12,105 research outputs found
Modified TAP equations for the SK spin glass
The stability of the TAP mean field equations is reanalyzed with the
conclusion that the exclusive reason for the breakdown at the spin glass
instability is an inconsistency for the value of the local susceptibility. A
new alternative approach leads to modified equations which are in complete
agreement with the original ones above the instability. Essentially altered
results below the instability are presented and the consequences for the
dynamical mean field equations are discussed.Comment: 7 pages, 2 figures, final revised version to appear in Europhys. Let
Semiclassical Evolution of Dissipative Markovian Systems
A semiclassical approximation for an evolving density operator, driven by a
"closed" hamiltonian operator and "open" markovian Lindblad operators, is
obtained. The theory is based on the chord function, i.e. the Fourier transform
of the Wigner function. It reduces to an exact solution of the Lindblad master
equation if the hamiltonian operator is a quadratic function and the Lindblad
operators are linear functions of positions and momenta.
Initially, the semiclassical formulae for the case of hermitian Lindblad
operators are reinterpreted in terms of a (real) double phase space, generated
by an appropriate classical double Hamiltonian. An extra "open" term is added
to the double Hamiltonian by the non-hermitian part of the Lindblad operators
in the general case of dissipative markovian evolution. The particular case of
generic hamiltonian operators, but linear dissipative Lindblad operators, is
studied in more detail. A Liouville-type equivariance still holds for the
corresponding classical evolution in double phase, but the centre subspace,
which supports the Wigner function, is compressed, along with expansion of its
conjugate subspace, which supports the chord function.
Decoherence narrows the relevant region of double phase space to the
neighborhood of a caustic for both the Wigner function and the chord function.
This difficulty is avoided by a propagator in a mixed representation, so that a
further "small-chord" approximation leads to a simple generalization of the
quadratic theory for evolving Wigner functions.Comment: 33 pages - accepted to J. Phys.
Significance of Ghost Orbit Bifurcations in Semiclassical Spectra
Gutzwiller's trace formula for the semiclassical density of states in a
chaotic system diverges near bifurcations of periodic orbits, where it must be
replaced with uniform approximations. It is well known that, when applying
these approximations, complex predecessors of orbits created in the bifurcation
("ghost orbits") can produce pronounced signatures in the semiclassical spectra
in the vicinity of the bifurcation. It is the purpose of this paper to
demonstrate that these ghost orbits themselves can undergo bifurcations,
resulting in complex, nongeneric bifurcation scenarios. We do so by studying an
example taken from the Diamagnetic Kepler Problem, viz. the period quadrupling
of the balloon orbit. By application of normal form theory we construct an
analytic description of the complete bifurcation scenario, which is then used
to calculate the pertinent uniform approximation. The ghost orbit bifurcation
turns out to produce signatures in the semiclassical spectrum in much the same
way as a bifurcation of real orbits would.Comment: 20 pages, 6 figures, LATEX (IOP style), submitted to J. Phys.
Teor de matéria seca das diferentes frações da planta de quatro híbridos de sorgo avaliados em quatro idades de corte.
Quantum Effects in Neural Networks
We develop the statistical mechanics of the Hopfield model in a transverse
field to investigate how quantum fluctuations affect the macroscopic behavior
of neural networks. When the number of embedded patterns is finite, the Trotter
decomposition reduces the problem to that of a random Ising model. It turns out
that the effects of quantum fluctuations on macroscopic variables play the same
roles as those of thermal fluctuations. For an extensive number of embedded
patterns, we apply the replica method to the Trotter-decomposed system. The
result is summarized as a ground-state phase diagram drawn in terms of the
number of patterns per site, , and the strength of the transverse
field, . The phase diagram coincides very accurately with that of the
conventional classical Hopfield model if we replace the temperature T in the
latter model by . Quantum fluctuations are thus concluded to be quite
similar to thermal fluctuations in determination of the macroscopic behavior of
the present model.Comment: 34 pages, LaTeX, 9 PS figures, uses jpsj.st
Replica field theory and renormalization group for the Ising spin glass in an external magnetic field
We use the generic replica symmetric cubic field-theory to study the
transition of short range Ising spin glasses in a magnetic field around the
upper critical dimension, d=6. A novel fixed-point is found, in addition to the
well-known zero magnetic field fixed-point, from the application of the
renormalization group. In the spin glass limit, n going to 0, this fixed-point
governs the critical behaviour of a class of systems characterised by a single
cubic interaction parameter. For this universality class, the spin glass
susceptibility diverges at criticality, whereas the longitudinal mode remains
massive. The third mode, the so-called anomalous one, however, behaves
unusually, having a jump at criticality. The physical consequences of this
unusual behaviour are discussed, and a comparison with the conventional de
Almeida-Thouless scenario presented.Comment: 5 pages written in revtex4. Accepted for publication in Phys. Rev.
Let
Importância do mel na composição da renda familiar em assentamentos rurais de Corumbá, MS.
Desde março de 2009 a Embrapa Pantanal e parceiros (AAPAN, AGRAER, Fundação Terra do Pantanal, assentados) vêm executando um projeto de pesquisa participativa em dois assentamentos rurais de Corumbá, MS, com o objetivo de avaliar as tecnologias apícolas adotadas e propor ajustes e inclusões ao sistema produtivo. A avaliação inclui um componente econômico que busca identificar a importância da apicultura na renda familiar total dos assentados. Para tanto, o produtor utiliza uma planilha onde anota diariamente as tarefas realizadas, produtos e insumos comprados e vendidos, entre outros. Os resultados do acompanhamento econômico dependem muito do adequado preenchimento desta planilha, sendo uma dificuldade inerente ao processo participativo. Observou-se que a apicultura é uma atividade complementar à renda familiar, mas pode vir a tornar-se a principal fonte de renda para alguns assentados, dependendo de certos requisitos e condicionantes do produtor, do lote e da própria cadeia produtiva.Disponível também em: Cadernos de Agroecologia, V. 5, n.1, 2010
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