225 research outputs found

    Low-temperature nucleation in a kinetic Ising model with soft stochastic dynamics

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    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

    Monte Carlo with Absorbing Markov Chains: Fast Local Algorithms for Slow Dynamics

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    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 nn-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 nn-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

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    We examine a square-lattice nearest-neighbor Ising quantum ferromagnet coupled to dd-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 H/J=2|H|/J=2 at low temperatures, the lifetime prefactor diverges because the transition rates between certain configurations approaches zero under these conditions. Near H/J=2|H|/J=2 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 H/J>2|H|/J>2 and one for H/J<2|H|/J<2.Comment: published versio

    Teor relativo de clorofila em folhas de milho inoculado com Azospirillum braziliense sob diferentes doses de nitrogênio e manejo com braquiária.

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    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

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    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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