7,966 research outputs found
On the computation of the term of the series defining the center manifold for a scalar delay differential equation
In computing the third order terms of the series of powers of the center
manifold at an equilibrium point of a scalar delay differential equation, with
a single constant delay some problems occur at the term
More precisely, in order to determine the values at 0,
respectively of the function an algebraic system of
equations must be solved. We show that the two equations are dependent, hence
the system has an infinity of solutions. Then we show how we can overcome this
lack of uniqueness and provide a formula for Comment: Presented at the Conference on Applied and Industrial Mathematics-
CAIM 2011, Iasi, Romania, 22-25 September, 2011. Preprin
Compensation for nonlinear effects due to high heat flux in thin-film thermometry
Compensation for nonlinear effects due to high heat flux in thin-film thermometr
BiSON data preparation: A correction for differential extinction and the weighted averaging of contemporaneous data
The Birmingham Solar Oscillations Network (BiSON) has provided high-quality
high-cadence observations from as far back in time as 1978. These data must be
calibrated from the raw observations into radial velocity and the quality of
the calibration has a large impact on the signal-to-noise ratio of the final
time series. The aim of this work is to maximise the potential science that can
be performed with the BiSON data set by optimising the calibration procedure.
To achieve better levels of signal-to-noise ratio we perform two key steps in
the calibration process: we attempt a correction for terrestrial atmospheric
differential extinction; and the resulting improvement in the calibration
allows us to perform weighted averaging of contemporaneous data from different
BiSON stations. The improvements listed produce significant improvement in the
signal-to-noise ratio of the BiSON frequency-power spectrum across all
frequency ranges. The reduction of noise in the power spectrum will allow
future work to provide greater constraint on changes in the oscillation
spectrum with solar activity. In addition, the analysis of the low-frequency
region suggests we have achieved a noise level that may allow us to improve
estimates of the upper limit of g-mode amplitudes.Comment: Accepted for publication in MNRAS; 10 pages, 7 figure
Hot phonon decay in supported and suspended exfoliated graphene
Near infrared pump-probe spectroscopy has been used to measure the ultrafast
dynamics of photoexcited charge carriers in monolayer and multilayer graphene.
We observe two decay processes occurring on 100 fs and 2 ps timescales. The
first is attributed to the rapid electron-phonon thermalisation in the system.
The second timescale is found to be due to the slow decay of hot phonons. Using
a simple theoretical model we calculate the hot phonon decay rate and show that
it is significantly faster in monolayer flakes than in multilayer ones. In
contrast to recent claims, we show that this enhanced decay rate is not due to
the coupling to substrate phonons, since we have also seen the same effect in
suspended flakes. Possible intrinsic decay mechanisms that could cause such an
effect are discussed.Comment: 4 pages, 3 figure
Performance of the Birmingham Solar-Oscillations Network (BiSON)
The Birmingham Solar-Oscillations Network (BiSON) has been operating with a
full complement of six stations since 1992. Over 20 years later, we look back
on the network history. The meta-data from the sites have been analysed to
assess performance in terms of site insolation, with a brief look at the
challenges that have been encountered over the years. We explain how the
international community can gain easy access to the ever-growing dataset
produced by the network, and finally look to the future of the network and the
potential impact of nearly 25 years of technology miniaturisation.Comment: 31 pages, 19 figures. Accepted by Solar Physics: 2015 October 20.
First online: 2015 December 7. Open Acces
The environment and host haloes of the brightest z~6 Lyman-break galaxies
By studying the large-scale structure of the bright high-redshift Lyman-break
galaxy (LBG) population it is possible to gain an insight into the role of
environment in galaxy formation physics in the early Universe. We measure the
clustering of a sample of bright (-22.7<M_UV<-21.125) LBGs at z~6 and use a
halo occupation distribution (HOD) model to measure their typical halo masses.
We find that the clustering amplitude and corresponding HOD fits suggests that
these sources are highly biased (b~8) objects in the densest regions of the
high-redshift Universe. Coupled with the observed rapid evolution of the number
density of these objects, our results suggest that the shape of high luminosity
end of the luminosity function is related to feedback processes or dust
obscuration in the early Universe - as opposed to a scenario where these
sources are predominantly rare instances of the much more numerous M_UV ~ -19
population of galaxies caught in a particularly vigorous period of star
formation. There is a slight tension between the number densities and
clustering measurements, which we interpret this as a signal that a refinement
of the model halo bias relation at high redshifts or the incorporation of
quasi-linear effects may be needed for future attempts at modelling the
clustering and number counts. Finally, the difference in number density between
the fields (UltraVISTA has a surface density ~1.8 times greater than UDS) is
shown to be consistent with the cosmic variance implied by the clustering
measurements.Comment: 19 pages, 8 figures, accepted MNRAS 23rd March 201
The Sun in transition? Persistence of near-surface structural changes through Cycle 24
We examine the frequency shifts in low-degree helioseismic modes from the
Birmingham Solar-Oscillations Network (BiSON) covering the period from 1985 -
2016, and compare them with a number of global activity proxies well as a
latitudinally-resolved magnetic index. As well as looking at frequency shifts
in different frequency bands, we look at a parametrization of the shift as a
cubic function of frequency. While the shifts in the medium- and highfrequency
bands are very well correlated with all of the activity indices (with the best
correlation being with the 10.7 cm radio flux), we confirm earlier findings
that there appears to have been a change in the frequency response to activity
during solar cycle 23, and the low frequency shifts are less correlated with
activity in the last two cycles than they were in Cycle 22. At the same time,
the more recent cycles show a slight increase in their sensitivity to activity
levels at medium and higher frequencies, perhaps because a greater proportion
of activity is composed of weaker or more ephemeral regions. This lends weight
to the speculation that a fundamental change in the nature of the solar dynamo
may be in progress.Comment: 9 pages, 6 figures. Accepted by MNRAS 24 May 201
Strong nonlinear optical response of graphene flakes measured by four-wave mixing
We present the first experimental investigation of nonlinear optical
properties of graphene flakes. We find that at near infrared frequencies a
graphene monolayer exhibits a remarkably high third-order optical nonlinearity
which is practically independent of the wavelengths of incident light. The
nonlinear optical response can be utilized for imaging purposes, with image
contrasts of graphene which are orders of magnitude higher than those obtained
using linear microscopy.Comment: 4 pages, 5 figure
Native American Weight Loss Movement: Pilot Test of a Culturally Tailored Weight Loss Program for American Indians
American Indians (AIs) have higher rates of obesity than other racial/ethnic groups, placing them at heightened risk for cardiovascular diseases, diabetes, and certain cancers. Culturally appropriate weight loss interventions may be the key to reducing risk. The most successful program used in AI communities has been the Diabetes Prevention Program (DPP), which limits enrollment to individuals with a clinical diagnosis of pre-diabetes. The purpose of this pilot project was to modify and culturally tailor a weight loss intervention to AI communities in Kansas to improve weight loss related behaviors among those who do not qualify for the DPP. The Native American Weight Loss Movement (NAWLM) was developed from 2012-2014 using an iterative process with 4 sequential modifications to the program. Group 1 received a slightly modified version of the DPP that was originally tailored to African Americans. Each group received an improved program based on modifications from the previous group. Our analysis shows 36.1% (95% CI: 25.7, 47.5) of all participants (n=72) lost weight; a majority (63.9%, 95% CI: 52.8-75.0) maintained weight, gained weight, or dropped out. Among individuals who completed the program (n=34), 76.5% lost weight (95% CI: 61.4, 91.5). These individuals lost an average of 2.98% body weight (95% CI: 1.58, 4.37), with 6 participants losing \u3e7% body weight. While most participants who completed the program lost weight, more research is needed to determine factors that discourage drop-out and promote behavioral changes. NAWLM shows promise as a weight loss program for AIs who do not qualify for the DPP
Atom in a coherently controlled squeezed vacuum
A broadband squeezed vacuum photon field is characterized by a complex
squeezing function. We show that by controlling the wavelength dependence of
its phase it is possible to change the dynamics of the atomic polarization
interacting with the squeezed vacuum. Such a phase modulation effectively
produces a finite range temporal interaction kernel between the two quadratures
of the atomic polarization yielding the change in the decay rates as well as
the appearance of additional oscillation frequencies. We show that decay rates
slower than the spontaneous decay rate can be achieved even for a squeezed bath
in the classic regime. For linear and quadratic phase modulations the power
spectrum of the scattered light exhibits narrowing of the central peak due to
the modified decay rates. For strong phase modulations side lobes appear
symmetrically around the central peak reflecting additional oscillation
frequencies.Comment: 4 pages, 4 figure
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