800 research outputs found
Weakly Nonlinear Theory of Pattern-Forming Systems with Spontaneously Broken Isotropy
Quasi two-dimensional pattern forming systems with spontaneously broken
isotropy represent a novel symmetry class, that is experimentally accessible in
electroconvection of homeotropically aligned liquid crystals. We present a
weakly nonlinear analysis leading to amplitude equations which couple the
short-wavelength patterning mode with the Goldstone mode resulting from the
broken isotropy. The new coefficients in these equations are calculated from
the hydrodynamics. Simulations exhibit a new type of spatio-temporal chaos at
onset. The results are compared with experiments.Comment: 4 pages, RevTeX, 4 PS-figures, to appear in PR
Specific heat and thermal conductivity in the mixed state of MgB2
The specific heat C and the electronic and phononic thermal conductivities
kappa_e and kappa_{ph} are calculated in the mixed state for magnetic fields H
near H_{c2}. The effects of supercurrent flow and Andreev scattering of the
Abrikosov vortex lattice on the quasiparticles are taken into account. The
resulting function C(H) is nearly linear while kappa_e(H) exhibits an upward
curvature near H_{c2}. The slopes decrease with impurity scattering which
improves the agreement with the data on MgB_2. The ratio of phonon relaxation
times tau_n/tau_s = g(omega_0,H) for phonon energy omega_0, which is nearly a
step function at omega_0 = 2Delta for the BCS state, is smeared out and tends
to one for increasing H. This leads to a rapid reduction of kappa_{ph}(H) in
MgB_2 for relatively small fields due to the rapid suppression of the smaller
energy gap.Comment: 8 pages, 4 figures, accepted for publication in Phys. Rev. Letter
Specific heat and thermal conductivity in the vortex state of the two-gap superconductor MgB_2
The specific heat coefficient gamma_s(H) and the electronic thermal
conductivity kappa_{es}(H) are calculated for Abrikosov's vortex lattice by
taking into account the effects of supercurrent flow and Andreev scattering.
First we solve the gap equation for the entire range of magnetic fields. We
take into account vertex corrections due to impurity scattering calculated in
the Born approximation. The function gamma_s(H)/gamma_n increases from zero and
becomes approximately linear above H/H_{c2} \sim 0.1. The dependence on
impurity scattering is substantially reduced by the vertex corrections. The
upward curvature of kappa_{es}(H)/kappa_{en}, which is caused by decreasing
Andreev scattering for increasing field, is reduced for increasing impurity
scattering. We also calculate the temperature dependence of the scattering
rates 1/tau_{ps}(H) of a phonon and 1/tau_{es}(H) of a quasiparticle due to
quasiparticle and phonon scattering, respectively. At low temperatures the
ratio tau_{pn}/tau_{ps}(H) increases rapidly to one as H tends to H_{c2} which
yields a rapid drop in the phononic thermal conductivity kappa_{ph}. Our
results are in qualitative agreement with the experiments on the two-gap
superconductor MgB_2.Comment: 12 pages, 5 figures, additions to figures 1, 2, and 3. Accepted by
Phys. Rev.
Defect chaos and bursts: Hexagonal rotating convection and the complex Ginzburg-Landau equation
We employ numerical computations of the full Navier-Stokes equations to investigate non-Boussinesq convection in a rotating system using water as the working fluid. We identify two regimes. For weak non- Boussinesq effects the Hopf bifurcation from steady to oscillating (whirling) hexagons is supercritical and typical states exhibit defect chaos that is systematically described by the cubic complex Ginzburg-Landau equation. For stronger non-Boussinesq effects the Hopf bifurcation becomes subcritical and the oscil- lations exhibit localized chaotic bursting, which is modeled by a quintic complex Ginzburg-Landau equation
Dynamics and Selection of Giant Spirals in Rayleigh-Benard Convection
For Rayleigh-Benard convection of a fluid with Prandtl number \sigma \approx
1, we report experimental and theoretical results on a pattern selection
mechanism for cell-filling, giant, rotating spirals. We show that the pattern
selection in a certain limit can be explained quantitatively by a
phase-diffusion mechanism. This mechanism for pattern selection is very
different from that for spirals in excitable media
“You’ve got to settle down!”: Mothers’ perceptions of physical activity in their young children
Abstract
Background
Mothers are important mediators of children’s physical activity (PA) level and risk of obesity, however previous studies of maternal perceptions of child PA have been limited. Furthermore, it is unknown if maternal perceptions of child PA are predicted by family, mother and child characteristics. Therefore objectives of this study were to 1) evaluate maternal perceptions of PA in their children and 2) test associations of family, mother and child characteristics with these perceptions.
Methods
278 low-income mothers of children (mean age 70.9 months) participated in an audio-taped semi-structured interview. Transcripts were systematically analyzed using the constant comparative method and themes were generated. A coding scheme to classify the themes appearing in each transcript was developed and reliably applied. Anthropometrics were measured. Demographics and questionnaires (the Confusion, Hubbub and Order Scale, The Parenting Scale, and the Child Behavior Questionnaire (CBQ)) were collected. Logistic regression models were used to test the associations of family, mother and child characteristics with each theme.
Results
In this sample of low-income United States mothers, two themes emerged: 1) Mothers perceive their children as already very active (87.8 %, n = 244), predicted by the child being younger, the child not being overweight, and higher child CBQ Activity Level; and 2) Mothers view their children’s high activity level as problematic (27.0 %, n = 75), predicted by lower Parenting Laxness, the child being male and lower child CBQ Inhibitory Control.
Conclusions
Low-income United States mothers have unique perceptions of PA in their children; these beliefs are associated with characteristics of the child and mother but not characteristics of the family. Further understanding of contributors to maternal perceptions of child PA may inform future childhood obesity interventions. The influence of these perceptions on physical activity outcomes in low-income children should be pursued in future research.http://deepblue.lib.umich.edu/bitstream/2027.42/114383/1/12887_2015_Article_466.pd
Whirling Hexagons and Defect Chaos in Hexagonal Non-Boussinesq Convection
We study hexagon patterns in non-Boussinesq convection of a thin rotating
layer of water. For realistic parameters and boundary conditions we identify
various linear instabilities of the pattern. We focus on the dynamics arising
from an oscillatory side-band instability that leads to a spatially disordered
chaotic state characterized by oscillating (whirling) hexagons. Using
triangulation we obtain the distribution functions for the number of pentagonal
and heptagonal convection cells. In contrast to the results found for defect
chaos in the complex Ginzburg-Landau equation and in inclined-layer convection,
the distribution functions can show deviations from a squared Poisson
distribution that suggest non-trivial correlations between the defects.Comment: 4 mpg-movies are available at
http://www.esam.northwestern.edu/~riecke/lit/lit.html submitted to New J.
Physic
ZAP-70, CTLA-4 and proximal T cell receptor signaling in cows infected with Mycobacterium avium subsp. paratuberculosis
Paratuberculosis is a chronic intestinal disease of ruminant animals caused by Mycobacterium avium subsp. paratuberculosis (MAP). A hallmark of paratuberculosis is a transition from a cell-mediated Th1 type response to a humoral Th2 response with the progression of disease from a subclinical to clinical state. The objective of this study was to investigate the expression of two crucial molecules in T cell function, ZAP-70 (zeta-chain-associated protein of 70kDa) and CTLA-4 (cytotoxic T-lymphocyte antigen-4), in cows naturally infected with MAP. Peripheral blood mononuclear cells (PBMCs) isolated from control non-infected cows (n = 5), and cows in subclinical (n = 6) and clinical stages of paratuberculosis (n = 6) were cultured alone (medium only), and with concanavalin A, and a whole cell sonicate of MAP for 24, 72 and 144 h to measure the dynamic changes of ZAP-70 and CTLA-4 expression on CD4, CD8, and gamma delta () T cells. Flow cytometry was also performed to measure ZAP-70 phosphorylation to examine proximal T cell receptor signaling in animals of different disease status. The surface expression of CTLA- 4 was increased in animals in subclinical stage of infection while levels of ZAP-70 were decreased in CD4+ T cells of both subclinical and clinical animals, indicating a change in T cell phenotype with disease state. Interestingly, proximal T cell receptor signaling was not altered in infected animals. This study demonstrated changes in crucial signaling molecules in animals infected with MAP, thereby elucidating T cell alterations associated with disease progression
Ultrasonic attenuation in magnetic fields for superconducting states with line nodes in Sr2RuO4
We calculate the ultrasonic attenuation in magnetic fields for
superconducting states with line nodes vertical or horizontal relative to the
RuO_2 planes. This theory, which is valid for fields near Hc2 and not too low
temperatures, takes into account the effects of supercurrent flow and Andreev
scattering by the Abrikosov vortex lattice. For rotating in-plane field
H(theta) the attenuation alpha(theta)exhibits variations of fourfold symmetry
in the rotation angle theta. In the case of vertical nodes, the transverse T100
sound mode yields the weakest(linear)H and T dependence of alpha, while the
longitudinal L100 mode yields a stronger (quadratic) H and T dependence. This
is in strong contrast to the case of horizontal line nodes where alpha is the
same for the T100 and L100 modes (apart from a shift of pi/4 in field
direction) and is roughly a quadratic function of H and T. Thus we conclude
that measurements of alpha in in-plane magnetic fields for different in-plane
sound modes may be an important tool for probing the nodal structure of the gap
in Sr_2RuO_4.Comment: 5 pages, 6 figures, replaced in non-preprint form, to appear in Phys.
Rev.
Four nearby L dwarfs
We present spectroscopic, photometric and astrometric observations of four
bright L dwarfs identified in the course of the 2MASS near-infrared survey. Our
spectroscopic data extend to wavelengths shortward of 5000\AA in the L0 dwarf
2MASSJ0746+2000 and the L4 dwarf 2MASSJ0036+1840, allowing the identification
of absorption bands due to MgH and CaOH. The atomic resonance lines Ca I
4227\AA and Na I 5890/5896\AA are extremely strong, with the latter having an
equivalent width of 240\AA in the L4 dwarf. By spectral type L5, the D lines
extend over \AA and absorb a substantial fraction of the flux emitted
in the V band, with a corresponding effect on the (V-I) broadband colour. The
KI resonance doublet at 7665/7699\AA increases in equivalent width from
spectral type M3 to M7, but decreases in strength from M7 to L0 before
broadening substantially at later types. These variations are likely driven by
dust formation in these cool atmospheres.Comment: to appear in AJ, January 2000; 27 pages, including 3 tables and 7
figures embedded in the tex
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