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2017-04-18 | Critical pairing fluctuations in the normal state of a superconductor: pseudogap and quasi-particle damping | We study the effect of critical pairing fluctuations on the electronic
properties in the normal state of a clean superconductor in three dimensions.
Using a functional renormalization group approach to take the non-Gaussian
nature of critical fluctuations into account, we show microscopically that in
the BCS regime, where the inverse coherence length is much smaller than the
Fermi wavevector, critical pairing fluctuations give rise to a non-analytic
contribution to the quasi-particle damping of order $ T_c \sqrt{Gi} \ln ( 80 /
Gi )$, where the Ginzburg-Levanyuk number $Gi$ is a dimensionless measure for
the width of the critical region. As a consequence, there is a temperature
window above $T_c$ where the quasiparticle damping due to critical pairing
fluctuations can be larger than the usual $T^2$-Fermi liquid damping due to
non-critical scattering processes. On the other hand, in the strong coupling
regime where $Gi$ is of order unity, we find that the quasiparticle damping due
to critical pairing fluctuations is proportional to the temperature. Moreover,
we show that in the vicinity of the critical temperature $T_c$ the electronic
density of states exhibits a fluctuation-induced pseudogap. We also use
functional renormalization group methods to derive and classify various types
of processes induced by the pairing interaction in Fermi systems close to the
superconducting instability. | 1704.05282v2 |
2018-10-16 | The Solution to the Differential Equation with Linear Damping describing a Physical Systems governed by a Cubic Energy Potential | An analytical solution to the nonlinear differential equation describing the
equation of motion of a particle moving in an unforced physical system with
linear damping, governed by a cubic potential well, is presented in terms of
the Jacobi elliptic functions. In the attractive region of the potential the
system becomes an anharmonic damped oscillator, however with asymmetric
displacement. An expression for the period of oscillation is derived, which for
a nonlinear damped system is time dependent, and in particular it contains a
quartic root of an exponentially decaying term in the denominator. Initially
the period is longer as compared to that of a linear oscillator, however
gradually it decreases to that of a linear damped oscillator.
Transforming the undamped nonlinear differential equation into the
differential equation describing orbital motion of planets, the perihelion
advance of Mercury can be estimated to 42.98 arcseconds/century, close to
present day observations of 43.1 arcseconds/century.
Some familiarity with the Jacobi elliptic functions is required, in
particular with respect to the differential behavior of these functions,
however, they are standard functions of advanced mathematical computer algebra
tools. The expression derived for the solution to the nonlinear physical
system, and in particular the expression for the period of oscillation, is
useful for an accurate evaluation of experiments in introductory and advanced
physics labs, but also of interest for specialists working with nonlinear
phenomena governed by the cubic potential well. | 1810.10336v1 |
2019-01-10 | Damping and softening of transverse acoustic phonons in colossal magnetoresistive La$_{0.7}$Ca$_{0.3}$MnO$_3$ and La$_{0.7}$Sr$_{0.3}$MnO$_3$ | Neutron spectroscopy is used to probe transverse acoustic phonons near the
(2, 2, 0) Bragg position in colossal magnetoresistive La0.7Ca0.3MnO3 and
La0.7Sr0.3MnO3. Upon warming to temperatures near Tc = 257 K the phonon peaks
in La0.7Ca0.3MnO3 soften and damp significantly with the phonon half width at
half maximum approaching 2.5 meV for phonons at a reduced wave vector of q =
(0.2, 0.2, 0). Concurrently a quasielastic component develops that dominates
the spectrum near the polaron position at high temperatures. This quasielastic
scattering is ~5 times more intense near Tc than in La0.7Sr0.3MnO3 despite
comparable structural distortions in the two. The damping becomes more
significant near the polaron position with a temperature dependence similar to
that of polaron structural distortions. An applied magnetic field of 9.5 T only
partially reverses the damping and quasielastic component, despite smaller
fields being sufficient to drive the colossal magnetoresistive effect. The
phonon energy, on the other hand, is unaffected by field. The damping in
La0.7Sr0.3MnO3 near Tc at a reduced wave vector of q = (0.25, 0.25, 0) is
significantly smaller but displays a similar trend with an applied magnetic
field. | 1901.03394v1 |
2012-09-15 | Damped kink oscillations of flowing prominence threads | Transverse oscillations of thin threads in solar prominences are frequently
reported in high-resolution observations. Two typical features of the
observations are that the oscillations are damped in time and that simultaneous
mass flows along the threads are detected. Flows cause the dense threads to
move along the prominence magnetic structure while the threads are oscillating.
The oscillations have been interpreted in terms of standing magnetohydrodynamic
(MHD) kink waves of the magnetic flux tubes which support the threads. The
damping is most likely due to resonant absorption caused by plasma
inhomogeneity. The technique of seismology uses the observations combined with
MHD wave theory to estimate prominence physical parameters. This paper presents
a theoretical study of the joint effect of flow and resonant absorption on the
amplitude of standing kink waves in prominence threads. We find that flow and
resonant absorption can either be competing effects on the amplitude or both
can contribute to damp the oscillations depending on the instantaneous position
of the thread within the prominence magnetic structure. The amplitude profile
deviates from the classic exponential profile of resonantly damped kink waves
in static flux tubes. Flow also introduces a progressive shift of the
oscillation period compared to the static case, although this effect is in
general of minor importance. We test the robustness of seismological estimates
by using synthetic data aiming to mimic real observations. The effect of the
thread flow can significantly affect the estimation of the transverse
inhomogeneity length scale. The presence of random background noise adds
uncertainty to this estimation. Caution needs to be paid to the seismological
estimates that do not take the influence of flow into account. | 1209.3382v1 |
2010-04-09 | Oscillations of weakly viscous conducting liquid drops in a strong magnetic field | We analyse small-amplitude oscillations of a weakly viscous electrically
conducting liquid drop in a strong uniform DC magnetic field. An asymptotic
solution is obtained showing that the magnetic field does not affect the shape
eigenmodes, which remain the spherical harmonics as in the non-magnetic case.
Strong magnetic field, however, constrains the liquid flow associated with the
oscillations and, thus, reduces the oscillation frequencies by increasing
effective inertia of the liquid. In such a field, liquid oscillates in a
two-dimensional (2D) way as solid columns aligned with the field. Two types of
oscillations are possible: longitudinal and transversal to the field. Such
oscillations are weakly damped by a strong magnetic field - the stronger the
field, the weaker the damping, except for the axisymmetric transversal and
inherently 2D modes. The former are overdamped because of being incompatible
with the incompressibility constraint, whereas the latter are not affected at
all because of being naturally invariant along the field. Since the magnetic
damping for all other modes decreases inversely with the square of the field
strength, viscous damping may become important in a sufficiently strong
magnetic field. The viscous damping is found analytically by a simple energy
dissipation approach which is shown for the longitudinal modes to be equivalent
to a much more complicated eigenvalue perturbation technique. This study
provides a theoretical basis for the development of new measurement methods of
surface tension, viscosity and the electrical conductivity of liquid metals
using the oscillating drop technique in a strong superimposed DC magnetic
field. | 1004.1548v2 |
2017-03-01 | The Plastic Scintillator Detector at DAMPE | he DArk Matter Particle Explorer (DAMPE) is a general purposed
satellite-borne high energy $\gamma-$ray and cosmic ray detector, and among the
scientific objectives of DAMPE are the searches for the origin of cosmic rays
and an understanding of Dark Matter particles. As one of the four detectors in
DAMPE, the Plastic Scintillator Detector (PSD) plays an important role in the
particle charge measurement and the photons/electrons separation. The PSD has
82 modules, each consists of a long organic plastic scintillator bar and two
PMTs at both ends for readout, in two layers and covers an overall active area
larger than 82 cm $\times$ 82 cm. It can identify the charge states for
relativistic ions from H to Fe, and the detector efficiency for Z=1 particles
can reach 0.9999. The PSD has been successfully launched with DAMPE on Dec. 17,
2015. In this paper, the design, the assembly, the qualification tests of the
PSD and some of the performance measured on the ground have been described in
detail. | 1703.00098v1 |
2017-03-22 | Observation of a strong coupling effect on electron-ion collisions in ultracold plasmas | Ultracold plasmas (UCP) provide a well-controlled system for studying
multiple aspects in plasma physics that include collisions and strong coupling
effects. By applying a short electric field pulse to a UCP, a plasma electron
center-of-mass (CM) oscillation can be initiated. In accessible parameter
ranges, the damping rate of this oscillation is determined by the electron-ion
collision rate. We performed measurements of the oscillation damping rate with
such parameters and compared the measured rates to both a molecular dynamic
(MD) simulation that includes strong coupling effects and to Monte-Carlo
collisional operator simulation designed to predict the damping rate including
only weak coupling considerations. We found agreement between experimentally
measured damping rate and the MD result. This agreement did require including
the influence of a previously unreported UCP heating mechanism whereby the
presence of a DC electric field during ionization increased the electron
temperature, but estimations and simulations indicate that such a heating
mechanism should be present for our parameters. The measured damping rate at
our coldest electron temperature conditions was much faster than the weak
coupling prediction obtained from the Monte-Carlo operator simulation, which
indicates the presence of significant strong coupling influence. The density
averaged electron strong coupling parameter $\Gamma$ measured at our coldest
electron temperature conditions was 0.35. | 1703.07852v2 |
2019-09-11 | Critical corrections to formulations of nonlinear energy dissipation of ultrasonically excited bubbles and a unifying parameter to asses and enhance bubble activity in applications | Nonlinear oscillations of bubbles can significantly increase the attenuation
of the host media. Optimization of bubble related applications needs a
realistic estimation of the medium attenuation and bubble activity. A correct
estimation of the wave attenuation in bubbly media requires an accurate
estimation of the power dissipated by nonlinear oscillations of bubbles.
Pioneering work of Louisnard \cite{1} meticulously derived the nonlinear energy
terms for viscous and thermal damping; however, radiation damping arising from
the compressibility of the liquid was neglected. Jamshidi $\&$ Brenner \cite{2}
have considered the effects of the compressibility of the liquid and showed
that damping due to radiation becomes the most significant factor at pressures
above the blake threshold. Despite the improvement in their formulation;
however, the radiation damping term estimates non-physical values for some
frequency and pressure regions including near resonance oscillations. Thus, the
new terms arising from the compressibility of the liquid needs critical
assessment. In this work, we provide critical corrections to the present
formulations. Importance of the new corrections are highlighted by the
scattering to damping ratio (STDR). We then introduce a unifying parameter to
assess the efficacy of applications; this parameter is defined as the
multiplication of maximum scattered pressure by STDR. | 1909.04864v1 |
2019-09-14 | Measurement-Based Wide-Area Damping of Inter-Area Oscillations based on MIMO Identification | Interconnected power grid exhibits oscillatory response after a disturbance
in the system. One such type of oscillations, the inter-area oscillations has
the oscillation frequency in the range of 0.1 to 1 Hz. The damping of
inter-area oscillations is difficult with local controllers, but it can be
achieved using a Wide Area Damping Controller (WADC). For effective control,
the input to the WADC should be the most observable signal and the WADC output
should be sent to the most controllable generator. This paper presents a
measurement-based novel algorithm for multi-input-multi-output (MIMO) transfer
function identification of the power system based on optimization to estimate
such oscillation frequencies. Based on the MIMO transfer function the optimal
control loop for WADC is estimated. The WADC design is based on the discrete
linear quadratic regulator (DLQR) and Kalman filtering for damping of
inter-area oscillations. Since the MIMO identification is based on actual
measurements, the proposed method can accurately monitor changes in the power
grid whereas the conventional methods are based on small-signal analysis of a
linearized model which does not consider changing operating conditions. The
overall algorithm is implemented and validated on a RTDS/RSCAD and MATLAB
real-time co-simulation platform using two-area and IEEE 39 bus power system
models. | 1909.06687v1 |
2020-03-16 | Spin-orbit torques originating from bulk and interface in Pt-based structures | We investigated spin-orbit torques in prototypical Pt-based spintronic
devices. We found that, in Pt/Ni and Pt/Fe bilayers, the damping-like torque
efficiency depends on the thickness of the Pt layer. We also found that the
damping-like torque efficiency is almost identical in the Pt/Ni and Pt/Fe
bilayers despite the stronger spin memory loss at the Pt/Fe interface. These
results suggest that although the dominant source of the damping-like torque is
the bulk spin Hall effect in the Pt layer, a sizable damping-like torque is
generated by the interface in the Pt/Fe bilayer due to the stronger interfacial
spin-orbit coupling. In contrast to the damping-like torque, whose magnitude
and sign are almost identical in the Pt/Ni and Pt/Fe bilayers, the field-like
torque strongly depends on the choice of the ferromagnetic layer. The sign of
the field-like torque originating from the bulk spin Hall effect in the Pt
layer is opposite between the Pt/Ni and Pt/Fe bilayers, which can be attributed
to the opposite sign of the imaginary part of the spin-mixing conductance.
These results demonstrate that the spin-orbit torques are quite sensitive to
the electronic structure of the FM layer. | 2003.07271v2 |
2020-03-23 | Heat-like and wave-like lifespan estimates for solutions of semilinear damped wave equations via a Kato's type lemma | In this paper we study several semilinear damped wave equations with
"subcritical" nonlinearities, focusing on demonstrating lifespan estimates for
energy solutions. Our main concern is on equations with scale-invariant damping
and mass. Under different assumptions imposed on the initial data, lifespan
estimates from above are clearly showed. The key fact is that we find
"transition surfaces", which distinguish lifespan estimates between "wave-like"
and "heat-like" behaviours. Moreover we conjecture that the lifespan estimates
on the "transition surfaces" can be logarithmically improved. As direct
consequences, we reorganize the blow-up results and lifespan estimates for the
massless case in which the "transition surfaces" degenerate to "transition
curves". Furthermore, we obtain improved lifespan estimates in one space
dimension, comparing to the known results. We also study semilinear wave
equations with the scattering damping and negative mass term, and find that if
the decay rate of the mass term equals to 2, the lifespan estimate is the same
as one special case of the equations with the scale-invariant damping and
positive mass. The main strategy of the proof consists of a Kato's type lemma
in integral form, which is established by iteration argument. | 2003.10578v1 |
2018-05-26 | Critical collapse of ultra-relativistic fluids: damping or growth of aspherical deformations | We perform fully nonlinear numerical simulations to study aspherical
deformations of the critical self-similar solution in the gravitational
collapse of ultra-relativistic fluids. Adopting a perturbative calculation,
Gundlach predicted that these perturbations behave like damped or growing
oscillations, with the frequency and damping (or growth) rates depending on the
equation of state. We consider a number of different equations of state and
degrees of asphericity and find very good agreement with the findings of
Gundlach for polar $\ell = 2$ modes. For sufficiently soft equations of state,
the modes are damped, meaning that, in the limit of perfect fine-tuning, the
spherically symmetric critical solution is recovered. We find that the degree
of asphericity has at most a small effect on the frequency and damping
parameter, or on the critical exponents in the power-law scalings. Our findings
also confirm, for the first time, Gundlach's prediction that the $\ell = 2$
modes become unstable for sufficiently stiff equations of state. In this regime
the spherically symmetric self-similar solution can no longer be recovered by
fine-tuning to the black-hole threshold, and one can no longer expect power-law
scaling to hold to arbitrarily small scales. | 1805.10442v1 |
2019-03-13 | Inference of magnetic field strength and density from damped transverse coronal waves | A classic application of coronal seismology uses transverse oscillations of
waveguides to obtain estimates of the magnetic field strength. The procedure
requires information on the density of the structures. Often, it ignores the
damping of the oscillations. We computed marginal posteriors for parameters
such as the waveguide density; the density contrast; the transverse
inhomogeneity length-scale; and the magnetic field strength, under the
assumption that the oscillations can be modelled as standing
magnetohydrodynamic (MHD) kink modes damped by resonant absorption. Our results
show that the magnetic field strength can be properly inferred, even if the
densities inside and outside the structure are largely unknown. Incorporating
observational estimates of plasma density further constrains the obtained
posteriors. The amount of information one is willing to include (a priori) for
the density and the density contrast influences their corresponding posteriors,
but very little the inferred magnetic field strength. The decision to include
or leave out the information on the damping and the damping time-scales have a
minimal impact on the obtained magnetic field strength. In contrast to the
classic method which provides with numerical estimates with error bars or
possible ranges of variation for the magnetic field strength, Bayesian methods
offer the full distribution of plausibility over the considered range of
possible values. The methods are applied to available datasets of observed
transverse loop oscillations, can be extended to prominence fine structures or
chromospheric spicules and implemented to propagating waves in addition to
standing oscillations. | 1903.05437v1 |
2019-03-14 | A metal-poor damped Ly-alpha system at redshift 6.4 | We identify a strong Ly-alpha damping wing profile in the spectrum of the
quasar P183+05 at z=6.4386. Given the detection of several narrow metal
absorption lines at z=6.40392, the most likely explanation for the absorption
profile is that it is due to a damped Ly-alpha system. However, in order to
match the data a contribution of an intergalactic medium 5-38% neutral or
additional weaker absorbers near the quasar is also required. The absorption
system presented here is the most distant damped Ly-alpha system currently
known. We estimate an HI column density ($10^{20.68\pm0.25}\,$cm$^{-2}$),
metallicity ([O/H]$=-2.92\pm 0.32$), and relative chemical abundances of a
system consistent with a low-mass galaxy during the first Gyr of the universe.
This object is among the most metal-poor damped Ly-alpha systems known and,
even though it is observed only ~850 Myr after the big bang, its relative
abundances do not show signatures of chemical enrichment by Population III
stars. | 1903.06186v2 |
2019-04-30 | DmpIRFs and DmpST: DAMPE Instrument Response Functions and Science Tools for Gamma-Ray Data Analysis | GeV gamma ray is an important observation target of DArk Matter Particle
Explorer (DAMPE) for indirect dark matter searching and high energy
astrophysics. We present in this work a set of accurate instrument response
functions of DAMPE (DmpIRFs) including the effective area, point-spread
function and energy dispersion that are crucial for the gamma-ray data analysis
based on the high statistics simulation data. A dedicated software named DmpST
is developed to facilitate the scientific analyses of DAMPE gamma-ray data.
Considering the limited number of photons and the angular resolution of DAMPE,
the maximum likelihood method is adopted in the DmpST to better disentangle
different source components. The basic mathematics and the framework regarding
this software are also introduced in this paper. | 1904.13098v1 |
2019-05-14 | Fractional damping through restricted calculus of variations | We deliver a novel approach towards the variational description of Lagrangian
mechanical systems subject to fractional damping by establishing a restricted
Hamilton's principle. Fractional damping is a particular instance of non-local
(in time) damping, which is ubiquitous in mechanical engineering applications.
The restricted Hamilton's principle relies on including fractional derivatives
to the state space, the doubling of curves (which implies an extra mirror
system) and the restriction of the class of varied curves. We will obtain the
correct dynamics, and will show rigorously that the extra mirror dynamics is
nothing but the main one in reversed time; thus, the restricted Hamilton's
principle is not adding extra physics to the original system. The price to pay,
on the other hand, is that the fractional damped dynamics is only a sufficient
condition for the extremals of the action. In addition, we proceed to
discretise the new principle. This discretisation provides a set of numerical
integrators for the continuous dynamics that we denote Fractional Variational
Integrators (FVIs). The discrete dynamics is obtained upon the same
ingredients, say doubling of discrete curves and restriction of the discrete
variations. We display the performance of the FVIs, which have local truncation
order 1, in two examples. As other integrators with variational origin, for
instance those generated by the discrete Lagrange-d'Alembert principle, they
show a superior performance tracking the dissipative energy, in opposition to
direct (order 1) discretisations of the dissipative equations, such as explicit
and implicit Euler schemes. | 1905.05608v1 |
2019-05-22 | Ultra-low magnetic damping in Co 2 Mn-based Heusler compounds: promising materials for spintronic | The prediction of ultra-low magnetic damping in Co 2 MnZ Heusler half-metal
thin-film magnets is explored in this study and the damping response is shown
to be linked to the underlying electronic properties. By substituting the Z
elements in high crystalline quality films (Co 2 MnZ with Z=Si, Ge, Sn, Al, Ga,
Sb), electronic properties such as the minority spin band gap, Fermi energy
position in the gap and spin polarization can be tuned and the consequence on
magnetization dynamics analyzed. The experimental results allow us to directly
explore the interplay of spin polarization, spin gap, Fermi energy position and
the magnetic damping obtained in these films, together with ab initio
calculation predictions. The ultra-low magnetic damping coefficients measured
in the range 4.1 10-4-9 10-4 for Co 2 MnSi, Ge, Sn, Sb are the lowest values
obtained on a conductive layer and offers a clear experimental demonstration of
theoretical predictions on Half-Metal Magnetic Heusler compounds and a pathway
for future materials design. | 1905.08987v1 |
2019-07-16 | Damping of slow magnetoacoustic oscillations by the misbalance between heating and cooling processes in the solar corona | Rapidly decaying slow magnetoacoustic waves are regularly observed in the
solar coronal structures, offering a promising tool for a seismological
diagnostics of the coronal plasma, including its thermodynamical properties.
The effect of damping of standing slow magnetoacoustic oscillations in the
solar coronal loops is investigated accounting for the field-aligned thermal
conductivity and a wave-induced misbalance between radiative cooling and some
unspecified heating rates. The non-adiabatic terms were allowed to be
arbitrarily large, corresponding to the observed values. The thermal
conductivity was taken in its classical form, and a power-law dependence of the
heating function on the density and temperature was assumed. The analysis was
conducted in the linear regime and in the infinite magnetic field
approximation. The wave dynamics is found to be highly sensitive to the
characteristic time scales of the thermal misbalance. Depending on certain
values of the misbalance time scales three regimes of the wave evolution were
identified, namely the regime of a suppressed damping, enhanced damping where
the damping rate drops down to the observational values, and acoustic
over-stability. The specific regime is determined by the dependences of the
radiative cooling and heating functions on thermodynamical parameters of the
plasma in the vicinity of the perturbed thermal equilibrium. The comparison of
the observed and theoretically derived decay times and oscillation periods
allows us to constrain the coronal heating function. For typical coronal
parameters, the observed properties of standing slow magnetoacoustic
oscillations could be readily reproduced with a reasonable choice of the
heating function. | 1907.07051v1 |
2019-07-29 | Breather arrest in a chain of damped oscillators with Hertzian contact | We explore breather propagation in the damped oscillatory chain with
essentially nonlinear (non-linearizable) nearest-neighbour coupling.
Combination of the damping and the substantially nonlinear coupling leads to
rather unusual two-stage pattern of the breather propagation. The first stage
occurs at finite fragment of the chain and is characterized by power-law decay
of the breather amplitude. The second stage is characterized by extremely small
breather amplitudes that decay hyper-exponentially with the site number. Thus,
practically, one can speak about finite penetration depth of the breather. This
phenomenon is referred to as breather arrest (BA). As particular example, we
explore the chain with Hertzian contacts. Dependencies of the breather
penetration depth on the initial excitation and on the damping coefficient on
the breather penetration depth obey power laws. The results are rationalized by
considering beating responses in a system of two damped linear oscillators with
strongly nonlinear (non-linearizable) coupling. Initial excitation of one of
these oscillators leads to strictly finite number of beating cycles. Then, the
beating cycle in this simplified system is associated with the passage of the
discrete breather between the neighbouring sites in the chain. Somewhat
surprisingly, this simplified model reliably predicts main quantitative
features of the breather arrest in the chain, including the exponents in
numerically observed power laws. | 1907.12462v1 |
2020-09-03 | Dynamics of magnetic collective modes in the square and triangular lattice Mott insulators at finite temperature | We study the equilibrium dynamics of magnetic moments in the Mott insulating
phase of the Hubbard model on the square and triangular lattice. We rewrite the
Hubbard interaction in terms of an auxiliary vector field and use a recently
developed Langevin scheme to study its dynamics. A thermal `noise', derivable
approximately from the Keldysh formalism, allows us to study the effect of
finite temperature. At strong coupling, $U \gg t$, where $U$ is the local
repulsion and $t$ the nearest neighbour hopping, our results reproduce the well
known dynamics of the nearest neighbour Heisenberg model with exchange $J \sim
{\cal O}(t^2/U)$. These include crossover from weakly damped dispersive modes
at temperature $T \ll J$ to strong damping at $T \sim {\cal O}(J)$, and
diffusive dynamics at $T \gg J$. The crossover temperatures are naturally
proportional to $J$. To highlight the progressive deviation from Heisenberg
physics as $U/t$ reduces we compute an effective exchange scale $J_{eff}(U)$
from the low temperature spin wave velocity. We discover two features in the
dynamical behaviour with decreasing $U/t$: (i)~the low temperature dispersion
deviates from the Heisenberg result, as expected, due to longer range and
multispin interactions, and (ii)~the crossovers between weak damping, strong
damping, and diffusion take place at noticeably lower values of $T/J_{eff}$. We
relate this to enhanced mode coupling, in particular to thermal amplitude
fluctuations, at weaker $U/t$. A comparison of the square and triangular
lattice reveals the additional effect of geometric frustration on damping. | 2009.01833v2 |
2020-09-15 | Classification of the mechanisms of wave energy dissipation in the nonlinear oscillations of coated and uncoated bubbles | Acoustic waves are dissipated when they pass through bubbly media.
Dissipation by bubbles takes place through thermal damping (Td), radiation
damping (Rd) and damping due to the friction of the liquid (Ld) and friction of
the coating (Cd). Knowledge of the contributions of the Td, Rd, Ld and Cd
during nonlinear bubble oscillations will help in optimizing bubble and
ultrasound exposure parameters for the relevant applications by maximizing a
desirable parameter. In this work we investigate the mechanisms of dissipation
in bubble oscillations and their contribution to the total damping (Wtotal) in
various nonlinear regimes. By using bifurcation analysis, we have classified
nonlinear dynamics of bubbles that are sonicated with their 3rd superharmonic
(SuH) and 2nd SuH resonance frequency (fr), pressure dependent resonance
frequency (PDfr), fr, subharmonic (SH) resonance (fsh=2fr), pressure dependent
SH resonance (PDfsh) and 1/3 order SH resonance. The corresponding Td, Rd, Ld,
Cd, Wtotal, scattering to dissipation ratio (STDR), maximum wall velocity and
maximum back-scattered pressure from non-destructive oscillations of bubbles
were calculated and analyzed using the bifurcation diagrams. We classified
different regimes of dissipation and provided parameter regions in which a
particular parameter of interest (e.g. Rd) can be enhanced. Afterwards enhanced
bubble activity is linked to some relevant applications in ultrasound. This
paper represents the first comprehensive analysis of the nonlinear oscillations
regimes and the corresponding damping mechanisms. | 2009.07380v1 |
2020-11-18 | The effect of redshift degeneracy and the damping effect of viscous medium on the information extracted from gravitational wave signals | Considering the cosmological redshift $z_c$ , the mass of GW source extracted
from GW signal is $1+z_c$ times larger than its intrinsic value, and distance
between detector and GW source should be regarded as luminosity distance.
However, besides cosmological redshift, there are other kinds of redshifts
should be considered, which is actually ignored, in the analysis of GW data,
such as Doppler redshift and gravitational redshift, so the parameters
extracted from GW may deviate from their intrinsic values. Another factor that
may affect GW is the viscous medium in propagation path of GW, which may damp
the GW with a damping rate of $16{\pi}G{\eta}$. Some studies indicate dark
matter may interact with each other, thus dark matter may be the origin of
viscosity of cosmic medium. Then the GW may be rapidly damped by the viscous
medium that is made of dark matter, such as dark matter "mini-spike" around
intermediate mass black hole. In this article, we mainly discuss how Doppler
and gravitational redshift, together with the damping effect of viscous medium,
affect the information, such as the mass and redshift of GW source, extracted
from GW signals. | 2011.09169v2 |
2020-12-28 | On the Efficacy of Friction Damping in the Presence of Nonlinear Modal Interactions | This work addresses friction-induced modal interactions in jointed
structures, and their effects on the passive mitigation of vibrations by means
of friction damping. Under the condition of (nearly) commensurable natural
frequencies, the nonlinear character of friction can cause so-called nonlinear
modal interactions. If harmonic forcing near the natural frequency of a
specific mode is applied, for instance, another mode may be excited due to
nonlinear energy transfer and thus contribute considerably to the vibration
response. We investigate how this phenomenon affects the performance of
friction damping. To this end, we study the steady-state, periodic forced
vibrations of a system of two beams connected via a local mechanical friction
joint. The system can be tuned to continuously adjust the ratio between the
first two natural frequencies in the range around the $1:3$ internal resonance,
in order to trigger or suppress the emergence of modal interactions. Due to the
re-distribution of the vibration energy, the vibration level can in fact be
reduced in certain situations. However, in other situations, the multi-harmonic
character of the vibration has detrimental effects on the effective damping
provided by the friction joint. The resulting response level can be
significantly larger than in the absence of modal interactions. Moreover, it is
shown that the vibration behavior is highly sensitive in the neighborhood of
internal resonances. It is thus concluded that the condition of internal
resonance should be avoided in the design of friction-damped systems. | 2101.03232v1 |
2021-06-30 | Origin of Nonlinear Damping due to Mode Coupling in Auto-Oscillatory Modes Strongly Driven by Spin-Orbit Torque | We investigate the physical origin of nonlinear damping due to mode coupling
between several auto-oscillatory modes driven by spin-orbit torque in
constricted Py/Pt heterostructures by examining the dependence of
auto-oscillation on temperature and applied field orientation. We observe a
transition in the nonlinear damping of the auto-oscillation modes extracted
from the total oscillation power as a function of drive current, which
coincides with the onset of power redistribution amongst several modes and the
crossover from linewidth narrowing to linewidth broadening in all individual
modes. This indicates the activation of another relaxation process by nonlinear
magnon-magnon scattering within the modes. We also find that both nonlinear
damping and threshold current in the mode-interaction damping regime at high
drive current after transition are temperature independent, suggesting that the
mode coupling occurs dominantly through a non-thermal magnon scattering process
via a dipole or exchange interaction rather than thermally excited
magnon-mediated scattering. This finding presents a promising pathway to
overcome the current limitations of efficiently controlling the interaction
between two highly nonlinear magnetic oscillators to prevent mode crosstalk or
inter-mode energy transfer and deepens understanding of complex nonlinear spin
dynamics in multimode spin wave systems. | 2107.00150v2 |
2021-07-15 | On the long-time behavior for a damped Navier-Stokes-Bardina model | In this paper, we consider a damped Navier-Stokes-Bardina model posed on the
whole three-dimensional. These equations have an important physical motivation
and they arise from some oceanic model. From the mathematical point of view,
they write down as the well-know Navier-Stokes equations with an additional
nonlocal operator in their nonlinear transport term, and moreover, with an
additional damping term depending of a parameter $\beta>0$. We study first the
existence and uniqueness of global in time weak solutions in the energy space.
Thereafter, our main objective is to describe the long time behavior of these
solutions. For this, we use some tools in the theory of dynamical systems to
prove the existence of a global attractor, which is a compact subset in the
energy space attracting all the weak solutions when the time goes to infinity.
Moreover, we derive an upper bound for the fractal dimension of the global
attractor associated to these equations.
Finally, we find a range of values for the damping parameter $\beta>0$, where
we are able to give an acutely description of the internal structure of the
global attractor. More precisely, we prove that the global attractor only
contains the stationary (time-independing) solution of the damped
Navier-Stokes-Bardina equations. | 2107.07070v2 |
2021-07-17 | Plasmon-Exciton Coupling Effect on Plasmon Damping | Plasmon decay via the surface or interface is a critical process for
practical energy conversion and plasmonic catalysis. However, the relationship
between plasmon damping and the coupling between the plasmon and 2D materials
is still unclear. The spectral splitting due to plasmon-exciton interaction
impedes the conventional single-particle method to evaluate the plasmon damping
rate by the spectral linewidth directly. Here, we investigated the interaction
between a single gold nanorod (GNR) and 2D materials using the single-particle
spectroscopy method assisted with in situ nanomanipulation technique by
comparing scattering intensity and linewidth together. Our approach allows us
to indisputably identify that the plasmon-exciton coupling in the GNR-WSe2
hybrid would induce plasmon damping. We can also isolate the contribution
between the charge transfer channel and resonant energy transfer channel for
the plasmon decay in the GNR-graphene hybrid by comparing that with thin hBN
layers as an intermediate medium to block the charge transfer. We find out that
the contact layer between the GNR and 2D materials contributes most of the
interfacial plasmon damping. These findings contribute to a deep understanding
of interfacial excitonic effects on the plasmon and 2D materials hybrid. | 2107.08230v1 |
2021-10-12 | Outflows in the presence of cosmic rays and waves with cooling | Plasma outflow from a gravitational potential well with cosmic rays and
self-excited Alfv\'en waves with cooling and wave damping is studied in the
hydrodynamics regime. We study outflows in the presence of cosmic ray and
Alfv\'en waves including the effect of cooling and wave damping. We seek
physically allowable steady-state subsonic-supersonic transonic solutions. We
adopted a multi-fluid hydrodynamical model for the cosmic ray plasma system.
Thermal plasma, cosmic rays, and self-excited Alfv\'en waves are treated as
fluids. Interactions such as cosmic-ray streaming instability, cooling, and
wave damping were fully taken into account. We considered one-dimensional
geometry and explored steady-state solutions. The model is reduced to a set of
ordinary differential equations, which we solved for subsonic-supersonic
transonic solutions with given boundary conditions at the base of the
gravitational potential well. We find that physically allowable
subsonic-supersonic transonic solutions exist for a wide range of parameters.
We studied the three-fluid system (considering only forward-propagating
Alfv\'en waves) in detail. We examined the cases with and without cosmic ray
diffusion separately. Comparisons of solutions with and without cooling and
with and without wave damping for the same set of boundary conditions (on
density, pressures of thermal gas, cosmic rays and waves) are presented. We
also present the interesting case of a four-fluid system (both forward- and
backward-propagating Alfv\'en waves are included), highlighting the intriguing
relation between different components. | 2110.06170v1 |
2021-11-19 | Finite time extinction for a class of damped Schr{ö}dinger equations with a singular saturated nonlinearity | We present some sharper finite extinction time results for solutions of a
class of damped nonlinear Schr{\"o}dinger equations when the nonlinear damping
term corresponds to the limit cases of some ``saturating non-Kerr law''
$F(|u|^2)u=\frac{a}{\varepsilon+(|u|^2)^\alpha}u,$ with $a\in\mathbb{C},$
$\varepsilon\geqslant0,$ $2\alpha=(1-m)$ and $m\in[0,1).$ To carry out the
improvement of previous results in the literature we present in this paper a
careful revision of the existence and regularity of weak solutions under very
general assumptions on the data. We prove that the problem can be solved in the
very general framework of the maximal monotone operators theory, even under a
lack of regularity of the damping term. This allows us to consider, among other
things, the singular case $m=0.$ We replace the above approximation of the
damping term by a different one which keeps the monotonicity for any
$\varepsilon\geqslant0$. We prove that, when $m=0,$ the finite extinction time
of the solution arises for merely bounded right hand side data $f(t,x).$ This
is specially useful in the applications in which the Schr{\"o}dinger equation
is coupled with some other functions satisfying some additional equations. | 2111.10136v2 |
2022-01-26 | Effect of Chiral Damping on the dynamics of chiral domain walls and skyrmions | Friction plays an essential role in most physical processes that we
experience in our everyday life. Examples range from our ability to walk or
swim, to setting boundaries of speed and fuel efficiency of moving vehicles. In
magnetic systems, the displacement of chiral domain walls (DW) and skyrmions
(SK) by Spin Orbit Torques (SOT), is also prone to friction. Chiral damping,
the dissipative counterpart of the Dzyaloshinskii Moriya Interaction (DMI),
plays a central role in these dynamics. Despite experimental observation, and
numerous theoretical studies confirming its existence, the influence of chiral
damping on DW and SK dynamics has remained elusive due to the difficulty of
discriminating from DMI. Here we unveil the effect that chiral damping has on
the flow motion of DWs and SKs driven by current and magnetic field. We use a
static in-plane field to lift the chiral degeneracy. As the in-plane field is
increased, the chiral asymmetry changes sign. When considered separately,
neither DMI nor chiral damping can explain the sign reversal of the asymmetry,
which we prove to be the result of their competing effects. Finally, numerical
modelling unveils the non-linear nature of chiral dissipation and its critical
role for the stabilization of moving SKs. | 2201.10742v1 |
2022-01-27 | A Study on Monte Carlo simulation of the radiation environment above GeV at the DAMPE orbit | The Dark Matter Particle Explorer (DAMPE) has been undergoing a stable
on-orbit operation for more than 6 years and acquired observation of over 11
billion events. And a better understanding of the overall radiation environment
on the DAMPE orbit is crucial for both simulation data production and flight
data analysis. In this work, we study the radiation environment at the low
Earth orbit and develop a simulation software package using the framework of
ATMNC3, in which state-of-the-art full 3D models of the Earth's atmospheric and
magnetic-field configurations is integrated. We consider in our Monte Carlo
procedure event-by-event propagation of the cosmic rays in the geomagnetic
field and their interaction with the Earth's atmosphere, focusing on the
particles above GeV that are able to trigger the DAMPE data acquisition system.
We compare the simulation results with the cosmic-ray electrons and positrons
(CREs) flux measurements made by DAMPE. The overall agreement on both the
spectral and angular distribution of the CREs flux demonstrates that our
simulation is well established. Our software package could be of more general
usage for the simulation of the radiation environment at the low Earth orbit of
various altitudes. | 2201.11364v1 |
2022-05-10 | Nonlinear damping quantification from phase-resonant tests under base excitation | The present work addresses the experimental identification of
amplitude-dependent modal parameters (modal frequency, damping ratio, Fourier
coefficients of periodic modal oscillation). Phase-resonant testing has emerged
as an important method for this task, as it substantially reduces the amount of
data required for the identification compared to conventional
frequency-response testing at different excitation/response levels. In the case
of shaker-stinger excitation, the applied excitation force is commonly measured
in order to quantify the amplitude-dependent modal damping ratio from the
phase-resonant test data. In the case of base excitation, however, the applied
excitation force is challenging or impossible to measure. In this work we
develop an original method for damping quantification from phase-resonant
tests. It relies solely on response measurement; it avoids the need to resort
to force measurement. The key idea is to estimate the power provided by the
distributed inertia force imposed by the base motion. We develop both a
model-free and a model-based variant of the method. We validate the developed
method first in virtual experiments of a friction-damped and a geometrically
nonlinear system, and then in a physical experiment involving a thin beam
clamped at both ends via bolted joints. We conclude that the method is highly
robust and provides high accuracy already for a reasonable number of sensors. | 2205.04735v1 |
2022-09-22 | Neutrino Fast Flavor Pendulum. Part 2: Collisional Damping | In compact astrophysical objects, the neutrino density can be so high that
neutrino-neutrino refraction can lead to fast flavor conversion of the kind
$\nu_e \bar\nu_e \leftrightarrow \nu_x \bar\nu_x$ with $x=\mu,\tau$, depending
on the neutrino angle distribution. Previously, we have shown that in a
homogeneous, axisymmetric two-flavor system, these collective solutions evolve
in analogy to a gyroscopic pendulum. In flavor space, its deviation from the
weak-interaction direction is quantified by a variable $\cos\vartheta$ that
moves between $+1$ and $\cos\vartheta_{\rm min}$, the latter following from a
linear mode analysis. As a next step, we include collisional damping of flavor
coherence, assuming a common damping rate $\Gamma$ for all modes. Empirically
we find that the damped pendular motion reaches an asymptotic level of pair
conversion $f=A+(1-A)\cos\vartheta_{\rm min}$ (numerically $A\simeq 0.370$)
that does not depend on details of the angular distribution (except for fixing
$\cos\vartheta_{\rm min}$), the initial seed, nor $\Gamma$. On the other hand,
even a small asymmetry between the neutrino and antineutrino damping rates
strongly changes this picture and can even enable flavor instabilities in
otherwise stable systems. | 2209.11235v3 |
2022-10-12 | Second order two-species systems with nonlocal interactions: existence and large damping limits | We study the mathematical theory of second order systems with two species,
arising in the dynamics of interacting particles subject to linear damping, to
nonlocal forces and to external ones, and resulting into a nonlocal version of
the compressible Euler system with linear damping. Our results are limited to
the $1$ space dimensional case but allow for initial data taken in a
Wasserstein space of probability measures. We first consider the case of smooth
nonlocal interaction potentials, not subject to any symmetry condition, and
prove existence and uniqueness. The concept of solutions relies on a stickiness
condition in case of collisions, in the spirit of previous works in the
literature. The result uses concepts from classical Hilbert space theory of
gradient flows (cf. Brezis [7]) and a trick used in [4]. We then consider a
large-time and large-damping scaled version of our system and prove convergence
to solutions to the corresponding first order system. Finally, we consider the
case of Newtonian potentials -- subject to symmetry of the cross-interaction
potentials -- and external convex potentials. After showing existence in the
sticky particles framework in the spirit of [4], we prove convergence for large
times towards Dirac delta solutions for the two densities. All the results
share a common technical framework in that solutions are considered in a
Lagrangian framework, which allows to estimate the behavior of solutions via
$L^2$ estimates of the pseudo-inverse variables corresponding to the two
densities. In particular, due to this technique, the large-damping result holds
under a rather weak condition on the initial data, which does not require
well-prepared initial velocities. We complement the results with numerical
simulations. | 2210.06162v1 |
2022-10-12 | Stability of the Néel quantum critical point in the presence of Dirac fermions | We investigate the stability of the N\'eel quantum critical point of
two-dimensional quantum antiferromagnets, described by a non-linear $\sigma$
model (NL$\sigma$M), in the presence of a Kondo coupling to $N_f$ flavours of
two-component Dirac fermion fields. The long-wavelength order parameter
fluctuations are subject to Landau damping by electronic particle-hole
fluctuations. Using momentum-shell RG, we demonstrate that the Landau damping
is weakly irrelevant at the N\'eel quantum critical point, despite the fact
that the corresponding self-energy correction dominates over the quadratic
gradient terms in the IR limit. In the ordered phase, the Landau damping
increases under the RG, indicative of damped spin-wave excitations. Although
the Kondo coupling is weakly relevant, sufficiently strong Landau damping
renders the N\'eel quantum critical point quasi-stable for $N_f\ge 4$ and
thermodynamically stable for $N_f<4$. In the latter case, we identify a new
multi-critical point which describes the transition between the N\'eel critical
and Kondo run-away regimes. The symmetry breaking at this fixed point results
in the opening of a gap in the Dirac fermion spectrum. Approaching the
multi-critical point from the disordered phase, the fermionic quasiparticle
residue vanishes, giving rise to non-Fermi-liquid behavior. | 2210.06577v3 |
2022-11-13 | Damping analysis of Floating Offshore Wind Turbine (FOWT): a new control strategy reducing the platform vibrations | In this paper, the coupled dynamics of the floating platform and the WTG
rotor is analysed. In particular, the damping is explicitly derived from the
coupled equations of rotor and floating platform. The analysis of the damping
leads to the study of the instability phenomena and it derives the explicit
conditions that lead to the Non Minimum Phase Zero (NMPZ). Two NMPZs, one
related to the rotor dynamics and the other one to the platform pitch dynamics,
are analysed. The latter is a novelty and it is analysed in this work,
providing the community of an explicit condition for its verification. The
domain of the instability of the platform is explicitly derived from the
coupled system of equations. In the second part of the paper, from the analysis
of the damping of the floating platform, a new strategy for the control of
FOWTs is proposed. This strategy allows one to impose to the controller an
explicit level of damping in the platform pitch motion without changing the
period of platform pitching. Finally the new strategy is compared to the one
without compensation by performing aero-hydro-servo-elastic numerical
simulations of the UMaine IEA15MW FOWT. Generated power, movements, blade pitch
and tower base fatigue are compared showing that the new control strategy can
reduce fatigue in the structure without affecting the power production. | 2211.10362v1 |
2022-11-22 | Universal Dynamics of Damped-Driven Systems: The Logistic Map as a Normal Form for Energy Balance | Damped-driven systems are ubiquitous in engineering and science. Despite the
diversity of physical processes observed in a broad range of applications, the
underlying instabilities observed in practice have a universal characterization
which is determined by the overall gain and loss curves of a given system. The
universal behavior of damped-driven systems can be understood from a
geometrical description of the energy balance with a minimal number of
assumptions. The assumptions on the energy dynamics are as follows: the energy
increases monotonically as a function of increasing gain, and the losses become
increasingly larger with increasing energy, i.e. there are many routes for
dissipation in the system for large input energy. The intersection of the gain
and loss curves define an energy balanced solution. By constructing an
iterative map between the loss and gain curves, the dynamics can be shown to be
homeomorphic to the logistic map, which exhibits a period doubling cascade to
chaos. Indeed, the loss and gain curves allow for a geometrical description of
the dynamics through a simple Verhulst diagram (cobweb plot). Thus irrespective
of the physics and its complexities, this simple geometrical description
dictates the universal set of logistic map instabilities that arise in complex
damped-driven systems. More broadly, damped-driven systems are a class of
non-equilibrium pattern forming systems which have a canonical set of
instabilities that are manifest in practice. | 2211.11748v1 |
2023-01-23 | Optimal Inter-area Oscillation Damping Control: A Transfer Deep Reinforcement Learning Approach with Switching Control Strategy | Wide-area damping control for inter-area oscillation (IAO) is critical to
modern power systems. The recent breakthroughs in deep learning and the broad
deployment of phasor measurement units (PMU) promote the development of
datadriven IAO damping controllers. In this paper, the damping control of IAOs
is modeled as a Markov Decision Process (MDP) and solved by the proposed Deep
Deterministic Policy Gradient (DDPG) based deep reinforcement learning (DRL)
approach. The proposed approach optimizes the eigenvalue distribution of the
system, which determines the IAO modes in nature. The eigenvalues are evaluated
by the data-driven method called dynamic mode decomposition. For a given power
system, only a subset of generators selected by participation factors needs to
be controlled, alleviating the control and computing burdens. A Switching
Control Strategy (SCS) is introduced to improve the transient response of IAOs.
Numerical simulations of the IEEE-39 New England power grid model validate the
effectiveness and advanced performance of the proposed approach as well as its
robustness against communication delays. In addition, we demonstrate the
transfer ability of the DRL model trained on the linearized power grid model to
provide effective IAO damping control in the non-linear power grid model
environment. | 2301.09321v1 |
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 |
2023-04-13 | Centralised Multimode Power Oscillation Damping Controller for Photovoltaic Plants with Communication Delay Compensation | Low-frequency oscillations are an inherent phenomena in transmission networks
and renewable energy plants should be configured to damp them. Commonly, a
centralised controller is used in PV plants to coordinate PV generators via
communication channels. However, the communication systems of PV plants
introduce delays of a stochastic nature that degrade the performance of
centralised control algorithms. Therefore, controllers for oscillation damping
may not operate correctly unless the communication channel characteristics are
not considered and compensated. In this paper, a centralised controller is
proposed for the oscillation damping that uses a PV plant with all the
realistic effects of communication channels taken into consideration. The
communication channels are modelled based on measurements taken in a laboratory
environment. The controller is designed to damp several modes of oscillation by
using the open-loop phase shift compensation. Theoretical developments were
validated in a laboratory using four converters acting as two PV inverters, a
battery and a STATCOM. A real-time processing platform was used to implement
the centralised controller and to deploy the communication infrastructure.
Experimental results show the communication channels impose severe restrictions
on the performance of centralised POD controllers, highlighting the importance
of their accurate modelling and consideration during the controller design
stage. | 2304.06415v1 |
2023-05-09 | Glassy heat capacity from overdamped phasons and a hypothetical phason-induced superconductivity in incommensurate structures | Phasons are collective low-energy modes that appear in disparate condensed
matter systems such as quasicrystals, incommensurate structures, fluctuating
charge density waves, and Moir\'e superlattices. They share several
similarities with acoustic phonon modes, but they are not protected by any
exact translational symmetry. As a consequence, they are subject to a
wavevector independent damping, and they develop a finite pinning frequency,
which destroy their acoustic linearly propagating dispersion. Under a few and
simple well-motivated assumptions, we compute the phason density of states, and
we derive the phason heat capacity as a function of the temperature. Finally,
imagining a hypothetical s-wave pairing channel with electrons, we compute the
critical temperature $T_c$ of the corresponding superconducting state as a
function of phason damping using the Eliashberg formalism. We find that for
large phason damping, the heat capacity is linear in temperature, showing a
distinctive glass-like behavior. Additionally, we observe that the phason
damping can strongly enhance the effective Eliashberg coupling, and we reveal a
sharp non-monotonic dependence of the superconducting temperature $T_c$ on the
phason damping, with a maximum located at the underdamped to overdamped
crossover scale. Our simple computations confirm the potential role of
overdamped modes in explaining the glassy properties of incommensurate
structures, but also in possibly inducing strongly-coupled superconductivity
therein, and enhancing the corresponding $T_c$. | 2305.05407v2 |
2023-08-03 | Flavor-wave theory with quasiparticle damping at finite temperatures: Application to chiral edge modes in the Kitaev model | We propose a theoretical framework to investigate elementary excitations at
finite temperatures within a localized electron model that describes the
interactions between multiple degrees of freedom, such as quantum spin models
and Kugel-Khomskii models. Thus far, their excitation structures have been
mainly examined using the linear flavor-wave theory, an SU($N$) generalization
of the linear spin-wave theory. These techniques introduce noninteracting
bosonic quasiparticles as elementary excitations from the ground state, thereby
elucidating numerous physical phenomena, including excitation spectra and
transport properties characterized by topologically nontrivial band structures.
Nevertheless, the interactions between quasiparticles cannot be ignored in
systems exemplified by $S=1/2$ quantum spin models, where strong quantum
fluctuations are present. Recent studies have investigated the effects of
quasiparticle damping at zero temperature in such models. In our study,
extending this approach to the flavor-wave theory for general localized
electron models, we construct a comprehensive method to calculate excitation
spectra with the quasiparticle damping at finite temperatures. We apply our
method to the Kitaev model under magnetic fields, a typical example of models
with topologically nontrivial magnon bands. Our calculations reveal that chiral
edge modes undergo significant damping in weak magnetic fields, amplifying the
damping rate by the temperature increase. This effect is caused by collisions
with thermally excited quasiparticles. Since our approach starts from a general
Hamiltonian, it will be widely applicable to other localized systems, such as
spin-orbital coupled systems derived from multi-orbital Hubbard models in the
strong correlation limit. | 2308.01711v1 |
2024-02-13 | Investigating the Effect of Noise on the Training Performance of Hybrid Quantum Neural Networks | In this paper, we conduct a comprehensively analyze the influence of
different quantum noise gates, including Phase Flip, Bit Flip, Phase Damping,
Amplitude Damping, and the Depolarizing Channel, on the performance of HyQNNs.
Our results reveal distinct and significant effects on HyQNNs training and
validation accuracies across different probabilities of noise. For instance,
the Phase Flip gate introduces phase errors, and we observe that HyQNNs exhibit
resilience at higher probability (p = 1.0), adapting effectively to consistent
noise patterns, whereas at intermediate probabilities, the performance
declines. Bit Flip errors, represented by the PauliX gate, impact HyQNNs in a
similar way to that Phase Flip error gate. The HyQNNs, can adapt such kind of
errors at maximum probability (p = 1.0). Unlike Phase and Bit Flip error gates,
Phase Damping and Amplitude Damping gates disrupt quantum information, with
HyQNNs demonstrating resilience at lower probabilities but facing challenges at
higher probabilities. Amplitude Damping error gate, in particular, poses
efficiency and accuracy issues at higher probabilities however with lowest
probability (p = 0.1),it has the least effect and the HyQNNs, however not very
effectively, but still tends to learn. The Depolarizing Channel proves most
detrimental to HyQNNs performance, with limited or no training improvements.
There was no training potential observed regardless of the probability of this
noise gate. These findings underscore the critical need for advanced quantum
error mitigation and resilience strategies in the design and training of
HyQNNs, especially in environments prone to depolarizing noise. This paper
quantitatively investigate that understanding the impact of quantum noise gates
is essential for harnessing the full potential of quantum computing in
practical applications. | 2402.08523v1 |
2024-03-03 | Magnonic $\varphi$ Josephson Junctions and Synchronized Precession | There has been a growing interest in non-Hermitian physics. One of its main
goals is to engineer dissipation and to explore ensuing functionality. In
magnonics, the effect of dissipation due to local damping on magnon transport
has been explored. However, the effects of non-local damping on the magnonic
analog of the Josephson effect remain missing, despite that non-local damping
is inevitable and has been playing a central role in magnonics. Here, we
uncover theoretically that a surprisingly rich dynamics can emerge in magnetic
junctions due to intrinsic non-local damping, using analytical and numerical
methods. In particular, under microwave pumping, we show that coherent spin
precession in the right and left insulating ferromagnet (FM) of the junction
becomes synchronized by non-local damping and thereby a magnonic analog of the
$\varphi$ Josephson junction emerges, where $\varphi$ stands here for the
relative precession phase of right and left FM in the stationary limit.
Remarkably, $\varphi$ decreases monotonically from $ \pi$ to $\pi/2$ as the
magnon-magnon interaction, arising from spin anisotropies, increases. Moreover,
we also find a magnonic diode effect giving rise to rectification of magnon
currents. Our predictions are readily testable with current device and
measurement technologies at room temperatures. | 2403.01625v1 |
1997-01-17 | Evidence for Rotation in the Galaxy at z=3.15 Responsible for a Damped Lyman-alpha Absorption System in the Spectrum of Q2233+1310 | Proof of the existence of a significant population of normal disk galaxies at
redshift z>2 would have profound implications for theories of structure
formation and evolution. We present evidence based on Keck HIRES observations
that the damped Lyman-alpha absorber at z=3.15 toward the quasar Q2233+1310 may
well be such an example. Djorgovski et al have recently detected the
Lyman-alpha emission from the absorber, which we assume is at the systemic
redshift of the absorbing galaxy. By examining the profiles of the metal
absorption lines arising from the absorbing galaxy in relation to its systemic
redshift, we find strong kinematical evidence for rotation. Therefore the
absorber is likely to be a disk galaxy. The inferred circular velocity for the
galaxy is >200 km/s. With a separation of ~17 kpc between the galaxy and the
quasar sightline, the implied dynamic mass for the galaxy is >1.6x10(11) solar
mass. The metallicity of the galaxy is found to be [Fe/H]=-1.4, typical of
damped Lyman-alpha galaxies at such redshifts. However, in another damped
galactic rotation is evident. In the latter case, the damped Lyman-alpha
absorber occurs near the background quasar in redshift so its properties may be
influenced by the background quasar. These represent the only two cases at
present for which the technique used here may be applied. Future applications
of the same technique to a large sample of damped Lyman-alpha galaxies may
allow us to determine if a significant population of disk galaxies already
existed only a few billion years after the Big Bang. | 9701116v2 |
1997-04-11 | The Metallicity of High Redshift Galaxies: The Abundance of Zinc in 34 Damped Lyman Alpha Systems from z = 0.7 to 3.4 | We report new observations of ZnII and CrII absorption lines in 10 damped
\lya systems (DLAs), mostly at redshift $z_{abs} \simgt 2.5$ . By combining
these results with those from our earlier survey (Pettini et al. 1994) and
other recent data, we construct a sample of 34 measurements (or upper limits)
of the Zn abundance relative to hydrogen [Zn/H]; the sample includes more than
one third of the total number of DLAs known.
The plot of the abundance of Zn as a function of redshift reinforces the two
main findings of our previous study. (1) Damped \lya systems are mostly
metal-poor, at all redshifts sampled; the column density weighted mean for the
whole data set is [Zn/H] $= -1.13 \pm 0.38$ (on a logarithmic scale), or
approximately 1/13 of solar. (2) There is a large spread, by up to two orders
of magnitude, in the metallicities we measure at essentially the same
redshifts. We propose that damped \lya systems are drawn from a varied
population of galaxies of different morphological types and at different stages
of chemical evolution, supporting the idea of a protracted epoch of galaxy
formation.
At redshifts $z \simgt 2$ the typical metallicity of the damped \lya systems
is in agreement with expectations based on the consumption of HI gas implied by
the recent measurements of $\Omega_{DLA}$ by Storrie-Lombardi et al. (1996a),
and with the metal ejection rates in the universe at these epochs deduced by
Madau (1996) from the ultraviolet luminosities of high redshift galaxies
revealed by deep imaging surveys. There are indications in our data for an
increase in the mean metallicity of the damped \lya systems from $z > 3$ to
$\approx 2$, consistent with the rise in the comoving star formation rate
indicated by the relative numbers of $U$ and $B$ drop-outs in the Hubble Deep
Field. Although such comparisons are still tentative, it appears that these
different avenues for exploring the early evolution of galaxies give a broadly
consistent picture. | 9704102v1 |
1997-04-17 | On the Kinematics of the Damped Lyman Alpha Protogalaxies | We present the first results of an ongoing program to investigate the
kinematic characteristics of high redshift damped lya systems. Because damped
lya systems are widely believed to be the progenitors of current massive
galaxies, an analysis of their kinematic history allows a direct test of galaxy
formation scenarios.
We have collected a kinematically unbiased sample of 17 high S/N ratio, high
resolution damped lya spectra taken with HIRES on the 10m W.M. Keck Telescope.
Our study focuses on the unsaturated, low-ion transitions of these systems
which reveal their kinematic traits. The profiles exhibit a nearly uniform
distribution of velocity widths ranging from 20 - 200 km/s and a relatively
high degree of asymmetry. In an attempt to explain these characteristics, we
introduce several physical models, which have previously been attributed to
damped lya systems, including rapidly rotating cold disks, slowly rotating hot
disks, massive isothermal halos, and a hydrodynamic spherical accretion model.
Using standard Monte Carlo techniques, we run sightlines through these model
systems to derive simulated low-ion profiles. Comparing statistical measures of
the simulated profiles with the observed profiles, we determine that the
rapidly rotating cold disk model is the only tested model consistent with the
data at high confidence levels. A Relative Likelihood Test of the rapidly
rotating cold disk model indicates the disks must have large rotation speeds; v
> 180 km/s at the 99% c.l. In turn, we demonstrate that the Cold Dark Matter
Model, as developed by Kauffmann (1996), is inconsistent with the damped lya
data at very high c.l. This is because the CDM Model does not predict a large
enough fraction of rapidly rotating disks at z approx 2.5. | 9704169v2 |
2000-11-20 | H-alpha Imaging with HST+NICMOS of An Elusive Damped Ly-alpha Cloud at z=0.6 | Despite previous intensive ground-based imaging and spectroscopic campaigns
and wide-band HST imaging of the z=0.927 QSO 3C336 field, the galaxy that hosts
the damped Ly-alpha system along this line-of-sight has eluded detection. We
present a deep narrow-band H-alpha image of the field of this z=0.656 damped
Ly-alpha absorber, obtained through the F108N filter of NICMOS 1 onboard the
Hubble Space Telescope. The goal of this project was to detect any H-alpha
emission 10 times closer than previous studies to unveil the damped absorber.
We do not detect H-alpha emission between 0.05'' and 6'' (0.24 and 30 $h^{-1}$
kpc) from the QSO, with a 3-sigma flux limit of $3.70 \times 10^{-17} h^{-2}$
erg/s/cm^2 for an unresolved source, corresponding to a star formation rate
(SFR) of $0.3 h^{-2}$ M_sun/yr. This leads to a 3-sigma upper limit of 0.15
M_sun/yr/kpc^2 on the SFR density, or a maximum SFR of 1.87 M_sun/yr assuming a
disk of 4 kpc in diameter. This result adds to the number of low redshift
damped Ly-alpha absorbers that are not associated with the central regions of
Milky-Way-like disks. Damped Ly-alpha absorption can arise from high density
concentrations in a variety of galactic environments including some that,
despite their high local HI densities, are not conducive to widespread star
formation. | 0011374v2 |
2005-08-17 | The SDSS Damped Lya Survey: Data Release 3 | We present the results from a damped Lya survey of the Sloan Digital Sky
Survey, Data Release 3 based on over 500 new damped Lya systems at z>2.2. We
measure the HI column density distribution f(N) and its zeroth and first
moments (the incidence l(X) and gas mass-density O_dla of damped Lya systems,
respectively) as a function of redshift. The key results include: (1) the f(N)
distribution is well fit by a Gamma-function with `break' column density log
N_g=10^21.5 and `faint-end' slope alpha=-1.8; (2) the shape of the f(N)
distributions do not show evolution with redshift; (3) l(X) and O_dla decrease
by 35% and 50% during ~1Gyr between redshift z=[3.,3.5] to z=[2.2,2.5]; and (4)
l(X) and O_dla in the lowest SDSS redshift bin (z=2.2) are consistent with the
current values. We investigate systematic errors in damped Lya analysis and
identify only one important effect: we measure 40 +/- 20% higher O_dla values
toward a subset of brighter quasars than toward a faint subset. This effect
runs contrary to the bias associated with dust obscuration and suggests that
gravitational lensing may be important. Comparing the results against models of
galaxy formation, we find all of the models significantly underpredict l(X) at
z=3 and only SPH models with significant feedback may reproduce O_dla at high
redshift. We argue that the Lyman limit systems contribute ~1/3 of the
universe's HI atoms at all redshifts z=2 to 5 and that the f(N) distribution
for N(HI)<10^20 has an inflection with slope >-1. We advocate a new mass
density definition -- the mass density of predominantly neutral gas O_neut --
to be contrasted with the mass density of gas associated with HI atoms. We
contend the damped Lya systems contribute >80% of O_neut at all redshifts and
therefore are the main reservoirs for star formation. [abridged] | 0508361v1 |
2010-03-11 | Damping of MHD turbulence in partially ionized gas and the observed difference of velocities of neutrals and ions | Theoretical and observational studies on the turbulence of the interstellar
medium developed fast in the past decades. The theory of supersonic magnetized
turbulence, as well as the understanding of projection effects of observed
quantities, are still in progress. In this work we explore the characterization
of the turbulent cascade and its damping from observational spectral line
profiles. We address the difference of ion and neutral velocities by clarifying
the nature of the turbulence damping in the partially ionized. We provide
theoretical arguments in favor of the explanation of the larger Doppler
broadening of lines arising from neutral species compared to ions as arising
from the turbulence damping of ions at larger scales. Also, we compute a number
of MHD numerical simulations for different turbulent regimes and explicit
turbulent damping, and compare both the 3-dimensional distributions of velocity
and the synthetic line profile distributions. From the numerical simulations,
we place constraints on the precision with which one can measure the 3D
dispersion depending on the turbulence sonic Mach number. We show that no
universal correspondence between the 3D velocity dispersions measured in the
turbulent volume and minima of the 2D velocity dispersions available through
observations exist. For instance, for subsonic turbulence the correspondence is
poor at scales much smaller than the turbulence injection scale, while for
supersonic turbulence the correspondence is poor for the scales comparable with
the injection scale. We provide a physical explanation of the existence of such
a 2D-3D correspondence and discuss the uncertainties in evaluating the damping
scale of ions that can be obtained from observations. However, we show that the
statistics of velocity dispersion from observed line profiles can provide the
spectral index and the energy transfer rate of turbulence. Also, comparing two
similar simulations with different viscous coefficients it was possible to
constrain the turbulent cut-off scale. This may especially prove useful since
it is believed that ambipolar diffusion may be one of the dominant dissipative
mechanism in star-forming regions. In this case, the determination of the
ambipolar diffusion scale may be used as a complementary method for the
determination of magnetic field intensity in collapsing cores. We discuss the
implications of our findings in terms of a new approach to magnetic field
measurement proposed by Li & Houde (2008). | 1003.2346v1 |
2011-09-07 | Weakly collisional Landau damping and three-dimensional Bernstein-Greene-Kruskal modes: New results on old problems | Landau damping and Bernstein-Greene-Kruskal (BGK) modes are among the most
fundamental concepts in plasma physics. While the former describes the
surprising damping of linear plasma waves in a collisionless plasma, the latter
describes exact undamped nonlinear solutions of the Vlasov equation. There does
exist a relationship between the two: Landau damping can be described as the
phase-mixing of undamped eigenmodes, the so-called Case-Van Kampen modes, which
can be viewed as BGK modes in the linear limit. While these concepts have been
around for a long time, unexpected new results are still being discovered. For
Landau damping, we show that the textbook picture of phase-mixing is altered
profoundly in the presence of collision. In particular, the continuous spectrum
of Case-Van Kampen modes is eliminated and replaced by a discrete spectrum,
even in the limit of zero collision. Furthermore, we show that these discrete
eigenmodes form a complete set of solutions. Landau-damped solutions are then
recovered as true eigenmodes (which they are not in the collisionless theory).
For BGK modes, our interest is motivated by recent discoveries of electrostatic
solitary waves in magnetospheric plasmas. While one-dimensional BGK theory is
quite mature, there appear to be no exact three-dimensional solutions in the
literature (except for the limiting case when the magnetic field is
sufficiently strong so that one can apply the guiding-center approximation). We
show, in fact, that two- and three-dimensional solutions that depend only on
energy do not exist. However, if solutions depend on both energy and angular
momentum, we can construct exact three-dimensional solutions for the
unmagnetized case, and two-dimensional solutions for the case with a finite
magnetic field. The latter are shown to be exact, fully electromagnetic
solutions of the steady-state Vlasov-Poisson-Amp\`ere system. | 1109.1353v1 |
2013-05-16 | Application of vibration-transit theory to distinct dynamic response for a monatomic liquid | We examine the distinct part of the density autocorrelation function Fd(q,t),
also called the intermediate scattering function, from the point of view of the
vibration-transit (V-T) theory of monatomic liquid dynamics. A similar study
has been reported for the self part, and we study the self and distinct parts
separately because their damping processes are not simply related. We begin
with the perfect vibrational system, which provides precise definitions of the
liquid correlations, and provides the vibrational approximation Fdvib(q,t) at
all q and t. Two independent liquid correlations are defined, motional and
structural, and these are decorrelated sequentially, with a crossover time
tc(q). This is done by two independent decorrelation processes: the first,
vibrational dephasing, is naturally present in Fdvib(q,t) and operates to damp
the motional correlation; the second, transit-induced decorrelation, is invoked
to enhance the damping of motional correlation, and then to damp the structural
correlation. A microscopic model is made for the "transit drift", the averaged
transit motion that damps motional correlation on 0 < t < tc(q). Following the
previously developed self-decorrelation theory, a microscopic model is also
made for the "transit random walk," which damps the structural correlation on t
> tc(q). The complete model incorporates a property common to both self and
distinct decorrelation: simple exponential decay following a delay period,
where the delay is tc(q, the time required for the random walk to emerge from
the drift. Our final result is an accurate expression for Fd(q,t) for all q
through the first peak in Sd(q). The theory is calibrated and tested using
molecular dynamics (MD) calculations for liquid Na at 395K; however, the theory
itself does not depend on MD, and we consider other means for calibrating it. | 1305.3954v2 |
2013-09-16 | Two-atom system as a nano-antenna for mode switching and light routing | We determine how a system composed of two nonidentical two-level atoms with
different resonance frequencies and different damping rates could work as a
nano-antenna for controlled mode switching and light routing. We calculate the
angular distribution of the emitted field detected in a far-field zone of the
system including the direct interatomic interactions and arbitrary linear
dimensions of the system. The calculation is carried out in terms of the
symmetric and antisymmetric modes of the two atom system. We find that as long
as the atoms are identical, the emission cannot be switched between the
symmetric and antisymmetric modes. The switching may occur when the atoms are
non-identical and the emission can then be routed to different modes by
changing the relative ratio of the atomic frequencies, or damping rates or by a
proper tuning of the laser frequency to the atomic resonance frequencies. It is
shown that in the case of atoms of different resonance frequencies but equal
damping rates, the light routing is independent of the frequency of the driving
laser field. It depends only on the sign of the detuning between the atomic
resonance frequencies. In the case of atoms of different damping rates, the
emission can be switched between different modes by changing the laser
frequency from the blue to red detuned from the atomic resonance. The effect of
the interatomic interactions is also considered and it is found that in the
case of unequal resonance frequencies of the atoms, the interactions slightly
modify the visibility of the intensity pattern. The case of unequal damping
rates of the atoms is affected rather more drastically, the light routing
becoming asymmetric under the dipole-dipole interaction with the enhanced
intensities of the modes turned towards the atom of smaller damping rate. | 1309.3924v1 |
2015-04-01 | Landau damping of Gardner solitons in a dusty bi-ion plasma | The effects of linear Landau damping on the nonlinear propagation of
dust-acoustic solitary waves (DASWs) are studied in a collisionless
unmagnetized dusty plasma with two species of positive ions. The extremely
massive, micron-seized, cold and negatively charged dust particles are
described by fluid equations, whereas the two species of positive ions, namely
the cold (heavy) and hot (light) ions are described by the kinetic Vlasov
equations. Following Ott and Sudan [Phys. Fluids {\bf 12}, 2388 (1969)], and by
considering lower and higher-order perturbations, the evolution of DASWs with
Landau damping is shown to be governed by Korteweg-de Vries (KdV), modified KdV
(mKdV) or Gardner (KdV-mKdV)-like equations. The properties of the phase
velocity and the Landau damping rate of DASWs are studied for different values
of the ratios of the temperatures $(\sigma)$ and the number densities $(\mu)$
of hot and cold ions as well the cold to hot ion mass ratio $m$. The
distinctive features of the decay rates of the amplitudes of the KdV, mKdV and
Gardner solitons with a small effect of Landau damping are also studied in
different parameter regimes. It is found that the Gardner soliton points to
lower wave amplitudes than the KdV and mKdV solitons. The results may be useful
for understanding the localization of solitary pulses and associated wave
damping (collisionless) in laboratory and space plasmas (e.g., the F-ring of
Saturn) in which the number density of free electrons is much smaller than that
of ions and the heavy, micron seized dust grains are highly charged. | 1504.00089v2 |
2018-06-27 | In-flight performance of the DAMPE silicon tracker | DAMPE (DArk Matter Particle Explorer) is a spaceborne high-energy cosmic ray
and gamma-ray detector, successfully launched in December 2015. It is designed
to probe astroparticle physics in the broad energy range from few GeV to 100
TeV. The scientific goals of DAMPE include the identification of possible
signatures of Dark Matter annihilation or decay, the study of the origin and
propagation mechanisms of cosmic-ray particles, and gamma-ray astronomy. DAMPE
consists of four sub-detectors: a plastic scintillator strip detector, a
Silicon-Tungsten tracKer-converter (STK), a BGO calorimeter and a neutron
detector. The STK is composed of six double layers of single-sided silicon
micro-strip detectors interleaved with three layers of tungsten for photon
conversions into electron-positron pairs. The STK is a crucial component of
DAMPE, allowing to determine the direction of incoming photons, to reconstruct
tracks of cosmic rays and to estimate their absolute charge (Z). We present the
in-flight performance of the STK based on two years of in-flight DAMPE data,
which includes the noise behavior, signal response, thermal and mechanical
stability, alignment and position resolution. | 1806.10355v1 |
2019-06-12 | Study of Alfven Eigenmodes stability in plasma with multiple NBI driven energetic particle specie | The aim of this study is to analyze the destabilization of Alfven Eigenmodes
(AE) by multiple energetic particles (EP) species in DIII-D and LHD discharges.
We use the reduced MHD equations to describe the linear evolution of the
poloidal flux and the toroidal component of the vorticity in a full 3D system,
coupled with equations of density and parallel velocity moments for the
energetic particles species, including the effect of the acoustic modes,
diamagnetic currents and helical couplings. We add the Landau damping and
resonant destabilization effects using a closure relation. The simulations with
multiple NBI lines show three different regimes: the non damped regime where
the multi beam AEs growth rate is larger compared to the growth rate of the AEs
destabilized by the individual NBI lines, the interaction regime where the
multi beam AEs growth rate is smaller than the single NBI AEs and the damped
regime where the AEs are suppressed. Operations in the damped regime requires
EP species with different density profile flatness or gradient locations. In
addition, the AEs growth rate in the interaction regime is further reduced if
the combined NBI lines have similar beam temperatures and the beta of the NBI
line with flatter EP density profile increases. Then, optimization trends are
identified in DIII-D high poloidal beta and LHD low density / magnetic field
discharges with multiple NBI lines as well as the configuration requirements to
operate in the damped and interaction regimes. DIII-D simulations show a
decrease of the n=2 to 6 AEs growth rate and n=1 AE are stabilized in the LHD
case. The helical coupling effects in LHD simulations lead to a transition from
the interaction to the damped regime of the n=2,-8,12 helical family. | 1906.05701v1 |
2012-11-06 | Torsional Alfvén waves in solar partially ionized plasma: effects of neutral helium and stratification | Ion-neutral collisions may lead to the damping of Alfven waves in
chromospheric and prominence plasmas. Neutral helium atoms enhance the damping
in certain temperature interval, where the ratio of neutral helium and neutral
hydrogen atoms is increased. Therefore, the height-dependence of ionization
degrees of hydrogen and helium may influence the damping rate of Alfven waves.
We aim to study the effect of neutral helium in the damping of Alfven waves in
stratified partially ionized plasma of the solar chromosphere. We consider a
magnetic flux tube, which is expanded up to 1000 km height and then becomes
vertical due to merging with neighboring tubes, and study the dynamics of
linear torsional Alfven waves in the presence of neutral hydrogen and neutral
helium atoms. We start with three-fluid description of plasma and consequently
derive single-fluid magnetohydrodynamic (MHD) equations for torsional Alfven
waves. Thin flux tube approximation allows to obtain the dispersion relation of
the waves in the lower part of tubes, while the spatial dependence of
steady-state Alfven waves is governed by Bessel type equation in the upper part
of tubes. Consecutive derivation of single-fluid MHD equations results in a new
Cowling diffusion coefficient in the presence of neutral helium which is
different from previously used one. We found that shorter-period (< 5 s)
torsional Alfven waves damp quickly in the chromospheric network due to
ion-neutral collision. On the other hand, longer-period (> 5 s) waves do not
reach the transition region as they become evanescent at lower heights in the
network cores. Propagation of torsional Alfven waves through the chromosphere
into the solar corona should be considered with caution: low-frequency waves
are evanescent due to the stratification, while high-frequency waves are damped
due to ion neutral collisions. | 1211.1348v2 |
2018-10-30 | Effect of Landau damping on ion acoustic solitary waves in a multi-species collisionless unmagnetized plasma consisting of nonthermal and isothermal electrons | A Korteweg-de Vries (KdV) equation including the effect of Landau damping is
derived to study the propagation of weakly nonlinear and weakly dispersive ion
acoustic waves in a collisionless unmagnetized plasma consisting of warm
adiabatic ions and two different species of electrons at different
temperatures. The hotter energetic electron species follows the nonthermal
velocity distribution of Cairns et al. [Geophys. Res. Lett. 22, 2709 (1995)]
whereas the cooler electron species obeys the Boltzmann distribution. It is
found that the coefficient of the nonlinear term of this KdV like evolution
equation vanishes along different family of curves in different parameter
planes. In this context, a modified KdV (MKdV) equation including the effect of
Landau damping effectively describes the nonlinear behaviour of ion acoustic
waves. It has also been observed that the coefficients of the nonlinear terms
of the KdV and MKdV like evolution equations including the effect of Landau
damping, are simultaneously equal to zero along a family of curves in the
parameter plane. In this situation, we have derived a further modified KdV
(FMKdV) equation including the effect of Landau damping to describe the
nonlinear behaviour of ion acoustic waves. In fact, different modified KdV like
evolution equations including the effect of Landau damping have been derived to
describe the nonlinear behaviour of ion acoustic waves in different region of
parameter space. The method of Ott & Sudan [Phys. Fluids 12, 2388 (1969)] has
been applied to obtain the solitary wave solution of the evolution equation
having the nonlinear term $(\phi^{(1)})^{r}\frac{\partial \phi^{(1)}}{\partial
\xi}$, where $\phi^{(1)}$ is the first order perturbed electrostatic potential
and $r =1,2,3$. We have found that the amplitude of the solitary wave solution
decreases with time for all $r =1,2,3$. | 1810.12739v1 |
2017-07-18 | Explanations of the DAMPE high energy electron/positron spectrum in the dark matter annihilation and pulsar scenarios | Many studies have shown that either the nearby astrophysical source or dark
matter (DM) annihilation/decay is required to explain the origin of high energy
cosmic ray (CR) $e^\pm$, which are measured by many experiments, such as PAMELA
and AMS-02. Recently, the Dark Matter Particle Explorer (DAMPE) collaboration
has reported its first result of the total CR $e^\pm$ spectrum from $25
\,\mathrm{GeV}$ to $4.6 \,\mathrm{TeV}$ with high precision. In this work, we
study the DM annihilation and pulsar interpretations of the DAMPE high energy
$e^\pm$ spectrum. In the DM scenario, the leptonic annihilation channels to
$\tau^+\tau^-$, $4\mu$, $4\tau$, and mixed charged lepton final states can well
fit the DAMPE result, while the $\mu^+\mu^-$ channel has been excluded. In
addition, we find that the mixed charged leptons channel would lead to a sharp
drop at $\sim$ $\mathrm{TeV}$. However, these DM explanations are almost
excluded by the observations of gamma-ray and CMB, unless some complicated DM
models are introduced. In the pulsar scenario, we analyze 21 nearby known
pulsars and assume that one of them is the primary source of high energy CR
$e^\pm$.Considering the constraint from the Fermi-LAT observation of the
$e^\pm$ anisotropy, we find that two pulsars are possible to explain the DAMPE
data. Our results show that it is difficult to distinguish between the DM
annihilation and single pulsar explanations of high energy $e^\pm$ with the
current DAMPE result. | 1707.05664v2 |
2019-03-28 | Improving convergence of volume penalised fluid-solid interactions | We analyse and improve the volume-penalty method, a simple and versatile way
to model objects in fluid flows. The volume-penalty method is a kind of
fictitious-domain method that approximates no-slip boundary conditions with
rapid linear damping inside the object. The method can then simulate complex,
moving objects in general numerical solvers without specialised algorithms or
boundary-conforming grids. Volume penalisation pays for this simplicity by
introducing an equation-level error, the $\textit{model error}$, that is
related to the damping time $\eta \ll 1$. While the model error has been proven
to vanish as the damping time tends to zero, previous work suggests convergence
at a slow rate of $\mathcal{O}(\eta^{1/2})$. The stiffness of the damping
implies conventional volume penalisation only achieves first order numerical
accuracy. We analyse the volume-penalty method using multiple-scales
matched-asymptotics with a signed-distance coordinate system valid for
arbitrary smooth geometries. We show the dominant model error stems from a
displacement length that is proportional to a Reynolds number $\text{Re}$
dependent boundary layer of size $\mathcal{O}(\eta^{1/2}\text{Re}^{-1/2})$. The
relative size of the displacement length and damping time leads to multiple
error regimes. Our key finding derives a simple smoothing prescription for the
damping that eliminates the displacement length and reduces the model error to
$\mathcal{O}(\eta)$ in all regimes. This translates to second order numerical
accuracy. We validate our findings in several comprehensive benchmark problems
and finally combine Richardson extrapolation of the model error with our
correction to further improve convergence to $\mathcal{O}(\eta^{2})$. | 1903.11914v4 |
2020-06-08 | Stochastic re-acceleration and magnetic-field damping in Tycho's supernova remnant | A number of studies suggest that shock acceleration with particle feedback
and very efficient magnetic-field amplification combined with Alfv\'{e}nic
drift are needed to explain the rather soft radio spectrum and the narrow rims
observed for Tycho's SNR. We show that the broadband spectrum of Tycho's SNR
can alternatively be well explained when accounting for stochastic acceleration
as a secondary process. The re-acceleration of particles in the turbulent
region immediately downstream of the shock should be efficient enough to impact
particle spectra over several decades in energy. The so-called Alfv\'{e}nic
drift and particle feedback on the shock structure are not required in this
scenario. Additionally, we investigate whether synchrotron losses or
magnetic-field damping play a more profound role in the formation of the
non-thermal filaments. We solve the full particle transport equation in
test-particle mode using hydrodynamic simulations of the SNR plasma flow. The
background magnetic field is either computed from the induction equation or
follows analytic profiles, depending on the model considered. Fast-mode waves
in the downstream region provide the diffusion of particles in momentum space.
We show that the broadband spectrum of Tycho can be well explained if
magnetic-field damping and stochastic re-acceleration of particles are taken
into account. Although not as efficient as standard DSA, stochastic
acceleration leaves its imprint on the particle spectra, which is especially
notable in the emission at radio wavelengths. We find a lower limit for the
post-shock magnetic-field strength $\sim330\,\mathrm{\mu G}$, implying
efficient amplification even for the magnetic-field damping scenario. For the
formation of the filaments in the radio range magnetic-field damping is
necessary, while the X-ray filaments are shaped by both the synchrotron losses
and magnetic-field damping. | 2006.04832v1 |
2021-02-23 | Influence of Ion-Neutral Damping on the Cosmic-Ray Streaming Instability: Magnetohydrodynamic Particle-in-cell Simulations | We explore the physics of the gyro-resonant cosmic ray streaming instability
(CRSI) including the effects of ion-neutral (IN) damping. This is the main
damping mechanism in (partially-ionized) atomic and molecular gas, which are
the primary components of the interstellar medium (ISM) by mass. Limitation of
CRSI by IN damping is important in setting the amplitude of Alfv\'en waves that
scatter cosmic rays and control galactic-scale transport. Our study employs the
MHD-PIC hybrid fluid-kinetic numerical technique to follow linear growth as
well as post-linear and saturation phases. During the linear phase of the
instability -- where simulations and analytical theory are in good agreement --
IN damping prevents wave growth at small and large wavelengths, with the
unstable bandwidth lower for higher ion-neutral collision rate $\nu_{\rm in}$.
Purely MHD effects during the post-linear phase extend the wave spectrum
towards larger $k$. In the saturated state, the cosmic ray distribution evolves
toward greater isotropy (lower streaming velocity) by scattering off of Alv\'en
waves excited by the instability. In the absence of low-$k$ waves, CRs with
sufficiently high momentum are not isotropized. The maximum wave amplitude and
rate of isotropization of the distribution function decreases at higher
$\nu_{\rm in}$. When the IN damping rate approaches the maximum growth rate of
CSRI, wave growth and isotropization is suppressed. Implications of our results
for CR transport in partially ionized ISM phases are discussed. | 2102.11878v3 |
2022-06-17 | Quantum Dynamics of Magnetic Skyrmions: Consistent Path Integral Formulation | We present a path integral formalism for the intrinsic quantum dynamics of
magnetic skyrmions coupled to a thermal background of magnetic fluctuations.
Upon promoting the skyrmion's collective coordinate $\boldsymbol{R}$ to a
dynamic variable and integrating out the magnonic heat bath, we derive the
generalized equation of motion for $\boldsymbol{R}$ with a non-local damping
term that describes a steady-state skyrmion dynamics at finite temperatures.
Being essentially temperature dependent, the intrinsic damping is shown to
originate from the coupling of thermally activated magnon modes to the
adiabatic potential driven by a rigid skyrmion motion, which can be regarded as
another manifestation of emergent electrodynamics inherent to topological
magnetic textures. We further argue that the diagonal components of the damping
term act as the source of dissipation and inertia, while its off-diagonal
components modify the gyrotropic motion of a magnetic skyrmion. By means of
numerical calculations for the lattice spin model of chiral ferromagnets, we
study the temperature behavior of the intrinsic damping as a function of
magnetic field in periodic and confined geometries. The intrinsic damping is
demonstrated to be highly non-local, revealing its quantum-mechanical nature,
that becomes more pronounced with increasing temperature. At high temperatures
when the magnon occupation factors are large, the intrinsic damping is shown to
yield a modified Thiele's equation with the additional non-local dissipative
and mass terms that exhibit an almost linear temperature behavior. Our results
provide a microscopic background for semiclassical magnetization dynamics and
establish a framework for understanding spin caloritronics effects in
topological magnetic textures. | 2206.08532v2 |
2024-02-05 | Revisiting the role of cosmic-ray driven Alfvén waves in pre-existing magnetohydrodynamic turbulence. I. Turbulent damping rates and feedback on background fluctuations | Alfv\'en waves (AWs) excited by the cosmic-ray (CR) streaming instability
(CRSI) are a fundamental ingredient for CR confinement. The effectiveness of
self-confinement relies on a balance between CRSI growth rate and damping
mechanisms acting on quasi-parallel AWs excited by CRs. One relevant mechanism
is the so-called turbulent damping, in which an AW packet injected in
pre-existing turbulence undergoes a cascade process due to its nonlinear
interaction with fluctuations of the background. The turbulent damping of an AW
packet in pre-existing magnetohydrodynamic turbulence is re-examined, revised,
and extended to include most-recent theories of MHD turbulence that account for
dynamic alignment and reconnection-mediated regime. The case in which the role
of feedback of CR-driven AWs on pre-existing turbulence is important will also
be discussed. Particular attention is given to the nonlinearity parameter
$\chi^w$ that estimates the strength of nonlinear interaction between CR-driven
AWs and background fluctuations. We point out the difference between $\chi^w$
and $\chi^z$ that instead describes the strength of nonlinear interactions
between pre-existing fluctuations. When $\chi^w$ is properly taken into
account, one finds that (i) the turbulent damping rate of quasi-parallel AWs in
anisotropic turbulence depends on the background-fluctuations' amplitude to the
third power, hence is strongly suppressed, and (ii) the dependence on the AW's
wavelength (and thus on the CR gyro-radius from which it is excited) is
different from what has been previously obtained. Finally, (iii) when dynamic
alignment of cascading fluctuations and the possibility of a
reconnection-mediated range is included in the picture, the turbulent damping
rate exhibits novel regimes and breaks. Finally, a criterion for CR-feedback is
derived and simple phenomenological models of CR-modified turbulent scaling are
provided. | 2402.02901v1 |
2006-10-24 | Logical contradictions of Landau damping | Landau damping/growing at boundary condition of excitation of a harmonic wave
in collisionless ion-electron-neutrals plasma contradicts to the law of energy
conservation of a wave damping/growing in space. There is also no criterion of
a choice either damping or growing solution in difference from always
non-damping in the direction of propagation Vlasov waves. Variety of other
incongruities as consequence of Landau damping is specified also. Absence of
explicit positivity and finiteness of wave solutions for electron distribution
function near singularity point leads to need of imposing additional cutting
off constraints with resulting positivity and finiteness of the electron
distribution function at the singularity points and finiteness of the complex
dispersion integral. Landau damping as a real physical phenomenon of
collisionless damping does not exist. A relation is established for the real
dispersion equation with real waves (see Appendices 2,4) between the averaged
over period wave damping decrement and the collisional energy-exchange term of
kinetic equation. Collisionless Vlasov-Landau damping is explained finally by
the usual wrong use of nonlinearly complex wave functions leading to complex
dispersion equation. All used solution of the complex dispersion equation for
the simultaneously existing collisionless both exponentially damping and
growing nonlinear complex waves is entirely, quantitatively and in its logical
sense, different from the solution of initially real dispersion equation for
real either damping or growing waves and should be discarded (see Appendices
2,4,5,6). Collisionless damping is caused by unreasonable use of wave functions
with complex frequency or complex wave number leading to complex dispersion
relation with unphysical binomial virtual complex roots. Thus finding roots of
the complex dispersion equation has only abstract mathematical interest. | 0610220v67 |
2000-05-31 | The afterglow of the short/intermediate-duration gamma-ray burst GRB 000301C: A jet at z=2.04 | We present Ulysses and NEAR data from the detection of the short or
intermediate duration (2 s) gamma-ray burst GRB000301C (2000 March 1.41 UT).
The gamma-ray burst (GRB) was localised by the Inter Planetary Network (IPN)
and RXTE to an area of 50 arcmin^2. A fading optical counterpart was
subsequently discovered with the Nordic Optical Telescope (NOT) about 42h after
the burst. The GRB lies at the border between the long-soft and the short-hard
classes of GRBs. If GRB000301C belongs to the latter class, this would be the
first detection of an afterglow to a short-hard burst. We present UBRI and JHK
photometry from the time of the discovery until 11 days after the burst.
Finally, we present spectroscopic observations of the optical afterglow
obtained with the ESO VLT Antu telescope 4 and 5 days after the burst. The
optical light curve is consistent with being achromatic from 2 to 11 days after
the burst and exhibits a break. A broken power-law fit yields a shallow
pre-break decay power-law slope of a_1=-0.72+-0.06, a break time of
t_b=4.39+-0.26 days after the burst, and a post-break slope of a_2=-2.29+-0.17,
which is best explained by a sideways expanding jet in an ambient medium of
constant mean density. In the optical spectrum we find absorption features that
are consistent with FeII, CIV, CII, SiII and Ly-a at a redshift of
2.0404+-0.0008. We find evidence for a curved shape of the spectral energy
distribution of the observed afterglow. It is best fitted with a power-law
spectral distribution with index b ~ -0.7 reddened by an SMC-like extinction
law with A_V~0.1 mag. Based on the Ly-a absorption line we estimate the HI
column density to be log(N(HI))=21.2+-0.5. This is the first direct indication
of a connection between GRB host galaxies and Damped Ly-a Absorbers. | 0005609v2 |
2011-05-16 | A Measurement of the Damping Tail of the Cosmic Microwave Background Power Spectrum with the South Pole Telescope | We present a measurement of the angular power spectrum of the cosmic
microwave background (CMB) using data from the South Pole Telescope (SPT). The
data consist of 790 square degrees of sky observed at 150 GHz during 2008 and
2009. Here we present the power spectrum over the multipole range 650 < ell <
3000, where it is dominated by primary CMB anisotropy. We combine this power
spectrum with the power spectra from the seven-year Wilkinson Microwave
Anisotropy Probe (WMAP) data release to constrain cosmological models. We find
that the SPT and WMAP data are consistent with each other and, when combined,
are well fit by a spatially flat, LCDM cosmological model. The SPT+WMAP
constraint on the spectral index of scalar fluctuations is ns = 0.9663 +/-
0.0112. We detect, at ~5-sigma significance, the effect of gravitational
lensing on the CMB power spectrum, and find its amplitude to be consistent with
the LCDM cosmological model. We explore a number of extensions beyond the LCDM
model. Each extension is tested independently, although there are degeneracies
between some of the extension parameters. We constrain the tensor-to-scalar
ratio to be r < 0.21 (95% CL) and constrain the running of the scalar spectral
index to be dns/dlnk = -0.024 +/- 0.013. We strongly detect the effects of
primordial helium and neutrinos on the CMB; a model without helium is rejected
at 7.7-sigma, while a model without neutrinos is rejected at 7.5-sigma. The
primordial helium abundance is measured to be Yp = 0.296 +/- 0.030, and the
effective number of relativistic species is measured to be Neff = 3.85 +/-
0.62. The constraints on these models are strengthened when the CMB data are
combined with measurements of the Hubble constant and the baryon acoustic
oscillation feature. Notable improvements include ns = 0.9668 +/- 0.0093, r <
0.17 (95% CL), and Neff = 3.86 +/- 0.42. The SPT+WMAP data show... | 1105.3182v2 |
2016-10-07 | The Atacama Cosmology Telescope: Two-Season ACTPol Spectra and Parameters | We present the temperature and polarization angular power spectra measured by
the Atacama Cosmology Telescope Polarimeter (ACTPol). We analyze night-time
data collected during 2013-14 using two detector arrays at 149 GHz, from 548
deg$^2$ of sky on the celestial equator. We use these spectra, and the spectra
measured with the MBAC camera on ACT from 2008-10, in combination with Planck
and WMAP data to estimate cosmological parameters from the temperature,
polarization, and temperature-polarization cross-correlations. We find the new
ACTPol data to be consistent with the LCDM model. The ACTPol
temperature-polarization cross-spectrum now provides stronger constraints on
multiple parameters than the ACTPol temperature spectrum, including the baryon
density, the acoustic peak angular scale, and the derived Hubble constant.
Adding the new data to planck temperature data tightens the limits on damping
tail parameters, for example reducing the joint uncertainty on the number of
neutrino species and the primordial helium fraction by 20%. | 1610.02360v1 |
1999-12-17 | Infrared Spectroscopy of a Massive Obscured Star Cluster in the Antennae Galaxies (NGC 4038/4039) with NIRSPEC | We present infrared spectroscopy of the Antennae Galaxies (NGC 4038/4039)
with NIRSPEC at the W. M. Keck Observatory. We imaged the star clusters in the
vicinity of the southern nucleus (NGC 4039) in 0.39" seeing in K-band using
NIRSPEC's slit-viewing camera. The brightest star cluster revealed in the
near-IR (M_K(0) = -17.9) is insignificant optically, but coincident with the
highest surface brightness peak in the mid-IR (12-18 micron) ISO image
presented by Mirabel et al. (1998). We obtained high signal-to-noise 2.03 -
2.45 micron spectra of the nucleus and the obscured star cluster at R ~ 1900.
The cluster is very young (4 Myr old), massive (16e6 M_sun), and compact
(density ~ 115 M_sun pc^(-3) within a 32 pc half-light radius), assuming a
Salpeter IMF (0.1 - 100 M_sun). Its hot stars have a radiation field
characterized by T_eff ~ 39,000 K, and they ionize a compact H II region with
n_e ~ 1e4 cm^(-3). The stars are deeply embedded in gas and dust (A_V ~ 9-10
mag), and their strong FUV field powers a clumpy photodissociation region with
densities n_H >= 1e5 cm^(-3) on scales of up to 200 pc, radiating L[H_2 1-0
S(1)] = 9600 L_sun. | 9912369v1 |
2000-09-07 | Practical Quantum Cryptography: A Comprehensive Analysis (Part One) | We perform a comprehensive analysis of practical quantum cryptography (QC)
systems implemented in actual physical environments via either free-space or
fiber-optic cable quantum channels for ground-ground, ground-satellite,
air-satellite and satellite-satellite links. (1) We obtain universal
expressions for the effective secrecy capacity and rate for QC systems taking
into account three important attacks on individual quantum bits, including
explicit closed-form expressions for the requisite amount of privacy
amplification. Our analysis also includes the explicit calculation in detail of
the total cost in bits of continuous authentication, thereby obtaining new
results for actual ciphers of finite length. (2) We perform for the first time
a detailed, explicit analysis of all systems losses due to propagation, errors,
noise, etc. as appropriate to both optical fiber cable- and satellite
communications-based implementations of QC. (3) We calculate for the first time
all system load costs associated to classical communication and computational
constraints that are ancillary to, but essential for carrying out, the pure QC
protocol itself. (4) We introduce an extended family of generalizations of the
Bennett-Brassard (BB84) QC protocol that equally provide unconditional secrecy
but allow for the possibility of optimizing throughput rates against specific
cryptanalytic attacks. (5) We obtain universal predictions for maximal rates
that can be achieved with practical system designs under realistic
environmental conditions. (6) We propose a specific QC system design that
includes the use of a novel method of high-speed photon detection that may be
able to achieve very high throughput rates for actual implementations in
realistic environments. | 0009027v5 |
2009-08-07 | The Dominance of Metal-Rich Streams in Stellar Halos: A Comparison Between Substructure in M31 and Lambda-CDM Models | Extensive photometric and spectroscopic surveys of the Andromeda galaxy (M31)
have discovered tidal debris features throughout M31's stellar halo. We present
stellar kinematics and metallicities in fields with identified substructure
from our on-going SPLASH survey of M31 red giant branch stars with the DEIMOS
spectrograph on the Keck II 10-m telescope. Radial velocity criteria are used
to isolate members of the kinematically-cold substructures. The substructures
are shown to be metal-rich relative to the rest of the dynamically hot stellar
population in the fields in which they are found. We calculate the mean
metallicity and average surface brightness of the various kinematical
components in each field, and show that, on average, higher surface brightness
features tend to be more metal-rich than lower surface brightness features.
Simulations of stellar halo formation via accretion in a cosmological context
are used to illustrate that the observed trend can be explained as a natural
consequence of the observed dwarf galaxy mass-metallicity relation. A
significant spread in metallicity at a given surface brightness is seen in the
data; we show that this is due to time effects, namely the variation in the
time since accretion of the tidal streams' progenitor onto the host halo. We
show that in this theoretical framework a relationship between the
alpha-enhancement and surface brightness of tidal streams is expected, which
arises from the varying times of accretion of the progenitor satellites onto
the host halo. Thus, measurements of the alpha-enrichment, metallicity, and
surface brightness of tidal debris can be used to reconstruct the luminosity
and time of accretion onto the host halo of the progenitors of tidal streams. | 0908.1111v1 |
2009-09-25 | The SPLASH Survey: A Spectroscopic Portrait of Andromeda's Giant Southern Stream | The giant southern stream (GSS) is the most prominent tidal debris feature in
M31's stellar halo. The GSS is composed of a relatively metal-rich, high
surface-brightness "core" and a lower metallicity, lower surface brightness
"envelope." We present Keck/DEIMOS spectroscopy of red giant stars in six
fields in the vicinity of M31's GSS and one field on Stream C, an arc-like
feature on M31's SE minor axis at R=60 kpc. Several GSS-related findings and
measurements are presented here. We present the innermost kinematical detection
of the GSS core to date (R=17 kpc). This field also contains the continuation
of a second kinematically cold component originally seen in a GSS core field at
R=21 kpc. The velocity gradients of the GSS and the second component in the
combined data set are parallel over a radial range of 7 kpc, suggesting a
possible bifurcation in the line-of-sight velocities of GSS stars. We also
present the first kinematical detection of substructure in the GSS envelope.
Using kinematically identified samples, we show that the envelope debris has a
~0.7 dex lower mean photometric metallicity and possibly higher intrinsic
velocity dispersion than the GSS core. The GSS is also identified in the field
of the M31 dSph satellite And I; the GSS in this field has a metallicity
distribution identical to that of the GSS core. We confirm the presence of two
kinematically cold components in Stream C, and measure intrinsic velocity
dispersions of ~10 and ~4 km/s. This compilation of the kinematical (mean
velocity, intrinsic velocity dispersion) and chemical properties of stars in
the GSS core and envelope, coupled with published surface brightness
measurements and wide-area star-count maps, will improve constraints on the
orbit and internal structure of the dwarf satellite progenitor. | 0909.4540v1 |
2011-05-28 | Transmission Control of Two-User Slotted ALOHA Over Gilbert-Elliott Channel: Stability and Delay Analysis | In this paper, we consider the problem of calculating the stability region
and average delay of two user slotted ALOHA over a Gilbert-Elliott channel,
where users have channel state information and adapt their transmission
probabilities according to the channel state. Each channel has two states,
namely, the 'good' and 'bad' states. In the 'bad' state, the channel is assumed
to be in deep fade and the transmission fails with probability one, while in
the 'good' state, there is some positive success probability. We calculate the
Stability region with and without Multipacket Reception capability as well as
the average delay without MPR. Our results show that the stability region of
the controlled S-ALOHA is always a superset of the stability region of
uncontrolled S-ALOHA. Moreover, if the channel tends to be in the 'bad' state
for long proportion of time, then the stability region is a convex Polyhedron
strictly containing the TDMA stability region and the optimal transmission
strategy is to transmit with probability one whenever the nodes have packets
and it is shown that this strategy is delay optimal. On the other hand, if the
channel tends to be in the 'good' state more often, then the stability region
is bounded by a convex curve and is strict subset of the TDMA stability region.
We also show that enhancing the physical layer by allowing MPR capability can
significantly enhance the performance while simplifying the MAC Layer design by
the lack of the need of scheduling under some conditions. Furthermore, it is
shown that transmission control not only allows handling higher stable arrival
rates but also leads to lower delay for the same arrival rate compared with
ordinary S-ALOHA. | 1105.5676v2 |
2014-09-12 | Global Properties of M31's Stellar Halo from the SPLASH Survey: II. Metallicity Profile | We present the metallicity distribution of red giant branch (RGB) stars in
M31's stellar halo, derived from photometric metallicity estimates for over
1500 spectroscopically confirmed RGB halo stars. The stellar sample comes from
38 halo fields observed with the Keck/DEIMOS spectrograph, ranging from 9 to
175 kpc in projected distance from M31's center, and includes 52 confirmed M31
halo stars beyond 100 kpc. While a wide range of metallicities is seen
throughout the halo, the metal-rich peak of the metallicity distribution
function becomes significantly less prominent with increasing radius. The
metallicity profile of M31's stellar halo shows a continuous gradient from 9 to
~100 kpc, with a magnitude of -0.01 dex/kpc. The stellar velocity distributions
in each field are used to identify stars that are likely associated with tidal
debris features. The removal of tidal debris features does not significantly
alter the metallicity gradient in M31's halo: a gradient is maintained in
fields spanning 10 to 90 kpc. We analyze the halo metallicity profile, as well
as the relative metallicities of stars associated with tidal debris features
and the underlying halo population, in the context of current simulations of
stellar halo formation. We argue that the large scale gradient in M31's halo
implies M31 accreted at least one relatively massive progenitor in the past,
while the field to field variation seen in the metallicity profile indicates
that multiple smaller progenitors are likely to have contributed substantially
to M31's outer halo. | 1409.3843v1 |
2016-07-15 | Solving the stochastic Landau-Lifshitz-Gilbert-Slonczewski equation for monodomain nanomagnets : A survey and analysis of numerical techniques | The stochastic Landau-Lifshitz-Gilbert-Slonczewski (s-LLGS) equation is
widely used to study the temporal evolution of the macrospin subject to spin
torque and thermal noise. The numerical simulation of the s-LLGS equation
requires an appropriate choice of stochastic calculus and numerical integration
scheme. In this paper, we comprehensively evaluate the accuracy and complexity
of various numerical techniques to solve the s-LLGS equation. We focus on
implicit midpoint, Heun, and Euler-Heun methods that converge to the
Stratonovich solution of the s-LLGS equation. By performing numerical tests for
both strong (path-wise) and weak (statistical) convergence, we quantify the
accuracy of various numerical schemes used to solve the s-LLGS equation. We
demonstrate a new method intended to solve Stochastic Differential Equations
(SDEs) with small noise (RK4-Heun), and test its capability to handle the
s-LLGS equation. We also discuss the circuit implementation of nanomagnets for
large-scale SPICE-based simulations. We evaluate the efficacy of SPICE in
handling the stochastic dynamics of the multiplicative noise in the s-LLGS
equation. Numerical schemes such as Euler and Gear, typically used by
SPICE-based circuit simulators do not yield the expected outcome when solving
the Stratonovich s-LLGS equation. While the trapezoidal method in SPICE does
solve for the Stratonovich solution, its accuracy is limited by the minimum
time step of integration in SPICE. We implement the s-LLGS equation in both its
cartesian and spherical coordinates form in SPICE and compare the stability and
accuracy of the two implementations. The results in this paper will serve as
guidelines for researchers to understand the tradeoffs between accuracy and
complexity of various numerical methods and the choice of appropriate calculus
to solve the s-LLGS equation. | 1607.04596v4 |
2016-11-30 | Low Energy Supergravity Revisited (I) | General forms of the K\"ahler and superpotenials that lead to consistent low
energy broken Supersymmetry originating from $N=1$ Supergravity have been
classified and used for model building since more than three decades. We point
out the incompleteness of this classification when hidden sector vacuum
expectation values are of the order of the Planck mass. Focusing in this paper
mainly on the case of minimal K\"ahler potential, we adopt a rigorous approach
that retrieves on the one hand the known forms, and demonstrate on the other
hand the existence of a whole set of new forms for the superpotential of which
we give a complete classification. The latter forms involve a new type of
chiral superfields having the unusual property of belonging neither to the
hidden sector nor to the conventional observable sector. Comparing the obtained
forms with the conventional ones, we argue how new possibilities for model
building can arise, and discuss the gravity mediation of soft as well as
additional hard (but parametrically small) Supersymmetry breaking, in the
presence of the new type of chiral superfields. In the simplest case, we study
the vacuum structure, characterize the masses and couplings of the scalar
components to the hidden and observable sectors and discuss briefly the
physical role they could play. In the generic case, we estimate the magnitude
and possible consequences of the hard breaking of Supersymmetry in terms of the
interplay between hidden and visible sectors mass scales. | 1611.10327v2 |
2019-06-20 | Ongoing Vaccine and Monoclonal Antibody HIV Prevention Efficacy Trials and Considerations for Sequel Efficacy Trial Designs | Four randomized placebo-controlled efficacy trials of a candidate vaccine or
passively infused monoclonal antibody for prevention of HIV-1 infection are
underway (HVTN 702 in South African men and women; HVTN 705 in sub-Saharan
African women; HVTN 703/HPTN 081 in sub-Saharan African women; HVTN 704/HPTN
085 in U.S., Peruvian, Brazilian, and Swiss men or transgender persons who have
sex with men). Several challenges are posed to the optimal design of the sequel
efficacy trials, including: (1) how to account for the evolving mosaic of
effective prevention interventions that may be part of the trial design or
standard of prevention; (2) how to define viable and optimal sequel trial
designs depending on the primary efficacy results and secondary 'correlates of
protection' results of each of the ongoing trials; and (3) how to define the
primary objective of sequel efficacy trials if HIV-1 incidence is expected to
be very low in all study arms such that a standard trial design has a steep
opportunity cost. After summarizing the ongoing trials, I discuss statistical
science considerations for sequel efficacy trial designs, both generally and
specifically to each trial listed above. One conclusion is that the results of
'correlates of protection' analyses, which ascertain how different host
immunological markers and HIV-1 viral features impact HIV-1 risk and prevention
efficacy, have an important influence on sequel trial design. This influence is
especially relevant for the monoclonal antibody trials because of the focused
pre-trial hypothesis that potency and coverage of serum neutralization
constitutes a surrogate endpoint for HIV-1 infection... (see manuscript for the
full abstract) | 1906.08409v1 |
2019-08-12 | Elemental Abundances in M31: First Alpha and Iron Abundance Measurements in M31's Giant Stellar Stream | We present the first measurements of [Fe/H] and [$\alpha$/Fe] abundances,
obtained using spectral synthesis modeling, for red giant branch stars in M31's
giant stellar stream. The spectroscopic observations, obtained at a projected
distance of 17 kpc from M31's center, yielded 61 stars with [Fe/H]
measurements, including 21 stars with [$\alpha$/Fe] measurements, from 112
targets identified as M31 stars. The [Fe/H] measurements confirm the
expectation from photometric metallicity estimates that stars in this region of
M31's halo are relatively metal-rich compared to stars in the MW's inner halo:
more than half the stars in the field, including those not associated with
kinematically identified substructure, have [Fe/H] abundances $> -1.0$. The
stars in this field are $\alpha$-enhanced at lower metallicities, while
[$\alpha$/Fe] decreases with increasing [Fe/H] above metallicities of [Fe/H]
$\gtrsim -0.9$. Three kinematical components have been previously identified in
this field: the giant stellar stream, a second kinematically cold feature of
unknown origin, and M31's kinematically hot halo. We compare probabilistic
[Fe/H] and [$\alpha$/Fe] distribution functions for each of the components. The
giant stellar stream and the second kinematically cold feature have very
similar abundance distributions, while the halo component is more metal-poor.
Although the current sample sizes are small, a comparison of the abundances of
stars in the giant stellar stream field with abundances of M31 halo and dSph
stars from the literature indicate that the progenitor of the stream was likely
more massive, and experienced a higher efficiency of star formation, than M31's
existing dSphs or the dEs NGC147 and NGC185. | 1908.04429v1 |
2012-10-11 | Global Properties of M31's Stellar Halo from the SPLASH Survey. I. Surface Brightness Profile | We present the surface brightness profile of M31's stellar halo out to a
projected radius of 175 kpc. The surface brightness estimates are based on
confirmed samples of M31 red giant branch stars derived from Keck/DEIMOS
spectroscopic observations. A set of empirical spectroscopic and photometric
M31 membership diagnostics is used to identify and reject foreground and
background contaminants. This enables us to trace the stellar halo of M31 to
larger projected distances and fainter surface brightnesses than previous
photometric studies. The surface brightness profile of M31's halo follows a
power-law with index -2.2 +/- 0.2 and extends to a projected distance of at
least ~175 kpc (~ 2/3 of M31's virial radius), with no evidence of a downward
break at large radii. The best-fit elliptical isophotes have b/a=0.94 with the
major axis of the halo aligned along the minor axis of M31's disk, consistent
with a prolate halo, although the data are also consistent with M31's halo
having spherical symmetry. The fact that tidal debris features are
kinematically cold is used to identify substructure in the spectroscopic fields
out to projected radii of 90 kpc, and investigate the effect of this
substructure on the surface brightness profile. The scatter in the surface
brightness profile is reduced when kinematically identified tidal debris
features in M31 are statistically subtracted; the remaining profile indicates a
comparatively diffuse stellar component to M31's stellar halo exists to large
distances. Beyond 90 kpc, kinematically cold tidal debris features can not be
identified due to small number statistics; nevertheless, the significant
field-to-field variation in surface brightness beyond 90 kpc suggests that the
outermost region of M31's halo is also comprised to a significant degree of
stars stripped from accreted objects. | 1210.3362v2 |
2018-09-24 | Ionic Tuning of Cobaltites at the Nanoscale | Control of materials through custom design of ionic distributions represents
a powerful new approach to develop future technologies ranging from spintronic
logic and memory devices to energy storage. Perovskites have shown particular
promise for ionic devices due to their high ion mobility and sensitivity to
chemical stoichiometry. In this work, we demonstrate a solid-state approach to
control of ionic distributions in (La,Sr)CoO$_{3}$ thin films. Depositing a Gd
capping layer on the perovskite film, oxygen is controllably extracted from the
structure, up-to 0.5 O/u.c. throughout the entire 36 nm thickness. Commensurate
with the oxygen extraction, the Co valence state and saturation magnetization
show a smooth continuous variation. In contrast, magnetoresistance measurements
show no-change in the magnetic anisotropy and a rapid increase in the
resistivity over the same range of oxygen stoichiometry. These results suggest
significant phase separation, with metallic ferromagnetic regions and
oxygen-deficient, insulating, non-ferromagnetic regions, forming percolated
networks. Indeed, X-ray diffraction identifies oxygen-vacancy ordering,
including transformation to a brownmillerite crystal structure. The unexpected
transformation to the brownmillerite phase at ambient temperature is further
confirmed by high-resolution scanning transmission electron microscopy which
shows significant structural - and correspondingly chemical - phase separation.
This work demonstrates room-temperature ionic control of magnetism, electrical
resistivity, and crystalline structure in a 36 nm thick film, presenting new
opportunities for ionic devices that leverage multiple material
functionalities. | 1809.08728v1 |
2019-04-10 | The Convergence of Iterative Delegations in Liquid Democracy in a Social Network | Liquid democracy is a collective decision making paradigm which lies between
direct and representative democracy. One of its main features is that voters
can delegate their votes in a transitive manner such that: A delegates to B and
B delegates to C leads to A indirectly delegates to C. These delegations can be
effectively empowered by implementing liquid democracy in a social network, so
that voters can delegate their votes to any of their neighbors in the network.
However, it is uncertain that such a delegation process will lead to a stable
state where all voters are satisfied with the people representing them. We
study the stability (w.r.t. voters preferences) of the delegation process in
liquid democracy and model it as a game in which the players are the voters and
the strategies are their possible delegations. We answer several questions on
the equilibria of this process in any social network or in social networks that
correspond to restricted types of graphs.
We show that a Nash-equilibrium may not exist, and that it is even
NP-complete to decide whether one exists or not. This holds even if the social
network is a complete graph or a bounded degree graph. We further show that
this existence problem is W[1]-hard w.r.t. the treewidth of the social network.
Besides these hardness results, we demonstrate that an equilibrium always
exists whatever the preferences of the voters iff the social network is a tree.
We design a dynamic programming procedure to determine some desirable
equilibria (e.g., minimizing the dissatisfaction of the voters) in polynomial
time for tree social networks. Lastly, we study the convergence of delegation
dynamics. Unfortunately, when an equilibrium exists, we show that a best
response dynamics may not converge, even if the social network is a path or a
complete graph. | 1904.05775v2 |
2019-11-06 | Doppler Spectrum Classification with CNNs via Heatmap Location Encoding and a Multi-head Output Layer | Spectral Doppler measurements are an important part of the standard
echocardiographic examination. These measurements give important insight into
myocardial motion and blood flow providing clinicians with parameters for
diagnostic decision making. Many of these measurements can currently be
performed automatically with high accuracy, increasing the efficiency of the
diagnostic pipeline. However, full automation is not yet available because the
user must manually select which measurement should be performed on each image.
In this work we develop a convolutional neural network (CNN) to automatically
classify cardiac Doppler spectra into measurement classes. We show how the
multi-modal information in each spectral Doppler recording can be combined
using a meta parameter post-processing mapping scheme and heatmaps to encode
coordinate locations. Additionally, we experiment with several state-of-the-art
network architectures to examine the tradeoff between accuracy and memory usage
for resource-constrained environments. Finally, we propose a confidence metric
using the values in the last fully connected layer of the network. We analyze
example images that fall outside of our proposed classes to show our confidence
metric can prevent many misclassifications. Our algorithm achieves 96% accuracy
on a test set drawn from a separate clinical site, indicating that the proposed
method is suitable for clinical adoption and enabling a fully automatic
pipeline from acquisition to Doppler spectrum measurements. | 1911.02407v2 |
2020-01-02 | The Effect of Treatment-Related Deaths and "Sticky" Diagnoses on Recorded Prostate Cancer Mortality | Background: Although recorded cancer mortality should include both deaths
from cancer and deaths from cancer treatment, there is evidence suggesting that
the measure may be incomplete. To investigate the completeness of recorded
prostate cancer mortality, we compared other-cause (non-prostate cancer)
mortality in men found and not found to have prostate cancer following a needle
biopsy.
Methods: We linked Medicare claims data to SEER data to analyze survival in
the population of men aged 65+ enrolled in Medicare who resided in a SEER area
and received a needle biopsy in 1993-2001. We compared other-cause mortality in
men found to have prostate cancer (n=53,462) to that in men not found to have
prostate cancer (n=103,659).
Results: The age-race adjusted other-cause mortality rate was 471 per 10,000
person-years in men found to have prostate cancer vs. 468 per 10,000 in men not
found to have prostate cancer (RR = 1.01;95% CI:0.98-1.03). The effect was
modified, however, by age. The RR declined in a stepwise fashion from 1.08 (95%
CI:1.03-1.14) in men age 65-69 to 0.89 (95% CI:0.83-0.95) in men age 85 and
older. If the excess (or deficit) in other-cause mortality were added to the
recorded prostate cancer mortality, prostate cancer mortality would rise 23% in
the youngest age group (from 90 to 111 per 10,000) and would fall 30% in the
oldest age group (from 551 to 388 per 10,000).
Conclusion: Although recorded prostate cancer mortality appears to be an
accurate measure overall, it systematically underestimates the mortality
associated with prostate cancer diagnosis and treatment in younger men and
overestimates it in the very old. We surmise that in younger men
treatment-related deaths are incompletely captured in recorded prostate cancer
mortality, while in older men the diagnosis "sticks"-- once diagnosed, they are
more likely to be said to have died from the disease. | 2001.00492v1 |
2020-06-09 | Elemental Abundances in M31: Iron and Alpha Element Abundances in M31's Outer Halo | We present [Fe/H] and [$\alpha$/Fe] abundances, derived using spectral
synthesis techniques, for stars in M31's outer stellar halo. The 21 [Fe/H]
measurements and 7 [$\alpha$/Fe] measurements are drawn from fields ranging
from 43 to 165 kpc in projected distance from M31. We combine our measurements
with existing literature measurements, and compare the resulting sample of 23
stars with [Fe/H] and 9 stars with [$\alpha$/Fe] measurements in M31's outer
halo with [$\alpha$/Fe] and [Fe/H] measurements, also derived from spectral
synthesis, in M31's inner stellar halo ($r < $26 kpc) and dSph galaxies. The
stars in M31's outer halo have [$\alpha$/Fe] patterns that are consistent with
the largest of M31's dSph satellites (And I and And VII). These abundances
provide tentative evidence that the [$\alpha$/Fe] abundances of stars in M31's
outer halo are more similar to the abundances of Milky Way halo stars than to
the abundances of stars in M31's inner halo. We also compare the spectral
synthesis-based [Fe/H] measurements of stars in M31's halo with previous
photometric [Fe/H] estimates, as a function of projected distance from M31. The
spectral synthesis-based [Fe/H] measurements are consistent with a large-scale
metallicity gradient previously observed in M31's stellar halo to projected
distances as large as 100 kpc. | 2006.05430v1 |
2021-03-30 | Equivalence between Sobolev spaces of first-order dominating mixed smoothness and unanchored ANOVA spaces on $\mathbb{R}^d$ | We prove that a variant of the classical Sobolev space of first-order
dominating mixed smoothness is equivalent (under a certain condition) to the
unanchored ANOVA space on $\mathbb{R}^d$, for $d \geq 1$. Both spaces are
Hilbert spaces involving weight functions, which determine the behaviour as
different variables tend to $\pm \infty$, and weight parameters, which
represent the influence of different subsets of variables. The unanchored ANOVA
space on $\mathbb{R}^d$ was initially introduced by Nichols & Kuo in 2014 to
analyse the error of quasi-Monte Carlo (QMC) approximations for integrals on
unbounded domains; whereas the classical Sobolev space of dominating mixed
smoothness was used as the setting in a series of papers by Griebel, Kuo &
Sloan on the smoothing effect of integration, in an effort to develop a
rigorous theory on why QMC methods work so well for certain non-smooth
integrands with kinks or jumps coming from option pricing problems. In this
same setting, Griewank, Kuo, Le\"ovey & Sloan in 2018 subsequently extended
these ideas by developing a practical smoothing by preintegration technique to
approximate integrals of such functions with kinks or jumps.
We first prove the equivalence in one dimension (itself a non-trivial task),
before following a similar, but more complicated, strategy to prove the
equivalence for general dimensions. As a consequence of this equivalence, we
analyse applying QMC combined with a preintegration step to approximate the
fair price of an Asian option, and prove that the error of such an
approximation using $N$ points converges at a rate close to $1/N$. | 2103.16075v3 |
2021-06-13 | Advantages of a semi-implicit scheme over a fully implicit scheme for Landau-Lifshitz-Gilbert equation | Magnetization dynamics in magnetic materials is modeled by the
Landau-Lifshitz-Gilbert (LLG) equation. In the LLG equation, the length of
magnetization is conserved and the system energy is dissipative. Implicit and
semi-implicit schemes have been used in micromagnetics simulations due to their
unconditional numerical stability. In more details, implicit schemes preserve
the properties of the LLG equation, but solve a nonlinear system of equations
per time step. In contrast, semi-implicit schemes only solve a linear system of
equations, while additional operations are needed to preserve the length of
magnetization. It still remains unclear which one shall be used if both
implicit and semi-implicit schemes are available. In this work, using the
implicit Crank-Nicolson (ICN) scheme as a benchmark, we propose to make this
implicit scheme semi-implicit. It can be proved that both schemes are
second-order accurate in space and time. For the unique solvability of
nonlinear systems of equations in the ICN scheme, we require that the temporal
step size scales quadratically with the spatial mesh size. It is numerically
verified that the convergence of the nonlinear solver becomes slower for larger
temporal step size and multiple magnetization profiles are obtained for
different initial guesses. The linear systems of equations in the semi-implicit
CN (SICN) scheme are unconditionally uniquely solvable, and the condition that
the temporal step size scales linearly with the spatial mesh size is needed in
the convergence of the SICN scheme. In terms of numerical efficiency, the SICN
scheme achieves the same accuracy as the ICN scheme with less computational
time. Based on these results, we conclude that a semi-implicit scheme is
superior to its implicit analog both theoretically and numerically, and we
recommend the semi-implicit scheme in micromagnetics simulations if both
methods are available. | 2106.06936v1 |
2021-08-06 | A second-order semi-implicit method for the inertial Landau-Lifshitz-Gilbert equation | Recent theoretical and experimental advances show that the inertia of
magnetization emerges at sub-picoseconds and contributes to the ultrafast
magnetization dynamics which cannot be captured intrinsically by the LLG
equation. Therefore, as a generalization, the inertial Landau-Lifshitz-Gilbert
(iLLG) equation is proposed to model the ultrafast magnetization dynamics.
Mathematically, the LLG equation is a nonlinear system of parabolic type with
(possible) degeneracy. However, the iLLG equation is a nonlinear system of
mixed hyperbolic-parabolic type with degeneracy, and exhibits more complicated
structures. It behaves like a hyperbolic system at the sub-picosecond scale
while behaves like a parabolic system at larger timescales. Such hybrid
behaviors impose additional difficulties on designing numerical methods for the
iLLG equation. In this work, we propose a second-order semi-implicit scheme to
solve the iLLG equation. The second temporal derivative of magnetization is
approximated by the standard centered difference scheme and the first
derivative is approximated by the midpoint scheme involving three time steps.
The nonlinear terms are treated semi-implicitly using one-sided interpolation
with the second-order accuracy. At each step, the unconditionally unique
solvability of the unsymmetric linear system of equations in the proposed
method is proved with a detailed discussion on the condition number.
Numerically, the second-order accuracy in both time and space is verified.
Using the proposed method, the inertial effect of ferromagnetics is observed in
micromagnetics simulations at small timescales, in consistency with the
hyperbolic property of the model at sub-picoseconds. For long time simulations,
the results of the iLLG model are in nice agreements with those of the LLG
model, in consistency with the parabolic feature of the iLLG model at larger
timescales. | 2108.03060v1 |
2021-09-08 | Flares, Rotation, and Planets of the AU Mic System from TESS Observations | AU Mic is a young ($\sim$24 Myr), pre-Main Sequence M~dwarf star that was
observed in the first month of science observations of the Transiting Exoplanet
Survey Satellite (TESS) and re-observed two years later. This target has
photometric variability from a variety of sources that is readily apparent in
the TESS light curves; spots induce modulation in the light curve, flares are
present throughout (manifesting as sharp rises with slow exponential decay
phases), and transits of AU Mic b may be seen by eye as dips in the light
curve. We present a combined analysis of both TESS Sector 1 and Sector 27 AU
Mic light curves including the new 20-second cadence data from TESS Year 3. We
compare flare rates between both observations and analyze the spot evolution,
showing that the activity levels increase slightly from Sector 1 to Sector 27.
Furthermore, the 20-second data collection allows us to detect more flares,
smaller flares, and better resolve flare morphology in white light as compared
to the 2-minute data collection mode. We also refine the parameters for AU Mic
b by fitting three additional transits of AU Mic b from Sector 27 using a model
that includes stellar activity. We show that the transits exhibit clear transit
timing variations (TTVs) with an amplitude of $\sim$80 seconds. We also detect
three transits of a 2.8 $R_\oplus$ planet, AU Mic c, which has a period of
18.86 days. | 2109.03924v1 |
2021-10-29 | The TREX Survey: Kinematical Complexity Throughout M33's Stellar Disk and Evidence for a Stellar Halo | We present initial results from a large spectroscopic survey of stars
throughout M33's stellar disk. We analyze a sample of 1667 red giant branch
(RGB) stars extending to projected distances of $\sim 11$ kpc from M33's center
($\sim 18$ kpc, or $\sim 10$ scale lengths, in the plane of the disk). The
line-of-sight velocities of RGB stars show the presence of two kinematical
components. One component is consistent with rotation in the plane of M33's HI
disk and has a velocity dispersion ($\sim 19$ km s$^{-1}$) consistent with that
observed in a comparison sample of younger stars, while the second component
has a significantly higher velocity dispersion. A two-component fit to the RGB
velocity distribution finds that the high dispersion component has a velocity
dispersion of $59.3^{+2.6}_{-2.5}$ km s$^{-1}$ and rotates very slowly in the
plane of the disk (consistent with no rotation at the $<1.5\sigma$ level),
which favors interpreting it as a stellar halo rather than a thick disk
population. A spatial analysis indicates that the fraction of RGB stars in the
high-velocity-dispersion component decreases with increasing radius over the
range covered by the spectroscopic sample. Our spectroscopic sample establishes
that a significant high-velocity-dispersion component is present in M33's RGB
population from near M33's center to at least the radius where M33's HI disk
begins to warp at 30$'$ ($\sim 7.5$ kpc) in the plane of the disk. This is the
first detection and spatial characterization of a kinematically hot stellar
component throughout M33's inner regions. | 2110.15773v1 |
2021-11-23 | Magnetism in Metastable and Annealed Compositionally Complex Alloys | Compositionally complex materials (CCMs) present a potential paradigm shift
in the design of magnetic materials. These alloys exhibit long-range structural
order coupled with limited or no chemical order. As a result, extreme local
environments exist with a large opposing magnetic energy term, which can
manifest large changes in the magnetic behavior. In the current work, the
magnetic properties of (Cr, Mn, Fe, Ni) alloys are presented. These materials
were prepared by room-temperature combinatorial sputtering, resulting in a
range of compositions with a single BCC structural phase and no chemical
ordering. The combinatorial growth technique allows CCMs to be prepared outside
of their thermodynamically stable phase, enabling the exploration of otherwise
inaccessible order. The mixed ferromagnetic and antiferromagnetic interactions
in these alloys causes frustrated magnetic behavior, which results in an
extremely low coercivity (<1 mT), which increases rapidly at 50 K. At low
temperatures, the coercivity achieves values of nearly 500 mT, which is
comparable to some high-anisotropy magnetic materials. Commensurate with the
divergent coercivity is an atypical drop in the temperature dependent
magnetization. These effects are explained by a mixed magnetic phase model,
consisting of ferro-, antiferro , and frustrated magnetic regions, and are
rationalized by simulations. A machine-learning algorithm is employed to
visualize the parameter space and inform the development of subsequent
compositions. Annealing the samples at 600 {\deg}C orders the sample, more-than
doubling the Curie temperature and increasing the saturation magnetization by
as much as 5x. Simultaneously, the large coercivities are suppressed, resulting
in magnetic behavior that is largely temperature independent over a range of
350 K. | 2111.12188v1 |
2021-11-23 | Controlling magnetic configuration in soft-hard bilayers probed by polarized neutron reflectometry | Hard/soft magnetic bilayer thin films have been widely used in data storage
technologies and permanent magnet applications. The magnetic configuration and
response to temperatures and magnetic fields in these heterostructures are
considered to be highly dependent on the interfacial coupling. However, the
intrinsic properties of each of the layers, such as the saturation
magnetization and layer thickness, also strongly influence the magnetic
configuration. Changing these parameters provides an effective method to tailor
magnetic properties in composite magnets. Here, we use polarized neutron
reflectometry (PNR) to experimentally probe the interfacial magnetic
configurations in hard/soft bilayer thin films: L10-FePt/A1-FePt, [Co/Pd]
/CoPd, [Co/Pt] /FeNi and L10-FePt/Fe, which all have a perpendicular magnetic
anisotropy in the hard layer. These films were designed with different soft and
hard layer thicknesses (t_soft and t_hard) and saturation magnetization
(M_s^soft and M_s^hard), respectively. The influences of an in-plane magnetic
field (H_ip) and temperature (T) are also studied using a L10 FePt/A1-FePt
bilayer sample. Comparing the PNR results to micromagnetic simulations reveals
that the interfacial magnetic configuration is highly dependent on t_soft,
M_s^soft and the external factors (H_ip and T), and has a relatively weak
dependence on t_hard and M_s^hard. Key among these results, for thin t_soft,
the hard and soft layers are rigidly coupled in the out-of-plane direction,
then undergo a transition to relax in-plane. This transition can be delayed to
larger t_soft by decreasing M_s^soft. Understanding the influence of these
parameters on the magnetic configuration is critical to designing functional
composite magnets for applications. | 2111.12191v1 |
2022-01-18 | On-demand electrical control of spin qubits | Once called a "classically non-describable two-valuedness" by Pauli , the
electron spin is a natural resource for long-lived quantum information since it
is mostly impervious to electric fluctuations and can be replicated in large
arrays using silicon quantum dots, which offer high-fidelity control.
Paradoxically, one of the most convenient control strategies is the integration
of nanoscale magnets to artificially enhance the coupling between spins and
electric field, which in turn hampers the spin's noise immunity and adds
architectural complexity. Here we demonstrate a technique that enables a
\emph{switchable} interaction between spins and orbital motion of electrons in
silicon quantum dots, without the presence of a micromagnet. The naturally weak
effects of the relativistic spin-orbit interaction in silicon are enhanced by
more than three orders of magnitude by controlling the energy quantisation of
electrons in the nanostructure, enhancing the orbital motion. Fast electrical
control is demonstrated in multiple devices and electronic configurations,
highlighting the utility of the technique. Using the electrical drive we
achieve coherence time $T_{2,{\rm Hahn}}\approx50 \mu$s, fast single-qubit
gates with ${T_{\pi/2}=3}$ ns and gate fidelities of 99.93 % probed by
randomised benchmarking. The higher gate speeds and better compatibility with
CMOS manufacturing enabled by on-demand electric control improve the prospects
for realising scalable silicon quantum processors. | 2201.06679v2 |
2022-04-28 | Anti-microbial properties of a multi-component alloy | High traffic touch surfaces such as doorknobs, countertops, and handrails can
be transmission points for the spread of pathogens, emphasizing the need to
develop materials that actively self-sanitize. Metals are frequently used for
these surfaces due to their durability, but many metals also possess
antimicrobial properties which function through a variety of mechanisms. This
work investigates metallic alloys comprised of several bioactive metals with
the target of achieving broad-spectrum, rapid bioactivity through synergistic
activity. An entropy-motivated stabilization paradigm is proposed to prepare
scalable alloys of copper, silver, nickel and cobalt. Using combinatorial
sputtering, thin-film alloys were prepared on 100 mm wafers with 50%
compositional grading of each element across the wafer. The films were then
annealed and investigated for alloy stability. Bioactivity testing was
performed on both the as-grown alloys and the annealed films using four
microorganisms -- Phi6, MS2, Bacillus subtilis and Escherichia coli -- as
surrogates for human viral and bacterial pathogens. Testing showed that after
30 s of contact with some of the test alloys, Phi6, an enveloped,
single-stranded RNA bacteriophage that serves as a SARS-CoV 2 surrogate, was
reduced up to 6.9 orders of magnitude (>99.9999%). Additionally, the
non-enveloped, double-stranded DNA bacteriophage MS2, and the Gram-negative E.
coli and Gram-positive B. subtilis bacterial strains showed a 5.0, 6.4, and 5.7
log reduction in activity after 30, 20 and 10 minutes, respectively.
Bioactivity in the alloy samples showed a strong dependence on the composition,
with the log reduction scaling directly with the Cu content. Concentration of
Cu by phase separation after annealing improved activity in some of the
samples. The results motivate a variety of themes which can be leveraged to
design ideal bioactive surfaces. | 2205.00886v1 |
2022-05-11 | Models of Advance Recording Systems: A Multi-timescale Micromagnetic code for granular thin film magnetic recording systems | Micromagnetic modelling provides the ability to simulate large magnetic
systems accurately without the computational cost limitation imposed by
atomistic modelling. Through micromagnetic modelling it is possible to simulate
systems consisting of thousands of grains over a time range of nanoseconds to
years, depending upon the solver used. Here we present the creation and release
of an open-source multi-timescale micromagnetic code combining three key
solvers: Landau-Lifshitz-Gilbert; Landau-Lifshitz-Bloch; Kinetic Monte Carlo.
This code, called MARS (Models of Advanced Recording Systems), is capable of
accurately simulating the magnetisation dynamics in large and structurally
complex single- and multi-layered granular systems. The short timescale
simulations are achieved for systems far from and close to the Curie point via
the implemented Landau-Lifshitz-Gilbert and Landau-Lifshitz-Bloch solvers
respectively. This enables read/write simulations for general perpendicular
magnetic recording and also state of the art heat assisted magnetic recording
(HAMR). The long timescale behaviour is simulated via the Kinetic Monte Carlo
solver, enabling investigations into signal-to-noise ratio and data longevity.
The combination of these solvers opens up the possibility of multi-timescale
simulations within a single software package. For example the entire HAMR
process from initial data writing and data read back to long term data storage
is possible via a single simulation using MARS. The use of atomistic
parameterisation for the material input of MARS enables highly accurate
material descriptions which provide a bridge between atomistic simulation and
real world experimentation. Thus MARS is capable of performing simulations for
all aspects of recording media research and development. This ranges from
material characterisation and optimisation to system design and implementation. | 2205.05263v1 |
2022-10-11 | Finite-time singularity formations for the Landau-Lifshitz-Gilbert equation in dimension two | We construct finite time blow-up solutions to the Landau-Lifshitz-Gilbert
equation (LLG) from ${\mathbb R}^2$ into $S^2$ \begin{equation*} \begin{cases}
u_t= a(\Delta u+|\nabla u|^2u) -b u\wedge \Delta u &\ \mbox{ in }\ {\mathbb
R}^2\times(0,T), u(\cdot,0) = u_0\in S^2 &\ \mbox{ in }\ {\mathbb R}^2,
\end{cases} \end{equation*} where $a^2+b^2=1,~a > 0,~ b\in {\mathbb R}$. Given
any prescribed $N$ points in $\mathbb{R}^2$ and small $T>0$, we prove that
there exists regular initial data such that the solution blows up precisely at
these points at finite time $t=T$, taking around each point the profile of
sharply scaled degree 1 harmonic map with the type II blow-up speed
\begin{equation*} \| \nabla u\|_{L^\infty } \sim \frac{|\ln(T-t)|^2}{ T-t } \
\mbox{ as } \ t\to T. \end{equation*} The proof is based on the {\em parabolic
inner-outer gluing method}, developed in \cite{17HMF} for Harmonic Map Flow
(HMF). However, a direct consequence of the presence of dispersion is the {\em
lack of maximum principle} for suitable quantities, which makes the analysis
more delicate even at the linearized level. To overcome this difficulty, we
make use of two key technical ingredients: first, for the inner problem we
employ the tool of {\em distorted Fourier transform}, as developed by Krieger,
Miao, Schlag and Tataru \cite{Krieger09Duke,KMS20WM}. Second, the linear theory
for the outer problem is achieved by means of the sub-Gaussian estimate for the
fundamental solution of parabolic system in non-divergence form with
coefficients of Dini mean oscillation in space ($\mathsf{DMO_x}$), which was
proved by Dong, Kim and Lee \cite{dong22-non-divergence} recently. | 2210.05800v1 |
2023-01-03 | Measuring Physical and Electrical Parameters in Free-Living Subjects: Motivating an Instrument to Characterize Analytes of Clinical Importance in Blood Samples | Significance: A path is described to increase the sensitivity and accuracy of
body-worn devices used to monitor patient health. This path supports improved
health management. A wavelength-choice algorithm developed at Mayo demonstrates
that critical biochemical analytes can be assessed using accurate optical
absorption curves over a wide range of wavelengths. Aim: Combine the
requirements for monitoring cardio/electrical, movement, activity, gait,
tremor, and critical biochemical analytes including hemoglobin makeup in the
context of body-worn sensors. Use the data needed to characterize clinically
important analytes in blood samples to drive instrument requirements. Approach:
Using data and knowledge gained over previously separate research threads, some
providing currently usable results from more than eighty years back, determine
analyte characteristics needed to design sensitive and accurate multiuse
measurement and recording units. Results: Strategies for wavelength selection
are detailed. Fine-grained, broad-spectrum measurement of multiple analytes
transmission, absorption, and anisotropic scattering are needed.
Post-Beer-Lambert, using the propagation of error from small variations, and
utility functions that include costs and systemic error sources, improved
measurements can be performed. Conclusions: The Mayo Double-Integrating Sphere
Spectrophotometer (referred hereafter as MDISS), as described in the companion
report arXiv:2212.08763, produces the data necessary for optimal component
choice. These data can provide for robust enhancement of the sensitivity, cost,
and accuracy of body-worn medical sensors. Keywords: Bio-Analyte,
Spectrophotometry, Body-worn monitor, Propagation of error, Double-Integrating
Sphere, Mt. Everest medical measurements, O2SAT
Please see also arXiv:2212.08763 | 2301.00938v2 |
2023-01-09 | A Second Earth-Sized Planet in the Habitable Zone of the M Dwarf, TOI-700 | We report the discovery of TOI-700 e, a 0.95 R$_\oplus$ planet residing in
the Optimistic Habitable Zone (HZ) of its host star. This discovery was enabled
by multiple years of monitoring from NASA's Transiting Exoplanet Survey
Satellite (TESS) mission. The host star, TOI-700 (TIC 150428135), is a nearby
(31.1 pc), inactive, M2.5 dwarf ($V_{mag} = 13.15$). TOI-700 is already known
to host three planets, including the small, HZ planet, TOI-700 d. The new
planet has an orbital period of 27.8 days and, based on its radius (0.95
R$_\oplus$), it is likely rocky. TOI-700 was observed for 21 sectors over Years
1 and 3 of the TESS mission, including 10 sectors at 20-second cadence in Year
3. Using this full set of TESS data and additional follow-up observations, we
identify, validate, and characterize TOI-700 e. This discovery adds another
world to the short list of small, HZ planets transiting nearby and bright host
stars. Such systems, where the stars are bright enough that follow-up
observations are possible to constrain planet masses and atmospheres using
current and future facilities, are incredibly valuable. The presence of
multiple small, HZ planets makes this system even more enticing for follow-up
observations. | 2301.03617v1 |
2023-01-17 | Introduction to Non-Invasive Current Estimation (NICE) | It is notoriously difficult to measure instantaneous supply current to a
device such as an ASIC, FPGA, or CPU without also affecting the instantaneous
supply voltage and compromising the operation of the device [21]. For decades
designers have relied on rough estimates of dynamic load currents that
stimulate a designed Power Delivery Network (PDN). The consequences of
inaccurate load-current characterization can range from excessive PDN cost and
lengthened development schedules to poor performance or functional failure.
This paper will introduce and describe a method to precisely determine
timedomain current waveforms from a pair of measured timedomain voltage
waveforms. This NonInvasive Current Estimation (NICE) method is based on
established twoport network theory along with component and board modeling
techniques that have been validated through measurements on demonstrative
circuits. This paper will show that the NICE method works for any transient
event that can be captured on a digital oscilloscope. Limitations of the method
and underlying measurements are noted where appropriate. The method is applied
to a simple PDN with an arbitrary load, and the NICE-derived current waveform
is verified against an independent measurement by sense resistor. With careful
component and board modeling, it is possible to calculate current waveforms
with a root mean square error of less than five percent compared to the
reference measurement. Current transients that were previously difficult or
impossible to characterize by any means can now be calculated and displayed
within seconds of an oscilloscope-trigger event by using NICE. ASIC and FPGA
manufacturers can now compute the startup current for their device and publish
the actual waveform, or provide a piecewiselinear SPICE model (PWL source) to
facilitate design and testing of the regulator and PDN required to support
their device. | 2301.10237v1 |
2023-02-08 | Weighted Edit Distance Computation: Strings, Trees and Dyck | Given two strings of length $n$ over alphabet $\Sigma$, and an upper bound
$k$ on their edit distance, the algorithm of Myers (Algorithmica'86) and Landau
and Vishkin (JCSS'88) computes the unweighted string edit distance in
$\mathcal{O}(n+k^2)$ time. Till date, it remains the fastest algorithm for
exact edit distance computation, and it is optimal under the Strong Exponential
Hypothesis (STOC'15). Over the years, this result has inspired many
developments, including fast approximation algorithms for string edit distance
as well as similar $\tilde{\mathcal{O}}(n+$poly$(k))$-time algorithms for
generalizations to tree and Dyck edit distances. Surprisingly, all these
results hold only for unweighted instances.
While unweighted edit distance is theoretically fundamental, almost all
real-world applications require weighted edit distance, where different weights
are assigned to different edit operations and may vary with the characters
being edited. Given a weight function $w: \Sigma \cup \{\varepsilon \}\times
\Sigma \cup \{\varepsilon \} \rightarrow \mathbb{R}_{\ge 0}$ (such that
$w(a,a)=0$ and $w(a,b)\ge 1$ for all $a,b\in \Sigma \cup \{\varepsilon\}$ with
$a\ne b$), the goal is to find an alignment that minimizes the total weight of
edits. Except for the vanilla $\mathcal{O}(n^2)$-time dynamic-programming
algorithm and its almost trivial $\mathcal{O}(nk)$-time implementation, none of
the aforementioned developments on the unweighted edit distance apply to the
weighted variant. In this paper, we propose the first
$\mathcal{O}(n+$poly$(k))$-time algorithm that computes weighted string edit
distance exactly, thus bridging a fundamental gap between our understanding of
unweighted and weighted edit distance. We then generalize this result to
weighted tree and Dyck edit distances, which lead to a deterministic algorithm
that improves upon the previous work for unweighted tree edit distance. | 2302.04229v1 |
2023-03-07 | Multilevel Monte Carlo methods for stochastic convection-diffusion eigenvalue problems | We develop new multilevel Monte Carlo (MLMC) methods to estimate the
expectation of the smallest eigenvalue of a stochastic convection-diffusion
operator with random coefficients. The MLMC method is based on a sequence of
finite element (FE) discretizations of the eigenvalue problem on a hierarchy of
increasingly finer meshes. For the discretized, algebraic eigenproblems we use
both the Rayleigh quotient (RQ) iteration and implicitly restarted Arnoldi
(IRA), providing an analysis of the cost in each case. By studying the variance
on each level and adapting classical FE error bounds to the stochastic setting,
we are able to bound the total error of our MLMC estimator and provide a
complexity analysis. As expected, the complexity bound for our MLMC estimator
is superior to plain Monte Carlo. To improve the efficiency of the MLMC
further, we exploit the hierarchy of meshes and use coarser approximations as
starting values for the eigensolvers on finer ones. To improve the stability of
the MLMC method for convection-dominated problems, we employ two additional
strategies. First, we consider the streamline upwind Petrov--Galerkin
formulation of the discrete eigenvalue problem, which allows us to start the
MLMC method on coarser meshes than is possible with standard FEs. Second, we
apply a homotopy method to add stability to the eigensolver for each sample.
Finally, we present a multilevel quasi-Monte Carlo method that replaces Monte
Carlo with a quasi-Monte Carlo (QMC) rule on each level. Due to the faster
convergence of QMC, this improves the overall complexity. We provide detailed
numerical results comparing our different strategies to demonstrate the
practical feasibility of the MLMC method in different use cases. The results
support our complexity analysis and further demonstrate the superiority over
plain Monte Carlo in all cases. | 2303.03673v2 |
2023-03-09 | Zonostrophic instabilities in magnetohydrodynamic Kolmogorov flow | This paper concerns the stability of Kolmogorov flow u = (0, sin x) in the
infinite (x,y)-plane. A mean magnetic field of strength B0 is introduced and
the MHD linear stability problem studied for modes with wave-number k in the
y-direction, and Bloch wavenumber l in the x-direction. The parameters
governing the problem are Reynolds number 1/nu, magnetic Prandtl number P, and
dimensionless magnetic field strength B0. The mean magnetic field can be taken
to have an arbitrary direction in the (x,y)-plane and a mean x-directed flow U0
can be incorporated.
First the paper considers Kolmogorov flow with y-directed mean magnetic
field, referred to as vertical. Taking l=0, the suppression of the pure
hydrodynamic instability is observed with increasing field strength B0. A
branch of strong-field instabilities occurs for magnetic Prandtl number P less
than unity, as found by A.E. Fraser, I.G. Cresser and P. Garaud (J. Fluid Mech.
949, A43, 2022). Analytical results using eigenvalue perturbation theory in the
limit k->0 support the numerics for both weak- and strong-field instabilities,
and originate in the coupling of large-scale modes with x-wavenumber n=0, to
smaller-scale modes.
The paper considers the case of horizontal or x-directed mean magnetic field.
The unperturbed state consists of steady, wavey magnetic field lines. As the
magnetic field is increased, the purely hydrodynamic instability is suppressed
again, but for stronger fields a new branch of instabilities appears. Allowing
a non-zero Bloch wavenumber l allows further instability, and in some
circumstances when the system is hydrodynamically stable, arbitrarily weak
magnetic fields can give growing modes. Numerical results are presented
together with eigenvalue perturbation theory in the limits k,l->0. The theory
gives analytical approximations for growth rates and thresholds in good
agreement with those computed. | 2303.05212v1 |
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