225 research outputs found
Influência da granulometria da serragem de madeira como material filtrante no tratamento de águas residuais.
Low-temperature nucleation in a kinetic Ising model with soft stochastic dynamics
We study low-temperature nucleation in kinetic Ising models by analytical and
simulational methods, confirming the general result for the average metastable
lifetime, = A*exp(beta*Gamma) (beta = 1/kT) [E. Jordao Neves and R.H.
Schonmann, Commun. Math. Phys. 137, 209 (1991)]. Contrary to common belief, we
find that both A and Gamma depend significantly on the stochastic dynamic. In
particular, for a ``soft'' dynamic, in which the effects of the interactions
and the applied field factorize in the transition rates, Gamma does NOT simply
equal the energy barrier against nucleation, as it does for the standard
Glauber dynamic, which does not have this factorization property.Comment: 4 pages RevTex4, 2 figures. Phys. Rev. Lett., in pres
Mapeamento Funcional por EEG e Ressonância Magnética Funcional na Avaliação para Cirurgia da Epilepsia
Monte Carlo with Absorbing Markov Chains: Fast Local Algorithms for Slow Dynamics
A class of Monte Carlo algorithms which incorporate absorbing Markov chains
is presented. In a particular limit, the lowest-order of these algorithms
reduces to the -fold way algorithm. These algorithms are applied to study
the escape from the metastable state in the two-dimensional square-lattice
nearest-neighbor Ising ferromagnet in an unfavorable applied field, and the
agreement with theoretical predictions is very good. It is demonstrated that
the higher-order algorithms can be many orders of magnitude faster than either
the traditional Monte Carlo or -fold way algorithms.Comment: ReVTeX, Request 3 figures from [email protected]
Scaling analysis of a divergent prefactor in the metastable lifetime of a square-lattice Ising ferromagnet at low temperatures
We examine a square-lattice nearest-neighbor Ising quantum ferromagnet
coupled to -dimensional phonon baths. Using the density-matrix equation, we
calculate the transition rates between configurations, which determines the
specific dynamic. Applying the calculated stochastic dynamic in Monte Carlo
simulations, we measure the lifetimes of the metastable state. As the magnetic
field approaches at low temperatures, the lifetime prefactor diverges
because the transition rates between certain configurations approaches zero
under these conditions. Near and zero temperature, the divergent
prefactor shows scaling behavior as a function of the field, temperature, and
the dimension of the phonon baths. With proper scaling, the simulation data at
different temperatures and for different dimensions of the baths collapse well
onto two master curves, one for and one for .Comment: published versio
Teor relativo de clorofila em folhas de milho inoculado com Azospirillum braziliense sob diferentes doses de nitrogênio e manejo com braquiária.
A cultura do milho safrinha, implantada no início dos anos 80, no Estado do Paraná, ganhou destaque no fim dessa década como mais uma alternativa econômica na entressafra. Com objetivo de avaliar o teor relativo de clorofila em folhas de milho em função de doses de nitrogênio, inoculação das sementes e diferentes manejos, foi realizado um experimento de campo em Nitossolo Vermelho distroférrico, em Maringá (PR). Utilizaram-se três doses de nitrogênio (25, 50 e 75 kg ha-1), inoculante contendo estirpes de Azospirillum brasiliense no tratamento das sementes de milho e dois manejos distintos de milho solteiro e consorciado com Brachiaria ruziziensis. Os tratamentos foram arranjados com fatorial nas parcelas (doses de nitrogênio x presença e ausência de Azospirillum brasiliense) e em faixa nas subparcelas o milho safrinha (milho solteiro e consorciado com braquiária), sob o delineamento blocos ao acaso, com quatro repetições dos blocos, totalizando 64 parcelas. O teor relativo de clorofila nas folhas aumentou linearmente com as doses de nitrogênio utilizadas. Da mesma forma, as médias de todos os tratamentos que receberam a inoculação das sementes com Azospirillum brasiliense foram maiores que os tratamentos não inoculados. Por outro lado, o manejo não influenciou no teor relativo de clorofila nas folhas
Symmetry Breaking and Finite Size Effects in Quantum Many-Body Systems
We consider a quantum many-body system on a lattice with a continuous
symmetry which exhibits a spontaneous symmetry breaking in its infinite volume
ground states, but in which the order operator does not commute with the
Hamiltonian. A typical example is the Heisenberg antiferromagnet with a Neel
order. In the corresponding finite system, the symmetry breaking is usually
"obscured" by "quantum fluctuation" and one gets a symmetric ground state with
a long range order. In such a situation, we prove that there exist ever
increasing numbers of low-lying eigenstates whose excitation energies are
bounded by a constant times 1/N, where N denotes the number of sites. By
forming linear combinations of these low-lying states and the (finite-volume)
ground state, and by taking infinite volume limits, we construct infinite
volume ground states with explicit symmetry breaking. Our general theorems do
not only shed light on the nature ofsymmetry breaking in quantum many-body
systems, but provide indispensable information for numerical approaches to
these systems. We also discuss applications of our general results to a variety
of examples. The present paper is intended to be accessible to the readers
without background in mathematical approaches to quantum many-body systems.Comment: LaTeX, 58 pages, no figures. Notes about Bose-Einstein condenstaion
are added after the publicatio
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