publicationDate
stringlengths 1
2.79k
| title
stringlengths 1
36.5k
⌀ | abstract
stringlengths 1
37.3k
⌀ | id
stringlengths 9
47
|
|---|---|---|---|
2016-08-03
|
Ultra-Low Dissipation Superfluid Micromechanical Resonator
|
Micro and nanomechanical resonators with ultra-low dissipation have great
potential as useful quantum resources. The superfluid micromechanical
resonators presented here possess several advantageous characteristics:
straightforward thermalization, dissipationless flow, and in situ tunability.
We identify and quantitatively model the various dissipation mechanisms in two
resonators, one fabricated from borosilicate glass and one from single crystal
quartz. As the resonators are cryogenically cooled into the superfluid state,
the damping from thermal effects and from the normal fluid component are
strongly suppressed. At our lowest temperatures, damping is limited solely by
internal dissipation in the substrate materials, and reach quality factors up
to 913,000 at 13 mK. By lifting this limitation through substrate material
choice and resonator design, modelling suggests that the resonators should
reach quality factors as high as 10$^8$ at 100 mK, putting this architecture in
an ideal position to harness mechanical quantum effects.
|
1608.01380v1
|
2016-08-14
|
A second order dynamical system with Hessian-driven damping and penalty term associated to variational inequalities
|
We consider the minimization of a convex objective function subject to the
set of minima of another convex function, under the assumption that both
functions are twice continuously differentiable. We approach this optimization
problem from a continuous perspective by means of a second order dynamical
system with Hessian-driven damping and a penalty term corresponding to the
constrained function. By constructing appropriate energy functionals, we prove
weak convergence of the trajectories generated by this differential equation to
a minimizer of the optimization problem as well as convergence for the
objective function values along the trajectories. The performed investigations
rely on Lyapunov analysis in combination with the continuous version of the
Opial Lemma. In case the objective function is strongly convex, we can even
show strong convergence of the trajectories.
|
1608.04137v1
|
2016-08-15
|
Power requirements for cosmic ray propagation models involving diffusive reacceleration; estimates and implications for the damping of interstellar turbulence
|
We make quantitative estimates of the power supplied to the Galactic cosmic
ray population by second-order Fermi acceleration in the interstellar medium,
or as it is usually termed in cosmic ray propagation studies, diffusive
reacceleration. Using recent results on the local interstellar spectrum from
the Voyager missions we show that for parameter values, in particular the
Alfv\'en speed, typically used in propagation codes such as Galprop to fit the
B/C ratio, the power contributed by diffusive reacceleration is significant and
can be of order 50\% of the total Galactic cosmic ray power. The implications
for the damping of interstellar turbulence are briefly considered.
|
1608.04227v2
|
2016-09-01
|
Observation of oscillatory radial electric field relaxation in a helical plasma
|
Measurements of the relaxation of a zonal electrostatic potential
perturbation in a non-axisymmetric magnetically confined plasma are presented.
A sudden perturbation of the plasma equilibrium is induced by the injection of
a cryogenic hydrogen pellet in the TJ-II stellarator, which is observed to be
followed by a damped oscillation in the electrostatic potential. The waveform
of the relaxation is consistent with theoretical calculations of zonal
potential relaxation in a non-axisymmetric magnetic geometry. The turbulent
transport properties of a magnetic confinement configuration are expected to
depend on the features of the collisionless damping of zonal flows, of which
the present letter is the first direct observation.
|
1609.00281v1
|
2016-09-10
|
Stochastic Sampling for Structural Topology Optimization with Many Load Cases: Density-Based and Ground Structure Approaches
|
We propose an efficient probabilistic method to solve a deterministic problem
-- we present a randomized optimization approach that drastically reduces the
enormous computational cost of optimizing designs under many load cases for
both continuum and truss topology optimization. Practical structural designs by
topology optimization typically involve many load cases, possibly hundreds or
more. The optimal design minimizes a, possibly weighted, average of the
compliance under each load case (or some other objective). This means that in
each optimization step a large finite element problem must be solved for each
load case, leading to an enormous computational effort. On the contrary, the
proposed randomized optimization method with stochastic sampling requires the
solution of only a few (e.g., 5 or 6) finite element problems (large linear
systems) per optimization step. Based on simulated annealing, we introduce a
damping scheme for the randomized approach. Through numerical examples in two
and three dimensions, we demonstrate that the stochastic algorithm drastically
reduces computational cost to obtain similar final topologies and results
(e.g., compliance) compared with the standard algorithms. The results indicate
that the damping scheme is effective and leads to rapid convergence of the
proposed algorithm.
|
1609.03099v1
|
2016-09-15
|
Low-damping sub-10-nm thin films of lutetium iron garnet grown by molecular-beam epitaxy
|
We analyze the structural and magnetic characteristics of (111)-oriented
lutetium iron garnet (Lu$_3$Fe$_5$O$_{12}$) films grown by molecular-beam
epitaxy, for films as thin as 2.8 nm. Thickness-dependent measurements of the
in- and out-of-plane ferromagnetic resonance allow us to quantify the effects
of two-magnon scattering, along with the surface anisotropy and the saturation
magnetization. We achieve effective damping coefficients of $11.1(9) \times
10^{-4}$ for 5.3 nm films and $32(3) \times 10^{-4}$ for 2.8 nm films, among
the lowest values reported to date for any insulating ferrimagnetic sample of
comparable thickness.
|
1609.04753v1
|
2016-09-18
|
Inertia effects in the real-time dynamics of a quantum spin coupled to a Fermi sea
|
Spin dynamics in the Kondo impurity model, initiated by suddenly switching
the direction of a local magnetic field, is studied by means of the
time-dependent density-matrix renormalization group. Quantum effects are
identified by systematic computations for different spin quantum numbers $S$
and by comparing with tight-binding spin-dynamics theory for the classical-spin
Kondo model. We demonstrate that, besides the conventional precessional motion
and relaxation, the quantum-spin dynamics shows nutation, similar to a spinning
top. Opposed to semiclassical theory, however, the nutation is efficiently
damped on an extremely short time scale. The effect is explained in the
large-$S$ limit as quantum dephasing of the eigenmodes in an emergent two-spin
model that is weakly entangled with the bulk of the system. We argue that,
apart from the Kondo effect, the damping of nutational motion is essentially
the only characteristics of the quantum nature of the spin. Qualitative
agreement between quantum and semiclassical spin dynamics is found down to
$S=1/2$.
|
1609.05526v1
|
2016-09-21
|
Force sensitivity of multilayer graphene optomechanical devices
|
Mechanical resonators based on low-dimensional materials are promising for
force and mass sensing experiments. The force sensitivity in these ultra-light
resonators is often limited by the imprecision in the measurement of the
vibrations, the fluctuations of the mechanical resonant frequency, and the
heating induced by the measurement. Here, we strongly couple multilayer
graphene resonators to superconducting cavities in order to achieve a
displacement sensitivity of $1.3$ fm Hz$^{-1/2}$. This coupling also allows us
to damp the resonator to an average phonon occupation of $7.2$. Our best force
sensitivity, $390$ zN Hz$^{-1/2}$ with a bandwidth of $200$ Hz, is achieved by
balancing measurement imprecision, optomechanical damping, and heating. Our
results hold promise for studying the quantum capacitance of graphene, its
magnetization, and the electron and nuclear spins of molecules adsorbed on its
surface.
|
1609.06517v1
|
2016-09-26
|
Ferromagnetic resonance study of composite Co/Ni - FeCoB free layers with perpendicular anisotropy
|
We study the properties of composite free layers with perpendicular
anisotropy. The free layers are made of a soft FeCoB layer ferromagnetically
coupled by a variable spacer (Ta, W, Mo) to a very anisotropic [Co/Ni]
multilayer embodied in a magnetic tunnel junction meant for spin torque memory
applications. For this we use broadband ferromagnetic resonance to follow the
field dependence of the acoustical and optical excitation of the composite free
layer in both in-plane and out-of-plane applied fields. The modeling provides
the interlayer exchange coupling, the anisotropies and the damping factors. The
popular Ta spacer are outperformed by W and even more by Mo, which combines the
strongest interlayer exchange coupling without sacrificing anisotropies,
damping factors and transport properties.
|
1609.07863v1
|
2016-10-03
|
Inertia and universality of avalanche statistics: The case of slowly deformed amorphous solids
|
By means of a finite elements technique we solve numerically the dynamics of
an amorphous solid under deformation in the quasistatic driving limit. We study
the noise statistics of the stress-strain signal in the steady state plastic
flow, focusing on systems with low internal dissipation. We analyze the
distributions of avalanche sizes and durations and the density of shear
transformations when varying the damping strength. In contrast to avalanches in
the overdamped case, dominated by the yielding point universal exponents,
inertial avalanches are controlled by a non-universal damping dependent
feedback mechanism; eventually turning negligible the role of correlations.
Still, some general properties of avalanches persist and new scaling relations
can be proposed.
|
1610.00533v2
|
2016-10-04
|
Statistical properties of damped Lyman-alpha systems from Sloan Digital Sky Survey DR12
|
We present new estimates for the statistical properties of damped
Lyman-$\alpha$ absorbers (DLAs). We compute the column density distribution
function at $z>2$, the line density, $\mathrm{d}N/\mathrm{d}X$, and the neutral
hydrogen density, $\Omega_\mathrm{DLA}$. Our estimates are derived from the DLA
catalogue of Garnett 2016, which uses the SDSS-III DR12 quasar spectroscopic
survey. This catalogue provides a probability that a given spectrum contains a
DLA, allowing us to use even the noisiest data without biasing our results and
thus substantially increase our sample size. We measure a non-zero column
density distribution function at $95\%$ confidence for all column densities
$N_\mathrm{HI} < 5\times 10^{22}$ cm$^{-2}$. We make the first measurements
from SDSS of $\mathrm{d}N/\mathrm{d}X$ and $\Omega_\mathrm{DLA}$ at $z>4$. We
show that our results are insensitive to the signal-to-noise ratio of the
spectra, but that there is a residual dependence on quasar redshift for
$z<2.5$, which may be due to remaining systematics in our analysis.
|
1610.01165v2
|
2016-10-05
|
Higher-Harmonic Collective Modes in a Trapped Gas from Second-Order Hydrodynamics
|
Utilizing a second-order hydrodynamics formalism, the dispersion relations
for the frequencies and damping rates of collective oscillations as well as
spatial structure of these modes up to the decapole oscillation in both two-
and three- dimensional gas geometries are calculated. In addition to
higher-order modes, the formalism also gives rise to purely damped
"non-hydrodynamic" modes. We calculate the amplitude of the various modes for
both symmetric and asymmetric trap quenches, finding excellent agreement with
an exact quantum mechanical calculation. We find that higher-order hydrodynamic
modes are more sensitive to the value of shear viscosity, which may be of
interest for the precision extraction of transport coefficients in Fermi gas
systems.
|
1610.01611v2
|
2016-10-10
|
Robust force sensing for a free particle in a dissipative optomechanical system with a parametric amplifier
|
We theoretically investigate optical detection of a weak classical force
acting on a free particle in a dissipative coupling optomechanical system with
a degenerate parametric amplifier (PA). We show that the PA allows one to
achieve the force sensitivity far better than the standard quantum limit (SQL)
over a broad range of the detection frequencies. The improvement depends on the
parametric gain and the driving power. Moreover, we discuss the effects of the
mechanical damping and the thermal noise on the force sensitivity. We find that
the robustness of the force sensitivity much better than the SQL against the
mechanical damping and the thermal noise is achievable in the presence of the
PA with a high parametric gain. For the temperature $T = 1$ K, the improvement
in sensitivity is better by a factor of about 7 when the driving power is set
at a value corresponding to the SQL with no PA.
|
1610.02761v2
|
2016-10-12
|
Stabilization of the Gear-Grimshaw system with weak damping
|
The aim of this work is to consider the internal stabilization of a nonlinear
coupled system of two Korteweg--de Vries equations in a finite interval under
the effect of a very weak localized damping. The system was introduced by Gear
and Grimshaw to model the interactions of two-dimensional, long, internal
gravity waves propagation in a stratified fluid. Considering feedback controls
laws and using Compactness--Uniqueness Argument, which reduce the problem to
use a unique continuation property, we establish the exponential stability of
the weak solutions when the exponent in the nonlinear term ranges over the
interval $[1,4)$.
|
1610.03829v4
|
2016-10-14
|
Nambu mechanics for stochastic magnetization dynamics
|
The Landau-Lifshitz-Gilbert (LLG) equation describes the dynamics of a damped
magnetization vector that can be understood as a generalization of Larmor spin
precession. The LLG equation cannot be deduced from the Hamiltonian framework,
by introducing a coupling to a usual bath, but requires the introduction of
additional constraints. It is shown that these constraints can be formulated
elegantly and consistently in the framework of dissipative Nambu mechanics.
This has many consequences for both the variational principle and for
topological aspects of hidden symmetries that control conserved quantities. We
particularly study how the damping terms of dissipative Nambu mechanics affect
the consistent interaction of magnetic systems with stochastic reservoirs and
derive a master equation for the magnetization. The proposals are supported by
numerical studies using symplectic integrators that preserve the topological
structure of Nambu equations. These results are compared to computations
performed by direct sampling of the stochastic equations and by using closure
assumptions for the moment equations, deduced from the master equation.
|
1610.04598v2
|
2016-10-19
|
Heavy-tailed response of structural systems subjected to stochastic excitation containing extreme forcing events
|
We characterize the complex, heavy-tailed probability distribution functions
(pdf) describing the response and its local extrema for structural systems
subjected to random forcing that includes extreme events. Our approach is based
on the recent probabilistic decomposition-synthesis technique, where we
decouple rare events regimes from the background fluctuations. The result of
the analysis has the form of a semi-analytical approximation formula for the
pdf of the response (displacement, velocity, and acceleration) and the pdf of
the local extrema. For special limiting cases (lightly damped or heavily damped
systems) our analysis provides fully analytical approximations. We also
demonstrate how the method can be applied to high dimensional structural
systems through a two-degrees-of-freedom structural system undergoing rare
events due to intermittent forcing. The derived formulas can be evaluated with
very small computational cost and are shown to accurately capture the
complicated heavy-tailed and asymmetrical features in the probability
distribution many standard deviations away from the mean, through comparisons
with expensive Monte-Carlo simulations.
|
1610.06110v3
|
2016-10-27
|
Thermally activated phase slips of one-dimensional Bose gases in shallow optical lattices
|
We study the decay of superflow via thermally activated phase slips in
one-dimensional Bose gases in a shallow optical lattice. By using the Kramers
formula, we numerically calculate the nucleation rate of a thermally activated
phase slip for various values of the filling factor and flow velocity in the
absence of a harmonic trapping potential. Within the local density
approximation, we derive a formula connecting the phase-slip nucleation rate
with the damping rate of a dipole oscillation of the Bose gas in the presence
of a harmonic trap. We use the derived formula to directly compare our theory
with the recent experiment done by the LENS group [L. Tanzi, et al., Sci. Rep.
{\bf 6}, 25965 (2016)]. From the comparison, the observed damping of dipole
oscillations in a weakly correlated and small velocity regime is attributed
dominantly to thermally activated phase slips rather than quantum phase slips.
|
1610.08982v3
|
2016-10-28
|
Damping-free collective oscillations of a driven two-component Bose gas in optical lattices
|
We explore quantum many-body physics of a driven Bose-Einstein condensate in
optical lattices. The laser field induces a gap in the generalized Bogoliubov
spectrum proportional to the effective Rabi frequency. The lowest lying modes
in a driven condensate are characterized by zero group velocity and non-zero
current. Thus, the laser field induces roton modes, which carry interaction in
a driven condensate. We show that collective excitations below the energy of
the laser-induced gap remain undamped, while above the gap they are
characterized by a significantly suppressed Landau damping rate.
|
1610.09076v5
|
2016-11-02
|
An extension of Bakhvalov's theorem for systems of conservation laws with damping
|
For $2\X2$ systems of conservation laws satisfying Bakhvalov conditions, we
present a class of damping terms that still yield the existence of global
solutions with periodic initial data of possibly large bounded total variation
per period. We also address the question of the decay of the periodic solution.
As applications we consider the systems of isentropic gas dynamics, with
pressure obeying a $\gamma$-law, for the physical range $\gamma\ge1$, and also
for the "non-physical" range $0<\gamma<1$, both in the classical Lagrangian and
Eulerian formulation, and in the relativistic setting. We give complete details
for the case $\gamma=1$, and also analyze the general case when $|\gamma-1|$ is
small. Further, our main result also establishes the decay of the periodic
solution.
|
1611.00698v1
|
2016-11-15
|
The damped stochastic wave equation on p.c.f. fractals
|
A p.c.f. fractal with a regular harmonic structure admits an associated
Dirichlet form, which is itself associated with a Laplacian. This Laplacian
enables us to give an analogue of the damped stochastic wave equation on the
fractal. We show that a unique function-valued solution exists, which has an
explicit formulation in terms of the spectral decomposition of the Laplacian.
We then use a Kolmogorov-type continuity theorem to derive the spatial and
temporal H\"older exponents of the solution. Our results extend the analogous
results on the stochastic wave equation in one-dimensional Euclidean space. It
is known that no function-valued solution to the stochastic wave equation can
exist in Euclidean dimension two or higher. The fractal spaces that we work
with always have spectral dimension less than two, and show that this is the
right analogue of dimension to express the "curse of dimensionality" of the
stochastic wave equation. Finally we prove some results on the convergence to
equilibrium of the solutions.
|
1611.04874v3
|
2016-11-29
|
Phase mixing importance for both Landau instability and damping
|
We discuss the self-consistent dynamics of plasmas by means of hamiltonian
formalism for a system of $N$ near-resonant electrons interacting with a single
Langmuir wave. The connection with the Vlasov description is revisited through
the numerical calculation of the van Kampen-like eigenfrequencies of the
linearized dynamics for many degrees of freedom. Both the exponential-like
growth as well as damping of the Langmuir wave are shown to emerge from a phase
mixing effect among beam modes, revealing unexpected similarities between the
stable and unstable regimes.
|
1611.09596v3
|
2016-11-29
|
Dark matter annihilation and jet quenching phenomena in the early universe
|
Dark-matter particles like neutralinos should decouple from the hot cosmic
plasma at temperatures of about 40 GeV. Later they can annihilate each other
into standard-model particles, which are injected into the dense primordial
plasma and quickly loose energy. This process is similar to jet quenching in
ultrarelativistic heavy-ion collisions, actively studied in RHIC and LHC
experiments. Using empirical information from heavy-ion experiments I show that
the cosmological (anti)quark and gluon jets are damped very quickly until the
plasma remains in the deconfined phase. The charged hadron and lepton jets are
strongly damped until the recombination of electrons and protons. The
consequences of energy transfer by the annihilation products to the cosmic
matter are discussed.
|
1611.09662v1
|
2016-11-30
|
Perspective on the cosmic-ray electron spectrum above TeV
|
The AMS-02 has measured the cosmic ray electron (plus positron) spectrum up
to ~TeV with an unprecedent precision. The spectrum can be well described by a
power law without any obvious features above 10 GeV. The satellite instrument
Dark Matter Particle Explorer (DAMPE), which was launched a year ago, will
measure the electron spectrum up to 10 TeV with a high energy resolution. The
cosmic electrons beyond TeV may be attributed to few local cosmic ray sources,
such as supernova remnants. Therefore, spectral features, such as cutoff and
bumps, can be expected at high energies. In this work we give a careful study
on the perspective of the electron spectrum beyond TeV. We first examine our
astrophysical source models on the latest leptonic data of AMS-02 to give a
self-consistent picture. Then we focus on the discussion about the candidate
sources which could be electron contributors above TeV. Depending on the
properties of the local sources (especially on the nature of Vela), DAMPE may
detect interesting features in the electron spectrum above TeV in the future.
|
1611.10292v1
|
2016-12-01
|
Field- and damping-like spin-transfer torque in magnetic multilayers
|
We investigate the spin-transfer torque in a magnetic multilayer structure by
means of a spin-diffusion model. The torque in the considered system,
consisting of two magnetic layers separated by a conducting layer, is caused by
a perpendicular-to-plane current. We compute the strength of the field-like and
the damping-like torque for different material parameters and geometries. Our
studies suggest that the field-like torque highly depends on the exchange
coupling strength of the itinerant electrons with the magnetization both in the
pinned and the free layer. While a low coupling leads to very high field-like
torques, a high coupling leads to low or even negative field-like torques. The
dependence of the different torque terms on system parameters is considered
very important for the development of applications such as STT MRAM and
spin-torque oscillators.
|
1612.00194v1
|
2016-12-04
|
Vector Approximate Message Passing for the Generalized Linear Model
|
The generalized linear model (GLM), where a random vector $\boldsymbol{x}$ is
observed through a noisy, possibly nonlinear, function of a linear transform
output $\boldsymbol{z}=\boldsymbol{Ax}$, arises in a range of applications such
as robust regression, binary classification, quantized compressed sensing,
phase retrieval, photon-limited imaging, and inference from neural spike
trains. When $\boldsymbol{A}$ is large and i.i.d. Gaussian, the generalized
approximate message passing (GAMP) algorithm is an efficient means of MAP or
marginal inference, and its performance can be rigorously characterized by a
scalar state evolution. For general $\boldsymbol{A}$, though, GAMP can
misbehave. Damping and sequential-updating help to robustify GAMP, but their
effects are limited. Recently, a "vector AMP" (VAMP) algorithm was proposed for
additive white Gaussian noise channels. VAMP extends AMP's guarantees from
i.i.d. Gaussian $\boldsymbol{A}$ to the larger class of rotationally invariant
$\boldsymbol{A}$. In this paper, we show how VAMP can be extended to the GLM.
Numerical experiments show that the proposed GLM-VAMP is much more robust to
ill-conditioning in $\boldsymbol{A}$ than damped GAMP.
|
1612.01186v1
|
2016-12-06
|
Quantum Dynamics of Skyrmions in Chiral Magnets
|
We study the quantum propagation of a Skyrmion in chiral magnetic insulators
by generalizing the micromagnetic equations of motion to a finite-temperature
path integral formalism, using field theoretic tools. Promoting the center of
the Skyrmion to a dynamic quantity, the fluctuations around the Skyrmionic
configuration give rise to a time-dependent damping of the Skyrmion motion.
From the frequency dependence of the damping kernel, we are able to identify
the Skyrmion mass, thus providing a microscopic description of the kinematic
properties of Skyrmions. When defects are present or a magnetic trap is
applied, the Skyrmion mass acquires a finite value proportional to the
effective spin, even at vanishingly small temperature. We demonstrate that a
Skyrmion in a confined geometry provided by a magnetic trap behaves as a
massive particle owing to its quasi-one-dimensional confinement. An additional
quantum mass term is predicted, independent of the effective spin, with an
explicit temperature dependence which remains finite even at zero temperature.
|
1612.01885v2
|
2016-12-06
|
Increased low-temperature damping in yttrium iron garnet thin films
|
We report measurements of the frequency and temperature dependence of
ferromagnetic resonance (FMR) for a 15-nm-thick yttrium iron garnet (YIG) film
grown by off-axis sputtering. Although the FMR linewidth is narrow at room
temperature (corresponding to a damping coefficient $\alpha$ = (9.0 $\pm$ 0.2)
$\times 10^{-4}$), comparable to previous results for high-quality YIG films of
similar thickness, the linewidth increases strongly at low temperatures, by a
factor of almost 30. This increase cannot be explained as due to two-magnon
scattering from defects at the sample interfaces. We argue that the increased
low-temperature linewidth is due to impurity relaxation mechanisms that have
been investigated previously in bulk YIG samples. We suggest that the
low-temperature linewidth is a useful figure of merit to guide the optimization
of thin-film growth protocols because it is a particularly sensitive indicator
of impurities.
|
1612.01954v1
|
2016-12-09
|
Slow motion for one-dimensional nonlinear damped hyperbolic Allen-Cahn systems
|
We consider a nonlinear damped hyperbolic reaction-diffusion system in a
bounded interval of the real line with homogeneous Neumann boundary conditions
and we study the metastable dynamics of the solutions. Using an "energy
approach" introduced by Bronsard and Kohn [CPAM 1990] to study slow motion for
Allen-Cahn equation and improved by Grant [SIAM J. Math. Anal. 1995] in the
study of Cahn-Morral systems, we improve and extend to the case of systems the
results valid for the hyperbolic Allen-Cahn equation. In particular, we study
the limiting behavior of the solutions as $\varepsilon\to0^+$, where
$\varepsilon^2$ is the diffusion coefficient, and we prove existence and
persistence of metastable states for a time
$T_\varepsilon>\exp(A/\varepsilon)$. Such metastable states have a transition
layer structure and the transition layers move with exponentially small
velocity.
|
1612.03203v5
|
2016-12-18
|
Entropically Damped Artificial Compressibility for SPH
|
In this paper, the Entropically Damped Artificial Compressibility (EDAC)
formulation of Clausen (2013) is used in the context of the Smoothed Particle
Hydrodynamics (SPH) method for the simulation of incompressible fluids.
Traditionally, weakly-compressible SPH (WCSPH) formulations have employed
artificial compressiblity to simulate incompressible fluids. EDAC is an
alternative to the artificial compressiblity scheme wherein a pressure
evolution equation is solved in lieu of coupling the fluid density to the
pressure by an equation of state. The method is explicit and is easy to
incorporate into existing SPH solvers using the WCSPH formulation. This is
demonstrated by coupling the EDAC scheme with the recently proposed Transport
Velocity Formulation (TVF) of Adami et al. (2013). The method works for both
internal flows and for flows with a free surface (a drawback of the TVF
scheme). Several benchmark problems are considered to evaluate the proposed
scheme and it is found that the EDAC scheme gives results that are as good or
sometimes better than those produced by the TVF or standard WCSPH. The scheme
is robust and produces smooth pressure distributions and does not require the
use of an artificial viscosity in the momentum equation although using some
artificial viscosity is beneficial.
|
1612.05901v2
|
2016-12-19
|
Kinetic-simulation study of propagation of Langmuir-like ionic waves in dusty plasma
|
The propagation of ionic perturbations in a dusty plasma is considered
through a three-species kinetic simulation approach, in which the temporal
evolution of all three elements i.e. electrons, ions and dust particles are
followed based on the Vlasov equation coupled with the Poisson equation. Two
cases are focused upon: firstly a fully electron depleted dusty plasma, i.e., a
plasma consisting of ions and dust-particles. The second case includes dusty
plasmas with large electron-to-ion temperature ratios. The main features of the
ionic waves in these two settings including the dispersion relation and the
Landau damping rate are studied. It is shown that the dispersion relation of
the ionic waves perfectly matches the dispersion relation of Langmuir waves and
hence are called Langmuir-like ionic waves and can be considered as
ion-Langmuir waves. These waves can be theoretically predicted by the
dispersion relation of the dust-ion-acoustic waves. The transition of ionic
waves from dust-ion-acoustic to Langmuir-like waves are shown to be
sharp/smooth in first/second case. The Landau damping rates based on simulation
results are presented and compared with theoretical predictions wherever
possible.
|
1612.06182v1
|
2016-12-21
|
Environment generated quantum correlations in bipartite qubit-qutrit systems
|
The dynamics of entanglement and quantum discord for qubit-qutrit systems are
studied in the presence of phase damping and amplitude damping noises. Both one
way and two couplings of the marginal systems with the environments are
considered. Entanglement sudden death is unavoidable under any setup, however,
the required time span depends on the way of coupling. On the other hand, the
dynamics of quantum discord strongly depends both on the nature of environment
and on the number of dimensions of the Hilbert space of the coupled marginal
system. We show that freezing and invariance of quantum discord, as previously
reported in the literature, are limited to some special cases. Most
importantly, it is noted that under some particular coupling the existence of
environment can guarantee the generation of nonclassical correlations.
|
1612.06981v1
|
2016-12-28
|
Stretching and Kibble scaling regimes for Hubble-damped defect networks
|
The cosmological evolution of topological defect networks can broadly be
divided into two stages. At early times they are friction-dominated due to
particle scattering and therefore non-relativistic, and may either be
conformally stretched or evolve in the Kibble regime. At late times they are
relativistic and evolve in the well known linear scaling regime. In this work
we show that a sufficiently large Hubble damping (that is a sufficiently fast
expansion rate) leads to a linear scaling regime where the network is
non-relativistic. This is therefore another realization of a Kibble scaling
regime, and also has a conformal stretching regime counterpart which we
characterize for the first time. We describe these regimes using analytic
arguments in the context of the velocity-dependent one-scale model, and we
confirm them using high-resolution $4096^3$ field theory simulations of domain
wall networks. We also use these simulations to improve the calibration of this
analytic model for the case of domain walls.
|
1612.08863v1
|
2016-12-29
|
Laser Pulse Compression Using Magnetized Plasmas
|
Proposals to reach the next generation of laser intensities through Raman or
Brillouin backscattering have centered on optical frequencies. Higher
frequencies are beyond the range of such methods mainly due to the wave damping
that accompanies the higher density plasmas necessary for compressing higher
frequency lasers. However, we find that an external magnetic field transverse
to the direction of laser propagation can reduce the required plasma density.
Using parametric interactions in magnetized plasmas to mediate pulse
compression both reduces the wave damping and alleviates instabilities, thereby
enabling higher frequency or lower intensity pumps to produce pulses at higher
intensity and longer duration. In addition to these theoretical advantages, our
new method, in which strong uniform magnetic fields lessen the need for
high-density uniform plasmas, also lessens key engineering challenges, or at
least exchanges them for different challenges.
|
1612.09214v1
|
2017-01-05
|
Liquid-like thermal conduction in a crystalline solid
|
A solid conducts heat through both transverse and longitudinal acoustic
phonons, but a liquid employs only longitudinal vibrations. Here, we report
that the crystalline solid AgCrSe2 has liquid-like thermal conduction. In this
compound, Ag atoms exhibit a dynamic duality that they are exclusively involved
in intense low-lying transverse acoustic phonons while they also undergo local
fluctuations inherent in an order-to-disorder transition occurring at 450 K. As
a consequence of this extreme disorder-phonon coupling, transverse acoustic
phonons become damped as approaching the transition temperature, above which
they are not defined anymore because their lifetime is shorter than the
relaxation time of local fluctuations. Nevertheless, the damped longitudinal
acoustic phonon survives for thermal transport. This microscopic insight might
reshape the fundamental idea on thermal transport properties of matter and
facilitates the optimization of thermoelectrics.
|
1701.01192v1
|
2017-01-09
|
Oscillations and damping in the fractional Maxwell materials
|
This paper examines the oscillatory behaviour of complex viscoelastic systems
with power law-like relaxation behaviour. Specifically, we use the fractional
Maxwell model, consisting of a spring and fractional dashpot in series, which
produces a power-law creep behaviour and a relaxation law following the
Mittag-Leffler function. The fractional dashpot is characterised by a parameter
beta, continuously moving from the pure viscous behaviour when beta=1 to the
purely elastic response when beta=0. In this work, we study the general
response function and focus on the oscillatory behaviour of a fractional
Maxwell system in four regimes: stress impulse, strain impulse, step stress,
and driven oscillations. The solutions are presented in a format analogous to
the classical oscillator, showing how the fractional nature of relaxation
changes the long-time equilibrium behaviour and the short-time transient
solutions. We specifically test the critical damping conditions in the
fractional regime, since these have a particular relevance in biomechanics.
|
1701.02155v2
|
2017-01-11
|
Localization in finite asymmetric vibro-impact chains
|
We explore the dynamics of strongly localized periodic solutions (discrete
solitons, or discrete breathers) in a finite one-dimensional chain of
asymmetric vibro-impact oscillators. The model involves a parabolic on-site
potential with asymmetric rigid constraints (the displacement domain of each
particle is finite), and a linear nearest-neighbor coupling. When the particle
approaches the constraint, it undergoes an impact (not necessarily elastic),
that satisfies Newton impact law. Nonlinearity of the system stems from the
impacts; their possible non-elasticity is the sole source of damping in the
system. We demonstrate that this vibro-impact model allows derivation of exact
analytic solutions for the asymmetric discrete breathers, both in conservative
and forced-damped settings. The asymmetry makes two types of breathers
possible: breathers that impact both or only one constraint. Transition between
these two types of the breathers corresponds to a grazing bifurcation. Special
character of the nonlinearity permits explicit derivation of a monodromy
matrix. Therefore, the stability of the obtained breather solutions can be
exactly studied in the framework of simple methods of linear algebra, and with
rather moderate computational efforts. All three generic scenarios of the loss
of stability (pitchfork, Neimark-Sacker and period doubling bifurcations) are
observed.
|
1701.03055v1
|
2017-01-11
|
The Cauchy problem for the Landau-Lifshitz-Gilbert equation in BMO and self-similar solutions
|
We prove a global well-posedness result for the Landau-Lifshitz equation with
Gilbert damping provided that the BMO semi-norm of the initial data is small.
As a consequence, we deduce the existence of self-similar solutions in any
dimension. In the one-dimensional case, we characterize the self-similar
solutions associated with an initial data given by some ($\mathbb{S}^2$-valued)
step function and establish their stability. We also show the existence of
multiple solutions if the damping is strong enough. Our arguments rely on the
study of a dissipative quasilinear Schr\"odinger obtained via the stereographic
projection and techniques introduced by Koch and Tataru.
|
1701.03083v2
|
2017-01-13
|
Characterizing Fluid and Kinetic Instabilities using Field-Particle Correlations on Single-Point Time Series
|
A recently proposed technique correlating electric fields and particle
velocity distributions is applied to single-point time series extracted from
linearly unstable, electrostatic numerical simulations. The form of the
correlation, which measures the transfer of phase-space energy density between
the electric field and plasma distributions and had previously been applied to
damped electrostatic systems, is modified to include the effects of drifting
equilibrium distributions of the type that drive counter-streaming and
bump-on-tail instabilities. By using single-point time series, the correlation
is ideal for diagnosing dynamics in systems where access to integrated
quantities, such as energy, is observationally infeasible. The velocity-space
structure of the field-particle correlation is shown to characterize the
underlying physical mechanisms driving unstable systems. The use of this
correlation in simple systems will assist in its eventual application to
turbulent, magnetized plasmas, with the ultimate goal of characterizing the
nature of mechanisms that damp turbulent fluctuations in the solar wind.
|
1701.03687v1
|
2017-01-17
|
Backward Raman Amplification in the Long-wavelength Infrared
|
The wealth of work in backward Raman amplification in plasma has focused on
the extreme intensity limit, however backward Raman amplification may also
provide an effective and practical mechanism for generating intense, broad
bandwidth, long-wavelength infrared radiation (LWIR). An electromagnetic
simulation coupled with a relativistic cold fluid plasma model is used to
demonstrate the generation of picosecond pulses at a wavelength of 10 microns
with terawatt powers through backward Raman amplification. The effects of
collisional damping, Landau damping, pump depletion, and wave breaking are
examined, as well as the resulting design considerations for a LWIR Raman
amplifier.
|
1701.04879v2
|
2017-01-27
|
Structural scale $q-$derivative and the LLG-Equation in a scenario with fractionality
|
In the present contribution, we study the Landau-Lifshitz-Gilbert equation
with two versions of structural derivatives recently proposed: the scale
$q-$derivative in the non-extensive statistical mechanics and the axiomatic
metric derivative, which presents Mittag-Leffler functions as eigenfunctions.
The use of structural derivatives aims to take into account long-range forces,
possible non-manifest or hidden interactions and the dimensionality of space.
Having this purpose in mind, we build up an evolution operator and a deformed
version of the LLG equation. Damping in the oscillations naturally show up
without an explicit Gilbert damping term.
|
1701.08076v2
|
2017-01-31
|
Lack of correlation between the spin mixing conductance and the ISHE-generated voltages in CoFeB/Pt,Ta bilayers
|
We investigate spin pumping phenomena in polycrystalline CoFeB/Pt and
CoFeB/Ta bilayers and the correlation between the effective spin mixing
conductance $g^{\uparrow\downarrow}_{\rm eff}$ and the obtained voltages
generated by the spin-to-charge current conversion via the inverse spin Hall
effect in the Pt and Ta layers. For this purpose we measure the in-plane
angular dependence of the generated voltages on the external static magnetic
field and we apply a model to separate the spin pumping signal from the one
generated by the spin rectification effect in the magnetic layer. Our results
reveal a dominating role of anomalous Hall effect for the spin rectification
effect with CoFeB and a lack of correlation between
$g^{\uparrow\downarrow}_{\rm eff}$ and inverse spin Hall voltages pointing to a
strong role of the magnetic proximity effect in Pt in understanding the
observed increased damping. This is additionally reflected on the presence of a
linear dependency of the Gilbert damping parameter on the Pt thickness.
|
1701.09110v1
|
2017-02-14
|
Calibrating damping rates with LEGACY linewidths
|
Linear damping rates of radial oscillation modes in selected $Kepler$ stars
are estimated with the help of a nonadiabatic stability analysis. The
convective fluxes are obtained from a nonlocal, time-dependent convection
model. The mixing-length parameter is calibrated to the surface-convection-zone
depth of a stellar model obtained from fitting adiabatic frequencies to the
LEGACY observations, and two of the three nonlocal convection parameters are
calibrated to the corresponding LEGACY linewidth measurements. The atmospheric
structure in the 1D stability analysis adopts a temperature-optical-depth
relation derived from 3D hydrodynamical simulations. Results from 3D
simulations are also used to calibrate the turbulent pressure and to guide the
functional form of the depth-dependence of the anisotropy of the turbulent
velocity field in the 1D stability computations.
|
1702.04251v1
|
2017-02-20
|
Resonant Scattering Characteristics of Homogeneous Dielectric Sphere
|
In the present article the classical problem of electromagnetic scattering by
a single homogeneous sphere is revisited. Main focus is the study of the
scattering behavior as a function of the material contrast and the size
parameters for all electric and magnetic resonances of a dielectric sphere.
Specifically, the Pad\'e approximants are introduced and utilized as an
alternative system expansion of the Mie coefficients. Low order Pad\'e
approximants can give compact and physically insightful expressions for the
scattering system and the enabled dynamic mechanisms. Higher order approximants
are used for predicting accurately the resonant pole spectrum. These results
are summarized into general pole formulae, covering up to fifth order magnetic
and forth order electric resonances of a small dielectric sphere. Additionally,
the connection between the radiative damping process and the resonant linewidth
is investigated. The results obtained reveal the fundamental connection of the
radiative damping mechanism with the maximum width occurring for each
resonance. Finally, the suggested system ansatz is used for studying the
resonant absorption maximum through a circuit-inspired perspective.
|
1702.05883v1
|
2017-02-22
|
A Retrofitting-based Supplementary Controller Design for Enhancing Damping Performance of Wind Power Systems
|
In this paper we address the growing concerns of wind power integration from
the perspective of power system dynamics and stability. We propose a new
retrofit control technique where an additional controller is designed at the
doubly-fed induction generator site inside the wind power plant. This
controller cancels the adverse impacts of the power flow from the wind side to
the grid side on the dynamics of the overall power system. The main advantage
of this controller is that it can be implemented by feeding back only the wind
states and wind bus voltage without depending on any of the other synchronous
machines in the rest of the system. Through simulations of a 4-machine Kundur
power system model we show that the retrofit can efficiently enhance the
damping performance of the system variable despite very high values of wind
penetration.
|
1702.06695v1
|
2017-02-23
|
Plasmon modes of bilayer molybdenum disulfide: A density functional study
|
We explore the collective electronic excitations of bilayer molybdenum
disulfide (MoS$_2$) using the density functional theory together with the
random phase approximation. The many-body dielectric function and electron
energy-loss spectra are calculated using an {\it ab initio} based model
involving material-realistic physical properties. The electron energy-loss
function of bilayer MoS$_2$ system is found to be sensitive to either electron
or hole doping and it is owing to the fact that the Kohn-Sham band dispersions
are not symmetric for energies above and below the zero Fermi level. Three
plasmon modes are predicted. A damped high-energy mode, one optical mode
(in-phase mode) for which the plasmon dispersion exhibits $\sqrt q $ in the
long wavelength limit originating from low-energy electron scattering and
finally a highly damped acoustic mode (out-of-phase mode).
|
1702.07170v2
|
2017-02-25
|
Low-energy photoelectron transmission through aerosol overlayers
|
The transmission of low-energy (<1.8eV) photoelectrons through the shell of
core-shell aerosol particles is studied for liquid squalane, squalene, and DEHS
shells. The photoelectrons are exclusively formed in the core of the particles
by two-photon ionization. The total photoelectron yield recorded as a function
of shell thickness (1-80nm) shows a bi-exponential attenuation. For all
substances, the damping parameter for shell thicknesses below 15nm lies between
8 and 9nm, and is tentatively assigned to the electron attenuation length at
electron kinetic energies of ~0.5-1eV. The significantly larger damping
parameters for thick shells (> 20nm) are presumably a consequence of distorted
core-shell structures. A first comparison of aerosol and traditional thin film
overlayer methods is provided.
|
1702.07865v1
|
2017-02-26
|
Magnon-photon coupling in antiferromagnets
|
Magnon-photon coupling in antiferromagnets has many attractive features that
do not exist in ferro- or ferrimagnets. We show quantum-mechanically that, in
the absence of an external field, one of the two degenerated spin wave bands
couples with photons while the other does not. The photon mode anticrosses with
the coupled spin waves when their frequencies are close to each other. Similar
to its ferromagnetic counterpart, the magnon-photon coupling strength is
proportional to the square root of number of spins $\sqrt{N}$ in
antiferromagnets. An external field removes the spin wave degeneracy and both
spin wave bands couple to the photons, resulting in two anticrossings between
the magnons and photons. Two transmission peaks were observed near the
anticrossing frequency. The maximum damping that allows clear discrimination of
the two transmission peaks is proportional to $\sqrt{N}$ and it's well below
the damping of antiferromagnetic insulators. Therefore the strong magnon-photon
coupling can be realized in antiferromagnets and the coherent information
transfer between the photons and magnons is possible.
|
1702.07977v1
|
2017-03-03
|
Global behaviour of bistable solutions for hyperbolic gradient systems in one unbounded spatial dimension
|
This paper is concerned with damped hyperbolic gradient systems of the form
\[ \alpha u_{tt} + u_t = -\nabla V(u) + u_{xx}\,, \] where the spatial domain
is the whole real line, the state variable $u$ is multidimensional, $\alpha$ is
a positive quantity, and the potential $V$ is coercive at infinity. For such
systems, under generic assumptions on the potential, the asymptotic behaviour
of every bistable solution (that is, every solution close at both ends of space
to stable homogeneous equilibria) is described. Every such solution approaches,
far to the left in space a stacked family of bistable fronts travelling to the
left, far to the right in space a stacked family of bistable fronts travelling
to the right, and in between a pattern of profiles of stationary solutions
homoclinic or heteroclinic to stable homogeneous equilibria, going slowly away
from one another. In the absence of maximum principle, the arguments are purely
variational. This extends previous results obtained in companion papers for
damped wave equations or parabolic gradient systems, in the spirit of the
program initiated in the late seventies by Fife and McLeod on the global
asymptotic behaviour of bistable solutions for parabolic equations.
|
1703.01221v2
|
2017-03-06
|
Polynomial Stabilization of Solutions to a Class of Damped Wave Equations
|
We consider a class of wave equations of the type $\partial_{tt} u + Lu +
B\partial_{t} u = 0$, with a self-adjoint operator $L$, and various types of
local damping represented by $B$. By establishing appropriate and raher precise
estimates on the resolvent of an associated operator $A$ on the imaginary axis
of ${{\Bbb C}}$, we prove polynomial decay of the semigroup $\exp(-tA)$
generated by that operator. We point out that the rate of decay depends
strongly on the concentration of eigenvalues and that of the eigenfunctions of
the operator $L$. We give several examples of application of our abstract
result, showing in particular that for a rectangle $\Omega := (0,L_{1})\times
(0,L_{2})$ the decay rate of the energy is different depending on whether the
ratio $L_{1}^2/L_{2}^2$ is rational, or irrational but algebraic.
|
1703.01735v1
|
2017-03-20
|
A quantum correlated twin atom laser from a Bose-Hubbard system
|
We propose and evaluate a method to construct a quantum correlated twin atom
laser using a pumped and damped Bose-Hubbard inline trimer which can operate in
a stationary regime. With pumping via a source condensate filling the middle
well and damping using either an electron beam or optical means at the two end
wells, we show that bipartite quantum correlations build up between the ends of
the chain, and that these can be measured either in situ or in the outcoupled
beams. While nothing similar to our system has yet been achieved
experimentally, recent advances mean that it should be practically realisable
in the near future.
|
1703.06607v2
|
2017-03-20
|
Non-universal velocity probability densities in two-dimensional turbulence: the effect of large-scale dissipation
|
We show that some statistical properties of forced two-dimensional turbulence
have an important sensitivity to the form of large-scale dissipation which is
required to damp the inverse cascade. We consider three models of large-scale
dissipation: linear "Ekman" drag, non-linear quadratic drag, and scale
selective hypo-drag that damps only low-wavenumber modes. In all cases, the
statistically steady vorticity field is dominated by almost axisymmetric
vortices, and the probability density function of vorticity is non-Gaussian.
However, in the case of linear and quadratic drag, we find that the velocity
statistics is close to Gaussian, with non-negligible contribution coming from
the background turbulent flow. On the other hand, with hypo-drag, the
probability density function of velocity is non-Gaussian and is predominantly
determined by the properties of the vortices. With hypo-drag, the relative
positions of the vortices and the exponential distribution of the vortex
extremum are important factors responsible for the non-Gaussian velocity
statistics.
|
1703.07000v1
|
2017-03-21
|
Numerical Range and Quadratic Numerical Range for Damped Systems
|
We prove new enclosures for the spectrum of non-selfadjoint operator matrices
associated with second order linear differential equations $\ddot{z}(t) + D
\dot{z} (t) + A_0 z(t) = 0$ in a Hilbert space. Our main tool is the quadratic
numerical range for which we establish the spectral inclusion property under
weak assumptions on the operators involved; in particular, the damping operator
only needs to be accretive and may have the same strength as $A_0$. By means of
the quadratic numerical range, we establish tight spectral estimates in terms
of the unbounded operator coefficients $A_0$ and $D$ which improve earlier
results for sectorial and selfadjoint $D$; in contrast to numerical range
bounds, our enclosures may even provide bounded imaginary part of the spectrum
or a spectral free vertical strip. An application to small transverse
oscillations of a horizontal pipe carrying a steady-state flow of an ideal
incompressible fluid illustrates that our new bounds are explicit.
|
1703.07447v1
|
2017-03-22
|
Kinetic simulations of ladder climbing by electron plasma waves
|
The energy of plasma waves can be moved up and down the spectrum using
chirped modulations of plasma parameters, which can be driven by external
fields. Depending on whether the wave spectrum is discrete (bounded plasma) or
continuous (boundless plasma), this phenomenon is called ladder climbing (LC)
or autoresonant acceleration of plasmons. It was first proposed by Barth
\textit{et al.} [PRL \textbf{115}, 075001 (2015)] based on a linear fluid
model. In this paper, LC of electron plasma waves is investigated using fully
nonlinear Vlasov-Poisson simulations of collisionless bounded plasma. It is
shown that, in agreement with the basic theory, plasmons survive substantial
transformations of the spectrum and are destroyed only when their wave numbers
become large enough to trigger Landau damping. Since nonlinear effects decrease
the damping rate, LC is even more efficient when practiced on structures like
quasiperiodic Bernstein-Greene-Kruskal (BGK) waves rather than on Langmuir
waves \textit{per~se}.
|
1703.07694v1
|
2017-04-03
|
Sensing Coherent Phonons with Two-photon Interference
|
Detecting coherent phonons pose different challenges compared to coherent
photons due to the much stronger interaction between phonons and matter. This
is especially true for high frequency heat carrying phonons, which are
intrinsic lattice vibrations experiencing many decoherence events with the
environment, and are thus generally assumed to be incoherent. Two photon
interference techniques, especially coherent population trapping (CPT) and
electromagnetically induced transparency (EIT), have led to extremely sensitive
detection, spectroscopy and metrology. Here, we propose the use of two photon
interference in a three level system to sense coherent phonons. Unlike prior
works which have treated phonon coupling as damping, we account for coherent
phonon coupling using a full quantum-mechanical treatment. We observe strong
asymmetry in absorption spectrum in CPT and negative dispersion in EIT
susceptibility in the presence of coherent phonon coupling which cannot be
accounted for if only pure phonon damping is considered. Our proposal has
application in sensing heat carrying coherent phonons effects and understanding
coherent bosonic multi-pathway interference effects in three coupled oscillator
systems.
|
1704.00446v1
|
2017-04-03
|
Simulating spin-boson models with trapped ions
|
We propose a method to simulate the dynamics of spin-boson models with small
crystals of trapped ions where the electronic degree of freedom of one ion is
used to encode the spin while the collective vibrational degrees of freedom are
employed to form an effective harmonic environment. The key idea of our
approach is that a single damped mode can be used to provide a harmonic
environment with Lorentzian spectral density. More complex spectral functions
can be tailored by combining several individually damped modes. We propose to
work with mixed-species crystals such that one species serves to encode the
spin while the other species is used to cool the vibrational degrees of freedom
to engineer the environment. The strength of the dissipation on the spin can be
controlled by tuning the coupling between spin and vibrational degrees of
freedom. In this way the dynamics of spin-boson models with macroscopic and
non-Markovian environments can be simulated using only a few ions. We
illustrate the approach by simulating an experiment with realistic parameters
and show by computing quantitative measures that the dynamics is genuinely
non-Markovian.
|
1704.00629v1
|
2017-04-07
|
Coherent-induced state ordering with fixed mixedness
|
In this paper, we study coherence-induced state ordering with Tsallis
relative entropy of coherence, relative entropy of coherence and $l_{1}$ norm
of coherence. Firstly, we show that these measures give the same ordering for
single-qubit states with a fixed mixedness or a fixed length along the
direction $\sigma_{z}$. Secondly, we consider some special cases of high
dimensional states, we show that these measures generate the same ordering for
the set of high dimensional pure states if any two states of the set satisfy
majorization relation. Moreover, these three measures generate the same
ordering for all $X$ states with a fixed mixedness. Finally, we discuss
dynamics of coherence-induced state ordering under Markovian channels. We find
phase damping channel don't change the coherence-induced state ordering for
some single-qubit states with fixed mixedness, instead amplitude damping
channel change the coherence-induced ordering even though for single-qubit
states with fixed mixedness.
|
1704.02244v1
|
2017-04-13
|
A possible connection between the spin temperature of damped Lyman-alpha absorption systems and star formation history
|
We present a comprehensive analysis of the spin temperature/covering factor
degeneracy, T/f, in damped Lyman-alpha absorption systems. By normalising the
upper limits and including these via a survival analysis, there is, as
previously claimed, an apparent increase in T/f with redshift at z > 1.
However, when we account for the geometry effects of an expanding Universe,
neglected by the previous studies, this increase in T/f at z > 1 is preceded by
a decrease at z < 1. Using high resolution radio images of the background
continuum sources, we can transform the T/f degeneracy to T/d^2, where d is the
projected linear size of the absorber. Again, there is no overall increase with
redshift, although a dip at z ~ 2 persists. Furthermore, we find d^2/T to
follow a similar variation with redshift as the star formation rate. This
suggests that, although the total hydrogen column density shows little relation
to the SFR, the fraction of the cold neutral medium may. Therefore, further
efforts to link the neutral gas with the star formation history should also
consider the cool component of the gas.
|
1704.04294v2
|
2017-04-17
|
Magnetic field line random walk in two-dimensional dynamical turbulence
|
The field line random walk (FLRW) of magnetic turbulence is one of the
important topics in plasma physics and astrophysics. In this article by using
the field line tracing method mean square displacements (MSD) of FLRW is
calculated in all possible length scales for pure two-dimensional turbulence
with damping dynamical model. We demonstrate that in order to describe FLRW
with damping dynamical model a new dimensionless quantity $R$ is needed to be
introduced. In different length scales dimensionless MSD shows different
relationship with the dimensionless quantity $R$. Although temporal effect
impacts MSD of FLRW and even changes regimes of FLRW, it dose not affect the
relationship between the dimensionless MSD and dimensionless quantity $R$ in
all possible length scales.
|
1704.05059v3
|
2017-04-19
|
Quantum simulation of quantum channels in nuclear magnetic resonance
|
We propose and experimentally demonstrate an efficient framework for the
quantum simulation of quantum channels in NMR. Our approach relies on the
suitable decomposition of non-unitary operators in a linear combination of $d$
unitary ones, which can be then experimentally implemented with the assistance
of a number of ancillary qubits that grows logarithmically in $d$. As a
proof-of-principle demonstration, we realize the quantum simulation of three
quantum channels for a single-qubit: phase damping (PD), amplitude damping
(AD), and depolarizing (DEP) channels. For these paradigmatic cases, we measure
key features, such as the fidelity of the initial state and the associated von
Neuman entropy for a qubit evolving through these channels. Our experiments are
carried out using nuclear spins in a liquid sample and NMR control techniques.
|
1704.05593v2
|
2017-04-24
|
Beating the Classical Limits of Information Transmission using a Quantum Decoder
|
Encoding schemes and error-correcting codes are widely used in information
technology to improve the reliability of data transmission over real-world
communication channels. Quantum information protocols can further enhance the
performance in data transmission by encoding a message in quantum states,
however, most proposals to date have focused on the regime of a large number of
uses of the noisy channel, which is unfeasible with current quantum technology.
We experimentally demonstrate quantum enhanced communication over an amplitude
damping noisy channel with only two uses of the channel per bit and a single
entangling gate at the decoder. By simulating the channel using a photonic
interferometric setup, we experimentally increase the reliability of
transmitting a data bit by greater than 20% for a certain damping range over
classically sending the message twice. We show how our methodology can be
extended to larger systems by simulating the transmission of a single bit with
up to eight uses of the channel and a two-bit message with three uses of the
channel, predicting a quantum enhancement in all cases.
|
1704.07036v2
|
2017-04-24
|
Quasilinear diffusion coefficients in a finite Larmor radius expansion for ion cyclotron heated plasmas
|
In this paper, a reduced model of quasilinear diffusion by a small Larmor
radius approximation is derived to couple the Maxwell's equations and the
Fokker-Planck equation self-consistently for ion cyclotron range of frequency
waves in a tokamak. The reduced model ensures the important properties of the
full model by Kennel-Engelmann diffusion, such as diffusion directions, wave
polarizations, and H-theorem. The kinetic energy change (W-dot) is used to
derive the reduced model diffusion coefficients for the fundamental damping and
the second harmonic damping to the lowest order of the finite Larmor radius
expansion. The quasilinear diffusion coefficients are implemented in a coupled
code (TORIC-CQL3D) with the equivalent reduced model of dielectric tensor. We
also present the simulations of the ITER minority heating scenario, in which
the reduced model is verified within the allowable errors from the full model
results.
|
1704.07283v1
|
2017-04-27
|
Chirality-induced Antisymmetry in Magnetic Domain-Wall Speed
|
In chiral magnetic materials, numerous intriguing phenomena such as built in
chiral magnetic domain walls (DWs) and skyrmions are generated by the
Dzyaloshinskii Moriya interaction (DMI). The DMI also results in asymmetric DW
speed under in plane magnetic field, which provides a useful scheme to measure
the DMI strengths. However, recent findings of additional asymmetries such as
chiral damping have disenabled unambiguous DMI determination and the underlying
mechanism of overall asymmetries becomes under debate. By extracting the
DMI-induced symmetric contribution, here we experimentally investigated the
nature of the additional asymmetry. The results revealed that the additional
asymmetry has a truly antisymmetric nature with the typical behavior governed
by the DW chirality. In addition, the antisymmetric contribution changes the DW
speed more than 100 times, which cannot be solely explained by the chiral
damping scenario. By calibrating such antisymmetric contributions, experimental
inaccuracies can be largely removed, enabling again the DMI measurement scheme.
|
1704.08751v1
|
2017-05-04
|
Phase-space mixing in dynamically unstable, integrable few-mode quantum systems
|
Quenches in isolated quantum systems are currently a subject of intense
study. Here, we consider quantum few-mode systems that are integrable in their
classical mean-field limit and become dynamically unstable after a quench of a
system parameter. Specifically, we study a Bose-Einstein condensate (BEC) in a
double-well potential and an antiferromagnetic spinor BEC constrained to a
single spatial mode. We study the time dynamics after the quench within the
truncated Wigner approximation (TWA) and find that system relaxes to a steady
state due to phase-space mixing. Using the action-angle formalism and a
pendulum as an illustration, we derive general analytical expressions for the
time evolution of expectation values of observables and their long-time limits.
We find that the deviation of the long-time expectation value from its
classical value scales as $1/O(\ln N )$, where $N$ is the number of atoms in
the condensate. Furthermore, the relaxation of an observable to its steady
state value is a damped oscillation and the damping is Gaussian in time. We
confirm our results with numerical TWA simulations.
|
1705.01702v1
|
2017-05-11
|
Quantum Correlations and Bell Inequality Violation under Decoherence
|
Quantum Correlations are studied extensively in quantum information domain.
Entanglement Measures and Quantum Discord are good examples of these actively
studied correlations. Detection of violation in Bell inequalities is also a
widely active area in quantum information theory world. In this work, we
revisit the problem of analyzing the behavior of quantum correlations and
violation of Bell inequalities in noisy channels. We extend the problem defined
in [1] by observing the changes in negativity measure, quantum discord and a
modified version of Horodecki measure for violation of Bell inequalities under
amplitude damping, phase damping and depolarizing channels. We report different
interesting results for each of these correlations and measures. All these
correlations and measures decrease under decoherence channels, but some changes
are very dramatical comparing to others. We investigate also separability
conditions of example studied states.
|
1705.03882v2
|
2017-05-18
|
Local and global existence of solutions to a strongly damped wave equation of the $p$-Laplacian type
|
This article focuses on a quasilinear wave equation of $p$-Laplacian type: $$
u_{tt} - \Delta_p u - \Delta u_t=0$$ in a bounded domain
$\Omega\subset\mathbb{R}^3$ with a sufficiently smooth boundary
$\Gamma=\partial\Omega$ subject to a generalized Robin boundary condition
featuring boundary damping and a nonlinear source term. The operator
$\Delta_p$, $2 < p < 3$, denotes the classical $p$-Laplacian. The nonlinear
boundary term $f (u)$ is a source feedback that is allowed to have a
supercritical exponent, in the sense that the associated Nemytskii operator is
not locally Lipschitz from $W^{1,p}(\Omega)$ into $L^2(\Gamma)$. Under suitable
assumptions on the parameters we provide a rigorous proof of existence of a
local weak solution which can be extended globally in time provided the source
term satisfies an appropriate growth condition.
|
1705.06696v2
|
2017-05-23
|
Unifying description of the damping regimes of a stochastic particle in a periodic potential
|
We analyze the classical problem of the stochastic dynamics of a particle
confined in a periodic potential, through the so called Il'in and Khasminskii
model, with a novel semi-analytical approach. Our approach gives access to the
transient and the asymptotic dynamics in all damping regimes, which are
difficult to investigate in the usual Brownian model. We show that the
crossover from the overdamped to the underdamped regime is associated with the
loss of a typical time scale and of a typical length scale, as signaled by the
divergence of the probability distribution of a certain dynamical event. In the
underdamped regime, normal diffusion coexists with a non Gaussian displacement
probability distribution for a long transient, as recently observed in a
variety of different systems. We rationalize the microscopic physical processes
leading to the non-Gaussian behavior, as well as the timescale to recover the
Gaussian statistics. The theoretical results are supported by numerical
calculations and are compared to those obtained for the Brownian model.
|
1705.08083v2
|
2017-05-27
|
Experimental Observation of Electron-Acoustic Wave Propagation in Laboratory Plasma
|
In the field of fundamental plasma waves, direct observation of
electron-acoustic wave (EAW) propagation in laboratory plasmas remains a
challenging problem, mainly because of heavy damping. In the MaPLE device, the
wave is observed and seen to propagate with phase velocity $\sim1.8$ times the
electron thermal velocity. A small amount of cold, drifting electrons, with
moderate bulk to cold temperature ratio ($\approx 2 - 3$), is present in the
device. It plays a crucial role in reducing the damping. Our calculation
reveals that the drift relaxes the stringent condition on the temperature ratio
for wave destabilization. Growth rate becomes positive above a certain drift
velocity even if the temperature ratio is moderate. The observed phase velocity
agrees well with the theoretical estimate. Experimental realization of the mode
may open up a new avenue in EAW research.
|
1705.09806v1
|
2017-06-08
|
Superconductivity around nematic quantum critical point in two-dimensional metals
|
We study the properties of $s$-wave superconductivity induced around a
nematic quantum critical point in two-dimensional metals. The strong Landau
damping and the Cooper pairing between incoherent fermions have dramatic mutual
influence on each other, and hence should be treated on an equal footing. This
problem is addressed by analyzing the self-consistent Dyson-Schwinger equations
for the superconducting gap and Landau damping rate. We solve the equations at
zero temperature without making any linearization, and show that the
superconducting gap is maximized at the quantum critical point and decreases
rapidly as the system departs from this point. The interplay between nematic
fluctuation and an additional pairing interaction, caused by phonon or other
boson mode, is also investigated. The total superconducting gap generated by
such interplay can be several times larger than the direct sum of the gaps
separately induced by these two pairing interactions. This provides a promising
way to achieve remarkable enhancement of superconductivity.
|
1706.02583v2
|
2017-06-16
|
Simulation of non-Pauli Channels
|
We consider the simulation of a quantum channel by two parties who share a
resource state and may apply local operations assisted by classical
communication (LOCC). One specific type of such LOCC is standard teleportation,
which is however limited to the simulation of Pauli channels. Here we show how
we can easily enlarge this class by means of a minimal perturbation of the
teleportation protocol, where we introduce noise in the classical communication
channel between the remote parties. By adopting this noisy protocol, we provide
a necessary condition for simulating a non-Pauli channel. In particular, we
characterize the set of channels that are generated assuming the Choi matrix of
an amplitude damping channel as a resource state. Within this set, we identify
a class of Pauli-damping channels for which we bound the two-way quantum and
private capacities.
|
1706.05384v2
|
2017-06-17
|
\emph{Ab initio} calculation of spin-orbit coupling for NV center in diamond exhibiting dynamic Jahn-Teller effect
|
Point defects in solids may realize solid state quantum bits. The spin-orbit
coupling in these point defects plays a key role in the magneto-optical
properties that determine the conditions of quantum bit operation. However,
experimental data and methods do not directly yield this highly important data,
particularly, for such complex systems where dynamic Jahn-Teller (DJT) effect
damps the spin-orbit interaction. Here, we show for an exemplary quantum bit,
nitrogen-vacancy (NV) center in diamond, that \emph{ab initio} supercell
density functional theory provide quantitative prediction for the spin-orbit
coupling damped by DJT. We show that DJT is responsible for the multiple
intersystem crossing rates of NV center at cryogenic temperatures. Our results
pave the way toward optimizing solid state quantum bits for quantum information
processing and metrology applications.
|
1706.05523v2
|
2017-06-20
|
Decoherence induced spin squeezing signatures in Greenberger-Horne-Zeilinger and W states
|
We reckon the behaviour of spin squeezing in tripartite unsqueezed maximally
entangled Green- berger-Horne-Zeilinger (GHZ) and W states under various
decoherence channels with Kitagawa- Ueda (KU) criteria. In order to search spin
squeezing sudden death (SSSD) and signatures of spin squeezing production we
use bit flip, phase flip, bit-phase-flip, amplitude damping, phase damping and
depolarization channels in the present study. In literature, the influence of
decoherence has been studied as a destroying element. On the contrary here we
investigate the positive aspect of decoherence, which produce spin squeezing in
unsqueezed GHZ and W states under certain channels. Our meticulous study shows
that GHZ state remain unsqueezed under aforementioned channels except
bit-phase-flip and depolarization channels. While all the decoherence channels
produce spin squeezing in W state. So we find, GHZ is more robust in comparison
to W state in the sense of spin squeezing production under decoherence. Most
importantly we find that none of the decoherence channel produce SSSD in any
one of the state.
|
1706.06273v2
|
2017-06-22
|
Adaptive recurrence quantum entanglement distillation for two-Kraus-operator channels
|
Quantum entanglement serves as a valuable resource for many important quantum
operations. A pair of entangled qubits can be shared between two agents by
first preparing a maximally entangled qubit pair at one agent, and then sending
one of the qubits to the other agent through a quantum channel. In this
process, the deterioration of entanglement is inevitable since the noise
inherent in the channel contaminates the qubit. To address this challenge,
various quantum entanglement distillation (QED) algorithms have been developed.
Among them, recurrence algorithms have advantages in terms of implementability
and robustness. However, the efficiency of recurrence QED algorithms has not
been investigated thoroughly in the literature. This paper put forth two
recurrence QED algorithms that adapt to the quantum channel to tackle the
efficiency issue. The proposed algorithms have guaranteed convergence for
quantum channels with two Kraus operators, which include phase-damping and
amplitude-damping channels. Analytical results show that the convergence speed
of these algorithms is improved from linear to quadratic and one of the
algorithms achieves the optimal speed. Numerical results confirm that the
proposed algorithms significantly improve the efficiency of QED.
|
1706.07461v1
|
2017-06-26
|
Landau Damping of Beam Instabilities by Electron Lenses
|
Modern and future particle accelerators employ increasingly higher intensity
and brighter beams of charged particles and become operationally limited by
coherent beam instabilities. Usual methods to control the instabilities, such
as octupole magnets, beam feedback dampers and use of chromatic effects, become
less effective and insufficient. We show that, in contrast, Lorentz forces of a
low-energy, a magnetically stabilized electron beam, or "electron lens", easily
introduces transverse nonlinear focusing sufficient for Landau damping of
transverse beam instabilities in accelerators. It is also important that,
unlike other nonlinear elements, the electron lens provides the frequency
spread mainly at the beam core, thus allowing much higher frequency spread
without lifetime degradation. For the parameters of the Future Circular
Collider, a single conventional electron lens a few meters long would provide
stabilization superior to tens of thousands of superconducting octupole
magnets.
|
1706.08477v1
|
2017-07-01
|
Amending entanglement-breaking channels via intermediate unitary operations
|
We report a bulk optics experiment demonstrating the possibility of restoring
the entanglement distribution through noisy quantum channels by inserting a
suitable unitary operation (filter) in the middle of the transmission process.
We focus on two relevant classes of single-qubit channels consisting in
repeated applications of rotated phase damping or rotated amplitude damping
maps, both modeling the combined Hamiltonian and dissipative dynamics of the
polarization state of single photons. Our results show that interposing a
unitary filter between two noisy channels can significantly improve
entanglement transmission. This proof-of-principle demonstration could be
generalized to many other physical scenarios where entanglement-breaking
communication lines may be amended by unitary filters.
|
1707.00161v1
|
2017-06-30
|
Damped oscillations of the energy of a bosonic bath due to spectral gaps and special initial correlations
|
The energy of the bosonic bath and the flow of quantum information are
analyzed over short and long times in local dephasing channels for special
correlated or factorized initial conditions, respectively, which involve
thermal states. The continuous distribution of frequency modes of the bosonic
bath exhibits a spectral gap over low frequencies. The bath energy shows
oscillatory behaviors around the asymptotic value and information is
alternatively lost and gained by the open system. Due to the low-frequency gap,
the damped oscillations become regular over long times and the frequency of the
oscillations coincides with the upper cut-off frequency of the spectral gap.
Sequences of long-time intervals are obtained over which the bath energy
increases (decreases), for the correlated initial conditions, and information
is lost (gained) by the open system, for the factorized initial configurations,
even at different temperatures. Such long-time correspondence between the
variations of the bath energy and of the information is reversed if compared to
the one obtained without the low-frequency gap. The correspondence fails if the
spectral density is tailored according to power laws with odd natural powers
near the upper cut-off frequency of the spectral gap.
|
1707.00604v1
|
2017-07-11
|
Andreev reflection assisted lasing in an electromagnetic resonator coupled to a hybrid-quantum-dot
|
A single mode electromagnetic resonator coupled to a two-level
hybrid-quantum-dot(hQD) is studied theoretically as a laser(maser), when the
hQD is driven out of equilibrium with external applied d.c. bias voltage. Using
the formalism of the non-equilibrium Green's functions for the hQD and the
semi-classical laser equations, we determine the relevant physical quantities
of the system. We find that due to the resonant Andreev reflections and the
formation of the Floquet-Andreev side-resonances in the sub-gap region, at
appropriate gate voltages and above a certain threshold bias voltage and
damping factor of the resonator, the two-level QD has non-zero gain spectrum
and lasing can happen in the system in the frequency range of superconducting
gap. Furthermore, our results show that depending on the damping factor of the
resonator and above a specific threshold bias voltages, the lasing can be
either due to single electron transitions or cascaded electron transitions
between the Andreev resonances and Floquet-Andreev side-resonances.
|
1707.03219v2
|
2017-07-13
|
The sharp estimate of the lifespan for the semilinear wave equation with time-dependent damping
|
We consider the following semilinear wave equation with time-dependent
damping. \begin{align} \tag{NLDW} \left\{ \begin{array}{ll} \partial_t^2 u -
\Delta u + b(t)\partial_t u = |u|^{p}, & (t,x) \in [0,T) \times \mathbb{R}^n,
\\ u(0,x)=\varepsilon u_0(x), u_t(0,x)=\varepsilon u_1(x), & x \in
\mathbb{R}^n, \end{array} \right. \end{align} where $n \in \mathbb{N}$, $p>1$,
$\varepsilon>0$, and $b(t)\thickapprox (t+1)^{-\beta}$ with $\beta \in [-1,1)$.
It is known that small data blow-up occurs when $1<p< p_F$ and, on the other
hand, small data global existence holds when $p>p_F$, where $p_F:=1+2/n$ is the
Fujita exponent. The sharp estimate of the lifespan was well studied when $1<p<
p_F$. In the critical case $p=p_F$, the lower estimate of the lifespan was also
investigated. Recently, Lai and Zhou obtained the sharp upper estimate of the
lifespan when $p=p_F$ and $b(t)=1$. In the present paper, we give the sharp
upper estimate of the lifespan when $p=p_F$ and $b(t)\thickapprox
(t+1)^{-\beta}$ with $\beta \in [-1,1)$ by the Lai--Zhou method.
|
1707.03950v1
|
2017-07-17
|
Candidate dust structures for starlight polarization
|
Rotation damping and alignment are discussed as prerequisites for
polarization power. An expression is derived from first principles, for the
damping time of the rotation of a particle in a magnetic field, under the
Faraday braking torque, provided its electrical properties are known. This
makes it possible to describe mathematically, in great detail, the motion of
the particle and determine its ultimate state of motion, if a steady state is
possible at all. This work defines, first, the necessary condition for the
Faraday braking to be effective: a) the net electronic charge distribution
should not be uniform throughout; b) the number of vibration modes should
exceed a few tens. Resonance of rotation frequency with any of these modes is
not a requirement. For alignment to be possible, the ratio of gyroscopic and
conservative magnetic to non-conservative (retarding) magnetic torques must be
low. Either dia-, para- or ferro-magnetism can do, and a small susceptibility
is enough and even preferable. This opens up a wide spectrum of possible
candidates. A few examples are given.
|
1707.05247v1
|
2017-07-21
|
Quasinormal ringing of black holes in Einstein aether theory
|
The gravitational consequence of local Lorentz violation (LV) should show
itself in derivation of the characteristic quasinormal ringing of black hole
mergers from their general relativity case. In this paper, we study quasinormal
modes (QNMs) of the scalar and electromagnetic field perturbations to Einstein
aether black holes. We find that quasinormal ringing of the first kind aether
black hole is similar to that of another Lorentz violation model---the
QED-extension limit of standard model extension. These similarities between
completely different backgrounds may imply that LV in gravity sector and LV in
matter sector have some connections between themself: damping quasinormal
ringing of black holes more rapidly and prolonging its oscillation period. By
compared to Schwarzschild black hole, both the first and the second kind aether
black holes have larger damping rate and smaller real oscillation frequency of
QNMs. And the differences are from 0.7 percent to 35 percents, those could be
detected by new generation of gravitational antennas.
|
1707.06747v2
|
2017-07-24
|
Self-concordant inclusions: A unified framework for path-following generalized Newton-type algorithms
|
We study a class of monotone inclusions called "self-concordant inclusion"
which covers three fundamental convex optimization formulations as special
cases. We develop a new generalized Newton-type framework to solve this
inclusion. Our framework subsumes three schemes: full-step, damped-step and
path-following methods as specific instances, while allows one to use inexact
computation to form generalized Newton directions. We prove a local quadratic
convergence of both the full-step and damped-step algorithms. Then, we propose
a new two-phase inexact path-following scheme for solving this monotone
inclusion which possesses an
$\mathcal{O}\left(\sqrt{\nu}\log(1/\varepsilon)\right)$-worst-case
iteration-complexity to achieve an $\varepsilon$-solution, where $\nu$ is the
barrier parameter and $\varepsilon$ is a desired accuracy. As byproducts, we
customize our scheme to solve three convex problems: convex-concave
saddle-point, nonsmooth constrained convex program, and nonsmooth convex
program with linear constraints. We also provide three numerical examples to
illustrate our theory and compare with existing methods.
|
1707.07403v1
|
2017-07-28
|
Finding stability domains and escape rates in kicked Hamiltonians
|
We use an effective Hamiltonian to characterize particle dynamics and find
escape rates in a periodically kicked Hamiltonian. We study a model of
particles in storage rings that is described by a chaotic symplectic map.
Ignoring the resonances, the dynamics typically has a finite region in phase
space where it is stable. Inherent noise in the system leads to particle loss
from this stable region. The competition of this noise with radiation damping,
which increases stability, determines the escape rate. Determining this
`aperture' and finding escape rates is therefore an important physical problem.
We compare the results of two different perturbation theories and a variational
method to estimate this stable region. Including noise, we derive analytical
estimates for the steady-state populations (and the resulting beam emittance),
for the escape rate in the small damping regime, and compare them with
numerical simulations.
|
1707.09336v1
|
2017-07-29
|
Empirical Bayes approaches to PageRank type algorithms for rating scientific journals
|
Following criticisms against the journal Impact Factor, new journal influence
scores have been developed such as the Eigenfactor or the Prestige Scimago
Journal Rank. They are based on PageRank type algorithms on the cross-citations
transition matrix of the citing-cited network. The PageRank algorithm performs
a smoothing of the transition matrix combining a random walk on the data
network and a teleportation to all possible nodes with fixed probabilities (the
damping factor being $\alpha= 0.85$). We reinterpret this smoothing matrix as
the mean of a posterior distribution of a Dirichlet-multinomial model in an
empirical Bayes perspective. We suggest a simple yet efficient way to make a
clear distinction between structural and sampling zeroes. This allows us to
contrast cases with self-citations included or excluded to avoid overvalued
journal bias. We estimate the model parameters by maximizing the marginal
likelihood with a Majorize-Minimize algorithm. The procedure ends up with a
score similar to the PageRank ones but with a damping factor depending on each
journal. The procedures are illustrated with an example about cross-citations
among 47 statistical journals studied by Varin et. al. (2016).
|
1707.09508v2
|
2017-08-09
|
Stable and unstable roots of ion temperature gradient driven mode using curvature modified plasma dispersion functions
|
Basic, local kinetic theory of ion temperature gradient driven (ITG) mode,
with adiabatic electrons is reconsidered. Standard unstable, purely oscillating
as well as damped solutions of the local dispersion relation are obtained using
a bracketing technique that uses the argument principle. This method requires
computing the plasma dielectric function and its derivatives, which are
implemented here using modified plasma dispersion functions with curvature and
their derivatives, and allows bracketing/following the zeros of the plasma
dielectric function which corresponds to different roots of the ITG dispersion
relation. We provide an open source implementation of the derivatives of
modified plasma dispersion functions with curvature, which are used in this
formulation. Studying the local ITG dispersion, we find that near the threshold
of instability the unstable branch is rather asymmetric with oscillating
solutions towards lower wave numbers (i.e. drift waves), and damped solutions
toward higher wave numbers. This suggests a process akin to inverse cascade by
coupling to the oscillating branch towards lower wave numbers may play a role
in the nonlinear evolution of the ITG, near the instability threshold. Also,
using the algorithm, the linear wave diffusion is estimated for the marginally
stable ITG mode.
|
1708.03026v1
|
2017-08-13
|
Observation of Spin Superfluidity in a Bose Gas Mixture
|
The spin dynamics of a harmonically trapped Bose-Einstein condensed binary
mixture of sodium atoms is experimentally investigated at finite temperature.
In the collisional regime the motion of the thermal component is shown to be
damped because of spin drag, while the two condensates exhibit a counter flow
oscillation without friction, thereby providing direct evidence for spin
superfluidity. Results are also reported in the collisionless regime where the
spin components of both the condensate and thermal part oscillate without
damping, their relative motion being driven by a mean field effect. We also
measure the static polarizability of the condensed and thermal parts and we
find a large increase of the condensate polarizability with respect to the T=0
value, in agreement with the predictions of theory.
|
1708.03923v2
|
2017-08-16
|
Multi-component plasmons in monolayer MoS$_2$ with circularly polarized optical pumping
|
By making use of circularly polarized light and electrostatic gating,
monolayer molybdenum disulfide (ML-MoS$_2$) can form a platform supporting
multiple types of charge carriers. They can be discriminated by their spin,
valley index or whether they're electrons or holes. We investigate the
collective properties of those charge carriers and are able to identify new
distinct plasmon modes. We analyze the corresponding dispersion relation,
lifetime and oscillator strength, and calculate the phase relation between the
oscillations in the different components of the plasmon modes. All platforms in
ML-MoS$_2$ support a long-wavelength $\sqrt{q}$ plasmon branch at zero Kelvin.
In addition to this, for an $n$-component system, $n-1$ distinct plasmon modes
appear as acoustic modes with linear dispersion in the long-wavelength limit.
These modes correspond to out-of-phase oscillations in the different Fermion
liquids and have, although being damped, a relatively long lifetime.
Additionally, we also find new distinct modes at large wave vector that are
stronger damped by intra-band processes.
|
1708.04940v1
|
2017-08-17
|
Viscous forces and bulk viscoelasticity near jamming
|
When weakly jammed packings of soft, viscous, non-Brownian spheres are probed
mechanically, they respond with a complex admixture of elastic and viscous
effects. While many of these effects are understood for specific, approximate
models of the particles' interactions, there are a number of proposed force
laws in the literature, especially for viscous interactions. We numerically
measure the complex shear modulus $G^*$ of jammed packings for various viscous
force laws that damp relative velocities between pairs of contacting particles
or between a particle and the continuous fluid phase. We find a surprising
sensitive dependence of $G^*$ on the viscous force law: the system may or may
not display dynamic critical scaling, and the exponents describing how $G^*$
scales with frequency can change. We show that this sensitivity is closely
linked to manner in which viscous damping couples to floppy-like, non-affine
motion, which is prominent near jamming.
|
1708.05241v2
|
2017-08-21
|
Diverging, but negligible power at Carnot efficiency: theory and experiment
|
We discuss the possibility of reaching the Carnot efficiency by heat engines
(HEs) out of quasi-static conditions at nonzero power output. We focus on
several models widely used to describe the performance of actual HEs. These
models comprise quantum thermoelectric devices, linear irreversible HEs,
minimally nonlinear irreversible HEs, HEs working in the regime of low
dissipation, over-damped stochastic HEs and an under-damped stochastic HE.
Although some of these HEs can reach the Carnot efficiency at nonzero and even
diverging power, the magnitude of this power is always negligible compared to
the maximum power attainable in these systems. We provide conditions for
attaining the Carnot efficiency in the individual models and explain practical
aspects connected with reaching the Carnot efficiency at large power output.
Furthermore, we show how our findings can be tested in practice using a
standard Brownian HE realizable with available micromanipulation techniques.
|
1708.06261v4
|
2017-08-24
|
The influence of small-scale magnetic field on the evolution of inclination angle and precession damping in the framework of 3-component model of neutron star
|
The evolution of inclination angle and precession damping of radio pulsars is
considered. It is assumed that the neutron star consists of 3 "freely" rotating
components: the crust and two core components, one of which contains pinned
superfluid vortices. We suppose that each component rotates as a rigid body.
Also the influence of the small-scale magnetic field on the star's braking
process is examined. Within the framework of this model the star simultaneously
can have glitch-like events combined with long-period precession (with periods
$10-10^{4}$ years). It is shown that the case of the small quantity of pinned
superfluid vortices seems to be more consistent with observations.
|
1708.07505v1
|
2017-08-25
|
On the ultimate energy bound of solutions to some forced second order evolution equations with a general nonlinear damping operator
|
Under suitable growth and coercivity conditions on the nonlinear damping
operator $g$ which ensure non-resonance, we estimate the ultimate bound of the
energy of the general solution to the equation $\ddot{u}(t) + Au(t) +
g(\dot{u}(t))=h(t),\quad t\in\mathbb{R}^+ ,$ where $A$ is a positive
selfadjoint operator on a Hilbert space $H$ and $h$ is a bounded forcing term
with values in $H$. In general the bound is of the form $ C(1+ ||h||^4)$ where
$||h||$ stands for the $L^\infty$ norm of $h$ with values in $H$ and the growth
of $g$ does not seem to play any role. If $g$ behaves lie a power for large
values of the velocity, the ultimate bound has a quadratic growth with respect
to $||h||$ and this result is optimal. If $h$ is anti periodic, we obtain a
much lower growth bound and again the result is shown to be optimal even for
scalar ODEs.
|
1708.07639v1
|
2017-08-25
|
On annihilation of the relativistic electron vortex pair in collisionless plasmas
|
In contrast to hydrodynamic vortices, vortices in plasma contain an electric
current circulating around the center of the vortex, which generates a magnetic
field localized inside. Using computer simulations, we demonstrate that the
magnetic field associated with the vortex gives rise to a mechanism of
dissipation of the vortex pair in a collisionless plasma, leading to fast
annihilation of the magnetic field with its energy transforming into the energy
of fast electrons, secondary vortices, and plasma waves. Two major contributors
to the energy damping of double vortex system, namely, magnetic field
annihilation and secondary vortex formation, are regulated by the size of the
vortex with respect to the electron skin depth, which scales with the electron
gamma-factor, $\gamma_e$, as $R/d_e \propto \gamma_e^{1/2}$. Magnetic field
annihilation appears to be dominant in mildly relativistic vortices, while for
the ultrarelativistic case, secondary vortex formation is the main channel for
damping of the initial double vortex system.
|
1708.07803v2
|
2017-08-30
|
Finite-temperature behavior of the Bose polaron
|
We consider a mobile impurity immersed in a Bose gas at finite temperature.
Using perturbation theory valid for weak coupling between the impurity and the
bosons, we derive analytical results for the energy and damping of the impurity
for low and high temperatures, as well as for temperatures close to the
critical temperature $T_c$ for Bose-Einstein condensation. These results show
that the properties of the impurity vary strongly with temperature. In
particular, the energy exhibits a non-monotonic behavior close to $T_c$, and
the damping rises sharply close to $T_c$. We argue that this behaviour is
generic for impurities immersed in an environment undergoing a phase transition
that breaks a continuous symmetry. Finally, we discuss how these effects can be
detected experimentally.
|
1708.09172v2
|
2017-09-01
|
Quantum reservoir engineering through quadratic optomechanical interaction in the reversed dissipation regime
|
We explore the electromagnetic field coupled to a mechanical resonator via
quadratic optomechanical interaction in the reversed dissipation regime where
the mechanical damping rate is much larger than the cavity field dissipation
rate. It is shown that in this regime, the cavity field effectively acquires an
additional reservoir which is conditioned by the temperature of the mechanical
bath as well as the mechanical damping rate. We analytically find the
steady-state mean photon number and the critical temperature of the mechanical
oscillator to cool or heat the coupled electromagnetic field. We also show that
in the case of quadratic coupling, the temperature of the mechanical oscillator
can be estimated in the quantum regime by observing the noise spectrum of the
cavity field.
|
1709.00279v1
|
2017-09-12
|
Green's function formalism for spin transport in metal-insulator-metal heterostructures
|
We develop a Green's function formalism for spin transport through
heterostructures that contain metallic leads and insulating ferromagnets. While
this formalism in principle allows for the inclusion of various magnonic
interactions, we focus on Gilbert damping. As an application, we consider
ballistic spin transport by exchange magnons in a metal-insulator-metal
heterostructure with and without disorder. For the former case, we show that
the interplay between disorder and Gilbert damping leads to spin current
fluctuations. For the case without disorder, we obtain the dependence of the
transmitted spin current on the thickness of the ferromagnet. Moreover, we show
that the results of the Green's function formalism agree in the clean and
continuum limit with those obtained from the linearized stochastic
Landau-Lifshitz-Gilbert equation. The developed Green's function formalism is a
natural starting point for numerical studies of magnon transport in
heterostructures that contain normal metals and magnetic insulators.
|
1709.03775v1
|
2017-09-13
|
Life-span of solutions to semilinear wave equation with time-dependent critical damping for specially localized initial data
|
This paper is concerned with the blowup phenomena for initial value problem
of semilinear wave equation with critical time-dependent damping term (DW). The
result is the sharp upper bound of lifespan of solution with respect to the
small parameter $\ep$ when $p_F(N)\leq p\leq p_0(N+\mu)$, where $p_F(N)$
denotes the Fujita exponent for the nonlinear heat equations and $p_0(n)$
denotes the Strauss exponent for nonlinear wave equation in $n$-dimension with
$\mu=0$. Consequently, by connecting the result of D'Abbicco--Lucente--Reissig
2015, our result clarifies the threshold exponent $p_0(N+\mu)$ for dividing
blowup phenomena and global existence of small solutions when $N=3$. The
crucial idea is to construct suitable test functions satisfying the conjugate
linear equation $\pa_t^2\Phi-\Delta \Phi-\pa_t(\frac{\mu}{1+t}\Phi)=0$ of (DW)
including the Gauss hypergeometric functions; note that the construction of
test functions is different from Zhou--Han in 2014.
|
1709.04406v1
|
2017-09-18
|
Optimal Energy Growth in Current Sheets
|
In this paper, we investigate the possibility of transient growth in the
linear perturbation of current sheets. The resistive magnetohydrodynamic (MHD)
operator for a background field consisting of a current sheet is non-normal,
meaning that associated eigenvalues and eigenmodes can be very sensitive to
perturbation. In a linear stability analysis of a tearing current sheet, we
show that modes that are damped as $t\rightarrow\infty$ can produce transient
energy growth, contributing faster growth rates and higher energy attainment
(within a fixed finite time) than the unstable tearing mode found from
normal-mode analysis. We determine the transient growth for tearing-stable and
tearing-unstable regimes and discuss the consequences of our results for
processes in the solar atmosphere, such as flares and coronal heating. Our
results have significant potential impact on how fast current sheets can be
disrupted. In particular, transient energy growth due to (asymptotically)
damped modes may lead to accelerated current sheet thinning and, hence, a
faster onset of the plasmoid instability, compared to the rate determined by
the tearing mode alone.
|
1709.05858v1
|
2017-09-20
|
Spin-rotation mode in a quantum Hall ferromagnet
|
A spin-rotation mode emerging in a quantum Hall ferromagnet due to laser
pulse excitation is studied. This state, macroscopically representing a
rotation of the entire electron spin-system to a certain angle, is not
microscopically equivalent to a coherent turn of all spins as a single-whole
and is presented in the form of a combination of eigen quantum states
corresponding to all possible S_z spin numbers. The motion of the macroscopic
quantum state is studied microscopically by solving a non-stationary
Schroedinger equation and by means of a kinetic approach where damping of the
spin-rotation mode is related to an elementary process, namely, transformation
of a `Goldstone spin exciton' to a `spin-wave exciton'. The system exhibits a
spin stochastizationa mechanism (determined by spatial fluctuations of the
Land'e g-factor) ensuring damping, transverse spin relaxation, but irrelevant
to decay of spin-wave excitons and thus not involving longitudinal relaxation,
i.e., recovery of the S_z number to its equilibrium value.
|
1709.06811v2
|
2017-09-22
|
Muon spin rotation study of the topological superconductor SrxBi2Se3
|
We report transverse-field (TF) muon spin rotation experiments on single
crystals of the topological superconductor Sr$_x$Bi$_2$Se$_3$ with nominal
concentrations $x=0.15$ and $0.18$ ($T_c \sim 3$ K). The TF spectra ($B= 10$
mT), measured after cooling to below $T_c$ in field, did not show any
additional damping of the muon precession signal due to the flux line lattice
within the experimental uncertainty. This puts a lower bound on the magnetic
penetration depth $\lambda \geq 2.3 ~\mu$m. However, when we induce disorder in
the vortex lattice by changing the magnetic field below $T_c$ a sizeable
damping rate is obtained for $T \rightarrow 0$. The data provide microscopic
evidence for a superconducting volume fraction of $\sim 70~ \%$ in the $x=0.18$
crystal and thus bulk superconductivity.
|
1709.07547v1
|
2017-09-22
|
Slowly damped quasinormal modes of the massive Dirac field in d-dimensional Tangherlini spacetime
|
We consider quasinormal modes of the massive Dirac field in the background of
a Schwarzschild- Tangherlini black hole. Different dimensions of the spacetime
are considered, from d = 4 to d = 9. The quasinormal modes are calculated using
two independent methods: WKB and continued fraction. We obtain the spectrum of
quasinormal modes for different values of the overtone number and angular
quantum number. An analytical approximation of the spectrum valid in the case
of large values of the angular quantum number and mass is calculated. Although
we don't find unstable modes in the spectrum, we show that for large values of
the mass, the quasinormal modes can become very slowly damped, giving rise to
quasistationary perturbations.
|
1709.07864v3
|
2017-09-20
|
Non-Adiabatic Vibrational Damping of Molecular Adsorbates: Insights into Electronic Friction and the Role of Electronic Coherence
|
We present a perturbation approach rooted in time-dependent
density-functional theory to calculate electron hole (eh)-pair excitation
spectra during the non-adiabatic vibrational damping of adsorbates on metal
surfaces. Our analysis for the benchmark systems CO on Cu(100) and Pt(111)
elucidates the surprisingly strong influence of rather short electronic
coherence times. We demonstrate how in the limit of short electronic coherence
times, as implicitly assumed in prevalent quantum nuclear theories for the
vibrational lifetimes as well as electronic friction, band structure effects
are washed out. Our results suggest that more accurate lifetime or
chemicurrent-like experimental measurements could characterize the electronic
coherence.
|
1709.08003v1
|
2017-09-28
|
Phenomenological model for predicting stationary and non-stationary spectra of wave turbulence in vibrating plates
|
A phenomenological model describing the time-frequency dependence of the
power spectrum of thin plates vibrating in a wave turbulence regime, is
introduced. The model equation contains as basic solutions the Rayleigh-Jeans
equipartition of energy, as well as the Kolmogorov-Zakharov spectrum of wave
turbulence. In the Wave Turbulence Theory framework, the model is used to
investigate the self-similar, non-stationary solutions of forced and free
turbulent vibrations. Frequency-dependent damping laws can easily be accounted
for. Their effects on the characteristics of the stationary spectra of
turbulence are then investigated. Thanks to this analysis, self-similar
universal solutions are given, relating the power spectrum to both the injected
power and the damping law.
|
1709.09884v1
|
Subsets and Splits
No community queries yet
The top public SQL queries from the community will appear here once available.