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2021-08-22 | Factors Enhancing E-Government Service Gaps in a Developing Country Context | Globally, the discourse of e-government has gathered momentum in public
service delivery. No country has been left untouched in the implementation of
e-government. Several government departments and agencies are now using
information and communication technology (ICTs) to deliver government services
and information to citizens, other government departments, and businesses.
However, most of the government departments have not provided all of their
services electronically or at least the most important ones. Thus, this creates
a phenomenon of e-government service gaps. The objective of this study was to
investigate the contextual factors enhancing e-government service gaps in a
developing country. To achieve this aim, the TOE framework was employed
together with a qualitative case study to guide data collection and analysis.
The data was collected through semi-structured interviews from government
employees who are involved in the implementation of e-government services in
Zimbabwe as well as from citizens and businesses. Eleven (11) factors were
identified and grouped under the TOE framework. This research contributes
significantly to the implementation and utilisation of e-government services in
Zimbabwe. The study also contributes to providing a strong theoretical
understanding of the factors that enhance e-government service gaps explored in
the research model. | 2108.09803v1 |
2021-09-23 | Cyclically presented groups as Labelled Oriented Graph groups | We use results concerning the Smith forms of circulant matrices to identify
when cyclically presented groups have free abelianisation and so can be
Labelled Oriented Graph (LOG) groups. We generalize a theorem of Odoni and
Cremona to show that for a fixed defining word, whose corresponding representer
polynomial has an irreducible factor that is not cyclotomic and not equal to
$\pm t$, there are at most finitely many $n$ for which the corresponding
$n$-generator cyclically presented group has free abelianisation. We classify
when Campbell and Robertson's generalized Fibonacci groups $H(r,n,s)$ are LOG
groups and when the Sieradski groups are LOG groups. We prove that amongst
Johnson and Mawdesley's groups of Fibonacci type, the only ones that can be LOG
groups are Gilbert-Howie groups $H(n,m)$. We conjecture that if a Gilbert-Howie
group is a LOG group, then it is a Sieradski group, and prove this in certain
cases (in particular, for fixed $m$, the conjecture can only be false for
finitely many $n$). We obtain necessary conditions for a cyclically presented
group to be a connected LOG group in terms of the representer polynomial and
apply them to the Prishchepov groups. | 2109.11463v1 |
2021-12-03 | Spectral reconstruction in NRQCD via the Backus-Gilbert method | We present progress results from the FASTSUM collaboration's programme to
determine the spectrum of the bottomonium system as a function of temperature
using a variety of approaches. In this contribution, the Backus Gilbert method
is used to reconstruct spectral functions from NRQCD meson correlator data from
FASTSUM's anisotropic ensembles at nonzero temperature. We focus in particular
on the resolving power of the method, providing a demonstration of how the
underlying resolution functions can be probed by exploiting the Laplacian
nature of the NRQCD kernel. We conclude with estimates of the bottomonium
ground state mass and widths at nonzero temperature. | 2112.02075v2 |
2021-12-23 | Energy minimizing maps with prescribed singularities and Gilbert-Steiner optimal networks | We investigate the relation between energy minimizing maps valued into
spheres having topological singularities at given points and optimal networks
connecting them (e.g. Steiner trees, Gilbert-Steiner irrigation networks). We
show the equivalence of the corresponding variational problems, interpreting in
particular the branched optimal transport problem as a homological Plateau
problem for rectifiable currents with values in a suitable normed group. This
generalizes the pioneering work by Brezis, Coron and Lieb [10]. | 2112.12511v4 |
2022-02-02 | Asymptotic stability of precessing domain walls for the Landau-Lifshitz-Gilbert equation in a nanowire with Dzyaloshinskii-Moriya interaction | We consider a ferromagnetic nanowire and we focus on an asymptotic regime
where the Dzyaloshinskii-Moriya interaction is taken into account. First we
prove a dimension reduction result via $\Gamma$-convergence that determines a
limit functional $E$ defined for maps $m:\mathbb{R}\to \mathbb{S}^2$ in the
direction $e_1$ of the nanowire. The energy functional $E$ is invariant under
translations in $e_1$ and rotations about the axis $e_1$. We fully classify the
critical points of finite energy $E$ when a transition between $-e_1$ and $e_1$
is imposed; these transition layers are called (static) domain walls. The
evolution of a domain wall by the Landau-Lifshitz-Gilbert equation associated
to $E$ under the effect of an applied magnetic field $h(t)e_1$ depending on the
time variable $t$ gives rise to the so-called precessing domain wall. Our main
result proves the asymptotic stability of precessing domain walls for small $h$
in $L^\infty([0, +\infty))$ and small $H^1(\mathbb{R})$ perturbations of the
static domain wall, up to a gauge which is intrinsic to invariances of the
functional $E$. | 2202.01005v1 |
2022-02-14 | The Higgs Boson Mass as Fundamental Parameter of the Minimal Supersymmetric Standard Model | In the Minimal Supersymmetric Standard Model (MSSM) the mass of the lightest
neutral Higgs boson is determined by the supersymmetric parameters. In the
$m_h$MSSM the precisely measured Higgs boson replaces the trilinear coupling
$A_t$ as input parameter. Expressions are derived to extract $A_t$ in a
semi-analytical form as a function of the light Higgs boson (pole) mass. An
algorithm is developed and implemented at two--loop precision, generalizable to
higher orders, to perform this inversion consistently. The result of the
algorithm, implemented in the SuSpect spectrum calculator, is illustrated on a
parameter set compatible with LHC measurements. | 2202.06919v2 |
2022-02-15 | Coding and Bounds for Partially Defective Memory Cells | This paper considers coding for so-called partially stuck (defect) memory
cells. Such memory cells can only store partial information as some of their
levels cannot be used fully due to, e.g., wearout. First, we present new
constructions that are able to mask $u$ partially stuck cells while correcting
at the same time $t$ random errors. The process of "masking" determines a word
whose entries coincide with writable levels at the (partially) stuck cells. For
$u>1$ and alphabet size $q>2$, our new constructions improve upon the required
redundancy of known constructions for $t=0$, and require less redundancy for
masking partially stuck cells than former works required for masking fully
stuck cells (which cannot store any information). Second, we show that treating
some of the partially stuck cells as erroneous cells can decrease the required
redundancy for some parameters. Lastly, we derive Singleton-like,
sphere-packing-like, and Gilbert--Varshamov-like bounds. Numerical comparisons
state that our constructions match the Gilbert--Varshamov-like bounds for
several code parameters, e.g., BCH codes that contain all-one word by our first
construction. | 2202.07541v1 |
2022-03-24 | Multi-platform Process Flow Models and Algorithms for Extraction and Documentation of Digital Forensic Evidence from Mobile Devices | The increasing need for the examination of evidence from mobile and portable
gadgets increases the essential need to establish dependable measures for the
investigation of these gadgets. Many differences exist while detailing the
requirement for the examination of each gadget, to help detectives and
examiners in guaranteeing that of any kind piece of evidence extracted/
collected from any mobile devices is well documented and the outcomes can be
repeatable, a reliable and well-documented investigation process must be
implemented if the results of the examination are to be repeatable and
defensible in courts of law. In this paper we developed a generic process flow
model for the extraction of digital evidence in mobile devices running on
android, Windows, iOs and Blackberry operating system. The research adopted
survey approach and extensive literature review a s means to collect data. The
models developed were validate through expert opinion. Results of this work can
guide solution developers in ensuring standardization of evidence extraction
tools for mobile devices. | 2203.13258v1 |
2022-06-07 | Implicit biases in transit models using stellar pseudo-density | The transit technique is responsible for the majority of exoplanet
discoveries to date. Characterizing these planets involves careful modeling of
their transit profiles. A common technique involves expressing the transit
duration using a density-like parameter, $\tilde{\rho}$, often called the
"circular density." Most notably, the Kepler project -- the largest analysis of
transit lightcurves to date -- adopted a linear prior on $\tilde{\rho}$. Here,
we show that such a prior biases measurements of impact parameter, $b$, due to
the non-linear relationship between $\tilde{\rho}$ and transit duration. This
bias slightly favors low values ($b \lesssim 0.3$) and strongly disfavors high
values ($b \gtrsim 0.7$) unless transit signal-to-noise ratio is sufficient to
provide an independent constraint on $b$, a criterion that is not satisfied for
the majority of Kepler planets. Planet-to-star radius ratio, $r$, is also
biased due to $r{-}b$ covariance. Consequently, the median Kepler DR25 target
suffers a $1.6\%$ systematic underestimate of $r$. We present a techniques for
correcting these biases and for avoiding them in the first place. | 2206.03432v1 |
2022-06-22 | Homogenization of the Landau-Lifshitz-Gilbert equation with natural boundary condition | The full Landau-Lifshitz-Gilbert equation with periodic material coefficients
and natural boundary condition is employed to model the magnetization dynamics
in composite ferromagnets. In this work, we establish the convergence between
the homogenized solution and the original solution via a Lax equivalence
theorem kind of argument. There are a few technical difficulties, including: 1)
it is proven the classic choice of corrector to homogenization cannot provide
the convergence result in the $H^1$ norm; 2) a boundary layer is induced due to
the natural boundary condition; 3) the presence of stray field give rise to a
multiscale potential problem. To keep the convergence rates near the boundary,
we introduce the Neumann corrector with a high-order modification. Estimates on
singular integral for disturbed functions and boundary layer are deduced, to
conduct consistency analysis of stray field. Furthermore, inspired by length
conservation of magnetization, we choose proper correctors in specific
geometric space. These, together with a uniform $W^{1,6}$ estimate on original
solution, provide the convergence rates in the $H^1$ sense. | 2206.10948v1 |
2022-09-12 | GenLoco: Generalized Locomotion Controllers for Quadrupedal Robots | Recent years have seen a surge in commercially-available and affordable
quadrupedal robots, with many of these platforms being actively used in
research and industry. As the availability of legged robots grows, so does the
need for controllers that enable these robots to perform useful skills.
However, most learning-based frameworks for controller development focus on
training robot-specific controllers, a process that needs to be repeated for
every new robot. In this work, we introduce a framework for training
generalized locomotion (GenLoco) controllers for quadrupedal robots. Our
framework synthesizes general-purpose locomotion controllers that can be
deployed on a large variety of quadrupedal robots with similar morphologies. We
present a simple but effective morphology randomization method that
procedurally generates a diverse set of simulated robots for training. We show
that by training a controller on this large set of simulated robots, our models
acquire more general control strategies that can be directly transferred to
novel simulated and real-world robots with diverse morphologies, which were not
observed during training. | 2209.05309v1 |
2022-10-11 | Element-Specific First Order Reversal Curves Measured by Magnetic Transmission X-ray Microscopy | The first order reversal curve (FORC) method is a macroscopic measurement
technique which can be used to extract quantitative, microscopic properties of
hysteretic systems. Using magnetic transmission X-ray microscopy (MTXM), local
element-specific FORC measurements are performed on a 20 nm thick film of CoTb.
The FORCs measured with microscopy reveal a step-by-step domain evolution under
the magnetic field cycling protocol, and provide a direct visualization of the
mechanistic interpretation of FORC diagrams. They are compared with
magnetometry FORCs and show good quantitative agreement. Furthermore, the high
spatial resolution and element-specific sensitivity of MTXM provide new
capabilities to measure FORCs on small regions or specific phases within
multicomponent systems, including buried layers in heterostructures. The
ability to perform FORCs on very small features is demonstrated with the
MTXM-FORC measurement of a rectangular microstructure with vortex-like Landau
structures. This work demonstrates the confluence of two uniquely powerful
techniques to achieve quantitative insight into nanoscale magnetic behavior. | 2210.05739v1 |
2022-11-08 | Landau-Lifshitz-Gilbert equations: Controllability by Low Modes Forcing for deterministic version and Support Theorems for Stochastic version | In this article, we study the controllability issues of the
Landau-Lifshitz-Gilbert Equations (LLGEs), accompanied with non-zero exchange
energy only, in an interval in one spatial dimension with Neumann boundary
conditions. The paper is of twofold. In the first part of the paper, we study
the controllability issues of the LLGEs. The control force acting here is
degenerate i.e., it acts through a few numbers of low mode frequencies. We
exploit the Fourier series expansion of the solution. We borrow methods of
differential geometric control theory (Lie bracket generating property) to
establish the global controllability of the finite-dimensional Galerkin
approximations of LLGEs. We show $L^2$ approximate controllability of the full
system. In the second part, we consider the LLGEs with lower-dimensional
degenerate random forcing (finite-dimensional Brownian motions) and study
support theorems. | 2211.04204v1 |
2022-11-18 | Knowledge Graph Refinement based on Triplet BERT-Networks | Knowledge graph embedding techniques are widely used for knowledge graph
refinement tasks such as graph completion and triple classification. These
techniques aim at embedding the entities and relations of a Knowledge Graph
(KG) in a low dimensional continuous feature space. This paper adopts a
transformer-based triplet network creating an embedding space that clusters the
information about an entity or relation in the KG. It creates textual sequences
from facts and fine-tunes a triplet network of pre-trained transformer-based
language models. It adheres to an evaluation paradigm that relies on an
efficient spatial semantic search technique. We show that this evaluation
protocol is more adapted to a few-shot setting for the relation prediction
task. Our proposed GilBERT method is evaluated on triplet classification and
relation prediction tasks on multiple well-known benchmark knowledge graphs
such as FB13, WN11, and FB15K. We show that GilBERT achieves better or
comparable results to the state-of-the-art performance on these two refinement
tasks. | 2211.10460v1 |
2023-01-11 | Thou Shalt not Pick all Items if Thou are First: of Strategyproof and Fair Picking Sequences | When allocating indivisible items to agents, it is known that the only
strategyproof mechanisms that satisfy a set of rather mild conditions are
constrained serial dictatorships: given a fixed order over agents, at each step
the designated agent chooses a given number of items (depending on her position
in the sequence). With these rules, also known as non-interleaving picking
sequences, agents who come earlier in the sequence have a larger choice of
items. However, this advantage can be compensated by a higher number of items
received by those who come later. How to balance priority in the sequence and
number of items received is a nontrivial question. We use a previous model,
parameterized by a mapping from ranks to scores, a social welfare functional,
and a distribution over preference profiles. For several meaningful choices of
parameters, we show that the optimal sequence can be computed in polynomial
time. Last, we give a simple procedure for eliciting scoring vectors and we
study the impact of the assignment from agents to positions on the ex-post
social welfare. | 2301.06086v1 |
2023-01-17 | Comparison of Optical and Electrical Links for Highly-Interconnected Systems | As data rates for multi-gigabit serial interfaces within multi-node compute
systems approach and exceed 10 Gigabits per second (Gbps), board-to-board and
chip-to-chip optical signaling solutions become more attractive, particularly
for longer (e.g. 50-100 cm) links. The transition to optical signaling will
potentially allow new high performance compute (HPC) system architectures that
benefit from characteristics unique to optical links.
To examine these characteristics, we built and tested several optical
demonstration vehicles; one based on dense wavelength division multiplexing
(DWDM), and others based on multiple point-to-point links carried across
multimode fibers. All test vehicles were constructed to evaluate applicability
to a multi-node compute system. Test results, combined with data from recent
research efforts are summarized and compared to equivalent electrical links and
the advantages and design characteristics unique to optical signaling are
identified. | 2301.10169v1 |
2023-01-17 | PWB Manufacturing Variability Effects on High Speed SerDes Links: Statistical Insights from Thousands of 4-Port SParameter Measurements | Variability analysis is important in successfully deploying multi-gigabit
backplane printed wiring boards (PWBs) with growing numbers of high-speed
SerDes links. We discuss the need for large sample sizes to obtain accurate
variability estimates of SI metrics (eye height, phase skew, etc).
Using a dataset of 11,961 S-parameters, we demonstrate statistical techniques
to extract accurate estimates of PWB SI performance variations. We cite
numerical examples illustrating how these variations may contribute to
underestimated or overestimated design criteria, causing unnecessary design
expense. Tabular summaries of performance variation and key findings of broad
interest to the general SI community are highlighted. | 2301.10176v1 |
2023-01-17 | A Zero Sum Signaling Method for High Speed, Dense Parallel Bus Communications | Complex digital systems such as high performance computers (HPCs) make
extensive use of high-speed electrical interconnects, in routing signals among
processing elements, or between processing elements and memory. Despite
increases in serializer/deserializer (SerDes) and memory interface speeds,
there is demand for higher bandwidth busses in constrained physical spaces
which still mitigate simultaneous switching noise (SSN). The concept of zero
sum signaling utilizes coding across a data bus to allow the use of
single-ended buffers while still mitigating SSN, thereby reducing the number of
physical channels (e.g. circuit board traces) by nearly a factor of two when
compared with traditional differential signaling. Through simulation and
analysis of practical (non-ideal) data bus and power delivery network
architectures, we demonstrate the feasibility of zero sum signaling and compare
performance with that of traditional (single-ended and differential) methods. | 2302.05427v1 |
2023-03-20 | Dynamic Documentation for AI Systems | AI documentation is a rapidly-growing channel for coordinating the design of
AI technologies with policies for transparency and accessibility. Calls to
standardize and enact documentation of algorithmic harms and impacts are now
commonplace. However, documentation standards for AI remain inchoate, and fail
to match the capabilities and social effects of increasingly impactful
architectures such as Large Language Models (LLMs). In this paper, we show the
limits of present documentation protocols, and argue for dynamic documentation
as a new paradigm for understanding and evaluating AI systems. We first review
canonical approaches to system documentation outside the context of AI,
focusing on the complex history of Environmental Impact Statements (EISs). We
next compare critical elements of the EIS framework to present challenges with
algorithmic documentation, which have inherited the limitations of EISs without
incorporating their strengths. These challenges are specifically illustrated
through the growing popularity of Model Cards and two case studies of
algorithmic impact assessment in China and Canada. Finally, we evaluate more
recent proposals, including Reward Reports, as potential components of fully
dynamic AI documentation protocols. | 2303.10854v1 |
2023-01-17 | 56 Gbps PCB Design Strategies for Clean, Low-Skew Channels | Although next generation (>28 Gbps) SerDes standards have been contemplated
for several years, it has not been clear whether PCB structures supporting 56
Gbps NRZ will be feasible and practical. In this paper, we assess a number of
specific PCB design strategies (related to pin-field breakouts, via stubs, and
fiber weave skew) both through simulation and through measurement of a wide
range of structures on a PCB test vehicle. We demonstrate that conventional
approaches in many cases will not be sufficient, but that modest
(manufacturable) design changes can enable low-skew 56 Gbps NRZ channels having
acceptable insertion and return loss. | 2304.01909v1 |
2023-01-17 | Inverting the SerDes Link Design Flow Process | The traditional SerDes link simulation process begins with the extraction of
printed circuit board (PCB) physical stripline and via models, followed by
channel modeling and link simulation. We invert this simulation flow by first
creating link performance curves across an array of hypothetical channels
defined with specially-developed, high level, equation-based models; limited
physical extraction is later undertaken to relate PCB channel implementation to
these performance curves. These curves allow us to determine the system-level
SerDes channel requirements and to become better informed in choosing PCB
technologies for lower cost and easier manufacturability. The inverted modeling
process is very efficient, allowing for the rapid identification and avoidance
of problematic channel topologies and the study of other potentially useful
channel designs. | 2304.01911v1 |
2023-01-17 | Plated-Through-Hole Via Design Specifications for 112G Serial Links | An earlier study of a high layer-count test board using plated-through-hole
(PTH) vias and a limited quantity of laser vias was shown to be capable of
supporting 112 Gb/s PAM-4 links (or equivalent signaling having 28 GHz
(Nyquist) bandwidth). This original board design was then rebuilt using a
different fabricator, and the test results revealed a significant decrease in
the bandwidth of the vias. These results led to the development of a set of
design specifications that PCB vendors can easily validate, which will ensure
that the use of high layer-count boards with PTH technology are viable for
emerging 112 Gb/s PAM-4 links. | 2304.01913v1 |
2023-04-18 | Sample-and-Hold Safety with Control Barrier Functions | A common assumption on the deployment of safeguarding controllers on the
digital platform is that high sampling frequency translates to a small
violation of safety. This paper investigates and formalizes this assumption
through the lens of Input-to-State Safety. From this perspective, and
leveraging control barrier functions (CBFs), we propose an alternative solution
for maintaining safety of sample-and-hold controlled systems without any
violation to the original safe set. Our approach centers around modulating the
sampled control input in order to guarantee a more robust safety condition. We
analyze both the time-triggered and the event-triggered sample-and-hold
implementations, including the characterization of sampling frequency
requirements and trigger conditions. We demonstrate the effectiveness of our
approach in the context of adaptive cruise control through simulations. | 2304.08685v2 |
2023-01-17 | Efficient Circuit-Level Implementation of Knuth-Based Balanced and Nearly-Balanced Codes | Coding schemes are often used in high-speed processor-processor or
processor-memory busses in digital systems. In particular, we have introduced
(in a 2012 DesignCon paper) a zero sum (ZS) signaling method which uses
balanced or nearly-balanced coding to reduce simultaneous switching noise (SSN)
in a single-ended bus to a level comparable to that of differential signaling.
While several balanced coding schemes are known, few papers exist that describe
the necessary digital hardware implementations of (known) balanced coding
schemes, and no algorithms had previously been developed for nearly-balanced
coding. In this work, we extend a known balanced coding scheme to accommodate
nearly-balanced coding and demonstrate a range of coding and decoding circuits
through synthesis in 65 nm CMOS. These hardware implementations have minimal
impact on the energy efficiency and area when compared to current
serializer/deserializers (SerDes) at clock rates which would support SerDes
integration. | 2304.13497v1 |
2023-01-17 | Capacitor Optimization in Power Distribution Networks Using Numerical Computation Techniques | This paper presents a power distribution network (PDN) decoupling capacitor
optimization application with three primary goals: reduction of solution times
for large networks, development of flexible network scoring routines, and a
concentration strictly on achieving the best network performance. Example
optimizations are performed using broadband models of a printed circuit board
(PCB), a chip-package, on-die networks, and candidate capacitors. A novel
worst-case time-domain optimization technique is presented as an alternative to
the traditional frequency-domain approach. The trade-offs and criteria for
scoring the computed network are presented. The output is a recommended set of
capacitors which can then be applied to the product design. | 2305.01540v1 |
2023-05-26 | Sphaleron rate from a modified Backus-Gilbert inversion method | We compute the sphaleron rate in quenched QCD for a temperature $T \simeq
1.24~T_c$ from the inversion of the Euclidean lattice time correlator of the
topological charge density. We explore and compare two different strategies:
one follows a new approach proposed in this study and consists in extracting
the rate from finite lattice spacing correlators, and then in taking the
continuum limit at fixed smoothing radius followed by a zero-smoothing
extrapolation; the other follows the traditional approach of extracting the
rate after performing such double extrapolation directly on the correlator. In
both cases the rate is obtained from a recently-proposed modification of the
standard Backus-Gilbert procedure. The two strategies lead to compatible
estimates within errors, which are then compared to previous results in the
literature at the same or similar temperatures; the new strategy permits to
obtain improved results, in terms of statistical and systematic uncertainties. | 2305.17120v2 |
2023-07-21 | Visibility graph-based covariance functions for scalable spatial analysis in nonconvex domains | We present a new method for constructing valid covariance functions of
Gaussian processes over irregular nonconvex spatial domains such as water
bodies, where the geodesic distance agrees with the Euclidean distance only for
some pairs of points. Standard covariance functions based on geodesic distances
are not positive definite on such domains. Using a visibility graph on the
domain, we use the graphical method of "covariance selection" to propose a
class of covariance functions that preserve Euclidean-based covariances between
points that are connected through the domain. The proposed method preserves the
partially Euclidean nature of the intrinsic geometry on the domain while
maintaining validity (positive definiteness) and marginal stationarity over the
entire parameter space, properties which are not always fulfilled by existing
approaches to construct covariance functions on nonconvex domains. We provide
useful approximations to improve computational efficiency, resulting in a
scalable algorithm. We evaluate the performance of competing state-of-the-art
methods using simulation studies on a contrived nonconvex domain. The method is
applied to data regarding acidity levels in the Chesapeake Bay, showing its
potential for ecological monitoring in real-world spatial applications on
irregular domains. | 2307.11941v2 |
2023-08-23 | Consistency of common spatial estimators under spatial confounding | This paper addresses the asymptotic performance of popular spatial regression
estimators on the task of estimating the effect of an exposure on an outcome in
the presence of an unmeasured spatially-structured confounder. This setting is
often referred to as "spatial confounding." We consider spline models, Gaussian
processes (GP), generalized least squares (GLS), and restricted spatial
regression (RSR) under two data generation processes: one where the confounder
is a fixed effect and one where it is a random effect. The literature on
spatial confounding is confusing and contradictory, and our results correct and
clarify several misunderstandings. We first show that, like an unadjusted OLS
estimator, RSR is asymptotically biased under any spatial confounding scenario.
We then prove a novel result on the consistency of the GLS estimator under
spatial confounding. We finally prove that estimators like GLS, GP, and
splines, that are consistent under confounding by a fixed effect will also be
consistent under confounding by a random effect. We conclude that, contrary to
much of the recent literature on spatial confounding, traditional estimators
based on partially linear models are amenable to estimating effects in the
presence of spatial confounding. We support our theoretical arguments with
simulation studies. | 2308.12181v1 |
2023-09-19 | Stochastic control of the Landau-Lifshitz-Gilbert equation | We consider the stochastic Landau-Lifshitz-Gilbert equation in dimension 1. A
control process is added to the effective field. We show the existence of a
weak martingale solution for the resulting controlled equation. The proof uses
the classical Faedo-Galerkin approximation, along with the Jakubowski version
of the Skorohod Theorem. We then show pathwise uniqueness for the obtained
solution, which is then coupled with the theory of Yamada and Watanabe to give
the existence of a unique strong solution. We then show, using some semigroup
techniques that the obtained solution satisfies the maximum regularity. We then
show the existence of an optimal control. A main ingredient of the proof is
using the compact embedding of a space into itself, albeit with the weak
topology. | 2309.10260v1 |
2023-10-13 | Unified framework of the microscopic Landau-Lifshitz-Gilbert equation and its application to Skyrmion dynamics | The Landau-Lifshitz-Gilbert (LLG) equation is widely used to describe
magnetization dynamics. We develop a unified framework of the microscopic LLG
equation based on the nonequilibrium Green's function formalism. We present a
unified treatment for expressing the microscopic LLG equation in several
limiting cases, including the adiabatic, inertial, and nonadiabatic limits with
respect to the precession frequency for a magnetization with fixed magnitude,
as well as the spatial adiabatic limit for the magnetization with slow
variation in both its magnitude and direction. The coefficients of those terms
in the microscopic LLG equation are explicitly expressed in terms of
nonequilibrium Green's functions. As a concrete example, this microscopic
theory is applied to simulate the dynamics of a magnetic Skyrmion driven by
quantum parametric pumping. Our work provides a practical formalism of the
microscopic LLG equation for exploring magnetization dynamics. | 2310.08807v1 |
2023-10-18 | Parallel-in-Time Integration of the Landau-Lifshitz-Gilbert Equation with the Parallel Full Approximation Scheme in Space and Time | Speeding up computationally expensive problems, such as numerical simulations
of large micromagnetic systems, requires efficient use of parallel computing
infrastructures. While parallelism across space is commonly exploited in
micromagnetics, this strategy performs poorly once a minimum number of degrees
of freedom per core is reached. We use magnum.pi, a finite-element
micromagnetic simulation software, to investigate the Parallel Full
Approximation Scheme in Space and Time (PFASST) as a space- and time-parallel
solver for the Landau-Lifshitz-Gilbert equation (LLG). Numerical experiments
show that PFASST enables efficient parallel-in-time integration of the LLG,
significantly improving the speedup gained from using a given number of cores
as well as allowing the code to scale beyond spatial limits. | 2310.11819v1 |
2023-12-29 | Chebyshev and Backus-Gilbert reconstruction for inclusive semileptonic $B_{(s)}$-meson decays from Lattice QCD | We present a study on the nonperturbative calculation of observables for
inclusive semileptonic decays of $B_{(s)}$ mesons using lattice QCD. We focus
on the comparison of two different methods to analyse the lattice data of
Euclidean correlation functions, specifically Chebyshev and Backus-Gilbert
approaches. This type of computation may eventually provide new insight into
the long-standing tension between the inclusive and exclusive determinations of
the Cabibbo-Kobayashi-Maskawa (CKM) matrix elements $|V_{cb}|$ and $|V_{ub}|$.
We report the results from a pilot lattice computation for the decay $B_s
\rightarrow X_c \, l\nu_l$, where the valence quark masses are approximately
tuned to their physical values using the relativistic-heavy quark action for
the $b$ quark and the domain-wall formalism for the other valence quarks. We
address the computation of the total decay rate as well as leptonic and
hadronic moments, discussing similarities and differences between the two
analysis techniques. | 2312.17401v1 |
2024-02-22 | Gilbert-Varshamov Bound for Codes in $L_1$ Metric using Multivariate Analytic Combinatorics | Analytic combinatorics in several variables refers to a suite of tools that
provide sharp asymptotic estimates for certain combinatorial quantities. In
this paper, we apply these tools to determine the Gilbert--Varshamov lower
bound on the rate of optimal codes in $L_1$ metric. Several different code
spaces are analyzed, including the simplex and the hypercube in $\mathbb{Z^n}$,
all of which are inspired by concrete data storage and transmission models such
as the sticky insertion channel, the permutation channel, the adjacent
transposition (bit-shift) channel, the multilevel flash memory channel, etc. | 2402.14712v1 |
2024-02-28 | Embodied Supervision: Haptic Display of Automation Command to Improve Supervisory Performance | A human operator using a manual control interface has ready access to their
own command signal, both by efference copy and proprioception. In contrast, a
human supervisor typically relies on visual information alone. We propose
supplying a supervisor with a copy of the operators command signal,
hypothesizing improved performance, especially when that copy is provided
through haptic display. We experimentally compared haptic with visual access to
the command signal, quantifying the performance of N equals 10 participants
attempting to determine which of three reference signals was being tracked by
an operator. Results indicate an improved accuracy in identifying the tracked
target when haptic display was available relative to visual display alone. We
conjecture the benefit follows from the relationship of haptics to the
supervisor's own experience, perhaps muscle memory, as an operator. | 2402.18707v1 |
2024-03-14 | Quantum analog of Landau-Lifshitz-Gilbert dynamics | The Landau-Lifshitz-Gilbert (LLG) and Landau-Lifshitz (LL) equations play an
essential role for describing the dynamics of magnetization in solids. While a
quantum analog of the LL dynamics has been proposed in [Phys. Rev. Lett. 110,
147201 (2013)], the corresponding quantum version of LLG remains unknown. Here,
we propose such a quantum LLG equation that inherently conserves purity of the
quantum state. We examine the quantum LLG dynamics of a dimer consisting of two
interacting spin-1/2 particles. Our analysis reveals that, in the case of
ferromagnetic coupling, the evolution of initially uncorrelated spins mirrors
the classical LLG dynamics. However, in the antiferromagnetic scenario, we
observe pronounced deviations from classical behavior, underscoring the unique
dynamics of becoming a spinless state, which is non-locally correlated.
Moreover, when considering spins that are initially correlated, our study
uncovers an unusual form of transient quantum correlation dynamics, which
differ significantly from what is typically seen in open quantum systems. | 2403.09255v1 |
2024-03-15 | Identification and estimation of mediational effects of longitudinal modified treatment policies | We demonstrate a comprehensive semiparametric approach to causal mediation
analysis, addressing the complexities inherent in settings with longitudinal
and continuous treatments, confounders, and mediators. Our methodology utilizes
a nonparametric structural equation model and a cross-fitted sequential
regression technique based on doubly robust pseudo-outcomes, yielding an
efficient, asymptotically normal estimator without relying on restrictive
parametric modeling assumptions. We are motivated by a recent scientific
controversy regarding the effects of invasive mechanical ventilation (IMV) on
the survival of COVID-19 patients, considering acute kidney injury (AKI) as a
mediating factor. We highlight the possibility of "inconsistent mediation," in
which the direct and indirect effects of the exposure operate in opposite
directions. We discuss the significance of mediation analysis for scientific
understanding and its potential utility in treatment decisions. | 2403.09928v1 |
2024-03-22 | Two-scale Analysis for Multiscale Landau-Lifshitz-Gilbert Equation: Theory and Numerical Methods | This paper discusses the theory and numerical method of two-scale analysis
for the multiscale Landau-Lifshitz-Gilbert equation in composite ferromagnetic
materials. The novelty of this work can be summarized in three aspects:
Firstly, the more realistic and complex model is considered, including the
effects of the exchange field, anisotropy field, stray field, and external
magnetic field. The explicit convergence orders in the $H^1$ norm between the
classical solution and the two-scale solution are obtained. Secondly, we
propose a robust numerical framework, which is employed in several
comprehensive experiments to validate the convergence results for the Periodic
and Neumann problems. Thirdly, we design an improved implicit numerical scheme
to reduce the required number of iterations and relaxes the constraints on the
time step size, which can significantly improve computational efficiency.
Specifically, the projection and the expansion methods are given to overcome
the inherent non-consistency in the initial data between the multiscale problem
and homogenized problem. | 2403.14957v1 |
2020-11-30 | Role of Compressive Viscosity and Thermal Conductivity on the Damping of Slow Waves in the Coronal Loops With and Without Heating-Cooling Imbalance | In the present paper, we derive a new dispersion relation for slow
magnetoacoustic waves invoking the effect of thermal conductivity, compressive
viscosity, radiation and unknown heating term along with the consideration of
heating cooling imbalance from linearized MHD equations. We solve the general
dispersion relation to understand role of compressive viscosity and thermal
conductivity in damping of the slow waves in coronal loops with and without
heating cooling imbalance. We have analyzed wave damping for the range of loop
length $L$=50-500 Mm, temperature $T$=5-30 MK, and density
$\rho$=10$^{-11}$-10$^{-9}$ kg m$^{-3}$. It was found that inclusion of
compressive viscosity along with thermal conductivity significantly enhances
the damping of fundamental mode oscillations in shorter (e.g., $L$=50 Mm) and
super-hot ($T>$10 MK) loops. However, role of the viscosity in damping is
insignificant in longer (e.g., $L$=500 Mm) and hot loops (T$\leq$10 MK) where,
instead, thermal conductivity along with the presence of heating cooling
imbalance plays a dominant role. For the shorter loops at the super-hot regime
of the temperature, increment in loop density substantially enhances damping of
the fundamental modes due to thermal conductivity when the viscosity is absent,
however, when the compressive viscosity is added the increase in density
substantially weakens damping. Thermal conductivity alone is found to play a
dominant role in longer loops at lower temperatures (T$\leq$10 MK), while
compressive viscosity dominates in damping at super-hot temperatures ($T>$10
MK) in shorter loops. The predicted scaling law between damping time ($\tau$)
and wave period ($P$) is found to better match to observed SUMER oscillations
when heating cooling imbalance is taken into account in addition to thermal
conductivity and compressive viscosity for the damping of the fundamental slow
mode oscillations. | 2011.14519v2 |
2013-10-23 | Fundamental constants and high resolution spectroscopy | Absorption-line systems detected in high resolution quasar spectra can be
used to compare the value of dimensionless fundamental constants such as the
fine-structure constant, alpha, and the proton-to-electron mass ratio, mu =
m_p/m_e, as measured in remote regions of the Universe to their value today on
Earth. In recent years, some evidence has emerged of small temporal and also
spatial variations in alpha on cosmological scales which may reach a fractional
level of 10 ppm . We are conducting a Large Programme of observations with VLT
UVES to explore these variations. We here provide a general overview of the
Large Programme and report on the first results for these two constants,
discussed in detail in Molaro et al. and Rahmani et al. A stringent bound for
Delta(alpha)/Alpha is obtained for the absorber at_abs = 1.6919 towards HE
2217-2818. The absorption profile is complex with several very narrow features,
and is modeled with 32 velocity components. The relative variation in alpha in
this system is +1.3+-2.4_{stat}+-1.0_{sys} ppm if Al II lambda 1670AA and three
Fe II transitions are used, and +1.1+-2.6_{stat} ppm in a lightly different
analysis with only Fe II transitions used. The expectation at this sky position
of the recently-reported dipolar variation of alpha is (3.2--5.4)+-1.7 ppm
depending on dipole model. This constraint of Delta(alpha)/alpha at face value
is not supporting this expectation but is not inconsistent with it at the 3
sigma level. For the proton-to-electron mass ratio the analysis of the H_2
absorption lines of the z_{abs}~2.4018 damped Ly alpha system towards HE 0027-
1836 provides Delta(mu)/mu = (-7.6 +- 8.1_{stat} +- 6.3_{sys}) ppm which is
also consistent with a null variation. (abridged) | 1310.6280v1 |
2012-10-26 | A Measurement of the Cosmic Microwave Background Damping Tail from the 2500-square-degree SPT-SZ survey | We present a measurement of the cosmic microwave background (CMB) temperature
power spectrum using data from the recently completed South Pole Telescope
Sunyaev-Zel'dovich (SPT-SZ) survey. This measurement is made from observations
of 2540 deg$^2$ of sky with arcminute resolution at $150\,$GHz, and improves
upon previous measurements using the SPT by tripling the sky area. We report
CMB temperature anisotropy power over the multipole range $650<\ell<3000$. We
fit the SPT bandpowers, combined with the seven-year Wilkinson Microwave
Anisotropy Probe (WMAP7) data, with a six-parameter LCDM cosmological model and
find that the two datasets are consistent and well fit by the model. Adding SPT
measurements significantly improves LCDM parameter constraints; in particular,
the constraint on $\theta_s$ tightens by a factor of 2.7. The impact of
gravitational lensing is detected at $8.1\, \sigma$, the most significant
detection to date. This sensitivity of the SPT+WMAP7 data to lensing by
large-scale structure at low redshifts allows us to constrain the mean
curvature of the observable universe with CMB data alone to be
$\Omega_k=-0.003^{+0.014}_{-0.018}$. Using the SPT+WMAP7 data, we measure the
spectral index of scalar fluctuations to be $n_s=0.9623 \pm 0.0097$ in the LCDM
model, a $3.9\,\sigma$ preference for a scale-dependent spectrum with $n_s<1$.
The SPT measurement of the CMB damping tail helps break the degeneracy that
exists between the tensor-to-scalar ratio $r$ and $n_s$ in large-scale CMB
measurements, leading to an upper limit of $r<0.18$ (95%,C.L.) in the LCDM+$r$
model. Adding low-redshift measurements of the Hubble constant ($H_0$) and the
baryon acoustic oscillation (BAO) feature to the SPT+WMAP7 data leads to
further improvements. The combination of SPT+WMAP7+$H_0$+BAO constrains
$n_s=0.9538 \pm 0.0081$ in the LCDM model, a $5.7\,\sigma$ detection of $n_s <
1$, ... [abridged] | 1210.7231v2 |
1993-06-22 | Weakly Damped Modes in Star Clusters and Galaxies | A perturber may excite a coherent mode in a star cluster or galaxy. If the
stellar system is stable, it is commonly assumed that such a mode will be
strongly damped and therefore of little practical consequence other than
redistributing momentum and energy deposited by the perturber. This paper
demonstrates that this assumption is false; weakly damped modes exist and may
persist long enough to have observable consequences. To do this, a method for
investigating the dispersion relation for spherical stellar systems and for
locating weakly damped modes in particular is developed and applied to King
models of varying concentration. This leads to the following remarkable result:
King models exhibit {\it very} weakly damped $m=1$ modes over a wide range of
concentration ($0.67\le c\le1.5$ have been examined). The predicted damping
time is tens to hundreds of crossing times. This mode causes the peak density
to shift from and slowly revolve about the initial center. The existence of the
mode is supported by n-body simulation. Higher order modes and possible
astronomical consequences are discussed. Weakly damped modes, for example, may
provide a natural explanation for observed discrepancies between density and
kinematic centers in galaxies, the location of velocity cusps due to massive
black holes, and $m=1$ disturbances of disks embedded in massive halos.
Gravitational shocking may excite the $m=1$ mode in globular clusters, which
could modify their subsequent evolution and displace the positions of exotic
remnants. | 9306020v1 |
1997-12-03 | On the Evolution of Damped Lyman Alpha Systems to Galactic Disks | The mean metallicity of the thick disk of the Galaxy is 0.5 dex higher than
that of the damped Lyman alpha systems. This has been interpreted to argue that
stars in the former do not arise out of gas in the latter. Using new
metallicity and H I column-density data we show the metal-rich damped systems
do contain sufficient baryons at the thick-disk metallicity to account for the
stellar masses of thick disks. Comparing our kinematic data with the
metallicities we show that damped Lyman alpha systems exhibiting the largest
profile velocity widths span a narrow range of high metallicities, while
systems with small velocity widths span a wider range of metallicities. This is
naturally explained by passage of the damped Lyman alpha sightlines through
rapidly rotating disks with negative radial gradients in metallicity. The
systematically lower N(H I) of systems with high velocity widths indicates (a)
the gaseous disks have centrally located holes, and (b) an apparent
inconsistency with the protogalactic clump model for damped Lyman alpha
systems. The higher metallicity of systems with low N(H I) further implies that
stars rather than gas dominate the baryonic content of the most metal-rich
damped systems. | 9712050v1 |
1998-10-23 | Chemical Abundances of the Damped Lya Systems at z>1.5 | We present chemical abundance measurements for 19 damped lya systems observed
with HIRES on the 10m W.M. Keck Telescope. Our principal goal is to investigate
the abundance patterns of the damped systems and thereby determine the
underlying physical processes which dominate their chemical evolution. We place
particular emphasis on gauging the relative importance of two complementary
effects often invoked to explain the damped lya abundances: (1) nucleosynthetic
enrichment from Type II supernovae and (2) an ISM-like dust depletion pattern.
Similar to the principal results of Lu et al. (1996), our observations lend
support both for dust depletion and Type II SN enrichment. Specifically, the
observed overabundance of Zn/Fe and underabundance of Ni/Fe relative to solar
abundances suggest significant dust depletion within the damped lya systems.
Meanwhile, the relative abundances of Al, Si, and Cr vs. Fe are consistent with
both dust depletion and Type II supernova enrichment. Our measurements of Ti/Fe
and the Mn/Fe measurements from Lu et al. (1996), however, cannot be explained
by dust depletion and indicate an underlying Type II SN pattern. Finally, the
observed values of [S/Fe] are inconsistent with the combined effects of dust
depletion and the nucleosynthetic yields expected for Type II supernovae. This
last result emphasizes the need for another physical process to explain the
damped lya abundance patterns.
We also examine the metallicity of the damped lya systems both with respect
to Zn/H and Fe/H. Our results confirm previous surveys by Pettini and
collaborators, i.e., [<Zn/H>] = -1.15 +/- 0.15 dex. [abridged] | 9810381v1 |
2002-04-03 | The role of damped Alfven waves on magnetospheric accretion models of young stars | We examine the role of Alfven wave damping in heating the plasma in the
magnetic funnels of magnetospheric accretion models of young stars. We study
four different damping mechanisms of the Alfven waves: nonlinear, turbulent,
viscous-resistive and collisional. Two different possible origins for the
Alfven waves are discussed: 1) Alfven waves generated at the surface of the
star by the shock produced by the infalling matter; and 2) Alfven waves
generated locally in the funnel by the Kelvin-Helmholtz instability. We find
that, in general, the damping lengths are smaller than the tube length. Since
thermal conduction in the tube is not efficient, Alfven waves generated only at
the star's surface cannot heat the tube to the temperatures necessary to fit
the observations. Only for very low frequency Alfven waves ~10^{-5} the ion
cyclotron frequency, is the viscous-resistive damping length greater than the
tube length. In this case, the Alfven waves produced at the surface of the star
are able to heat the whole tube. Otherwise, local production of Alfven waves is
required to explain the observations. The turbulence level is calculated for
different frequencies for optically thin and thick media. We find that
turbulent velocities varies greatly for different damping mechanisms, reaching
\~100 km s^{-1} for the collisional damping of small frequency waves. | 0204056v1 |
2009-09-19 | Resonantly Damped Kink Magnetohydrodynamic Waves in a Partially Ionized Filament Thread | Transverse oscillations of solar filament and prominence threads have been
frequently reported. These oscillations have the common features of being of
short period (2-10 min) and being damped after a few periods. Kink
magnetohydrodynamic (MHD) wave modes have been proposed as responsible for the
observed oscillations, whereas resonant absorption in the Alfven continuum and
ion-neutral collisions are the best candidates to be the damping mechanisms.
Here, we study both analytically and numerically the time damping of kink MHD
waves in a cylindrical, partially ionized filament thread embedded in a coronal
environment. The thread model is composed of a straight and thin, homogeneous
filament plasma, with a transverse inhomogeneous transitional layer where the
plasma physical properties vary continuously from filament to coronal
conditions. The magnetic field is homogeneous and parallel to the thread axis.
We find that the kink mode is efficiently damped by resonant absorption for
typical wavelengths of filament oscillations, the damping times being
compatible with the observations. Partial ionization does not affect the
process of resonant absorption, and the filament plasma ionization degree is
only important for the damping for wavelengths much shorter than those
observed. To our knowledge, this is the first time that the phenomenon of
resonant absorption is studied in a partially ionized plasma. | 0909.3599v1 |
2009-10-15 | Time damping of non-adiabatic magnetohydrodynamic waves in a partially ionized prominence plasma: Effect of helium | Prominences are partially ionized, magnetized plasmas embedded in the solar
corona. Damped oscillations and propagating waves are commonly observed. These
oscillations have been interpreted in terms of magnetohydrodynamic (MHD) waves.
Ion-neutral collisions and non-adiabatic effects (radiation losses and thermal
conduction) have been proposed as damping mechanisms. We study the effect of
the presence of helium on the time damping of non-adiabatic MHD waves in a
plasma composed by electrons, protons, neutral hydrogen, neutral helium (He I),
and singly ionized helium (He II) in the single-fluid approximation. The
dispersion relation of linear non-adiabatic MHD waves in a homogeneous,
unbounded, and partially ionized prominence medium is derived. The period and
the damping time of Alfven, slow, fast, and thermal waves are computed. A
parametric study of the ratio of the damping time to the period with respect to
the helium abundance is performed. The efficiency of ion-neutral collisions as
well as thermal conduction is increased by the presence of helium. However, if
realistic abundances of helium in prominences (~10%) are considered, this
effect has a minor influence on the wave damping. The presence of helium can be
safely neglected in studies of MHD waves in partially ionized prominence
plasmas. | 0910.2883v1 |
2009-12-21 | The effect of longitudinal flow on resonantly damped kink oscillations | The most promising mechanism acting towards damping the kink oscillations of
coronal loops is resonant absorption. In this context most of previous studies
neglected the effect of the obvious equilibrium flow along magnetic field
lines. The flows are in general sub-Alfv\'enic and hence comparatively slow.
Here we investigate the effect of an equilibrium flow on the resonant
absorption of linear kink MHD waves in a cylindrical magnetic flux tube with
the aim of determining the changes in the frequency of the forward and backward
propagating waves and in the modification of the damping times due to the flow.
A loop model with both the density and the longitudinal flow changing in the
radial direction is considered. We use the thin tube thin boundary (TTTB)
approximation in order to calculate the damping rates. The full resistive
eigenvalue problem is also solved without assuming the TTTB approximation.
Using the small ratio of flow and Alfv\'en speeds we derive simple analytical
expressions to the damping rate. The analytical expressions are in good
agreement with the resistive eigenmode calculations. Under typical coronal
conditions the effect of the flow on the damped kink oscillations is small when
the characteristic scale of the density layer is similar or smaller than the
characteristic width of the velocity layer. However, in the opposite situation
the damping rates can be significantly altered, specially for the backward
propagating wave which is undamped while the forward wave is overdamped. | 0912.4136v1 |
2010-07-12 | Seismology of Standing Kink Oscillations of Solar Prominence Fine Structures | We investigate standing kink magnetohydrodynamic (MHD) oscillations in a
prominence fine structure modeled as a straight and cylindrical magnetic tube
only partially filled with the prominence material, and with its ends fixed at
two rigid walls representing the solar photosphere. The prominence plasma is
partially ionized and a transverse inhomogeneous transitional layer is included
between the prominence thread and the coronal medium. Thus, ion-neutral
collisions and resonant absorption are the considered damping mechanisms.
Approximate analytical expressions of the period, the damping time, and their
ratio are derived for the fundamental mode in the thin tube and thin boundary
approximations. We find that the dominant damping mechanism is resonant
absorption, which provides damping ratios in agreement with the observations,
whereas ion-neutral collisions are irrelevant for the damping. The values of
the damping ratio are independent of both the prominence thread length and its
position within the magnetic tube, and coincide with the values for a tube
fully filled with the prominence plasma. The implications of our results in the
context of the MHD seismology technique are discussed, pointing out that the
reported short-period (2 - 10 min) and short-wavelength (700 - 8,000 km) thread
oscillations may not be consistent with a standing mode interpretation and
could be related to propagating waves. Finally, we show that the inversion of
some prominence physical parameters, e.g., Alfv\'en speed, magnetic field
strength, transverse inhomogeneity length-scale, etc., is possible using
observationally determined values of the period and damping time of the
oscillations along with the analytical approximations of these quantities. | 1007.1959v2 |
2014-09-19 | Highly confined low-loss plasmons in graphene-boron nitride heterostructures | Graphene plasmons were predicted to possess ultra-strong field confinement
and very low damping at the same time, enabling new classes of devices for deep
subwavelength metamaterials, single-photon nonlinearities, extraordinarily
strong light-matter interactions and nano-optoelectronic switches. While all of
these great prospects require low damping, thus far strong plasmon damping was
observed, with both impurity scattering and many-body effects in graphene
proposed as possible explanations. With the advent of van der Waals
heterostructures, new methods have been developed to integrate graphene with
other atomically flat materials. In this letter we exploit near-field
microscopy to image propagating plasmons in high quality graphene encapsulated
between two films of hexagonal boron nitride (h-BN). We determine dispersion
and particularly plasmon damping in real space. We find unprecedented low
plasmon damping combined with strong field confinement, and identify the main
damping channels as intrinsic thermal phonons in the graphene and dielectric
losses in the h-BN. The observation and in-depth understanding of low plasmon
damping is the key for the development of graphene nano-photonic and
nano-optoelectronic devices. | 1409.5674v1 |
2015-09-02 | Energy Dependence of Synchrotron X-Ray Rims in Tycho's Supernova Remnant | Several young supernova remnants exhibit thin X-ray bright rims of
synchrotron radiation at their forward shocks. Thin rims require strong
magnetic field amplification beyond simple shock compression if rim widths are
only limited by electron energy losses. But, magnetic field damping behind the
shock could produce similarly thin rims with less extreme field amplification.
Variation of rim width with energy may thus discriminate between competing
influences on rim widths. We measured rim widths around Tycho's supernova
remnant in 5 energy bands using an archival 750 ks Chandra observation. Rims
narrow with increasing energy and are well described by either loss-limited or
damped scenarios, so X-ray rim width-energy dependence does not uniquely
specify a model. But, radio counterparts to thin rims are not loss-limited and
better reflect magnetic field structure. Joint radio and X-ray modeling favors
magnetic damping in Tycho's SNR with damping lengths ~1--5% of remnant radius
and magnetic field strengths ~50--400 $\mu$G assuming Bohm diffusion. X-ray rim
widths are ~1% of remnant radius, somewhat smaller than inferred damping
lengths. Electron energy losses are important in all models of X-ray rims,
suggesting that the distinction between loss-limited and damped models is
blurred in soft X-rays. All loss-limited and damping models require magnetic
fields $\gtrsim$ 20 $\mu$G, affirming the necessity of magnetic field
amplification beyond simple compression. | 1509.00877v1 |
2016-02-02 | Forward Modelling of Propagating Slow Waves in Coronal Loops and Their Frequency-Dependent Damping | Propagating slow waves in coronal loops exhibit a damping which depends upon
the frequency of the waves. In this study we aim to investigate the
relationship of the damping length (L$_d$) with the frequency of the
propagating wave. We present a 3-D coronal loop model with uniform density and
temperature and investigate the frequency dependent damping mechanism for the
four chosen wave periods. We include the thermal conduction to damp the waves
as they propagate through the loop. The numerical model output has been forward
modelled to generate synthetic images of SDO/AIA 171 \r{A} and 193 \r{A}
channels. The use of forward modelling, which incorporates the atomic emission
properties into the intensity images, allows us to directly compare our results
with the real observations. The results show that the damping lengths vary
linearly with the periods. We also measure the contributions of the emission
properties on the damping lengths by using density values from the simulation.
In addition to that} we have also calculated the theoretical dependence of
L$_d$ with wave periods and showed that it is consistent with the results we
obtained from the numerical modelling and earlier observations. | 1602.00787v1 |
2016-05-11 | Damping of prominence longitudinal oscillations due to mass accretion | We study the damping of longitudinal oscillations of a prominence thread
caused by the mass accretion. In this model we considered a thin curved
magnetic tube filled with the plasma. The parts of the tube at the two sides of
the thread are filled with hot rarefied plasma. We assume that there are flows
of rarefied plasma toward the thread caused by the plasma evaporation at the
magnetic tube footpoints. Our main assumption is that the hot plasma is
instantaneously accommodated by the thread when it arrives at the thread, and
its temperature and density become equal to those of the thread. Then we derive
the system of ordinary differential equations describing the thread dynamics.
We consider linear and nonlinear oscillation. The nonlinearity reduces the
damping time, however this reduction is small. The damping time is inversely
proportional to the accretion rate. We also obtain that the oscillation periods
decrease with time. However even for the largest initial oscillation amplitude
considered in our article the period reduction does not exceed 20%. We conclude
that the mass accretion can damp the motion of the threads rapidly. Thus, this
mechanism can explain the observed strong damping of large-amplitude
longitudinal oscillations. In addition, the damping time can be used to
determine the mass accretion rate and indirectly the coronal heating. | 1605.03376v1 |
2016-11-17 | Inductive detection of field-like and damping-like AC inverse spin-orbit torques in ferromagnet/normal metal bilayers | Functional spintronic devices rely on spin-charge interconversion effects,
such as the reciprocal processes of electric field-driven spin torque and
magnetization dynamics-driven spin and charge flow. Both damping-like and
field-like spin-orbit torques have been observed in the forward process of
current-driven spin torque and damping-like inverse spin-orbit torque has been
well-studied via spin pumping into heavy metal layers. Here we demonstrate that
established microwave transmission spectroscopy of ferromagnet/normal metal
bilayers under ferromagnetic resonance can be used to inductively detect the AC
charge currents driven by the inverse spin-charge conversion processes. This
technique relies on vector network analyzer ferromagnetic resonance (VNA-FMR)
measurements. We show that in addition to the commonly-extracted spectroscopic
information, VNA-FMR measurements can be used to quantify the magnitude and
phase of all AC charge currents in the sample, including those due to spin
pumping and spin-charge conversion. Our findings reveal that
Ni$_{80}$Fe$_{20}$/Pt bilayers exhibit both damping-like and field-like inverse
spin-orbit torques. While the magnitudes of both the damping-like and
field-like inverse spin-orbit torque are of comparable scale to prior reported
values for similar material systems, we observed a significant dependence of
the damping-like magnitude on the order of deposition. This suggests interface
quality plays an important role in the overall strength of the damping-like
spin-to-charge conversion. | 1611.05798v2 |
2017-01-04 | Controlling plasmon modes and damping in buckled two-dimensional material open systems | Full ranges of both hybrid plasmon-mode dispersions and their damping are
studied systematically by our recently developed mean-field theory in open
systems involving a conducting substrate and a two-dimensional (2D) material
with a buckled honeycomb lattice, such as silicene, germanene, and a group
\rom{4} dichalcogenide as well. In this hybrid system, the single plasmon mode
for a free-standing 2D layer is split into one acoustic-like and one
optical-like mode, leading to a dramatic change in the damping of plasmon
modes. In comparison with gapped graphene, critical features associated with
plasmon modes and damping in silicene and molybdenum disulfide are found with
various spin-orbit and lattice asymmetry energy bandgaps, doping types and
levels, and coupling strengths between 2D materials and the conducting
substrate. The obtained damping dependence on both spin and valley degrees of
freedom is expected to facilitate measuring the open-system dielectric property
and the spin-orbit coupling strength of individual 2D materials. The unique
linear dispersion of the acoustic-like plasmon mode introduces additional
damping from the intraband particle-hole modes which is absent for a
free-standing 2D material layer, and the use of molybdenum disulfide with a
large bandgap simultaneously suppresses the strong damping from the interband
particle-hole modes. | 1701.01084v1 |
2017-08-16 | Damping of an oscillating scalar field indirectly coupled to a thermal bath | The damping process of a homogeneous oscillating scalar field that indirectly
interacts with a thermal bath through a mediator field is investigated over a
wide range of model parameters. We consider two types of mediator fields, those
that can decay to the thermal bath and those that are individually stable but
pair annihilate. The former case has been extensively studied in the literature
by treating the damping as a local effect after integrating out the assumed
close-to-equilibrium mediator field. The same approach does not apply if the
mediator field is stable and freezes out of equilibrium. To account for the
latter case, we adopt a non-local description of damping that is only
meaningful when we consider full half-oscillations of the field being damped.
The damping rates of the oscillating scalar field and the corresponding heating
rate of the thermal bath in all bulk parameter regions are calculated in both
cases, corroborating previous results in the direct decay case. Using the
obtained results, the time it takes for the amplitude of the scalar field to be
substantially damped is estimated. | 1708.04865v2 |
2012-10-30 | Mode- and size-dependent Landau-Lifshitz damping in magnetic nanostructures: Evidence for non-local damping | We demonstrate a strong dependence of the effective damping on the nanomagnet
size and the particular spin-wave mode that can be explained by the theory of
intralayer transverse-spin-pumping. The effective Landau-Lifshitz damping is
measured optically in individual, isolated nanomagnets as small as 100 nm. The
measurements are accomplished by use of a novel heterodyne magneto-optical
microwave microscope with unprecedented sensitivity. Experimental data reveal
multiple standing spin-wave modes that we identify by use of micromagnetic
modeling as having either localized or delocalized character, described
generically as end- and center-modes. The damping parameter of the two modes
depends on both the size of the nanomagnet as well as the particular spin-wave
mode that is excited, with values that are enhanced by as much as 40% relative
to that measured for an extended film. Contrary to expectations based on the ad
hoc consideration of lithography-induced edge damage, the damping for the
end-mode decreases as the size of the nanomagnet decreases. The data agree with
the theory for damping caused by the flow of intralayer transverse
spin-currents driven by the magnetization curvature. These results have serious
implications for the performance of nanoscale spintronic devices such as
spin-torque-transfer magnetic random access memory. | 1210.8118v3 |
2012-11-21 | Kinetic theory of surface plasmon polariton in semiconductor nanowires | Based on the semiclassical model Hamiltonian of the surface plasmon polariton
and the nonequilibrium Green-function approach, we present a microscopic
kinetic theory to study the influence of the electron scattering on the
dynamics of the surface plasmon polariton in semiconductor nanowires. The
damping of the surface plasmon polariton originates from the resonant
absorption by the electrons (Landau damping), and the corresponding damping
exhibits size-dependent oscillations and distinct temperature dependence
without any scattering. The scattering influences the damping by introducing a
broadening and a shifting to the resonance. To demonstrate this, we investigate
the damping of the surface plasmon polariton in InAs nanowires in the presence
of the electron-impurity, electron-phonon and electron-electron Coulomb
scatterings. The main effect of the electron-impurity and electron-phonon
scatterings is to introduce a broadening, whereas the electron-electron Coulomb
scattering can not only cause a broadening, but also introduce a shifting to
the resonance. For InAs nanowires under investigation, the broadening due to
the electron-phonon scattering dominates. As a result, the scattering has a
pronounced influence on the damping of the surface plasmon polariton: The
size-dependent oscillations are smeared out and the temperature dependence is
also suppressed in the presence of the scattering. These results demonstrate
the the important role of the scattering on the surface plasmon polariton
damping in semiconductor nanowires. | 1211.5055v2 |
2017-04-05 | Stimulated Brillouin scattering behaviors in different species ignition hohlraum plasmas in high-temperature and high-density region | The presence of multiple ion species can add additional branches to the IAW
dispersion relation and change the Landau damping significantly. Different IAW
modes excited by stimulated Brillouin scattering (SBS) and different SBS
behaviors in several typical ignition hohlraum plasmas in the high-temperature
and high-density region have been researched by Vlasov-Maxwell simulation. The
slow mode in HeH or CH plasmas is the least damped mode and will be excited in
SBS, while the fast mode in AuB plasmas is the least damped mode and will be
excited in SBS. Due to strong Landau damping, the SBS in H or HeH plasmas is
strong convective instability, while the SBS in AuB plasmas is absolute
instability due to the weak Landau damping. However, although the SBS in CH
plasmas is weak convective instability in the linear theory, the SBS will
transform into absolute instability due to decreasing linear Landau damping by
particles trapping. These results give a detail research of the IAW modes
excitation and the properties of SBS in different species plasmas, thus
providing the possibility of controlling SBS by increasing the linear Landau
damping of the IAW by changing ion species. | 1704.02317v1 |
2017-06-29 | Resonant Absorption of Axisymmetric Modes in Twisted Magnetic Flux Tubes | It has been shown recently that magnetic twist and axisymmetric MHD modes are
ubiquitous in the solar atmosphere and therefore, the study of resonant
absorption for these modes have become a pressing issue as it can have
important consequences for heating magnetic flux tubes in the solar atmosphere
and the observed damping. In this investigation, for the first time, we
calculate the damping rate for axisymmetric MHD waves in weakly twisted
magnetic flux tubes. Our aim is to investigate the impact of resonant damping
of these modes for solar atmospheric conditions. This analytical study is based
on an idealized configuration of a straight magnetic flux tube with a weak
magnetic twist inside as well as outside the tube. By implementing the
conservation laws derived by \cite{Sakurai:1991aa} and the analytic solutions
for weakly twisted flux tubes obtained recently by \cite{Giagkiozis:2015apj},
we derive a dispersion relation for resonantly damped axisymmetric modes in the
spectrum of the Alfv\'{e}n continuum. We also obtain an insightful analytical
expression for the damping rate in the long wavelength limit. Furthermore, it
shown that both the longitudinal magnetic field and the density, which are
allowed to vary continuously in the inhomogeneous layer, have a significant
impact on the damping time. Given the conditions in the solar atmosphere,
resonantly damped axisymmetric modes are highly likely to be ubiquitous and
play an important role in energy dissipation.
We also suggest that given the character of these waves, it is likely that
they have already been observed in the guise of Alfv\'{e}n waves. | 1706.09665v1 |
2018-09-14 | Continuous and discrete damping reduction for systems with quadratic interaction | We study the connection between Lagrangian and Hamiltonian descriptions of
closed/open dynamics, for a collection of particles with quadratic interaction
(closed system) and a sub-collection of particles with linear damping (open
system). We consider both continuous and discrete versions of mechanics. We
define the Damping Reduction as the mapping from the equations of motion of the
closed system to those of the open one. As variational instruments for the
obtention of these equations we use the Hamilton's principle (closed dynamics)
and Lagrange-d'Alembert principle (open dynamics). We establish the
commutativity of the branches Legendre transform + Damping Reduction and
Damping Reduction+Legendre transform, where the Legendre transform is the usual
mapping between Lagrangian and Hamiltonian mechanics. At a discrete level, this
commutativity provides interesting insight about the resulting integrators.
More concretely, Discrete Damping Reduction yields particular numerical schemes
for linearly damped systems which are not symplectic anymore, but preserve some
of the features of their symplectic counterparts from which they proceed (for
instance the semi-implicitness in some cases). The theoretical results are
illustrated with the examples of the heat bath and transmission lines. In the
latter case some simulations are displayed, showing a better performance of the
integrators with variational origin. | 1809.05532v1 |
2021-04-13 | Apparent nonlinear damping triggered by quantum fluctuations | Nonlinear damping, the change in damping rate with the amplitude of
oscillations plays an important role in many electrical, mechanical and even
biological oscillators. In novel technologies such as carbon nanotubes,
graphene membranes or superconducting resonators, the origin of nonlinear
damping is sometimes unclear. This presents a problem, as the damping rate is a
key figure of merit in the application of these systems to extremely precise
sensors or quantum computers. Through measurements of a superconducting
resonator, we show that from the interplay of quantum fluctuations and the
nonlinearity of a Josephson junction emerges a power-dependence in the
resonator response which closely resembles nonlinear damping. The phenomenon
can be understood and visualized through the flow of quasi-probability in phase
space where it reveals itself as dephasing. Crucially, the effect is not
restricted to superconducting circuits: we expect that quantum fluctuations or
other sources of noise give rise to apparent nonlinear damping in systems with
a similar conservative nonlinearity, such as nano-mechanical oscillators or
even macroscopic systems. | 2104.06464v2 |
2013-11-12 | Damping filter method for obtaining spatially localized solutions | Spatially localized structures are key components of turbulence and other
spatio-temporally chaotic systems. From a dynamical systems viewpoint, it is
desirable to obtain corresponding exact solutions, though their existence is
not guaranteed. A damping filter method is introduced to obtain variously
localized solutions, and adopted into two typical cases. This method introduces
a spatially selective damping effect to make a good guess at the exact
solution, and we can obtain an exact solution through a continuation with the
damping amplitude. First target is a steady solution to Swift-Hohenberg
equation, which is a representative of bi-stable systems in which localized
solutions coexist, and a model for span-wisely localized cases. Not only
solutions belonging to the well-known snaking branches but also those belonging
to an isolated branch known as "isolas" are found with a continuation paths
between them in phase space extended with the damping amplitude. This indicates
that this spatially selective excitation mechanism has an advantage in
searching spatially localized solutions. Second target is a spatially localized
traveling-wave solution to Kuramoto-Sivashinsky equation, which is a model for
stream-wisely localized cases. Since the spatially selective damping effect
breaks Galilean and translational invariances, the propagation velocity cannot
be determined uniquely while the damping is active, and a singularity arises
when these invariances are recovered. We demonstrate that this singularity can
be avoided by imposing a simple condition, and a localized traveling-wave
solution is obtained with a specific propagation speed. | 1311.2792v2 |
2020-05-31 | Optimal decay rates of the compressible Euler equations with time-dependent damping in $\mathbb R^n$: (II) over-damping case | This paper is concerned with the multi-dimensional compressible Euler
equations with time-dependent over-damping of the form
$-\frac{\mu}{(1+t)^\lambda}\rho\boldsymbol u$ in $\mathbb R^n$, where $n\ge2$,
$\mu>0$, and $\lambda\in[-1,0)$. This continues our previous work dealing with
the under-damping case for $\lambda\in[0,1)$. We show the optimal decay
estimates of the solutions such that for $\lambda\in(-1,0)$ and $n\ge2$,
$\|\rho-1\|_{L^2(\mathbb R^n)}\approx(1+t)^{-\frac{1+\lambda}{4}n}$ and
$\|\boldsymbol u\|_{L^2(\mathbb R^n)}\approx
(1+t)^{-\frac{1+\lambda}{4}n-\frac{1-\lambda}{2}}$, which indicates that a
stronger damping gives rise to solutions decaying optimally slower. For the
critical case of $\lambda=-1$, we prove the optimal logarithmical decay of the
perturbation of density for the damped Euler equations such that
$\|\rho-1\|_{L^2(\mathbb R^n)}\approx |\ln(e+t)|^{-\frac{n}{4}}$ and
$\|\boldsymbol u\|_{L^2(\mathbb R^n)}\approx
(1+t)^{-1}\cdot|\ln(e+t)|^{-\frac{n}{4}-\frac{1}{2}}$ for $n\ge7$. The
over-damping effect reduces the decay rates of the solutions to be slow, which
causes us some technical difficulty in obtaining the optimal decay rates by the
Fourier analysis method and the Green function method. Here, we propose a new
idea to overcome such a difficulty by artfully combining the Green function
method and the time-weighted energy method. | 2006.00403v1 |
2020-07-07 | Nonlinear viscoelastic isolation for seismic vibration mitigation | The aim of this paper is to assess the effectiveness of nonlinear
viscoelastic damping in controlling base-excited vibrations. Specifically, the
focus is on investigating the robustness of the nonlinear base isolation
performance in controlling the system response due to a wide set of possible
excitation spectra. The dynamic model is derived to study a simple structure
whose base isolation is provided via a Rubber-Layer Roller Bearing (RLRB)
(rigid cylinders rolling on rigid plates with highly damping rubber coatings)
equipped with a nonlinear cubic spring, thus presenting both nonlinear damping
and stiffness. We found that, under periodic loading, due to the non-monotonic
bell-shaped viscoelastic damping arising from the viscoelastic rolling
contacts, different dynamic regimes occur mostly depending on whether the
damping peak is overcome or not. Interestingly, in the former case, poorly
damped self-excited vibrations may be triggered by the steep damping decrease.
Moreover, in order to investigate the robustness of the isolation performance,
we consider a set of real seismic excitations, showing that tuned nonlinear
RLRB provide loads isolation in a wider range of excitation spectra, compared
to generic linear isolators. This is peculiarly suited for applications (such
as seismic and failure engineering) in which the specific excitation spectrum
is unknown a priori, and blind design on statistical data has to be employed. | 2007.04378v1 |
2021-01-20 | Damped perturbations in stellar systems: Genuine modes and Landau-damped waves | This research was stimulated by the recent studies of damping solutions in
dynamically stable spherical stellar systems. Using the simplest model of the
homogeneous stellar medium, we discuss nontrivial features of stellar systems.
Taking them into account will make it possible to correctly interpret the
results obtained earlier and will help to set up decisive numerical experiments
in the future. In particular, we compare the initial value problem versus the
eigenvalue problem. It turns out that in the unstable regime, the Landau-damped
waves can be represented as a superposition of van Kampen modes {\it plus} a
discrete damped mode, usually ignored in the stability study. This mode is a
solution complex conjugate to the unstable Jeans mode. In contrast, the
Landau-damped waves are not genuine modes: in modes, eigenfunctions depend on
time as $\exp (-{\rm i} \omega t)$, while the waves do not have eigenfunctions
on the real $v$-axis at all. However, `eigenfunctions' on the complex
$v$-contours do exist. Deviations from the Landau damping are common and can be
due to singularities or cut-off of the initial perturbation above some fixed
value in the velocity space. | 2101.08287v2 |
2021-03-10 | Dynamical Pose Estimation | We study the problem of aligning two sets of 3D geometric primitives given
known correspondences. Our first contribution is to show that this primitive
alignment framework unifies five perception problems including point cloud
registration, primitive (mesh) registration, category-level 3D registration,
absolution pose estimation (APE), and category-level APE. Our second
contribution is to propose DynAMical Pose estimation (DAMP), the first general
and practical algorithm to solve primitive alignment problem by simulating
rigid body dynamics arising from virtual springs and damping, where the springs
span the shortest distances between corresponding primitives. We evaluate DAMP
in simulated and real datasets across all five problems, and demonstrate (i)
DAMP always converges to the globally optimal solution in the first three
problems with 3D-3D correspondences; (ii) although DAMP sometimes converges to
suboptimal solutions in the last two problems with 2D-3D correspondences, using
a scheme for escaping local minima, DAMP always succeeds. Our third
contribution is to demystify the surprising empirical performance of DAMP and
formally prove a global convergence result in the case of point cloud
registration by charactering local stability of the equilibrium points of the
underlying dynamical system. | 2103.06182v3 |
2023-07-26 | Improving frequency response with synthetic damping available from fleets of distributed energy resources | With the increasing use of renewable generation in power systems, responsive
resources will be necessary to support primary frequency control in future
low-inertia/under-damped power systems. Flexible loads can provide
fast-frequency response services if coordinated effectively. However, practical
implementations of such synthetic damping services require both effective local
sensing and control at the device level and an ability to accurately estimate
online and predict the available synthetic damping from a fleet. In addition,
the inherent trade-off between a fleet being available for fast frequency
response while providing other ancillary services needs to be characterized. In
this context, the manuscript presents a novel, fully decentralized,
packet-based controller for diverse flexible loads that dynamically prioritizes
and interrupts loads to engender synthetic damping suitable for primary
frequency control. Moreover, the packet-based control methodology is shown to
accurately characterize the available synthetic damping in real-time, which is
useful to aggregators and system operators. Furthermore, spectral analysis of
historical frequency regulation data is used to produce a probabilistic bound
on the expected available synthetic damping for primary frequency control from
a fleet and the trade-off from concurrently providing secondary frequency
control services. Finally, numerical simulation on IEEE test networks
demonstrates the effectiveness of the proposed methodology. | 2307.14498v1 |
2023-12-11 | Possible Contamination of the Intergalactic Medium Damping Wing in ULAS J1342+0928 by Proximate Damped Ly$α$ Absorption | The red damping wing from neutral hydrogen in the intergalactic medium is a
smoking-gun signal of ongoing reionization. One potential contaminant of the
intergalactic damping wing signal is dense gas associated with foreground
galaxies, which can give rise to proximate damped Ly$\alpha$ absorbers. The
Ly$\alpha$ imprint of such absorbers on background quasars is indistinguishable
from the intergalactic medium within the uncertainty of the intrinsic quasar
continuum, and their abundance at $z\gtrsim7$ is unknown. Here we show that the
complex of low-ionization metal absorption systems recently discovered by deep
JWST/NIRSpec observations in the foreground of the $z=7.54$ quasar
ULAS~J1342$+$0928 can potentially reproduce the quasar's spectral profile close
to rest-frame Ly$\alpha$ without invoking a substantial contribution from the
intergalactic medium, but only if the absorbing gas is extremely metal-poor
($[{\rm O}/{\rm H}]\sim-3.5$). Such a low oxygen abundance has never been
observed in a damped Ly$\alpha$ absorber at any redshift, but this possibility
still complicates the interpretation of the spectrum. Our analysis highlights
the need for deep spectroscopy of high-redshift quasars with JWST or ELT to
"purify" damping wing quasar samples, an exercise which is impossible for much
fainter objects like galaxies. | 2312.06747v1 |
2024-02-13 | Forecasts for Constraining Lorentz-violating Damping of Gravitational Waves from Compact Binary Inspirals | Violation of Lorentz symmetry can result in two distinct effects in the
propagation of the gravitational waves (GWs). One is a modified dispersion
relation and another is a frequency-dependent damping of GWs. While the former
has been extensively studied in the literature, in this paper we concentrate on
the frequency-dependent damping effect that arises from several specific
Lorentz-violating theories, such as spatial covariant gravities,
Ho\v{r}ava-Lifshitz gravities, etc. This Lorentz-violating damping effect
changes the damping rate of GWs at different frequencies and leads to an
amplitude correction to the GW waveform of compact binary inspiral systems.
With this modified waveform, we then use the Fisher information matrix to
investigate the prospects of constraining the Lorentz-violating damping effect
with GW observations. We consider both ground-based and space-based GW
detectors, including the advanced LIGO, Einstein Telescope, Cosmic Explorer
(CE), Taiji, TianQin, and LISA. Our results indicate that the ground-based
detectors in general give tighter constraints than those from the space-based
detectors. Among the considered three ground-based detectors, CE can give the
tightest constraints on the Lorentz-violating damping effect, which improves
the current constraint from LIGO-Virgo-KAGRA events by about 8 times. | 2402.08240v1 |
2024-03-13 | Thermal Hall effect incorporating magnon damping in localized spin systems | We propose a theory for thermal Hall transport mediated by magnons to address
the impact of their damping resulting from magnon-magnon interactions in
insulating magnets. This phenomenon is anticipated to be particularly
significant in systems characterized by strong quantum fluctuations,
exemplified by spin-1/2 systems. Employing a nonlinear flavor-wave theory, we
analyze a general model for localized electron systems and develop a
formulation for thermal conductivity based on a perturbation theory, utilizing
bosonic Green's functions with a nonzero self-energy. We derive the expression
of the thermal Hall conductivity incorporating magnon damping. To demonstrate
the applicability of the obtained representation, we adopt it to two $S=1/2$
quantum spin models on a honeycomb lattice. In calculations for these systems,
we make use of the self-consistent imaginary Dyson equation approach at finite
temperatures for evaluating the magnon damping rate. In both systems, the
thermal Hall conductivity is diminished due to the introduction of magnon
damping over a wide temperature range. This effect arises due to the smearing
of magnon spectra with nonzero Berry curvatures. We also discuss the relation
to the damping of chiral edge modes of magnons. Our formulation can be applied
to various localized electron systems as we begin with a general Hamiltonian
for these systems. Our findings shed light on a new aspect of topological
magnonics emergent from many-body effects and will stimulate further
investigations on the impact of magnon damping on topological phenomena. | 2403.08478v1 |
2024-04-02 | A recipe for eccentricity and inclination damping for partial gap opening planets in 3D disks | In a previous paper we showed that, like the migration speed, the
eccentricity damping efficiency is modulated linearly by the depth of the
partial gap a planet carves in the disk surface density profile, resulting in
less efficient $e$-damping compared to the prescription commonly used in
population synthesis works. Here, we extend our analysis to 3D, refining our
$e$-damping formula and studying how the inclination damping efficiency is also
affected. We perform high resolution 3D locally isothermal hydrodynamical
simulations of planets with varying masses embedded in disks with varying
aspect ratios and viscosities. We extract the gap profile and orbital damping
timescales for fixed eccentricities and inclinations up to the disk scale
height. The limit in gap depths below which vortices appear, in the
low-viscosity case, happens roughly at the transition between classical type-I
and type-II migration regimes. The orbital damping timescales can be described
by two linear trends with a break around gap depths $\sim80\%$ and with slopes
and intercepts depending on the eccentricity and inclination. These trends are
understood on physical grounds and are reproduced by simple fitting formulas
whose error is within the typically uncertainty of type-I torque formulas.
Thus, our recipes for the gap depth and orbital damping efficiencies yield a
simple description for planet-disk interactions to use in N-body codes in the
case of partial gap opening planets that is consistent with high-resolution 3D
hydro-simulations. Finally, we show examples of how our novel orbital damping
prescription can affect the outcome of population synthesis experiments. | 2404.02247v1 |
2009-08-21 | Surface Alfven Wave Damping in a 3D Simulation of the Solar Wind | Here we investigate the contribution of surface Alfven wave damping to the
heating of the solar wind in minima conditions. These waves are present in
regions of strong inhomogeneities in density or magnetic field (e. g., the
border between open and closed magnetic field lines). Using a 3-dimensional
Magnetohydrodynamics (MHD) model, we calculate the surface Alfven wave damping
contribution between 1-4 solar radii, the region of interest for both
acceleration and coronal heating. We consider waves with frequencies lower than
those that are damped in the chromosphere and on the order of those dominating
the heliosphere. In the region between open and closed field lines, within a
few solar radii of the surface, no other major source of damping has been
suggested for the low frequency waves we consider here. This work is the first
to study surface Alfven waves in a 3D environment without assuming a priori a
geometry of field lines or magnetic and density profiles. We determine that
waves with frequencies >2.8x10^-4 Hz are damped between 1-4 solar radii. In
quiet sun regions, surface Alfven waves are damped at further distances
compared to active regions, thus carrying additional wave energy into the
corona. We compare the surface Alfven wave contribution to the heating by a
variable polytropic index and find that it an order of magnitude larger than
needed for quiet sun regions. For active regions the contribution to the
heating is twenty percent. As it has been argued that a variable gamma acts as
turbulence, our results indicate that surface Alfven wave damping is comparable
to turbulence in the lower corona. This damping mechanism should be included
self consistently as an energy driver for the wind in global MHD models. | 0908.3146v1 |
2017-11-21 | Determination of spin Hall effect and spin diffusion length of Pt from self-consistent fitting of damping enhancement and inverse spin-orbit torque measurements | Understanding the evolution of spin-orbit torque (SOT) with increasing
heavy-metal thickness in ferromagnet/normal metal (FM/NM) bilayers is critical
for the development of magnetic memory based on SOT. However, several
experiments have revealed an apparent discrepancy between damping enhancement
and damping-like SOT regarding their dependence on NM thickness. Here, using
linewidth and phase-resolved amplitude analysis of vector network analyzer
ferromagnetic resonance (VNA-FMR) measurements, we simultaneously extract
damping enhancement and both field-like and damping-like inverse SOT in
Ni$_{80}$Fe$_{20}$/Pt bilayers as a function of Pt thickness. By enforcing an
interpretation of the data which satisfies Onsager reciprocity, we find that
both the damping enhancement and damping-like inverse SOT can be described by a
single spin diffusion length ($\approx$ 4 nm), and that we can separate the
spin pumping and spin memory loss (SML) contributions to the total damping.
This analysis indicates that less than 40% of the angular momentum pumped by
FMR through the Ni$_{80}$Fe$_{20}$/Pt interface is transported as spin current
into the Pt. On account of the SML and corresponding reduction in total spin
current available for spin-charge transduction in the Pt, we determine the Pt
spin Hall conductivity ($\sigma_\mathrm{SH} = (2.36 \pm 0.04)\times10^6
\Omega^{-1} \mathrm{m}^{-1}$) and bulk spin Hall angle
($\theta_\mathrm{SH}=0.387 \pm0.008$) to be larger than commonly-cited values.
These results suggest that Pt can be an extremely useful source of SOT if the
FM/NM interface can be engineered to minimize SML. Lastly, we find that
self-consistent fitting of the damping and SOT data is best achieved by a model
with Elliott-Yafet spin relaxation and extrinsic inverse spin Hall effect, such
that both the spin diffusion length and spin Hall conductivity are proportional
to the Pt charge conductivity. | 1711.07654v2 |
2019-09-19 | Nonlinear energy loss in the oscillations of coated and uncoated bubbles: Role of thermal, radiation damping and encapsulating shell at various excitation pressures | A simple generalized model (GM) for coated bubbles accounting for the effect
of compressibility of the liquid is presented. The GM was then coupled with
nonlinear ODEs that account for the thermal effects. Starting with mass and
momentum conservation equations for a bubbly liquid and using the GM, nonlinear
pressure dependent terms were derived for energy dissipation due to thermal
damping (Td), radiation damping (Rd) and dissipation due to the viscosity of
liquid (Ld) and coating (Cd). The dissipated energies were solved for uncoated
and coated 2- 20 $\mu m$ bubbles over a frequency range of $0.25f_r-2.5f_r$
($f_r$ is the bubble resonance) and for various acoustic pressures
(1kPa-300kPa). Thermal effects were examined for air and C3F8 gas cores in each
case. For uncoated bubbles with an air gas core and a diameter larger than 4
$\mu m$, thermal damping is the strongest damping factor. When pressure
increases, the contributions of Rd grow faster and become the dominant damping
mechanism for pressure dependent resonance frequencies (e.g. fundamental and
super harmonic resonances). For coated bubbles, Cd is the strongest damping
mechanism. As pressure increases Rd contributes more to damping compared to Ld
and Td. In case of air bubbles, as pressure increases, the linear thermal model
largely deviates from the nonlinear model and accurate modeling requires
inclusion of the full thermal model. However, for coated C3F8 bubbles of
diameter 1-8 $\mu m$, typically used in medical ultrasound, thermal effects
maybe neglected even at higher pressures. We show that the scattering to
damping ratio (STDR), a measure of the effectiveness of the bubble as contrast
agent, is pressure dependent and can be maximized for specific frequency ranges
and pressures. | 1909.08793v1 |
2020-11-20 | The effect of magnetic field on the damping of slow waves in the solar corona | Slow magnetoacoustic waves are routinely observed in astrophysical plasma
systems such as the solar corona. As a slow wave propagates through a plasma,
it modifies the equilibrium quantities of density, temperature, and magnetic
field. In the corona and other plasma systems, the thermal equilibrium is
comprised of a balance between continuous heating and cooling processes, the
magnitudes of which vary with density, temperature and magnetic field. Thus the
wave may induce a misbalance between these competing processes. Its back
reaction on the wave has been shown to lead to dispersion, and amplification or
damping, of the wave. In this work the importance of the effect of magnetic
field in the rapid damping of slow waves in the solar corona by heating/cooling
misbalance is evaluated and compared to the effects of thermal conduction. The
two timescales characterising the effect of misbalance are derived and
calculated for plasma systems with a range of typical coronal conditions. The
predicted damping times of slow waves from thermal misbalance in the solar
corona are found to be of the order of 10-100 minutes, coinciding with the wave
periods and damping times observed. Moreover the slow wave damping by thermal
misbalance is found to be comparable to the damping by field-aligned thermal
conduction. We show that in the infinite field limit, the wave dynamics is
insensitive to the dependence of the heating function on the magnetic field,
and this approximation is found to be valid in the corona so long as the
magnetic field strength is greater than 10G for quiescent loops and plumes and
100G for hot and dense loops. In summary thermal misbalance may damp slow
magnetoacoustic waves rapidly in much of the corona, and its inclusion in our
understanding of slow mode damping may resolve discrepancies between
observations and theory relying on compressive viscosity and thermal conduction
alone. | 2011.10437v1 |
1997-11-25 | Abundances of Heavy Elements and CO Molecules in High Redshift Damped Lyman-alpha Galaxies | Damped Lyman-alpha systems seen in spectra of background quasars are
generally thought to represent high redshift counterparts of present-day
galaxies. We summarize observations of heavy element abundances in damped
Lyman-alpha systems. The results of a systematic search for CO and C II*
absorption in 17 damped Lyman-alpha systems are also presented using
observations obtained with the 10m Keck telescopes. The latter provides a
useful constraint on the expected strength of [C II] 158 micron emission from
damped Lyman-alpha galaxies. It is hoped that these results will be useful for
planning future radio to millimeter wave observations of high redshift galaxies
using next generation instruments which are now being built. | 9711298v1 |
1997-12-05 | Magnetohydrodynamics in the Early Universe and the Damping of Non-linear Alfven Waves | The evolution and viscous damping of cosmic magnetic fields in the early
universe, is analysed. Using the fact that the fluid, electromagnetic, and
shear viscous energy-momentum tensors are all conformally invariant, the
evolution is transformed from the expanding universe setting into that in flat
spacetime. Particular attention is paid to the evolution of nonlinear Alfven
modes. For a small enough magnetic field, which satisfies our observational
constraints, these wave modes either oscillate negligibly or, when they do
oscillate, become overdamped. Hence they do not suffer Silk damping on galactic
and subgalactic scales. The smallest scale which survives damping depends on
the field strength and is of order a dimensionless Alfven velocity times the
usual baryon-photon Silk damping scale. After recombination, nonlinear effects
can convert the Alfven mode into compressional, gravitationally unstable waves
and seed cosmic structures if the cosmic magnetic field is sufficiently strong. | 9712083v1 |
2001-08-09 | Are Simulations of CDM Consistent with Galactic-Scale Observations at High Redshift? | We compare new observations on the kinematic characteristics of the damped
Lya systems against results from numerical SPH simulations to test the
predictions of hierarchical galaxy formation. This exercise is particularly
motivated by recent numerical results on the cross-section of damped Lya
systems. Our analysis focuses on the velocity widths of ~50 low-ion absorption
profiles from our sample of z>1.5 damped Lya systems. The results indicate that
current numerical simulations fail to match the damped Lya observations at high
confidence levels (>99.9%). Although we do not believe that our results present
an insurmountable challenge to the paradigm of hierarchical cosmology, the
damped Lya observations suggest that current numerical SPH simulations overlook
an integral aspect of galaxy formation. | 0108154v1 |
2003-03-19 | Distinct Abundance Patterns in Multiple Damped Ly-alpha Galaxies: Evidence for Truncated Star Formation? | (abridged) Following our previous work on metal abundances of a double damped
Ly-alpha system with a line-of-sight separation ~2000 km/s (Ellison & Lopez
2001), we present VLT UVES abundances of 3 new systems spanning a total of
\~6000 km/s at z~2.5 toward the southern QSO CTQ247. These abundances are
supplemented with echelle observations of another `double' damped Ly-alpha
system in the literature. We propose a definition in terms of velocity shift of
the sub-class 'multiple damped Ly-alpha system', which is motivated by its
possible connection with large-scale structure. We find that the abundance
ratio alpha/Fe is systematically low in multiple systems compared with single
systems, and with a small scatter. The same behavior is found in 2 more single
DLA systems taken from the literature that show evidence of belonging to a
galaxy group. After a careful investigation of possible sources of systematic
errors, we conclude that the low alpha/Fe ratios in multiple DLAs have a
nucleosynthetic origin. We suggest that they could be explained by reduced star
formation in multiple damped Ly-alpha systems, possibly due to environmental
effects. | 0303441v1 |
2003-05-16 | New Damped Lya Metallicities from ESI Spectroscopy of Five Palomar Sky Survey Quasars | This paper presents chemical abundance measurements for 12 new z>3 damped Lya
systems discovered toward five quasars from the Palomar Sky Survey. We
determine HI column densities from profile fits to the observed damped Lya
profiles and measure ionic column densities and limits for all observed
metal-line transitions. This dataset, acquired with the Echellette Spectrograph
and Imager on the KeckII telescope, adds to the rapidly growing database of
damped Lya abundances. It will impact studies of chemical evolution in the
early universe and help identify candidates for detailed follow-up observations
with echelle spectrographs. We report the discovery of the first quasar
sightline with four cosmologically distinct damped Lya systems. | 0305313v1 |
2006-07-06 | Ekman layer damping of r-modes revisited | We investigate the damping of neutron star r-modes due to the presence of a
viscous boundary (Ekman) layer at the interface between the crust and the core.
Our study is motivated by the possibility that the gravitational-wave driven
instability of the inertial r-modes may become active in rapidly spinning
neutron stars, eg. in low-mass X-ray binaries, and the fact that a viscous
Ekman layer at the core-crust interface provides an efficient damping mechanism
for these oscillations. We review various approaches to the problem and carry
out an analytic calculation of the effects due to the Ekman layer for a rigid
crust. Our analytic estimates support previous numerical results, and provide
further insight into the intricacies of the problem. We add to previous work by
discussing the effect that compressibility and composition stratification have
on the boundary layer damping. We show that, while stratification is
unimportant for the r-mode problem, composition suppresses the damping rate by
about a factor of two (depending on the detailed equation of state). | 0607105v2 |
1997-11-05 | Hydrodynamic damping in trapped Bose gases | Griffin, Wu and Stringari have derived the hydrodynamic equations of a
trapped dilute Bose gas above the Bose-Einstein transition temperature. We give
the extension which includes hydrodynamic damping, following the classic work
of Uehling and Uhlenbeck based on the Chapman-Enskog procedure. Our final
result is a closed equation for the velocity fluctuations $\delta v$ which
includes the hydrodynamic damping due to the shear viscosity $\eta$ and the
thermal conductivity $\kappa$. Following Kavoulakis, Pethick and Smith, we
introduce a spatial cutoff in our linearized equations when the density is so
low that the hydrodynamic description breaks down. Explicit expressions are
given for $\eta$ and $\kappa$, which are position-dependent through dependence
on the local fugacity when one includes the effect of quantum degeneracy of the
trapped gas. We also discuss a trapped Bose-condensed gas, generalizing the
work of Zaremba, Griffin and Nikuni to include hydrodynamic damping due to the
(non-condensate) normal fluid. | 9711036v4 |
1998-05-01 | Finite Temperature Perturbation Theory for a Spatially Inhomogeneous Bose-condensed Gas | We develop a finite temperature perturbation theory (beyond the mean field)
for a Bose-condensed gas and calculate temperature-dependent damping rates and
energy shifts for Bogolyubov excitations of any energy. The theory is
generalized for the case of excitations in a spatially inhomogeneous (trapped)
Bose-condensed gas, where we emphasize the principal importance of
inhomogeneouty of the condensate density profile and develop the method of
calculating the self-energy functions. The use of the theory is demonstrated by
calculating the damping rates and energy shifts of low-energy quasiclassical
excitations, i.e. the quasiclassical excitations with energies much smaller
than the mean field interaction between particles. In this case the boundary
region of the condensate plays a crucial role, and the result for the damping
rates and energy shifts is completely different from that in spatially
homogeneous gases. We also analyze the frequency shifts and damping of sound
waves in cylindrical Bose condensates and discuss the role of damping in the
recent MIT experiment on the sound propagation. | 9805015v2 |
2004-03-25 | XMCD characterization of rare-earth dopants in Ni$_{81}$Fe$_{19}$(50nm): microscopic basis of engineered damping | We present direct evidence for the contribution of local orbital moments to
the damping of magnetization precession in magnetic thin films. Using x-ray
magnetic circular dichroism (XMCD) characterization of rare-earth (RE)
M$_{4,5}$ edges in Ni$_{81}$Fe$_{19}$ doped with $<$ 2% Gd and Tb, we show that
the enhancement of GHz precessional relaxation is accompanied by a significant
orbital moment fraction on the RE site. Tb impurities, which enhance the
Landau-Lifshitz(-Gilbert) LL(-G) damping $\lambda(\alpha)$, show a spin to
orbital number ratio of 1.5$\pm$0.3; Gd impurities, which have no effect on
damping, show a spin to orbital number ratio of zero within experimental error.
The results indicate that the dopant-based control of magnetization damping in
RE-doped ferromagnets is an atomistic effect, arising from spin-lattice
coupling, and thus scalable to nanometer dimensions. | 0403627v1 |
2005-02-08 | Landau Damping of Spin Waves in Trapped Boltzmann Gases | A semiclassical method is used to study Landau damping of transverse
pseudo-spin waves in harmonically trapped ultracold gases in the collisionless
Boltzmann limit. In this approach, the time evolution of a spin is calculated
numerically as it travels in a classical orbit through a spatially dependent
mean field. This method reproduces the Landau damping results for spin-waves in
unbounded systems obtained with a dielectric formalism. In trapped systems, the
simulations indicate that Landau damping occurs for a given spin-wave mode
because of resonant phase space trajectories in which spins are "kicked out" of
the mode (in spin space). A perturbative analysis of the resonant and nearly
resonant trajectories gives the Landau damping rate, which is calculated for
the dipole and quadrupole modes as a function of the interaction strength. The
results are compared to a numerical solution of the kinetic equation by Nikuni
et al. | 0502189v1 |
2005-06-01 | Landau damping of Bogoliubov excitations in optical lattices at finite temperature | We study the damping of Bogoliubov excitations in an optical lattice at
finite temperatures. For simplicity, we consider a Bose-Hubbard tight-binding
model and limit our analysis to the lowest excitation band. We use the Popov
approximation to calculate the temperature dependence of the number of
condensate atoms $n^{\rm c 0}(T)$ in each lattice well. We calculate the Landau
damping of a Bogoliubov excitation in an optical lattice due to coupling to a
thermal cloud of excitations. While most of the paper concentrates on 1D
optical lattices, we also briefly present results for 2D and 3D lattices. For
energy conservation to be satisfied, we find that the excitations in the
collision process must exhibit anomalous dispersion ({\it i.e.} the excitation
energy must bend upward at low momentum), as also exhibited by phonons in
superfluid $^4\rm{He}$. This leads to the sudden disappearance of all damping
processes in $D$-dimensional simple cubic optical lattice when $U n^{\rm c
0}\ge 6DJ$, where $U$ is the on-site interaction, and $J$ is the hopping matrix
element. Beliaev damping in a 1D optical lattice is briefly discussed. | 0506016v1 |
2006-06-15 | Landau damping: instability mechanism of superfluid Bose gases moving in optical lattices | We investigate Landau damping of Bogoliubov excitations in a dilute Bose gas
moving in an optical lattice at finite temperatures. Using a 1D tight-binding
model, we explicitly obtain the Landau damping rate, the sign of which
determines the stability of the condensate. We find that the sign changes at a
certain condensate velocity, which is exactly the same as the critical velocity
determined by the Landau criterion of superfluidity. This coincidence of the
critical velocities reveals the microscopic mechanism of the Landau
instability. This instability mechanism is also consistent with the recent
experiment suggesting that a thermal cloud plays a crucial role in breakdown of
superfluids, since the thermal cloud is also vital in the Landau damping
process. We also examine the possibility of simultaneous disappearance of all
damping processes. | 0606398v2 |
1999-09-24 | Gauge Invariance of Nonlinear Landau Damping Rate of Bose Excitations in Quark-Gluon Plasma | On the basis of the approximate dynamical equations describing the behavior
of quark-gluon plasma (QGP) in the semiclassical limit and Yang-Mills equation,
the kinetic equation for longitudinal waves (plasmons) is obtained. With the
Ward identities the gauge invariance of obtained nonlinear Landau damping rate
is proved. The physical mechanisms defining nonlinear scattering of a plasmon
by QGP particles are analyzed. The problem on a connection of nonlinear Landau
damping rate of longitudinal oscillations with damping rate, obtained in the
framework of hard thermal loops approximation, is considered. It is shown that
the gauge-dependent part of nonlinear Landau damping rate for the plasmons with
zero momentum vanishes on mass-shell. | 9909505v1 |
2005-07-16 | Sharp estimates for the number of degrees of freedom for the damped-driven 2D Navier--Stokes equations | We derive upper bounds for the number of asymptotic degrees (determining
modes and nodes) of freedom for the two-dimensional Navier--Stokes system and
Navier-Stokes system with damping. In the first case we obtain the previously
known estimates in an explicit form, which are larger than the fractal
dimension of the global attractor. However, for the Navier--Stokes system with
damping our estimates for the number of the determining modes and nodes are
comparable to the sharp estimates for the fractal dimension of the global
attractor. Our investigation of the damped-driven 2D Navier--Stokes system is
inspired by the Stommel--Charney barotropic model of ocean circulation where
the damping represents the Rayleigh friction. We remark that our results
equally apply to the Stommel--Charney model. | 0507327v1 |
2006-12-04 | A singular perturbation approach for choosing PageRank damping factor | The choice of the PageRank damping factor is not evident. The Google's choice
for the value c=0.85 was a compromise between the true reflection of the Web
structure and numerical efficiency. However, the Markov random walk on the
original Web Graph does not reflect the importance of the pages because it
absorbs in dead ends. Thus, the damping factor is needed not only for speeding
up the computations but also for establishing a fair ranking of pages. In this
paper, we propose new criteria for choosing the damping factor, based on the
ergodic structure of the Web Graph and probability flows. Specifically, we
require that the core component receives a fair share of the PageRank mass.
Using singular perturbation approach we conclude that the value c=0.85 is too
high and suggest that the damping factor should be chosen around 1/2. As a
by-product, we describe the ergodic structure of the OUT component of the Web
Graph in detail. Our analytical results are confirmed by experiments on two
large samples of the Web Graph. | 0612079v1 |
1998-10-26 | Microscopic Structure of Rotational Damping | The damping of collective rotational motion is studied microscopically,
making use of shell model calculations based on the cranked Nilsson deformed
mean-field and on residual two-body interactions, and focusing on the shape of
the gamma-gamma correlation spectra and on its systematic behavior. It is shown
that the spectral shape is directly related to the damping width of collective
rotation, \Gammarot, and to the spreading width of many-particle many-hole
configurations, \Gammamu. The rotational damping width is affected by the shell
structure, and is very sensitive to the position of the Fermi surface, besides
mass number, spin and deformation. This produces a rich variety of features in
the rotational damping phenomena. | 9810066v1 |
2004-07-25 | Rotational damping in a multi-$j$ shell particles-rotor model | The damping of collective rotational motion is investigated by means of
particles-rotor model in which the angular momentum coupling is treated exactly
and the valence nucleons are in a multi-$j$ shell mean-field. It is found that
the onset energy of rotational damping is around 1.1 MeV above yrast line, and
the number of states which form rotational band structure is thus limited. The
number of calculated rotational bands around 30 at a given angular momentum
agrees qualitatively with experimental data. The onset of rotational damping
takes place gradually as a function of excitation energy. It is shown that the
pairing correlation between valence nucleons has a significant effect on the
appearance of rotational damping. | 0407089v3 |
2001-07-19 | Manifold Damping of Transverse Wakefields in High Phase Advance Traveling Wave Structures and Local Damping of Dipole Wakefields in Standing Wave Accelerators | Operating the SLAC/KEK DDS (Damped Detuned Structure) X-band linacs at high
gradients (in excess of 70MV/m) has recently been found to be limited by the
accelerator structures breaking down and as a consequence severe damage occurs
to the cells which makes the structures inoperable. A series of recent
experiments at SLAC indicates that arcing in the structures is significantly
reduced if the group velocity of the accelerating mode is reduced and
additionally it has been discovered that reducing the length of the
accelerating structure also limits the number and intensity of breakdown events
[1]. However, in designing new accelerating structures care must be taken to
ensure that the beam-induced transverse wakefields do not cause the beam to
become unstable. Here, we report on damping transverse wakefields in two
different short structures: a 90cm traveling wave structure in which the
wakefield is coupled out to four attached manifolds and secondly, in a standing
wave structure in which a limited number of cells heavily damp down the
wakefield.
[1] C. Adolphsen, ROAA003, this conf. | 0107048v1 |
2002-06-28 | Manifold Damping Of Wakefields In High Phase Advance Linacs For The NLC | Earlier RDDS (Rounded Damped Detuned Structures) [1,2], designed, fabricated
and tested at SLAC, in collaboration with KEK, have been shown to damp
wakefields successfully. However, electrical breakdown has been found to occur
in these structures and this makes them inoperable at the desired gradient.
Recent results [3] indicate that lowering the group velocity of the
accelerating mode reduces electrical breakdown events. In order to preserve the
filling time of each structure a high synchronous phase advance (150 degrees as
opposed to 120 used in previous NLC designs) has been chosen. Here, damping of
the wakefield is analyzed. Manifold damping and interleaving of structure cell
frequencies is discussed. These wakefields impose alignment tolerances on the
cells and on the structure as a whole. Tolerance calculations are performed and
these are compared with analytic estimations. | 0206090v1 |
2006-06-30 | Nonlinear Damping of the LC Circuit using Anti-parallel Diodes | We investigate a simple variation of the series RLC circuit in which
anti-parallel diodes replace the resistor. This results in a damped harmonic
oscillator with a nonlinear damping term that is maximal at zero current and
decreases with an inverse current relation for currents far from zero. A set of
nonlinear differential equations for the oscillator circuit is derived and
integrated numerically for comparison with circuit measurements. The agreement
is very good for both the transient and steady-state responses. Unlike the
standard RLC circuit, the behavior of this circuit is amplitude dependent. In
particular for the transient response the oscillator makes a transition from
under-damped to over-damped behavior, and for the driven oscillator the
resonance response becomes sharper and stronger as drive source amplitude
increases. The equipment is inexpensive and common to upper level physics labs. | 0606261v1 |
1995-11-11 | A New Look at the Landau's Theory of Spreading and Damping of Waves in Collisionless Plasmas | The theory of plasma waves and Landau damping in Maxwellian plasmas, Landau's
``rule of pass around poles'' include doubtful statements, particularly related
to an artificial ``constructing'' of the dispersion equation, what should allow
the possibility of its solution otherwise not existing at all, and the
possibility of analytical continuations of corresponding very specific ruptured
functions in the one-dimensional Laplace transformation, used by Landau, what
is the base of his theory.
We represent, as an accessible variant, a more general alternative theory
based on a two-dimensional Laplace transformation, leading to an asymptotical
in time and space solution as a complicated superposition of coupled damping
and {\em non-damping \/} plane waves and oscillations with different dispersion
laws for every constituent mode. This theory naturally and very simply explains
paradoxes of the phenomenon of plasma echo. We propose for discussion a new
ideology of plasma waves (both electron and ion-acoustic waves) qualitatively
different from the traditional theory of Landau damping for non-collisional as
well as for low-collisional plasmas. | 9511001v1 |
2001-07-27 | Quantum limits of cold damping with optomechanical coupling | Thermal noise of a mirror can be reduced by cold damping. The displacement is
measured with a high-finesse cavity and controlled with the radiation pressure
of a modulated light beam. We establish the general quantum limits of noise in
cold damping mechanisms and we show that the optomechanical system allows to
reach these limits. Displacement noise can be arbitrarily reduced in a narrow
frequency band. In a wide-band analysis we show that thermal fluctuations are
reduced as with classical damping whereas quantum zero-point fluctuations are
left unchanged. The only limit of cold damping is then due to zero-point energy
of the mirror | 0107138v2 |
2005-05-20 | A symmetric treatment of damped harmonic oscillator in extended phase space | Extended phase space (EPS) formulation of quantum statistical mechanics
treats the ordinary phase space coordinates on the same footing and thereby
permits the definite the canonical momenta conjugate to these coordinates . The
extended lagrangian and extended hamiltonian are defined in EPS by the same
procedure as one does for ordinary lagrangian and hamiltonian. The combination
of ordinary phase space and their conjugate momenta exhibits the evolution of
particles and their mirror images together. The resultant evolution equation in
EPS for a damped harmonic oscillator, is such that the energy dissipated by the
actual oscillator is absorbed in the same rate by the image oscillator leaving
the whole system as a conservative system. We use the EPS formalism to obtain
the dual hamiltonian of a damped harmonic oscillator, first proposed by
Batemann, by a simple extended canonical transformations in the extended phase
space. The extended canonical transformations are capable of converting the
damped system of actual and image oscillators to an undamped one, and transform
the evolution equation into a simple form. The resultant equation is solved and
the eigenvalues and eigenfunctions for damped oscillator and its mirror image
are obtained. The results are in agreement with those obtained by Bateman. At
last, the uncertainty relation are examined for above system. | 0505147v1 |
2007-08-28 | Pattern formation in the damped Nikolaevskiy equation | The Nikolaevskiy equation has been proposed as a model for seismic waves,
electroconvection and weak turbulence; we show that it can also be used to
model transverse instabilities of fronts. This equation possesses a large-scale
"Goldstone" mode that significantly influences the stability of spatially
periodic steady solutions; indeed, all such solutions are unstable at onset,
and the equation exhibits so-called soft-mode turbulence. In many applications,
a weak damping of this neutral mode will be present, and we study the influence
of this damping on solutions to the Nikolaevskiy equation. We examine the
transition to the usual Eckhaus instability as the damping of the large-scale
mode is increased, through numerical calculation and weakly nonlinear analysis.
The latter is accomplished using asymptotically consistent systems of coupled
amplitude equations. We find that there is a critical value of the damping
below which (for a given value of the supercriticality parameter) all periodic
steady states are unstable. The last solutions to lose stability lie in a cusp
close to the left-hand side of the marginal stability curve. | 0708.3735v1 |
2008-01-12 | Strong and weak coupling limits in optics of quantum well excitons | A transition between the strong (coherent) and weak (incoherent) coupling
limits of resonant interaction between quantum well (QW) excitons and bulk
photons is analyzed and quantified as a function of the incoherent damping rate
caused by exciton-phonon and exciton-exciton scattering. For confined QW
polaritons, a second, anomalous, damping-induced dispersion branch arises and
develops with increasing damping. In this case, the strong-weak coupling
transition is attributed to a critical damping rate, when the intersection of
the normal and damping-induced dispersion branches occurs. For the radiative
states of QW excitons, i.e., for radiative QW polaritons, the transition is
described as a qualitative change of the photoluminescence spectrum at grazing
angles along the QW structure. Furthermore, we show that the radiative
corrections to the QW exciton states with in-plane wavevector approaching the
photon cone are universally scaled by an energy parameter rather than diverge.
The strong-weak coupling transition rates are also proportional to the same
energy parameter. The numerical evaluations are given for a GaAs single quantum
well with realistic parameters. | 0801.1895v2 |
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