1,291 research outputs found
Magnetic charge and ordering in kagome spin ice
We present a numerical study of magnetic ordering in spin ice on kagome, a
two-dimensional lattice of corner-sharing triangles. The magnet has six ground
states and the ordering occurs in two stages, as one might expect for a
six-state clock model. In spin ice with short-range interactions up to second
neighbors, there is an intermediate critical phase separated from the
paramagnetic and ordered phases by Kosterlitz-Thouless transitions. In dipolar
spin ice, the intermediate phase has long-range order of staggered magnetic
charges. The high and low-temperature phase transitions are of the Ising and
3-state Potts universality classes, respectively. Freeze-out of defects in the
charge order produces a very large spin correlation length in the intermediate
phase. As a result of that, the lower-temperature transition appears to be of
the Kosterlitz-Thouless type.Comment: 20 pages, 12 figures, accepted version with minor change
Circulating Glycated Albumin and Glomerular Anionic Charges
Aiming to discern the mechanisms by which circulating
glycated albumin alters the glomerular filtration properties
that lead to glomerular dysfunction in diabetes, the authors
studied the distribution and densities of anionic charges
through the rat glomerular wall upon intravascular infusion
of Amadori products, as well as in various conditions of
increased glomerular permselectivity. Polylysine-gold was
used as the probe to reveal the anionic charges. The study
was carried on renal tissue sections of bovine serum albumin
(BSA)- and glycated BSA–injected, normoglycemic
animals. Results were generated through morphometrical
evaluations of the gold labeling. Changes in glomerular anionic
distribution were corroborated on renal tissue sections
of short- and long-term diabetic rats and of normal newborn
rats, situations known for abnormal glomerular filtration.
Altered renal function in these conditions was clearly
associated with changes in glomerular anionic charges. On
the other hand, the infusion of glycated albumin in the circulation
of normal rats, though altering glomerular filtration
properties, did not modify the distribution and density of
the polylysine-gold labeling through the glomerular basement
membrane. Thus, anionic charges seem not to be the
factor involved in the early changes of glomerular permeability
induced by circulating glycated albumin
Stability of the Bragg glass phase in a layered geometry
We study the stability of the dislocation-free Bragg glass phase in a layered
geometry consisting of coupled parallel planes of d=1+1 vortex lines lying
within each plane, in the presence of impurity disorder. Using renormalization
group, replica variational calculations and physical arguments we show that at
temperatures the 3D Bragg glass phase is always stable for weak
disorder. It undergoes a weakly first order transition into a decoupled 2D
vortex glass upon increase of disorder.Comment: RevTeX. Submitted to EP
Adhesion-independent synergy of monocytes and endothelial cells in cytokine production: regulation of IL-6 and GM–CSF production by PAF
Co-Cultures of monocytes (MO) and endothelial cells (EC) were studied for their capacity to synergize in the production of interleukin-6 (IL-6) and granulocyte-macrophage colony-stimulating factor (GM–CSF), two cytokines potentially important in vascular physiopathology. Resting monocytes produced detectable amounts of IL-6 but no GM–CSF, whereas confluent EC produced significant quantities of GM–CSF, but minimal IL-6. In co-cultures without stimuli, additive synthesis of both cytokines was observed. When EC were pretreated, however, with either PAF, TNF or both stimuli, before addition of MO, synergistic production of IL-6 was observed. In contrast, GM–CSF production was not enhanced by coculture of monocytes with activated EC. When either cell population was fixed with paraformaldehyde or killed by freeze-thawing before addition to the co-culture, cytokine levels reverted to those produced by the unaffected population alone. On the other hand, separating the two cell populations by a cell-impermeable membrane in transwell cultures did not affect the synergistic production of the cytokines. Taken together, our data suggest that EC and MO can synergize in response to stimuli by producing IL-6 and that this synergy is dependent on the integrity of both cell populations, but independent of cell-cell contact
Ferroelectric and Dipolar Glass Phases of Non-Crystalline Systems
In a recent letter [Phys. Rev. Lett. {\bf 75}, 2360 (1996)] we briefly
discussed the existence and nature of ferroelectric order in positionally
disordered dipolar materials. Here we report further results and give a
complete description of our work. Simulations of randomly frozen and
dynamically disordered dipolar soft spheres are used to study ferroelectric
ordering in non-crystalline systems. We also give a physical interpretation of
the simulation results in terms of short- and long-range interactions. Cases
where the dipole moment has 1, 2, and 3 components (Ising, XY and XYZ models,
respectively) are considered. It is found that the Ising model displays
ferroelectric phases in frozen amorphous systems, while the XY and XYZ models
form dipolar glass phases at low temperatures. In the dynamically disordered
model the equations of motion are decoupled such that particle translation is
completely independent of the dipolar forces. These systems spontaneously
develop long-range ferroelectric order at nonzero temperature despite the
absence of any fined-tuned short-range spatial correlations favoring dipolar
order. Furthermore, since this is a nonequilibrium model we find that the
paraelectric to ferroelectric transition depends on the particle mass. For the
XY and XYZ models, the critical temperatures extrapolate to zero as the mass of
the particle becomes infinite, whereas, for the Ising model the critical
temperature is almost independent of mass and coincides with the ferroelectric
transition found for the randomly frozen system at the same density. Thus in
the infinite mass limit the results of the frozen amorphous systems are
recovered.Comment: 25 pages (LATEX, no macros). 11 POSTSCRIPT figures enclosed.
Submitted to Phisical Review E. Contact: [email protected]
Non-trivial fixed point structure of the two-dimensional +-J 3-state Potts ferromagnet/spin glass
The fixed point structure of the 2D 3-state random-bond Potts model with a
bimodal (J) distribution of couplings is for the first time fully
determined using numerical renormalization group techniques. Apart from the
pure and T=0 critical fixed points, two other non-trivial fixed points are
found. One is the critical fixed point for the random-bond, but unfrustrated,
ferromagnet. The other is a bicritical fixed point analogous to the bicritical
Nishimori fixed point found in the random-bond frustrated Ising model.
Estimates of the associated critical exponents are given for the various fixed
points of the random-bond Potts model.Comment: 4 pages, 2 eps figures, RevTex 3.0 format requires float and epsfig
macro
Fractal Droplets in Two Dimensional Spin Glasses
The two-dimensional Edwards-Anderson model with Gaussian bond distribution is
investigated at T=0 with a numerical method. Droplet excitations are directly
observed. It turns out that the averaged volume of droplets is proportional to
l^D with D = 1.80(2) where l is the spanning length of droplets, revealing
their fractal nature. The exponent characterizing the l dependence of the
droplet excitation energy is estimated to be -0.42(4), clearly different from
the stiffness exponent for domain wall excitations.Comment: 4 pages 4 figure
On the theory of diamagnetism in granular superconductors
We study a highly disordered network of superconducting granules linked by
weak Josephson junctions in magnetic field and develop a mean field theory for
this problem. The diamagnetic response to a slow {\it variations} of magnetic
field is found to be analogous to the response of a type-II superconductor with
extremely strong pinning. We calculate an effective penetration depth
and critical current and find that both and
are non-zero but are strongly suppressed by frustration.Comment: REVTEX, 12 pages, two Postscript figure
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