301 research outputs found
Axions Scattering From a Quadrupole Magnetic Field
We study the 2D scattering of axions from an accelerator like quadrupole
magnet using the eikonal approximation in order to learn whether or not such a
setup could serve as a new possible method for detecting axions on terrestrial
experiments. The eikonal approximation in 2D is introduced and explained. We
also apply the eikonal approximation to two known cases in order to compare it
with previous results, obtained using Born's approximation, and discuss its
correctness
Sparsity-Based Super Resolution for SEM Images
The scanning electron microscope (SEM) produces an image of a sample by
scanning it with a focused beam of electrons. The electrons interact with the
atoms in the sample, which emit secondary electrons that contain information
about the surface topography and composition. The sample is scanned by the
electron beam point by point, until an image of the surface is formed. Since
its invention in 1942, SEMs have become paramount in the discovery and
understanding of the nanometer world, and today it is extensively used for both
research and in industry. In principle, SEMs can achieve resolution better than
one nanometer. However, for many applications, working at sub-nanometer
resolution implies an exceedingly large number of scanning points. For exactly
this reason, the SEM diagnostics of microelectronic chips is performed either
at high resolution (HR) over a small area or at low resolution (LR) while
capturing a larger portion of the chip. Here, we employ sparse coding and
dictionary learning to algorithmically enhance LR SEM images of microelectronic
chips up to the level of the HR images acquired by slow SEM scans, while
considerably reducing the noise. Our methodology consists of two steps: an
offline stage of learning a joint dictionary from a sequence of LR and HR
images of the same region in the chip, followed by a fast-online
super-resolution step where the resolution of a new LR image is enhanced. We
provide several examples with typical chips used in the microelectronics
industry, as well as a statistical study on arbitrary images with
characteristic structural features. Conceptually, our method works well when
the images have similar characteristics. This work demonstrates that employing
sparsity concepts can greatly improve the performance of SEM, thereby
considerably increasing the scanning throughput without compromising on
analysis quality and resolution.Comment: Final publication available at ACS Nano Letter
Antiphase Synchronization in a Flagellar-Dominance Mutant of Chlamydomonas
Groups of beating flagella or cilia often synchronize so that neighboring
filaments have identical frequencies and phases. A prime example is provided by
the unicellular biflagellate Chlamydomonas reinhardtii, which typically
displays synchronous in-phase beating in a low-Reynolds number version of
breaststroke swimming. We report here the discovery that ptx1, a flagellar
dominance mutant of C. reinhardtii, can exhibit synchronization in precise
antiphase, as in the freestyle swimming stroke. Long-duration high-speed
imaging shows that ptx1 flagella switch stochastically between in-phase and
antiphase states, and that the latter has a distinct waveform and significantly
higher frequency, both of which are strikingly similar to those found during
phase slips that stochastically interrupt in-phase beating of the wildtype.
Possible mechanisms underlying these observations are discussed.Comment: 5 pages, 4 figure
Runaway electrification of friable self-replicating granular matter
We establish that the nonlinear dynamics of collisions between particles
favors the charging of a insulating, friable, self-replicating granular
material that undergoes nucleation, growth, and fission processes; we
demonstrate with a minimal dynamical model that secondary nucleation produces a
positive feedback in an electrification mechanism that leads to runaway
charging. We discuss ice as an example of such a self-replicating granular
material: We confirm with laboratory experiments in which we grow ice from the
vapor phase in situ within an environmental scanning electron microscope that
charging causes fast-growing and easily breakable palm-like structures to form,
which when broken off may form secondary nuclei. We propose that thunderstorms,
both terrestrial and on other planets, and lightning in the solar nebula are
instances of such runaway charging arising from this nonlinear dynamics in
self-replicating granular matter
Testing three hypotheses about effects of sensitive-insensitive parenting on telomeres.
Telomeres are the protective DNA-protein sequences appearing at the ends of chromosomes; they shorten with each cell division and are considered a biomarker of aging. Shorter telomere length and greater erosion have been associated with compromised physical and mental health and are hypothesized to be affected by early life stress. In the latter case, most work has relied on retrospective measures of early life stressors. The Dutch research (n = 193) presented herein tested 3 hypotheses prospectively regarding effects of sensitive-insensitive parenting during the first 2.5 years on telomere length at age 6, when first measured, and change over the following 4 years. It was predicted that (1) less sensitive parenting would predict shorter telomeres and greater erosion and that such effects would be most pronounced in children (2) exposed to prenatal stress and/or (3) who were highly negatively emotional as infants. Results revealed, only, that prenatal stress amplified parenting effects on telomere change-in a differential-susceptibility-related manner: Prenatally stressed children displayed more erosion when they experienced insensitive parenting and less erosion when they experienced sensitive parenting. Mechanisms that might initiate greater postnatal plasticity as a result of prenatal stress are highlighted and future work outlined. (PsycINFO Database Record (c) 2020 APA, all rights reserved)
Direct measurement of the flow field around swimming microorganisms
Swimming microorganisms create flows that influence their mutual interactions
and modify the rheology of their suspensions. While extensively studied
theoretically, these flows have not been measured in detail around any
freely-swimming microorganism. We report such measurements for the microphytes
Volvox carteri and Chlamydomonas reinhardtii. The minute ~0.3% density excess
of V. carteri over water leads to a strongly dominant Stokeslet contribution,
with the widely-assumed stresslet flow only a correction to the subleading
source dipole term. This implies that suspensions of V. carteri have features
similar to suspensions of sedimenting particles. The flow in the region around
C. reinhardtii where significant hydrodynamic interaction is likely to occur
differs qualitatively from a "puller" stresslet, and can be described by a
simple three-Stokeslet model.Comment: 4 pages, 4 figures, accepted for publication in PR
Gravitational Trapping Near Domain Walls and Stable Solitons
In this work, the behavior of test particles near a domain wall of a stable
false vacuum bubble is studied. It is shown that matter is naturally trapped in
the vicinity of a static domain wall, and also, that there is a discontinuity
in the test particle's velocity when crossing the domain wall. The latter is
unexpected as it stands in contrast to Newtonian theory, where infinite forces
are not allowed. The weak field limit is defined in order to show that there is
no conflict with the non-relativistic behavior of gravitational fields and
particle motions under these conditions.Comment: 8 pages, 1 figure, problem is reanalyzed using a continuous
coordinate syste
Tidal Disruption Events and Gravitational Waves from Intermediate-mass Black Holes in Evolving Globular Clusters across Space and Time
We present a semi-analytic model for self-consistently evolving a population of globular clusters (GCs) in a given host galaxy across cosmic time. We compute the fraction of GCs still hosting intermediate-mass black holes (IMBHs) at a given redshift in early and late -type galaxies of different masses and sizes, and the corresponding rate of tidal disruption events (TDEs), both main-sequence (MS) and white dwarf (WD) stars. We find that the integrated TDE rate for the entire GC population can exceed the corresponding rate in a given galactic nucleus and that similar to 90% of the TDEs reside in GCs within a maximum radius of similar to 2-15 kpc from the host galaxy's center. This suggests that observational efforts designed to identify TDEs should not confine themselves to galactic nuclei alone, but should also consider the outer galactic halo where massive old GCs hosting IMBHs would reside. Indeed, such off-center TDEs as predicted here may already have been observed. MS TDE rates are more common than WD TDE rates by a factor of 30 (100) at z less than or similar to 0.5 (z = 2). We also calculate the rate of IMBH-SBH mergers across cosmic time, finding that the typical IMRI rate at low redshift is of the order of similar to 0.5-3 Gpc(-3) yr(-1), which becomes as high as similar to 100 Gpc(-3) yr(-1) near the peak of GC formation. Advanced LIGO, combined with VIRGO, KAGRA, the Einstein Telescope, and LISA will be able to observe the bottom end and top end of the IMBH population
Antiphase synchronization in a flagellar-dominance mutant of Chlamydomonas
Groups of beating flagella or cilia often synchronize so that neighboring filaments have identical frequencies and phases. A prime example is provided by the unicellular biflagellate Chlamydomonas reinhardtii, which typically displays synchronous in-phase beating in a low-Reynolds number version of breaststroke swimming. We report the discovery that ptx1, a flagellar-dominance mutant of C. reinhardtii, can exhibit synchronization in precise antiphase, as in the freestyle swimming stroke. High-speed imaging shows that ptx1 flagella switch stochastically between in-phase and antiphase states, and that the latter has a distinct waveform and significantly higher frequency, both of which are strikingly similar to those found during phase slips that stochastically interrupt in-phase beating of the wild-type. Possible mechanisms underlying these observations are discussed. Published by the American Physical Society under the terms of the Creative Commons Attribution 3.0 License. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.Support is acknowledged from the Spanish Ministerio de Ciencia y Innovación Grant No. FIS2010-22322-C01 and a Ramón y Cajal Fellowship (I. T.), an EPSRC postdoctoral Fellowship (M. P.), the BBSRC, the EPSRC, ERC Advanced Investigator Grant No. 247333, and a Senior Investigator Award from the Wellcome Trust (R. E. G.)Peer Reviewe
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