13,246 research outputs found
Synchronization by Reactive Coupling and Nonlinear Frequency Pulling
We present a detailed analysis of a model for the synchronization of
nonlinear oscillators due to reactive coupling and nonlinear frequency pulling.
We study the model for the mean field case of all-to-all coupling, deriving
results for the initial onset of synchronization as the coupling or
nonlinearity increase, and conditions for the existence of the completely
synchronized state when all the oscillators evolve with the same frequency.
Explicit results are derived for Lorentzian, triangular, and top-hat
distributions of oscillator frequencies. Numerical simulations are used to
construct complete phase diagrams for these distributions
Who Contributes to the Knowledge Sharing Economy?
Information sharing dynamics of social networks rely on a small set of
influencers to effectively reach a large audience. Our recent results and
observations demonstrate that the shape and identity of this elite, especially
those contributing \emph{original} content, is difficult to predict.
Information acquisition is often cited as an example of a public good. However,
this emerging and powerful theory has yet to provably offer qualitative
insights on how specialization of users into active and passive participants
occurs.
This paper bridges, for the first time, the theory of public goods and the
analysis of diffusion in social media. We introduce a non-linear model of
\emph{perishable} public goods, leveraging new observations about sharing of
media sources. The primary contribution of this work is to show that
\emph{shelf time}, which characterizes the rate at which content get renewed,
is a critical factor in audience participation. Our model proves a fundamental
\emph{dichotomy} in information diffusion: While short-lived content has simple
and predictable diffusion, long-lived content has complex specialization. This
occurs even when all information seekers are \emph{ex ante} identical and could
be a contributing factor to the difficulty of predicting social network
participation and evolution.Comment: 15 pages in ACM Conference on Online Social Networks 201
Atomic oxygen degradation of Intelsat 4-type solar array interconnects: Laboratory investigations
A Hughes 506 type communication satellite belonging to the Intelsat organization was marooned in low Earth orbit on March 14, 1990, following failure of the Titan third stage to separate properly. The satellite, Intelsat VI, was designed for service in geosynchronous orbit and contains several material configurations which are susceptible to attack by atomic oxygen. Analysis showed the silver foil interconnects in the satellite photovoltaic array to be the key materials issue because the silver is exposed directly to the atomic oxygen ram flux. The results are reported of atomic oxygen degradation testing of Intelsat VI type silver foil interconnects both as virgin material and in a configured solar cell element. Test results indicate that more than 80 pct. of the original thickness of silver in the Intelsat VI solar array interconnects should remain after completion of the proposed Space Shuttle rescue and/or reboost mission
The prevalence of occult hepatitis B virus (hbv) infection in a large multi-ethnic haemodialysis cohort.
Haemodialysis patients are at increased risk of exposure to blood borne viruses. To reduce transmission in the UK, all haemodialysis patients are regularly screened, and if susceptible to Hepatitis B virus (HBV) infection, vaccinated
Aging in the Relaxor Ferroelectric PMN/PT
The relaxor ferroelectric
(PbMnNbO)(PbTiO), ,
(PMN/PT(90/10)) is found to exhibit several regimes of complicated aging
behavior. Just below the susceptibility peak there is a regime exhibiting
rejuvenation but little memory. At lower temperature, there is a regime with
mainly cumulative aging, expected for simple domain-growth. At still lower
temperature, there is a regime with both rejuvenation and memory, reminiscent
of spin glasses. PMN/PT (88/12) is also found to exhibit some of these aging
regimes. This qualitative aging behavior is reminiscent of that seen in
reentrant ferromagnets, which exhibit a crossover from a domain-growth
ferromagnetic regime into a reentrant spin glass regime at lower temperatures.
These striking parallels suggest a picture of competition in PMN/PT (90/10)
between ferroelectric correlations formed in the domain-growth regime with
glassy correlations formed in the spin glass regime. PMN/PT (90/10) is also
found to exhibit frequency-aging time scaling of the time-dependent part of the
out-of-phase susceptibility for temperatures 260 K and below. The stability of
aging effects to thermal cycles and field perturbations is also reported.Comment: 8 pages RevTeX4, 11 figures; submitted to Phys. Rev.
High intensity 5 eV O-atom exposure facility for material degradation studies
An atomic oxygen exposure facility was developed for studies of material degradation. The goal of these studies is to provide design criteria and information for the manufacture of long life (20 to 30 years) construction materials for use in low Earth orbit. The studies that are being undertaken will provide: (1) absolute reaction cross sections for the engineering design problems, (2) formulations of reaction mechanisms for use in the selection of suitable existing materials and the design of new more resistant ones, and (3) the calibration of flight hardware (mass spectrometers, etc.) in order to directly relate experiments performed in low Earth orbit to ground based investigations. The facility consists of a CW laser sustained discharge source of O-atoms, an atomic beam formation and diagnostics system, a spinning rotor viscometer, and provision for using the system for calibration of actual flight instruments
EXAFS study of lead-free relaxor ferroelectric BaTi(1-x)Zr(x)O3 at the Zr K-edge
Extended X-ray absorption fine structure (EXAFS) experiments at the Zr K-edge
were carried out on perovskite relaxor ferroelectrics BaTi(1-x)Zr(x)O3 (BTZ) (x
= 0.25, 0.30, 0.35), and on BaZrO3 for comparison. Structural information up to
4.5 A around the Zr atoms is obtained, revealing that the local structure
differs notably from the average Pm-3m cubic structure deduced from X-ray
diffraction. In particular, our results show that the distance between Zr atoms
and their first oxygen neighbors is independent of the Zr substitution rate x
and equal to that measured in BaZrO3, while the X-ray cubic cell parameter
increases linearly with x. Furthermore, we show that the Zr atoms tend to
segregate in Zr-rich regions. We propose that the relaxor behavior in BTZ is
linked to random elastic fields generated by this particular chemical
arrangement, rather than to random electric fields as is the case in most
relaxors.Comment: 13 pages, 12 figures, 4 tables. Submitted to Phys. Rev.
Coexistence and competition of local- and long-range polar orders in a ferroelectric relaxor
We have performed a series of neutron diffuse scattering measurements on a
single crystal of the solid solution Pb(ZnNb)O (PZN) doped
with 8% PbTiO (PT), a relaxor compound with a Curie temperature T K, in an effort to study the change in local polar orders from the polar
nanoregions (PNR) when the material enters the ferroelectric phase. The diffuse
scattering intensity increases monotonically upon cooling in zero field, while
the rate of increase varies dramatically around different Bragg peaks. These
results can be explained by assuming that corresponding changes occur in the
ratio of the optic and acoustic components of the atomic displacements within
the PNR. Cooling in the presence of a modest electric field oriented
along the [111] direction alters the shape of diffuse scattering in reciprocal
space, but does not eliminate the scattering as would be expected in the case
of a classic ferroelectric material. This suggests that a field-induced
redistribution of the PNR has taken place
Universality of the Gunn effect: self-sustained oscillations mediated by solitary waves
The Gunn effect consists of time-periodic oscillations of the current flowing
through an external purely resistive circuit mediated by solitary wave dynamics
of the electric field on an attached appropriate semiconductor. By means of a
new asymptotic analysis, it is argued that Gunn-like behavior occurs in
specific classes of model equations. As an illustration, an example related to
the constrained Cahn-Allen equation is analyzed.Comment: 4 pages,3 Post-Script figure
The stochastic dynamics of micron and nanoscale elastic cantilevers in fluid: fluctuations from dissipation
The stochastic dynamics of micron and nanoscale cantilevers immersed in a
viscous fluid are quantified. Analytical results are presented for long slender
cantilevers driven by Brownian noise. The spectral density of the noise force
is not assumed to be white and the frequency dependence is determined from the
fluctuation-dissipation theorem. The analytical results are shown to be useful
for the micron scale cantilevers that are commonly used in atomic force
microscopy. A general thermodynamic approach is developed that is valid for
cantilevers of arbitrary geometry as well as for arrays of multiple cantilevers
whose stochastic motion is coupled through the fluid. It is shown that the
fluctuation-dissipation theorem permits the calculation of stochastic
quantities via straightforward deterministic methods. The thermodynamic
approach is used with deterministic finite element numerical simulations to
quantify the autocorrelation and noise spectrum of cantilever fluctuations for
a single micron scale cantilever and the cross-correlations and noise spectra
of fluctuations for an array of two experimentally motivated nanoscale
cantilevers as a function of cantilever separation. The results are used to
quantify the noise reduction possible using correlated measurements with two
closely spaced nanoscale cantilevers.Comment: Submitted to Nanotechnology April 26, 200
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