257 research outputs found
Tunneling and Non-Universality in Continuum Percolation Systems
The values obtained experimentally for the conductivity critical exponent in
numerous percolation systems, in which the interparticle conduction is by
tunnelling, were found to be in the range of and about , where
is the universal conductivity exponent. These latter values are however
considerably smaller than those predicted by the available ``one
dimensional"-like theory of tunneling-percolation. In this letter we show that
this long-standing discrepancy can be resolved by considering the more
realistic "three dimensional" model and the limited proximity to the
percolation threshold in all the many available experimental studiesComment: 4 pages, 2 figure
Theory of continuum percolation I. General formalism
The theoretical basis of continuum percolation has changed greatly since its
beginning as little more than an analogy with lattice systems. Nevertheless,
there is yet no comprehensive theory of this field. A basis for such a theory
is provided here with the introduction of the Potts fluid, a system of
interacting -state spins which are free to move in the continuum. In the limit, the Potts magnetization, susceptibility and correlation functions
are directly related to the percolation probability, the mean cluster size and
the pair-connectedness, respectively. Through the Hamiltonian formulation of
the Potts fluid, the standard methods of statistical mechanics can therefore be
used in the continuum percolation problem.Comment: 26 pages, Late
The influence of strength of hyperon-hyperon interactions on neutron star properties
An equation of state of neutron star matter with strange baryons has been
obtained. The effects of the strength of hyperon-hyperon interactions on the
equations of state constructed for the chosen parameter sets have been
analyzed. Numerous neutron star models show that the appearance of hyperons is
connected with the increasing density in neutron star interiors. The performed
calculations have indicated that the change of the hyperon-hyperon coupling
constants affects the chemical composition of a neutron star. The obtained
numerical hyperon star models exclude large population of strange baryons in
the star interior.Comment: 18 pages, 22 figures, accepted to be published in Journal of Physics
G: Nuclear and Particle Physic
Exact solution of a one-dimensional continuum percolation model
I consider a one dimensional system of particles which interact through a
hard core of diameter \si and can connect to each other if they are closer
than a distance . The mean cluster size increases as a function of the
density until it diverges at some critical density, the percolation
threshold. This system can be mapped onto an off-lattice generalization of the
Potts model which I have called the Potts fluid, and in this way, the mean
cluster size, pair connectedness and percolation probability can be calculated
exactly. The mean cluster size is S = 2 \exp[ \rho (d -\si)/(1 - \rho \si)] -
1 and diverges only at the close packing density \rho_{cp} = 1 / \si . This
is confirmed by the behavior of the percolation probability. These results
should help in judging the effectiveness of approximations or simulation
methods before they are applied to higher dimensions.Comment: 21 pages, Late
Theory of continuum percolation III. Low density expansion
We use a previously introduced mapping between the continuum percolation
model and the Potts fluid (a system of interacting s-states spins which are
free to move in the continuum) to derive the low density expansion of the pair
connectedness and the mean cluster size. We prove that given an adequate
identification of functions, the result is equivalent to the density expansion
derived from a completely different point of view by Coniglio et al. [J. Phys A
10, 1123 (1977)] to describe physical clustering in a gas. We then apply our
expansion to a system of hypercubes with a hard core interaction. The
calculated critical density is within approximately 5% of the results of
simulations, and is thus much more precise than previous theoretical results
which were based on integral equations. We suggest that this is because
integral equations smooth out overly the partition function (i.e., they
describe predominantly its analytical part), while our method targets instead
the part which describes the phase transition (i.e., the singular part).Comment: 42 pages, Revtex, includes 5 EncapsulatedPostscript figures,
submitted to Phys Rev
Are Radio Pulsars Strange Stars ?
A remarkably precise observational relation for pulse core component widths
of radio pulsars is used to derive stringent limits on pulsar radii, strongly
indicating that pulsars are strange stars rather than neutron stars. This is
achieved by inclusion of general relativistic effects due to the pulsar mass on
the size of the emission region needed to explain the observed pulse widths,
which constrain the pulsar masses to be less than 2.5 Solar masses and radii to
be smaller than 10.5 km.Comment: v.2 : 12 pages including 3 figures and 2 tables, LaTex, uses epsfig.
This version has one extra figure, few lines of new text and typos fixe
Solution of the tunneling-percolation problem in the nanocomposite regime
We noted that the tunneling-percolation framework is quite well understood at
the extreme cases of percolation-like and hopping-like behaviors but that the
intermediate regime has not been previously discussed, in spite of its
relevance to the intensively studied electrical properties of nanocomposites.
Following that we study here the conductivity of dispersions of particle
fillers inside an insulating matrix by taking into account explicitly the
filler particle shapes and the inter-particle electron tunneling process. We
show that the main features of the filler dependencies of the nanocomposite
conductivity can be reproduced without introducing any \textit{a priori}
imposed cut-off in the inter-particle conductances, as usually done in the
percolation-like interpretation of these systems. Furthermore, we demonstrate
that our numerical results are fully reproduced by the critical path method,
which is generalized here in order to include the particle filler shapes. By
exploiting this method, we provide simple analytical formulas for the composite
conductivity valid for many regimes of interest. The validity of our
formulation is assessed by reinterpreting existing experimental results on
nanotube, nanofiber, nanosheet and nanosphere composites and by extracting the
characteristic tunneling decay length, which is found to be within the expected
range of its values. These results are concluded then to be not only useful for
the understanding of the intermediate regime but also for tailoring the
electrical properties of nanocomposites.Comment: 13 pages with 8 figures + 10 pages with 9 figures of supplementary
material (Appendix B
Segregated tunneling-percolation model for transport nonuniversality
We propose a theory of the origin of transport nonuniversality in disordered
insulating-conducting compounds based on the interplay between microstructure
and tunneling processes between metallic grains dispersed in the insulating
host. We show that if the metallic phase is arranged in quasi-one dimensional
chains of conducting grains, then the distribution function of the chain
conductivities g has a power-law divergence for g -> 0 leading to nonuniversal
values of the transport critical exponent t. We evaluate the critical exponent
t by Monte Carlo calculations on a cubic lattice and show that our model can
describe universal as well nonuniversal behavior of transport depending on the
value of few microstructural parameters. Such segregated tunneling-percolation
model can describe the microstructure of a quite vast class of materials known
as thick-film resistors which display universal or nonuniversal values of t
depending on the composition.Comment: 8 pages, 5 figures (Phys. Rev. B - 1 August 2003)(fig1 replaced
Hyperon-hyperon interactions and properties of neutron matter
We present results from Brueckner-Hartree-Fock calculatons for beta stable
neutron star matter with nucleonic and hyperonic degress degrees of freedom,
employing the most recent parametrizations of the baryon-baryon interaction of
the Nijmegen group. It is found that the only strange baryons emergin in beta
stable matter up to total barionic densities of 1.2 fm^-3 are and
. The corresponding equations of state are then used to compute
properties of neutron stars such as masses and radii.Comment: 27 pages, LateX, includes 8 PostScript figures, (submitted to PRC
The fading of SN 1997D
We present a new set of spectroscopic and photometric data extending the
observations of SN 1997D to over 400 days after the explosion. These
observations confirm the peculiar properties of SN 1997D, such as the very low
abundance of 56Co (0.002 Msolar) and the low expansion velocity of the ejecta
(~1000 km/s). We discuss the implications of these observations for the
character of the progenitor and the nature of the remnant showing that a
Crab-like pulsar or an accreting neutron star formed in the explosion of a low
mass progenitor should already have produced a detectable luminosity at this
epoch, in contrast with photometric data. On the other hand, the explosion of a
high mass progenitor with the formation of a black hole is consistent with the
available observations. The consequences of this conclusion regarding the
nature of the explosion and the prospects of directly identifying the black
hole are also addressed.Comment: 9 pages, 5 figures. Accepted for publication in MNRAS. Few typos have
been correcte
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