409 research outputs found
Conformal Invariance in (2+1)-Dimensional Stochastic Systems
Stochastic partial differential equations can be used to model second order
thermodynamical phase transitions, as well as a number of critical
out-of-equilibrium phenomena. In (2+1) dimensions, many of these systems are
conjectured (and some are indeed proved) to be described by conformal field
theories. We advance, in the framework of the Martin-Siggia-Rose field
theoretical formalism of stochastic dynamics, a general solution of the
translation Ward identities, which yields a putative conformal energy-momentum
tensor. Even though the computation of energy-momentum correlators is
obstructed, in principle, by dimensional reduction issues, these are bypassed
by the addition of replicated fields to the original (2+1)-dimensional model.
The method is illustrated with an application to the Kardar-Parisi-Zhang (KPZ)
model of surface growth. The consistency of the approach is checked by means of
a straightforward perturbative analysis of the KPZ ultraviolet region, leading,
as expected, to its conformal fixed point.Comment: Title, abstract and part of the text have been rewritten. To be
published in Physical Review E
Supersymmetric Reflection Matrices
We briefly review the general structure of integrable particle theories in
1+1 dimensions having N=1 supersymmetry. Examples are specific perturbed
superconformal field theories (of Yang-Lee type) and the N=1 supersymmetric
sine-Gordon theory. We comment on the modifications that are required when the
N=1 supersymmetry algebra contains non-trivial topological charges.Comment: 7 pages, Revtex, 2 figures, talk given at the International Seminar
on Supersymmetry and Quantum Field Theory, dedicated to the memory of
D.V.Volkov, Kharkov (Ukraine), January 5-7, 199
Non-perturbative approach to backscattering off a dynamical impurity in 1D Fermi systems
We investigate the problem of backscattering off a time-dependent impurity in
a one-dimensional electron gas. By combining the Schwinger-Keldysh method with
an adiabatic approximation in order to deal with the corresponding out of
equilibrium Dirac equation, we compute the total energy density (TED) of the
system. We show how the free fermion TED is distorted by the backscattering
amplitude and the geometry of the impurity.Comment: 5 pages, 2 figures, RevTex4. Appendix and some text added. Results
and conclusions did not change. Version accepted for publication in Phys.
Rev.
Instantons and Fluctuations in a Lagrangian Model of Turbulence
We perform a detailed analytical study of the Recent Fluid Deformation (RFD)
model for the onset of Lagrangian intermittency, within the context of the
Martin-Siggia-Rose-Janssen-de Dominicis (MSRJD) path integral formalism. The
model is based, as a key point, upon local closures for the pressure Hessian
and the viscous dissipation terms in the stochastic dynamical equations for the
velocity gradient tensor. We carry out a power counting hierarchical
classification of the several perturbative contributions associated to
fluctuations around the instanton-evaluated MSRJD action, along the lines of
the cumulant expansion. The most relevant Feynman diagrams are then integrated
out into the renormalized effective action, for the computation of velocity
gradient probability distribution functions (vgPDFs). While the subleading
perturbative corrections do not affect the global shape of the vgPDFs in an
appreciable qualitative way, it turns out that they have a significant role in
the accurate description of their non-Gaussian cores.Comment: 32 pages, 9 figure
The Onset of Intermittency in Stochastic Burgers Hydrodynamics
We study the onset of intermittency in stochastic Burgers hydrodynamics, as
characterized by the statistical behavior of negative velocity gradient
fluctuations. The analysis is based on the response functional formalism, where
specific velocity configurations - the viscous instantons - are assumed to play
a dominant role in modeling the left tails of velocity gradient probability
distribution functions. We find, as expected on general grounds, that the field
theoretical approach becomes meaningful in practice only if the effects of
fluctuations around instantons are taken into account. Working with a
systematic cumulant expansion, it turns out that the integration of
fluctuations yields, in leading perturbative order, to an effective description
of the Burgers stochastic dynamics given by the renormalization of its
associated heat kernel propagator and the external force-force correlation
function.Comment: 10 pages, 6 figure
Langevin Simulation of the Chirally Decomposed Sine-Gordon Model
A large class of quantum and statistical field theoretical models,
encompassing relevant condensed matter and non-abelian gauge systems, are
defined in terms of complex actions. As the ordinary Monte-Carlo methods are
useless in dealing with these models, alternative computational strategies have
been proposed along the years. The Langevin technique, in particular, is known
to be frequently plagued with difficulties such as strong numerical
instabilities or subtle ergodic behavior. Regarding the chirally decomposed
version of the sine-Gordon model as a prototypical case for the failure of the
Langevin approach, we devise a truncation prescription in the stochastic
differential equations which yields numerical stability and is assumed not to
spoil the Berezinskii-Kosterlitz-Thouless transition. This conjecture is
supported by a finite size scaling analysis, whereby a massive phase ending at
a line of critical points is clearly observed for the truncated stochastic
model.Comment: 6 pages, 4 figure
Avaliação econômica da implantação e manutenção de um sistema agroflorestal com cultivo diversificado.
Resumo: Este trabalho apresenta a análise dos custos de implantação e manutenção de um sistema agroflorestal com cultivos diversificados. Esta avaliação é uma etapa preliminar de uma análise integrada que considerará, além dos fatores socioeconômicos, a recuperação ambiental da área. São apresentados o modelo empregado no sistema, alguns resultados iniciais e os custos de implantação e manutenção. A análise dos dados mostra que houve uma concentração dos gastos na implantação e no primeiro ano deste sistema. Na implantação, o custo principal foi com a aquisição de mudas, enquanto na manutenção os custos se concentraram na mão de obra. Abstract: This paper presents an analysis of the costs of implementation and maintenance of a agroforestry system with diversified crops. This evaluation is a preliminary step in an integrated analysis that will consider also the environmental restoration of the area. The model used in the system, some initial results and the costs of implementation and maintenance are presented. The data analysis indicated that there was a concentration of spending in the implementation and first year of this system. The seedlings was the main cost in the deployment of the system, differently the costs are concentrated in manpower in themaintenance stage
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