Decay of metastable phases in a model for the catalytic oxidation of CO

We study by kinetic Monte Carlo simulations the dynamic behavior of a Ziff-Gulari-Barshad model with CO desorption for the reaction CO + O $\to$ CO$_2$ on a catalytic surface. Finite-size scaling analysis of the fluctuations and the fourth-order order-parameter cumulant show that below a critical CO desorption rate, the model exhibits a nonequilibrium first-order phase transition between low and high CO coverage phases. We calculate several points on the coexistence curve. We also measure the metastable lifetimes associated with the transition from the low CO coverage phase to the high CO coverage phase, and {\it vice versa}. Our results indicate that the transition process follows a mechanism very similar to the decay of metastable phases associated with {\it equilibrium} first-order phase transitions and can be described by the classic Kolmogorov-Johnson-Mehl-Avrami theory of phase transformation by nucleation and growth. In the present case, the desorption parameter plays the role of temperature, and the distance to the coexistence curve plays the role of an external field or supersaturation. We identify two distinct regimes, depending on whether the system is far from or close to the coexistence curve, in which the statistical properties and the system-size dependence of the lifetimes are different, corresponding to multidroplet or single-droplet decay, respectively. The crossover between the two regimes approaches the coexistence curve logarithmically with system size, analogous to the behavior of the crossover between multidroplet and single-droplet metastable decay near an equilibrium first-order phase transition.Comment: 27 pages, 22 figures, accepted by Physical Review

Seletividade e eficácia das formulações com os herbicidas atrazine/dimethenamid-p e atrazine/dimethenamid, no controle de plantas daninhas na cultura do milho

A trial was carried out to test the efficacy and selective of formulation and doses of atrazine/ dimethenamid-p andatrazine/dimethenamid, in pre-emergence on corn (cv. Primaiz BR 3101). The following treatments were tested in a sandy Red Yellow Latossol soil, in Uberaba, Brazil: atrazine 396 g/L/dimethenamid-p 204 g/L (1800, 2100, 2400 and 2700 g/ha) and atrazine 320 g/L/ dimethenamid 289 g/L (2100, 2400, 2700 and 3000 g/ha), s-metolachlor 960 g/L (2880 g/ha) and hand weeding and weeded. The mixture of atrazine/dimethenamid-p and atrazine/ dimethenamid, independently of the rates provided good control of the weeds Bidens pilosa, Portulaca oleracea, Commelina benghalensis and Digitaria horizontalis. No significant differences was observed in the productivities of the chemical treatments, but weeded treatment reduced 72% of the crop yield in relation to the hand weeding treatment. These was no difference among treatments concerning to the crops density, cobs numbers and first cob height and seed weight. The harvest index for mechanical harvesting in the chemical treatments was excellent. All the tested treatments were selective to corn. O objetivo do ensaio foi estudar a seletividade e o controle de plantas daninhas anuais das misturas formuladas de dimethenamid-p 204 g/L/atrazine 396 g/L e dimethenamid 280 g/L/ atrazine 320 g/L, em pré-emergência, na cultura do milho (cv. Primaiz BR 3101 híbrido triplo). O solo foi um Latossolo Vermelho-Amarelo, textura média, em Uberaba, MG. Os tratamentos testados foram: dimethenamid-p 204 g/L/atrazine 396 g/L, nas doses de 1800, 2100, 2400 e 2700 g/há do i. a.; dimethenamid 280 g/L/atrazine 320 g/L, nas doses de 2100, 2400, 2700 e 3000 g/há do i. a.; s-metolachlor 960 g/L, na dose de 2880 g/há do i. a. e testemunha capinada e testemunha sem capina. As misturas formuladas de dimethenamid-p/atrazine e dimethenamid/atrazine, independentemente das doses utilizadas proporcionaram eficiente controle de Bidens pilosa (picão-preto), Portulaca oleracea (beldroega), Commelina benghalensis (trapoeraba) e Digitaria horizontalis (capim-colchão). Não foram observadas diferenças significativas entre as produtividades nos diversos tratamentos químicos, mas o rendimento foi menor na testemunha sem capina 75% em relação a testemunha capinada. A aplicação dos tratamentos não afetou significativamente a população final, altura da planta, altura de inserção da primeira espiga e o peso de 100 sementes. No índice de colheita mecânica nos tratamentos químicos verificou-se que os maiores valores foram alcançados com as misturas formuladas de dimethenamid-p/ atrazine, na dose de 2700 g/ha do i. a., com 4,5 e dimethenamid/atrazine, na dose de 2700 g/ha, com 3,8, sendo que nas outras doses testadasdas misturas de dimethenamid-p/atrazine e dimethenamid/atrazine, apresentaram índice de colheita entre 3,2 a 3,8. Todos os tratamentos químicos testados foram seletivos para o milho cv.Primaiz

On the potential vegetation feedbacks that enhance phosphorus availability – insights from a process-based model linking geological and ecological timescales

In old and heavily weathered soils, the availability of P might be so small that the primary production of plants is limited. However, plants have evolved several mechanisms to actively take up P from the soil or mine it to overcome this limitation. These mechanisms involve the active uptake of P mediated by mycorrhiza, biotic de-occlusion through root clusters, and the biotic enhancement of weathering through root exudation. The objective of this paper is to investigate how and where these processes contribute to alleviate P limitation on primary productivity. To do so, we propose a process-based model accounting for the major processes of the carbon, water, and P cycles including chemical weathering at the global scale. Implementing P limitation on biomass synthesis allows the assessment of the efficiencies of biomass production across different ecosystems. We use simulation experiments to assess the relative importance of the different uptake mechanisms to alleviate P limitation on biomass production. We find that active P uptake is an essential mechanism for sustaining P availability on long timescales, whereas biotic de-occlusion might serve as a buffer on timescales shorter than 10 000 yr. Although active P uptake is essential for reducing P losses by leaching, humid lowland soils reach P limitation after around 100 000 yr of soil evolution. Given the generalized modelling framework, our model results compare reasonably with observed or independently estimated patterns and ranges of P concentrations in soils and vegetation. Furthermore, our simulations suggest that P limitation might be an important driver of biomass production efficiency (the fraction of the gross primary productivity used for biomass growth), and that vegetation on old soils has a smaller biomass production rate when P becomes limiting. With this study, we provide a theoretical basis for investigating the responses of terrestrial ecosystems to P availability linking geological and ecological timescales under different environmental settings

Numerical Study of a Mixed Ising Ferrimagnetic System

We present a study of a classical ferrimagnetic model on a square lattice in which the two interpenetrating square sublattices have spins one-half and one. This model is relevant for understanding bimetallic molecular ferrimagnets that are currently being synthesized by several experimental groups. We perform exact ground-state calculations for the model and employ Monte Carlo and numerical transfer-matrix techniques to obtain the finite-temperature phase diagram for both the transition and compensation temperatures. When only nearest-neighbor interactions are included, our nonperturbative results indicate no compensation point or tricritical point at finite temperature, which contradicts earlier results obtained with mean-field analysis.Comment: Figures can be obtained by request to [email protected] or [email protected]

Infinite-range Ising ferromagnet in a time-dependent transverse field: quench and ac dynamics near the quantum critical point

We study an infinite range ferromagnetic Ising model in the presence of a transverse magnetic field which exhibits a quantum paramagnetic-ferromagnetic phase transition at a critical value of the transverse field. In the thermodynamic limit, the low-temperature properties of this model are dominated by the behavior of a single large classical spin governed by an anisotropic Hamiltonian. Using this property, we study the quench and AC dynamics of the model both numerically and analytically, and develop a correspondence between the classical phase space dynamics of a single spin and the quantum dynamics of the infinite-range ferromagnetic Ising model. In particular, we compare the behavior of the equal-time order parameter correlation function both near to and away from the quantum critical point in the presence of a quench or AC transverse field. We explicitly demonstrate that a clear signature of the quantum critical point can be obtained by studying the AC dynamics of the system even in the classical limit. We discuss possible realizations of our model in experimental systems.Comment: Revtex4, 10 pages including 10 figures; corrected a sign error in Eq. 32; this is the final published versio

Postemergence broadleafweed control in soybean with bentazon 400/acifluorfen 170 and bentazon 400/acifluorfen 140

o objetivo do ensaio foi avaliar a eficácia dos herbicidas bentazon 400/acifluorfen 170 e bentazon 400/acifluorfen 140, isolados e em mistura com óleo mineral (Assist), em pós-emergência inicial, para o controle de plantas daninhas latifoliadas na cultura da soja, cv. 'Garimpo', em Latossolo Vermelho-Escuro, textura média, em área de Cerrado. Os tratamentos foram: bentazon 400/acifluorfen 170 (570, 684 e 855 g/ha), bentazon 400/acifluorfen 170+óleo mineral (684+378 glha), bentazon 400/ acifluorfen 140 (540, 648 e 810 glha), bentazon 400/acifluorfen 140+óleo mineral (648+378 glha), fomesafenlfluazifop-butyl (500 g/ha) e testemunha capinada e sem capina. Na produtividade de grãos nenhum dos herbicidas estudados apresentou diferenças com relação à testemunha capinada. Porém a testemunha sem capina apresentou diferenças em relação a todos os demais tratamentos, com a produção de 813 kg/ha. Para a densidade de plantas por ocasião da colheita, altura média das plantas, altura média de inserção da primeira vagem e peso de 100 sementes, não foram detectadas diferenças entre os tratamentos. Verificouse que Amaranthus viridis (caruru) foi controlado por todos os herbicidas, com índices de controle acima de 95%, até os 21 dias após a aplicação. Para Nicandra physaloides (joá-de-capote), todos os herbicidas apresentaram um excelente controle, com índice de 100% desde os 7 até os 21 dias após a aplicação. Para Sida rhombifolia (vassoura), todos os herbicidas apresentaram índices de controle superiores a 97%, até 21 dias depois da aplicação. Na primeira avaliação verificou-se leve toxicidade às plantas causada pelas misturas fomesafen/fluazifop-butyl, bentazon 400/acifluorfen 140+óleo mineral (684+378 glha) e bentazon 400/acifluorfen 140. Na segunda avaliação, 15 dias após a aplicação, não era visível qualquer sintoma de toxicidade. The objective ofthe experiment was to evaluate the efficacy of hérbicides bentazon 400/acifluorfen 170 and bentazon 400/acifluorfen 140, alone and in mixture with mineral oil (Assist) in early postemergence control ofweeds in soybean. Treatrnents were: bentazon 400/acifluorfen 140 (570, 684 and 855 glha), bentazon 400/acifluorfen 170+mineral oil (684+378 glha), bentazon 400/acifluorfen 140 (540, 648 and 810 glha), bentazon 400/acifluorfen 140+mineral oil (648+378 glha) and fomesafenlfluazifopbutyl (500 g/ha) and a weedy and a weeded control. The results showed no difference in grain productivity between the herbicide treatments and the weeded control. However the weedy control showed differences in relation to alI treatments, with productivity of 813 kglha. Soybean density at harvesting, plant height, first pod insertion height and weight of 100 seeds, showed no difference between treatments. Amaranthus viridis was controled by ali herbicides, with 95% control or more until 21 days after aplication. Nicandra physaloides was also controlled by ali herbicides from 7th to 21 st day after spraying. In Sida rhombifolia, ali herbicides showed index control higher than 97% until 21 days after aplication. Seven days after herbicide spraying it was observed negligible toxicity to soybean plants. Seven days later there was no toxicity.

Avaliação da seletividade e eficácia de misturas formuladas de bentazon + acifluorfen em soja

A trial was carried out to test the efficacy and selectivity of differentformulations and doses of bentazon with acifluorfen, applied in post emergence ofsoybean (cv. Doko). The following treatments were tested in a sandy Latosoil, inPrudente de Morais, Brazil: bentazon+acifluorfen (480+204 g/ha and 600+255 g/ha)with or without mineral oil (0,25% v/v); bentazon+acifluorfen (480+168 g/ha and600+210 g/ha) with or without mineral oil (0,25%v/v); and bentazon +acifluorfen600+160 g/ha and weeded and no weeding checks. Independently of the doses andformulations (with or without mineral oil) all chemicals provided efficient control ofAmaranthus hybridus, Galinsoga parviflora and Solanum americanum. Significantdifferences were not observed among the productivities for the several chemicaltreatments, but reduced the interference of weeds caused an average decrease in yield of1445 kg/ha in relation to the weed check. These was no difference between treatments concerning plant density, plant height, height of first pod and weight of 100 seeds. Alltested treatments were selective for the soybean.     O objetivo do ensaio foi avaliar a seletividade e eficácia de misturas formuladas dos herbicidas bentazon 400 g/L+acifluorfen sal de sódio 170 g/L; bentazon 400 g/L+acifluorfen sal de sódio 140 g/L e bentazon 300 g/L+acifluorfen sal de sódio 80 g/L, em pós-emergência na cultura da soja (cv. Doko). O solo foi um Latossolo- Vermelho-Escuro, textura argilo-siltoso, no município de Prudente de Morais, MG. Os tratamentos testados foram: [480+204] e [600+255] g/ha do i. a., com e sem mistura de óleo mineral; [480+168] e [600+210] g/ha do i. a., com e sem mistura de óleo mineral; [600+160 g/la do i. a.; testemunha capinada e testemunha sem capina. Todos os herbicidas testados independente da dose a adição de óleo mineral 0,25% v/v apresentaram controle eficiente de Amaranthus hybridus (caruru), Galinsoga parviflora (mentrasto) e Solanum americanum (maria-pretinha). Não foram observadas diferenças significativas entre produtividade nos diferentes tratamentos químicos testados com a testemunha capinada, sendo que a interferência das plantas daninhas diminuiu o rendimento em 1445 kg/ha em média. As aplicações dos herbicidas não afetou a população final, altura da planta, altura de inserção da primeira vagem e o peso de 100 sementes. Todas as misturas formuladas de herbicidas testadas foram seletivas para a cultura da soja

Magnetic Behavior of a Mixed Ising Ferrimagnetic Model in an Oscillating Magnetic Field

The magnetic behavior of a mixed Ising ferrimagnetic system on a square lattice, in which the two interpenetrating square sublattices have spins +- 1/2 and spins +-1,0, in the presence of an oscillating magnetic field has been studied with Monte Carlo techniques. The model includes nearest and next-nearest neighbor interactions, a crystal field and the oscillating external field. By studying the hysteretic response of this model to an oscillating field we found that it qualitatively reproduces the increasing of the coercive field at the compensation temperature observed in real ferrimagnets, a crucial feature for magneto-optical applications. This behavior is basically independent of the frequency of the field and the size of the system. The magnetic response of the system is related to a dynamical transition from a paramagnetic to a ferromagnetic phase and to the different temperature dependence of the relaxation times of both sublattices.Comment: 10 figures. To be published in Phys.Rev

Phase diagram of a random-anisotropy mixed-spin Ising model

We investigate the phase diagram of a mixed spin-1/2--spin-1 Ising system in the presence of quenched disordered anisotropy. We carry out a mean-field and a standard self-consistent Bethe--Peierls calculation. Depending on the amount of disorder, there appear novel transition lines and multicritical points. Also, we report some connections with a percolation problem and an exact result in one dimension.Comment: 8 pages, 4 figures, accepted for publication in Physical Review