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2013-09-21
|
Patterns formation in axially symmetric Landau-Lifshitz-Gilbert-Slonczewski equations
|
The Landau-Lifshitz-Gilbert-Slonczewski equation describes magnetization
dynamics in the presence of an applied field and a spin polarized current. In
the case of axial symmetry and with focus on one space dimension, we
investigate the emergence of space-time patterns in the form of wavetrains and
coherent structures, whose local wavenumber varies in space. A major part of
this study concerns existence and stability of wavetrains and of front- and
domain wall-type coherent structures whose profiles asymptote to wavetrains or
the constant up-/down-magnetizations. For certain polarization the Slonczewski
term can be removed which allows for a more complete charaterization, including
soliton-type solutions. Decisive for the solution structure is the polarization
parameter as well as size of anisotropy compared with the difference of field
intensity and current intensity normalized by the damping.
|
1309.5523v4
|
2013-10-09
|
Symmetry Analysis in Linear Hydrodynamic Stability Theory: Classical and New Modes in Linear Shear
|
We present a symmetry classification of the linearised Navier-Stokes
equations for a two-dimensional unbounded linear shear flow of an
incompressible fluid. The full set of symmetries is employed to systematically
derive invariant ansatz functions. The symmetry analysis grasps three
approaches. Two of them are existing ones, representing the classical normal
modes and the Kelvin modes, while the third is a novel approach and leads to a
new closed-form solution of traveling modes, showing qualitatively different
behaviour in energetics, shape and kinematics when compared to the classical
approaches. The last modes are energy conserving in the inviscid case. They are
localized in the cross-stream direction and periodic in the streamwise
direction. As for the kinematics, they travel at constant velocity in the
cross-stream direction, whilst in the streamwise direction they are accelerated
by the base flow. In the viscous case, the modes break down due to damping of
high wavenumber contributions.
|
1310.2650v1
|
2013-10-23
|
Spectroscopic investigation of local mechanical impedance of living cells
|
The mechanical properties of PC12 living cells have been studied at the
nanoscale with a Force Feedback Microscope using two experimental approaches.
Firstly, the local mechanical impedance of the cell membrane has been mapped
simultaneously to the cell morphology at constant force. As the force of the
interaction is gradually increased, we observed the appearance of the
sub-membrane cytoskeleton. We shall compare the results obtained with this
method with the measurement of other existing techniques. Secondly, a
spectroscopic investigation has been performed varying the indentation of the
tip in the cell membrane and consequently the force applied on it. In contrast
with conventional dynamic atomic force microscopy techniques, here the small
oscillation amplitude of the tip is not necessarily imposed at the cantilever
first eigenmode. This allows the user to arbitrarily choose the excitation
frequency in developing spectroscopic AFM techniques. The mechanical response
of the PC12 cell membrane is found to be frequency dependent in the 1 kHz - 10
kHz range. The damping coefficient is reproducibly observed to decrease when
the excitation frequency is increased.
|
1310.6201v1
|
2013-10-23
|
Using an artificial electric field to create the analog of the red spot of Jupiter in light-heavy Fermi-Fermi mixtures of ultracold atoms
|
Time-of-flight images are a common tool in ultracold atomic experiments,
employed to determine the quasimomentum distribution of the interacting
particles. If one introduces a constant artificial electric field, then the
quasimomentum distribution evolves in time as Bloch oscillations are generated
in the system and then damped showing a complex series of patterns.
Surprisingly, in different mass Fermi-Fermi mixtures, these patterns can
survive for long times, and resemble the stability of the red spot of Jupiter
in classical nonlinear hydrodynamics. In this work, we illustrate the rich
phenomena that can be seen in these driven quantum systems.
|
1310.6350v1
|
2013-11-21
|
Note on the super inflation in loop quantum cosmology
|
Phenomenological effect of the super-inflation in loop quantum cosmology
(LQC) is discussed. We investigate the case that the Universe is filled with
the interacting field between massive scalar field and radiation. Considering
the damping coefficient $\Gamma$ as a constant, the changes of the scale factor
during super-inflation with four different initial conditions are discussed,
and we find that the changes of the scale factor depends on the initial values
of energy density of the scalar field and radiation at the bounce point. But no
matter which initial condition is chosen, the radiation always dominated at the
late time. Moreover, we investigate whether the super-inflation can provide
enough e-folding number. For the super-inflation starts from the quantum bounce
point, the initial value of Hubble parameter $H(t_i)\sim0$, then it is possible
to solve the flatness problem and horizon problem. As an example, following the
method of \cite{Amoros-prd} to calculate particle horizon on the condition that
the radiation dominated at bounce point, and we find that the Universe has had
enough time to be homogeneous and isotopic.
|
1311.5325v1
|
2013-12-10
|
Delaying the waterfall transition in warm hybrid inflation
|
We analyze the dynamics and observational predictions of supersymmetric
hybrid inflation in the warm regime, where dissipative effects are mediated by
the waterfall fields and their subsequent decay into light degrees of freedom.
This produces a quasi-thermal radiation bath with a slowly-varying temperature
during inflation and further damps the inflaton's motion, thus prolonging
inflation. As in the standard supercooled scenario, inflation ends when the
waterfall fields become tachyonic and can no longer sustain a nearly constant
vacuum energy, but the interaction with the radiation bath makes the waterfall
fields effectively heavier and delays the phase transition to the
supersymmetric minimum. In this work, we analyze for the first time the effects
of finite temperature corrections and SUSY mass splittings on the quantum
effective potential and the resulting dissipation coefficient. We show, in
particular, that dissipation can significantly delay the onset of the tachyonic
instability to yield 50-60 e-folds of inflation and an observationally
consistent primordial spectrum, which is not possible in the standard
supercooled regime when inflation is driven by radiative corrections.
|
1312.2961v1
|
2013-12-11
|
Modelling of the optical properties of silver with use of six fitting parameters
|
We propose a realistic model of the optical properties of silver, in which
inter-band transition with a threshold energy of ~ 4 eV is described
phenomenologically by an ensemble of oscillators with same damping constant and
a certain distribution of resonant frequencies in the interband transition
threshold to infinity. The contribution of the conduction electrons in the
dielectric function is determined by the Drude formula. The proposed model
actually contains the features of both the Drude-Lorentz model (Raki\'c et al.
1998) and Tauc-Lorentz model (Jian-Hong Qiu et al. 2005). However, unlike these
works proposed model contains only six fitting parameters, with the square root
of the mean square deviation of the absorption coefficient and refractive index
of silver from the experimental values in the range of 0.6 nm - 6.0 nm being of
the order of 0.05.
|
1312.3100v1
|
2014-02-13
|
Tailoring optical fields emitted by nanometric sources
|
Here we study a simple way of controlling the emitted fields of
sub-wavelength nanometric sources. The system consists of arrays of
nanoparticles (NPs) embedded in optical active media. The key concept is the
careful tuning of NP's damping factors, which changes the eigenmode's decay
rates of the whole array. This, at long time, leads to a locking of relative
phases and frequencies of individual localized-surfaces-plasmons (LSPs) and,
thus, controlls the emitted field. The amplitude of the LSP's oscillations can
be kept constant by embedding the system in optical active media. In the case
of full loss compensation, this implies that, not only the relative phases, but
also the amplitudes of the LSPs remain fixed, leading us, additionally, to
interpret the process as a new example of synchronization. The proposed
approach can be used as a general way of controlling and designing the
electromagnetic fields emitted by nanometric sources, which can find
applications in optoelectronic, nanoscale lithography and probing microscopy.
|
1402.3184v1
|
2014-02-25
|
Kinetic theory for a mobile impurity in a degenerate Tonks-Girardeau gas
|
A kinetic theory describing the motion of an impurity particle in a
degenerate Tonks-Girardeau gas is presented. The theory is based on the
one-dimensional Boltzmann equation. An iterative procedure for solving this
equation is proposed, leading to the exact solution in number of special cases
and to an approximate solution with the explicitly specified precision in a
general case. Previously we have reported that the impurity reaches a
non-thermal steady state, characterized by an impurity momentum $p_\infty$
depending on its initial momentum $p_0$. In the present paper the detailed
derivation of $p_\infty(p_0)$ is provided. We also study the motion of an
impurity under the action of a constant force $F$. It is demonstrated that if
the impurity is heavier than the host particles, $m_i>m_h$, damped oscillations
of the impurity momentum develop, while in the opposite case, $m_i<m_h$,
oscillations are absent. The steady state momentum as a function of the applied
force is determined. In the limit of weak force it is found to be force
independent for a light impurity and proportional to $\sqrt{F}$ for a heavy
impurity.
|
1402.6362v2
|
2014-02-28
|
Probing the critical behavior in the evolution of GDR width at very low temperatures in A~100 mass region
|
The influence of giant dipole resonance (GDR) induced quadrupole moment on
GDR width at low temperatures is investigated experimentally by measuring GDR
width systematically in the unexplored temperature range $T$=0.8-1.5 MeV, for
the first time, in $A$ $\sim$ 100 mass region. The measured GDR widths, using
alpha induced fusion reaction, for $^{97}$Tc confirm that the GDR width remains
constant at the ground state value up to a critical temperature and increases
sharply thereafter with increase in $T$. The data have been compared with the
adiabatic Thermal Shape Fluctuation Model (TSFM), phenomenological Critical
Temperature Fluctuation Model (CTFM) and microscopic Phonon Damping Model
(PDM). Interestingly, CTFM and PDM give similar results and agree with the
data, whereas the TSFM differs significantly even after incorporating the shell
effects.
|
1402.7210v1
|
2014-03-19
|
Kinetic description of wave induced plasma flow in the radio frequency domain
|
A model for ICRH induced flows in the presence of a strong magnetic field is
presented. These flows are the finite temperature counterpart of flows existing
in cold plasmas described e.g. in [D. Van Eester et al., Plasma Phys. Control.
Fusion 55 (2013) 025002] and thus do not rely on the waves being damped. The
kinetic corrections offer insight in what happens at cyclotron resonances.
Authors commonly either rely on the confining magnetic field $\vec{B}_o$-field
to be strong, or the electric field $\vec{E}$-field to be rapidly varying but
are not accounting for both when writing down the solution of the equation of
motion on the slow time scale. In this paper, the equation of motion is solved
for constant $B_o$ to keep the discussion as simple as possible. The
simultaneous presence of $\vec{B}_o$ and the $\vec{E}$-field inhomogeneity
causes drifts perpendicular to the $\vec{B}_o$ and to other slow time scale
accelerations, the Ponderomotive acceleration being one of them. Because of the
first and having tokamak applications in mind, these flows - although small in
magnitude - cause drifts that enter in competition with transport induced
flows.
|
1403.4770v1
|
2014-04-04
|
Irreversible Thermodynamics of the Universe: Constraints from Planck Data
|
The present work deals with irreversible Universal thermodynamics. The
homogenous and isotropic flat model of the universe is chosen as open
thermodynamical system and non-equilibrium thermodynamics comes into picture
due to the mechanism of particle creation. For simplicity, entropy flow is
considered only due to heat conduction. Further, due to Maxwell-Cattaneo
modified Fourier law for non-equilibrium phenomenon, the temperature satisfies
damped wave equation instead of heat conduction equation. Validity of
generalized second law of thermodynamics (GSLT) has been investigated for
Universe bounded by apparent or event horizon with cosmic substrutum as perfect
fluid with constant or variable equation of state or interacting dark species.
Finally, we have used three Planck data sets to constrain the thermal
conductivity \lambda and the coupling parameter b^2. These constraints must be
satisfied in order for GSLT to hold for Universe bounded by apparent or event
horizons.
|
1404.1220v1
|
2014-04-18
|
Transport Properties of Dirac Ferromagnet
|
We propose a model ferromagnet based on the Dirac Hamiltonian in three
spatial dimensions, and study its transport properties which include
anisotropic magnetoresistance (AMR) and anomalous Hall (AH) effect. This
relativistic extension allows two kinds of ferromagnetic order parameters,
denoted by $\bm{M}$ and $\bm{S}$, which are distinguished by the relative sign
between the positive- and negative-energy states (at zero momentum) and become
degenerate in the non-relativistic limit. Because of the relativistic coupling
between the spin and the orbital motion, both $\bm{M}$ and $\bm{S}$ induce
anisotropic deformations of the energy dispersion (and the Fermi surfaces) but
in mutually opposite ways. The AMR is determined primarily by the anisotropy of
the Fermi surface (group velocity), and secondarily by the anisotropy of the
damping; the latter becomes important for ${\bm M}=\pm{\bm S}$, where the Fermi
surfaces are isotropic. Even when the chemical potential lies in the gap, the
AH conductivity is found to take a finite non-quantized value, $\sigma_{ij} =
-(\alpha /3\pi^2 \hbar) \epsilon_{ijk} S_k $, where $\alpha$ is the (effective)
fine structure constant. This offers an example of Hall insulator in three
spatial dimensions.
|
1404.4741v2
|
2014-05-21
|
Spectroscopy of Rindler Modified Schwarzschild Black Hole
|
We study quasinormal modes (QNMs) of uncharged Grumiller black hole (GBH).
This massive BH has a Rindler acceleration $a$, and hence it is also called
Rindler modified Schwarzschild BH. After reducing the radial equation of the
massless Klein-Gordon equation to the Zerilli equation, we compute the complex
frequencies of the QNMs of the GBH. To this end, an approximation method which
considers small perturbation around its horizon is used. Considering the highly
damped QNMs in the process proposed by Maggiore, the quantum entropy and area
spectra of these BHs are found. Although the QNM frequencies are tuned by the
Rindler term, we show in detail that the spacing does not depend on it. Here,
dimensionless constant {\epsilon} of the area spectrum is found to be double of
its Schwarzschild value. The latter result is also discussed.
|
1405.5388v1
|
2014-08-08
|
Ambipolar diffusion in smoothed particle magnetohydrodynamics
|
In partially ionised plasmas, the magnetic field can become decoupled from
the neutral gas and diffuse through it in a process known as ambipolar
diffusion. Although ambipolar diffusion has been implemented in several grid
codes, we here provide an implementation in smoothed particle
magnetohydrodynamics (SPMHD). We use the strong coupling approximation in which
the ion density is negligible, allowing a single fluid approach. The equations
are derived to conserve energy, and to provide a positive definite contribution
to the entropy. We test the implementation in both a simple 1D SPMHD code and
the fully 3D code PHANTOM. The wave damping test yields agreement within 0.03-2
per cent of the analytical result, depending on the value of the collisional
coupling constant. The oblique C-shocks test yields results that typically
agree within 4 per cent of the semi-analytical result. Our algorithm is
therefore suitable for exploring the effect ambipolar diffusion has on physical
processes, such as the formation of stars from molecular clouds.
|
1408.1807v1
|
2014-10-05
|
Finite-time stabilization of a network of strings
|
We investigate the finite-time stabilization of a tree-shaped network of
strings. Transparent boundary conditions are applied at all the external nodes.
At any internal node, in addition to the usual continuity conditions, a
modified Kirchhoff law incorporating a damping term $\alpha u_t$ with a
coefficient $\alpha$ that may depend on the node is considered. We show that
for a convenient choice of the sequence of coefficients $\alpha$, any solution
of the wave equation on the network becomes constant after a finite time. The
condition on the coefficients proves to be sharp at least for a star-shaped
tree. Similar results are derived when we replace the transparent boundary
condition by the Dirichlet (resp. Neumann) boundary condition at one external
node.
|
1410.1122v1
|
2014-10-22
|
Spin current generation from sputtered Y3Fe5O12 films
|
Spin current injection from sputtered yttrium iron garnet (YIG) films into an
adjacent platinum layer has been investigated by means of the spin pumping and
the spin Seebeck effects. Films with a thickness of 83 and 96 nanometers were
fabricated by on-axis magnetron rf sputtering at room temperature and
subsequent post-annealing. From the frequency dependence of the ferromagnetic
resonance linewidth, the damping constant has been estimated to be
$(7.0\pm1.0)\times 10^{-4}$. Magnitudes of the spin current generated by the
spin pumping and the spin Seebeck effect are of the same order as values for
YIG films prepared by liquid phase epitaxy. The efficient spin current
injection can be ascribed to a good YIG|Pt interface, which is confirmed by the
large spin-mixing conductance $(2.0\pm0.2)\times 10^{18}$ m$^{-2}$.
|
1410.5987v1
|
2014-11-03
|
Experimental Demonstration of the Co-existence of the Spin Hall and Rashba Effects in beta-Tantalum/Ferromagnet Bilayers
|
We have measured the spin torques of beta-Tantalum / Co20Fe60B20 bilayers
versus Ta thickness at room temperature using an FMR technique. The spin Hall
coefficient was calculated both from the observed change in damping coefficient
of the ferromagnet with Ta thickness, and from the ratio of the symmetric and
anti-symmetric components of the FMR signal. Results from these two methods
yielded values for the spin Hall coefficient of -0.090+/-0.005 and
-0.11+/-0.01, respectively. We have also identified a significant out-of-plane
spin torque originating from Ta, which is constant with Ta thickness. We
ascribe this to an interface spin orbit coupling, or Rashba effect, due to the
strength and constancy of the torque with Ta thickness. From fitting measured
data to a model including interface spin orbit coupling, we have determined the
spin diffusion length for beta-Tantalum to be ~2.5 nm.
|
1411.0601v1
|
2014-12-01
|
Dissipation due to pure spin-current generated by spin pumping
|
Based on spin-dependent transport theory and thermodynamics, we develop a
generalized theory of the Joule heating in the presence of a spin current.
Along with the conventional Joule heating consisting of an electric current and
electrochemical potential, it is found that the spin current and spin
accumulation give an additional dissipation because the spin-dependent
scatterings inside bulk and ferromagnetic/nonmagnetic interface lead to a
change of entropy. The theory is applied to investigate the dissipation due to
pure spin-current generated by spin pumping across a
ferromagnetic/nonmagnetic/ferromagnetic multilayer. The dissipation arises from
an interface because the spin pumping is a transfer of both the spin angular
momentum and the energy from the ferromagnet to conduction electrons near the
interface. It is found that the dissipation is proportional to the enhancement
of the Gilbert damping constant by spin pumping.
|
1412.0688v1
|
2014-12-08
|
Higgs mode in the quench dynamics of a confined ultracold Fermi gas in the BCS regime
|
The Higgs amplitude mode of the order parameter of an ultracold confined
Fermi gas in the BCS regime after a quench of the coupling constant is analyzed
theoretically. Characteristic features are a damped oscillation which at a
certain transition time changes into a rather irregular dynamics. We compare
the numerical solution of the full set of nonlinear equations of motion for the
normal and anomalous Bogoliubov quasiparticle excitations with a linearized
approximation. In doing so the transition time as well as the difference
between resonant systems, i.e., systems where the Fermi energy is close to a
subband minimum, and off-resonant systems can be well understood and traced
back to the system and geometry parameters.
|
1412.2591v1
|
2014-12-16
|
Dynamics of reversals and condensates in 2D Kolmogorov flows
|
We present direct numerical simulations of the different two-dimensional flow
regimes generated by a constant spatially periodic forcing balanced by viscous
dissipation and large scale drag with a dimensionless damping rate $1/Rh$. The
linear response to the forcing is a $6\times6$ square array of counter-rotating
vortices, which is stable when the Reynolds number $Re$ or $Rh$ are small.
After identifying the sequence of bifurcations that lead to a spatially and
temporally chaotic regime of the flow when $Re$ and $Rh$ are increased, we
study the transitions between the different turbulent regimes observed for
large $Re$ by varying $Rh$. A large scale circulation at the box size (the
condensate state) is the dominant mode in the limit of vanishing large scale
drag ($Rh$ large). When $Rh$ is decreased, the condensate becomes unstable and
a regime with random reversals between two large scale circulations of opposite
signs is generated. It involves a bimodal probability density function of the
large scale velocity that continuously bifurcates to a Gaussian distribution
when $Rh$ is decreased further.
|
1412.4959v1
|
2015-01-14
|
Predissociation dynamics of lithium iodide
|
The predissociation dynamics of lithium iodide (LiI) in the first excited
A-state is investigated for molecules in the gas phase and embedded in helium
nanodroplets, using femtosecond pump-probe photoionization spectroscopy. In the
gas phase, the transient Li+ and LiI+ ion signals feature damped oscillations
due to the excitation and decay of a vibrational wave packet. Based on
high-level ab initio calculations of the electronic structure of LiI and
simulations of the wave packet dynamics, the exponential signal decay is found
to result from predissociation predominantly at the lowest avoided X-A
potential curve crossing, for which we infer a coupling constant V=650(20)
reciprocal cm. The lack of a pump-probe delay dependence for the case of LiI
embedded in helium nanodroplets indicates fast droplet-induced relaxation of
the vibrational excitation.
|
1501.03327v1
|
2015-01-15
|
Dynamics and performance of clock pendulums
|
We analyze the dynamics of a driven, damped pendulum as used in mechanical
clocks. We derive equations for the amplitude and phase of the oscillation, on
time scales longer than the pendulum period. The equations are first order ODEs
and permit fast simulations of the joint effects of circular and escapement
errors, friction, and other disturbances for long times. The equations contain
two averages of the driving torque over a period, so that the results are not
very sensitive to the fine structure of the driving. We adopt a constant-torque
escapement and study the stationary pendulum rate as a function of driving
torque and friction. We also study the reaction of the pendulum to a sudden
change in the driving torque, and to stationary noisy driving. The equations
for the amplitude and phase are shown to describe the pendulum dynamics quite
well on time scales of one period and longer. Our emphasis is on a clear
exposition of the physics.
|
1501.03673v1
|
2015-01-21
|
Controllable chaotic dynamics in a nonlinear fiber ring resonators with balanced gain and loss
|
We show the possibility of controlling the dynamical behavior of a single
fiber ring (SFR) resonator system with the fiber being an amplified (gain)
channel and the ring being attenuated (loss) nonlinear dielectric medium. Our
model is based on the simple alterations in the parity time symmetric synthetic
coupler structures proposed recently [A. Regensburger et al., Nature 488, 167
(2012)]. The system has been modeled using the transfer matrix formalism. We
find that this results in a dynamically controllable algorithm for the chaotic
dynamics inherent in the system. We have also shown the dependence of the
period doubling point on the input amplitude, emphasizing on the dynamical
aspects. Moreover, the fact that the resonator essentially plays the role of a
damped harmonic oscillator has been elucidated with the non-zero intensity
inside the resonator due to constant influx of input light.
|
1501.05187v2
|
2015-01-30
|
Head-to-Head Domain Wall Structures in Wide Permalloy Strips
|
We analyze the equilibrium micromagnetic domain wall structures encountered
in Permalloy strips of a wide range of thicknesses and widths, with strip
widths up to several micrometers. By performing an extensive set of
micromagnetic simulations, we show that the equilibrium phase diagram of the
domain wall structures exhibits in addition to the previously found structures
(symmetric and asymmetric transverse walls, vortex wall) also double vortex and
triple vortex domain walls for large enough strip widths and thicknesses. Also
several metastable domain wall structures are found for wide and/or thick
strips. We discuss the details of the relaxation process from random
magnetization initial states towards the stable domain wall structure, and show
that our results are robust with respect to changes of e.g. the magnitude of
the Gilbert damping constant and details of the initial conditions.
|
1501.07731v1
|
2015-03-19
|
On the efficiency of heat engines at the micro-scale and below
|
We investigate the thermodynamic efficiency of sub-micro-scale heat engines
operating under the conditions described by over-damped stochastic
thermodynamics. We prove that at maximum power the efficiency obeys for
constant isotropic mobility the universal law $\eta=2\,\eta_{C}/(4-\eta_{C})$
where $\eta_{C}$ is the efficiency of an ideal Carnot cycle. The corresponding
power optimizing protocol is specified by the solution of an optimal mass
transport problem. Such solution can be determined explicitly using well known
Monge--Amp\`ere--Kantorovich reconstruction algorithms. Furthermore, we show
that the same law describes the efficiency of heat engines operating at maximum
work over short time periods. Finally, we illustrate the straightforward
extension of these results to cases when the mobility is anisotropic and
temperature dependent.
|
1503.05788v2
|
2015-03-26
|
Thermophoresis of an Antiferromagnetic Soliton
|
We study dynamics of an antiferromagnetic soliton under a temperature
gradient. To this end, we start by phenomenologically constructing the
stochastic Landau-Lifshitz-Gilbert equation for an antiferromagnet with the aid
of the fluctuation-dissipation theorem. We then derive the Langevin equation
for the soliton's center of mass by the collective coordinate approach. An
antiferromagentic soliton behaves as a classical massive particle immersed in a
viscous medium. By considering a thermodynamic ensemble of solitons, we obtain
the Fokker-Planck equation, from which we extract the average drift velocity of
a soliton. The diffusion coefficient is inversely proportional to a small
damping constant $\alpha$, which can yield a drift velocity of tens of m/s
under a temperature gradient of $1$ K/mm for a domain wall in an easy-axis
antiferromagnetic wire with $\alpha \sim 10^{-4}$.
|
1503.07854v2
|
2015-04-09
|
Thinning and thickening in active microrheology
|
When pulling a probe particle in a many-particle system with fixed velocity,
the probe's effective friction, defined as average pulling force over its
velocity, $\gamma_{eff}:=\langle F_{ex}\rangle/u$, first keeps constant (linear
response), then decreases (thinning) and finally increases (thickening). We
propose a three-time-scales picture (TTSP) to unify thinning and thickening
behaviour. The points of the TTSP are that there are three distinct time scales
of bath particles: diffusion, damping, and single probe-bath (P-B) collision;
the dominating time scales, which are controlled by the pulling velocity,
determine the behaviour of the probe's friction. We confirm the TTSP by
Langevin dynamics simulation. Microscopically, we find that for computing the
effective friction, Maxwellian distribution of bath particles' velocities works
in low Reynolds number (Re) but fails in high Re. It can be understood based on
the microscopic mechanism of thickening obtained in the $T=0$ limit. Based on
the TTSP, we explain different thinning and thickening observations in some
earlier literature.
|
1504.02277v1
|
2015-06-09
|
Sensitivity analysis for shape optimization of a focusing acoustic lens in lithotripsy
|
We are interested in shape sensitivity analysis for an optimization problem
arising in medical applications of high intensity focused ultrasound. The goal
is to find the optimal shape of a focusing acoustic lens so that the desired
acoustic pressure at a kidney stone is achieved. Coupling of the silicone
acoustic lens and nonlinearly acoustic fluid region is modeled by the
Westervelt equation with nonlinear strong damping and piecewise constant
coefficients. We follow the variational approach to calculating the shape
derivative of the cost functional which does not require computing the shape
derivative of the state variable; however assumptions of certain spatial
regularity of the primal and the adjoint state are needed to obtain the
derivative, in particular for its strong form according to the
Delfour-Hadamard-Zol\' esio Structure Theorem.
|
1506.02781v1
|
2015-06-27
|
The Quasi-normal Modes of Charged Scalar Fields in Kerr-Newman black hole and Its Geometric Interpretation
|
It is well-known that there is a geometric correspondence between
high-frequency quasi-normal modes (QNMs) and null geodesics (spherical photon
orbits). In this paper, we generalize such correspondence to charged scalar
field in Kerr-Newman space-time. In our case, the particle and black hole are
all charged, so one should consider non-geodesic orbits. Using the WKB
approximation, we find that the real part of quasi-normal frequency corresponds
to the orbits frequency, the imaginary part of the frequency corresponds to the
Lyapunov exponent of these orbits and the eigenvalue of angular equation
corresponds to carter constant. From the properties of the imaginary part of
quasi-normal frequency of charged massless scalar field, we can still find that
the QNMs of charged massless scalar field possess the zero damping modes in
extreme Kerr-Newman spacetime under certain condition which has been fixed in
this paper.
|
1506.08276v2
|
2015-07-05
|
Representation of the Fourier transform as a weighted sum of the complex error functions
|
In this paper we show that a methodology based on a sampling with the
Gaussian function of kind $h\,{e^{ - {{\left( {t/c} \right)}^2}}}/\left(
{{c}\sqrt \pi } \right)$, where ${c}$ and $h$ are some constants, leads to the
Fourier transform that can be represented as a weighted sum of the complex
error functions. Due to remarkable property of the complex error function, the
Fourier transform based on the weighted sum can be significantly simplified and
expressed in terms of a damping harmonic series. In contrast to the
conventional discrete Fourier transform, this methodology results in a
non-periodic wavelet approximation. Consequently, the proposed approach may be
useful and convenient in algorithmic implementation.
|
1507.01241v3
|
2015-07-17
|
Oscillatory Growth: A Phenomenological View
|
In this communication, the approach of phenomenological universalities of
growth are considered to describe the behaviour of a system showing oscillatory
growth. Two phenomenological classes are proposed to consider the behaviour of
a system in which oscillation of a property may be observed. One of them is
showing oscillatory nature with constant amplitude and the other represents
oscillatory nature with a change in amplitude. The term responsible for damping
in the proposed class is also been identified. The variations in the nature of
oscillation with dependent parameters are studied in detail. In this
connection, the variation of a specific growth rate is also been considered.
The significance of presence and absence of each term involved in
phenomenological description are also taken into consideration in the present
communication. These proposed classes might be useful for the experimentalists
to extract characteristic features from the dataset and to develop a suitable
model consistent with their data set.
|
1507.04833v1
|
2015-07-29
|
Gravitational, shear and matter waves in Kantowski-Sachs cosmologies
|
A general treatment of vorticity-free, perfect fluid perturbations of
Kantowski-Sachs models with a positive cosmological constant are considered
within the framework of the 1+1+2 covariant decomposition of spacetime. The
dynamics is encompassed in six evolution equations for six harmonic
coefficients, describing gravito-magnetic, kinematic and matter perturbations,
while a set of algebraic expressions determine the rest of the variables. The
six equations further decouple into a set of four equations sourced by the
perfect fluid, representing forced oscillations and two uncoupled damped
oscillator equations.
The two gravitational degrees of freedom are represented by pairs of
gravito-magnetic perturbations. In contrast with the Friedmann case one of them
is coupled to the matter density perturbations, becoming decoupled only in the
geometrical optics limit. In this approximation, the even and odd tensorial
perturbations of the Weyl tensor evolve as gravitational waves on the
anisotropic Kantowski-Sachs background, while the modes describing the shear
and the matter density gradient are out of phase dephased by $\pi /2$ and share
the same speed of sound.
|
1507.08300v2
|
2015-08-04
|
Scalar Perturbations of two-dimensional Horava-Lifshitz Black Holes
|
In this article, we study the stability of black hole solutions found in the
context of dilatonic Horava-Lifshitz gravity in $1+1$ dimensions by means of
the quasinormal modes approach. In order to find the corresponding quasinormal
modes, we consider the perturbations of massive and massless scalar fields
minimally coupled to gravity. In both cases, we found that the quasinormal
modes have a discrete spectrum and are completely imaginary, which leads to
damping modes. For a massive scalar field and a non-vanishing cosmological
constant, our results suggest unstable behaviour for large values of the scalar
field mass.
|
1508.00650v2
|
2015-09-16
|
Outgoing electromagnetic power induced from pair plasma falling into a rotating black hole
|
We examine energy conversion from accreting pair plasma to outgoing Poynting
flux by black hole rotation. Our approach is based on a two-fluid model
consisting of collisionless pair plasma. The electric potential is not constant
along magnetic field lines, unlike an ideal magnetohydrodynamics approximation.
We show how and where longitudinal electric fields and toroidal magnetic fields
are generated by the rotation, whereas they vanish everywhere for radial flow
in a split monopole magnetic field in a Schwarzschild black hole. Outgoing
electromagnetic power in a steady state is calculated by applying the WKB
method to the perturbation equations for a small spin parameter. In our model,
the luminosity has a peak in the vicinity of the black hole, but is damped
toward the event horizon and infinity. The power at the peak is of the same
order as that in the Blandford--Znajek process, although the physical mechanism
is different.
|
1509.04793v1
|
2015-09-16
|
Hydrodynamics, resurgence and trans-asymptotics
|
The second-order hydrodynamical description of a homogeneous conformal plasma
that undergoes a boost- invariant expansion is given by a single nonlinear
ordinary differential equation, whose resurgent asymptotic properties we study,
developing further the recent work of Heller and Spalinski [Phys. Rev. Lett.
115, 072501 (2015)]. Resurgence clearly identifies the non-hydrodynamic modes
that are exponentially suppressed at late times, analogous to the
quasi-normal-modes in gravitational language, organizing these modes in terms
of a trans-series expansion. These modes are analogs of instantons in
semi-classical expansions, where the damping rate plays the role of the
instanton action. We show that this system displays the generic features of
resurgence, with explicit quantitative relations between the fluctuations about
different orders of these non-hydrodynamic modes. The imaginary part of the
trans-series parameter is identified with the Stokes constant, and the real
part with the freedom associated with initial conditions.
|
1509.05046v1
|
2015-10-30
|
Anisotropic characteristics of the Kraichnan direct cascade in two-dimensional hydrodynamic turbulence
|
Statistical characteristics of the Kraichnan direct cascade for
two-dimensional hydrodynamic turbulence are numerically studied (with spatial
resolution $8192\times 8192$) in the presence of pumping and viscous-like
damping. It is shown that quasi-shocks of vorticity and their Fourier
partnerships in the form of jets introduce an essential influence in turbulence
leading to strong angular dependencies for correlation functions. The energy
distribution as a function of modulus $k$ for each angle in the inertial
interval has the Kraichnan behavior, $\sim k^{-4}$, and simultaneously a strong
dependence on angles. However, angle average provides with a high accuracy the
Kraichnan turbulence spectrum $E_k=C_K\eta^{2/3} k^{-3}$ where $\eta$ is
enstrophy flux and the Kraichnan constant $C_K\simeq 1.3$, in correspondence
with the previous simulations. Familiar situation takes place for third-order
velocity structure function $S_3^L$ which, as for the isotropic turbulence,
gives the same scaling with respect to separation length $R$ and $\eta$,
$S_3^L=C_3\eta R^3$, but the mean over angles and time $\bar {C_3}$ differs
from its isotropic value.
|
1510.09052v1
|
2015-11-10
|
On the global solution of 3-D MHD system with initial data near equilibrium
|
In this paper, we prove the global existence of smooth solutions to the
three-dimensional incompressible magneto-hydrodynamical system with initial
data close enough to the equilibrium state, $(e_3,0).$ Compared with the the
previous works \cite{XLZMHD1, XZ15}, here we present a new Lagrangian
formulation of the system, which is a damped wave equation and which is
non-degenerate only in the direction of the initial magnetic field.
Furthermore, we remove the admissible condition on the initial magnetic field,
which was required in \cite{XLZMHD1, XZ15}. By using Frobenius Theorem and
anisotropic Littlewood-Paley theory for the Lagrangian formulation of the
system, we achieve the global $L^1$ in time Lipschwitz estimate of the velocity
field, which allows us to conclude the global existence of solutions to this
system. In the case when the initial magnetic field is a constant vector, the
large time decay rate of the solution is also obtained.
|
1511.02978v1
|
2016-01-10
|
Interfacial Dzyaloshinskii-Moriya interaction, surface anisotropy energy,and spin pumping at spin orbit coupled Ir/Co interface
|
The interfacial Dzyaloshinskii-Moriya interaction (iDMI), surface anisotropy
energy, and spin pumping at the Ir/Co interface are experimentally investigated
by performing Brillouin light scattering. Contrary to previous reports, we
suggest that the sign of the iDMI at the Ir/Co interface is the same as in the
case of the Pt/Co interface. We also find that the magnitude of the iDMI energy
density is relatively smaller than in the case of the Pt/Co interface, despite
the large strong spin-orbit coupling (SOC) of Ir. The saturation magnetization
and the perpendicular magnetic anisotropy (PMA) energy are significantly
improved due to a strong SOC. Our findings suggest that an SOC in an Ir/Co
system behaves in different ways for iDMI and PMA. Finally, we determine the
spin pumping effect at the Ir/Co interface, and it increases the Gilbert
damping constant from 0.012 to 0.024 for 1.5 nmthick Co.
|
1601.02210v3
|
2016-01-16
|
Spin-orbit torque in Cr/CoFeAl/MgO and Ru/CoFeAl/MgO epitaxial magnetic heterostructures
|
We study the spin-orbit torque (SOT) effective fields in Cr/CoFeAl/MgO and
Ru/CoFeAl/MgO magnetic heterostructures using the adiabatic harmonic Hall
measurement. High-quality perpendicular-magnetic-anisotropy CoFeAl layers were
grown on Cr and Ru layers. The magnitudes of the SOT effective fields were
found to significantly depend on the underlayer material (Cr or Ru) as well as
their thicknesses. The damping-like longitudinal effective field ({\Delta}H_L)
increases with increasing underlayer thickness for all heterostructures. In
contrast, the field-like transverse effective field ({\Delta}H_T) increases
with increasing Ru thickness while it is almost constant or slightly decreases
with increasing Cr thickness. The sign of {\Delta}H_L observed in the
Cr-underlayer devices is opposite from that in the Ru-underlayer devices while
{\Delta}H_T shows the same sign with a small magnitude. The opposite directions
of {\Delta}HL indicate that the signs of spin Hall angle in Cr and Ru are
opposite, which are in good agreement with theoretical predictions. These
results show sizable contribution from SOT even for elements with small spin
orbit coupling such as 3d Cr and 4d Ru.
|
1601.04164v1
|
2016-01-31
|
Role of epistasis on the fixation probability of a non-mutator in an adapted asexual population
|
The mutation rate of a well adapted population is prone to reduction so as to
have a lower mutational load. We aim to understand the role of epistatic
interactions between the fitness affecting mutations in this process. Using a
multitype branching process, the fixation probability of a single non-mutator
emerging in a large asexual mutator population is analytically calculated here.
The mutator population undergoes deleterious mutations at constant, but at a
much higher rate than that of the non-mutator. We find that antagonistic
epistasis lowers the chances of mutation rate reduction, while synergistic
epistasis enhances it. Below a critical value of epistasis, the fixation
probability behaves non-monotonically with variation in mutation rate of the
background population. Moreover, the variation of this critical value of the
epistasis parameter with the strength of the mutator is discussed in the
Appendix. For synergistic epistasis, when selection is varied, the fixation
probability reduces overall, with damped oscillations.
|
1602.00282v2
|
2016-02-21
|
Global Finite-Time Attitude Tracking via Quaternion Feedback
|
This paper addresses the attitude tracking of a rigid body using a quaternion
description. Global finite-time attitude controllers are designed with three
types of measurements, namely, full states, attitude plus constant-biased
angular velocity, and attitude only. In all three scenarios hybrid control
techniques are utilized to overcome the well-known topological constraint on
the attitude manifold, while coupled nonsmooth feedback inputs are designed via
homogeneous theory to achieve finite-time stability. Specially, a finite-time
bias observer is derived in the second scenario and a quaternion filter is
constructed to provide damping in the absence of velocity feedback. The
proposed methods ensure bounded control torques a priori and, in particular,
include several existing attitude controllers as special cases.
|
1602.06492v3
|
2016-04-12
|
Quasinormal modes and a new instability of Einstein-Gauss-Bonnet black holes in the de Sitter world
|
Analysis of time-domain profiles for gravitational perturbations shows that
Gauss-Bonnet black holes in a de Sitter world possess a new kind of dynamical
instability which does not take place for asymptotically flat
Einstein-Gauss-Bonnet black holes. The new instability is in the gravitational
perturbations of the scalar type and is due to the nonvanishing cosmological
constant. Analysis of the quasinormal spectrum in the stability sector shows
that although the scalar type of gravitational perturbations alone does not
obey Hod's conjectural bound, connecting the damping rate and the Hawking
temperature, the vector and tensor types (and thereby the gravitational
spectrum as a whole) do obey it.
|
1604.03604v2
|
2016-04-29
|
Friedmann dynamics recovered from compactified Einstein-Gauss-Bonnet cosmology
|
In this paper cosmological dynamics in Einstein-Gauss-Bonnet gravity with a
perfect fluid source in arbitrary dimension is studied. A systematic analysis
is performed for the case that the theory does not admit maximally symmetric
solutions. Considering two independent scale factors, namely one for the three
dimensional space and one for the extra dimensional space, is found that a
regime exists where the two scale factors tend to a constant value via damped
oscillations for not too negative pressure of the fluid, so that asymptotically
the evolution of the $(3+1)$-dimensional Friedmann model with perfect fluid is
recovered. At last, it is worth emphasizing that the present numerical results
strongly support a 't Hooft-like interpretation of the parameter $1/D$ (where
$D$ is the number of extra dimensions) as a small expansion parameter in very
much the same way as it happens in the large $N$ expansion of gauge theories
with $1/N$. Indeed, the dependence on $D$ of many of the relevant physical
quantities computed here manifests a clear WKB-like pattern, as expected on the
basis of large $N$ arguments.
|
1605.00041v1
|
2016-05-05
|
Pulse excitation to continuous-wave excitation in a low-dimensional interacting quantum system
|
Real-time dynamics in one-dimensional transverse Ising model coupled with the
time-dependent oscillating field is analyzed by using the infinite
time-evolving block decimation algorithm and the Floquet theory. In particular,
the transient dynamics induced by the pulse field and their connections to the
dynamics by a continuous-wave field are focused on. During the pulse-field
irradiation, the order parameter shows a characteristic oscillation, in which
the frequency shifts from the pulse-field frequency. This is considered as a
kind of the Rabi oscillation, but the frequency strongly depends on the
intersite Ising interaction. After turning off the pulse field, the oscillation
remains with a frequency $\mathit{\Omega}$ and a damping constant $\gamma$. In
the case of low fluence, both $\mathit{\Omega}$ and $\gamma$ are scaled by the
pulse amplitude in a wide range of the parameter values of the model. In the
case of high fluence, $\mathit{\Omega}$ and $\gamma$ are arranged by a product
of the pulse amplitude and the pulse width. This implies that the dynamics
after turning off the pulse field are decided by a population of the excited
state when the pulse field is turned off.
|
1605.01537v1
|
2016-06-20
|
Electronic friction near metal surfaces: a case where molecule-metal couplings depend on nuclear coordinates
|
We derive an explicit form for the electronic friction as felt by a molecule
near a metal surface for the general case that molecule-metal couplings depend
on nuclear coordinates. Our work generalizes a previous study by von Oppen et
al [Beilstein Journal of Nanotechnology, 3, 144, 2012], where we now go beyond
the Condon approximation (i.e. molecule-metal couplings are not held constant).
Using a non-equilibrium Green's function formalism in the adiabatic limit, we
show that fluctuating metal-molecule couplings lead to new frictional damping
terms and random forces, plus a correction to the potential of mean force.
Numerical tests are performed and compared with a modified classical master
equation; our results indicate that violating the Condon approximation can have
a large effect on dynamics.
|
1606.06110v1
|
2016-07-13
|
Inductive intrinsic localized modes in a 1D nonlinear electric transmission line
|
The experimental properties of intrinsic localized modes (ILM) have long been
compared with theoretical dynamical lattice models that make use of nonlinear
onsite and/or nearest neighbor intersite potentials. Here it is shown for a 1-D
lumped electrical transmission line a nonlinear inductive component in an
otherwise linear parallel capacitor lattice makes possible a new kind of ILM
outside the plane wave spectrum. To simplify the analysis the nonlinear
inductive current equations are transformed to flux transmission line equations
with analogue onsite hard potential nonlinearities. Approximate analytic
results compare favorably with those obtained from a driven damped lattice
model and with eigenvalue simulations. For this mono-element lattice ILMs above
the top of the plane wave spectrum are the result. We find that the current ILM
is spatially compressed relative to the corresponding flux ILM. Finally this
study makes the connection between the dynamics of mass and force constant
defects in the harmonic lattice and ILMs in a strongly anharmonic lattice.
|
1607.03962v1
|
2016-08-10
|
Glassy Dynamics of Brownian Particles with Velocity-Dependent Friction
|
We consider a two-dimensional model system of Brownian particles in which
slow particles are accelerated while fast particles are damped. The motion of
the individual particles are described by a Langevin equation with
Rayleigh-Helmholtz velocity dependent friction. In case of noninteracting
particles, the time evolution equations lead to a non-Gaussian velocity
distribution. The velocity dependent friction allows negative values of the
friction or energy intakes by slow particles which we consider as active
motion, and also causes breaking of the fluctuation dissipation relation.
Defining the effective temperature proportional to the second moment of
velocity, it is shown that for a constant effective temperature the higher the
noise strength, the lower are the number of active particles in the system.
Using the Mori-Zwanzig formalism and the mode-coupling approximation, the
equation of motion for the density auto-correlation function are derived. The
equations are solved using the equilibrium structure factors. The
integration-through-transients approach is used to derive a relation between
the structure factor in the stationary state considering the interacting
forces, and the conventional equilibrium static structure factor.
|
1608.03162v1
|
2016-09-05
|
Coarsening dynamics of topological defects in thin Permalloy films
|
We study the dynamics of topological defects in the magnetic texture of
rectangular Permalloy thin film elements during relaxation from random
magnetization initial states. Our full micromagnetic simulations reveal complex
defect dynamics during relaxation towards the stable Landau closure domain
pattern, manifested as temporal power-law decay, with a system-size dependent
cut-off time, of various quantities. These include the energy density of the
system, and the number densities of the different kinds of topological defects
present in the system. The related power-law exponents assume non-trivial
values, and are found to be different for the different defect types. The
exponents are robust against a moderate increase in the Gilbert damping
constant and introduction of quenched structural disorder. We discuss details
of the processes allowed by conservation of the winding number of the defects,
underlying their complex coarsening dynamics.
|
1609.01094v1
|
2016-09-27
|
Anomalous Feedback and Negative Domain Wall Resistance
|
Magnetic induction can be regarded as a negative feedback effect, where the
motive-force opposes the change of magnetic flux that generates the
motive-force. In artificial electromagnetics emerging from spintronics,
however, this is not necessarily the case. By studying the current-induced
domain wall dynamics in a cylindrical nanowire, we show that the spin
motive-force exerting on electrons can either oppose or support the applied
current that drives the domain wall. The switching into the anomalous feedback
regime occurs when the strength of the dissipative torque {\beta} is about
twice the value of the Gilbert damping constant {\alpha}. The anomalous
feedback manifests as a negative domain wall resistance, which has an analogy
with the water turbine.
|
1609.08250v1
|
2016-10-19
|
Coupling of Damped and Growing Modes in Unstable Shear Flow
|
Analysis of the saturation of the Kelvin-Helmholtz (KH) instability is
undertaken to determine the extent to which the conjugate linearly stable mode
plays a role. For a piecewise-continuous mean flow profile with constant shear
in a fixed layer, it is shown that the stable mode is nonlinearly excited,
providing an injection-scale sink of the fluctuation energy similar to what has
been found for gyroradius-scale drift-wave turbulence. Quantitative evaluation
of the contribution of the stable mode to the energy balance at the onset of
saturation shows that nonlinear energy transfer to the stable mode is as
significant as energy transfer to small scales in balancing energy injected
into the spectrum by the instability. The effect of the stable mode on momentum
transport is quantified by expressing the Reynolds stress in terms of stable
and unstable mode amplitudes at saturation, from which it is found that the
stable mode can produce a sizable reduction in the momentum flux.
|
1610.06142v4
|
2016-11-05
|
Number of Wavevectors for Each Frequency in a Periodic Structure
|
Periodic structures have interesting acoustic and vibration properties making
them suitable for a wide variety of applications. In a periodic structure, the
number of frequencies for each wavevector depends on the degree of freedom of
the unit cell. In this paper, we investigate the number of wavevectors for each
frequency. This analysis defines the upper bound for the maximum number of
wavevectors for each frequency in a general periodic structure which might
include damping. Investigation presented in this paper can also provide an
insight for designing materials in which the interaction between unit cells is
not limited to the closest neighbor. As an example application of this work, we
investigate phonon dispersion curves in hexagonal form of Boron Nitride to show
that first neighbor interaction is not sufficient to model dispersion curves
with force-constant-model.
|
1611.01677v1
|
2016-11-22
|
On the symplectic covariance and interferences of time-frequency distributions
|
We study the covariance property of quadratic time-frequency distributions
with respect to the action of the extended symplectic group. We show how
covariance is related, and in fact in competition, with the possibility of
damping the interferences which arise due to the quadratic nature of the
distributions. We also show that the well known fully covariance property of
the Wigner distribution in fact characterizes it (up to a constant factor)
among the quadratic distributions $L^{2}(\mathbb{R}^{n})\rightarrow C_{0}({
\mathbb{R}^{2n}})$. A similar characterization for the closely related Weyl
transform is given as well. The results are illustrated by several numerical
experiments for the Wigner and Born-Jordan distributions of the sum of four
Gaussian functions in the so-called "diamond configuration".
|
1611.07442v1
|
2017-01-05
|
Excitation and detection of short-waved spin waves in ultrathin Ta/CoFeB/MgO-layer system suitable for spin-orbit-torque magnonics
|
We report on the excitation and detection of short-waved spin waves with wave
vectors up to about $40\,\mathrm{rad}\,\mu\mathrm{m}^{-1}$ in spin-wave
waveguides made from ultrathin, in-plane magnetized Co$_{8}$Fe$_{72}$B$_{20}$
(CoFeB). The CoFeB is incorporated in a layer stack of Ta/CoFeB/Mgo, a layer
system featuring large spin orbit torques and a large perpendicular magnetic
anisotropy constant. The short-waved spin waves are excited by nanometric
coplanar waveguides and are detected via spin rectification and microfocussed
Brillouin light scattering spectroscopy. We show that the large perpendicular
magnetic anisotropy benefits the spin-wave lifetime greatly, resulting in a
lifetime comparable to bulk systems without interfacial damping. The presented
results pave the way for the successful extension of magnonics to ultrathin
asymmetric layer stacks featuring large spin orbit torques.
|
1701.01399v1
|
2017-03-28
|
Temperature dependent magnetic damping of yttrium iron garnet spheres
|
We investigate the temperature dependent microwave absorption spectrum of an
yttrium iron garnet sphere as a function of temperature (5 K to 300 K) and
frequency (3 GHz to 43.5 GHz). At temperatures above 100 K, the magnetic
resonance linewidth increases linearly with temperature and shows a
Gilbert-like linear frequency dependence. At lower temperatures, the
temperature dependence of the resonance linewidth at constant external magnetic
fields exhibits a characteristic peak which coincides with a non-Gilbert-like
frequency dependence. The complete temperature and frequency evolution of the
linewidth can be modeled by the phenomenology of slowly relaxing rare-earth
impurities and either the Kasuya-LeCraw mechanism or the scattering with
optical magnons. Furthermore, we extract the temperature dependence of the
saturation magnetization, the magnetic anisotropy and the g-factor.
|
1703.09444v2
|
2017-03-31
|
Spin Seebeck effect in Y-type hexagonal ferrite thin films
|
Spin Seebeck effect (SSE) has been investigated in thin films of two
Y-hexagonal ferrites Ba$_2$Zn$_{2}$Fe$_{12}$O$_{22}$ (Zn2Y) and
Ba$_2$Co$_{2}$Fe$_{12}$O$_{22}$ (Co2Y) deposited by a spin-coating method on
SrTiO$_3$(111) substrate. The selected hexagonal ferrites are both
ferrimagnetic with similar magnetic moments at room temperature and both
exhibit easy magnetization plane normal to $c$-axis. Despite that, SSE signal
was only observed for Zn2Y, whereas no significant SSE signal was detected for
Co2Y. We tentatively explain this different behavior by a presence of two
different magnetic ions in Co2Y, whose random distribution over octahedral
sites interferes the long range ordering and enhances the Gilbert damping
constant. The temperature dependence of SSE for Zn2Y was measured and analyzed
with regard to the heat flux and temperature gradient relevant to the SSE
signal.
|
1703.10903v1
|
2017-04-05
|
Superfluid Field response to Edge dislocation motion
|
We study the dynamic response of a superfluid field to a moving edge
dislocation line to which the field is minimally coupled. We use a dissipative
Gross-Pitaevskii equation, and determine the initial conditions by solving the
equilibrium version of the model. We consider the subsequent time evolution of
the field for both glide and climb dislocation motion and analyze the results
for a range of values of the constant speed $V_D$ of the moving dislocation. We
find that the type of motion of the dislocation line is very important in
determining the time evolution of the superfluid field distribution associated
with it. Climb motion of the dislocation line induces increasing asymmetry, as
function of time, in the field profile, with part of the probability being, as
it were, left behind. On the other hand, glide motion has no effect on the
symmetry properties of the superfluid field distribution. Damping of the
superfluid field due to excitations associated with the moving dislocation line
occurs in both cases.
|
1704.01600v1
|
2017-04-29
|
Multiple steady states and the form of response functions to antigen in a model for the initiation of T cell activation
|
The aim of this paper is to study the qualitative behaviour predicted by a
mathematical model for the initial stage of T cell activation. The state
variables in the model are the concentrations of phosphorylation states of the
T cell receptor complex and the phosphatase SHP-1 in the cell. It is shown that
these quantities cannot approach zero and that the model possesses more than
one positive steady state for certain values of the parameters. It can also
exhibit damped oscillations. It is proved that the chemical concentration which
represents the degree of activation of the cell, that of the maximally
phosphorylated form of the T cell receptor complex, is in general a
non-monotone function of the activating signal. In particular there are cases
where there is a value of the dissociation constant of the ligand from the
receptor which produces an optimal activation of the T cell. In this way the
results of certain simulations in the literature have been confirmed rigorously
and some important features which had not previously been seen have been
discovered.
|
1705.00149v1
|
2017-05-12
|
From an Action Principle for Action-dependent Lagrangians toward non-conservative Gravity: accelerating Universe without dark energy
|
In the present work, we propose an Action Principle for Action-dependent
Lagrangians by generalizing the Herglotz variational problem for several
independent variables. This Action Principle enables us to formulate Lagrangian
densities for non-conservative fields. In special, from a Lagrangian depending
linearly on the Action, we obtain a generalized Einstein's field equations for
a non-conservative gravity and analyze some consequences of their solutions to
cosmology and gravitational waves. We show that the non-conservative part of
the field equations depends on a constant cosmological four-vector. Depending
on this four-vector, the theory displays damped/amplified gravitational waves
and an accelerating Universe without dark energy.
|
1705.04604v1
|
2017-07-21
|
Temperature dependence of nuclear fission time in heavy-ion fusion-fission reactions
|
Accounting for viscous damping within Fokker-Planck equations led to various
improvements in the understanding and analysis of nuclear fission of heavy
nuclei. Analytical expressions for the fission time are typically provided by
Kramers' theory, which improves on the Bohr-Wheeler estimate by including the
time-scale related to many-particle dissipative processes along the deformation
coordinate. However, Kramers' formula breaks down for sufficiently high
excitation energies where Kramers' assumption of a large barrier no longer
holds. In the regime $T>1$ MeV, Kramers' theory should be replaced by a new
theory based on the Ornstein-Uhlenbeck first-passage time method that is
proposed here. The theory is applied to fission time data from fusion-fission
experiments on $^{16}$O+$^{208}$Pb $\rightarrow$ $^{224}$Th. The proposed model
provides an internally consistent one-parameter fitting of fission data with a
constant nuclear friction as the fitting parameter, whereas Kramers' fitting
requires a value of friction which falls out of the allowed range. The theory
provides also an analytical formula that in future work can be easily
implemented in numerical codes such as CASCADE or JOANNE4.
|
1707.06836v1
|
2017-09-18
|
Stability of traveling, pre-tensioned, heavy cables
|
We study the dynamics of an inclined tensioned, heavy cable traveling with a
constant speed in the vertical plane. The cable is modeled as a beam resisting
bending and shear. The governing equation for the transverse in-plane
vibrations of the cable are derived through the Newton-Euler method. The cable
dynamics is also studied in the limit of zero bending stiffness. In all cases,
application of en- ergy balance reveals that the total energy of the system
fluctuates even though the oscillations are small and bounded in time,
indicating that the system is nonconser- vative. A comprehensive stability
analysis is carried out in the parameter space of inclination, traveling speed,
pre-tension, bending rigidity and the slenderness of the cable. Effect of
damping is also considered. We conclude that, while pre-tension, rigidity and
slenderness enhance the stability of the traveling cable, the angle of
inclination affects the stability adversely. These results may act as
guidelines for safer design and operation.
|
1709.09516v1
|
2017-10-17
|
New results on disturbance rejection for energy-shaping controlled port-Hamiltonian systems
|
In this paper we present a method to robustify energy-shaping controllers for
port-Hamiltonian (pH) systems by adding an integral action that rejects unknown
additive disturbances. The proposed controller preserves the pH structure and,
by adding to the new energy function a suitable cross term between the plant
and the controller coordinates, it avoids the unnatural coordinate
transformation used in the past. This paper extends our previous work by
relaxing the requirement that the systems Hamiltonian is strictly convex and
separable, which allows the controller to be applied to a large class of
mechanical systems, including underactuated systems with non-constant mass
matrix. Furthermore, it is shown that the proposed integral action control is
robust against unknown damping in the case of fully-actuated systems.
|
1710.06070v1
|
2017-11-15
|
Probing Split-Ring Resonator Permeabilities with Loop-Gap Resonators
|
A method is proposed to experimentally determine the effective complex
permeability of split-ring resonator (SRR) arrays used in the design of
metamaterials at microwave frequencies. We analyze the microwave response of a
loop-gap resonator (LGR) whose bore has been partially loaded with one or more
SRRs. Our analysis reveals that the resonance frequency, magnetic plasma
frequency, and damping constant of the effective permeability of the SRR array
can be extracted from fits to the reflection coefficient (S11) of an
inductively-coupled LGR. We propose LGR designs that would allow both a
one-dimensional array of SRRs and small three-dimensional arrays of SRRs to be
characterized. Finally, we demonstrate the method using a toroidal LGR loaded
with a single extended SRR of length z.
|
1711.05819v1
|
2017-11-23
|
Graphene multi-mode parametric oscillators
|
In the field of nanomechanics, parametric excitations are of interest since
they can greatly enhance sensing capabilities and eliminate cross-talk.
However, parametric excitations often rely on externally tuned springs, which
limits their application to high quality factor resonators and usually does not
allow excitation of multiple higher modes into parametric resonance. Here we
demonstrate parametric amplification and resonance of suspended single-layer
graphene membranes by an efficient opto-thermal drive that modulates the
intrinsic spring constant. With a large amplitude of the optical drive, a
record number of 14 mechanical modes can be brought into parametric resonance
by modulating a single parameter: the pretension. In contrast to conventional
mechanical resonators, it is shown that graphene membranes demonstrate an
interesting combination of both strong nonlinear stiffness and nonlinear
damping.
|
1711.08798v1
|
2017-12-10
|
Magnetic field gradient driven dynamics of isolated skyrmions and antiskyrmions in frustrated magnets
|
The study of skyrmion/antiskyrmion motion in magnetic materials is very
important in particular for the spintronics applications. In this work, we
study the dynamics of isolated skyrmions and antiskyrmions in frustrated
magnets driven by magnetic field gradient, using the Landau-Lifshitz-Gilbert
simulations on the frustrated classical Heisenberg model on the triangular
lattice. A Hall-like motion induced by the gradient is revealed in bulk system,
similar to that in the well-studied chiral magnets. More interestingly, our
work suggests that the lateral confinement in nano-stripes of the frustrated
system can completely suppress the Hall motion and significantly speed up the
motion along the gradient direction. The simulated results are well explained
by the Thiele theory. It is demonstrated that the acceleration of the motion is
mainly determined by the Gilbert damping constant, which provides useful
information for finding potential materials for skyrmion-based spintronics.
|
1712.03550v1
|
2018-01-21
|
Microwave fields driven domain wall motions in antiferromagnetic nanowires
|
In this work, we study the microwave field driven antiferromagnetic domain
wall motion in an antiferromagnetic nanowire, using the numerical calculations
based on a classical Heisenberg spin model. We show that a proper combination
of a static magnetic field plus an oscillating field perpendicular to the
nanowire axis is sufficient to drive the domain wall propagation along the
nanowire with the axial magnetic anisotropy. More importantly, the drift
velocity at the resonance frequency is comparable to that induced by
temperature gradients, suggesting that microwave field can be a very promising
tool to control domain wall motions in antiferromagnetic nanostructures.
Furthermore, the dependences of resonance frequency and drift velocity on the
static and oscillating fields, the axial anisotropy, and the damping constant
are discussed in details. This work provides useful information for the spin
dynamics in antiferromagnetic nanostructures for spintronics applications.
|
1801.06748v1
|
2018-02-07
|
Breaking the current density threshold in spin-orbit-torque magnetic random access memory
|
Spin-orbit-torque magnetic random access memory (SOT-MRAM) is a promising
technology for the next generation of data storage devices. The main bottleneck
of this technology is the high reversal current density threshold. This
outstanding problem of SOT-MRAM is now solved by using a current density of
constant magnitude and varying flow direction that reduces the reversal current
density threshold by a factor of more than the Gilbert damping coefficient. The
Euler-Lagrange equation for the fastest magnetization reversal path and the
optimal current pulse are derived for an arbitrary magnetic cell. The
theoretical limit of minimal reversal current density and current density for a
GHz switching rate of the new reversal strategy for CoFeB/Ta SOT-MRAMs are
respectively of the order of $10^5$ A/cm$^2$ and $10^6$ A/cm$^2$ far below
$10^7$ A/cm$^2$ and $10^8$ A/cm$^2$ in the conventional strategy. Furthermore,
no external magnetic field is needed for a deterministic reversal in the new
strategy.
|
1802.02415v1
|
2018-02-20
|
High-temperature structural phase transition and infrared dielectric features of La2CoMnO6
|
Temperature-dependent FAR-infrared reflectivity spectra of partially ordered
magnetodielectric La2CoMnO6 is presented, from room temperature up to 675 K. A
clear first-ordered structural phase transition (SPT) from a monoclinic
structure with P2_1/n symmetry to a rhombohedral phase with R-3 symmetry was
evidenced from the behaviour of polar phonon modes at TC~590 K. The temperature
dependences of the transversal and longitudinal phonon branches, dielectric
strengths, and damping of the strongest dielectric modes confirm the
significant contribution of the phonon modes on the SPT, and revealed an
important lattice anharmonicity, particularly for the low frequency modes. In
addition, these investigations showed that structural ordering does not inhibit
the SPT, and provided valuable information towards the polar phonons, their
implications on intrinsic dielectric constant in double perovskites and in
related compounds.
|
1802.07318v1
|
2018-03-26
|
Force spectroscopy analysis in polymer translocation
|
This paper reports the force spectroscopy analysis of a polymer that
translocates from one side of a membrane to the other side through an extended
pore, pulled by a cantilever that moves with constant velocity against the
damping and the potential barrier generated by the reaction of the membrane
walls. The polymer is modeled as a beads-springs chain with both excluded
volume and bending contributions, and moves in a stochastic three dimensional
environment described by a Langevin dynamics at fixed temperature. The force
trajectories recorded at different velocities reveal two unexplored exponential
regimes: the force increases when the first part of the chain enters the pore,
and then decreases when the first monomer reaches the trans region. The
spectroscopy analysis of the force values permit the estimation of the free
energy barrier as well as the limit force to permit the translocation. The
stall force to maintain the polymer fixed has been also calculated
independently, and its value confirms the force spectroscopy outcomes.
|
1803.09798v2
|
2018-03-26
|
Diffusive stability against nonlocalized perturbations of planar wave trains in reaction-diffusion systems
|
Planar wave trains are traveling wave solutions whose wave profiles are
periodic in one spatial direction and constant in the transverse direction. In
this paper, we investigate the stability of planar wave trains in
reaction-diffusion systems. We establish nonlinear diffusive stability against
perturbations that are bounded along a line in $\mathbb{R}^2$ and decay
exponentially in the distance from this line. Our analysis is the first to
treat spatially nonlocalized perturbations that do not originate from a phase
modulation. We also consider perturbations that are fully localized and
establish nonlinear stability with better decay rates, suggesting a trade-off
between spatial localization of perturbations and temporal decay rate. Our
stability analysis utilizes pointwise estimates to exploit the spatial
structure of the perturbations. The nonlocalization of perturbations prevents
the use of damping estimates in the nonlinear iteration scheme; instead, we
track the perturbed solution in two different coordinate systems.
|
1803.09812v2
|
2018-04-07
|
Chemotaxis effect vs logistic damping on boundedness in the 2-D minimal Keller-Segel model
|
In this paper, we study chemotaxis effect vs logistic dampening on
boundedness for the two-dimensional minimal Keller-Segel model with logistic
source in a 2-D smooth and bounded domain. It is well-known that this model
allows only for global and uniform-in-time bounded solutions for any
chemotactic strength and logistic dampening. Here, we carefully employ a simple
and new method to regain its boundedness and, with particular attention to how
boundedness depends qualitatively on the coefficient of chemotactic strength
and logistic dampening rate. Up to a scaling constant depending only on initial
data and the domain, we provide explicit upper bounds for the the solution
components of the corresponding initial-boundary value problem. This
qualitative boundedness results seems to be the first result in the regard.
|
1804.02501v1
|
2018-04-18
|
Large perpendicular magnetic anisotropy in epitaxial Fe/MgAl2O4(001) heterostructures
|
We investigated perpendicular magnetic anisotropy (PMA) and related
properties of epitaxial Fe (0.7 nm)/MgAl2O4(001) heterostructures prepared by
electron-beam evaporation. Using an optimized structure, we obtained a large
PMA energy ~1 MJ/m3 at room temperature, comparable to that in
ultrathin-Fe/MgO(001) heterostructures. Both the PMA energy and saturation
magnetization show weak temperature dependence, ensuring wide working
temperature in application. The effective magnetic damping constant of the 0.7
nm Fe layer was ~0.02 using time-resolved magneto-optical Kerr effect. This
study demonstrates capability of the Fe/MgAl2O4 heterostructure for
perpendicular magnetic tunnel junctions, as well as a good agreement with
theoretical predictions.
|
1804.06526v2
|
2018-04-20
|
Regular solutions to the fractional Euler alignment system in the Besov spaces framework
|
We here construct (large) local and small global-in-time regular unique
solutions to the fractional Euler alignment system in the whole space ${\mathbb
R}^d$, in the case where the deviation of the initial density from a constant
is sufficiently small. Our analysis strongly relies on the use of Besov spaces
of the type $L^1(0,T;\dot B^s_{p,1})$, which allow to get time independent
estimates for the density even though it satisfies a transport equation with no
damping. Our choice of a functional setting is not optimal but aims at
providing a transparent and accessible argumentation.
|
1804.07611v2
|
2018-06-01
|
Dirac-Surface-State Modulated Spin Dynamics in a Ferrimagnetic Insulator at Room Temperature
|
This work demonstrates dramatically modified spin dynamics of magnetic
insulator (MI) by the spin-momentum locked Dirac surface states of the adjacent
topological insulator (TI) which can be harnessed for spintronic applications.
As the Bi-concentration x is systematically tuned in 5 nm thick (BixSb1-x)2Te3
TI film, the weight of the surface relative to bulk states peaks at x = 0.32
when the chemical potential approaches the Dirac point. At this concentration,
the Gilbert damping constant of the precessing magnetization in 10 nm thick
Y3Fe5O12 MI film in the MI/TI heterostructures is enhanced by an order of
magnitude, the largest among all concentrations. In addition, the MI acquires
additional strong magnetic anisotropy that favors the in-plane orientation with
similar Bi-concentration dependence. These extraordinary effects of the Dirac
surface states distinguish TI from other materials such as heavy metals in
modulating spin dynamics of the neighboring magnetic layer.
|
1806.00151v1
|
2018-06-27
|
Non normal amplification of stochastic quasi-cycles
|
Stochastic quasi-cycles for a two species model of the excitatory-inhibitory
type, arranged on a triangular loop, are studied. By increasing the strength of
the inter-nodes coupling, one moves the system towards the Hopf bifurcation and
the amplitude of the stochastic oscillations are consequently magnified. When
the system is instead constrained to evolve on specific manifolds, selected so
as to return a constant rate of deterministic damping for the perturbations,
the observed amplification correlates with the degree of non normal reactivity,
here quantified by the numerical abscissa. The thermodynamics of the reactive
loop is also investigated and the degree of inherent reactivity shown to
facilitate the out-of-equilibrium exploration of the available phase space.
|
1806.10566v1
|
2018-08-15
|
Collective excitations in two-component one-dimensional massless Dirac plasma
|
We study spectra of long wavelength plasma oscillations in a system of two
energy splitted one-dimensional (1D) massless Dirac fermion subbands coupled by
spin-orbit interaction. Such a system may be formed by edge subbands in
semiconducting transition metal dichalcogenide monolayers. Intrasubband
transitions of massless Dirac fermions give rise to optical and acoustic
gapless branches of intrasubband 1D plasmons. We reveal that the optical branch
is of quantum character with group velocity being inverse proportional to
square root of the Planck constant, whereas the acoustic branch is classical
one with group velocity proportional to geometric mean of the edge subband
velocities. Spin-orbit interaction, allowing intersubband transitions in the
system, results in emergence of two branches of intersubband 1D plasmons: upper
and lower ones. The upper and lower branches are gapped at small wave vectors
and evolve with positive and negative group velocities, respectively, from
energy splitting of the edge subbands at Fermi-level. The both intersubband
branches adjoin intersubband single particle excitation continuum from above,
while in case of the edge subbands with unequal velocities the lower one
experiences Landau damping at small wave vectors. In addition, the lower
branch, attaining zero frequency at a non-zero wave vector, alters its group
velocity from negative to positive one.
|
1808.05180v1
|
2018-09-17
|
On the speed of domain walls in thin nanotubes: the transition from the linear to the magnonic regime
|
Numerical simulations of domain wall propagation in thin nanotubes when an
external magnetic field is applied along the nanotube axis have shown an
unexpected behavior described as a transition from a linear to a magnonic
regime. As the applied magnetic field increases, the initial regime of linear
growth of the speed with the field is followed by a sudden change in slope
accompanied by the emission of spin waves. In this work an analytical formula
for the speed of the domain wall that explains this behavior is derived by
means of an asymptotic study of the Landau Lifshitz Gilbert equation for thin
nanotubes. We show that the dynamics can be reduced to a one dimensional
hyperbolic reaction diffusion equation, namely, the damped double Sine Gordon
equation, which shows the transition to the magnonic regime as the domain wall
speed approaches the speed of spin waves. This equation has been previously
found to describe domain wall propagation in weak ferromagnets with the
mobility proportional to the Dzyaloshinskii-Moriya interaction constant, for
Permalloy nanotubes the mobility is proportional to the nanotube radius.
|
1809.06278v3
|
2018-10-04
|
Effect of bunch shape on its TMCI spectrum and threshold with high space charge
|
Transverse mode coupling instability of bunched beam is investigated in the
paper at different form of the bunches with space charge included. Equation of
transverse motion of the bunch in parabolic potential well of synchrotron
oscillations is derived and analysed. The bunch of constant density (flat
bunch) is examined in detail to make comparison with the square well model. It
is shown that both models result in very close instability thresholds of the
flat bunch. Then different form bunches are investigated in the parabolic
potential well. It is shown that decrease of the bunch r.m.s length leads to
the growth of its threshold, that is the flat bunch model gives only a minimal
estimation of the threshold. The results are treated in terms of Landau damping
due to spread of the space charge tune shift.
|
1810.02458v4
|
2018-10-11
|
Stability of scrape-off layer plasma: a modified Rayleigh-Benard problem
|
We present a linear stability analysis of a two-dimensional fluid model used
to study the plasma dynamics in the scrape-off layer of tokamaks. The model
equations are based on the Braginskii fluid equations under the assumptions of
drift ordering and an electrostatic plasma. The model also employs the common
slab geometry approximation, whereby the magnetic field is assumed constant and
straight, with the effects of curvature reintroduced as effective gravitational
terms. We demonstrate that the governing plasma equations for the scrape-off
layer can be viewed as describing a thermal convection problem with additional
effects. The new features include a non-uniform basic state gradient, linear
damping terms, and additional advective terms. We characterise the conditions
at the onset of instability, and perform an extensive parameter scan to
describe how the stability threshold varies as a function of plasma parameters.
|
1810.05111v1
|
2018-10-30
|
Role of acoustic phonons in exotic conductivity of two-dimensional Dirac electrons
|
We examine the effect of acoustic phonon scattering on the conductivity of
two-dimensional Dirac electrons. The temperature ($T$) dependence of the
conductivity ($\sigma$) is calculated using the electron Green's function with
damping by both the impurity ($\Gamma_0$) and phonon ($\Gamma_{\rm ph}$).For
zero or small doping, on which the present Rapid Communication focuses, $\sigma
(T)$ increases and becomes almost constant due to the competition between the
Dirac electrons and the phonon scattering. Such strange behavior of $\sigma
(T)$ is ascribed to an exotic mechanism of phonon scattering, whose momentum
space is strongly reduced in the presence of a Dirac cone. For large doping,
$\sigma$ decreases due to the interplay of the Fermi surface and the phonon.
The unconventional $T$ dependence of the resistivity $\rho (=1/\sigma)$ for
small doping is compared with that of the experiment of Dirac electrons in an
organic conductor.
|
1810.12875v1
|
2018-11-05
|
Out of Time Ordered Quantum Dissipation
|
We consider a quantum Brownian particle interacting with two harmonic baths,
which is then perturbed by a cubic coupling linking the particle and the baths.
This cubic coupling induces non-linear dissipation and noise terms in the
influence functional/master equation of the particle. Its effect on the
Out-of-Time-Ordered Correlators (OTOCs) of the particle cannot be captured by
the conventional Feynman-Vernon formalism.We derive the generalised influence
functional which correctly encodes the physics of OTO fluctuations, response,
dissipation and decoherence. We examine an example where Markovian
approximation is valid for the OTO dynamics. If the original cubic coupling has
a definite time-reversal parity, the leading order OTO influence functional is
completely determined by the couplings in the usual master equation via OTO
generalisation of Onsager-Casimir relations. New OTO fluctuation-dissipation
relations connect the non-Gaussianity of the thermal noise to the thermal
jitter in the damping constant of the Brownian particle.
|
1811.01513v4
|
2018-12-03
|
Microscopic theory of magnon-drag electron flow in ferromagnetic metals
|
A temperature gradient applied to a ferromagnetic metal induces not only
independent flows of electrons and magnons but also drag currents because of
their mutual interaction. In this paper, we present a microscopic study of the
electron flow induced by the drag due to magnons. The analysis is based on the
$s$-$d$ model, which describes conduction electrons and magnons coupled via the
$s$-$d$ exchange interaction. Magnetic impurities are introduced in the
electron subsystem as a source of spin relaxation. The obtained magnon-drag
electron current is proportional to the entropy of magnons and to $\alpha -
\beta$ (more precisely, to $1 - \beta/\alpha$), where $\alpha$ is the Gilbert
damping constant and $\beta$ is the dissipative spin-transfer torque parameter.
This result almost coincides with the previous phenomenological result based on
the magnonic spin-motive forces, and consists of spin-transfer and
momentum-transfer contributions, but with a slight disagreement in the former.
The result is interpreted in terms of the nonequilibrium spin chemical
potential generated by nonequilibrium magnons.
|
1812.00720v1
|
2018-12-06
|
Towards surface diffusion potential mapping on atomic length scale
|
The surface diffusion potential landscape plays an essential role in a number
of physical and chemical processes such as self-assembly and catalysis.
Diffusion energy barriers can be calculated theoretically for simple systems,
but there is currently no experimental technique to systematically measure them
on the relevant atomic length scale. Here, we introduce an atomic force
microscopy based method to semiquantitatively map the surface diffusion
potential on an atomic length scale. In this proof of concept experiment, we
show that the atomic force microscope damping signal at constant
frequency-shift can be linked to nonconservative processes associated with the
lowering of energy barriers and compared with calculated single-atom diffusion
energy barriers.
|
1812.02512v3
|
2019-01-01
|
Gravitational Waves in the Presence of Viscosity
|
We analyze gravitational waves propagating in an isotropic cosmic fluid
endowed with a bulk viscosity $\zeta$ and a shear viscosity $\eta$, assuming
these coefficients to vary with fluid density $\rho$ as $\rho^\lambda$, with
$\lambda=1/2$ favored by experimental evidence. We give the general governing
equation for the gravitational waves, and focus thereafter on two examples. The
first concerns waves in the very late universe, close to the Big Rip, where the
fate of the comic fluid is dependent highly on the values of the parameters.
Our second example considers the very early universe, the lepton era; the
motivation for this choice being that the microscopical bulk viscosity as
calculated from statistical mechanics is then at maximum. We find that the
gravitational waves on such an underlying medium are damped, having a decay
constant equal to the inverse of the conformal Hubble parameter. Our results
turn out to be in good agreement with other viscosity-based approaches.
|
1901.00767v3
|
2019-01-17
|
Spin transport parameters of NbN thin films characterised by spin pumping experiments
|
We present measurements of ferromagnetic-resonance - driven spin pumping and
inverse spin-Hall effect in NbN/Y3Fe5O12 (YIG) bilayers. A clear enhancement of
the (effective) Gilbert damping constant of the thin-film YIG was observed due
to the presence of the NbN spin sink. By varying the NbN thickness and
employing spin-diffusion theory, we have estimated the room temperature values
of the spin diffusion length and the spin Hall angle in NbN to be 14 nm and
-1.1 10-2, respectively. Furthermore, we have determined the spin-mixing
conductance of the NbN/YIG interface to be 10 nm-2. The experimental
quantification of these spin transport parameters is an important step towards
the development of superconducting spintronic devices involving NbN thin films.
|
1901.05753v1
|
2019-04-28
|
Memory and mutualism in species sustainability: a time-fractional Lotka-Volterra model with harvesting
|
We first present a predator-prey model for two species and then extend the
model to three species where the two predator species engage in mutualistic
predation. Constant effort harvesting and the impact of by-catch issue are also
incorporated. Necessary sufficient conditions for the existence and stability
of positive equilibrium points are examined. It is shown that harvesting is
sustainable, and the memory concept of the fractional derivative damps out
oscillations in the population numbers so that the system as a whole settles on
an equilibrium quicker than it would with integer time derivatives. Finally,
some possible physical explanations are given for the obtained results. It is
shown that the stability requires the memory concept in the model.
|
1904.12340v3
|
2019-05-29
|
Prospects of reinforcement learning for the simultaneous damping of many mechanical modes
|
We apply adaptive feedback for the partial refrigeration of a mechanical
resonator, i.e. with the aim to simultaneously cool the classical thermal
motion of more than one vibrational degree of freedom. The feedback is obtained
from a neural network parametrized policy trained via a reinforcement learning
strategy to choose the correct sequence of actions from a finite set in order
to simultaneously reduce the energy of many modes of vibration. The actions are
realized either as optical modulations of the spring constants in the so-called
quadratic optomechanical coupling regime or as radiation pressure induced
momentum kicks in the linear coupling regime. As a proof of principle we
numerically illustrate efficient simultaneous cooling of four independent modes
with an overall strong reduction of the total system temperature.
|
1905.12344v2
|
2019-06-10
|
An automatic dynamic balancer in a rotating mechanism with time-varying angular velocity
|
We consider the system of a two ball automatic dynamic balancer attached to a
rotating disc with nonconstant angular velocity. We directly compare the
scenario of constant angular velocity with that when the acceleration of the
rotor is taken into consideration. In doing so we show that there are cases
where one must take the acceleration phase into consideration to obtain an
accurate picture of the dynamics. Similarly we identify cases where the
acceleration phase of the disc may be ignored. Finally, we briefly consider
nonmonotonic variations of the angular velocity, with a view of maximising the
basin of attraction of the desired solution, corresponding to damped
vibrations.
|
1906.04823v1
|
2019-06-13
|
Magnetoelasticity of $\mathrm{Co_{25}}\mathrm{Fe_{75}}$ thin films
|
We investigate the magnetoelastic properties of
$\mathrm{Co_{25}}\mathrm{Fe_{75}}$ and $\mathrm{Co_{10}}\mathrm{Fe_{90}}$ thin
films by measuring the mechanical properties of a doubly clamped string
resonator covered with multi-layer stacks containing these films. For the
magnetostrictive constants we find
$\lambda_{\mathrm{Co_{25}}\mathrm{Fe_{75}}}=(-20.68\pm0.25)\times10^{-6}$ and
$\lambda_{\mathrm{Co_{10}}\mathrm{Fe_{90}}}=(-9.80\pm0.12)\times10^{-6}$ at
room temperature. In stark contrast to the positive magnetostriction previously
found in bulk CoFe crystals. $\mathrm{Co_{25}}\mathrm{Fe_{75}}$ thin films
unite low damping and sizable magnetostriction and are thus a prime candidate
for micromechanical magnonic applications, such as sensors and hybrid
phonon-magnon systems.
|
1906.05543v1
|
2019-07-25
|
Model-independent constraints on the hydrogen-ionizing emissivity at z>6
|
Modelling reionization often requires significant assumptions about the
properties of ionizing sources. Here, we infer the total output of
hydrogen-ionizing photons (the ionizing emissivity, $\dot{N}_\textrm{ion}$) at
$z=4-14$ from current reionization constraints, being maximally agnostic to the
properties of ionizing sources. We use a Bayesian analysis to fit for a
non-parametric form of $\dot{N}_\textrm{ion}$, allowing us to flexibly explore
the entire prior volume. We infer a declining $\dot{N}_\textrm{ion}$ with
redshift at $z>6$, which can be used as a benchmark for reionization models.
Model-independent reionization constraints from the CMB optical depth and
Ly$\alpha$ and Ly$\beta$ forest dark pixel fraction produce
$\dot{N}_\textrm{ion}$ evolution
($d\log_{10}\dot{N}_\textrm{ion}/dz|_{z=6\rightarrow8} = -0.31\pm0.35$ dex)
consistent with the declining UV luminosity density of galaxies, assuming
constant ionizing photon escape fraction and efficiency. Including measurements
from Ly$\alpha$ damping of galaxies and quasars produces a more rapid decline:
$d\log_{10}\dot{N}_\textrm{ion}/dz|_{z=6\rightarrow8} =-0.44\pm0.22$ dex,
steeper than the declining galaxy luminosity density (if extrapolated beyond
$M_\mathrm{UV} \lesssim -13$), and constrains the mid-point of reionization to
$z = 6.93\pm0.14$.
|
1907.11332v1
|
2019-09-10
|
Viscoelasticity-induced pulsatile motion of 2D roll cell in laminar wall-bounded shear flow
|
For the clarification of the routes to elasto-inertial turbulence (EIT), it
is essential to understand how viscoelasticity modulates coherent flow
structures including the longitudinal vortices. We focused on a rotating plane
Couette flow that provides two-dimensional (2D) roll cells for the steady
laminar Newtonian-fluid case, and we investigated how the steady longitudinal
vortices are modulated by viscoelasticity at different Weissenberg numbers. The
viscoelasticity was found to induce an unsteady flow state where the 2D
roll-cell structure was periodically enhanced and damped with a constant
period, keeping the homogeneity in the streamwise direction. This pulsatile
motion of the roll cell was caused by a time lag in the response of the
viscoelastic force to the vortex development. Both the pulsation period and
time lag were found to be scaled by the turnover time of cell rotation rather
than by the relaxation time, despite the viscoelasticity-induced instability.
We also discuss the counter torque on the roll cell and the net energy balance,
considering their relevance to polymer drag reduction and EIT.
|
1909.04257v1
|
2019-09-16
|
On spatial Gevrey regularity for some strongly dissipative second order evolution equations
|
Let A be a positive self-adjoint linear operator acting on a real Hilbert
space H and $\alpha$, c be positive constants. We show that all solutions of
the evolution equation u + Au + cA $\alpha$ u = 0 with u(0) $\in$ D(A 1 2), u
(0) $\in$ H belong for all t > 0 to the Gevrey space G(A, $\sigma$) with
$\sigma$ = min{ 1 $\alpha$ , 1 1--$\alpha$ }. This result is optimal in the
sense that $\sigma$ can not be reduced in general. For the damped wave equation
(SDW) $\alpha$ corresponding to the case where A = --$\Delta$ with domain D(A)
= {w $\in$ H 1 0 ($\Omega$), $\Delta$w $\in$ L 2 ($\Omega$)} with $\Omega$ any
open subset of R N and (u(0), u (0)) $\in$ H 1 0 ($\Omega$)xL 2 ($\Omega$), the
unique solution u of (SDW) $\alpha$ satisfies $\forall$t > 0, u(t) $\in$ G s
($\Omega$) with s = min{ 1 2$\alpha$ , 1 2(1--$\alpha$) }, and this result is
also optimal. Mathematics Subject Classification 2010 (MSC2010): 35L10, 35B65,
47A60.
|
1909.07067v1
|
2019-12-05
|
Optomechanical cavity without a Stokes side-band
|
We investigate a theoretical demonstration of perfect frequency conversion in
an optomechanical system in the weak coupling regime without a Stokes
side-band. An optomechanical cavity illuminated by a weak probe field generates
two side-modes, differentiating from the original signal by a phonon frequency.
We report the presence of a special combination of parameters in the
weak-coupling regime, where Stokes side-mode vanishes exactly. Only the
anti-Stokes mode is observed with a few hundreds Hz spectral bandwidth of the
probe field. Emergence of this special point is totally unrelated with the
electromagnetically induced transparency (EIT) condition, where absorption
(dip) cancellation is limited with the damping rate of the mechanical
oscillator. Emergence is independent of the cavity type, i.e. single or
double-sided, and takes place only for a single value of the effective coupling
strength constant which is specific to the system parameters. At a specific
effective coupling strength between the mirror and the cavity field, which can
be tunable via the coupling field, only the anti-Stokes band is generated. At
that specific coupling there appears no Stokes field. Hence, a filter, to
eliminate the Stokes field, does not necessitate.
|
1912.02530v1
|
2019-12-11
|
Uncertainty Principles Associated to Sets Satisfying the Geometric Control Condition
|
In this paper, we study forms of the uncertainty principle suggested by
problems in control theory. We obtain a version of the classical
Paneah-Logvinenko-Sereda theorem for the annulus. More precisely, we show that
a function with spectrum in an annulus of a given thickness can be bounded, in
$L^2$-norm, from above by its restriction to a neighborhood of a GCC set, with
constant independent of the radius of the annulus. We apply this result to
obtain energy decay rates for damped fractional wave equations, extending the
work of Malhi and Stanislavova to both the higher-dimensional and non-periodic
setting.
|
1912.05077v3
|
2020-01-04
|
Observation of spin-motive force in ferrimagnetic GdFeCo alloy films
|
Non-uniform magnetic structures produce emergent electromagnetic phenomena
such as the topological Hall effect and the spin-motive force (SMF). The
experimental reports on the SMF, however, are very few and the relationship
between the SMF and material parameters is still unclear. In this study, we
investigated the SMF in ferrimagnetic GdFeCo alloy films using the
spin-torque-induced ferromagnetic resonance method and clarified the
relationship. The amplitude of the detected SMF becomes larger than that of the
transition metal alloy FeCo by the Gd doping and reaches the maximum near a Gd
composition of the boundary between in-plane and perpendicularly magnetized
films. According to the analytical calculation, the enhancement is related to
the trajectory of the magnetization precession. Moreover, we find that the SMF
induced by the magnetic resonance is inversely proportional to the square of
the damping constant.
|
2001.01042v2
|
2020-01-06
|
Suppression of Oscillations in Two-Class Traffic by Full-State Feedback
|
This paper develops a full-state feedback controller that damps out
oscillations in traffic density and traffic velocity whose dynamical behavior
is governed by the linearized two-class Aw-Rascle (AR) model. Thereby, the
traffic is considered to be in the congested regime and subdivided in two
classes whereas each class represents vehicles with the same size and driver's
behavior. The macroscopic second-order two-class AR model consists of four
first order hyperbolic partial differential equations (PDEs) and introduces a
concept of area occupancy to depict the mixed density of two-class vehicles in
the traffic. Moreover, the linearized model equations show heterodirectional
behavior with both positive and negative characteristic speeds in the congested
regime. The control objective is to achieve convergence to a constant
equilibrium in finite time. The control input is realized by ramp metering
acting at the outlet of the considered track section. The backstepping method
is employed to design full-state feedback for the $4\times 4$ hyperbolic PDEs.
The performance of the full-state feedback controller is verified by
simulation.
|
2001.01504v1
|
2020-03-26
|
Bipolar spin Hall nano-oscillators
|
We demonstrate a novel type of spin Hall nano-oscillator (SHNO) that allows
for efficient tuning of magnetic auto-oscillations over an extended range of
gigahertz frequencies, using bipolar direct currents at constant magnetic
fields. This is achieved by stacking two distinct ferromagnetic layers with a
platinum interlayer. In this device, the orientation of the spin polarised
electrons accumulated at the top and bottom interfaces of the platinum layer is
switched upon changing the polarity of the direct current. As a result, the
effective anti-damping required to drive large amplitude auto-oscillations can
appear either at the top or bottom magnetic layer. Tuning of the
auto-oscillation frequencies by several gigahertz can be obtained by combining
two materials with sufficiently different saturation magnetization. Here we
show that the combination of NiFe and CoFeB can result in 3 GHz shifts in the
auto-oscillation frequencies. Bipolar SHNOs as such may bring enhanced
synchronisation capabilities to neuromorphic computing applications.
|
2003.11776v1
|
2020-04-01
|
Quasinormal modes of the Dirac field in the novel 4D Einstein-Gauss-Bonnet gravity
|
While quasinormal modes of bosonic fields for the non-trivial $4$-dimensional
Einstein-Gauss-Bonnet theory of gravity suggested in [D.~Glavan and C.~Lin,
Phys.\ Rev.\ Lett.\ {\bf 124}, 081301 (2020)] have been recently studied, there
is no such study for fermionic fields. Here we calculate quasinormal modes of
the Dirac field for spherically symmetric asymptotically flat black hole in
this novel $4D$ Einstein-Gauss-Bonnet theory. The values of the quasinormal
frequencies, calculated by the 6th order WKB method with Pad\'{e} approximants
and the time-domain integration, show that the real part of the quasinormal
modes is considerably increased, while the damping rate is usually decreasing
when the coupling constant is growing.
|
2004.00513v2
|
2020-04-10
|
Dirichlet boundary valued problems for linear and nonlinear wave equations on arbitrary and fractal domains
|
The weak well-posedness results of the strongly damped linear wave equation
and of the non linear Westervelt equation with homogeneous Dirichlet boundary
conditions are proved on arbitrary three dimensional domains or any two
dimensional domains which can be obtained by a limit of NTA domains
caractarized by the same geometrical constants. The two dimensional result is
obtained thanks to the Mosco convergence of the functionals corresponding to
the weak formulations for the Westervelt equation with the homogeneous
Dirichlet boundary condition. The non homogeneous Dirichlet condition is also
treated in the class of admissible domains composed on Sobolev extension
domains of $\mathbb{R}^n$ with a $d$-set boundary $n-1\leq d<n$ preserving
Markov's local inequality.The obtained Mosco convergence also alows to
approximate the solution of the Westervelt equation on an arbitrary domain by
solutions on a converging sequence of domains without additional conditions on
their boundary regularity in $\mathbb{R}^3$, or on a converging sequence of NTA
domains in $\mathbb{R}^2$.
|
2004.05055v1
|
2020-05-09
|
Self-consistent T-matrix approach to gap renormalization in quantum magnets with bond disorder
|
Based on the self-consistent T-matrix approximation (SCTMA), analytical
theory of density of states (DOS) in three-dimensional quantum magnets with the
bond disorder is proposed. It successfully describes DOS in both cases of
resonant and non-resonant scattering which appearance is governed by the ratio
of scattering length and the average distance between impurities. Corrections
to the quasiparticles band gap in these cases are shown to be $\propto c^{2/3}$
and $\propto c$, respectively. Moreover, the theory yields a semi-circle form
of DOS for the bound states inside the gap which results in highly nontrivial
DOS in the intermediate parameters region between two limiting cases when the
band DOS and the semi-circle are overlapped. Long-wavelength excitations are
discussed. In the resonant regime their damping is almost constant $\propto
c^{2/3}$, which according to Ioffe-Regel criterion means their localization.
Applicability of the theory is illustrated by a quantitative description of the
recent experimental data on spin-dimer system Ba$_{3-x}$Sr$_x$Cr$_2$O$_8$.
|
2005.04438v2
|
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