54 research outputs found
First Passage Time in a Two-Layer System
As a first step in the first passage problem for passive tracer in stratified
porous media, we consider the case of a two-dimensional system consisting of
two layers with different convection velocities. Using a lattice generating
function formalism and a variety of analytic and numerical techniques, we
calculate the asymptotic behavior of the first passage time probability
distribution. We show analytically that the asymptotic distribution is a simple
exponential in time for any choice of the velocities. The decay constant is
given in terms of the largest eigenvalue of an operator related to a half-space
Green's function. For the anti-symmetric case of opposite velocities in the
layers, we show that the decay constant for system length crosses over from
behavior in diffusive limit to behavior in the convective
regime, where the crossover length is given in terms of the velocities.
We also have formulated a general self-consistency relation, from which we have
developed a recursive approach which is useful for studying the short time
behavior.Comment: LaTeX, 28 pages, 7 figures not include
Sliding blocks with random friction and absorbing random walks
With the purpose of explaining recent experimental findings, we study the
distribution of distances traversed by a block that
slides on an inclined plane and stops due to friction. A simple model in which
the friction coefficient is a random function of position is considered.
The problem of finding is equivalent to a First-Passage-Time
problem for a one-dimensional random walk with nonzero drift, whose exact
solution is well-known. From the exact solution of this problem we conclude
that: a) for inclination angles less than \theta_c=\tan(\av{\mu})
the average traversed distance \av{\lambda} is finite, and diverges when
as \av{\lambda} \sim (\theta_c-\theta)^{-1}; b) at
the critical angle a power-law distribution of slidings is obtained:
. Our analytical results are confirmed by
numerical simulation, and are in partial agreement with the reported
experimental results. We discuss the possible reasons for the remaining
discrepancies.Comment: 8 pages, 8 figures, submitted to Phys. Rev.
Diffusion and Trapping on a one-dimensional lattice
The properties of a particle diffusing on a one-dimensional lattice where at
each site a random barrier and a random trap act simultaneously on the particle
are investigated by numerical and analytical techniques. The combined effect of
disorder and traps yields a decreasing survival probability with broad
distribution (log-normal). Exact enumerations, effective-medium approximation
and spectral analysis are employed. This one-dimensional model shows rather
rich behaviours which were previously believed to exist only in higher
dimensionality. The possibility of a trapping-dominated super universal class
is suggested.Comment: 20 pages, Revtex 3.0, 13 figures in compressed format using uufiles
command, to appear in Phys. Rev. E, for an hard copy or problems e-mail to:
[email protected]
Harmonic Vibrational Excitations in Disordered Solids and the "Boson Peak"
We consider a system of coupled classical harmonic oscillators with spatially
fluctuating nearest-neighbor force constants on a simple cubic lattice. The
model is solved both by numerically diagonalizing the Hamiltonian and by
applying the single-bond coherent potential approximation. The results for the
density of states are in excellent agreement with each other. As
the degree of disorder is increased the system becomes unstable due to the
presence of negative force constants. If the system is near the borderline of
stability a low-frequency peak appears in the reduced density of states
as a precursor of the instability. We argue that this peak
is the analogon of the "boson peak", observed in structural glasses. By means
of the level distance statistics we show that the peak is not associated with
localized states
Hole-doping dependence of percolative phase separation in Pr_(0.5-delta)Ca_(0.2+delta)Sr_(0.3)MnO_(3) around half doping
We address the problem of the percolative phase separation in polycrystalline
samples of PrCaSrMnO for (hole doping between 0.46 and 0.54). We perform
measurements of X-ray diffraction, dc magnetization, ESR, and electrical
resistivity. These samples show at a paramagnetic (PM) to ferromagnetic
(FM) transition, however, we found that for there is a coexistence of
both of these phases below . On lowering below the charge-ordering
(CO) temperature all the samples exhibit a coexistence between the FM
metallic and CO (antiferromagnetic) phases. In the whole range the FM phase
fraction () decreases with increasing . Furthermore, we show that only
for the metallic fraction is above the critical percolation
threshold . As a consequence, these samples show very
different magnetoresistance properties. In addition, for we
observe a percolative metal-insulator transition at , and for
the insulating-like behavior generated by the enlargement of
with increasing is well described by the percolation law , where is a critical exponent. On the basis of
the values obtained for this exponent we discuss different possible percolation
mechanisms, and suggest that a more deep understanding of geometric and
dimensionality effects is needed in phase separated manganites. We present a
complete vs phase diagram showing the magnetic and electric properties
of the studied compound around half doping.Comment: 9 text pages + 12 figures, submitted to Phys. Rev.
Effective one-dimensionality of AC hopping conduction in the extreme disorder limit
It is argued that in the limit of extreme disorder AC hopping is dominated by
"percolation paths". Modelling a percolation path as a one-dimensional path
with a sharp jump rate cut-off leads to an expression for the universal AC
conductivity, that fits computer simulations in two and three dimensions better
than the effective medium approximation.Comment: 6 postscript figure
Magnetic relaxation in La0.250Pr0.375Ca0.375MnO3 with varying phase separation
We have studied the magnetic relaxation properties of the phase-separated
manganite compound La0.250Pr0.375Ca0.375MnO3 . A series of polycrystalline
samples was prepared with different sintering temperatures, resulting in a
continuous variation of phase fraction between metallic (ferromagnetic) and
charge-ordered phases at low temperatures. Measurements of the magnetic
viscosity show a temperature and field dependence which can be correlated to
the static properties. Common to all the samples, there appears to be two types
of relaxation processes - at low fields associated with the reorientation of
ferromagnetic domains and at higher fields associated with the transformation
between ferromagnetic and non-ferromagnetic phases.Comment: 30 pages with figures, PDF, accepted to be published in Physical
Review
Effective dielectric function of a metal-dielectric composite with nonrandomly distributed particles
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