Comment on ``Validity of Feynman's prescription of disregarding the Pauli principle in intermediate states''

In a recent paper Coutinho, Nogami and Tomio [Phys. Rev. A 59, 2624 (1999); quant-ph/9812073] presented an example in which, they claim, Feynman's prescription of disregarding the Pauli principle in intermediate states of perturbation theory fails. We show that, contrary to their claim, Feynman's prescription is consistent with the exact solution of their example.Comment: 1 pag

Doped AB_2 Hubbard Chain: Spiral, Nagaoka and RVB States, Phase Separation and Luttinger Liquid Behavior

We present an extensive numerical study of the Hubbard model on the doped AB$_2$ chain, both in the weak coupling and the infinite-U limit. Due to the special unit cell topology, this system displays a rich variety of phases as function of hole doping ($\delta$) away from half-filling. Near half-filling, spiral states develop in the weak coupling regime, while Nagaoka itinerant ferromagnetism is observed in the infinite-U limit. For higher doping the system phase-separates before reaching a Mott insulating phase of short-range RVB states at $\delta=1/3$. Moreover, for $\delta>1/3$ we observe a crossover, which anticipates the Luttinger liquid behavior for $\delta > 2/3$.Comment: 11 pages, 13 figure

Conservation of Orbital Angular Momentum in Stimulated Down-Conversion

We report on an experiment demonstrating the conservation of orbital angular momentum in stimulated down-conversion. The orbital angular momentum is not transferred to the individual beams of the spontaneous down-conversion, but it is conserved when twin photons are taken individually. We observe the conservation law for an individual beam of the down-conversion through cavity-free stimulated emission.Comment: Submitted for publication in Phys. Rev. Let

Avaliação dos efeitos de um protocolo com a plataforma NINTENDO WII® nas dimensões cognitivas e físicas /equilíbrio e coordenação) em idosos

Objetivos: Avaliar e quantificar o impacto do tratamento de fisioterapia, utilizando exclusivamente a Nintendo Wii®, na Qualidade de Vida relacionada com a saúde dos sujeitos com 65 anos ou mais e as mudanças na cognição, equilíbrio, coordenação e desempenho das Atividades da Vida Diária após o protocolo; Inquirir o grau de adesão ao treino. Material e Métodos: 30 sujeitos em dois momentos de avaliação. Medidas aplicadas: SF-12, MSE, TUG e com tarefa cognitiva, Escala de Equilíbrio de Berg, avaliação da coordenação e Escala de Barthel. Intervenção durante 4 semanas, 2 por semana, 60 minutos, em 8 sessões. Resultados: Ganhos em todas as escalas, com significado estatístico na Escala de Saúde Mental Geral, componente do SF-12 (p=0,008), MMSE (p=0,007), TUG normal (p=0,003), TUG com tarefa cognitiva (p=0,028) e Protocolo de Avaliação da Coordenação proposto por O’Neil et al. (p=0,008). Da amostra 46,67% teve um Baixo Grau de Adesão, 30% Médio Grau de Adesão, e 23,33% Alto Grau de Adesão. Conclusões: Um protocolo de intervenção de 4 semanas com a Nintendo Wii Sports® (bowling) em sujeitos institucionalizados, apresenta benefícios na QV relacionada com a Saúde Metal e de cognição, e dimensão física da coordenação e capacidade para a dupla-tarefa

Magnetism and Electronic Correlations in Quasi-One-Dimensional Compounds

In this contribution on the celebration of the 80th birthday anniversary of Prof. Ricardo Ferreira, we present a brief survey on the magnetism of quasi-one-dimensional compounds. This has been a research area of intense activity particularly since the first experimental announcements of magnetism in organic and organometallic polymers in the mid 80s. We review experimental and theoretical achievements on the field, featuring chain systems of correlated electrons in a special AB2 unit cell structure present in inorganic and organic compounds

Inducing energy gaps in graphene monolayer and bilayer

In this paper we propose a mechanism for the induction of energy gaps in the spectrum of graphene and its bilayer, when both these materials are covered with water and ammonia molecules. The energy gaps obtained are within the range 20-30 meV, values compatible to those found in experimental studies of graphene bilayer. We further show that the binding energies are large enough for the adsorption of the molecules to be maintained even at room temperature