59 research outputs found

### Second Harmonic Generation for a Dilute Suspension of Coated Particles

We derive an expression for the effective second-harmonic coefficient of a
dilute suspension of coated spherical particles. It is assumed that the coating
material, but not the core or the host, has a nonlinear susceptibility for
second-harmonic generation (SHG). The resulting compact expression shows the
various factors affecting the effective SHG coefficient. The effective SHG per
unit volume of nonlinear coating material is found to be greatly enhanced at
certain frequencies, corresponding to the surface plasmon resonance of the
coated particles. Similar expression is also derived for a dilute suspension of
coated discs. For coating materials with third-harmonic (THG) coefficient,
results for the effective THG coefficients are given for the cases of coated
particles and coated discs.Comment: 11 pages, 3 figures; accepted for publication in Phys. Rev.

### Quantum diffusion on a cyclic one dimensional lattice

The quantum diffusion of a particle in an initially localized state on a
cyclic lattice with N sites is studied. Diffusion and reconstruction time are
calculated. Strong differences are found for even or odd number of sites and
the limit N->infinit is studied. The predictions of the model could be tested
with micro - and nanotechnology devices.Comment: 17 pages, 5 figure

### Dimensional Crossover in the Effective Second Harmonic Generation of Films of Random Dielectrics

The effective nonlinear response of films of random composites consisting of
a binary composite with nonlinear particles randomly embedded in a linear host
is theoretically and numerically studied. A theoretical expression for the
effective second harmonic generation susceptibility, incorporating the
thickness of the film, is obtained by combining a modified effective-medium
approximation with the general expression for the effective second harmonic
generation susceptibility in a composite. The validity of the thoretical
results is tested against results obtained by numerical simulations on random
resistor networks. Numerical results are found to be well described by our
theory. The result implies that the effective-medium approximation provides a
convenient way for the estimation of the nonlinear response in films of random
dielectrics.Comment: 9 pages, 2 figures; accepted for publication in Phys. Rev.

### Comment on "Mean First Passage Time for Anomalous Diffusion"

We correct a previously erroneous calculation [Phys. Rev. E 62, 6065 (2000)]
of the mean first passage time of a subdiffusive process to reach either end of
a finite interval in one dimension. The mean first passage time is in fact
infinite.Comment: To appear in Phys. Rev.

### Reaction-controlled diffusion: Monte Carlo simulations

We study the coupled two-species non-equilibrium reaction-controlled
diffusion model introduced by Trimper et al. [Phys. Rev. E 62, 6071 (2000)] by
means of detailed Monte Carlo simulations in one and two dimensions. Particles
of type A may independently hop to an adjacent lattice site provided it is
occupied by at least one B particle. The B particle species undergoes
diffusion-limited reactions. In an active state with nonzero, essentially
homogeneous B particle saturation density, the A species displays normal
diffusion. In an inactive, absorbing phase with exponentially decaying B
density, the A particles become localized. In situations with algebraic decay
rho_B(t) ~ t^{-alpha_B}, as occuring either at a non-equilibrium continuous
phase transition separating active and absorbing states, or in a power-law
inactive phase, the A particles propagate subdiffusively with mean-square
displacement ~ t^{1-alpha_A}. We find that within the accuracy of
our simulation data, \alpha_A = \alpha_B as predicted by a simple mean-field
approach. This remains true even in the presence of strong spatio-temporal
fluctuations of the B density. However, in contrast with the mean-field
results, our data yield a distinctly non-Gaussian A particle displacement
distribution n_A(x,t) that obeys dynamic scaling and looks remarkably similar
for the different processes investigated here. Fluctuations of effective
diffusion rates cause a marked enhancement of n_A(x,t) at low displacements
|x|, indicating a considerable fraction of practically localized A particles,
as well as at large traversed distances.Comment: Revtex, 19 pages, 27 eps figures include

### Absence of self-averaging in the complex admittance for transport through random media

A random walk model in a one dimensional disordered medium with an
oscillatory input current is presented as a generic model of boundary
perturbation methods to investigate properties of a transport process in a
disordered medium. It is rigorously shown that an admittance which is equal to
the Fourier-Laplace transform of the first-passage time distribution is
non-self-averaging when the disorder is strong. The low frequency behavior of
the disorder-averaged admittance, $-1 \sim \omega^{\mu}$ where $\mu <
1$, does not coincide with the low frequency behavior of the admittance for any
sample, $\chi - 1 \sim \omega$. It implies that the Cole-Cole plot of $$
appears at a different position from the Cole-Cole plots of $\chi$ of any
sample. These results are confirmed by Monte-Carlo simulations.Comment: 7 pages, 2 figures, published in Phys. Rev.

### Anisotropic thermally activated diffusion in percolation systems

We present a study of static and frequency-dependent diffusion with
anisotropic thermally activated transition rates in a two-dimensional bond
percolation system. The approach accounts for temperature effects on diffusion
coefficients in disordered anisotropic systems. Static diffusion shows an
Arrhenius behavior for low temperatures with an activation energy given by the
highest energy barrier of the system. From the frequency-dependent diffusion
coefficients we calculate a characteristic frequency $\omega_{c}\sim 1/t_{c}$,
related to the time $t_c$ needed to overcome a characteristic barrier. We find
that $\omega_c$ follows an Arrhenius behavior with different activation
energies in each direction.Comment: 5 pages, 4 figure

### MOBILITY IN A ONE-DIMENSIONAL DISORDER POTENTIAL

In this article the one-dimensional, overdamped motion of a classical
particle is considered, which is coupled to a thermal bath and is drifting in a
quenched disorder potential. The mobility of the particle is examined as a
function of temperature and driving force acting on the particle. A framework
is presented, which reveals the dependence of mobility on spatial correlations
of the disorder potential. Mobility is then calculated explicitly for new
models of disorder, in particular with spatial correlations. It exhibits
interesting dynamical phenomena. Most markedly, the temperature dependence of
mobility may deviate qualitatively from Arrhenius formula and a localization
transition from zero to finite mobility may occur at finite temperature.
Examples show a suppression of this transition by disorder correlations.Comment: 10 pages, latex, with 3 figures, to be published in Z. Phys.

### Reaction-controlled diffusion

The dynamics of a coupled two-component nonequilibrium system is examined by
means of continuum field theory representing the corresponding master equation.
Particles of species A may perform hopping processes only when particles of
different type B are present in their environment. Species B is subject to
diffusion-limited reactions. If the density of B particles attains a finite
asymptotic value (active state), the A species displays normal diffusion. On
the other hand, if the B density decays algebraically ~t^{-a} at long times
(inactive state), the effective attractive A-B interaction is weakened. The
combination of B decay and activated A hopping processes gives rise to
anomalous diffusion, with mean-square displacement ~ t^{1-a} for a
< 1. Such algebraic subdiffusive behavior ensues for n-th order B annihilation
reactions (n B -> 0) with n >=3, and n = 2 for d < 2. The mean-square
displacement of the A particles grows only logarithmically with time in the
case of B pair annihilation (n = 2) and d >= 2 dimensions. For radioactive B
decay (n = 1), the A particles remain localized. If the A particles may hop
spontaneously as well, or if additional random forces are present, the A-B
coupling becomes irrelevant, and conventional diffusion is recovered in the
long-time limit.Comment: 7 pages, revtex, no figures; latest revised versio

### Harmonic generation from metal-oxide and metal-metal boundaries

We explore the outcomes of detailed microscopic models by calculating second- and third-harmonic generation from thin-film surfaces with discontinuous free-electron densities. These circumstances can occur in structures consisting of a simple metal mirror, or arrangements composed of either different metals or a metal and a free-electron system like a conducting oxide. Using a hydrodynamic approach we highlight the case of a gold mirror and that of a two-layer system containing indium tin oxide (ITO) and gold. We assume the gold mirror surface is characterized by a free-electron cloud of varying density that spills into the vacuum, which as a result of material dispersion exhibits epsilon-near-zero conditions and local-field enhancement at the surface. For a bilayer consisting of a thin ITO and gold film, if the wave is incident from the ITO side the electromagnetic field is presented with a free-electron discontinuity at the ITO-gold interface, and wavelength-dependent epsilon-near-zero conditions that enhance local fields and conversion efficiencies and determine the surface's emission properties. We evaluate the relative significance of additional nonlinear sources that arise when a free-electron discontinuity is present, and show that harmonic generation can be sensitive to the density of the screening free-electron cloud, and not its thickness. Our findings also suggest the possibility to control surface harmonic generation through surface charge engineering.Peer ReviewedPostprint (author's final draft

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