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2012-06-05
|
Effects of Variable Newton Constant During Inflation
|
In this paper the effects of time-dependent Newton constant G during
inflation are studied. We present the formalism of curvature perturbations in
an inflationary system with a time-dependent Newton constant. As an example we
consider a toy model in which G undergoes a sudden change during inflation. By
imposing the appropriate matching conditions the imprints of this sharp change
in G on curvature perturbation power spectrum are studied. We show that if G
increases (decreases) during the transition the amplitude of curvature
perturbations on large scales decreases (increases). In our model with a sudden
change in G a continuous sinusoidal modulations on curvature power spectrum is
induced. However, in a realistic scenario in which the change in G has some
finite time scale we expect these sinusoidal modulations to be damped on short
scales. The generated features may be used to explain the observed glitches on
CMB power spectrum. This puts a bound on $\Delta G$ during inflation of roughly
the same order as current bounds on $\Delta G$ during the entire observed age
of the universe.
|
1206.0903v2
|
2013-09-05
|
Spherical steady accretion flows -- dependence on the cosmological constant, exact isothermal solutions and applications to cosmology
|
We investigate spherical, isothermal and polytropic steady accretion models
in the presence of the cosmological constant. Exact solutions are found for
three classes of isothermal fluids, assuming the test gas approximation. The
cosmological constant damps the mass accretion rate and - above certain limit -
completely stops the steady accretion onto black holes. A "homoclinic-type"
accretion flow of polytropic gas has been discovered in AdS spacetimes in the
test-gas limit. These results can have cosmological connotation, through the
Einstein--Straus vacuole model of embedding local structures into
Friedman-Lemaitre-Robertson-Walker spacetimes. In particular one infers that
steady accretion would not exist in the late phases of the Penrose's scenario
of the evolution of the Universe, known as the Weyl curvature hypothesis.
|
1309.1252v1
|
2014-07-02
|
Basins of attraction in forced systems with time-varying dissipation
|
We consider dissipative periodically forced systems and investigate cases in
which having information as to how the system behaves for constant dissipation
may be used when dissipation varies in time before settling at a constant final
value. First, we consider situations where one is interested in the basins of
attraction for damping coefficients varying linearly between two given values
over many different time intervals: we outline a method to reduce the
computation time required to estimate numerically the relative areas of the
basins and discuss its range of applicability. Second, we observe that
sometimes very slight changes in the time interval may produce abrupt large
variations in the relative areas of the basins of attraction of the surviving
attractors: we show how comparing the contracted phase space at a time after
the final value of dissipation has been reached with the basins of attraction
corresponding to that value of constant dissipation can explain the presence of
such variations. Both procedures are illustrated by application to a pendulum
with periodically oscillating support.
|
1407.0556v1
|
2017-12-21
|
An Adaptive Passivity-Based Controller of a Buck-Boost Converter With a Constant Power Load
|
This paper addresses the problem of regulating the output voltage of a DC-DC
buck-boost converter feeding a constant power load,which is a problem of
current practical interest. Designing a stabilising controller is theoretically
challenging because its average model is a bilinear second order system that,
due to the presence of the constant power load,is non- minimum phase with
respect to both states.Moreover,to design a high performance controller, the
knowledge of the extracted load power, which is difficult to measure in
industrial applications, is required. In this paper, an adaptive
interconnection and damping assignment passivity based control that
incorporates the immersion and invariance parameter estimator for the load
power is proposed to solve the problem. Some detailed simulations are provided
to validate the transient behaviour of the proposed controller and compare it
with the performance of a classical PD scheme.
|
1712.07792v1
|
2023-08-03
|
Quasinormal modes of the spherical bumblebee black holes with a global monopole
|
The bumblebee model is an extension of the Einstein-Maxwell theory that
allows for the spontaneous breaking of the Lorentz symmetry of the spacetime.
In this paper, we study the quasinormal modes of the spherical black holes in
this model that are characterized by a global monopole. We analyze the two
cases with a vanishing cosmological constant or a negative one (the anti-de
Sitter case). We find that the black holes are stable under the perturbation of
a massless scalar field. However, both the Lorentz symmetry breaking and the
global monopole have notable impacts on the evolution of the perturbation. The
Lorentz symmetry breaking may prolong or shorten the decay of the perturbation
according to the sign of the breaking parameter. The global monopole, on the
other hand, has different effects depending on whether a nonzero cosmological
constant presences: it reduces the damping of the perturbations for the case
with a vanishing cosmological constant, but has little influence for the
anti-de Sitter case.
|
2308.01575v1
|
2009-08-12
|
Linear Fractionally Damped Oscillator
|
In this paper the linearly damped oscillator equation is considered with the
damping term generalized to a Caputo fractional derivative. The order of the
derivative being considered is 0 less than or equal to nu which is less than or
equal to 1 . At the lower end, nu = 0, the equation represents an un-damped
oscillator and at the upper end, nu = 1, the ordinary linearly damped
oscillator equation is recovered. A solution is found analytically and a
comparison with the ordinary linearly damped oscillator is made. It is found
that there are nine distinct cases as opposed to the usual three for the
ordinary equation (damped, over-damped, and critically damped). For three of
these cases it is shown that the frequency of oscillation actually increases
with increasing damping order before eventually falling to the limiting value
given by the ordinary damped oscillator equation. For the other six cases the
behavior is as expected, the frequency of oscillation decreases with increasing
order of the derivative (damping term).
|
0908.1683v1
|
1998-02-23
|
Shell Effects on Rotational Damping in Superdeformed Nuclei
|
Damping of rotational motion in superdeformed Hg and Dy-region nuclei is
studied by means of cranked shell model diagonalization. It is shown that a
shell oscillation in single-particle alignments affects significantly
properties of rotational damping. Onset properties of damping and damping width
for Hg are quite different from those for Dy-region superdeformed nuclei.
|
9802065v1
|
2003-08-29
|
Influence of radiative damping on the optical-frequency susceptibility
|
Motivated by recent discussions concerning the manner in which damping
appears in the electric polarizability, we show that (a) there is a dependence
of the nonresonant contribution on the damping and that (b) the damping enters
according to the "opposite sign prescription." We also discuss the related
question of how the damping rates in the polarizability are related to
energy-level decay rates.
|
0309001v1
|
2024-03-19
|
Weakly elliptic damping gives sharp decay
|
We prove that weakly elliptic damping gives sharp energy decay for the
abstract damped wave semigroup, where the damping is not in the functional
calculus. In this case, there is no overdamping. We show applications in
linearised water waves and Kelvin--Voigt damping.
|
2403.13067v1
|
2015-05-15
|
Reliable Damping of Free Surface Waves in Numerical Simulations
|
This paper generalizes existing approaches for free-surface wave damping via
momentum sinks for flow simulations based on the Navier-Stokes equations. It is
shown in 2D flow simulations that, to obtain reliable wave damping, the
coefficients in the damping functions must be adjusted to the wave parameters.
A scaling law for selecting these damping coefficients is presented, which
enables similarity of the damping in model- and full-scale. The influence of
the thickness of the damping layer, the wave steepness, the mesh fineness and
the choice of the damping coefficients are examined. An efficient approach for
estimating the optimal damping setup is presented. Results of 3D ship
resistance computations show that the scaling laws apply to such simulations as
well, so the damping coefficients should be adjusted for every simulation to
ensure convergence of the solution in both model and full scale. Finally,
practical recommendations for the setup of reliable damping in flow simulations
with regular and irregular free surface waves are given.
|
1505.04087v2
|
2019-02-25
|
Resonant absorption as a damping mechanism for the transverse oscillations of the coronal loops observed by SDO/AIA
|
Solar coronal loops represent the variety of fast, intermediate, and slow
normal mode oscillations. In this study, the transverse oscillations of the
loops with a few-minutes period and also with damping caused by the resonant
absorption were analyzed using extreme ultraviolet (EUV) images of the Sun. We
employed the 171 $\AA$ data recorded by Solar Dynamic Observatory
(SDO)/Atmospheric Imaging Assembly (AIA) to analyze the parameters of coronal
loop oscillations such as period, damping time, loop length, and loop width.
For the loop observed on 11 October 2013, the period and the damping of this
loop are obtained to be 19 and 70 minutes, respectively. The damping quality,
the ratio of the damping time to the period, is computed about 3.6. The period
and damping time for the extracted loop recorded on 22 January 2013 are about
81 and 6.79 minutes, respectively. The damping quality is also computed as 12.
It can be concluded that the damping of the transverse oscillations of the
loops is in the strong damping regime, so resonant absorption would be the main
reason for the damping.
|
1902.09649v1
|
2016-08-08
|
Damping Functions correct over-dissipation of the Smagorinsky Model
|
This paper studies the time-averaged energy dissipation rate $\langle
\varepsilon_{SMD} (u)\rangle$ for the combination of the Smagorinsky model and
damping function. The Smagorinsky model is well known to over-damp. One common
correction is to include damping functions that reduce the effects of model
viscosity near walls. Mathematical analysis is given here that allows
evaluation of $\langle \varepsilon_{SMD} (u)\rangle $ for any damping function.
Moreover, the analysis motivates a modified van Driest damping. It is proven
that the combination of the Smagorinsky with this modified damping function
does not over dissipate and is also consistent with Kolmogorov phenomenology.
|
1608.02655v2
|
2018-03-19
|
Fundamental Solutions and Gegenbauer Expansions of Helmholtz Operators in Riemannian Spaces of Constant Curvature
|
We perform global and local analysis of oscillatory and damped spherically
symmetric fundamental solutions for Helmholtz operators
$\big({-}\Delta\pm\beta^2\big)$ in $d$-dimensional, $R$-radius hyperbolic
${\mathbf H}_R^d$ and hyperspherical ${\mathbf S}_R^d$ geometry, which
represent Riemannian manifolds with positive constant and negative constant
sectional curvature respectively. In particular, we compute closed-form
expressions for fundamental solutions of $\big({-}\Delta \pm \beta^2\big)$ on
${\mathbf H}_R^d$, $\big({-}\Delta+\beta^2\big)$ on ${\mathbf S}_R^d$, and
present two candidate fundamental solutions for $\big({-}\Delta-\beta^2\big)$
on ${\mathbf S}_R^d$. Flat-space limits, with their corresponding asymptotic
representations, are used to restrict proportionality constants for these
fundamental solutions. In order to accomplish this, we summarize and derive new
large degree asymptotics for associated Legendre and Ferrers functions of the
first and second kind. Furthermore, we prove that our fundamental solutions on
the hyperboloid are unique due to their decay at infinity. To derive Gegenbauer
polynomial expansions of our fundamental solutions for Helmholtz operators on
hyperspheres and hyperboloids, we derive a collection of infinite series
addition theorems for Ferrers and associated Legendre functions which are
generalizations and extensions of the addition theorem for Gegenbauer
polynomials. Using these addition theorems, in geodesic polar coordinates for
dimensions greater than or equal to three, we compute Gegenbauer polynomial
expansions for these fundamental solutions, and azimuthal Fourier expansions in
two-dimensions.
|
1803.07149v2
|
2005-01-02
|
Effect of dipolar interactions on the magnetization of a cubic array of nanomagnets
|
We investigated the effect of intermolecular dipolar interactions on a cubic
3D ensemble of 5X5X4=100 nanomagnets, each with spin $S = 5$. We employed the
Landau-Lifshitz-Gilbert equation to solve for the magnetization $M(B)$ curves
for several values of the damping constant $\alpha$, the induction sweep rate,
the lattice constant $a$, the temperature $T$, and the magnetic anisotropy
field $H_A$. We find that the smaller the $\alpha$, the stronger the maximum
induction required to produce hysteresis. The shape of the hysteresis loops
also depends on the damping constant. We find further that the system
magnetizes and demagnetizes at decreasing magnetic field strengths with
decreasing sweep rates, resulting in smaller hysteresis loops. Variations of
$a$ within realistic values (1.5 nm - 2.5 nm) show that the dipolar interaction
plays an important role in the magnetic hysteresis by controlling the
relaxation process. The $T$ dependencies of $\alpha$ and of $M$ are presented
and discussed with regard to recent experimental data on nanomagnets. $H_A$
enhances the size of the hysteresis loops for external fields parallel to the
anisotropy axis, but decreases it for perpendicular external fields. Finally,
we reproduce and test an $M(B)$ curve for a 2D-system [M. Kayali and W. Saslow,
Phys. Rev. B {\bf 70}, 174404 (2004)]. We show that its hysteretic behavior is
only weakly dependent on the shape anisotropy field and the sweep rate, but
depends sensitively upon the dipolar interactions. Although in 3D systems,
dipole-dipole interactions generally diminish the hysteresis, in 2D systems,
they strongly enhance it. For both square 2D and rectangular 3D lattices with
${\bm B}||(\hat{\bm x}+\hat{\bm y})$, dipole-dipole interactions can cause
large jumps in the magnetization.
|
0501006v2
|
2015-03-16
|
Habitability of waterworlds: runaway greenhouses, atmospheric expansion and multiple climate states of pure water atmospheres
|
There are four different stable climate states for pure water atmospheres, as
might exist on so-called "waterworlds". I map these as a function of solar
constant for planets ranging in size from Mars size to 10 Earth-mass. The
states are: globally ice covered (Ts< 245K), cold and damp (270 < Ts< 290K),
hot and moist (350< Ts< 550K) and very hot and dry (Ts< 900K). No stable
climate exists for 290< Ts < 350K or 550 < Ts < 900K. The union of hot moist
and cold damp climates describe the liquid water habitable zone, the width and
location of which depends on planet mass. At each solar constant, two or three
different climate states are stable. This is a consequence of strong
non-linearities in both thermal emission and the net absorption of sunlight.
Across the range of planet sizes, I account for the atmospheres expanding to
high altitudes as they warm. The emitting and absorbing surfaces (optical depth
of unity) move to high altitude, making their area larger than the planet
surface, so more thermal radiation is emitted and more sunlight absorbed (the
former dominates). The atmospheres of small planets expand more due to weaker
gravity: the effective runaway greenhouse threshold is about 35Wm-2 higher for
Mars, 10Wm-2 higher for Earth or Venus but only a few Wm-2 higher for a 10
Earth-mass planet. There is an underlying (expansion neglected) trend of
increasing runaway greenhouse threshold with planetary size (40Wm-2 higher for
a 10 Earth-mass planet than for Mars). Summing these opposing trends means that
Venus-size (or slightly smaller) planets are most susceptible to a runaway
greenhouse.
The habitable zone for pure water atmospheres is very narrow, with an
insolation range of 0.07 times the solar constant. A wider habitable zone
requires background gas and greenhouse gas; N2 and CO2 on Earth, which are
biologically controlled. Thus, habitability depends on inhabitance.
|
1503.04835v1
|
2001-08-07
|
Dynamics and Origin of the 2:1 Orbital Resonances of the GJ 876 Planets
|
(Abridged) A dynamical fit has placed the two planets about the star GJ 876
in coplanar orbits deep in 3 resonances at the 2:1 mean-motion commensurability
with small libration amplitudes. The libration of both lowest order mean-motion
resonance variables, theta_1 and theta_2, and the secular resonance variable,
theta_3, about 0 deg. differs from the familiar geometry of the Io-Europa pair,
where theta_2 and theta_3 librate about 180 deg. By considering a condition for
stable simultaneous librations of theta_1 and theta_2, we show that the GJ 876
geometry results because of the large orbital eccentricities e_i, whereas the
very small e_i in the Io-Europa system lead to the latter's geometry.
Surprisingly, the GJ 876 resonance configuration remains stable for e_1 up to
0.86 and for amplitude of libration of theta_1 approaching 45 deg. with the
current e_i. We find that inward migration of the outer planet of the GJ 876
system results in certain capture into the observed resonances if initially e_1
<0.06 and e_2<0.03 and the migration rate |(da_2/dt)/a_2| < 0.03(a_2/AU)^{-3/2}
yr^{-1}. The bound on the migration rate is easily satisfied by migration due
to planet-nebula interaction. If there is no eccentricity damping, eccentricity
growth is rapid with continued migration within the resonance, with e_i
exceeding the observed values after a further reduction in the semi-major axes
a_i of only 7%. With eccentricity damping (de_i/dt)/e_i = -K|(da_i/dt)/a_i|,
the e_i reach equilibrium values that remain constant for arbitrarily long
migration within the resonances. The equilibrium e_i are close to the observed
e_i for K=100 (K=10) if there is migration and damping of the outer planet only
(of both planets). It is as yet unclear that planet-nebula interaction can
produce the large value of K required to obtain the observed eccentricities.
|
0108104v2
|
2007-09-25
|
On the Structure of Dark Matter Halos at the Damping Scale of the Power Spectrum with and without Relict Velocities
|
We report a series of high-resolution cosmological N-body simulations
designed to explore the formation and properties of dark matter halos with
masses close to the damping scale of the primordial power spectrum of density
fluctuations. We further investigate the effect that the addition of a random
component, v_rms, into the particle velocity field has on the structure of
halos. We adopted as a fiducial model the Lambda Warm Dark Matter cosmology
with a non-thermal sterile neutrino mass of 0.5 keV. The filtering mass
corresponds then to M_f = 2.6x10^12 M_sun/h. Halos of masses close to M_f were
simulated with several million of particles. The results show that, on one
hand, the inner density slope of these halos (at radii <~0.02 the virial radius
Rvir) is systematically steeper than the one corresponding to the NFW fit or to
the CDM counterpart. On the other hand, the overall density profile (radii
larger than 0.02Rvir) is less curved and less concentrated than the NFW fit,
with an outer slope shallower than -3. For simulations with v_rms, the inner
halo density profiles flatten significantly at radii smaller than 2-3 kpc/h
(<~0.010-0.015Rvir). A constant density core is not detected in our
simulations, with the exception of one halo for which the flat core radius is
~1 kpc/h. Nevertheless, if ``cored'' density profiles are used to fit the halo
profiles, the inferred core radii are ~0.1-0.8 kpc/h, in rough agreement with
theoretical predictions based on phase-space constrains, and on dynamical
models of warm gravitational collapse. A reduction of v_rms by a factor of 3
produces a modest decrease in core radii, less than a factor of 1.5. We discuss
the extension of our results into several contexts, for example, to the
structure of the cold DM micro-halos at the damping scale of this model.
|
0709.4027v1
|
2009-11-18
|
Slow Diffusive Gravitational Instability Before Decoupling
|
Radiative diffusion damps acoustic modes at large comoving wavenumber (k)
before decoupling (``Silk damping''). In a simple WKB analysis, neglecting
moments of the temperature distribution beyond the quadrupole, damping appears
in the acoustic mode as a term of order ik^2/(taudot) where taudot is the
scattering rate per unit conformal time. Although the Jeans instability is
stabilized on scales smaller than the adiabatic Jeans length, I show that the
medium is linearly unstable to first order in (1/taudot) to a slow diffusive
mode. At large comoving wavenumber, the characteristic growth rate becomes
independent of spatial scale and constant: (t_{KH}a)^-1 ~ (128 pi G/9 kappa_T
c)(rho_m/rho_b), where "a" is the scale factor, rho_m and rho_b are the matter
and baryon energy density, respectively, and kappa_T is the Thomson opacity.
This is the characteristic timescale for a fluid parcel to radiate away its
thermal energy content at the Eddington limit, analogous to the Kelvin-Helmholz
(KH) time for a massive star or the Salpeter time for black hole growth.
Although this mode grows at all times prior to decoupling and on scales smaller
than the horizon, the growth time is long, about 100 times the age of the
universe at decoupling. Thus, it modifies the density and temperature
perturbations on small scales only at the percent level. The physics of this
mode is already accounted for in the popular codes CMBFAST and CAMB, but is
typically neglected in analytic studies of the growth of primordial
perturbations. This work clarifies the physics of this instability in the epoch
before decoupling, and emphasizes that the universe is formally unstable on
scales below the horizon, even in the limit of large taudot. Analogous
instabilities at yet earlier epochs are also mentioned. (Abridged)
|
0911.3665v1
|
2010-04-02
|
Modeling the Time Variability of SDSS Stripe 82 Quasars as a Damped Random Walk
|
We model the time variability of ~9,000 spectroscopically confirmed quasars
in SDSS Stripe 82 as a damped random walk. Using 2.7 million photometric
measurements collected over 10 years, we confirm the results of Kelly et al.
(2009) and Koz{\l}owski et al. (2010) that this model can explain quasar light
curves at an impressive fidelity level (0.01-0.02 mag). The damped random walk
model provides a simple, fast [O(N) for N data points], and powerful
statistical description of quasar light curves by a characteristic time scale
(tau) and an asymptotic rms variability on long time scales (SF_inf). We
searched for correlations between these two variability parameters and physical
parameters such as luminosity and black hole mass, and rest-frame wavelength.
We find that tau increases with increasing wavelength with a power law index of
0.17, remains nearly constant with redshift and luminosity, and increases with
increasing black hole mass with power law index of 0.21+/-0.07. The amplitude
of variability is anti-correlated with the Eddington ratio, which suggests a
scenario where optical fluctuations are tied to variations in the accretion
rate. The radio-loudest quasars have systematically larger variability
amplitudes by about 30%, when corrected for the other observed trends, while
the distribution of their characteristic time scale is indistinguishable from
that of the full sample. We do not detect any statistically robust differences
in the characteristic time scale and variability amplitude between the full
sample and the small subsample of quasars detected by ROSAT. Our results
provide a simple quantitative framework for generating mock quasar light
curves, such as currently used in LSST image simulations. (abridged)
|
1004.0276v2
|
2013-01-21
|
Mass-metallicity relation from z=5 to the present: Evidence for a transition in the mode of galaxy growth at z=2.6 due to the end of sustained primordial gas infall
|
We analyze the redshift evolution of the mass-metallicity relation in a
sample of 110 Damped Ly$\alpha$ absorbers spanning the redshift range
$z=0.11-5.06$ and find that the zero-point of the correlation changes
significantly with redshift. The evolution is such that the zero-point is
constant at the early phases of galaxy growth (i.e. no evolution) but then
features a sharp break at $z=2.6\pm 0.2$ with a rapid incline towards lower
redshifts such that damped absorbers of identical masses are more metal rich at
later times than earlier. The slope of this mass metallicity correlation
evolution is $0.35 \pm 0.07$ dex per unit redshift.
We compare this result to similar studies of the redshift evolution of
emission selected galaxy samples and find a remarkable agreement with the slope
of the evolution of galaxies of stellar mass log$(M_{*}/M_\odot) \approx 8.5$.
This allows us to form an observational tie between damped absorbers and
galaxies seen in emission.
We use results from simulations to infer the virial mass of the dark matter
halo of a typical DLA galaxy and find a ratio $(M_{vir}/M_{*}) \approx 30$.
We compare our results to those of several other studies that have reported
strong transition-like events at redshifts around $z=2.5-2.6$ and argue that
all those observations can be understood as the consequence of a transition
from a situation where galaxies were fed more unprocessed infalling gas than
they could easily consume to one where they suddenly become infall starved and
turn to mainly processing, or re-processing, of previously acquired gas.
|
1301.5013v2
|
2013-02-13
|
Low mass planets in protoplanetary disks with net vertical magnetic fields: the Planetary Wake and Gap Opening
|
We study wakes and gap opening by low mass planets in gaseous protoplanetary
disks threaded by net vertical magnetic fields which drive
magnetohydrodynamical (MHD) turbulence through the magnetorotational instabilty
(MRI), using three dimensional simulations in the unstratified local shearing
box approximation. The wakes, which are excited by the planets, are damped by
shocks similar to the wake damping in inviscid hydrodynamic (HD) disks. Angular
momentum deposition by shock damping opens gaps in both MHD turbulent disks and
inviscid HD disks even for low mass planets, in contradiction to the "thermal
criterion" for gap opening. To test the "viscous criterion", we compared gap
properties in MRI-turbulent disks to those in viscous HD disks having the same
stress, and found that the same mass planet opens a significantly deeper and
wider gap in net vertical flux MHD disks than in viscous HD disks. This
difference arises due to the efficient magnetic field transport into the gap
region in MRI disks, leading to a larger effective \alpha within the gap. Thus,
across the gap, the Maxwell stress profile is smoother than the gap density
profile, and a deeper gap is needed for the Maxwell stress gradient to balance
the planetary torque density. We also confirmed the large excess torque close
to the planet in MHD disks, and found that long-lived density features (termed
zonal flows) produced by the MRI can affect planet migration. The comparison
with previous results from net toroidal flux/zero flux MHD simulations
indicates that the magnetic field geometry plays an important role in the gap
opening process. Overall, our results suggest that gaps can be commonly
produced by low mass planets in realistic protoplanetary disks, and caution the
use of a constant \alpha-viscosity to model gaps in protoplanetary disks.
|
1302.3239v1
|
2015-08-03
|
Using coronal seismology to estimate the magnetic field strength in a realistic coronal model
|
Coronal seismology is extensively used to estimate properties of the corona,
e.g. the coronal magnetic field strength are derived from oscillations observed
in coronal loops. We present a three-dimensional coronal simulation including a
realistic energy balance in which we observe oscillations of a loop in
synthesised coronal emission. We use these results to test the inversions based
on coronal seismology.
From the simulation of the corona above an active region we synthesise
extreme ultraviolet (EUV) emission from the model corona. From this we derive
maps of line intensity and Doppler shift providing synthetic data in the same
format as obtained from observations. We fit the (Doppler) oscillation of the
loop in the same fashion as done for observations to derive the oscillation
period and damping time.
The loop oscillation seen in our model is similar to imaging and
spectroscopic observations of the Sun. The velocity disturbance of the kink
oscillation shows an oscillation period of 52.5s and a damping time of 125s,
both being consistent with the ranges of periods and damping times found in
observation. Using standard coronal seismology techniques, we find an average
magnetic field strength of $B_{\rm kink}=79$G for our loop in the simulation,
while in the loop the field strength drops from some 300G at the coronal base
to 50G at the apex. Using the data from our simulation we can infer what the
average magnetic field derived from coronal seismology actually means. It is
close to the magnetic field strength in a constant cross-section flux tube that
would give the same wave travel time through the loop.
Our model produced not only a realistic looking loop-dominated corona, but
also provides realistic information on the oscillation properties that can be
used to calibrate and better understand the result from coronal seismology.
|
1508.00593v1
|
2021-08-05
|
Small-scale clumping at recombination and the Hubble tension
|
Despite the success of the standard $\Lambda$CDM model of cosmology, recent
data improvements have made tensions emerge between low- and high-redshift
observables, most importantly in determinations of the Hubble constant $H_0$
and the (rescaled) clustering amplitude $S_8$. The high-redshift data, from the
cosmic microwave background (CMB), crucially relies on recombination physics
for its interpretation. Here we study how small-scale baryon inhomogeneities
(i.e., clumping) can affect recombination and consider whether they can relieve
both the $H_0$ and $S_8$ tensions. Such small-scale clumping, which may be
caused by primordial magnetic fields or baryon isocurvature below kpc scales,
enhances the recombination rate even when averaged over larger scales, shifting
recombination to earlier times. We introduce a flexible clumping model,
parametrized via three spatial zones with free densities and volume fractions,
and use it to study the impact of clumping on CMB observables. We find that
increasing $H_0$ decreases both $\Omega_m$ and $S_8$, which alleviates the
$S_8$ tension. On the other hand, the shift in $\Omega_m$ is disfavored by the
low-$z$ baryon-acoustic-oscillations measurements. We find that the clumping
parameters that can change the CMB sound horizon enough to explain the $H_0$
tension also alter the damping tail, so they are disfavored by current Planck
2018 data. We test how the CMB damping-tail information rules out changes to
recombination by first removing $\ell>1000$ multipoles in Planck data, where we
find that clumping could resolve the $H_0$ tension. Furthermore, we make
predictions for future CMB experiments, as their improved damping-tail
precision can better constrain departures from standard recombination. Both the
Simons Observatory and CMB-S4 will provide decisive evidence for or against
clumping as a resolution to the $H_0$ tension.
|
2108.02747v3
|
2022-05-29
|
Modeling the Dynamics of the Coronavirus SARS-CoV-2 Pandemic using Modified SIR Model with the 'Damped-Oscillator' Dynamics of the Effective Reproduction Number
|
The COVID-19 pandemic has been a great catastrophe that upended human lives
and caused millions of deaths all over the world. The rapid spread of the
virus, with its early-stage exponential growth and subsequent 'waves', caught
many medical professionals and decision-makers unprepared. Even though
epidemiological models have been known for almost a century (since the 'Spanish
Influenza' pandemic of 1918-20), the real-life spread of the SARS-CoV-2 virus
often confounded the modelers. While the general framework of epidemiological
models like SEIR (susceptible-exposed-infected-recovered) or SIR
(susceptible-exposed-infected) was not in question, the behavior of model
parameters turned out to be unpredictable and complicated. In particular, while
the 'basic' reproduction number, R0, can be considered a constant (for the
original SARS-CoV-2 virus, prior to the emergence of variants, R0 is between
2.5 and 3.0), the 'effective' reproduction number, R(t), was a complex function
of time, influenced by human behavior in response to the pandemic (e.g.,
masking, lockdowns, transition to remote work, etc.) To better understand these
phenomena, we model the first year of the pandemic (between February 2020 and
February 2021) for a number of localities (fifty US states, as well as several
countries) using a simple SIR model. We show that the evolution of the pandemic
can be described quite successfully by assuming that R(t) behaves in a
'viscoelastic' manner, as a sum of two or three 'damped oscillators' with
different natural frequencies and damping coefficients. These oscillators
likely correspond to different sub-populations having different reactions to
proposed mitigation measures. The proposed approach can offer future data
modelers new ways to fit the reproduction number evolution with time (as
compared to the purely data-driven approaches most prevalent today).
|
2205.14747v1
|
2023-08-05
|
Modulating Spin Current Induced Effective Damping in $β-W/Py$ Heterostructures by a Systematic Variation in Resistivity of the Sputtered Deposited $β-W$ films
|
Utilizing the spin-induced pumping from a ferromagnet (FM) into a heavy metal
(HM) under the ferromagnetic resonance (FMR) condition, we report an
enhancement in effective damping in $\beta$- W/Py bilayers by systematically
varying resistivity ($\rho_{W}$) of $\beta$-W films. Different resistivity
ranging from 100 $\mu\Omega$-cm to 1400 $\mu\Omega$-cm with a thickness of 8 nm
can be achieved by varying the argon pressure ($P_{Ar}$) during the growth by
the method of sputtering. The coefficient of effective damping $\alpha_{eff}$
is observed to increase from 0.010 to 0.025 with $\rho_{W}$, which can be
modulated by $P_{Ar}$. We observe a modest dependence of $\alpha_{eff}$ on the
sputtering power ($p_{S}$) while keeping the $P_{Ar}$ constant. $\alpha_{eff}$
dependence on both $P_{Ar}$ and $p_{S}$ suggests that there exists a strong
correlation between $\alpha_{eff}$ and $\rho_{W}$. It is thus possible to
utilize $\rho_{W}$ as a tuning parameter to regulate the $\alpha_{eff}$, which
can be advantageous for faster magnetization dynamics switching. The thickness
dependence study of Py in the aforementioned bilayers manifests a higher spin
mixing conductance ($g^{\uparrow\downarrow}_{eff}$) which suggests a strong
spin pumping from Py into the $\beta$-W layer. The effective spin current
($J_{S(eff)}$) is also evaluated by considering the spin-back flow in this
process. Intrinsic spin mixing conductance ($g^{\uparrow\downarrow}_{W}$) and
spin diffusion length ($\lambda_{SD}$) of $\beta$-W are additionally
investigated using thickness variations in $\beta$-W. Furthermore, the
low-temperature study in $\beta$-W/Py reveals an intriguing temperature
dependence in $\alpha_{eff}$ which is quite different from $\alpha_{b}$ of
single Py layer and the enhancement in $\alpha_{eff}$ at low temperature can be
attributed to the spin-induced pumping from Py layer into $\beta$-W.
|
2308.02939v1
|
2023-12-26
|
Observation of Magnon Damping Minimum Induced by Kondo Coupling in a van der Waals Ferromagnet Fe$_{3-x}$GeTe$_{2}$
|
In heavy-fermion systems with $f$ electrons, there is an intricate interplay
between Kondo screening and magnetic correlations, which can give rise to
various exotic phases. Recently, similar interplay appears to also occur in
$d$-electron systems, but the underlying mechanism remains elusive. Here, using
inelastic neutron scattering, we investigate the temperature evolution of the
low-energy spin waves in a metallic van der Waals ferromagnet
Fe$_{3-x}$GeTe$_{2}$ (Curie temperature $T_{\rm C}\sim160$ K), where the
Kondo-lattice behavior emerges in the ferromagnetic phase below a
characteristic temperature $T^*\sim90$ K. We observe that the magnon damping
constant diverges at both low and high temperatures, exhibiting a minimum
coincidentally around $T^*$. Such an observation is analogous to the
resistivity minimum as due to the single-impurity Kondo effect. This unusual
behavior is described by a formula that combines logarithmic and power terms,
representing the dominant contributions from Kondo screening and thermal
fluctuations, respectively. Furthermore, we find that the magnon damping
increases with momentum below $T_{\rm C}$. These findings can be explained by
considering spin-flip electron-magnon scattering, which serves as a magnonic
analog of the Kondo-impurity scattering, and thus provides a measure of the
Kondo coupling through magnons. Our results provide critical insights into how
Kondo coupling manifests itself in a system with magnetic ordering and shed
light on the coexistence of and interplay between magnetic order and Kondo
effect in itinerant 3$d$-electron systems.
|
2312.15961v1
|
2022-07-13
|
Probing Bardeen-Kiselev black hole with cosmological constant caused by Einstein equations coupled with nonlinear electrodynamics using quasinormal modes and greybody bounds
|
In this work, we investigate a static and spherically symmetric
Bardeen-Kiselev black hole with cosmological constant which is a solution of
the Einstein-non-linear Maxwell field equations along with a quintessential
field. We compute the quasinormal frequencies for Bardeen-Kiselev black
hole(BH) with cosmological constant due to electromagnetic and gravitational
perturbations. By varying the BH parameters, we discuss the behaviour of both
real and imaginary parts of the BH quasinormal frequencies and compare
frequencies with Reissner-Nordstr\"om-de Sitter BH surrounded by quintessence
(RN-dSQ). Interestingly, it shows that the response of Bardeen-Kiselev BH with
cosmological constant and RN-dSQ under electromagnetic perturbations are
different when the charge parameter $q$, the state parameter $w$ and the
normalization factor $c$ are varied, but for the gravitational perturbations,
the response of Bardeen-Kiselev BH with cosmological constant and RN-dSQ are
different only when the charge parameter $q$ is varied. Therefore, compared
with the gravitational perturbations, the electromagnetic perturbations can be
used to understand nonlinear and linear electromagnetic fields in curved
spacetime separately. Another interesting observation is that due to the
presence of quintessence, the electromagnetic perturbations around the
Bardeen-Kiselev BH with cosmological constant damps faster and oscillates
slowly, and for the gravitational perturbations, the quasinormal mode decays
slowly and oscillates slowly. We also study the reflection and transmission
coefficients along with absorption cross section for the Bardeen-Kiselev BH
with cosmological constant, it shows that the transmission coefficients will
increase due to the presence of quintessence.
|
2207.05907v4
|
1997-10-23
|
Power Spectra for Cold Dark Matter and its Variants
|
The bulk of recent cosmological research has focused on the adiabatic cold
dark matter model and its simple extensions. Here we present an accurate
fitting formula that describes the matter transfer functions of all common
variants, including mixed dark matter models. The result is a function of
wavenumber, time, and six cosmological parameters: the massive neutrino
density, number of neutrino species degenerate in mass, baryon density, Hubble
constant, cosmological constant, and spatial curvature. We show how
observational constraints---e.g. the shape of the power spectrum, the abundance
of clusters and damped Lyman-alpha systems, and the properties of the
Lyman-alpha forest--- can be extended to a wide range of cosmologies, including
variations in the neutrino and baryon fractions in both high-density and
low-density universes.
|
9710252v1
|
2004-11-13
|
K-shell Photoabsorption of Oxygen Ions
|
Extensive calculations of the atomic data required for the spectral modelling
of the K-shell photoabsorption of oxygen ions have been carried out in a
multi-code approach. The present level energies and wavelengths for the highly
ionized species (electron occupancies 2 <= N <= 4) are accurate to within 0.5
eV and 0.02 A, respectively. For N > 4, lack of measurements, wide experimental
scatter, and discrepancies among theoretical values are handicaps in reliable
accuracy assessments. The radiative and Auger rates are expected to be accurate
to 10% and 20%, respectively, except for transitions involving strongly mixed
levels. Radiative and Auger dampings have been taken into account in the
calculation of photoabsorption cross sections in the K-threshold region,
leading to overlapping lorentzian shaped resonances of constant widths that
cause edge smearing. The behavior of the improved opacities in this region has
been studied with the XSTAR modelling code using simple constant density slab
models, and is displayed for a range of ionization parameters.
|
0411374v2
|
2005-12-12
|
Most precise single redshift bound to the variability of the fine-structure constant
|
Verification of theoretical predictions of an oscillating behavior of the
fine-structure constant, alpha, with cosmic time requires high precision
measurements at individual redshifts, while in earlier studies the mean Delta
alpha/alpha values averaged over wide redshift intervals were usually reported.
This requirement can be met via the Single Ion Differential alpha Measurement
(SIDAM) procedure. We apply SIDAM to the FeII lines associated with the damped
Ly-alpha system observed at z=1.15 in the spectrum of HE0515-4414. The weighted
mean calculated on base of carefully selected 34 FeII pairs is <Delta
alpha/alpha>=(-0.07+/-0.84)10^{-6}. The precision of this estimate represents
the absolute improvement with respect to what has been done in the measurements
of Delta alpha/alpha.
|
0512287v1
|
2000-11-29
|
Corrugation of Roads
|
We present a one dimensional model for the development of corrugations in
roads subjected to compressive forces from a flux of cars. The cars are modeled
as damped harmonic oscillators translating with constant horizontal velocity
across the surface, and the road surface is subject to diffusive relaxation. We
derive dimensionless coupled equations of motion for the positions of the cars
and the road surface H(x,t), which contain two phenomenological variables: an
effective diffusion constant Delta(H) that characterizes the relaxation of the
road surface, and a function alpha(H) that characterizes the plasticity or
erodibility of the road bed. Linear stability analysis shows that corrugations
grow if the speed of the cars exceeds a critical value, which decreases if the
flux of cars is increased. Modifying the model to enforce the simple fact that
the normal force exerted by the road can never be negative seems to lead to
restabilized, quasi-steady road shapes, in which the corrugation amplitude and
phase velocity remain fixed.
|
0011510v2
|
2005-07-28
|
Theory of transverse spin dynamics in a polarized Fermi liquid and an itinerant ferromagnet
|
The linear equations for transverse spin dynamics in a weakly polarized
degenerate Fermi liquid with arbitrary relationship between temperature and
polarization are derived from Landau-Silin phenomenological kinetic equation
with general form of two-particle collision integral. Unlike the previous
treatment where Fermi velocity and density of states have been taken as
constants independent of polarization here we made derivation free from this
assumption. The obtained equations are applicable for description of spin
dynamics in paramagnetic Fermi liquid with finite polarization as well in an
itinerant ferromagnet. In both cases transverse spin wave frequency is found to
be proportional to the square of the wave vector with complex constant of
proportionality (diffusion coefficient) such that the damping has a finite
value at T=0. The polarization dependence of the diffusion coefficient is found
to be different for a polarized Fermi liquid and for an itinerant ferromagnet.
These conclusions are confirmed by derivation of transverse spin wave
dispersion law in frame of field theoretical methods from the integral equation
for the vortex function. It is shown that similar derivation taking into
consideration the divergency of static transverse susceptibility also leads to
the same attenuating spin wave spectrum.
|
0507675v1
|
2006-04-21
|
Dynamic approach for micromagnetics close to the Curie temperature
|
In conventional micromagnetism magnetic domain configurations are calculated
based on a continuum theory for the magnetization which is assumed to be of
constant length in time and space. Dynamics is usually described with the
Landau-Lifshitz-Gilbert (LLG) equation the stochastic variant of which includes
finite temperatures. Using simulation techniques with atomistic resolution we
show that this conventional micromagnetic approach fails for higher
temperatures since we find two effects which cannot be described in terms of
the LLG equation: i) an enhanced damping when approaching the Curie temperature
and, ii) a magnetization magnitude that is not constant in time. We show,
however, that both of these effects are naturally described by the
Landau-Lifshitz-Bloch equation which links the LLG equation with the theory of
critical phenomena and turns out to be a more realistic equation for
magnetization dynamics at elevated temperatures.
|
0604508v1
|
2004-10-08
|
An Effective Field Theory at Finite Density
|
An effective theory to treat the dense nuclear medium by the perturbative
expansion method is proposed as a natural extension of the Heavy Baryon Chiral
Perturbation Theory (HBChPT). Treating the Fermi momentum scale as a separate
scale of the system, we get an improved convergence and the conceptually clear
interpretation. We compute the pion decay constant and the pion velocity in the
nuclear medium, and find their characters different from what the usual HBChPT
predicts. We also obtain the Debye screening scale at the normal nuclear matter
density, and the damping scale of the pion wave. Those results indicate that
the present theory, albeit its improvement over the HBChPT, has the limitation
yet to go over to the medium of about 1.3 times of normal matter density due to
the absence of the intrinsic density dependence of the coupling constants. We
discuss how we overcome this limitation in terms of the renormalization method.
|
0410124v1
|
2006-10-16
|
Multichannel calculation of the very narrow $D_{s0}^*(2317)$ and the very broad $D_0^*(2300-2400)$
|
The narrow $D_{s0}^{\ast}$(2317) and broad $D_0^{\ast}$(2300-2400) charmed
scalar mesons and their radial excitations are described in a coupled-channel
quark model that also reproduces the properties of the light scalar nonet. All
two-meson channels containing ground-state pseudoscalars and vectors are
included. The parameters are chosen fixed at published values, except for the
overall coupling constant $\lambda$, which is fine-tuned to reproduce the
$D_{s0}^{\ast}$(2317) mass, and a damping constant $\alpha$ for subthreshold
contributions. Variations of $\lambda$ and $D_0^{\ast}$(2300-2400) pole
postions are studied for different $\alpha$ values. Calculated cross sections
for $S$-wave $DK$ and $D\pi$ scattering, as well as resonance pole positions,
are given for the value of $\alpha$ that fits the light scalars. The thus
predicted radially excited state ${D_{s0}^*}'$(2850), with a width of about 50
MeV, seems to have been observed already.
|
0610188v1
|
1997-10-03
|
Zero-Temperature Casimir Fluctuations and the Limits of Force Microscope Sensitivity
|
It is predicted that in force microscopy the quantum fluctuations responsible
for the Casimir force can be directly observed as temperature-independent force
fluctuations having spectral density $9\pi/(40\ln(4/e)) \hbar \delta k$, where
$\hbar$ is Planck's constant and $\delta k$ is the observed change in spring
constant as the microscope tip approaches a sample. For typical operating
parameters the predicted force noise is of order $10^{-18}$ Newton in one Hertz
of bandwidth. The Second Law is respected via the fluctuation-dissipation
theorem. For small tip-sample separations the cantilever damping is predicted
to increase as temperature is reduced, a behavior that is reminiscent of the
Kondo effect.
|
9710017v3
|
2001-01-16
|
Transient QED effects in absorbing dielectrics
|
The spontaneous emission rate of a radiating atom reaches its
time-independent equilibrium value after an initial transient regime. In this
paper we consider the associated relaxation effects of the spontaneous decay
rate of atoms in dispersive and absorbing dielectric media for atomic
transition frequencies near material resonances. A quantum mechanical
description of such media is furnished by a damped-polariton model, in which
absorption is taken into account through coupling to a bath. We show how all
field and matter operators in this theory can be expressed in terms of the bath
operators at an initial time. The consistency of these solutions for the field
and matter operators are found to depend on the validity of certain velocity
sum rules. The transient effects in the spontaneous decay rate are studied with
the help of several specific models for the dielectric constant, which are
shown to follow from the general theory by adopting particular forms of the
bath coupling constant.
|
0101075v1
|
2009-02-16
|
A polycrystalline graphite model for the 2175 Angstrom interstellar extinction band
|
A random, hydrogen-free, assembly of microscopic sp2 carbon chips, forming a
macroscopically homogeneous and isotropic solid, is proposed as a model carrier
for the UV interstellar extinction band . The validity of this model is based
on the calculation of the Bruggeman average dielectric function of a mixture of
the known parallel and perpendicular dielectric functions of graphite. The pi
absorption feature of Rayleigh-sized spheres of this mixture falls near 4.6
mu-1 (2175 Angstroms), but its width is 1.5 mu-1, somewhat larger than the
astronomically observed average, 1 mu-1. This is confirmed by measurements of
the reflectance of an industrial material, polycrystalline graphite. A better
fit to the IS feature position and width is obtained with a hypothetical
material, having the same dielectric functions as natural graphite, except for
less extended wings of the pi resonance. Physically, this could result from
changes in the electronic band structure due to previous thermal histories. On
this model, the Frolich feature central wavelength depends only on the pi
resonance frequency, while its width depends only on the damping constant of
the same resonance. This explains the range of observed feature widths at
constant feature wavelength.
|
0902.2637v1
|
2009-04-18
|
Brownian Dynamics of charged particles in a constant magnetic field
|
Numerical algorithms are proposed for simulating the Brownian dynamics of
charged particles in an external magnetic field, taking into account the
Brownian motion of charged particles, damping effect and the effect of magnetic
field self-consistently. Performance of these algorithms is tested in terms of
their accuracy and long-time stability by using a three-dimensional Brownian
oscillator model with constant magnetic field. Step-by-step recipes for
implementing these algorithms are given in detail. It is expected that these
algorithms can be directly used to study particle dynamics in various dispersed
systems in the presence of a magnetic field, including polymer solutions,
colloidal suspensions and, particularly complex (dusty) plasmas. The proposed
algorithms can also be used as thermostat in the usual molecular dynamics
simulation in the presence of magnetic field.
|
0904.2849v1
|
2009-05-18
|
Far-infrared optical excitations in multiferroic TbMnO_3
|
We provide a detailed study of the reflectivity of multiferroic TbMnO_3 for
wave numbers from 40 cm^{-1} to 1000 cm^{-1} and temperatures 5 K < T < 300 K.
Excitations are studied for polarization directions E || a, the polarization
where electromagnons are observed, and for E || c, the direction of the
spontaneous polarization in this material. The temperature dependencies of
eigenfrequencies, damping constants and polar strengths of all modes are
studied and analyzed. For E || a and below the spiral ordering temperature of
about 27 K we observe a transfer of optical weight from phonon excitations to
electromagnons, which mainly involves low-frequency phonons. For E || c an
unusual increase of the total polar strength and hence of the dielectric
constant is observed indicating significant transfer of dynamic charge probably
within manganese-oxygen bonds on decreasing temperatures.
|
0905.2921v1
|
2009-11-08
|
Complete characterization and synthesis of the response function of elastodynamic networks
|
The response function of a network of springs and masses, an elastodynamic
network, is the matrix valued function $W(\omega)$, depending on the frequency
$\omega$, mapping the displacements of some accessible or terminal nodes to the
net forces at the terminals. We give necessary and sufficient conditions for a
given function $W(\omega)$ to be the response function of an elastodynamic
network, assuming there is no damping. In particular we construct an
elastodynamic network that can mimic a suitable response in the frequency or
time domain. Our characterization is valid for networks in three dimensions and
also for planar networks, which are networks where all the elements,
displacements and forces are in a plane. The network we design can fit within
an arbitrarily small neighborhood of the convex hull of the terminal nodes,
provided the springs and masses occupy an arbitrarily small volume.
Additionally, we prove stability of the network response to small changes in
the spring constants and/or addition of springs with small spring constants.
|
0911.1501v1
|
2012-05-31
|
Resonance Plasmon Linewidth Oscillations in Spheroidal Metallic Nanoparticle Embedded in a Dielectric Matrix
|
The kinetic approach is applied to calculate oscillations of a surface
plasmon linewidth in a spheroidal metal nanoparticle embedded in any dielectric
media. The principal attention is focused on the case, when the free electron
path is much greater than the particle size.
The linewidth of the plasmon resonance as a function of the particle radius,
shape, dielectric constant of the surrounding medium, and the light frequency
is studied in detail. It is found that the resonance plasmon linewidth
oscillates with increasing both the particle size and the dielectric constant
of surrounding medium.
The main attention is paid to the electron surface-scattering contribution to
the plasmon decay.
All calculations the plasmon resonance linewidth are illustrated by the
example of the Na nanoparticles with different radii.
The results obtained in the kinetic approach are compared with the known ones
from other models.
The role of the radiative damping is discussed as well.
|
1205.6959v2
|
2012-06-21
|
Direct Observation of Massless Domain Wall Dynamics in Nanostripes with Perpendicular Magnetic Anisotropy
|
Domain wall motion induced by nanosecond current pulses in nanostripes with
perpendicular magnetic anisotropy (Pt/Co/AlO$_x$) is shown to exhibit
negligible inertia. Time-resolved magnetic microscopy during current pulses
reveals that the domain walls start moving, with a constant speed, as soon as
the current reaches a constant amplitude, and no or little motion takes place
after the end of the pulse. The very low 'mass' of these domain walls is
attributed to the combination of their narrow width and high damping parameter
$\alpha$. Such a small inertia should allow accurate control of domain wall
motion, by tuning the duration and amplitude of the current pulses.
|
1206.4967v1
|
2015-06-10
|
Tunable inertia of chiral magnetic domain walls
|
The time it takes to accelerate an object from zero to a given velocity
depends on the applied force and the environment. If the force ceases, it takes
exactly the same time to completely decelerate. A magnetic domain wall (DW) is
a topological object that has been observed to follow this behavior. Here we
show that acceleration and deceleration times of chiral Neel walls driven by
current are different in a system with low damping and moderate
Dzyaloshinskii-Moriya (DM) exchange constant. The time needed to accelerate a
DW with current via the spin Hall torque is much faster than the time it needs
to decelerate once the current is turned off. The deceleration time is defined
by the DM exchange constant whereas the acceleration time depends on the spin
Hall torque, enabling tunable inertia of chiral DWs. Such unique feature of
chiral DWs can be utilized to move and position DWs with lower current, key to
the development of storage class memory devices.
|
1506.03490v2
|
2015-06-23
|
The Vlasov-Poisson System for Stellar Dynamics in Spaces of Constant Curvature
|
We obtain a natural extension of the Vlasov-Poisson system for stellar
dynamics to spaces of constant Gaussian curvature $\kappa\ne 0$: the unit
sphere $\mathbb S^2$, for $\kappa>0$, and the unit hyperbolic sphere $\mathbb
H^2$, for $\kappa<0$. These equations can be easily generalized to higher
dimensions. When the particles move on a geodesic, the system reduces to a
1-dimensional problem that is more singular than the classical analogue of the
Vlasov-Poisson system. In the analysis of this reduced model, we study the
well-posedness of the problem and derive Penrose-type conditions for linear
stability around homogeneous solutions in the sense of Landau damping.
|
1506.07090v1
|
2015-06-24
|
Holographic Tunneling Wave Function
|
The Hartle-Hawking wave function in cosmology can be viewed as a decaying
wave function with anti-de Sitter (AdS) boundary conditions. We show that the
growing wave function in AdS familiar from Euclidean AdS/CFT is equivalent,
semiclassically and up to surface terms, to the tunneling wave function in
cosmology. The cosmological measure in the tunneling state is given by the
partition function of certain relevant deformations of CFTs on a locally AdS
boundary. We compute the partition function of finite constant mass
deformations of the O(N) vector model on the round three sphere and show this
qualitatively reproduces the behaviour of the tunneling wave function in
Einstein gravity coupled to a positive cosmological constant and a massive
scalar. We find the amplitudes of inhomogeneities are not damped in the
holographic tunneling state.
|
1506.07374v2
|
2015-11-19
|
Memory effects and active Brownian diffusion
|
A self-propelled artificial microswimmer is often modeled as a ballistic
Brownian particle moving with constant speed aligned along one of its axis, but
changing direction due to random collisions with the environment. Similarly to
thermal noise, its angular randomization is described as a memoryless
stochastic process. Here, we speculate that finite-time correlations in the
orientational dynamics can affect the swimmer's diffusivity. To this purpose we
propose and solve two alternative models. In the first one we simply assume
that the environmental fluctuations governing the swimmer's propulsion are
exponentially correlated in time, whereas in the second one we account for
possible damped fluctuations of the propulsion velocity around the swimmer's
axis. The corresponding swimmer's diffusion constants are predicted to get,
respectively, enhanced or suppressed upon increasing the model memory time.
Possible consequences of this effect on the interpretation of the experimental
data are discussed.
|
1511.06113v1
|
2016-05-12
|
Phase coexistence and spatial correlations in reconstituting k-mer models
|
In reconstituting k-mer models, extended objects which occupy several sites
on a one dimensional lattice, undergo directed or undirected diffusion, and
reconstitute -when in contact- by transferring a single monomer unit from one
k-mer to the other; the rates depend on the size of participating k-mers. This
polydispersed system has two conserved quantities, the number of k-mers and the
packing fraction. We provide a matrix product method to write the steady state
of this model and to calculate the spatial correlation functions analytically.
We show that for a constant reconstitution rate, the spatial correlation
exhibits damped oscillations in some density regions separated, from other
regions with exponential decay, by a disorder surface. In a specific limit,
this constant-rate reconstitution model is equivalent to a single dimer model
and exhibits a phase coexistence similar to the one observed earlier in totally
asymmetric simple exclusion process on a ring with a defect.
|
1605.03859v2
|
2016-10-20
|
Evolving Planck Mass in Classically Scale-Invariant Theories
|
We consider classically scale-invariant theories with non-minimally coupled
scalar fields, where the Planck mass and the hierarchy of physical scales are
dynamically generated. The classical theories possess a fixed point, where
scale invariance is spontaneously broken. In these theories, however, the
Planck mass becomes unstable in the presence of explicit sources of scale
invariance breaking, such as non-relativistic matter and cosmological constant
terms. We quantify the constraints on such classical models from Big Bang
Nucleosynthesis that lead to an upper bound on the non-minimal coupling and
require trans-Planckian field values. We show that quantum corrections to the
scalar potential can stabilise the fixed point close to the minimum of the
Coleman-Weinberg potential. The time-averaged motion of the evolving fixed
point is strongly suppressed, thus the limits on the evolving gravitational
constant from Big Bang Nucleosynthesis and other measurements do not presently
constrain this class of theories. Field oscillations around the fixed point, if
not damped, contribute to the dark matter density of the Universe.
|
1610.06571v3
|
2017-01-30
|
Regularized solutions for some backward nonlinear parabolic equations with statistical data
|
In this paper, we study the backward problem of determining initial condition
for some class of nonlinear parabolic equations in multidimensional domain
where data are given under random noise. This problem is ill-posed, i.e., the
solution does not depend continuously on the data. To regularize the instable
solution, we develop some new methods to construct some new regularized
solution. We also investigate the convergence rate between the regularized
solution and the solution of our equations. In particular, we establish results
for several equations with constant coefficients and time dependent
coefficients. The equations with constant coefficients include heat equation,
extended Fisher-Kolmogorov equation, Swift-Hohenberg equation and many others.
The equations with time dependent coefficients include Fisher type Logistic
equations, Huxley equation, Fitzhugh-Nagumo equation. The methods developed in
this paper can also be applied to get approximate solutions to several other
equations including 1-D Kuramoto-Sivashinsky equation, 1-D modified
Swift-Hohenberg equation, strongly damped wave equation and 1-D Burger's
equation with randomly perturbed operator.
|
1701.08459v2
|
2017-11-17
|
Cosmological abundance of the QCD axion coupled to hidden photons
|
We study the cosmological evolution of the QCD axion coupled to hidden
photons. For a moderately strong coupling, the motion of the axion field leads
to an explosive production of hidden photons by tachyonic instability. We use
lattice simulations to evaluate the cosmological abundance of the QCD axion. In
doing so, we incorporate the backreaction of the produced hidden photons on the
axion dynamics, which becomes significant in the non-linear regime. We find
that the axion abundance is suppressed by at most ${\cal O}(10^{2})$ for the
decay constant $f_a = 10^{16}$ GeV, compared to the case without the coupling.
For a sufficiently large coupling, the motion of the QCD axion becomes strongly
damped, and as a result, the axion abundance is enhanced. Our results show that
the cosmological upper bound on the axion decay constant can be relaxed by a
few hundred for a certain range of the coupling to hidden photons.
|
1711.06590v2
|
2017-12-28
|
Eddy diffusivity of quasi-neutrally-buoyant inertial particles
|
We investigate the large-scale transport properties of
quasi-neutrally-buoyant inertial particles carried by incompressible zero-mean
periodic or steady ergodic flows. We show how to compute large-scale indicators
such as the inertial-particle terminal velocity and eddy diffusivity from first
principles in a perturbative expansion around the limit of added-mass factor
close to unity. Physically, this limit corresponds to the case where the mass
density of the particles is constant and close in value to the mass density of
the fluid which is also constant. Our approach differs from the usual
over-damped expansion inasmuch we do not assume a separation of time scales
between thermalization and small-scale convection effects. For general
incompressible flows, we derive closed-form cell equations for the auxiliary
quantities determining the terminal velocity and effective diffusivity. In the
special case of parallel flows these equations admit explicit analytic
solution. We use parallel flows to show that our approach enables to shed light
onto the behavior of terminal velocity and effective diffusivity for Stokes
numbers of the order of unity.
|
1712.10049v1
|
2018-06-28
|
Transient thermal characterization of suspended monolayer MoS$_2$
|
We measure the thermal time constants of suspended single layer molybdenum
disulfide drums by their thermomechanical response to a high-frequency
modulated laser. From this measurement the thermal diffusivity of single layer
MoS$_2$ is found to be 1.14 $\times$ 10$^{-5}$ m$^2$/s on average. Using a
model for the thermal time constants and a model assuming continuum heat
transport, we extract thermal conductivities at room temperature between 10 to
40 W/(m$\cdot$K). Significant device-to-device variation in the thermal
diffusivity is observed. Based on statistical analysis we conclude that these
variations in thermal diffusivity are caused by microscopic defects that have a
large impact on phonon scattering, but do not affect the resonance frequency
and damping of the membrane's lowest eigenmode. By combining the experimental
thermal diffusivity with literature values of the thermal conductivity, a
method is presented to determine the specific heat of suspended 2D materials,
which is estimated to be 255 $\pm$ 104 J/(kg$\cdot$K) for single layer MoS$_2$.
|
1806.10769v1
|
2018-09-12
|
The relaxation time of OH bond for hydrogen impurity in LiNbO3
|
The one dimensional model for the dynamic of hydrogen in lithium niobate is
explained by adopting Morse potential. The diffused hydrogen substitutes
Lithium and it makes bonding with one oxygen atom of a facet of
oxygen-triangle. The bonds will be stretched to set up anharmonic vibration.
The damped anharmonic oscillation is derived to explain the dynamics of
hydrogen as an impurity. The thermal fluctuation is studied by Fokker Planck
equation has an important role to determine the diffusion constant for
substitutional hydrogen. The hydrogen diffusion constant and relaxation time
are calculated to support the proposed theory and existing experimental
results. The concentration of substitutional hydrogens is studied with the help
of Boltzmann distribution.
|
1810.01959v1
|
2019-04-19
|
Semi-implicit methods for the dynamics of elastic sheets
|
Recent applications (e.g. active gels and self-assembly of elastic sheets)
motivate the need to efficiently simulate the dynamics of thin elastic sheets.
We present semi-implicit time stepping algorithms to improve the time step
constraints that arise in explicit methods while avoiding much of the
complexity of fully-implicit approaches. For a triangular lattice
discretization with stretching and bending springs, our semi-implicit approach
involves discrete Laplacian and biharmonic operators, and is stable for all
time steps in the case of overdamped dynamics. For a more general
finite-difference formulation that can allow for general elastic constants, we
use the analogous approach on a square grid, and find that the largest stable
time step is two to three orders of magnitude greater than for an explicit
scheme. For a model problem with a radial traveling wave form of the reference
metric, we find transitions from quasi-periodic to chaotic dynamics as the
sheet thickness is reduced, wave amplitude is increased, and damping constant
is reduced.
|
1904.09198v1
|
2019-06-02
|
Analytical prediction of logarithmic Rayleigh scattering in amorphous solids from tensorial heterogeneous elasticity with power-law disorder
|
The damping or attenuation coefficient of sound waves in solids due to
impurities scales with the wavevector to the fourth power, also known as
Rayleigh scattering. In amorphous solids, Rayleigh scattering may be enhanced
by a logarithmic factor although computer simulations offer conflicting
conclusions regarding this enhancement and its microscopic origin. We present a
tensorial replica field-theoretic derivation based on heterogeneous or
fluctuating elasticity (HE), which shows that long-range (power-law) spatial
correlations of the elastic constants, is the origin of the logarithmic
enhancement to Rayleigh scattering of phonons in amorphous solids. We also
consider the case of zero spatial fluctuations in the elastic constants, and of
power-law decaying fluctuations in the internal stresses. Also in this case the
logarithmic enhancement to the Rayleigh scattering law can be derived from the
proposed tensorial HE framework.
|
1906.00372v3
|
2019-09-10
|
Voltage regulation in buck--boost coniverters feeding an unknown constant power load: an adaptive passivity-based control
|
Rapid developments in power distribution systems and renewable energy have
widened the applications of dc--dc buck--boost converters in dc voltage
regulation. Applications include vehicular power systems, renewable energy
sources that generate power at a low voltage, and dc microgrids. It is noted
that the cascade--connection of converters in these applications may cause
instability due to the fact that converters acting as loads have a constant
power load (CPL) behavior. In this paper, the output voltage regulation problem
of a buck--boost converter feeding a CPL is addressed. The construction of the
feedback controller is based on the interconnection and damping assignment
control technique. Additionally, an immersion and invariance parameter
estimator is proposed to compute online the extracted load power, which is
difficult to measure in practical applications. It is ensured through the
design that the desired operating point is (locally) asymptotically stable with
a guaranteed domain of attraction. The approach is validated via computer
simulations and experimental prototyping.
|
1909.04438v1
|
2020-06-18
|
Quasinormal modes and Hawking radiation of black holes in cubic gravity
|
We consider quasinormal modes and Hawking radiation of four-dimensional
asymptotically flat black holes in the most general up
to-cubic-order-in-curvature dimension-independent Einsteinian theory of gravity
that shares its graviton spectrum with the Einstein theory on constant
curvature backgrounds. We show that damping rate and real oscillation
frequencies of quasinormal modes for scalar, electromagnetic and Dirac fields
are suppressed once the coupling with the cubic term is on. The intensity of
Hawking radiation is suppressed as well, leading to, roughly, one order longer
lifetime at a sufficiently large coupling constant.
|
2006.10462v4
|
2020-07-28
|
Pair emission from a relativistic domain wall in antiferromagnets
|
Magnon emission and excitation by a relativistic domain wall at a constant
velocity in antiferromagnet is theoretically studied. A pair emission due to a
quadratic magnon coupling is shown to be dominant. The emission corresponds in
the comoving frame to a vacuum polarization induced by a zero-energy
instability of the Lorentz-boosted anomalous response function. The emission
rate is sensitive to the magnon dispersion and wall profile, and is
significantly enhanced for a thin wall with velocity close to the effective
light velocity. The Ohmic damping constant due to magnon excitation at low
velocity is calculated.
|
2007.13939v1
|
2020-09-25
|
Exciton-polariton mediated interaction between two nitrogen-vacancy color centers in diamond using two-dimensional transition metal dichalcogenides
|
In this paper, starting from a quantum master equation, we discuss the
interaction between two negatively charged Nitrogen-vacancy color centers in
diamond via exciton-polaritons propagating in a two-dimensional transition
metal dichalcogenide layer in close proximity to a diamond crystal. We focus on
the optical 1.945 eV transition and model the Nitrogen-vacancy color centers as
two-level (artificial) atoms. We find that the interaction parameters and the
energy levels renormalization constants are extremely sensitive to the distance
of the Nitrogen-vacancy centers to the transition metal dichalcogenide layer.
Analytical expressions are obtained for the spectrum of the exciton-polaritons
and for the damping constants entering the Lindblad equation. The conditions
for occurrence of exciton mediated superradiance are discussed.
|
2009.12194v1
|
2020-10-11
|
Coordinate-space representation of a charged scalar particle propagator in a constant magnetic field expanded as a sum over the Landau levels
|
A coordinate-space representation for a charged scalar particle propagator in
a constant magnetic field was obtained as a series over the Landau levels.
Using the recently developed modified Fock-Schwinger method, an intermediate
expression was constructed and symmetrized, thus, allowing for factorization of
the series terms into two factors. The first one, a sum of Bessel functions,
depends on time and $z$-coordinate, where the $z$-axis is chosen to be a
direction of the magnetic field, and has a structure similar to the propagator
of a free field. The second one, a product of a Laguerre polynomial and a
damping exponential, depends on $x,y$-coordinates, which form a plane
perpendicular to the direction of the magnetic field, and ensures the localized
propagation in the $x,y$-plane.
|
2010.05195v2
|
2020-10-12
|
Robustness of the Cosmological Constant Damping Mechanism Through Matter Eras
|
A dynamical resolution to the cosmological constant fine-tuning problem has
been previously put forward, based on a scalar-tensor gravitational theory
possessing de Sitter attractor solutions characterized by a small Hubble
expansion rate, irrespective of an initially large vacuum energy. We show that
a technically natural subregion of the parameter space yields a cosmological
evolution through radiation- and matter-dominated eras that is essentially
indistinguishable from that predicted by General Relativity. Similarly, the
proposed model automatically satisfies the observational constraints on a fifth
force mediated by the new scalar degree of freedom.
|
2010.05927v2
|
2021-05-25
|
Gravitational Waves in a Closed Spacetime via Deviation Equation
|
Within the closed universe, we obtain the amplitude and frequency of
gravitational waves in the terms of discrete wave numbers, wave propagation
time, and cosmological constant using the deviation equation in the first-order
perturbed metric. We demonstrate that the cosmological constant effect on GWs
is only seen in the early universe. Also, by considering the time evolution of
a gravitational wave in a closed spacetime, we investigate its effect on a
circle of nearby massless particles, which will be compared with this case in
the flat spacetime. Expanding the universe has effective damping on GWs; thus,
we suggest it can be used as a tool to characterize the large-scale curvature
of the universe
|
2105.11690v1
|
2022-03-14
|
The influence of the medium physical conditions and atomic constants on the Stokes profiles of absorption lines in the solar spectrum
|
The Stokes profiles of Fe I lines in the photosphere of the Sun are
calculated within the Unno-Beckers-Landi-Dagl`Innocenti theory. Estimates of
the magnetic strengthening of the lines were obtained. The changes in the
Stokes profiles depending on the excitation potential, wavelength, equivalent
width, Lande factor, micro-macroturbulent velocities, radial velocity, damping
constant, atmospheric model, magnetic field strength and direction are
considered. The graphically presented variations of the Stokes profiles make it
possible to determine the initial values of the input parameters for solving
the problems of magnetic field vector reconstruction by the inversion method.
The presented dependencies of the magnetic strengthening on the line parameters
will help to correctly select magnetically sensitive lines for the
investigation of sunspots, flux tubes, plages, and other magnetic features.
|
2203.06867v1
|
2022-10-06
|
Information scrambling of the dilute Bose gas at low temperature
|
We calculate the quantum Lyapunov exponent $\lambda_L$ and butterfly velocity
$v_B$ in the dilute Bose gas at temperature $T$ deep in the Bose-Einstein
condensation phase. The generalized Boltzmann equation approach is used for
calculating out-of-time ordered correlators, from which $\lambda_L$ and $v_B$
are extracted. At very low temperature where elementary excitations are
phonon-like, we find $\lambda_L\propto T^5$ and $v_B\sim c$, the sound
velocity. At relatively high temperature, we have $\lambda_L\propto T$ and
$v_B\sim c(T/T_*)^{0.23}$. We find $\lambda_L$ is always comparable to the
damping rate of a quasiparticle, whose energy depends suitably on $T$. The
chaos diffusion constant $D_L=v_B^2/\lambda_L$, on the other hand, differs from
the energy diffusion constant $D_E$. We find $D_E\ll D_L$ at very low
temperature and $D_E\gg D_L$ otherwise.
|
2210.03025v2
|
2023-06-16
|
Short-term evolution of electron wave packet in a constant crossed field with radiative corrections
|
We study the dynamics of an electron wave packet in a strong constant crossed
electromagnetic field with account for radiative corrections due to interaction
of the electron with the vacuum fluctuations. We evaluate a wave packet
composed of the solutions to the Dyson-Schwinger equation, which describes
electron propagation without emission of real photons. Spacetime dependence of
the wave packet is obtained analytically for a short time interval, the more
restricted from above the wider is the packet in momentum space. The radiative
corrections alter the electron wavefunction, resulting in particular in a
damping of the wave packet. The expectation value of the Dirac spin operator
also gets modified.
|
2306.09932v1
|
2023-12-11
|
Accelerated Gradient Dynamics on Riemannian Manifolds: Faster Rate and Trajectory Convergence
|
In order to minimize a differentiable geodesically convex function, we study
a second-order dynamical system on Riemannian manifolds with an asymptotically
vanishing damping term of the form $\alpha/t$. For positive values of $\alpha$,
convergence rates for the objective values and convergence of trajectory is
derived. We emphasize the crucial role of the curvature of the manifold for the
distinction of the modes of convergence. There is a clear correspondence to the
results that are known in the Euclidean case. When $\alpha$ is larger than a
certain constant that depends on the curvature of the manifold, we improve the
convergence rate of objective values compared to the previously known rate and
prove the convergence of the trajectory of the dynamical system to an element
of the set of minimizers. For $\alpha$ smaller than this curvature-dependent
constant, the best known sub-optimal rates for the objective values and the
trajectory are transferred to the Riemannian setting. We present computational
experiments that corroborate our theoretical results.
|
2312.06366v1
|
1994-02-02
|
Constraints on the Models for Structure Formation from the Abundance of Damped Lyman Alpha Systems
|
Models for structure formation attempt to predict the power spectrum of
density perturbations in the present universe from the initial power spectrum
and the nature of dark matter. Observational constraints on the power spectrum
at different scales in the present epoch can, therefore, be used to eliminate
(or choose between) different theoretical models. Such a comparison is fairly
easy at large scales (at which linear theory is valid), and one can use
observations like the MBR anisotropy, large scale steaming motions etc to
constrain the models. But to discriminate between the models effectivley, it is
necessay to constrain the power spectrum at small scales. The most reliable
constraints on the power spectra at small scales come from the predicted
abundance of bound systems which can be estimated reasonably accurately using
Press-Schecter (or similar) methods$^1$. In the past, this method has been used
in conjunction with the quasar abundance$^{2-4}$ and cluster abundance$^{5-7}$.
We show here that the abundance of damped lyman alpha systems (DLAS,
hereafter), provides a far stronger constraint on the models for structure
formation. Models with a mixture of hot and cold dark matter $^{8-11}$ (which
are consistent with large scale observations) are strongly ruled out by the
DLAS constraints while models with cosmological constant $^{12}$ are marginally
inconsistent. It is also possible to combine the constraints from the abundance
of clusters, DLAS and QSO's to obtain model-independent bounds on the power
spectrum at the nonlinear scales. These bounds are to be respected by any
viable model for structure formation.
|
9402006v1
|
1994-02-04
|
Damped Lyman Alpha Systems and Galaxy Formation
|
We examine the constraints on theories of galaxy formation that are obtained
from observations of damped $\lya$ (DL) systems, assuming they are gaseous
protodisks in dark matter halos. Using the Press-Schechter formalism, we find
that the mixed dark matter model, with $\ohdm = 0.3$, $\ocdm = 0.65$, $\obaryon
= 0.05$, and $h=0.5$, is ruled out because the number of galactic halos at
$z\simeq 3$ is too small to account for the total gaseous mass in DL systems,
even under the assumption that all the gas in collapsed halos has settled into
disks of neutral gas. The standard CDM model can account for the gas in DL
systems if the bias is $b\lsim 2$; the same is true for the CDM model with a
cosmological constant, if $b\lsim 1.5$ for $\Lambda = 0.8$. However, one still
needs to assume that a fraction $\gsim 0.4$ of the baryons in collapsed halos
at $z\simeq 3$ is in the form of neutral gas in disks. We also calculate the
column density distribution $f(\nhi)$ of the DL systems, in terms of the
surface density profiles of disks and the distribution of their central column
densities. It is shown that the form of $f(\nhi)$ at the high end of column
density is a diagnostic for the nature of DL systems.
|
9402014v1
|
1997-09-02
|
Estimating the mass density of neutral gas at $z < 1$
|
We use the relationships between galactic HI mass and B-band luminosity
determined by Rao & Briggs to recalculate the mass density of neutral gas at
the present epoch based on more recent measures of the galaxy luminosity
function than were available to those authors. We find $\Omega_{gas}(z=0)
\simeq 5 \times 10^{-4}$ in good agreement with the original Rao & Briggs
value, suggesting that this quantity is now reasonably secure. We then show
that, if the scaling between H I mass and B-band luminosity has remained
approximately constant since $z = 1$, the evolution of the luminosity function
found by the Canada-France redshift survey translates to an increase of
$\Omega_{gas}$ by a factor of $\approx 3$ at $z = 0.5 - 1$ . A similar value is
obtained quite independently from consideration of the luminosity function of
Mg II absorbers at $z = 0.65$. By combining these new estimates with data from
damped \lya systems at higher redshift, it is possible to assemble a rough
sketch of the evolution of $\Omega_{gas}$ over the last 90% of the age of the
universe. The consumption of H I gas with time is in broad agreement with
models of chemical evolution which include the effects of dust, although more
extensive samples of damped \lya systems at low and intermediate redshift are
required for a quantitative assessment of the dust bias.
|
9709014v1
|
1997-10-02
|
Dust and elemental abundances in Damped Ly alpha absorbers
|
The effects of the dust on the determination of elemental abundances in
damped Ly alpha (DLA) absorbers are investigated. Relations between the
observed abundances measured in the gas phase and the overall abundances (gas
plus dust) are derived as a function of dust-to-gas ratio, metallicity,
element-to-element abundance pattern, average extinction coefficient of dust
grains, and chemical composition of dust grains. A method is presented for
determining dust-to-gas ratios, dust-to-metals ratios, and dust-corrected
relative abundances in DLA absorbers by assuming dust of Galactic type and
constant abundance ratios between iron-peak elements. The method is applied to
a sample of 17 DLA absorbers with available Zn, Cr and/or Fe measurements. The
resulting dust-to-gas ratios are mostly distributed between 2% and 25% of the
Galactic value, in good quantitative agreement with the results from reddening
studies of QSOs with foreground DLA absorption. A correlation is found between
dust-to-gas ratio and metallicity in DLA galaxies, with a typical
dust-to-metals ratio of ~ 60% the Galactic value. The derived dust-to-metals
ratios are then used to correct from the effects of dust the abundance ratios
[Si/Fe], [S/Fe], [Ti/Fe], [Mn/Fe], [Ni/Fe] available for a sub-sample of 9
absorbers. The [alpha/Fe] ratios corrected from dust do not show the
enhancement characteristic of metal-poor Galactic stars, but instead have
essentially solar values, within +/- 0.2 dex. This suggests that the chemical
history of DLA absorbers is different from that experienced by the Milky Way.
Evidences that point to dwarf galaxies, rather than to spiral galaxies, as
important contributors to the DLA phenomenon are summarized.
|
9710026v1
|
1998-10-29
|
Runaway Heating By R-modes of Neutron Stars in Low Mass X-ray Binaries
|
Recently Andersson et. al., and Bildsten have independently suggested that an
r-mode instability might be responsible for stalling the neutron-star spin-up
in strongly accreting, Low Mass X-ray Binaries (LMXBs). We show that if this
does occur, then there are two possibilities for the resulting neutron-star
evolution:
If the r-mode damping is a decreasing function of temperature, then the star
undergoes a cyclic evolution: (i) accretional spin-up triggers the instability
near the observed maximum spin rate; (ii) the r-modes become highly excited
through gravitational-radiation reaction, and in a fraction of a year they
viscously heat the star; (iii) r-mode gravitational-radiation reaction then
spins the star down in a fraction of a year to some limiting rotational
frequency; (iv) the r-mode instability shuts off; (v) the neutron star slowly
cools and is spun up by accretion, until it once again reaches the instability
point, closing the cycle. The shortness of the epoch of r-mode activity makes
it unlikely that r-modes are currently excited in the neutron star of any
galactic LMXBs. Nevertheless, this cyclic evolution could be responsible for
keeping the rotational frequencies within the observed LMXB frequency range.
If, on the other hand, the r-mode damping is temperature independent, then a
steady state with constant angular velocity and $T_{\rm core}\simeq 4\times
10^8$K is reached, in which r-mode viscous heating is balanced by neutrino
cooling and accretional spin-up torque is balanced by
gravitational-radiation-reaction spin-down torque. In this case the neutron
stars in LMXBs could be potential sources of periodic gravitational waves,
detectable by enhanced LIGO interferometers.
|
9810471v1
|
2000-04-04
|
Radiative Precession of an Isolated Neutron Star
|
Euler's equations of motion are derived exactly for a rigid, triaxial,
internally frictionless neutron star spinning down electromagnetically in
vacuo. It is shown that the star precesses, but not freely: its regular
precession relative to the principal axes of inertia couples to the component
of the radiation torque associated with the near-zone radiation fields and is
modified into an anharmonic wobble. The wobble period \tau_1 typically
satisfies \tau_1 < 10^{-2}\tau_0, where \tau_0 is the braking time-scale; the
wobble amplitude evolves towards a constant non-zero value, oscillates, or
decreases to zero, depending on the degree of oblateness or prolateness of the
star and its initial spin state; and the (negative) angular frequency
derivative d{\omega}/dt oscillates as well, exhibiting quasi-periodic spikes
for triaxial stars of a particular figure. In light of these properties, a
young, Crab-like pulsar ought to display fractional changes of order unity in
the space of a few years in its pulse profile, magnetic inclination angle, and
d{\omega}/dt. Such changes are not observed, implying that the wobble is damped
rapidly by internal friction, if its amplitude is initially large upon
crystallization of the stellar crust. If the friction is localized in the inner
and outer crusts, the thermal luminosity of the neutron star increases by a
minimum amount \Delta L = 3*10^{31} (\epsilon / 10^{-12}) (\omega / 10^3 rad
s^{-1})^2 (\tau_d / 1 yr)^{-1} erg s^{-1}, where epsilon is the ellipticity and
\tau_d is the damping time-scale, with the actual value of \Delta L determined
in part by the thermal conduction time \tau_cond. The increased luminosity is
potentially detectable as thermal X-rays lasting for a time max(tau_d,tau_cond)
following crystallization of the crust.
|
0004035v1
|
2002-08-22
|
Effects of Resonance in Quasiperiodic Oscillators of Neutron Star Binaries
|
Using a large quantity of Rossi X-ray Timing Explorer data presented in the
literature I offer a detailed investigation into the accuracy of quasiperiodic
oscillations (QPO) frequency determination. The QPO phenomenon seen in X-ray
binaries is possibly a result of the resonance of the intrinsic (eigen)
oscillations and harmonic driving forces of the system. I show that the
resonances, in the presence of the damping of oscillations, occur at the
frequencies which are systematically and randomly shifted with respect to the
eigenfrequencies of the system. The shift value strongly depends on the damping
rate which is measured by the halfwidth of the QPO feature. Taking into account
this effect I analyze the QPO data for four Z-sources: Sco X-1, GX 340+0, GX
5-1, GX 17+2 and two atoll sources: 4U 1728-34, 4U 0614+09. The transition
layer model (TLM) predicts the existence of the invariant quantity: delta, an
inclination angle of the magnetospheric axis with respect to the normal to the
disk. I calculate delta and the error bars of delta using the resonance shift
and I find that the inferred delta-values are consistent with constants for
these four Z-sources, where horizontal branch oscillation and kilohertz
frequencies have been detected and correctly identified. It is shown that the
inferred delta are in the range between 5.5 and 6.5 degrees. I conclude that
the TLM seems to be compatible with data.
|
0208423v1
|
2004-03-10
|
The early build-up of dust in galaxies: A study of Damped Ly alpha Systems
|
We present a study of the early build-up of dust in high redshift galaxies.
The study is based on the analysis of 38 Damped Ly alpha systems (DLAs) for
which we derive the fraction of iron atoms in dust form, f_{Fe}. The sample is
representative of metal-poor galaxies in the redshift range 0.6 </= z </= 3.4
selected on the basis of their absorption HI column density (N(HI) >/= 2 x
10^{20} atoms cm^{-2}). We find that the dust fraction increases with
metallicity, from f_{Fe}~0 at [Fe/H] ~ -2 dex, up to f_{Fe} ~ 0.9 at solar
metallicity; the increase is fast below [Fe/H] ~ -1 dex and mild at higher
metallicities. We also find some evidence for an increase of f_{Fe} with cosmic
time; a large fraction of the systems younger than ~3 Gyr has f_{Fe} </~ 0.5.
These results indicate the dust-to-metal ratio increases in the course of
chemical evolution, at variance with the hypothesis of an approximately
constant dust-to-metal ratio, commonly adopted in models of galactic evolution.
This hypothesis is consistent with local and high-redshift data only when the
metallicity is relatively high ([Fe/H] >/~ -1 dex). The results of this work
suggest that the main mechanisms of dust formation may be rather sensitive to
the level of metallicity attained by a galaxy in the course of its chemical
evolution. A metallicity-dependent dust production by SNe II seems to be the
most promising mechanism for explaining the rise of f_{Fe} at [Fe/H] </~ -1
dex.
|
0403237v1
|
2004-07-22
|
The Chemical Composition of Alpha Centauri A: Strong Lines and the ABO Theory of Collisional Line Broadening
|
The mean abundances of Mg, Si, Ca, Ti, Cr and Fe based on both strong and
weak lines of Alpha Centauri A are determined by matching the observed line
profiles with those synthesized from stellar atmospheric models and comparing
these results with a similar analysis for the Sun. There is good agreement
between the abundances from strong and weak lines.
Strong lines should generally be an excellent indicator of abundance and far
easier to measure than the weak lines normally used. Until the development of
the Anstee, Barklem and O'Mara theory for collisional line broadening, the
uncertainty in the value of the damping constant prevented strong lines being
used for abundance determinations other than in close differential analyses.
We found that Alpha Centauri A has a mean overabundance of 0.12+/-0.06 dex
compared to solar mean abundances. This result agrees remarkably well with
previous studies that did not use strong lines or the Anstee, Barklem and
O'Mara theory for collisional line broadening. Our result support the
conclusion that reliable abundances can be derived from strong lines provided
this new theory for line broadening is used to calculate the van der Waal's
damping.
|
0407468v1
|
2006-08-07
|
The Complex Gap in Color Superconductivity
|
We solve the gap equation for color-superconducting quark matter in the 2SC
phase, including both the energy and the momentum dependence of the gap,
\phi=\phi(k_0,\vk). For that purpose a complex Ansatz for \phi is made. The
calculations are performed within an effective theory for cold and dense quark
matter. The solution of the complex gap equation is valid to subleading order
in the strong coupling constant g and in the limit of zero temperature. We find
that, for momenta sufficiently close to the Fermi surface and for small
energies, the dominant contribution to the imaginary part of $\phi$ arises from
Landau-damped magnetic gluons. Further away from the Fermi surface and for
larger energies the other gluon sectors have to be included into Im\phi. We
confirm that Im$ \phi$ contributes a correction of order g to the prefactor of
\phi for on-shell quasiquarks sufficiently close to the Fermi surface, whereas
further away from the Fermi surface Im\phi and Re\phi are of the same order.
Finally, we discuss the relevance of Im\phi for the damping of quasiquark
excitations.
|
0608020v2
|
2006-10-11
|
Superfluid-normal phase transition in finite systems and its effect on damping of hot giant resonances
|
Thermal fluctuations of quasiparticle number are included making use of the
secondary Bogolyubov's transformation, which turns quasiparticles operators
into modified-quasiparticle ones. This restores the unitarity relation for the
generalized single-particle density operator, which is violated within the
Hartree-Fock-Bogolyubov (HFB) theory at finite temperature. The resulting
theory is called the modified HFB (MHFB) theory, whose limit of a constant
pairing interaction yields the modified BCS (MBCS) theory. Within the MBCS
theory, the pairing gap never collapses at finite temperature T as it does
within the BCS theory, but decreases monotonously with increasing T. It is
demonstrated that this non-vanishing thermal pairing is the reason why the
width of the giant dipole resonance (GDR) does not increase with T up to T
around 1 MeV. At higher T, when the thermal pairing is small, the GDR width
starts to increase with T. The calculations within the phonon-damping model
yield the results in good agreement with the most recent experimental
systematic for the GDR width as a function of T. A similar effect, which causes
a small GDR width at low T, is also seen after thermal pairing is included in
the thermal fluctuation model.
|
0610047v1
|
1995-03-31
|
Dissipative Quantum Systems with Potential Barrier. General Theory and Parabolic Barrier
|
We study the real time dynamics of a quantum system with potential barrier
coupled to a heat-bath environment. Employing the path integral approach an
evolution equation for the time dependent density matrix is derived. The time
evolution is evaluated explicitly near the barrier top in the temperature
region where quantum effects become important. It is shown that there exists a
quasi-stationary state with a constant flux across the potential barrier. This
state generalizes the Kramers flux solution of the classical Fokker-Planck
equation to the quantum regime. In the temperature range explored the quantum
flux state depends only on the parabolic approximation of the anharmonic
barrier potential near the top. The parameter range within which the solution
is valid is investigated in detail. In particular, by matching the flux state
onto the equilibrium state on one side of the barrier we gain a condition on
the minimal damping strength. For very high temperatures this condition reduces
to a known result from classical rate theory. Within the specified parameter
range the decay rate out of a metastable state is calculated from the flux
solution. The rate is shown to coincide with the result of purely thermodynamic
methods. The real time approach presented can be extended to lower temperatures
and smaller damping.
|
9503027v1
|
2007-07-17
|
Is a gas of strongly interacting atomic fermions a nearly perfect fluid?
|
We use all-optical methods to produce a highly-degenerate Fermi gas of
spin-1/2 $^6$Li atoms. A magnetic field tunes the gas near a collisional
(Feshbach) resonance, producing strong interactions between spin-up and
spin-down atoms. This atomic gas is a paradigm for strong interactions in
nature, and provides tests of current quantum many-body calculational methods
for diverse systems, including very high temperature superconductors, nuclear
matter in neutron stars, and the quark-gluon plasma of the Big Bang. We have
measured properties of a breathing mode over a wide range of temperatures. At
temperatures both below and well above the superfluid transition, the frequency
of the mode is nearly constant and very close to the hydrodynamic value.
However, explaining both the frequency and the damping rate in the normal
collisional regime has not been achieved. Our measurements of the damping rate
as a function of the energy of the gas are used to estimate an upper bound on
the viscosity. Using our new measurements of the entropy of the gas, we
estimate the ratio of the shear viscosity to the entropy density, and compare
the result with the lower bound for quantum viscosity recently predicted using
string theory methods.
|
0707.2574v1
|
2009-07-14
|
Multi-mode entanglement of N harmonic oscillators coupled to a non-Markovian reservoir
|
Multi-mode entanglement is investigated in the system composed of $N$ coupled
identical harmonic oscillators interacting with a common environment. We treat
the problem very general by working with the Hamiltonian without the
rotating-wave approximation and by considering the environment as a
non-Markovian reservoir to the oscillators. We invoke an $N$-mode unitary
transformation of the position and momentum operators and find that in the
transformed basis the system is represented by a set of independent harmonic
oscillators with only one of them coupled to the environment. Working in the
Wigner representation of the density operator, we find that the covariance
matrix has a block diagonal form that it can be expressed in terms of multiples
of $3\times 3$ and $4\times 4$ matrices. This simple property allows to treat
the problem to some extend analytically. We illustrate the advantage of working
in the transformed basis on a simple example of three harmonic oscillators and
find that the entanglement can persists for long times due to presence of
constants of motion for the covariance matrix elements. We find that, in
contrast to what one could expect, a strong damping of the oscillators leads to
a better stationary entanglement than in the case of a weak damping.
|
0907.2298v2
|
2009-10-20
|
New searches for HI 21-cm in damped Lyman-alpha absorption systems
|
We present the results of three separate searches for HI 21-cm absorption in
a total of twelve damped Lyman-alpha absorption systems (DLAs) and sub-DLAs
over the redshift range z = 0.86-3.37. We find no absorption in the five
systems for which we obtain reasonable sensitivities and add the results to
those of other recent surveys in order to investigate factors which could have
an effect on the detection rate: We provide evidence that the mix of spin
temperature/covering factor ratios seen at low redshift may also exist at high
redshift, with a correlation between the 21-cm line strength and the total
neutral hydrogen column density, indicating a roughly constant spin
temperature/covering factor ratio for all of the DLAs searched. Also, by
considering the geometry of a flat expanding Universe together with the
projected sizes of the background radio emission regions, we find, for the
detections, that the 21-cm line strength is correlated with the size of the
absorber. For the non-detections it is apparent that larger absorbers (covering
factors) are required in order to exhibit 21-cm absorption, particularly if
these DLAs do not arise in spiral galaxies. We also suggest that the recent z =
2.3 detection towards TXS 0311+430 arises in a spiral galaxy, but on the basis
of a large absorption cross-section and high metallicity, rather than a low
spin temperature
|
0910.3742v2
|
2010-09-30
|
An efficient numerical algorithm for the L2 optimal transport problem with applications to image processing
|
We present a numerical method to solve the optimal transport problem with a
quadratic cost when the source and target measures are periodic probability
densities. This method is based on a numerical resolution of the corresponding
Monge-Amp\`ere equation. We extend the damped Newton algorithm of Loeper and
Rapetti \cite{LR} to the more general case of a non uniform density which is
relevant to the optimal transport problem, and we show that our algorithm
converges for sufficiently large damping coefficients. The main idea consists
of designing an iterative scheme where the fully nonlinear equation is
approximated by a non-constant coefficient linear elliptic PDE that we solve
numerically. We introduce several improvements and some new techniques for the
numerical resolution of the corresponding linear system. Namely, we use a Fast
Fourier Transform (FFT) method by Strain \cite{St}, which allows to increase
the efficiency of our algorithm against the standard finite difference method.
Moreover, we use a fourth order finite difference scheme to approximate the
partial derivatives involved in the nonlinear terms of the Newton algorithm,
which are evaluated once at each iteration; this leads to a significant
improvement of the accuracy of the method, but does not sacrifice its
efficiency. Finally, we present some numerical experiments which demonstrate
the robustness and efficiency of our method on several examples of image
processing, including an application to multiple sclerosis disease detection.
|
1009.6039v2
|
2011-06-28
|
A refined empirical stability criterion for nonlinear Schroedinger solitons under spatiotemporal forcing
|
We investigate the dynamics of travelling oscillating solitons of the cubic
NLS equation under an external spatiotemporal forcing of the form $f(x,t) = a
\exp[iK(t)x]$. For the case of time-independent forcing a stability criterion
for these solitons, which is based on a collective coordinate theory, was
recently conjectured. We show that the proposed criterion has a limited
applicability and present a refined criterion which is generally applicable, as
confirmed by direct simulations. This includes more general situations where
$K(t)$ is harmonic or biharmonic, with or without a damping term in the NLS
equation. The refined criterion states that the soliton will be unstable if the
"stability curve" $p(\v)$, where $p(t)$ and $\v(t)$ are the normalized momentum
and the velocity of the soliton, has a section with a negative slope. Moreover,
for the case of constant $K$ and zero damping we use the collective coordinate
solutions to compute a "phase portrait" of the soliton where its dynamics is
represented by two-dimensional projections of its trajectories in the
four-dimensional space of collective coordinates. We conjecture, and confirm by
simulations, that the soliton is unstable if a section of the resulting closed
curve on the portrait has a negative sense of rotation.
|
1106.5609v1
|
2012-11-06
|
Low-Energy Effective Field Theory for Chromo-Natural Inflation
|
Chromo-natural inflation is a novel model of inflation which relies on the
existence of non-abelian gauge fields interacting with an axion. In its
simplest realization, an SU(2) gauge field is assumed to begin inflation in a
rotationally invariant VEV. The dynamics of the gauge fields significantly
modifies the equations of motion for the axion, providing an additional damping
term that supports slow-roll inflation, without the need to fine tune the axion
decay constant. We demonstrate that in an appropriate slow-roll limit it is
possible to integrate out the massive gauge field fluctuations whilst still
maintaining the nontrivial modifications of the gauge field to the axion. In
this slow-roll limit, chromo-natural inflation is exactly equivalent to a
single scalar field effective theory with a non-minimal kinetic term, i.e. a
P(X,\chi) model. This occurs through a precise analogue of the gelaton
mechanism, whereby heavy fields can have unsuppressed effects on the light
field dynamics without contradicting decoupling. The additional damping effect
of the gauge fields can be completely captured by the non-minimal kinetic term
of the single scalar field effective theory. We utilize the single scalar field
effective theory to infer the power spectrum and non-gaussianities in
chromo-natural inflation and confirm that the mass squared of all the gauge
field fluctuations is sufficiently large and positive that they completely
decouple during inflation. These results confirm that chromo-natural inflation
is a viable, stable and compelling model for the generation of inflationary
perturbations.
|
1211.1396v2
|
2013-11-22
|
Multi-dimensional fractional wave equation and some properties of its fundamental solution
|
In this paper, a multi-dimensional fractional wave equation that describes
propagation of the damped waves is introduced and analyzed. In contrast to the
fractional diffusion-wave equation, the fractional wave equation contains
fractional derivatives of the same order $\alpha,\ 1\le \alpha \le 2$ both in
space and in time. This feature is a decisive factor for inheriting some
crucial characteristics of the wave equation like e.g. a constant phase
velocity of the damped waves that are described by the fractional wave
equation. Some new integral representations of the fundamental solution of the
multi-dimensional wave equation are presented. In the one- and
three-dimensional cases, the fundamental solution is obtained in explicit form
in terms of elementary functions. In the one-dimensional case, the fundamental
solution is shown to be a spatial probability density function evolving in
time. However, for the dimensions grater than one, the fundamental solution can
be negative and therefore does not allow a probabilistic interpretation
anymore. To illustrate analytical findings, results of numerical calculations
and numerous plots are presented.
|
1311.5920v1
|
2014-03-18
|
The nature of Damped Lyman-α and MgII absorbers explored with their dust contents
|
We estimate the abundance of dust in damped Lyman-{\alpha} absorbers (DLAs)
by statistically measuring the excess reddening they induce on their background
quasars. We detect systematic reddening behind DLA consistent with the SMC type
reddening curve, but it is inconsistent with the Milky Way type reddening. We
find that the derived dust-to-gas ratio is, on average, inversely proportional
to the column density of neutral hydrogen, implying that the amount of dust is
constant, irrespective of the column density of hydrogen. It means that the
average metallicity is inversely proportional to the column density of
hydrogen, unless the average dust-to-metal ratio varies with the hydrogen
column density. This indicates that the prime origin of metals seen in DLAs is
not by in situ star formation, with which Z ~ N_{HI}^{0.4} is expected from the
empirical star formation law, contrary to our observation. We interpret the
metals observed in absorbers being deposited dominantly from nearby galaxies by
galactic winds ubiquitous in intergalactic space. When extrapolating the
relation between dust-to-gas ratio and HI column density to lower column
density, we find a value which is consistent with what is observed for Mg II
absorbers.
|
1403.4339v2
|
2014-06-15
|
Dynamical topological phases in quenched spin-orbit coupled degenerate Fermi gas
|
The spin-orbit coupled degenerate Fermi gas provides a totally new platform
to realize topological superfluids and related topological excitations.
Previous studies have mainly focused on the properties of the ground state.
Here we consider a two-dimensional Fermi gas with Rashba spin-orbit coupling
subject to a perpendicular Zeeman field. For this system, we have found that
its ground state topological structure is captured by the spin texture, which
is readily measurable in experiments. We show that, when the Zeeman field is
suddenly quenched, dynamical topological phases can be realized. More
specifically, three post-quench dynamical phases can be identified according to
the asymptotic behavior of the order parameter. In the undamped phase, a
persistent oscillation of the order parameter may support a topological Floquet
state with multiple edge states. In the Landau damped phase, the magnitude of
the order parameter approaches a constant via a power-law decay, and this
system can support a dynamical topological phase with a pair of edge states at
the boundary. In the over-damped phase, the order parameter decays to zero
exponentially although the condensate fraction remains finite. These
predictions can be observed in the strong coupling regime of ultracold Fermi
gas.
|
1406.3821v2
|
2014-08-02
|
Tunnel magnetoresistance and spin-transfer-torque switching in polycrystalline Co2FeAl full-Heusler alloy magnetic tunnel junctions on Si/SiO2 amorphous substrates
|
We studied polycrystalline B2-type Co2FeAl (CFA) full-Heusler alloy based
magnetic tunnel junctions (MTJs) fabricated on a Si/SiO2 amorphous substrate.
Polycrystalline CFA films with a (001) orientation, a high B2 ordering, and a
flat surface were achieved using a MgO buffer layer. A tunnel magnetoresistance
(TMR) ratio up to 175% was obtained for an MTJ with a CFA/MgO/CoFe structure on
a 7.5-nm-thick MgO buffer. Spin-transfer torque induced magnetization switching
was achieved in the MTJs with a 2-nm-thick polycrystalline CFA film as a
switching layer. Using a thermal activation model, the intrinsic critical
current density (Jc0) was determined to be 8.2 x 10^6 A/cm^2, which is lower
than 2.9 x 10^7 A/cm^2, the value for epitaxial CFA-MTJs [Appl. Phys. Lett.
100, 182403 (2012)]. We found that the Gilbert damping constant evaluated using
ferromagnetic resonance measurements for the polycrystalline CFA film was
~0.015 and was almost independent of the CFA thickness (2~18 nm). The low Jc0
for the polycrystalline MTJ was mainly attributed to the low damping of the CFA
layer compared with the value in the epitaxial one (~0.04).
|
1408.0341v1
|
2014-08-12
|
Landau Damping in Relativistic Plasmas
|
We examine the phenomenon of Landau Damping in relativistic plasmas via a
study of the relativistic Vlasov-Poisson system (rVP) on the torus for initial
data sufficiently close to a spatially uniform steady state. We find that if
the steady state is regular enough (essentially in a Gevrey class of degree in
a specified range) and that the deviation of the initial data from this steady
state is small enough in a certain norm, the evolution of the system is such
that its spatial density approaches a uniform constant value sub-exponentially
fast (i.e. like $\exp(-C|t|^{\overline{\nu}})$ for $\overline{\nu} \in (0,1)$).
We take as \emph{a priori} assumptions that solutions launched by such initial
data exist for all times (by no means guaranteed with rVP, but reasonable since
we are close to a spatially uniform state) and that the various norms in
question are continuous in time (which should be a consequence of an abstract
version of the Cauchy-Kovalevskaya Theorem). In addition, we must assume a kind
of "reverse Poincar\'e inequality" on the Fourier transform of the solution. In
spirit, this assumption amounts to the requirement that there exists
$0<\varkappa<1$ so that the mass in the annulus $\varkappa \le |v| < 1$ for the
solution launched by the initial data is uniformly small for all $t$.
|
1408.2666v2
|
2014-10-02
|
Localized and propagating excitations in gapped phases of spin systems with bond disorder
|
Using the conventional $T$-matrix approach, we discuss gapped phases in 1D,
2D, and 3D spin systems (both with and without a long range magnetic order)
with bond disorder and with weakly interacting bosonic elementary excitations.
This work is motivated by recent experimental and theoretical activity in
spin-liquid-like systems with disorder and in the disordered interacting boson
problem. In particular, we apply our theory to both paramagnetic low-field and
fully polarized high-field phases in dimerized spin-$\frac12$ systems and in
integer-spin magnets with large single-ion easy-plane anisotropy $\cal D$ with
disorder in exchange coupling constants (and/or $\cal D$). The elementary
excitation spectrum and the density of states are calculated in the first order
in defects concentration $c\ll1$. In 2D and 3D systems, the scattering on
defects leads to a finite damping of all propagating excitations in the band
except for states lying near its edges. We demonstrate that the analytical
approach is inapplicable for states near the band edges and our numerical
calculations reveal their localized nature. We find that the damping of
propagating excitations can be much more pronounced in considered systems than
in magnetically ordered gapless magnets with impurities. In 1D systems, the
disorder leads to localization of all states in the band, while those lying far
from the band edges (short-wavelength excitations) can look like conventional
wavepackets.
|
1410.0712v1
|
2014-11-12
|
Fundamental limits in heat assisted magnetic recording and methods to overcome it with exchange spring structures
|
The switching probability of magnetic elements for heat assisted recording is
investigated. It is found that FePt elements with a diameter of 5 nm and a
height of 10nm show, at a field of 0.5 T, thermally written in errors of 12
percent, which is significant too large for bit patterned magnetic recording.
Thermally written in errors can be decreased if larger head fields are applied.
However, larger fields lead to an increase the fundamental thermal jitter. This
leads to a dilemma between thermally written in errors and fundamental thermal
jitter. This dilemma can be partly relaxed by increasing the thickness of the
FePt film up to 30nm. For realistic head fields, it is found that the
fundamental thermal jitter is in the same order of magnitude of the fundamental
thermal jitter in conventional recording, which is about 0.5 to 0.8 nm.
Composite structures consisting of high Curie top layer and FePt as hard
magnetic storage layer can reduce the thermally written in errors to be smaller
than 10-4 if the damping constant is increased in the soft layer. Large damping
may be realized by doping with rare earth elements. Similar to single FePt
grains also in composite structure an increase of switching probability is
sacrifices by an increase of thermal jitter. Structures utilizing first order
phase transitions breaking the thermal jitter and writeability dilemma are
discussed.
|
1411.3052v1
|
2014-11-25
|
A framework for studying the effect of compliant surfaces on wall turbulence
|
This paper extends the resolvent formulation proposed by McKeon & Sharma
(2010) to consider turbulence-compliant wall interactions. Under this
formulation, the turbulent velocity field is expressed as a linear
superposition of propagating modes, identified via a gain-based decomposition
of the Navier-Stokes equations. Compliant surfaces, modeled as a complex
wall-admittance linking pressure and velocity, affect the gain and structure of
these modes. With minimal computation, this framework accurately predicts the
emergence of the quasi-2D propagating waves observed in recent direct numerical
simulations. Further, the analysis also enables the rational design of
compliant surfaces, with properties optimized to suppress flow structures
energetic in wall turbulence. It is shown that walls with unphysical negative
damping are required to interact favorably with modes resembling the energetic
near-wall cycle, which could explain why previous studies have met with limited
success. Positive-damping walls are effective for modes resembling the
so-called very large-scale motions (VLSMs), indicating that compliant surfaces
may be better suited for application at higher Reynolds number. Unfortunately,
walls that suppress structures energetic in natural turbulence are also
predicted to have detrimental effects elsewhere in spectral space. Consistent
with previous experiments and simulations, slow-moving spanwise-constant
structures are particularly susceptible to further amplification. Mitigating
these adverse effects will be central to the development of compliant coatings
that have a net positive influence on the flow.
|
1411.6690v2
|
2015-03-29
|
ETEAPOT: symplectic orbit/spin tracking code for all-electric storage rings
|
Proposed methods for measuring the electric dipole moment (EDM) of the proton
use an intense, polarized proton beam stored in an all-electric storage ring
"trap". At the "magic" kinetic energy of 232.792 MeV, proton spins are
"frozen", for example always parallel to the instantaneous particle momentum.
This paper describes an accelerator simulation code, ETEAPOT, a new component
of the Unified Accelerator Libraries (UAL), to be used for long term tracking
of particle orbits and spins in electric bend accelerators, in order to
simulate EDM storage ring experiments. Though qualitatively much like magnetic
rings, the non-constant particle velocity in electric rings give them
significantly different properties, especially in weak focusing rings. Like the
earlier code TEAPOT (for magnetic ring simulation) this code performs
\emph{exact tracking in an idealized (approximate) lattice} rather than the
more conventional approach, which is \emph{approximate tracking in a more
nearly exact lattice.} The BMT equation describing the evolution of spin
vectors through idealized bend elements is also solved exactly---original to
this paper. Furthermore the idealization permits the code to be exactly
symplectic (with no artificial "symplectification"). Any residual spurious
damping or anti-damping is sufficiently small to permit reliable tracking for
the long times, such as the 1000 seconds assumed in estimating the achievable
EDM precision.
|
1503.08468v1
|
2015-06-03
|
The anomalous transport of axial charge: topological vs non-topological fluctuations
|
Axial charge imbalance is an essential ingredient in novel effects associated
with chiral anomaly such as chiral magnetic effects (CME). In a non-Abelian
plasma with chiral fermions, local axial charge can be generated a) by
topological fluctuations which would create domains with non-zero winding
number b) by conventional non-topological thermal fluctuations. We provide a
holographic evaluations of medium's response to dynamically generated axial
charge density in hydrodynamic limit and examine if medium's response depends
on the microscopic origins of axial charge imbalance. We show a local domain
with non-zero winding number would induce a non-dissipative axial current due
to chiral anomaly. We illustrate holographically that a local axial charge
imbalance would be damped out with the damping rate related to Chern-Simon
diffusive constant. By computing chiral magnetic current in the presence of
dynamically generated axial charge density, we found that the ratio of CME
current over the axial charge density is independent of the origin of axial
charge imbalance in low frequency and momentum limit. Finally, a stochastic
hydrodynamic equation of the axial charge is formulated by including both types
of fluctuations.
|
1506.01384v2
|
2015-08-06
|
Characteristics of hybrid compact stars with a sharp hadron-quark interface
|
We describe two aspects of the physics of hybrid stars that have a sharp
interface between a core of quark matter and a mantle of nuclear matter.
Firstly, we analyze the mass-radius relation. We describe a generic "Constant
Speed of Sound" (CSS) parameterization of the quark matter equation of state
(EoS), in which the speed of sound is independent of density. In terms of the
three parameters of the CSS EoS we obtain the phase diagram of possible forms
of the hybrid star mass-radius relation, and we show how observational
constraints on the maximum mass and typical radius of neutron stars can be
expressed as constraints on the CSS parameters.
Secondly, we propose a mechanism for the damping of density oscillations,
including r-modes, in hybrid stars with a sharp interface. The dissipation
arises from the periodic conversion between quark matter and nuclear matter
induced by the pressure oscillations in the star. We find the damping grows
nonlinearly with the amplitude of the oscillation and is powerful enough to
saturate an r-mode at very low saturation amplitude, of order $10^{-10}$, which
is compatible with currently-available observations of neutron star spin
frequencies and temperatures.
|
1508.01261v2
|
2016-03-08
|
Global effect of local skin friction drag reduction in spatially developing turbulent boundary layer
|
A numerical investigation of two locally applied drag reducing control
schemes is carried out in the configuration of a spatially developing turbulent
boundary layer (TBL). One control is designed to damp near-wall turbulence and
the other induces constant mass flux in the wall-normal direction. Both control
schemes yield similar local drag reduction rates within the control region.
However, the flow development downstream of the control significantly differs:
persistent drag reduction is found for the uniform blowing case whereas drag
increase is found for the turbulence damping case. In order to account for this
difference the formulation of a global drag reduction rate is suggested. It
represents the reduction of the streamwise force exerted by the fluid on a
finite length plate. Furthermore, it is shown that the far downstream
development of the TBL after the control region can be described by a single
quantity, namely a streamwise shift of the uncontrolled boundary layer, i.e. a
changed virtual origin. Based on this result, a simple model is developed that
allows relating for the local drag reduction rate to the global one without the
need of conducting expensive simulations or measurements far downstream of the
control region.
|
1603.02427v2
|
2016-09-23
|
Influence of heavy metal materials on magnetic properties of Pt/Co/heavy metal tri-layered structures
|
Pt/Co/heavy metal (HM) tri-layered structures with interfacial perpendicular
magnetic anisotropy (PMA) are currently under intensive research for several
emerging spintronic effects, such as spinorbit torque, domain wall motion, and
room temperature skyrmions. HM materials are used as capping layers to generate
the structural asymmetry and enhance the interfacial effects. For instance, the
Pt/Co/Ta structure attracts a lot of attention as it may exhibit large
Dzyaloshinskii-Moriya interaction. However, the dependence of magnetic
properties on different capping materials has not been systematically
investigated. In this paper, we experimentally show the interfacial PMA and
damping constant for Pt/Co/HM tri-layered structures through time-resolved
magneto-optical Kerr effect measurements as well as magnetometry measurements,
where the capping HM materials are W, Ta, and Pd. We found that the Co/HM
interface plays an important role on the magnetic properties. In particular,
the magnetic multilayers with a W capping layer features the lowest effective
damping value, which may be attributed to the different spin-orbit coupling and
interfacial hybridization between Co and HM materials. Our findings allow a
deep understanding of the Pt/Co/HM tri-layered structures. Such structures
could lead to a better era of data storage and processing devices.
|
1609.07320v2
|
2016-12-15
|
Plasmons at the LaAlO$_3$/SrTiO$_3$ interface and Graphene-LaAlO$_3$/SrTiO$_3$ double layer
|
We study plasmon modes of the two-dimensional electron gas residing at the
interface of band insulators $\rm{LaAlO_3}$ and $\rm{SrTiO_3}$ (LAO/STO) and
the plasmon excitations of graphene-LAO/STO double layer as well. Considering
the electron-electron interaction within random phase approximation, we
calculate the plasmon dispersions of both systems numerically and in the long
wavelength limit analytical expressions for collective modes are found. One
optical mode and two (three) acoustic modes are predicted for the LAO/STO
(graphene-LAO/STO) system where only the uppermost acoustic mode of both
systems can emerge above the electron-hole continuum depending on the
characteristics of each system. In the case of LAO/STO interface, thanks to the
spatial separation between $\rm{t_{2g}}$ orbitals, the upper acoustic mode
might be undamped at the long wavelength limit depending on the exact value of
the dielectric constant of $\rm{SrTiO_3}$. Same as other double layer systems,
the interlayer distance for the graphene-LAO/STO system plays a crucial role in
damping the upper acoustic mode. Faster damping of all plasmon modes of the
present double layer system in comparison with the ones with conventional 2DEG
instead of $\rm{t_{2g}}$ electron gas is also found due to heavier effective
masses of the gas and also stronger interlayer Coulomb interaction.
|
1612.04953v2
|
2017-06-22
|
Full-counting statistics of transient energy current in mesoscopic systems
|
We investigate the full-counting statistics (FCS) of energy flow carried by
electrons in the transient regime. Based on two measurement scheme we formulate
a non-equilibrium Keldysh Green's function theory to compute the generating
function for FCS of energy transport. Specifically, we express the generating
function using the path integral along Keldysh contour and obtain exact
solution of the generating function using the Grassmann algebra. With this
formalism, we calculate the transient energy current and higher order cumulants
for both single and double quantum dot (QD) systems in the transient regime. To
examine finite bandwidth effect of leads to FCS of energy transport, we have
used an exact solvable model with a Lorentizian linewidth where all
non-equilibrium Green's functions can be solved exactly in the time domain. It
is found that the transient energy current exhibits damped oscillatory
behavior. For the single quantum dot system the frequency of oscillation is
independent of bandwidth of the leads while the decay rate of the oscillation
amplitude is determined by the lifetime of resonant state which increases as
the bandwidth decreases. At short times, a universal scaling of maximum
amplitude of normalized cumulants is identified for the single QD system. For
the double QD system, the damped oscillation of energy current is dominated by
Rabi oscillation with frequency approximately proportional to the coupling
constant between two quantum dots. In general, the transient energy current
increases when the coupling between two QDs is stronger. However, when the
interdot coupling is larger than half of the external bias the transient energy
current is suppressed significantly. All these results can be understood
analytically.
|
1706.07182v1
|
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