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2014-11-22
|
Quantification of the spin-Hall anti-damping torque with a resonance spectrometer
|
We present a simple technique using a cavity-based resonance spectrometer to
quantify the anti-damping torque due to the spin Hall effect. Modification of
ferromagnetic resonance is observed as a function of small DC current in
sub-mm-wide strips of bilayers, consisting of magnetically soft FeGaB and
strong spin-Hall metal Ta. From the detected current-induced linewidth change,
we obtain an effective spin Hall angle of 0.08-0.09 independent of the magnetic
layer thickness. Our results demonstrate that a sensitive resonance
spectrometer can be a general tool to investigate spin Hall effects in various
material systems, even those with vanishingly low conductivity and
magnetoresistance.
|
1411.6166v1
|
2014-11-28
|
Quantifying entanglement of a two-qubit system via measurable and invariant moments of its partially transposed density matrix
|
We describe a direct method to determine the negativity of an arbitrary
two-qubit state in experiments. The method is derived by analyzing the relation
between the purity, negativity, and a universal entanglement witness for
two-qubit entanglement. We show how the negativity of a two-qubit state can be
calculated from just three experimentally accessible moments of the partially
transposed density matrix of a two-photon state. Moreover, we show that the
negativity can be given as a function of only six invariants, which are linear
combinations of nine invariants from the complete set of 21 fundamental and
independent two-qubit invariants. We analyze the relation between these moments
and the concurrence for some classes of two-qubit states (including the X
states, as well as pure states affected by the amplitude-damping and
phase-damping channels). We also discuss the possibility of using the universal
entanglement witness as an entanglement measure for various classes of
two-qubit states. Moreover, we analyze how noise affects the estimation of
entanglement via this witness.
|
1411.7977v2
|
2014-12-05
|
Adaptive Damping and Mean Removal for the Generalized Approximate Message Passing Algorithm
|
The generalized approximate message passing (GAMP) algorithm is an efficient
method of MAP or approximate-MMSE estimation of $x$ observed from a noisy
version of the transform coefficients $z = Ax$. In fact, for large zero-mean
i.i.d sub-Gaussian $A$, GAMP is characterized by a state evolution whose fixed
points, when unique, are optimal. For generic $A$, however, GAMP may diverge.
In this paper, we propose adaptive damping and mean-removal strategies that aim
to prevent divergence. Numerical results demonstrate significantly enhanced
robustness to non-zero-mean, rank-deficient, column-correlated, and
ill-conditioned $A$.
|
1412.2005v1
|
2014-12-11
|
Deviation From the Landau-Lifshitz-Gilbert equation in the Inertial regime of the Magnetization
|
We investigate in details the inertial dynamics of a uniform magnetization in
the ferromagnetic resonance (FMR) context. Analytical predictions and numerical
simulations of the complete equations within the Inertial
Landau-Lifshitz-Gilbert (ILLG) model are presented. In addition to the usual
precession resonance, the inertial model gives a second resonance peak
associated to the nutation dynamics provided that the damping is not too large.
The analytical resolution of the equations of motion yields both the precession
and nutation angular frequencies. They are function of the inertial dynamics
characteristic time $\tau$, the dimensionless damping $\alpha$ and the static
magnetic field $H$. A scaling function with respect to $\alpha\tau\gamma H$ is
found for the nutation angular frequency, also valid for the precession angular
frequency when $\alpha\tau\gamma H\gg 1$. Beyond the direct measurement of the
nutation resonance peak, we show that the inertial dynamics of the
magnetization has measurable effects on both the width and the angular
frequency of the precession resonance peak when varying the applied static
field. These predictions could be used to experimentally identify the inertial
dynamics of the magnetization proposed in the ILLG model.
|
1412.3783v1
|
2014-12-14
|
An adaptive selective frequency damping method
|
The selective frequency damping (SFD) method is an alternative to classical
Newton's method to obtain unstable steady-state solutions of dynamical systems.
However this method has two main limitations: it does not converge for
arbitrary control parameters; and when it does converge, the time necessary to
reach the steady-state solution may be very long. In this paper we present an
adaptive algorithm to address these two issues. We show that by evaluating the
dominant eigenvalue of a "partially converged" steady flow, we can select a
control coefficient and a filter width that ensure an optimum convergence of
the SFD method. We apply this adaptive method to several classical test cases
of computational fluid dynamics and we show that a steady-state solution can be
obtained without any a priori knowledge of the flow stability properties.
|
1412.4372v1
|
2014-12-23
|
Photoacoustic elastic oscillation and characterization
|
Photoacoustic imaging and sensing have been studied extensively to probe the
optical absorption of biological tissue in multiple scales ranging from large
organs to small molecules. However, its elastic oscillation characterization is
rarely studied and has been an untapped area to be explored. In literature,
photoacoustic signal induced by pulsed laser is commonly modelled as a bipolar
"N-shape" pulse from an optical absorber. In this paper, the photoacoustic
damped oscillation is predicted and modelled by an equivalent mass-spring
system by treating the optical absorber as an elastic oscillator. The
photoacoustic simulation incorporating the proposed oscillation model shows
better agreement with the measured signal from an elastic phantom, than
conventional photoacoustic simulation model. More interestingly, the
photoacoustic damping oscillation effect could potentially be a useful
characterization approach to evaluate biological tissue's mechanical properties
in terms of relaxation time, peak number and ratio beyond optical absorption
only, which is experimentally demonstrated in this paper.
|
1412.7284v1
|
2015-01-02
|
Inertia, diffusion and dynamics of a driven skyrmion
|
Skyrmions recently discovered in chiral magnets are a promising candidate for
magnetic storage devices because of their topological stability, small size
($\sim 3-100$nm), and ultra-low threshold current density ($\sim
10^{6}$A/m$^2$) to drive their motion. However, the time-dependent dynamics has
hitherto been largely unexplored. Here we show, by combining the numerical
solution of the Landau-Lifshitz-Gilbert equation and the analysis of a
generalized Thiele's equation, that inertial effects are almost completely
absent in skyrmion dynamics driven by a time-dependent current. In contrast,
the response to time-dependent magnetic forces and thermal fluctuations depends
strongly on frequency and is described by a large effective mass and a (anti-)
damping depending on the acceleration of the skyrmion. Thermal diffusion is
strongly suppressed by the cyclotron motion and is proportional to the Gilbert
damping coefficient $\alpha$. This indicates that the skyrmion position is
stable, and its motion responds to the time-dependent current without delay or
retardation even if it is fast. These findings demonstrate the advantages of
skyrmions as information carriers.
|
1501.00444v1
|
2015-01-08
|
Landau Damping of Geodesic Acoustic Mode in Toroidally Rotating Tokamaks
|
Geodesic acoustic mode (GAM) is analyzed by using modified gyro-kinetic (MGK)
equation applicable to low-frequency microinstabilities in a rotating
axisymmetric plasma. Dispersion relation of GAM in the presence of arbitrary
Mach number is analytically derived. Toroidal rotation plays the same effects
on the GAM regardless of the orientation of equilibrium flow. It is shown that
the toroidal Mach number $M$ increases the GAM frequency and dramatically
decreases the Landau damping rate. The valid of classical gyro-kinetic (CGK)
equation is also examined. For zero electron temperature, CGK is identical with
MGK. For non-zero electron temperature, CGK gives the same real frequency of
GAM as MGK but induces an instability with a growth rate proportional to
$M^3/q$, where $q$ is the safety factor.
|
1501.01750v2
|
2015-01-17
|
Applications of quantum cryptographic switch: Various tasks related to controlled quantum communication can be performed using Bell states and permutation of particles
|
Recently, several aspects of controlled quantum communication (e.g.,
bidirectional controlled state teleportation, controlled quantum secure direct
communication, controlled quantum dialogue, etc.) have been studied using
$n$-qubit ($n\geq3$) entanglement. Specially, a large number of schemes for
bidirectional controlled state teleportation are proposed using $m$-qubit
entanglement ($m\in\{5,6,7\}$). Here, we propose a set of protocols to
illustrate that it is possible to realize all these tasks related to controlled
quantum communication using only Bell states and permutation of particles
(PoP). As the generation and maintenance of a Bell state is much easier than a
multi-partite entanglement, the proposed strategy has a clear advantage over
the existing proposals. Further, it is shown that all the schemes proposed here
may be viewed as applications of the concept of quantum cryptographic switch
which was recently introduced by some of us. The performances of the proposed
protocols as subjected to the amplitude damping and phase damping noise on the
channels are also discussed.
|
1501.04187v1
|
2015-02-06
|
pQCD approach to Charmonium regeneration in QGP at the LHC
|
We analyze the applicability of perturbative QCD (pQCD) approach to the issue
of $J/\psi$ recombination at the Large Hadron Collider (LHC), and calculate the
recombination cross section for $c\bar{c}$ recombination to form $J/\psi$ as a
function of temperature. The charmonium wavefunction is obtained by employing a
temperature dependent phenomenological potential between the $c\bar{c}$ pair.
The temperature dependent formation time of charmonium is also employed in the
current work. A set of coupled rate equations is established which incorporates
color screening, gluonic dissociation, collisional damping and recombination of
uncorrelated $c\bar{c}$ pair in the quark-gluon plasma (QGP) medium. The final
$J/\psi$ suppression, thus determined as a function of centrality is compared
with the ALICE experimental data at both mid and forward rapidity and CMS
experimental data at mid rapidity obtained from the Large Hadron Collider (LHC)
at center of mass energy $\sqrt{s_{NN}} = 2.76$ TeV.
Keywords : Color screening, Recombination, Gluonic dissociation, Collisional
damping, Survival probability, pQCD, Charmonium
PACS numbers : 12.38.Mh, 12.38.Gc, 25.75.Nq, 24.10.Pa
|
1502.01790v2
|
2015-02-06
|
Biological control via "ecological" damping: An approach that attenuates non-target effects
|
In this work we develop and analyze a mathematical model of biological
control to prevent or attenuate the explosive increase of an invasive species
population in a three-species food chain. We allow for finite time blow-up in
the model as a mathematical construct to mimic the explosive increase in
population, enabling the species to reach "disastrous" levels, in a finite
time. We next propose various controls to drive down the invasive population
growth and, in certain cases, eliminate blow-up. The controls avoid chemical
treatments and/or natural enemy introduction, thus eliminating various
non-target effects associated with such classical methods. We refer to these
new controls as "ecological damping", as their inclusion dampens the invasive
species population growth. Further, we improve prior results on the regularity
and Turing instability of the three-species model that were derived in earlier
work. Lastly, we confirm the existence of spatio-temporal chaos.
|
1502.02010v1
|
2015-02-11
|
On the mass determination in liquid utilizing measurement of only the fundamental flexural resonances of the micro-/nanomechanical based mass sensors
|
Micro-/nanomechanical mass sensors are capable to quantitatively determine
molecule mass from only first three (two) measured cantilever (bridge) resonant
frequencies. However, in liquid solutions that are relevant to most of the
biological systems, the mass determination is challenging because the Q-factor
due to fluid damping decreases and, as a result, usually just the fundamental
resonant frequencies can be correctly identified. Moreover, for higher modes
the resonance coupling, noise and internal damping have been proven to strongly
affect the measured resonant frequencies and, correspondingly, the accuracy of
the estimated masses. Here, we derive the easy accessible expressions enabling
the quantitative mass(es) determination just from the fundamental resonant
frequencies of the micro/nanomechanical mass sensor under intentionally applied
axial tension, which can be easily created and controlled by the electrostatic
or magnetostatic forces. We also show that typically achievable force
resolution has a negligible impact on the mass determination and the mass
sensitivity.
|
1502.03232v1
|
2015-02-13
|
A Dynamical Model of Plasma Turbulence in the Solar Wind
|
A dynamical approach, rather than the usual statistical approach, is taken to
explore the physical mechanisms underlying the nonlinear transfer of energy,
the damping of the turbulent fluctuations, and the development of coherent
structures in kinetic plasma turbulence. It is argued that the linear and
nonlinear dynamics of Alfven waves are responsible, at a very fundamental
level, for some of the key qualitative features of plasma turbulence that
distinguish it from hydrodynamic turbulence, including the anisotropic cascade
of energy and the development of current sheets at small scales. The first
dynamical model of kinetic turbulence in the weakly collisional solar wind
plasma that combines self-consistently the physics of Alfven waves with the
development of small-scale current sheets is presented and its physical
implications are discussed. This model leads to a simplified perspective on the
nature of turbulence in a weakly collisional plasma: the nonlinear interactions
responsible for the turbulent cascade of energy and the formation of current
sheets are essentially fluid in nature, while the collisionless damping of the
turbulent fluctuations and the energy injection by kinetic instabilities are
essentially kinetic in nature.
|
1502.04109v1
|
2015-03-19
|
Vertical Oscillations of Fluid and Stellar Disks
|
A satellite galaxy or dark matter subhalo that passes through a stellar disk
may excite coherent oscillations in the disk perpendicular to its plane. We
determine the properties of these modes for various self-gravitating plane
symmetric systems (Spitzer sheets) using the matrix method of Kalnajs. In
particular, we find an infinite series of modes for the case of a barotropic
fluid. In general, for a collisionless system, there is a double series of
modes, which include normal modes and/or Landau-damped oscillations depending
on the phase space distribution function of the stars. Even Landau-damped
oscillations may decay slowly enough to persist for several hundred Myr. We
discuss the implications of these results for the recently discovered vertical
perturbations in the kinematics of solar neighborhood stars and for broader
questions surrounding secular phenomena such as spiral structure in disk
galaxies.
|
1503.05741v1
|
2015-03-24
|
Linearized nonequilibrium dynamics in nonconformal plasma
|
We investigate the behaviour of the lowest nonhydrodynamic modes in a class
of holographic models which exhibit an equation of state closely mimicking the
one determined from lattice QCD. We calculate the lowest quasinormal mode
frequencies for a range of scalar self-interaction potentials and find that the
damping of the quasinormal modes at the phase transition/crossover falls off by
a factor of around two from conformality after factoring out standard conformal
temperature dependence. The damping encoded in the imaginary part of the
frequencies turns out to be correlated with the speed of sound and is basically
independent of the UV details of the model. We also find that the dynamics of
the nonhydrodynamic degrees of freedom remains ultralocal, even to a higher
degree, as we deviate from conformality. These results indicate that the role
of nonhydrodynamic degrees of freedom in the vicinity of the crossover
transition may be enhanced.
|
1503.07149v1
|
2015-03-25
|
The Conversion of CESR to Operate as the Test Accelerator, CesrTA, Part 1: Overview
|
Cornell's electron/positron storage ring (CESR) was modified over a series of
accelerator shutdowns beginning in May 2008, which substantially improves its
capability for research and development for particle accelerators. CESR's
energy span from 1.8 to 5.6 GeV with both electrons and positrons makes it
ideal for the study of a wide spectrum of accelerator physics issues and
instrumentation related to present light sources and future lepton damping
rings. Additionally a number of these are also relevant for the beam physics of
proton accelerators. This paper outlines the motivation, design and conversion
of CESR to a test accelerator, CesrTA, enhanced to study such subjects as low
emittance tuning methods, electron cloud (EC) effects, intra-beam scattering,
fast ion instabilities as well as general improvements to beam instrumentation.
While the initial studies of CesrTA focussed on questions related to the
International Linear Collider (ILC) damping ring design, CesrTA is a very
flexible storage ring, capable of studying a wide range of accelerator physics
and instrumentation questions. This paper contains the outline and the basis
for a set of papers documenting the reconfiguration of the storage ring and the
associated instrumentation required for the studies described above. Further
details may be found in these papers.
|
1503.07451v2
|
2015-04-10
|
Fission barrier, damping of shell correction and neutron emission in the fission of A$\sim$200
|
Decay of $^{210}$Po compound nucleus formed in light and heavy-ion induced
fusion reactions has been analyzed simultaneously using a consistent
prescription for fission barrier and nuclear level density incorporating shell
correction and its damping with excitation energy. Good description of all the
excitation functions have been achieved with a fission barrier of 21.9 $\pm$
0.2 MeV. For this barrier height, the predicted statistical pre-fission
neutrons in heavy-ion fusion-fission are much smaller than the experimental
values, implying the presence of dynamical neutrons due to dissipation even at
these low excitation energies ($\sim$ 50~MeV) in the mass region A $\sim$ 200.
When only heavy-ion induced fission excitation functions and the pre-fission
neutron multiplicities are included in the fits, the deduced best fit fission
barrier depends on the assumed fission delay time during which dynamical
neutrons can be emitted. A fission delay of (0.8 $\pm$ 0.1 )$\times 10^{-19}$ s
has been estimated corresponding to the above fission barrier height assuming
that the entire excess neutrons over and above the statistical model
predictions are due to the dynamics. The present observation has implication on
the study of fission time scale/ nuclear viscosity using neutron emission as a
probe.
|
1504.02599v1
|
2015-04-10
|
Enhancement of the Anti-Damping Spin Torque Efficacy of Platinum by Interface Modification
|
We report a strong enhancement of the efficacy of the spin Hall effect (SHE)
of Pt for exerting anti-damping spin torque on an adjacent ferromagnetic layer
by the insertion of $\approx$ 0.5 nm layer of Hf between a Pt film and a thin,
< 2 nm, Fe$_{60}$Co$_{20}$B$_{20}$ ferromagnetic layer. This enhancement is
quantified by measurement of the switching current density when the
ferromagnetic layer is the free electrode in a magnetic tunnel junction. The
results are explained as the suppression of spin pumping through a substantial
decrease in the effective spin-mixing conductance of the interface, but without
a concomitant reduction of the ferromagnet\' s absorption of the SHE generated
spin current.
|
1504.02806v1
|
2015-04-21
|
Effect of assortative mixing in the second-order Kuramoto model
|
In this paper we analyze the second-order Kuramoto model presenting a
positive correlation between the heterogeneity of the connections and the
natural frequencies in scale-free networks. We numerically show that
discontinuous transitions emerge not just in disassortative but also in
assortative networks, in contrast with the first-order model. We also find that
the effect of assortativity on network synchronization can be compensated by
adjusting the phase damping. Our results show that it is possible to control
collective behavior of damped Kuramoto oscillators by tuning the network
structure or by adjusting the dissipation related to the phases movement.
|
1504.05447v1
|
2015-04-27
|
Controlled merging and annihilation of localized dissipative structures in an AC-driven damped nonlinear Schrödinger system
|
We report studies of controlled interactions of localized dissipative
structures in a system described by the AC-driven damped nonlinear
Schr\"odinger equation. Extensive numerical simulations reveal a diversity of
interaction scenarios that are governed by the properties of the system driver.
In our experiments, performed with a nonlinear optical Kerr resonator, the
phase profile of the driver is used to induce interactions on demand. We
observe both merging and annihilation of localized structures, i.e.,
interactions governed by the dissipative, out-of-equilibrium nature of the
system.
|
1504.07231v1
|
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
|
2015-08-04
|
A Critical Analysis of the Feasibility of Pure Strain-Actuated Giant Magnetostrictive Nanoscale Memories
|
Concepts for memories based on the manipulation of giant magnetostrictive
nanomagnets by stress pulses have garnered recent attention due to their
potential for ultra-low energy operation in the high storage density limit.
Here we discuss the feasibility of making such memories in light of the fact
that the Gilbert damping of such materials is typically quite high. We report
the results of numerical simulations for several classes of toggle precessional
and non-toggle dissipative magnetoelastic switching modes. Material candidates
for each of the several classes are analyzed and forms for the anisotropy
energy density and ranges of material parameters appropriate for each material
class are employed. Our study indicates that the Gilbert damping as well as the
anisotropy and demagnetization energies are all crucial for determining the
feasibility of magnetoelastic toggle-mode precessional switching schemes. The
roles of thermal stability and thermal fluctuations for stress-pulse switching
of giant magnetostrictive nanomagnets are also discussed in detail and are
shown to be important in the viability, design, and footprint of
magnetostrictive switching schemes.
|
1508.00629v2
|
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
|
2015-10-05
|
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
|
2015-10-17
|
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
|
2015-12-04
|
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
|
Parity-time symmetry breaking in magnetic systems
|
The understanding of out-of-equilibrium physics, especially dynamic
instabilities and dynamic phase transitions, is one of the major challenges of
contemporary science, spanning the broadest wealth of research areas that range
from quantum optics to living organisms. Focusing on nonequilibrium dynamics of
an open dissipative spin system, we introduce a non-Hermitian Hamiltonian
approach, in which non-Hermiticity reflects dissipation and deviation from
equilibrium. The imaginary part of the proposed spin Hamiltonian describes the
effects of Gilbert damping and applied Slonczewski spin-transfer torque. In the
classical limit, our approach reproduces Landau-Lifshitz-Gilbert-Slonczewski
dynamics of a large macrospin. We reveal the spin-transfer torque-driven
parity-time symmetry-breaking phase transition corresponding to a transition
from precessional to exponentially damped spin dynamics. Micromagnetic
simulations for nanoscale ferromagnetic disks demonstrate the predicted effect.
Our findings can pave the way to a general quantitative description of
out-of-equilibrium phase transitions driven by spontaneous parity-time symmetry
breaking.
|
1512.05408v2
|
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
|
2016-04-16
|
A broadband Ferromagnetic Resonance dipper probe for magnetic damping measurements from 4.2 K to 300 K
|
A dipper probe for broadband Ferromagnetic Resonance (FMR) operating from 4.2
K to room temperature is described. The apparatus is based on a 2-port
transmitted microwave signal measurement with a grounded coplanar waveguide.
The waveguide generates a microwave field and records the sample response. A
3-stage dipper design is adopted for fast and stable temperature control. The
temperature variation due to FMR is in the milli-Kelvin range at liquid helium
temperature. We also designed a novel FMR probe head with a spring-loaded
sample holder. Improved signal-to-noise ratio and stability compared to a
common FMR head are achieved. Using a superconducting vector magnet we
demonstrate Gilbert damping measurements on two thin film samples using a
vector network analyzer with frequency up to 26 GHz: 1) A Permalloy film of 5
nm thickness and 2) a CoFeB film of 1.5 nm thickness. Experiments were
performed with the applied magnetic field parallel and perpendicular to the
film plane.
|
1604.04688v1
|
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
|
2016-05-03
|
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
|
2016-05-10
|
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
|
2016-05-27
|
A reduced model for precessional switching of thin-film nanomagnets under the influence of spin-torque
|
We study the magnetization dynamics of thin-film magnetic elements with
in-plane magnetization subject to a spin-current flowing perpendicular to the
film plane. We derive a reduced partial differential equation for the in-plane
magnetization angle in a weakly damped regime. We then apply this model to
study the experimentally relevant problem of switching of an elliptical element
when the spin-polarization has a component perpendicular to the film plane,
restricting the reduced model to a macrospin approximation. The macrospin
ordinary differential equation is treated analytically as a weakly damped
Hamiltonian system, and an orbit-averaging method is used to understand
transitions in solution behaviors in terms of a discrete dynamical system. The
predictions of our reduced model are compared to those of the full
Landau--Lifshitz--Gilbert--Slonczewski equation for a macrospin.
|
1605.08698v1
|
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
|
2016-06-21
|
Torsion Effects and LLG Equation
|
Based on the non-relativistic regime of the Dirac equation coupled to a
torsion pseudo-vector, we study the dynamics of magnetization and how it is
affected by the presence of torsion. We consider that torsion interacting terms
in Dirac equation appear in two ways one of these is thhrough the covariant
derivative considering the spin connection and gauge magnetic field and the
other is through a non-minimal spin torsion coupling. We show within this
framework, that it is possible to obtain the most general Landau, Lifshitz and
Gilbert (LLG) equation including the torsion effects, where we refer to torsion
as a geometric field playing an important role in the spin coupling process. We
show that the torsion terms can give us two important landscapes in the
magnetization dynamics: one of them related with damping and the other related
with the screw dislocation that give us a global effect like a helix damping
sharped. These terms are responsible for changes in the magnetization
precession dynamics.
|
1606.06610v1
|
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
|
2016-07-05
|
Magnetic moment of inertia within the breathing model
|
An essential property of magnetic devices is the relaxation rate in magnetic
switching which strongly depends on the energy dissipation and magnetic inertia
of the magnetization dynamics. Both parameters are commonly taken as a
phenomenological entities. However very recently, a large effort has been
dedicated to obtain Gilbert damping from first principles. In contrast, there
is no ab initio study that so far has reproduced measured data of magnetic
inertia in magnetic materials. In this letter, we present and elaborate on a
theoretical model for calculating the magnetic moment of inertia based on the
torque-torque correlation model. Particularly, the method has been applied to
bulk bcc Fe, fcc Co and fcc Ni in the framework of the tight-binding
approximation and the numerical values are comparable with recent experimental
measurements. The theoretical results elucidate the physical origin of the
moment of inertia based on the electronic structure. Even though the moment of
inertia and damping are produced by the spin-orbit coupling, our analysis shows
that they are caused by undergo different electronic structure mechanisms.
|
1607.01307v1
|
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