3,192 research outputs found
Impurity and boundary effects in one and two-dimensional inhomogeneous Heisenberg antiferromagnets
We calculate the ground-state energy of one and two-dimensional spatially
inhomogeneous antiferromagnetic Heisenberg models for spins 1/2, 1, 3/2 and 2.
Our calculations become possible as a consequence of the recent formulation of
density-functional theory for Heisenberg models. The method is similar to
spin-density-functional theory, but employs a local-density-type approximation
designed specifically for the Heisenberg model, allowing us to explore
parameter regimes that are hard to access by traditional methods, and to
consider complications that are important specifically for nanomagnetic
devices, such as the effects of impurities, finite-size, and boundary geometry,
in chains, ladders, and higher-dimensional systems.Comment: 4 pages, 4 figures, accepted by Phys. Rev.
Poynting Vector Flow in a Circular Circuit
A circuit is considered in the shape of a ring, with a battery of negligible
size and a wire of uniform resistance. A linear charge distribution along the
wire maintains an electrostatic field and a steady current, which produces a
constant magnetic field. Earlier studies of the Poynting vector and the rate of
flow of energy considered only idealized geometries in which the Poynting
vector was confined to the space within the circuit. But in more realistic
cases the Poynting vector is nonzero outside as well as inside the circuit. An
expression is obtained for the Poynting vector in terms of products of
integrals, which are evaluated numerically to show the energy flow. Limiting
expressions are obtained analytically. It is shown that the total power
generated by the battery equals the energy flowing into the wire per unit time.Comment: 19 pages, 8 figure
An infinite-period phase transition versus nucleation in a stochastic model of collective oscillations
A lattice model of three-state stochastic phase-coupled oscillators has been
shown by Wood et al (2006 Phys. Rev. Lett. 96 145701) to exhibit a phase
transition at a critical value of the coupling parameter, leading to stable
global oscillations. We show that, in the complete graph version of the model,
upon further increase in the coupling, the average frequency of collective
oscillations decreases until an infinite-period (IP) phase transition occurs,
at which point collective oscillations cease. Above this second critical point,
a macroscopic fraction of the oscillators spend most of the time in one of the
three states, yielding a prototypical nonequilibrium example (without an
equilibrium counterpart) in which discrete rotational (C_3) symmetry is
spontaneously broken, in the absence of any absorbing state. Simulation results
and nucleation arguments strongly suggest that the IP phase transition does not
occur on finite-dimensional lattices with short-range interactions.Comment: 15 pages, 8 figure
Energy of bond defects in quantum spin chains obtained from local approximations and from exact diagonalization
We study the influence of ferromagnetic and antiferromagnetic bond defects on
the ground-state energy of antiferromagnetic spin chains. In the absence of
translational invariance, the energy spectrum of the full Hamiltonian is
obtained numerically, by an iterative modification of the power algorithm. In
parallel, approximate analytical energies are obtained from a local-bond
approximation, proposed here. This approximation results in significant
improvement upon the mean-field approximation, at negligible extra
computational effort.Comment: 3 pages, 2 figures. Manuscript accepted by Journal of Magnetism and
Magnetic Materials, special issue for LAWMMM 2007 conferenc
Dynamic range of hypercubic stochastic excitable media
We study the response properties of d-dimensional hypercubic excitable
networks to a stochastic stimulus. Each site, modelled either by a three-state
stochastic susceptible-infected-recovered-susceptible system or by the
probabilistic Greenberg-Hastings cellular automaton, is continuously and
independently stimulated by an external Poisson rate h. The response function
(mean density of active sites rho versus h) is obtained via simulations (for
d=1, 2, 3, 4) and mean field approximations at the single-site and pair levels
(for all d). In any dimension, the dynamic range of the response function is
maximized precisely at the nonequilibrium phase transition to self-sustained
activity, in agreement with a reasoning recently proposed. Moreover, the
maximum dynamic range attained at a given dimension d is a decreasing function
of d.Comment: 7 pages, 4 figure
Indicadores de risco do desenvolvimento das nanotecnologias: uma ferramenta de apoio à decisão.
Resumo: A nanotecnologia representa atualmente um negócio mundial que movimenta mais de 100 bilhões de dólares e atrai cada vez mais recursos humanos e financeiros, devido ao seu enorme potencial de aplicação. Assim, metodologias que permitam avaliar a segurança dos nanoprodutos, especialmente aqueles que já encontram-se no mercado, são de fundamental importância. No entanto, a área do conhecimento que envolve as avaliações de riscos das nanotecnologias encontra-se ainda bastante incipiente no Brasil. A criação de uma metodologia para a Avaliação dos Riscos das nanotecnologias representa uma medida mitigatória eficaz para enfrentar os desafios cada vez maiores identificados pelos cientistas e legisladores, podendo atuar em três momentos: prevenindo, monitorando e restaurando. Desta forma, são apresentados nesse trabalho 14 indicadores para a avaliar o potencial de risco. Esses foram validados pelos especialistas da área de nanotecnologia por meio da Técnica Delphi de consulta aos especialistas. Segundo eles, esses indicadores representam de modo mais completo os aspectos mais críticos relacionados ao desenvolvimento da Nanotecnologia. Estes indicadores, de modo geral, têm a finalidade de auxiliar os desenvolvedores desta tecnologia para que reavaliem as metodologias empregadas para o desenvolvimento das nanotecnologias com a finalidade de mitigar um provável risco
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