947 research outputs found
Universality of ac-conduction in anisotropic disordered systems: An effective medium approximation study
Anisotropic disordered system are studied in this work within the random
barrier model. In such systems the transition probabilities in different
directions have different probability density functions. The
frequency-dependent conductivity at low temperatures is obtained using an
effective medium approximation. It is shown that the isotropic universal
ac-conduction law, , is recovered if properly scaled
conductivity () and frequency () variables are used.Comment: 5 pages, no figures, final form (with corrected equations
Anisotropic dielectric function in polar nano-regions of relaxor ferroelectrics
The paper suggests to treat the infrared reflectivity spectra of single
crystal perovskite relaxors as fine-grained ferroelectric ceramics: locally
frozen polarization makes the dielectric function strongly anisotropic in the
phonon frequency range and the random orientation of the polarization at
nano-scopic scale requires to take into account the inhomogeneous
depolarization field. Employing a simple effective medium approximation
(Bruggeman symmetrical formula) to dielectric function describing the polar
optic modes as damped harmonic oscillators turns out to be sufficient for
reproducing all principal features of room temperature reflectivity of PMN. One
of the reflectivity bands is identified as a geometrical resonance entirely
related to the nanoscale polarization inhomogeneity. The approach provides a
general guide for systematic determination of the polar mode frequencies split
by the inhomogeneous polarization at nanometer scale.Comment: 5 pages, 2 figure
Effective Elastic Moduli in Solids with High Crack Density
We investigate the weakening of elastic materials through randomly
distributed circles and cracks numerically and compare the results to
predictions from homogenization theories. We find a good agreement for the case
of randomly oriented cracks of equal length in an isotropic plane-strain medium
for lower crack densities; for higher densities the material is weaker than
predicted due to precursors of percolation. For a parallel alignment of cracks,
where percolation does not occur, we analytically predict a power law decay of
the effective elastic constants for high crack densities, and confirm this
result numerically.Comment: 8 page
Spectral Representation for the Effective Macroscopic Response of a Polycrystal: Application to Third-Order Nonlinear Susceptibility
Erratum:
In our paper, we show that the spectral representation for isotropic
two-component composites also applies to uniaxial polycrystals. We have learned
that this result was, in fact, first conjectured by G.W. Milton. While our
derivation is more detailed, our result for the spectral function is the same
as Milton's. We very much regret not having been aware of this work at the time
of writing our paper.
Original abstract:
We extend the spectral theory used for the calculation of the effective
linear response functions of composites to the case of a polycrystalline
material with uniaxially anisotropic microscopic symmetry. As an application,
we combine these results with a nonlinear decoupling approximation as modified
by Ma et al., to calculate the third-order nonlinear optical susceptibility of
a uniaxial polycrystal, assuming that the effective dielectric function of the
polycrystal can be calculated within the effective-medium approximation.Comment: v2 includes erratum and the original preprin
British Bryological Society expedition to Mulanje Mountain, Malawi : 13., new and other unpublished records
Further results are provided of the 1991 British Bryological Society Expedition to Mulanje Mt., Malawi including 168 taxa of bryophyte, comprising 72 taxa of liverwort (38 new to Malawi) and 96 taxa of mosses (45 new to Malawi)
A Superlens Based on Metal-Dielectric Composites
Pure noble metals are typically considered to be the materials of choice for
a near-field superlens that allows subwavelength resolution by recovering both
propagating and evanescent waves. However, a superlens based on bulk metal can
operate only at a single frequency for a given dielectric host. In this Letter,
it is shown that a composite metal-dielectric film, with an appropriate metal
filling factor, can operate at practically any desired wavelength in the
visible and near-infrared ranges. Theoretical analysis and simulations verify
the feasibility of the proposed lens.Comment: 15 pages, 4 figure
Self consistent determination of plasmonic resonances in ternary nanocomposites
We have developed a self consistent technique to predict the behavior of
plasmon resonances in multi-component systems as a function of wavelength. This
approach, based on the tight lower bounds of the Bergman-Milton formulation, is
able to predict experimental optical data, including the positions, shifts and
shapes of plasmonic peaks in ternary nanocomposites without using any ftting
parameters. Our approach is based on viewing the mixing of 3 components as the
mixing of 2 binary mixtures, each in the same host. We obtained excellent
predictions of the experimental optical behavior for mixtures of Ag:Cu:SiO2 and
alloys of Au-Cu:SiO2 and Ag-Au:H2 O, suggesting that the essential physics of
plasmonic behavior is captured by this approach.Comment: 7 pages and 4 figure
Plasmon tunability in metallodielectric metamaterials
The dielectric properties of metamaterials consisting of periodically
arranged metallic nanoparticles of spherical shape are calculated by rigorously
solving Maxwell's equations. Effective dielectric functions are obtained by
comparing the reflectivity of planar surfaces limiting these materials with
Fresnel's formulas for equivalent homogeneous media, showing mixing and
splitting of individual-particle modes due to inter-particle interaction.
Detailed results for simple cubic and fcc crystals of aluminum spheres in
vacuum, silver spheres in vacuum, and silver spheres in a silicon matrix are
presented. The filling fraction of the metal f is shown to determine the
position of the plasmon modes of these metamaterials. Significant deviations
are observed with respect to Maxwell-Garnett effective medium theory for large
f, and multiple plasmons are predicted to exist in contrast to Maxwell-Garnett
theory.Comment: 6 pages, 4 figure
Optical conductivity of a granular metal at not very low temperatures
We study the finite-temperature optical conductivity, sigma(omega,T), of a
granular metal using a simple model consisting of a array of spherical metallic
grains. It is necessary to include quantum tunneling and Coulomb blockade
effects to obtain the correct temperature dependence of sigma(omega, T), and to
consider polarization oscillations to obtain the correct frequency dependence.
We have therefore generalized the Ambegaokar-Eckern-Schoen (AES) model for
granular metals to obtain an effective field theory incorporating the
polarization fluctuations of the individual metallic grains. In contrast to the
DC conductivity, which is determined by inter-grain charge transfer and obeys
an Arrhenius law at low temperature, the AC conductivity is dominated by a
resonance peak for intra-grain polarization oscillations, which has a power-law
tail at low frequencies. More importantly, although the resonance frequency
agrees with the classical prediction, the resonance width depends on intergrain
quantum tunneling and Coulomb blockade parameters, in addition to the classical
Drude relaxation within the grain. This additional damping is due to inelastic
cotunneling of polarization fluctuations to neighbouring grains and it
qualitatively differs from the DC conductivity in its temperature dependence
quite unlike the expectation from Drude theory.Comment: Added figures, published version, 16 pages, REVTe
Effective conductivity in association with model structure and spatial inhomogeneity of polymer/carbon black composites
The relationship between effective conductivity and cell structure of
polyethylene/carbon composites as well as between effective conductivity and
spatial distribution of carbon black are discussed. Following Yoshida's model
both structures can, in a way, be said to be intermediate between the well
known Maxwell-Garnett (MG) and Bruggeman (BR) limiting structures. Using TEM
photographs on composites with various carbon blacks we have observed that the
larger is Garncarek's inhomogeneity measure H of two-dimensional (2D)
representative distribution of the carbon black, the smaller is the effective
conductivity of the composite.Comment: 7 pages, 9 figure
- …