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Search Results: 1 - 10 of 8691 matches for " Wanderleia Elizabeth Brinckmann "
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Desarrollo y desigualdad regional: el ejemplo del Valle de Río Pardo (Río Grande do Sul, Brasil)
Wanderleia Elizabeth Brinckmann
Papeles de Geografía , 2001,
Abstract: En este artículo se pretende realizar una reflexión sobre la desigualdad existente en el proceso de desarrollo económico en la región del Valle del Río Pardo, Río Grande do Sul, Brasil, que actualmente intenta reducir este proceso a través de proyectos que permitan trabajar con los diferenciales regionales de competitividad.
La cuenca del Río Pardo (Brasil) y sus estrategias de desarrollo rural sostenible
Wanderleia Elizabeth Brinckmann,Carlos André Brinckmann,Frederico Bonnenberger
Papeles de Geografía , 2002,
Abstract: La finalidad del presente artículo consiste en estudiar el cuadro socioeconómico y ambiental de la agricultura familiar en la Cuenca Hidrográfica del Río Pardo, Brasil. Se busca analizar la producción agroecológica, sus dificultades y potencialidades. El análisis se refiere a las estrategias de desarrollo rural sostenible en curso en la Cuenca Hidrográfica del Río Pardo.
Desarrollo, complejidad y turismo sostenible. El uso del territorio frente a los retos del siglo XXI
Wanderléia Elizabeth Brinckmann,Mauricio Neison Brinckmann,Débora Cristina Mueller
Papeles de Geografía , 2010,
Abstract: El siglo XXI trae consigo grandes retos desafiando a los investigadores de diferentes áreas del conocimiento a estudiar la complejidad del territorio. Entender pues, el movimiento de re-organización y re-funcionalización del territorio a partir de algunos ejes vertebradores como sean el desarrollo y el turismo sostenible, es el reto que proponemos en esta reflexión.
Wick's Theorem and a New Perturbation Theory Around the Atomic Limit of Strongly Correlated Electron Systems
Jan Brinckmann
Physics , 1994, DOI: 10.1209/0295-5075/28/3/006
Abstract: A new type of perturbation expansion in the mixing $V$ of localized orbitals with a conduction-electron band in the $U\to\infty$ Anderson model is presented. It is built on Feynman diagrams obeying standard rules. The local correlations of the unperturbed system (the atomic limit) are included exactly, no auxiliary particles are introduced. As a test, an infinite-order ladder-type resummation is analytically treated in the Kondo regime, recovering the correct energy scale. An extension to the Anderson-lattice model is obtained via an effective-site approximation through a cumulant expansion in $V$ on the lattice. Relation to treatments in infinite spatial dimensions are indicated.
Self-Consistent Strong-Coupling-Perturbation Theory for the Anderson Model, Based on Wicks Theorem
Jan Brinckmann
Physics , 1996, DOI: 10.1103/PhysRevB.54.10465
Abstract: A strong-coupling-perturbation theory around the Atomic Limit of the Anderson model with large $U$ for a localized $f$-orbital coupled to a conduction-electron band is presented. Although an auxiliary-particle representation is {\em not} used, application of the canonical Wick's theorem is possible and yields an expansion in the hybridization $V$ via dressed skeleton-Feynman diagrams. The Self-Consistent T-Approximation is constructed as a $\Phi$-derivable approximation. From a numerical solution of self-consistency equations the $f$-electron-excitation spectrum is investigated. Comparison to the Non-Crossing Approximation is made in virtue of exact formal relations and numerical results. An extension of this Feynman-diagram approach to the Anderson-lattice model is indicated, and application within the Local-Approximation scheme (limit of infinite spatial dimension) is given.
Strong-coupling approach to ground-state properties of the Anderson lattice-model
Jan Brinckmann
Physics , 1996, DOI: 10.1016/S0921-4526(97)81586-4
Abstract: A Slave-Boson perturbational approach to ground-state properties of the $U\to\infty$ periodic Anderson model is derived as an expansion around the Atomic Limit ($V=0$). In the case of zero temperature any constraint-integral or limiting procedure can be avoided, a gauge-symmetry broken Mean-Field phase is not involved. Physical quantities like the wave vector $k$ dependent Green's function obey a direct representation in Feynman-skeleton diagrams in $k$-space. A self-consistent $1/N$-expansion is derived, and its relation to the limit of infinite spatial dimension $d\to\infty$ is pointed out.
On Perturbation Theory Around the Atomic Limit of Strongly Correlated Electron Systems: A New Approach Based on Wick's Theorem
Jan Brinckmann
Physics , 1994, DOI: 10.1016/0921-4526(94)00398-F
Abstract: A new perturbational approach to spectral and thermal properties of strongly correlated electron systems is presented: The Anderson model is reexamined for $U\to\infty$\,, and it is shown that an expansion of Green's functions with respect to the hybridization $V$ built on Feynman diagrams obeying standard rules is possible. The local correlations of the unperturbed system (the atomic limit) are included exactly through a two-particle vertex. No auxiliary particles are introduced into the theory. As an example and test the small energy scale and many-body resonance of the Kondo problem are reproduced analytically.
Auxiliary-Fermion Approach to Critical Fluctuations in the 2D Quantum AF Heisenberg Model
Jan Brinckmann,Peter Woelfle
Physics , 2004, DOI: 10.1103/PhysRevB.70.174445
Abstract: The nearest-neighbor quantum-antiferromagnetic (AF) Heisenberg model for spin 1/2 on a two-dimensional square lattice is studied in the auxiliary-fermion representation. Expressing spin operators by canonical fermionic particles requires a constraint on the fermion charge Q=1 on each lattice site, which is imposed approximately through the thermal average. The resulting interacting fermion system is first treated in mean-field theory (MFT), which yields an AF ordered ground state and spin waves in quantitative agreement with conventional spin-wave theory. At finite temperature a self-consistent approximation beyond mean field is required in order to fulfill the Mermin-Wagner theorem. We first discuss a fully self-consistent approximation, where fermions are renormalized due to fluctuations of their spin density, in close analogy to FLEX. While static properties like the correlation length come out correctly, the dynamical response lacks the magnon-like peaks which would reflect the appearance of short-range order at low T. This drawback, which is caused by overdamping, is overcome in a `minimal self-consistent approximation' (MSCA), which we derive from the equations of motion. The MSCA features dynamical scaling at small energy and temperature and is qualitatively correct both in the regime of order-parameter relaxation at long wavelengths and in the short-range-order regime. We also discuss the impact of vertex corrections and the problem of pseudo-gap formation in the single-particle density of states due to long-range fluctuations. Finally we show that the (short-range) magnetic order in MFT and MSCA helps to fulfill the constraint on the local fermion occupancy.
Diagrammatic approximations for the 2d quantum antiferromagnet: exact projection of auxiliary fermions
Jan Brinckmann,Peter Woelfle
Physics , 2008,
Abstract: We present diagrammatic approximations to the spin dynamics of the 2d Heisenberg antiferromagnet for all temperatures, employing an auxiliary-fermion representation. The projection onto the physical subspace is effected by introducing an imaginary-valued chemical potential as proposed by Popov and Fedotov. The method requires that the fermion number at any lattice site is strictly conserved. We compare results obtained within a self-consistent approximation using two different auxiliary-particle projection schemes, (1) exact and (2) on average. Significant differences between the two are found at higher temperatures, whereas in the limit of zero temperature (approaching the magnetically ordered ground state) identical results emerge from (1) and (2), providing the qualitatively correct dynamical scaling behavior. An interpretation of these findings is given. We also present in some detail the derivation of the approximation, which goes far beyond mean-field theory and is formulated in terms of complex-valued spectral functions of auxiliary fermions.
Spin susceptibility and the pi-excitation in underdoped cuprates
Jan Brinckmann,Patrick A. Lee
Physics , 1997, DOI: 10.1016/S0022-3697(98)00111-5
Abstract: The dynamical spin susceptibility chi'' at wave vector (pi, pi) and the spectrum pi'' of the spin-triplet particle--particle excitation with center of mass momentum (pi, pi) (pi-excitation) are considered in the slave-boson formulation of the t--J-model. Propagators are calculated in a diagrammatic t-matrix approximation in the d-wave superconducting state for a wide doping range. The resulting spectra chi'' and pi'' both show a resonance at a doping dependent energy, in qualitative agreement with recent numerical cluster calculations. In underdoped systems, the peak position is comparable to that found in neutron scattering experiments. The peak in chi'' as well as pi'' is at low doping entirely caused by spin fluctuations, whereas the triplet particle--particle channel does not contribute as a collective mode.
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