174 research outputs found
Anisotropy effects in a mixed quantum-classical Heisenberg model in two dimensions
We analyse a specific two dimensional mixed spin Heisenberg model with
exchange anisotropy, by means of high temperature expansions and Monte Carlo
simulations. The goal is to describe the magnetic properties of the compound
(NBu_{4})_{2}Mn_{2}[Cu(opba)]_{3}\cdot 6DMSO\cdot H_{2}O which exhibits a
ferromagnetic transition at . Extrapolating our analysis on the
basis of renormalisation group arguments, we find that this transition may
result from a very weak anisotropy effect.Comment: 8 pages, 10 Postscript figure
Thermodynamics of a mixed quantum-classical Heisenberg model in two dimensions
We study the planar antiferromagnetic Heisenberg model on a decorated
hexagonal lattice, involving both classical spins (occupying the vertices) and
quantum spins (occupying the middle of the links). This study is motivated by
the description of a recently synthesized molecular magnetic compound. First,
we trace out the spin 1/2 degrees of freedom to obtain a fully classical model
with an effective ferromagnetic interaction. Then, using high temperature
expansions and Monte Carlo simulations, we analyse its thermal and magnetic
properties. We show that it provides a good quantitative description of the
magnetic susceptibility of the molecular magnet in its paramagnetic phase.Comment: Revtex, 6 pages, 4 included postscript figures, fig.1 upon request to
[email protected] . To appear in J. of Physic C (condensed matter
Random sequential adsorption on a dashed line
We study analytically and numerically a model of random sequential adsorption
(RSA) of segments on a line, subject to some constraints suggested by two kinds
of physical situations:
- deposition of dimers on a lattice where the sites have a spatial extension;
- deposition of extended particles which must overlap one (or several)
adsorbing sites on the substrate.
Both systems involve discrete and continuous degrees of freedom, and, in one
dimension, are equivalent to our model, which depends on one length parameter.
When this parameter is varied, the model interpolates between a variety of
known situations : monomers on a lattice, "car-parking" problem, dimers on a
lattice. An analysis of the long-time behaviour of the coverage as a function
of the parameter exhibits an anomalous 1/t^2 approach to the jamming limit at
the transition point between the fast exponential kinetics, characteristic of
the lattice model, and the 1/t law of the continuous one.Comment: 14 pages (Latex) + 4 Postscript figure
Adsorption of Line Segments on a Square Lattice
We study the deposition of line segments on a two-dimensional square lattice.
The estimates for the coverage at jamming obtained by Monte-Carlo simulations
and by -order time-series expansion are successfully compared. The
non-trivial limit of adsorption of infinitely long segments is studied, and the
lattice coverage is consistently obtained using these two approaches.Comment: 19 pages in Latex+5 postscript files sent upon request ; PTB93_
Critical properties of loop percolation models with optimization constraints
We study loop percolation models in two and in three space dimensions, in
which configurations of occupied bonds are forced to form closed loop. We show
that the uncorrelated occupation of elementary plaquettes of the square and the
simple cubic lattice by elementary loops leads to a percolation transition that
is in the same universality class as the conventional bond percolation. In
contrast to this an optimization constraint for the loop configurations, which
then have to minimize a particular generic energy function, leads to a
percolation transition that constitutes a new universality class, for which we
report the critical exponents. Implication for the physics of solid-on-solid
and vortex glass models are discussed.Comment: 8 pages, 8 figure
Universal Short-Time Dynamics in the Kosterlitz-Thouless Phase
We study the short-time dynamics of systems that develop ``quasi long-range
order'' after a quench to the Kosterlitz-Thouless phase. With the working
hypothesis that the ``universal short-time behavior'', previously found in
Ising-like systems, also occurs in the Kosterlitz-Thouless phase, we explore
the scaling behavior of thermodynamic variables during the relaxational process
following the quench. As a concrete example, we investigate the two-dimensional
-state clock model by Monte Carlo simulation. The exponents governing the
magnetization, the second moment, and the autocorrelation function are
calculated. From them, by means of scaling relations, estimates for the
equilibrium exponents and are derived. In particular, our estimates
for the temperature-dependent anomalous dimension that governs the
static correlation function are consistent with existing analytical and
numerical results and, thus, confirm our working hypothesis.Comment: 16 pages, 9 postscript figures, REVTEX 3.0, submitted to Phys. Rev.
Magnetic and thermal properties of 4f-3d ladder-type molecular compounds
We report on the low-temperature magnetic susceptibilities and specific heats
of the isostructural spin-ladder molecular complexes L[M(opba)]_{3\cdot
xDMSOHO, hereafter abbreviated with LM (where L =
La, Gd, Tb, Dy, Ho and M = Cu, Zn). The results show that the Cu containing
complexes (with the exception of LaCu) undergo long range magnetic
order at temperatures below 2 K, and that for GdCu this ordering is
ferromagnetic, whereas for TbCu and DyCu it is probably
antiferromagnetic. The susceptibilities and specific heats of TbCu
and DyCu above have been explained by means of a model
taking into account nearest as well as next-nearest neighbor magnetic
interactions. We show that the intraladder L--Cu interaction is the predominant
one and that it is ferromagnetic for L = Gd, Tb and Dy. For the cases of Tb, Dy
and Ho containing complexes, strong crystal field effects on the magnetic and
thermal properties have to be taken into account. The magnetic coupling between
the (ferromagnetic) ladders is found to be very weak and is probably of dipolar
origin.Comment: 13 pages, 15 figures, submitted to Phys. Rev.
Circulating Mesenchymal Stem Cells Microparticles in Patients with Cerebrovascular Disease
Preclinical and clinical studies have shown that the application of CD105+ mesenchymal stem cells (MSCs) is feasible and may lead to recovery after stroke. In addition, circulating microparticles are reportedly functional in various disease conditions. We tested the levels of circulating CD105+ microparticles in patients with acute ischemic stroke. The expression of CD105 (a surface marker of MSCs) and CXCR4 (a CXC chemokine receptor for MSC homing) on circulating microparticles was evaluated by flow cytometry of samples from 111 patients and 50 healthy subjects. The percentage of apoptotic CD105 microparticles was determined based on annexin V (AV) expression. The relationship between serum levels of CD105+/AV− microparticles, stromal cells derived factor-1α (SDF-1α), and the extensiveness of cerebral infarcts was also evaluated. CD105+/AV− microparticles were higher in stroke patients than control subjects. Correlation analysis showed that the levels of CD105+/AV− microparticles increased as the baseline stroke severity increased. Multivariate testing showed that the initial severity of stroke was independently associated with circulating CD105+/AV− microparticles (OR, 1.103 for 1 point increase in the NIHSS score on admission; 95% CI, 1.032–1.178) after adjusting for other variables. The levels of CD105+/CXCR4+/AV− microparticles were also increased in patients with severe disability (r = 0.192, p = 0.046 for NIHSS score on admission), but were decreased with time after stroke onset (r = −0.204, p = 0.036). Risk factor profiles were not associated with the levels of circulating microparticles or SDF-1α. In conclusion, our data showed that stroke triggers the mobilization of MSC-derived microparticles, especially in patients with extensive ischemic stroke
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