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2015-11-25
|
A Proposal of a Damping Term for the Relativistic Euler Equations
|
We introduce a damping term for the special relativistic Euler equations in
$3$-D and show that the equations reduce to the non-relativistic damped Euler
equations in the Newtonian limit. We then write the equations as a symmetric
hyperbolic system for which local-in-time existence of smooth solutions can be
shown.
|
1511.08183v1
|
2016-01-27
|
Concatenated Codes for Amplitude Damping
|
We discuss a method to construct quantum codes correcting amplitude damping
errors via code concatenation. The inner codes are chosen as asymmetric
Calderbank-Shor-Steane (CSS) codes. By concatenating with outer codes
correcting symmetric errors, many new codes with good parameters are found,
which are better than the amplitude damping codes obtained by any previously
known construction.
|
1601.07423v1
|
2016-03-29
|
Generalized damped Milne-Pinney equation and Chiellini method
|
We adopt the Chiellini integrability method to find the solutions of various
generalizations of the damped Milne-Pinney equations. In particular, we find
the solution of the damped Ermakov-Painlev\'e II equation and generalized
dissipative Milne-Pinney equation.
|
1603.08747v2
|
2017-12-07
|
Damped wave equations on compact hyperbolic surfaces
|
We prove exponential decay of energy for solutions of the damped wave
equation on compact hyperbolic surfaces with regular initial data as long as
the damping is nontrivial. The proof is based on a similar strategy as in
Dyatlov-Jin and in particular, uses the fractal uncertainty principle proved in
Bourgain-Dyatlov.
|
1712.02692v1
|
2018-03-20
|
Stability of the wave equations on a tree with local Kelvin-Voigt damping
|
In this paper we study the stability problem of a tree of elastic strings
with local Kelvin-Voigt damping on some of the edges. Under the compatibility
condition of displacement and strain and continuity condition of damping
coefficients at the vertices of the tree, exponential/polynomial stability are
proved.
|
1803.07280v1
|
2018-09-10
|
Logarithmic Decay of a Wave Equation with Kelvin-Voigt Damping
|
In this paper we analyze the long time behavior of a wave equation with local
Kelvin-Voigt Damping. Through introducing proper class symbol and
pseudo-differential calculus, we obtain a Carleman estimate, and then establish
an estimate on the corresponding resolvent operator. As a result, we show the
logarithmic decay rate for energy of the system without any geometric
assumption on the subdomain on which the damping is effective.
|
1809.03196v1
|
2018-11-07
|
Slow-dissipation limit of the harmonic oscillator with general power-law damping
|
An approximate solution is presented for simple harmonic motion in the
presence of damping by a force which is a general power-law function of the
velocity. The approximation is shown to be quite robust, allowing for a simple
way to investigate amplitude decay in the presence of general types of weak,
nonlinear damping.
|
1811.02953v2
|
2019-09-25
|
Forced Coupled Duffing Oscillators with Nonlinear Damping: Resonance and Antiresonance
|
In this work, we investigate resonance and antiresonance behaviour in forced
coupled Duffing oscillators with nonlinear damping. Further, we will analyse
the parameter dependence of the frequency response and stability. In the course
of all the analysis, emphasis shall be on how different damping mechanisms
contrast against each other.
|
1909.11390v1
|
2020-04-21
|
Damping rate limitations for transverse dampers in large hadron colliders
|
The paper focuses on two issues important for design and operation of
bunch-by-bunch transverse damper in a very large hadron collider, where fast
damping is required to suppress beam instabilities and noise induced emittance
growth. The first issue is associated with kick variation along a bunch which
affects the damping of head-tail modes. The second issue is associated with
affect of damper noise on the instability threshold.
|
2004.10249v2
|
2021-08-17
|
Spectral enclosures for the damped elastic wave equation
|
In this paper we investigate spectral properties of the damped elastic wave
equation. Deducing a correspondence between the eigenvalue problem of this
model and the one of Lam\'e operators with non self-adjoint perturbations, we
provide quantitative bounds on the location of the point spectrum in terms of
suitable norms of the damping coefficient.
|
2108.07676v1
|
2022-02-10
|
Stochastic optimal control for nonlinear damped network dynamics
|
We present a stochastic optimal control problem for a tree network. The
dynamics of the network are governed by transport equations with a special
emphasis on the non-linear damping function. Demand profiles at the network
sinks are modelled by a stochastic differential equations. An explicit optimal
inflow into the network is determined and numerical simulations are presented
to show the effects for different choices of the non-linear damping.
|
2202.05114v1
|
2022-03-03
|
Conformal symmetry in damped Pais-Uhlenbeck oscillator
|
Two Lagrangian formulations for describing of the damped harmonic oscillator
have been introduced by Bateman. For these models we construct higher
derivative generalization which enjoys the l-conformal Newton-Hooke symmetry.
The dynamics of generalized systems corresponds to the damped Pais-Uhlenbeck
oscillator for a particular choice of its frequencies.
|
2203.01651v1
|
2022-05-26
|
Ergodic results for the stochastic nonlinear Schrödinger equation with large damping
|
We study the nonlinear Schr\"odinger equation with linear damping, i.e. a
zero order dissipation, and additive noise. Working in $R^d$ with d = 2 or d =
3, we prove the uniqueness of the invariant measure when the damping
coefficient is sufficiently large.
|
2205.13364v1
|
2022-10-31
|
An adaptive damped Newton method for strongly monotone and Lipschitz continuous operator equations
|
We will consider the damped Newton method for strongly monotone and Lipschitz
continuous operator equations in a variational setting. We will provide a very
accessible justification why the undamped Newton method performs better than
its damped counterparts in a vicinity of a solution. Moreover, in the given
setting, an adaptive step-size strategy will be presented, which guarantees the
global convergence and favours an undamped update if admissible.
|
2210.17107v1
|
2022-11-19
|
Blow up and lifespan estimates for systems of semi-linear wave equations with damping and potential
|
In this paper, we consider the semi-linear wave systems with
power-nonlinearities and a large class of space-dependent damping and
potential. We obtain the same blow-up regions and the lifespan estimates for
three types wave systems, compared with the systems without damping and
potential.
|
2211.10639v1
|
2023-08-10
|
Pathwise uniqueness for stochastic heat and damped equations with Hölder continuous drift
|
In this paper, we prove pathwise uniqueness for stochastic differential
equations in infinite dimension. Under our assumptions, we are able to consider
the stochastic heat equation up to dimension $3$, the stochastic damped wave
equation in dimension $1$ and the stochastic Euler-Bernoulli damped beam
equation up to dimension $3$. We do not require that the so-called {\it
structure condition} holds true.
|
2308.05415v1
|
2023-10-30
|
Beliaev damping in Bose gas
|
According to the Bogoliubov theory the low energy behaviour of the Bose gas
at zero temperature can be described by non-interacting bosonic quasiparticles
called phonons. In this work the damping rate of phonons at low momenta, the
so-called Beliaev damping, is explained and computed with simple arguments
involving the Fermi Golden Rule and Bogoliubov's quasiparticles.
|
2310.20070v1
|
2023-11-25
|
Energy scattering for the unsteady damped nonlinear Schrodinger equation
|
We investigate the large time behavior of the solutions to the nonlinear
focusing Schr\"odinger equation with a time-dependent damping in the energy
sub-critical regime. Under non classical assumptions on the unsteady damping
term, we prove some scattering results in the energy space.
|
2311.14980v2
|
1995-03-13
|
Tidal Excitation of Modes in Binary Systems with Applications to Binary Pulsars
|
We consider the tidal excitation of modes in a binary system of arbitrary
eccentricity. For a circular orbit, the modes generally undergo forced
oscillation with a period equal to the orbital period ($T$). For an eccentric
orbit, the amplitude of each tidally excited mode can be written approximately
as the sum of an oscillatory term that varies sinusoidally with the mode
frequency and a `static' term that follows the time dependence of the tidal
forcing function. The oscillatory term falls off exponentially with increasing
$\b$ (defined as the ratio of the periastron passage time to the mode period),
whereas the `static' term is independent of $\b$. For small $\b$ modes ($\b
\approx 1$), the two terms are comparable, and the magnitude of the mode
amplitude is nearly constant over the orbit. For large $\b$ modes ($\b \gta$ a
few), the oscillatory term is very small compared to the `static' term, in
which case the mode amplitude, like the tidal force, varies as the distance
cubed. For main sequence stars, $p$, $f$, and low order $g$-modes generally
have large $\b$ and hence small amplitudes of oscillation. High overtone
$g$-modes, however, have small overlap with the tidal forcing function. Thus,
we expect an intermediate overtone $g$-mode with $\b \sim 1$ to have the
largest oscillation amplitude. The dependence on mode damping and the stellar
rotation rate is considered, as well as the effects of orbital evolution. We
apply our work to the two binary pulsar system: PSR J0045-7319 and PSR
B1259-63.
|
9503053v1
|
1997-09-05
|
The Formation of Cosmic Structures in a Light Gravitino Dominated Universe
|
We analyse the formation of cosmic structures in models where the dark matter
is dominated by light gravitinos with mass of $ 100$ eV -- 1 keV, as predicted
by gauge-mediated supersymmetry (SUSY) breaking models. After evaluating the
number of degrees of freedom at the gravitinos decoupling ($g_*$), we compute
the transfer function for matter fluctuations and show that gravitinos behave
like warm dark matter (WDM) with free-streaming scale comparable to the galaxy
mass scale. We consider different low-density variants of the WDM model, both
with and without cosmological constant, and compare the predictions on the
abundances of neutral hydrogen within high-redshift damped Ly--$\alpha$ systems
and on the number density of local galaxy clusters with the corresponding
observational constraints. We find that none of the models satisfies both
constraints at the same time, unless a rather small $\Omega_0$ value ($\mincir
0.4$) and a rather large Hubble parameter ($\magcir 0.9$) is assumed.
Furthermore, in a model with warm + hot dark matter, with hot component
provided by massive neutrinos, the strong suppression of fluctuation on scales
of $\sim 1\hm$ precludes the formation of high-redshift objects, when the
low--$z$ cluster abundance is required. We conclude that all different variants
of a light gravitino DM dominated model show strong difficulties for what
concerns cosmic structure formation.
This gives a severe cosmological constraint on the gauge-mediated SUSY
breaking scheme.
|
9709047v1
|
1998-01-05
|
Calculation of the Ostriker-Vishniac Effect in Cold Dark Matter Models
|
We present a new derivation of the cosmic microwave background anisotropy
spectrum from the Ostriker-Vishniac effect for an open, flat, or closed
Universe, and calculate the anisotropy expected in cold dark-matter (CDM)
models. We provide simple semi-analytic fitting formulas for the Vishniac power
spectrum that can be used to evaluate the expected anisotropy in CDM models for
any arbitrary ionization history. In a flat Universe, CDM models normalized to
cluster abundances produce rms temperature anisotropies of 0.8--2.4 $\mu$K on
arcminute angular scales for a constant ionization fraction of unity, whereas
an ionization fraction of 0.2 yields rms anisotropies of 0.3--0.8 $\mu$K. In an
open and/or high-baryon-density Universe, the level of anisotropy is somewhat
higher. The signal in some of these models may be detectable with planned
interferometry experiments. The damping of the acoustic peaks in the
primary-anisotropy spectrum at degree angular scales depends primarily on the
optical depth and only secondarily on the epoch of reionization. On the other
hand, the amplitude of Ostriker-Vishniac anisotropies depends sensitively on
the epoch of reionization. Therefore, when combined with the estimate of the
reionization optical depth provided by maps of degree-scale anisotropies, the
Ostriker-Vishniac effect can provide a unique probe of the epoch of
reionization.
|
9801022v3
|
1999-06-08
|
Constrained Violent Relaxation to a Spherical Halo
|
Violent relaxation during the collapse of a galaxy halo is known to be
incomplete in realistic cases such as cosmological infall or mergers. We adopt
a physical picture of strong but short lived interactions between potential
fluctuations and particle orbits, using the broad framework outlined by
Tremaine (1987) for incorporating incompleteness of the relaxation. We are
guided by results from plasma physics, viz. the quasilinear theory of Landau
damping, but allow for significant differences in our case. Crucially, wave
particle scattering does not drive the system to an equilibrium distribution
function of the exponential type, even in regions of phase space allowed by the
constraints. The physical process is mixing without friction in ``action''
space, for which the simplest possible model is a constant phase space density
modulated by the constraints. Our distribution function does not use the
exponential functions of the energy prevalent in other work, which we regard as
inappropriate to collisionless systems. The halo of the self-consistent,
parameter-free solutions show an r^(-4) behavior in density at large r, an
r^(1/4) surface brightness profile in the region 0.1-8 r_e, and a radially
anisotropic velocity dispersion profile outside an isotropic core. The energy
distribution seen in simulations N(E) singles out the pericenter cutoff model
as the most realistic among the variants we have explored.
|
9906133v1
|
2000-02-21
|
Star Formation and Chemical Evolution of Lyman-Break Galaxies
|
The number density and clustering properties of Lyman-break galaxies (LBGs)
observed at redshift $z\sim 3$ are best explained by assuming that they are
associated with the most massive haloes at $z\sim 3$ predicted in hierarchical
models of structure formation. In this paper we study, under the same
assumption, how star formation and chemical enrichment may have proceeded in
the LBG population. A consistent model, in which the amount of cold gas
available for star formation must be regulated, is suggested. It is found that
gas cooling in dark haloes provides a natural regulation process. In this
model, the star formation rate in an LBG host halo is roughly constant over
about 1 Gyr. The predicted star formation rates and effective radii are
consistent with observations. The metallicity of the gas associated with an LBG
is roughly equal to the chemical yield, or about the order of $1 Z_{\odot}$ for
a Salpeter IMF. The contribution to the total metals of LBGs is roughly
consistent with that obtained from the observed cosmic star formation history.
The model predicts a marked radial metallicity gradient in a galaxy, with the
gas in the outer region having much lower metallicity. As a result, the
metallicities for the damped Lyman-alpha absorption systems expected from the
LBG population are low. Since LBG halos are filled with hot gas in this model,
their contributions to the soft X-ray background and to the UV ionization
background are calculated and discussed.
|
0002388v1
|
2000-03-12
|
Gravity-Modes in ZZ Ceti Stars: IV. Amplitude Saturation by Parametric Instability
|
ZZ Ceti stars exhibit small amplitude photometric pulsations in multiple
gravity-modes. We demonstrate that parametric instability, a form of resonant
3-mode coupling, limits overstable modes to amplitudes similar to those
observed. In particular, it reproduces the observed trend that longer period
modes have larger amplitudes. Parametric instability involves the
destabilization of a pair of stable daughter modes by an overstable parent
mode. The 3-modes must satisfy exact angular selection rules and approximate
frequency resonance. The lowest instability threshold for each parent mode is
provided by the daughter pair that minimizes
$(\delta\omega^2+\gamma_d^2)/\kappa^2$, where $\kappa$ is the nonlinear
coupling constant, $\delta\omega$ is the frequency mismatch, and $\gamma_d$ is
the energy damping rate of the daughter modes. The overstable mode's amplitude
is maintained at close to the instability threshold value.
Although parametric instability defines an upper envelope for the amplitudes
of overstable modes in ZZ Ceti stars, other nonlinear mechanisms are required
to account for the irregular distribution of amplitudes of similar modes and
the non-detection of modes with periods longer than $1,200\s$. Resonant 3-mode
interactions involving more than one excited mode may account for the former.
Our leading candidate for the latter is Kelvin-Helmholtz instability of the
mode-driven shear layer below the convection zone.
|
0003163v1
|
2000-10-18
|
Magnetic Helicity Conservation and Astrophysical Dynamos
|
We construct a magnetic helicity conserving dynamo theory which incorporates
a calculated magnetic helicity current. In this model the fluid helicity plays
a small role in large scale magnetic field generation. Instead, the dynamo
process is dominated by a new quantity, derived from asymmetries in the second
derivative of the velocity correlation function, closely related to the `twist
and fold' dynamo model. The turbulent damping term is, as expected, almost
unchanged. Numerical simulations with a spatially constant fluid helicity and
vanishing resistivity are not expected to generate large scale fields in
equipartition with the turbulent energy density. The prospects for driving a
fast dynamo under these circumstances are uncertain, but if it is possible,
then the field must be largely force-free. On the other hand, there is an
efficient analog to the $\alpha-\Omega$ dynamo. Systems whose turbulence is
driven by some anisotropic local instability in a shearing flow, like real
stars and accretion disks, and some computer simulations, may successfully
drive the generation of strong large scale magnetic fields, provided that
$\partial_r\Omega< \partial_\theta v_z\omega_\theta>>0$. We show that this
criterion is usually satisfied. Such dynamos will include a persistent,
spatially coherent vertical magnetic helicity current with the same sign as
$-\partial_r\Omega$, that is, positive for an accretion disk and negative for
the Sun. We comment on the role of random magnetic helicity currents in storing
turbulent energy in a disordered magnetic field, which will generate an
equipartition, disordered field in a turbulent medium, and also a declining
long wavelength tail to the power spectrum. As a result, calculations of the
galactic `seed' field are largely irrelevant.
|
0010373v2
|
2001-01-12
|
The Scaling of the Redshift Power Spectrum: Observations from the Las Campanas Redshift Survey
|
In a recent paper we have studied the redshift power spectrum $P^S(k,\mu)$ in
three CDM models with the help of high resolution simulations. Here we apply
the method to the largest available redshift survey, the Las Campanas Redshift
Survey (LCRS). The basic model is to express $P^S(k,\mu)$ as a product of three
factors P^S(k,\mu)=P^R(k)(1+\beta\mu^2)^2 D(k,\mu). Here $\mu$ is the cosine of
the angle between the wave vector and the line of sight. The damping function
$D$ for the range of scales accessible to an accurate analysis of the LCRS is
well approximated by the Lorentz factor D=[1+{1\over
2}(k\mu\sigma_{12})^2]^{-1}. We have investigated different values for $\beta$
($\beta=0.4$, 0.5, 0.6), and measured $P^R(k)$ and $\sigma_{12}(k)$ from
$P^S(k,\mu)$ for different values of $\mu$. The velocity dispersion
$\sigma_{12}(k)$ is nearly a constant from $k=0.5$ to 3 $\mpci$. The average
value for this range is $510\pm 70 \kms$. The power spectrum $P^R(k)$ decreases
with $k$ approximately with $k^{-1.7}$ for $k$ between 0.1 and 4 $\mpci$. The
statistical significance of the results, and the error bars, are found with the
help of mock samples constructed from a large set of high resolution
simulations. A flat, low-density ($\Omega_0=0.2$) CDM model can give a good fit
to the data, if a scale-dependent special bias scheme is used which we have
called the cluster-under-weighted bias (Jing et al.).
|
0101211v2
|
2002-07-15
|
Atomic data for the K-vacancy states of Fe XXIV
|
As part of a project to compute improved atomic data for the spectral
modeling of iron K lines, we report extensive calculations and comparisons of
atomic data for K-vacancy states in Fe XXIV. The data sets include: (i) energy
levels, line wavelengths, radiative and Auger rates; (ii) inner-shell electron
impact excitation rates and (iii) fine structure inner-shell photoionization
cross sections. The calculations of energy levels and radiative and Auger rates
have involved a detailed study of orbital representations, core relaxation,
configuration interaction, relativistic corrections, cancellation effects and
semi-empirical corrections. It is shown that a formal treatment of the Breit
interaction is essential to render the important magnetic correlations that
take part in the decay pathways of this ion. As a result, the accuracy of the
present A-values is firmly ranked at better than 10% while that of the Auger
rates at only 15%. The calculations of collisional excitation and
photoionization cross sections take into account the effects of radiation and
spectator Auger dampings. In the former, these effects cause significant
attenuation of resonances leading to a good agreement with a simpler method
where resonances are excluded. In the latter, resonances converging to the K
threshold display symmetric profiles of constant width that causes edge
smearing.
|
0207323v2
|
2002-12-17
|
Probing the Friedmann equation during recombination with future CMB experiments
|
We show that by combining measurements of the temperature and polarization
anisotropies of the Cosmic Microwave Background (CMB), future experiments will
tightly constrain the expansion rate of the universe during recombination. A
change in the expansion rate modifies the way in which the recombination of
hydrogen proceeds, altering the shape of the acoustic peaks and the level of
CMB polarization. The proposed test is similar in spirit to the examination of
abundances of light elements produced during Big Bang Nucleosynthesis and it
constitutes a way to study possible departures from standard recombination. For
simplicity we parametrize the change in the Friedmann equation by changing the
gravitational constant $G$. The main effect on the temperature power spectrum
is a change in the degree of damping of the acoustic peaks on small angular
scales. The effect can be compensated by a change in the shape of the
primordial power spectrum. We show that this degeneracy between the expansion
rate and the primordial spectrum can be broken by measuring CMB polarization.
In particular we show that the MAP satellite could obtain a constraint for the
expansion rate $H$ during recombination of $\delta H/H \simeq 0.09$ or $\delta
G/G \simeq 0.18$ after observing for four years, whereas Planck could obtain
$\delta H/H \leq 0.014$ or $\delta G/G \leq 0.028$ within two years, even after
allowing for further freedom in the shape of the power spectrum of primordial
fluctuations.
|
0212360v2
|
2003-10-08
|
The Sizes and Kinematic Structure of Absorption Systems Towards the Lensed Quasar APM08279+5255
|
We have obtained spatially resolved spectra of the z=3.911 triply imaged QSO
APM08279+5255 using the Space Telescope Imaging Spectrograph (STIS) on board
the Hubble Space Telescope (HST). We study the line of sight equivalent width
(EW) differences and velocity shear of high and low ionization absorbers
(including a damped Lyman alpha [DLA] system identified in a spatially
unresolved ground based spectrum) in the three lines of sight. We find that
high ionization systems (primarily CIV absorbers) do not exhibit strong EW
variations on scales <0.4 kpc; their fractional EW differences are typically
less than 30%. When combined with previous work on other QSO pairs, we find
that the fractional variation increases steadily with separation out to at
least ~100 kpc. Conversely, low ionization systems (primarily MgII absorbers)
show strong variations (often > 80%) over kpc scales. A minimum radius for
strong (EW > 0.3 A) MgII systems of > 1.4 kpc is inferred from absorption
coincidences in all lines of sight. For weak MgII absorbers (EW < 0.3 A), a
maximum likelihood analysis indicates a most probable coherence scale of 2.0
kpc for a uniform spherical geometry, with 95% confidence limits ranging
between 1.5 and 4.4 kpc. Finally, for systems with weak absorption that can be
confidently converted to column densities, we find constant N(CIV)/N(SiIV)
across the three lines of sight. Similarly, the [Al/Fe] ratios in the z = 2.974
DLA are consistent with solar relative abundances over a transverse distance of
\~0.35 kpc. (abrdiged)
|
0310221v1
|
2004-05-11
|
Photoionization Modeling and the K Lines of Iron
|
We calculate the efficiency of iron K line emission and iron K absorption in
photoionized models using a new set of atomic data. These data are more
comprehensive than those previously applied to the modeling of iron K lines
from photoionized gases, and allow us to systematically examine the behavior of
the properties of line emission and absorption as a function of the ionization
parameter, density and column density of model constant density clouds. We show
that, for example, the net fluorescence yield for the highly charged ions is
sensitive to the level population distribution produced by photoionization, and
these yields are generally smaller than those predicted assuming the population
is according to statistical weight. We demonstrate that the effects of the many
strongly damped resonances below the K ionization thresholds conspire to smear
the edge, thereby potentially affecting the astrophysical interpretation of
absorption features in the 7-9 keV energy band. We show that the centroid of
the ensemble of K$\alpha$ lines, the K$\beta$ energy, and the ratio of the
K$\alpha_1$ to K$\alpha_2$ components are all diagnostics of the ionization
parameter of our model slabs
|
0405210v1
|
2005-05-23
|
Damped Lyman Alpha Surveys and Statistics - A Review
|
The history and progress of DLA research over the past several decades is
reviewed here. Larger datasets and deeper surveys, particularly over the last
couple of years, have improved our knowledge of the neutral gas content and
distribution in the universe at all observable redshifts, including the present
epoch. New results on the statistics of DLAs at z<1.65 from our HST-UV surveys
are presented and discussed in the context of recent results at z=0 and at high
redshift. We find that Omega_{DLA}(z>0) remains roughly constant to within the
uncertainties; the z=0 value of the neutral gas mass density, Omega_{g}, is a
factor of ~2 less than Omega_{DLA}. The DLA incidence, n(z), undergoes rapid
evolution between redshifts 5 and 2, but is consistent with the no-evolution
curve in the current concordance cosmology for z ~< 2. We also show that if the
local Schmidt law relating surface density of gas and star formation rate (SFR)
is valid at the DLA redshifts, then the DLA SFR density is too low for them to
provide a significant contribution to the cosmic star formation history (SFH)
at z >~ 1. This implies that the DLAs are unlikely to be the same population as
the star forming galaxies (i.e., the Lyman break and sub-millimeter galaxies)
that dominate the SFH of the high redshift universe. We suggest that this
discrepancy and the DLA ``missing metals'' problem could be the result of
missing very high column density gas due to its very small absorption cross
section. (abridged)
|
0505479v1
|
2005-06-28
|
The Metallicity - Kinematics Relation in Large-N(HI) Absorbers
|
Sloan Digital Sky Survey (SDSS) quasar spectroscopy is yielding a database of
strong low-ionization MgII absorbers over the redshift interval 0.36<z<2.28
which is over two orders of magnitude larger than anything previously
assembled. HST UV spectroscopy has been used to measure neutral hydrogen column
densities for a small subset of them. These data empirically show that MgII
absorbers with rest equivalent widths $W^{\lambda2796}_0 \ge 0.6$ \AA have a
mean neutral hydrogen column density that is roughly constant at $N(HI) \approx
4 \times 10^{20}$ atoms cm$^{-2}$, with individual systems lying in the damped
Ly-alpha (DLA) and sub-DLA regimes. Since the MgII doublets generally exhibit
saturation, the $W^{\lambda2796}_0$ values are an indication of the absorbers'
velocity spreads. Thus, we can study neutral-gas-phase metallicities as a
function of kinematics by forming SDSS composite spectra and measuring weak
unsaturated metal lines that form in neutral gas (e.g., CrII, FeII, MnII, SiII,
ZnII) as a function of $W^{\lambda2796}_0$. We use this method on SDSS
composite spectra to show how metallicity and kinematics are positively
correlated for large N(HI) absorbers, including trends related to dust
depletion and the enhancement of alpha-elements. We also discuss the need to
account for selection effects in DLA surveys, and we make inferences about
models for DLA absorption and their contribution to cosmic star formation.
|
0506701v1
|
2006-11-28
|
Detecting cold gas at intermediate redshifts: GMRT survey using Mg II systems
|
Intervening HI 21-cm absorption systems at z > 1.0 are very rare and only 4
confirmed detections have been reported in the literature. Despite their
scarcity, they provide interesting and unique insights into the physical
conditions in the interstellar medium of high-z galaxies. Moreover, they can
provide independent constraints on the variation of fundamental constants. We
report 3 new detections based on our ongoing Giant Metrewave Radio Telescope
(GMRT) survey for 21-cm absorbers at 1.10< z_abs< 1.45 from candidate damped
Lyman_alpha systems. The 21-cm lines are narrow for the z_abs = 1.3710 system
towards SDSS J0108-0037 and z_abs = 1.1726 system toward SDSS J2358-1020. Based
on line full-width at half maximum, the kinetic temperatures are <= 5200 K and
<=800 K, respectively. The 21-cm absorption profile of the third system, z_abs
=1.1908 system towards SDSS J0804+3012, is shallow, broad and complex,
extending up to 100 km/s. The centroids of the 21-cm lines are found to be
shifted with respect to the corresponding centroids of the metal lines derived
from SDSS spectra. This may mean that the 21-cm absorption is not associated
with the strongest metal line component.
|
0611836v1
|
1998-10-01
|
Finite temperature dynamics of vortices in the two dimensional anisotropic Heisenberg model
|
We study the effects of finite temperature on the dynamics of non-planar
vortices in the classical, two-dimensional anisotropic Heisenberg model with
XY- or easy-plane symmetry. To this end, we analyze a generalized
Landau-Lifshitz equation including additive white noise and Gilbert damping.
Using a collective variable theory with no adjustable parameters we derive an
equation of motion for the vortices with stochastic forces which are shown to
represent white noise with an effective diffusion constant linearly dependent
on temperature. We solve these stochastic equations of motion by means of a
Green's function formalism and obtain the mean vortex trajectory and its
variance. We find a non-standard time dependence for the variance of the
components perpendicular to the driving force. We compare the analytical
results with Langevin dynamics simulations and find a good agreement up to
temperatures of the order of 25% of the Kosterlitz-Thouless transition
temperature. Finally, we discuss the reasons why our approach is not
appropriate for higher temperatures as well as the discreteness effects
observed in the numerical simulations.
|
9810011v1
|
2001-02-09
|
The influence of the oscillations of the chemical potential on the de Haas - van Alphen effect in quasi-two-dimensional compounds
|
The de Haas - van Alphen effect in quasi-two-dimensional metals is studied at
arbitrary parameters. The oscillations of the chemical potential may
substantially change the temperature dependence of harmonic amplitudes that is
usually used to determine the effective electron mass. Hence, the processing of
the experimental data using the standard Lifshitz-Kosevich formula (that
assumes the chemical potential to be constant) may lead to substantial errors
even in the limit of strong harmonic damping. This fact may explain the
difference between the effective electron masses, determined from the de Haas -
van Alphen effect and the cyclotron resonance measurements. The oscillations of
the chemical potential and the deviations from the Lifshitz-Kosevich formula
depend on the reservoir density of states, that exists in organic metals due to
open sheets of Fermi surface. This dependence can be used to determine the
density of electron states on open sheets of Fermi surface. We present the
analytical results of the calculations of harmonic amplitudes in some limiting
cases that show the importance of the oscillations of the chemical potential.
The algorithm of the simple numerical calculation of the harmonic amplitudes at
arbitrary reservoir density of states, arbitrary warping, spin-splitting,
temperature and Dingle temperature is also described.
|
0102177v1
|
2003-11-11
|
Ornstein-Zernike equation and Percus-Yevick theory for molecular crystals
|
We derive the Ornstein-Zernike equation for molecular crystals of axially
symmetric particles and apply the Percus-Yevick approximation to this system.
The one-particle orientational distribution function has a nontrivial
dependence on the orientation and is needed as an input. Despite some
differences, the Ornstein-Zernike equation for molecular crystals has a similar
structure as for liquids. We solve both equations for hard ellipsoids on a sc
lattice. Compared to molecular liquids, the tensorial orientational correlators
exhibit less structure. However, depending on the lengths a and b of the
rotation axis and the perpendicular axes of the ellipsoids, different behavior
is found. For oblate and prolate ellipsoids with b >= 0.35 (units of the
lattice constant), damped oscillations in distinct directions of direct space
occur for some correlators. They manifest themselves in some correlators in
reciprocal space as a maximum at the Brillouin zone edge, accompanied by maxima
at the zone center for other correlators. The oscillations indicate alternating
orientational fluctuations, while the maxima at the zone center originate from
nematic-like orientational fluctuations. For a <= 2.5 and b <= 0.35, the
oscillations are weaker. For a >= 3.0 and b <= 0.35, no oscillations occur any
longer. For many of the correlators in reciprocal space, an increase of a at
fixed b leads to a divergence at the zone center q = 0, consistent with
nematic-like long range fluctuations, and for some oblate and prolate systems
with b ~< 1.0 a simultaneous tendency to divergence of few other correlators at
the zone edge is observed. Comparison with correlators from MC simulations
shows satisfactory agreement. We also obtain a phase boundary for
order-disorder transitions.
|
0311253v1
|
2004-07-12
|
Microwave-induced flow of vortices in long Josephson junctions
|
We report experimental and numerical study of microwave-induced flow of
vortices in long Josephson junctions at zero dc magnetic field. Our intriguing
observation is that applying an ac-bias of a small frequency $f \ll f_p $ and
sufficiently large amplitude changes the current-voltage characteristics
($I$-$V$ curve) of the junction in a way similar to the effect of dc magnetic
field, well known as the flux-flow behavior. The characteristic voltage $V$ of
this low voltage branch increases with the power $P$ of microwave radiation as
$V_{s}\propto P^{\alpha}$ with the index $\alpha \simeq 0.5 $. Experiments
using a low-temperature laser scanning microscope unambiguously indicate the
motion of Josephson vortices driven by microwaves. Numerical simulations agree
with the experimental data and show strongly {\it irregular} vortex motion. We
explain our results by exploiting an analogy between the microwave-induced
vortex flow in long Josephson junctions and incoherent multi-photon absorption
in small Josephson junctions in the presence of large thermal fluctuations. In
the case of long Josephson junctions the spatially-temporal chaos in the vortex
motion mimics the thermal fluctuations. In accordance with this analogy, a
control of the intensity of chaos in a long junction by changing its damping
constant leads to a pronounced change in the shape of the $I$-$V$ curve. Our
results provide a possible explanation to previously measured but not yet
understood microwave-driven properties of intrinsic Josephson junctions in
high-temperature superconductors.
|
0407290v1
|
2005-07-29
|
Frustrated two-level impurities in two-dimensional antiferromagnets
|
Dynamical properties of the impurity spin-$\frac12$ in 2D and quasi-2D
Heisenberg antiferromagnets (AFs) at $T\ge0$ are discussed. The specific case
of an impurity coupled symmetrically to two neighboring host spins is
considered. The specific feature of this problem is that the defect is
degenerate (frustrated) being located in zero molecular field. It is shown that
this problem can be described by spin-boson model without tunneling term and
with a more complex interaction. We demonstrate that the effect of the host
system on the defect is completely described by the spectral function. It is
found within the spin-wave approximation that for not too small $\omega$ the
spectral function is proportional to $\omega^2/J^3$, where $J$ is the exchange
constant between the host spins. The defect dynamical susceptibility is derived
using Abrikosov's pseudofermion technique and diagrammatic expansion. The
calculations are performed within the fourth order of the dimensionless
coupling parameter $f$. It is found that transverse impurity susceptibility
$\chi_\perp(\omega)$ has a Lorenz peak with the width proportional to
$f^4J(T/J)^3$ which disappears at T=0, and a non-resonant term. The later term
diverges logarithmically as $\omega,T \to 0$. The static susceptibility
$\chi(0)$ has the free-spin-like contribution $1/(4T)$, and a logarithmic
correction proportional to $f^2\ln(J/T)$. The influence of finite concentration
of the defects $n$ on the low-temperature properties of AF is also
investigated. A logarithmic correction to spin-wave velocity of the form
$nf^4\ln|J/\omega|$ and an anomalous damping of spin waves proportional to
$nf^4|\omega|$ are obtained.
|
0507704v2
|
2006-10-11
|
Miscibility in a degenerate fermionic mixture induced by linear coupling
|
We consider a one-dimensional mean-field-hydrodynamic model of a
two-component degenerate Fermi gas in an external trap, each component
representing a spin state of the same atom. We demonstrate that the
interconversion between them (linear coupling), imposed by a resonant
electromagnetic wave, transforms the immiscible binary gas into a miscible
state, if the coupling constant, $\kappa $, exceeds a critical value, $ \kappa
_{\mathrm{cr}}$. The effect is predicted in a variational approximation, and
confirmed by numerical solutions. Unlike the recently studied model of a binary
BEC with the linear coupling, the components in the immiscible phase of the
binary fermion mixture never fill two separated domains with a wall between
them, but rather form anti-locked ($\pi $ -phase-shifted) density waves.
Another difference from the bosonic mixture is spontaneous breaking of symmetry
between the two components in terms of numbers of atoms in them, $N_{1}$ and
$N_{2}$. The latter effect is characterized by the parameter $\nu \equiv
(N_{1}-N_{2})/(N_{1}+N_{2}) $ (only $N_{1}+N_{2}$ is a conserved quantity), the
onset of miscibility at $\kappa \geq \kappa_{\mathrm{cr}}$ meaning a transition
to $\nu \equiv 0$. At $\kappa <\kappa_{\mathrm{cr}}$, $\nu $ features damped
oscillations as a function of $\kappa $. We also briefly consider an asymmetric
model, with a chemical-potential difference between the two components.
|
0610317v1
|
2002-03-12
|
Gravitational Waves from the Merger of Binary Neutron Stars in a Fully General Relativistic Simulation
|
We performed 3D numerical simulations of the merger of equal-mass binary
neutron stars in full general relativity using a new large scale supercomputer.
We take the typical grid size as (505,505,253) for (x,y,z) and the maximum grid
size as (633,633,317). These grid numbers enable us to put the outer boundaries
of the computational domain near the local wave zone and hence to calculate
gravitational waveforms of good accuracy (within $\sim 10%$ error) for the
first time. To model neutron stars, we adopt a $\Gamma$-law equation of state
in the form $P=(\Gamma-1)\rho\epsilon$, where P, $\rho$, $\varep$ and $\Gamma$
are the pressure, rest mass density, specific internal energy, and adiabatic
constant. It is found that gravitational waves in the merger stage have
characteristic features that reflect the formed objects. In the case that a
massive, transient neutron star is formed, its quasi-periodic oscillations are
excited for a long duration, and this property is reflected clearly by the
quasi-periodic nature of waveforms and the energy luminosity. In the case of
black hole formation, the waveform and energy luminosity are likely damped
after a short merger stage. However, a quasi-periodic oscillation can still be
seen for a certain duration, because an oscillating transient massive object is
formed during the merger. This duration depends strongly on the initial
compactness of neutron stars and is reflected in the Fourier spectrum of
gravitational waves. To confirm our results and to calibrate the accuracy of
gravitational waveforms, we carried out a wide variety of test simulations,
changing the resolution and size of the computational domain.
|
0203037v1
|
2002-06-20
|
Gravitational Radiation Damping and the Three-Body Problem
|
A model of three-body motion is developed which includes the effects of
gravitational radiation reaction. The radiation reaction due to the emission of
gravitational waves is the only post-Newtonian effect that is included here.
For simplicity, all of the motion is taken to be planar. Two of the masses are
viewed as a binary system and the third mass, whose motion will be a fixed
orbit around the center-of-mass of the binary system, is viewed as a
perturbation. This model aims to describe the motion of a relativistic binary
pulsar that is perturbed by a third mass. Numerical integration of this
simplified model reveals that given the right initial conditions and parameters
one can see resonances. These (m,n) resonances are defined by the resonance
condition, $m\omega=2n\Omega$, where $m$ and $n$ are relatively prime integers
and $\omega$ and $\Omega$ are the angular frequencies of the binary orbit and
third mass orbit, respectively. The resonance condition consequently fixes a
value for the semimajor axis of the binary orbit for the duration of the
resonance; therefore, the binary energy remains constant on the average while
its angular momentum changes during the resonance.
|
0206059v2
|
1997-10-15
|
Domain Walls Out of Equilibrium
|
We study the non-equilibrium dynamics of domain walls in real time for
$\phi^4$ and Sine Gordon models in 1+1 dimensions in the dilute regime. The
equation of motion for the collective coordinate is obtained by integrating out
the meson excitations around the domain wall to one-loop order. The real-time
non-equilibrium relaxation is studied analytically and numerically to this
order. The constant friction coefficient vanishes but there is dynamical
friction and relaxation caused by off-shell non-Markovian effects. The validity
of a Markovian description is studied in detail. The proper Langevin equation
is obtained to this order, the noise is Gaussian and additive but colored. We
analyze the classical and hard thermal loop contributions to the self-energy
and noise kernels and show that at temperatures larger than the meson mass the
hard contributions are negligible and the finite temperature contribution to
the dynamics is governed by the classical soft modes of the meson bath. The
long time relaxational dynamics is completely dominated by classical Landau
damping resulting in that the corresponding time scales are not set by the
temperature but by the meson mass. The noise correlation function and the
dissipative kernel obey a generalized form of the Fluctuation-Dissipation
relation.
|
9710359v2
|
2000-07-18
|
Further studies on relic neutrino asymmetry generation I: the adiabatic Boltzmann limit, non-adiabatic evolution, and the classical harmonic oscillator analogue of the quantum kinetic equations
|
We demonstrate that the relic neutrino asymmetry evolution equation derived
from the quantum kinetic equations (QKEs) reduces to the Boltzmann limit that
is dependent only on the instantaneous neutrino number densities, in the
adiabatic limit in conjunction with sufficient damping. An original physical
and/or geometrical interpretation of the adiabatic approximation is given,
which serves as a convenient visual aid to understanding the sharply
contrasting resonance behaviours exhibited by the neutrino ensemble in opposing
collision regimes. We also present a classical analogue for the evolution of
the difference in $\nu_{\alpha}$ and $\nu_s$ number densities which, in the
Boltzmann limit, is akin to the behaviour of the generic reaction $A
\rightleftharpoons B$ with equal forward and reverse reaction rate constants. A
new characteristic quantity, the matter and collision-affected mixing angle of
the neutrino ensemble, is identified here for the first time. The role of
collisions is revealed to be twofold: (i) to wipe out the inherent
oscillations, and (ii) to equilibrate the $\nu_{\alpha}$ and $\nu_s$ number
densities in the long run. Studies on non-adiabatic evolution and its possible
relation to rapid oscillations in lepton number generation also feature, with
the introduction of an adiabaticity parameter for collision-affected
oscillations.
|
0007185v3
|
2002-09-27
|
Lorentz-Invariant Non-Commutative QED
|
Lorentz-invariant non-commutative QED (NCQED) is constructed such that it
should be a part of Lorentz-invariant non-commutative standard model (NCSM), a
subject to be treated in later publications. Our NCSM is based on Connes'
observation that the total fermion field in the standard model may be regarded
as a bi-module over a flavor-color algebra. In this paper, it is shown that
there exist two massless gauge fields in NCQED which are interchanged by $C'$
transformation. Since $C'$ is reduced to the conventional charge conjugation
$C$ in the commutative limit, the two gauge fields become identical to the
photon field in the same limit, which couples to only four spinors with charges
$\pm 2,\pm 1.$ Following Carlson-Carone-Zobin, our NCQED respects Lorentz
invariance employing Doplicher-Fredenhagen-Roberts' algebra instead of the
usual algebra with constant $\theta^{\mu\nu}$. In the new version
$\theta^{\mu\nu}$ becomes an integration variable. We show using a simple NC
scalar model that the $\theta$ integration gives an {\it invariant} damping
factor instead of the oscillating one to the nonplanar self-energy diagram in
the one-loop approximation. Seiberg-Witten map shows that the $\theta$
expansion of NCQED generates exotic but well-motivated derivative interactions
beyond QED with allowed charges being only $0, \pm 1, \pm 2$.
|
0209234v2
|
2005-11-12
|
Elliptic Curves, Algebraic Geometry Approach in Gravity Theory and Some Applications in Theories with Extra Dimensions I
|
Motivated by the necessity to find exact solutions with the elliptic
Weierstrass function of the Einstein's equations (see gr-qc/0105022),the
present paper develops further the proposed approach in hep-th/0107231,
concerning the s.c. cubic algebraic equation for effective parametrization.
Obtaining an ''embedded'' sequence of cubic equations, it is shown that it is
possible to parametrize also a multi-variable cubic curve, which is not the
standardly known case from algebraic geometry. Algebraic solutions for the
contravariant metric tensor components are derived and the parametrization is
extended in respect to the covariant components as well. It has been speculated
that corrections to the extradimensional volume in theories with extra
dimensions should be taken into account, due to the non-euclidean nature of the
Lobachevsky space. It was shown that the mechanism of exponential "damping" of
the physical mass in the higher-dimensional brane theory may be more
complicated due to the variety of contravariant metric components for a
spacetime with a given constant curvature. The invariance of the low-energy
type I string theory effective action is considered in respect not only to the
known procedure of compactification to a four-dimensional spacetime, but also
in respect to rescaling the contravariant metric components. As a result,
instead of the simple algebraic relations between the parameters in the string
action, quasilinear differential equations in partial derivatives are obtained,
which have been solved for the most simple case. In the Appendix, a new block
structure method is presented for solving the well known system of operator
equations in gravity theory in the N-dimensional case.
|
0511136v1
|
2002-04-11
|
High-fidelity teleportation of entanglements of running-wave field states
|
We describe a scheme for the teleportation of entanglements of zero- and
one-photon running-wave field states. In addition to linear optical elements,
Kerr nonlinearity is also employed so as to achieve a 100% probability of
success in the ideal case. A comprehensive phenomenological treatment of errors
in the domain of running-wave physics, for linear and nonlinear optical
elements, is also given, making it possible to calculate the fidelity of the
teleportation process. A strategy for carrying out the Bell-type measurement
which is able to probe the absorption of photons in the optical elements is
adopted. Such strategy, combined with usually small damping constants
characterizing the optical devices, results in a high fidelity for the
teleportation process. The feasibility of the proposed scheme relies on the
fact that the Kerr nonlinearity it demands can be achieved through the recently
reported ultraslow light propagation in cold atomic media [Phys. Rev. Lett. 84,
1419 (2000); Phys. Rev. A 65, 033833 (2002)].
|
0204057v2
|
2007-04-25
|
Zakharov simulation study of spectral features of on-demand Langmuir turbulence in an inhomogeneous plasma
|
We have performed a simulation study of Langmuir turbulence in the Earth's
ionosphere by means of a Zakharov model with parameters relevant for the F
layer. The model includes dissipative terms to model collisions and Landau
damping of the electrons and ions, and a linear density profile, which models
the ionospheric plasma inhomogeneity whose length scale is of the order 10--100
km. The injection of energy into the system is modeled by a constant source
term in the Zakharov equation. Langmuir turbulence is excited ``on-demand'' in
controlled ionospheric modification experiments where the energy is provided by
an HF radio beam injected into the overhead ionospheric plasma. The ensuing
turbulence can be studied with radars and in the form of secondary radiation
recorded by ground-based receivers. We have analyzed spectral signatures of the
turbulence for different sets of parameters and different altitudes relative to
the turning point of the linear Langmuir mode where the Langmuir frequency
equals the local plasma frequency. By a parametric analysis, we have derived a
simple scaling law, which links the spectral width of the turbulent frequency
spectrum to the physical parameters in the ionosphere. The scaling law provides
a quantitative relation between the physical parameters (temperatures, electron
number density, ionospheric length scale, etc.) and the observed frequency
spectrum. This law may be useful for interpreting experimental results.
|
0704.3419v2
|
2007-06-04
|
Quantum theory of exciton-photon coupling in photonic crystal slabs with embedded quantum wells
|
A theoretical description of radiation-matter coupling for
semiconductor-based photonic crystal slabs is presented, in which quantum wells
are embedded within the waveguide core layer. A full quantum theory is
developed, by quantizing both the electromagnetic field with a spatial
modulation of the refractive index and the exciton center of mass field in a
periodic piecewise constant potential. The second-quantized hamiltonian of the
interacting system is diagonalized with a generalized Hopfield method, thus
yielding the complex dispersion of mixed exciton-photon modes including losses.
The occurrence of both weak and strong coupling regimes is studied, and it is
concluded that the new eigenstates of the system are described by
quasi-particles called photonic crystal polaritons, which can occur in two
situations: (i) below the light line, when a resonance between exciton and
non-radiative photon levels occurs (guided polaritons), (ii) above the light
line, provided the exciton-photon coupling is larger than the intrinsic
radiative damping of the resonant photonic mode (radiative polaritons). For a
square lattice of air holes, it is found that the energy minimum of the lower
polariton branch can occur around normal incidence. The latter result has
potential implications for the realization of polariton parametric interactions
in photonic crystal slabs.
|
0706.0396v1
|
2007-06-05
|
Strong field effects on binary systems in Einstein-aether theory
|
"Einstein-aether" theory is a generally covariant theory of gravity
containing a dynamical preferred frame. This article continues an examination
of effects on the motion of binary pulsar systems in this theory, by
incorporating effects due to strong fields in the vicinity of neutron star
pulsars. These effects are included through an effective approach, by treating
the compact bodies as point particles with nonstandard, velocity dependent
interactions parametrized by dimensionless "sensitivities". Effective
post-Newtonian equations of motion for the bodies and the radiation damping
rate are determined. More work is needed to calculate values of the
sensitivities for a given fluid source, so precise constraints on the theory's
coupling constants cannot yet be stated. It is shown, however, that strong
field effects will be negligible given current observational uncertainties if
the dimensionless couplings are less than roughly 0.01 and two conditions that
match the PPN parameters to those of pure general relativity are imposed. In
this case, weak field results suffice and imply one further condition on the
couplings. Thus, there exists a one-parameter family of Einstein-aether
theories with "small-enough" couplings that passes all current observational
tests. No conclusion can yet be reached for large couplings.
|
0706.0704v2
|
2007-07-19
|
Structure formation in the presence of dark energy perturbations
|
We study non-linear structure formation in the presence of dark energy. The
influence of dark energy on the growth of large-scale cosmological structures
is exerted both through its background effect on the expansion rate, and
through its perturbations as well. In order to compute the rate of formation of
massive objects we employ the Spherical Collapse formalism, which we generalize
to include fluids with pressure. We show that the resulting non-linear
evolution equations are identical to the ones obtained in the Pseudo-Newtonian
approach to cosmological perturbations, in the regime where an equation of
state serves to describe both the background pressure relative to density, and
the pressure perturbations relative to the density perturbations as well. We
then consider a wide range of constant and time-dependent equations of state
(including phantom models) parametrized in a standard way, and study their
impact on the non-linear growth of structure. The main effect is the formation
of dark energy structure associated with the dark matter halo: non-phantom
equations of state induce the formation of a dark energy halo, damping the
growth of structures; phantom models, on the other hand, generate dark energy
voids, enhancing structure growth. Finally, we employ the Press-Schechter
formalism to compute how dark energy affects the number of massive objects as a
function of redshift.
|
0707.2882v3
|
2007-07-25
|
Chromoelectric response functions for quark-gluon plasma
|
We determine the chromoelectric response of quark-gluon plasma (QGP)
systematically within the framework of classical transport equations. The
transport equations are set up in the phase space which includes the SU(3)
group space corresponding to color (which is a dynamical degree of freedom), in
addition to the position - momentum variables. The distribution functions are
defined by projecting the density operators for the quarks and the gluons to
their respective coherent states (defined over the extended phase space). The
full import of the Yang-Mills(YM) dynamics is shown to manifest through the
emergence of an intrinsic nonlinear, nonlocal response, whose behavior we
determine in the long wavelength limit. It also manifests as a tensor response
which is a characteristic of gluons. The response functions are shown to have a
natural interpretation in terms of the renormalizations of the Abelian and the
non-Abelian coupling constants. A detailed analysis of the screening of heavy
quark potential and of the exact role played by the Debye mass screening in the
case of the Cornell potential, is performed. We also discuss the non-Abelian
contribution to Landau damping in QGP.
|
0707.3697v3
|
2007-07-26
|
Coarse-Grained Finite-Temperature Theory for the Condensate in Optical Lattices
|
In this work, we derive a coarse-grained finite-temperature theory for a Bose
condensate in a one-dimensional optical lattice, in addition to a confining
harmonic trap potential. We start from a two-particle irreducible (2PI)
effective action on the Schwinger-Keldysh closed-time contour path. In
principle, this action involves all information of equilibrium and
non-equilibrium properties of the condensate and noncondensate atoms. By
assuming an ansatz for the variational function, i.e., the condensate order
parameter in an effective action, we derive a coarse-grained effective action,
which describes the dynamics on the length scale much longer than a lattice
constant. Using the variational principle, coarse-grained equations of motion
for the condensate variables are obtained. These equations include a
dissipative term due to collisions between condensate and noncondensate atoms,
as well as noncondensate mean-field. To illustrate the usefulness of our
formalism, we discuss a Landau instability of the condensate in optical
lattices by using the coarse-grained generalized Gross-Pitaevskii
hydrodynamics. We found that the collisional damping rate due to collisions
between the condensate and noncondensate atoms changes sign when the condensate
velocity exceeds a renormalized sound velocity, leading to a Landau instability
consistent with the Landau criterion. Our results in this work give an insight
into the microscopic origin of the Landau instability.
|
0707.3984v1
|
2007-10-04
|
Detailed Examination of Transport Coefficients in Cubic-Plus-Quartic Oscillator Chains
|
We examine the thermal conductivity and bulk viscosity of a one-dimensional
(1D) chain of particles with cubic-plus-quartic interparticle potentials and no
on-site potentials. This system is equivalent to the FPU-alpha beta system in a
subset of its parameter space. We identify three distinct frequency regimes
which we call the hydrodynamic regime, the perturbative regime and the
collisionless regime. In the lowest frequency regime (the hydrodynamic regime)
heat is transported ballistically by long wavelength sound modes. The model
that we use to describe this behaviour predicts that as the frequency goes to
zero the frequency dependent bulk viscosity and the frequency dependent thermal
conductivity should diverge with the same power law dependence on frequency.
Thus, we can define the bulk Prandtl number as the ratio of the bulk viscosity
to the thermal conductivity (with suitable prefactors to render it
dimensionless). This dimensionless ratio should approach a constant value as
frequency goes to zero. We use mode-coupling theory to predict the zero
frequency limit. Values of the bulk Prandtl number from simulations are in
agreement with these predictions over a wide range of system parameters. In the
middle frequency regime, which we call the perturbative regime, heat is
transported by sound modes which are damped by four-phonon processes. We call
the highest frequency regime the collisionless regime since at these
frequencies the observing times are much shorter than the characteristic
relaxation times of phonons. The perturbative and collisionless regimes are
discussed in detail in the appendices.
|
0710.1066v1
|
2007-10-16
|
A generalization of Snoek's law to ferromagnetic films and composites
|
The present paper establishes characteristics of the relative magnetic
permeability spectrum $\mu$(f) of magnetic materials at microwave frequencies.
The integral of the imaginary part of $\mu$(f) multiplied with the frequency f
gives remarkable properties. A generalisation of Snoek's law consists in this
quantity being bounded by the square of the saturation magnetization multiplied
with a constant. While previous results have been obtained in the case of
non-conductive materials, this work is a generalization to ferromagnetic
materials and ferromagnetic-based composites with significant skin effect. The
influence of truncating the summation to finite upper frequencies is
investigated, and estimates associated to the finite summation are provided. It
is established that, in practice, the integral does not depend on the damping
model under consideration. Numerical experiments are performed in the exactly
solvable case of ferromagnetic thin films with uniform magnetization, and these
numerical experiments are found to confirm our theoretical results. Microwave
permeability measurements on soft amorphous films are reported. The relation
between the integral and the saturation magnetization is verified
experimentally, and some practical applications of the theoretical results are
introduced. The integral can be used to determine the average magnetization
orientation in materials with complex configurations of the magnetization, and
furthermore to demonstrate the accuracy of microwave measurement systems. For
certain applications, such as electromagnetic compatibility or radar absorbing
materials, the relations established herein provide useful indications for the
design of efficient materials, and simple figures of merit to compare the
properties measured on various materials.
|
0710.2980v2
|
2007-11-28
|
Scalar Casimir Effect on a D-dimensional Einstein Static Universe
|
We compute the renormalised energy momentum tensor of a free scalar field
coupled to gravity on an (n+1)-dimensional Einstein Static Universe (ESU),
RxS^n, with arbitrary low energy effective operators (up to mass dimension
n+1). A generic class of regulators is used, together with the Abel-Plana
formula, leading to a manifestly regulator independent result. The general
structure of the divergences is analysed to show that all the gravitational
couplings (not just the cosmological constant) are renormalised for an
arbitrary regulator. Various commonly used methods (damping function,
point-splitting, momentum cut-off and zeta function) are shown to, effectively,
belong to the given class. The final results depend strongly on the parity of
n. A detailed analytical and numerical analysis is performed for the behaviours
of the renormalised energy density and a quantity `sigma' which determines if
the strong energy condition holds for the `quantum fluid'. We briefly discuss
the quantum fluid back-reaction problem, via the higher dimensional Friedmann
and Raychaudhuri equations, observe that equilibrium radii exist and unveil the
possibility of a `Casimir stabilisation of Einstein Static Universes'.
|
0711.4564v2
|
2008-01-04
|
Study of the preheating phase of chaotic inflation
|
Particle production and its effects on the inflaton field are investigated
during the preheating phase of chaotic inflation using a model consisting of a
massive scalar inflaton field coupled to N massless quantum scalar fields. The
effects of spacetime curvature and interactions between the quantum fields are
ignored. A large N expansion is used to obtain a coupled set of equations
including a backreaction equation for the classical inflaton field. Previous
studies of preheating using these equations have been done. Here the first
numerical solutions to the full set of equations are obtained for various
values of the coupling constant and the initial amplitude of the inflaton
field. States are chosen so that initially the backreaction effects on the
inflaton field are small and the mode equations for the quantum fields take the
form of Mathieu equations. Potential problems relating to the parametric
amplification of certain modes of the quantum fields are identified and
resolved. A detailed study of the damping of the inflaton field is undertaken.
Some predictions of previous studies are verified and some new results are
obtained.
|
0801.0730v3
|
2008-01-07
|
Collective plasmonic modes of metal nano-particles in two-dimensional periodic regular arrays
|
We investigate the collective plasmonic modes of metal nano-particles in
periodic two-dimensional (2D) arrays within a point-dipole description. As an
open system, the full-dynamic dispersion relations of the 2D arrays are
obtained through an efficient method which gives an effective polarizability
describing the collective response of a system. Both the dispersion relations
and mode qualities are simultaneously related to the imaginary part of the
effective polarizability, which has contributions from the single-particle
response as well as the inter-particle coupling. The transversal long-range
dipolar interaction is dominated by a wave term together with a purely
geometrical constant representing the static geometrical contribution to
resonant frequencies. As concrete examples, we considered small Ag spheres
arranged in a square lattice. We find that inside the light-cone, the
transverse quasi-mode has a reasonably high mode quality while the two in-plane
modes show significant radiation damping. Near the light-line, we observe
strong coupling with free photons for the bands of the transverse mode and the
transversal in-plane mode, and the longitudinal in-plane mode exhibits a
negative group-velocity inside the light-cone. Vanishing group velocities in
the light-cone for all the quasi-modes are found to be intrinsic properties of
the 2D metal nano-sphere dense arrays.
|
0801.0906v1
|
2008-01-28
|
Doebner-Goldin Equation for Electrodynamic Particle. The Implied Applications
|
We set up the Maxwell's equations and the corresponding classical wave
equations for the electromagnetic waves which together with the generating
source, a traveling oscillatory charge of zero rest mass, comprise a particle
traveling in the force field of an usual conservative potential and an
additional frictional force $f$. At the de Broglie wavelength scale and in the
classic-velocity limit, the total wave equation decomposes into a component
equation describing the particle kinetic motion, which for $f=0$ identifies
with the usual linear Schr\"odinger equation as previously. The $f$-dependent
probability density presents generally an observable diffusion current of a
real diffusion constant; this and the particle's usual quantum diffusion
current as a whole are under adiabatic condition conserved and obey the
Fokker-Planck equation. The corresponding extra, $f$-dependent term in the
Hamiltonian operator identifies with that obtained by H.-D. Doebner and G.A.
Goldin. The friction produces to the particle's wave amplitude a damping that
can describe well the effect due to a radiation (de)polarization field, which
is always by-produced by the particle's oscillatory charge in a (nonpolar)
dielectric medium. The radiation depolarization field in a dielectric vacuum
has two separate significances: it participates to exert on another particle an
attractive, depolarization radiation force which resembles in overall respects
Newton's universal gravity as we showed earlier, and it exerts on the particle
itself an attractive, self depolarization radiation force whose time rate gives
directly the frictional force $f$.
|
0801.4279v2
|
2008-03-17
|
Fermionic Casimir effect in toroidally compactified de Sitter spacetime
|
We investigate the fermionic condensate and the vacuum expectation values of
the energy-momentum tensor for a massive spinor field in de Sitter spacetime
with spatial topology $\mathrm{R}^{p}\times (\mathrm{S}^{1})^{q}$. Both cases
of periodicity and antiperiodicity conditions along the compactified dimensions
are considered. By using the Abel-Plana formula, the topological parts are
explicitly extracted from the vacuum expectation values. In this way the
renormalization is reduced to the renormalization procedure in uncompactified
de Sitter spacetime. It is shown that in the uncompactified subspace the
equation of state for the topological part of the energy-momentum tensor is of
the cosmological constant type. Asymptotic behavior of the topological parts in
the expectation values is investigated in the early and late stages of the
cosmological expansion. In the limit when the comoving length of a compactified
dimension is much smaller than the de Sitter curvature radius the topological
part in the expectation value of the energy-momentum tensor coincides with the
corresponding quantity for a massless field and is conformally related to the
corresponding flat spacetime result. In this limit the topological part
dominates the uncompactified de Sitter part. In the opposite limit, for a
massive field the asymptotic behavior of the topological parts is damping
oscillatory for both fermionic condensate and the energy-momentum tensor.
|
0803.2413v1
|
2008-03-27
|
Towards Understanding the Mass-Metallicity relation of Quasar Absorbers: Evidence for bimodality and consequences
|
One way to characterize and understand HI-selected galaxies is to study their
metallicity properties. In particular, we show that the metallicity of
absorbers is a bivariate function of the HI column density (NHI) and the MgII
equivalent width (Wr(2796)). Thus, a selection upon Wr(2796) is not equivalent
to a HI selection for intervening absorbers. A direct consequence for damped
absorbers with NHI>20.3 from the bivariate metallicity distribution is that any
correlation between the metallicity [X/H] and velocity width (or Wr(2796))
arises from the HI cut and therefore can not be interpreted as a signature of
the mass-metallicity relation akin to normal field galaxies. Thus DLA samples
are intrinsically heterogeneous. On the other hand, a sample of MgII-selected
absorbers, which are statistically dominated by lowest NHI systems (sub-DLAs),
are found to have a more uniform metallicity distribution. We postulate that
the bivariate metallicity distribution can be explained by two different
physical origins of absorbers, namely sight-lines through the ISM of small
galaxies and sight-lines through out-flowing material.
Several published results follow from this bivariate [X/H] distribution such
as (a) the two classes of DLAs, reported by Wolfe et al., and (b) the constant
dust-to-gas ratio for MgII-absorbers.
|
0803.3944v3
|
2008-04-21
|
Precision calculations of the gravitational wave background spectrum from inflation
|
The spectrum of the gravitational wave background originating from quantum
fluctuations during inflation is calculated numerically for various inflation
models over a wide range of frequencies. We take into account four ingredients
: the scalar field dynamics during inflation making no use of the slow-roll
approximation, the fermionic decay of the scalar field with a small coupling
constant during the reheating process, the change of the effective number of
degrees of freedom g_* in the radiation-dominated era, and the anisotropic
stress of free-streaming neutrinos. By numerically solving the evolution of
gravitational waves during and after inflation up to the present, all of these
effects can be examined comprehensively and accurately over a broad spectrum,
even at very high frequencies. We find that the spectrum shows (i) a large
deviation from the spectrum less accurate obtained by Taylor expanding around
the CMB scale using the slow-roll approximation (ii) a characteristic frequency
dependence due to the reheating effect, and (iii) damping due to the g_*
changes and the neutrino anisotropic stress. We suggest that future analysis of
the gravitational wave background should take into consideration the fact that
analytical estimates using the Taylor expansion overestimate the amplitude of
the spectrum.
|
0804.3249v3
|
2008-04-21
|
Gravitational Instability in Radiation Pressure Dominated Backgrounds
|
I consider the physics of gravitational instabilities in the presence of
dynamically important radiation pressure and gray radiative diffusion, governed
by a constant opacity, kappa. For any non-zero radiation diffusion rate on an
optically-thick scale, the medium is unstable unless the classical gas-only
isothermal Jeans criterion is satisfied. When diffusion is "slow," although the
dynamical Jeans instability is stabilized by radiation pressure on scales
smaller than the adiabatic Jeans length, on these same spatial scales the
medium is unstable to a diffusive mode. In this regime, neglecting gas
pressure, the characteristic timescale for growth is independent of spatial
scale and given by (3 kappa c_s^2)/(4 pi G c), where c_s is the adiabatic sound
speed. This timescale is that required for a fluid parcel to radiate away its
thermal energy content at the Eddington limit, the Kelvin-Helmholz timescale
for a radiation pressure supported self-gravitating object. In the limit of
"rapid" diffusion, radiation does nothing to suppress the Jeans instability and
the medium is dynamically unstable unless the gas-only Jeans criterion is
satisfied. I connect with treatments of Silk damping in the early universe. I
discuss several applications, including photons diffusing in regions of extreme
star formation (starburst galaxies & pc-scale AGN disks), and the diffusion of
cosmic rays in normal galaxies and galaxy clusters. The former (particularly,
starbursts) are "rapidly" diffusing and thus cannot be supported against
dynamical instability in the linear regime by radiation pressure alone. The
latter are more nearly "slowly" diffusing. I speculate that the turbulence in
starbursts may be driven by the dynamical coupling between the radiation field
and the self-gravitating gas, perhaps mediated by magnetic fields. (Abridged)
|
0804.3403v1
|
2008-04-26
|
Spatial Periodicity of Galaxy Number Counts, CMB Anisotropy, and SNIa Hubble Diagram Based on the Universe Accompanied by a Non-Minimally Coupled Scalar Field
|
We have succeeded in establishing a cosmological model with a non-minimally
coupled scalar field $\phi$ that can account not only for the spatial
periodicity or the {\it picket-fence structure} exhibited by the galaxy $N$-$z$
relation of the 2dF survey but also for the spatial power spectrum of the
cosmic microwave background radiation (CMB) temperature anisotropy observed by
the WMAP satellite. The Hubble diagram of our model also compares well with the
observation of Type Ia supernovae. The scalar field of our model universe
starts from an extremely small value at around the nucleosynthesis epoch,
remains in that state for sufficiently long periods, allowing sufficient time
for the CMB temperature anisotropy to form, and then starts to grow in
magnitude at the redshift $z$ of $\sim 1$, followed by a damping oscillation
which is required to reproduce the observed picket-fence structure of the
$N$-$z$ relation. To realize such behavior of the scalar field, we have found
it necessary to introduce a new form of potential $V(\phi)\propto
\phi^2\exp(-q\phi^2)$, with $q$ being a constant. Through this parameter $q$,
we can control the epoch at which the scalar field starts growing.
|
0804.4240v1
|
2008-09-09
|
Turbulent Convection in Stellar Interiors. II. The Velocity Field
|
We analyze stellar convection with the aid of 3D hydrodynamic simulations,
introducing the turbulent cascade into our theoretical analysis. We devise
closures of the Reynolds-decomposed mean field equations by simple physical
modeling of the simulations (we relate temperature and density fluctuations via
coefficients); the procedure (CABS, Convection Algorithms Based on Simulations)
is terrestrially testable and is amenable to systematic improvement. We develop
a turbulent kinetic energy equation which contains both nonlocal and time
dependent terms, and is appropriate if the convective transit time is shorter
than the evolutionary time scale. The interpretation of mixing-length theory
(MLT) as generally used in astrophysics is incorrect; MLT forces the mixing
length to be an imposed constant. Direct tests show that the damping associated
with the flow is that suggested by Kolmogorov. The eddy size is approximately
the depth of the convection zone, and this dissipation length corresponds to
the "mixing length". New terms involving local heating by turbulent dissipation
should appear in the stellar evolution equations. The enthalpy flux
("convective luminosity") is directly connected to the buoyant acceleration,
and hence the velocity scale. MLT tends to systematically underestimate this
velocity scale. Quantitative comparison with a variety of 3D simulations
reveals a previously recognized consistency. Examples of application to stellar
evolution will be presented in subsequent papers in this series.
|
0809.1625v2
|
2009-02-25
|
Propagating slow magnetoacoustic waves in coronal loops observed by Hinode/EIS
|
We present the first Hinode/EIS observations of 5 min quasi-periodic
oscillations detected in a transition-region line (He II) and five coronal
lines (Fe X, Fe XII, Fe XIII, Fe XIV, and Fe XV) at the footpoint of a coronal
loop. The oscillations exist throughout the whole observation, characterized by
a series of wave packets with nearly constant period, typically persisting for
4-6 cycles with a lifetime of 20-30 min. There is an approximate in-phase
relation between Doppler shift and intensity oscillations. This provides
evidence for slow magnetoacoustic waves propagating upwards from the transition
region into the corona. We find that the oscillations detected in the five
coronal lines are highly correlated, and the amplitude decreases with
increasing temperature. The amplitude of Doppler shift oscillations decrease by
a factor of about 3, while that of relative intensity decreases by a factor of
about 4 from Fe X to Fe XV. These oscillations may be caused by the leakage of
the photospheric p-modes through the chromosphere and transition region into
the corona, which has been suggested as the source for intensity oscillations
previously observed by TRACE. The temperature dependence of the oscillation
amplitudes can be explained by damping of the waves traveling along the loop
with multithread structure near the footpoint. Thus, this property may have
potential value for coronal seismology in diagnostic of temperature structure
in a coronal loop.
|
0902.4480v1
|
2009-04-22
|
The Rotation of Janus and Epimetheus
|
Epimetheus, a small moon of Saturn, has a rotational libration (an
oscillation about synchronous rotation) of 5.9 +- 1.2 degrees, placing
Epimetheus in the company of Earth's Moon and Mars' Phobos as the only natural
satellites for which forced rotational libration has been detected. The forced
libration is caused by the satellite's slightly eccentric orbit and
non-spherical shape.
Detection of a moon's forced libration allows us to probe its interior by
comparing the measured amplitude to that predicted by a shape model assuming
constant density. A discrepancy between the two would indicate internal density
asymmetries. For Epimetheus, the uncertainties in the shape model are large
enough to account for the measured libration amplitude. For Janus, on the other
hand, although we cannot rule out synchronous rotation, a permanent offset of
several degrees between Janus' minimum moment of inertia (long axis) and the
equilibrium sub-Saturn point may indicate that Janus does have modest internal
density asymmetries.
The rotation states of Janus and Epimetheus experience a perturbation every
four years, as the two moons "swap" orbits. The sudden change in the orbital
periods produces a free libration about synchronous rotation that is
subsequently damped by internal friction. We calculate that this free libration
is small in amplitude (<0.1 degree) and decays quickly (a few weeks, at most),
and is thus below the current limits for detection using Cassini images.
|
0904.3515v2
|
2009-09-18
|
Stochastic Electron Acceleration in the TeV Supernova Remnant RX J1713.7-3946: The High-Energy Cut-off
|
In the leptonic scenario for TeV emission from a few well-observed shell-type
TeV supernova remnants (STTSNRs), very weak magnetic fields are inferred. If
fast-mode waves are produced efficiently in the shock downstream, we show that
they are viable agents for acceleration of relativistic electrons inferred from
the observed spectra even in the subsonic phase, in spite that these waves are
subject to strong damping by thermal background ions at small dissipation
scales. Strong collisionless non-relativistic astrophysical shocks are studied
with the assumption of a constant Aflven speed in the downstream. The
turbulence evolution is modeled with both the Kolmogorov and Kraichnan
phenomenology. Processes determining the high-energy cutoff of nonthermal
electron distributions are examined. The Kraichnan models lead to a shallower
high-energy cutoff of the electron distribution and require a lower downstream
density than the Kolmogorov models to fit a given emission spectrum. With
reasonable parameters, the model explains observations of STTSNRs, including
recent data obtained with the Fermi gamma-ray telescope. More detailed studies
of the turbulence generation and dissipation processes, supernova explosions
and progenitors are warranted for better understanding the nature of supernova
shocks.
|
0909.3349v2
|
2009-10-10
|
The Electron Propagator in External Electromagnetic Fields in Lower Dimensions
|
We study the electron propagator in quantum electrodynamics in lower
dimensions. In the case of free electrons, it is well known that the propagator
in momentum space takes the simple form $S_F(p)=1/(\gamma\cdot p-m)$. In the
presence of external electromagnetic fields, electron asymptotic states are no
longer plane-waves, and hence the propagator in the basis of momentum
eigenstates has a more intricate form. Nevertheless, in the basis of the
eigenfunctions of the operator $(\gamma\cdot \Pi)^2$, where $\Pi_\mu$ is the
canonical momentum operator, it acquires the free form $S_F(p)=1/(\gamma\cdot
\bar{p}-m)$ where $\bar{p}_\mu$ depends on the dynamical quantum numbers. We
construct the electron propagator in the basis of the $(\gamma\cdot \Pi)^2$
eigenfunctions. In the (2+1)-dimensional case, we obtain it in an irreducible
representation of the Clifford algebra incorporating to all orders the effects
of a magnetic field of arbitrary spatial shape pointing perpendicularly to the
plane of motion of the electrons. Such an exercise is of relevance in graphene
in the massless limit. The specific examples considered include the uniform
magnetic field and the exponentially damped static magnetic field. We further
consider the electron propagator for the massive Schwinger model incorporating
the effects of a constant electric field to all orders within this framework.
|
0910.1881v1
|
2009-10-12
|
Effects on the two-point correlation function from the coupling of quintessence to dark matter
|
We investigate the effects of the nonminimal coupling between the scalar
field dark energy (quintessence) and the dark matter on the two- point
correlation function. It is well known that this coupling shifts the turnover
scale as well as suppresses the amplitude of the matter power spectrum.
However, these effects are too small to be observed when we limit the coupling
strength to be consistent with observations. Since the coupling of quintessence
to baryons is strongly constrained, species dependent coupling may arise. This
results in a baryon bias that is dif- ferent from unity. Thus, we look over the
correlation function in this coupled model. We find that even the non-coupled
quintessence model gives the better fit to the correlation function compared to
the cosmo- logical constant model. We are also able to observe the enhancement
of the baryon acoustic oscillation (BAO) peak due to the increasing bias factor
of baryon from this species dependent coupling. In order to avoid the damping
effect of the BAO signature in the matter power spectrum due to nonlinear
clustering, we consider the coupling effect on the BAO bump in the linear
regime. This provides an alternative method to constrain the coupling of dark
energy to dark matter.
|
0910.2175v2
|
2009-11-10
|
A Comparison between Two Simple Models of a Slug Flow in a Long Flexible Marine Riser
|
Slug flows are extremely interesting multiphase regime phenomena which
frequently occur in flexible marine risers used by the petroleum industry in
offshore environments and have both a liquid and gaseous phase. This paper
describes two simple models of the slug flow regime by means of an equivalent
monophase flow with a non-constant density. The slug regime is modelled as a
monophase density-varying flow with a sinusoidal density, travelling along the
pipe itself towards the top end node of the riser. Starting from the bottom
end, it is characterized by adiabatic processes and energy loss along the
entire length of the pipe. In the first model, the slug wavelength is supposed
to be independent of the riser inclination, while in the second one a simple
linear relationship between the slug wavelength and the pipe inclination was
imposed. The global equation of the motion of the riser (written in a
two-dimensional domain throughout the plane containing the riser) was solved
using a Matlab code in the time domain. In particular, the axial tensile force,
the bending moment, the viscous structural damping, the wave-induced forces and
the riser-seabed interaction are all modelled here. This work presents a
comparison between the two models in two main kinds of configuration (a very
long riser with and without seabed presence) and it allows the authors to make
some considerations on general pipe behaviour.
|
0911.1873v1
|
2009-11-13
|
Two-resonator circuit QED: Dissipative Theory
|
We present a theoretical treatment for the dissipative two-resonator circuit
quantum electrodynamics setup referred to as quantum switch. There, switchable
coupling between two superconducting resonators is mediated by a
superconducting qubit operating in the dispersive regime, where the qubit
transition frequency is far detuned from those of the resonators. We derive an
effective Hamiltonian for the quantum switch beyond the rotating wave
approximation and study the dissipative dynamics within a Bloch-Redfield
quantum master equation approach. We derive analytically how the qubit affects
the quantum switch even if the qubit has no dynamics, and we estimate the
strength of this influence. The analytical results are corroborated by
numerical calculations, where coherent oscillations between the resonators, the
decay of coherent and Fock states, and the decay of resonator-resonator
entanglement are studied. Finally, we suggest an experimental protocol for
extracting the damping constants of qubit and resonators by measuring the
quadratures of the resonator fields.
|
0911.2657v4
|
2009-12-02
|
Eulerian and Lagrangian propagators for the adhesion model (Burgers dynamics)
|
Motivated by theoretical studies of gravitational clustering in the Universe,
we compute propagators (response functions) in the adhesion model. This model,
which is able to reproduce the skeleton of the cosmic web and includes
nonlinear effects in both Eulerian and Lagrangian frameworks, also corresponds
to the Burgers equation of hydrodynamics. Focusing on the one-dimensional case
with power-law initial conditions, we obtain exact results for Eulerian and
Lagrangian propagators. We find that Eulerian propagators can be expressed in
terms of the one-point velocity probability distribution and show a strong
decay at late times and high wavenumbers, interpreted as a "sweeping effect"
but not a genuine damping of small-scale structures. By contrast, Lagrangian
propagators can be written in terms of the shock mass function -- which would
correspond to the halo mass function in cosmology -- and saturate to a constant
value at late times. Moreover, they show a power-law dependence on scale or
wavenumber which depends on the initial power-spectrum index and is directly
related to the low-mass tail of the shock mass function. These results strongly
suggest that Lagrangian propagators are much more sensitive probes of nonlinear
structures in the underlying density field and of relaxation processes than
their Eulerian counterparts.
|
0912.0356v1
|
2010-02-18
|
Average luminosity distance in inhomogeneous universes
|
Using numerical ray tracing, the paper studies how the average distance
modulus in an inhomogeneous universe differs from its homogeneous counterpart.
The averaging is over all directions from a fixed observer not over all
possible observers (cosmic), thus it is more directly applicable to our
observations. Unlike previous studies, the averaging is exact,
non-perturbative, and includes all possible non-linear effects. The
inhomogeneous universes are represented by Sweese-cheese models containing
random and simple cubic lattices of mass-compensated voids. The Earth observer
is in the homogeneous cheese which has an Einstein - de Sitter metric. For the
first time, the averaging is widened to include the supernovas inside the voids
by assuming the probability for supernova emission from any comoving volume is
proportional to the rest mass in it. Despite the well known argument for photon
flux conservation, the average distance modulus correction at low redshifts is
not zero due to the peculiar velocities. A formula for the maximum possible
average correction as a function of redshift is derived and shown to be in
excellent agreement with the numerical results. The actual average correction
calculated in random and simple cubic void lattices is severely damped below
the predicted maximal average. That is traced to cancelations between the
corrections coming from the fronts and backs of different voids at the same
redshift from the observer. The calculated correction at low redshifts allows
one to readily predict the redshift at which the averaged fluctuation in the
Hubble diagram is below a required precision and suggests a method to extract
the background Hubble constant from low redshift data without the need to
correct for peculiar velocities.
|
1002.3408v1
|
2010-05-25
|
Structural, static and dynamic magnetic properties of CoMnGe thin films on a sapphire a-plane substrate
|
Magnetic properties of CoMnGe thin films of different thicknesses (13, 34,
55, 83, 100 and 200 nm), grown by RF sputtering at 400{\deg}C on single crystal
sapphire substrates, were studied using vibrating sample magnetometry (VSM) and
conventional or micro-strip line (MS) ferromagnetic resonance (FMR). Their
behavior is described assuming a magnetic energy density showing twofold and
fourfold in-plane anisotropies with some misalignment between their principal
directions. For all the samples, the easy axis of the fourfold anisotropy is
parallel to the c-axis of the substrate while the direction of the twofold
anisotropy easy axis varies from sample to sample and seems to be strongly
influenced by the growth conditions. Its direction is most probably monitored
by the slight unavoidable angle of miscut the Al2O3 substrate. The twofold
in-plane anisotropy field is almost temperature independent, in contrast with
the fourfold field which is a decreasing function of the temperature. Finally,
we study the frequency dependence of the observed line-width of the resonant
mode and we conclude to a typical Gilbert damping constant of 0.0065 for the
55-nm-thick film.
|
1005.4595v3
|
2010-09-28
|
Accurate interaction energies at DFT level by means of an efficient dispersion correction
|
This paper presents an approach for obtaining accurate interaction energies
at the DFT level for systems where dispersion interactions are important. This
approach combines Becke and Johnson's [J. Chem. Phys. 127, 154108 (2007)]
method for the evaluation of dispersion energy corrections and a Hirshfeld
method for partitioning of molecular polarizability tensors into atomic
contributions. Due to the availability of atomic polarizability tensors, the
method is extended to incorporate anisotropic contributions, which prove to be
important for complexes of lower symmetry. The method is validated for a set of
eighteen complexes, for which interaction energies were obtained with the
B3LYP, PBE and TPSS functionals combined with the aug-cc-pVTZ basis set and
compared with the values obtained at CCSD(T) level extrapolated to a complete
basis set limit. It is shown that very good quality interaction energies can be
obtained by the proposed method for each of the examined functionals, the
overall performance of the TPSS functional being the best, which with a slope
of 1.00 in the linear regression equation and a constant term of only 0.1
kcal/mol allows to obtain accurate interaction energies without any need of a
damping function for complexes close to their exact equilibrium geometry.
|
1009.5631v1
|
2010-11-17
|
On the Critical Coupling for Kuramoto Oscillators
|
The Kuramoto model captures various synchronization phenomena in biological
and man-made systems of coupled oscillators. It is well-known that there exists
a critical coupling strength among the oscillators at which a phase transition
from incoherency to synchronization occurs. This paper features four
contributions. First, we characterize and distinguish the different notions of
synchronization used throughout the literature and formally introduce the
concept of phase cohesiveness as an analysis tool and performance index for
synchronization. Second, we review the vast literature providing necessary,
sufficient, implicit, and explicit estimates of the critical coupling strength
for finite and infinite-dimensional, and for first and second-order Kuramoto
models. Third, we present the first explicit necessary and sufficient condition
on the critical coupling to achieve synchronization in the finite-dimensional
Kuramoto model for an arbitrary distribution of the natural frequencies. The
multiplicative gap in the synchronization condition yields a practical
stability result determining the admissible initial and the guaranteed ultimate
phase cohesiveness as well as the guaranteed asymptotic magnitude of the order
parameter. Fourth and finally, we extend our analysis to multi-rate Kuramoto
models consisting of second-order Kuramoto oscillators with inertia and viscous
damping together with first-order Kuramoto oscillators with multiple time
constants. We prove that the multi-rate Kuramoto model is locally topologically
conjugate to a first-order Kuramoto model with scaled natural frequencies, and
we present necessary and sufficient conditions for almost global phase
synchronization and local frequency synchronization. Interestingly, these
conditions do not depend on the inertiae which contradicts prior observations
on the role of inertiae in synchronization of second-order Kuramoto models.
|
1011.3878v2
|
2011-01-04
|
Universal Spin Transport in a Strongly Interacting Fermi Gas
|
Transport of fermions is central in many fields of physics. Electron
transport runs modern technology, defining states of matter such as
superconductors and insulators, and electron spin, rather than charge, is being
explored as a new carrier of information [1]. Neutrino transport energizes
supernova explosions following the collapse of a dying star [2], and
hydrodynamic transport of the quark-gluon plasma governed the expansion of the
early Universe [3]. However, our understanding of non-equilibrium dynamics in
such strongly interacting fermionic matter is still limited. Ultracold gases of
fermionic atoms realize a pristine model for such systems and can be studied in
real time with the precision of atomic physics [4, 5]. It has been established
that even above the superfluid transition such gases flow as an almost perfect
fluid with very low viscosity [3, 6] when interactions are tuned to a
scattering resonance. However, here we show that spin currents, as opposed to
mass currents, are maximally damped, and that interactions can be strong enough
to reverse spin currents, with opposite spin components reflecting off each
other. We determine the spin drag coeffcient, the spin diffusivity, and the
spin susceptibility, as a function of temperature on resonance and show that
they obey universal laws at high temperatures. At low temperatures, the spin
diffusivity approaches a minimum value set by the ratio of the reduced Planck's
constant to the atomic mass. For repulsive interactions, our measurements
appear to exclude a metastable ferromagnetic state [7-9].
|
1101.0780v1
|
2011-02-02
|
Galaxy N-z Relation and CMB Spectrum Based on Cosmological Model with Scalar Field Having Modified Potential Form
|
We have succeeded in establishing a cosmological model with a non-minimally
coupled scalar field $\phi$ that can account not only for the spatial
periodicity or the picket-fence structure exhibited by the galaxy $N$-$z$
relation of the 2dF survey, but also for the spatial power spectrum of the
cosmic microwave background radiation (CMB) temperature anisotropy observed by
the WMAP satellite. The scalar field of our model universe starts from an
extremely small value at around the nucleosynthesis epoch, remains in that
state for sufficiently long periods, allowing sufficient time for the CMB
temperature anisotropy to form, and then starts to grow in magnitude at the
redshift $z$ of $\sim 1$, followed by a damping oscillation which is required
to reproduce the observed picket-fence structure of the $N$-$z$ relation. To
realize such behavior of the scalar field, we have found it necessary to
introduce a new form of potential $V(\phi)\propto \phi^2\exp(-q\phi^2)$, with
$q$ being a constant. Through this parameter $q$, we can control the epoch at
which the scalar field starts growing.
|
1102.0341v1
|
2011-03-10
|
Compressive high-frequency waves riding on an Alfvén/ion-cyclotron wave in a multi-fluid plasma
|
In this paper, we study the weakly-compressive high-frequency plasma waves
which are superposed on a large-amplitude Alfv\'en wave in a multi-fluid plasma
consisting of protons, electrons, and alpha particles. For these waves, the
plasma environment is inhomogenous due to the presence of the low-frequency
Alfv\'en wave with a large amplitude, a situation that may apply to space
plasmas such as the solar corona and solar wind. The dispersion relation of the
plasma waves is determined from a linear stability analysis using a new
eigenvalue method that is employed to solve the set of differential wave
equations which describe the propagation of plasma waves along the direction of
the constant component of the Alfv\'en wave magnetic field. This approach also
allows one to consider weak compressive effects. In the presence of the
background Alfv\'en wave, the dispersion branches obtained differ significantly
from the situation of a uniform plasma. Due to compressibility, acoustic waves
are excited and couplings between various modes occur, and even an instability
of the compressive mode. In a kinetic treatment, these plasma waves would be
natural candidates for Landau-resonant wave-particle interactions, and may thus
via their damping lead to particle heating.
|
1103.2029v3
|
2011-03-17
|
Phonon Spectra, Nearest Neighbors, and Mechanical Stability of Disordered Colloidal Clusters with Attractive Interactions
|
We investigate the influence of morphology and size on the vibrational
properties of disordered clusters of colloidal particles with attractive
interactions. From measurements of displacement correlations between particles
in each cluster, we extract vibrational properties of the corresponding
"shadow" glassy cluster, with the same geometric configuration and interactions
as the "source" cluster but without damping. Spectral features of the
vibrational modes are found to depend strongly on the average number of nearest
neighbors, $\bar{NN}$, but only weakly on the number of particles in each
glassy cluster. In particular, the median phonon frequency, $\omega_{med}$, is
essentially constant for $\bar{NN}$ $<2$ and then grows linearly with
$\bar{NN}$ for $\bar{NN}$ $>2$. This behavior parallels concurrent observations
about local isostatic structures, which are absent in clusters with $\bar{NN}$
$<2$ and then grow linearly in number for $\bar{NN}$$>2$. Thus, cluster
vibrational properties appear to be strongly connected to cluster mechanical
stability (i.e., fraction of locally isostatic regions), and the scaling of
$\omega_{med}$ with $\bar{NN}$ is reminiscent of the behavior of packings of
spheres with repulsive interactions at the jamming transition. Simulations of
random networks of springs corroborate observations and suggest that
connections between phonon spectra and nearest neighbor number are generic to
disordered networks.
|
1103.3535v1
|
2011-06-15
|
Chiral symmetry breaking in QCD-like gauge theories with a confining propagator and dynamical gauge boson mass generation
|
We study chiral symmetry breaking in QCD-like gauge theories introducing a
confining effective propagator, as proposed recently by Cornwall, and
considering the effect of dynamical gauge boson mass generation. The effective
confining propagator has the form $1/(k^2+m^2)^2$ and we study the bifurcation
equation finding limits on $m$ below which a satisfactory fermion mass solution
is generated. Since the coupling constant and gauge boson propagator are damped
in the infrared, due to the presence of dynamically massive gauge bosons, the
major part of the chiral breaking is only due to the confining propagator. We
study the asymptotic behavior of the gap equation containing confinement and
massive gauge boson exchange, and find that the symmetry breaking can be
approximated at some extent by an effective four-fermion interaction generated
by the confining propagator. We compute some QCD chiral parameters as a
function of $m$, finding values compatible with the experimental data. Within
this approach we expect that lattice simulations should not see large
differences between the confinement and chiral symmetry breaking scales
independent of the fermionic representation and we find a simple approximate
relation between the fermion condensate and dynamical mass for a given
representation as a function of the parameters appearing in the effective
confining propagator.
|
1106.2860v3
|
2011-06-21
|
Symmetry and the macroscopic dynamics of antiferromagnetic materials in the presence of spin-polarized current
|
Antiferromagnetic (AFM) materials with zero or vanishingly small macroscopic
magnetization are nowadays the constituent elements of spintronic devices.
However, possibility to use them as active elements that show nontrivial
controllable magnetic dynamics is still discussible. In the present paper we
extend the theory [A.F.Andreev, V.I.Marchenko, Sov. Phys. --- Uspekhi, 23
(1980), 21] of macroscopic dynamics in AFMs for the cases typical for
spin-valve devices. In particular, we consider the solid-like magnetic dynamics
of AFMs with strong exchange coupling in the presence of spin-polarized current
and give an expression for the current-induced Rayleigh dissipation function in
terms of the rotation vector for different types %generalized potential of
AFMs. Basing on the analysis of linearized equations of motion we predict the
current-induced reorientation and AFM resonance, and found the values of
critical currents in terms of AFMR frequencies and damping constants. We show
the possibility of current-induced spin-diode effect and second-harmonic
generation in AF layer. The proposed approach is generalized for the
description of current-related phenomena in inhomogeneous AFMs.
|
1106.4231v3
|
2011-07-05
|
Black-hole hair loss: learning about binary progenitors from ringdown signals
|
Perturbed Kerr black holes emit gravitational radiation, which (for the
practical purposes of gravitational-wave astronomy) consists of a superposition
of damped sinusoids termed quasi-normal modes. The frequencies and
time-constants of the modes depend only on the mass and spin of the black hole
- a consequence of the no-hair theorem. It has been proposed that a measurement
of two or more quasi-normal modes could be used to confirm that the source is a
black hole and to test if general relativity continues to hold in ultra-strong
gravitational fields. In this paper we propose a practical approach to testing
general relativity with quasi-normal modes. We will also argue that the
relative amplitudes of the various quasi-normal modes encode important
information about the origin of the perturbation that caused them. This helps
in inferring the nature of the perturbation from an observation of the emitted
quasi-normal modes. In particular, we will show that the relative amplitudes of
the different quasi-normal modes emitted in the process of the merger of a pair
of nonspinning black holes can be used to measure the component masses of the
progenitor binary.
|
1107.0854v3
|
2011-08-01
|
Symplectic integrators with adaptive time steps
|
In recent decades, there have been many attempts to construct symplectic
integrators with variable time steps, with rather disappointing results. In
this paper we identify the causes for this lack of performance, and find that
they fall into two categories. In the first, the time step is considered a
function of time alone, \Delta=\Delta(t). In this case, backwards error
analysis shows that while the algorithms remain symplectic, parametric
instabilities arise because of resonance between oscillations of \Delta(t) and
the orbital motion. In the second category the time step is a function of phase
space variables \Delta=\Delta(q,p). In this case, the system of equations to be
solved is analyzed by introducing a new time variable \tau with dt=\Delta(q,p)
d\tau. The transformed equations are no longer in Hamiltonian form, and thus
are not guaranteed to be stable even when integrated using a method which is
symplectic for constant \Delta. We analyze two methods for integrating the
transformed equations which do, however, preserve the structure of the original
equations. The first is an extended phase space method, which has been
successfully used in previous studies of adaptive time step symplectic
integrators. The second, novel, method is based on a non-canonical
mixed-variable generating function. Numerical trials for both of these methods
show good results, without parametric instabilities or spurious growth or
damping. It is then shown how to adapt the time step to an error estimate found
by backward error analysis, in order to optimize the time-stepping scheme.
Numerical results are obtained using this formulation and compared with other
time-stepping schemes for the extended phase space symplectic method.
|
1108.0322v1
|
2011-11-02
|
Self-consistent size and velocity distributions of collisional cascades
|
The standard theoretical treatment of collisional cascades derives a
steady-state size distribution assuming a single constant velocity dispersion
for all bodies regardless of size. Here we relax this assumption and solve
self-consistently for the bodies' steady-state size and size-dependent velocity
distributions. Specifically, we account for viscous stirring, dynamical
friction, and collisional damping of the bodies' random velocities in addition
to the mass conservation requirement typically applied to find the size
distribution in a steady-state cascade. The resulting size distributions are
significantly steeper than those derived without velocity evolution. For
example, accounting self-consistently for the velocities can change the
standard q=3.5 power-law index of the Dohnanyi (1969) differential size
spectrum to an index as large as q=4. Similarly, for bodies held together by
their own gravity, the corresponding power-law index range 2.88<q<3.14 of Pan &
Sari (2005) can steepen to values as large as q=3.26. Our velocity results
allow quantitative predictions of the bodies' scale heights as a function of
size. Together with our predictions, observations of the scale heights for
different sized bodies for the Kuiper belt, the asteroid belt, and extrasolar
debris disks may constrain the mass and number of large bodies stirring the
cascade as well as the colliding bodies' internal strengths.
|
1111.0667v1
|
2011-11-21
|
Ultrasoft Fermionic Modes at High Temperature
|
A possible collective fermionic excitation in the ultrasoft energy-momentum
region is examined in Yukawa model with scalar coupling and quantum
electrodynamics (QED) with g being coupling constant at extremely high
temperature T where the fermion mass is negligible. We analytically sum up the
ladder diagrams for the vertex correction in the leading order in QED, which is
not necessary in the Yukawa model, and find that the fermion pole exists at
\omega = \pm p/3-i\zeta with ultrasoft momentum p both for the Yukawa model and
QED; \zeta is the sum of the damping rates of fermion and boson with hard
momenta. We also obtain the expression of the residue of the pole, which is as
small as of order g^2. We show that the fermion propagator and the vertex
function satisfy the Ward-Takahashi identity in QED. Thus we establish the
existence of an ultrasoft fermionic mode at extremely high temperature, which
was originally called phonino and was suggested in the context of supersymmetry
and its breaking at finite T. We discuss the possible origin of such an
ultrasoft fermionic mode without recourse to supersymmetry. The case of QCD is
briefly mentioned.
|
1111.5015v1
|
2011-12-05
|
Chiral symmetry breaking with a confining propagator and dynamically massive gluons
|
Chiral symmetry breaking in QCD is studied introducing a confining effective
propagator, as proposed recently by Cornwall, and considering the effect of
dynamically massive gluons. The effective confining propagator has the form
$1/(k^2+m^2)^2$ and we study the bifurcation equation finding limits on the
parameter $m$ below which a satisfactory fermion mass solution is generated.
Since the coupling constant and gluon propagator are damped in the infrared,
due to the presence of a dynamical gluon mass, the major part of the chiral
breaking is only due to the confining propagator and related to the low
momentum region of the gap equation. We study the asymptotic behavior of the
gap equation containing this confinement effect and massive gluon exchange, and
find that the symmetry breaking can be approximated by an effective
four-fermion interaction generated by the confining propagator. We compute some
QCD chiral parameters as a function of $m$, finding values compatible with the
experimental data. We find a simple approximate relation between the fermion
condensate and dynamical mass for a given representation as a function of the
parameters appearing in the effective confining propagator.
|
1112.0926v1
|
2012-01-03
|
Hydrodynamic model for electron-hole plasma in graphene
|
We propose a hydrodynamic model describing steady-state and dynamic electron
and hole transport properties of graphene structures which accounts for the
features of the electron and hole spectra. It is intended for electron-hole
plasma in graphene characterized by high rate of intercarrier scattering
compared to external scattering (on phonons and impurities), i.e., for
intrinsic or optically pumped (bipolar plasma), and gated graphene (virtually
monopolar plasma). We demonstrate that the effect of strong interaction of
electrons and holes on their transport can be treated as a viscous friction
between the electron and hole components. We apply the developed model for the
calculations of the graphene dc conductivity, in particular, the effect of
mutual drag of electrons and holes is described. The spectra and damping of
collective excitations in graphene in the bipolar and monopolar limits are
found. It is shown that at high gate voltages and, hence, at high electron and
low hole densities (or vice-versa), the excitations are associated with the
self-consistent electric field and the hydrodynamic pressure (plasma waves). In
intrinsic and optically pumped graphene, the waves constitute quasineutral
perturbations of the electron and hole densities (electron-hole sound waves)
with the velocity being dependent only on the fundamental graphene constants.
|
1201.0592v1
|
2012-07-02
|
Establishing micromagnetic parameters of ferromagnetic semiconductor (Ga,Mn)As
|
(Ga,Mn)As is at the forefront of research exploring the synergy of magnetism
with the physics and technology of semiconductors, and has led to discoveries
of new spin-dependent phenomena and functionalities applicable to a wide range
of material systems. Its recognition and utility as an ideal model material for
spintronics research has been undermined by the large scatter in reported
semiconducting doping trends and micromagnetic parameters. In this paper we
establish these basic material characteristics by individually optimizing the
highly non-equilibrium synthesis for each Mn-doping level and by simultaneously
determining all micromagnetic parameters from one set of magneto-optical
pump-and-probe measurements. Our (Ga,Mn)As thin-film epilayers, spannig the
wide range of accessible dopings, have sharp thermodynamic Curie point
singularities typical of uniform magnetic systems. The materials show
systematic trends of increasing magnetization, carrier density, and Curie
temperature (reaching 188 K) with increasing doping, and monotonous doping
dependence of the Gilbert damping constant of ~0.1-0.01 and the spin stiffness
of ~2-3 meVnm^2. These results render (Ga,Mn)As well controlled degenerate
semiconductor with basic magnetic characteristics comparable to common band
ferromagnets.
|
1207.0310v1
|
2012-10-30
|
Transverse kink oscillations in the presence of twist
|
Magnetic twist is thought to play an important role in coronal loops. The
effects of magnetic twist on stable magnetohydrodynamic (MHD) waves is poorly
understood because they are seldom studied for relevant cases. The goal of this
work is to study the fingerprints of magnetic twist on stable transverse kink
oscillations. We numerically calculated the eigenmodes of propagating and
standing MHD waves for a model of a loop with magnetic twist. The azimuthal
component of the magnetic field was assumed to be small in comparison to the
longitudinal component. We did not consider resonantly damped modes or kink
instabilities in our analysis. For a nonconstant twist the frequencies of the
MHD wave modes are split, which has important consequences for standing waves.
This is different from the degenerated situation for equilibrium models with
constant twist, which are characterised by an azimuthal component of the
magnetic field that linearly increases with the radial coordinate. In the
presence of twist standing kink solutions are characterised by a change in
polarisation of the transverse displacement along the tube. For weak twist, and
in the thin tube approximation, the frequency of standing modes is unaltered
and the tube oscillates at the kink speed of the corresponding straight tube.
The change in polarisation is linearly proportional to the degree of twist.
This has implications with regard to observations of kink modes, since the
detection of this variation in polarisation can be used as an indirect method
to estimate the twist in oscillating loops.
|
1210.8093v1
|
2013-02-07
|
Scalar and fermionic vacuum currents in de Sitter spacetime with compact dimensions
|
Vacuum expectation values (VEVs) of the current densities for charged scalar
and Dirac spinor fields are investigated in (D+1)-dimensional de Sitter (dS)
spacetime with toroidally compactified spatial dimensions. Along compact
dimensions we impose quasiperiodicity conditions with arbitrary phases. In
addition, the presence of a classical constant gauge field is assumed. The VEVs
of the charge density and of the components for the current density along
noncompact dimensions vanish. The gauge field leads to Aharonov-Bohm-like
oscillations of the components along compact dimensions as functions of the
magnetic flux. For small values of the comoving length of a compact dimension,
compared with the dS curvature scale, the current density is related to the
corresponding current in the Minkowski spacetime by a conformal relation. For
large values of the comoving length and for a scalar field, depending on the
mass of the field, two different regimes are realized with the monotonic and
oscillatory damping of the current density. For a massive spinor field, the
decay of the current density is always oscillatory. In supersymmetric models on
the background of Minkowski spacetime with equal number of scalar and fermionic
degrees of freedom and with the same phases in the periodicity conditions, the
total current density vanishes due to the cancellation between the scalar and
fermionic parts. The background gravitational field modifies the current
densities for scalar and fermionic fields in different ways and for massive
fields there is no cancellation in the dS spacetime.
|
1302.1688v2
|
2013-04-17
|
Theoretical characterization of excitation energy transfer in chlorosome light-harvesting antennae from green sulfur bacteria
|
Chlorosomes are the largest and most efficient natural light-harvesting
antenna systems. They contain thousands of pigment molecules -
bacteriochlorophylls (BChls)- that are organized into supramolecular aggregates
and form a very efficient network for excitonic energy migration. Here, we
present a theoretical study of excitation energy transfer (EET) in the
chlorosome based on experimental evidence of the molecular assembly. Our model
for the exciton dynamics throughout the antenna combines a stochastic time
propagation of the excitonic wave function with molecular dynamics simulations
of supramolecular structure, and electronic structure calculations of the
excited states. The simulation results reveal a detailed picture of the EET in
the chlorosome. Coherent energy transfer is significant only for the first 50
fs after the initial excitation, and the wavelike motion of the exciton is
completely damped at 100 fs. Characteristic time constants of incoherent energy
transfer, subsequently, vary from 1 ps to several tens of ps. We assign the
time scales of the EET to specific physical processes by comparing our results
with the data obtained from time-resolved spectroscopy experiments.
|
1304.4902v2
|
2013-07-12
|
Study of Conduction Cooling Effects in Long Aspect Ratio Penning-Malmberg Micro-Traps
|
A first order perturbation with respect to velocity has been employed to find
the frictional damping force imposed on a single moving charge due to a
perturbative electric field, inside a long circular cylindrical trap. We find
that the electric field provides a cooling effect, has a tensorial relationship
with the velocity of the charge. A mathematical expression for the tensor field
has been derived and numerically estimated. The corresponding drag forces for a
charge moving close to the wall in a cylindrical geometry asymptotically
approaches the results for a flat surface geometry calculated in the
literature. Many particle conduction cooling power dissipation is formulated
using the single particle analysis. Also the cooling rate for a weakly
interacting ensemble is estimated. It is suggested that a pre-trap section with
relatively high electrical resistivity can be employed to cool down low density
ensembles of electrons/positrons before being injected into the trap. For a
micro-trap with tens of thousands of micro-tubes, hundreds of thousands of
particles can be cooled down in each cooling cycle. For example, tens of
particles per micro-tube in a $5 cm$ long pre-trap section with the resistivity
of $0.46 \Omega m$ and the micro-tubes of radius $50 \mu m$ can be cooled down
with the time constant of $106\mu s$.
|
1307.3357v1
|
2013-07-17
|
Analysis of the Taylor dissipation surrogate in forced isotropic turbulence
|
From the energy balance in wavenumber space expressed by the Lin equation, we
derive a new form for the local Karman-Howarth equation for forced isotropic
turbulence in real space. This equation is then cast into a dimensionless form,
from which a combined analytical and numerical study leads us to deduce a new
model for the scale-independent nondimensional dissipation rate $\Ceps$, which
takes the form $\Ceps = \Cinf + C_L/R_L$, where the asymptotic value $\Cinf$
can be evaluated from the third-order structure function. This is found to fit
the numerical data with $\Cinf = 0.47 \pm 0.01$ and $C_L= 18.5 \pm 1.3$. By
considering $\Ceps - \Cinf$ on logarithmic scales, we show that $R_L^{-1}$ is
indeed the correct Reynolds number behaviour. The model is compared to previous
attempts in the literature, with encouraging agreement. The effects of the
scale-dependence of the inertial and viscous terms due to finite forcing are
then considered and shown to compensate one another, such that the model
equation is applicable for systems subject to finite forcing. In addition, we
also show that, contrary to the case of freely decaying turbulence, the
characteristic decline in $\Ceps$ with increasing Reynolds number is due to the
\emph{increase} in the surrogate expression $U^3/L$; the dissipation rate being
maintained constant as a consequence of the fixed rate of forcing. A long-time
non-turbulent stable state is found to exist for low Reynolds number numerical
simulations which use negative damping as a means of energy injection.
|
1307.4574v1
|
2013-07-26
|
The history force on a small particle in a linearly stratified fluid
|
The hydrodynamic force experienced by a small spherical particle undergoing
an arbitrary time-dependent motion in a density-stratified fluid is
investigated theoretically. The study is carried out under the
Oberbeck-Boussinesq approximation, and in the limit of small Reynolds and small
P\'eclet numbers. The force acting on the particle is obtained by using matched
asymptotic expansions in which the small parameter is given by a/l where a is
the particle radius and l is the stratification length defined by Ardekani &
Stocker (2010), which depends on the Brunt-Vaisala frequency, on the fluid
kinematic viscosity and on the thermal or the concentration diffusivity
(depending on the case considered). The matching procedure used here, which is
based on series expansions of generalized functions, slightly differs from that
generally used in similar problems. In addition to the classical Stokes drag,
it is found the particle experiences a memory force given by two convolution
products, one of which involves, as usual, the particle acceleration and the
other one, the particle velocity. Owing to the stratification, the transient
behaviour of this memory force, in response to an abrupt motion, consists of an
initial fast decrease followed by a damped oscillation with an
angular-frequency corresponding to the Brunt-Vaisala frequency. The
perturbation force eventually tends to a constant which provides us with
correction terms that should be added to the Stokes drag to accurately predict
the settling time of a particle in a diffusive stratified-fluid.
|
1307.6934v2
|
2013-08-27
|
Mechanical fluidity of fully suspended biological cells
|
Mechanical characteristics of single biological cells are used to identify
and possibly leverage interesting differences among cells or cell populations.
Fluidity---hysteresivity normalized to the extremes of an elastic solid or a
viscous liquid---can be extracted from, and compared among, multiple
rheological measurements of cells: creep compliance vs. time, complex modulus
vs. frequency, and phase lag vs. frequency. With multiple strategies available
for acquisition of this nondimensional property, fluidity may serve as a useful
and robust parameter for distinguishing cell populations, and for understanding
the physical origins of deformability in soft matter. Here, for three disparate
eukaryotic cell types deformed in the suspended state via optical stretching,
we examine the dependence of fluidity on chemical and environmental influences
around a time scale of 1 s. We find that fluidity estimates are consistent in
the time and the frequency domains under a structural damping (power-law or
fractional derivative)model, but not under an equivalent-complexity
lumpedcomponent (spring-dashpot) model; the latter predicts spurious time
constants. Although fluidity is suppressed by chemical crosslinking, we find
that adenosine triphosphate (ATP) depletion in the cell does not measurably
alter the parameter, and thus conclude that active ATP-driven events are not a
crucial enabler of fluidity during linear viscoelastic deformation of a
suspended cell. Finally, by using the capacity of optical stretching to produce
near-instantaneous increases in cell temperature, we establish that fluidity
increases with temperature---now measured in a fully suspended, sortable cell
without the complicating factor of cell-substratum adhesion.
|
1308.6004v1
|
2013-09-11
|
Numerical Boson Stars with a Single Killing Vector II: the D=3 Case
|
We complete the analysis of part I in this series (Ref.
\cite{Stotyn:2013yka}) by numerically constructing boson stars in 2+1
dimensional Einstein gravity with negative cosmological constant, minimally
coupled to a complex scalar field. These lower dimensional boson stars have
strikingly different properties than their higher dimensional counterparts,
most noticeably that there exists a finite central energy density, above which
an extremal BTZ black hole forms. In this limit, all of the scalar field
becomes enclosed by the horizon; it does not contract to a singularity, but
rather the origin remains smooth and regular and the solution represents a
spinning boson star trapped inside a degenerate horizon. Additionally, whereas
in higher dimensions the mass, angular momentum, and angular velocity all
display damped harmonic oscillations as functions of the central energy
density, in $D=3$ these quantities change monotonically up to the bound on the
central energy density. Some implications for the holographic dual of these
objects are discussed and it is argued that the boson star and extremal BTZ
black hole phases are dual to a spontaneous symmetry breaking at zero
temperature but finite energy scale.
|
1309.2911v3
|
2013-11-13
|
Convergent perturbative nuclear effective field theory
|
We consider the nuclear effective field theory including pions in the
two-nucleon sector in the S waves up to including the next-to-next-to-leading
order (NNLO) terms according to the power counting suggested by the Wilsonian
renormalization group analysis done in a previous paper. We treat only the
leading contact interaction nonperturbatively, and the rest, including the
long-distance part of pion exchange, are treated as perturbations. To define
the long-distance part, it is important to introduce a separation scale, or a
cutoff. We employ a hybrid regularization, in which the loops with only contact
interactions are regularized with Power Divergence Subtraction (PDS), while the
loops with (long-distance part of) pion exchange are regularized with a
Gaussian damping factor (GDF), to simplify the (nonperturbative) leading-order
amplitudes. The scale introduced by PDS is identified with the cutoff of GDF up
to a numerical factor. We emphasize that the introduction of the GDF requires a
careful definition of the coupling constant for the pion exchange. We obtain
the analytic expressions for the phase shifts for the $^1S_0$ and
$^3S_1$-$^3D_1$ channels. By fitting them to the Nijmegen partial wave analysis
data, it is shown that the effective theory expansion with perturbative
long-distance part of pion exchange is converging.
|
1311.3063v1
|
2013-11-14
|
Electric field gradient wave (EFGW) in iron-based superconductor Ba(0.6)K(0.4)Fe2As2 studied by Mössbauer spectroscopy
|
The optimally doped 122 iron-based superconductor Ba(0.6)K(0.4)Fe2As2 has
been studied by 57Fe Moessbauer spectroscopy versus temperature ranging from
4.2 K till 300 K with particular attention paid to the superconducting
transition around 38 K. The spectra do not contain magnetic components and they
exhibit quasi-continuous distribution of quadrupole split doublets. A
distribution follows the electric field gradient (EFG) spatial modulation
(wave) - EFGW. The EFGW is accompanied by some charge density wave (CDW) having
about an order of magnitude lesser influence on the spectrum. The EFGW could be
modeled as widely separated narrow sheets with the EFG increasing from small
till maximum value almost linearly and subsequently dropping back to the
original value in a similar fashion - across the sheet. One encounters very
small and almost constant EFG between sheets. The EFGW shape and amplitude as
well as the amplitude of CDW are strongly affected by a superconducting
transition. All modulations are damped significantly at transition (38 K) and
recover at a temperature being about 14 K lower. The maximum quadrupole
splitting at 4.2 K amounts to about 2.1 mm/s, while the dispersion of CDW seen
on the iron nuclei could be estimated far away from the superconducting gap
opening and at low temperature as 0.5 el./a.u.^3. It drops to about 0.3
el./a.u.^3 just below transition to the superconducting state.
|
1311.3503v4
|
2013-12-24
|
Slow Light in Metamaterial Waveguides
|
Metamaterials, which are materials engineered to possess novel optical
properties, have been increasingly studied. The ability to fabricate
metamaterials has sparked an interest in determining possible applications. We
investigate using a metamaterial for boundary engineering in waveguides.
A metamaterial-clad cylindrical waveguide is used to provide confinement for
an optical signal, thereby increasing the local electromagnetic energy density.
We show that metamaterial-clad waveguides have unique optical properties,
including new modes, which we call hybrid modes. These modes have properties of
both ordinary guided modes and surface plasmon-polariton modes.
We show that for certain metamaterial parameters, the surface
plasmon-polariton modes of a metamaterial-clad waveguide have less propagation
loss than those of a metal-clad guide with the same permittivity. This low-loss
mode is exploited for all-optical control of weak fields. Embedding three-level
{\Lambda} atoms in the dielectric core of a metamaterial-clad waveguide allows
the use of electromagnetically induced transparency to control an optical
signal. Adjusting the pump field alters the group velocity of the signal,
thereby controllably delaying pulses.
Using the low-loss surface mode of a metamaterial-clad guide reduces losses
by 20% over a metal cladding without sacrificing the group velocity reduction
or confinement. In addition, we show that losses can be reduced by as much as
40% with sufficient reduction of the magnetic damping constant of the
metamaterial.
As this work aims for applications, practical considerations for fabricating
and testing metamaterial-clad waveguides are discussed. An overview of the
benefits and drawbacks for two different dielectric core materials is given.
Also, a short discussion of other modes that could be used is given.
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1312.6892v1
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