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2008-02-14
|
Influence of an inner disc on the orbital evolution of massive planets migrating in resonance
|
The formation of resonant pairs of planets in exoplanetary systems involves
planetary migration in the protoplanetary disc. After a resonant capture, the
subsequent migration in this configuration leads to a large increase of
planetary eccentricities if no damping mechanism is applied. This has led to
the conclusion that the migration of resonant planetary systems cannot occur
over large radial distances and has to be terminated sufficiently rapidly
through disc dissipation.
In this study, we investigate whether the presence of an inner disc might
supply an eccentricity damping of the inner planet, and if this effect could
explain the observed eccentricities in some systems. To investigate the
influence of an inner disc, we first compute hydrodynamic simulations of giant
planets orbiting with a given eccentricity around an inner gas disc, and
measure the effect of the latter on the planetary orbital parameters. We then
perform detailed long term calculations of the GJ 876 system. We also run
N-body simulations with artificial forces on the planets mimicking the effects
of the inner and outer discs.
We find that the influence of the inner disc can not be neglected, and that
it might be responsible for the observed eccentricities. In particular, we
reproduce quite well the orbital parameters of a few systems engaged in 2:1
mean motion resonances: GJ 876, HD 73526, HD 82943 and HD 128311. Finally, we
derive analytically the effect that the inner disc should have on the inner
planet to reach a specific orbital configuration with a given damping effect of
the outer disc on the outer planet.
|
0802.2014v1
|
2008-04-10
|
Embedded Oscillating Starless Cores
|
In a previous paper we demonstrated that non-radial hydrodynamic oscillations
of a thermally-supported (Bonnor-Ebert) sphere embedded in a low-density,
high-temperature medium persist for many periods. The predicted column density
variations and molecular spectral line profiles are similar to those observed
in the Bok globule B68 suggesting that the motions in some starless cores may
be oscillating perturbations on a thermally supported equilibrium structure.
Such oscillations can produce molecular line maps which mimic rotation,
collapse or expansion, and thus could make determining the dynamical state from
such observations alone difficult.
However, while B68 is embedded in a very hot, low-density medium, many
starless cores are not, having interior/exterior density contrasts closer to
unity. In this paper we investigate the oscillation damping rate as a function
of the exterior density. For concreteness we use the same interior model
employed in Broderick et al. (2007), with varying models for the exterior gas.
We also develop a simple analytical formalism, based upon the linear
perturbation analysis of the oscillations, which predicts the contribution to
the damping rates due to the excitation of sound waves in the external medium.
We find that the damping rate of oscillations on globules in dense molecular
environments is always many periods, corresponding to hundreds of thousands of
years, and persisting over the inferred lifetimes of the globules.
|
0804.1790v1
|
2008-05-07
|
Design of parametrically forced patterns and quasipatterns
|
The Faraday wave experiment is a classic example of a system driven by
parametric forcing, and it produces a wide range of complex patterns, including
superlattice patterns and quasipatterns. Nonlinear three-wave interactions
between driven and weakly damped modes play a key role in determining which
patterns are favoured. We use this idea to design single and multi-frequency
forcing functions that produce examples of superlattice patterns and
quasipatterns in a new model PDE with parametric forcing. We make quantitative
comparisons between the predicted patterns and the solutions of the PDE.
Unexpectedly, the agreement is good only for parameter values very close to
onset. The reason that the range of validity is limited is that the theory
requires strong damping of all modes apart from the driven pattern-forming
modes. This is in conflict with the requirement for weak damping if three-wave
coupling is to influence pattern selection effectively. We distinguish the two
different ways that three-wave interactions can be used to stabilise
quasipatterns, and present examples of 12-, 14- and 20-fold approximate
quasipatterns. We identify which computational domains provide the most
accurate approximations to 12-fold quasipatterns, and systematically
investigate the Fourier spectra of the most accurate approximations.
|
0805.0878v1
|
2008-06-19
|
Itinerant spin excitations near the hidden order transition in URu2Si2
|
By means of neutron scattering we show that the high-temperature precursor to
the hidden order state of the heavy fermion superconductor URu$_{2}$Si$_{2}$
exhibits heavily damped incommensurate paramagnons whose strong energy
dispersion is very similar to that of the long-lived longitudinal f-spin
excitations that appear below T$_{0}$. Since the underlying local f-exchange is
preserved we expect only the f-d interactions to change across the phase
transition and to cause the paramagnetic damping. The damping exhibits
single-ion behavior independent of wave vector and vanishes below the hidden
order transition. We suggest that this arises from a transition from valence
fluctuations to a hybridized f-d state below T$_{0}$. Here we present evidence
that the itinerant excitations, like those in chromium, are due to Fermi
surface nesting of hole and electron pockets so that the hidden order phase
likely originates from a Fermi-surface instability. We identify wave vectors
that span nested regions of a band calculation and that match the neutron spin
crossover from incommensurate to commensurate on approach to the hidden order
phase.
|
0806.3137v2
|
2008-11-21
|
Specific heat anomalies of open quantum systems
|
The evaluation of the specific heat of an open, damped quantum system is a
subtle issue. One possible route is based on the thermodynamic partition
function which is the ratio of the partition functions of system plus bath and
of the bath alone. For the free damped particle it has been shown, however,
that the ensuing specific heat may become negative for appropriately chosen
environments. Being an open system this quantity then naturally must be
interpreted as the change of the specific heat obtained as the difference
between the specific heat of the heat bath coupled to the system degrees of
freedom and the specific heat of the bath alone. While this difference may
become negative, the involved specific heats themselves are always positive;
thus, the known thermodynamic stability criteria are perfectly guaranteed. For
a damped quantum harmonic oscillator, instead of negative values, under
appropriate conditions one can observe a dip in the difference of specific
heats as a function of temperature. Stylized minimal models containing a single
oscillator heat bath are employed to elucidate the occurrence of the anomalous
temperature dependence of the corresponding specific heat values. Moreover, we
comment on the consequences for the interpretation of the density of states
based on the thermal partitionfunction.
|
0811.3509v2
|
2008-11-26
|
Three-dimensional simulations of multiple protoplanets embedded in a protostellar disc
|
Protoplanet eccentricities of e >~ H/r can slow or reverse migration, but
previous 2D studies have shown that gravitational scattering cannot maintain
significant planet eccentricities against disc-induced damping. We simulate the
evolution of low-mass protoplanetary swarms in three dimensions. The aim is to
examine both protoplanet survival rates and the dynamical structure of the
resulting planetary systems, and to compare them with 2D simulations. We
present results from a 3D hydrodynamic simulation of eight protoplanets
embedded in a protoplanetary disc. We also present a suite of simulations
performed using an N-body code, modified to include prescriptions for planetary
migration and for eccentricity and inclination damping. These prescriptions
were obtained by fitting analytic formulae to hydrodynamic simulations of
planets embedded in discs with initially eccentric and/or inclined orbits. As
was found in two dimensions, differential migration produces groups of
protoplanets in stable, multiple mean-motion resonances that migrate in
lockstep, preventing prolonged periods of gravitational scattering. In almost
all simulations, this leads to large-scale migration of the protoplanet swarm
into the central star in the absence of a viable stopping mechanism. The
evolution involves mutual collisions, occasional instances of large-scale
scattering, and the frequent formation of the long-lived, co-orbital planet
systems that arise in > 30% of all runs. Disc-induced damping overwhelms
eccentricity and inclination growth due to planet-planet interactions.
Co-orbital planets are a natural outcome of dynamical relaxation in a strongly
dissipative environment, and if observed in nature would imply that such a
period of evolution commonly arises during planetary formation.
|
0811.4322v1
|
2008-12-15
|
Swift-UVOT captures the earliest ultraviolet spectrum of a Gamma Ray Burst
|
We present the earliest ever ultraviolet spectrum of a gamma-ray burst (GRB)
as observed with the Swift-UVOT. The GRB 081203A spectrum was observed for 50
seconds with the UV grism starting 251 seconds after the Swift-BAT trigger when
the GRB was of u ~13.4 mag and still rising to its peak optical brightness. The
UV grism spectrum shows a damped Ly-alpha line, Ly-beta, and the Lyman
continuum break at a redshift z = 2.05 +/- 0.01. A model fit to the Lyman
absorption implies log N(HI) = 22.0 +/- 0.2 cm-2, which is typical for GRB host
galaxies with damped Ly-alpha absorbers. This observation of GRB 081203A
demonstrates that for GRBs brighter than v ~14 mag and with 0.5 < z < 3.5 the
UVOT will be able to provide redshifts, and probe for damped Ly-alpha absorbers
within 4-6 minutes from the time of the Swift-BAT trigger.
|
0812.2943v2
|
2008-12-16
|
On the oscillations of dissipative superfluid neutron stars
|
We investigate the oscillations of slowly rotating superfluid stars, taking
into account the vortex mediated mutual friction force that is expected to be
the main damping mechanism in mature neutron star cores. Working to linear
order in the rotation of the star, we consider both the fundamental f-modes and
the inertial r-modes. In the case of the (polar) f-modes, we work out an
analytic approximation of the mode which allows us to write down a closed
expression for the mutual friction damping timescale. The analytic result is in
good agreement with previous numerical results obtained using an energy
integral argument. We extend previous work by considering the full range of
permissible values for the vortex drag, e.g. the friction between each
individual vortex and the electron fluid. This leads to the first ever results
for the f-mode in the strong drag regime. Our estimates provide useful insight
into the dependence on, and relevance of, various equation of state parameters.
In the case of the (axial) r-modes, we confirm the existence of two classes of
modes. However, we demonstrate that only one of these sets remains purely axial
in more realistic neutron star models. Our analysis lays the foundation for
companion studies of the mutual friction damping of the r-modes at second order
in the slow-rotation approximation, the first time evolutions for superfluid
neutron star perturbations and also the first detailed attempt at studying the
dynamics of superfluid neutron stars with both a relative rotation between the
components and mutual friction.
|
0812.3023v1
|
2009-03-27
|
Cascade and Damping of Alfvén-Cyclotron Fluctuations: Application to Solar Wind Turbulence
|
It is well-recognized that the presence of magnetic fields will lead to
anisotropic energy cascade and dissipation of astrophysical turbulence. With
the diffusion approximation and linear dissipation rates, we study the cascade
and damping of Alfv\'en-cyclotron fluctuations in solar plasmas numerically.
For an isotropic case the steady-state turbulence spectra are nearly isotropic
in the inertial range and can be fitted by a single power-law function with a
spectral index of -3/2, similar to the Iroshnikov-Kraichnan phenomenology.
Beyond the MHD regime the kinetic effects make the spectrum softer at higher
wavenumbers. In the dissipation range the turbulence spectrum cuts off at the
wavenumber, where the damping rate becomes comparable to the cascade rate, and
the cutoff wavenumber changes with the wave propagation direction. The angle
averaged turbulence spectrum of the isotropic model resembles a broken
power-law. Taking into account the Doppler effects, the model naturally
reproduces the broken power-law turbulence spectra observed in the solar wind
and predicts that a higher break frequency always comes along with a softer
dissipation range spectrum that may be caused by the increase of the turbulence
intensity, the reciprocal of the plasma \beta, and/or the angle between the
solar wind velocity and the mean magnetic field. These predictions can be
tested by detailed comparisons with more accurate observations.
|
0903.4904v1
|
2009-04-17
|
Plasmons and polaritons in a semi-infinite plasma and a plasma slab
|
Plasmon and polariton modes are derived for an ideal semi-infinite
(half-space) plasma and an ideal plasma slab by using a general, unifying
procedure, based on equations of motion, Maxwell's equations and suitable
boundary conditions. Known results are re-obtained in much a more direct manner
and new ones are derived. The approach consists of representing the charge
disturbances by a displacement field in the positions of the moving particles
(electrons). The dielectric response and the electron energy loss are computed.
The surface contribution to the energy loss exhibits an oscillatory behaviour
in the transient regime near the surfaces. The propagation of an
electromagnetic wave in these plasmas is treated by using the retarded
electromagnetic potentials. The resulting integral equations are solved and the
reflected and refracted waves are computed, as well as the reflection
coefficient. For the slab we compute also the transmitted wave and the
transmission coefficient. Generalized Fresnel's relations are thereby obtained
for any incidence angle and polarization. Bulk and surface plasmon-polariton
modes are identified. As it is well known, the field inside the plasma is
either damped (evanescent) or propagating (transparency regime), and the
reflection coefficient for a semi-infinite plasma exhibits an abrupt
enhancement on passing from the propagating regime to the damped one (total
reflection). Similarly, apart from characteristic oscillations, the reflection
and transmission coefficients for a plasma slab exhibit an appreciable
enhancement in the damped regime.
|
0904.2662v1
|
2009-09-30
|
Dynamic polarization of graphene by moving external charges: random phase approximation
|
We evaluate the stopping and image forces on a charged particle moving
parallel to a doped sheet of graphene by using the dielectric response
formalism for graphene's $\pi$-electron bands in the random phase approximation
(RPA). The forces are presented as functions of the particle speed and the
particle distance for a broad range of charge-carrier densities in graphene. A
detailed comparison with the results from a kinetic equation model reveal the
importance of inter-band single-particle excitations in the RPA model for high
particle speeds. We also consider the effects of a finite gap between graphene
and a supporting substrate, as well as the effects of a finite damping rate
that is included through the use of Mermin's procedure. The damping rate is
estimated from a tentative comparison of the Mermin loss function with a HREELS
experiment. In the limit of low particle speeds, several analytical results are
obtained for the friction coefficient that show an intricate relationship
between the charge-carrier density, the damping rate, and the particle
distance, which may be relevant to surface processes and electrochemistry
involving graphene.
|
0909.5598v3
|
2010-02-05
|
Implementation of an Innovative Bio Inspired GA and PSO Algorithm for Controller design considering Steam GT Dynamics
|
The Application of Bio Inspired Algorithms to complicated Power System
Stability Problems has recently attracted the researchers in the field of
Artificial Intelligence. Low frequency oscillations after a disturbance in a
Power system, if not sufficiently damped, can drive the system unstable. This
paper provides a systematic procedure to damp the low frequency oscillations
based on Bio Inspired Genetic (GA) and Particle Swarm Optimization (PSO)
algorithms. The proposed controller design is based on formulating a System
Damping ratio enhancement based Optimization criterion to compute the optimal
controller parameters for better stability. The Novel and contrasting feature
of this work is the mathematical modeling and simulation of the Synchronous
generator model including the Steam Governor Turbine (GT) dynamics. To show the
robustness of the proposed controller, Non linear Time domain simulations have
been carried out under various system operating conditions. Also, a detailed
Comparative study has been done to show the superiority of the Bio inspired
algorithm based controllers over the Conventional Lead lag controller.
|
1002.1184v1
|
2010-03-12
|
Improving the model of emission from spinning dust: effects of grain wobbling and transient spin-up
|
Observations continue to support the interpretation of the anomalous
microwave foreground as electric dipole radiation from spinning dust grains as
proposed by Draine and Lazarian (1998ab). In this paper we present a refinement
of the original model by improving the treatment of a number of physical
effects. First, we consider a disk-like grain rotating with angular velocity at
an arbitrary angle with respect to the grain symmetry axis and derive the
rotational damping and excitation coefficients arising from infrared emission,
plasma-grain interactions and electric dipole emission. The angular velocity
distribution and the electric dipole emission spectrum for grains is calculated
using the Langevin equation, for cases both with and without fast internal
relaxation. Our results show that, the peak emissivity of spinning dust,
compared to earlier studies, increases by a factor of ~2 for the Warm Neutral
Medium (WNM), the Warm Ionized Medium (WIM), the Cold Neutral Medium (CNM) and
the Photodissociation Region (PDR), and by a factor ~4 for Reflection Nebulae
(RN). The frequency at the emission peak also increases by factors ~1.4 to ~2
for these media. The increased emission and peak frequency result from the
non-sphericity of grain shape and from the anisotropy in damping and excitation
along directions parallel and perpendicular to the grain symmetry axis. Second,
we provide a detailed numerical study including transient spin-up of grains by
single-ion collisions. The impulses broaden the emission spectrum and increase
the peak emissivity for the CNM, WNM and WIM. In addition, we present an
improved treatment of rotational excitation and damping by infrared emission.
|
1003.2638v2
|
2010-03-15
|
Small BGK waves and nonlinear Landau damping
|
Consider 1D Vlasov-poisson system with a fixed ion background and periodic
condition on the space variable. First, we show that for general homogeneous
equilibria, within any small neighborhood in the Sobolev space W^{s,p}
(p>1,s<1+(1/p)) of the steady distribution function, there exist nontrivial
travelling wave solutions (BGK waves) with arbitrary minimal period and
traveling speed. This implies that nonlinear Landau damping is not true in
W^{s,p}(s<1+(1/p)) space for any homogeneous equilibria and any spatial period.
Indeed, in W^{s,p} (s<1+(1/p)) neighborhood of any homogeneous state, the long
time dynamics is very rich, including travelling BGK waves, unstable
homogeneous states and their possible invariant manifolds. Second, it is shown
that for homogeneous equilibria satisfying Penrose's linear stability
condition, there exist no nontrivial travelling BGK waves and unstable
homogeneous states in some W^{s,p} (p>1,s>1+(1/p)) neighborhood. Furthermore,
when p=2,we prove that there exist no nontrivial invariant structures in the
H^{s} (s>(3/2)) neighborhood of stable homogeneous states. These results
suggest the long time dynamics in the W^{s,p} (s>1+(1/p)) and particularly, in
the H^{s} (s>(3/2)) neighborhoods of a stable homogeneous state might be
relatively simple. We also demonstrate that linear damping holds for initial
perturbations in very rough spaces, for linearly stable homogeneous state. This
suggests that the contrasting dynamics in W^{s,p} spaces with the critical
power s=1+(1/p) is a trully nonlinear phenomena which can not be traced back to
the linear level.
|
1003.3005v1
|
2010-04-06
|
Electronic screening and damping in magnetars
|
We calculate the screening of the ion-ion potential due to electrons in the
presence of a large background magnetic field, at densities of relevance to
neutron star crusts. Using the standard approach to incorporate electron
screening through the one-loop polarization function, we show that the magnetic
field produces important corrections both at short and long distances. In
extreme fields, realized in highly magnetized neutron stars called magnetars,
electrons occupy only the lowest Landau levels in the relatively low density
region of the crust. Here our results show that the screening length for
Coulomb interactions between ions can be smaller than the inter-ion spacing.
More interestingly, we find that the screening is anisotropic and the screened
potential between two static charges exhibits long range Friedel oscillations
parallel to the magnetic field. This long-range oscillatory behavior is likely
to affect the lattice structure of ions, and can possibly create rod-like
structures in the magnetar crusts. We also calculate the imaginary part of the
electron polarization function which determines the spectrum of electron-hole
excitations and plays a role in damping lattice phonon excitations. We
demonstrate that even for modest magnetic fields this damping is highly
anisotropic and will likely lead to anisotropic phonon heat transport in the
outer neutron star crust.
|
1004.0926v1
|
2010-06-25
|
Decoherence window and electron-nuclear cross-relaxation in the molecular magnet V 15
|
Rabi oscillations in the V_15 Single Molecule Magnet (SMM) embedded in the
surfactant DODA have been studied at different microwave powers. An intense
damping peak is observed when the Rabi frequency Omega_R falls in the vicinity
of the Larmor frequency of protons w_N, while the damping time t_R of
oscillations reaches values 10 times shorter than the phase coherence time t_2
measured at the same temperature. The experiments are interpreted by the N-spin
model showing that t_R is directly associated with the decoherence via
electronic/nuclear spin cross-relaxation in the rotating reference frame. It is
shown that this decoherence is accompanied with energy dissipation in the range
of the Rabi frequencies w_N - sigma_e < Omega_R < w_N, where sigma_e is the
mean super-hyperfine field (in frequency units) induced by protons at SMMs.
Weaker damping without dissipation takes place outside this dissipation window.
Simple local field estimations suggest that this rapid cross-relaxation in
resonant microwave field observed for the first time in SMMV_15 should take
place in other SMMs like Fe_8 and Mn_12 containing protons, too.
|
1006.4960v3
|
2010-08-31
|
A SINFONI Integral Field Spectroscopy Survey for Galaxy Counterparts to Damped Lyman-alpha Systems - II. Dynamical Properties of the Galaxies towards Q0302-223 and Q1009-0026
|
Details of processes through which galaxies convert their gas into stars need
to be studied in order to obtain a complete picture of galaxy formation. One
way to tackle these phenomena is to relate the HI gas and the stars in
galaxies. Here, we present dynamical properties of Damped and sub-Damped
Lyman-alpha Systems identified in H-alpha emission with VLT/SINFONI at near
infra-red wavelengths. While the DLA towards Q0302-223 is found to be
dispersion-dominated, the sub-DLA towards Q1009-0026 shows clear signatures of
rotation. We use a proxy to circular velocity to estimate the mass of the halo
in which the sub-DLA resides and find M_halo=10^12.6 M_sun. We also derive
dynamical masses of these objects, and find M_dyn=10^10.3 M_sun and 10^10.9
M_sun. For one of the two systems (towards Q0302-223), we are able to derive a
stellar mass of M_*=10^9.5 M_sun from Spectral Energy Distribution fit. The gas
fraction in this object is 1/3rd, comparable to similar objects at these
redshifts. Our work illustrates that detailed studies of quasar absorbers can
offer entirely new insights into our knowledge of the interaction between stars
and the interstellar gas in galaxies.
|
1009.0027v1
|
2010-11-10
|
Standing Slow-Mode Waves in Hot Coronal Loops: Observations, Modeling, and Coronal Seismology
|
Strongly damped Doppler shift oscillations are observed frequently associated
with flarelike events in hot coronal loops. In this paper, a review of the
observed properties and the theoretical modeling is presented. Statistical
measurements of physical parameters (period, decay time, and amplitude) have
been obtained based on a large number of events observed by SOHO/SUMER and
Yohkoh/BCS. Several pieces of evidence are found to support their
interpretation in terms of the fundamental standing longitudinal slow mode. The
high excitation rate of these oscillations in small- or micro-flares suggest
that the slow mode waves are a natural response of the coronal plasma to
impulsive heating in closed magnetic structure. The strong damping and the
rapid excitation of the observed waves are two major aspects of the waves that
are poorly understood, and are the main subject of theoretical modeling. The
slow waves are found mainly damped by thermal conduction and viscosity in hot
coronal loops. The mode coupling seems to play an important role in rapid
excitation of the standing slow mode. Several seismology applications such as
determination of the magnetic field, temperature, and density in coronal loops
are demonstrated. Further, some open issues are discussed.
|
1011.2483v1
|
2010-11-29
|
Long-time dynamics of Kirchhoff wave models with strong nonlinear damping
|
We study well-posedness and long-time dynamics of a class of quasilinear wave
equations with a strong damping. We accept the Kirchhoff hypotheses and assume
that the stiffness and damping coefficients are $C^1$ functions of the
$L_2$-norm of the gradient of the displacement. We first prove the existence
and uniqueness of weak solutions and study their properties for a rather wide
class of nonlinearities which covers the case of possible degeneration (or even
negativity) of the stiffness coefficient and the case of a supercritical source
term. Our main results deal with global attractors. In the case of strictly
positive stiffness factors we prove that in the natural energy space endowed
with a partially strong topology there exists a global attractor whose fractal
dimension is finite. In the non-supercritical case the partially strong
topology becomes strong and a finite dimensional attractor exists in the strong
topology of the energy space. Moreover, in this case we also establish the
existence of a fractal exponential attractor and give conditions that guarantee
the existence of a finite number of determining functionals. Our arguments
involve a recently developed method based on "compensated" compactness and
quasi-stability estimates.
|
1011.6271v3
|
2010-12-01
|
Dissipative processes in superfluid neutron stars
|
We present some results about a novel damping mechanism of r-mode
oscillations in neutron stars due to processes that change the number of
protons, neutrons and electrons. Deviations from equilibrium of the number
densities of the various species lead to the appearance in the Euler equations
of the system of a dissipative mechanism, the so-called rocket effect. The
evolution of the r-mode oscillations of a rotating neutron star are influenced
by the rocket effect and we present estimates of the corresponding damping
timescales. In the description of the system we employ a two-fluid model, with
one fluid consisting of all the charged components locked together by the
electromagnetic interaction, while the second fluid consists of superfluid
neutrons. Both components can oscillate however the rocket effect can only
efficiently damp the countermoving r-mode oscillations, with the two fluids
oscillating out of phase. In our analysis we include the mutual friction
dissipative process between the neutron superfluid and the charged component.
We neglect the interaction between the two r-mode oscillations as well as
effects related with the crust of the star. Moreover, we use a simplified model
of neutron star assuming a uniform mass distribution.
|
1012.0345v1
|
2011-01-19
|
Numerical simulation of fundamental trapped sausage modes
|
Context: We integrate the 2D MHD ideal equations of a straight slab to
simulate observational results associated with fundamental sausage trapped
modes. Aims: Starting from a non-equilibrium state with a dense chromospheric
layer, we analyse the evolution of the internal plasma dynamics of magnetic
loops, subject to line-tying boundary conditions, and with the coronal
parameters described in Asai et al. (2001) and Melnikov et al. (2002) to
investigate the onset and damping of sausage modes. Methods: To integrate the
equations we used a high resolution shock-capturing (HRSC) method specially
designed to deal appropriately with flow discontinuities. Results: Due to
non-linearities and inhomogeneities, pure modes are difficult to sustain and
always occur coupled among them so as to satisfy, e.g., the line-tying
constraint. We found that, in one case, the resonant coupling of the sausage
fundamental mode with a slow one results in a non-dissipative damping of the
former. Conclusions: In scenarios of thick and dense loops, where the
analytical theory predicts the existence of fundamental trapped sausage modes,
the coupling of fast and slow quasi-periodic modes -with a node at the center
of the longitudinal speed- occur contributing to the damping of the fast mode.
If a discontinuity in the total pressure between the loop and the corona is
assumed, a fundamental fast sausage transitory leaky regime is spontaneously
produced and an external compressional Alfv\'en wave takes away the magnetic
energy.
|
1101.3782v1
|
2011-03-25
|
Dissipation and Vertical Energy Transport in Radiation-Dominated Accretion Disks
|
Standard models of radiation supported accretion disks generally assume that
diffusive radiation flux is solely responsible for vertical heat transport.
This requires that heat must be generated at a critical rate per unit volume if
the disk is to be in hydrostatic and thermal equilibrium. This raises the
question of how heat is generated and how energy is transported in MHD
turbulence. By analysis of a number of radiation/MHD stratified shearing-box
simulations, we show that the divergence of the diffusive radiation flux is
indeed capped at the critical rate, but deep inside the disk, substantial
vertical energy flux is also carried by advection of radiation. Work done by
radiation pressure is a significant part of the energy budget, and much of this
work is dissipated later through damping by radiative diffusion. We show how
this damping can be measured in the simulations, and identify its physical
origins. Radiative damping accounts for as much as tens of percent of the total
dissipation, and is the only realistic physical mechanism for dissipation of
turbulence that can actually be resolved in numerical simulations of accretion
disks. Buoyancy associated with dynamo-driven, highly magnetized,
nearly-isobaric nonlinear slow magnetosonic fluctuations is responsible for the
radiation advection flux, and also explains the persistent periodic magnetic
upwelling seen at all values of the radiation to gas pressure ratio. The
intimate connection between radiation advection and magnetic buoyancy is the
first example we know of in astrophysics in which a dynamo has direct impact on
the global energetics of a system.
|
1103.5052v1
|
2011-06-09
|
Planet-disk interaction in highly inclined systems
|
We study the interaction of a proto-planetary disk and a planet on a highly
inclined orbit in the linear regime. The evolution of the planet is dominated
by dynamical friction for planet masses above several Earth-masses. Smaller
planets are dominated by aerodynamic drag, especially for very high
inclinations and retrograde orbits.
The time-scales associated with migration and inclination damping are
calculated. For certain values of the inclination, the inclination damping
time-scale is longer than the migration time-scale and the disk lifetime. This
result shows that highly inclined planets can not (re-)align with the
proto-planetary disk.
We discuss the dependence of numerical simulations on the gravitational
softening parameter. We find only a logarithmic dependence, making global three
dimensional simulations of this process computationally feasible.
A large fraction of Hot Jupiters is on highly inclined orbits with respect to
the rotation axis of the star. On the other hand small-mass planetary systems
discovered by the Kepler mission have low mutual inclinations. This shows that
there are two distinct formation mechanisms at work. The process that creates
inclined Hot Jupiters does not operate on small mass planets because the
damping timescales are so long that these systems would still be inclined
today.
|
1106.1869v2
|
2011-08-16
|
Brownian force noise from molecular collisions and the sensitivity of advanced gravitational wave observatories
|
We present an analysis of Brownian force noise from residual gas damping of
reference test masses as a fundamental sensitivity limit in small force
experiments. The resulting acceleration noise increases significantly when the
distance of the test mass to the surrounding experimental apparatus is smaller
than the dimension of the test mass itself. For the Advanced LIGO
interferometric gravitational wave observatory, where the relevant test mass is
a suspended 340 mm diameter cylindrical end mirror, the force noise power is
increased by roughly a factor 40 by the presence of a similarly shaped reaction
mass at a nominal separation of 5 mm. The force noise, of order 20 fN\rthz\ for
$2 \times 10^{-6}$ Pa of residual H$_2$ gas, rivals quantum optical
fluctuations as the dominant noise source between 10 and 30 Hz. We present here
a numerical and analytical analysis for the gas damping force noise for
Advanced LIGO, backed up by experimental evidence from several recent
measurements. Finally, we discuss the impact of residual gas damping on the
gravitational wave sensitivity and possible mitigation strategies.
|
1108.3254v2
|
2011-09-22
|
Hole spin relaxation and coefficients in Landau-Lifshitz-Gilbert equation in ferromagnetic GaMnAs
|
We investigate the temperature dependence of the coefficients in the
Landau-Lifshitz-Gilbert equation in ferromagnetic GaMnAs by employing the Zener
model. We first calculate the hole spin relaxation time based on the
microscopic kinetic equation. We find that the hole spin relaxation time is
typically several tens femtoseconds and can present a nonmonotonic temperature
dependence due to the variation of the interband spin mixing, influenced by the
temperature related Zeeman splitting. With the hole spin relaxation time, we
are able to calculate the coefficients in the Landau-Lifshitz-Gilbert equation,
such as the Gilbert damping, nonadiabatic spin torque, spin stiffness and
vertical spin stiffness coefficients. We find that the nonadiabatic spin torque
coefficient $\beta$ is around $0.1\sim 0.3$ at low temperature, which is
consistent with the experiment [Adam {\em et al.}, Phys. Rev. B {\bf 80},
193204 (2009)]. As the temperature increases, $\beta$ monotonically increases
and can exceed one in the vicinity of the Curie temperature. In the low
temperature regime with $\beta<1$, the Gilbert damping coefficient $\alpha$
increases with temperature, showing good agreement with the experiments [Sinova
{\em et al.}, Phys. Rev. B {\bf 69}, 085209 (2004); Khazen {\em et al.}, {\em
ibid.} {\bf 78}, 195210 (2008)]. Furthermore, we predict that $\alpha$
decreases with increasing temperature once $\beta>1$ near the Curie
temperature. We also find that the spin stiffness decreases with increasing
temperature, especially near the Curie temperature due to the modification of
the finite $\beta$. Similar to the Gilbert damping, the vertical spin stiffness
coefficient is also found to be nonmonotonically dependent on the temperature.
|
1109.4964v1
|
2011-09-23
|
Accretion of Rocky Planets by Hot Jupiters
|
The observed population of Hot Jupiters displays a stunning variety of
physical properties, including a wide range of densities and core sizes for a
given planetary mass. Motivated by the observational sample, this paper studies
the accretion of rocky planets by Hot Jupiters, after the Jovian planets have
finished their principal migration epoch and become parked in $\sim4$-day
orbits. In this scenario, rocky planets form later and then migrate inward due
to torques from the remaining circumstellar disk, which also damps the orbital
eccentricity. This mechanism thus represents one possible channel for
increasing the core masses and metallicities of Hot Jupiters. This paper
determines probabilities for the possible end states for the rocky planet:
collisions with the Jovian planets, accretion onto the star, ejection from the
system, and long-term survival of both planets. These probabilities depend on
the mass of the Jovian planet and its starting orbital eccentricity, as well as
the eccentricity damping rate for the rocky planet. Since these systems are
highly chaotic, a large ensemble ($N\sim10^3$) of simulations with effectively
equivalent starting conditions is required. Planetary collisions are common
when the eccentricity damping rate is sufficiently low, but are rare otherwise.
For systems that experience planetary collisions, this work determines the
distributions of impact velocities -- both speeds and impact parameters -- for
the collisions. These velocity distributions help determine the consequences of
the impacts, e.g., where energy and heavy elements are deposited within the
giant planets.
|
1109.5104v1
|
2011-09-27
|
Dispersion and damping of zone-boundary magnons in the noncentrosymmetric superconductor CePt3Si
|
Inelastic neutron scattering (INS) is employed to study damped spin-wave
excitations in the noncentrosymmetric heavy-fermion superconductor CePt3Si
along the antiferromagnetic Brillouin-zone boundary in the low-temperature
magnetically ordered state. Measurements along the (1/2 1/2 L) and (H H 1/2-H)
reciprocal-space directions reveal deviations in the spin-wave dispersion from
the previously reported model. Broad asymmetric shape of the peaks in energy
signifies strong spin-wave damping by interactions with the particle-hole
continuum. Their energy width exhibits no evident anomalies as a function of
momentum along the (1/2 1/2 L) direction, which could be attributed to
Fermi-surface nesting effects, implying the absence of pronounced commensurate
nesting vectors at the magnetic zone boundary. In agreement with a previous
study, we find no signatures of the superconducting transition in the magnetic
excitation spectrum, such as a magnetic resonant mode or a superconducting spin
gap, either at the magnetic ordering wavevector (0 0 1/2) or at the zone
boundary. However, the low superconducting transition temperature in this
material still leaves the possibility of such features being weak and therefore
hidden below the incoherent background at energies ~0.1 meV, precluding their
detection by INS.
|
1109.5784v1
|
2011-11-15
|
Spin waves in nanosized magnetic films
|
We have studied spin excitations in nanosized magnetic films in the
Heisenberg model with magnetic dipole and exchange interactions by the spin
operator diagram technique. Dispersion relations of spin waves in thin magnetic
films (in two-dimensional magnetic monolayers and in two-layer magnetic films)
and the spin-wave resonance spectrum in N-layer structures are found. For thick
magnetic films generalized Landau-Lifshitz equations are derived from first
principles. Landau-Lifshitz equations have the integral (pseudodifferential)
form, but not differential one. Spin excitations are determined by simultaneous
solution of the Landau-Lifshitz equations and the equation for the
magnetostatic potential. For normal magnetized ferromagnetic films the spin
wave damping has been calculated in the one-loop approximation for a diagram
expansion of the Green functions at low temperature. In thick magnetic films
the magnetic dipole interaction makes a major contribution to the relaxation of
long-wavelength spin waves. Thin films have a region of low relaxation of
long-wavelength spin waves. In thin magnetic films four-spin-wave processes
take place and the exchange interaction makes a major contribution to the
damping. It is found that the damping of spin waves propagating in magnetic
monolayer is proportional to the quadratic dependence on the temperature and is
very low for spin waves with small wavevectors. Spin-wave devices on the base
of nanosized magnetic films are proposed -- tunable narrow-band spin-wave
filters with high quality at the microwave frequency range and field-effect
transistor (FET) structures contained nanosized magnetic films under the gate
electrode. Spin-wave resonances in nanosized magnetic films can be used to
construct FET structures operating in Gigahertz and Terahertz frequency bands.
|
1111.3532v1
|
2011-12-16
|
Nonlinear viscous damping and gravitational wave detectability of the f-mode instability in neutron stars
|
We study the damping of the gravitational radiation-driven f-mode instability
in rotating neutron stars by nonlinear bulk viscosity in the so-called
supra-thermal regime. In this regime the dissipative action of bulk viscosity
is known to be enhanced as a result of nonlinear contributions with respect to
the oscillation amplitude. Our analysis of the f-mode instability is based on a
time-domain code that evolves linear perturbations of rapidly rotating
polytropic neutron star models. The extracted mode frequency and eigenfunctions
are subsequently used in standard energy integrals for the gravitational wave
growth and viscous damping. We find that nonlinear bulk viscosity has a
moderate impact on the size of the f-mode instability window, becoming an
important factor and saturating the mode's growth at a relatively large
oscillation amplitude. We show similarly that nonlinear bulk viscosity leads to
a rather high saturation amplitude even for the r-mode instability. In
addition, we show that the action of bulk viscosity can be significantly
mitigated by the presence of superfluidity in neutron star matter. Apart from
revising the f-mode's instability window we provide results on the mode's
gravitational wave detectability. Considering an f-mode-unstable neutron star
located in the Virgo cluster and assuming a mode amplitude at the level allowed
by bulk viscosity, we find that the emitted gravitational wave signal could be
detectable by advanced ground-based detectors such as Advanced LIGO/Virgo and
the Einstein Telescope.
|
1112.3931v2
|
2012-01-23
|
PageRank and rank-reversal dependence on the damping factor
|
PageRank (PR) is an algorithm originally developed by Google to evaluate the
importance of web pages. Considering how deeply rooted Google's PR algorithm is
to gathering relevant information or to the success of modern businesses, the
question of rank-stability and choice of the damping factor (a parameter in the
algorithm) is clearly important. We investigate PR as a function of the damping
factor d on a network obtained from a domain of the World Wide Web, finding
that rank-reversal happens frequently over a broad range of PR (and of d). We
use three different correlation measures, Pearson, Spearman, and Kendall, to
study rank-reversal as d changes, and show that the correlation of PR vectors
drops rapidly as d changes from its frequently cited value, $d_0=0.85$.
Rank-reversal is also observed by measuring the Spearman and Kendall rank
correlation, which evaluate relative ranks rather than absolute PR.
Rank-reversal happens not only in directed networks containing rank-sinks but
also in a single strongly connected component, which by definition does not
contain any sinks. We relate rank-reversals to rank-pockets and bottlenecks in
the directed network structure. For the network studied, the relative rank is
more stable by our measures around $d=0.65$ than at $d=d_0$.
|
1201.4787v1
|
2012-02-28
|
Planetesimal Dynamics in Inclined Binary Systems: The Role of Gas-Disk Gravity
|
We investigate the effects of gas-disk gravity on the planetesimal dynamics
in inclined binary systems, where the circumprimary disk plane is tilted by a
significant angle ($i_B$) with respect to the binary disk plane. Our focus is
on the Lidov-Kozai mechanism and the evolution of planetesimal eccentricity and
inclination. Using both analytical and numerical methods, we find that, on one
hand, the disk gravity generally narrows down the Kozai-on region, i.e., the
Lidov-Kozai effect can be suppressed in certain parts of (or even the whole of)
the disk, depending on various parameters. In the Kozai-off region,
planetesimals would move on orbits close to the mid-plane of gas-disk, with the
relative angle ($i^{'}$) following a small amplitude periodical oscillation. On
the other hand, when we include the effects of disk gravity, we find that the
Lidov-Kozai effect can operate even at arbitrarily low inclinations ($i_B$),
although lower $i_B$ leads to a smaller Kozai-on region. Furthermore, in the
Kozai-on region, most planetesimals' eccentricities can be excited to extremely
high values ($\sim 1$), and such extreme high eccentricities usually accompany
orbital flipping, i.e., planetesimal orbit flips back and forth between
anterograde and retrograde. Once a planetesimal reaches very high orbital
eccentricity, gas drag damping will shrink the planetesimal orbit, forming a
"hot planetesimal" on a near circular orbit very close to the primary star.
Such a mechanism, if replacing the planetesimals and gas drag damping with
Jupiters and tidal damping respectively, may lead to frequent production of
hot-Jupiters.
|
1202.6102v1
|
2012-04-20
|
The Stability of Massive Main Sequence Stars as a Function of Metallicity
|
We investigate the pulsational stability of massive (M >~ 120 Msun) main
sequence stars of a range of metallicities, including primordial, Population
III stars. We include a formulation of convective damping motivated by
numerical simulations of the interaction between convection and periodic shear
flows. We find that convective viscosity is likely strong enough to stabilize
radial pulsations whenever nuclear-burning (the epsilon-mechanism) is the
dominant source of driving. This suggests that massive main sequence stars with
Z <~ 2 x 10^-3 are pulsationally stable and are unlikely to experience
pulsation-driven mass loss on the main sequence. These conclusions are,
however, sensitive to the form of the convective viscosity and highlight the
need for further high-resolution simulations of the convection-oscillation
interaction. For more metal-rich stars (Z >~ 2 x 10^-3), the dominant
pulsational driving arises due to the kappa-mechanism arising from the
iron-bump in opacity and is strong enough to overcome convective damping. Our
results highlight that even for oscillations with periods a few orders of
magnitude shorter than the outer convective turnover time, the "frozen-in"
approximation for the convection-oscillation interaction is inappropriate, and
convective damping should be taken into account when assessing mode stability.
|
1204.4741v1
|
2012-05-14
|
Dynamics of multipartite quantum correlations under decoherence
|
Quantum discord is an optimal resource for the quantification of classical
and non-classical correlations as compared to other related measures. Geometric
measure of quantum discord is another measure of quantum correlations.
Recently, the geometric quantum discord for multipartite states has been
introduced by Jianwei Xu [arxiv:quant/ph.1205.0330]. Motivated from the recent
study [Ann. Phys. 327 (2012) 851] for the bipartite systems, I have
investigated global quantum discord (QD) and geometric quantum discord (GQD)
under the influence of external environments for different multipartite states.
Werner-GHZ type three-qubit and six-qubit states are considered in inertial and
non-inertial settings. The dynamics of QD and GQD is investigated under
amplitude damping, phase damping, depolarizing and flipping channels. It is
seen that the quantum discord vanishes for p>0.75 in case of three-qubit GHZ
states and for p>0.5 for six qubit GHZ states. This implies that multipartite
states are more fragile to decoherence for higher values of N. Surprisingly, a
rapid sudden death of discord occurs in case of phase flip channel. However,
for bit flip channel, no sudden death happens for the six-qubit states. On the
other hand, depolarizing channel heavily influences the QD and GQD as compared
to the amplitude damping channel. It means that the depolarizing channel has
the most destructive influence on the discords for multipartite states. From
the perspective of accelerated observers, it is seen that effect of environment
on QD and GQD is much stronger than that of the acceleration of non-inertial
frames. The degradation of QD and GQD happens due to Unruh effect. Furthermore,
QD exhibits more robustness than GQD when the multipartite systems are exposed
to environment.
|
1205.3133v1
|
2012-06-04
|
In search of random uncorrelated particle motion (RUM) in a simple random flow field
|
DNS studies of dispersed particle motion in isotropic homogeneous turbulence
[1] have revealed the existence of a component of random uncorrelated motion
(RUM)dependent on the particle inertia {\tau}p(normalised particle response
time or Stoke number). This paper reports the presence of RUM in a simple
linear random smoothly varying flow field of counter rotating vortices where
the two-particle velocity correlation was measured as a function of spatial
separation. Values of the correlation less than one for zero separation
indicated the presence of RUM. In terms of Stokes number, the motion of the
particles in one direction corresponds to either a heavily damped ({\tau}p <
0.25) or lightly damped ({\tau}p > 0.25)harmonic oscillator. In the lightly
damped case the particles overshoot the stagnation lines of the flow and are
projected from one vortex to another (the so-called sling-shot effect). It is
shown that RUM occurs only when {\tau}p > 0.25, increasing monotonically with
increasing Stokes number. Calculations of the particle pair separation
distribution function show that equilibrium of the particle concentration field
is never reached, the concentration at zero separation increasing monotonically
with time. This is consistent with the calculated negative values of the
average Liapounov exponent (finite compressibility) of the particle velocity
field.
|
1206.0572v1
|
2012-06-11
|
Solitons in a parametrically driven damped discrete nonlinear Schrödinger equation
|
We consider a parametrically driven damped discrete nonlinear Schr\"odinger
(PDDNLS) equation. Analytical and numerical calculations are performed to
determine the existence and stability of fundamental discrete bright solitons.
We show that there are two types of onsite discrete soliton, namely onsite type
I and II. We also show that there are four types of intersite discrete soliton,
called intersite type I, II, III, and IV, where the last two types are
essentially the same, due to symmetry. Onsite and intersite type I solitons,
which can be unstable in the case of no dissipation, are found to be stabilized
by the damping, whereas the other types are always unstable. Our further
analysis demonstrates that saddle-node and pitchfork (symmetry-breaking)
bifurcations can occur. More interestingly, the onsite type I, intersite type
I, and intersite type III-IV admit Hopf bifurcations from which emerge periodic
solitons (limit cycles). The continuation of the limit cycles as well as the
stability of the periodic solitons are computed through the numerical
continuation software Matcont. We observe subcritical Hopf bifurcations along
the existence curve of the onsite type I and intersite type III-IV. Along the
existence curve of the intersite type I we observe both supercritical and
subcritical Hopf bifurcations.
|
1206.2405v1
|
2012-07-13
|
Axion as a cold dark matter candidate: low-mass case
|
Axion as a coherently oscillating scalar field is known to behave as a cold
dark matter in all cosmologically relevant scales. For conventional axion mass
with 10^{-5} eV, the axion reveals a characteristic damping behavior in the
evolution of density perturbations on scales smaller than the solar system
size. The damping scale is inversely proportional to the square-root of the
axion mass. We show that the axion mass smaller than 10^{-24} eV induces a
significant damping in the baryonic density power spectrum in cosmologically
relevant scales, thus deviating from the cold dark matter in the scale smaller
than the axion Jeans scale. With such a small mass, however, our basic
assumption about the coherently oscillating scalar field is broken in the early
universe. This problem is shared by other dark matter models based on the
Bose-Einstein condensate and the ultra-light scalar field. We introduce a
simple model to avoid this problem by introducing evolving axion mass in the
early universe, and present observational effects of present-day low-mass axion
on the baryon density power spectrum, the cosmic microwave background radiation
(CMB) temperature power spectrum, and the growth rate of baryon density
perturbation. In our low-mass axion model we have a characteristic small-scale
cutoff in the baryon density power spectrum below the axion Jeans scale. The
small-scale deviations from the cold dark matter model in both matter and CMB
power spectra clearly differ from the ones expected in the cold dark matter
model mixed with the massive neutrinos as a hot dark matter component.
|
1207.3124v1
|
2012-09-04
|
Solving Vlasov Equations Using NRxx Method
|
In this paper, we propose a moment method to numerically solve the Vlasov
equations using the framework of the NRxx method developed in [6, 8, 7] for the
Boltzmann equation. Due to the same convection term of the Boltzmann equation
and the Vlasov equation, it is very convenient to use the moment expansion in
the NRxx method to approximate the distribution function in the Vlasov
equations. The moment closure recently presented in [5] is applied to achieve
the globally hyperbolicity so that the local well-posedness of the moment
system is attained. This makes our simulations using high order moment
expansion accessible in the case of the distribution far away from the
equilibrium which appears very often in the solution of the Vlasov equations.
With the moment expansion of the distribution function, the acceleration in the
velocity space results in an ordinary differential system of the macroscopic
velocity, thus is easy to be handled. The numerical method we developed can
keep both the mass and the momentum conserved. We carry out the simulations of
both the Vlasov-Poisson equations and the Vlasov-Poisson-BGK equations to study
the linear Landau damping. The numerical convergence is exhibited in terms of
the moment number and the spatial grid size, respectively. The variation of
discretized energy as well as the dependence of the recurrence time on moment
order is investigated. The linear Landau damping is well captured for different
wave numbers and collision frequencies. We find that the Landau damping rate
linearly and monotonically converges in the spatial grid size. The results are
in perfect agreement with the theoretic data in the collisionless case.
|
1209.0527v1
|
2012-10-09
|
A close-pair analysis of damp mergers at intermediate redshifts
|
We have studied the kinematics of ~2800 candidate close pair galaxies at
0.1<z<1.2 identified from the Canada-France-Hawaii Telescope Legacy Survey
fields. Spectra of these systems were obtained using spectrometers on the 6.5m
Magellan and 5m Hale telescopes. These data allow us to constrain the rate of
dry mergers at intermediate redshifts and to test the `hot halo' model for
quenching of star formation. Using virial radii estimated from the correlation
between dynamical and stellar masses published by Leauthaud et al. (2011), we
find that around 1/5 of our candidate pairs are likely to share a common dark
matter halo (our metric for close physical association). These pairs are
divided into red-red, blue-red and blue-blue systems using the rest-frame
colors classification method introduced in Chou et al. (2011). Galaxies
classified as red in our sample have very low star-formation rates, but they
need not be totally quiescent, and hence we refer to them as `damp', rather
than `dry', systems. After correcting for known selection effects, the fraction
of blue-blue pairs is significantly greater than that of red-red and blue-red
pairs. Red-red pairs are almost entirely absent from our sample, suggesting
that damp mergers are rare at z~0.5. Our data supports models with a short
merging timescale (<0.5 Gyr) in which star-formation is enhanced in the early
phase of mergers, but quenched in the late phase. Hot halo models may explain
this behaviour, but only if virial shocks that heat gas are inefficient until
major mergers are nearly complete.
|
1210.2692v1
|
2012-10-24
|
Dual Trigger of Transverse Oscillations in a Prominence by EUV Fast and Slow Coronal Waves: SDO/AIA and STEREO/EUVI Observations
|
We analyze flare-associated transverse oscillations in a quiescent solar
prominence on 8-9 September, 2010. Both the flaring active region and the
prominence were located near the West limb, with a favorable configuration and
viewing angle. The fulldisk extreme ultraviolet (EUV) images of the Sun
obtained with high spatial and temporal resolution by the Atmospheric Imaging
Assembly (AIA) aboard the Solar Dynamics Observatory, show flare-associated
lateral oscillations of the prominence sheet. The STEREO-A spacecraft, 81.5
degrees ahead of the Sun-Earth line, provides on-disk view of the
flare-associated coronal disturbances. We derive the temporal profile of the
lateral displacement of the prominence sheet by using the image
cross-correlation technique. The displacement curve was de-trended and the
residual oscillatory pattern was derived. We fit these oscillations with a
damped cosine function with a variable period and find that the period is
increasing. The initial oscillation period (P0) is 28.2 minutes and the damping
time (t_D) of 44 minutes. We confirm the presence of fast and slow EUV wave
components. Using STEREO-A observations we derive a propagation speed of 250
km/s for the slow EUV wave by applying time-slice technique to the running
difference images. We propose that the prominence oscillations are excited by
the fast EUV wave while the increase in oscillation period of the prominence is
an apparent effect, related to a phase change due to the slow EUV wave acting
as a secondary trigger. We discuss implications of the dual trigger effect for
coronal prominence seismology and scaling law studies of damping mechanisms.
|
1210.6690v1
|
2012-10-25
|
Molecular dissipation in the nonlinear eddy viscosity in the Navier-Stokes equations: modelling of accretion discs
|
Physical damping, regarding the nonlinear Navier-Stokes viscous flow
dynamics, refers to a tensorial turbulent dissipation term, attributed to
adjacent moving macroscopic flow components. Mutual dissipation among these
parts of fluid is described by a braking term in the momentum equation together
with a heating term in the energy equation, both responsible of the damping of
the momentum variation and of the viscous conversion of mechanical energy into
heat. A macroscopic mixing scale length is currently the only characteristic
length needed in the nonlinear modelling of viscous fluid dynamics describing
the nonlinear eddy viscosity through the kinematic viscosity coefficient in the
viscous stress tensor, without any reference to the chemical composition and to
the atomic dimensions. Therefore, in this paper, we write a new formulation for
the kinematic viscosity coefficient to the turbulent viscous physical
dissipation in the Navier-Stokes equations, where molecular parameters are also
included. Results of 2D tests are shown, where comparisons among flow
structures are made on 2D shockless radial viscous transport and on 2D damping
of collisional chaotic turbulence. An application to the 3D accretion disc
modelling in low mass cataclysmic variables is also discussed. Consequences of
the kinematic viscosity coefficient reformulation in a more strictly physical
terms on the thermal conductivity coefficient for dilute gases are also
discussed. The physical nature of the discussion here reported excludes any
dependence by the pure mathematical aspect of the numerical modelling.
|
1210.6848v3
|
2012-11-07
|
Gaussification through decoherence
|
We investigate the loss of nonclassicality and non-Gaussianity of a
single-mode state of the radiation field in contact with a thermal reservoir.
The damped density matrix for a Fock-diagonal input is written using the Weyl
expansion of the density operator. Analysis of the evolution of the
quasiprobability densities reveals the existence of two successive
characteristic times of the reservoir which are sufficient to assure the
positivity of the Wigner function and, respectively, of the $P$ representation.
We examine the time evolution of non-Gaussianity using three recently
introduced distance-type measures. They are based on the Hilbert-Schmidt
metric, the relative entropy, and the Bures metric. Specifically, for an
$M$-photon-added thermal state, we obtain a compact analytic formula of the
time-dependent density matrix that is used to evaluate and compare the three
non-Gaussianity measures. We find a good consistency of these measures on the
sets of damped states. The explicit damped quasiprobability densities are shown
to support our general findings regarding the loss of negativities of Wigner
and $P$ functions during decoherence. Finally, we point out that Gaussification
of the attenuated field mode is accompanied by a nonmonotonic evolution of the
von Neumann entropy of its state conditioned by the initial value of the mean
photon number.
|
1211.1701v3
|
2013-02-05
|
Complex fragment emission in low energy light-ion reactions
|
Inclusive energy spectra of the complex fragments (3 $\leq$ Z $\leq$ 5)
emitted in the reactions $^{12}$C (77 MeV)+ $^{28}$Si, $^{11}$B (64 MeV)+
$^{28}$Si and $^{12}$C (73 MeV)+ $^{27}$Al (all having the same excitation
energy of $ \sim$ 67 MeV), have been measured in the angular range of
10$^\circ$ $\lesssim \theta_{lab} \lesssim$ 60$^\circ$. The fully energy damped
(fusion-fission) and the partially energy damped (deep inelastic) components of
the fragment energy spectra have been extracted. It has been found that the
yields of the fully energy damped fragments for all the above reactions are in
conformity with the respective statistical model predictions. The time scales
of various deep inelastic fragment emissions have been extracted from the
angular distribution data. The angular momentum dissipation in deep inelastic
collisions has been estimated from the data and it has been found to be close
to the corresponding sticking limit value.
|
1302.1002v1
|
2013-02-11
|
Propagation of Alfvénic Waves From Corona to Chromosphere and Consequences for Solar Flares
|
How do magnetohydrodynamic waves travel from the fully ionized corona, into
and through the underlying partially ionized chromosphere, and what are the
consequences for solar flares? To address these questions, we have developed a
2-fluid model (of plasma and neutrals) and used it to perform 1D simulations of
Alfv\'en waves in a solar atmosphere with realistic density and temperature
structure. Studies of a range of solar features (faculae, plage, penumbra and
umbra) show that energy transmission from corona to chromosphere can exceed 20%
of incident energy for wave periods of one second or less. Damping of waves in
the chromosphere depends strongly on wave frequency: waves with periods 10
seconds or longer pass through the chromosphere with relatively little damping,
however, for periods of 1 second or less, a substantial fraction (37%-100%) of
wave energy entering the chromosphere is damped by ion-neutral friction in the
mid and upper chromosphere, with electron resistivity playing some role in the
lower chromosphere and in umbras. We therefore conclude that Alfv\'enic waves
with periods of a few seconds or less are capable of heating the chromosphere
during solar flares, and speculate that they could also contribute to electron
acceleration or exciting sunquakes.
|
1302.2458v1
|
2013-02-25
|
Non-classical Correlations in the Quantum Search Algorithm
|
Entanglement lies at the heart of quantum mechanics and has no classical
analogue. It is central to the speed up achieved by quantum algorithms over
their classical counterparts. The Grover's search algorithm is one such
algorithm which enables us to achieve a quadratic speed up over any known
classical algorithm that searches for an element in an unstructured database.
Here, we analyse and quantify the effects of entanglement in the generalized
version of this algorithm for two qubits. By 'generalized', it is meant that
the use of any arbitrary single qubit unitary gate is permitted to create
superposed states. Our analysis has been firstly on a noise free environment
and secondly in the presence of noise. In the absence of noise, we establish a
relation between the concurrence and the amplitude of the final state thereby
showing the explicit effects of entanglement on the same. Moreover, the effects
of noisy channels, namely amplitude and phase damping channels are studied. We
investigate the amount of quantum correlation in the states obtained after the
phase inversion stage of the algorithm followed by interaction of those states
with the noisy environment. The quantum correlations are quantified by
geometric discord. It has been revealed that the states generated after the
effect of amplitude damping on the phase inverted states of the quantum search
algorithm possess non-zero quantum correlation even when entanglement is
absent. However, this is absent in the phase damping scenario.
|
1302.6005v1
|
2013-03-01
|
Generation of Electrostatic Waves via Parametric Instability and Heating of Solar Corona
|
In the upper layers of the solar atmosphere the temperature increases
sharply. We studied possibility of the transfer of neutrals motion energy into
the electrostatic waves.Electrostatic waves could damp in the upper layers of
the solar atmosphere and their energy could be transformed into the thermal
energy of the solar atmosphere plasma. When studying the plasma dynamics in the
low altitudes of the solar atmosphere, we investigated hydrodynamics of the
plasma which consists of thee components-electrons, ions and neutrals. In order
to study evolution of disturbances of high amplitudes the parametric resonance
technique is used. The dispersion relation for the electrostatic waves excited
due tot he motion of neutrals is derived. The frequencies of electromagnetic
waves which could be excited due to existence of the acoustic wave are found.
The increment of excited electrostatic waves are determined. The motion of the
neutrals in the lower solar atmosphere, where ionization rate is low, could
excite electrostatic waves. In the upper solar atmosphere the ionization rate
increases and motion of the neutrals could not support electrostatic waves and
these waves could damp due to the collision of the charged particles. The
energy of the damping waves could be transformed into the thermal energy of the
plasma in the upper atmosphere.
|
1303.0151v1
|
2013-03-19
|
Cosmic Ray Streaming in Clusters of Galaxies
|
The observed bimodality in radio luminosity in galaxy clusters is puzzling.
We investigate the possibility that cosmic-ray (CR) streaming in the
intra-cluster medium can 'switch off' hadronically induced radio and gamma-ray
emission. For self-confined CRs, this depends on the source of MHD wave
damping: if only non-linear Landau damping operates, then CRs stream on the
slow Alfvenic timescale, but if turbulent wave damping operates, super-Alfvenic
streaming is possible. As turbulence increases, it promotes outward streaming
more than it enables inward turbulent advection. Curiously, the CR flux is
independent of $\nabla f$ (as long as it is non-zero) and depends only on
plasma parameters; this enables radio halos with flat inferred CR profiles to
turn off. We perform 1D time-dependent calculations of a radio mini-halo
(Perseus) and giant radio halo (Coma) and find that both diminish in radio
luminosity by an order of magnitude in several hundred Myr, given plausible
estimates for the magnetic field in the outskirts of the cluster. Due to the
energy dependence of CR streaming, spectral curvature develops, and radio halos
turn off more slowly at low frequencies -- properties consistent with
observations. Similarly, CR streaming rapidly turns off gamma-ray emission at
the high-energies probed by Cherenkov telescopes, but not at the low energies
probed by Fermi. CR mediated wave-heating of the ICM is unaffected, as it is
dominated by ~GeV CRs which stream Alfvenically.
|
1303.4746v1
|
2013-05-13
|
New features of ion acoustic waves in inhomogeneous and permeating plasmas
|
It is generally accepted that the ion acoustic (IA) wave in plasmas
containing ions and electrons with the same temperature is of minor importance
due to strong damping of the wave by hot resonant ions. In this work it will be
shown that the IA wave is susceptible to excitation even in plasmas with hot
ions when both an electromagnetic transverse wave and a background density
gradient are present in the plasma, and in addition the wave is in fact
unstable (i.e., growing) in the case of permeating homogeneous plasmas. The
multi-component fluid theory is used to describe the IA wave susceptibility for
excitation in inhomogeneous plasmas and its coupling with electromagnetic
waves. The growing IA wave in permeating homogeneous plasmas is described by
the kinetic theory. In plasmas with density and temperature gradients the IA
wave is effectively coupled with the electromagnetic waves. In comparison to
ordinary IA wave in homogeneous plasma, the Landau damping of the present wave
is much smaller, and to demonstrate this effect a simple but accurate fluid
model is presented for the Landau damping. In the case of permeating plasmas, a
kinetic mechanism for the current-less IA wave instability is presented, with a
very low threshold for excitation as compared with ordinary
electron-current-driven kinetic instability. Such growing IA waves can
effectively heat plasma in the upper solar atmosphere by a stochastic heating
mechanism presented in the work. The results of this work suggest that the IA
wave role in the heating of the solar atmosphere (chromosphere and corona)
should be reexamined.
|
1305.2739v1
|
2013-05-15
|
Beam energy dependence of the viscous damping of anisotropic flow
|
The flow harmonics $v_{2,3}$ for charged hadrons, are studied for a broad
range of centrality selections and beam collision energies in Au+Au
($\sqrt{s_{NN}}= 7.7 - 200$ GeV) and Pb+Pb ($\sqrt{s_{NN}}= 2.76$ TeV)
collisions. They validate the characteristic signature expected for the system
size dependence of viscous damping at each collision energy studied. The
extracted viscous coefficients, that encode the magnitude of the ratio of shear
viscosity to entropy density $\eta/s$, are observed to decrease to an apparent
minimum as the collision energy is increased from $\sqrt{s_{NN}}= 7.7$ to
approximately 62.4 GeV; thereafter, they show a slow increase with
$\sqrt{s_{NN}}$ up to 2.76 TeV. This pattern of viscous damping provides the
first experimental constraint for $\eta/s$ in the temperature-baryon chemical
potential ($T, \mu_B$) plane, and could be an initial indication for decay
trajectories which lie close to the critical end point in the phase diagram for
nuclear matter.
|
1305.3341v3
|
2013-07-19
|
Exact 3D solution for static and damped harmonic response of simply supported general laminates
|
The state-space method is adapted to obtain three dimensional exact solutions
for the static and damped dynamic behaviors of simply supported general
laminates. The state-space method is written in a general form that permits to
handle both cross-ply and antisymmetric angle-ply laminates. This general form
also permits to obtain exact solutions for general laminates, albeit with some
constraints. For the general case and for the static behavior, either an
additive term is added to the load to simulate simply supported boundary
conditions, or the plate bends in a particular way. For the dynamic behavior,
the general case leads to pairs of natural frequencies for each order, with
associated mode shapes. Finite element simulations have been performed to
validate most of the results presented in this study. As the boundary
conditions needed for the general case are not so straightforward, a specific
discussion has been added. It is shown that these boundary conditions also work
for the two aforementioned laminate classes. The damped harmonic response of a
non symmetrical isotropic sandwich is studied for different frequencies around
the fundamental frequency. The static and undamped dynamic behaviors of the
[-15/15], [0/30/0] and [-10/0/40] laminates are studied for various
length-to-thickness ratios.
|
1307.5285v2
|
2013-07-25
|
Lyman-alpha Heating of Inhomogeneous High-redshift Intergalactic Medium
|
The intergalactic medium (IGM) prior to the epoch of reionization consists
mostly of neutral hydrogen gas. Ly-alpha photons produced by early stars
resonantly scatter off hydrogen atoms, causing energy exchange between the
radiation field and the gas. This interaction results in moderate heating of
the gas due to the recoil of the atoms upon scattering, which is of great
interest for future studies of the pre-reionization IGM in the HI 21 cm line.
We investigate the effect of this Ly-alpha heating in the IGM with linear
density, temperature, and velocity perturbations. Perturbations smaller than
the diffusion length of photons could be damped due to heat conduction by
Ly-alpha photons. The scale at which damping occurs and the strength of this
effect depend on various properties of the gas, the flux of Ly-alpha photons
and the way in which photon frequencies are redistributed upon scattering. To
find the relevant length scale and the extent to which Ly-alpha heating affects
perturbations, we calculate the gas heating rates by numerically solving
linearized Boltzmann equations in which scattering is treated by the
Fokker-Planck approximation. We find that (1) perturbations add a small
correction to the gas heating rate, and (2) the damping of temperature
perturbations occurs at scales with comoving wavenumber k>10^4 Mpc^{-1}, which
are much smaller than the Jeans scale and thus unlikely to substantially affect
the observed 21 cm signal.
|
1307.6859v2
|
2013-08-17
|
Thickness and power dependence of the spin-pumping effect in Y3Fe5O12/Pt heterostructures measured by the inverse spin Hall effect
|
The dependence of the spin-pumping effect on the yttrium iron garnet
(Y3Fe5O12, YIG) thickness detected by the inverse spin Hall effect (ISHE) has
been investigated quantitatively. Due to the spin-pumping effect driven by the
magnetization precession in the ferrimagnetic insulator YIG film a
spin-polarized electron current is injected into the Pt layer. This spin
current is transformed into electrical charge current by means of the ISHE. An
increase of the ISHE-voltage with increasing film thickness is observed and
compared to the theoretically expected behavior. The effective damping
parameter of the YIG/Pt samples is found to be enhanced with decreasing YIG
film thickness. The investigated samples exhibit a spin mixing conductance of
g=(7.43 \pm 0.36) \times 10^{18} m^{-2} and a spin Hall angle of theta_{ISHE} =
0.009 \pm 0.0008. Furthermore, the influence of nonlinear effects on the
generated voltage and on the Gilbert damping parameter at high excitation
powers are revealed. It is shown that for small YIG film thicknesses a
broadening of the linewidth due to nonlinear effects at high excitation powers
is suppressed because of a lack of nonlinear multi-magnon scattering channels.
We have found that the variation of the spin-pumping efficiency for thick YIG
samples exhibiting pronounced nonlinear effects is much smaller than the
nonlinear enhancement of the damping.
|
1308.3787v1
|
2013-09-13
|
Analytical and experimental stability investigation of a hardware-in-the-loop satellite docking simulator
|
The European Proximity Operation Simulator (EPOS) of the DLR-German Aerospace
Center is a robotics-based simulator that aims at validating and verifying a
satellite docking phase. The generic concept features a robotics tracking
system working in closed loop with a force/torque feedback signal. Inherent
delays in the tracking system combined with typical high stiffness at contact
challenge the stability of the closed-loop system. The proposed concept of
operations is hybrid: the feedback signal is a superposition of a measured
value and of a virtual value that can be tuned in order to guarantee a desired
behavior. This paper is concerned with an analytical study of the system's
closed-loop stability, and with an experimental validation of the hybrid
concept of operations in one dimension (1D). The robotics simulator is modeled
as a second-order loop-delay system and closed-form expressions for the
critical delay and associated frequency are derived as a function of the
satellites' mass and the contact dynamics stiffness and damping parameters. A
numerical illustration sheds light on the impact of the parameters on the
stability regions. A first-order Pade approximation provides additional means
of stability investigation. Experiments were performed and tests results are
described for varying values of the mass and the damping coefficients. The
empirical determination of instability is based on the coefficient of
restitution and on the observed energy. There is a very good agreement between
the critical damping values predicted by the analysis and observed during the
tests...
|
1309.3512v1
|
2013-11-12
|
Investigation into electron cloud effects in the International Linear Collider positron damping ring
|
We report modeling results for electron cloud buildup and instability in the
International Linear Collider positron damping ring. Updated optics, wiggler
magnets, and vacuum chamber designs have recently been developed for the 5 GeV,
3.2-km racetrack layout. An analysis of the synchrotron radiation profile
around the ring has been performed, including the effects of diffuse and
specular photon scattering on the interior surfaces of the vacuum chamber. The
results provide input to the cloud buildup simulations for the various magnetic
field regions of the ring. The modeled cloud densities thus obtained are used
in the instability threshold calculations. We conclude that the mitigation
techniques employed in this model will suffice to allow operation of the
damping ring at the design operational specifications.
|
1311.2890v4
|
2013-12-18
|
The behavior of transverse waves in nonuniform solar flux tubes. II. Implications for coronal loop seismology
|
Seismology of coronal loops using observations of damped transverse
oscillations in combination with results from theoretical models is a tool to
indirectly infer physical parameters in the solar atmospheric plasma. Existing
seismology schemes based on approximations to the period and damping time of
kink oscillations are often used beyond their theoretical range of
applicability. These approximations assume that the variation of density across
the loop is confined to a nonuniform layer much thinner than the radius of the
loop, but the results of the inversion problem often do not satisfy this
preliminary hypothesis. Here, we determine the accuracy of the analytic
approximations to the period and damping time, and its impact on seismology
estimates, when largely nonuniform loops are considered. We find that the
accuracy of the approximations when used beyond their range of applicability is
strongly affected by the form of the density profile across the loop, that is
observationally unknown and so must be arbitrarily imposed as part of the
theoretical model. The error associated with the analytic approximations can be
larger than 50% even for relatively thin nonuniform layers. This error directly
affects the accuracy of approximate seismology estimates compared to actual
numerical inversions. In addition, assuming different density profiles can
produce noncoincident intervals of the seismic variables in inversions of the
same event. The ignorance about the true shape of density variation across the
loop is an important source of error that may dispute the reliability of
parameters seismically inferred assuming an ad hoc density profile.
|
1312.5079v1
|
2014-01-02
|
Quasi-Normal Modes for Subtracted Rotating and Magnetised Geometries
|
We obtain explicit separable solutions of the wave equation of massless
minimally coupled scalar fields in the subtracted geometry of four-dimensional
rotating and Melvin (magnetised) four-charge black holes of the STU model, a
consistent truncation of maximally supersymmetric supergravity with four types
of electromagnetic fields. These backgrounds possess a hidden SL(2,R) x SL(2,R)
x SO(3) symmetry and faithfully model the near horizon geometry of these black
holes, but locate them in a confining asymptotically conical box. For each
subtracted geometry we obtain two branches of quasi-normal modes, given in
terms of hypergeometric functions and spherical harmonics. One branch is
over-damped and the other under-damped and they exhibit rotational splitting.
No black hole bomb is possible because the Killing field which co-rotates with
the horizon is everywhere timelike outside the black hole. A five-dimensional
lift of these geometries is given locally by the product of a BTZ black hole
with a two-sphere. This allows an explicit analysis of the minimally coupled
massive five-dimensional scalar field. Again, there are two branches, both
damped, however now their oscillatory parts are shifted by the quantised wave
number $k$ along the fifth circle direction.
|
1401.0544v3
|
2014-03-12
|
Enhancing robustness of multiparty quantum correlations using weak measurement
|
Multipartite quantum correlations are important resources for the development
of quantum information and computation protocols. However, the resourcefulness
of multipartite quantum correlations in practical settings is limited by its
fragility under decoherence due to environmental interactions. Though there
exist protocols to protect bipartite entanglement under decoherence, the
implementation of such protocols for multipartite quantum correlations has not
been sufficiently explored. Here, we study the effect of local amplitude
damping channel on the generalized Greenberger-Horne-Zeilinger state, and use a
protocol of optimal reversal quantum weak measurement to protect the
multipartite quantum correlations. We observe that the weak measurement
reversal protocol enhances the robustness of multipartite quantum correlations.
Further it increases the critical damping value that corresponds to
entanglement sudden death. To emphasize the efficacy of the technique in
protection of multipartite quantum correlation, we investigate two proximately
related quantum communication tasks, namely, quantum teleportation in a one
sender, many receivers setting and multiparty quantum information splitting,
through a local amplitude damping channel. We observe an increase in the
average fidelity of both the quantum communication tasks under the weak
measurement reversal protocol. The method may prove beneficial, for combating
external interactions, in other quantum information tasks using multipartite
resources.
|
1403.2939v1
|
2014-04-11
|
Functional Regression for Quasar Spectra
|
The Lyman-alpha forest is a portion of the observed light spectrum of distant
galactic nuclei which allows us to probe remote regions of the Universe that
are otherwise inaccessible. The observed Lyman-alpha forest of a quasar light
spectrum can be modeled as a noisy realization of a smooth curve that is
affected by a `damping effect' which occurs whenever the light emitted by the
quasar travels through regions of the Universe with higher matter
concentration. To decode the information conveyed by the Lyman-alpha forest
about the matter distribution, we must be able to separate the smooth
`continuum' from the noise and the contribution of the damping effect in the
quasar light spectra. To predict the continuum in the Lyman-alpha forest, we
use a nonparametric functional regression model in which both the response and
the predictor variable (the smooth part of the damping-free portion of the
spectrum) are function-valued random variables. We demonstrate that the
proposed method accurately predicts the unobservable continuum in the
Lyman-alpha forest both on simulated spectra and real spectra. Also, we
introduce distribution-free prediction bands for the nonparametric functional
regression model that have finite sample guarantees. These prediction bands,
together with bootstrap-based confidence bands for the projection of the mean
continuum on a fixed number of principal components, allow us to assess the
degree of uncertainty in the model predictions.
|
1404.3168v1
|
2014-06-04
|
Constraints on the gas masses of low-z damped Lyman-$α$ systems
|
We report a deep search for redshifted HI 21 cm emission from three damped
and sub-damped Lyman-$\alpha$ absorbers (DLAs) at $z \approx 0.1$ with the
Green Bank Telescope (GBT). No evidence for a redshifted HI 21 cm emission
signal was obtained in the GBT spectra of two absorbers, with the data on the
third rendered unusable by terrestrial interference. The non-detections of HI
21 cm emission yield strong constraints on the HI masses of the associated
galaxies, M$_{\rm HI} < 2.3 \times 10^9 \times (\Delta V/100)^{1/2}$ M$_\odot$
for the sub-DLA at $z = 0.0830$ towards J1553+3548, and M$_{\rm HI} < 2.7
\times 10^9 \times (\Delta V/100)^{1/2}$ M$_\odot$ for the DLA at $z = 0.0963$
towards J1619+3342, where $\Delta V$ is the HI 21 cm line width, in km
s$^{-1}$. This continues the trend of low HI masses found in all low-$z$ DLAs
and sub-DLAs that have been searched for redshifted HI 21 cm emission.
Low-redshift absorbers with relatively low HI column densities, $\lesssim few
\times 10^{20}$ cm$^{-2}$, thus do not typically arise in massive gas-rich
galaxies.
|
1406.0991v2
|
2014-06-09
|
Evolution of eccentricity and orbital inclination of migrating planets in 2:1 mean motion resonance
|
We determine, analytically and numerically, the conditions needed for a
system of two migrating planets trapped in a 2:1 mean motion resonance to enter
an inclination-type resonance. We provide an expression for the asymptotic
equilibrium value that the eccentricity $e_{\rm i}$ of the inner planet reaches
under the combined effects of migration and eccentricity damping. We also show
that, for a ratio $q$ of inner to outer masses below unity, $e_{\rm i}$ has to
pass through a value $e_{\rm i,res}$ of order 0.3 for the system to enter an
inclination-type resonance. Numerically, we confirm that such a resonance may
also be excited at another, larger, value $e_{\rm i, res} \simeq 0.6$, as found
by previous authors. A necessary condition for onset of an inclination-type
resonance is that the asymptotic equilibrium value of $e_{\rm i}$ is larger
than $e_{\rm i,res}$. We find that, for $q \le 1$, the system cannot enter an
inclination-type resonance if the ratio of eccentricity to semimajor axis
damping timescales $t_e/t_a$ is smaller than 0.2. This result still holds if
only the eccentricity of the outer planet is damped and $q \lesssim 1$. As the
disc/planet interaction is characterized by $t_e/t_a \sim 10^{-2}$, we conclude
that excitation of inclination through the type of resonance described here is
very unlikely to happen in a system of two planets migrating in a disc.
|
1406.2189v1
|
2014-06-13
|
Magnetic-Field Amplification in the Thin X-ray Rims of SN1006
|
Several young supernova remnants (SNRs), including SN1006, emit synchrotron
X-rays in narrow filaments, hereafter thin rims, along their periphery. The
widths of these rims imply 50 to 100 $\mu$G fields in the region immediately
behind the shock, far larger than expected for the interstellar medium
compressed by unmodified shocks, assuming electron radiative losses limit rim
widths. However, magnetic-field damping could also produce thin rims. Here we
review the literature on rim width calculations, summarizing the case for
magnetic-field amplification. We extend these calculations to include an
arbitrary power-law dependence of the diffusion coefficient on energy, $D
\propto E^{\mu}$. Loss-limited rim widths should shrink with increasing photon
energy, while magnetic-damping models predict widths almost independent of
photon energy. We use these results to analyze Chandra observations of SN 1006,
in particular the southwest limb. We parameterize the full widths at half
maximum (FWHM) in terms of energy as FWHM $\propto E^{m_E}_{\gamma}$. Filament
widths in SN1006 decrease with energy; $m_E \sim -0.3$ to $-0.8$, implying
magnetic field amplification by factors of 10 to 50, above the factor of 4
expected in strong unmodified shocks. For SN 1006, the rapid shrinkage rules
out magnetic damping models. It also favors short mean free paths (small
diffusion coefficients) and strong dependence of $D$ on energy ($\mu \ge 1$).
|
1406.3630v2
|
2014-07-02
|
Spin Waves in Ferromagnetic Insulators Coupled via a Normal Metal
|
Herein, we study the spin-wave dispersion and dissipation in a ferromagnetic
insulator--normal metal--ferromagnetic insulator system. Long-range dynamic
coupling because of spin pumping and spin transfer lead to collective magnetic
excitations in the two thin-film ferromagnets. In addition, the dynamic dipolar
field contributes to the interlayer coupling. By solving the
Landau-Lifshitz-Gilbert-Slonczewski equation for macrospin excitations and the
exchange-dipole volume as well as surface spin waves, we compute the effect of
the dynamic coupling on the resonance frequencies and linewidths of the various
modes. The long-wavelength modes may couple acoustically or optically. In the
absence of spin-memory loss in the normal metal, the spin-pumping-induced
Gilbert damping enhancement of the acoustic mode vanishes, whereas the optical
mode acquires a significant Gilbert damping enhancement, comparable to that of
a system attached to a perfect spin sink. The dynamic coupling is reduced for
short-wavelength spin waves, and there is no synchronization. For intermediate
wavelengths, the coupling can be increased by the dipolar field such that the
modes in the two ferromagnetic insulators can couple despite possible small
frequency asymmetries. The surface waves induced by an easy-axis surface
anisotropy exhibit much greater Gilbert damping enhancement. These modes also
may acoustically or optically couple, but they are unaffected by thickness
asymmetries.
|
1407.0635v1
|
2014-07-15
|
C$ν$B damping of primordial gravitational waves and the fine-tuning of the C$γ$B temperature anisotropy
|
Damping of primordial gravitational waves due to the anisotropic stress
contribution owing to the cosmological neutrino background (C$\nu$B) is
investigated in the context of a radiation-to-matter dominated Universe.
Besides its inherent effects on the gravitational wave propagation, the
inclusion of the C$\nu$B anisotropic stress into the dynamical equations also
affects the tensor mode contribution to the anisotropy of the cosmological
microwave background (C$\gamma$B) temperature. Given that the fluctuations of
the C$\nu$B temperature in the (ultra)relativistic regime are driven by a
multipole expansion, the mutual effects on the gravitational waves and on the
C$\gamma$B are obtained through a unified prescription for a
radiation-to-matter dominated scenario. The results are confronted with some
preliminary results for the radiation dominated scenario. Both scenarios are
supported by a simplified analytical framework, in terms of a scale independent
dynamical variable, $k \eta$, that relates cosmological scales, $k$, and the
conformal time, $\eta$. The background relativistic (hot dark) matter
essentially works as an effective dispersive medium for the gravitational waves
such that the damping effect is intensified for the Universe evolving to the
matter dominated era. Changes on the temperature variance owing to the
inclusion of neutrino collision terms into the dynamical equations result into
spectral features that ratify that the multipole expansion coefficients
$C_{l}^{T}$'s die out for $l \sim 100$.
|
1407.4058v1
|
2014-08-08
|
Stable Magnetic Droplet Solitons in Spin Transfer Nanocontacts
|
Magnetic thin films with perpendicular magnetic anisotropy (PMA) have
localized excitations that correspond to reversed dynamically precessing
magnetic moments, known as magnetic droplet solitons. Fundamentally, these
excitations are associated with an attractive interaction between elementary
spin-excitations (i.e., magnons) and were predicted to occur in PMA materials
in the absence of damping [1,2]. While damping, present in all magnetic
materials, suppresses these excitations, it is now possible to compensate
damping by spin transfer torques through electrical current flow in nanometer
scale contacts to ferromagnetic thin films [3,4]. A theory predicts the
appearance of magnetic droplet solitons at a threshold current in nanocontacts
[5] and, recently, experimental signatures of droplet nucleation have been
reported [6]. However, thus far, they have been observed to be nearly
reversible excitations, with only partially reversed magnetization and to be
subject to instabilities that cause them to drift away from the nanocontacts
(i.e., drift instabilities) [6]. Here we show that magnetic droplet solitons
can be stabilized in a spin transfer nanocontact. Further, they exhibit a
strong hysteretic response to fields and currents and a nearly fully reversed
magnetization in the contact. These observations, in addition to their
fundamental interest, open up new applications for magnetic droplet solitons as
multi-state high frequency current and field tunable oscillators.
|
1408.1902v1
|
2014-08-19
|
Probing Electron Interactions in a Two-Dimensional System by Quantum Magneto-Oscillations
|
We have experimentally studied the renormalized effective mass m* and Dingle
temperature T_D in two spin subbands with essentially different electron
populations. Firstly, we found that the product (m*T_D) that determines damping
of quantum oscillations, to the first approximation, is the same in the
majority and minority subbands even at the spin polarization degree as high as
66\%. This result confirms the theoretical predictions that the interaction
takes place at high energies ~ E_F rather than within a narrow strip of
energies E_F\pm k_BT. Secondly, to the next approximation, we revealed a
difference in the damping factor of the two spin subbands, which causes
skewness of the oscillation lineshape. In the absence of the in-plane magnetic
field, the damping factor (m*T_D) is systematically smaller in the
spin-majority subband. The difference, quantified with the skew factor \gamma =
(T_{D\downarrow}-T_{D\uparrow})/2T_{D0} can be as large as 20%. The skew factor
tends to decrease as B_\parallel or temperature grow, or perpendicular field
decreases; for low electron densities and high in-plane fields the skew factor
even changes sign. Finally, we compared the temperature and magnetic field
dependencies of the magneto-oscillations amplitude with predictions of the
interaction correction theory, and found, besides some qualitative
similarities, several quantitative and qualitative differences. To explain
qualitatively our results, we suggested an empirical model that assumes the
existence of easily magnetized triplet scatterers on the Si/SiO_2 interface.
|
1408.4393v1
|
2014-09-30
|
Collective modes in two- and three-dimensional electron systems with Rashba spin-orbit coupling
|
In addition to charge plasmons, a 2D electron system with Rashba-type
spin-orbit coupling (SOC) also supports three collective modes in the spin
sector: the chiral-spin modes. We study the dispersions of the charge and spin
modes and their coupling to each other within a generalized Random Phase
Approximation for arbitrarily strong SOC, and both in 2D and 3D systems. In
both 2D and 3D, we find that the charge plasmons are coupled to only one of the
three chiral-spin modes. This coupling is shown to affect the dispersions of
the modes at finite but not at zero wavenumbers. In 3D, the chiral-spin modes
are strongly damped by particle-hole excitations and disappear for weak
electron-electron interaction. Landau damping of the chiral-spin modes in 3D is
directly related to the fact that, in contrast to 2D, there is no gap for
particle-hole excitations between spin-split subbands. The gapless continuum is
also responsible for Landau damping of the charge plasmon in 3D - a
qualitatively new feature of the SOC system. We also discuss the optical
conductivity of clean 2D and 3D systems and show that SOC introduces spectral
weight at finite frequency in a such way that the sum rule is satisfied. The
in-plane tranverse chiral-spin mode shows up as dispersing peak in the optical
conductivity at finite number which can can be measured in the presence of
diffraction grating. We also discuss possible experimental manifestations of
chiral-spin modes in semiconductor quantum wells such InGaAs/AlGaAs and 3D
giant Rashba materials of the BiTeI family.
|
1409.8666v1
|
2014-11-15
|
Spin-transfer-torque through antiferromagnetic IrMn
|
Spin-transfer-torque, a transfer of angular momentum between the electron
spin and the local magnetic moments, is a promising and key mechanism to
control ferromagnetic materials in modern spintronic devices . However, much
less attention has been paid to the same effect in antiferromagnets. For the
sake of investigating how the spin current interacts with the magnetic moments
in antiferromagnets, we perform spin-torque ferromagnetic resonance
measurements on Co20Fe60B20 4nm/Ir25Mn75 tIrMn nm/Pt 4 nm multilayers under a
spin Hall effect of Pt. The effective magnetic damping in Co20Fe60B20 is
modified by the spin current injected from the Pt layer via the IrMn layer. The
results indicate that the spin current interacts with IrMn magnetic moments and
exerts the anti-damping torque on the magnetic moments of Co20Fe60B20 through
the IrMn. It is also found that the reduction of the exchange bias in the
IrMn/Pt interface degrades the anti-damping torque exerted on the Co20Fe60B20
layer, suggesting the transmission of the spin torque becomes less efficient as
the interface exchange coupling degrades. Our work infers that the magnetic
moments in IrMn can be manipulated by spin torque similarly to the one in a
ferromagnetic layer.
|
1411.4100v4
|
2014-11-28
|
On damped second-order gradient systems
|
Using small deformations of the total energy, as introduced in [31], we
establish that damped second order gradient systems
$$u^{\prime\prime}(t)+\gamma u^\prime(t)+\nabla G(u(t))=0,$$may be viewed as
quasi-gradient systems. In order to study the asymptotic behavior of these
systems, we prove that any (nontrivial) desingularizing function appearing in
KL inequality satisfies $\varphi(s)\ge c\sqrt s$ whenever the original function
is definable and $C^2.$ Variants to this result are given. These facts are used
in turn to prove that a desingularizing function of the potential $G$ also
desingularizes the total energy and its deformed versions. Our approach brings
forward several results interesting for their own sake: we provide an
asymptotic alternative for quasi-gradient systems, either a trajectory
converges, or its norm tends to infinity. The convergence rates are also
analyzed by an original method based on a one-dimensional worst-case gradient
system.We conclude by establishing the convergence of solutions of damped
second order systems in various cases including the definable case. The
real-analytic case is recovered and some results concerning convex functions
are also derived.
|
1411.8005v5
|
2014-12-08
|
Variable frequency characterization of interaction at nanoscale in linear dynamic AFM: an FFM primer
|
Using electrostatic coupling between an AFM tip and a metallic surface as a
test interaction, we here present the measurement of the force between the tip
and the surface, together with the measurement of the interaction stiffness and
the associated dissipation. These three quantities constitute a full
characterization of the interaction at nanoscale. They are measured
independently, simultaneously and quantitatively at the same place. This is
made possible thanks to a force feedback method that ensures the DC immobility
of the tip and to the simultaneous application of a sub-nanometer oscillation
to the tip. In this established linear regime, stiffness and damping are
directly obtained from amplitude and phase change measurements. The needed
information for this linear transformation is solely the lever properties in
the experimental context. Knowledge of k, its stiffness, its damping
coefficient and Q0, its first resonance frequency is shown to be sufficient in
the frequency range we are here exploring. Finally, we demonstrate that this
method is not restricted to the lever resonance frequency. To the contrary,
this interaction characterization whose resolution is limited by the Brownian
motion, can be used at any frequencies with essentially the same performances.
We believe that simultaneous and independent measurements of force, stiffness
and damping, out of lever resonance, at nanoscale, and within the context of
linear response define a new AFM paradigm that we call Force Feedback
Microscopy (FFM). This article details the use of FFM using a well known and
easy to implement electrostatic interaction between a regular AFM tip and a
metallic surface in air.
|
1412.2640v1
|
2015-01-12
|
Standing Slow MHD Waves in Radiatively Cooling Coronal Loops
|
The standing slow magneto-acoustic oscillations in cooling coronal loops are
investigated. There are two damping mechanisms which are considered to generate
the standing acoustic modes in coronal magnetic loops namely thermal conduction
and radiation. The background temperature is assumed to change temporally due
to optically thin radiation. In particular, the background plasma is assumed to
be radiatively cooling. The effects of cooling on longitudinal slow MHD modes
is analytically evaluated by choosing a simple form of radiative function that
ensures the temperature evolution of the background plasma due to radiation
coincides with the observed cooling profile of coronal loops. The assumption of
low-beta plasma leads to neglect the magnetic field perturbation and eventually
reduces the MHD equations to a 1D system modelling longitudinal MHD
oscillations in a cooling coronal loop. The cooling is assumed to occur on a
characteristic time scale much larger than the oscillation period that
subsequently enables using the WKB theory to study the properties of standing
wave. The governing equation describing the time-dependent amplitude of waves
is obtained and solved analytically. The analytically derived solutions are
numerically evaluated to give further insight into the evolution of the
standing acoustic waves. We find that the plasma cooling gives rise to a
decrease in the amplitude of oscillations. In spite of the reduction in damping
rate caused by rising the cooling, the damping scenario of slow standing MHD
waves strongly increases in hot coronal loops.
|
1501.02689v1
|
2015-01-12
|
Beta decay of 252Cf on the way to scission from the exit point
|
Upon increasing significantly the nuclear elongation, the beta-decay energy
grows. This paper investigates within a simple yet partly microscopic approach,
the transition rate of the beta decay of the 252Cf nucleus on the way to
scission from the exit point for a spontaneous fission process. A rather crude
classical approximation is made for the corresponding damped collective motion
assumed to be one dimensional. Given these assumptions, we only aim in this
paper at providing the order of magnitudes of such a phenomenon. At each
deformation the energy available for beta decay, is determined from such a
dynamical treatment. Then, for a given elongation, transition rates for the
allowed (Fermi) beta decay are calculated from pair correlated wave functions
obtained within a macroscopic-microscopic approach and then integrated over the
time corresponding to the whole descent from exit to scission. The results are
presented as a function of the damping factor (inverse of the characteristic
damping time) in use in our classical dynamical approach. For instance, in the
case of a descent time from the exit to the scission points of about $10^{-
20}$ second, one finds a total rate of beta decay corresponding roughly to 20
events per year and per milligram of 252Cf. The inclusion of pairing
correlations does not affect much these results.
|
1501.02701v1
|
2015-01-13
|
Fundamental oscillation modes of neutron stars: validity of universal relations
|
We study the $f$-mode frequencies and damping times of nonrotating neutron
stars (NS) in general relativity (GR) by solving the linearized perturbation
equations, with the aim to establish "universal" relations that depend only
weakly on the equations of state (EOS). Using a more comprehensive set of EOSs,
we re-examine some proposed linearizations that describe the $f$-mode
parameters in terms of mass and radius of the neutron star (NS), and we test a
more recent proposal for expressing the $f$-mode parameters as quadratic
functions of the effective compactness. Our extensive results for each equation
of state considered allow us to study the accuracy of each proposal. In
particular, we find that the damping time deviates quite considerably from the
proposed linearization. We introduce a new universal relation for the product
of the $f$-mode frequency and damping time as a function of the (ordinary)
compactness, which proved to be more accurate. The relations using the
effective compactness on the other hand also fit our data accurately. Our
results show that the maximum oscillation frequency depends strongly on the
EOS, such that the measurement of a high oscillation frequency would rule out
several EOSs. Lastly, we compare the exact mode frequencies to those obtained
in the Cowling approximation, and also to results obtained with a nonlinear
evolution code, validating the implementations of the different approaches.
|
1501.02970v2
|
2015-02-13
|
Magnetohydrodynamic kink waves in nonuniform solar flux tubes: phase mixing and energy cascade to small scales
|
Magnetohydrodynamic (MHD) kink waves are ubiquitously observed in the solar
atmosphere. The propagation and damping of these waves may play relevant roles
for the transport and dissipation of energy in the solar atmospheric medium.
However, in the atmospheric plasma dissipation of transverse MHD wave energy by
viscosity or resistivity needs very small spatial scales to be efficient. Here,
we theoretically investigate the generation of small scales in nonuniform solar
magnetic flux tubes due to phase mixing of MHD kink waves. We go beyond the
usual approach based on the existence of a global quasi-mode that is damped in
time due to resonant absorption. Instead, we use a modal expansion to express
the MHD kink wave as a superposition of Alfv\'en continuum modes that are phase
mixed as time evolves. The comparison of the two techniques evidences that the
modal analysis is more physically transparent and describes both the damping of
global kink motions and the building up of small scales due to phase mixing. In
addition, we discuss that the processes of resonant absorption and phase mixing
are intimately linked. They represent two aspects of the same underlying
physical mechanism: the energy cascade from large scales to small scales due to
naturally occurring plasma and/or magnetic field inhomogeneities. This process
may provide the necessary scenario for efficient dissipation of transverse MHD
wave energy in the solar atmospheric plasma.
|
1502.03949v1
|
2015-03-25
|
Wave Propagation at Oblique Shocks: How Did Tycho Get Its Stripes?
|
We describe a new model for the "stripes" of synchrotron radiation seen in
the remnant of Tycho's supernova. In our picture, cosmic rays streaming ahead
of the forward shock generate parallel (with respect to the local magnetic
field direction) circularly polarized Alfven waves that are almost free of
dissipation, and due to being circularly polarized exhibit no spatial variation
of magnetic field strength. Following interaction with the SNR shock with
nonzero obliquity, these parallel waves become obliquely propagating, due the
the wave refraction (different in principle for the different plane wave
components), and dissipation sets in. The magnetosonic polarization decays
faster, due to transit time damping, leaving only the Alfven mode. This
surviving mode now exhibits a spatial variation of the magnetic field, leading
to local maxima and minima in the synchrotron emission, i.e. the stripes. We
attribute the initial wave generation to the Bell instability, which in
contrast to the resonant generation of upstream Alfven waves, gives rise to a
preferred wavelength, and hence the single wave period at which the stripes are
seen. Based on estimates for damping rates due to turbulent cascade and transit
time damping, we estimate the dependence of the visibility of the stripes on
the shock obliquity, and determine a maximum cosmic ray energy in Tycho's SNR
in the range $6\times 10^{14} - 1\times 10^{15}$ eV.
|
1503.07497v1
|
2015-03-30
|
Superradiance and instability of small rotating charged AdS black holes in all dimensions
|
Rotating small AdS black holes exhibit the superradiant instability to
low-frequency scalar perturbations, which is amenable to a complete analytic
description in four dimensions. In this paper, we extend this description to
all higher dimensions, focusing on slowly rotating charged AdS black holes with
a single angular momentum. We divide the spacetime of these black holes into
the near-horizon and far regions and find solutions to the scalar wave equation
in each of these regions. Next, we perform the matching of these solutions in
the overlap between the regions, by employing the idea that the orbital quantum
number $ \ell $ can be thought of as an approximate integer. Thus, we obtain
the complete low-frequency solution that allows us to calculate the complex
frequency spectrum of quasinormal modes, whose imaginary part is determined by
a small damping parameter. Finally, we find a remarkably instructive expression
for the damping parameter, which appears to be a complex quantity in general.
We show that the real part of the damping parameter can be used to give a {\it
universal} analytic description of the superradiant instability for slowly
rotating charged AdS black holes in all spacetime dimensions.
|
1503.08607v2
|
2015-04-12
|
Evolution of Kinetic and Magnetic Energy in Intra Cluster Media
|
Intra Cluster Media (ICMs) located at galaxy clusters is in the state of hot,
tenuous, magnetized, and highly ionized X-ray emitting plasmas. This overall
collisionless, viscous, and conductive magnetohydrodynamic (MHD) turbulence in
ICM is simulated using hyper and physical magnetic diffusivity. The results
show that fluctuating random plasma motion amplifies the magnetic field, which
cascades toward the diffusivity scale passing through the viscous scale. The
kinetic eddies in the subviscous scale are driven and constrained by the
magnetic tension which finally gets balanced with the highly damping effect of
the kinetic eddies. However, the saturated kinetic energy spectrum is deeper
than that of the incompressible or compressible hydrodynamics fluid. To explain
this unusual field profile we set up two simultaneous differential equations
for the kinetic and magnetic energy spectrum using an Eddy Damped Quasi Normal
Markovianized (EDQNM) approximation. The analytic solution tells us that the
magnetic energy in addition to the viscous damping effect constrains the plasma
motion leading to the power spectra: kinetic energy spectrum $E_V^k\sim k^{-3}$
and corresponding representative magnetic energy spectrum $E_M^k\sim k^{-1/2}$.
Also the comparison of simulation results with different resolutions and
magnetic diffusivities implies the role of small scale magnetic energy in
dynamo.
|
1504.02940v3
|
2015-04-17
|
Effective Action for Cosmological Scalar Fields at Finite Temperature
|
Scalar fields appear in many theories beyond the Standard Model of particle
physics. In the early universe, they are exposed to extreme conditions,
including high temperature and rapid cosmic expansion. Understanding their
behavior in this environment is crucial to understand the implications for
cosmology. We calculate the finite temperature effective action for the field
expectation value in two particularly important cases, for damped oscillations
near the ground state and for scalar fields with a flat potential. We find that
the behavior in both cases can in good approximation be described by a complex
valued effective potential that yields Markovian equations of motion. Near the
potential minimum, we recover the solution to the well-known Langevin equation.
For large field values we find a very different behavior, and our result for
the damping coefficient differs from the expressions frequently used in the
literature. We illustrate our results in a simple scalar model, for which we
give analytic approximations for the effective potential and damping
coefficient. We also provide various expressions for loop integrals at finite
temperature that are useful for future calculations in other models.
|
1504.04444v2
|
2015-05-19
|
The elusive HI-> H2 transition in high-z damped Lyman-alpha systems
|
We study the H2 molecular content in high redshift damped Lyman-alpha systems
(DLAs) as a function of the HI column density. We find a significant increase
of the H2 molecular content around log N(HI) (cm^-2)~21.5-22, a regime unprobed
until now in intervening DLAs, beyond which the majority of systems have log
N(H2) > 17. This is in contrast with lines of sight towards nearby stars, where
such H2 column densities are always detected as soon as log N(HI)>20.7. This
can qualitatively be explained by the lower average metallicity and possibly
higher surrounding UV radiation in DLAs. However, unlike in the Milky Way, the
overall molecular fractions remain modest, showing that even at a large N(HI)
only a small fraction of overall HI is actually associated with the
self-shielded H2 gas. Damped Lyman-alpha systems with very high-N(HI) probably
arise along quasar lines of sight passing closer to the centre of the host
galaxy where the gas pressure is higher. We show that the colour changes
induced on the background quasar by continuum (dust) and line absorption (HI
Lyman and H2 Lyman & Werner bands) in DLAs with log N(HI)~22 and metallicity
~1/10 solar is significant, but not responsible for the long-discussed lack of
such systems in optically selected samples. Instead, these systems are likely
to be found towards intrinsically fainter quasars that dominate the quasar
luminosity function. Colour biasing should in turn be severe at higher
metallicities.
|
1505.04997v1
|
2015-06-11
|
Dynamics near the subcritical transition of the 3D Couette flow I: Below threshold case
|
We study small disturbances to the periodic, plane Couette flow in the 3D
incompressible Navier-Stokes equations at high Reynolds number $\textbf{Re}$.
We prove that for sufficiently regular initial data of size $\epsilon \leq
c_0\textbf{Re}^{-1}$ for some universal $c_0 > 0$, the solution is global,
remains within $O(c_0)$ of the Couette flow in $L^2$, and returns to the
Couette flow as $t \rightarrow \infty$. For times $t \gtrsim
\textbf{Re}^{1/3}$, the streamwise dependence is damped by a mixing-enhanced
dissipation effect and the solution is rapidly attracted to the class of "2.5
dimensional" streamwise-independent solutions referred to as streaks. Our
analysis contains perturbations that experience a transient growth of kinetic
energy from $O(\textbf{Re}^{-1})$ to $O(c_0)$ due to the algebraic linear
instability known as the lift-up effect. Furthermore, solutions can exhibit a
direct cascade of energy to small scales. The behavior is very different from
the 2D Couette flow, in which stability is independent of $\textbf{Re}$,
enstrophy experiences a direct cascade, and inviscid damping is dominant
(resulting in a kind of inverse energy cascade). In 3D, inviscid damping will
play a role on one component of the velocity, but the primary stability
mechanism is the mixing-enhanced dissipation. Central to the proof is a
detailed analysis of the interplay between the stabilizing effects of the
mixing and enhanced dissipation and the destabilizing effects of the lift-up
effect, vortex stretching, and weakly nonlinear instabilities connected to the
non-normal nature of the linearization.
|
1506.03720v1
|
2015-06-12
|
Casimir Friction Between Polarizable Particle and Half-Space with Radiation Damping and Image Damping at Zero Temperature
|
Casimir friction between a polarizable particle and a semi-infinite space is
a delicate physical phenomenon, as it concerns the interaction between a
microscopic quantum particle and a semi-infinite reservoir. Not unexpectedly,
results obtained in the past about the friction force obtained via different
routes are sometimes, at least apparently, wildly different from each other.
Recently, we considered the Casimir friction force for two dielectric
semi-infinite plates moving parallel to each other [J. S. H{\o}ye and I.
Brevik, Eur. Phys. J. D {\bf 68}, 61 (2014)], and managed to get essential
agreement with results obtained by Pendry (1997), Volokitin and Persson (2007),
and Barton (2011). Our method was based upon use of the Kubo formalism. In the
present paper we focus on the interaction between a polarizable particle and a
dielectric half-space again, and calculate the friction force using the same
basic method as before. The new ingredient in the present analysis is that we
take into account radiative damping, and derive the modifications thereof. Some
comparisons are also made with works from others. Essential agreement with the
results of Intravaia, Behunin, and Dalvit can also be achieved using the
modification of the atomic polarizability by the metallic plate.
|
1506.03937v2
|
2015-07-08
|
Periods and damping rates of fast sausage oscillations in multi-shelled coronal loops
|
Standing sausage modes are important in interpreting quasi-periodic
pulsations in the lightcurves of solar flares. Their periods and damping times
play an important role in seismologically diagnosing key parameters like the
magnetic field strength in regions where flare energy is released. Usually such
applications are based on theoretical results neglecting unresolved fine
structures in magnetized loops. However, the existence of fine structuring is
suggested on both theoretical and observational grounds. Adopting the framework
of cold magnetohydrodynamics (MHD), we model coronal loops as magnetized
cylinders with a transverse equilibrium density profile comprising a monolithic
part and a modulation due to fine structuring in the form of concentric shells.
The equation governing the transverse velocity perturbation is solved with an
initial-value-problem approach, and the effects of fine structuring on the
periods $P$ and damping times $\tau$ of global, leaky, standing sausage modes
are examined. A parameter study shows that fine structuring, be it periodically
or randomly distributed, brings changes of only a few percent to $P$ and $\tau$
when there are more than about ten shells. The monolithic part, its steepness
in particular, plays a far more important role in determining $P$ and $\tau$.
We conclude that when measured values of $P$ and $\tau$ of sausage modes are
used for seismological purposes, it is justified to use theoretical results
where the effects due to fine structuring are neglected.
|
1507.02169v1
|
2015-07-23
|
Asteroseismology of rapidly rotating neutron stars - an alternative approach
|
In the present paper we examine gravitational wave asteroseismology relations
for f-modes of rapidly rotating neutron stars. An approach different to the
previous studies is employed - first, the moment of inertia is used instead of
the stellar radius, and second, the normalization of the oscillation
frequencies and damping times is different. It was shown that in the
non-rotating case this can lead to a much stronger equation of state
independence and our goal is to generalize the static relations to the rapidly
rotating case and values of the spherical mode number $l\ge2$. We employ
realistic equations of state that cover a very large range of stiffness in
order to check better the universality of the relations. At the end we explore
the inverse problem, i.e. obtain the neutron star parameters from the observed
gravitational frequencies and damping times. It turns out that with this new
set of relations we can solve the inverse problem with a very good accuracy
using three frequencies that was not possible in the previous studies where one
needs also the damping times. The asteroseismology relations are also
particularly good for the massive rapidly rotating models that are subject to
secular instabilities.
|
1507.06606v1
|
2015-07-31
|
Eccentricity Evolution Through Accretion of Protoplanets
|
Most of super-Earths detected by the radial velocity (RV) method have
significantly smaller eccentricities than the eccentricities corresponding to
velocity dispersion equal to their surface escape velocity ("escape
eccentricities"). If orbital instability followed by giant impacts among
protoplanets that have migrated from outer region is considered, it is usually
considered that eccentricities of the merged bodies become comparable to those
of orbital crossing bodies, which are excited up to their escape eccentricities
by close scattering. However, the eccentricity evolution in the {\it in situ}
accretion model has not been studied in detail. Here, we investigate the
eccentricity evolution through {\it N}-body simulations. We have found that the
merged planets tend to have much smaller eccentricities than the escape
eccentricities due to very efficient collision damping. If the protoplanet
orbits are initially well separated and their eccentricities are securely
increased, an inner protoplanet collides at its apocenter with an outer
protoplanet at its pericenter. The eccentricity of the merged body is the
smallest for such configuration. Orbital inclinations are also damped by this
mechanism and planets tend to share a same orbital plane, which is consistent
with {\it Kepler} data. Such efficient collision damping is not found when we
start calculations from densely packed orbits of the protoplanets. If the
protoplanets are initially in the mean-motion resonances, which corresponds to
well separated orbits, the {\it in situ} accretion model well reproduces the
features of eccentricities and inclinations of multiple super-Earths/Earth
systems discovered by RV and {\it Kepler} surveys.
|
1507.08809v1
|
2015-08-25
|
Effects of inertia on the steady-shear rheology of disordered solids
|
We study the finite-shear-rate rheology of disordered solids by means of
molecular dynamics simulations in two dimensions. By systematically varying the
damping magnitude $\zeta$ in the low-temperature limit, we identify two well
defined flow regimes, separated by a thin (temperature-dependent) crossover
region. In the overdamped regime, the athermal rheology is governed by the
competition between elastic forces and viscous forces, whose ratio gives the
Weissenberg number $Wi= \zeta \dot\gamma$ (up to elastic parameters); the
macroscopic stress $\Sigma$ follows the frequently encountered Herschel-Bulkley
law $\Sigma= \Sigma\_0 + k \sqrt{Wi}$, with yield stress
$\Sigma\_0\textgreater{}0$. In the underdamped (inertial) regime, dramatic
changes in the rheology are observed for low damping: the flow curve becomes
non-monotonic. This change is not caused by longer-lived correlations in the
particle dynamics at lower damping; instead, for weak dissipation, the sample
heats up considerably due to, and in proportion to, the driving. By suitably
thermostatting more or less underdamped systems, we show that their rheology
only depends on their kinetic temperature and the shear rate, rescaled with
Einstein's vibration frequency.
|
1508.06067v3
|
2015-09-01
|
Persistence of a Non-Equilibrium State: Observation of a Boltzmannian Special Case
|
Well before the atomistic nature of matter was experimentally established,
Ludwig Boltzmann's audacious effort to explain the macroscopic world of human
experience in terms of the workings of an unseen microscopic world met with
vigorous opposition. A contentious point was the problem of irreversibility:
the microscopic equations of motion are reversible, yet friction and viscosity
cause things always to slow down and warm up, never to speed up and cool down.
What was worse, Boltzmann himself discovered that his transport equation
predicts special cases in which gases never come to thermal equilibrium, a
particular example being that the monopole "breathe" mode of gas will never
damp if it is confined in 3D to a perfectly isotropic harmonic potential. Such
absences of damping were not observed in nature. Nondamping of a monopole mode
in lower dimensional systems has only very recently been observed, using cold
atoms. Kinoshita et al. and Chevy et al. have experimentally observed
suppressed relaxation in highly elongated geometries. The difficulty in
generating sufficiently spherical harmonic confinement for ultracold atoms,
however, has meant that Boltzmann's fully 3D, isotropic case has never been
observed. With the development of a new magnetic trap capable of producing
near-spherical harmonic confinement for ultracold atoms, we have been able to
make the first observation of this historically significant oddity. We observe
a monopole mode for which the collisional contribution to damping vanishes, a
long-delayed vindication for Boltzmann's microscopic theory.
|
1509.00366v1
|
2015-09-14
|
Nonlinear Energetic Particle Transport in the Presence of Multiple Alfvenic Waves in ITER
|
This work presents the results of a multi mode ITER study on Toroidal Alfven
Eigenmodes, using the nonlinear hybrid HAGIS-LIGKA model. It is found that main
conclusions from earlier studies of ASDEX Upgrade discharges can be transferred
to the ITER scenario: global, nonlinear effects are crucial for the evolution
of the multi mode scenario. This work focuses on the ITER 15 MA baseline
scenario with with a safety factor at the magnetic axis of $q_0 =$ 0.986. The
least damped eigenmodes of the system are identified with the gyrokinetic,
non-perturbative LIGKA solver, concerning mode structure, frequency and
damping. Taking into account all weakly damped modes that can be identified
linearly, nonlinear simulations with HAGIS reveal strong multi mode behavior:
while in some parameter range, quasi-linear estimates turn out to be reasonable
approximations for the nonlinearly relaxed energetic particle profile, under
certain conditions low-n TAE branches can be excited. As a consequence, not
only grow amplitudes of all modes to (up to orders of magnitude) higher values
compared to the single mode cases but also, strong redistribution is triggered
in the outer radial area between $\rho_\mathrm{pol} =$ 0.6 and 0.85, far above
quasi-linear estimates.
|
1509.04010v1
|
2015-09-30
|
Dynamic Quantum Tomography Model for Phase-Damping Channels
|
In this article we propose a dynamic quantum tomography model for open
quantum systems with evolution given by phase-damping channels. Mathematically,
these channels correspond to completely positive trace-preserving maps defined
by the Hadamard product of the initial density matrix with a time-dependent
matrix which carries the knowledge about the evolution. Physically, there is a
strong motivation for considering this kind of evolution because such channels
appear naturally in the theory of open quantum systems. The main idea behind a
dynamic approach to quantum tomography claims that by performing the same kind
of measurement at some time instants one can obtain new data for state
reconstruction. Thus, this approach leads to a decrease in the number of
distinct observables which are required for quantum tomography; however, the
exact benefit for employing the dynamic approach depends strictly on how the
quantum system evolves in time. Algebraic analysis of phase-damping channels
allows one to determine optimal criteria for quantum tomography of systems in
question. General theorems and observations presented in the paper are
accompanied by a specific example, which shows step by step how the theory
works. The results introduced in this article can potentially be applied in
experiments where there is a tendency a look at quantum tomography from the
point of view of economy of measurements, because each distinct kind of
measurement requires, in general, preparing a separate setup.
|
1509.09318v3
|
2015-10-24
|
Propagation of GeV neutrinos through Earth
|
We have studied the Earth matter effect on the oscillation of upward going
GeV neutrinos by taking into account the three active neutrino flavors. For
neutrino energy in the range 3 to 12 GeV we observed three distinct resonant
peaks for the oscillation process $\nu_e\leftrightarrow \nu_{\mu,\tau}$ in
three \textit{distinct} densities. However, according to the most realistic
density profile of the Earth, the second peak at neutrino energy 6.18 GeV
corresponding to the density $6.6\,g/cm^3$ does not exist. So the resonance at
this energy can not be of MSW-type. For the calculation of observed flux of
these GeV neutrinos on Earth, we considered two different flux ratios at the
source, the standard scenario with the flux ratio $1:2:0$ and the muon damped
scenario with $0:1:0$. It is observed that at the detector while the standard
scenario gives the observed flux ratio $1:1:1$, the muon damped scenario has a
different ratio. For muon damped case with $E_{\nu} < 20$ GeV, we always get
observed neutrino fluxes as $\Phi_{\nu_e} < \Phi_{\nu_\mu}\simeq
\Phi_{\nu_\tau}$ and for $E_{\nu} > 20$ GeV, we get the average
$\Phi_{\nu_e}\sim 0$ and $\Phi_{\nu_\mu}\simeq \Phi_{\nu_\tau}\simeq 0.45$. The
upcoming PINGU will be able to shed more light on the nature of the resonance
in these GeV neutrinos and hopefully will also be able to discriminate among
different processes of neutrino production at the source in GeV energy range.
|
1510.07103v2
|
2015-11-18
|
Surface waves propagation on a turbulent flow forced electromagnetically
|
We study the propagation of monochromatic surface waves on a turbulent flow.
The flow is generated in a layer of liquid metal by an electromagnetic forcing.
This forcing creates a quasi two-dimensional (2D) turbulence with strong
vertical vorticity. The turbulent flow contains much more energy than the
surface waves. In order to focus on the surface wave, the deformations induced
by the turbulent flow are removed. This is done by performing a coherent phase
averaging. For wavelengths smaller than the forcing lengthscale, we observe a
significant increase of the wavelength of the propagating wave that has not
been reported before. We suggest that it can be explained by the random
deflection of the wave induced by the velocity gradient of the turbulent flow.
Under this assumption, the wavelength shift is an estimate of the fluctuations
of deflection angle. The local measurements of the wave frequency far from the
wavemaker do not reveal such systematic behavior, although a small shift is
measured. Finally we quantify the damping enhancement induced by the turbulent
flow. We review various theoretical scaling laws proposed previously. Most of
them propose a damping that increases as the square of Froude number. In
contrast, our experimental results show a turbulent damping increasing linearly
with the Froude number. We interpret this linear behaviour as a balance between
the time spent by a wave to cross a turbulent structure with the turbulent
mixing time. The larger is the ratio of these 2 times, the more energy is
extracted from the progressive wave. Finally, mechanisms of energy exchange and
open issues are discussed and further studies are proposed.
|
1511.05900v1
|
2015-12-15
|
Correlations between compositions and orbits established by the giant impact era of planet formation
|
The giant impact phase of terrestrial planet formation establishes
connections between super-Earths' orbital properties (semimajor axis spacings,
eccentricities, mutual inclinations) and interior compositions (the presence or
absence of gaseous envelopes). Using N-body simulations and analytic arguments,
we show that spacings derive not only from eccentricities, but also from
inclinations. Flatter systems attain tighter spacings, a consequence of an
eccentricity equilibrium between gravitational scatterings, which increase
eccentricities, and mergers, which damp them. Dynamical friction by residual
disk gas plays a critical role in regulating mergers and in damping
inclinations and eccentricities. Systems with moderate gas damping and high
solid surface density spawn gas-enveloped super-Earths with tight spacings,
small eccentricities, and small inclinations. Systems in which super-Earths
coagulate without as much ambient gas, in disks with low solid surface density,
produce rocky planets with wider spacings, larger eccentricities, and larger
mutual inclinations. A combination of both populations can reproduce the
observed distributions of spacings, period ratios, transiting planet
multiplicities, and transit duration ratios exhibited by Kepler super-Earths.
The two populations, both formed in situ, also help to explain observed trends
of eccentricity vs. planet size, and bulk density vs. method of mass
measurement (radial velocities vs. transit timing variations). Simplifications
made in this study --- including the limited timespan of the simulations, and
the approximate treatments of gas dynamical friction and gas depletion history
--- should be improved upon in future work to enable a detailed quantitative
comparison to the observations.
|
1512.04951v2
|
2015-12-18
|
Seismic waves damping with arrays of inertial resonators
|
We investigate the elastic stop band properties of a theoretical cubic array
of iron spheres connected to a bulk of concrete via iron or rubber ligaments.
Each sphere can move freely within a surrounding air cavity, but ligaments
couple it to the bulk and further facilitate bending and rotational motions.
Associated low frequency local resonances are well predicted by an asymptotic
formula. We find complete stop bands (for all wave-polarizations) in the
frequency range $[16,21]$ Hertz (resp. $[6,11]$ Hertz) for $7.4$-meter (resp.
$0.74$-meter) diameter iron spheres with a $10$-meter (resp. $1$-meter)
center-to-center spacing, when they are connected to concrete via steel (resp.
rubber) ligaments. The scattering problem shows that only bending modes are
responsible for damping and that rotational modes are totally overwritten by
bending modes. Regarding seismic applications, we further consider soil as a
bulk medium, in which case the relative bandwidth of the low frequency stop
band can be enlarged through ligaments of different sizes that allow for well
separated bending and rotational modes. We finally achieve some damping of
elastodynamic waves from $8$ to $49$ Hertz (relative stop band of $143$
percent) for iron spheres $0.74$-meter in diameter that are connected to soil
with six rubber ligaments of optimized shapes. These results represent a
preliminary step in the design of seismic shields placed around, or underneath,
the foundations of large civil infrastructures.
|
1512.06078v2
|
2016-01-08
|
Kinetic Simulation of Slow Magnetosonic Waves and Quasi-periodic Upflows in the Solar Corona
|
Quasi-periodic disturbances of emission-line parameters are frequently
observed in the corona. These disturbances propagate upward along the magnetic
field with speeds $\sim100~\rm{km~s}^{-1}$. This phenomenon has been
interpreted as evidence of the propagation of slow magnetosonic waves or argued
to be signature of the intermittent outflows superposed on the background
plasmas. Here we aim to present a new "wave + flow" model to interpret these
observations. In our scenario, the oscillatory motion is a slow mode wave, and
the flow is associated with a beam created by the wave-particle interaction
owing to Landau resonance. With the help of a Vlasov model, we simulate the
propagation of the slow mode wave and the generation of the beam flow. We find
that weak periodic beam flows can be generated owing to Landau resonance in the
solar corona, and the phase with strongest blueward asymmetry is ahead of that
with strongest blueshift by about 1/4 period. We also find that the slow wave
damps to the level of 1/e after the transit time of two wave periods, owing to
Landau damping and Coulomb collisions in our simulation. This damping time
scale is similar to that resulting from thermal-conduction in the
magnetohydrodynamics regime. The beam flow is weakened/attenuated with
increasing wave period and decreasing wave amplitude since Coulomb collision
becomes more and more dominant over the wave action. We suggest that this "wave
+ flow" kinetic model provides an alternative explanation for the observed
quasi-periodic propagating perturbations in various parameters in the solar
corona.
|
1601.01823v1
|
2016-01-25
|
Comprehensive evaluation of the linear stability of Alfvén eigenmodes driven by alpha particles in an ITER baseline scenario
|
The linear stability of Alfv\'en eigenmodes in the presence of fusion-born
alpha particles is thoroughly assessed for two variants of an ITER baseline
scenario, which differ significantly in their core and pedestal temperatures. A
systematic approach is used that considers all possible eigenmodes for a given
magnetic equilibrium and determines their growth rates due to alpha-particle
drive and Landau damping on fuel ions, helium ashes and electrons. This
extensive stability study is efficiently conducted through the use of a
specialized workflow that profits from the performance of the hybrid MHD
drift-kinetic code $\mbox{CASTOR-K}$ (Borba D. and Kerner W. 1999 J. Comput.
Phys. ${\bf 153}$ 101; Nabais F. ${\it et\,al}$ 2015 Plasma Sci. Technol. ${\bf
17}$ 89), which can rapidly evaluate the linear growth rate of an eigenmode. It
is found that the fastest growing instabilities in the aforementioned ITER
scenario are core-localized, low-shear toroidal Alfv\'en eigenmodes. The
largest growth-rates occur in the scenario variant with higher core
temperatures, which has the highest alpha-particle density and density
gradient, for eigenmodes with toroidal mode numbers $n\approx30$. Although
these eigenmodes suffer significant radiative damping, which is also evaluated,
their growth rates remain larger than those of the most unstable eigenmodes
found in the variant of the ITER baseline scenario with lower core
temperatures, which have $n\approx15$ and are not affected by radiative
damping.
|
1601.06621v1
|
2016-01-27
|
Flash ionisation signature in coherent cyclotron emission from Brown Dwarfs
|
Brown dwarfs form mineral clouds in their atmospheres, where charged
particles can produce large-scale discharges in form of lightning resulting in
a substantial sudden increase of local ionisation. Brown dwarfs are observed to
emit cyclotron radio emission. We show that signatures of strong transient
atmospheric ionisation events (flash ionisation) can be imprinted on a
pre-existing radiation. Detection of such flash ionisation events will open
investigations into the ionisation state and atmospheric dynamics. Such
ionisation events can also result from explosion shock waves, bursts or
eruptions. We present an analytical model that describes the modulation of a
pre-existing electromagnetic radiation by a time-dependent (flash) conductivity
that is characteristic for flash ionisation events like lightning. Our
conductivity model reproduces the conductivity function derived from
observations of Terrestrial Gamma Ray Flashes, and is applicable to
astrophysical objects with strong temporal variations in the local ionization,
as in planetary atmospheres and protoplanetary disks. We show that the field
responds with a characteristic flash-shaped pulse to a conductivity flash of
intermediate intensity. More powerful ionisation events result in smaller
variations of the initial radiation, or in its damping. We show that the
characteristic damping of the response field for high-power initial radiation
carries information about the ionisation flash magnitude and duration. The
duration of the pulse amplification or the damping is consistently shorter for
larger conductivity variations and can be used to evaluate the intensity of the
flash ionisation. Our work suggests that cyclotron emission could be probe
signals for electrification processes inside BD atmosphere.
|
1601.07474v1
|
2016-02-02
|
Sudden-quench dynamics of Bardeen-Cooper-Schrieffer states in deep optical lattices
|
We determine the exact dynamics of an initial Bardeen-Cooper-Schrieffer (BCS)
state of ultra-cold atoms in a deep hexagonal optical lattice. The dynamical
evolution is triggered by a quench of the lattice potential, such that the
interaction strength $U_f$ is much larger than the hopping amplitude $J_f$. The
quench initiates collective oscillations with frequency $|U_f|/(2\pi)$ in the
momentum occupation numbers and imprints an oscillating phase with the same
frequency on the BCS order parameter $\Delta$. The oscillation frequency of
$\Delta$ is not reproduced by treating the time evolution in mean-field theory.
In our theory, the momentum noise (i.e. density-density) correlation functions
oscillate at frequency $|U_f|/2\pi$ as well as at its second harmonic. For a
very deep lattice, with zero tunneling energy, the oscillations of momentum
occupation numbers are undamped. Non-zero tunneling after the quench leads to
dephasing of the different momentum modes and a subsequent damping of the
oscillations. The damping occurs even for a finite-temperature initial BCS
state, but not for a non-interacting Fermi gas. Furthermore, damping is
stronger for larger order parameter and may therefore be used as a signature of
the BCS state. Finally, our theory shows that the noise correlation functions
in a honeycomb lattice will develop strong anti-correlations near the Dirac
point.
|
1602.00979v2
|
2016-03-09
|
Conclusive Identification of Quantum Channels via Monogamy of Quantum Correlations
|
We investigate the action of local and global noise on monogamy of quantum
correlations, when monogamy scores are considered as observables, and
three-qubit systems are subjected to global noise and various local noisy
channels, namely, amplitude-damping, phase-damping, and depolarizing channels.
We show that the dynamics of monogamy scores corresponding to negativity and
quantum discord, in the case of generalized W states, as inputs to the noisy
channels, can exhibit non-monotonic dynamics with respect to increasing noise
parameter, which is in contrast to the monotonic decay of monogamy scores when
generalized Greenberger-Horne-Zeilinger states are exposed to noise. We
quantify the persistence of monogamy against noise via a characteristic value
of the noise parameter, and show that depolarizing noise destroys monogamy of
quantum correlation faster compared to other noisy channels. We demonstrate
that the negativity monogamy score is more robust than the quantum discord
monogamy score, when the noise is of the phase-damping type. We also
investigate the variation of monogamy with increasing noise for arbitrary
three-qubit pure states as inputs. Finally, depending on these results, we
propose a two-step protocol, which can conclusively identify the type of noise
applied to the quantum system, by using generalized Greenberger-Horne-Zeilinger
and generalized W states as resource states. We discuss a possible
generalization of the results to higher number of parties.
|
1603.02801v3
|
2016-05-05
|
Electromagnetic fields in the exterior of an oscillating relativistic star -- II. Electromagnetic damping
|
An important issue in the asteroseismology of compact and magnetized stars is
the determination of the dissipation mechanism which is most efficient in
damping the oscillations when these are produced. In a linear regime and for
low-multipolarity modes these mechanisms are confined to either
gravitational-wave or electromagnetic losses. We here consider the latter and
compute the energy losses in the form of Poynting fluxes, Joule heating and
Ohmic dissipation in a relativistic oscillating spherical star with a dipolar
magnetic field in vacuum. While this approach is not particularly realistic for
rapidly rotating stars, it has the advantage that it is fully analytic and that
it provides expressions for the electric and magnetic fields produced by the
most common modes of oscillation both in the vicinity of the star and far away
from it. In this way we revisit and extend to a relativistic context the
classical estimates of McDermott et al. Overall, we find that
general-relativistic corrections lead to electromagnetic damping time-scales
that are at least one order of magnitude smaller than in Newtonian gravity.
Furthermore, with the only exception of $g$ (gravity) modes, we find that $f$
(fundamental), $p$ (pressure), $i$ (interface) and $s$ (shear) modes are
suppressed more efficiently by gravitational losses than by electromagnetic
ones.
|
1605.01709v1
|
2016-05-22
|
Low Gilbert damping in Co2FeSi and Fe2CoSi films
|
Thin highly textured Fe$_{\mathrm{1+x}}$Co$_{\mathrm{2-x}}$Si ($0 \leq$ x
$\leq 1$) films were prepared on MgO (001) substrates by magnetron
co-sputtering. The magneto-optic Kerr effect (MOKE) and ferromagnetic resonance
(FMR) measurements were used to investigate the composition dependence of the
magnetization, the magnetic anisotropy, the gyromagnetic ratio and the
relaxation of the films. The effective magnetization for the thin
Fe$_{\mathrm{1+x}}$Co$_{\mathrm{2-x}}$Si films, determined by FMR measurements,
are consistent with the Slater Pauling prediction. Both MOKE and FMR
measurements reveal a pronounced fourfold anisotropy distribution for all
films. In addition we found a strong influence of the stoichiometry on the
anisotropy as the cubic anisotropy strongly increases with increasing Fe
concentration. The gyromagnetic ratio is only weakly dependent on the
composition. We find low Gilbert damping parameters for all films with values
down to $0.0012\pm0.00012$ for Fe$_{1.75}$Co$_{1.25}$Si. The effective damping
parameter for Co$_2$FeSi is found to be $0.0018\pm 0.0004$. We also find a
pronounced anisotropic relaxation, which indicates significant contributions of
two-magnon scattering processes that is strongest along the easy axes of the
films. This makes thin Fe$_{\mathrm{1+x}}$Co$_{\mathrm{2-x}}$Si films ideal
materials for the application in STT-MRAM devices.
|
1605.06797v1
|
2016-06-14
|
Landau damping to partially locked states in the Kuramoto model
|
In the Kuramoto model of globally coupled oscillators, partially locked
states (PLS) are stationary solutions that incorporate the emergence of partial
synchrony when the interaction strength increases. While PLS have long been
considered, existing results on their stability are limited to neutral
stability of the linearized dynamics in strong topology, or to specific
invariant subspaces (obtained via the so-called Ott-Antonsen (OA) ansatz) with
specific frequency distributions for the oscillators. In the mean field limit,
the Kuramoto model shows various ingredients of the Landau damping mechanism in
the Vlasov equation. This analogy has been a source of inspiration for
stability proofs of regular Kuramoto equilibria. Besides, the major
mathematical issue with PLS asymptotic stability is that these states consist
of heterogeneous and singular measures. Here, we establish an explicit
criterion for their spectral stability and we prove their local asymptotic
stability in weak topology, for a large class of analytic frequency marginals.
The proof strongly relies on a suitable functional space that contains (Fourier
transforms of) singular measures, and for which the linearized dynamics is well
under control. For illustration, the stability criterion is evaluated in some
standard examples. We show in particular that no loss of generality results in
assuming the OA ansatz. To our best knowledge, our result provides the first
proof of Landau damping to heterogeneous and irregular equilibria, in absence
of dissipation.
|
1606.04470v1
|
2016-07-20
|
Electromagnon in the Z-type hexaferrite $({\rm Ba}_{x}{\rm Sr}_{1-x})_3\rm Co_2Fe_{24}O_{41}$
|
We studied experimentally the high-temperature magnetoelectric $({\rm
Ba}_{x}{\rm Sr}_{1-x})_3\rm Co_2Fe_{24}O_{41}$ prepared as ceramics (x = 0,
0.2) and a single crystal (x = 0.5) using inelastic neutron scattering, THz
time-domain, Raman and far-infrared spectroscopies. The spectra, measured with
varying temperature and magnetic field, reveal rich information about the
collective spin and lattice excitations. In the ceramics, we observed an
infrared-active magnon which is absent in $E^{\omega}\perp z$ polarized THz
spectra of the crystal, and we assume that it is an electromagnon active in
$E^{\omega} \| z$ polarized spectra. On heating from 7 to 250 K, the frequency
of this electromagnon drops from 36 to 25 cm$^{-1}$ and its damping gradually
increases, so it becomes overdamped at room temperature. Applying external
magnetic field has a similar effect on the damping and frequency of the
electromagnon, and the mode is no more observable in the THz spectra above 2 T,
as the transverse-conical magnetic structure transforms into a collinear one.
Raman spectra reveal another spin excitation with a slightly different
frequency and much higher damping. Upon applying magnetic field higher than 3
T, in the low-frequency part of the THz spectra, a narrow excitation appears
whose frequency linearly increases with magnetic field. We interpret this
feature as the ferromagnetic resonance.
|
1607.05878v1
|
2016-07-28
|
Damping of the Milky Way bar by manifold-driven spirals
|
We describe a new phenomenon of `bar damping' that may have played an
important role in shaping the Milky Way bar and bulge as well as its spiral
structure. We use a collisionless N-body simulation of a Milky Way-like galaxy
initially composed of a dark matter halo and an exponential disk with Toomre
parameter slightly above unity. In this configuration, dominated by the disk in
the center, a bar forms relatively quickly, after 1 Gyr of evolution. This is
immediately followed by the formation of two manifold-driven spiral arms and
the outflow of stars that modifies the potential in the vicinity of the bar,
apparently shifting the position of the L_1/L_2 Lagrange points. This
modification leads to the shortening of the bar and the creation of a next
generation of manifold-driven spiral arms at a smaller radius. The process
repeats itself a few times over the next 0.5 Gyr resulting in further
substantial weakening and shortening of the bar. The time when the damping
comes to an end coincides with the first buckling episode in the bar which
rebuilds the orbital structure so that no more new spiral arms are formed. The
morphology of the bar and the spiral structure at this time show remarkable
similarity to the present properties of the Milky Way. Later on, the bar starts
to grow rather steadily again, weakened only by subsequent buckling episodes
occurring at more distant parts of the disk.
|
1607.08339v2
|
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