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2015-04-29
|
Wide-Range Tunable Dynamic Property of Carbon Nanotube-Based Fibers
|
Carbon nanotube (CNT) fiber is formed by assembling millions of individual
tubes. The assembly feature provides the fiber with rich interface structures
and thus various ways of energy dissipation, as reflected by the non-zero loss
tangent (>0.028--0.045) at low vibration frequencies. A fiber containing
entangled CNTs possesses higher loss tangents than a fiber spun from aligned
CNTs. Liquid densification and polymer infiltration, the two common ways to
increase the interfacial friction and thus the fiber's tensile strength and
modulus, are found to efficiently reduce the damping coefficient. This is
because the sliding tendency between CNT bundles can also be well suppressed by
the high packing density and the formation of covalent polymer cross-links
within the fiber. The CNT/bismaleimide composite fiber exhibited the smallest
loss tangent, nearly as the same as that of carbon fibers. At a higher level of
the assembly structure, namely a multi-ply CNT yarn, the inter-fiber friction
and sliding tendency obviously influence the yarn's damping performance, and
the loss tangent can be tuned within a wide range, as similar to carbon fibers,
nylon yarns, or cotton yarns. The wide-range tunable dynamic properties allow
new applications ranging from high quality factor materials to dissipative
systems.
|
1504.07881v1
|
2015-05-10
|
Effects of electron drift on the collisionless damping of kinetic Alfvén waves in the solar wind
|
The collisionless dissipation of anisotropic Alfv\'enic turbulence is a
promising candidate to solve the solar wind heating problem. Extensive studies
examined the kinetic properties of Alfv\'en waves in simple Maxwellian or
bi-Maxwellian plasmas. However, the observed electron velocity distribution
functions in the solar wind are more complex. In this study, we analyze the
properties of kinetic Alfv\'en waves in a plasma with two drifting electron
populations. We numerically solve the linearized Maxwell-Vlasov equations and
find that the damping rate and the proton-electron energy partition for kinetic
Alfv\'en waves are significantly modified in such plasmas, compared to plasmas
without electron drifts. We suggest that electron drift is an important factor
to take into account when considering the dissipation of Alfv\'enic turbulence
in the solar wind or other $\beta \sim 1$ astrophysical plasmas.
|
1505.02328v1
|
2015-05-13
|
The effect of a reversible shear transformation on plastic deformation of an amorphous solid
|
Molecular dynamics simulations are performed to investigate the plastic
response of a model glass to a local shear transformation in a quiescent
system. The deformation of the material is induced by a spherical inclusion
that is gradually strained into an ellipsoid of the same volume and then
reverted back into the sphere. We show that the number of cage-breaking events
increases with increasing strain amplitude of the shear transformation. The
results of numerical simulations indicate that the density of cage jumps is
larger in the cases of weak damping or slow shear transformation. Remarkably,
we also found that, for a given strain amplitude, the peak value of the density
profiles is a function of the ratio of the damping coefficient and the time
scale of the shear transformation.
|
1505.03488v1
|
2015-05-14
|
Local large deviations principle for occupation measures of the damped nonlinear wave equation perturbed by a white noise
|
We consider the damped nonlinear wave (NLW) equation driven by a spatially
regular white noise. Assuming that the noise is non-degenerate in all Fourier
modes, we establish a large deviations principle (LDP) for the occupation
measures of the trajectories. The lower bound in the LDP is of a local type,
which is related to the weakly dissipative nature of the equation and seems to
be new in the context of randomly forced PDE's. The proof is based on an
extension of methods developed in \cite{JNPS-2012} and \cite{JNPS-2014} in the
case of kick forced dissipative PDE's with parabolic regularisation property
such as, for example, the Navier-Stokes system and the complex Ginzburg-Landau
equations. We also show that a high concentration towards the stationary
measure is impossible, by proving that the rate function that governs the LDP
cannot have the trivial form (i.e., vanish on the stationary measure and be
infinite elsewhere).
|
1505.03686v1
|
2015-05-19
|
Undamped nonequilibrium dynamics of a nondegenerate Bose gas in a 3D isotropic trap
|
We investigate anomalous damping of the monopole mode of a non-degenerate 3D
Bose gas under isotropic harmonic confinement as recently reported by the JILA
TOP trap experiment [D. S. Lob- ser, A. E. S. Barentine, E. A. Cornell, and H.
J. Lewandowski (in preparation)]. Given a realistic confining potential, we
develop a model for studying collective modes that includes the effects of
anharmonic corrections to a harmonic potential. By studying the influence of
these trap anharmonicities throughout a range of temperatures and collisional
regimes, we find that the damping is caused by the joint mechanisms of
dephasing and collisional relaxation. Furthermore, the model is complimented by
Monte Carlo simulations which are in fair agreement with data from the JILA
experiment.
|
1505.04841v1
|
2015-05-21
|
Control and stabilization of degenerate wave equations
|
We study a wave equation in one space dimension with a general diffusion
coefficient which degenerates on part of the boundary. Degeneracy is measured
by a real parameter $\mu_a>0$. We establish observability inequalities for
weakly (when $\mu_a \in [0,1[$) as well as strongly (when $\mu_a \in [1,2[$)
degenerate equations. We also prove a negative result when the diffusion
coefficient degenerates too violently (i.e. when $\mu_a>2$) and the blow-up of
the observability time when $\mu_a$ converges to $2$ from below. Thus, using
the HUM method we deduce the exact controllability of the corresponding
degenerate control problem when $\mu_a \in [0,2[$. We conclude the paper by
studying the boundary stabilization of the degenerate linearly damped wave
equation and show that a suitable boundary feedback stabilizes the system
exponentially. We extend this stability analysis to the degenerate nonlinearly
boundary damped wave equation, for an arbitrarily growing nonlinear feedback
close to the origin. This analysis proves that the degeneracy does not affect
the optimal energy decay rates at large time. We apply the optimal-weight
convexity method of \cite{alaamo2005, alajde2010} together with the results of
the previous section, to perform this stability analysis.
|
1505.05720v1
|
2015-05-23
|
Existence, blow-up and exponential decay of solutions for a porous-elastic system with damping and source terms
|
In this paper we consider a porous-elastic system consisting of nonlinear
boundary/interior damping and nonlinear boundary/interior sources. Our interest
lies in the theoretical understanding of the existence, finite time blow-up of
solutions and their exponential decay using non-trivial adaptations of
well-known techniques. First, we apply the conventional Faedo-Galerkin method
with standard arguments of density on the regularity of initial conditions to
establish two local existence theorems of weak solutions. Moreover, we detail
the uniqueness result in some specific cases. In the second theme, we prove
that any weak solution possessing negative initial energy has the latent
blow-up in finite time. Finally, we obtain the so-called exponential decay
estimates for the global solution under the construction of a suitable Lyapunov
functional. In order to corroborate our theoretical decay, a numerical example
is provided.
|
1505.06373v2
|
2015-05-26
|
Quantum Particle Motion in Absorbing Harmonic Trap
|
The motivation of this work is to get an additional insight into the
irreversible energy dissipation on the quantum level. The presented examination
procedure is based on the Feynman path integral method that is applied and
widened towards the calculation of the kernel of a quantum mechanical damped
oscillator. Here, it is shown that the energy loss of the oscillator can be
generated by the introduced harmonic complex potential. The related damped wave
function, however, does not pertain to the probability meaning as it is usual
in the case of complex absorbing potentials. This decrease of the wave function
is evaluated, moreover, the energy dissipation and the measure the
irreversibility are expressed.
|
1505.06874v1
|
2015-06-03
|
Non-uniqueness of Admissible Weak Solutions to Compressible Euler Systems with Source Terms
|
We consider admissible weak solutions to the compressible Euler system with
source terms, which include rotating shallow water system and the Euler system
with damping as special examples. In the case of anti-symmetric sources such as
rotations, for general piecewise Lipschitz initial densities and some suitably
constructed initial momentum, we obtain infinitely many global admissible weak
solutions. Furthermore, we construct a class of finite-states admissible weak
solutions to the Euler system with anti-symmetric sources. Under the additional
smallness assumption on the initial densities, we also obtain multiple
global-in-time admissible weak solutions for more general sources including
damping. The basic framework are based on the convex integration method
developed by De~Lellis and Sz\'{e}kelyhidi \cite{dLSz1,dLSz2} for the Euler
system. One of the main ingredients of this paper is the construction of
specified localized plane wave perturbations which are compatible with a given
source term.
|
1506.01103v1
|
2015-06-03
|
The effect of perpendicular electric field on Temperature-induced plasmon excitations for intrinsic silicene
|
We use the tight-binding model and the random-phase approximation to
investigate the intrinsic plasmon in silicene. At finite temperatures, an
undamped plasmon is generated from the interplay between the intraband and the
interband-gap transitions. The extent of the plasmon existence range in terms
of momentum and temperature, which is dependent on the size of
single-particle-excitation gap, is further tuned by applying a perpendicular
electric field. The plasmon becomes damped in the interband-excitation region.
A low damped zone is created by the field-induced spin split. The
field-dependent plasmon spectrum shows a strong tunability in plasmon intensity
and spectral bandwidth. This could make silicene a very suitable candidate for
plasmonic applications.
|
1506.01140v1
|
2015-06-09
|
Transport Barrier generation at the interface of regions with different zonal flows dynamics
|
A novel and generic understanding of spontaneous generation of transport
barriers and zonation regimes in turbulent self-organization is presented. It
associates the barrier onset to the development of a spectral gap between large
scale flows and turbulence modes leading to a zonation regime. A robust barrier
builds-up at the interface of such a region and a neighboring one with reduced
zonal flow generation. This more complex and generic transition paradigm could
fit the numerous and sometimes conflicting observations as in fusion plasma
experiments. Barrier relaxation by bursts of turbulence regenerate the zonal
flows that are eroded by viscous (collisional-like) damping. The duration of
the quiescent phase between the quasi-periodic relaxation events is governed by
this damping process, hence the barrier collision frequency for fusion plasmas.
|
1506.02942v2
|
2015-06-11
|
Fighting noise with noise in realistic quantum teleportation
|
We investigate how the efficiency of the quantum teleportation protocol is
affected when the qubits involved in the protocol are subjected to noise or
decoherence. We study all types of noise usually encountered in real world
implementations of quantum communication protocols, namely, the bit flip, phase
flip (phase damping), depolarizing, and amplitude damping noise. Several
realistic scenarios are studied in which a part or all of the qubits employed
in the execution of the quantum teleportation protocol are subjected to the
same or different types of noise. We find noise scenarios not yet known in
which more noise or less entanglement lead to more efficiency. Furthermore, we
show that if noise is unavoidable it is better to subject the qubits to
different noise channels in order to obtain an increase in the efficiency of
the protocol.
|
1506.03803v2
|
2015-07-08
|
Low-Dimensional Stochastic Projected Gross-Pitaevskii Equation
|
We present reduced-dimensional stochastic projected Gross-Pitaevskii
equations describing regimes of confinement and temperature where a 1D or 2D
superfluid is immersed in a 3D thermal cloud. The projection formalism provides
both a formally rigorous and physically natural way to effect the dimensional
reduction. The 3D form of the number-damping (growth) terms is unchanged by the
dimensional reduction. Projection of the energy-damping (scattering) terms
leads to modified stochastic equations of motion describing energy exchange
with the thermal reservoir. The regime of validity of the dimensional reduction
is investigated via variational analysis. Paying particular attention to 1D, we
validate our variational treatment by comparing numerical simulations of a
trapped oblate system in 3D with the 1D theory, and establish a consistent
choice of cutoff for the 1D theory. We briefly discuss the scenario involving
two-components with different degeneracy, suggesting that a wider regime of
validity exists for systems in contact with a buffer-gas reservoir.
|
1507.02023v2
|
2015-07-08
|
Low-emittance storage rings
|
The effects of synchrotron radiation on particle motion in storage rings are
discussed. In the absence of radiation, particle motion is symplectic, and the
beam emittances are conserved. The inclusion of radiation effects in a
classical approximation leads to emittance damping: expressions for the damping
times are derived. Then, it is shown that quantum radiation effects lead to
excitation of the beam emittances. General expressions for the equilibrium
longitudinal and horizontal (natural) emittances are derived. The impact of
lattice design on the natural emittance is discussed, with particular attention
to the special cases of FODO, achromat, and TME style lattices. Finally, the
effects of betatron coupling and vertical dispersion (generated by magnet
alignment and lattice tuning errors) on the vertical emittance are considered.
|
1507.02213v1
|
2015-07-09
|
Controlling the stability of steady states in continuous variable quantum systems
|
For the paradigmatic case of the damped quantum harmonic oscillator we
present two measurement-based feedback schemes to control the stability of its
fixed point. The first scheme feeds back a Pyragas-like time-delayed reference
signal and the second uses a predetermined instead of time-delayed reference
signal. We show that both schemes can reverse the effect of the damping by
turning the stable fixed point into an unstable one. Finally, by taking the
classical limit $\hbar\rightarrow0$ we explicitly distinguish between inherent
quantum effects and effects, which would be also present in a classical noisy
feedback loop. In particular, we point out that the correct description of a
classical particle conditioned on a noisy measurement record is given by a
non-linear stochastic Fokker-Planck equation and not a Langevin equation, which
has observable consequences on average as soon as feedback is considered.
|
1507.02538v1
|
2015-07-14
|
WIMP isocurvature perturbation and small scale structure
|
The adiabatic perturbation of dark matter is damped during the kinetic
decoupling due to the collision with relativistic component on sub-horizon
scales. However the isocurvature part is free from damping and could be large
enough to make a substantial contribution to the formation of small scale
structure. We explicitly study the weakly interacting massive particles as dark
matter with an early mater dominated period before radiation domination and
show that the isocurvature perturbation is generated during the phase
transition and leaves imprint in the observable signatures for small scale
structure.
|
1507.03871v2
|
2015-07-17
|
Pairwise quantum discord for a symmetric multi-qubit system in different types of noisy channels
|
We study the pairwise quantum discord (QD) for a symmetric multi-qubit system
in different types of noisy channels, such as phase-flip, amplitude damping,
phase-damping, and depolarizing channels. Using the QD and geometric measure of
quantum discord (GMQD) to quantify quantum correlations, some analytical or
numerical results are presented. The results show that, the dynamics of the
pairwise QD is related to the number of spin particles $N$ as well as initial
parameter $\theta$ of the one-axis twisting collective state. With the number
of spin particles $N$ increasing, the amount of the pairwise QD increases.
However, when the amount of the pairwise QD arrives at a stable maximal value,
the pairwise QD is independence of the number of spin particles $N$ increasing.
The behavior of the pairwise QD is symmetrical during a period $0\leq \theta
\leq 2\pi$. Moreover, we compare the pairwise QD dynamics with the GMQD for a
symmetric multi-qubit system in different types of noisy channels.
|
1507.04826v1
|
2015-07-20
|
Landau damping effects in the synchronization of conformist and contrarian oscillators
|
Two decades ago, a phenomenon resembling Landau damping was described in the
synchronization of globally coupled oscillators: the evidence of a regime where
the order parameter decays when linear theory predicts neutral stability for
the incoherent state. We here show that such an effect is far more generic, as
soon as phase oscillators couple to their mean field according to their natural
frequencies, being then grouped into two distinct populations of conformists
and contrarians. We report the analytical solution of this latter situation,
which allows determining the critical coupling strength and the stability of
the incoherent state, together with extensive numerical simulations that fully
support all theoretical predictions. The relevance of our results is discussed
in relationship to collective phenomena occurring in polarized social systems.
|
1507.05383v1
|
2015-07-28
|
Attractors for Strongly Damped Wave Equations with Nonlinear Hyperbolic Dynamic Boundary Conditions
|
We establish the well-posedness of a strongly damped semilinear wave equation
equipped with nonlinear hyperbolic dynamic boundary conditions. Results are
carried out with the presence of a parameter distinguishing whether the
underlying operator is analytic, $\alpha>0$, or only of Gevrey class,
$\alpha=0$. We establish the existence of a global attractor for each
$\alpha\in[0,1],$ and we show that the family of global attractors is
upper-semicontinuous as $\alpha\rightarrow0.$ Furthermore, for each
$\alpha\in[0,1]$, we show the existence of a weak exponential attractor. A weak
exponential attractor is a finite dimensional compact set in the weak topology
of the phase space. This result insures the corresponding global attractor also
possess finite fractal dimension in the weak topology; moreover, the dimension
is independent of the perturbation parameter $\alpha$. In both settings,
attractors are found under minimal assumptions on the nonlinear terms.
|
1507.07971v1
|
2015-07-29
|
Neimark--Sacker bifurcation and evidence of chaos in a discrete dynamical model of walkers
|
Bouncing droplets on a vibrating fluid bath can exhibit wave-particle
behavior, such as being propelled by interacting with its own wave field. These
droplets seem to walk across the bath, and thus are dubbed walkers. Experiments
have shown that walkers can exhibit exotic dynamical behavior indicative of
chaos. While the integro-differential models developed for these systems agree
well with the experiments, they are difficult to analyze mathematically. In
recent years, simpler discrete dynamical models have been derived and studied
numerically. The numerical simulations of these models show evidence of exotic
dynamics such as period doubling bifurcations, Neimark--Sacker (N--S)
bifurcations, and even chaos. For example, in [Gilet, PRE 2014], based on
simulations Gilet conjectured the existence of a supercritical N-S bifurcation
as the damping factor in his one-dimensional path model. We prove Gilet's
conjecture and more; in fact, both supercritical and subcritical (N-S)
bifurcations are produced by separately varying the damping factor and
wave-particle coupling for all eigenmode shapes. Then we compare our
theoretical results with some previous and new numerical simulations, and find
complete qualitative agreement. Furthermore, evidence of chaos is shown by
numerically studying a global bifurcation.
|
1507.08057v2
|
2015-07-30
|
Beliaev damping of the Goldstone mode in atomic Fermi superfluids
|
Beliaev damping in a superfluid is the decay of a collective excitation into
two lower frequency collective excitations; it represents the only decay mode
for a bosonic collective excitation in a superfluid at T = 0. The standard
treatment for this decay assumes a linear spectrum, which in turn implies that
the final state momenta must be collinear to the initial state. We extend this
treatment, showing that the inclusion of a gradient term in the Hamiltonian
yields a realistic spectrum for the bosonic excitations; we then derive a
formula for the decay rate of such excitations, and show that even moderate
nonlinearities in the spectrum can yield substantial deviations from the
standard result. We apply our result to an attractive Fermi gas in the BCS-BEC
crossover: here the low-energy bosonic collective excitations are density
oscillations driven by the phase of the pairing order field. These collective
excitations, which are gapless modes as a consequence of the Goldstone
mechanism, have a spectrum which is well established both theoretically and
experimentally, and whose linewidth, we show, is determined at low temperatures
by the Beliaev decay mechanism.
|
1507.08535v1
|
2015-08-09
|
Small quantum absorption refrigerator in the transient regime: time scales, enhanced cooling and entanglement
|
A small quantum absorption refrigerator, consisting of three qubits, is
discussed in the transient regime. We discuss time scales for coherent
dynamics, damping, and approach to the steady state, and we study cooling and
entanglement. We observe that cooling can be enhanced in the transient regime,
in the sense that lower temperatures can be achieved compared to the
steady-state regime. This is a consequence of coherent dynamics, but can occur
even when this dynamics is strongly damped by the dissipative thermal
environment, and we note that precise control over couplings or timing is not
needed to achieve enhanced cooling. We also show that the amount of
entanglement present in the refrigerator can be much larger in the transient
regime compared to the steady-state. These results are of relevance to future
implementations of quantum thermal machines.
|
1508.02025v2
|
2015-08-13
|
Hybridization of Higgs modes in a bond-density-wave state in cuprates
|
Recently, several groups have reported observations of collective modes of
the charge order present in underdoped cuprates. Motivated by these
experiments, we study theoretically the oscillations of the order parameters,
both in the case of pure charge order, and for charge order coexisting with
superconductivity. Using a hot-spot approximation we find in the coexistence
regime two Higgs modes arising from hybridization of the amplitude oscillations
of the different order parameters. One of them has a minimum frequency that is
within the single particle energy gap and which is a non-monotonic function of
temperature. The other -- high-frequency -- mode is smoothly connected to the
Higgs mode in the single-order-parameter region, but quickly becomes overdamped
in the case of coexistence. We explore an unusual low-energy damping channel
for the collective modes, which relies on the band reconstruction caused by the
coexistence of the two orders. For completeness, we also consider the damping
of the collective modes originating from the nodal quasiparticles. At the end
we discuss some experimental consequences of our results.
|
1508.03335v1
|
2015-08-18
|
Negative nonlinear damping of a graphene mechanical resonator
|
We experimentally investigate the nonlinear response of a multilayer graphene
resonator using a superconducting microwave cavity to detect its motion. The
radiation pressure force is used to drive the mechanical resonator in an
optomechanically induced transparency configuration. By varying the amplitudes
of drive and probe tones, the mechanical resonator can be brought into a
nonlinear limit. Using the calibration of the optomechanical coupling, we
quantify the mechanical Duffing nonlinearity. By increasing the drive force, we
observe a decrease in the mechanical dissipation rate at large amplitudes,
suggesting a negative nonlinear damping mechanism in the graphene resonator.
Increasing the optomechanical backaction, we observe a nonlinear regime not
described by a Duffing response that includes new instabilities of the
mechanical response.
|
1508.04298v1
|
2015-08-21
|
Formulation and solutions of fractional continuously variable order mass spring damper systems controlled by viscoelastic and viscous-viscoelastic dampers
|
The article presents the formulation and a new approach to find analytic
solutions for fractional continuously variable order dynamic models viz.
Fractional continuously variable order mass-spring damper systems. Here, we use
the viscoelastic and viscous-viscoelastic dampers for describing the damping
nature of the oscillating systems, where the order of fractional derivative
varies continuously. Here, we handle the continuous changing nature of
fractional order derivative for dynamic systems, which has not been studied
yet. By successive iteration method, here we find the solution of fractional
continuously variable order mass-spring damper systems, and then give a close
form solution. We then present and discuss the solutions obtained in the cases
with continuously variable order of damping for this oscillator with graphical
plots.
|
1508.06202v1
|
2015-08-25
|
Simultaneous cooling of coupled mechanical oscillators using whispering gallery mode resonances
|
We demonstrate simultaneous center-of-mass cooling of two coupled
oscillators, consisting of a microsphere-cantilever and a tapered optical
fiber. Excitation of a whispering gallery mode (WGM) of the microsphere, via
the evanescent field of the taper, provides a transduction signal that
continuously monitors the relative motion between these two microgram objects
with a sensitivity of 3 pm. The cavity enhanced optical dipole force is used to
provide feedback damping on the motion of the micron-diameter taper, whereas a
piezo stack is used to damp the motion of the much larger (up to $180\,\mu$m in
diameter), heavier (up to $1.5\times 10^{-7}\,$kg) and stiffer
microsphere-cantilever. In each feedback scheme multiple mechanical modes of
each oscillator can be cooled, and mode temperatures below 10 K are reached for
the dominant mode, consistent with limits determined by the measurement noise
of our system. This represents stabilization on the picometer level and is the
first demonstration of using WGM resonances to cool the mechanical modes of
both the WGM resonator and its coupling waveguide.
|
1508.06217v2
|
2015-08-31
|
Damping of a system of linear oscillators using the generalized dry friction
|
The problem of damping a system of linear oscillators is considered. The
problem is solved by using a control in the form of dry friction. The motion of
the system under the control is governed by a system of differential equations
with discontinuous right-hand side. A uniqueness and continuity theorem is
proved for the phase flow of this system. Thus, the control in the form of
generalized dry friction defines the motion of the system of oscillators
uniquely.
|
1508.07775v2
|
2015-09-02
|
Smoothing and Global Attractors for the Majda-Biello System on the Torus
|
In this paper, we consider the Majda-Biello system, a coupled KdV-type
system, on the torus. In the first part of the paper, it is shown that, given
initial data in a Sobolev space, the difference between the linear and the
nonlinear evolution almost always resides in a smoother space. The smoothing
index depends on number-theoretic properties of the coupling parameter in the
system which control the behavior of the resonant sets. In the second part of
the paper, we consider the forced and damped version of the system and obtain
similar smoothing estimates. These estimates are used to show the existence of
a global attractor in the energy space. We also show that when the damping is
large in relation to the forcing terms, the attractor is trivial.
|
1509.00776v1
|
2015-09-07
|
Non-linear Cosmic Ray propagation close to the acceleration site
|
Recent advances on gamma-ray observations from SuperNova Remnants and
Molecular Clouds offer the possibility to study in detail the properties of the
propagation of escaping Cosmic Rays (CR). However, a complete theory for CR
transport outside the acceleration site has not been developed yet. Two
physical processes are thought to be relevant to regulate the transport: the
growth of waves caused by streaming instability, and possible wave damping
mechanisms that reduce the growth of the turbulence. Only a few attempts have
been made so far to incorporate these mechanisms in the theory of CR diffusion.
In this work we present recent advances in this subject. In particular, we show
results obtained by solving the coupled equations for the diffusion of CRs and
the evolution of Alfven waves. We discuss the importance of streaming
instabilities and wave damping in different ISM phases.
|
1509.02174v1
|
2015-09-10
|
Strain-dependent damping in nanomechanical resonators from thin $\mathrm{MoS_2}$ crystals
|
We investigate the effect of mechanical strain on the dynamics of thin
$\mathrm{MoS_2}$ nanodrum resonators. Using a piezoelectric crystal,
compressive and tensile biaxial strain is induced in initially flat and buckled
devices. In the flat device, we observe a remarkable strain-dependence of the
resonance line width, while the change in the resonance frequency is relatively
small. In the buckled device, the strain-dependence of the damping is less
pronounced, and a clear hysteresis is observed. The experiment suggests that
geometric imperfections, such as microscopic wrinkles, could play a role in the
strong dissipation observed in nanoresonators fabricated from 2-D materials.
|
1509.03080v1
|
2015-09-16
|
Variational principle for magnetisation dynamics in a temperature gradient
|
By applying a variational principle on a magnetic system within the framework
of extended irreversible thermodynamics, we find that the presence of a
temperature gradient in a ferromagnet leads to a generalisation of the
Landau-Lifshitz equation with an additional magnetic induction field
proportional to the temperature gradient. This field modulates the damping of
the magnetic excitation. It can increase or decrease the damping, depending on
the orientation of the magnetisation wave-vector with respect to the
temperature gradient. This variational approach confirms the existence of the
Magnetic Seebeck effect which was derived from thermodynamics and provides a
quantitative estimate of the strength of this effect.
|
1509.04825v2
|
2015-10-01
|
Dilapidation of nonlocal correlations of two qubit states in noisy environment
|
Composite quantum systems exhibit non-local correlations. These counter
intuitive correlations form a resource for quantum information processing and
quantum computation. In our previous work on two qubit maximally entangled
mixed states, we observed that entangled states, states that can be used for
quantum teleportaion, states that violate Bell-CHSH inequality and states that
do not admit local hidden variable description is the hierarchy in terms of the
order of nonlocal correlations. In order to establish this hierarchy, in the
present work, we investigate the effect of noise on two quibt states that
exhibit higher order nonlocal correlations. We find that dilapidation of
nonlocal correlations in the presence of noise follow the same hierarchy, that
is, higher order nonlocal correlation disappears for small strength of noise,
where as lower order nonlocal correlations survive strong noisy environment. We
show the results for decoherence due to amplitude damping channel on various
quantum states. However, we observe that same hierarchy is followed by states
undergoing decoherence due to phase damping as well as depolarizing channels.
|
1510.00119v1
|
2015-10-01
|
Production of charged Higgs boson pairs in the $pp \to ppH^{+}H^{-}$ reaction at the LHC and FCC
|
We present differential cross sections for the $pp \to ppH^{+}H^{-}$ reaction
via photon-photon fusion with exact kinematics. We show predictions for
$\sqrt{s}$ = 14 TeV (LHC) and at the Future Circular Collider (FCC) for
$\sqrt{s}$ = 100 TeV. The integrated cross section for $\sqrt{s}$ = 14~TeV
(LHC) is about 0.1~fb and about 0.9~fb at the FCC for $\sqrt{s}$ = 100~TeV when
assuming $m_{H^{\pm}} = 150$~GeV. We present distributions in diHiggs boson
invariant mass. The results are compared with those obtained within
equivalent-photon approximation. We discuss also first calculations of cross
section for exclusive diffractive pQCD mechanism with estimated limits on the
$g_{hH^{+}H^{-}}$ coupling constant within 2HDM based on the LHC experimental
data. The diffractive contribution is much smaller than the $\gamma \gamma$
one. Absorption corrections are calculated differentially for various
distributions. In general, they lead to a damping of the cross section. The
damping depends on $M_{H^{+}H^{-}}$ invariant mass and on four-momentum
transfers squared in the proton line. We discuss a possibility to measure the
exclusive production of $H^{\pm}$ bosons.
|
1510.00171v1
|
2015-10-05
|
Universality in antiferromagnetic strange metals
|
We propose a theory of metals at the spin-density wave quantum critical point
in spatial dimension $d=2$. We provide a first estimate of the full set of
critical exponents (dynamical exponent $z=2.13$, correlation length $\nu
=1.02$, spin susceptibility $\gamma = 0.96$, electronic non-Fermi liquid
$\eta^f_\tau = 0.53$, spin-wave Landau damping $\eta^b_\tau = 1.06$), which
determine the universal power-laws in thermodynamics and response functions in
the quantum-critical regime relevant for experiments in heavy-fermion systems
and iron pnictides. We present approximate numerical and analytical solutions
of Polchinski-Wetterich type flow equations with soft frequency regulators for
an effective action of electrons coupled to spin-wave bosons. Performing the
renormalization group in frequency -instead of momentum- space allows to track
changes of the Fermi surface shape and to capture Landau damping during the
flow. The technique is easily generalizable from models retaining only patches
of the Fermi surface to full, compact Fermi surfaces.
|
1510.01331v3
|
2015-10-09
|
Spin-orbit torque in Pt/CoNiCo/Pt symmetric devices
|
Current induced magnetization switching by spin-orbit torques offers an
energy-efficient means of writing information in heavy metal/ferromagnet (FM)
multilayer systems. The relative contributions of field-like torques and
damping-like torques to the magnetization switching induced by the electrical
current are still under debate. Here, we describe a device based on a symmetric
Pt/FM/Pt structure, in which we demonstrate a strong damping-like torque from
the spin Hall effect and unmeasurable field-like torque from Rashba effect. The
spin-orbit effective fields due to the spin Hall effect were investigated
quantitatively and were found to be consistent with the switching effective
fields after accounting for the switching current reduction due to thermal
fluctuations from the current pulse. A non-linear dependence of deterministic
switching of average Mz on the in-plane magnetic field was revealed, which
could be explained and understood by micromagnetic simulation.
|
1510.02555v1
|
2015-10-15
|
On the global existence and blowup of smooth solutions of 3-D compressible Euler equations with time-depending damping
|
In this paper, we are concerned with the global existence and blowup of
smooth solutions of the 3-D compressible Euler equation with time-depending
damping $$
\partial_t\rho+\operatorname{div}(\rho u)=0, \quad
\partial_t(\rho u)+\operatorname{div}\left(\rho u\otimes
u+p\,I_{3}\right)=-\,\frac{\mu}{(1+t)^{\lambda}}\,\rho u, \quad
\rho(0,x)=\bar \rho+\varepsilon\rho_0(x),\quad u(0,x)=\varepsilon u_0(x), $$
where $x\in\mathbb R^3$, $\mu>0$, $\lambda\geq 0$, and $\bar\rho>0$ are
constants, $\rho_0,\, u_0\in C_0^{\infty}(\mathbb R^3)$, $(\rho_0,
u_0)\not\equiv 0$, $\rho(0,\cdot)>0$, and $\varepsilon>0$ is sufficiently
small. For $0\leq\lambda\leq1$, we show that there exists a global smooth
solution $(\rho, u)$ when $\operatorname{curl} u_0\equiv 0$, while for
$\lambda>1$, in general, the solution $(\rho, u)$ will blow up in finite time.
Therefore, $\lambda=1$ appears to be the critical value for the global
existence of small amplitude smooth solutions.
|
1510.04613v1
|
2015-10-17
|
Direct evidence for minority spin gap in the Co2MnSi Heusler alloy
|
Half Metal Magnets are of great interest in the field of spintronics because
of their potential full spin-polarization at the Fermi level and low
magnetization damping. The high Curie temperature and predicted 0.7eV minority
spin gap make the Heusler alloy Co2MnSi very promising for applications.We
investigated the half-metallic magnetic character of this alloy using
spin-resolved photoemission, ab initio calculation and ferromagnetic resonance.
At the surface of Co2MnSi, a gap in the minority spin channel is observed,
leading to 100% spin polarization. However, this gap is 0.3 eV below the Fermi
level and a minority spin state is observed at the Fermi level. We show that a
minority spin gap at the Fermi energy can nevertheless be recovered either by
changing the stoichiometry of the alloy or by covering the surface by Mn, MnSi
or MgO. This results in extremely small damping coefficients reaching values as
low as 7x 10-4.
|
1510.05085v1
|
2015-10-18
|
Information transmission over an amplitude damping channel with an arbitrary degree of memory
|
We study the performance of a partially correlated amplitude damping channel
acting on two qubits. We derive lower bounds for the single-shot classical
capacity by studying two kinds of quantum ensembles, one which allows to
maximize the Holevo quantity for the memoryless channel and the other allowing
the same task but for the full-memory channel. In these two cases, we also show
the amount of entanglement which is involved in achieving the maximum of the
Holevo quantity. For the single-shot quantum capacity we discuss both a lower
and an upper bound, achieving a good estimate for high values of the channel
transmissivity. We finally compute the entanglement-assisted classical channel
capacity.
|
1510.05313v1
|
2015-10-29
|
A Semismooth Newton Method for Tensor Eigenvalue Complementarity Problem
|
In this paper, we consider the tensor eigenvalue complementarity problem
which is closely related to the optimality conditions for polynomial
optimization, as well as a class of differential inclusions with nonconvex
processes. By introducing an NCP-function, we reformulate the tensor eigenvalue
complementarity problem as a system of nonlinear equations. We show that this
function is strongly semismooth but not differentiable, in which case the
classical smoothing methods cannot apply. Furthermore, we propose a damped
semismooth Newton method for tensor eigenvalue complementarity problem. A new
procedure to evaluate an element of the generalized Jocobian is given, which
turns out to be an element of the B-subdifferential under mild assumptions. As
a result, the convergence of the damped semismooth Newton method is guaranteed
by existing results. The numerical experiments also show that our method is
efficient and promising.
|
1510.08570v1
|
2015-11-15
|
Mode coupling in solar spicule oscillations
|
In a real medium which has oscillations, the perturbations can cause the
energy transfer between different modes. The perturbation interpreted as an
interaction between the modes is inferred as mode coupling. Mode coupling
process in an inhomogeneous medium such as solar spicules may lead to the
coupling of kink waves to local Alfven waves. This coupling occurs practically
in any conditions when there is smooth variation in density in the radial
direction. This process is seen as the decay of transverse kink waves in the
medium. To study the damping of kink waves due to mode coupling, a
2.5-dimensional numerical simulation of the initial wave is considered in
spicules. The initial perturbation is assumed to be in a plane perpendicular to
the spicule axis. The considered kink wave is a standing wave which shows an
exponential damping in the inhomogeneous layer after occurrence of the mode
coupling.
|
1511.05135v1
|
2015-11-19
|
Resurgence in Extended Hydrodynamics
|
It has recently been understood that the hydrodynamic series generated by the
M\"uller-Israel-Stewart theory is divergent, and that this large order
behaviour is consistent with the theory of resurgence. Furthermore, it was
observed, that the physical origin of this is the presence of a purely damped
nonhydrodynamic mode. It is very interesting to ask whether this picture
persists in cases where the spectrum of nonhydrodynamic modes is richer. We
take the first step in this direction by considering the simplest hydrodynamic
theory which, instead of the purely damped mode, contains a pair of
nonhydrodynamic modes of complex conjugate frequencies. This mimics the pattern
of black brane quasinormal modes which appear on the gravity side of the
AdS/CFT description of \symm\ plasma. We find that the resulting hydrodynamic
series is divergent in a way consistent with resurgence and precisely encodes
information about the nonhydrodynamic modes of the theory.
|
1511.06358v2
|
2015-12-02
|
Flow of colloidal solids and fluids through constrictions: dynamical density functional theory versus simulation
|
Using both dynamical density functional theory and particle-resolved Brownian
dynamics simulations, we explore the flow of two-dimensional colloidal solids
and fluids driven through a linear channel with a geometric constriction. The
flow is generated by a constant external force acting on all colloids. The
initial configuration is equilibrated in the absence of flow and then the
external force is switched on instantaneously. Upon starting the flow, we
observe four different scenarios: a complete blockade, a monotonic decay to a
constant particle flux (typical for a fluid), a damped oscillatory behaviour in
the particle flux, and a long-lived stop-and-go behaviour in the flow (typical
for a solid). The dynamical density functional theory describes all four
situations but predicts infinitely long undamped oscillations in the flow which
are always damped in the simulations. We attribute the mechanisms of the
underlying stop-and-go flow to symmetry conditions on the flowing solid. Our
predictions are verifiable in real-space experiments on magnetic colloidal
monolayers which are driven through structured microchannels and can be
exploited to steer the flow throughput in microfluidics.
|
1512.00751v1
|
2015-12-04
|
Numerical solution of the stationary multicomponent nonlinear Schrödinger equation with a constraint on the angular momentum
|
We formulate a damped oscillating particle method to solve the stationary
nonlinear Schr\"{o}dinger equation (NLSE). The ground state solutions are found
by a converging damped oscillating evolution equation that can be discretized
with symplectic numerical techniques. The method is demonstrated for three
different cases: for the single-component NLSE with an attractive
self-interaction, for the single-component NLSE with a repulsive self
interaction and a constraint on the angular momentum, and for the two-component
NLSE with a constraint on the total angular momentum. We reproduce the so
called yrast curve for the single-component case, described in [A. D. Jackson
et al., Europhys. Lett. 95, 30002 (2011)], and produce for the first time an
analogous curve for the two-component NLSE. The numerical results are compared
with analytic solutions and competing numerical methods. Our method is well
suited to handle a large class of equations and can easily be adapted to
further constraints and components.
|
1512.01441v2
|
2015-12-08
|
Influence of kinetic effects on the spectrum of a parallel electrode probe
|
Active Plasma Resonance Spectroscopy (APRS) denotes a class of diagnostic
techniques which utilize the natural ability of plasmas to resonate on or near
the electron plasma frequency. One particular class of APRS can be described in
an abstract notation based on functional analytic methods in electrostatic
approximation. These methods allow for a general solution of the kinetic model
in arbitrary geometry. This solution is given as the response function of the
probe-plasma system and is defined by the resolvent of an appropriate dynamical
operator. The general response predicts an additional damping due to kinetic
effects. This manuscript provides the derivation of an explicit response
function of the kinetic APRS model in a simple geometry. Therefore, the
resolvent is determined by its matrix representation based on an expansion in
orthogonal basis functions. This allows to compute an approximated response
function. The resulting spectra show clearly a stronger damping due to kinetic
effects.
|
1512.02450v1
|
2015-12-09
|
Long time behaviors for 3D cubic damped Klein-Gordon equations in inhomogeneous mediums
|
In this paper, we study the asymptotic dynamics of global solutions to damped
Klein-Gordon equations in inhomogeneous mediums (KGI). In the defocusing case,
we prove for any initial data, the solution is globally define in forward time
and it will converge to an equilibrium. In the focusing case, for global
solutions, we prove the solutions converge to the superposition of equilibriums
among which there exists at most one equilibrium to KGI and the other
equilibriums are solutions to stationary nonlinear Klein-Gordon equations. The
core ingredients of our proof are the existence of the "concentration-compact
attractor" and the gradient system theory.
|
1512.02755v3
|
2015-12-10
|
Strong Frequency Dependence in Over-damped Systems
|
Strong frequency dependence is unlikely in diffusive or over-damped systems.
When exceptions do occur, such as in the case of stochastic resonance, it
signals an interesting underlying phenomenon. We find that such a case appears
in the motion of a particle in a diffusive environment under the effect of
periodically oscillating retarded force emanating from the boundaries. The
amplitude for the expectation value of position has an oscillating frequency
dependence, quite unlike a typical resonance. We first present an analysis of
the associated Fokker-Planck equation, then report the results of a Monte Carlo
simulation of the effect of a periodic perturbation on a totally asymmetric
simple exclusion process (TASEP) model with single species. This model is known
to exhibit a randomly moving shock profile, dynamics of which is a discrete
realization of the Fokker-Planck equation. Comparison of relevant quantities
from the two analyses indicate that the same phenomenon is apparent in both
systems.
|
1512.03187v1
|
2015-12-16
|
Back to Maupertuis' least action principle for dissipative systems: not all motions in Nature are most energy economical
|
It is shown that an oldest form of variational calculus of mechanics, the
Maupertuis least action principle, can be used as a simple and powerful
approach for the formulation of the variational principle for damped motions,
allowing a simple derivation of the Lagrangian mechanics for any dissipative
systems and an a connection of the optimization of energy dissipation to the
least action principles. On this basis, it is shown that not all motions of
classical mechanics obey the rule of least energy dissipation or follow the
path of least resistance, and that the least action is equivalent to least
dissipation for two kinds of motions : all stationary motions with constant
velocity and all motions damped by Stokes drag.
|
1512.05339v1
|
2016-01-11
|
Damping of Landau levels in neutral graphene at low magnetic fields: A phonon Raman scattering study
|
Landau level broadening mechanisms in electrically neutral and quasineutral
graphene were investigated through micro-magneto-Raman experiments in three
different samples, namely, a natural single-layer graphene flake and a
back-gated single-layer device, both deposited over Si/SiO2 substrates, and a
multilayer epitaxial graphene employed as a reference sample. Interband Landau
level transition widths were estimated through a quantitative analysis of the
magnetophonon resonances associated with optically active Landau level
transitions crossing the energy of the E_2g Raman-active phonon. Contrary to
multilayer graphene, the single-layer graphene samples show a strong damping of
the low-field resonances, consistent with an additional broadening contribution
of the Landau level energies arising from a random strain field. This extra
contribution is properly quantified in terms of a pseudomagnetic field
distribution Delta_B = 1.0-1.7 T in our single-layer samples.
|
1601.02663v3
|
2016-01-21
|
Negative frequencies in wave propagation: a microscopic model
|
A change in the sign of the frequency of a wave between two inertial
reference frames corresponds to a reversal of the phase velocity. Yet from the
point of view of the relation $E=\hbar\omega$, a positive quantum of energy
apparently becomes a negative energy one. This is physically distinct from a
change in the sign of the wave-vector, and has been associated with various
effects such as Cherenkov radiation, quantum friction, and the Hawking effect.
In this work we provide a more detailed understanding of these negative
frequency modes based on a simple microscopic model of a dielectric medium as a
lattice of scatterers. We calculate the classical and quantum mechanical
radiation damping of an oscillator moving through such a lattice and find that
the modes where the frequency has changed sign contribute negatively. In terms
of the lattice of scatterers we find that this negative radiation damping
arises due to phase of the periodic force experienced by the oscillator due to
the relative motion of the lattice.
|
1601.05636v1
|
2016-01-26
|
On the wave equation with hyperbolic dynamical boundary conditions, interior and boundary damping and supercritical sources
|
The aim of the paper is to study the problem $$ \begin{cases} u_{tt}-\Delta
u+P(x,u_t)=f(x,u) \qquad &\text{in $(0,\infty)\times\Omega$,} u=0 &\text{on
$(0,\infty)\times \Gamma_0$,} u_{tt}+\partial_\nu u-\Delta_\Gamma
u+Q(x,u_t)=g(x,u)\qquad &\text{on $(0,\infty)\times \Gamma_1$,}
u(0,x)=u_0(x),\quad u_t(0,x)=u_1(x) & \text{in $\bar{\Omega}$,} \end{cases}$$
where $\Omega$ is a bounded open $C^1$ subset of $\mathbb{R}^N$, $N\ge 2$,
$\Gamma=\partial\Omega$, $(\Gamma_0,\Gamma_1)$ is a measurable partition of
$\Gamma$, $\Delta_\Gamma$ denotes the Laplace--Beltrami operator on $\Gamma$,
$\nu$ is the outward normal to $\Omega$, and the terms $P$ and $Q$ represent
nonlinear damping terms, while $f$ and $g$ are nonlinear source, or sink,
terms.
In the paper we establish local and existence, uniqueness and Hadamard
well--posedness results when source terms can be supercritical or
super-supercritical.
|
1601.07075v3
|
2016-01-29
|
Numerical Simulation of Tidal Evolution of a Viscoelastic Body Modelled with a Mass-Spring Network
|
We use a damped mass-spring model within an N-body code to simulate the tidal
evolution of the spin and orbit of a self-gravitating viscoelastic spherical
body moving around a point-mass perturber. The damped mass-spring model
represents a Kelvin-Voigt viscoelastic solid. We measure the tidal quality
function (the dynamical Love number $\,k_2\,$ divided by the tidal quality
factor $\,Q\,$) from the numerically computed tidal drift of the semimajor axis
of the binary. The shape of $\,k_2/Q\,$, as a function of the principal tidal
frequency, reproduces the kink shape predicted by Efroimsky (2012a; CeMDA
112$\,:\,$283) for the tidal response of near-spherical homogeneous
viscoelastic rotators. We demonstrate that we can directly simulate the tidal
evolution of spinning viscoelastic objects. In future, the mass-spring N-body
model can be generalised to inhomogeneous and/or non-spherical bodies.
|
1601.08222v2
|
2016-02-04
|
Highly efficient two photon generation from a coherently pumped quantum dot embedded in a microcavity
|
We propose a scheme to realize a highly efficient solid state source of
photon pairs using cavity-assisted stimulated Raman adiabatic passage (STIRAP)
in a single quantum dot, where a single photon from pump pulse and two stokes
photons from cavity mode drives the Raman transition. The Autler-Townes
doublet, generated by using a resonant continuous wave laser between biexciton
and exciton states, and two-photon-resonant transition through strongly coupled
cavity mode are utilized to facilitate (1+2)type Raman transition in the
quantum dot. We show in the case of weak pump although the probability of
generating two photons in cavity mode is small without cavity damping but
two-photon-resonant emission is enhanced by cavity damping within strong
coupling regime. We also discuss spectrum of the generated photon pair and
photon-photon correlations in the generated photon pair. The efficiency of two
photon source could be more than 80\% in current experimental conditions.
|
1602.01562v1
|
2016-02-09
|
Preservation of quantum key rate in the presence of decoherence
|
It is well known that the interaction of quantum systems with the environment
reduces the inherent quantum correlations. Under special circumstances the
effect of decoherence can be reversed, for example, the interaction modeled by
an amplitude damping channel can boost the teleportation fidelity from the
classical to the quantum region for a bipartite quantum state. Here, we first
show that this phenomena fails in the case of a quantum key distribution
protocol. We further show that the technique of weak measurement can be used to
slow down the process of decoherence, thereby helping to preserve the quantum
key rate when one or both systems are interacting with the environment via an
amplitude damping channel. Most interestingly, in certain cases weak
measurement with post-selection where one considers both success and failure of
the technique is shown to be more useful than without it when both systems
interact with the environment.
|
1602.02903v1
|
2016-02-19
|
An accelerated exponential time integrator for semi-linear stochastic strongly damped wave equation with additive noise
|
This paper is concerned with the strong approximation of a semi-linear
stochastic wave equation with strong damping, driven by additive noise. Based
on a spatial discretization performed by a spectral Galerkin method, we
introduce a kind of accelerated exponential time integrator involving linear
functionals of the noise. Under appropriate assumptions, we provide error
bounds for the proposed full-discrete scheme. It is shown that the scheme
achieves higher strong order in time direction than the order of temporal
regularity of the underlying problem, which allows for higher convergence rate
than usual time-stepping schemes. For the space-time white noise case in two or
three spatial dimensions, the scheme still exhibits a good convergence
performance. Another striking finding is that, even for the velocity with low
regularity the scheme always promises first order strong convergence in time.
Numerical examples are finally reported to confirm our theoretical findings.
|
1602.06050v1
|
2016-02-23
|
The calibration and electron energy reconstruction of the BGO ECAL of the DAMPE detector
|
The DArk Matter Particle Explorer (DAMPE) is a space experiment designed to
search for dark matter indirectly by measuring the spectra of photons,
electrons, and positrons up to 10 TeV. The BGO electromagnetic calorimeter
(ECAL) is its main sub-detector for energy measurement. In this paper, the
instrumentation and development of the BGO ECAL is briefly described. The
calibration on the ground, including the pedestal, minimum ionizing particle
(MIP) peak, dynode ratio, and attenuation length with the cosmic rays and beam
particles is discussed in detail. Also, the energy reconstruction results of
the electrons from the beam test are presented.
|
1602.07015v1
|
2016-02-25
|
Order-disorder transition in repulsive self-propelled particle systems
|
We study the collective dynamics of repulsive self-propelled particles. The
particles are governed by coupled equations of motion that include polar
self-propulsion, damping of velocity and of polarity, repulsive
particle-particle interaction, and deterministic dynamics. Particle dynamics
simulations show that the collective coherent motion with large density
fluctuations spontaneously emerges from a disordered, isotropic state. In the
parameter region where the rotational damping of polarity is strong, the
systems undergoes an abrupt shift to the absorbing ordered state after a
waiting period in the metastable disordered state. In order to obtain a simple
understanding of the mechanism underlying the collective behavior, we analyze
binary particle scattering process. We show that this approach correctly
predicts the order-disorder transition at dilute limit. The same approach is
expanded for finite densities, although it disagrees with the result from
many-particle simulations due to many-body correlations and density
fluctuations.
|
1602.07971v2
|
2016-03-01
|
Ferromagnetic resonance and magnetic damping in C-doped Mn5Ge3
|
Ferromagnetic resonance (FMR) was used to investigate the static and dynamic
magnetic properties of carbon-doped Mn5Ge3 (C$_{0.1}$ and C$_{0.2}$) thin films
grown on Ge(111). The temperature dependence of magnetic anisotropy shows an
increased perpendicular magneto-crystalline contribution at 80K with an
in-plane easy axis due to the large shape contribution. We find that our
samples show a small FMR linewidth (corresponding to an intrinsic magnetic
damping parameter $\alpha$=0.005), which is a measure of the spin relaxation
and directly related with the magnetic and structural quality of the material.
In the perpendicular-to-plane geometry, the FMR linewidth shows a minimum at
around 200K for all the samples, which seems to be not correlated to the
C-doping. The magnetic relaxation parameters have been determined and indicate
the two-magnon scattering as the main extrinsic contribution. We observe a
change in the main contribution from scattering centres in Mn5Ge3C0.2 at low
temperatures, which could be related to the minimum in linewidth.
|
1603.00190v1
|
2016-03-01
|
Probing superfluidity in a quasi two-dimensional Bose gas through its local dynamics
|
We report direct evidence of superfluidity in a quasi two-dimensional Bose
gas by observing its dynamical response to a collective excitation. Relying on
a novel local correlation analysis, we are able to probe inhomogeneous clouds
and reveal their local dynamics. We identify in this way the superfluid and
thermal phases inside the gas and locate the boundary at which the
Berezinskii--Kosterlitz--Thouless crossover occurs. This new analysis also
allows to evidence the coupling of the two fluids which induces at finite
temperatures damping rates larger than the usual Landau damping.
|
1603.00434v2
|
2016-03-03
|
Backreaction Effects on Nonequilibrium Spectral Function
|
We show how to compute the spectral function for a scalar theory in two
different scenarios: one which disregards back-reaction i.e. the response of
the environment to the external particle, and the other one where back-reaction
is considered. The calculation was performed using the Kadanoff-Baym equation
through the Keldysh formalism. When back-reaction is neglected, the spectral
function is equal to the equilibrium one, which can be represented as a
Breit-Wigner distribution. When back-reaction is introduced we observed a
damping in the spectral function of the thermal bath. Such behavior modifies
the damping rate for particles created within the bath. This certainly implies
phenomenological consequences right after the Big-Bang, when the primordial
bath was created.
|
1603.01118v1
|
2016-04-06
|
Brownian motion of a matter-wave bright soliton: realizing a quantum pollen grain
|
Taking an open quantum systems approach, we derive a collective equation of
motion for the dynamics of a matter-wave bright soliton moving through a
thermal cloud of a distinct atomic species. The reservoir interaction involves
energy transfer without particle transfer between the soliton and thermal
cloud, thus damping the soliton motion without altering its stability against
collapse. We derive a Langevin equation for the soliton centre of mass velocity
in the form of an Ornstein-Uhlenbeck process with analytical drift and
diffusion coefficients. This collective motion is confirmed by simulations of
the full stochastic projected Gross-Pitaevskii equation for the matter-wave
field. The system offers a pathway for experimentally observing the elusive
energy-damping reservoir interaction, and a clear realization of collective
Brownian motion for a mesoscopic superfluid droplet.
|
1604.01487v1
|
2016-04-06
|
Quantum backaction and noise interference in asymmetric two-cavity optomechanical systems
|
We study the effect of cavity damping asymmetries on backaction in a
"membrane-in-the-middle" optomechanical system, where a mechanical mode
modulates the coupling between two photonic modes. We show that in the
adiabatic limit, this system generically realizes a dissipative optomechanical
coupling, with an effective position-dependent photonic damping rate. The
resulting quantum noise interference can be used to ground-state cool a
mechanical resonator in the unresolved sideband regime. We explicitly
demonstrate how quantum noise interference controls linear backaction effects,
and show that this interference persists even outside the adiabatic limit. For
a one-port cavity in the extreme bad-cavity limit, the interference allows one
to cancel all linear backaction effects. This allows continuous measurements of
position-squared, with no stringent constraints on the single-photon
optomechanical coupling strength. In contrast, such a complete cancellation is
not possible in the good cavity limit. This places strict bounds on the
optomechanical coupling required for quantum non-demolition measurements of
mechanical energy, even in a one-port device.
|
1604.01703v1
|
2016-04-08
|
Discrete Breathers and Multi-Breathers in Finite Vibro-Impact Chain
|
We explore dynamics of discrete breathers and multi-breathers in finite
one-dimensional chain. The model involves parabolic on-site potential with
rigid constraints and linear nearest-neighbor coupling. The rigid non-ideal
impact constraints are the only source of nonlinearity and damping in the
model. The model allows derivation of exact analytic solutions for the
breathers and multi-breathers with arbitrary set of localization sites, both in
conservative and forced-damped settings. We choose periodic boundary
conditions; exact solutions for other types of the boundary conditions are also
possible. Local character of the nonlinearity allows explicit derivation of a
monodromy matrix for the breather solutions. Consequently, a stability of the
derived breather and multi-breather solutions can be efficiently studied in the
framework of simple methods of linear algebra, and with rather moderate
computational efforts. We demonstrate that finitness of the chain fragment and
proximity of the localization sites strongly effect existence and stability
patterns of these localized solutions.
|
1604.02296v1
|
2016-04-22
|
A Liapunov function approach to the stabilization of second order coupled systems
|
In 2002, Fatiha Alabau, Piermarco Cannarsa and Vilmos Komornik investigated
the extent of asymptotic stability of the null solution for weakly coupled
partially damped equations of the second order in time. The main point is that
the damping operator acts only on the first component and, whenever it is
bounded, the coupling is not strong enough to produce an exponential decay in
the energy space associated to the conservative part of the system. As a
consequence, for initial data in the energy space, the rate of decay is not
exponential. Due to the nature of the result it seems at first sight impossible
to obtain the asymptotic stability result by the classical Liapunov method.
Surprisingly enough, this turns out to be possible and we exhibit, under some
compatibility conditions on the operators, an explicit class of Liapunov
functions which allows to do 3 different things: 1) When the problem is reduced
to a stable finite dimensional space, we recover the exponential decay by a
single differential inequality and we estimate the logarithmic decrement of the
solutions with worst (slowest) decay. The estimate is optimal at least for some
values of the parameters.
|
1604.06547v1
|
2016-04-28
|
Temperature Dependence of Viscosity in Normal Fluid $^3$He Below 800mK Determined by a Micro-electro-mechanical Oscillator
|
A micro-electro-mechanical system vibrating in its shear mode was used to
study the viscosity of normal liquid $^3$He from 20mK to 770mK at 3bar, 21bar,
and 29bar. The damping coefficient of the oscillator was determined by
frequency sweeps through its resonance at each temperature. Using a slide film
damping model, the viscosity of the fluid was obtained. Our viscosity values
are compared with previous measurements and with calculated values from Fermi
liquid theory. The crossover from the classical to the Fermi liquid regime is
manifest in the temperature dependence of viscosity. In the Fermi liquid
regime, the temperature dependence of viscosity changes from $T^{-1}$ to
$T^{-2}$ on cooling, indicating a transition from the Stokes flow to the
Couette flow regime.
|
1604.08554v1
|
2016-05-01
|
Optical trapping by Laguerre-Gaussian beams: Symmetries, stability and equilibria
|
We use the T-matrix formalism in combination with the method of far-field
matching to evaluate the optical force exerted by Laguerre-Gaussian (LG) light
beams on a spherical (Mie) particle. For both non-vortex and optical vortex LG
beams, the theoretical results are used to analyze the optical-force-induced
dynamics of the scatterer near the trapping points represented by the
equilibrium (zero-force) positions. The regimes of linearized dynamics are
described in terms of the stiffness matrix spectrum and the damping constant of
the ambient medium. For the purely azimuthal LG beams, the dynamics is found to
be locally non-conservative and is characterized by the presence of
conditionally stable equilibria (unstable zero-force points that can be
stabilized by the ambient damping). The effects related to the Mie resonances
that under certain conditions manifest themselves as the points changing the
trapping properties of the particles are discussed.
|
1605.00243v2
|
2016-05-03
|
Including atomic vibrations in XANES calculations: polarization-dependent damping of the fine structure at the Cu K edge of (creat)$_{2}$CuCl$_{4}$
|
Atomic vibrations are usually not taken into account when analyzing x-ray
absorption near edge structure (XANES) spectra. One of the reasons is that
including the vibrations in a formally exact way is quite complicated while the
effect of vibrations is supposed to be small in the XANES region. By analyzing
polarized Cu K edge x-ray absorption spectra of creatinium tetrachlorocuprate
[(creat)$_{2}$CuCl$_{4}$], we demonstrate that a technically simple method,
consisting in calculating the XANES via the same formula as for static systems
but with a modified free-electron propagator which accounts for fluctuations of
interatomic distances, may substantially help in understanding XANES of some
layered systems. In particular we show that the difference in the damping of
the x-ray absorption fine structure oscillations for different polarisations of
the incoming x-rays cannot be reproduced by calculations which rely on a static
lattice but it can be described if atomic vibrations are accounted for in such
a way that individual creatinium and CuCl$_{4}$ molecular blocks are treated as
semi-rigid entities while the mutual positions of these blocks are subject to
large mean relative displacements.
|
1605.00861v1
|
2016-05-05
|
Relaxation of Ferroelectric States in 2D Distributions of quantum Dots:EELS Simulation
|
The relaxation time of collective electronic states in a 2D distribution of
quantum dots is investigated theoretically by simulating EELS experiments. From
the numerical calculation of the probability of energy loss of an electron
beam, traveling parallel to the distribution, it is possible to estimate the
damping time of ferroelectric-like states. We generate this collective response
of the distribution by introducing a mean field interaction among the quantum
dots, and then, the model is extended incorporating effects of long-range
correlations through a Bragg-Williams approximation. The behavior of the
dielectric function, the energy loss function, and the relaxation time of
ferroelectric-like states is then investigated as a function of the temperature
of the distribution and the damping constant of the electronic states in the
single quantum dots. The robustness of the trends and tendencies of our results
indicate that this scheme of analysis can guide experimentalists to develop
tailored quantum dots distributions for specific applications.
|
1605.01642v1
|
2016-05-10
|
Damped wave systems on networks: Exponential stability and uniform approximations
|
We consider a damped linear hyperbolic system modelling the propagation of
pressure waves in a network of pipes. Well-posedness is established via
semi-group theory and the existence of a unique steady state is proven in the
absence of driving forces. Under mild assumptions on the network topology and
the model parameters, we show exponential stability and convergence to
equilibrium. This generalizes related results for single pipes and
multi-dimensional domains to the network context. Our proof of the exponential
stability estimate is based on a variational formulation of the problem, some
graph theoretic results, and appropriate energy estimates. The main arguments
are rather generic and can be applied also for the analysis of Galerkin
approximations. Uniform exponential stability can be guaranteed for the
resulting semi-discretizations under mild compatibility conditions on the
approximation spaces. A particular realization by mixed finite elements is
discussed and the theoretical results are illustrated by numerical tests in
which also bounds for the decay rate are investigated.
|
1605.03066v1
|
2016-05-14
|
The damped oscillating propagation of the compensating self-accelerating beams
|
We report a new form of compensating accelerating beam generated by amplitude
modulation of the symmetric Airy beam (SAB) caustics with an exponential
apodization mask. Our numerical study manifests that the compensating beam is
with one main-lobe beam structure and can maintain the mean-intensity invariant
both in the free space and loss media. Specially, the beam inherits the
beamlets structure from the SAB and owns a novel damped oscillating propagation
property. We also conduct a comparative study of its propagation property with
that of the Airy beam theoretically. And by altering the signs of 2D masks, the
main lobe of the compensating beam can be modulated to orientate in four
different quadrants flexibly. The proposed compensating accelerating beam is
anticipated to get special applications in particle manipulation or plasmas
regions.
|
1605.04392v1
|
2016-05-14
|
Detecting Damped Lyman-$α$ Absorbers with Gaussian Processes
|
We develop an automated technique for detecting damped Lyman-$\alpha$
absorbers (DLAs) along spectroscopic lines of sight to quasi-stellar objects
(QSOs or quasars). The detection of DLAs in large-scale spectroscopic surveys
such as SDSS-III sheds light on galaxy formation at high redshift, showing the
nucleation of galaxies from diffuse gas. We use nearly 50 000 QSO spectra to
learn a novel tailored Gaussian process model for quasar emission spectra,
which we apply to the DLA detection problem via Bayesian model selection. We
propose models for identifying an arbitrary number of DLAs along a given line
of sight. We demonstrate our method's effectiveness using a large-scale
validation experiment, with excellent performance. We also provide a catalog of
our results applied to 162 858 spectra from SDSS-III data release 12.
|
1605.04460v2
|
2016-05-17
|
Direct observation of dynamic modes excited in a magnetic insulator by pure spin current
|
Excitation of magnetization dynamics by pure spin currents has been recently
recognized as an enabling mechanism for spintronics and magnonics, which allows
implementation of spin-torque devices based on low-damping insulating magnetic
materials. Here we report the first spatially-resolved study of the dynamic
modes excited by pure spin current in nanometer-thick microscopic insulating
Yttrium Iron Garnet disks. We show that these modes exhibit nonlinear
self-broadening preventing the formation of the self-localized magnetic bullet,
which plays a crucial role in the stabilization of the single-mode
magnetization oscillations in all-metallic systems. This peculiarity associated
with the efficient nonlinear mode coupling in low-damping materials can be
among the main factors governing the interaction of pure spin currents with the
dynamic magnetization in high-quality magnetic insulators.
|
1605.05211v1
|
2016-05-18
|
Damped Arrow-Hurwicz algorithm for sphere packing
|
We consider algorithms that, from an arbitrarily sampling of $N$ spheres
(possibly overlapping), find a close packed configuration without overlapping.
These problems can be formulated as minimization problems with non-convex
constraints. For such packing problems, we observe that the classical iterative
Arrow-Hurwicz algorithm does not converge. We derive a novel algorithm from a
multi-step variant of the Arrow-Hurwicz scheme with damping. We compare this
algorithm with classical algorithms belonging to the class of linearly
constrained Lagrangian methods and show that it performs better. We provide an
analysis of the convergence of these algorithms in the simple case of two
spheres in one spatial dimension. Finally, we investigate the behaviour of our
algorithm when the number of spheres is large.
|
1605.05473v1
|
2016-05-17
|
Dynamics of sessile drops. Part 3. Theory of forced oscillations
|
A partially-wetting sessile drop is driven by a sinusoidal pressure field
that produces capillary waves on the liquid/gas interface. The analysis
presented in Part 1 of this series (Bostwick & Steen 2014) is extended by
computing response diagrams and phase shifts for the viscous droplet, whose
three phase contact-line moves with contact-angle that is a smooth function of
the contact line speed. Viscous dissipation is incorporated through the viscous
potential flow approximation and the critical Ohnesorge number bounding regions
beyond which a given mode becomes over-damped is computed. Davis dissipation
originating from the contact-line speed condition leads to damped oscillations
for drops with finite contact-line mobility, even for inviscid fluids. The
critical mobility and associated driving frequency to generate the largest
Davis dissipation is computed. Lastly, regions of modal coexistence where two
modes can be simultaneously excited by a single forcing frequency are
identified. Predictions compare favorably to related experiments on vibrated
drops.
|
1605.05533v1
|
2016-05-18
|
Electrical control over perpendicular magnetization switching driven by spin-orbit torques
|
Flexible control of magnetization switching by electrical manners is crucial
for applications of spin-orbitronics. Besides of a switching current that is
parallel to an applied field, a bias current that is normal to the switching
current is introduced to tune the magnitude of effective damping-like and
field-like torques and further to electrically control magnetization switching.
Symmetrical and asymmetrical control over the critical switching current by the
bias current with opposite polarities is both realized in Pt/Co/MgO and
$\alpha$-Ta/CoFeB/MgO systems, respectively. This research not only identifies
the influences of field-like and damping-like torques on switching process but
also demonstrates an electrical method to control it.
|
1605.05569v1
|
2016-05-25
|
Quadratic band touching with long range interactions in and out of equilibrium
|
Motivated by recent advances in cold atomic systems, we study the equilibrium
and quench properties of two dimensional fermions with quadratic band touching
at the Fermi level, in the presence of infinitely long range interactions.
Unlike when only short range interactions are present, both nematic and quantum
anomalous Hall (QAH) states state appear at weak interactions, separated by a
narrow coexistence region, whose boundaries mark second and third order quantum
phase transitions. After an interaction quench, the QAH order exhibits three
distinct regions: persistent or damped oscillations and exponential decay to
zero. In contrast, the nematic order always reaches a non-zero stationary value
through power law damped oscillations, due to the interplay of the symmetry of
the interaction and the specific topology of the quadratic band touching.
|
1605.07932v1
|
2016-05-26
|
Asymmetric Quantum Dialogue in Noisy Environment
|
A notion of asymmetric quantum dialogue (AQD) is introduced. Conventional
protocols of quantum dialogue are essentially symmetric as both the users
(Alice and Bob) can encode the same amount of classical information. In
contrast, the scheme for AQD introduced here provides different amount of
communication powers to Alice and Bob. The proposed scheme, offers an
architecture, where the entangled state and the encoding scheme to be shared
between Alice and Bob depends on the amount of classical information they want
to exchange with each other. The general structure for the AQD scheme has been
obtained using a group theoretic structure of the operators introduced in
(Shukla et al., Phys. Lett. A, 377 (2013) 518). The effect of different types
of noises (e.g., amplitude damping and phase damping noise) on the proposed
scheme is investigated, and it is shown that the proposed AQD is robust and
uses optimized amount of quantum resources.
|
1605.08363v1
|
2016-06-11
|
Parameter identification in a semilinear hyperbolic system
|
We consider the identification of a nonlinear friction law in a
one-dimensional damped wave equation from additional boundary measurements.
Well-posedness of the governing semilinear hyperbolic system is established via
semigroup theory and contraction arguments. We then investigte the inverse
problem of recovering the unknown nonlinear damping law from additional
boundary measurements of the pressure drop along the pipe. This coefficient
inverse problem is shown to be ill-posed and a variational regularization
method is considered for its stable solution. We prove existence of minimizers
for the Tikhonov functional and discuss the convergence of the regularized
solutions under an approximate source condition. The meaning of this condition
and some arguments for its validity are discussed in detail and numerical
results are presented for illustration of the theoretical findings.
|
1606.03580v1
|
2016-06-16
|
Design of the readout electronics for the DAMPE Silicon Tracker detector
|
The Silicon Tracker (STK) is a detector of the DAMPE satellite to measure the
incidence direction of high energy cosmic ray. It consists of 6 X-Y double
layers of silicon micro-strip detectors with 73,728 readout channels. It's a
great challenge to readout the channels and process the huge volume of data in
the critical space environment. 1152 Application Specific Integrated Circuits
(ASIC) and 384 ADCs are adopted to readout the detector channels. The 192
Tracker Front-end Hybrid (TFH) modules and 8 identical Tracker Readout Board
(TRB) modules are designed to control and digitalize the front signals. In this
paper, the design of the readout electronics for STK and its performance will
be presented in detail.
|
1606.05080v1
|
2016-06-20
|
Classical analogue of the continuous transition between the Weisskopf-Wigner exponential decay and the Rabi oscillation
|
When a discrete state is coupled to a continuum, the dynamics can be
described either by the Weisskopf-Wigner exponential decay or by the Rabi
oscillation, depending on the relative magnitudes of the continuum width and of
the Rabi frequency. A continuous transition between these two regimes exists,
as demonstrated in 1977 by C. Cohen-Tannoudji and P. Avan. Here, we describe a
fully analogous transition in classical mechanics, by studying the dynamics of
two coupled mechanical oscillators in the presence of damping. By varying the
relative magnitudes of the damping and coupling terms, we observe a continuous
transition between a regime analogous to the Rabi oscillation and a regime
analogous to the Weisskopf-Wigner exponential decay.
|
1606.06153v1
|
2016-07-05
|
Quantum Zeno and Anti-Zeno Effects on the Entanglement Dynamics of Qubits Dissipating into a Common and non-Markovian Environment
|
We investigate the quantum Zeno and anti-Zeno effects on pairwise
entanglement dynamics of a collective of non-interacting qubits which have been
initially prepared in a Werner state and are off-resonantly coupled to a common
and non-Markovian environment. We obtain the analytical expression of the
concurrence in the absence and presence of the non-selective measurements. In
particular, we express our results in the strong and weak coupling regimes and
examine the role of the system size, and the effect of the detuning from the
cavity field frequency on the temporal behaviour of the pairwise entanglement.
We show that, the detuning parameter has a positive role in the protection of
entanglement in the absence of the measurement for weak coupling regime. We
find that for the values of detuning parameter less than the cavity damping
rate, the quantum Zeno effect is always dominant, while for the values greater
than the cavity damping rate, both Zeno and anti-Zeno effects can occur,
depending on the measurement intervals. We also find that the anti-Zeno effect
can occur in the pairwise entanglement dynamics in the absence and presence of
the detuning in the strong coupling regime.
|
1607.01160v1
|
2016-07-27
|
Linear and nonlinear viscoelastic arterial wall models: application on animals
|
This work deals with the viscoelasticity of the arterial wall and its
influence on the pulse waves. We describe the viscoelasticity by a non-linear
Kelvin-Voigt model in which the coefficients are fitted using experimental time
series of pressure and radius measured on a sheep's arterial network. We
obtained a good agreement between the results of the nonlinear Kelvin-Voigt
model and the experimental measurements. We found that the viscoelastic
relaxation time-defined by the ratio between the viscoelastic coefficient and
the Young's modulus-is nearly constant throughout the network. Therefore, as it
is well known that smaller arteries are stiffer, the viscoelastic coefficient
rises when approaching the peripheral sites to compensate the rise of the
Young's modulus, resulting in a higher damping effect. We incorporated the
fitted viscoelastic coefficients in a nonlinear 1D fluid model to compute the
pulse waves in the network. The damping effect of viscoelasticity on the high
frequency waves is clear especially at the peripheral sites.
|
1607.07973v1
|
2016-08-03
|
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
|
2016-08-19
|
Cooling a harmonic oscillator by optomechanical modification of its bath
|
Optomechanical systems show tremendous promise for high sensitivity sensing
of forces and modification of mechanical properties via light. For example,
similar to neutral atoms and trapped ions, laser cooling of mechanical motion
by radiation pressure can take single mechanical modes to their ground state.
Conventional optomechanical cooling is able to introduce additional damping
channel to mechanical motion, while keeping its thermal noise at the same
level, and as a consequence, the effective temperature of the mechanical mode
is lowered. However, the ratio of temperature to quality factor remains roughly
constant, preventing dramatic advances in quantum sensing using this approach.
Here we propose an approach for simultaneously reducing the thermal load on a
mechanical resonator while improving its quality factor. In essence, we use the
optical interaction to dynamically modify the dominant damping mechanism,
providing an optomechanically-induced effect analogous to a phononic band gap.
The mechanical mode of interest is assumed to be weakly coupled to its heat
bath but strongly coupled to a second mechanical mode, which is cooled by
radiation pressure coupling to a red detuned cavity field. We also identify a
realistic optomechanical design that has the potential to realize this novel
cooling scheme.
|
1608.05717v1
|
2016-09-01
|
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
|
2016-09-30
|
Origin of the effective mobility in non-linear active micro-rheology
|
The distinction between the damping coefficient and the effective non-linear
mobility of driven particles in active micro-rheology of supercooled liquids is
explained in terms of individual and collective dynamics. The effective
mobility arises as a collective effect which gives insight into the energy
landscape of the system. On the other hand, the damping coefficient is a
constant that modulates the effect of external forces over the thermal energy
which particles have at their disposition to perform Brownian motion. For long
times, these thermal fluctuations become characterized in terms of an effective
temperature that is a consequence of the dynamic coupling between kinetic and
configurational degrees of freedom induced by the presence of the strong
external force. The interplay between collective mobility and effective
temperature allows to formulate a generalized Stokes-Einstein relation that may
be used to determine the collective diffusion coefficient. The explicit
relations we deduce reproduce simulation data remarkably well.
|
1609.09853v1
|
2016-10-03
|
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
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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
|
2016-10-16
|
Pulse-noise approach for classical spin systems
|
For systems of classical spins interacting with the bath via damping and
thermal noise, the approach is suggested to replace the white noise by a pulse
noise acting at regular time intervals $\Delta t$, within which the system
evolves conservatively. The method is working well in the typical case of a
small dimensionless damping constant $\lambda$ and allows a considerable
speed-up of computations by using high-order numerical integrators with a large
time step $\delta t$ (up to a fraction of the precession period), while keeping
$\delta t\ll\Delta t$ to reduce the relative contribution of noise-related
operations. In cases when precession can be discarded, $\delta t$ can be
increased up to a fraction of the relaxation time $\propto1/\lambda$ that leads
to a further speed-up. This makes equilibration speed comparable with that of
Metropolis Monte Carlo. The pulse-noise approach is tested on single-spin and
multi-spin models.
|
1610.04914v2
|
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
|
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