2,695 research outputs found
Connectivity-dependent properties of diluted sytems in a transfer-matrix description
We introduce a new approach to connectivity-dependent properties of diluted
systems, which is based on the transfer-matrix formulation of the percolation
problem. It simultaneously incorporates the connective properties reflected in
non-zero matrix elements and allows one to use standard random-matrix
multiplication techniques. Thus it is possible to investigate physical
processes on the percolation structure with the high efficiency and precision
characteristic of transfer-matrix methods, while avoiding disconnections. The
method is illustrated for two-dimensional site percolation by calculating (i)
the critical correlation length along the strip, and the finite-size
longitudinal DC conductivity: (ii) at the percolation threshold, and (iii) very
near the pure-system limit.Comment: 4 pages, no figures, RevTeX, Phys. Rev. E Rapid Communications (to be
published
Quantum Fields with Noncommutative Target Spaces
Quantum field theories (QFT's) on noncommutative spacetimes are currently
under intensive study. Usually such theories have world sheet noncommutativity.
In the present work, instead, we study QFT's with commutative world sheet and
noncommutative target space. Such noncommutativity can be interpreted in terms
of twisted statistics and is related to earlier work of Oeckl [1], and others
[2,3,4,5,6,7,8]. The twisted spectra of their free Hamiltonians has been found
earlier by Carmona et al [9,10]. We review their derivation and then compute
the partition function of one such typical theory. It leads to a deformed black
body spectrum, which is analysed in detail. The difference between the usual
and the deformed black body spectrum appears in the region of high frequencies.
Therefore we expect that the deformed black body radiation may potentially be
used to compute a GZK cut-off which will depend on the noncommutative parameter
.Comment: 20 pages, 5 figures; Abstract changed. Changes and corrections in the
text. References adde
Coletor simplificado de sedimentos do fundo de viveiros de aqüicultura.
bitstream/CNPMA/5821/1/comunicado_16.pd
Protocolo de coleta e preparação de amostras de macroinvertebrados bentônicos em riachos.
O biomonitoramento de corpos hídricos através do uso de macroinvertebrados bentônicos é cada vez mais usado e aceito como uma importante ferramenta na avaliação da qualidade da água. Embora seja uma ferramenta utilizada desde o início do século XX na Europa e na América do Norte, no Brasil esta técnica tem apenas algumas décadas e não existem publicações de protocolos específicos para a coleta de macroinvertebrados em rios tropicais. Na América do Norte, em particular, a publicação de Rosenberg & Resh (1993) constitui uma das principais obras sobre o tema, sendo uma boa fonte de consulta para os iniciantes no estudo do biomonitoramento da qualidade das águas. Por ser uma metodologia de baixo custo e de aparato técnico simples, é interessante que se estabeleça um protocolo padrão para utilização em riachos nos países em desenvolvimento, a fim de que estudos futuros possam ser comparados, desde que desenvolvidos em áreas de clima e geografia semelhantes. Esta metodologia apresenta várias vantagens quando empregada em associação à tradicional análise de parâmetros físicos, químicos e físico-químicos da água.bitstream/CNPMA/5841/1/comunicado_19.pd
Coleta e preparação de amostras de sedimentos de viveiros de aqüicultura.
bitstream/CNPMA/5820/1/comunicado_17.pd
Surface crossover exponent for branched polymers in two dimensions
Transfer-matrix methods on finite-width strips with free boundary conditions
are applied to lattice site animals, which provide a model for randomly
branched polymers in a good solvent. By assigning a distinct fugacity to sites
along the strip edges, critical properties at the special (adsorption) and
ordinary transitions are assessed. The crossover exponent at the adsorption
point is estimated as , consistent with recent
predictions that exactly for all space dimensionalities.Comment: 10 pages, LaTeX with Institute of Physics macros, to appear in
Journal of Physics
Noncommutative field gas driven inflation
We investigate early time inflationary scenarios in an Universe filled with a
dilute noncommutative bosonic gas at high temperature. A noncommutative bosonic
gas is a gas composed of bosonic scalar field with noncommutative field space
on a commutative spacetime. Such noncommutative field theories was recently
introduced as a generalization of quantum mechanics on a noncommutative
spacetime. As key features of these theories are Lorentz invariance violation
and CPT violation. In the present study we use a noncommutative bosonic field
theory that besides the noncommutative parameter shows up a further
parameter . This parameter controls the range of the
noncommutativity and acts as a regulator for the theory. Both parameters play a
key role in the modified dispersion relations of the noncommutative bosonic
field, leading to possible striking consequences for phenomenology. In this
work we obtain an equation of state for the
noncommutative bosonic gas relating pressure and energy density , in
the limit of high temperature. We analyse possible behaviours for this gas
parameters , and , so that , which
is the region where the Universe enters an accelerated phase.Comment: Reference added. Version to appear in Journal of Cosmology and
Astroparticle Physics - JCA
Development of maize cultivars for higher zinc and iron content and bioavailability in Brazil.
Caracterização de linhagens de milho quanto aos teores de ácido fítico, Zn, Fe e às razões molares ácido fítico: Fe e ácido fítico: Zn.
On surface properties of two-dimensional percolation clusters
The two-dimensional site percolation problem is studied by transfer-matrix
methods on finite-width strips with free boundary conditions. The relationship
between correlation-length amplitudes and critical indices, predicted by
conformal invariance, allows a very precise determination of the surface
decay-of-correlations exponent, , consistent with
the analytical value . It is found that a special transition does
not occur in the case, corroborating earlier series results. At the ordinary
transition, numerical estimates are consistent with the exact value
for the irrelevant exponent.Comment: 8 pages, LaTeX with Institute of Physics macros, to appear in Journal
of Physics
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