2,266 research outputs found
Optical microrheology using rotating laser-trapped particles
We demonstrate an optical system that can apply and accurately measure the
torque exerted by the trapping beam on a rotating birefringent probe particle.
This allows the viscosity and surface effects within liquid media to be
measured quantitatively on a micron-size scale using a trapped rotating
spherical probe particle. We use the system to measure the viscosity inside a
prototype cellular structure.Comment: 5 pages, 4 figures. v2: bibliographic details, minor text correction
Long-time discrete particle effects versus kinetic theory in the self-consistent single-wave model
The influence of the finite number N of particles coupled to a monochromatic
wave in a collisionless plasma is investigated. For growth as well as damping
of the wave, discrete particle numerical simulations show an N-dependent long
time behavior resulting from the dynamics of individual particles. This
behavior differs from the one due to the numerical errors incurred by Vlasov
approaches. Trapping oscillations are crucial to long time dynamics, as the
wave oscillations are controlled by the particle distribution inhomogeneities
and the pulsating separatrix crossings drive the relaxation towards thermal
equilibrium.Comment: 11 pages incl. 13 figs. Phys. Rev. E, in pres
Abelian symmetries in multi-Higgs-doublet models
N-Higgs doublet models (NHDM) are a popular framework to construct
electroweak symmetry breaking mechanisms beyond the Standard model. Usually,
one builds an NHDM scalar sector which is invariant under a certain symmetry
group. Although several such groups have been used, no general analysis of
symmetries possible in the NHDM scalar sector exists. Here, we make the first
step towards this goal by classifying the elementary building blocks, namely
the abelian symmetry groups, with a special emphasis on finite groups. We
describe a strategy that identifies all abelian groups which are realizable as
symmetry groups of the NHDM Higgs potential. We consider both the groups of
Higgs-family transformations only and the groups which also contain generalized
CP transformations. We illustrate this strategy with the examples of 3HDM and
4HDM and prove several statements for arbitrary N.Comment: 33 pages, 2 figures; v2: conjecture 3 is proved and becomes theorem
3, more explanations of the main strategy are added, matches the published
versio
L-Edge Spectroscopy of Dilute, Radiation-Sensitive Systems Using a Transition-Edge-Sensor Array
We present X-ray absorption spectroscopy and resonant inelastic X-ray
scattering (RIXS) measurements on the iron L-edge of 0.5 mM aqueous
ferricyanide. These measurements demonstrate the ability of high-throughput
transition-edge-sensor (TES) spectrometers to access the rich soft X-ray
(100-2000eV) spectroscopy regime for dilute and radiation-sensitive samples.
Our low-concentration data are in agreement with high-concentration
measurements recorded by conventional grating-based spectrometers. These
results show that soft X-ray RIXS spectroscopy acquired by high-throughput TES
spectrometers can be used to study the local electronic structure of dilute
metal-centered complexes relevant to biology, chemistry and catalysis. In
particular, TES spectrometers have a unique ability to characterize frozen
solutions of radiation- and temperature-sensitive samples.Comment: 19 pages, 4 figure
Evidence for early life in Earth’s oldest hydrothermal vent precipitates
Although it is not known when or where life on Earth began, some of the earliest habitable environments may have been submarine-hydrothermal vents. Here we describe putative fossilized microorganisms that are at least 3,770 million and possibly 4,280 million years old in ferruginous sedimentary rocks, interpreted as seafloor-hydrothermal vent-related precipitates, from the Nuvvuagittuq belt in Quebec, Canada. These structures occur as micrometre-scale haematite tubes and filaments with morphologies and mineral assemblages similar to those of filamentous microorganisms from modern hydrothermal vent precipitates and analogous microfossils in younger rocks. The Nuvvuagittuq rocks contain isotopically light carbon in carbonate and carbonaceous material, which occurs as graphitic inclusions in diagenetic carbonate rosettes, apatite blades intergrown among carbonate rosettes and magnetite–haematite granules, and is associated with carbonate in direct contact with the putative microfossils. Collectively, these observations are consistent with an oxidized biomass and provide evidence for biological activity in submarine-hydrothermal environments more than 3,770 million years ago
Physics on the edge: contour dynamics, waves and solitons in the quantum Hall effect
We present a theoretical study of the excitations on the edge of a
two-dimensional electron system in a perpendicular magnetic field in terms of a
contour dynamics formalism. In particular, we focus on edge excitations in the
quantum Hall effect. Beyond the usual linear approximation, a non-linear
analysis of the shape deformations of an incompressible droplet yields soliton
solutions which correspond to shapes that propagate without distortion. A
perturbative analysis is used and the results are compared to analogous
systems, like vortex patches in ideal hydrodynamics. Under a local induction
approximation we find that the contour dynamics is described by a non-linear
partial differential equation for the curvature: the modified Korteweg-de Vries
equation.
PACS number(s): 73.40.Hm, 02.40.Ma, 03.40.Gc, 11.10.LmComment: 15 pages, 12 embedded figures, submitted to Phys. Rev.
Small BGK waves and nonlinear Landau damping
Consider 1D Vlasov-poisson system with a fixed ion background and periodic
condition on the space variable. First, we show that for general homogeneous
equilibria, within any small neighborhood in the Sobolev space W^{s,p}
(p>1,s<1+(1/p)) of the steady distribution function, there exist nontrivial
travelling wave solutions (BGK waves) with arbitrary minimal period and
traveling speed. This implies that nonlinear Landau damping is not true in
W^{s,p}(s<1+(1/p)) space for any homogeneous equilibria and any spatial period.
Indeed, in W^{s,p} (s<1+(1/p)) neighborhood of any homogeneous state, the long
time dynamics is very rich, including travelling BGK waves, unstable
homogeneous states and their possible invariant manifolds. Second, it is shown
that for homogeneous equilibria satisfying Penrose's linear stability
condition, there exist no nontrivial travelling BGK waves and unstable
homogeneous states in some W^{s,p} (p>1,s>1+(1/p)) neighborhood. Furthermore,
when p=2,we prove that there exist no nontrivial invariant structures in the
H^{s} (s>(3/2)) neighborhood of stable homogeneous states. These results
suggest the long time dynamics in the W^{s,p} (s>1+(1/p)) and particularly, in
the H^{s} (s>(3/2)) neighborhoods of a stable homogeneous state might be
relatively simple. We also demonstrate that linear damping holds for initial
perturbations in very rough spaces, for linearly stable homogeneous state. This
suggests that the contrasting dynamics in W^{s,p} spaces with the critical
power s=1+(1/p) is a trully nonlinear phenomena which can not be traced back to
the linear level
AI-Driven Assessment of Students: Current Uses and Research Trends
During the last decade, the use of AIs is being incorporated into the
educational field whether to support the analysis of human behavior in teachinglearning
contexts, as didactic resource combined with other technologies or as a
tool for the assessment of the students.
This proposal presents a Systematic Literature Review and mapping study on
the use of AIs for the assessment of students that aims to provide a general
overview of the state of the art and identify the current areas of research by
answering 6 research questions related with the evolution of the field, and the
geographic and thematic distribution of the studies.
As a result of the selection process this study identified 20 papers focused on
the research topic in the repositories SCOPUS and Web of Science from an
initial amount of 129.
The analysis of the papers allowed the identification of three main thematic
categories: assessment of student behaviors, assessment of student sentiments
and assessment of student achievement as well as several gaps in the literature
and future research lines addressed in the discussion
Global existence for a system of non-linear and non-local transport equations describing the dynamics of dislocation densities
In this paper, we study the global in time existence problem for the
Groma-Balogh model describing the dynamics of dislocation densities. This model
is a two-dimensional model where the dislocation densities satisfy a system of
transport equations such that the velocity vector field is the shear stress in
the material, solving the equations of elasticity. This shear stress can be
expressed as some Riesz transform of the dislocation densities. The main tool
in the proof of this result is the existence of an entropy for this syste
Cancer risk in childhood-onset systemic lupus
INTRODUCTION: The aim of this study was to assess cancer incidence in childhood-onset systemic lupus erythematosus (SLE). METHODS: We ascertained cancers within SLE registries at 10 pediatric centers. Subjects were linked to cancer registries for the observational interval, spanning 1974 to 2009. The ratio of observed to expected cancers represents the standardized incidence ratio (SIR) or relative cancer risk in childhood-onset SLE, versus the general population. RESULTS: There were 1020 patients aged <18 at cohort entry. Most (82%) were female and Caucasian; mean age at cohort entry was 12.6 years (standard deviation (SD) = 3.6). Subjects were observed for a total of 7,986 (average 7.8) patient-years. Within this interval, only three invasive cancers were expected. However, 14 invasive cancers occurred with an SIR of 4.7, 95% confidence interval (CI) 2.6 to 7.8. Three hematologic cancers were found (two non-Hodgkin’s lymphoma, one leukemia), for an SIR of 5.2 (95% CI 1.1 to 15.2). The SIRs stratified by age group and sex, were similar across these strata. There was a trend for highest cancer occurrence 10 to 19 years after SLE diagnosis. CONCLUSIONS: These results suggest an increased cancer risk in pediatric onset SLE versus the general population. In absolute terms, this represents relatively few events. Of note, risk may be highest only after patients have transferred to adult care
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