6,403 research outputs found
Enhancement of mobilities in a pinned multidomain crystal
Mobility properties inside and around degenerate domains of an elastic
lattice partially pinned on a square array of traps are explored by means of a
fully controllable model system of macroscopic particles. We focus on the
different configurations obtained for filling ratios equal to 1 or 2 when the
pinning strength is lowered. These theoretically expected but never observed
configurations are degenerated, which implies the existence of a multidomain
crystal. We show that the distinction between trapped and untrapped particles
that is made in the case of strong pinning is not relevant for such a weaker
pinning. Indeed, one ought to distinguish between particles inside or around
the domains associated to positional degeneracies. The possible consequences on
the depinning dynamics of the lattice are discussed.Comment: 7 pages, 10 figures Version 2 : longer versio
The dead ringer/retained transcriptional regulatory gene is required for positioning of the longitudinal glia in the Drosophila embryonic CNS
The Drosophila dead ringer (dri, also known as retained, retn) gene encodes a nuclear protein with a conserved DNA-binding domain termed the ARID (AT-rich interaction domain). We show here that dri is expressed in a subset of longitudinal glia in the Drosophila embryonic central nervous system and that dri forms part of the transcriptional regulatory cascade required for normal development of these cells. Analysis of mutant embryos revealed a role for dri in formation of the normal embryonic CNS. Longitudinal glia arise normally in dri mutant embryos, but they fail to migrate to their final destinations. Disruption of the spatial organization of the dri-expressing longitudinal glia accounts for the mild defects in axon fasciculation observed in the mutant embryos. Consistent with the late phenotypes observed, expression of the glial cells missing (gcm) and reversed polarity (repo) genes was found to be normal in dri mutant embryos. However, from stage 15 of embryogenesis, expression of locomotion defects (loco) and prospero (pros) was found to be missing in a subset of LG. This suggests that loco and pros are targets of DRI transcriptional activation in some LG. We conclude that dri is an important regulator of the late development of longitudinal glia
Neutron scattering study of ferroelectric Sn2P2S6 under pressure
Ferroelectric phase transition in the semiconductor Sn2P2S6 single crystal
has been studied by means of neutron scattering in the pressure-temperature
range adjacent to the anticipated tricritical Lifshitz point (p=0.18GPa,
T=296K). The observations reveal a direct ferroelectric-paraelectric phase
transition in the whole investigated pressure range (0.18 - 0.6GPa). These
results are in a clear disagreement with phase diagrams assumed in numerous
earlier works, according to which a hypothetical intermediate incommensurate
phase extends over several or even tens of degrees in the 0.5GPa pressure
range. Temperature dependence of the anisotropic quasielastic diffuse
scattering suggests that polarization fluctuations present above TC are
strongly reduced in the ordered phase. Still, the temperature dependence of the
(200) Bragg reflection intensity at p=0.18GPa can be remarkably well modeled
assuming the order-parameter amplitude growth according to the power law with
logarithmic corrections predicted for a uniaxial ferroelectric transition at
the tricritical Lifshitz point
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Assessing the Influence of Letter Position in Reading Normal and Transposed Texts Using a Letter Detection Task
During word recognition, some letters appear to play a more important role than others. Although some studies have suggested that the first and last letters of a word have a privileged status, there is no consensus with regards to the importance of the different letter positions when reading connected text. In the current experiments, we used a simple letter search task to examine the impact of letter position on word identification in connected text using a classic paper and pencil procedure (Experiment 1) and an eye movement monitoring procedure (Experiment 2). In Experiments 3 and 4, a condition with transposed letters was included. Our results show that the first letter of a word is detected more easily than the other letters, and transposing letters in a word revealed the importance of the final letter. It is concluded that both the initial and final letters play a special role in word identification during reading but that the underlying processes might differ
Paramagnetic Meissner effect in superconductors from self-consistent solutions of Ginzburg-Landau equations
The paramagnetic Meissner effect (PME) is observed in small superconducting
samples, and a number of controversial explanations of this effect are
proposed, but there is as yet no clear understanding of its nature. In the
present paper PME is considered on the base of the Ginzburg-Landau theory (GL).
The one-dimensional solutions are obtained in a model case of a long
superconducting cylinder for different cylinder radii R, the GL-parameters
\kappa and vorticities m. Acording to GL-theory, PME is caused by the presence
of vortices inside the sample. The superconducting current flows around the
vortex to screeen the vortex own field from the bulk of the sample. Another
current flows at the boundary to screen the external field H from entering the
sample. These screening currents flow in opposite directions and contribute
with opposite signs to the total magnetic moment (or magnetization) of the
sample. Depending on H, the total magnetization M may be either negative
(diamagnetism), or positive (paramagnetism). A very complicated saw-like
dependence M(H) (and other characteristics), which are obtained on the base of
self-consistent solutions of the GL-equations, are discussed.Comment: 6 pages, 5 figures, RevTex, submitted to Phys. Rev.
Vortices in small superconducting disks
We study the Ginzburg-Landau equations in order to describe a two-dimensional
superconductor in a bounded domain. Using the properties of a particular
integrability point () of these nonlinear equations which
allows vortex solutions, we obtain a closed expression for the energy of the
superconductor. The presence of the boundary provides a selection mechanism for
the number of vortices.
A perturbation analysis around enables us to include the
effects of the vortex interactions and to describe quantitatively the
magnetization curves recently measured on small superconducting disks.
We also calculate the optimal vortex configuration and obtain an expression
for the confining potential away from the London limit.Comment: 4 pages, to be published in Physica C (Superconductivity
Association of Cumulative Lead Exposure with Parkinson's Disease
BACKGROUND. Research using reconstructed exposure histories has suggested an association between heavy metal exposures, including lead, and Parkinson's disease (PD), but the only study that used bone lead, a biomarker of cumulative lead exposure, found a nonsignificant increase in risk of PD with increasing bone lead. OBJECTIVES. We sought to assess the association between bone lead and PD. METHODS. Bone lead concentrations were measured using 109Cd excited K-shell X-ray fluorescence from 330 PD patients (216 men, 114 women) and 308 controls (172 men, 136 women) recruited from four clinics for movement disorders and general-community cohorts. Adjusted odds ratios (ORs) for PD were calculated using logistic regression. RESULTS. The average age of cases and controls at bone lead measurement was 67 (SD = 10) and 69 (SD = 9) years of age, respectively. In primary analyses of cases and controls recruited from the same groups, compared with the lowest quartile of tibia lead, the OR for PD in the highest quartile was 3.21 [95% confidence interval (CI), 1.17-8.83]. Results were similar but slightly weaker in analyses restricted to cases and controls recruited from the movement disorders clinics only (fourth-quartile OR = 2.57; 95% CI, 1.11-5.93) or when we included controls recruited from sites that did not also contribute cases (fourth-quartile OR = 1.91; 95% CI, 1.01-3.60). We found no association with patella bone lead. CONCLUSIONS. These findings, using an objective biological marker of cumulative lead exposure among typical PD patients seen in our movement disorders clinics, strengthen the evidence that cumulative exposure to lead increases the risk of PD.National Institutes of Health (R01-ES010798, K01-ES01265
Determination of the critical current density in the d-wave superconductor YBCO under applied magnetic fields by nodal tunneling
We have studied nodal tunneling into YBa2Cu3O7-x (YBCO) films under magnetic
fields. The films' orientation was such that the CuO2 planes were perpendicular
to the surface with the a and b axis at 450 form the normal. The magnetic field
was applied parallel to the surface and perpendicular to the CuO2 planes. The
Zero Bias Conductance Peak (ZBCP) characteristic of nodal tunneling splits
under the effect of surface currents produced by the applied fields. Measuring
this splitting under different field conditions, zero field cooled and field
cooled, reveals that these currents have different origins. By comparing the
field cooled ZBCP splitting to that taken in decreasing fields we deduce a
value of the Bean critical current superfluid velocity, and calculate a Bean
critical current density of up to 3*10^7 A/cm2 at low temperatures. This
tunneling method for the determination of critical currents under magnetic
fields has serious advantages over the conventional one, as it avoids having to
make high current contacts to the sample.Comment: 8 pages, 2 figure
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