colors independent of galaxy
age. A scenario in which the red GC subpopulation is a combination of old and
new GCs (formed in the same event as the central galaxy starburst) can not be
ruled out; although this would require an age-metallicity relation for the
newly formed GCs that is steeper than the Galactic relation. However, the data
are also well described by a scenario in which most red GCs are old, and few,
if any, are formed in recent gaseous mergers. This is consistent with the old
ages inferred from some spectroscopic studies of GCs in external systems. The
event that induced the central galaxy starburst may have therefore involved
insufficient gas mass for significant GC formation. We term such gas-poor
events "damp" mergers.
|
0612415v1
|
1998-02-24
|
Resonant steps and spatiotemporal dynamics in the damped dc-driven Frenkel-Kontorova chain
|
Kink dynamics of the damped Frenkel-Kontorova (discrete sine-Gordon) chain
driven by a constant external force are investigated. Resonant steplike
transitions of the average velocity occur due to the competitions between the
moving kinks and their radiated phasonlike modes. A mean-field consideration is
introduced to give a precise prediction of the resonant steps. Slip-stick
motion and spatiotemporal dynamics on those resonant steps are discussed. Our
results can be applied to studies of the fluxon dynamics of 1D
Josephson-junction arrays and ladders, dislocations, tribology and other
fields.
|
9802251v1
|
1999-03-11
|
Thermally activated escape rates of uniaxial spin systems with transverse field
|
Classical escape rates of uniaxial spin systems are characterized by a
prefactor differing from and much smaller than that of the particle problem,
since the maximum of the spin energy is attained everywhere on the line of
constant latitude: theta=const, 0 =< phi =< 2*pi. If a transverse field is
applied, a saddle point of the energy is formed, and high, moderate, and low
damping regimes (similar to those for particles) appear. Here we present the
first analytical and numerical study of crossovers between the uniaxial and
other regimes for spin systems. It is shown that there is one HD-Uniaxial
crossover, whereas at low damping the uniaxial and LD regimes are separated by
two crossovers.
|
9903192v2
|
2006-11-18
|
Distributions of switching times of single-domain particles using a time quantified Monte Carlo method
|
Using a time quantified Monte Carlo scheme we performed simulations of the
switching time distribution of single mono-domain particles in the
Stoner-Wohlfarth approximation. We considered uniaxial anisotropy and different
conditions for the external applied field. The results obtained show the
switching time distribution can be well described by two relaxation times,
either when the applied field is parallel to the easy axis or for an oblique
external field and a larger damping constant. We found that in the low barrier
limit these relaxation times are in very good agreement with analytical results
obtained from solutions of the Fokker-Planck equation related to this problem.
When the damping is small and the applied field is oblique the shape of the
distribution curves shows several peaks and resonance effects.
|
0611494v2
|
2006-11-22
|
Magnetization damping in a local-density approximation
|
The linear response of itinerant transition metal ferromagnets to transverse
magnetic fields is studied in a self-consistent adiabatic local-density
approximation. The susceptibility is calculated from a microscopic Hamiltonian,
including spin-conserving impurities, impurity induced spin-orbit interaction
and magnetic impurities using the Keldysh formalism. The Gilbert damping
constant in the Landau-Lifshitz-Gilbert equation is identified, parametrized by
an effective transverse spin dephasing rate, and is found to be inversely
proportional to the exchange splitting. Our result justify the phenomenological
treatment of transverse spin dephasing in the study of current-induced
magnetization dynamics in weak, itinerant ferromagnets by Tserkovnyak
\textit{et al.}. We show that neglect of gradient corrections in the
quasiclassical transport equations leads to incorrect results when the exchange
potential becomes of the order of the Fermi energy.
|
0611588v1
|
2004-01-13
|
Highly Damped Quasinormal Modes of Kerr Black Holes: A Complete Numerical Investigation
|
We compute for the first time very highly damped quasinormal modes of the
(rotating) Kerr black hole. Our numerical technique is based on a decoupling of
the radial and angular equations, performed using a large-frequency expansion
for the angular separation constant_{s}A_{l m}. This allows us to go much
further in overtone number than ever before. We find that the real part of the
quasinormal frequencies approaches a non-zero constant value which does not
depend on the spin s of the perturbing field and on the angular index l:
\omega_R=m\varpi(a). We numerically compute \varpi(a). Leading-order
corrections to the asymptotic frequency are likely to be of order 1/\omega_I.
The imaginary part grows without bound, the spacing between consecutive modes
being a monotonic function of a.
|
0401052v1
|
1992-06-21
|
Gauge Dependence of the Resummed Thermal Gluon Self Energy
|
The gauge dependence of the hot gluon self energy is examined in the context
of Pisarski's method for resumming hard thermal loops. Braaten and Pisarski
have used the Ward identities satisfied by the hard corrections to the n-point
functions to argue the gauge fixing independence of the leading order resummed
QCD plasma damping rate in covariant and strict Coulomb gauges. We extend their
analysis to include all linear gauges that preserve rotational invariance and
display explicitly the conditions required for gauge fixing independence. It is
shown that in covariant gauges the resummed damping constant is gauge fixing
independent only if an infrared regulator is explicitly maintained throughout
the calculation.
|
9206239v1
|
1993-05-07
|
Thermal quark production in pure glue and quark gluon plasmas
|
We calculate production rates for massless $(u,d)$ and massive $(s,c,b)$
quarks in pure glue and quark gluon plasmas to leading order in the strong
coupling constant $g$. The leading contribution comes from gluon decay into
$q\bar q$ pairs, using a thermal gluon propagator with finite thermal mass and
damping rate. The rate behaves as $\alpha_S^2(\ln 1/\alpha_S)^2 T^4$ when $m,
\alpha_S \rightarrow 0$ and depends linearly on the transverse gluon damping
rate for all values of the quark mass $m$. The light quark ($u$, $d$, $s$)
chemical equilibration time is approximately 10-100 $T^{-1}$ for $g=$2-3, so
that quarks are likely to remain far from chemical equilibrium in
ultrarelativistic nuclear collisions.
|
9305227v1
|
2005-06-28
|
Liouville Decoherence in a Model of Flavour Oscillations in the presence of Dark Energy
|
We study in some detail the master equation, and its solution in a simplified
case modelling flavour oscillations of a two-level system, stemming from the
Liouville-string approach to quantum space time foam. In this framework we
discuss the appearance of diffusion terms and decoherence due to the
interaction of low-energy string matter with space-time defects, such as
D-particles in the specific model of ``D-particle foam'', as well as dark
energy contributions. We pay particular attention to contrasting the decoherent
role of a cosmological constant in inducing exponential quantum damping in the
evolution of low-energy observables, such as the probability of flavour
oscillations, with the situation where the dark energy relaxes to zero for
asymptotically large times, in which case such a damping is absent. Our
findings may be of interest to (astrophysical) tests of quantum space-time foam
models in the not-so-distant future.
|
0506242v1
|
2006-04-07
|
Quasi-periodic attractors, Borel summability and the Bryuno condition for strongly dissipative systems
|
We consider a class of ordinary differential equations describing
one-dimensional analytic systems with a quasi-periodic forcing term and in the
presence of damping. In the limit of large damping, under some generic
non-degeneracy condition on the force, there are quasi-periodic solutions which
have the same frequency vector as the forcing term. We prove that such
solutions are Borel summable at the origin when the frequency vector is either
any one-dimensional number or a two-dimensional vector such that the ratio of
its components is an irrational number of constant type. In the first case the
proof given simplifies that provided in a previous work of ours. We also show
that in any dimension $d$, for the existence of a quasi-periodic solution with
the same frequency vector as the forcing term, the standard Diophantine
condition can be weakened into the Bryuno condition. In all cases, under a
suitable positivity condition, the quasi-periodic solution is proved to
describe a local attractor.
|
0604162v1
|
1998-02-27
|
New collective mode due to collisional coupling
|
Starting from a nonmarkovian conserving relaxation time approximation for
collisions we derive coupled dispersion relations for asymmetric nuclear
matter. The isovector and isoscalar modes are coupled due to asymmetric nuclear
meanfield acting on neutrons and protons differently. A further coupling is
observed by collisional correlations. The latter one leads to the appearance of
a new soft mode besides isoscalar and isovector modes in the system. We suggest
that this mode might be observable in asymmetric systems. This soft mode
approaches the isovector mode for high temperatures. At the same time the
isovector mode remains finite and approaches a constant value at higher
temperatures showing a transition from zero sound like damping to first sound.
The damping of the new soft mode is first sound like at all temperatures.
|
9802083v1
|
2000-08-14
|
Design of a 3 GHz Accelerator Structure for the CLIC Test Facility (CTF 3) Drive Beam
|
For the CLIC two-beam scheme, a high-current, long-pulse drive beam is
required for RF power generation. Taking advantage of the 3 GHz klystrons
available at the LEP injector once LEP stops, a 180 MeV electron accelerator is
being constructed for a nominal beam current of 3.5 A and 1.5 microsecond pulse
length. The high current requires highly effective suppression of dipolar
wakes. Two concepts are investigated for the accelerating structure design: the
"Tapered Damped Structure" developed for the CLIC main beam, and the "Slotted
Iris - Constant Aperture" structure. Both use 4 SiC loads per cell for
effective higher-order mode damping. A full-size prototype of the TDS structure
has been built and tested successfully at full power. A first prototype of the
SICA structure is being built.
|
0008052v1
|
2004-11-02
|
Supersymmetric free-damped oscillators: Adaptive observer estimation of the Riccati parameter
|
A supersymmetric class of free damped oscillators with three parameters has
been obtained in 1998 by Rosu and Reyes through the factorization of the Newton
equation. The supplementary parameter is the integration constant of the
general Riccati solution. The estimation of the latter parameter is performed
here by employing the recent adaptive observer scheme of Besancon et al., but
applied in a nonstandard form in which a time-varying quantity containing the
unknown Riccati parameter is estimated first. Results of computer simulations
are presented to illustrate the good feasibility of this approach for a case in
which the estimation is not easily accomplished by other means
|
0411019v2
|
2007-01-30
|
Charge Fluctuation of Dust Grain and Its Impact on Dusty-Acoustic Wave Damping
|
We consider the influence of dust charge fluctuations on damping of the
dust-ion-acoustic waves. It is assumed that all grains have equal masses but
charges are not constant in time - they may fluctuate in time. The dust charges
are not really independent of the variations in the plasma potentials. All
modes will influence the charging mechanism, and feedback will lead to several
new interesting and unexpected phenomena. The charging of the grains depends on
local plasma characteristics. If the waves disturb these characteristic, then
charging of the grains is affected and the grain charge is modified, with a
resulting feedback on the wave mode. In the case considered here, when the
temperature of electrons is much greater than the temperature of the ions and
the temperature of electrons is not great enough for further ionization of the
ions, we show that attenuation of the acoustic wave depends only on one
phenomenological coefficient
|
0701336v1
|
2004-01-28
|
Bloch Equations and Completely Positive Maps
|
The phenomenological dissipation of the Bloch equations is reexamined in the
context of completely positive maps. Such maps occur if the dissipation arises
from a reduction of a unitary evolution of a system coupled to a reservoir. In
such a case the reduced dynamics for the system alone will always yield
completely positive maps of the density operator. We show that, for Markovian
Bloch maps, the requirement of complete positivity imposes some Bloch
inequalities on the phenomenological damping constants. For non-Markovian Bloch
maps some kind of Bloch inequalities involving eigenvalues of the damping basis
can be established as well. As an illustration of these general properties we
use the depolarizing channel with white and colored stochastic noise.
|
0401177v1
|
2006-01-10
|
Quantum Brownian motion and the Third Law of thermodynamics
|
The quantum thermodynamic behavior of small systems is investigated in
presence of finite quantum dissipation. We consider the archetype cases of a
damped harmonic oscillator and a free quantum Brownian particle. A main finding
is that quantum dissipation helps to ensure the validity of the Third Law. For
the quantum oscillator, finite damping replaces the zero-coupling result of an
exponential suppression of the specific heat at low temperatures by a power-law
behavior. Rather intriguing is the behavior of the free quantum Brownian
particle. In this case, quantum dissipation is able to restore the Third Law:
Instead of being constant down to zero temperature, the specific heat now
vanishes proportional to temperature with an amplitude that is inversely
proportional to the ohmic dissipation strength. A distinct subtlety of finite
quantum dissipation is the result that the various thermodynamic functions of
the sub-system do not only depend on the dissipation strength but depend as
well on the prescription employed in their definition.
|
0601056v1
|
2007-08-26
|
Geodesic plasma flows instabilities of Riemann twisted solar loops
|
Riemann and sectional curvatures of magnetic twisted flux tubes in Riemannian
manifold are computed to investigate the stability of the plasma astrophysical
tubes. The geodesic equations are used to show that in the case of thick
magnetic tubes, the curvature of planar (Frenet torsion-free) tubes have the
effect ct of damping the flow speed along the tube. Stability of geodesic flows
in the Riemannian twisted thin tubes (almost filaments), against constant
radial perturbations is investigated by using the method of negative sectional
curvature for unstable flows. No special form of the flow like Beltrami flows
is admitted, and the proof is general for the case of thin magnetic flux tubes.
In the magnetic equilibrium state, the twist of the tube is shown to display
also a damping effect on the toroidal velocity of the plasma flow. It is found
that for positive perturbations and angular speed of the flow, instability is
achieved, since the sectional Ricci curvature of the magnetic twisted tube
metric is negative. Solar flare production may appear from these geometrical
instabilities of the twisted solar loops.
|
0708.3473v1
|
2009-01-28
|
Location- and observation time-dependent quantum-tunneling
|
We investigate quantum tunneling in a translation invariant chain of
particles. The particles interact harmonically with their nearest neighbors,
except for one bond, which is anharmonic. It is described by a symmetric double
well potential. In the first step, we show how the anharmonic coordinate can be
separated from the normal modes. This yields a Lagrangian which has been used
to study quantum dissipation. Elimination of the normal modes leads to a
nonlocal action of Caldeira-Leggett type. If the anharmonic bond defect is in
the bulk, one arrives at Ohmic damping, i.e. there is a transition of a
delocalized bond state to a localized one if the elastic constant exceeds a
critical value $C_{crit}$. The latter depends on the masses of the bond defect.
Superohmic damping occurs if the bond defect is in the site $M$ at a finite
distance from one of the chain ends. If the observation time $T$ is smaller
than a characteristic time $\tau_M \sim M$, depending on the location M of the
defect, the behavior is similar to the bulk situation. However, for $T \gg
\tau_M$ tunneling is never suppressed.
|
0901.4518v1
|
2010-01-06
|
Freezing of spin dynamics and omega/T scaling in underdoped cuprates
|
The memory function approach to spin dynamics in doped antiferromagnetic
insulator combined with the assumption of temperature independent static spin
correlations and constant collective mode damping leads to omega/T scaling in a
broad range. The theory involving a non universal scaling parameter is used to
analyze recent inelastic neutron scattering results for underdoped cuprates.
Adopting modified damping function also the emerging central peak in low-doped
cuprates at low temperatures can be explained within the same framework.
|
1001.0837v1
|
2010-02-02
|
Inertial Oscillations of Pinned Dislocations
|
Dislocation pinning plays a vital role in the plastic behaviour of a
crystalline solid. Here we report the first observation of the damped
oscillations of a mobile dislocation after it gets pinned at an obstacle in the
presence of a constant static shear load. These oscillations are found to be
inertial, instead of forced as obtained in the studies of internal friction of
solid. The rate of damping enables us to determine the effective mass of the
dislocation. Nevertheless, the observed relation between the oscillation
frequency and the link length is found to be anomalous, when compared with the
theoretical results in the framework of Koehler's vibrating string model. We
assign this anomaly to the improper boundary conditions employed in the
treatment. Finally, we propose that the inertial oscillations may offer a
plausible explanation of the electromagnetic emissions during material
deformation and seismic activities.
|
1002.0422v1
|
2010-05-20
|
Line Solutions for the Euler and Euler-Poisson Equations with Multiple Gamma Law
|
In this paper, we study the Euler and Euler-Poisson equations in $R^{N}$,
with multiple $\gamma$-law for pressure function: \begin{equation}
P(\rho)=e^{s}\sum_{j=1}^{m}\rho^{\gamma_{j}}, \end{equation} where all
$\gamma_{i+1}>\gamma_{i}\geq1$, is the constants. The analytical line solutions
are constructed for the systems. It is novel to discover the analytical
solutions to handle the systems with mixed pressure function. And our solutions
can be extended to the systems with the generalized multiple damping and
pressure function.
|
1005.3651v1
|
2010-07-12
|
Ferromagnetic Excitations in La$_{0.82}$Sr$_{0.18}$CoO$_{3}$ Observed Using Neutron Inelastic Scattering
|
Polarized neutron inelastic scattering has been used to measure spin
excitations in ferromagnetic La$_{0.82}$Sr$_{0.18}$CoO$_{3}$. The magnon
spectrum of these spin excitations is well defined at low energies but becomes
heavily damped at higher energies, and can be modeled using a quadratic
dispersion. We determined a spin wave stiffness constant of $D=94\pm
3$\,meV\,\AA$^{2}$. Assuming a nearest-neighbor Heisenberg model we find
reasonable agreement between the exchange determined from D and the bulk Curie
temperature. Several possible mechanisms to account for the observed spin-wave
damping are discussed.
|
1007.1919v1
|
2010-09-15
|
A discontinuous Galerkin method for the Vlasov-Poisson system
|
A discontinuous Galerkin method for approximating the Vlasov-Poisson system
of equations describing the time evolution of a collisionless plasma is
proposed. The method is mass conservative and, in the case that piecewise
constant functions are used as a basis, the method preserves the positivity of
the electron distribution function and weakly enforces continuity of the
electric field through mesh interfaces and boundary conditions. The performance
of the method is investigated by computing several examples and error estimates
associated system's approximation are stated. In particular, computed results
are benchmarked against established theoretical results for linear advection
and the phenomenon of linear Landau damping for both the Maxwell and Lorentz
distributions. Moreover, two nonlinear problems are considered: nonlinear
Landau damping and a version of the two-stream instability are computed. For
the latter, fine scale details of the resulting long-time BGK-like state are
presented. Conservation laws are examined and various comparisons to theory are
made. The results obtained demonstrate that the discontinuous Galerkin method
is a viable option for integrating the Vlasov-Poisson system.
|
1009.3046v2
|
2010-10-03
|
Measurement of damping and temperature: Precision bounds in Gaussian dissipative channels
|
We present a comprehensive analysis of the performance of different classes
of Gaussian states in the estimation of Gaussian phase-insensitive dissipative
channels. In particular, we investigate the optimal estimation of the damping
constant and reservoir temperature. We show that, for two-mode squeezed vacuum
probe states, the quantum-limited accuracy of both parameters can be achieved
simultaneously. Moreover, we show that for both parameters two-mode squeezed
vacuum states are more efficient than either coherent, thermal or single-mode
squeezed states. This suggests that at high energy regimes two-mode squeezed
vacuum states are optimal within the Gaussian setup. This optimality result
indicates a stronger form of compatibility for the estimation of the two
parameters. Indeed, not only the minimum variance can be achieved at fixed
probe states, but also the optimal state is common to both parameters.
Additionally, we explore numerically the performance of non-Gaussian states for
particular parameter values to find that maximally entangled states within
D-dimensional cutoff subspaces perform better than any randomly sampled states
with similar energy. However, we also find that states with very similar
performance and energy exist with much less entanglement than the maximally
entangled ones.
|
1010.0442v1
|
2010-10-18
|
K-shell photoionization of Na-like to Cl-like ions of Mg, Si, S, Ar, and Ca
|
We present $R$-matrix calculations of photoabsorption and photoionization
cross sections across the K-edge of Mg, Si, S, Ar, and Ca ions with more than
10 electrons. The calculations include the effects of radiative and Auger
damping by means of an optical potential. The wave functions are constructed
from single-electron orbital bases obtained using a Thomas--Fermi--Dirac
statistical model potential. Configuration interaction is considered among all
states up to $n=3$. The damping processes affect the resonances converging to
the K-thresholds causing them to display symmetric profiles of constant width
that smear the otherwise sharp edge at the photoionization threshold. These
data are important for modeling of features found in photoionized plasmas.
|
1010.3734v1
|
2010-10-19
|
A possible signature of cosmic neutrino decoupling in the nHz region of the spectrum of primordial gravitational waves
|
In this paper we study the effect of cosmic neutrino decoupling on the
spectrum of cosmological gravitational waves (GWs). At temperatures T>>1 MeV,
neutrinos constitute a perfect fluid and do not hinder GW propagation, while
for T<<1 MeV they free-stream and have an effective viscosity that damps
cosmological GWs by a constant amount. In the intermediate regime,
corresponding to neutrino decoupling, the damping is frequency-dependent. GWs
entering the horizon during neutrino decoupling have a frequency f ~ 1 nHz,
corresponding to a frequency region that will be probed by Pulsar Timing Arrays
(PTAs). In particular, we show how neutrino decoupling induces a spectral
feature in the spectrum of cosmological GWs just below 1 nHz. We briefly
discuss the conditions for a detection of this feature and conclude that it is
unlikely to be observed by PTAs.
|
1010.3849v2
|
2011-04-25
|
Exactly Solvable Nonhomogeneous Burgers Equations with Variable Coefficients
|
We consider a nonhomogeneous Burgers equation with time variable
coefficients, and obtain an explicit solution of the general initial value
problem in terms of solution to a corresponding linear ODE. Special exact
solutions such as generalized shock and multi-shock solitary waves, triangular
wave, N-wave and rational type solutions are found and discussed. As exactly
solvable models, we study forced Burgers equations with constant damping and an
exponentially decaying diffusion coefficient. Different type of exact solutions
are obtained for the critical, over and under damping cases, and their behavior
is illustrated explicitly. In particular, the existence of inelastic type of
collisions is observed by constructing multi-shock solitary wave solutions, and
for the rational type solutions the motion of the pole singularities is
described.
|
1104.4717v1
|
2011-07-15
|
K-shell photoionization of Nickel ions using R-matrix
|
We present R-matrix calculations of photoabsorption and photoionization cross
sections across the K edge of the Li-like to Ca-like ions stages of Ni.
Level-resolved, Breit-Pauli calculations were performed for the Li-like to
Na-like stages. Term-resolved calculations, which include the mass-velocity and
Darwin relativistic corrections, were performed for the Mg-like to Ca-like ion
stages. This data set is extended up to Fe-like Ni using the distorted wave
approximation as implemented by AUTOSTRUCTURE. The R-matrix calculations
include the effects of radiative and Auger dampings by means of an optical
potential. The damping processes affect the absorption resonances converging to
the K thresholds causing them to display symmetric profiles of constant width
that smear the otherwise sharp edge at the K-shell photoionization threshold.
These data are important for the modeling of features found in photoionized
plasmas.
|
1107.3146v1
|
2011-12-21
|
A numerical method for computing radially symmetric solutions of a dissipative nonlinear modified Klein-Gordon equation
|
In this paper we develop a finite-difference scheme to approximate radially
symmetric solutions of the initial-value problem with smooth initial conditions
in an open sphere around the origin, where the internal and external damping
coefficients are constant, and the nonlinear term follows a power law. We prove
that our scheme is consistent of second order when the nonlinearity is
identically equal to zero, and provide a necessary condition for it to be
stable order n. Part of our study will be devoted to compare the physical
effects of the damping coefficients.
|
1112.4921v1
|
2012-02-25
|
Fractional Order Phase Shaper Design with Routh's Criterion for Iso-damped Control System
|
Phase curve of an open loop system is flat in nature if the derivative of
phase with respect to frequency is zero. With a flat phase curve, the
corresponding closed-loop system exhibits an iso-damped property i.e. maintains
constant overshoot with the change of gain and with other parametric
variations. In recent past application, fractional order (FO) phase shapers
have been proposed by contemporary researchers to achieve enhanced parametric
robustness. In this paper, a simple Routh tabulation based methodology is
proposed to design an appropriate FO phase shaper to achieve phase flattening
in a control loop, comprising a system, controlled by a classical PID
controller. The method is demonstrated using MATLAB simulation of a second
order DC motor plant and also a first order with time delay system.
|
1202.5667v1
|
2012-07-18
|
Attractiveness of periodic orbits in parametrically forced systemswith time-increasing friction
|
We consider dissipative one-dimensional systems subject to a periodic force
and study numerically how a time-varying friction affects the dynamics. As a
model system, particularly suited for numerical analysis, we investigate the
driven cubic oscillator in the presence of friction. We find that, if the
damping coefficient increases in time up to a final constant value, then the
basins of attraction of the leading resonances are larger than they would have
been if the coefficient had been fixed at that value since the beginning. From
a quantitative point of view, the scenario depends both on the final value and
the growth rate of the damping coefficient. The relevance of the results for
the spin-orbit model are discussed in some detail.
|
1207.4319v1
|
2012-07-19
|
Acoustic damping and dispersion in vitreous germanium oxide
|
New Brillouin scattering measurements of velocity and attenuation of sound in
the hypersonic regime are presented. The data are analyzed together with the
literature results at sonic and ultrasonic frequencies. As usual, thermally
activated relaxation of structural entities describes the attenuation at sonic
and ultrasonic frequencies. As already shown in vitreous silica, we conclude
that the damping by network viscosity, resulting from relaxation of thermal
phonons, must be taken into account to describe the attenuation at hypersonic
frequencies. In addition, the bare velocity obtained by subtracting to the
experimental data the effect of the two above mechanisms is constant for
temperatures below 250 K, but increases almost linearly above, up to the glass
transition temperature. This might indicate the presence of a progressive local
polyamorphic transition, as already suggested for vitreous silica.
|
1207.4582v1
|
2012-08-21
|
Brownian transport in corrugated channels with inertia
|
The transport of suspended Brownian particles dc-driven along corrugated
narrow channels is numerically investigated in the regime of finite damping. We
show that inertial corrections cannot be neglected as long as the width of the
channel bottlenecks is smaller than an appropriate particle diffusion length,
which depends on the the channel corrugation and the drive intensity. Being
such a diffusion length inversely proportional to the damping constant,
transport through sufficiently narrow obstructions turns out to be always
sensitive to the viscosity of the suspension fluid. The inertia corrections to
the transport quantifiers, mobility and diffusivity, markedly differ for
smoothly and sharply corrugated channels.
|
1208.4401v2
|
2012-09-26
|
Damping of giant dipole resonance in highly excited nuclei
|
The giant dipole resonance's (GDR) width and shape at finite temperature and
angular momentum are described within the phonon damping model (PDM), which
predicts an overall increase in the GDR's total width at low and moderate
temperature T, and its saturation at high T. At T< 1 MeV the GDR width remains
nearly constant because of thermal pairing. The PDM description is compared
with the experimental systematics obtained from heavy-ion fusion, inelastic
scattering of light particles on heavy targets, and alpha induced fusion
reactions, as well as with predictions by other theoretical approaches. The
results obtained within the PDM and GDR's experimental data are also employed
to predict the viscosity of hot medium and heavy nuclei.
|
1209.5820v2
|
2012-10-12
|
Threshold current for switching of a perpendicular magnetic layer induced by spin Hall effect
|
We theoretically investigate the switching of a perpendicular magnetic layer
by in-plane charge current due to the spin Hall effect. We find that, in the
high damping regime, the threshold switching current is independent of the
damping constant, and is almost linearly proportional to both effective
perpendicular magnetic anisotropy field and external in-plane field applied
along the current direction. We obtain an analytic expression of the threshold
current, in excellent agreement with numerical results. This expression can be
used to determine the physical quantities associated with spin Hall effect, and
to design relevant magnetic devices based on the switching of perpendicular
magnetic layers.
|
1210.3442v2
|
2012-10-15
|
Symmetries of the quantum damped harmonic oscillator
|
For the non-conservative Caldirola-Kanai system, describing a quantum damped
harmonic oscillator, a couple of constant-of-motion operators generating the
Heisenberg-Weyl algebra can be found. The inclusion of the standard time
evolution generator (which is not a symmetry) as a symmetry in this algebra, in
a unitary manner, requires a non-trivial extension of this basic algebra and
hence of the physical system itself. Surprisingly, this extension leads
directly to the so-called Bateman dual system, which now includes a new
particle acting as an energy reservoir. In addition, the Caldirola-Kanai
dissipative system can be retrieved by imposing constraints. The algebra of
symmetries of the dual system is presented, as well as a quantization that
implies, in particular, a first-order Schr\"odinger equation. As opposed to
other approaches, where it is claimed that the spectrum of the Bateman
Hamiltonian is complex and discrete, we obtain that it is real and continuous,
with infinite degeneracy in all regimes.
|
1210.4058v1
|
2013-01-23
|
Characterization of magnetostatic surface spin waves in magnetic thin films: evaluation for microelectronic applications
|
The authors have investigated the possibility of utilizing spin waves for
inter- and intra-chip communications, and as logic elements using both
simulations and experimental techniques. Through simulations it has been shown
that the decay lengths of magnetostatic spin waves are affected most by the
damping parameter, and least by the exchange stiffness constant. The damping
and dispersion properties of spin waves limit the attenuation length to several
tens of microns. Thus, we have ruled out the possibility of inter-chip
communications via spin waves. Experimental techniques for the extraction of
the dispersion relationship have also been demonstrated, along with
experimental demonstrations of spin wave interference for amplitude modulation.
The effectiveness of spin wave modulation through interference, along with the
capability of determining the spin wave dispersion relationships electrically
during manufacturing and testing phase of chip production may pave the way for
using spin waves in analog computing wherein the circuitry required for
performing similar functionality becomes prohibitive.
|
1301.5395v1
|
2013-04-15
|
Energy dissipation in DC-field driven electron lattice coupled to fermion baths
|
Electron transport in electric-field-driven tight-binding lattice coupled to
fermion baths is comprehensively studied. We reformulate the problem by using
the scattering state method within the Coulomb gauge. Calculations show that
the formulation justifies direct access to the steady-state bypassing the
time-transient calculations, which then makes the steady-state methods
developed for quantum dot theories applicable to lattice models. We show that
the effective temperature of the hot-electron induced by a DC electric field
behaves as $T_{\rm eff}=C\gamma(\Omega/\Gamma)$ with a numerical constant $C$,
tight-binding parameter $\gamma$, the Bloch oscillation frequency $\Omega$ and
the damping parameter $\Gamma$. In the small damping limit $\Gamma/\Omega\to
0$, the steady-state has a singular property with the electron becoming
extremely hot in an analogy to the short-circuit effect. This leads to the
conclusion that the dissipation mechanism cannot be considered as an implicit
process, as treated in equilibrium theories. Finally, using the energy flux
relation, we derive a steady-state current for interacting models where only
on-site Green's functions are necessary.
|
1304.4269v1
|
2013-05-07
|
Micromagnetic modelling of anisotropic damping in ferromagnet
|
We report a numerical implementation of the Landau-Lifshitz-Baryakhtar
theory, which dictates that the micromagnetic relaxation term obeys the
symmetry of the magnetic crystal, i. e. replacing the single intrinsic damping
constant with a tensor of corresponding symmetry. The effect of anisotropic
relaxation is studied in thin saturated ferromagnetic disk and ellipse with and
without uniaxial magneto-crystalline anisotropy. We investigate the angular
dependency of the linewidth of magnonic resonances with respect to the given
structure of the relaxation tensor. The simulations suggest that the anisotropy
of the magnonic linewidth is determined by only two factors: the projection of
the relaxation tensor onto the plane of precession and the ellipticity of the
later.
|
1305.1641v2
|
2013-07-14
|
Asteroseismic effects in close binary stars
|
Turbulent processes in the convective envelopes of the sun and stars have
been shown to be a source of internal acoustic excitations. In single stars,
acoustic waves having frequencies below a certain cutoff frequency propagate
nearly adiabatically and are effectively trapped below the photosphere where
they are internally reflected. This reflection essentially occurs where the
local wavelength becomes comparable to the pressure scale height. In close
binary stars, the sound speed is a constant on equipotentials, while the
pressure scale height, which depends on the local effective gravity, varies on
equipotentials and may be much greater near the inner Lagrangian point (L_1).
As a result, waves reaching the vicinity of L_1 may propagate unimpeded into
low density regions, where they tend to dissipate quickly due to non-linear and
radiative effects. We study the three dimensional propagation and enhanced
damping of such waves inside a set of close binary stellar models using a WKB
approximation of the acoustic field. We find that these waves can have much
higher damping rates in close binaries, compared to their non-binary
counterparts. We also find that the relative distribution of acoustic energy
density at the visible surface of close binaries develops a ring-like feature
at specific acoustic frequencies and binary separations.
|
1307.3709v1
|
2013-07-31
|
Dynamics of ions in the selectivity filter of the KcsA channel: Towards a coupled Brownian particle description
|
The statistical and dynamical properties of ions in the selectivity filter of
the KcsA ion channel are considered on the basis of molecular dynamics (MD)
simulations of the KcsA protein embedded in a lipid membrane surrounded by an
ionic solution. A new approach to the derivation of a Brownian dynamics (BD)
model of ion permeation through the filter is discussed, based on unbiased MD
simulations. It is shown that depending on additional assumptions, ion's
dynamics can be described either by under-damped Langevin equation with
constant damping and white noise or by Langevin equation with a fractional
memory kernel. A comparison of the potential of the mean force derived from
unbiased MD simulations with the potential produced by the umbrella sampling
method demonstrates significant differences in these potentials. The origin of
these differences is an open question that requires further clarifications.
|
1307.8298v1
|
2013-10-09
|
Improved Coincident and Coherent Detection Statistics for Searches for Gravitational Wave Ringdown Signals
|
We study an improved method for detecting gravitational wave (GW) signals
from perturbed black holes by earth-based detectors in the quest for searching
for intermediate-mass black holes (IMBHs). Such signals, called ringdowns, are
damped sinusoids whose frequency and damping constant can be used to measure a
black hole's mass and spin. Utilizing the output from a matched filter analysis
pipeline, we present an improved statistic for the detection of a ringdown
signal that is found to be coincident in multiple detectors. The statistic
addresses the non-Gaussianity of the data without the use of an additional
signal-based waveform consistency test. We also develop coherent network
statistics to check for consistency of signal amplitudes and phases in the
different detectors with their different orientations and signal arrival times.
We find that the detection efficiency can be improved at least by a few tens of
percent by applying these multi-detector statistics primarily because of the
ineffectiveness of single-detector based discriminators of non-stationary
noise, such as the chi-square test, in the case of ringdown signals studied
here.
|
1310.2341v2
|
2014-04-25
|
Nonlinear and Linear Timescales near Kinetic Scales in Solar Wind Turbulence
|
The application of linear kinetic treatments to plasma waves, damping, and
instability requires favorable inequalities between the associated linear
timescales and timescales for nonlinear (e.g., turbulence) evolution. In the
solar wind these two types of timescales may be directly compared using
standard Kolmogorov-style analysis and observational data. The estimated local
nonlinear magnetohydrodynamic cascade times, evaluated as relevant kinetic
scales are approached, remain slower than the cyclotron period, but comparable
to, or faster than, the typical timescales of instabilities, anisotropic waves,
and wave damping. The variation with length scale of the turbulence timescales
is supported by observations and simulations. On this basis the use of linear
theory - which assumes constant parameters to calculate the associated kinetic
rates - may be questioned. It is suggested that the product of proton
gyrofrequency and nonlinear time at the ion gyroscales provides a simple
measure of turbulence influence on proton kinetic behavior.
|
1404.6569v1
|
2014-09-01
|
Damping of Bloch oscillations: Variational solutions of the Boltzmann equation beyond linear response
|
Variational solutions of the Boltzmann equation usually rely on the concept
of linear response. We extend the variational approach for tight-binding models
at high entropies to a regime far beyond linear response. We analyze both
weakly interacting fermions and incoherent bosons on a lattice. We consider a
case where the particles are driven by a constant force, leading to the
well-known Bloch oscillations, and we consider interactions that are weak
enough not to overdamp these oscillations. This regime is computationally
demanding and relevant for ultracold atoms in optical lattices. We derive a
simple theory in terms of coupled dynamic equations for the particle density,
energy density, current and heat current, allowing for analytic solutions. As
an application, we identify damping coefficients for Bloch oscillations in the
Hubbard model at weak interactions and compute them for a one-dimensional toy
model. We also approximately solve the long-time dynamics of a weakly
interacting, strongly Bloch-oscillating cloud of fermionic particles in a
tilted lattice, leading to a subdiffusive scaling exponent.
|
1409.0560v2
|
2014-12-12
|
Spin waves in micro-structured yttrium iron garnet nanometer-thick films
|
We investigated the spin-wave propagation in a micro-structured yttrium iron
garnet waveguide of $40$ nm thickness. Utilizing spatially-resolved Brillouin
light scattering microscopy, an exponential decay of the spin-wave amplitude of
$(10.06 \pm 0.83)$ $\mu$m was observed. This leads to an estimated Gilbert
damping constant of $\alpha=(8.79\pm 0.73)\times 10^{-4}$, which is larger than
damping values obtained through ferromagnetic resonance measurements in
unstructured films. The theoretically calculated spatial interference of
waveguide modes was compared to the spin-wave pattern observed experimentally
by means of Brillouin light scattering spectroscopy.
|
1412.4032v1
|
2015-10-01
|
Production of charged Higgs boson pairs in the $pp \to ppH^{+}H^{-}$ reaction at the LHC and FCC
|
We present differential cross sections for the $pp \to ppH^{+}H^{-}$ reaction
via photon-photon fusion with exact kinematics. We show predictions for
$\sqrt{s}$ = 14 TeV (LHC) and at the Future Circular Collider (FCC) for
$\sqrt{s}$ = 100 TeV. The integrated cross section for $\sqrt{s}$ = 14~TeV
(LHC) is about 0.1~fb and about 0.9~fb at the FCC for $\sqrt{s}$ = 100~TeV when
assuming $m_{H^{\pm}} = 150$~GeV. We present distributions in diHiggs boson
invariant mass. The results are compared with those obtained within
equivalent-photon approximation. We discuss also first calculations of cross
section for exclusive diffractive pQCD mechanism with estimated limits on the
$g_{hH^{+}H^{-}}$ coupling constant within 2HDM based on the LHC experimental
data. The diffractive contribution is much smaller than the $\gamma \gamma$
one. Absorption corrections are calculated differentially for various
distributions. In general, they lead to a damping of the cross section. The
damping depends on $M_{H^{+}H^{-}}$ invariant mass and on four-momentum
transfers squared in the proton line. We discuss a possibility to measure the
exclusive production of $H^{\pm}$ bosons.
|
1510.00171v1
|
2015-10-15
|
On the global existence and blowup of smooth solutions of 3-D compressible Euler equations with time-depending damping
|
In this paper, we are concerned with the global existence and blowup of
smooth solutions of the 3-D compressible Euler equation with time-depending
damping $$
\partial_t\rho+\operatorname{div}(\rho u)=0, \quad
\partial_t(\rho u)+\operatorname{div}\left(\rho u\otimes
u+p\,I_{3}\right)=-\,\frac{\mu}{(1+t)^{\lambda}}\,\rho u, \quad
\rho(0,x)=\bar \rho+\varepsilon\rho_0(x),\quad u(0,x)=\varepsilon u_0(x), $$
where $x\in\mathbb R^3$, $\mu>0$, $\lambda\geq 0$, and $\bar\rho>0$ are
constants, $\rho_0,\, u_0\in C_0^{\infty}(\mathbb R^3)$, $(\rho_0,
u_0)\not\equiv 0$, $\rho(0,\cdot)>0$, and $\varepsilon>0$ is sufficiently
small. For $0\leq\lambda\leq1$, we show that there exists a global smooth
solution $(\rho, u)$ when $\operatorname{curl} u_0\equiv 0$, while for
$\lambda>1$, in general, the solution $(\rho, u)$ will blow up in finite time.
Therefore, $\lambda=1$ appears to be the critical value for the global
existence of small amplitude smooth solutions.
|
1510.04613v1
|
2015-12-16
|
Back to Maupertuis' least action principle for dissipative systems: not all motions in Nature are most energy economical
|
It is shown that an oldest form of variational calculus of mechanics, the
Maupertuis least action principle, can be used as a simple and powerful
approach for the formulation of the variational principle for damped motions,
allowing a simple derivation of the Lagrangian mechanics for any dissipative
systems and an a connection of the optimization of energy dissipation to the
least action principles. On this basis, it is shown that not all motions of
classical mechanics obey the rule of least energy dissipation or follow the
path of least resistance, and that the least action is equivalent to least
dissipation for two kinds of motions : all stationary motions with constant
velocity and all motions damped by Stokes drag.
|
1512.05339v1
|
2016-05-01
|
Optical trapping by Laguerre-Gaussian beams: Symmetries, stability and equilibria
|
We use the T-matrix formalism in combination with the method of far-field
matching to evaluate the optical force exerted by Laguerre-Gaussian (LG) light
beams on a spherical (Mie) particle. For both non-vortex and optical vortex LG
beams, the theoretical results are used to analyze the optical-force-induced
dynamics of the scatterer near the trapping points represented by the
equilibrium (zero-force) positions. The regimes of linearized dynamics are
described in terms of the stiffness matrix spectrum and the damping constant of
the ambient medium. For the purely azimuthal LG beams, the dynamics is found to
be locally non-conservative and is characterized by the presence of
conditionally stable equilibria (unstable zero-force points that can be
stabilized by the ambient damping). The effects related to the Mie resonances
that under certain conditions manifest themselves as the points changing the
trapping properties of the particles are discussed.
|
1605.00243v2
|
2016-05-05
|
Relaxation of Ferroelectric States in 2D Distributions of quantum Dots:EELS Simulation
|
The relaxation time of collective electronic states in a 2D distribution of
quantum dots is investigated theoretically by simulating EELS experiments. From
the numerical calculation of the probability of energy loss of an electron
beam, traveling parallel to the distribution, it is possible to estimate the
damping time of ferroelectric-like states. We generate this collective response
of the distribution by introducing a mean field interaction among the quantum
dots, and then, the model is extended incorporating effects of long-range
correlations through a Bragg-Williams approximation. The behavior of the
dielectric function, the energy loss function, and the relaxation time of
ferroelectric-like states is then investigated as a function of the temperature
of the distribution and the damping constant of the electronic states in the
single quantum dots. The robustness of the trends and tendencies of our results
indicate that this scheme of analysis can guide experimentalists to develop
tailored quantum dots distributions for specific applications.
|
1605.01642v1
|
2016-07-27
|
Linear and nonlinear viscoelastic arterial wall models: application on animals
|
This work deals with the viscoelasticity of the arterial wall and its
influence on the pulse waves. We describe the viscoelasticity by a non-linear
Kelvin-Voigt model in which the coefficients are fitted using experimental time
series of pressure and radius measured on a sheep's arterial network. We
obtained a good agreement between the results of the nonlinear Kelvin-Voigt
model and the experimental measurements. We found that the viscoelastic
relaxation time-defined by the ratio between the viscoelastic coefficient and
the Young's modulus-is nearly constant throughout the network. Therefore, as it
is well known that smaller arteries are stiffer, the viscoelastic coefficient
rises when approaching the peripheral sites to compensate the rise of the
Young's modulus, resulting in a higher damping effect. We incorporated the
fitted viscoelastic coefficients in a nonlinear 1D fluid model to compute the
pulse waves in the network. The damping effect of viscoelasticity on the high
frequency waves is clear especially at the peripheral sites.
|
1607.07973v1
|
2016-08-19
|
Cooling a harmonic oscillator by optomechanical modification of its bath
|
Optomechanical systems show tremendous promise for high sensitivity sensing
of forces and modification of mechanical properties via light. For example,
similar to neutral atoms and trapped ions, laser cooling of mechanical motion
by radiation pressure can take single mechanical modes to their ground state.
Conventional optomechanical cooling is able to introduce additional damping
channel to mechanical motion, while keeping its thermal noise at the same
level, and as a consequence, the effective temperature of the mechanical mode
is lowered. However, the ratio of temperature to quality factor remains roughly
constant, preventing dramatic advances in quantum sensing using this approach.
Here we propose an approach for simultaneously reducing the thermal load on a
mechanical resonator while improving its quality factor. In essence, we use the
optical interaction to dynamically modify the dominant damping mechanism,
providing an optomechanically-induced effect analogous to a phononic band gap.
The mechanical mode of interest is assumed to be weakly coupled to its heat
bath but strongly coupled to a second mechanical mode, which is cooled by
radiation pressure coupling to a red detuned cavity field. We also identify a
realistic optomechanical design that has the potential to realize this novel
cooling scheme.
|
1608.05717v1
|
2016-09-30
|
Origin of the effective mobility in non-linear active micro-rheology
|
The distinction between the damping coefficient and the effective non-linear
mobility of driven particles in active micro-rheology of supercooled liquids is
explained in terms of individual and collective dynamics. The effective
mobility arises as a collective effect which gives insight into the energy
landscape of the system. On the other hand, the damping coefficient is a
constant that modulates the effect of external forces over the thermal energy
which particles have at their disposition to perform Brownian motion. For long
times, these thermal fluctuations become characterized in terms of an effective
temperature that is a consequence of the dynamic coupling between kinetic and
configurational degrees of freedom induced by the presence of the strong
external force. The interplay between collective mobility and effective
temperature allows to formulate a generalized Stokes-Einstein relation that may
be used to determine the collective diffusion coefficient. The explicit
relations we deduce reproduce simulation data remarkably well.
|
1609.09853v1
|
2016-10-16
|
Pulse-noise approach for classical spin systems
|
For systems of classical spins interacting with the bath via damping and
thermal noise, the approach is suggested to replace the white noise by a pulse
noise acting at regular time intervals $\Delta t$, within which the system
evolves conservatively. The method is working well in the typical case of a
small dimensionless damping constant $\lambda$ and allows a considerable
speed-up of computations by using high-order numerical integrators with a large
time step $\delta t$ (up to a fraction of the precession period), while keeping
$\delta t\ll\Delta t$ to reduce the relative contribution of noise-related
operations. In cases when precession can be discarded, $\delta t$ can be
increased up to a fraction of the relaxation time $\propto1/\lambda$ that leads
to a further speed-up. This makes equilibration speed comparable with that of
Metropolis Monte Carlo. The pulse-noise approach is tested on single-spin and
multi-spin models.
|
1610.04914v2
|
2017-03-22
|
New versions of Newton method: step-size choice, convergence domain and under-determined equations
|
Newton method is one of the most powerful methods for finding solutions of
nonlinear equations and for proving their existence. In its "pure" form it has
fast convergence near the solution, but small convergence domain. On the other
hand damped Newton method has slower convergence rate, but weaker conditions on
the initial point. We provide new versions of Newton-like algorithms, resulting
in combinations of Newton and damped Newton method with special step-size
choice, and estimate its convergence domain. Under some assumptions the
convergence is global. Explicit complexity results are also addressed. The
adaptive version of the algorithm (with no a priori constants knowledge) is
presented. The method is applicable for under-determined equations (with $m<n$,
$m$ being the number of equations and $n$ being the number of variables). The
results are specified for systems of quadratic equations, for composite
mappings and for one-dimensional equations and inequalities.
|
1703.07810v2
|
2017-05-21
|
Plasmon modes in graphene-GaAs heterostructures
|
We investigate the plasmon dispersion relation and damping rate of collective
excitations in a double-layer system consisting of bilayer graphene and GaAs
quantum well, separated by a distance, at zero temperature with no interlayer
tunneling. We use the random-phase-approximation dielectric function and take
into account the nonhomogeneity of the dielectric background of the system. We
show that the plasmon frequencies and damping rates depend considerably on
interlayer correlation parameters, electron densities and dielectric constants
of the contacting media.
|
1705.07389v1
|
2017-06-05
|
Consistent microscopic analysis of spin pumping effects
|
We present a consistent microscopic study of spin pumping effects for both
metallic and insulating ferromagnets. As for metallic case, we present a simple
quantum mechanical picture of the effect as due to the electron spin flip as a
result of a nonadiabatic (off-diagonal) spin gauge field. The effect of
interface spin-orbit interaction is briefly discussed. We also carry out
field-theoretic calculation to discuss on the equal footing the spin current
generation and torque effects such as enhanced Gilbert damping constant and
shift of precession frequency both in metallic and insulating cases. For thick
ferromagnetic metal, our study reproduces results of previous theories such as
the correspondence between the dc component of the spin current and enhancement
of the damping. For thin metal and insulator, the relation turns out to be
modified. For the insulating case, driven locally by interface $sd$ exchange
interaction due to magnetic proximity effect, physical mechanism is distinct
from the metallic case. Further study of proximity effect and interface
spin-orbit interaction would be crucial to interpret experimental results in
particular for insulators.
|
1706.01185v1
|
2017-08-02
|
Global existence of solutions for semi-linear wave equation with scale-invariant damping and mass in exponentially weighted spaces
|
In this paper we consider the following Cauchy problem for the semi-linear
wave equation with scale-invariant dissipation and mass and power
non-linearity: \begin{align}\label{CP abstract} \begin{cases} u_{tt}-\Delta
u+\dfrac{\mu_1}{1+t} u_t+\dfrac{\mu_2^2}{(1+t)^2}u=|u|^p, \\ u(0,x)=u_0(x),
\,\, u_t(0,x)=u_1(x), \end{cases}\tag{$\star$} \end{align} where $\mu_1,
\mu_2^2$ are nonnegative constants and $p>1$. On the one hand we will prove a
global (in time) existence result for \eqref{CP abstract} under suitable
assumptions on the coefficients $\mu_1, \mu_2^2$ of the damping and the mass
term and on the exponent $p$, assuming the smallness of data in exponentially
weighted energy spaces. On the other hand a blow-up result for \eqref{CP
abstract} is proved for values of $p$ below a certain threshold, provided that
the data satisfy some integral sign conditions. Combining these results we find
the critical exponent for \eqref{CP abstract} in all space dimensions under
certain assumptions on $\mu_1$ and $\mu_2^2$. Moreover, since the global
existence result is based on a contradiction argument, it will be shown firstly
a local (in time) existence result.
|
1708.00738v1
|
2017-08-21
|
Solar Line Asymmetries: Modelling the Effect of Granulation on the Solar Spectrum
|
A parametric model of granulation employing a small number of parameters was
developed. Synthetic spectra calculated using this model closely match observed
spectra and, in particular, reproduce the asymmetries observed in spectral
lines. Both the microturbulent motions and the large-scale flow velocity
decrease exponentially with a scale height of 368 km as the height within the
photosphere increases. The model agrees with observations of the solar
granulation (from which it was derived).
The horizontal motions associated with granulation were found and used to
calculate spectra emergent away from disk centre. These calculated spectra were
compared to observed spectra, with the agreement supporting the accuracy of the
granular model.
Also in the course of this work, the Brueckner-O'Mara damping theory was
found to predict damping constants accurately. The photospheric abundances of a
number of elements were determined. The abundance obtained for iron agrees with
the meteoric iron abundance. Astrophysical f-values for some lines were also
determined.
|
1708.06408v1
|
2017-11-01
|
Plasmon modes in bilayer-monolayer graphene heterostructures
|
We investigate the dispersion relation and damping of plasmon modes in a
bilayer-monolayer graphene heterostructure with carrier densities and at zero
temperature within the random-phase-approximation taking into account the
nonhomogeneity of the dielectric background of the system. We derive analytical
expressions for plasmon frequencies by using long wavelength expansion of
response and bare Coulomb interaction functions. We show that optical plasmon
dispersion curve of the bilayer-monolayer system lies slightly below that of
double-layer graphene (DLG) and the acoustic one lies much lower than that of
DLG. We find that while decay rates of acoustic modes of the system and DLG are
remarkably different, those of optical modes in both double-layer systems are
similar. Except the damping rate of acoustic mode, properties of plasmon
excitations in considered system depend remarkably on the interlayer distance,
inhomogeneity of the background, density ratio and spacer dielectric constant,
especially at large wave-vectors.
|
1711.00334v1
|
2018-07-15
|
Asymptotic profile of solutions for semilinear wave equations with structural damping
|
This paper is concerned with the initial value problem for semilinear wave
equation with structural damping $u_{tt}+(-\Delta)^{\sigma}u_t -\Delta u
=f(u)$, where $\sigma \in (0,\frac{1}{2})$ and $f(u) \sim |u|^p$ or $u
|u|^{p-1}$ with $p> 1 + {2}/(n - 2 \sigma)$. We first show the global existence
for initial data small in some weighted Sobolev spaces on $\mathcal R^n$ ($n
\ge 2$). Next, we show that the asymptotic profile of the solution above is
given by a constant multiple of the fundamental solution of the corresponding
parabolic equation, provided the initial data belong to weighted $L^1$ spaces.
|
1807.05509v3
|
2018-12-28
|
Axion Misalignment Driven to the Bottom
|
Several theoretical motivations point to ultralight QCD axions with large
decay constants $f_a \simeq \mathcal{O}(10^{16}-10^{17})$ GeV, to which
experimental proposals are dedicated. This regime is known to face the problem
of overproduction of axion dark matter from the misalignment mechanism unless
the misalignment angle $\theta_{\rm mis}$ is as small as
$\mathcal{O}(10^{-3}-10^{-4})$, which is generally considered a fine-tuning
problem. We investigate a dynamical explanation for a small $\theta_{\rm mis}$.
The axion mass arises from strong dynamics and may be sufficiently enhanced by
early dynamics so as to overcome Hubble friction and drive the field value to
the bottom of the potential long before the QCD phase transition. Together with
an approximate CP symmetry in the theory, this minimum is very closely related
to today's value and thus $\theta_{\rm mis}$ can automatically be well under
unity. Owing to such efficient relaxation, the isocurvature perturbations are
essentially damped. As an existence proof, using supersymmetric theories we
illustrate that the Higgs coupling with the inflaton energy can successfully
achieve this axion damping in a consistent inflationary cosmology.
|
1812.11186v2
|
2019-04-09
|
Ferromagnetic Resonance Studies of Strain tuned Bi:YIG Films
|
Bismuth-doped Yttrium iron garnet (Bi:YIG) thin films known for large
Magneto-optical activity with low losses still needs to get probed for its
magnetization dynamics. We demonstrate a controlled tuning of
magnetocrystalline anisotropy in Bi-doped Y_3 Fe_5 O_12 (Bi:YIG) films of high
crystalline quality using growth induced epitaxial strain on [111]-oriented
Gd_3 Ga_5 O_12 (GGG) substrate. We optimize a growth protocol to get thick
highly-strained epitaxial films showing large magneto-crystalline anisotropy,
compare to thin films prepared using a different protocol. Ferromagnetic
resonance measurements establish a linear dependence of the out-of-plane
uniaxial anisotropy on the strain induced rhombohedral distortion of Bi:YIG
lattice. Interestingly, the enhancement in the magnetoelastic constant due to
an optimum substitution of Bi^(3+) ions with strong spin orbit coupling does
not strongly affect the precessional damping (~2x10^(-3) ). Large
magneto-optical activity, reasonably low damping, large magnetocrystalline
anisotropy and large magnetoelastic coupling in BiYIG are the properties that
may help BiYIG emerge as a possible material for photo-magnonics and other
spintronics applications.
|
1904.04800v2
|
2019-04-25
|
Low damping magnetic properties and perpendicular magnetic anisotropy with strong volume contribution in the Heusler alloy Fe1.5CoGe
|
We present a study of the dynamic magnetic properties of TiN-buffered
epitaxial thin films of the Heusler alloy Fe$_{1.5}$CoGe. Thickness series
annealed at different temperatures are prepared and the magnetic damping is
measured, a lowest value of $\alpha=2.18\times 10^{-3}$ is obtained. The
perpendicular magnetic anisotropy properties in Fe$_{1.5}$CoGe/MgO are also
characterized. The evolution of the interfacial perpendicular anisotropy
constant $K^{\perp}_{\rm S}$ with the annealing temperature is shown and
compared with the widely used CoFeB/MgO interface. A large volume contribution
to the perpendicular anisotropy of $(4.3\pm0.5)\times 10^{5}$ $\rm J/m^3$ is
also found, in contrast with vanishing bulk contribution in common Co- and
Fe-based Heusler alloys.
|
1904.11247v1
|
2019-04-26
|
Terahertz spin dynamics driven by a field-derivative torque
|
Efficient manipulation of magnetization at ultrashort time scales is of
particular interest for future technology. Here, we numerically investigate the
influence of the so-called field-derivative torque, which was derived earlier
based on relativistic Dirac theory [Mondal et al., Phys. Rev. B 94, 144419
(2016)], on the spin dynamics triggered by ultrashort laser pulses. We find
that only considering the THz Zeeman field can underestimate the spin
excitation in antiferromagnetic oxide systems as, e.g., NiO and CoO. However,
accounting for both, the THz Zeeman torque and the field-derivative torque, the
amplitude of the spin excitation increases significantly. Studying the damping
dependence of field-derivative torque we observe larger effects for materials
having larger damping constants.
|
1904.11768v2
|
2019-05-30
|
Predicting New Iron Garnet Thin Films with Perpendicular Magnetic Anisotropy
|
Perpendicular magnetic anisotropy (PMA) is a necessary condition for many
spintronic applications like spin-orbit torques switching, logic and memory
devices. An important class of magnetic insulators with low Gilbert damping at
room temperature are iron garnets, which only have a few PMA types such as
terbium and samarium iron garnet. More and stable PMA garnet options are
necessary for researchers to be able to investigate new spintronic phenomena.
In this study, we predict 20 new substrate/magnetic iron garnet film pairs with
stable PMA at room temperature. The effective anisotropy energies of 10
different garnet films that are lattice-matched to 5 different commercially
available garnet substrates have been calculated using shape, magnetoelastic
and magnetocrystalline anisotropy terms. Strain type, tensile or compressive
depending on substrate choice, as well as the sign and the magnitude of the
magnetostriction constants of garnets determine if a garnet film may possess
PMA. We show the conditions in which Samarium, Gadolinium, Terbium, Holmium,
Dysprosium and Thulium garnets may possess PMA on the investigated garnet
substrate types. Guidelines for obtaining garnet films with low damping are
presented. New PMA garnet films with tunable saturation moment and field may
improve spin-orbit torque memory and compensated magnonic thin film devices.
|
1905.13042v1
|
2019-05-30
|
Intrinsically Undamped Plasmon Modes in Narrow Electron Bands
|
Surface plasmons in 2-dimensional electron systems with narrow Bloch bands
feature an interesting regime in which Landau damping (dissipation via
electron-hole pair excitation) is completely quenched. This surprising behavior
is made possible by strong coupling in narrow-band systems characterized by
large values of the "fine structure" constant $\alpha=e^2/\hbar \kappa v_{\rm
F}$. Dissipation quenching occurs when dispersing plasmon modes rise above the
particle-hole continuum, extending into the forbidden energy gap that is free
from particle-hole excitations. The effect is predicted to be prominent in
moir\'e graphene, where at magic twist-angle values, flat bands feature
$\alpha\gg1$. The extinction of Landau damping enhances spatial optical
coherence. Speckle-like interference, arising in the presence of disorder
scattering, can serve as a telltale signature of undamped plasmons directly
accessible in near-field imaging experiments.
|
1905.13088v2
|
2019-06-21
|
Control of eigenfunctions on surfaces of variable curvature
|
We prove a microlocal lower bound on the mass of high energy eigenfunctions
of the Laplacian on compact surfaces of negative curvature, and more generally
on surfaces with Anosov geodesic flows. This implies controllability for the
Schr\"odinger equation by any nonempty open set, and shows that every
semiclassical measure has full support. We also prove exponential energy decay
for solutions to the damped wave equation on such surfaces, for any nontrivial
damping coefficient. These results extend previous works [arXiv:1705.05019],
[arXiv:1712.02692], which considered the setting of surfaces of constant
negative curvature.
The proofs use the strategy of [arXiv:1705.05019], [arXiv:1712.02692] and
rely on the fractal uncertainty principle of [arXiv:1612.09040]. However, in
the variable curvature case the stable/unstable foliations are not smooth, so
we can no longer associate to these foliations a pseudodifferential calculus of
the type used in [arXiv:1504.06589]. Instead, our argument uses Egorov's
Theorem up to local Ehrenfest time and the hyperbolic parametrix of
[arXiv:0706.3242], together with the $C^{1+}$ regularity of the stable/unstable
foliations.
|
1906.08923v2
|
2019-07-17
|
Inhomogeneous domain walls in spintronic nanowires
|
In case of a spin-polarized current, the magnetization dynamics in nanowires
are governed by the classical Landau-Lifschitz equation with Gilbert damping
term, augmented by a typically non-variational Slonczewski term. Taking axial
symmetry into account, we study the existence of domain wall type coherent
structure solutions, with focus on one space dimension and spin-polarization,
but our results also apply to vanishing spin-torque term. Using methods from
bifurcation theory for arbitrary constant applied fields, we prove the
existence of domain walls with non-trivial azimuthal profile, referred to as
inhomogeneous. We present an apparently new type of domain wall, referred to as
non-flat, whose approach of the axial magnetization has a certain oscillatory
character. Additionally, we present the leading order mechanism for the
parameter selection of flat and non-flat inhomogeneous domain walls for an
applied field below a threshold, which depends on anisotropy, damping, and
spin-transfer. Moreover, numerical continuation results of all these domain
wall solutions are presented.
|
1907.07470v2
|
2019-09-06
|
The interplay of large two-magnon ferromagnetic resonance linewidths and low Gilbert damping in Heusler thin films
|
We report on broadband ferromagnetic resonance linewidth measurements
performed on epitaxial Heusler thin films. A large and anisotropic two-magnon
scattering linewidth broadening is observed for measurements with the
magnetization lying in the film plane, while linewidth measurements with the
magnetization saturated perpendicular to the sample plane reveal low Gilbert
damping constants of $(1.5\pm0.1)\times 10^{-3}$, $(1.8\pm0.2)\times 10^{-3}$,
and $<8\times 10^{-4}$ for Co$_2$MnSi/MgO, Co$_2$MnAl/MgO, and Co$_2$FeAl/MgO,
respectively. The in-plane measurements are fit to a model combining Gilbert
and two-magnon scattering contributions to the linewidth, revealing a
characteristic disorder lengthscale of 10-100 nm.
|
1909.02738v2
|
2019-12-10
|
Stability of traveling waves in a driven Frenkel-Kontorova model
|
In this work we revisit a classical problem of traveling waves in a damped
Frenkel-Kontorova lattice driven by a constant external force. We compute these
solutions as fixed points of a nonlinear map and obtain the corresponding
kinetic relation between the driving force and the velocity of the wave for
different values of the damping coefficient. We show that the kinetic curve can
become non-monotone at small velocities, due to resonances with linear modes,
and also at large velocities where the kinetic relation becomes multivalued.
Exploring the spectral stability of the obtained waveforms, we identify, at the
level of numerical accuracy of our computations, a precise criterion for
instability of the traveling wave solutions: monotonically decreasing portions
of the kinetic curve always bear an unstable eigendirection. We discuss why the
validity of this criterion in the {\it dissipative} setting is a rather
remarkable feature offering connections to the Hamiltonian variant of the model
and of lattice traveling waves more generally. Our stability results are
corroborated by direct numerical simulations which also reveal the possible
outcomes of dynamical instabilities.
|
1912.05052v2
|
2020-05-20
|
Dynamic Peach-Koehler self-force, inertia, and radiation damping of a regularized dislocation
|
The elastodynamic Peach-Koehler force is computed for a fully-regularized
straight dislocation with isotropic core in continuum isotropic elastic
elasticity, in compact forms involving partial mass or impulsion functions
relative to shear and compressional waves. The force accounts for both dynamic
radiation damping and inertia. The expressions are valid indifferently for
subsonic or supersonic velocities. Results are compared with the case of a
flat-core dislocation of the Peierls-Eshelby type, for a motion of jump from
rest to constant velocity. In the steady-state limit, the Lagrangian function
relevant to expressing the force in the flat-core case must be replaced by a
related but different function for the regularized dislocation. However, by
suitably defining the regularizing dislocation width, the steady-state limits
of the force for the fully-regularized and flat-core dislocations can be
matched exactly.
|
2005.12704v2
|
2020-06-29
|
Collective excitations in spin-polarized bilayer graphene
|
We calculate the plasmon frequency and damping rate of plasma oscillations in
a spin-polarized BLG system. Using the long wavelength approximation for
dynamical dielectric function, we obtain an analytical expression for plasmon
frequency showing that the degree of spin polarization P has negligible effect
on the long wavelength plasmon frequency. Numerical calculations demonstrate
that the degree of spin polarization affects slightly (strongly) plasmon
frequency at small (large) wave-vectors and the maximum value of damping rate
increases with increasing P. We also study the effects of carrier density and
substrate dielectric constant on plasmon properties for different value of spin
polarization. The numerically calculated critical wave-vector, at which the
plasmon dispersion curve hits the edge of electron-hole continuum, decreases
with P and can be used to determine experimentally the degree of spin
polarization.
|
2006.16042v2
|
2020-08-14
|
Testing Dissipative Collapse Models with a Levitated Micromagnet
|
We present experimental tests of dissipative extensions of spontaneous wave
function collapse models based on a levitated micromagnet with ultralow
dissipation. The spherical micromagnet, with radius $R=27$ $\mu$m, is levitated
by Meissner effect in a lead trap at $4.2$ K and its motion is detected by a
SQUID. We perform accurate ringdown measurements on the vertical translational
mode with frequency $57$ Hz, and infer the residual damping at vanishing
pressure $\gamma/2\pi<9$ $\mu$Hz. From this upper limit we derive improved
bounds on the dissipative versions of the CSL (continuous spontaneous
localization) and the DP (Di\'{o}si-Penrose) models with proper choices of the
reference mass. In particular, dissipative models give rise to an intrinsic
damping of an isolated system with the effect parameterized by a temperature
constant; the dissipative CSL model with temperatures below 1 nK is ruled out,
while the dissipative DP model is excluded for temperatures below $10^{-13}$ K.
Furthermore, we present the first bounds on dissipative effects in a more
recent model, which relates the wave function collapse to fluctuations of a
generalized complex-valued spacetime metric.
|
2008.06245v2
|
2020-08-14
|
Large enhancement of spin pumping due to the surface bound states in normal metal/superconductor structures
|
We show that the spin pumping from ferromagnetic insulator into the adjacent
metallic spin sink can be strongly stimulated by the superconducting
correlations.
The key physical mechanism responsible for this effect is the presence of
quasiparticle surface states at the ferromagnetic insulator/superconductor
interface. We consider the minimal model when these states appear because of
the suppressed pairing constant within the interfacial normal layer. For thin
normal layers we obtain a strongly peaked temperature dependence of the Gilbert
damping coefficient which has been recently observed in such systems. For
thicker normal layers the Gilbert damping monotonically increases down to the
temperatures much smaller than the critical one. The suggested model paves the
way to controlling the temperature dependence of the spin pumping by
fabricating hybrid normal metal/superconductor spin sinks.
|
2008.06253v1
|
2020-10-28
|
Spin-valley collective modes of the electron liquid in graphene
|
We develop the theory of collective modes supported by a Fermi liquid of
electrons in pristine graphene. Under reasonable assumptions regarding the
electron-electron interaction, all the modes but the plasmon are over-damped.
In addition to the $SU(2)$ symmetric spin mode, these include also the valley
imbalance modes obeying a $U(1)$ symmetry, and a $U(2)$ symmetric valley spin
imbalance mode. We derive the interactions and diffusion constants
characterizing the over-damped modes. The corresponding relaxation rates set
fundamental constraints on graphene valley- and spintronics applications.
|
2010.15154v2
|
2020-11-14
|
Oscillating charge currents of one-dimensional Hubbard model in an electric field
|
The time evolution properties of charge current for the one-dimensional
Hubbard model in an electric field have been studied in a rigorous manner. We
find that there is a complete and orthonormal set of time-evolution states for
which the charge current can only keep zero or oscillate constantly, differing
from the possible picture of damped or over-damped Bloch oscillations due to
strong correlations. It is also found that, associated with these states, there
is a set of constant phase factors, which are uniquely determined and are very
useful on discussing the long-time evolution behaviors of the system.
|
2011.07220v2
|
2021-01-15
|
Efficient Spin-Orbit Torque Generation in Semiconducting WTe2 with Hopping Transport
|
Spin-orbit torques (SOTs) from transition metal dichalcogenides systems
(TMDs) in conjunction with ferromagnetic materials are recently attractive in
spintronics for their versatile features. However, most of the previously
studied crystalline TMDs are prepared by mechanical exfoliation, which limits
their potentials for industrial applications. Here we show that amorphous WTe2
heterostructures deposited by magnetron sputtering possess a sizable
damping-like SOT efficiency {\xi}_DL^WTe2 ~ 0.20 and low damping constant
{\alpha} = 0.009/pm0.001. Only an extremely low critical switching current
density J_c ~ 7.05\times10^9 A/m^2 is required to achieve SOT-driven
magnetization switching. The SOT efficiency is further proved to depend on the
W and Te relative compositions in the co-sputtered W_100-xTe_x samples, from
which a sign change of {\xi}_DL^WTe2 is observed. Besides, the electronic
transport in amorphous WTe2 is found to be semiconducting and is governed by a
hopping mechanism. With the above advantages and rich tunability, amorphous and
semiconducting WTe2 serves as a unique SOT source for future spintronics
applications.
|
2101.06047v1
|
2021-03-13
|
Dissipative structures in a parametrically driven dissipative lattice: chimera, localized disorder, continuous-wave, and staggered state
|
Discrete dissipative coupled systems exhibit complex behavior such as chaos,
spatiotemporal intermittence, chimera among others. We construct and
investigate chimera states, in the form of confined stationary and dynamical
states in a chain of parametrically driven sites with onsite damping and cubic
nonlinearity. The system is modeled by the respective discrete parametrically
driven damped nonlinear Schrodinger equation. Chimeras feature quasi-periodic
or chaotic dynamic in the filled area, quantified by time dependence of the
total norm (along with its power spectrum), and by the largest Lyapunov
exponent. Systematic numerical simulations, in combination with some analytical
results, reveal regions in the parameter space populated by stable localized
states of different types. A phase transition from the stationary disorder
states to spatially confined dynamical chaotic one is identified. Essential
parameters of the system are the strength and detuning of the forcing, as well
as the lattice's coupling constant.
|
2103.07748v1
|
2021-05-31
|
Machine-Learning Non-Conservative Dynamics for New-Physics Detection
|
Energy conservation is a basic physics principle, the breakdown of which
often implies new physics. This paper presents a method for data-driven "new
physics" discovery. Specifically, given a trajectory governed by unknown
forces, our Neural New-Physics Detector (NNPhD) aims to detect new physics by
decomposing the force field into conservative and non-conservative components,
which are represented by a Lagrangian Neural Network (LNN) and a universal
approximator network (UAN), respectively, trained to minimize the force
recovery error plus a constant $\lambda$ times the magnitude of the predicted
non-conservative force. We show that a phase transition occurs at $\lambda$=1,
universally for arbitrary forces. We demonstrate that NNPhD successfully
discovers new physics in toy numerical experiments, rediscovering friction
(1493) from a damped double pendulum, Neptune from Uranus' orbit (1846) and
gravitational waves (2017) from an inspiraling orbit. We also show how NNPhD
coupled with an integrator outperforms previous methods for predicting the
future of a damped double pendulum.
|
2106.00026v2
|
2021-07-29
|
$n$-dimensional PDM-damped harmonic oscillators: Linearizability, and exact solvability
|
We consider position-dependent mass (PDM) Lagrangians/Hamiltonians in their
standard textbook form, where the long-standing \emph{gain-loss balance}
between the kinetic and potential energies is kept intact to allow conservation
of total energy (i.e., $L=T-V$, $H=T+V$, and $dH/dt=dE/dt=0$). Under such
standard settings, we discuss and report on $n$-dimensional PDM damped harmonic
oscillators (DHO). We use some $n$-dimensional point canonical transformation
to facilitate the linearizability of their $n$-PDM dynamical equations into
some $n$-linear DHOs' dynamical equations for constant mass setting.
Consequently, the well know exact solutions for the linear DHOs are mapped,
with ease, onto the exact solutions for PDM DHOs. A set of one-dimensional and
a set of $n$-dimensional PDM-DHO illustrative examples are reported along with
their phase-space trajectories.
|
2107.14617v1
|
2021-11-16
|
Ultrathin ferrimagnetic GdFeCo films with very low damping
|
Ferromagnetic materials dominate as the magnetically active element in
spintronic devices, but come with drawbacks such as large stray fields, and low
operational frequencies. Compensated ferrimagnets provide an alternative as
they combine the ultrafast magnetization dynamics of antiferromagnets with a
ferromagnet-like spin-orbit-torque (SOT) behavior. However to use ferrimagnets
in spintronic devices their advantageous properties must be retained also in
ultrathin films (t < 10 nm). In this study, ferrimagnetic Gdx(Fe87.5Co12.5)1-x
thin films in the thickness range t = 2-20 nm were grown on high resistance
Si(100) substrates and studied using broadband ferromagnetic resonance
measurements at room temperature. By tuning their stoichiometry, a nearly
compensated behavior is observed in 2 nm Gdx(Fe87.5Co12.5)1-x ultrathin films
for the first time, with an effective magnetization of Meff = 0.02 T and a low
effective Gilbert damping constant of {\alpha} = 0.0078, comparable to the
lowest values reported so far in 30 nm films. These results show great promise
for the development of ultrafast and energy efficient ferrimagnetic spintronic
devices.
|
2111.08768v1
|
2021-11-30
|
First and second order magnetic anisotropy and damping of europium iron garnet under high strain
|
Understanding and tailoring static and dynamic properties of magnetic
insulator thin films is important for spintronic device applications. Here, we
grow atomically flat epitaxial europium iron garnet (EuIG) thin films by pulsed
laser deposition on (111)-oriented garnet substrates with a range of lattice
parameters. By controlling the lattice mismatch between EuIG and the
substrates, we tune the strain in EuIG films from compressive to tensile
regime, which is characterized by X-ray diffraction. Using ferromagnetic
resonance, we find that in addition to the first-order perpendicular magnetic
anisotropy which depends linearly on the strain, there is a significant
second-order one that has a quadratic strain dependence. Inhomogeneous
linewidth of the ferromagnetic resonance increases notably with increasing
strain, while the Gilbert damping parameter remains nearly constant (~
2x10^-2). These results provide valuable insight into the spin dynamics in
ferrimagnetic insulators and useful guidance for material synthesis and
engineering of next-generation spintronics applications.
|
2111.15142v1
|
2022-07-17
|
Locational Aspect of Fast Frequency Reserves in Low-Inertia Systems -- Control Performance Analysis
|
This paper evaluates the frequency performance of an AC system when primary
frequency response is provided by inverter-based resources located at
remote-areas. Due to potentially larger wave propagation constants over longer
lines, fast active power response from inverter based resources may have a
negative impact on the system frequency response. Within this context, this
paper presents a control performance analysis is presented in order to identify
limitations for improving the frequency stability when inverter-based resources
in remote locations use local frequency measurements. Our results suggest that
there exists a trafeoff between disturbance rejection and stability robustness
when allocating primary frequency control. In particular, fast frequency
control can have a negative impact on the damping ratio of poorly damped
electromechanical modes.
|
2207.08188v1
|
2022-08-17
|
Linking fluctuation and dissipation in spatially extended out-of-equilibrium systems
|
For systems in equilibrium at a temperature $T$, thermal noise and energy
damping are related to $T$ through the fluctuation-dissipation theorem (FDT).
We study here an extension of the FDT to an out of equilibrium steady state: a
microcantilever subject to a constant heat flux. The resulting thermal profile
in this spatially extended system interplays with the local energy dissipation
field to prescribe the amplitude of mechanical fluctuations. Using three
samples with different damping profiles (localized or distributed), we probe
this approach and experimentally demonstrate the link between fluctuations and
dissipation. The thermal noise can therefore be predicted a priori from the
measurement of the dissipation as a function of the maximum temperature of the
micro-oscillator.
|
2208.08356v2
|
2022-09-07
|
Classical correlations for Generic States are Fragile under Decoherence
|
Quantum correlations typically decrease with increasing noise, although
classical correlators (CCors) may rise for a particular class of states with
noise. To analyse the behavior of classical correlation (CC) in the presence of
local noise, we scrutinize the set of classical correlators, axiomatic CC
measures like classical discord, and local work for Haar uniformly generated
states. Like quantum correlation measures, we illustrate that when noise levels
rise, the average value of the CC measures for noisy output states obtained
from random input states decreases for most of the channels. We also
demonstrate a connection between the CCors of the noise-affected multipartite
states that are produced and the CCors of the initial states that exhibit
exponential, polynomial, and constant behavior as the noise level changes.
Moreover, based on CCors of the generalised N-qubit W state as input, we
determine a method to discriminate between the quantum channels, namely phase
damping, depolarizing, and amplitude damping channels. We also relate
classical, quantum, and total correlation measures that exhibit a comparable
reaction to decoherence for generic states.
|
2209.03334v1
|
2022-10-19
|
Global well-posedness of the partially damped 2D MHD equations via a direct normal mode method for the anisotropic linear operator
|
We prove the global well-posedness of the 2D incompressible non-resistive MHD
equations with a velocity damping term near the non-zero constant background
magnetic field. To this end, we newly design a normal mode method of
effectively leveraging the anisotropy of the linear propagator that encodes
both the partially dissipative nature of the non-resistive MHD system and the
stabilizing mechanism of the underlying magnetic field. Isolating new key
quantities and estimating them with themselves in an entangling way via the
eigenvalue analysis based on Duhamel's formulation, we establish the global
well-posedness for any initial data $(v_0,B_0)$ that is sufficiently small in a
space rougher than $H^{4}\cap L^1$. This improves the recent work in SIAM J.
Math. Anal. 47, 2630-2656 (2015) where the similar result was obtained provided
that $(v_0,B_0)$ was small enough in a space strictly embedded in $H^{20}\cap
W^{6,1}$.
|
2210.10283v1
|
2022-11-07
|
On Vacuum Free Boundary Problem of the Spherically Symmetric Euler Equations with Damping and Solid Core
|
In this paper, the global existence of smooth solution and the long-time
asymptotic stability of the equilibrium to vacuum free boundary problem of the
spherically symmetric Euler equations with damping and solid core have been
obtained for arbitrary finite positive gas constant $A$ in the state equation
$p=A \rho^\gamma$ with $p$ being the pressure and $\rho$ the density, provided
that $\gamma>4/3,$ initial perturbation is small and the radius of the
equilibrium $R$ is suitably larger than the radius of the solid core $r_0$.
Moreover, we obtain the pointwise convergence from the smooth solution to the
equilibrium in a surprisingly exponential time-decay rate. The proof is mainly
based on weighted energy method in Lagrangian coordinate.
|
2211.03347v2
|
2022-11-16
|
Endemic Oscillations for SARS-CoV-2 Omicron -- A SIRS model analysis
|
The SIRS model with constant vaccination and immunity waning rates is well
known to show a transition from a disease-free to an endemic equilibrium as the
basic reproduction number $r_0$ is raised above threshold. It is shown that
this model maps to Hethcote's classic endemic model originally published in
1973. In this way one obtains unifying formulas for a whole class of models
showing endemic bifurcation. In particular, if the vaccination rate is smaller
than the recovery rate and $r_- < r_0 < r_+$ for certain upper and lower bounds
$r_\pm$, then trajectories spiral into the endemic equilibrium via damped
infection waves. Latest data of the SARS-CoV-2 Omicron variant suggest that
according to this simplified model continuous vaccination programs will not be
capable to escape the oscillating endemic phase. However, in view of the strong
damping factors predicted by the model, in reality these oscillations will
certainly be overruled by time-dependent contact behaviors.
|
2211.09005v2
|
Subsets and Splits
No community queries yet
The top public SQL queries from the community will appear here once available.