Anomalous Hall conductivity: local orbitals approach

A theory of the anomalous Hall conductivity based on the properties of single site orbitals is presented. Effect of the finite electron life time is modeled by energy fluctuations of atomic-like orbitals. Transition from the ideal Bloch system for which the conductivity is determined by the Berry phase curvatures to the case of strong disorder for which the conductivity becomes dependent on the relaxation time is analyzed. Presented tight-binding model gives by the unified way experimentally observed qualitative features of the anomalous conductivity in the so called good metal regime and that called as bad metal or hopping regime.Comment: 8 pages, 3 figure

Equilibrium Current and Orbital Magnetization in a Quantum Hall Fluid

We present a general theory for the equilibrium current distribution in an interacting two-dimensional electron gas subjected to a perpendicular magnetic field, and confined by a potential that varies slowly on the scale of the magnetic length. The distribution is found to consist of strips or channels of current, which alternate in direction, and which have universal integrated strength.Comment: 13 pages, Revtex, to appear in the proceedings of the "Workshop on Novel Physics in Low-Dimensional Electron Systems" held in Madra

Síndrome de down e o determinismo da deficiência intelectual : refutando certezas, propondo outra lógica

Esta tese tem como problema central a associação direta entre síndrome de Down e deficiência intelectual como resultado do determinismo biológico, genético. Trata-se de um estudo teórico em forma de ensaio que faz dialogar diferentes áreas do conhecimento, com o objetivo principal de refutar a certeza e a generalização de um aprender restrito para pessoas com a síndrome de Down. A partir da compreensão das condições de produção dos enunciados científicos e culturais que associam a síndrome de Down à deficiência intelectual desde o século XIX, buscamos, na história das ideias, nos estudos sobre deficiência e nas proposições da epigenética, argumentos que sustentem a crítica à ideia de uma natureza determinista do indivíduo e ao reducionismo imposto pelo modelo médico de deficiência, bem como argumentos que contestem o status que o centrismo genético ocupa na conceituação da síndrome de Down. Por fim, oferecemos uma discussão sobre a inteligência e a aprendizagem propondo uma lógica que considera a complexidade do desenvolvimento humano e de suas capacidades intelectuais. Com isso, pretendemos não apenas reforçar a ideia de que alguém com a síndrome de Down pode aprender para além das restrições culturalmente aceitas como invariáveis, mas também defender a tese de que alguém com síndrome de Down não pode ser considerado deficiente intelectual antes de que — além dos fatores biológicos ou genéticos — os fatores culturais, sociais, educacionais, intersubjetivos e inconscientes possam entrar em jogo.The main problem discussed in this paper is the direct association between Down syndrome and intellectual disability as a result of biological (genetic) determinism. This is a theoretical study presented as an essay that combines different subject areas in an attempt to refute the belief and generalization around restricted learning for people with Down syndrome. We seek to understand the production conditions of the scientific and cultural statements that associate Down syndrome with intellectual disability since the 19th century. In doing so, we use the history of ideas, studies about disability, and epigenetics to gather arguments to review the idea that individuals have a deterministic nature and to refute the reductionism imposed by the medical model of disability. Furthermore, we propose arguments against the gene-centrism in the conceptualization of Down syndrome. Finally, we offer a discussion on intelligence and learning and propose an understanding that takes into consideration the complexity of human development and its intellectual capacities. Thus, we intend not only to reinforce the idea that someone with Down syndrome can learn, beyond the culturally accepted restrictions as unvarying, but also to defend the thesis that someone with Down syndrome cannot be considered intellectually disabled disregarding cultural, social, educational, intersubjective and unconscious factors, besides biological or genetic factors.Esa tesis tiene como problema central la asociación directa entre síndrome de Down y la deficiencia intelectual, como resultado del determinismo biológico, genético. Se trata de un estudio teórico, en forma de ensayo, que pretende relacionar diferentes áreas del conocimiento, cuyo objetivo principal de refutar la certeza y la generalización de un aprendizaje restrictivo para personas con el síndrome de Down. A partir de la comprensión de las condiciones de producción de los enunciados científicos y culturales que asocian el síndrome de Down con la deficiencia intelectual desde el siglo XIX, buscamos, en la historia de las ideas, en los estudios sobre discapacidad y en las proposiciones de la epigenética, argumentos que sostienen la crítica a la idea de una naturaleza determinada del individuo como así también al reduccionismo impuesto por el modelo médico de discapacidad, y argumentos que cuestionan el status que el centrismo genético ocupa en la conceptualización del síndrome de Down. Por último, ofrecemos una discusión sobre la inteligencia y el aprendizaje proponiendo una lógica que conduce a la complejidad del desarrollo humano y de sus capacidades intelectuales. Con ello pretendemos no solo reforzar la idea de que alguien con el síndrome de Down puede aprender más allá de las restricciones culturalmente aceptadas como invariables; y defender la tesis sosteniendo que alguien con síndrome de Down no puede ser considerado discapacitado intelectual antes que — además de los factores biológicos o genéticos — los factores culturales, sociales, educativos, intersubjetivos e inconscientes puedan entrar en juego

Beating of the oscillations in the transport coefficients of a one-dimensionally periodically modulated two-dimensional electron gas in the presence of spin-orbit interaction

Transport properties of a two-dimensional electron gas (2DEG) are studied in the presence of a perpendicular magnetic field $B$, of a {\it weak} one-dimensional (1D) periodic potential modulation, and of the spin-orbit interaction (SOI) described only by the Rashba term. In the absence of the modulation the SOI mixes the spin-up and spin-down states of neighboring Landau levels into two new, unequally spaced energy branches. The levels of these branches broaden into bands in the presence of the modulation and their bandwidths oscillate with the field $B$. Evaluated at the Fermi energy, the $n$-th level bandwidth of each series has a minimum or vanishes at different values of the field $B$. In contrast with the 1D-modulated 2DEG without SOI, for which only one flat-band condition applies, here there are two flat-band conditions that can change considerably as a function of the SOI strength $\alpha$ and accordingly influence the transport coefficients of the 2DEG. The phase and amplitude of the Weiss and Shubnikov-de Haas (SdH) oscillations depend on the strength $\alpha$. For small values of $\alpha$ both oscillations show beating patterns. Those of the former are due to the independently oscillating bandwidths whereas those of the latter are due to modifications of the density of states, exhibit an even-odd filling factor transition, and are nearly independent of the modulation strength. For strong values of $\alpha$ the SdH oscillations are split in two

ELES PODEM FAZER MUITAS COISAS...” A Síndrome de Down e o aprender em cena

Culturalmente, a síndrome de Down aparece associada à impossibilidade de aprender. Como efeito, surgem os limites sociais impostos à aprendizagem e aos espaços escolares por onde circulam os sujeitos. Em tempos de inclusão escolar, é fundamental que esta questão seja problematizada no contexto de formação de professores. Como disparador das questões que compõe este artigo, utiliza-se a narrativa de uma propaganda veiculada na mídia no fim do século XX, que traz essa associação. O conceito de modalidad de aprendizaje da psicopedagoga argentina Alicia Fernández operacionaliza a discussão. Desnaturalizar aquilo que se antecipa como limite posto pela condição genética se torna um movimento necessário para que o professor possa reconhecer seu aluno com síndrome de Down como um sujeito pode aprender

Magnetic Miniband Structure and Quantum Oscillations in Lateral Semiconductor Superlattices

We present fully quantum-mechanical magnetotransport calculations for short-period lateral superlattices with one-dimensional electrostatic modulation. A non-perturbative treatment of both magnetic field and modulation potential proves to be necessary to reproduce novel quantum oscillations in the magnetoresistance found in recent experiments in the resistance component parallel to the modulation potential. In addition, we predict oscillations of opposite phase in the component perpendicular to the modulation not yet observed experimentally. We show that the new oscillations originate from the magnetic miniband structure in the regime of overlapping minibands.Comment: 6 pages with 4 figure

Spin Hall effect in Sr2RuO4 and transition metals (Nb,Ta)

We study the intrinsic spin Hall conductivity (SHC) and the $d$-orbital Hall conductivity (OHC) in metallic $d$-electron systems based on the multiorbital tight-binding model. The obtained Hall conductivities are much larger than that in $p$-type semiconductors. The origin of these huge Hall effects is the "effective Aharonov-Bohm phase" induced by the signs of inter-orbital hopping integrals as well as atomic spin-orbit interaction. Huge SHC and OHC due to this mecahnism is ubiquitous in multiorbital transition metals.Comment: 4 pages, 3 figures, Proceedings of SNS conference in Sendai, 200

Visibility diagrams and experimental stripe structure in the quantum Hall effect

We analyze various properties of the visibility diagrams that can be used in the context of modular symmetries and confront them to some recent experimental developments in the Quantum Hall Effect. We show that a suitable physical interpretation of the visibility diagrams which permits one to describe successfully the observed architecture of the Quantum Hall states gives rise naturally to a stripe structure reproducing some of the experimental features that have been observed in the study of the quantum fluctuations of the Hall conductance. Furthermore, we exhibit new properties of the visibility diagrams stemming from the structure of subgroups of the full modular group.Comment: 8 pages in plain TeX, 7 figures in a single postscript fil