653 research outputs found
Nonlinear resonance in a three-terminal carbon nanotube resonator
The RF-response of a three-terminal carbon nanotube resonator coupled to
RF-transmission lines is studied by means of perturbation theory and direct
numerical integration. We find three distinct oscillatory regimes, including
one regime capable of exhibiting very large hysteresis loops in the frequency
response. Considering a purely capacitive transduction, we derive a set of
algebraic equations which can be used to find the output power (S-parameters)
for a device connected to transmission lines with characteristic impedance
.Comment: 16 pages, 8 figure
Troll and Divide: The Language of Online Polarization
The affective animosity between the political left and right has grown steadily in many countries over the past few years, posing a threat to democratic practices and public health. There is a rising concern over the role that âbad actorsâ or trolls may play in the polarization of online networks. In this research, we examined the processes by which trolls may sow intergroup conflict through polarized rhetoric. We developed a dictionary to assess online polarization by measuring language associated with communications that display partisan bias in their diffusion. We validated the polarized language dictionary in four different contexts and across multiple time periods. The polarization dictionary made out-of-set predictions, generalized to both new political contexts (#BlackLivesMatter) and a different social media platform (Reddit), and predicted partisan differences in public opinion polls about COVID-19. Then we analyzed tweets from a known Russian troll source (N = 383,510) and found that their use of polarized language has increased over time. We also compared troll tweets from three countries (N = 798,33) and found that they all utilize more polarized language than regular Americans (N = 1,507,300) and trolls have increased their use of polarized rhetoric over time. We also find that polarized language is associated with greater engagement, but this association only holds for politically engaged users (both trolls and regular users). This research clarifies how trolls leverage polarized language and provides an open-source, simple tool for exploration of polarized communications on social media
Slightly Two or Three Dimensional Self-Similar Solutions
Self similarity allows for analytic or semi-analytic solutions to many
hydrodynamics problems. Most of these solutions are one dimensional. Using
linear perturbation theory, expanded around such a one-dimensional solution, we
find self-similar hydrodynamic solutions that are two- or three-dimensional.
Since the deviation from a one-dimensional solution is small, we call these
slightly two-dimensional and slightly three-dimensional self-similar solutions,
respectively. As an example, we treat strong spherical explosions of the second
type. A strong explosion propagates into an ideal gas with negligible
temperature and density profile of the form
rho(r,theta,phi)=r^{-omega}[1+sigma*F(theta,phi)], where omega>3 and sigma <<
1. Analytical solutions are obtained by expanding the arbitrary function
F(theta,phi) in spherical harmonics. We compare our results with two
dimensional numerical simulations, and find good agreement.Comment: Submitted to Physics of Fluid
Radiative Transfer Effects in He I Emission Lines
We consider the effect of optical depth of the 2 ^{3}S level on the nebular
recombination spectrum of He I for a spherically symmetric nebula with no
systematic velocity gradients. These calculations, using many improvements in
atomic data, can be used in place of the earlier calculations of Robbins. We
give representative Case B line fluxes for UV, optical, and IR emission lines
over a range of physical conditions: T=5000-20000 K, n_{e}=1-10^{8} cm^{-3},
and tau_{3889}=0-100. A FORTRAN program for calculating emissivities for all
lines arising from quantum levels with n < 11 is also available from the
authors.
We present a special set of fitting formulae for the physical conditions
relevant to low metallicity extragalactic H II regions: T=12,000-20,000 K,
n_{e}=1-300 cm^{-3}, and tau_{3889} < 2.0. For this range of physical
conditions, the Case B line fluxes of the bright optical lines 4471 A, 5876 A,
and 6678 A, are changed less than 1%, in agreement with previous studies.
However, the 7065 A corrections are much smaller than those calculated by
Izotov & Thuan based on the earlier calculations by Robbins. This means that
the 7065 A line is a better density diagnostic than previously thought. Two
corrections to the fitting functions calculated in our previous work are also
given.Comment: To be published in 10 April 2002 ApJ; relevant code available at
ftp://wisp.physics.wisc.edu/pub/benjamin/Heliu
870 micron observations of nearby 3CRR radio galaxies
We present submillimeter continuum observations at 870 microns of the cores
of low redshift 3CRR radio galaxies, observed at the Heinrich Hertz
Submillimeter Telescope. The cores are nearly flat spectrum between the radio
and submillimeter which implies that the submillimeter continuum is likely to
be synchrotron emission and not thermal emission from dust. The emitted power
from nuclei detected at optical wavelengths and in the X-rays is similar in the
submillimeter, optical and X-rays. The submillimeter to optical and X-ray power
ratios suggest that most of these sources resemble misdirected BL Lac type
objects with synchrotron emission peaking at low energies. However we find
three exceptions, the FR I galaxy 3C264 and the FR II galaxies 3C390.3 and
3C338 with high X-ray to submillimeter luminosity ratios. These three objects
are candidate high or intermediate energy peaked BL Lac type objects. With
additional infrared observations and from archival data, we compile spectral
energy distributions (SEDs) for a subset of these objects. The steep dips
observed near the optical wavelengths in many of these objects suggest that
extinction inhibits the detection and reduces the flux of optical continuum
core counterparts. High resolution near or mid-infrared imaging may provide
better measurements of the underlying synchrotron emission peak.Comment: accepted for publication in A
A class of well-posed parabolic final value problems
This paper focuses on parabolic final value problems, and well-posedness is
proved for a large class of these. The clarification is obtained from Hilbert
spaces that characterise data that give existence, uniqueness and stability of
the solutions. The data space is the graph normed domain of an unbounded
operator that maps final states to the corresponding initial states. It induces
a new compatibility condition, depending crucially on the fact that analytic
semigroups always are invertible in the class of closed operators. Lax--Milgram
operators in vector distribution spaces constitute the main framework. The
final value heat conduction problem on a smooth open set is also proved to be
well posed, and non-zero Dirichlet data are shown to require an extended
compatibility condition obtained by adding an improper Bochner integral.Comment: 16 pages. To appear in "Applied and numerical harmonic analysis"; a
reference update. Conference contribution, based on arXiv:1707.02136, with
some further development
Dust Formation and He II 4686 emission in the Dense Shell of the Peculiar Type Ib Supernova 2006jc
We present evidence for the formation of dust grains in an unusual Type Ib SN
based on late-time spectra of SN 2006jc. The progenitor suffered an LBV-like
outburst just 2 yr earlier, and we propose that the dust formation is a
consequence of the SN blast wave overtaking that LBV-like shell. The key
evidence for dust formation is (a) the appearance of a red/near-IR continuum
source fit by 1600 K graphite grains, and (b) fading of the redshifted sides of
He I emission lines, yielding progressively more asymmetric blueshifted lines
as dust obscures receding material. This provides the strongest case yet for
dust formation in any SN Ib/c. Both developments occurred between 51 and 75 d
after peak, while other SNe observed to form dust did so after a few hundred
days. Geometric considerations indicate that dust formed in the dense swept-up
shell between the forward and reverse shocks, and not in the freely expanding
SN ejecta. Rapid cooling leading to dust formation may have been aided by
extremely high shell densities, as indicated by He I line ratios. The brief
epoch of dust formation is accompanied by He II 4686 emission and enhanced
X-ray emission. These clues suggest that the unusual dust formation in this
object was not due to properties of the SN itself, but instead -- like most
peculiarities of SN 2006jc -- was a consequence of the dense environment
created by an LBV-like eruption 2 yr before the SN.Comment: ApJ, accepted. added some discussion and 2 figures, better title,
conclusions same as previous version. 12 pages, 4 color fig
Full-field swept-source optical coherence tomography and neural tissue classification for deep brain imaging
Optical coherence tomography can differentiate brain regions with intrinsic contrast and at a micron scale resolution. Such a device can be particularly useful as a realtime neurosurgical guidance tool. We present, to our knowledge, the first full-field swept-source optical coherence tomography system operating near a wavelength of 1310 nm. The proof-of-concept system was integrated with an endoscopic probe tip, that is compatible with deep brain stimulation keyhole neurosurgery. Neuroimaging experiments were performed on ex vivo brain tissues and in vivo in rat brains. Using classification algorithms involving texture features and optical attenuation, images were successfully classified into three brain tissue types
Direct Measurement of the System-Environment Coupling as a Tool For Understanding Decoherence and Dynamical Decoupling
Decoherence is a major obstacle to any practical implementation of quantum
information processing. One of the leading strategies to reduce decoherence is
dynamical decoupling --- the use of an external field to average out the effect
of the environment. The decoherence rate under any control field can be
calculated if the spectrum of the coupling to the environment is known. We
present a direct measurement of the bath coupling spectrum in an ensemble of
optically trapped ultracold atoms, by applying a spectrally narrow-band control
field. The measured spectrum follows a Lorentzian shape at low frequencies, but
exhibits non-monotonic features at higher frequencies due to the oscillatory
motion of the atoms in the trap. These features agree with our analytical
models and numerical Monte-Carlo simulations of the collisional bath. From the
inferred bath-coupling spectrum, we predict the performance of well-known
dynamical decoupling sequences: CPMG, UDD and CDD. We then apply these
sequences in experiment and compare the results to predictions, finding good
agreement in the weak-coupling limit. Thus, our work establishes experimentally
the validity of the overlap integral formalism, and is an important step
towards the implementation of an optimal dynamical decoupling sequence for a
given measured bath spectrum.Comment: 9 pages, 6 figure
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