Navegação Ciências Exatas e da Terra por Autor "Theumann, Alba Graciela Rivas de"
Resultados 1-14 de 14
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Anisotropic magnetoconductivity of a three-dimensional disordered electron gas
Theumann, Alba Graciela Rivas de; Magalhães, Sergio Garcia (1989) [Artigo de periódico]The magnetoconductivity of a three-dimensional disordered electron gas is calculated from first principies with use of the exact electron eigenstates in a magnetic field, thus avoiding the usual semiclassical approximation ... -
Categorization in a Hopfield network trained with weighted examples : extensive number of concepts
Costa, Rogerio Adeodato Lima; Theumann, Alba Graciela Rivas de (2000) [Artigo de periódico]We consider the categorization problem in a Hopfield network with an extensive number of concepts p =αN and trained with s examples of weight λτ, T=1, . . . ,s in the presence of synaptic noise represented by a dimensionless ... -
Conductivity of a metal with inverse-power-law correlated impurities
Varriale, Maria Cristina; Theumann, Alba Graciela Rivas de (1995) [Artigo de periódico]We calculate the cooperon for a system of noninteracting electrons in the presence of random potentials with correlations W(r)= W08(r)+ W,r-<d+u> in d dimensions, for arbitrary values a and d, to first order in W1. Our ... -
Effect of a finite-range impurity potential on two-dimensional anderson localization
Beal-Monod, M.T.; Theumann, Alba Graciela Rivas de; Forgacs, Gabor (1992) [Artigo de periódico]We show that in two-dimensional weakly disordered metais a finite-range impurity-scattering potential reinforces the localization of the electronic states. -
Magnetoconductivity of a disordered electron system with n orbitals persite
Magalhães, Sergio Garcia; Theumann, Alba Graciela Rivas de (1994) [Artigo de periódico]In the absence of a magnetic field, two formally equivalent expressions for the conductivity contribute different sets of Feynman diagrams, due to cancellations. We use this result to calculate the magnetoconductivity of ... -
Multiband and Mass-Anisotropy Effects in the Magnetoconductivity of a Disordered Electron Gas
Magalhães, Sergio Garcia; Theumann, Alba Graciela Rivas de (1993) [Artigo de periódico]We investigate the magnetoconductance of a two-band electron system and a model with effectivemass anisotropy by means of the method based on the exact electron eigenstates in a magnetic field proposed formerly by us to ... -
Quantum critical point in the spin glass-Kondo transition in heavy-fermion systems
Theumann, Alba Graciela Rivas de; Coqblin, Bernard (2004) [Artigo de periódico]The Kondo–spin glass competition is studied in a theoretical model of a Kondo lattice with an intrasite Kondo-type exchange interaction treated within the mean-field approximation, an intersite quantum Ising exchange ... -
Quantum spherical spin glass : supersymmetry and annealing
Silva, Pedro Castro Menezes Xavier de Mello e; Theumann, Alba Graciela Rivas de (2007) [Artigo de periódico]We show that the effective action of the quantum spherical spin glass is invariant under a generalized form of Becchi-Rouet-Stora-Tyutin supersymmetry. The Ward identities associated with this invariance indicate that the ... -
Quantum spin glass : a replica-symmetric theory with positive entropy
Theumann, Alba Graciela Rivas de (1986) [Artigo de periódico]In the present paper we present a detailed analysis of quantum effects in the spin-glass transition as described by a quantum Heisenberg analogue of the Sherrington-Kirkpatrick model. The spin operators are represented in ... -
Replica symmetry breaking in the transverse-field Ising spin-glass model : two fermionic representations
Santos, Eduardo Muccillo Monteiro; Theumann, Alba Graciela Rivas de (2005) [Artigo de periódico]We analyze the infinite range Ising spin glass in a transverse-field below the critical temperature by a one step replica symmetry breaking theory. The set of n replicas is divided in r blocks of m replicas each. We present ... -
Singularities in the paramagnetism of two-dimensional nearly magnetic itinerant-fermion systems at very low temperature : application to degenerate two-dimensional liquid- 3 He films
Theumann, Alba Graciela Rivas de; Beal-Monod, M.T. (1984) [Artigo de periódico]It is shown that two-dimensional (2D) paramagnon problems exhibit strong algebraic singularities |q2-4K 2/f| -a, when some relevant momentum q is close to twice the Fermi momentum KF. As ali q values from O to 2KF are ... -
Space of interactions with definite symmetry in neural networks with biased patterns as a spin-glass problem
Theumann, Alba Graciela Rivas de (1996) [Artigo de periódico]We study the space of interactions of a connected neural network with biased patterns, when the synaptic interactions satisfy a symmetry constraint. We show that the solution to the problem requires the calculation of a ... -
Spin-glass freezing in Kondo-lattice compounds
Theumann, Alba Graciela Rivas de; Coqblin, Bernard; Magalhães, Sergio Garcia; Schmidt, Alex Andre (2001) [Artigo de periódico]A theory is presented that describes a spin-glass phase at finite temperatures in Kondo-lattice systems with an additional Ruderman–Kittel–Kasuya–Yosida interaction represented by long range, random couplings among localized ... -
Weak instability of frustated fermionic models
Theumann, Alba Graciela Rivas de (1997) [Artigo de periódico]We study the Almeida-Thouless instability of two fermionic models analogous to spin glasses that exhibit frustration and that were solved some time ago with a replica symmetric ansatz. In the first model (I) we consider ...