publicationDate stringlengths 1 2.79k | title stringlengths 1 36.5k ⌀ | abstract stringlengths 1 37.3k ⌀ | id stringlengths 9 47 |
|---|---|---|---|
2017-06-07 | Adiabatic and nonadiabatic spin torques induced by spin-triplet supercurrent | We study spin transfer torques induced by a spin-triplet supercurrent in a
magnet with the superconducting proximity effect. By a perturbative approach,
we show that spin-triplet correlations realize new types of torques, which are
analogous to the adiabatic and non-adiabatic ($\beta$) torques, without
extrinsic spin-flip scattering. Remarkable advantages compared to conventional
spin-transfer torques are highlighted in domain wall manipulation. Oscillatory
motions of a domain wall do not occur for a small Gilbert damping, and the
threshold current density to drive its motion becomes zero in the absence of
extrinsic pinning potentials due to the nonadiabatic torque controlled by the
triplet correlations. | 1706.02296v2 |
2017-06-26 | Perpendicular magnetic anisotropy in insulating ferrimagnetic gadolinium iron garnet thin films | We present experimental control of the magnetic anisotropy in a gadolinium
iron garnet (GdIG) thin film from in-plane to perpendicular anisotropy by
simply changing the sample temperature. The magnetic hysteresis loops obtained
by SQUID magnetometry measurements unambiguously reveal a change of the
magnetically easy axis from out-of-plane to in-plane depending on the sample
temperature. Additionally, we confirm these findings by the use of temperature
dependent broadband ferromagnetic resonance spectroscopy (FMR). In order to
determine the effective magnetization, we utilize the intrinsic advantage of
FMR spectroscopy which allows to determine the magnetic anisotropy independent
of the paramagnetic substrate, while magnetometry determines the combined
magnetic moment from film and substrate. This enables us to quantitatively
evaluate the anisotropy and the smooth transition from in-plane to
perpendicular magnetic anisotropy. Furthermore, we derive the temperature
dependent $g$-factor and the Gilbert damping of the GdIG thin film. | 1706.08488v1 |
2017-09-07 | Tunable spin pumping in exchange coupled magnetic trilayers | Magnetic thin films at ferromagnetic resonance (FMR) leak angular momentum,
which may be absorbed by adjacent layers. This phenomenon, known as spin
pumping, is manifested by an increase in the resonance linewidth ($\Delta H$),
and the closely related Gilbert damping. Another effect of this transfer of
spin currents is a dynamical and long-range coupling that can drive two
magnetic layers into a collective precession when their FMR frequencies
coincide. A collective behavior is also found in magnetic trilayers with
interlayer exchange coupling (IEC). In this study we investigate the interplay
between IEC and spin pumping, using Co/Cu/Py pseudo-spin values. We employ
broadband FMR spectroscopy to explore both the frequency and coupling-strength
dependence of $\Delta H$. Our observations show that there exists a cut-off
frequency, set by the IEC strength, below which the precession is truly
collective and the spin pumping is suppressed. These results demonstrate that
it is possible to control the spin pumping efficiency by varying the frequency
or the interlayer exchange coupling. | 1709.02295v1 |
2017-10-30 | Probe of Spin Dynamics in Superconducting NbN Thin Films via Spin Pumping | The emerging field of superconductor (SC) spintronics has attracted intensive
attentions recently. Many fantastic spin dependent properties in SC have been
discovered, including the observation of large magnetoresistance, long spin
lifetimes and the giant spin Hall effect in SC, as well as spin supercurrent in
Josephson junctions, etc. Regarding the spin dynamic in SC films, few studies
has been reported yet. Here, we report the investigation of the spin dynamics
in an s-wave superconducting NbN film via spin pumping from an adjacent
insulating ferromagnet GdN layer. A profound coherence peak of the Gilbert
damping is observed slightly below the superconducting critical temperature of
the NbN layer, which is consistent with recent theoretical studies. Our results
further indicate that spin pumping could be a powerful tool for investigating
the spin dynamics in 2D crystalline superconductors. | 1710.10833v2 |
2017-11-17 | Shot noise of charge and spin transport in a junction with a precessing molecular spin | Magnetic molecules and nanomagnets can be used to influence the electronic
transport in mesoscopic junction. In a magnetic field the precessional motion
leads to resonances in the dc- and ac-transport properties of a nanocontact, in
which the electrons are coupled to the precession. Quantities like the
dc-conductance or the ac-response provide valuable information like the level
structure and the coupling parameters. Here, we address the current noise
properties of such contacts. This encompasses the charge current and
spin-torque shot noise, which both show a step-like behavior as functions of
bias voltage and magnetic field. The charge current noise shows pronounced dips
around the steps, which we trace back to interference effects of electron in
quasienergy levels coupled by the molecular spin precession. We show that some
components of the noise of the spin-torque currents are directly related to the
Gilbert damping and, hence, are experimentally accessible. Our results show
that the noise characteristics allow to investigate in more detail the
coherence of spin transport in contacts containing magnetic molecules. | 1711.06759v2 |
2018-02-05 | Cooper-Pair Spin Current in a Strontium Ruthenate Heterostructure | It has been recognized that the condensation of spin-triplet Cooper pairs
requires not only the broken gauge symmetry but also the spin ordering as well.
One consequence of this is the possibility of the Cooper-pair spin current
analogous to the magnon spin current in magnetic insulators, the analogy also
extending to the existence of the Gilbert damping of the collective
spin-triplet dynamics. The recently fabricated heterostructure of the thin film
of the itinerant ferromagnet SrRuO3 on the bulk Sr2RuO4, the best-known
candidate material for the spin-triplet superconductor, offers a promising
platform for generating such spin current. We will show how such
heterostructure allows us to not only realize the long-range spin valve but
also electrically drive the collective spin mode of the spin-triplet order
parameter. Our proposal represents both a new realization of the spin
superfluidity and a transport signature of the spin-triplet superconductivity. | 1802.01599v1 |
2018-02-12 | Spin-orbit torque and spin pumping in YIG/Pt with interfacial insertion layers | We experimentally investigate spin-orbit torque and spin pumping in
Y$_3$Fe$_5$O$_{12}$(YIG)/Pt bilayers with ultrathin insertion layers at the
interface. An insertion layer of Cu suppresses both spin-orbit torque and spin
pumping, whereas an insertion layer of Ni$_{80}$Fe$_{20}$ (permalloy, Py)
enhances them, in a quantitatively consistent manner with the reciprocity of
the two spin transmission processes. However, we observe a large enhancement of
Gilbert damping with the insertion of Py that cannot be accounted for solely by
spin pumping, suggesting significant spin-memory loss due to the interfacial
magnetic layer. Our findings indicate that the magnetization at the YIG-metal
interface strongly influences the transmission and depolarization of pure spin
current. | 1802.03865v3 |
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 |
2018-08-23 | Reduced thermal stability of antiferromagnetic nanostructures | Antiferromagnetic materials hold promising prospects in novel types of
spintronics applications. Assessing the stability of antiferromagnetic
nanostructures against thermal excitations is a crucial aspect of designing
devices with a high information density. Here we use theoretical calculations
and numerical simulations to determine the mean switching time of
antiferromagnetic nanoparticles in the superparamagnetic limit. It is
demonstrated that the thermal stability is drastically reduced compared to
ferromagnetic particles in the limit of low Gilbert damping, attributed to the
exchange enhancement of the attempt frequencies. It is discussed how the system
parameters have to be engineered in order to optimize the switching rates in
antiferromagnetic nanoparticles. | 1808.07665v3 |
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 |
2018-10-19 | Magnon properties of random alloys | We study magnon properties in terms of spin stiffness, Curie temperatures and
magnon spectrum of Fe-Ni, Co-Ni and Fe-Co random alloys using a combination of
electronic structure calculations and atomistic spin dynamics simulations.
Influence of the disorder are studied in detail by use of large supercells with
random atomic arrangement. It is found that disorder affects the magnon
spectrum in vastly different ways depending on the system. Specifically, it is
more pronounced in Fe-Ni alloys compared to Fe-Co alloys. In particular, the
magnon spectrum at room temperature in Permalloy (Fe$_{20}$Ni$_{80}$) is found
to be rather diffuse in a large energy interval while in Fe$_{75}$Co$_{25}$ it
forms sharp branches. Fe-Co alloys are very interesting from a technological
point of view due to the combination of large Curie temperatures and very low
calculated Gilbert damping of $\sim$0.0007 at room temperature for Co
concentrations around 20--30\%. | 1810.08487v1 |
2018-11-09 | Switching of biaxial synthetic antiferromagnets: a micromagentic study | We simulate the switching behavior of nanoscale synthetic antiferromagnets
(SAFs), inspired by recent experimental progress in spin-orbit-torque switching
of crystal antiferromagnets. The SAF consists of two ferromagnetic thin films
with in-plane biaxial anisotropy and interlayer exchange coupling. Staggered
field-like Rashba spin-orbit torques from the opposite surfaces of the SAF
induce a canted net magnetization, which triggers an orthogonal torque that
drives 90$^\circ$ switching of the N\'eel vector. Such dynamics driven by the
field-like spin-orbit torque allows for faster switching with increased Gilbert
damping, without a significant detrimental increase of the threshold switching
current density. Our results point to the potential of SAFs as model systems,
based on simple ferromagnetic metals, to mimic antiferromagnetic device
physics. | 1811.04094v2 |
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 |
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 |
2019-02-12 | Characterization of spin wave propagation in (111) YIG thin films with large anisotropy | We report on long-range spin wave (SW) propagation in nanometer-thick yttrium
iron garnet (YIG) film with an ultralow Gilbert damping. The knowledge of a
wavenumber value $|\vec{k}|$ is essential for designing SW devices. Although
determining the wavenumber $|\vec{k}|$ in experiments like Brillouin light
scattering spectroscopy is straightforward, quantifying the wavenumber in
all-electrical experiments has not been widely commented upon so far. We
analyze magnetostatic spin wave (SW) propagation in YIG films in order to
determine the SW wavenumber $|\vec{k}|$ excited by the coplanar waveguide. We
show that it is crucial to consider the influence of magnetic anisotropy fields
present in YIG thin films for precise determination of SW wavenumber. With the
proposed methods we find that experimentally derived values of $|\vec{k}|$ are
in perfect agreement with that obtained from electromagnetic simulation only if
anisotropy fields are included. | 1902.04608v1 |
2019-03-20 | Nonlinear magnetization dynamics driven by strong terahertz fields | We present a comprehensive experimental and numerical study of magnetization
dynamics triggered in a thin metallic film by single-cycle terahertz pulses of
$\sim20$ MV/m electric field amplitude and $\sim1$ ps duration. The
experimental dynamics is probed using the femtosecond magneto-optical Kerr
effect (MOKE), and it is reproduced numerically using macrospin simulations.
The magnetization dynamics can be decomposed in three distinct processes: a
coherent precession of the magnetization around the terahertz magnetic field,
an ultrafast demagnetization that suddenly changes the anisotropy of the film,
and a uniform precession around the equilibrium effective field that is relaxed
on the nanosecond time scale, consistent with a Gilbert damping process.
Macrospin simulations quantitatively reproduce the observed dynamics, and allow
us to predict that novel nonlinear magnetization dynamics regimes can be
attained with existing table-top terahertz sources. | 1903.08395v2 |
2019-04-11 | Measurement of spin mixing conductance in Ni$_{81}$Fe$_{19}$/$α$-W and Ni$_{81}$Fe$_{19}$/$β$-W heterostrucutures via ferromagnetic resonance | We present measurements of interfacial Gilbert damping due to the spin
pumping effect in Ni$_{81}$Fe$_{19}$/W heterostructures. Measurements were
compared for heterostructures in which the crystallographic phase of W, either
$\alpha$(bcc)-W or $\beta$(A15)-W, was enriched through deposition conditions
and characterized using X-ray diffraction (XRD) and high-resolution
cross-sectional transmission electron microscopy (HR-XTEM). Single phase
Ni$_{81}$Fe$_{19}$/$\alpha$-W heterostructures could be realized, but
heterostructures with $\beta$-W were realized as mixed $\alpha$-$\beta$ phase.
The spin mixing conductances (SMC) for W at interfaces with Ni$_{81}$Fe$_{19}$
were found to be significantly lower than those for similarly heavy metals such
as Pd and Pt, but comparable to those for Ta, and independent of enrichment in
the $\beta$ phase. | 1904.05950v2 |
2019-05-26 | Influence of field-like torque in synchronization of spin torque oscillators | The magnetization dynamics of two parallelly coupled spin torque oscillators,
destabilization of steady states and removal of multistability, are
investigated by taking into account the influence of field-like torque. It is
shown that the existence of such torque can cancel the effect of damping and
can, therefore, cause the oscillators to exhibit synchronized oscillations in
response to direct current. Further, our results show that the presence of
field-like torque enhances the power and Q-factor of the synchronized
oscillations. The validity of the above results is confirmed by numerical and
analytical studies based on the stochastic Landau-Lifshitz-Gilbert-Slonczewski
equation. | 1905.10804v2 |
2019-05-30 | Sub-nanosecond switching in a cryogenic spin-torque spin-valve memory element with a dilute permalloy free layer | We present a study of the pulsed current switching characteristics of
spin-valve nanopillars with in-plane magnetized dilute permalloy and undiluted
permalloy free layers in the ballistic regime at low temperature. The dilute
permalloy free layer device switches much faster: the characteristic switching
time for a permalloy free (Ni0.83Fe0.17) layer device is 1.18 ns, while that
for a dilute permalloy ([Ni0.83Fe0.17]0.6Cu0.4) free layer device is 0.475 ns.
A ballistic macrospin model can capture the data trends with a reduced spin
torque asymmetry parameter, reduced spin polarization and increased Gilbert
damping for the dilute permalloy free layer relative to the permalloy devices.
Our study demonstrates that reducing the magnetization of the free layer
increases the switching speed while greatly reducing the switching energy and
shows a promising route toward even lower power magnetic memory devices
compatible with superconducting electronics. | 1905.13262v1 |
2019-07-05 | Theory for shift current of bosons: Photogalvanic spin current in ferrimagnetic and antiferromagnetic insulators | We theoretically study the optical generation of dc spin current (i.e., a
spin-current solar cell) in ordered antiferromagnetic and ferrimagnetic
insulators, motivated by a recent study on the laser-driven spinon spin current
in noncentrosymmetric quantum spin chains [H. Ishizuka and M. Sato, Phys. Rev.
Lett. 122, 197702 (2019)]. Using a non-linear response theory for magnons, we
analyze the dc spin current generated by a linearly-polarized electromagnetic
wave (typically, terahertz or gigahertz waves). Considering noncentrosymmetric
two-sublattice magnets as an example, we find a finite dc spin current
conductivity at $T=0$, where no thermally-excited magnons exist; this is in
contrast to the case of the spinon spin current, in which the optical
transition of the Fermi degenerate spinons plays an essential role. We find
that the dc spin-current conductivity is insensitive to the Gilbert damping,
i.e., it may be viewed as a shift current carried by bosonic particles
(magnons). Our estimate shows that an electric-field intensity of
$E\sim10^4-10^6$ V/cm is sufficient for an observable spin current. Our theory
indicates that the linearly-polarized electromagnetic wave generally produces a
dc spin current in noncentrosymmetric magnetic insulators. | 1907.02734v1 |
2019-07-10 | Temperature dependence of magnetic resonance in ferrimagnetic GdFeCo alloys | We provide a macroscopic theory and experimental results for magnetic
resonances of antiferromagnetically-coupled ferrimagnets. Our theory, which
interpolates the dynamics of antiferromagnets and ferromagnets smoothly, can
describe ferrimagnetic resonances across the angular momentum compensation
point. We also present experimental results for spin-torque induced
ferrimagnetic resonance at several temperatures. The spectral analysis based on
our theory reveals that the Gilbert damping parameter, which has been
considered to be strongly temperature dependent, is insensitive to temperature.
We envision that our work will facilitate further investigation of
ferrimagnetic dynamics by providing a theoretical framework suitable for a
broad range of temperatures. | 1907.04540v1 |
2019-07-11 | Improving the Signal-to-noise Ratio for Heat-Assisted Magnetic Recording by Optimizing a High/Low Tc bilayer structure | We optimize the recording medium for heat-assisted magnetic recording by
using a high/low $T_{\mathrm{c}}$ bilayer structure to reduce AC and DC noise.
Compared to a former work, small Gilbert damping $\alpha=0.02$ is considered
for the FePt like hard magnetic material. Atomistic simulations are performed
for a cylindrical recording grain with diameter $d=5\,$nm and height $h=8\,$nm.
Different soft magnetic material compositions are tested and the amount of hard
and soft magnetic material is optimized. The results show that for a soft
magnetic material with $\alpha_{\mathrm{SM}}=0.1$ and
$J_{ij,\mathrm{SM}}=7.72\times 10^{-21}\,$J/link a composition with $50\%$ hard
and $50\%$ soft magnetic material leads to the best results. Additionally, we
analyse how much the areal density can be improved by using the optimized
bilayer structure compared to the pure hard magnetic recording material. It
turns out that the optimized bilayer design allows an areal density that is
$1\,$Tb/in$^2$ higher than that of the pure hard magnetic material while
obtaining the same SNR. | 1907.05027v1 |
2019-07-19 | A cryogenic memory element based on an anomalous Josephson junction | We propose a non-volatile memory element based on a lateral ferromagnetic
Josephson junction with spin-orbit coupling and out-of-plane magnetization. The
interplay between the latter and the intrinsic exchange field of the
ferromagnet leads to a magnetoelectric effect that couples the charge current
through the junction and its magnetization, such that by applying a current
pulse the direction of the magnetic moment in F can be switched. The two memory
states are encoded in the direction of the out-of-plane magnetization. With the
aim to determine the optimal working temperature for the memory element, we
explore the noise-induced effects on the averaged stationary magnetization by
taking into account thermal fluctuations affecting both the Josephson phase and
the magnetic moment dynamics. We investigate the switching process as a
function of intrinsic parameters of the ferromagnet, such as the Gilbert
damping and strength of the spin-orbit coupling, and proposed a non-destructive
readout scheme based on a dc-SQUID. Additionally, we analyze a way to protect
the memory state from external perturbations by voltage gating in systems with
a both linear-in-momentum Rashba and Dresselhaus spin-orbit coupling. | 1907.08454v2 |
2019-07-23 | Electron transport in high-entropy alloys: Al$_{x}$CrFeCoNi as a case study | The high-entropy alloys Al$_{x}$CrFeCoNi exist over a broad range of Al
concentrations ($0 < x < 2$). With increasing Al content their structure is
changed from the fcc to bcc phase. We investigate the effect of such structural
changes on transport properties including the residual resistivity and the
anomalous Hall resistivity. We have performed a detailed comparison of the
first-principles simulations with available experimental data. We show that the
calculated residual resistivities for all studied alloy compositions are in a
fair agreement with available experimental data as concerns both the
resistivity values and concentration trends. We emphasize that a good agreement
with experiment was obtained also for the anomalous Hall resistivity. We have
completed study by estimation of the anisotropic magnetoresistance,
spin-disorder resistivity, and Gilbert damping. The obtained results prove that
the main scattering mechanism is due to the intrinsic chemical disorder whereas
the effect of spin polarization on the residual resistivity is appreciably
weaker. | 1907.09731v1 |
2019-09-11 | Chaos in nanomagnet via feedback current | Nonlinear magnetization dynamics excited by spin-transfer effect with
feedback current is studied both numerically and analytically. The numerical
simulation of the Landau-Lifshitz-Gilbert equation indicates the positive
Lyapunov exponent for a certain range of the feedback rate, which identifies
the existence of chaos in a nanostructured ferromagnet. Transient behavior from
chaotic to steady oscillation is also observed in another range of the feedback
parameter. An analytical theory is also developed, which indicates the
appearance of multiple attractors in a phase space due to the feedback current.
An instantaneous imbalance between the spin-transfer torque and damping torque
causes a transition between the attractors, and results in the complex
dynamics. | 1909.05315v2 |
2019-11-27 | Ellipticity and Dissipation Effects in Magnon Spin Valves | We consider alignment-dependent spin and heat transport across a magnon spin
valve in the tunneling regime, i.e., a junction consisting of two weakly
coupled ferromagnetic insulators. We determine the difference in spin and heat
conductance between the parallel and antiparallel configuration of the
magnetization direction. The dependence of these conductances on both the
Gilbert damping and ellipticity is studied. We find that both magnon
ellipticity and dissipation open channels for magnons to tunnel through in the
antiparallel configuration. Our results highlight an important difference
between electronic and magnon spin transport in spin-valve structures and may
be important for the development of devices based on magnetic insulators. | 1911.12017v2 |
2020-03-24 | Spin-transfer torque driven intrinsic localized spin excitations in the presence of field-like torque | We study the existence of intrinsic localized one-spin excitation in the
Heisenberg one-dimensional ferromagnetic spin chain in the presence of
perpendicular and parallel external magnetic fields and current with
spin-transfer torque and field-like torque. The
Landau-Lifshitz-Gilbert-Slonczewski(LLGS) equation is exactly solved for the
one spin excitation in the absence of onsite anisotropy for the excitations of
spin with fields perpendicular and parallel to the chain. We show the removal
of damping in the spin excitations by appropriately introducing current and
also the enhancement of angular frequency of the oscillations due to field-like
torque in the case of both perpendicular and parallel field. The exactness of
the analytical results is verified by matching with numerical counterparts.
Further, we numerically confirm the existence of in-phase and anti-phase stable
synchronized oscillations for two spin-excitations in the presence of current
with perpendicular field and field-like torque. | 2003.11023v2 |
2020-04-02 | Stable solitons in a nearly PT-symmetric ferromagnet with spin-transfer torque | We consider the Landau-Lifshitz equation for the spin torque oscillator - a
uniaxial ferromagnet in an external magnetic field with polarised spin current
driven through it. In the absence of the Gilbert damping, the equation turns
out to be PT-symmetric. We interpret the PT-symmetry as a balance between gain
and loss - and identify the gaining and losing modes. In the vicinity of the
bifurcation point of a uniform static state of magnetisation, the PT-symmetric
Landau-Lifshitz equation with a small dissipative perturbation reduces to a
nonlinear Schr\"odinger equation with a quadratic nonlinearity. The analysis of
the Schr\"odinger dynamics demonstrates that the spin torque oscillator
supports stable magnetic solitons. The PT near-symmetry is crucial for the
soliton stability: the addition of a finite dissipative term to the
Landau-Lifshitz equation destabilises all solitons that we have found. | 2004.01245v2 |
2020-05-11 | Manipulating 1-dimensinal skyrmion motion by external magnetic field gradient | We have investigated an analytic formula of the 1-dimensional magnetic
skyrmion dynamics under external magnetic field gradient. We find excellent
agreement between the analytical model and micromagnetic simulation results for
various magnetic parameters such as the magnetic field gradient, Gilbert
damping constant. We also observe much faster velocity of the chiral domain
wall (DW) motion. The chiral DW is exist with smaller interfacial
Dzyaloshinskii-Moriya interaction energy density cases. These results provide
to develop efficient control of skyrmion for spintronic devices. | 2005.05011v1 |
2020-07-08 | Finite-frequency spin susceptibility and spin pumping in superconductors with spin-orbit relaxation | Static spin susceptibility of superconductors with spin-orbit relaxation has
been calculated in the seminal work of A.A. Abrikosov and L.P. Gor'kov [Sov.
Phys. JETP, {\bf 15}, 752 (1962)]. Surprisingly the generalization of this
result to finite frequencies has not been done despite being quite important
for the modern topic of superconducting spintronics. The present paper fills
this gap by deriving the analytical expression for spin susceptibility. The
time-dependent spin response is shown to be captured by the quasiclassical
Eilenberger equation with collision integrals corresponding to the ordinary and
spin-orbit scattering. Using the developed formalism we study the linear spin
pumping effect between the ferromagnet and the adjacent superconducting film.
The consequences for understanding recent experiments demonstrating the
modification of Gilbert damping by the superconducting correlations are
discussed. | 2007.04372v2 |
2020-07-16 | Thermal noise effects on the magnetization switching of a ferromagnetic anomalous Josephson junction | We discuss the effects of thermal noise on the magnetic response of a lateral
ferromagnetic Josephson junction with spin-orbit coupling and out-of-plane
magnetization. The direction of the magnetic moment in the ferromagnetic layer
can be inverted by using controlled current pulses. This phenomenon is due to
the magnetoelectric effect that couples the flowing charge current and the
magnetization of the ferromagnet. We investigate the magnetization reversal
effect versus intrinsic parameters of the ferromagnet, such as the Gilbert
damping and strength of the spin-orbit coupling. We estimate the magnetization
reversing time and find the optimal values of the parameters for fast
switching. With the aim of increasing the operation temperature we study the
effects induced by thermal fluctuations on the averaged stationary
magnetization, and find the conditions that make the system more robust against
noise. | 2007.08414v3 |
2020-08-21 | Integration and characterization of micron-sized YIG structures with very low Gilbert damping on arbitrary substrates | We present a novel process that allows the transfer of monocrystalline
yttrium-iron-garnet microstructures onto virtually any kind of substrate. The
process is based on a recently developed method that allows the fabrication of
freestanding monocrystalline YIG bridges on gadolinium-gallium-garnet. Here the
bridges' spans are detached from the substrate by a dry etching process and
immersed in a watery solution. Using drop casting the immersed YIG platelets
can be transferred onto the substrate of choice, where the structures finally
can be reattached and thus be integrated into complex devices or experimental
geometries. Using time resolved scanning Kerr microscopy and inductively
measured ferromagnetic resonance we can demonstrate that the structures retain
their excellent magnetic quality. At room temperature we find a ferromagnetic
resonance linewidth of $\mu_0\Delta H_{HWHM}\approx 195\,\mu T$ and we were
even able to inductively measure magnon spectra on a single micron-sized
yttrium-iron-garnet platelet at a temperature of 5 K. The process is flexible
in terms of substrate material and shape of the structure. In the future this
approach will allow for new types of spin dynamics experiments up to now
unthinkable. | 2008.09390v1 |
2020-09-01 | Quantum Brownian Motion for Magnets | Spin precession in magnetic materials is commonly modelled with the classical
phenomenological Landau-Lifshitz-Gilbert (LLG) equation. Based on a quantized
spin+environment Hamiltonian, we here derive a general spin operator equation
of motion that describes three-dimensional precession and damping and
consistently accounts for effects arising from memory, coloured noise and
quantum statistics. The LLG equation is recovered as its classical, Ohmic
approximation. We further introduce resonant Lorentzian system--reservoir
couplings that allow a systematic comparison of dynamics between Ohmic and
non--Ohmic regimes. Finally, we simulate the full non-Markovian dynamics of a
spin in the semi--classical limit. At low temperatures, our numerical results
demonstrate a characteristic reduction and flattening of the steady state spin
alignment with an external field, caused by the quantum statistics of the
environment. The results provide a powerful framework to explore general
three-dimensional dissipation in quantum thermodynamics. | 2009.00600v2 |
2020-09-30 | Quantum hydrodynamics of spin winding | An easy-plane spin winding in a quantum spin chain can be treated as a
transport quantity, which propagates along the chain but has a finite lifetime
due to phase slips. In a hydrodynamic formulation for the winding dynamics, the
quantum continuity equation acquires a source term due to the transverse
vorticity flow. The latter reflects the phase slips and generally compromises
the global conservation law. A linear-response formalism for the nonlocal
winding transport then reduces to a Kubo response for the winding flow along
the spin chain, in conjunction with the parasitic vorticity flow transverse to
it. One-dimensional topological hydrodynamics can be recovered when the
vorticity flow is asymptotically small. Starting with a microscopic spin-chain
formulation, we focus on the asymptotic behavior of the winding transport based
on the renormalized sine-Gordon equation, incorporating phase slips as well as
Gilbert damping. A generic electrical device is proposed to manifest this
physics. We thus suggest winding conductivity as a tangible concept that can
characterize low-energy dynamics in a broad class of quantum magnets. | 2010.00144v1 |
2020-11-29 | Cross-sublattice Spin Pumping and Magnon Level Attraction in van der Waals Antiferromagnets | We theoretically study spin pumping from a layered van der Waals
antiferromagnet in its canted ground state into an adjacent normal metal. We
find that the resulting dc spin pumping current bears contributions along all
spin directions. Our analysis allows for detecting intra- and cross-sublattice
spin-mixing conductances via measuring the two in-plane spin current
components. We further show that sublattice symmetry-breaking Gilbert damping
can be realized via interface engineering and induces a dissipative coupling
between the optical and acoustic magnon modes. This realizes magnon level
attraction and exceptional points in the system. Furthermore, the dissipative
coupling and cross-sublattice spin pumping contrive to produce an
unconventional spin current in the out-of-plane direction. Our findings provide
a route to extract the spin mixing conductance matrix and uncovers the unique
opportunities, such as level attraction, offered by van der Waals
antiferromagnet-normal metal hybrids. | 2011.14314v1 |
2021-01-18 | Topological electric driving of magnetization dynamics in insulators | Established forms of electromagnetic coupling are usually conservative (in
insulators) or dissipative (in metals and semiconductors). Here we point out
the possibility of nondissipative electric driving of magnetization dynamics,
if the valence electronic states have nontrivial topology in the combined space
of crystal momentum and magnetization configuration. We provide a hybrid
insulator system to demonstrate that the topology-based nonconservative
electrical generalized force is capable of supporting sustained magnetization
motion in the presence of Gilbert damping, with quantized and steady energy
pumping into magnetization motion from the electric field. We also generalize
our results to magnetic textures, and discuss electric field induced
Dzyaloshinskii-Moriya interaction which can be nonconservative. | 2101.07164v3 |
2021-02-07 | Spinterface Induced Modification in Magnetic Properties in Co40Fe40B20/Fullerene Bilayers | Organic semiconductor/ferromagnetic bilayer thin films can exhibit novel
properties due to the formation of the spinterface at the interface.
Buckminsterfullerene (C60) has been shown to exhibit ferromagnetism at the
interface when it is placed next to a ferromagnet (FM) such as Fe or Co.
Formation of spinterface occurs due to the orbital hybridization and spin
polarized charge transfer at the interface. In this work, we have demonstrated
that one can enhance the magnetic anisotropy of the low Gilbert damping alloy
CoFeB by introducing a C60 layer. We have shown that anisotropy increases by
increasing the thickness of C60 which might be a result of the formation of
spinterface. However, the magnetic domain structure remains same in the bilayer
samples as compared to the reference CoFeB film. | 2102.03914v4 |
2021-02-15 | Magnetodynamic properties of dipole-coupled 1D magnonic crystals | Magnonic crystals are magnetic metamaterials, that provide a promising way to
manipulate magnetodynamic properties by controlling the geometry of the
patterned structures. Here, we study the magnetodynamic properties of 1D
magnonic crystals consisting of parallel NiFe strips with different strip
widths and separations. The strips couple via dipole-dipole interactions. As an
alternative to experiments and/or micromagnetic simulations, we investigate the
accuracy of a simple macrospin model. For the case of simple strips, a model
with a single free parameter to account for an overestimation of the
out-of-plane demagnetization of the magnonic lattice is described. By adjusting
this parameter a good fit with experimental as well as micromagnetic results is
obtained. Moreover, the Gilbert damping is found independent of the lattice
constant however the inhomogeneous linewidth broadening found to increase with
decreasing stripe separation. | 2102.07712v2 |
2021-05-24 | Spin pumping of two-dimensional electron gas with Rashba and Dresselhaus spin-orbit interactions | We theoretically consider spin pumping in a junction between a ferromagnetic
insulator (FI) and a two-dimensional electron gas (2DEG) in which the Rashba
and Dresselhaus spin-orbit interactions coexist. Using second-order
perturbation theory, we derive an increase in linewidth in the case of an
interfacial exchange coupling in a ferromagnetic resonance (FMR) experiment. We
clarify how the enhancement of Gilbert damping depends on the resonant
frequency and spin orientation of the FI. We show that this setup of an FMR
experiment can provide information on the spin texture of 2DEG at the Fermi
surface. | 2105.11193v3 |
2021-08-05 | Spin-transfer torque driven localized spin excitations in the presence of field-like torque | We study the existence of localized one-spin excitation in the Heisenberg
one-dimensional ferromagnetic spin chain in the presence of perpendicular and
parallel external magnetic fields and current with spin-transfer torque and
field-like torque. The Landau-Lifshitz-Gilbert-Slonczewski (LLGS) equation is
exactly solved for the one spin excitation in the absence of onsite anisotropy
for the excitations of spin with fields perpendicular and parallel to the
chain. We show the removal of damping in the spin excitations by appropriately
introducing current and also the enhancement of angular frequency of the
oscillations due to field-like torque in the case of both perpendicular and
parallel field. The exactness of the analytical results is verified by matching
with numerical counterparts. Further, we numerically confirm the existence of
in-phase and anti-phase stable synchronized oscillations for two
spin-excitations in the presence of current with perpendicular field and
field-like torque. We also show that the one-spin excitation is stable against
thermal noise and gets only slightly modified against thermal fluctuations. | 2108.02380v1 |
2021-09-07 | Inertial spin dynamics in epitaxial cobalt films | We investigate the spin dynamics driven by terahertz magnetic fields in
epitaxial thin films of cobalt in its three crystalline phases. The terahertz
magnetic field generates a torque on the magnetization which causes it to
precess for about 1 ps, with a sub-picosecond temporal lag from the driving
force. Then, the magnetization undergoes natural damped THz oscillations at a
frequency characteristic of the crystalline phase. We describe the experimental
observations solving the inertial Landau-Lifshitz-Gilbert equation. Using the
results from the relativistic theory of magnetic inertia, we find that the
angular momentum relaxation time $\eta$ is the only material parameter needed
to describe all the experimental evidence. Our experiments suggest a
proportionality between $\eta$ and the strength of the magneto-crystalline
anisotropy. | 2109.03076v2 |
2021-09-26 | Transition state dynamics of a driven magnetic free layer | Magnetization switching in ferromagnetic structures is an important process
for technical applications such as data storage in spintronics, and therefore
the determination of the corresponding switching rates becomes essential. We
investigate a free-layer system in an oscillating external magnetic field
resulting in an additional torque on the spin. The magnetization dynamics
including inertial damping can be described by the phenomenological Gilbert
equation. The magnetization switching between the two stable orientations on
the sphere then requires the crossing of a potential region characterized by a
moving rank-1 saddle. We adopt and apply recent extensions of transition state
theory for driven systems to compute both the time-dependent and average
switching rates of the activated spin system in the saddle region. | 2109.12605v1 |
2021-12-24 | Skyrmion nucleation on the surface of a topological insulator | Skyrmion nucleation induced by spin-transfer torques at an interface of a
topological insulator and a ferromagnetic insulator is investigated. Due to
strong spin-orbit coupling on a surface of topological insulators, which
enhances the effect of spin torques, efficient manipulation of skyrmions is
expected, and therefore, topological insulators could provide the ideal
platform to achieve high-performance skyrmionic devices. Using micromagnetic
simulations and energetics, we evaluate properties of the skyrmion nucleation
on a surface of topological insulators, such as nucleation time, critical
electric field, and skyrmion numbers. We show that the nucleation time is
inversely proportional to the applied electric field. We also identify the
Gilbert damping and temperature dependencies of the critical field.
Furthermore, we analytically evaluate the effect of the Dzyaloshinskii-Moriya
interaction and demonstrate that the temperature dependence can be explained by
the reduction of a magnon excitation gap due to the self-energy corrections. | 2112.12967v2 |
2021-12-10 | Enhanced Planar Antenna Efficiency Through Magnetic Thin-Films | This work proposes to use magnetic material as the substrate of planar
antennas to overcome the platform effect caused by the conducting ground plane.
The upper bound of the radiation efficiency of an electric-current-driven
low-profile antenna is theoretically derived, which is inversely proportional
to the Gilbert damping factor of the magnetic material. Meanwhile, the
improvement of radiation due to the use of magnetic material is demonstrated by
a three-dimensional (3D) multiphysics and multiscale time-domain model. The
simulation results match the theoretical derivation, showing 25% radiation
efficiency from a planar antenna backed by a FeGaB thin film with 2.56 um
thickness. Furthermore, for conductive ferromagnetic materials, it is shown
that the eddy current loss can be well suppressed by laminating the thin film
into multiple layers. The radiation efficiency of the modeled antenna with a
conductive ferromagnetic substrate is improved from 2.2% to 11.8% by dividing
the substrate into 10 layers, with a ferromagnetic material fill factor of 93%. | 2201.04932v1 |
2022-03-07 | Ultrafast optical observation of spin-pumping induced dynamic exchange coupling in ferromagnetic semiconductor/metal bilayer | Spin angular momentum transfer in magnetic bilayers offers the possibility of
ultrafast and low-loss operation for next-generation spintronic devices. We
report the field- and temperature- dependent measurements on the magnetization
precessions in Co$_2$FeAl/(Ga,Mn)As by time-resolved magneto-optical Kerr
effect (TRMOKE). Analysis of the effective Gilbert damping and phase shift
indicates a clear signature of an enhanced dynamic exchange coupling between
the two ferromagnetic (FM) layers due to the reinforced spin pumping at
resonance. The temperature dependence of the dynamic exchange-coupling reveals
a primary contribution from the ferromagnetism in (Ga,Mn)As. | 2203.03225v2 |
2022-04-21 | Transport theory for topological Josephson junctions with a Majorana qubit | We construct a semiclassical theory for the transport of topological
junctions starting from a microscopic Hamiltonian that comprehensively includes
the interplay among the Majorana qubit, the Josephson phase, and the
dissipation process. With the path integral approach, we derive a set of
semiclassical equations of motion that can be used to calculate the time
evolution of the Josephson phase and the Majorana qubit. In the equations we
reveal rich dynamical phenomena such as the qubit induced charge pumping, the
effective spin-orbit torque, and the Gilbert damping. We demonstrate the
influence of these dynamical phenomena on the transport signatures of the
junction. We apply the theory to study the Shapiro steps of the junction, and
find the suppression of the first Shapiro step due to the dynamical feedback of
the Majorana qubit. | 2204.09923v1 |
2022-11-04 | Derivation of Interacting Two-Qubit Dynamics from Spin-Boson Model | We derive damping equations of motion for interacting two-spin states from a
spin-boson model in order to examine qubit dynamics in quantum computers. On
the basis of the composite operator method, we develop the Caldeira-Leggett
approach for open quantum systems so that the entanglement dynamics originated
from the two-spin correlation can be taken. We demonstrate numerical results
for time dependence on the two-spin dynamics. We find that the relaxation of
the total spin is described by a quantum version of the Landau-Lifshitz-Gilbert
equation for magnetic materials. We also find that a two-spin composite mode
keeps oscillation even after the total spin has been fully relaxed. We thus
conclude that the two-spin correlation due to the presence of the composite
mode is stable against dissipation. We consider the mechanism of why the
correlation is maintained. | 2211.02490v1 |
2023-02-06 | Global solutions of the Landau--Lifshitz--Baryakhtar equation | The Landau--Lifshitz--Baryakhtar (LLBar) equation is a generalisation of the
Landau--Lifshitz--Gilbert and the Landau--Lifshitz--Bloch equations which takes
into account contributions from nonlocal damping and is valid at moderate
temperature below the Curie temperature. Therefore, it is used to explain some
discrepancies between the experimental observations and the known theories in
various problems on magnonics and magnetic domain-wall dynamics. In this paper,
the existence and uniqueness of global weak, strong, and regular solutions to
LLBar equation are proven. H\"older continuity of the solution is also
discussed. | 2302.02556v3 |
2023-03-22 | Twisted bilayer graphene reveals its flat bands under spin pumping | The salient property of the electronic band structure of twisted bilayer
graphene (TBG), at the so-called magic angle (MA), is the emergence of flat
bands around the charge neutrality point. These bands are associated with the
observed superconducting phases and the correlated insulating states. Scanning
tunneling microscopy combined with angle resolved photoemission spectroscopy
are usually used to visualize the flatness of the band structure of TBG at the
MA. Here, we theoretically argue that spin pumping (SP) provides a direct probe
of the flat bands of TBG and an accurate determination of the MA. We consider a
junction separating a ferromagnetic insulator and a heterostructure of TBG
adjacent to a monolayer of a transition metal dichalcogenide. We show that the
Gilbert damping of the ferromagnetic resonance experiment, through this
junction, depends on the twist angle of TBG, and exhibits a sharp drop at the
MA. We discuss the experimental realization of our results which open the way
to a twist switchable spintronics in twisted van der Waals heterostructures. | 2303.12380v2 |
2023-05-01 | Coherent and incoherent magnons induced by strong ultrafast demagnetization in thin permalloy films | Understanding spin dynamics on femto- and picosecond timescales offers new
opportunities for faster and more efficient spintronic devices. Here, we
experimentally investigate the coherent spin dynamics after ultrashort laser
excitation by time-resolved magneto optical Kerr effect (TR-MOKE) in thin
Ni80Fe20 films. We provide a detailed study of the magnetic field and pump
fluence dependence of the coherent precessional dynamics. We show that the
coherent precession lifetime increases with the applied external magnetic field
which cannot be understood by viscous Gilbert damping of the coherent magnons.
Instead, it can be explained by nonlinear magnon interactions and by the change
in the fraction of incoherent magnons. This interpretation is in agreement with
the observed trends of the coherent magnon amplitude and lifetime as a function
of the exciting laser fluence. Our results provide a new insight into the
magnetization relaxation processes in ferromagnetic thin films, which is of
great importance for further spintronic applications. | 2305.00814v2 |
2023-07-15 | Switching current distributions in ferromagnetic anomalous Josephson junctions | We investigate the switching current distributions of ferromagnetic anomalous
Josephson junctions subjected to a linearly increasing bias current. Our study
uncovers a significant correlation between the position of the switching
current distributions and crucial system parameters, such as the strength of
the spin-orbit coupling and the Gilbert damping parameter. This indicates that
these parameters can be directly determined through experimental measurements.
By conducting a comprehensive analysis of the interplay among noise,
magnetization, phase dynamics, and the statistical properties of the switching
current distribution, we deepen our understanding of these intriguing cryogenic
spintronics devices. These findings hold potential for applications in the
field of quantum computing architectures and information processing
technologies. | 2307.07751v2 |
2023-07-26 | Oscillatory Edge Modes in Two Dimensional Spin-Torque Oscillator Arrays | Spin torque oscillators (STOs) are dissipative magnetic systems that provide
a natural platform for exploring non-Hermitian phenomena. We theoretically
study a two-dimensional (2d) array of STOs and show that its dynamics can be
mapped to a 2d, non-Hermitian Su-Schrieffer-Heeger (SSH) model. We calculate
the energy spectrum and identify the one-dimensional (1d) edge states of our
model, corresponding to auto-oscillation of STOs on the boundary of the system
while the bulk oscillators do not activate. We show that tuning the Gilbert
damping, injected spin current, and coupling between STOs allows for exploring
the edge state properties under different parameter regimes. Furthermore, this
system admits 1d edge states with non-uniform probability density, and we
explore their properties in systems of different sizes. Additional symmetry
analysis indicates that these states are not topologically protected but are
nevertheless confined to the edge of the system, as the bulk is protected by
PT-symmetry. These results indicate that 2d arrays of STOs may be useful to
explore novel edge state behavior in dissipative systems. | 2307.13876v1 |
2024-03-25 | Detection of spin pumping free of rectification and thermal artefacts in molecular-based ferromagnetic insulator V[TCNE]x~2 | The molecular-based ferrimagnetic insulator V(TCNE)x has gained recent
interest for efficient spin-wave excitation due to its low Gilbert damping
ratio a=4E-5, and narrow ferromagnetic resonance linewidth f=1Oe. Here we
report a clean spin pumping signal detected on V(TCNE)x/metal bilayer
structures, free from spin rectification or thermal artifacts. On-chip coupling
of microwave power is achieved via a coplanar waveguide to measure the in-plane
angle-dependence of the inverse spin-Hall effect under ferromagnetic resonance
conditions with respect to a constant external magnetic field. A signature of
pure spin current from V(TCNE)x is observed in both platinum and permalloy
metal layers, demonstrating the utility of V(TCNE)x for magnon spintronics
studies in molecule/solid-state heterostructures. | 2403.16429v2 |
2024-04-01 | Harnessing Interlayer Magnetic Coupling for Efficient, Field-Free Current-Induced Magnetization Switching in a Magnetic Insulator | Owing to the unique features of low Gilbert damping, long spin-diffusion
lengths and zero Ohmic losses, magnetic insulators are promising candidate
materials for next-generation spintronic applications. However, due to the
localized magnetic moments and the complex metal-oxide interface between
magnetic insulators and heavy metals, spin-functional Dzyaloshinskii-Moriya
interactions or spin Hall and Edelstein effects are weak, which diminishes the
performance of these typical building blocks for spintronic devices. Here, we
exploit the exchange coupling between metallic and insulating magnets for
efficient electrical manipulation of heavy metal/magnetic insulator
heterostructures. By inserting a thin Co layer, we enhance the spin-orbit
torque efficiency by more than 20 times, which significantly reduces the
switching current density. Moreover, we demonstrate field-free current-induced
magnetization switching caused by a symmetry-breaking non-collinear magnetic
texture. Our work launches magnetic insulators as an alternative platform for
low-power spintronic devices. | 2404.00845v1 |
1999-02-01 | Damping Rates and Mean Free Paths of Soft Fermion Collective Excitations in a Hot Fermion-Gauge-Scalar Theory | We study the transport coefficients, damping rates and mean free paths of
soft fermion collective excitations in a hot fermion-gauge-scalar plasma with
the goal of understanding the main physical mechanisms that determine transport
of chirality in scenarios of non-local electroweak baryogenesis. The focus is
on identifying the different transport coefficients for the different branches
of soft collective excitations of the fermion spectrum. These branches
correspond to collective excitations with opposite ratios of chirality to
helicity and different dispersion relations. By combining results from the hard
thermal loop (HTL) resummation program with a novel mechanism of fermion
damping through heavy scalar decay, we obtain a robust description of the
different damping rates and mean free paths for the soft collective excitations
to leading order in HTL and lowest order in the Yukawa coupling. The space-time
evolution of wave packets of collective excitations unambiguously reveals the
respective mean free paths. We find that whereas both the gauge and scalar
contribution to the damping rates are different for the different branches, the
difference of mean free paths for both branches is mainly determined by the
decay of the heavy scalar into a hard fermion and a soft collective excitation.
We argue that these mechanisms are robust and are therefore relevant for
non-local scenarios of baryogenesis either in the Standard Model or extensions
thereof. | 9902218v2 |
2004-11-26 | Open quantum systems | The damping of the harmonic oscillator is studied in the framework of the
Lindblad theory for open quantum systems. A generalization of the fundamental
constraints on quantum mechanical diffusion coefficients which appear in the
master equation for the damped quantum oscillator is presented; the
Schr\"odinger, Heisenberg and Weyl-Wigner-Moyal representations of the Lindblad
equation are given explicitly. On the basis of these representations it is
shown that various master equations for the damped quantum oscillator used in
the literature are particular cases of the Lindblad equation and that not all
of these equations are satisfying the constraints on quantum mechanical
diffusion coefficients. The master equation is transformed into Fokker-Planck
equations for quasiprobability distributions and a comparative study is made
for the Glauber $P$ representation, the antinormal ordering $Q$ representation
and the Wigner $W$ representation. The density matrix is represented via a
generating function, which is obtained by solving a time-dependent linear
partial differential equation derived from the master equation. The damped
harmonic oscillator is applied for the description of the charge equilibration
mode observed in deep inelastic reactions. For a system consisting of two
harmonic oscillators the time dependence of expectation values, Wigner function
and Weyl operator are obtained and discussed. In addition models for the
damping of the angular momentum are studied. Using this theory to the quantum
tunneling through the nuclear barrier, besides Gamow's transitions with energy
conservation, additional transitions with energy loss, are found. When this
theory is used to the resonant atom-field interaction, new optical equations
describing the coupling through the environment are obtained. | 0411189v1 |
2015-04-16 | Attenuation of short strongly nonlinear stress pulses in dissipative granular chains | Attenuation of short, strongly nonlinear stress pulses in chains of spheres
and cylinders was investigated experimentally and numerically for two ratios of
their masses keeping their contacts identical. The chain with mass ratio 0.98
supports solitary waves and another one (with mass ratio 0.55) supports
nonstationary pulses which preserve their identity only on relatively short
distances, but attenuate on longer distances because of radiation of small
amplitude tails generated by oscillating small mass particles. Pulse
attenuation in experiments in the chain with mass ratio 0.55 was faster at the
same number of the particles from the entrance than in the chain with mass
ratio 0.98. It is in quantitative agreement with results of numerical
calculations with effective damping coefficient 6 kg/s. This level of damping
was critical for eliminating the gap openings between particles in the system
with mass ratio 0.55 present at lower or no damping. However with increase of
dissipation numerical results show that the chain with mass ratio 0.98 provides
faster attenuation than chain with mass ratio 0.55 due to the fact that the
former system supports the narrower pulse with the larger difference between
velocities of neighboring particles. The investigated chains demonstrated
different wave structure at zero dissipation and at intermediate damping
coefficients and the similar behavior at large damping. | 1504.04344v1 |
2023-03-15 | Blow-up and decay for a class of variable coefficient wave equation with nonlinear damping and logarithmic source | In this paper, we consider the long time behavior for the solution of a class
of variable coefficient wave equation with nonlinear damping and logarithmic
source. The existence and uniqueness of local weak solution can be obtained by
using the Galerkin method and contraction mapping principle. However, the long
time behavior of the solution is usually complicated and it depends on the
balance mechanism between the damping and source terms. When the damping
exponent $(p+1)$ (see assumption (H3)) is greater than the source term exponent
$(q-1)$ (see equation (1.1)), namely, $p+2>q$, we obtain the global existence
and accurate decay rates of the energy for the weak solutions with any initial
data. Moreover, whether the weak solution exists globally or blows up in finite
time, it is closely related to the initial data. In the framework of modified
potential well theory, we construct the stable and unstable sets (see (2.8))
for the initial data. For the initial data belonging to the stable set, we
prove that the weak solution exists globally and has similar decay rates as the
previous results. For $p+2<q$ and the initial data belonging to the unstable
set, we prove that the weak solution blows up in finite time for a little
special damping $g(u_{t})=|u_{t}|^{p}u_{t}$. | 2303.08629v1 |
2023-07-12 | Exponential stability of damped Euler-Bernoulli beam controlled by boundary springs and dampers | In this paper, the vibration model of an elastic beam, governed by the damped
Euler-Bernoulli equation
$\rho(x)u_{tt}+\mu(x)u_{t}$$+\left(r(x)u_{xx}\right)_{xx}=0$, subject to the
clamped boundary conditions $u(0,t)=u_x(0,t)=0$ at $x=0$, and the boundary
conditions $\left(-r(x)u_{xx}\right)_{x=\ell}=k_r u_x(\ell,t)+k_a
u_{xt}(\ell,t)$, $\left(-\left(r(x)u_{xx}\right)_{x}\right )_{x=\ell}$$=- k_d
u(\ell,t)-k_v u_{t}(\ell,t)$ at $x=\ell$, is analyzed. The boundary conditions
at $x=\ell$ correspond to linear combinations of damping moments caused by
rotation and angular velocity and also, of forces caused by displacement and
velocity, respectively. The system stability analysis based on well-known
Lyapunov approach is developed. Under the natural assumptions guaranteeing the
existence of a regular weak solution, uniform exponential decay estimate for
the energy of the system is derived. The decay rate constant in this estimate
depends only on the physical and geometric parameters of the beam, including
the viscous external damping coefficient $\mu(x) \ge 0$, and the boundary
springs $k_r,k_d \ge 0$ and dampers $k_a,k_v \ge 0$. Some numerical examples
are given to illustrate the role of the damping coefficient and the boundary
dampers. | 2307.06170v2 |
2017-04-12 | The gradient condition and the contribution of the dynamical part of Green-Kubo formula to the diffusion coefficient | In the diffusive hydrodynamic limit for a symmetric interacting particle
system (such as the exclusion process, the zero range process, the stochastic
Ginzburg-Landau model, the energy exchange model), a possibly non-linear
diffusion equation is derived as the hydrodynamic equation. The bulk diffusion
coefficient of the limiting equation is given by Green-Kubo formula and it can
be characterized by a variational formula. In the case the system satisfies the
gradient condition, the variational problem is explicitly solved and the
diffusion coefficient is given from the Green-Kubo formula through a static
average only. In other words, the contribution of the dynamical part of
Green-Kubo formula is 0. In this paper, we consider the converse, namely if the
contribution of the dynamical part of Green-Kubo formula is 0, does it imply
the system satisfies the gradient condition or not. We show that if the
equilibrium measure {\mu} is product and {L^2} space of its single site
marginal is separable, then the converse also holds. As an application of the
result, we consider a class of stochastic models for energy transport studied
by Gaspard and Gilbert in [1, 2], where the exact problem is discussed for this
specific model. | 1704.03745v2 |
2010-06-25 | Perturbations of Mathieu equations with parametric excitation of large period | We consider a linear differential system of Mathieu equations with periodic
coefficients over periodic closed orbits and we prove that, arbitrarily close
to this system, there is a linear differential system of Hamiltonian damped
Mathieu equations with periodic coefficients over periodic closed orbits such
that, all but a finite number of closed periodic coefficients, have unstable
solutions. The perturbations will be peformed in the periodic coefficients. | 1006.5025v1 |
2008-02-07 | Cascade and Damping of Alfvén-Cyclotron Fluctuations: Application to Solar Wind Turbulence Spectrum | With the diffusion approximation, we study the cascade and damping of
Alfv\'{e}n-cyclotron fluctuations in solar plasmas numerically. Motivated by
wave-wave couplings and nonlinear effects, we test several forms of the
diffusion tensor. For a general locally anisotropic and inhomogeneous diffusion
tensor in the wave vector space, the turbulence spectrum in the inertial range
can be fitted with power-laws with the power-law index varying with the wave
propagation direction. For several locally isotropic but inhomogeneous
diffusion coefficients, the steady-state turbulence spectra are nearly
isotropic in the absence of damping and can be fitted by a single power-law
function. However, the energy flux is strongly polarized due to the
inhomogeneity that leads to an anisotropic cascade. Including the anisotropic
thermal damping, the turbulence spectrum cuts off at the wave numbers, where
the damping rates become comparable to the cascade rates. The combined
anisotropic effects of cascade and damping make this cutoff wave number
dependent on the wave propagation direction, and the propagation direction
integrated turbulence spectrum resembles a broken power-law, which cuts off at
the maximum of the cutoff wave numbers or the $^4$He cyclotron frequency.
Taking into account the Doppler effects, the model can naturally reproduce the
broken power-law wave spectra observed in the solar wind and predicts that a
higher break frequency is aways accompanied with a greater spectral index
change that may be caused by the increase of the Alfv\'{e}n Mach number, the
reciprocal of the plasma beta, and/or the angle between the solar wind velocity
and the mean magnetic field. These predictions can be tested by future
observations. | 0802.0910v1 |
2023-10-07 | OEDG: Oscillation-eliminating discontinuous Galerkin method for hyperbolic conservation laws | Controlling spurious oscillations is crucial for designing reliable numerical
schemes for hyperbolic conservation laws. This paper proposes a novel, robust,
and efficient oscillation-eliminating discontinuous Galerkin (OEDG) method on
general meshes, motivated by the damping technique in [Lu, Liu, and Shu, SIAM
J. Numer. Anal., 59:1299-1324, 2021]. The OEDG method incorporates an OE
procedure after each Runge-Kutta stage, devised by alternately evolving
conventional semidiscrete DG scheme and a damping equation. A novel damping
operator is carefully designed to possess scale-invariant and
evolution-invariant properties. We rigorously prove optimal error estimates of
the fully discrete OEDG method for linear scalar conservation laws. This might
be the first generic fully-discrete error estimates for nonlinear DG schemes
with automatic oscillation control mechanism. The OEDG method exhibits many
notable advantages. It effectively eliminates spurious oscillations for
challenging problems across various scales and wave speeds, without
problem-specific parameters. It obviates the need for characteristic
decomposition in hyperbolic systems. It retains key properties of conventional
DG method, such as conservation, optimal convergence rates, and
superconvergence. Moreover, it remains stable under normal CFL condition. The
OE procedure is non-intrusive, facilitating integration into existing DG codes
as an independent module. Its implementation is easy and efficient, involving
only simple multiplications of modal coefficients by scalars. The OEDG approach
provides new insights into the damping mechanism for oscillation control. It
reveals the role of damping operator as a modal filter and establishes close
relations between the damping and spectral viscosity techniques. Extensive
numerical results confirm the theoretical analysis and validate the
effectiveness and advantages of the OEDG method. | 2310.04807v1 |
2018-08-08 | Analysis of quasi-Monte Carlo methods for elliptic eigenvalue problems with stochastic coefficients | We consider the forward problem of uncertainty quantification for the
generalised Dirichlet eigenvalue problem for a coercive second order partial
differential operator with random coefficients, motivated by problems in
structural mechanics, photonic crystals and neutron diffusion. The PDE
coefficients are assumed to be uniformly bounded random fields, represented as
infinite series parametrised by uniformly distributed i.i.d. random variables.
The expectation of the fundamental eigenvalue of this problem is computed by
(a) truncating the infinite series which define the coefficients; (b)
approximating the resulting truncated problem using lowest order conforming
finite elements and a sparse matrix eigenvalue solver; and (c) approximating
the resulting finite (but high dimensional) integral by a randomly shifted
quasi-Monte Carlo lattice rule, with specially chosen generating vector. We
prove error estimates for the combined error, which depend on the truncation
dimension $s$, the finite element mesh diameter $h$, and the number of
quasi-Monte Carlo samples $N$. Under suitable regularity assumptions, our
bounds are of the particular form $\mathcal{O}(h^2+N^{-1+\delta})$, where
$\delta>0$ is arbitrary and the hidden constant is independent of the
truncation dimension, which needs to grow as $h\to 0$ and $N\to\infty$.
Although the eigenvalue problem is nonlinear, which means it is generally
considered harder than the analogous source problem, in almost all cases we
obtain error bounds that converge at the same rate as the corresponding rate
for the source problem. The proof involves a detailed study of the regularity
of the fundamental eigenvalue as a function of the random parameters. As a key
intermediate result in the analysis, we prove that the spectral gap (between
the fundamental and the second eigenvalues) is uniformly positive over all
realisations of the random problem. | 1808.02639v3 |
2021-01-22 | Measurements and analysis of response function of cold atoms in optical molasses | We report our experimental measurements and theoretical analysis of the
position response function of a cloud of cold atoms residing in the viscous
medium of an optical molasses and confined by a magneto-optical trap (MOT). We
measure the position response function by applying a transient homogeneous
magnetic field as a perturbing force. We observe a transition from a damped
oscillatory motion to an over-damped relaxation, stemming from a competition
between the viscous drag provided by the optical molasses and the restoring
force of the MOT. Our observations are in both qualitative and quantitative
agreement with the predictions of a theoretical model based on the Langevin
equation. As a consistency check, and as a prototype for future experiments, we
also study the free diffusive spreading of the atomic cloud in our optical
molasses with the confining magnetic field of the MOT turned off. We find that
the measured value of the diffusion coefficient agrees with the value predicted
by our Langevin model, using the damping coefficient. The damping coefficient
was deduced from our measurements of the position response function at the same
temperature. | 2101.09118v2 |
2020-12-08 | Sparse Correspondence Analysis for Contingency Tables | Since the introduction of the lasso in regression, various sparse methods
have been developed in an unsupervised context like sparse principal component
analysis (s-PCA), sparse canonical correlation analysis (s-CCA) and sparse
singular value decomposition (s-SVD). These sparse methods combine feature
selection and dimension reduction. One advantage of s-PCA is to simplify the
interpretation of the (pseudo) principal components since each one is expressed
as a linear combination of a small number of variables. The disadvantages lie
on the one hand in the difficulty of choosing the number of non-zero
coefficients in the absence of a well established criterion and on the other
hand in the loss of orthogonality for the components and/or the loadings. In
this paper we propose sparse variants of correspondence analysis (CA)for large
contingency tables like documents-terms matrices used in text mining, together
with pPMD, a deation technique derived from projected deflation in s-PCA. We
use the fact that CA is a double weighted PCA (for rows and columns) or a
weighted SVD, as well as a canonical correlation analysis of indicator
variables. Applying s-CCA or s-SVD allows to sparsify both rows and columns
weights. The user may tune the level of sparsity of rows and columns and
optimize it according to some criterium, and even decide that no sparsity is
needed for rows (or columns) by relaxing one sparsity constraint. The latter is
equivalent to apply s-PCA to matrices of row (or column) profiles. | 2012.04271v1 |
1996-01-09 | Relaxation of Collective Excitations in LJ-13 Cluster | We have performed classical molecular dynamics simulation of $Ar_{13}$
cluster to study the behavior of collective excitations. In the solid ``phase''
of the cluster, the collective oscillation of the monopole mode can be well
fitted to a damped harmonic oscillator. The parameters of the equivalent damped
harmonic oscillator-- the damping coefficient, spring constant, time period of
oscillation and the mass of the oscillator -- all show a sharp change in
behavior at a kinetic temperature of about $7.0^oK$. This marks yet another
characteristic temperature of the system, a temperature $T_s$ below which
collective excitations are very stable, and at higher temperatures the single
particle excitations cause the damping of the collective oscillations. We argue
that so long as the cluster remains confined within the global potential energy
minimum the collective excitations do not decay; and once the cluster comes out
of this well, the local potential energy minima pockets act as single particle
excitation channels in destroying the collective motion. The effect is manifest
in almost all the physical observables of the cluster. | 9601026v2 |
2003-09-11 | Frequency and damping of hydrodynamic modes in a trapped Bose-condensed gas | Recently it was shown that the Landau-Khalatnikov two-fluid hydrodynamics
describes the collision-dominated region of a trapped Bose condensate
interacting with a thermal cloud. We use these equations to discuss the low
frequency hydrodynamic collective modes in a trapped Bose gas at finite
temperatures. We derive a variational expressions based on these equations for
both the frequency and damping of collective modes. A new feature is our use of
frequency-dependent transport coefficients, which produce a natural cutoff by
eliminating the collisionless low-density tail of the thermal cloud. Above the
superfluid transition, our expression for the damping in trapped inhomogeneous
gases is analogous to the result first obtained by Landau and Lifshitz for
uniform classical fluids. We also use the moment method to discuss the
crossover from the collisionless to the hydrodynamic region. Recent data for
the monopole-quadrupole mode in the hydrodynamic region of a trapped gas of
metastable $^4$He is discussed. We also present calculations for the damping of
the analogous $m=0$ monopole-quadrupole condensate mode in the superfluid
phase. | 0309269v1 |
2012-01-31 | Parametric amplification and self-oscillation in a nanotube mechanical resonator | A hallmark of mechanical resonators made from a single nanotube is that the
resonance frequency can be widely tuned. Here, we take advantage of this
property to realize parametric amplification and self-oscillation. The gain of
the parametric amplification can be as high as 18.2 dB and tends to saturate at
high parametric pumping due to nonlinear damping. These measurements allow us
to determine the coefficient of the linear damping force. The corresponding
damping rate is lower than the one obtained from the lineshape of the resonance
(without pumping), supporting the recently reported scenario that describes
damping in nanotube resonators by a nonlinear force. The possibility to combine
nanotube resonant mechanics and parametric amplification holds promise for
future ultra-low force sensing experiments. | 1201.6557v1 |
2012-07-12 | Damping of phase fluctuations in superfluid Bose gases | Using Popov's hydrodynamic approach we derive an effective Euclidean action
for the long-wavelength phase fluctuations of superfluid Bose gases in D
dimensions. We then use this action to calculate the damping of phase
fluctuations at zero temperature as a function of D. For D >1 and wavevectors |
k | << 2 mc (where m is the mass of the bosons and c is the sound velocity) we
find that the damping in units of the phonon energy E_k = c | k | is to leading
order gamma_k / E_k = A_D (k_0^D / 2 pi rho) (| k | / k_0)^{2 D -2}, where rho
is the boson density and k_0 =2 mc is the inverse healing length. For D -> 1
the numerical coefficient A_D vanishes and the damping is proportional to an
additional power of |k | /k_0; a self-consistent calculation yields in this
case gamma_k / E_k = 1.32 (k_0 / 2 pi rho)^{1/2} |k | / k_0. In one dimension,
we also calculate the entire spectral function of phase fluctuations. | 1207.3002v3 |
2014-10-13 | Relaxation damping in oscillating contacts | If a contact of two purely elastic bodies with no sliding (infinite
coefficient of friction) is subjected to superimposed oscillations in the
normal and tangential directions, then a specific damping appears, that is not
dependent on friction or dissipation in the material. We call this effect
"relaxation damping". The rate of energy dissipation due to relaxation damping
is calculated in a closed analytic form for arbitrary axially-symmetric
contacts. In the case of equal frequency of normal and tangential oscillations,
the dissipated energy per cycle is proportional to the square of the amplitude
of tangential oscillation and to the absolute value of the amplitude of normal
oscillation, and is dependent on the phase shift between both oscillations. In
the case of low frequency tangential motion with superimposed high frequency
normal oscillations, the system acts as a tunable linear damper. Generalization
of the results for macroscopically planar, randomly rough surfaces is
discussed. | 1410.3238v1 |
2016-04-29 | Nonlinear Landau damping of wave envelopes in a quantum plasma | The nonlinear theory of Landau damping of electrostatic wave envelopes (WEs)
is revisited in a quantum electron-positron (EP) pair plasma. Starting from a
Wigner-Moyal equation coupled to the Poisson equation and applying the multiple
scale technique, we derive a nonlinear Schr{\"o}dinger (NLS) equation which
governs the evolution of electrostatic WEs. It is shown that the coefficients
of the NLS equation, including the nonlocal nonlinear term, which appears due
to the resonant particles having group velocity of the WEs, are significantly
modified by the particle dispersion. The effects of the quantum parameter $H$
(the ratio of the plasmon energy to the thermal energy densities), associated
with the particle dispersion, are examined on the Landau damping rate of
carrier waves, as well as on the modulational instability of WEs. It is found
that the Landau damping rate and the decay rate of the solitary wave amplitude
are greatly reduced compared to their classical values $(H=0)$. | 1604.08751v4 |
2017-06-11 | Absorbing boundary layers for spin wave micromagnetics | Micromagnetic simulations are used to investigate the effects of different
absorbing boundary layers (ABLs) on spin waves (SWs) reflected from the edges
of a magnetic nano-structure. We define the conditions that a suitable ABL must
fulfill and compare the performance of abrupt, linear, polynomial and tan
hyperbolic damping profiles in the ABL. We first consider normal incidence in a
permalloy stripe and propose a transmission line model to quantify reflections
and calculate the loss introduced into the stripe due to the ABL. We find that
a parabolic damping profile absorbs the SW energy efficiently and has a low
reflection coefficient, thus performing much better than the commonly used
abrupt damping profile. We then investigated SWs that are obliquely incident at
26.6, 45 and 63.4 degrees on the edge of a yttrium-iron-garnet film. The
parabolic damping profile again performs efficiently by showing a high SW
energy transfer to the ABL and a low reflected SW amplitude. | 1706.03325v1 |
2017-10-17 | Entropic uncertainty relation under quantum channels with memory | Recently, Xu et al. [Phys. Rev. A 86, 012113(2012)] explored the behavior of
the entropic uncertainty relation under the influence of local unital and
nonunital noisy channels for a class of Bell-diagonal states. We here reform
their results and investigate the entropic uncertainty relation under the
influence of unital and nonunital noisy channels with memory. Different types
of noisy channels with memory, such as amplitude damping channel(nonunitary),
phase-damping and depolarizing channels(unitary) have been taken into account.
Some analytical or numerical results are presented. The effect of channels with
memory on dynamics of the entropic uncertainties (or their lower bounds) has
been discussed in detail. Compare with previous results, our results show that,
the entropic uncertainties (or their lower bounds) subjecting to amplitude
damping channel with memory will be reduced at first and then be lifted with
the memory coefficient of channel $\mu$ increasing, however they will be only
reduced under phase-damping and depolarizing channels with memory. Especially,
in the limit of $\mu\rightarrow1$, the entropic uncertainties (or their lower
bounds) could be well protected and immune to decoherence of channle. Moreover,
the mechanism behind these phenomena are also explored by using the purity of
state. | 1710.06344v1 |
2018-04-15 | Reevaluation of radiation reaction and consequences for light-matter interactions at the nanoscale | In the context of electromagnetism and nonlinear optical interactions damping
is generally introduced as a phenomenological, viscous term that dissipates
energy, proportional to the temporal derivative of the polarization. Here, we
follow the radiation reaction method presented in [G. W. Ford and R. F.
O'Connell, Phys. Lett. A, 157, 217 (1991)], which applies to non-relativistic
electrons of finite size, to introduce an explicit reaction force in the
Newtonian equation of motion, and derive a hydrodynamic equation that offers
new insight on the influence of damping in generic plasmas, metal-based and/or
dielectric structures. In these settings, we find new damping-dependent linear
and nonlinear source terms that suggest the damping coefficient is proportional
to the local charge density, and nonlocal contributions that stem from the
spatial derivative of the magnetic field and discuss the conditions that could
modify both linear and nonlinear electromagnetic responses. | 1804.05369v1 |
2018-12-04 | Atmospheric oscillations provide simultaneous measurement of neutron star mass and radius | Neutron stars with near-Eddington observable luminosities were shown to
harbor levitating atmospheres, suspended above their surface. We report a new
method to simultaneously measure the mass and radius of a neutron star based on
oscillations of such atmospheres. In this paper, we present an analytic
derivation of a family of relativistic, oscillatory, spherically symmetric
eigenmodes of the optically and geometrically thin levitating atmospheres,
including the damping effects induced by the radiation drag. We discover
characteristic maxima in the frequencies of the damped oscillations and show
that using the frequency maxima, one can estimate mass and radius of the
neutron star, given the observed frequency and the corresponding luminosity of
the star during the X-ray burst. Thus, our model provides a new way to probe
the stellar parameters. We also show that the ratio of any two undamped
eigenfrequencies depends only on the adiabatic index of the atmosphere, while
for the damped eigenfrequencies, this ratio varies with the luminosity. The
damping coefficient is independent of the mode number of the oscillations.
Signatures of these atmospheres' dynamics will be reflected in the source's
X-ray light curves. | 1812.01299v2 |
2019-01-13 | Nonexistence of global solutions for a weakly coupled system of semilinear damped wave equations in the scattering case with mixed nonlinear terms | In this paper we consider the blow-up of solutions to a weakly coupled system
of semilinear damped wave equations in the scattering case with nonlinearities
of mixed type, namely, in one equation a power nonlinearity and in the other a
semilinear term of derivative type. The proof of the blow-up results is based
on an iteration argument. As expected, due to the assumptions on the
coefficients of the damping terms, we find as critical curve in the p-q plane
for the pair of exponents (p,q) in the nonlinear terms the same one found by
Hidano-Yokoyama and, recently, by Ikeda-Sobajima-Wakasa for the weakly coupled
system of semilinear wave equations with the same kind of nonlinearities. In
the critical and not-damped case we provide a different approach from the test
function method applied by Ikeda-Sobajima-Wakasa to prove the blow-up of the
solution on the critical curve, improving in some cases the upper bound
estimate for the lifespan. More precisely, we combine an iteration argument
with the so-called slicing method to show the blow-up dynamic of a weighted
version of the functionals used in the subcritical case. | 1901.04038v1 |
2019-03-03 | Spin wave damping in periodic and quasiperiodic magnonic structures | We investigated the lifetime of spin wave eigenmodes in periodic and
quasiperiodic sequences of Py and Co wires. Those materials differ
significantly in damping coefficients, therefore, the spatial distribution of
the mode amplitude within the structure is important for the lifetime of
collective spin wave excitations. Modes of the lower frequencies prefer to
concentrate in Py wires, because of the lower FMR frequency for this material.
This inhomogeneous distribution of amplitude of modes (with lower amplitude in
material of higher damping and with higher amplitude in material of lower
damping) is preferable for extending the lifetime of the collective excitations
beyond the volume average of lifetimes for solid materials. We established the
relation between the profile of the mode and its lifetime for periodic and
quasiperiodic structures. We performed also the comparative studies in order to
find the differences resulting from complexity of the structure and enhancement
of localization in quasiperiodic system on the lifetime of spin waves. | 1903.00856v1 |
2019-05-23 | Strauss exponent for semilinear wave equations with scattering space dependent damping | It is believed or conjectured that the semilinear wave equations with
scattering space dependent damping admit the Strauss critical exponent, see
Ikehata-Todorova-Yordanov \cite{ITY}(the bottom in page 2) and
Nishihara-Sobajima-Wakasugi \cite{N2}(conjecture iii in page 4). In this work,
we are devoted to showing the conjecture is true at least when the decay rate
of the space dependent variable coefficients before the damping is larger than
2. Also, if the nonlinear term depends only on the derivative of the solution,
we may prove the upper bound of the lifespan is the same as that of the
solution of the corresponding problem without damping. This shows in another
way the \lq\lq hyperbolicity" of the equation. | 1905.09445v2 |
2020-06-09 | Logarithmic decay for damped hypoelliptic wave and Schr{ö}dinger equations | We consider damped wave (resp. Schr{\"o}dinger and plate) equations driven by
a hypoelliptic "sum of squares" operator L on a compact manifold and a damping
function b(x). We assume the Chow-Rashevski-H{\"o}rmander condition at rank k
(at most k Lie brackets needed to span the tangent space) together with
analyticity of M and the coefficients of L. We prove decay of the energy at
rate $log(t)^{-1/k}$ (resp. $log(t)^{-2/k}$ ) for data in the domain of the
generator of the associated group. We show that this decay is optimal on a
family of Grushin-type operators. This result follows from a perturbative
argument (of independent interest) showing, in a general abstract setting, that
quantitative approximate observability/controllability results for wave-type
equations imply a priori decay rates for associated damped wave,
Schr{\"o}dinger and plate equations. The adapted quantitative approximate
observability/controllability theorem for hypoelliptic waves is obtained by the
authors in [LL19, LL17]. | 2006.05122v1 |
2020-11-15 | A Random Matrix Theory Approach to Damping in Deep Learning | We conjecture that the inherent difference in generalisation between adaptive
and non-adaptive gradient methods in deep learning stems from the increased
estimation noise in the flattest directions of the true loss surface. We
demonstrate that typical schedules used for adaptive methods (with low
numerical stability or damping constants) serve to bias relative movement
towards flat directions relative to sharp directions, effectively amplifying
the noise-to-signal ratio and harming generalisation. We further demonstrate
that the numerical damping constant used in these methods can be decomposed
into a learning rate reduction and linear shrinkage of the estimated curvature
matrix. We then demonstrate significant generalisation improvements by
increasing the shrinkage coefficient, closing the generalisation gap entirely
in both logistic regression and several deep neural network experiments.
Extending this line further, we develop a novel random matrix theory based
damping learner for second order optimiser inspired by linear shrinkage
estimation. We experimentally demonstrate our learner to be very insensitive to
the initialised value and to allow for extremely fast convergence in
conjunction with continued stable training and competitive generalisation. | 2011.08181v5 |
2021-04-09 | Nonexistence result for the generalized Tricomi equation with the scale-invariant damping, mass term and time derivative nonlinearity | In this article, we consider the damped wave equation in the
\textit{scale-invariant case} with time-dependent speed of propagation, mass
term and time derivative nonlinearity. More precisely, we study the blow-up of
the solutions to the following equation: $$ (E) \quad u_{tt}-t^{2m}\Delta
u+\frac{\mu}{t}u_t+\frac{\nu^2}{t^2}u=|u_t|^p, \quad \mbox{in}\
\mathbb{R}^N\times[1,\infty), $$ that we associate with small initial data.
Assuming some assumptions on the mass and damping coefficients, $\nu$ and
$\mu>0$, respectively, that the blow-up region and the lifespan bound of the
solution of $(E)$ remain the same as the ones obtained for the case without
mass, {\it i.e.} $\nu=0$ in $(E)$. The latter case constitutes, in fact, a
shift of the dimension $N$ by $\frac{\mu}{1+m}$ compared to the problem without
damping and mass. Finally, we think that the new bound for $p$ is a serious
candidate to the critical exponent which characterizes the threshold between
the blow-up and the global existence regions. | 2104.04393v2 |
2021-07-29 | A N-dimensional elastic\viscoelastic transmission problem with Kelvin-Voigt damping and non smooth coefficient at the interface | We investigate the stabilization of a multidimensional system of coupled wave
equations with only one Kelvin Voigt damping. Using a unique continuation
result based on a Carleman estimate and a general criteria of Arendt Batty, we
prove the strong stability of the system in the absence of the compactness of
the resolvent without any geometric condition. Then, using a spectral analysis,
we prove the non uniform stability of the system. Further, using frequency
domain approach combined with a multiplier technique, we establish some
polynomial stability results by considering different geometric conditions on
the coupling and damping domains. In addition, we establish two polynomial
energy decay rates of the system on a square domain where the damping and the
coupling are localized in a vertical strip. | 2107.13785v1 |
2021-12-27 | Trajectory attractors for 3D damped Euler equations and their approximation | We study the global attractors for the damped 3D Euler--Bardina equations
with the regularization parameter $\alpha>0$ and Ekman damping coefficient
$\gamma>0$ endowed with periodic boundary conditions as well as their damped
Euler limit $\alpha\to0$. We prove that despite the possible non-uniqueness of
solutions of the limit Euler system and even the non-existence of such
solutions in the distributional sense, the limit dynamics of the corresponding
dissipative solutions introduced by P.\,Lions can be described in terms of
attractors of the properly constructed trajectory dynamical system. Moreover,
the convergence of the attractors $\Cal A(\alpha)$ of the regularized system to
the limit trajectory attractor $\Cal A(0)$ as $\alpha\to0$ is also established
in terms of the upper semicontinuity in the properly defined functional space. | 2112.13691v1 |
2023-01-02 | Fast convex optimization via closed-loop time scaling of gradient dynamics | In a Hilbert setting, for convex differentiable optimization, we develop a
general framework for adaptive accelerated gradient methods. They are based on
damped inertial dynamics where the coefficients are designed in a closed-loop
way. Specifically, the damping is a feedback control of the velocity, or of the
gradient of the objective function. For this, we develop a closed-loop version
of the time scaling and averaging technique introduced by the authors. We thus
obtain autonomous inertial dynamics which involve vanishing viscous damping and
implicit Hessian driven damping. By simply using the convergence rates for the
continuous steepest descent and Jensen's inequality, without the need for
further Lyapunov analysis, we show that the trajectories have several
remarkable properties at once: they ensure fast convergence of values, fast
convergence of the gradients towards zero, and they converge to optimal
solutions. Our approach leads to parallel algorithmic results, that we study in
the case of proximal algorithms. These are among the very first general results
of this type obtained using autonomous dynamics. | 2301.00701v1 |
2023-04-22 | Video analysis of the damped oscillations of Pohl's pendulum | In this paper problems that arose with the introduction of distance learning
in physics at the Technical University of Sofia due to the COVID-19 pandemic
and the imposition of video recording of laboratory exercises are indicated. It
was found that the video for the ''Damped Mechanical Oscillations'' exercise
provides enough information for a more detailed and in-depth analysis of the
studied phenomenon compared to the standard way of capturing the data. The
Video Editor program was used to view the video frame by frame and statistical
processing - non-linear regression - was performed with the recorded data. The
laboratory results are compared with the theoretical function, the parameters
of which are optimized as a result of the specified processing. A theoretical
model of the damped oscillation is described and the dependence of the damping
coefficient on the current through the electromagnetic brake is theoretically
investigated. | 2304.11390v1 |
2023-11-23 | Friction of a driven chain: Role of momentum conservation, Goldstone and radiation modes | We analytically study friction and dissipation of a driven bead in a 1D
harmonic chain, and analyze the role of internal damping mechanism as well as
chain length. Specifically, we investigate Dissipative Particle Dynamics and
Langevin Dynamics, as paradigmatic examples that do and do not display
translational symmetry, with distinct results: For identical parameters, the
friction forces can differ by many orders of magnitude. For slow driving, a
Goldstone mode traverses the entire system, resulting in friction of the driven
bead that grows arbitrarily large (Langevin) or gets arbitrarily small
(Dissipative Particle Dynamics) with system size. For a long chain, the
friction for DPD is shown to be bound, while it shows a singularity (i.e. can
be arbitrarily large) for Langevin damping. For long underdamped chains, a
radiation mode is recovered in either case, with friction independent of
damping mechanism. For medium length chains, the chain shows the expected
resonant behavior. At the resonance, friction is non-analytic in damping
parameter $\gamma$, depending on it as $\gamma^{-1}$. Generally, no zero
frequency bulk friction coefficient can be determined, as the limits of small
frequency and infinite chain length do not commute, and we discuss the regimes
where "simple" macroscopic friction occurs. | 2311.14075v1 |
2023-12-14 | Smoluchowski-Kramers diffusion approximation for systems of stochastic damped wave equations with non-constant friction | We consider systems of damped wave equations with a state-dependent damping
coefficient and perturbed by a Gaussian multiplicative noise. Initially, we
investigate their well-posedness, under quite general conditions on the
friction. Subsequently, we study the validity of the so-called
Smoluchowski-Kramers diffusion approximation. We show that, under more
stringent conditions on the friction, in the small-mass limit the solution of
the system of stochastic damped wave equations converges to the solution of a
system of stochastic quasi-linear parabolic equations. In this convergence, an
additional drift emerges as a result of the interaction between the noise and
the state-dependent friction. The identification of this limit is achieved by
using a suitable generalization of the classical method of perturbed test
functions, tailored to the current infinite dimensional setting. | 2312.08925v1 |
2024-03-25 | Radiation damping of a Rayleigh scatterer illuminated by a plane wave | We investigate the radiation damping experienced by a dielectric spherical
particle when it is illuminated by an electromagnetic plane wave within the
Rayleigh regime. We derive the equivalent electric dipole of the moving
particle and subsequently calculate the electromagnetic force acting on it from
two different approaches. In the first approach, we calculate the force from
the integration of stress tensor and field momentum. In the second one, we
calculate the force directly from the integration of the force density. Our
derivations reveal that the damping coefficient is equal to $6P_{scat}/mc^2$
along the propagation direction, whereas it is $P_{scat}/mc^2$ along
perpendicular directions. Here, $P_{scat}$ denotes the power scattered by the
particle, and $mc^2$ represents the particle's mass energy. The radiation
damping derived in this study sets upper limits on the quality factor of
optically levitated objects and ensures the existence of a steady-state
solution of the particle's dynamics. | 2403.16618v1 |
2022-06-30 | Effect of a viscous fluid shell on the propagation of gravitational waves | In this paper we show that there are circumstances in which the damping of
gravitational waves (GWs) propagating through a viscous fluid can be highly
significant; in particular, this applies to Core Collapse Supernovae (CCSNe).
In previous work, we used linearized perturbations on a fixed background within
the Bondi-Sachs formalism, to determine the effect of a dust shell on GW
propagation. Here, we start with the (previously found) velocity field of the
matter, and use it to determine the shear tensor of the fluid flow. Then, for a
viscous fluid, the energy dissipated is calculated, leading to an equation for
GW damping. It is found that the damping effect agrees with previous results
when the wavelength $\lambda$ is much smaller than the radius $r_i$ of the
matter shell; but if $\lambda\gg r_i$, then the damping effect is greatly
increased.
Next, the paper discusses an astrophysical application, CCSNe. There are
several different physical processes that generate GWs, and many models have
been presented in the literature. The damping effect thus needs to be evaluated
with each of the parameters $\lambda,r_i$ and the coefficient of shear
viscosity $\eta$, having a range of values. It is found that in most cases
there will be significant damping, and in some cases that it is almost
complete.
We also consider the effect of viscous damping on primordial gravitational
waves (pGWs) generated during inflation in the early Universe. Two cases are
investigated where the wavelength is either much shorter than the shell radii
or much longer; we find that there are conditions that will produce significant
damping, to the extent that the waves would not be detectable. | 2206.15103v2 |
2005-07-15 | A Theory of Physical Quantum Computation: The Quantum Computer Condition | In this paper we present a new unified theoretical framework that describes
the full dynamics of quantum computation. Our formulation allows any questions
pertaining to the physical behavior of a quantum computer to be framed, and in
principle, answered. We refer to the central organizing principle developed in
this paper, on which our theoretical structure is based, as the *Quantum
Computer Condition* (QCC), a rigorous mathematical statement that connects the
irreversible dynamics of the quantum computing machine, with the reversible
operations that comprise the quantum computation intended to be carried out by
the quantum computing machine. Armed with the QCC, we derive a powerful result
that we call the *Encoding No-Go Theorem*. This theorem gives a precise
mathematical statement of the conditions under which fault-tolerant quantum
computation becomes impossible in the presence of dissipation and/or
decoherence. In connection with this theorem, we explicitly calculate a
universal critical damping value for fault-tolerant quantum computation. In
addition we show that the recently-discovered approach to quantum error
correction known as "operator quantum error-correction" (OQEC) is a special
case of our more general formulation. Our approach furnishes what we will refer
to as "operator quantum fault-tolerance" (OQFT). In particular, we show how the
QCC allows one to derive error thresholds for fault tolerance in a completely
general context. We prove the existence of solutions to a class of
time-dependent generalizations of the Lindblad equation. Using the QCC, we also
show that the seemingly different circuit, graph- (including cluster-) state,
and adiabatic paradigms for quantum computing are in fact all manifestations of
a single, universal paradigm for all physical quantum computation. | 0507141v2 |
2018-12-22 | Spin dynamics of $3d$ and $4d$ impurities embedded in prototypical topological insulators | Topological insulators are insulating bulk materials hosting conducting
surface states. Their magnetic doping breaks time-reversal symmetry and
generates numerous interesting effects such as dissipationless transport.
Nonetheless, their dynamical properties are still poorly understood. Here, we
perform a systematic investigation of transverse spin excitations of $3d$ and
$4d$ single impurities embedded in two prototypical topological insulators
(Bi$_2$Te$_3$ and Bi$_2$Se$_3$). The impurity-induced states within the bulk
gap of the topological insulators are found to have a drastic impact on the
spin excitation spectra, resulting in very high lifetimes reaching up to
${microseconds}$. An intuitive picture of the spin dynamics is obtained by
mapping onto a generalized Landau-Lifshitz-Gilbert phenomenological model. The
first quantity extracted from this mapping procedure is the magnetic anisotropy
energy, which is then compared to the one provided by the magnetic force
theorem. This uncovers some difficulties encountered with the latter, which can
provide erroneous results for impurities with a high density of states at the
Fermi energy. Moreover, the Gilbert damping and nutation tensors are obtained.
The nutation effects can lead to a non-negligible shift in the spin excitation
resonance in the high-frequency regime. Finally, we study the impact of the
surface state on the spin dynamics, which may be severely altered due to the
repositioning of the impurity-induced state in comparison to the bulk case. Our
systematic investigation of this series of magnetic impurities sheds light on
their spin dynamics within topological insulators, with implications for
available and future experimental studies as, for instance, on the viability of
using such impurities for solid-state qubits. | 1812.09596v1 |
2016-07-27 | Linear and nonlinear viscoelastic arterial wall models: application on animals | This work deals with the viscoelasticity of the arterial wall and its
influence on the pulse waves. We describe the viscoelasticity by a non-linear
Kelvin-Voigt model in which the coefficients are fitted using experimental time
series of pressure and radius measured on a sheep's arterial network. We
obtained a good agreement between the results of the nonlinear Kelvin-Voigt
model and the experimental measurements. We found that the viscoelastic
relaxation time-defined by the ratio between the viscoelastic coefficient and
the Young's modulus-is nearly constant throughout the network. Therefore, as it
is well known that smaller arteries are stiffer, the viscoelastic coefficient
rises when approaching the peripheral sites to compensate the rise of the
Young's modulus, resulting in a higher damping effect. We incorporated the
fitted viscoelastic coefficients in a nonlinear 1D fluid model to compute the
pulse waves in the network. The damping effect of viscoelasticity on the high
frequency waves is clear especially at the peripheral sites. | 1607.07973v1 |
2018-09-26 | The influence of oscillations on energy estimates for damped wave models with time-dependent propagation speed and dissipation | The aim of this paper is to derive higher order energy estimates for
solutions to the Cauchy problem for damped wave models with time-dependent
propagation speed and dissipation. The model of interest is \begin{equation*}
u_{tt}-\lambda^2(t)\omega^2(t)\Delta u +\rho(t)\omega(t)u_t=0, \quad
u(0,x)=u_0(x), \,\, u_t(0,x)=u_1(x). \end{equation*} The coefficients
$\lambda=\lambda(t)$ and $\rho=\rho(t)$ are shape functions and
$\omega=\omega(t)$ is an oscillating function. If $\omega(t)\equiv1$ and
$\rho(t)u_t$ is an "effective" dissipation term, then $L^2-L^2$ energy
estimates are proved in [2]. In contrast, the main goal of the present paper is
to generalize the previous results to coefficients including an oscillating
function in the time-dependent coefficients. We will explain how the interplay
between the shape functions and oscillating behavior of the coefficient will
influence energy estimates. | 1809.10179v2 |
2009-01-12 | Nonlinear Wigner solid transport over superfluid helium under AC conditions | Nonlinear transport properties of the two-dimensional Wigner solid of surface
electrons on superfluid helium are studied for alternating current conditions.
For time-averaged quantities like Fourier coefficients, the field-velocity
characteristics are shown to be qualitatively different as compared to that
found in the DC theory. For a spatially uniform current we found a general
solution for the field-velocity relationship which appears to be strongly
dependent on the current frequency. If the current frequency is much lower than
the ripplon damping parameter, the Bragg-Cherenkov resonances which appear at
high enough drift velocities acquire a distinctive saw-tooth shape with long
right-side tails independent of small damping. For current frequencies which
are close or higher than the ripplon damping coefficient, the interference of
ripplons excited at different time intervals results in a new oscillatory (in
drift velocity) regime of Bragg-Cherenkov scattering. | 0901.1508v1 |
2012-10-03 | Effect of temperature and velocity on superlubricity | We study the effects of temperature and sliding velocity on superlubricity in
numerical simulations of the Frenkel-Kontorova model. We show that resonant
excitations of the phonons in an incommensurate sliding body lead to an
effective friction and to thermal equilibrium with energy distributed over the
internal degrees of freedom. For finite temperature, the effective friction can
be described well by a viscous damping force, with a damping coefficient that
emerges naturally from the microscopic dynamics. This damping coefficient is a
non-monotonic function of the sliding velocity which peaks around resonant
velocities and increases with temperature. At low velocities, it remains finite
and nonzero, indicating the preservation of superlubricity in the zero-velocity
limit. Finally, we propose experimental systems in which our results could be
verified. | 1210.1124v1 |
2015-04-09 | Periodic-coefficient damping estimates, and stability of large-amplitude roll waves in inclined thin film flow | A technical obstruction preventing the conclusion of nonlinear stability of
large-Froude number roll waves of the St. Venant equations for inclined thin
film flow is the "slope condition" of Johnson-Noble-Zumbrun, used to obtain
pointwise symmetrizability of the linearized equations and thereby
high-frequency resolvent bounds and a crucial H s nonlinear damping estimate.
Numerically, this condition is seen to hold for Froude numbers 2 \textless{} F
3.5, but to fail for 3.5 F. As hydraulic engineering applications typically
involve Froude number 3 F 5, this issue is indeed relevant to practical
considerations. Here, we show that the pointwise slope condition can be
replaced by an averaged version which holds always, thereby completing the
nonlinear theory in the large-F case. The analysis has potentially larger
interest as an extension to the periodic case of a type of weighted
"Kawashima-type" damping estimate introduced in the asymptotically-constant
coefficient case for the study of stability of large-amplitude viscous shock
waves. | 1504.02292v1 |
2015-07-24 | Effect of Landau damping on alternative ion-acoustic solitary waves in a magnetized plasma consisting of warm adiabatic ions and non-thermal electrons | Bandyopadhyay and Das [Phys. Plasmas, 9, 465-473, 2002] have derived a
nonlinear macroscopic evolution equation for ion acoustic wave in a magnetized
plasma consisting of warm adiabatic ions and non-thermal electrons including
the effect of Landau damping. In that paper they have also derived the
corresponding nonlinear evolution equation when coefficient of the nonlinear
term of the above mentioned macroscopic evolution equation vanishes, the
nonlinear behaviour of the ion acoustic wave is described by a modified
macroscopic evolution equation. But they have not considered the case when the
coefficient is very near to zero. This is the case we consider in this paper
and we derive the corresponding evolution equation including the effect of
Landau damping. Finally, a solitary wave solution of this macroscopic evolution
is obtained, whose amplitude is found to decay slowly with time. | 1507.06733v1 |
2017-03-28 | Singularity formation for the 1D compressible Euler equation with variable damping coefficient | In this paper, we consider some blow-up problems for the 1D Euler equation
with time and space dependent damping. We investigate sufficient conditions on
initial data and the rate of spatial or time-like decay of the coefficient of
damping for the occurrence of the finite time blow-up. In particular, our
sufficient conditions ensure that the derivative blow-up occurs in finite time
with the solution itself and the pressure bounded. Our method is based on
simple estimates with Riemann invariants. Furthermore, we give sharp lower and
upper estimates of the lifespan of solutions, when initial data are small
perturbations of constant states. | 1703.09821v3 |
2018-06-22 | Optimal Design of Virtual Inertia and Damping Coefficients for Virtual Synchronous Machines | Increased penetration of inverter-connected renewable energy sources (RES) in
the power system has resulted in a decrease in available rotational inertia
which serves as an immediate response to frequency deviation due to
disturbances. The concept of virtual inertia has been proposed to combat this
decrease by enabling the inverters to produce active power in response to a
frequency deviation like a synchronous generator. In this paper, we present an
algorithm to optimally design the inertia and damping coefficient required for
an inverter-based virtual synchronous machine (VSM) to participate efficiently
in the inertia response portion of primary frequency control. We design the
objective function to explicitly trade-off between competing objectives such as
the damping rate the the frequency nadir. Specifically, we formulate the design
problem as a constrained and regularized H2 norm minimization problem, and
develop an efficient gradient algorithm for this non-convex problem. This
proposed algorithm is applied to a test case to demonstrate its performance
against existing methods. | 1806.08488v1 |
Subsets and Splits
No community queries yet
The top public SQL queries from the community will appear here once available.