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1.
ABSTRACTIn this paper, we discuss broadband dielectric spectroscopy from mHz up to the infrared range mainly for materials with inhomogeneous weak conductivity, including conductor-dielectric nanocomposites. Our discussion is based on the effective medium approximation (EMA) and experiments modeled by this approach are reviewed. We discuss core–shell composites modeled by coated-spheres (Hashin–Shtrikman model) and normal composites with a possible percolation of the conductor component resulting in sharp or smeared percolation threshold of the DC conductivity and diverging static permittivity in the former case. The sharp percolation threshold is modeled by the Bruggeman EMA or by general EMA with arbitrary percolation threshold and arbitrary critical exponents of the DC conductivity and static permittivity. For the case of smeared percolation threshold in the case of complex topologies, we use the Lichtenecker model allowing for partial percolation of both the components. Finally, numerous papers reporting negative permittivity in weakly conducting materials are criticized and concluded to be due to spurious effects. 相似文献
2.
We study continuum percolation of overlapping circular discs of two sizes. We propose a phenomenological scaling equation
for the increase in the effective size of the larger discs due to the presence of the smaller discs. The critical percolation
threshold as a function of the ratio of sizes of discs, for different values of the relative areal densities of two discs,
can be described in terms of a scaling function of only one variable. The recent accurate Monte Carlo estimates of critical
threshold by Quintanilla and Ziff [Phys. Rev.
E76, 051115 (2007)] are in very good agreement with the proposed scaling relation. 相似文献
3.
For fields above a critical value the expansion of the conductivity in powers of the field ceases to be valid and the weak-nonlinearity
approximation no longer works. The density behavior of the critical fields in strongly inhomogeneous media near the percolation
threshold is found on the basis of two criteria—an average criterion and a local criterion. The parameter values of the medium
for which crossover—a change of the critical behavior—occurs are determined. Similar calculations are performed for the critical
currents.
Zh. Tekh. Fiz. 68, 5–8 (June 1998) 相似文献
4.
The problem of a nonlinear current flow in a heterophase medium formed by a random mixture of linear and nonlinear phases is investigated. The duality relation is derived for the critical indices describing the effective response of a heterogeneous system. The critical index is calculated at the percolation threshold (for equal concentrations of the phases). The nonlinear percolation problem is simulated numerically for degrees k = 3, 5, and 7 of the nonlinear phase. The existence of a duality relation for critical indices is established in a range of phase concentrations. 相似文献
5.
A variety of studies have modeled the physics of material deformation and damage as examples of generalized phase transitions, involving either critical phenomena or spinodal nucleation. Here we study a model for frictional sliding with long-range interactions and recurrent damage that is parameterized by a process of damage and partial healing during sliding. We introduce a failure threshold weakening parameter into the cellular automaton slider-block model which allows blocks to fail at a reduced failure threshold for all subsequent failures during an event. We show that a critical point is reached beyond which the probability of a system-wide event scales with this weakening parameter. We provide a mapping to the percolation transition, and show that the values of the scaling exponents approach the values for mean-field percolation (spinodal nucleation) as lattice size L is increased for fixed R. We also examine the effect of the weakening parameter on the frequency-magnitude scaling relationship and the ergodic behavior of the model. 相似文献
6.
B. A. Aronzon E. Z. Meilikhov D. A. Bakaushin A. S. Vedeneev V. V. Ryl’kov 《JETP Letters》1997,66(10):668-674
A threshold vanishing of the Hall emf with decreasing gate voltage is observed at ≈ 77 K in semiconductor systems which are
disordered as a result of a high built-in charge density near the plane of the 2D-electron channel. The effect is observed
at a channel conductivity σ ≈e
2/h and is due to a transition to nondegenerate-electron transport via a 2D percolation cluster having a quasi-1D character of
the conduction. We have established that the conductance of “short” structures, having a length of the order of the correlation
length of a percolation cluster, equals ≈e
2/h per electron and is determined by isolated percolation paths having a lowered percolation threshold. These phenomena are
a general property of disordered 2D systems.
Pis’ma Zh. éksp. Teor. Fiz. 66, No. 10, 633–638 (25 November 1997) 相似文献
7.
The deviation from the Nordheim-Kurnakov rule and the anomalous behavior of spin-disordered electrical resistivity in quasi-binary
GdZn (T
C
=268 K)-GdCu (T
N
=142 K) solid solutions is explained in effective medium approximation within percolation theory for the case of three phases,
viz., ferro-, antiferro-, and paramagnetic. The strong increase of ρ at zinc concentrations x∼0.45 is attributed to the closeness of the system to the percolation threshold. The phase volumes calculated for the random-distribution
case fit well to the concentration dependence of magnetic susceptibility.
Fiz. Tverd. Tela (St. Petersburg) 40, 974–979 (June 1998) 相似文献
8.
The stochastic star formation model of galactic evolution can be cast as a problem of directed percolation, the time dimension being that along which the directed bonds exist. We study various aspects of this percolation, those of general interest for the percolation phase transition and those of particular importance for the astrophysical application. Both analytical calculations and computer simulations are provided and the results compared. Among the properties are: value of the percolation threshold, critical indices, percolation probability (star density) near and away from the critical point, local density, cluster sizes, effects of rotation (for disk galaxy models) on the percolation threshold. Astrophysical consequences of some of these properties are discussed, in particular the way in which general phase transition behavior contributes to spiral arm morphology. We look at 1 (space) + 1 (time), 2 + 1 and + 1 dimensions, the 2 + 1 case being of interest for disk galaxies. 相似文献
9.
A. A. Klochikhin S. A. Permogorov A. N. Reznitskii 《Journal of Experimental and Theoretical Physics》1999,88(3):574-585
We propose an approach to describing the density of fluctuation states in a disordered solid solution with a strong perturbation
introduced by isoelectronic substitution in the range of attraction-center concentrations below the threshold of percolation
along the sites of a disordered sublattice. To estimate the number of localized states we use the results of lattice percolation
theory. We describe a method for distinguishing, within the continuum percolation theory, among the various “radiating” states
of the fluctuation-induced tail, states that form the luminescence band at weak excitation. We also establish the position
of the band of radiating states in relation to the absorption band of the excitonic ground state and the mobility edge of
the system. The approach is used to describe the optical spectra of the solid solution ZnSe1−c
Tec, which at low Te concentrations can be interpreted as a system with strong scattering. We take into account the exciton-phonon
interaction and show that the calculated and observed luminescence spectra of localized excitons are in good agreement with
each other.
Zh. éksp. Teor. Fiz. 115, 1039–1062 (March 1999) 相似文献
10.
11.
A. G. Hunt 《哲学杂志》2013,93(29):3409-3434
12.
Uma Divakaran 《Physica A》2007,384(1):39-43
In this article, we briefly review the critical behaviour of a long-range percolation model in which any two sites are connected with a probability that falls off algebraically with the distance. The results of this percolation transition are used to describe the quantum phase transitions in a dilute transverse Ising model at the percolation threshold pc of the long-range connected lattice. In the similar spirit, we propose a new model of a contact process defined on the same long-range diluted lattice and explore the transitions at pc. The long-range nature of the percolation transition allows us to evaluate some critical exponents exactly in both the above models. Moreover, mean field theory is valid for a wide region of parameter space. In either case, the strength of Griffiths McCoy singularities are tunable as the range parameter is varied. 相似文献
13.
An array of Josephson tunnel junctions is studied as a model for granular superconductors. We determine the critical line in the (H, T)-plane, separating the phase-coherent and incoherent region as a function of the magnetic field. Both positional disorder and junction-dilution are treated. Approaching the percolation threshold from above, the critical exponent of the upper critical field is found to be 0.60±0.03. 相似文献
14.
R. Cloots A. Dang Ph. Vanderbemden A. Vanderschueren H. Bougrine H. W. Vanderschueren A. Rulmont M. Ausloos 《Zeitschrift für Physik B Condensed Matter》1996,100(4):551-555
We measured the AC susceptibility of magnetically textured (123) 80%/211(20%) DyBaCuO composite in a special set-up in order to enhance the intergrain contribution. The synthesis process led to very clean “weak links” at grain boundaries. At the percolation threshold bulk shielding paths were such that the intergrain critical current density Jc was above 105 A/cm2. The field dependence of Jc was understood through an analytical form indicating a distribution of currents similar to the law of clusters at fracture/percolation thresholds. 相似文献
15.
Random percolation theory is a common basis for modelling intergranular phenomena such as cracking, corrosion or diffusion. However, crystallographic constraints in real microstructures dictate that grain boundaries are not assembled at random. In this work a Monte Carlo method is used to construct physically realistic networks composed of high-angle grain boundaries that are susceptible to intergranular attack, as well as twin-variant boundaries that are damage resistant. When crystallographic constraints are enforced, the simulated networks exhibit triple-junction distributions that agree with experiment and reveal the non-random nature of grain-boundary connectivity. The percolation threshold has been determined for several constrained boundary networks and is substantially different from the classical result of percolation theory; compared with a randomly assembled network, about 50-75% more resistant boundaries are required to break up the network of susceptible boundaries. Triple-junction distributions are also shown to capture many details of the correlated percolation problem and to provide a simple means of ranking microstructures. 相似文献
16.
On the basis of a fractal model the macroscopic elastic properties of an inhomogeneous medium with random structure have been
determined. It is shown that if the ratio of the bulk moduli of the phases K
2/K
1→0, then the percolation threshold p
c the Poisson coefficient is equal to 0.2. A study of the behavior of a two-phase medium with negative Poisson coefficient
is carried out.
Fiz. Tverd. Tela (St. Petersburg) 41, 2147–2153 (December 1999) 相似文献
17.
Multiplicity of Phase Transitions and Mean-Field Criticality on Highly Non-Amenable Graphs 总被引:1,自引:0,他引:1
Roberto H. Schonmann 《Communications in Mathematical Physics》2001,219(2):271-322
We consider independent percolation, Ising and Potts models, and the contact process, on infinite, locally finite, connected
graphs.
It is shown that on graphs with edge-isoperimetric Cheeger constant sufficiently large, in terms of the degrees of the vertices
of the graph, each of the models exhibits more than one critical point, separating qualitatively distinct regimes. For unimodular
transitive graphs of this type, the critical behaviour in independent percolation, the Ising model and the contact process
are shown to be mean-field type.
For Potts models on unimodular transitive graphs, we prove the monotonicity in the temperature of the property that the free
Gibbs measure is extremal in the set of automorphism invariant Gibbs measures, and show that the corresponding critical temperature
is positive if and only if the threshold for uniqueness of the infinite cluster in independent bond percolation on the graph
is less than 1.
We establish conditions which imply the finite-island property for independent percolation at large densities, and use those
to show that for a large class of graphs the q-state Potts model has a low temperature regime in which the free Gibbs measure decomposes as the uniform mixture of the q ordered phases.
In the case of non-amenable transitive planar graphs with one end, we show that the q-state Potts model has a critical point separating a regime of high temperatures in which the free Gibbs measure is extremal
in the set of automorphism-invariant Gibbs measures from a regime of low temperatures in which the free Gibbs measure decomposes
as the uniform mixture of the q ordered phases.
Received: 27 March 2000 / Accepted: 7 December 2000 相似文献
18.
A theory is presented of how orienting fields and steric interactions conspire against the formation of a percolating network of, in some sense, connected elongated colloidal particles in fluid dispersions. We find that the network that forms above a critical loading breaks up again at higher loadings due to interaction-induced enhancement of the particle alignment. Upon approach of the percolation threshold, the cluster dimensions diverge with the same critical exponent parallel and perpendicular to the field direction, implying that connectedness percolation is not in the universality class of directed percolation. 相似文献
19.
We review theoretical and experimental studies of the AC dielectric response of inhomogeneous materials, modelled as bond percolation networks, with a binary (conductor-dielectric) distribution of bond conductances. We first summarize the key results of percolation theory, concerning mostly geometrical and static (DC) transport properties, with emphasis on the scaling properties of the critical region around the percolation threshold. The frequency-dependent (AC) response of a general binary model is then studied by means of various approaches, including the effective-medium approximation, a scaling theory of the critical region, numerical computations using the transfer-matrix algorithm, and several exactly solvable deterministic fractal models. Transient regimes, related to singularities in the complex-frequency plane, are also investigated. Theoretical predictions are made more explicit in two specific cases, namely R-C and RL-C networks, and compared with a broad variety of experimental results, concerning, for example, granular composites, thin films, powders, microemulsions, cermets, porous ceramics and the viscoelastic properties of gels. 相似文献
20.
E. Del Gado L. de Arcangelis A. Coniglio 《The European physical journal. E, Soft matter》2000,2(4):359-365
Within a recently introduced model based on the bond-fluctuation dynamics, we study the viscoelastic behaviour of a polymer
solution at the gelation threshold. We here present the results of the numerical simulation of the model on a cubic lattice:
the percolation transition, the diffusion properties and the time autocorrelation functions have been studied. From both the
diffusion coefficients and the relaxation times critical behaviour a critical exponent k for the viscosity coefficient has been extracted: the two results are comparable within the errors giving , in close agreement with the Rouse model prediction and with some experimental results. In the critical region below the
transition threshold the time autocorrelation functions show a long-time tail which is well fitted by a stretched exponential
decay.
Received 20 December 1999 and Received in final form 18 February 2000 相似文献