The Magnetic Ordering of the 3d Wigner Crystal

Using Path Integral Monte Carlo, we have calculated exchange frequencies as electrons undergo ring exchanges of 2, 3 and 4 electrons in a ``clean'' 3d Wigner crystal (bcc lattice) as a function of density. We find pair exchange dominates and estimate the critical temperature for the transition to antiferromagnetic ordering to be roughly $1 \times 10^{-8}$Ry at melting. In contrast to the situation in 2d, the 3d Wigner crystal is different from the solid bcc 3He in that the pair exchange dominates because of the softer interparticle potential. We discuss implications for the magnetic phase diagram of the electron gas

A mechanism for detecting normally hyperbolic invariant tori in differential equations

Determining the existence of compact invariant manifolds is a central quest in the qualitative theory of differential equations. Singularities, periodic solutions, and invariant tori are examples of such invariant manifolds. A classical and useful result from the averaging theory relates the existence of isolated periodic solutions of non-autonomous periodic differential equations, given in a specific standard form, with the existence of simple singularities of the so-called guiding system, which is an autonomous differential equation given in terms of the first non-vanishing higher order averaged function. In this paper, we provide an analogous result for the existence of invariant tori. Namely, we show that a non-autonomous periodic differential equation, given in the standard form, has a normally hyperbolic invariant torus in the extended phase space provided that the guiding system has a hyperbolic limit cycle. We apply this result to show the existence of normally hyperbolic invariant tori in a family of jerk differential equations

Formation energy and interaction of point defects in two-dimensional colloidal crystals

The manipulation of individual colloidal particles using optical tweezers has allowed vacancies to be created in two-dimensional (2d) colloidal crystals, with unprecedented possibility of real-time monitoring the dynamics of such defects (Nature {\bf 413}, 147 (2001)). In this Letter, we employ molecular dynamics (MD) simulations to calculate the formation energy of single defects and the binding energy between pairs of defects in a 2d colloidal crystal. In the light of our results, experimental observations of vacancies could be explained and then compared to simulation results for the interstitial defects. We see a remarkable similarity between our results for a 2d colloidal crystal and the 2d Wigner crystal (Phys. Rev. Lett. {\bf 86}, 492 (2001)). The results show that the formation energy to create a single interstitial is $12$ lower than that of the vacancy. Because the pair binding energies of the defects are strongly attractive for short distances, the ground state should correspond to bound pairs with the interstitial bound pairs being the most probable.Comment: 5 pages, 2 figure

Classical artificial two-dimensional atoms: the Thomson model

The ring configurations for classical two-dimensional atoms are calculated within the Thomson model and compared with the results from `exact' numerical simulations. The influence of the functional form of the confinement potential and the repulsive interaction potential between the particles on the configurations is investigated. We also give exact results on those eigenmodes of the system whose frequency does not depend on the number of particles in the system.Comment: 9 pages, RevTeX, 4 figure

Modelling of the IPS buoy wave energy converter including the effect of non-uniform tube cross-section

An important class of floating wave energy converters (that includes the IPS buoy, the Wavebob and the PowerBuoy) comprehends devices in which the energy is converted from the relative (essentially heaving) motion between two bodies oscillating differently. The paper considers the case of the IPS buoy, consisting of a floater rigidly connected to a fully submerged vertical (acceleration) tube open at both ends. The tube contains a piston whose motion relative to the floater-tube system (motion originated by wave action on the floater and by the inertia of the water enclosed in the tube) drives a power take-off mechanism (PTO) (assumed to be a linear damper). To solve the problem of the end-stops, the central part of the tube, along which the piston slides, bells out at either end to limit the stroke of the piston. The use of a hydraulic turbine inside the tube is examined as an alternative to the piston. A frequency domain analysis of the device in regular waves is developed, combined with a one-dimensional unsteady flow model inside the tube (whose cross-section is in general nonuniform). Numerical results are presented for a cylindrical buoy in regular waves, including the optimization of the acceleration tube geometry and PTO damping coefficient for several wave periods

Supersymmetric and Shape-Invariant Generalization for Nonresonant and Intensity-Dependent Jaynes-Cummings Systems

A class of shape-invariant bound-state problems which represent transition in a two-level system introduced earlier are generalized to include arbitrary energy splittings between the two levels as well as intensity-dependent interactions. We show that the couple-channel Hamiltonians obtained correspond to the generalizations of the nonresonant and intensity-dependent nonresonant Jaynes-Cummings Hamiltonians, widely used in quantized theories of laser. In this general context, we determine the eigenstates, eigenvalues, the time evolution matrix and the population inversion matrix factor.Comment: A combined version of quant-ph/0005045 and quant-ph/0005046. 24 pages, LATE

Eficiência de uso da água para produção de leite de cabra em pasto de capim-tanzânia (Panicum maximum cv. Tanzânia) no Nordeste Brasileiro.

O uso de pastagens cultivadas e irrigadas tem sido uma alternativa utilizada para distribuir melhor a oferta de forragem ao longo do ano, melhorando o desempenho dos sistemas pecuários de produção. O objetivo deste estudo foi determinar a eficiência de uso da água para a produção de forragem e a produtividade da água para produção de leite de cabra em pasto irrigado de capim-tanzânia, submetido a diferentes manejos. Os manejos testados foram combinações entre níveis de adubação nitrogenada e alturas resíduais pós-pastejo (Altr) de modo a imprimir ao pasto diferentes manejos: intensivo (600kg N/ha ano-1 e 33cm de Altr); moderado (600kg N/ha ano-1 e 47cm de Altr); leve (0 kg N/ha ano-1 e 47 cm de Altr) e convencional (0 kg N/ha ano-1 e 33cm de Altr). As maiores eficiências de uso da água foram obtidas nos manejos adubados. Nestas condições, um mm de água foi capaz de produzir até 19 kg de forragem e 10l de leite de cabra. A quantidade de água para produzir um litro de leite de cabra variou de 2.000 a 6000L

Componentes da biomassa pós-pastejo do dossel de capim-tifton 85 submetido a diferentes manejos sob lotação rotativa.

Resumo: Avaliaram-se componentes de biomassa pós-pastejo do capim-tifton 85 submetido a diferentes manejos. Os manejos consistiam em Convencional (altura residual 10 cm e sem adubação), Intensivo (altura residual 10 cm e adubação equivalente a 600 kg de N/ha x ano), Leve (altura residual 20 cm e sem adubação) e Moderado (altura residual 20 cm e adubação equivalente a 300 kg de N/ha x ano). Utilizou-se um delineamento inteiramente casualizado com medidas repetidas no tempo em dois períodos do ano com quatro repetições. Tanto a Massa Seca de Forragem Total residual (MSFTr), quanto a de Forragem Verde residual (MSFVr) e Forragem Morta residual (MSFMr) apresentaram efeito dos manejos e do período do ano, com maiores médias de MSFTr e MSFMr para os manejos Moderado e Leve e para a MSFVr no manejo Moderado. O período seco apresentou os maiores valores de MSFTr, MSFVr e MSFMr com 3674, 2416 e 1258 kg/ha, respectivamente. A relação Material Vivo/Material Morto residual foi maior nos manejos Intensivo e Moderado. A Massa Seca de Lâmina foliar Verde residual (MSLVr) e a de Colmo Verde residual (MSCVr) comportaram-se de forma semelhante com maior média para o manejo Moderado. Na MSCVr houve maior produção de colmo no período seco. A relação Folha/Colmo residual foi menor no manejo Convencional e maior no período chuvoso. Os manejos Moderado e Intensivo apresentam uma maior quantidade de massas de forragem residual que garantem maior vigor na rebrotação da pastagem de capim-tifton 85. [Biomass components Tifton 85 grass canopy post-grazing, under different managements in intermittent stocking]. Abstract: Was evaluated the structural characteristics pos grazing of tifton 85 bermudagrass, subjected to different intensities of management. The management consisted in Extensive (residual height 10 cm and no fertilization), Intensive (residual height 10 cm and equivalent fertilization 600 kg of N/ha x year), Light (residual height 20 cm and no fertilization) e Moderate (residual height 20 cm and equivalent fertilization 300 kg of N/ha x year). We used entirely randomized with repeated measurements over time in two seasons with four replicates. Both residual Dry Mass Total Herbage (DMTHr), as the residual Green Herbage (DMGHr) and residual Dead Herbage (DMDHr) had the effect of the managements and the season of year, with higher average DMTHr and DMDHr for the Moderate and Light managements and for the DMGHr in the Moderate management. The dry season had the higest values of DMTHr, DMGHr and DMDHr with 3674, 2416 e 1258 kg/ha, respectively. The residual Live Material/Dead Material ratio was highest in the Intensive and Moderate management. The residual Dry Mass of Green Leaf (DMGLr) and the residual Green Stems (DMGSr) behaved similarly with highest average for Moderate management. In DMGSr had highest stem production in the rainy season. The residual Leaf/Stem ration was lowest in the Conventional management and the rainy season. The Moderate and Intensive management present a higher amount residual forage mass to ensure greater strength in the pasture regrowth of Tifton 85 grass

Efficiency of water use and nitrogen for goat milk production in irrigated pasture to different management.

Abstract: The aim was to determine the efficiency of use of water and nitrogen for forage production and goat-milk production on an irrigated Tanzania Guineagrass (Panicum maximum cv. Tanzânia) pasture subjected to different management practices. The management levels tested were combinations among nitrogen fertilization levels and post-grazing residual heights (ResH): Intensive (ResH = 33.0cm and 600.0kg N/ha.year-1); Moderate (ResH = 47.0cm and 300.0kg N/ha.year-1); Light (ResH = 47cm and 0kg N/ha.year-1); and Conventional (ResH = 33cm and 0kg N/ha year-1). The efficiency of water use for forage production was higher in intensive and Moderate management. The Conventional management was recommended only for forage production since there is no nitrogen input available because this result was similar to Intensive management in water efficiency. The efficiency of water use to produce goat milk was higher in Intensive management. Moderate management presented higher efficiency of nitrogen to produce forage. On the other hand, Intensive management was more efficient using nitrogen in goat milk production. The amount of water needed to produce one liter of goat milk varied from 893.20 to 3,933.50L. In the moderate management, up to 121.48kg forage and 21.56kg of milk were produced for every kilogram of N utilized. Intensive management is advantageous for water use efficiency as well nitrogen efficiency to produce goat milk in cultivated pasture. [Eficiência de uso de água e nitrogênio para produção de leite de cabra em pastagem irrigada sob diferentes manejos]. Resumo: Objetivou-se determinar a eficiência de uso de água e nitrogênio para a produção de forragem e de leite de cabra em pasto irrigado de capim-tanzânia, submetido a diferentes manejos. Os manejos testados foram combinações entre níveis de adubação nitrogenada e alturas residuais pós-pastejo (Altr): intensivo (Altr = 33,0cm e 600,0kg N/ha.ano-1); moderado (Altr = 47,0cm e 300,0kg N/ha.ano-1); leve (Altr = 47,0cm e 0kg N/ha.ano-1) e convencional (Altr = 33,0cm e 0kg N/ha.ano-1). A eficiência do uso de água para produção de forragem foi obtida nos manejos intensivo e moderado. O manejo convencional só foi interessante no caso da produção de forragem em situação de ausência de nitrogênio, não diferindo do manejo intensivo para a eficiência de uso de água. A eficiência de uso de água para a produção de leite foi maior no intensivo. A maior eficiência do uso de nitrogênio para produzir forragem foi no moderado, enquanto a maior eficiência de nitrogênio para a produção de leite foi no intensivo. A quantidade de água para produzir um litro de leite de cabra variou de 893,20 em manejo intensivo a 3.933,50L em convencional. Manejo intensivo é vantajoso para eficiência de uso de água e nitrogênio para a produção de forragem e de leite de cabra