Topology of Almost Complex Structures on Six-Manifolds

We study the space of (orthogonal) almost complex structures on closed six-dimensional manifolds as the space of sections of the twistor space for a given metric. For a connected six-manifold with vanishing first Betti number, we express the space of almost complex structures as a quotient of the space of sections of a seven-sphere bundle over the manifold by a circle action, and then use this description to compute the rational homotopy theoretic minimal model of the components that satisfy a certain Chern number condition. We further obtain a formula for the homological intersection number of two sections of the twistor space in terms of the Chern classes of the corresponding almost complex structures

Abrupt field-induced transition triggered by magnetocaloric effect in phase-separated manganites

The occurrence at low temperatures of an ultrasharp field-induced transition in phase separated manganites is analyzed. Experimental results show that magnetization and specific heat step-like transitions below 5 K are correlated with an abrupt change of the sample temperature, which happens at a certain critical field. This temperature rise, a magnetocaloric effect, is interpreted as produced by the released energy at the transition point, and is the key to understand the existence of the abrupt field-induced transition. A qualitative analysis of the results suggests the existence of a critical growing rate of the ferromagnetic phase, beyond which an avalanche effect is triggered.Comment: 6 pages, 4 figures included. Acepted for publication in Phys. Rev.

Structural and energetic properties of nickel clusters: 2≤N≤1502 \le N \le 150

The four most stable structures of Ni$_N$ clusters with $N$ from 2 to 150 have been determined using a combination of the embedded-atom method in the version of Daw, Baskes and Foiles, the {\it variable metric/quasi-Newton} method, and our own {\it Aufbau/Abbau} method. A systematic study of energetics, structure, growth, and stability of also larger clusters has been carried through without more or less severe assumptions on the initial geometries in the structure optimization, on the symmetry, or on bond lengths. It is shown that cluster growth is predominantly icosahedral with $islands$ of {\it fcc}, {\it tetrahedral} and {\it decahedral} growth. For the first time in unbiased computations it is found that Ni$_{147}$ is the multilayer (third Mackay) icosahedron. Further, we point to an enhanced ability of {\it fcc} clusters to compete with the icosahedral and decahedral structures in the vicinity of N=79. In addition, it is shown that conversion from the {\it hcp}/anti-Mackay kind of icosahedral growth to the {\it fcc}/Mackay one occurs within a transition layer including several cluster sizes. Moreover, we present and apply different analytical tools in studying structural and energetic properties of such a large class of clusters. These include means for identifying the overall shape, the occurrence of atomic shells, the similarity of the clusters with, e.g., fragments of the {\it fcc} crystal or of a large icosahedral cluster, and a way of analysing whether the $N$-atom cluster can be considered constructed from the $(N-1)$-atom one by adding an extra atom. In addition, we compare in detail with results from chemical-probe experiment. Maybe the most central result is that first for clusters with $N$ above 80 general trends can be identified.Comment: 37 pages, 11 figure

Porosidade e densidade de solos sob uso agrícola no município de Sobradinho-BA.

Considerando que o manejo do solo altera suas propriedades físicas, imprescindíveis ao oferecimento de condições favoráveis ao desenvolvimento produtivo, objetivou-se com este estudo avaliar a porosidade e a densidade de quatro solos sob uso agrícola, localizados as margens do Lago de Sobradinho, no município de Sobradinho-BA. Foram selecionadas quatro propriedades rurais onde foram coletadas amostras de solos nas profundidades de 0,00?0,10; 0,10-0,20 e 0,20-0,40 m tanto na área agrícola como na área sob caatinga, utilizada como referência. Em laboratório avaliou-se granulometria, argila dispersa em água e densidade do solo. Além disso, foram calculados: índice de floculação, macroporosidade e microporosidade. O intenso processo de mecanização adotado no uso agrícola proporcionou comportamentos distintos aos solos, em relação aos índices analisados, sendo que houve aumento na sua densidade e redução da macroporosidade

Atributos físicos e químicos de argissolo amarelo sob cultivo orgânico.

A utilização de sistemas orgânicos de produção agrícola tem ganhado destaque no cenário mundial principalmente devido à busca por alimentos de maior qualidade, mas também por outros benefícios como a conservação/melhoria de características físicas, químicas e biológicas do solo, aliada ao menor impacto ambiental. O presente trabalho propõe a avaliar a qualidade física e química de um Argissolo após dois anos de cultivo orgânico de videira de vinho, tendo como referência uma área de pousio. A avaliação se deu através da análise da estabilidade de agregados, dos cálculos dos índices de e de dispersão floculação do solo, da densidade do solo e do teor de carbono orgânico, a partir de amostras coletadas nas linhas e entrelinhas da videira e área de pousio, nas profundidades de 0-0,10, 0,10-0,20; 0,20-0,30 m e 0,30-0,60 m. Observou-se que o sistema influenciou a agregação do solo; o manejo nas linhas com a implantação de coquetéis vegetais possivelmente elevou o conteúdo de matéria orgânica quando comparado à entrelinha e pousio; e foram encontrados maiores teores dos elementos Mg e Ca na área de pousio e na linha de plantio, o mesmo não ocorrendo para K e Na

Low attenuation frequency bands for Lamb waves immersed in viscous fluids: theoretical analysis and experimental validation.

6 páginas, 3 figuras. Proceedings of the 2013 International Congress on Ultrasonics (ICU 2013). 2-5 may 2013, SingaporeThe application of Lamb waves is a promising technique i n ultrasonic NDE techniques for inspection and fluid characterization due to multi modal and dispersive characteristics. When a plate is in contact with a viscous fluid these waves are strongly attenuated. However, for most of the Lamb wave modes there is a low at tenuation frequency band that could be used for non-destructive testing or fluid characteriz ation. In order to explore this feature the phase velocity and attenuation curves of Lamb modes are expe rimentally measured in these low attenuation frequency bands, showing good agreement with theory .The present work was supported by the Brazilian National Council for Scientific and Technological Development – CNPq and the Spanish Ministery of Economy and Competitivity through the projects 560825/2010-2 and PIB2010BZ-00570, respectively.Peer reviewe

Comparison of ratioing and RCNA methods in the detection of flooded areas using Sentinel 2 Imagery (case study: Tulun, Russia)

Climate change and natural disasters caused by hydrological, meteorological, and climatic phenomena have a significant impact on cities. Russia, a continental country with a vast territory of complex geographic–ecological environments and highly variable climatic conditions, is subject to substantial and frequent natural disasters. On 29 June 2019, an extreme precipitation event occurred in the city of Tulun in the Irkutsk oblast, Russian Federation, which caused flooding due to the increase in the water level of the Iya River that passes through the city, leaving many infrastructures destroyed and thousands of people affected. This study aims to determine the flooded areas in the city of Tulun based on two change detection methods: Radiometric Rotation Controlled by No-change Axis (RCNA) and Ratioing, using Sentinel 2 images obtained before the event (19 June 2019) and during the flood peak (29 June 2019). The results obtained by the two methodologies were compared through cross-classification, and a 98% similarity was found in the classification of the areas. The study was validated based on photointerpretation of Google Earth images. The methodology presented proved to be useful for the automatic precession of flooded areas in a straightforward, but rigorous, manner. This allows stakeholders to efficiently manage areas that are buffeted by flooding episodes.LA/P/0069/2020info:eu-repo/semantics/publishedVersio