6,177 research outputs found
Frequency-dependent (ac) Conduction in Disordered Composites: a Percolative Study
In a recent paper [Phys. Rev. B{\bf57}, 3375 (1998)], we examined in detail
the nonlinear (electrical) dc response of a random resistor cum tunneling bond
network (, introduced by us elsewhere to explain nonlinear response of
metal-insulator type mixtures). In this work which is a sequel to that paper,
we consider the ac response of the -based correlated () model.
Numerical solutions of the Kirchoff's laws for the model give a power-law
exponent (= 0.7 near ) of the modulus of the complex ac conductance at
moderately low frequencies, in conformity with experiments on various types of
disordered systems. But, at very low frequencies, it gives a simple quadratic
or linear dependence on the frequency depending upon whether the system is
percolating or not. We do also discuss the effective medium approximation
() of our and the traditional random network model, and discuss
their comparative successes and shortcomings.Comment: Revised and reduced version with 17 LaTeX pages plus 8 JPEG figure
Nonlinear DC-response in Composites: a Percolative Study
The DC-response, namely the - and - charateristics, of a variety
of composite materials are in general found to be nonlinear. We attempt to
understand the generic nature of the response charactersistics and study the
peculiarities associated with them. Our approach is based on a simple and
minimal model bond percolative network. We do simulate the resistor network
with appropritate linear and nonlinear bonds and obtain macroscopic nonlinear
response characteristics. We discuss the associated physics. An effective
medium approximation (EMA) of the corresponding resistor network is also given.Comment: Text written in RevTEX, 15 pages (20 postscript figures included),
submitted to Phys. Rev. E. Some minor corrections made in the text, corrected
one reference, the format changed (from 32 pages preprint to 15 pages
Cosmology with decaying tachyon matter
We investigate the case of a homogeneous tachyon field coupled to gravity in
a spatially flat Friedman-Robertson-Walker spacetime. Assuming the field
evolution to be exponentially decaying with time we solve the field equations
and show that, under certain conditions, the scale factor represents an
accelerating universe, following a phase of decelerated expansion. We make use
of a model of dark energy (with p=-\rho) and dark matter (p=0) where a single
scalar field (tachyon) governs the dynamics of both the dark components. We
show that this model fits the current supernova data as well as the canonical
\LambdaCDM model. We give the bounds on the parameters allowed by the current
data.Comment: 14 pages, 6 figures, v2, Discussions and references addede
Wigner delay time from a random passive and active medium
We consider the scattering of electron by a one-dimensional random potential
(both passive and active medium) and numerically obtain the probability
distribution of Wigner delay time (). We show that in a passive medium
our probability distribution agrees with the earlier analytical results based
on random phase approximation. We have extended our study to the strong
disorder limit, where random phase approximation breaks down. The delay time
distribution exhibits the long time tail () due to resonant states,
which is independent of the nature of disorder indicating the universality of
the tail of the delay time distribution. In the presence of coherent absorption
(active medium) we show that the long time tail is suppressed exponentially due
to the fact that the particles whose trajectories traverse long distances in
the medium are absorbed and are unlikely to be reflected.Comment: 13 pages RevTex, 5 EPS figures included, communicated to PR
Amplification or Reduction of Backscattering in a Coherently Amplifying or Absorbing Disordered Chain
We study localization properties of a one-dimensional disordered system
characterized by a random non-hermitean hamiltonian where both the randomness
and the non-hermiticity arises in the local site-potential; its real part being
random, and a constant imaginary part implying the presence of either a
coherent absorption or amplification at each site. While the two-probe
transport properties behave seemingly very differently for the amplifying and
the absorbing chains, the logarithmic resistance = ln where
is the 4-probe resistance gives a unified description of both the cases. It is
found that the ensemble-averaged increases linearly with length
indicating exponential growth of resistance. While in contrast to the case of
Anderson localization (random hermitean matrix), the variance of could be
orders of magnitude smaller in the non-hermitean case, the distribution of
still remains non-Gaussian even in the large length limit.Comment: 11 LaTeX pages plus 14 EPS figure
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