A Embrapa e a comunicação para a Agricultura Familiar: uma pesquisa exploratória no Distrito Federal.

Este artigo tem como propósito apresentar os resultados de uma pesquisa exploratória realizada com agricultores familiares do Distrito Federal acerca da imagem da Empresa Brasileira de Pesquisa Agropecuária (Embrapa) no que se refere à relação e a comunicação com a agricultura familiar, tendo como ponto inicial a identificação de entraves na comunicação da empresa com esse segmento de agricultores

Influence of non-local exchange on RKKY interactions in III-V diluted magnetic semiconductors

The RKKY interaction between substitutional Mn local moments in GaAs is both spin-direction-dependent and spatially anisotropic. In this Letter we address the strength of these anisotropies using a semi-phenomenological tight-binding model which treats the hybridization between Mn d-orbitals and As p-orbitals perturbatively and accounts realistically for the non-local exchange interaction between their spins. We show that exchange non-locality, valence-band spin-orbit coupling, and band-structure anisotropy all play a role in determining the strength of both effects. We use these results to estimate the degree of ground-state magnetization suppression due to frustrating interactions between randomly located Mn ions.Comment: 4 pages RevTeX, 2 figures included, v2: replacement because of font proble

Clustering in disordered ferromagnets: The Curie temperature in diluted magnetic semiconductors

We theoretically investigate impurity correlation and magnetic clustering effects on the long-range ferromagnetic ordering in diluted magnetic semiconductors, such as $\textrm{Ga}_{1-x}\textrm{Mn}_{x}\textrm{As}$, using analytical arguments and direct Monte Carlo simulations. We obtain an analytic formula for the ferromagnetic transition temperature $T_{c}$ which becomes asymptotically exact in the strongly disordered, highly dilute (i.e. small $x$) regime. We establish that impurity correlations have only small effects on $T_{c}$ with the neutrally correlated random disorder producing the nominally highest $T_{c}$. We find that the ferromagnetic order is approached from the high temperature paramagnetic side through a random magnetic clustering phenomenon consistent with the percolation transition scenario.Comment: 5 pages, 4 figure

Doping dependence of the Neel temperature in Mott-Hubbard antiferromagnets: Effect of vortices

The rapid destruction of long-range antiferromagnetic order upon doping of Mott-Hubbard antiferromagnetic insulators is studied within a generalized Berezinskii-Kosterlitz-Thouless renormalization group theory in accordance with recent calculations suggesting that holes dress with vortices. We calculate the doping-dependent Neel temperature in good agreement with experiments for high-Tc cuprates. Interestingly, the critical doping where long-range order vanishes at zero temperature is predicted to be xc ~ 0.02, independently of any energy scales of the system.Comment: 4 pages with 3 figures included, minor revisions, to be published in PR

Mathieu Moonshine and N=2 String Compactifications

There is a `Mathieu moonshine' relating the elliptic genus of K3 to the sporadic group M_{24}. Here, we give evidence that this moonshine extends to part of the web of dualities connecting heterotic strings compactified on K3 \times T^2 to type IIA strings compactified on Calabi-Yau threefolds. We demonstrate that dimensions of M_{24} representations govern the new supersymmetric index of the heterotic compactifications, and appear in the Gromov--Witten invariants of the dual Calabi-Yau threefolds, which are elliptic fibrations over the Hirzebruch surfaces F_n.Comment: 28 pages; v2: minor changes, published versio

Embrapa e agricultura familiar: um novo desafio para a comunicação organizacional.

IBERCOM

Resonant and Kondo tunneling through molecular magnets

Transport through molecular magnets is studied in the regime of strong coupling to the leads. We consider a resonant-tunneling model where the electron spin in a quantum dot or molecule is coupled to an additional local, anisotropic spin via exchange interaction. The two opposite regimes dominated by resonant tunneling and by Kondo transport, respectively, are considered. In the resonant-tunneling regime, the stationary state of the impurity spin is calculated for arbitrarily strong molecule-lead coupling using a master-equation approach, which treats the exchange interaction perturbatively. We find that the characteristic fine structure in the differential conductance persists even if the hybridization energy exceeds thermal energies. Transport in the Kondo regime is studied within a diagrammatic approach. We show that magnetic anisotropy gives rise to a splitting of the Kondo peak at low bias voltages.Comment: 13 pages, 5 figures, version as publishe

Quantum Monte Carlo simulation of thin magnetic films

The stochastic series expansion quantum Monte Carlo method is used to study thin ferromagnetic films, described by a Heisenberg model including local anisotropies. The magnetization curve is calculated, and the results compared to Schwinger boson and many-body Green's function calculations. A transverse field is introduced in order to study the reorientation effect, in which the magnetization changes from out-of-plane to in-plane. Since the approximate theoretical approaches above differ significantly from each other, and the Monte Carlo method is free of systematic errors, the calculation provides an unbiased check of the approximate treatments. By studying quantum spin models with local anisotropies, varying spin size, and a transverse field, we also demonstrate the general applicability of the recent cluster-loop formulation of the stochastic series expansion quantum Monte Carlo method.Comment: 9 pages, 12 figure

Stochastic Assembly of Bacteria in Microwell Arrays Reveals the Importance of Confinement in Community Development

Citation: Hansen, R. H., Timm, A. C., Timm, C. M., Bible, A. N., Morrell-Falvey, J. L., Pelletier, D. A., . . . Retterer, S. T. (2016). Stochastic Assembly of Bacteria in Microwell Arrays Reveals the Importance of Confinement in Community Development. Plos One, 11(5), 18. doi:10.1371/journal.pone.0155080The structure and function of microbial communities is deeply influenced by the physical and chemical architecture of the local microenvironment and the abundance of its community members. The complexity of this natural parameter space has made characterization of the key drivers of community development difficult. In order to facilitate these characterizations, we have developed a microwell platform designed to screen microbial growth and interactions across a wide variety of physical and initial conditions. Assembly of microbial communities into microwells was achieved using a novel biofabrication method that exploits well feature sizes for control of innoculum levels. Wells with incrementally smaller size features created populations with increasingly larger variations in inoculum levels. This allowed for reproducible growth measurement in large (20 mu m diameter) wells, and screening for favorable growth conditions in small (5, 10 mu m diameter) wells. We demonstrate the utility of this approach for screening and discovery using 5 mu m wells to assemble P. aeruginosa colonies across a broad distribution of innoculum levels, and identify those conditions that promote the highest probability of survivial and growth under spatial confinement. Multi-member community assembly was also characterized to demonstrate the broad potential of this platform for studying the role of member abundance on microbial competition, mutualism and community succession