Polaron effects in electron channels on a helium film

Using the Feynman path-integral formalism we study the polaron effects in quantum wires above a liquid helium film. The electron interacts with two-dimensional (2D) surface phonons, i.e. ripplons, and is confined in one dimension (1D) by an harmonic potential. The obtained results are valid for arbitrary temperature ($T$), electron-phonon coupling strength ($\alpha$), and lateral confinement ($\omega_{0}$). Analytical and numerical results are obtained for limiting cases of $T$, $\alpha$, and $\omega_{0}$. We found the surprising result that reducing the electron motion from 2D to quasi-1D makes the self-trapping transition more continuous.Comment: 6 pages, 7 figures, submitted to Phys. Rev.

Aplicação localizada de herbicidas para controle de alecrim-do-campo (Vernonia nudiflora) em pastagem natural.

Resumo: Para avaliar o controle de alecrim-do-campo (Vernonia nudiflora) com aplicação de herbicidas de contato, foi conduzido durante 353 dias, um experimento com delineamento de parcelas subdivididas, dispostas em blocos ao acaso, com três repetições em um arranjo fatorial (5x3)+1, sendo cinco herbicidas em três doses mais a testemunha. As parcelas corresponderam aos herbicidas, com doses de 1,5; 3,0 e 4,5 L ha-1 de Tordon, Aminol 806, Garlon, Grazon e Plenum. A abundância de V. nudiflora foi avaliada antes da aplicação e 60 e 353 dias após. A aplicação foi feita com ?roçadeira química? tratorizada, consistindo em uma plataforma com bandejas com altura regulável, que entra em contato somente com as plantas alvo. Os herbicidas não diferiram quanto à redução na abundância do alecrim e todos reduziram significativamente mais que a testemunha. Os tratamentos não interferiram na frequência na composição de espécies desejáveis no campo nativo. O uso do aplicador químico é eficaz para o controle, mesmo com baixas concentrações de herbicidas, tendo eficácia na redução do alecrim-do-campo e preservando as espécies desejáveis para a produção animal. [Located herbicide application to control alecrim-do-campo (Vernonia nudiflora) in natural grassland]. Abstract: To evaluate the control of alecrim-do-campo (Vernonia nudiflora) with contact herbicides, an experiment was conducted during 353 days, following a split plot block design, with three replicates in a (5x3)+1 factorial arrangement, being five herbicides in three doses and the control. The plots corresponded to the herbicides, with doses of 1.5, 3.0 and 4.5 L ha-1 of Tordon, Aminol 806, Garlon, Grazon e Plenum. The presence of V. nudiflora was evaluated prior to application and 60 and 353 days after. The application was made using a chemical applicator, which consists of a platform of adjustable height that contacts only the target plant. The herbicides did not differ in reducing the abundance of V. nudiflora and all of them reduced significantly more than the control. The chemical applicator was effective to control alecrim-do-campo, even with low herbicide concentrations, reducing the Vernonia abundance and not being harmful to the native desirable species

Controle de espécies indesejáveis e preservação de leguminosas de campo nativo com aplicação localizada de herbicidas.

Resumo: Para avaliar o efeito da aplicação localizada de herbicidas por contato, em Desmodium incanum, foi conduzido este estudo pelo período de 353 dias. O delineamento experimental foi de parcelas subdivididas, dispostas em blocos ao acaso, com três repetições em um arranjo fatorial [(5x3)+1], sendo cinco herbicidas mais a testemunha e três épocas de avaliação. As parcelas corresponderam aos herbicidas, locados na dose de 1,5 L ha-1 de Tordon, Aminol 806, Garlon, Grazon e Plenum, sendo compostas por oito quadros dispostos sobre uma transecta fixa. Realizou-se um levantamento prévio ao experimento, e dois levantamentos subseqüentes aos 60 e 365 dias apos a aplicação. A planta indesejável mais frequente era Vernonia nudiflora. Para aplicação dos herbicidas utilizou-se uma 'roçadeira química' tratorizada, consistindo em uma plataforma com bandejas com altura regulável, que entra em contato somente com as plantas alvo. O uso do aplicador químico foi eficaz para o controle de espécies indesejáveis além de não interferir na frequência do Desmodium incanum, sendo interessante a limpeza seletiva e preservação da flora nativa. [Weed control and preservation of native legumes species in natural grassland by herbicide localized application] Abstract: To evaluate the effect of localized application of herbicides by contact, in Desmodium incanum, this trial was conducted during a period of 353 days. The experimental design was a split plot arranged in a randomized block design with three replications in a factorial arrangement [(5x3) +1], five herbicides plus the control and three evaluation periods. The plots corresponded to herbicides, doses were locate: 1.5, 3.0 and 4.5 L ha -1 of Tordon, Aminol 806, Garlon, Grazon e Plenum, being composed by 8 tables arranged on a fixed line. Prior to application, was made a survey, another in 60 and 353 days after application. For the herbicides application, was used a chemical applicator, which consists in an adjustable height platform, and it is contact is only with the target plant. The use of chemical applicator was effective for controlling undesirable species besides not interfere on Desmodium incanum frequency besides being interesting for selective cleaning and preservation of the native vegetation

Yukawa particles confined in a channel and subject to a periodic potential: ground state and normal modes

We consider a classical system of two-dimensional (2D) charged particles, which interact through a repulsive Yukawa potential $exp(-r/\lambda)/r$, confined in a parabolic channel which limits the motion of the particles in the $y$-direction. Along the $x$-direction, the particles are also subject to a periodic potential substrate. The ground state configurations and the normal mode spectra of the system are obtained as function of the periodicity and strength of the periodic potential ($V_0$), and density. An interesting set of tunable ground state configurations are found, with first and second order structural transitions between them. A magic configuration with particles aligned in each minimum of the periodic potential is obtained for V_0 larger than some critical value which has a power law dependence on the density. The phonon spectrum of different configurations were also calculated. A localization of the modes into a small frequency interval is observed for a sufficient strength of the periodic potential. A tunable band-gap is found as a function of $V_0$. This model system can be viewed as a generalization of the Frenkel and Kontorova model.Comment: Submmited to PR

Performance and emission of pollutants of an agricultural engine with two-power take-off settings

Farming operations are usually performed implements driven by a power take-off (PTO) from tractors. Some manufacturers offer PTO settings named as economic (540E). Such configuration drives implements at lower tractor’s engine rotation, thus providing lower fuel consumption. An engine performance using standard (540) or 540E settings are different and poorly studied. The amounts of pollutants emitted by engines vary mainly with the applied load and rotation. The aim of this study was to compare the performance and exhaust emissions of a farming tractor engine under different PTO loads, using both standard (540) and 540E settings. Specific fuel consumption using 540E was an average 14.7% less than the standard setting. However, 540E promoted increasing levels of gas opacity, CO, CO2, NOx, and HC for the highest PTO power demands.Postprint (published version

Spatial distribution pattern of Euxylophora paraensis Huber in a natural managed forest in the Eastern Amazon.

O objetivo desse trabalho foi caracterizar o padrão de distribuição espacial de Euxylophora paraensis (pau-amarelo), para subsidiar estratégias de conservação dessa espécie em floresta de terra firme manejada no estado do Pará. A área de estudo está localizada na Fazenda Rio Capim, pertencente a CKBV Florestal Ltda., no município de Paragominas. Para a análise da distribuição espacial das árvores utilizou-se a geoestatística, a partir da modelagem de semivariograma e confecção de mapas de krigagem. Todas as avaliações tiveram melhor ajuste ao modelo esférico, apresentando o maior coeficiente de determinação em relação aos outros modelos testados. Euxylophora paraensis apresentou padrão de distribuição agregada na floresta estudada, com dependência espacial descrita pelo modelo esférico, formando reboleiras de 300 a 1000 m