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2019-08-21 | Generalized Metric Repair on Graphs | Many modern data analysis algorithms either assume or are considerably more
efficient if the distances between the data points satisfy a metric. These
algorithms include metric learning, clustering, and dimension reduction. As
real data sets are noisy, distances often fail to satisfy a metric. For this
reason, Gilbert and Jain and Fan et al. introduced the closely related sparse
metric repair and metric violation distance problems. The goal of these
problems is to repair as few distances as possible to ensure they satisfy a
metric. Three variants were considered, one admitting a polynomial time
algorithm. The other variants were shown to be APX-hard, and an
$O(OPT^{1/3})$-approximation was given, where $OPT$ is the optimal solution
size.
In this paper, we generalize these problems to no longer consider all
distances between the data points. That is, we consider a weighted graph $G$
with corrupted weights $w$, and our goal is to find the smallest number of
weight modifications so that the resulting weighted graph distances satisfy a
metric. This is a natural generalization and is more flexible as it takes into
account different relationships among the data points. As in previous work, we
distinguish among the types of repairs permitted and focus on the increase only
and general versions. We demonstrate the inherent combinatorial structure of
the problem, and give an approximation-preserving reduction from MULTICUT.
Conversely, we show that for any fixed constant $\varsigma$, for the large
class of $\varsigma$-chordal graphs, the problems are fixed parameter
tractable. Call a cycle broken if it contains an edge whose weight is larger
than the sum of all its other edges, and call the amount of this difference its
deficit. We present approximation algorithms, one which depends on the maximum
number of edges in a broken cycle, and one which depends on the number of
distinct deficit values. | 1908.08411v1 |
2019-09-19 | Parameterized Complexity of Manipulating Sequential Allocation | The sequential allocation protocol is a simple and popular mechanism to
allocate indivisible goods, in which the agents take turns to pick the items
according to a predefined sequence. While this protocol is not strategy-proof,
it has been shown recently that finding a successful manipulation for an agent
is an NP-hard problem (Aziz et al., 2017). Conversely, it is also known that
finding an optimal manipulation can be solved in polynomial time in a few
cases: if there are only two agents or if the manipulator has a binary or a
lexicographic utility function. In this work, we take a parameterized approach
to provide several new complexity results on this manipulation problem. More
precisely, we give a complete picture of its parameterized complexity w.r.t.
the following three parameters: the number $n$ of agents, the number $\mu(a_1)$
of times the manipulator $a_1$ picks in the picking sequence, and the maximum
range $\mathtt{rg}^{\max}$ of an item. This third parameter is a correlation
measure on the preference rankings of the agents. In particular, we show that
the problem of finding an optimal manipulation can be solved in polynomial time
if $n$ or $\mu(a_1)$ is a constant, and that it is fixed-parameter tractable
w.r.t. $\mathtt{rg}^{\max}$ and $n+\mu(a_1)$. Interestingly enough, we show
that w.r.t. the single parameters $n$ and $\mu(a_1)$ it is W[1]-hard. Moreover,
we provide an integer program and a dynamic programming scheme to solve the
manipulation problem and we show that a single manipulator can increase the
utility of her bundle by a multiplicative factor which is at most 2. | 1909.08920v4 |
2019-11-06 | Doppler Spectrum Classification with CNNs via Heatmap Location Encoding and a Multi-head Output Layer | Spectral Doppler measurements are an important part of the standard
echocardiographic examination. These measurements give important insight into
myocardial motion and blood flow providing clinicians with parameters for
diagnostic decision making. Many of these measurements can currently be
performed automatically with high accuracy, increasing the efficiency of the
diagnostic pipeline. However, full automation is not yet available because the
user must manually select which measurement should be performed on each image.
In this work we develop a convolutional neural network (CNN) to automatically
classify cardiac Doppler spectra into measurement classes. We show how the
multi-modal information in each spectral Doppler recording can be combined
using a meta parameter post-processing mapping scheme and heatmaps to encode
coordinate locations. Additionally, we experiment with several state-of-the-art
network architectures to examine the tradeoff between accuracy and memory usage
for resource-constrained environments. Finally, we propose a confidence metric
using the values in the last fully connected layer of the network. We analyze
example images that fall outside of our proposed classes to show our confidence
metric can prevent many misclassifications. Our algorithm achieves 96% accuracy
on a test set drawn from a separate clinical site, indicating that the proposed
method is suitable for clinical adoption and enabling a fully automatic
pipeline from acquisition to Doppler spectrum measurements. | 1911.02407v2 |
2020-01-02 | The Effect of Treatment-Related Deaths and "Sticky" Diagnoses on Recorded Prostate Cancer Mortality | Background: Although recorded cancer mortality should include both deaths
from cancer and deaths from cancer treatment, there is evidence suggesting that
the measure may be incomplete. To investigate the completeness of recorded
prostate cancer mortality, we compared other-cause (non-prostate cancer)
mortality in men found and not found to have prostate cancer following a needle
biopsy.
Methods: We linked Medicare claims data to SEER data to analyze survival in
the population of men aged 65+ enrolled in Medicare who resided in a SEER area
and received a needle biopsy in 1993-2001. We compared other-cause mortality in
men found to have prostate cancer (n=53,462) to that in men not found to have
prostate cancer (n=103,659).
Results: The age-race adjusted other-cause mortality rate was 471 per 10,000
person-years in men found to have prostate cancer vs. 468 per 10,000 in men not
found to have prostate cancer (RR = 1.01;95% CI:0.98-1.03). The effect was
modified, however, by age. The RR declined in a stepwise fashion from 1.08 (95%
CI:1.03-1.14) in men age 65-69 to 0.89 (95% CI:0.83-0.95) in men age 85 and
older. If the excess (or deficit) in other-cause mortality were added to the
recorded prostate cancer mortality, prostate cancer mortality would rise 23% in
the youngest age group (from 90 to 111 per 10,000) and would fall 30% in the
oldest age group (from 551 to 388 per 10,000).
Conclusion: Although recorded prostate cancer mortality appears to be an
accurate measure overall, it systematically underestimates the mortality
associated with prostate cancer diagnosis and treatment in younger men and
overestimates it in the very old. We surmise that in younger men
treatment-related deaths are incompletely captured in recorded prostate cancer
mortality, while in older men the diagnosis "sticks"-- once diagnosed, they are
more likely to be said to have died from the disease. | 2001.00492v1 |
2020-06-09 | Elemental Abundances in M31: Iron and Alpha Element Abundances in M31's Outer Halo | We present [Fe/H] and [$\alpha$/Fe] abundances, derived using spectral
synthesis techniques, for stars in M31's outer stellar halo. The 21 [Fe/H]
measurements and 7 [$\alpha$/Fe] measurements are drawn from fields ranging
from 43 to 165 kpc in projected distance from M31. We combine our measurements
with existing literature measurements, and compare the resulting sample of 23
stars with [Fe/H] and 9 stars with [$\alpha$/Fe] measurements in M31's outer
halo with [$\alpha$/Fe] and [Fe/H] measurements, also derived from spectral
synthesis, in M31's inner stellar halo ($r < $26 kpc) and dSph galaxies. The
stars in M31's outer halo have [$\alpha$/Fe] patterns that are consistent with
the largest of M31's dSph satellites (And I and And VII). These abundances
provide tentative evidence that the [$\alpha$/Fe] abundances of stars in M31's
outer halo are more similar to the abundances of Milky Way halo stars than to
the abundances of stars in M31's inner halo. We also compare the spectral
synthesis-based [Fe/H] measurements of stars in M31's halo with previous
photometric [Fe/H] estimates, as a function of projected distance from M31. The
spectral synthesis-based [Fe/H] measurements are consistent with a large-scale
metallicity gradient previously observed in M31's stellar halo to projected
distances as large as 100 kpc. | 2006.05430v1 |
2020-11-11 | Unique Decoding of Explicit $ε$-balanced Codes Near the Gilbert-Varshamov Bound | The Gilbert-Varshamov bound (non-constructively) establishes the existence of
binary codes of distance $1/2 -\epsilon$ and rate $\Omega(\epsilon^2)$ (where
an upper bound of $O(\epsilon^2\log(1/\epsilon))$ is known). Ta-Shma [STOC
2017] gave an explicit construction of $\epsilon$-balanced binary codes, where
any two distinct codewords are at a distance between $1/2 -\epsilon/2$ and
$1/2+\epsilon/2$, achieving a near optimal rate of
$\Omega(\epsilon^{2+\beta})$, where $\beta \to 0$ as $\epsilon \to 0$.
We develop unique and list decoding algorithms for (essentially) the family
of codes constructed by Ta-Shma. We prove the following results for
$\epsilon$-balanced codes with block length $N$ and rate
$\Omega(\epsilon^{2+\beta})$ in this family:
- For all $\epsilon, \beta > 0$ there are explicit codes which can be
uniquely decoded up to an error of half the minimum distance in time
$N^{O_{\epsilon, \beta}(1)}$.
- For any fixed constant $\beta$ independent of $\epsilon$, there is an
explicit construction of codes which can be uniquely decoded up to an error of
half the minimum distance in time $(\log(1/\epsilon))^{O(1)} \cdot
N^{O_\beta(1)}$.
- For any $\epsilon > 0$, there are explicit $\epsilon$-balanced codes with
rate $\Omega(\epsilon^{2+\beta})$ which can be list decoded up to error $1/2 -
\epsilon'$ in time $N^{O_{\epsilon,\epsilon',\beta}(1)}$, where $\epsilon',
\beta \to 0$ as $\epsilon \to 0$.
The starting point of our algorithms is the list decoding framework from Alev
et al. [SODA 2020], which uses the Sum-of-Squares SDP hierarchy. The rates
obtained there were quasipolynomial in $\epsilon$. Here, we show how to
overcome the far from optimal rates of this framework obtaining unique decoding
algorithms for explicit binary codes of near optimal rate. These codes are
based on simple modifications of Ta-Shma's construction. | 2011.05500v1 |
2021-03-30 | Equivalence between Sobolev spaces of first-order dominating mixed smoothness and unanchored ANOVA spaces on $\mathbb{R}^d$ | We prove that a variant of the classical Sobolev space of first-order
dominating mixed smoothness is equivalent (under a certain condition) to the
unanchored ANOVA space on $\mathbb{R}^d$, for $d \geq 1$. Both spaces are
Hilbert spaces involving weight functions, which determine the behaviour as
different variables tend to $\pm \infty$, and weight parameters, which
represent the influence of different subsets of variables. The unanchored ANOVA
space on $\mathbb{R}^d$ was initially introduced by Nichols & Kuo in 2014 to
analyse the error of quasi-Monte Carlo (QMC) approximations for integrals on
unbounded domains; whereas the classical Sobolev space of dominating mixed
smoothness was used as the setting in a series of papers by Griebel, Kuo &
Sloan on the smoothing effect of integration, in an effort to develop a
rigorous theory on why QMC methods work so well for certain non-smooth
integrands with kinks or jumps coming from option pricing problems. In this
same setting, Griewank, Kuo, Le\"ovey & Sloan in 2018 subsequently extended
these ideas by developing a practical smoothing by preintegration technique to
approximate integrals of such functions with kinks or jumps.
We first prove the equivalence in one dimension (itself a non-trivial task),
before following a similar, but more complicated, strategy to prove the
equivalence for general dimensions. As a consequence of this equivalence, we
analyse applying QMC combined with a preintegration step to approximate the
fair price of an Asian option, and prove that the error of such an
approximation using $N$ points converges at a rate close to $1/N$. | 2103.16075v3 |
2021-06-13 | Advantages of a semi-implicit scheme over a fully implicit scheme for Landau-Lifshitz-Gilbert equation | Magnetization dynamics in magnetic materials is modeled by the
Landau-Lifshitz-Gilbert (LLG) equation. In the LLG equation, the length of
magnetization is conserved and the system energy is dissipative. Implicit and
semi-implicit schemes have been used in micromagnetics simulations due to their
unconditional numerical stability. In more details, implicit schemes preserve
the properties of the LLG equation, but solve a nonlinear system of equations
per time step. In contrast, semi-implicit schemes only solve a linear system of
equations, while additional operations are needed to preserve the length of
magnetization. It still remains unclear which one shall be used if both
implicit and semi-implicit schemes are available. In this work, using the
implicit Crank-Nicolson (ICN) scheme as a benchmark, we propose to make this
implicit scheme semi-implicit. It can be proved that both schemes are
second-order accurate in space and time. For the unique solvability of
nonlinear systems of equations in the ICN scheme, we require that the temporal
step size scales quadratically with the spatial mesh size. It is numerically
verified that the convergence of the nonlinear solver becomes slower for larger
temporal step size and multiple magnetization profiles are obtained for
different initial guesses. The linear systems of equations in the semi-implicit
CN (SICN) scheme are unconditionally uniquely solvable, and the condition that
the temporal step size scales linearly with the spatial mesh size is needed in
the convergence of the SICN scheme. In terms of numerical efficiency, the SICN
scheme achieves the same accuracy as the ICN scheme with less computational
time. Based on these results, we conclude that a semi-implicit scheme is
superior to its implicit analog both theoretically and numerically, and we
recommend the semi-implicit scheme in micromagnetics simulations if both
methods are available. | 2106.06936v1 |
2021-08-06 | A second-order semi-implicit method for the inertial Landau-Lifshitz-Gilbert equation | Recent theoretical and experimental advances show that the inertia of
magnetization emerges at sub-picoseconds and contributes to the ultrafast
magnetization dynamics which cannot be captured intrinsically by the LLG
equation. Therefore, as a generalization, the inertial Landau-Lifshitz-Gilbert
(iLLG) equation is proposed to model the ultrafast magnetization dynamics.
Mathematically, the LLG equation is a nonlinear system of parabolic type with
(possible) degeneracy. However, the iLLG equation is a nonlinear system of
mixed hyperbolic-parabolic type with degeneracy, and exhibits more complicated
structures. It behaves like a hyperbolic system at the sub-picosecond scale
while behaves like a parabolic system at larger timescales. Such hybrid
behaviors impose additional difficulties on designing numerical methods for the
iLLG equation. In this work, we propose a second-order semi-implicit scheme to
solve the iLLG equation. The second temporal derivative of magnetization is
approximated by the standard centered difference scheme and the first
derivative is approximated by the midpoint scheme involving three time steps.
The nonlinear terms are treated semi-implicitly using one-sided interpolation
with the second-order accuracy. At each step, the unconditionally unique
solvability of the unsymmetric linear system of equations in the proposed
method is proved with a detailed discussion on the condition number.
Numerically, the second-order accuracy in both time and space is verified.
Using the proposed method, the inertial effect of ferromagnetics is observed in
micromagnetics simulations at small timescales, in consistency with the
hyperbolic property of the model at sub-picoseconds. For long time simulations,
the results of the iLLG model are in nice agreements with those of the LLG
model, in consistency with the parabolic feature of the iLLG model at larger
timescales. | 2108.03060v1 |
2021-09-08 | Flares, Rotation, and Planets of the AU Mic System from TESS Observations | AU Mic is a young ($\sim$24 Myr), pre-Main Sequence M~dwarf star that was
observed in the first month of science observations of the Transiting Exoplanet
Survey Satellite (TESS) and re-observed two years later. This target has
photometric variability from a variety of sources that is readily apparent in
the TESS light curves; spots induce modulation in the light curve, flares are
present throughout (manifesting as sharp rises with slow exponential decay
phases), and transits of AU Mic b may be seen by eye as dips in the light
curve. We present a combined analysis of both TESS Sector 1 and Sector 27 AU
Mic light curves including the new 20-second cadence data from TESS Year 3. We
compare flare rates between both observations and analyze the spot evolution,
showing that the activity levels increase slightly from Sector 1 to Sector 27.
Furthermore, the 20-second data collection allows us to detect more flares,
smaller flares, and better resolve flare morphology in white light as compared
to the 2-minute data collection mode. We also refine the parameters for AU Mic
b by fitting three additional transits of AU Mic b from Sector 27 using a model
that includes stellar activity. We show that the transits exhibit clear transit
timing variations (TTVs) with an amplitude of $\sim$80 seconds. We also detect
three transits of a 2.8 $R_\oplus$ planet, AU Mic c, which has a period of
18.86 days. | 2109.03924v1 |
2021-10-29 | The TREX Survey: Kinematical Complexity Throughout M33's Stellar Disk and Evidence for a Stellar Halo | We present initial results from a large spectroscopic survey of stars
throughout M33's stellar disk. We analyze a sample of 1667 red giant branch
(RGB) stars extending to projected distances of $\sim 11$ kpc from M33's center
($\sim 18$ kpc, or $\sim 10$ scale lengths, in the plane of the disk). The
line-of-sight velocities of RGB stars show the presence of two kinematical
components. One component is consistent with rotation in the plane of M33's HI
disk and has a velocity dispersion ($\sim 19$ km s$^{-1}$) consistent with that
observed in a comparison sample of younger stars, while the second component
has a significantly higher velocity dispersion. A two-component fit to the RGB
velocity distribution finds that the high dispersion component has a velocity
dispersion of $59.3^{+2.6}_{-2.5}$ km s$^{-1}$ and rotates very slowly in the
plane of the disk (consistent with no rotation at the $<1.5\sigma$ level),
which favors interpreting it as a stellar halo rather than a thick disk
population. A spatial analysis indicates that the fraction of RGB stars in the
high-velocity-dispersion component decreases with increasing radius over the
range covered by the spectroscopic sample. Our spectroscopic sample establishes
that a significant high-velocity-dispersion component is present in M33's RGB
population from near M33's center to at least the radius where M33's HI disk
begins to warp at 30$'$ ($\sim 7.5$ kpc) in the plane of the disk. This is the
first detection and spatial characterization of a kinematically hot stellar
component throughout M33's inner regions. | 2110.15773v1 |
2021-11-23 | Magnetism in Metastable and Annealed Compositionally Complex Alloys | Compositionally complex materials (CCMs) present a potential paradigm shift
in the design of magnetic materials. These alloys exhibit long-range structural
order coupled with limited or no chemical order. As a result, extreme local
environments exist with a large opposing magnetic energy term, which can
manifest large changes in the magnetic behavior. In the current work, the
magnetic properties of (Cr, Mn, Fe, Ni) alloys are presented. These materials
were prepared by room-temperature combinatorial sputtering, resulting in a
range of compositions with a single BCC structural phase and no chemical
ordering. The combinatorial growth technique allows CCMs to be prepared outside
of their thermodynamically stable phase, enabling the exploration of otherwise
inaccessible order. The mixed ferromagnetic and antiferromagnetic interactions
in these alloys causes frustrated magnetic behavior, which results in an
extremely low coercivity (<1 mT), which increases rapidly at 50 K. At low
temperatures, the coercivity achieves values of nearly 500 mT, which is
comparable to some high-anisotropy magnetic materials. Commensurate with the
divergent coercivity is an atypical drop in the temperature dependent
magnetization. These effects are explained by a mixed magnetic phase model,
consisting of ferro-, antiferro , and frustrated magnetic regions, and are
rationalized by simulations. A machine-learning algorithm is employed to
visualize the parameter space and inform the development of subsequent
compositions. Annealing the samples at 600 {\deg}C orders the sample, more-than
doubling the Curie temperature and increasing the saturation magnetization by
as much as 5x. Simultaneously, the large coercivities are suppressed, resulting
in magnetic behavior that is largely temperature independent over a range of
350 K. | 2111.12188v1 |
2021-11-23 | Controlling magnetic configuration in soft-hard bilayers probed by polarized neutron reflectometry | Hard/soft magnetic bilayer thin films have been widely used in data storage
technologies and permanent magnet applications. The magnetic configuration and
response to temperatures and magnetic fields in these heterostructures are
considered to be highly dependent on the interfacial coupling. However, the
intrinsic properties of each of the layers, such as the saturation
magnetization and layer thickness, also strongly influence the magnetic
configuration. Changing these parameters provides an effective method to tailor
magnetic properties in composite magnets. Here, we use polarized neutron
reflectometry (PNR) to experimentally probe the interfacial magnetic
configurations in hard/soft bilayer thin films: L10-FePt/A1-FePt, [Co/Pd]
/CoPd, [Co/Pt] /FeNi and L10-FePt/Fe, which all have a perpendicular magnetic
anisotropy in the hard layer. These films were designed with different soft and
hard layer thicknesses (t_soft and t_hard) and saturation magnetization
(M_s^soft and M_s^hard), respectively. The influences of an in-plane magnetic
field (H_ip) and temperature (T) are also studied using a L10 FePt/A1-FePt
bilayer sample. Comparing the PNR results to micromagnetic simulations reveals
that the interfacial magnetic configuration is highly dependent on t_soft,
M_s^soft and the external factors (H_ip and T), and has a relatively weak
dependence on t_hard and M_s^hard. Key among these results, for thin t_soft,
the hard and soft layers are rigidly coupled in the out-of-plane direction,
then undergo a transition to relax in-plane. This transition can be delayed to
larger t_soft by decreasing M_s^soft. Understanding the influence of these
parameters on the magnetic configuration is critical to designing functional
composite magnets for applications. | 2111.12191v1 |
2022-01-18 | On-demand electrical control of spin qubits | Once called a "classically non-describable two-valuedness" by Pauli , the
electron spin is a natural resource for long-lived quantum information since it
is mostly impervious to electric fluctuations and can be replicated in large
arrays using silicon quantum dots, which offer high-fidelity control.
Paradoxically, one of the most convenient control strategies is the integration
of nanoscale magnets to artificially enhance the coupling between spins and
electric field, which in turn hampers the spin's noise immunity and adds
architectural complexity. Here we demonstrate a technique that enables a
\emph{switchable} interaction between spins and orbital motion of electrons in
silicon quantum dots, without the presence of a micromagnet. The naturally weak
effects of the relativistic spin-orbit interaction in silicon are enhanced by
more than three orders of magnitude by controlling the energy quantisation of
electrons in the nanostructure, enhancing the orbital motion. Fast electrical
control is demonstrated in multiple devices and electronic configurations,
highlighting the utility of the technique. Using the electrical drive we
achieve coherence time $T_{2,{\rm Hahn}}\approx50 \mu$s, fast single-qubit
gates with ${T_{\pi/2}=3}$ ns and gate fidelities of 99.93 % probed by
randomised benchmarking. The higher gate speeds and better compatibility with
CMOS manufacturing enabled by on-demand electric control improve the prospects
for realising scalable silicon quantum processors. | 2201.06679v2 |
2022-04-28 | Anti-microbial properties of a multi-component alloy | High traffic touch surfaces such as doorknobs, countertops, and handrails can
be transmission points for the spread of pathogens, emphasizing the need to
develop materials that actively self-sanitize. Metals are frequently used for
these surfaces due to their durability, but many metals also possess
antimicrobial properties which function through a variety of mechanisms. This
work investigates metallic alloys comprised of several bioactive metals with
the target of achieving broad-spectrum, rapid bioactivity through synergistic
activity. An entropy-motivated stabilization paradigm is proposed to prepare
scalable alloys of copper, silver, nickel and cobalt. Using combinatorial
sputtering, thin-film alloys were prepared on 100 mm wafers with 50%
compositional grading of each element across the wafer. The films were then
annealed and investigated for alloy stability. Bioactivity testing was
performed on both the as-grown alloys and the annealed films using four
microorganisms -- Phi6, MS2, Bacillus subtilis and Escherichia coli -- as
surrogates for human viral and bacterial pathogens. Testing showed that after
30 s of contact with some of the test alloys, Phi6, an enveloped,
single-stranded RNA bacteriophage that serves as a SARS-CoV 2 surrogate, was
reduced up to 6.9 orders of magnitude (>99.9999%). Additionally, the
non-enveloped, double-stranded DNA bacteriophage MS2, and the Gram-negative E.
coli and Gram-positive B. subtilis bacterial strains showed a 5.0, 6.4, and 5.7
log reduction in activity after 30, 20 and 10 minutes, respectively.
Bioactivity in the alloy samples showed a strong dependence on the composition,
with the log reduction scaling directly with the Cu content. Concentration of
Cu by phase separation after annealing improved activity in some of the
samples. The results motivate a variety of themes which can be leveraged to
design ideal bioactive surfaces. | 2205.00886v1 |
2022-05-11 | Models of Advance Recording Systems: A Multi-timescale Micromagnetic code for granular thin film magnetic recording systems | Micromagnetic modelling provides the ability to simulate large magnetic
systems accurately without the computational cost limitation imposed by
atomistic modelling. Through micromagnetic modelling it is possible to simulate
systems consisting of thousands of grains over a time range of nanoseconds to
years, depending upon the solver used. Here we present the creation and release
of an open-source multi-timescale micromagnetic code combining three key
solvers: Landau-Lifshitz-Gilbert; Landau-Lifshitz-Bloch; Kinetic Monte Carlo.
This code, called MARS (Models of Advanced Recording Systems), is capable of
accurately simulating the magnetisation dynamics in large and structurally
complex single- and multi-layered granular systems. The short timescale
simulations are achieved for systems far from and close to the Curie point via
the implemented Landau-Lifshitz-Gilbert and Landau-Lifshitz-Bloch solvers
respectively. This enables read/write simulations for general perpendicular
magnetic recording and also state of the art heat assisted magnetic recording
(HAMR). The long timescale behaviour is simulated via the Kinetic Monte Carlo
solver, enabling investigations into signal-to-noise ratio and data longevity.
The combination of these solvers opens up the possibility of multi-timescale
simulations within a single software package. For example the entire HAMR
process from initial data writing and data read back to long term data storage
is possible via a single simulation using MARS. The use of atomistic
parameterisation for the material input of MARS enables highly accurate
material descriptions which provide a bridge between atomistic simulation and
real world experimentation. Thus MARS is capable of performing simulations for
all aspects of recording media research and development. This ranges from
material characterisation and optimisation to system design and implementation. | 2205.05263v1 |
2023-01-03 | Measuring Physical and Electrical Parameters in Free-Living Subjects: Motivating an Instrument to Characterize Analytes of Clinical Importance in Blood Samples | Significance: A path is described to increase the sensitivity and accuracy of
body-worn devices used to monitor patient health. This path supports improved
health management. A wavelength-choice algorithm developed at Mayo demonstrates
that critical biochemical analytes can be assessed using accurate optical
absorption curves over a wide range of wavelengths. Aim: Combine the
requirements for monitoring cardio/electrical, movement, activity, gait,
tremor, and critical biochemical analytes including hemoglobin makeup in the
context of body-worn sensors. Use the data needed to characterize clinically
important analytes in blood samples to drive instrument requirements. Approach:
Using data and knowledge gained over previously separate research threads, some
providing currently usable results from more than eighty years back, determine
analyte characteristics needed to design sensitive and accurate multiuse
measurement and recording units. Results: Strategies for wavelength selection
are detailed. Fine-grained, broad-spectrum measurement of multiple analytes
transmission, absorption, and anisotropic scattering are needed.
Post-Beer-Lambert, using the propagation of error from small variations, and
utility functions that include costs and systemic error sources, improved
measurements can be performed. Conclusions: The Mayo Double-Integrating Sphere
Spectrophotometer (referred hereafter as MDISS), as described in the companion
report arXiv:2212.08763, produces the data necessary for optimal component
choice. These data can provide for robust enhancement of the sensitivity, cost,
and accuracy of body-worn medical sensors. Keywords: Bio-Analyte,
Spectrophotometry, Body-worn monitor, Propagation of error, Double-Integrating
Sphere, Mt. Everest medical measurements, O2SAT
Please see also arXiv:2212.08763 | 2301.00938v2 |
2023-01-09 | A Second Earth-Sized Planet in the Habitable Zone of the M Dwarf, TOI-700 | We report the discovery of TOI-700 e, a 0.95 R$_\oplus$ planet residing in
the Optimistic Habitable Zone (HZ) of its host star. This discovery was enabled
by multiple years of monitoring from NASA's Transiting Exoplanet Survey
Satellite (TESS) mission. The host star, TOI-700 (TIC 150428135), is a nearby
(31.1 pc), inactive, M2.5 dwarf ($V_{mag} = 13.15$). TOI-700 is already known
to host three planets, including the small, HZ planet, TOI-700 d. The new
planet has an orbital period of 27.8 days and, based on its radius (0.95
R$_\oplus$), it is likely rocky. TOI-700 was observed for 21 sectors over Years
1 and 3 of the TESS mission, including 10 sectors at 20-second cadence in Year
3. Using this full set of TESS data and additional follow-up observations, we
identify, validate, and characterize TOI-700 e. This discovery adds another
world to the short list of small, HZ planets transiting nearby and bright host
stars. Such systems, where the stars are bright enough that follow-up
observations are possible to constrain planet masses and atmospheres using
current and future facilities, are incredibly valuable. The presence of
multiple small, HZ planets makes this system even more enticing for follow-up
observations. | 2301.03617v1 |
2023-01-17 | Introduction to Non-Invasive Current Estimation (NICE) | It is notoriously difficult to measure instantaneous supply current to a
device such as an ASIC, FPGA, or CPU without also affecting the instantaneous
supply voltage and compromising the operation of the device [21]. For decades
designers have relied on rough estimates of dynamic load currents that
stimulate a designed Power Delivery Network (PDN). The consequences of
inaccurate load-current characterization can range from excessive PDN cost and
lengthened development schedules to poor performance or functional failure.
This paper will introduce and describe a method to precisely determine
timedomain current waveforms from a pair of measured timedomain voltage
waveforms. This NonInvasive Current Estimation (NICE) method is based on
established twoport network theory along with component and board modeling
techniques that have been validated through measurements on demonstrative
circuits. This paper will show that the NICE method works for any transient
event that can be captured on a digital oscilloscope. Limitations of the method
and underlying measurements are noted where appropriate. The method is applied
to a simple PDN with an arbitrary load, and the NICE-derived current waveform
is verified against an independent measurement by sense resistor. With careful
component and board modeling, it is possible to calculate current waveforms
with a root mean square error of less than five percent compared to the
reference measurement. Current transients that were previously difficult or
impossible to characterize by any means can now be calculated and displayed
within seconds of an oscilloscope-trigger event by using NICE. ASIC and FPGA
manufacturers can now compute the startup current for their device and publish
the actual waveform, or provide a piecewiselinear SPICE model (PWL source) to
facilitate design and testing of the regulator and PDN required to support
their device. | 2301.10237v1 |
2023-02-08 | Weighted Edit Distance Computation: Strings, Trees and Dyck | Given two strings of length $n$ over alphabet $\Sigma$, and an upper bound
$k$ on their edit distance, the algorithm of Myers (Algorithmica'86) and Landau
and Vishkin (JCSS'88) computes the unweighted string edit distance in
$\mathcal{O}(n+k^2)$ time. Till date, it remains the fastest algorithm for
exact edit distance computation, and it is optimal under the Strong Exponential
Hypothesis (STOC'15). Over the years, this result has inspired many
developments, including fast approximation algorithms for string edit distance
as well as similar $\tilde{\mathcal{O}}(n+$poly$(k))$-time algorithms for
generalizations to tree and Dyck edit distances. Surprisingly, all these
results hold only for unweighted instances.
While unweighted edit distance is theoretically fundamental, almost all
real-world applications require weighted edit distance, where different weights
are assigned to different edit operations and may vary with the characters
being edited. Given a weight function $w: \Sigma \cup \{\varepsilon \}\times
\Sigma \cup \{\varepsilon \} \rightarrow \mathbb{R}_{\ge 0}$ (such that
$w(a,a)=0$ and $w(a,b)\ge 1$ for all $a,b\in \Sigma \cup \{\varepsilon\}$ with
$a\ne b$), the goal is to find an alignment that minimizes the total weight of
edits. Except for the vanilla $\mathcal{O}(n^2)$-time dynamic-programming
algorithm and its almost trivial $\mathcal{O}(nk)$-time implementation, none of
the aforementioned developments on the unweighted edit distance apply to the
weighted variant. In this paper, we propose the first
$\mathcal{O}(n+$poly$(k))$-time algorithm that computes weighted string edit
distance exactly, thus bridging a fundamental gap between our understanding of
unweighted and weighted edit distance. We then generalize this result to
weighted tree and Dyck edit distances, which lead to a deterministic algorithm
that improves upon the previous work for unweighted tree edit distance. | 2302.04229v1 |
2023-03-09 | Zonostrophic instabilities in magnetohydrodynamic Kolmogorov flow | This paper concerns the stability of Kolmogorov flow u = (0, sin x) in the
infinite (x,y)-plane. A mean magnetic field of strength B0 is introduced and
the MHD linear stability problem studied for modes with wave-number k in the
y-direction, and Bloch wavenumber l in the x-direction. The parameters
governing the problem are Reynolds number 1/nu, magnetic Prandtl number P, and
dimensionless magnetic field strength B0. The mean magnetic field can be taken
to have an arbitrary direction in the (x,y)-plane and a mean x-directed flow U0
can be incorporated.
First the paper considers Kolmogorov flow with y-directed mean magnetic
field, referred to as vertical. Taking l=0, the suppression of the pure
hydrodynamic instability is observed with increasing field strength B0. A
branch of strong-field instabilities occurs for magnetic Prandtl number P less
than unity, as found by A.E. Fraser, I.G. Cresser and P. Garaud (J. Fluid Mech.
949, A43, 2022). Analytical results using eigenvalue perturbation theory in the
limit k->0 support the numerics for both weak- and strong-field instabilities,
and originate in the coupling of large-scale modes with x-wavenumber n=0, to
smaller-scale modes.
The paper considers the case of horizontal or x-directed mean magnetic field.
The unperturbed state consists of steady, wavey magnetic field lines. As the
magnetic field is increased, the purely hydrodynamic instability is suppressed
again, but for stronger fields a new branch of instabilities appears. Allowing
a non-zero Bloch wavenumber l allows further instability, and in some
circumstances when the system is hydrodynamically stable, arbitrarily weak
magnetic fields can give growing modes. Numerical results are presented
together with eigenvalue perturbation theory in the limits k,l->0. The theory
gives analytical approximations for growth rates and thresholds in good
agreement with those computed. | 2303.05212v1 |
2023-03-30 | Fate of entanglement in magnetism under Lindbladian or non-Markovian dynamics and conditions for their transition to Landau-Lifshitz-Gilbert classical dynamics | It is commonly assumed in spintronics and magnonics that localized spins
within antiferromagnets are in the N\'{e}el ground state (GS), as well as that
such state evolves, when pushed out of equilibrium by current or external
fields, according to the Landau-Lifshitz-Gilbert (LLG) equation viewing
localized spins as classical vectors of fixed length. On the other hand, the
true GS of antiferromagnets is highly entangled, as confirmed by very recent
neutron scattering experiments witnessing their entanglement. Although GS of
ferromagnets is always unentangled, their magnonic low-energy excitation are
superpositions of many-body spin states and, therefore, entangled. In this
study, we initialize quantum Heisenberg ferro- or antiferromagnetic chains
hosing localized spins $S=1/2$, $S=1$ or $S=5/2$ into unentangled pure state
and then evolve them by quantum master equations (QMEs) of Lindblad or
non-Markovian type, derived by coupling localized spins to a bosonic bath (such
as due to phonons) or by using additional ``reaction coordinate'' in the latter
case. The time evolution is initiated by applying an external magnetic field,
and entanglement of time-evolving {\em mixed} quantum states is monitored by
computing its logarithmic negativity. We find that non-Markovian dynamics
maintains some degree of entanglement, which shrinks the length of the vector
of spin expectation values, thereby making the LLG equation inapplicable.
Conversely, Lindbladian (i.e., Markovian) dynamics ensures that entanglement
goes to zero, thereby enabling quantum-to-classical (i.e., to LLG) transition
in all cases -- $S=1/2$, $S=1$ and $S=5/2$ ferromagnet or $S=5/2$
antiferromagnet -- {\em except} for $S=1/2$ and $S=1$ antiferromagnet. We also
investigate the stability of entangled antiferromagnetic GS upon suddenly
coupling it to the bosonic bath. | 2303.17596v3 |
2024-02-07 | Item-Level Heterogeneous Treatment Effects of Selective Serotonin Reuptake Inhibitors (SSRIs) on Depression: Implications for Inference, Generalizability, and Identification | In analysis of randomized controlled trials (RCTs) with patient-reported
outcome measures (PROMs), Item Response Theory (IRT) models that allow for
heterogeneity in the treatment effect at the item level merit consideration.
These models for ``item-level heterogeneous treatment effects'' (IL-HTE) can
provide more accurate statistical inference, allow researchers to better
generalize their results, and resolve critical identification problems in the
estimation of interaction effects. In this study, we extend the IL-HTE model to
polytomous data and apply the model to determine how the effect of selective
serotonin reuptake inhibitors (SSRIs) on depression varies across the items on
a depression rating scale. We first conduct a Monte Carlo simulation study to
assess the performance of the polytomous IL-HTE model under a range of
conditions. We then apply the IL-HTE model to item-level data from 28 RCTs
measuring the effect of SSRIs on depression using the 17-item Hamilton
Depression Rating Scale (HDRS-17) and estimate potential heterogeneity by
subscale (HDRS-6). Our results show that the IL-HTE model provides more
accurate statistical inference, allows for generalizability of results to
out-of-sample items, and resolves identification problems in the estimation of
interaction effects. Our empirical application shows that while the average
effect of SSRIs on depression is beneficial (i.e., negative) and statistically
significant, there is substantial IL-HTE, with estimates of the standard
deviation of item-level effects nearly as large as the average effect. We show
that this substantial IL-HTE is driven primarily by systematically larger
effects on the HDRS-6 subscale items. The IL-HTE model has the potential to
provide new insights for the inference, generalizability, and identification of
treatment effects in clinical trials using patient reported outcome measures. | 2402.04487v1 |
1995-02-16 | Lyman alpha Emission from High-Redshift Galaxies | We summarise the results of a deep search for Lyman alpha emission from
star-forming regions associated with damped Lyman alpha absorption systems and
conclude that the Lyman alpha luminosity of high redshift galaxies is generally
less than 10^(42) erg/s . We also present a newly discovered case, in the field
of the QSO Q2059-360, where the emission is unusually strong, possibly because
the damped system is close in redshift to the QSO. | 9502076v1 |
1995-10-12 | Limits on diffusive shock acceleration in dense and incompletely ionised media | The limits imposed on diffusive shock acceleration by upstream ion-neutral
Alfven wave damping, and by ionisation and Coulomb losses of low energy
particles, are calculated. Analytic solutions are given for the steady upstream
wave excitation problem with ion-neutral damping and the resulting escaping
upstream flux calculated. The time dependent problem is discussed and numerical
solutions presented. Finally the significance of these results for possible
observational tests of shock acceleration in supernova remnants is discussed. | 9510066v2 |
1995-11-28 | Damping of GRR instability by direct URCA reactions | The role of direct URCA reactions in damping of the gravitational radiation
driven instability is discussed. The temperature at which bulk viscosity
suppresses completely this instability is calculated.
The results are obtained analytically using recent calculations performed in
the case of bulk viscosity due to the modified URCA processes (Lindblom 1995;
Yoshida & Eriguchi 1995).
The bulk viscosity caused by direct URCA reactions is found to reduce
significantly the region of temperatures and rotation frequencies where a
neutron star is subject to GRR instability. | 9511136v1 |
1997-10-31 | Abundances in Damped Lyman-alpha Systems and Chemical Evolution of High Redshift Galaxies | Recent abundance measurements in damped Lyman-alpha galaxies, supplemented
with unpublished Keck observations, are discussed. The metallicity distribution
with cosmic time is examined for clues about the degree of enrichment, the
onset of initial star formation, and the nature of the galxies. The relative
abundances of the elements are compared with the abundnce pattern in Galactic
halo stars and in the Sun, taking into account of the effects of dust
depletion, in order to gain insight into the stellar processes and the time
scales by which the enrichment occurred. | 9710370v1 |
1998-05-08 | Exploring the Damped Lyman-alpha Clouds with AXAF | The High Energy Transmission Grating (HETG) Spectrometer on the Advanced
X-ray Astrophysics Facility (AXAF) (scheduled for launch in August, 1998) will
provide a new tool for the study of absorption in the X-ray spectra of high
redshift quasars due to the material along the line of sight. In this paper we
try to explore the possibility of using AXAF HETG to detect resonance
absorption lines from the Damped Lyman-alpha (DLA) clouds. | 9805110v1 |
1998-05-28 | Photon Damping of Waves in Accretion Disks | MHD turbulence is generally believed to have two important functions in
accretion disks: it transports angular momentum outward, and the energy in its
shortest wavelength modes is dissipated into the heat that the disks radiate.
In this paper we examine a pair of mechanisms which may play an important role
in regulating the amplitude and spectrum of this turbulence: photon diffusion
and viscosity. We demonstrate that in radiation pressure-dominated disks,
photon damping of compressive MHD waves is so rapid that it likely dominates
all other dissipation mechanisms. | 9805358v1 |
1998-06-11 | Damping of differential rotation in neutron stars | We derive the transport relaxation times for quasiparticle-vortex scattering
processes via nuclear force, relevant for the damping of differential rotation
of superfluids in the quantum liquid core of a neutron star. The proton
scattering off the neutron vortices provides the dominant resistive force on
the vortex lattice at all relevant temperatures in the phase where neutrons
only are in the paired state. If protons are superconducting, a small fraction
of hyperons and resonances in the normal state would be the dominant source of
friction on neutron and proton vortex lattices at the core temperatures $T\ge
10^{7}$ K. | 9806156v1 |
1999-03-10 | Elemental abundances at early times: the nature of Damped Lyman-alpha systems | The distribution of element abundances with redshift in Damped Ly-alpha (DLA)
systems can be adequately reproduced by the same model reproducing the halo and
disk components of the Milky Way Galaxy at different galactocentric distances:
DLA systems are well represented by normal spiral galaxies in their early
evolutionary stages. | 9903150v1 |
1999-07-26 | Are Damped Ly-alpha Systems Large, Galactic Disks ? | The hypothesis that the Damped Ly-alpha systems (DLAs) are large, galactic
disks (Milky Way sized) is tested by confronting predictions of models of the
formation and evolution of (large) disk galaxies with observations, in
particular the Zinc abundance distribution with neutral hydrogen column density
found for DLAs. A pronounced mismatch is found strongly hinting that the
majority of DLAs may not be large, galactic disks. | 9907349v1 |
1999-08-26 | Oscillator Strengths and Damping Constants for Atomic Lines in the J and H Bands | We have built a line list in the near-infrared J and H bands (1.00-1.34,
1.49-1.80 um) by gathering a series of laboratory and computed line lists.
Oscillator strengths and damping constants were computed or obtained by fitting
the solar spectrum.
The line list presented in this paper is, to our knowledge, the most complete
one now available, and supersedes previous lists. | 9908296v1 |
1999-11-25 | Probing Solar Convection | In the solar convection zone acoustic waves are scattered by turbulent sound
speed fluctuations. In this paper the scattering of waves by convective cells
is treated using Rytov's technique. Particular care is taken to include
diffraction effects which are important especially for high-degree modes that
are confined to the surface layers of the Sun. The scattering leads to damping
of the waves and causes a phase shift. Damping manifests itself in the width of
the spectral peak of p-mode eigenfrequencies. The contribution of scattering to
the line widths is estimated and the sensitivity of the results on the assumed
spectrum of the turbulence is studied. Finally the theoretical predictions are
compared with recently measured line widths of high-degree modes. | 9911469v1 |
1999-12-14 | The Gas Reservoir for present day Galaxies : Damped Ly-alpha Absorption Systems | We present results from an ongoing search for galaxy counterparts of a
subgroup of Quasar Absorption Line Systems called Damped Ly-alpha Absorbers
(DLAs). DLAs have several characteristics that make them essential in the
process of understanding how galaxies formed in the early universe and evolved
to the galaxies we see today in the local universe.
Finally we compare DLAs with recent findings of a population of starforming
galaxies at high redshifts, so called Lyman-break galaxies. | 9912268v1 |
2000-06-22 | Nuclear Reaction Rates in a Plasma: The Effect of Highly Damped Modes | The fluctuation-dissipation theorem is used to evaluate the screening factor
of nuclear reactions due to the electromagnetic fluctuations in a plasma. We
show that the commonly used Saltpeter factor is obtained if only fluctuations
near the plasma eigenfrequency are assumed to be important (\omega \sim
\omega_{pe}\ll T (\hbar=k_{B}=1)). By taking into account all the fluctuations,
the highly damped ones, with \omega >\omega_{pe}, as well as those with
\omega\leq\omega_{pe}, we find that nuclear reaction rates are higher than
those obtained using the Saltpeter factor, for many interesting plasmas. | 0006326v1 |
2000-09-06 | The Cosmological Evolution of Quasar Damped Lyman-Alpha Systems | We present results from an efficient, non-traditional survey to discover
damped Lyman-alpha (DLA) absorption-line systems with neutral hydrogen column
densities N(HI)>2x10^{20} atoms cm^{-2} and redshifts z<1.65. Contrary to
previous studies at higher redshift that showed a decrease in the cosmological
mass density of neutral gas in DLA absorbers, Omega_{DLA}, with time, our
results indicate that Omega_{DLA} is consistent with remaining constant from
redshifts z \approx 4 to z \approx 0.5. There is no evidence that Omega_{DLA}
is approaching the value at z=0. Other interesting results from the survey are
also presented. | 0009098v1 |
2001-01-13 | Measuring Feedback in Damped Lyman Alpha Systems | We measure feedback (heating rates) in damped Lyman alpha systems from the
cooling rate of the neutral gas. Since cooling occurs through [C II] 158 micron
emission, we infer cooling from C II^{*} 1335.7 absorption lines detected with
HIRES on the Keck I telescope. The inferred heating rates are about 30 times
lower than for the Galaxy ISM. At z = 2.8, the implied star formation rate per
unit area is 10^{-2.4+-0.3} solar masses per kpc^{2} per year, and the the star
formation rate per unit comoving volume is 10^{-0.8+-0.2} solar masses per
Mpc^{3} per year. This is the first measurement of star formation rates in
objects likely to be the progenitors of current galaxies. | 0101218v1 |
2001-04-18 | The First Detection of Cobalt in a Damped Lyman Alpha System | We present the first ever detection of Cobalt in a Damped Lyman Alpha system
(DLA) at z = 1.92. In addition to providing important clues to the star
formation history of these high redshift galaxies, we discuss how studying the
Co abundance in DLAs may also help to constrain models of stellar
nucleosynthesis in a regime not probed by Galactic stars. | 0104301v1 |
2001-05-09 | Nuclear reaction rates and energy in stellar plasmas : The effect of highly damped modes | The effects of the highly damped modes in the energy and reaction rates in a
plasma are discussed. These modes, with wavenumbers $k \gg k_{D}$, even being
only weakly excited, with less than $k_{B}T$ per mode, make a significant
contribution to the energy and screening in a plasma. When the de Broglie
wavelength is much less than the distance of closest approach of thermal
electrons, a classical analysis of the plasma can | 0105153v1 |
2001-07-03 | The HI Content and Extent of Low Surface Brightness Galaxies - Could LSB Galaxies be Responsible for Damped Ly-alpha Absorption? | Low surface brightness galaxies, those galaxies with a central surface
brightness at least one magnitude fainter than the night sky, are often not
included in discussions of extragalactic gas at z < 0.1. In this paper we
review many of the properties of low surface brightness galaxies, including
recent studies which indicate low surface brightness systems may contribute far
more to the local HI luminosity function than previously thought. Additionally,
we use the known (HI) gas properties of low surface brightness galaxies to
consider their possible contribution to nearby damped Lyman-alpha absorbers. | 0107064v1 |
2001-09-10 | H_2 molecules in damped systems | Damped Lyman alpha systems seen in the spectra of high-z QSOs arise in
high-density neutral gas in which molecular hydrogen (H_2) should be
conspicuous. Systematic searches to detect the H_2 lines redshifted into the
Lyman alpha forest at <3400\AA are now possible thanks to the unique
capabilities of UVES on the VLT. Here we summarise the present status of our on
going programme to search for H_2 in DLAs, discuss the physical conditions in
the systems where H_2 is detected and the implications of non-detections. | 0109155v1 |
2001-10-23 | A scaling law of interstellar depletions as a tool for abundance studies of Damped Ly alpha systems | An analytical expression is presented that allows dust depletions to be
estimated in different types of interstellar environments, including Damped Ly
alpha systems. The expression is a scaling law of a reference depletion pattern
and takes into account the possibility that the dust chemical composition may
vary as a function of the dust-to-metals ratio and of the intrinsic abundances
of the medium. Preliminary tests and applications of the proposed scaling law
are briefly reported. | 0110499v1 |
2002-09-23 | Outflows in Galaxies and Damped Ly-alpha System | Although quasar absorbers, and in particular Damped Lyman-alpha systems
(DLAs) have proven a valuable tool to study the early Universe, their exact
nature is so far poorly constrained. It has been suggested that outflows in
galaxies might account for at least part of the DLA population. Observational
evidences and models in support of this hypothesis are reviewed, including
recent observations of Lyman Break Galaxies (LBGs). Observational
counter-arguments and theoretical limitations are also given. Finally,
implications of such a model for the environment of galaxies at high-redshifts
are discussed. | 0209463v1 |
2004-03-15 | The Damping Wing of the Gunn-Peterson Absorption and Lyman-Alpha Emitters in the Pre-Reionization Era | We use a numerical simulation of cosmological reionization to estimate the
likelihood of detecting Lyman-alpha emitting galaxies during the
pre-reionization era. We show that it is possible to find galaxies even at z~9
that are barely affected by the dumping wing of the Gunn-Peterson absorption
from the neutral IGM outside of their HII regions. The damping wing becomes
rapidly more significant at z>9, but even at z>10 is it not inconceivable
(although quite hard) to see a Lyman-alpha emission line from a star-forming
galaxy. | 0403345v1 |
2005-05-28 | Cosmic ray transport in MHD turbulence | Recent advances in understanding of magnetohydrodynamic (MHD) turbulence call
for revisions in the picture of cosmic ray transport. In this paper we use
recently obtained scaling laws for MHD modes to obtain the scattering frequency
for cosmic rays. We account for the turbulence cutoff arising from both
collisional and collisionless damping. We obtain the scattering rate and show
that fast modes provide the dominant contribution to cosmic ray scattering for
the typical interstellar conditions in spite of the fact that fast modes are
subjected to damping. We determine how the efficiency of the scattering depends
on the characteristics of ionized media, e.g. plasma $\beta$. We show that
streaming instability is suppressed by the ambient MHD turbulence. | 0505575v1 |
2005-06-09 | Phantom damping of matter perturbations | Cosmological scaling solutions are particularly important in solving the
coincidence problem of dark energy. We derive the equations of sub-Hubble
linear matter perturbations for a general scalar-field Lagrangian--including
quintessence, tachyon, dilatonic ghost condensate and k-essence--and solve them
analytically for scaling solutions. We find that matter perturbations are
always damped if a phantom field is coupled to dark matter and identify the
cases in which the gravitational potential is constant. This provides an
interesting possibility to place stringent observational constraints on scaling
dark energy models. | 0506222v1 |
2005-06-22 | A Damped Ly-alpha Absorption-line System in an Apparent Void at Redshift 2.38 | We study the contents of an apparent void in the distribution of Ly-alpha
emitting galaxies at redshift 2.38. We show that this void is not empty, but
contains a damped Ly-alpha absorption-line system, seen in absorption against
background QSO 2138-4427. Imaging does not reveal any galaxy associated with
this absorption-line system, but it contains metals (Fe/H ~ -1.3), and its
large velocity range (~ 180 km/s) implies a significant mass. | 0506525v1 |
2005-08-08 | Fluorescence in damp air and comments on the radiative life time | Photon yields in damp air excited by an electron using a Sr90 $\beta$ source
are compared withthose in dry air. Water vapors considerably reduce the yields,
however, a further study is needed to evaluate the effects on the energy
estimation of ultrahigh-energy cosmic rays. The relation of fluorescence
efficiency to the life time of de-excitation by radiation is discussed. | 0508183v1 |
2006-08-17 | Electron thermal conductivity owing to collisions between degenerate electrons | We calculate the thermal conductivity of electrons produced by
electron-electron Coulomb scattering in a strongly degenerate electron gas
taking into account the Landau damping of transverse plasmons. The Landau
damping strongly reduces this conductivity in the domain of ultrarelativistic
electrons at temperatures below the electron plasma temperature. In the inner
crust of a neutron star at temperatures T < 1e7 K this thermal conductivity
completely dominates over the electron conductivity due to electron-ion
(electron-phonon) scattering and becomes competitive with the the electron
conductivity due to scattering of electrons by impurity ions. | 0608371v1 |
2006-09-19 | Dust, Metals and Diffuse Interstellar Bands in Damped Lyman Alpha Systems | Although damped Lyman alpha (DLA) systems are usually considered metal-poor,
it has been suggested that this could be due to observational bias against
metal-enriched absorbers. I review recent surveys to quantify the particular
issue of dust obscuration bias and demonstrate that there is currently no
compelling observational evidence to support a widespread effect due to
extinction. On the other hand, a small sub-set of DLAs may be metal-rich and I
review some recent observations of these metal-rich absorbers and the detection
of diffuse interstellar bands in one DLA at z ~ 0.5. | 0609530v1 |
2006-11-08 | Comments on Viscous Damping of Non-Adiabatic MHD Waves in an Unbounded Solar Coronal Plasma by Kumar and Kumar | Considering thermal conduction, compressive viscosity and optically thin
radiation as damping mechanisms for MHD waves, we derive a six-order general
dispersion relation. We point out a fundamental flaw in the derivation of
five-order dispersion relation by Kumar and Kumar (2006) who adopt as a basis
vector. The correct definition of the motion in the x-z plane (2-D vector
space) stems from the two independent variables, namely . | 0611252v2 |
2007-01-10 | Non-gaussianity in fluctuations from warm inflation | The scalar mode density perturbations in a the warm inflationary scenario are
analysed with a view to predicting the amount of non-gaussianity produced by
this scenario. The analysis assumes that the inflaton evolution is strongly
damped by the radiation, with damping terms that are temperature independent.
Entropy fluctuations during warm inflation play a crucial role in generating
non-gaussianity and result in a distinctive signal which should be observable
by the Planck satellite. | 0701302v2 |
1998-05-22 | WKB for a damped spin | The master equation for a damped spin well known from the theory of
superradiance, is written as a finite-difference equation and solved by a
WKB-like method. The propagator thus obtained looks like the van Vleck
propagator of a certain classical Hamiltonian system with one degree of
freedom. A new interpretation is provided of the temporal broadening of
initially sharp probability distributions as the analogue of the spreading of
the quantum mechanical wave packet. | 9805018v1 |
1998-11-04 | Cascades of energy and helicity in the GOY shell model of turbulence | The effect of extreme hyperviscous damping, $\nu k_n^p, p=\infty$ is studied
numerically in the GOY shell model of turbulence. It has resently been
demonstrated [Leveque and She, Phys. Rev. Lett, 75,2690 (1995)] that the
inertial range scaling in the GOY model is non-universal and depending on the
viscous damping. The present study shows that the deviation from Kolmogorov
scaling is due to the cascade of the second inviscid invariant. This invariant
is non-positive definite and in this sense analogous to the helicity of 3D
turbulent flow. | 9811009v1 |
1994-02-04 | Effects of Disorder in a Dilute Bose Gas | We discuss the effects of a weak random external potential on the properties
of the dilute Bose gas at zero temperature. The results recently obtained by
Huang and Meng for the depletion of the condensate and of the superfluid
density are recovered. Results for the shift of the velocity of sound as well
as for its damping due to collisions with the external field are presented. The
damping of phonons is calculated also for dense superfluids. (submitted to
Phys.Rev.B) | 9402015v1 |
1995-02-10 | The influence of structure disorder on mean atomic momentum fluctuations and a spin-wave spectrum | The relation between atomic momenta fluctuations and density fluctuations is
obtained in frames of mean-field approximation. Using two-time temperature
Green functions within Tyablikov approximation the equations for spin
excitation energy and damping are obtained. The asymptotics of energy and
damping in the long-wave limit are investigated and the anomalous behaviour of
spin-wave stiffness constant is discussed. | 9502042v1 |
1997-02-13 | Comment on "Collective Excitations of a Bose-Einstein Condensate in a Magnetic Trap" | We calculate the damping rate of collective excitations for a nearly pure
Bose-Einstein condensate regarding the recent experiments in MIT [M.-O. Mews et
al, Phys. Rev. Lett. 77, 988 (1996)]. The decay time of collective excitations
obtained in our theoretical calculations agrees well with their experimental
result. We argue that the damping of collective excitations is due to thermal
contributions rather than interactions between collective modes. | 9702122v1 |
1997-08-14 | Landau damping in dilute Bose gases | Landau damping in weakly interacting Bose gases is investigated by means of
perturbation theory. Our approach points out the crucial role played by
Bose-Einstein condensation and yields an explicit expression for the decay rate
of elementary excitations in both uniform and non uniform gases. Systematic
results are derived for the phonon width in homogeneous gases interacting with
repulsive forces. Special attention is given to the low and high temperature
regimes. | 9708104v1 |
1997-11-07 | Coulomb suppression of NMR coherence peak in fullerene superconductors | The suppressed NMR coherence peak in the fullerene superconductors is
explained in terms of the dampings in the superconducting state induced by the
Coulomb interaction between conduction electrons. The Coulomb interaction,
modelled in terms of the onsite Hubbard repulsion, is incorporated into the
Eliashberg theory of superconductivity with its frequency dependence considered
self-consistently at all temperatures. The vertex correction is also included
via the method of Nambu. The frequency dependent Coulomb interaction induces
the substantial dampings in the superconducting state and, consequently,
suppresses the anticipated NMR coherence peak of fullerene superconductors as
found experimentally. | 9711060v2 |
1997-12-09 | The Sound of Sonoluminescence | We consider an air bubble in water under conditions of single bubble
sonoluminescence (SBSL) and evaluate the emitted sound field nonperturbatively
for subsonic gas-liquid interface motion. Sound emission being the dominant
damping mechanism, we also implement the nonperturbative sound damping in the
Rayleigh-Plesset equation for the interface motion. We evaluate numerically the
sound pulse emitted during bubble collapse and compare the nonperturbative and
perturbative results, showing that the usual perturbative description leads to
an overestimate of the maximal surface velocity and maximal sound pressure. The
radius vs. time relation for a full SBSL cycle remains deceptively unaffected. | 9712097v1 |
1998-07-02 | Linear systems with adiabatic fluctuations | We consider a dynamical system subjected to weak but adiabatically slow
fluctuations of external origin. Based on the ``adiabatic following''
approximation we carry out an expansion in \alpha/|\mu|, where \alpha is the
strength of fluctuations and 1/|\mu| refers to the time scale of evolution of
the unperturbed system to obtain a linear differential equation for the average
solution. The theory is applied to the problems of a damped harmonic oscillator
and diffusion in a turbulent fluid. The result is the realization of
`renormalized' diffusion constant or damping constant for the respective
problems. The applicability of the method has been critically analyzed. | 9807031v1 |
1998-12-02 | Vortex lattice melting and the damping of the dHvA oscillations in the mixed state | Phase fluctuations in the superconducting order parameter, which are
responsible for the melting of the Abrikosov vortex lattice below the mean
field $H_{c2}$, are shown to dramatically enhance the scattering of
quasi-particles by the fluctuating pair potential, thus leading to enhanced
damping of the dHvA oscillations in the liquid mixed state. This effect is
shown to quantitatively account for the detailed field dependence of the dHvA
amplitude observed recently in the mixed state of a Quasi 2D organic SC. | 9812040v1 |
1999-01-19 | Damping of Growth Oscillations | Computer simulations and scaling theory are used to investigate the damping
of oscillations during epitaxial growth on high-symmetry surfaces. The
crossover from smooth to rough growth takes place after the deposition of
(D/F)^\delta monolayers, where D and F are the surface diffusion constant and
the deposition rate, respectively, and the exponent \delta=2/3 on a
two-dimensional surface. At the transition, layer-by-layer growth becomes
desynchronized on distances larger than a layer coherence length proportional
l^2, where l is a typical distance between two-dimensional islands in the
submonolayer region of growth. | 9901178v1 |
1999-06-15 | Temperature-induced resonances and Landau damping of collective modes in Bose-Einstein condensed gases in spherical traps | Interaction between collective monopole oscillations of a trapped
Bose-Einstein condensate and thermal excitations is investigated by means of
perturbation theory. We assume spherical symmetry to calculate the matrix
elements by solving the linearized Gross-Pitaevskii equations. We use them to
study the resonances of the condensate induced by temperature when an external
perturbation of the trapping frequency is applied and to calculate the Landau
damping of the oscillations. | 9906214v1 |
1999-08-03 | Kinetic Theory of Collective Modes in Atomic Clouds above the Bose-Einstein Transition Temperature | We calculate frequencies and damping rates of the lowest collective modes of
a dilute Bose gas confined in an anisotropic trapping potential above the
Bose-Einstein transition temperature. From the Boltzmann equation with a
simplified collision integral we derive a general dispersion relation that
interpolates between the collisionless and hydrodynamic regimes. In the case of
axially symmetric traps we obtain explicit expressions for the frequencies and
damping rates of the lowest modes in terms of a phenomenological collision
time. Our results are compared with microscopic calculations and experiments. | 9908043v1 |
1999-09-01 | Normal Fermi Liquid Behavior of Quasiholes in the Spin-Polaron Model for Copper Oxides | Based on the t-J model and the self-consistent Born approximation, the
damping of quasiparticle hole states near the Fermi surface is calculated in a
low doping regime. Renormalization of spin-wave excitations due to hole doping
is taken into account. The damping is shown to be described by a familiar form
$\text{Im}\Sigma({\bf k}^{\prime},\epsilon)\propto (\epsilon^{2}/
\epsilon_{F})\ln(\epsilon/ \epsilon_{F})$ characteristic of the 2-dimensional
Fermi liquid, in contrast with the earlier statement reported by Li and Gong
[Phys. Rev. B {\bf 51}, 6343 (1995)] on the marginal Fermi liquid behavior of
quasiholes. | 9909020v1 |
1999-12-01 | Impurity relaxation mechanism for dynamic magnetization reversal in a single domain grain | The interaction of coherent magnetization rotation with a system of two-level
impurities is studied. Two different, but not contradictory mechanisms, the
`slow-relaxing ion' and the `fast-relaxing ion' are utilized to derive a system
of integro-differential equations for the magnetization. In the case that the
impurity relaxation rate is much greater than the magnetization precession
frequency, these equations can be written in the form of the Landau-Lifshitz
equation with damping. Thus the damping parameter can be directly calculated
from these microscopic impurity relaxation processes. | 9912014v1 |
2000-02-16 | Dissipative dynamics of Bose condensates in optical cavities | We study the zero temperature dynamics of Bose-Einstein condensates in driven
high-quality optical cavities in the limit of large atom-field detuning. We
calculate the stationary ground state and the spectrum of coupled atom and
field mode excitations for standing wave cavities as well as for travelling
wave cavities. Finite cavity response times lead to damping or controlled
amplification of these excitations. Analytic solutions in the Lamb-Dicke
expansion are in good agreement with numerical results for the full problem and
show that oscillation frequencies and the corresponding damping rates are
qualitatively different for the two cases. | 0002247v1 |
2000-03-27 | Effect of memory and dynamical chaos in long Josephson junctions | A long Josephson junction in a constant external magnetic field and in the
presence of a dc bias current is investigated. It is shown that the system,
simulated by the sine-Gorgon equation, "remembers" a rapidly damping initial
perturbation and final asymptotic states are determined exactly with this
perturbation. Numerical solving of the boundary sine-Gordon problem and
calculations of Lyapunov indices show that this system has a memory even when
it is in a state of dynamical chaos, i.e., dynamical chaos does not destroy
initial information having a character of rapidly damping perturbation. | 0003421v1 |
2000-09-13 | Oscillations of the superconducting order parameter in a ferromagnet | Planar tunneling spectroscopy reveals damped oscillations of the
superconducting order parameter induced into a ferromagnetic thin film by the
proximity effect. The oscillations are due to the finite momentum transfer
provided to Cooper pairs by the splitting of the spin up and down bands in the
ferromagnet. As a consequence, for negative values of the superconducting order
parameter the tunneling spectra are capsized ("$\pi$-state"). The oscillations'
damping and period are set by the same length scale, which depends on the spin
polarization. | 0009192v1 |
2000-09-29 | Damping and revivals of collective oscillations in a finite-temperature model of trapped Bose-Einstein condensation | We utilize a two-gas model to simulate collective oscillations of a
Bose-Einstein condensate at finite temperatures. The condensate is described
using a generalized Gross-Pitaevskii equation, which is coupled to a thermal
cloud modelled by a Monte Carlo algorithm. This allows us to include the
collective dynamics of both the condensed and non-condensed components
self-consistently. We simulate quadrupolar excitations, and measure the damping
rate and frequency as a function of temperature. We also observe revivals in
condensate oscillations at high temperatures, and in the thermal cloud at low
temperature. Extensions of the model to include non-equilibrium effects and
describe more complex phenomena are discussed. | 0009468v1 |
2001-04-18 | Effective rate equations for the over-damped motion in fluctuating potentials | We discuss physical and mathematical aspects of the over-damped motion of a
Brownian particle in fluctuating potentials. It is shown that such a system can
be described quantitatively by fluctuating rates if the potential fluctuations
are slow compared to relaxation within the minima of the potential, and if the
position of the minima does not fluctuate. Effective rates can be calculated;
they describe the long-time dynamics of the system. Furthermore, we show the
existence of a stationary solution of the Fokker-Planck equation that describes
the motion within the fluctuating potential under some general conditions. We
also show that a stationary solution of the rate equations with fluctuating
rates exists. | 0104330v1 |
2001-09-05 | Spin Excitations in a Fermi Gas of Atoms | We have experimentally investigated a spin excitation in a quantum degenerate
Fermi gas of atoms. In the hydrodynamic regime the damping time of the
collective excitation is used to probe the quantum behavior of the gas. At
temperatures below the Fermi temperature we measure up to a factor of 2
reduction in the excitation damping time. In addition we observe a strong
excitation energy dependence for this quantum statistical effect. | 0109098v2 |
2001-10-09 | Freezing of a Stripe Liquid | The existence of a stripe-liquid phase in a layered nickelate,
La(1.725)Sr(0.275)NiO(4), is demonstrated through neutron scattering
measurements. We show that incommensurate magnetic fluctuations evolve
continuously through the charge-ordering temperature, although an abrupt
decrease in the effective damping energy is observed on cooling through the
transition. The energy and momentum dependence of the magnetic scattering are
parametrized with a damped-harmonic-oscillator model describing overdamped
spin-waves in the antiferromagnetic domains defined instantaneously by charge
stripes. | 0110191v2 |
2001-12-13 | Magnon softening and damping in the ferromagnetic manganites due to orbital correlations | We present a theory for spin excitations in ferromagnetic metallic manganites
and demonstrate that orbital fluctuations have strong effects on the magnon
dynamics in the case these compounds are close to a transition to an orbital
ordered state. In particular we show that the scattering of the spin
excitations by low-lying orbital modes with cubic symmetry causes both the
magnon softening and damping observed experimentally. | 0112252v2 |
2002-01-16 | Quantum Spin dynamics of the Bilayer Ferromagnet La(1.2)Sr(1.8)Mn2O7 | We construct a theory of spin wave excitations in the bilayer manganite
La(1.2)Sr(1.8)Mn2O7 based on the simplest possible double-exchange model, but
including leading quantum corrections to the spin wave dispersion and damping.
Comparison is made with recent inelastic neutron scattering experiments. We
find that quantum effects account for some part of the measured damping of spin
waves, but cannot by themselves explain the observed softening of spin waves at
the zone boundary. Furthermore a doping dependence of the total spin wave
dispersion and the optical spin wave gap is predicted. | 0201269v1 |
2002-02-21 | Dynamics of a Bose-Einstein condensate at finite temperature in an atomoptical coherence filter | The macroscopic coherent tunneling through the barriers of a periodic
potential is used as an atomoptical filter to separate the condensate and the
thermal components of a $^{87}$Rb mixed cloud. We condense in the combined
potential of a laser standing-wave superimposed on the axis of a cigar-shape
magnetic trap and induce condensate dipole oscillation in the presence of a
static thermal component. The oscillation is damped due to interaction with the
thermal fraction and we investigate the role played by the periodic potential
in the damping process. | 0202369v1 |
2002-03-11 | A Damping of the de Haas-van Alphen Oscillations in the superconducting state | Deploying a recently developed semiclassical theory of quasiparticles in the
superconducting state we study the de Haas-van Alphen effect. We find that the
oscillations have the same frequency as in the normal state but their amplitude
is reduced. We find an analytic formulae for this damping which is due to
tunnelling between semiclassical quasiparticle orbits comprising both
particle-like and hole-like segments. The quantitative predictions of the
theory are consistent with the available data. | 0203224v1 |
2002-03-26 | Measurement induced quantum-classical transition | A model of an electrical point contact coupled to a mechanical system
(oscillator) is studied to simulate the dephasing effect of measurement on a
quantum system. The problem is solved at zero temperature under conditions of
strong non-equilibrium in the measurement apparatus. For linear coupling
between the oscillator and tunneling electrons, it is found that the oscillator
dynamics becomes damped, with the effective temperature determined by the
voltage drop across the junction. It is demonstrated that both the quantum
heating and the quantum damping of the oscillator manifest themselves in the
current-voltage characteristic of the point contact. | 0203521v3 |
2002-07-04 | Fluctuations and correlations in hexagonal optical patterns | We analyze the influence of noise in transverse hexagonal patterns in
nonlinear Kerr cavities. The near field fluctuations are determined by the
neutrally stable Goldstone modes associated to translational invariance and by
the weakly damped soft modes. However these modes do not contribute to the far
field intensity fluctuations which are dominated by damped perturbations with
the same wave vectors than the pattern. We find strong correlations between the
intensity fluctuations of any arbitrary pair of wave vectors of the pattern.
Correlation between pairs forming 120 degrees is larger than between pairs
forming 180 degrees, contrary to what a naive interpretation of emission in
terms of twin photons would suggest. | 0207127v2 |
2002-09-19 | Damping of long-wavelength collective excitations in quasi-onedimensional Fermi liquids | The imaginary part of the exchange-correlation kernel in the longitudinal
current-current response function of a quasi-onedimensional Fermi liquid is
evaluated by an approximate decoupling in the equation of motion for the
current density, which accounts for processes of excitation of two
particle-hole pairs. The two-pair spectrum determines the intrinsic damping
rate of long-wavelength collective density fluctuations, which is calculated
and contrasted with a result previously obtained for a clean Luttinger liquid. | 0209455v1 |
2002-11-05 | Magnetic fluctuations and resonant peak in cuprates: a microscopic theory | The theory for the dynamical spin susceptibility within the t-J model is
developed, as relevant for the resonant magnetic peak and normal-state magnetic
response in superconducting (SC) cuprates. The analysis is based on the
equations of motion for spins and the memory-function presentation of magnetic
response where the main damping of the low-energy spin collective mode comes
from the decay into fermionic degrees of freedom. It is shown that the damping
function at low doping is closely related to the c-axis optical conductivity.
The analysis reproduces doping-dependent features of the resonant magnetic
scattering. | 0211090v1 |
2002-11-20 | Damping of Nodal Fermions Caused by a Dissipative Mode | Using a $d_{x^2 - y^2}$ superconductor in 2+1 dimensions we show that the
Nambu Goldstone fluctuations are replaced by dissipative excitations. We find
that the nodal quasi-particles damping is caused by the strong dissipative
excitations near the nodal points. As a result we find that the scattering
rates are linear in frequency and not cubic as predicted in the literature for
the ``d'' wave superconductors. Our results explain the recent angle resolved
photoemission spectroscopy and optical conductivity in the BSCCO high $T_c$
compounds. | 0211440v1 |
2003-05-27 | Dynamics of a classical gas including dissipative and mean field effects | By means of a scaling ansatz, we investigate an approximated solution of the
Boltzmann-Vlasov equation for a classical gas. Within this framework, we derive
the frequencies and the damping of the collective oscillations of a
harmonically trapped gas and we investigate its expansion after release of the
trap. The method is well suited to studying the collisional effects taking
place in the system and in particular to discussing the crossover between the
hydrodynamic and the collisionless regimes. An explicit link between the
relaxation times relevant for the damping of the collective oscillations and
for the expansion is established. | 0305624v1 |
2003-07-21 | Chaotic scattering of a quantum particle weakly coupled to a very complicated background | Effect of a complicated many-body environment is analyzed on the chaotic
motion of a quantum particle in a mesoscopic ballistic structure. The dephasing
and absorption phenomena are treated on the same footing in the framework of a
model which is free of the ambiguities inherent to earlier models. The
single-particle doorway resonance states excited via an external channel are
damped not only because of the escape onto such channels but also due to
ulterior population of long-lived background states, the resulting internal
damping being uniquely characterized by the spreading width. On the other hand,
the formation of the fine-structure resonances strongly enhances the delay time
fluctuations thus broadening the delay time distribution. | 0307496v1 |
2003-09-24 | Landau Damping in a 2D Electron Gas with Imposed Quantum Grid | Dielectric properties of semiconductor substrate with imposed two dimensional
(2D) periodic grid of quantum wires or nanotubes (quantum crossbars, QCB) are
studied. It is shown that a capacitive contact between QCB and semiconductor
substrate does not destroy the Luttinger liquid character of the long wave QCB
excitations. However, the dielectric losses of a substrate surface are
drastically modified due to diffraction processes on the QCB superlattice.
QCB-substrate interaction results in additional Landau damping regions of the
substrate plasmons. Their existence, form and the density of losses are
strongly sensitive to the QCB lattice constant. | 0309546v2 |
2003-11-21 | Self-stabilised fractality of sea-coasts through damped erosion | Erosion of rocky coasts spontaneously creates irregular seashores. But the
geometrical irregularity, in turn, damps the sea-waves, decreasing the average
wave amplitude. There may then exist a mutual self-stabilisation of the waves
amplitude together with the irregular morphology of the coast. A simple model
of such stabilisation is studied. It leads, through a complex dynamics of the
earth-sea interface, to the appearance of a stationary fractal seacoast with
dimension close to 4/3. Fractal geometry plays here the role of a morphological
attractor directly related to percolation geometry. | 0311509v1 |
2003-12-10 | Exciton-LO-phonon dynamics in InAs/GaAs quantum dots: Effects of zone-edge phonon damping | The dynamics of an exciton-LO-phonon system after an ultrafast optical
excitation in an InAs/GaAs quantum dot is studied theoretically. Influence of
anharmonic phonon damping and its interplay with the phonon dispersion is
analyzed. The signatures of the zone-edge decay process in the absorption
spectrum and time evolution are highlighted, providing a possible way of
experimental investigation on phonon anharmonicity effects. | 0312256v2 |
2004-01-13 | Vortex waves and the onset of turbulence in $^3$He-B | In a recent experiment Finne et al. discovered an intrinsic condition for the
onset of quantum turbulence in $^3$He-B, that q=alpha/(1-alpha')<1, where alpha
and alpha' are mutual friction parameters. The authors argued that this
condition corresponds to Kelvin waves which are marginally damped, so for q>1
Kelvin waves cannot grow in amplitude and trigger vortex reconnections and
turbulence. By analysing both axisymmetric and non-axisymmetric modes of
oscillations of a rotating superfluid, we confirm that in the long axial
wavelength limit the simple condition q=1 is indeed the crossover between
damped and propagating Kelvin waves. | 0401212v1 |
2004-01-28 | Long lived acoustic vibrational modes of an embedded nanoparticle | Classical continuum elastic calculations show that the acoustic vibrational
modes of an embedded nanoparticle can be lightly damped even when the
longitudinal plane wave acoustic impedances $Z_o=\rho v_L$ of the nanoparticle
and the matrix are the same. It is not necessary for the matrix to be less
dense or softer than the nanoparticle in order to have long lived vibrational
modes. Continuum boundary conditions do not always accurately reflect the
microscropic nature of the interface between nanoparticle and matrix, and a
multi-layer model of the interface reveals the possibility of additional
reduction of mode damping. | 0401579v2 |
2004-07-20 | Dynamics of a trapped ultracold two-dimensional atomic gas | This article is devoted to the study of two-dimensional Bose gases
harmonically confined. We first summarize their equilibrium properties. For
such a gas above the critical temperature, we also derive the frequencies and
the damping of the collective oscillations and we investigate its expansion
after releasing of the trap. The method is well suited to study the collisional
effects taking place in the system and in particular to discuss the crossover
between the hydrodynamic and the collisionless regimes. We establish the link
between the relaxation times relevant for the damping of the collective
oscillations and for the time-of-flight expansion. We also evaluate the
collision rate and its relationship with the relaxation time. | 0407522v1 |
2004-12-06 | Thermal wave packets induced by attosecond laser pulses | In this paper the dynamics of the interaction of attosecond laser pulses with
matter is investigated. It will be shown that the master equation: modified
Klein-Gordon equation describes the propagation of the heatons. Heatons are the
thermal wave packets. When the duration of the laser pulsees \delta t is of the
order of attosecond the heaton-thermal wave packets are nondispersive objects.
For \delta t \to \infty, the heatons are damped with damping factor of the
order of relaxation time for thermal processes.
Key words: Temperature fields; Attosecond laser pulses; Heatons; Modified
Klein-Gordon equation. | 0412126v1 |
2005-04-12 | Nonlinear response and discrete breather excitation in driven micro-mechanical cantilever arrays | We explain the origin of the generation of discrete breathers (DBs) in
experiments on damped and driven micromechanical cantilever arrays (M.Sato et
al. Phys. Rev. Lett. {\bf 90}, 044102, 2003). Using the concept of the
nonlinear response manifold (NLRM) we provide a systematic way to find the
optimal parameter regime in damped and driven lattices where DBs exist. Our
results show that DBs appear via a new instability of the NLRM different from
the anticipated modulational instability (MI) known for conservative systems.
We present several ways of exciting DBs, and compare also to experimental
studies of exciting and destroying DBs in antiferromagnetic layered systems. | 0504298v1 |
2005-05-14 | Monopole Oscillations and Dampings in Boson and Fermion Mixture in the Time-Dependent Gross-Pitaevskii and Vlasov Equations | We construct a dynamical model for the time evolution of the boson-fermion
coexistence system. The dynamics of bosons and fermions are formulated with the
time-dependent Gross-Pitaevsky equation and the Vlasov equation. We thus study
the monopole oscillation in the bose-fermi mixture. We find that large damping
exists for fermion oscillations in the mixed system even at zero temperature. | 0505357v1 |
2005-10-13 | Superconducting Flywheel Model for Energy Storage Applications | In order to explore the complexity and diversity of the flywheels' dynamics,
we have developed the real-physics computer model of a universal mechanical
rotor. Due to an arbitrary external force concept, the model can be adjusted to
operate identical to the real experimental prototype. Taking the high-speed
magnetic rotor on superconducting bearings as the prototype, the law for the
energy loss in real high temperature superconducting bearings has been derived.
Varying the laws of damping and elasticity in the system, we have found a way
to effectively damp the parasitic resonances and minimize the loss of energy
storage. | 0510346v1 |
2005-11-05 | Ratchet Effect in Magnetization Reversal of Stoner Particles | A new strategy is proposed aimed at substantially reducing the minimal
magnetization switching field for a Stoner particle. Unlike the normal method
of applying a static magnetic field which must be larger than the magnetic
anisotropy, a much weaker field, proportional to the damping constant in the
weak damping regime, can be used to switch the magnetization from one state to
another if the field is along the motion of the magnetization. The concept is
to constantly supply energy to the particle from the time-dependent magnetic
field to allow the particle to climb over the potential barrier between the
initial and the target states. | 0511135v1 |
2005-12-03 | Apparent vibrational side-bands in pi-conjugated systems: the case of distyrylbenzene | The photoluminescence (PL) spectra of dilute solution and single crystals of
distyrylbenzene show unique temperature dependent vibronic structures. The
characteristic single frequency progression at high temperatures is modulated
by a low frequency progression series at low temperatures. None of the series
side band modes corresponds to any of the distyrylbenzene Raman frequencies. We
explain these PL properties using a time dependent model with temperature
dependent damping, in which the many-mode system is effectively transformed to
two- and then to a single "apparent" mode as damping increases. | 0512067v1 |
2006-05-26 | Thermo-Plasma Polariton within Scaling Theory of Single-Layer Graphene | Electrodynamics of single-layer graphene is studied in the scaling regime. At
any finite temperature, there is a weakly damped collective thermo-plasma
polariton mode whose dispersion and wavelength dependent damping is determined
analytically. The electric and magnetic fields associated with this mode decay
exponentially in the direction perpendicular to the graphene layer, but unlike
the surface plasma polariton modes of metals, the decay length and the mode
frequency are strongly temperature dependent. This may lead to new ways of
generation and manipulation of these modes. | 0605642v1 |
2006-12-18 | Shear viscosity and damping for a Fermi gas in the unitarity limit | The shear viscosity of a two-component Fermi gas in the normal phase is
calculated as a function of temperature in the unitarity limit, taking into
account strong-coupling effects that give rise to a pseudogap in the spectral
density for single-particle excitations. The results indicate that recent
measurements of the damping of collective modes in trapped atomic clouds can be
understood in terms of hydrodynamics, with a decay rate given by the viscosity
integrated over an effective volume of the cloud. | 0612460v2 |
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