6,930 research outputs found
Ab initio calculation of the dynamical properties of PPP and PPV
In this work, we have calculated the vibrational modes and frequencies of the
crystalline PPP (in both the Pbam and Pnnm symmetries) and PPV (in the P21/c
symmetry). Our results are in good agreement with the available experimental
data. Also, we have calculated the temperature dependence of their specific
heats at constant volume, and of their vibrational entropies. Based on our
results, at high temperatures, the PPP is more stable in the Pnnm structure
than in the Pbam one.Comment: 5 pages, 7 figures, accepted for publication in Braz. J. Phys.,
special number, Proceedings of BWSP-12, 12th Brazilian Workshop on
Semiconductor Physic
Specific heat amplitude ratios for anisotropic Lifshitz critical behaviors
We determine the specific heat amplitude ratio near a -axial Lifshitz
point and show its universal character. Using a recent renormalization group
picture along with new field-theoretical -expansion techniques,
we established this amplitude ratio at one-loop order. We estimate the
numerical value of this amplitude ratio for and . The result is in
very good agreement with its experimental measurement on the magnetic material
. It is shown that in the limit it trivially reduces to the
Ising-like amplitude ratio.Comment: 8 pages, RevTex, accepted as a Brief Report in Physical Review
Leptodactylus ocellatus (Amphibia): mechanism of defense in the skin and molecular phylogenetic relationships.
Logarithmic periodicities in the bifurcations of type-I intermittent chaos
The critical relations for statistical properties on saddle-node bifurcations
are shown to display undulating fine structure, in addition to their known
smooth dependence on the control parameter. A piecewise linear map with the
type-I intermittency is studied and a log-periodic dependence is numerically
obtained for the average time between laminar events, the Lyapunov exponent and
attractor moments. The origin of the oscillations is built in the natural
probabilistic measure of the map and can be traced back to the existence of
logarithmically distributed discrete values of the control parameter giving
Markov partition. Reinjection and noise effect dependences are discussed and
indications are given on how the oscillations are potentially applicable to
complement predictions made with the usual critical exponents, taken from data
in critical phenomena.Comment: 4 pages, 6 figures, accepted for publication in PRL (2004
A new picture of the Lifshitz critical behavior
New field theoretic renormalization group methods are developed to describe
in a unified fashion the critical exponents of an m-fold Lifshitz point at the
two-loop order in the anisotropic (m not equal to d) and isotropic (m=d close
to 8) situations. The general theory is illustrated for the N-vector phi^4
model describing a d-dimensional system. A new regularization and
renormalization procedure is presented for both types of Lifshitz behavior. The
anisotropic cases are formulated with two independent renormalization group
transformations. The description of the isotropic behavior requires only one
type of renormalization group transformation. We point out the conceptual
advantages implicit in this picture and show how this framework is related to
other previous renormalization group treatments for the Lifshitz problem. The
Feynman diagrams of arbitrary loop-order can be performed analytically provided
these integrals are considered to be homogeneous functions of the external
momenta scales. The anisotropic universality class (N,d,m) reduces easily to
the Ising-like (N,d) when m=0. We show that the isotropic universality class
(N,m) when m is close to 8 cannot be obtained from the anisotropic one in the
limit d --> m near 8. The exponents for the uniaxial case d=3, N=m=1 are in
good agreement with recent Monte Carlo simulations for the ANNNI model.Comment: 48 pages, no figures, two typos fixe
Análise estrutural em floresta ombrófila densa de terra firme não explorada, Amazônia Oriental.
O estudo teve como objetivo analisar a composição florÃstica e as estruturas horizontal, interna e diamétrica da floresta. A pesquisa foi conduzida na Unidade de Manejo Florestal (UMF) da Fazenda Tracajás (02o35?53?S e 47o47?10?W), MunicÃpio de Paragominas, Estado do Pará, Brasil. A floresta foi estratificada em três áreas homogêneas, denominadas classes I, II e III de estoques volumétricos, empregando-se análise multivariada: análises de agrupamento e discriminante. Em cada classe de estoque foram instaladas, aleatoriamente, cinco parcelas de 100 x 100 m (1,0 ha) cada uma, para medição dos indivÃduos com dap ? 15 cm. No centro de cada parcela de 100 x 100 m foi instalada uma subparcela de 10 x 100 m (0,1 ha), para medição dos indivÃduos com 5 cm ? dap < 15 cm. Nas classes I, II e III de estoques volumétricos das árvores com dap ? 15 cm (nÃvel I de inclusão), respectivamente, estimou-se uma densidade total de 322,4; 309,0; e 313,8 indivÃduos por hectare, bem como dominância total de 27,36; 27,45; e 25,88 m2/ha e volume de fuste total de 358,69; 328,33; e 308,69 m3/ha. Nas classes I, II e III de estoques volumétricos das árvores com 5 cm ? dap < 15 cm (nÃvel II de inclusão), respectivamente, estimaram-se densidade total de 846; 854; e 886 indivÃduos por hectare, dominância total de 4,80; 4,93; e 5,46 m2/ha e volume de fuste total de 93,98; 91,23; e 97,61 m3/ha. As espécies de maior valor de importância relativa, Lecythis idatimon (potencial), Rinorea guianensis (não comercial) e Pouteria guianensis (potencial), ocorreram em todos os nÃveis de inclusão e classes de estoque. As distribuições de diâmetros de todas as espécies e das espécies comerciais com dap ? 5 cm, estimadas pela equação de Meyer, confirmaram a tendência exponencial negativa (?J-invertido?). A análise da estrutura da floresta em classes de estoque permitiu melhor conhecimento da composição de espécies e da estrutura fitossociológica, sendo útil na tomada de decisões em planos de manejo de rendimento sustentável
Predicting carbon dynamics in integrated production systems in Brazil using the CQESTR model.
The CQESTR model was used to examine the effect of different soil management practices, including integrated crop-livestock system (iCLS), and various scenarios on soil C dynamics over time and to validate its use for tropical ecosystems
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