5,067 research outputs found

    The phase transition of the diffusive pair contact process revisited

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    The restricted diffusive pair contact process 2A->3A, 2A->0 (PCPD) and the classification of its critical behavior continues to be a challenging open problem of non-equilibrium statistical mechanics. Recently Kockelkoren and Chate [Phys. Rev. Lett. 90, 125701 (2003)] suggested that the PCPD in one spatial dimension represents a genuine universality class of non-equilibrium phase transitions which differs from previously known classes. To this end they introduced an efficient lattice model in which the number of particles per site is unrestricted. In numerical simulations this model displayed clean power laws, indicating ordinary critical behavior associated with certain non-trivial critical exponents. In the present work, however, we arrive at a different conclusion. Increasing the numerical effort, we find a slow drift of the effective exponents which is of the same type as observed in previously studied fermionic realizations. Analyzing this drift we discuss the possibility that the asymptotic critical behavior of the PCPD may be governed by an ordinary directed percolation fixed point.Comment: 6 pages, 1 figur

    On possible experimental realizations of directed percolation

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    Directed percolation is one of the most prominent universality classes of nonequilibrium phase transitions and can be found in a large variety of models. Despite its theoretical success, no experiment is known which clearly reproduces the critical exponents of directed percolation. The present work compares suggested experiments and discusses possible reasons why the observation of the critical exponents of directed percolation is obscured or even impossible.Comment: RevTeX, 13 pages, 11 eps figure

    Roughening transition driven by a binary spreading process

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    We introduce a solid-on-solid growth process which evolves by random deposition of dimers, surface diffusion, and evaporation of monomers from the edges of plateaus. It is shown that the model exhibits a robust transition from a smooth to a rough phase. The roughening transition is driven by an absorbing phase transition at the bottom layer of the interface, which displays the same type of critical behavior as the pair contact process with diffusion 2A->3A, 2A->0.Comment: RevTeX, 6 pages, 6 eps figure

    Logarithmic roughening in a growth process with edge evaporation

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    Roughening transitions are often characterized by unusual scaling properties. As an example we investigate the roughening transition in a solid-on-solid growth process with edge evaporation [Phys. Rev. Lett. 76, 2746 (1996)], where the interface is known to roughen logarithmically with time. Performing high-precision simulations we find appropriate scaling forms for various quantities. Moreover we present a simple approximation explaining why the interface roughens logarithmically.Comment: revtex, 6 pages, 7 eps figure

    Benefits and conflicts from integration of livestock into the organic agricultural system in Sub-Saharan Africa

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    Modern organic agriculture is a concept, that have evolved since the 1920s, when the first pioneers of organic agriculture (Rudolf Steiner, Robert McCarrison and Sir Albert Howard), expressed their concern about the use of inorganic fertiliser in agriculture, and faced a turning point with the publication of “Silent spring” by Rachel Carson in 1962 (Kristiansen & Merfield, 2006). The International Federation of Organic Agriculture Movement (IFOAM) was founded in 1972 (IFOAM, 2010), and has formulated four key principles on which organic agriculture build upon: Health, Ecology, Fairness and Care (IFOAM, 2005). These principles should be adopted world-wide, inspire the organic movement (IFOAM, 2005) and ensure that the organic production are adapted to the local conditions of the individual farm (Kristiansen & Merfield, 2006). Organic agriculture are very often, in the global south, confused with traditional farming, farming without chemical inputs or certified organic farming for export (Vaarst, 2010). In this assignment organic agriculture refers to agriculture based on the organic principles and is defined as followed: “Organic agriculture is a production system that sustains the health of soils, ecosystems and people. It relies on ecological processes, biodiversity and cycles adapted to local conditions, rather than the use of inputs with adverse effects. Organic agriculture combines tradition, innovation and science to benefit the shared environment and promote fair relationships and good quality of life for all involved.” (IFOAM, 2008). This leads to a concept of organic agriculture, in which certification not necessarily is implied. Soil fertility and biodiversity will be enhanced, and land degradation, erosion, poisoning and other negative effects of chemical activities will be minimised, if the principles of organic is followed in practice (Vaarst, 2010). This is achieved by using agro-ecological farming methods, such as incorporation of legumes and compost to improve soil fertility, crop diversity to prevent pest and diseases and inter-cropping (Anonymous, 2010; Vaarst, 2010). The farmers can intensify and increase production, without necessarily depending on chemical pesticides and fertilisers, by using the agro-ecological farming methods (Anonymous, 2010). This leads to reduces risk of becoming indebted or poisoned by pesticides. Agriculture are the basis for millions of peoples livelihood (Anonymous, 2010), and population pressure and low yields affects the farmers, so that they abandon the traditional farming system, with fallow and crop rotations, which has maintain their livelihood for millennia (Belay & Edwards, 2003). Livestock keeping are an important livelihood strategy for African smallholders (Descheemaeker et al., 2010). Soil fertility is general low in Africa (Lobe, 2008), and therefore, have there been approached to increase the soil fertility through chemical inputs (Belay & Edwards, 2003; Lobe, 2008). Some farmers have been fallen into the dept trap, because of the cost of purchasing chemical fertilisers (Belay & Edwards, 2003; Lobe, 2008). The organic agriculture, is an alternative to farming with chemical inputs, which can contribute to productivity increase and to reduce vulnerability and poverty among African farmers (Anonymous, 2010). With the use of organic farming practice the farmer does not have to purchase chemical inputs for the farm. The organic agricultural production system, is a system which are well-adapted to smallholders (Anonymous, 2010) and are a way of improving food security among smallholder farmers in developing countries (Halberg et al., 2009). In developing countries lives 60% of the population in rural areas and of those is 85% agriculturalist (Dixon et al., 2001). The rural areas are also where 70% of the total poverty are found, with the poverty concentrated in South Asia and Sub-Saharan Africa (Dixon et al., 2001). Broad-based agricultural are in general a way of reducing poverty (Dixon et al., 2001), and especially livestock keeping are a way of escaping poverty in the rural areas (Peden et al., 2007) and have been a part of the culture of some communities for millennia (Omore et al., 2009). Livestock are a way to attain the organic principles of ecology, which says that “organic agriculture should be based on living ecological systems and cycles. Work with them, emulate them and help sustain them” (IFOAM, 2005). It means that the organic farm should base it production on ecological processes and recycling. Livestock production should therefore form an integral part of many organic farms, because of its role in nutrient recycling on the farm (Hermansen, 2003; Powell et al., 2004). In a natural ecosystem the wild herbivores scatter their faeces and urine in the environment when they are grazing, which gives nutrient for plant growth, which provide food for the herbivores, which then produce more faeces and so on. When livestock are integrated into the whole system, is creates a situation where the livestock contribute to the system and at the same time the system should contribute to sure that the principles of fairness to obtained. Which means that “animals should be provided with the conditions and opportunities of life that accord with their physiology, natural behaviour and well-being” (IFOAM, 2005). The integration of livestock into the whole system can both give benefits and conflicts. Therefore is the aim of this assignment is to illustrate (explore, discuss and analyse) some of the benefits and conflicts from integration livestock, especially ruminants, into the whole farm, with a specific focus on Sub-Saharan Africa. The first section of this assignment gives some highlights of the livestock in Sub-Saharan Africa, followed by a description of benefits from integration of livestock with focus on nutrient recycling and a description of conflicts, with focus on breeds and water. A case from Ethiopia will be reviewed and the final section will be a discussion of the case in relation to integration of livestock
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