Pathogenic variation of Phakopsora pachyrhizi populations in Brazil.

The obligate basidiomycete Phakopsora pachyrhizi is the causal agent of soybean rust that has potential to reduce the yield of soybean drastically. Soybean production in Brazil has been threatened by the rust since the pathogen was first discovered in 2001. To understand pathogenic variation of the rust populations in South America, an evaluation system for soybean rust resistance has been constructed using a set of 16 differential varieties. In this study, the evaluation system was used to investigate pathogenic variation among the P. pachyrhizi populations in Brazil. Samples of P. pachyrhizi were collected from the diseased soybeans in Brazil in the 2007-2008 and 2008-2009 soybean cultivation seasons. In the first season, two rust samples showed similar pattern of the infection types on the differential set, suggesting that the same or similar pathogen population was present in the two locations. The other samples were likely different pathogenic populations. In the second season, different patterns of the infection types were found among the samples. Comparison of the evaluation data from the two seasons demonstrated that pathogenic variation between the seasons was detected in the populations from Rio Grande do Sul and Paraná but was not remarkable in those from Rondônia. This study provides useful knowledge about P. pachyrhizi populations in Brazil to identify the resistant soybean genotypes and target effective cultivars against certain pathogen populations.Edição do Proceedings of the National Soybean Rust Symposium, New Orleans, 2009

Atomic Configuration of Nitrogen Doped Single-Walled Carbon Nanotubes

Having access to the chemical environment at the atomic level of a dopant in a nanostructure is crucial for the understanding of its properties. We have performed atomically-resolved electron energy-loss spectroscopy to detect individual nitrogen dopants in single-walled carbon nanotubes and compared with first principles calculations. We demonstrate that nitrogen doping occurs as single atoms in different bonding configurations: graphitic-like and pyrrolic-like substitutional nitrogen neighbouring local lattice distortion such as Stone-Thrower-Wales defects. The stability under the electron beam of these nanotubes has been studied in two extreme cases of nitrogen incorporation content and configuration. These findings provide key information for the applications of these nanostructures.Comment: 25 pages, 13 figure

Cones, pringles, and grain boundary landscapes in graphene topology

A polycrystalline graphene consists of perfect domains tilted at angle {\alpha} to each other and separated by the grain boundaries (GB). These nearly one-dimensional regions consist in turn of elementary topological defects, 5-pentagons and 7-heptagons, often paired up into 5-7 dislocations. Energy G({\alpha}) of GB computed for all range 0<={\alpha}<=Pi/3, shows a slightly asymmetric behavior, reaching ~5 eV/nm in the middle, where the 5's and 7's qualitatively reorganize in transition from nearly armchair to zigzag interfaces. Analysis shows that 2-dimensional nature permits the off-plane relaxation, unavailable in 3-dimensional materials, qualitatively reducing the energy of defects on one hand while forming stable 3D-landsapes on the other. Interestingly, while the GB display small off-plane elevation, the random distributions of 5's and 7's create roughness which scales inversely with defect concentration, h ~ n^(-1/2)Comment: 9 pages, 4 figure

On the merit of a Central Limit Theorem-based approximation in statistical physics

The applicability conditions of a recently reported Central Limit Theorem-based approximation method in statistical physics are investigated and rigorously determined. The failure of this method at low and intermediate temperature is proved as well as its inadequacy to disclose quantum criticalities at fixed temperatures. Its high temperature predictions are in addition shown to coincide with those stemming from straightforward appropriate expansions up to (k_B T)^(-2). Our results are clearly illustrated by comparing the exact and approximate temperature dependence of the free energy of some exemplary physical systems.Comment: 12 pages, 1 figur

Synthesis effects on the magnetic and superconducting properties of RuSr2GdCu2O8

A systematic study on the synthesis of the Ru-1212 compound by preparing a series of samples that were annealed at increasing temperatures and then quenched has been performed. It results that the optimal temperature for the annealing lies around 1060-1065 C; a further temperature increase worsens the phase formation. Structural order is very important and the subsequent grinding and annealing improves it. Even if from the structural point of view the samples appear substantially similar, the physical characterization highlight great differences both in the electrical and magnetic properties related to intrinsic properties of the phase as well as to the connection between the grains as inferred from the resistive and the Curie Weiss behaviour at high temperature as well as in the visibility of ZFC anf FC magnetic signals.Comment: 17 pages, 12 figures. Proc. Int. Workshop " Ruthenate and rutheno-cuprate materials: theory and experiments", Vietri, October 2001. To be published on LNP Series, Springer Verlag, Berlin, C. Noce, A. Vecchione, M. Cuoco, A. Romano Eds, 200

Studies of structural damage in high-{Tc} superconductors by high-energy heavy-ion irradiation

The results of studies of structural damage by high-energy (MeV) Si{sup +13}, Cu{sup +18}, Ag{sup +21}, and Au{sup +24} ions, using transmission electron microscopy techniques, revealed that the size of the damaged area (amorphous) is strongly dependent on: (1) the stopping power [dE/dx (keV/nm)] of the irradiating ions, (2) the thermal diffusivity of the crystal, (3) the degree of oxygenation, in the case of YBa{sub 2}Cu{sub 3}O{sub 7-{delta}}, and (4) the direction of the ion beam with respect to the crystallographic axis

Imaging and Dynamics of Light Atoms and Molecules on Graphene

Observing the individual building blocks of matter is one of the primary goals of microscopy. The invention of the scanning tunneling microscope [1] revolutionized experimental surface science in that atomic-scale features on a solid-state surface could finally be readily imaged. However, scanning tunneling microscopy has limited applicability due to restrictions, for example, in sample conductivity, cleanliness, and data aquisition rate. An older microscopy technique, that of transmission electron microscopy (TEM) [2, 3] has benefited tremendously in recent years from subtle instrumentation advances, and individual heavy (high atomic number) atoms can now be detected by TEM [4 - 7] even when embedded within a semiconductor material [8, 9]. However, detecting an individual low atomic number atom, for example carbon or even hydrogen, is still extremely challenging, if not impossible, via conventional TEM due to the very low contrast of light elements [2, 3, 10 - 12]. Here we demonstrate a means to observe, by conventional transmision electron microscopy, even the smallest atoms and molecules: On a clean single-layer graphene membrane, adsorbates such as atomic hydrogen and carbon can be seen as if they were suspended in free space. We directly image such individual adatoms, along with carbon chains and vacancies, and investigate their dynamics in real time. These techniques open a way to reveal dynamics of more complex chemical reactions or identify the atomic-scale structure of unknown adsorbates. In addition, the study of atomic scale defects in graphene may provide insights for nanoelectronic applications of this interesting material.Comment: 9 pages manuscript and figures, 9 pages supplementary informatio