371 research outputs found
Modulated structures in electroconvection in nematic liquid crystals
Motivated by experiments in electroconvection in nematic liquid crystals with
homeotropic alignment we study the coupled amplitude equations describing the
formation of a stationary roll pattern in the presence of a weakly-damped mode
that breaks isotropy. The equations can be generalized to describe the planarly
aligned case if the orienting effect of the boundaries is small, which can be
achieved by a destabilizing magnetic field. The slow mode represents the
in-plane director at the center of the cell. The simplest uniform states are
normal rolls which may undergo a pitchfork bifurcation to abnormal rolls with a
misaligned in-plane director.We present a new class of defect-free solutions
with spatial modulations perpendicular to the rolls. In a parameter range where
the zig-zag instability is not relevant these solutions are stable attractors,
as observed in experiments. We also present two-dimensionally modulated states
with and without defects which result from the destabilization of the
one-dimensionally modulated structures. Finally, for no (or very small)
damping, and away from the rotationally symmetric case, we find static chevrons
made up of a periodic arrangement of defect chains (or bands of defects)
separating homogeneous regions of oblique rolls with very small amplitude.
These states may provide a model for a class of poorly understood stationary
structures observed in various highly-conducting materials ("prechevrons" or
"broad domains").Comment: 13 pages, 13 figure
Three-dimensional pattern formation, multiple homogeneous soft modes, and nonlinear dielectric electroconvection
Patterns forming spontaneously in extended, three-dimensional, dissipative
systems are likely to excite several homogeneous soft modes (
hydrodynamic modes) of the underlying physical system, much more than quasi
one- and two-dimensional patterns are. The reason is the lack of damping
boundaries. This paper compares two analytic techniques to derive the patten
dynamics from hydrodynamics, which are usually equivalent but lead to different
results when applied to multiple homogeneous soft modes. Dielectric
electroconvection in nematic liquid crystals is introduced as a model for
three-dimensional pattern formation. The 3D pattern dynamics including soft
modes are derived. For slabs of large but finite thickness the description is
reduced further to a two-dimensional one. It is argued that the range of
validity of 2D descriptions is limited to a very small region above threshold.
The transition from 2D to 3D pattern dynamics is discussed. Experimentally
testable predictions for the stable range of ideal patterns and the electric
Nusselt numbers are made. For most results analytic approximations in terms of
material parameters are given.Comment: 29 pages, 2 figure
Understanding the local structure of Eu3+- and Y3+-stabilized zirconia: insights from luminescence and X-ray absorption spectroscopic investigations
This study combines bulk structural and spectroscopic investigations of Eu- or Y/Eu co-doped tetragonal and cubic zirconia polymorphs to gain an indepth understanding of the solid solution formation process. Our bulk structural characterizations show that the dopant is homogenously distributed in the ZrO host structure resulting in an increase of the bulk symmetry with increasing dopant substitution (from 8 to 26 mol%). The local site symmetry around the Eu dopant, however, determined with luminescence spectroscopy (TRLFS), remains low in all samples. Results obtained with X-ray pair distribution function and X-ray absorption spectroscopy show that the average coordination environment in the stabilized zirconia structures remains practically unchanged. Despite this very constant average dopant environment, siteselective TRLFS data show the presence of three nonequivalent Eu environments in the ZrO solid structures. These Eu environments are assumed to arise from Eu incorporation at superficial sites, which increase in abundance as the size of the crystallites decrease, and incorporation on two bulk sites differing in the location of the oxygen vacancies with respect to the dopant cation
1D-confinement of polyiodides inside single-wall carbon nanotubes
International audience1D-confinement of polyiodides inside single-wall carbon nanotubes (SWCNT) is investigated. Structural arrangement of iodine species as a function of the SWCNT diameters is studied. Evidence for long range one dimensional ordering of the iodine species is shown by X-ray and electron diffraction experiments independently of the tube diameter. The structure of the confined polyiodides is investigated by X-ray absorption spectroscopy. The confinement influences the local arrangement of the chains. Below a critical diameter Fc of 1 nm, long linear polyiodides are evidenced leading to a weaker charge transfer than for nanotube diameter above Fc. A shortening of the polyiodides is exhibited with the increase of the nanotube diameter leading to a more efficient charge transfer. This point reflects the 1D-confinement of the polyiodides inside the nanotubes
Structural defects in Hg1âxCdxI2 layers grown on CdTe substrates by vapor phase epitaxy
Hg1âxCdxI2 20â25-ÎŒm-thick layers with a uniform composition in the range of x = 0.1â0.2 were grown on CdTe substrates by vapor phase epitaxy (VPE). The growth was carried out using an α-HgI2 polycrystalline source at 200â°C and in the time range of 30â100 h. The layers were studied by scanning electron microscopy (SEM) and high resolution synchrotron x-ray topography (SXRT). The SEM and SXRT images of Hg1âxCdxI2 VPE layers allow one to identify the defects affecting the layer structure. The two main types of structural defects in the layers are subgrain boundaries and densely spaced striations similar to those referred generally to as vapor grown HgI2 bulk crystals. The effect of the growth time on these defects has been analyzed and on the basis of this it has been possible to grow Hg1âxCdxI2 layers with low defect [email protected]
Analysis of cross-correlations in electroencephalogram signals as an approach to proactive diagnosis of schizophrenia
We apply flicker-noise spectroscopy (FNS), a time series analysis method
operating on structure functions and power spectrum estimates, to study the
clinical electroencephalogram (EEG) signals recorded in children/adolescents
(11 to 14 years of age) with diagnosed schizophrenia-spectrum symptoms at the
National Center for Psychiatric Health (NCPH) of the Russian Academy of Medical
Sciences. The EEG signals for these subjects were compared with the signals for
a control sample of chronically depressed children/adolescents. The purpose of
the study is to look for diagnostic signs of subjects' susceptibility to
schizophrenia in the FNS parameters for specific electrodes and
cross-correlations between the signals simultaneously measured at different
points on the scalp. Our analysis of EEG signals from scalp-mounted electrodes
at locations F3 and F4, which are symmetrically positioned in the left and
right frontal areas of cerebral cortex, respectively, demonstrates an essential
role of frequency-phase synchronization, a phenomenon representing specific
correlations between the characteristic frequencies and phases of excitations
in the brain. We introduce quantitative measures of frequency-phase
synchronization and systematize the values of FNS parameters for the EEG data.
The comparison of our results with the medical diagnoses for 84 subjects
performed at NCPH makes it possible to group the EEG signals into 4 categories
corresponding to different risk levels of subjects' susceptibility to
schizophrenia. We suggest that the introduced quantitative characteristics and
classification of cross-correlations may be used for the diagnosis of
schizophrenia at the early stages of its development.Comment: 36 pages, 6 figures, 2 tables; to be published in "Physica A
Data driven optimal filtering for phase and frequency of noisy oscillations: application to vortex flowmetering
A new method for extracting the phase of oscillations from noisy time series
is proposed. To obtain the phase, the signal is filtered in such a way that the
filter output has minimal relative variation in the amplitude (MIRVA) over all
filters with complex-valued impulse response. The argument of the filter output
yields the phase. Implementation of the algorithm and interpretation of the
result are discussed. We argue that the phase obtained by the proposed method
has a low susceptibility to measurement noise and a low rate of artificial
phase slips. The method is applied for the detection and classification of mode
locking in vortex flowmeters. A novel measure for the strength of mode locking
is proposed.Comment: 12 pages, 10 figure
A Grassmann integral equation
The present study introduces and investigates a new type of equation which is
called Grassmann integral equation in analogy to integral equations studied in
real analysis. A Grassmann integral equation is an equation which involves
Grassmann integrations and which is to be obeyed by an unknown function over a
(finite-dimensional) Grassmann algebra G_m. A particular type of Grassmann
integral equations is explicitly studied for certain low-dimensional Grassmann
algebras. The choice of the equation under investigation is motivated by the
effective action formalism of (lattice) quantum field theory. In a very general
setting, for the Grassmann algebras G_2n, n = 2,3,4, the finite-dimensional
analogues of the generating functionals of the Green functions are worked out
explicitly by solving a coupled system of nonlinear matrix equations. Finally,
by imposing the condition G[{\bar\Psi},{\Psi}] = G_0[{\lambda\bar\Psi},
{\lambda\Psi}] + const., 0<\lambda\in R (\bar\Psi_k, \Psi_k, k=1,...,n, are the
generators of the Grassmann algebra G_2n), between the finite-dimensional
analogues G_0 and G of the (``classical'') action and effective action
functionals, respectively, a special Grassmann integral equation is being
established and solved which also is equivalent to a coupled system of
nonlinear matrix equations. If \lambda \not= 1, solutions to this Grassmann
integral equation exist for n=2 (and consequently, also for any even value of
n, specifically, for n=4) but not for n=3. If \lambda=1, the considered
Grassmann integral equation has always a solution which corresponds to a
Gaussian integral, but remarkably in the case n=4 a further solution is found
which corresponds to a non-Gaussian integral. The investigation sheds light on
the structures to be met for Grassmann algebras G_2n with arbitrarily chosen n.Comment: 58 pages LaTeX (v2: mainly, minor updates and corrections to the
reference section; v3: references [4], [17]-[21], [39], [46], [49]-[54],
[61], [64], [139] added
A general framework for combining ecosystem models
When making predictions about ecosystems, we often have available a number of different ecosystem models that attempt to represent their dynamics in a detailed mechanistic way. Each of these can be used as a simulator of large-scale experiments and make projections about the fate of ecosystems under different scenarios to support the development of appropriate management strategies. However, structural differences, systematic discrepancies and uncertainties lead to different models giving different predictions. This is further complicated by the fact that the models may not be run with the same functional groups, spatial structure or time scale. Rather than simply trying to select a âbestâ model, or taking some weighted average, it is important to exploit the strengths of each of the models, while learning from the differences between them. To achieve this, we construct a flexible statistical model of the relationships between a collection of mechanistic models and their biases, allowing for structural and parameter uncertainty and for different ways of representing reality. Using this statistical meta-model, we can combine prior beliefs, model estimates and direct observations using Bayesian methods and make coherent predictions of future outcomes under different scenarios with robust measures of uncertainty. In this study, we take a diverse ensemble of existing North Sea ecosystem models and demonstrate the utility of our framework by applying it to answer the question what would have happened to demersal fish if fishing was to stop
Steeper size spectra with decreasing phytoplankton biomass indicate strong trophic amplification and future fish declines
Under climate change, model ensembles suggest that declines in phytoïżœplankton biomass amplify into greater reductions at higher trophic levels, with
serious implications for fisheries and carbon storage. However, the extent and
mechanisms of this trophic amplification vary greatly among models, and
validation is problematic. In situ size spectra offer a novel alternative, comïżœparing biomass of small and larger organisms to quantify the net efficiency of
energy transfer through natural food webs that are already challenged with
multiple climate change stressors. Our global compilation of pelagic size
spectrum slopes supports trophic amplification empirically, independently
from model simulations. Thus, even a modest (16%) decline in phytoplankton
this century would magnify into a 38% decline in supportable biomass of fish
within the intensively-fished mid-latitude ocean. We also show that this
amplification stems not from thermal controls on consumers, but mainly from
temperature or nutrient controls that structure the phytoplankton baseline of
the food web. The lack of evidence for direct thermal effects on size structure
contrasts with most current thinking, based often on more acute stress
experiments or shorter-timescale responses. Our synthesis of size spectra
integrates these short-term dynamics, revealing the net efficiency of food
webs acclimating and adapting to climatic stressor
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