10,064 research outputs found
Electric field distortions in structures of the twist bend nematic (NTB) phase of a bent-core liquid crystal
Dielectric spectroscopy of a twist bend nematic phase of an achiral bent core
liquid crystalline compound under DC bias is used to investigate its response
to electric field. Two collective relaxation processes are revealed, these are
assigned to distortions of helicoidal structure by the external bias field.
Frequency of the mode depends primarily on the helicoidal angle and has
anomalous, softening- like behaviour at the nematic to the twist bend nematic
transition. A coupling of dielectric anisotropy with electric field gives rise
to a new equilibrium periodic structure in the time scale involved. The modulus
of the wave vector gradually vanishes on increasing the bias field (except for
the initial behaviour, which is just the opposite). Transition from the twist
bend to the splay bend structure is clearly observed by a sudden drop in the
frequency of this mode, which decreases almost linearly with increasing field.
Results agree with predictions from current models for the periodically
distorted a twist bend nematic phase.Comment: 14 PAGES, 7 FIGURES, submitted to Physical Review Letter
Coccolithophore fluxes in the open tropical North Atlantic: influence of thermocline depth, Amazon water, and Saharan dust
Coccolithophores are calcifying phytoplankton and major contributors to both the organic and inorganic oceanic carbon pumps. Their export fluxes, species composition, and seasonal patterns were determined in two sediment trap moorings (M4 at 12 degrees N, 49 degrees W and M2 at 14 degrees N, 37 degrees W) collecting settling particles synchronously from October 2012 to November 2013 at 1200 m of water depth in the open equatorial North Atlantic. The two trap locations showed a similar seasonal pattern in total coccolith export fluxes and a predominantly tropical coccolithophore settling assemblage. Species fluxes were dominated throughout the year by lower photic zone (LPZ) taxa (Florisphaera profunda, Gladiolithus flabellatus) but also included upper photic zone (UPZ) taxa (Umbellosphaera spp., Rhabdosphaera spp., Umbilicosphaera spp., Helicosphaera spp.). The LPZ flora was most abundant during fall 2012, whereas the UPZ flora was more important during summer. In spite of these similarities, the western part of the study area produced persistently higher fluxes, averaging 241 x 10(7) +/- 76 x 10(7) coccoliths m(-2) d(-1) at station M4 compared to only 66 x 10(7) +/- 31 x 10(7) coccoliths m(-2) d(-1) at station M2. Higher fluxes at M4 were mainly produced by the LPZ species, favoured by the westward deepening of the thermocline and nutricline. Still, most UPZ species also contributed to higher fluxes, reflecting enhanced productivity in the western equatorial North Atlantic. Such was the case of two marked flux peaks of the more opportunistic species Gephyrocapsa muellerae and Emiliania huxleyi in January and April 2013 at M4, indicating a fast response to the nutrient enrichment of the UPZ, probably by wind-forced mixing. Later, increased fluxes of G. oceanica and E. huxleyi in October-November 2013 coincided with the occurrence of Amazon-River-affected surface waters. Since the spring and fall events of 2013 were also accompanied by two dust flux peaks, we propose a scenario in which atmospheric dust also provided fertilizing nutrients to this area. Enhanced surface buoyancy associated with the river plume indicates that the Amazon acted not only as a nutrient source, but also as a surface density retainer for nutrients supplied from the atmosphere. Nevertheless, lower total coccolith fluxes during these events compared to the maxima recorded in November 2012 and July 2013 indicate that transient productivity by opportunistic species was less important than "background" tropical productivity in the equatorial North Atlantic. This study illustrates how two apparently similar sites in the tropical open ocean actually differ greatly in ecological and oceanographic terms. The results presented here provide valuable insights into the processes governing the ecological dynamics and the downward export of coccolithophores in the tropical North Atlantic.Netherlands Organization for Scientific Research (NWO) [822.01.008]; European Research Council (ERC) [311152]; University of Bremen; European Union [600411]info:eu-repo/semantics/publishedVersio
funcX: A Federated Function Serving Fabric for Science
Exploding data volumes and velocities, new computational methods and
platforms, and ubiquitous connectivity demand new approaches to computation in
the sciences. These new approaches must enable computation to be mobile, so
that, for example, it can occur near data, be triggered by events (e.g.,
arrival of new data), be offloaded to specialized accelerators, or run remotely
where resources are available. They also require new design approaches in which
monolithic applications can be decomposed into smaller components, that may in
turn be executed separately and on the most suitable resources. To address
these needs we present funcX---a distributed function as a service (FaaS)
platform that enables flexible, scalable, and high performance remote function
execution. funcX's endpoint software can transform existing clouds, clusters,
and supercomputers into function serving systems, while funcX's cloud-hosted
service provides transparent, secure, and reliable function execution across a
federated ecosystem of endpoints. We motivate the need for funcX with several
scientific case studies, present our prototype design and implementation, show
optimizations that deliver throughput in excess of 1 million functions per
second, and demonstrate, via experiments on two supercomputers, that funcX can
scale to more than more than 130000 concurrent workers.Comment: Accepted to ACM Symposium on High-Performance Parallel and
Distributed Computing (HPDC 2020). arXiv admin note: substantial text overlap
with arXiv:1908.0490
GINA - A Polarized Neutron Reflectometer at the Budapest Neutron Centre
The setup, capabilities and operation parameters of the neutron reflectometer
GINA, the recently installed "Grazing Incidence Neutron Apparatus" at the
Budapest Neutron Centre, are introduced. GINA, a dance-floor-type,
constant-energy, angle-dispersive reflectometer is equipped with a 2D
position-sensitive detector to study specular and off-specular scattering.
Wavelength options between 3.2 and 5.7 {\AA} are available for unpolarized and
polarized neutrons. Spin polarization and analysis are achieved by magnetized
transmission supermirrors and radio-frequency adiabatic spin flippers. As a
result of vertical focusing by the five-element (pyrolytic graphite)
monochromator the reflected intensity from a 20x20 mm sample has doubled. GINA
is dedicated to studies of magnetic films and heterostructures, but unpolarized
options for non-magnetic films, membranes and other surfaces are also provided.
Shortly after its startup, reflectivity values as low as 3x10-5 have been
measured on the instrument. The facility is now open for the international user
community, but its development is continuing mainly to establish new sample
environment options, the spin analysis of off-specularly scattered radiation
and further decrease of the background
Inelastic scattering of light by a cold trapped atom: Effects of the quantum center-of-mass motion
The light scattered by a cold trapped ion, which is in the stationary state
of laser cooling, presents features due to the mechanical effects of
atom-photon interaction. These features appear as additional peaks (sidebands)
in the spectrum of resonance fluorescence. Among these sidebands the literature
has discussed the Stokes and anti-Stokes components, namely the sidebands of
the elastic peak. In this manuscript we show that the motion also gives rise to
sidebands of the inelastic peaks. These are not always visible, but, as we
show, can be measured in parameter regimes which are experimentally accessible.Comment: 10 pages, 4 figures, submitted to Phys. Rev.
Testing a bead-rod contact with a nonlinear resonance method
International audienceWe study the dynamics of an elastic structure composed of a cylindrical rod in contact with a bead at one extremity. Wave propagation within the cylindrical rod is considered linear and dispersionless while the bead-rod contact shows a highly nonlinear behavior as expected from the Hertz's model of contact. The resonance curves of the nonlinear contact depend on the excitation amplitude, where a downshift of the resonance frequency with increasing exci-tation amplitude is observed. The prediction of the resonance frequency shift by the Hertz's model is compared to the experimental results and shows a disagreement. A better agreement is found by considering the losses with a viscoelastic model, namely the Kuwabara and Kono or Brilliantov model. The observation of the nonlinear effects linked to the resonance of the mass-spring system can lead to the design of nonlinear elastic metamaterials, where the wave propagation is controlled by nonlinear isolated resonators
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