36,802 research outputs found
Chaos and a Resonance Mechanism for Structure Formation in Inflationary Models
We exhibit a resonance mechanism of amplification of density perturbations in
inflationary mo-dels, using a minimal set of ingredients (an effective
cosmological constant, a scalar field minimally coupled to the gravitational
field and matter), common to most models in the literature of inflation. This
mechanism is based on the structure of homoclinic cylinders, emanating from an
unstable periodic orbit in the neighborhood of a saddle-center critical point,
present in the phase space of the model. The cylindrical structure induces
oscillatory motions of the scales of the universe whenever the orbit visits the
neighborhood of the saddle-center, before the universe enters a period of
exponential expansion. The oscillations of the scale functions produce, by a
resonance mechanism, the amplification of a selected wave number spectrum of
density perturbations, and can explain the hierarchy of scales observed in the
actual universe. The transversal crossings of the homoclinic cylinders induce
chaos in the dynamics of the model, a fact intimately connected to the
resonance mechanism occuring immediately before the exit to inflation.Comment: 4 pages. This essay received an Honorable Mention from the Gravity
Research Foundation, 1998-Ed. To appear in Mod. Phys. Lett.
On Galois-Division Multiple Access Systems: Figures of Merit and Performance Evaluation
A new approach to multiple access based on finite field transforms is
investigated. These schemes, termed Galois-Division Multiple Access (GDMA),
offer compact bandwidth requirements. A new digital transform, the Finite Field
Hartley Transform (FFHT) requires to deal with fields of characteristic p, p
\neq 2. A binary-to-p-ary (p \neq 2) mapping based on the opportunistic
secondary channel is introduced. This allows the use of GDMA in conjunction
with available digital systems. The performance of GDMA is also evaluated.Comment: 6 pages, 4 figures. In: XIX Simposio Brasileiro de Telecomunicacoes,
2001, Fortaleza, CE, Brazi
Broad Histogram Monte Carlo
We propose a new Monte Carlo technique in which the degeneracy of energy
states is obtained with a Markovian process analogous to that of Metropolis
used currently in canonical simulations. The obtained histograms are much
broader than those of the canonical histogram technique studied by Ferrenberg
and Swendsen. Thus we can reliably reconstruct thermodynamic functions over a
much larger temperature scale also away from the critical point. We show for
the two-dimensional Ising model how our new method reproduces exact results
more accurately and using less computer time than the conventional histogram
method. We also show data in three dimensions for the Ising ferromagnet and the
Edwards Anderson spin glass.Comment: 6 pages of a TeX file with 4 PS figures. Related papers at
http://www.if.uff.br/~tjp
A Flexible Implementation of a Matrix Laurent Series-Based 16-Point Fast Fourier and Hartley Transforms
This paper describes a flexible architecture for implementing a new fast
computation of the discrete Fourier and Hartley transforms, which is based on a
matrix Laurent series. The device calculates the transforms based on a single
bit selection operator. The hardware structure and synthesis are presented,
which handled a 16-point fast transform in 65 nsec, with a Xilinx SPARTAN 3E
device.Comment: 4 pages, 4 figures. IEEE VI Southern Programmable Logic Conference
201
Study of the Fully Frustrated Clock Model using the Wang-Landau Algorithm
Monte Carlo simulations using the newly proposed Wang-Landau algorithm
together with the broad histogram relation are performed to study the
antiferromagnetic six-state clock model on the triangular lattice, which is
fully frustrated. We confirm the existence of the magnetic ordering belonging
to the Kosterlitz-Thouless (KT) type phase transition followed by the chiral
ordering which occurs at slightly higher temperature. We also observe the lower
temperature phase transition of KT type due to the discrete symmetry of the
clock model. By using finite-size scaling analysis, the higher KT temperature
and the chiral critical temperature are respectively estimated as
and . The results are in favor of the double
transition scenario. The lower KT temperature is estimated as .
Two decay exponents of KT transitions corresponding to higher and lower
temperatures are respectively estimated as and
, which suggests that the exponents associated with the KT
transitions are universal even for the frustrated model.Comment: 7 pages including 9 eps figures, RevTeX, to appear in J. Phys.
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