1,449 research outputs found

### Gas Transport in Porous Media: Simulations and Experiments on Partially Densified Aerogels

The experimental density dependence of gas (argon and nitrogen) permeability
of partially densified silica aerogels in the Knudsen regime is quantitatively
accounted for by a computer model. The model simulates both the structure of
the sintered material and the random ballistic motion of a point particle
inside its voids. The same model is also able to account for the densit y
dependence of the specific pore surface as measured from nitrogen adsorption
experiments.Comment: RevTex, 11 pages + 5 postscript figures appended using "uufiles".
Published in Europhys. Lett. 29, p. 567 (1995

### Classical molecular dynamics simulations of amorphous silica surfaces

We have adapted classical molecular dynamics to study the structural and
dynamical properties of amorphous silica surfaces. Concerning the structure,
the density profile exhibits oscillations perpendicularly to the surface as
observed in liquid metal surfaces and the pair correlation functions as well as
the angle distributions show features (absent in the interior of the films)
that can be attributed to the presence of 2-fold rings which are perpendicular
to the surface. From the mean-squared displacement of the non bridging oxygen
atoms we find that in the interior region they move perpendicular to the
surface while they move parallel to it in the surface region.Comment: 7 pages,5 figures - To be published in J. Phys. C.

### Phase diagram of the frustrated spin ladder

We re-visit the phase diagram of the frustrated spin-1/2 ladder with two
competing inter-chain antiferromagnetic exchanges, rung coupling J_\perp and
diagonal coupling J_\times. We suggest, based on the accurate renormalization
group analysis of the low-energy Hamiltonian of the ladder, that marginal
inter-chain current-current interaction plays central role in destabilizing
previously predicted intermediate columnar dimer phase in the vicinity of
classical degeneracy line J_\perp = 2J_\times. Following this insight we then
suggest that changing these competing inter-chain exchanges from the previously
considered antiferromagnetic to the ferromagnetic ones eliminates the issue of
the marginal interactions altogether and dramatically expands the region of
stability of the columnar dimer phase. This analytical prediction is
convincingly confirmed by the numerical density matrix renormalization group
and exact diagonalization calculations as well as by the perturbative
calculation in the strong rung-coupling limit. The phase diagram for
ferromagnetic J_\perp and J_\times is determined.Comment: 12 pages, 12 figures, 1 Table. v2: version to appear in Phys. Rev.

### Influence of surfactants on the structure of titanium oxide gels : experiments and simulations

We report here on experimental and numerical studies of the influence of
surfactants on mineral gel synthesis. The modification of the gel structure
when the ratios water-precursor and water-surfactant vary is brought to the
fore by fractal dimension measures. A property of {\em polydispersity of the
initial hydrolysis} is proposed to explain these results, and is successfuly
tested through numerical experiments of three dimensional chemically limited
aggregation.Comment: 12 pages, 4 Postscript figures, uses RevTe

### Small Angle Scattering by Fractal Aggregates: A Numerical Investigation of the Crossover Between the Fractal Regime and the Porod Regime

Fractal aggregates are built on a computer using off-lattice cluster-cluster
aggregation models. The aggregates are made of spherical particles of different
sizes distributed according to a Gaussian-like distribution characterised by a
mean $a_0$ and a standard deviation $\sigma$. The wave vector dependent
scattered intensity $I(q)$ is computed in order to study the influence of the
particle polydispersity on the crossover between the fractal regime and the
Porod regime. It is shown that, given $a_0$, the location $q_c$ of the
crossover decreases as $\sigma$ increases. The dependence of $q_c$ on $\sigma$
can be understood from the evolution of the shape of the center-to-center
interparticle-distance distribution function.Comment: RevTex, 4 pages + 6 postscript figures, compressed using "uufiles",
published in Phys. Rev. B 50, 1305 (1994

### Discontinuous percolation transitions in real physical systems

We study discontinuous percolation transitions (PT) in the diffusion-limited
cluster aggregation model of the sol-gel transition as an example of real
physical systems, in which the number of aggregation events is regarded as the
number of bonds occupied in the system. When particles are Brownian, in which
cluster velocity depends on cluster size as $v_s \sim s^{\eta}$ with
$\eta=-0.5$, a larger cluster has less probability to collide with other
clusters because of its smaller mobility. Thus, the cluster is effectively more
suppressed in growth of its size. Then the giant cluster size increases
drastically by merging those suppressed clusters near the percolation
threshold, exhibiting a discontinuous PT. We also study the tricritical
behavior by controlling the parameter $\eta$, and the tricritical point is
determined by introducing an asymmetric Smoluchowski equation.Comment: 5 pages, 5 figure

### Kinetic Antiferromagnetism in the Triangular Lattice

We show that the motion of a single hole in the infinite $U$ Hubbard model
with frustrated hopping leads to weak metallic antiferromagnetism of kinetic
origin. An intimate relationship is demonstrated between the simplest versions
of this problem in 1 and 2 dimensions, and two of the most subtle many body
problems, namely the Heisenberg Bethe ring in 1-d and the 2-dimensional
triangular lattice Heisenberg antiferromagnet.Comment: 10 pages, 2 figures, 5 supplementary figures; Figures fixe

### Stochastic Model for the Motion of a Particle on an Inclined Rough Plane and the Onset of Viscous Friction

Experiments on the motion of a particle on an inclined rough plane have
yielded some surprising results. For example, it was found that the frictional
force acting on the ball is viscous, {\it i.e.} proportional to the velocity
rather than the expected square of the velocity. It was also found that, for a
given inclination of the plane, the velocity of the ball scales as a power of
its radius. We present here a one dimensional stochastic model based on the
microscopic equations of motion of the ball, which exhibits the same behaviour
as the experiments. This model yields a mechanism for the origins of the
viscous friction force and the scaling of the velocity with the radius. It also
reproduces other aspects of the phase diagram of the motion which we will
discuss.Comment: 19 pages, latex, 11 postscript figures in separate uuencoded fil

### A new conjecture extends the GM law for percolation thresholds to dynamical situations

The universal law for percolation thresholds proposed by Galam and Mauger
(GM) is found to apply also to dynamical situations. This law depends solely on
two variables, the space dimension d and a coordinance numberq. For regular
lattices, q reduces to the usual coordination number while for anisotropic
lattices it is an effective coordination number. For dynamical percolation we
conjecture that the law is still valid if we use the number q_2 of second
nearest neighbors instead of q. This conjecture is checked for the dynamic
epidemic model which considers the percolation phenomenon in a mobile
disordered system. The agreement is good.Comment: 8 pages, latex, 3 figures include

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