Cloaking resonant scatterers and tuning electron flow in graphene

We consider resonant scatterers with large scattering cross-sections in graphene that are produced by a gated disk or a vacancy, and show that a gated ring can be engineered to produce an efficient electron cloak. We also demonstrate that this same scheme can be applied to tune the direction of electron flow. Our analysis is based on a partial-wave expansion of the electronic wave-functions in the continuum approximation, described by the Dirac equation. Using a symmetrized version of the massless Dirac equation, we derive a general condition for the cloaking of a scatterer by a potential with radial symmetry. We also perform tight-binding calculations to show that our findings are robust against the presence of disorder in the gate potential.NMRP acknowledges support from EC under Graphene Flagship (Contract No. CNECT-ICT604391), the hospitality of the Instituto de Física of the UFRJ, and stimulating discussions with Bruno Amorim on the Lippamnn-Schwinger equation for Dirac electrons. TGR thanks the Brazilian agencies CNPq and FAPERJ and Brazil Science without Borders program for nancial support. FAP acknowledges CAPES (Grant No. BEX 1497/14-6) and CNPq (Grant No. 303286/2013-0) for financial suppor

Sparse precision matrix estimation in phenotypic trait evolution models

Phylogenetic trait evolution models allow for the estimation of evolutionary correlations between a set of traits observed in a sample of related organisms. By directly modeling the evolution of the traits along an estimable phylogenetic tree, the model's structure effectively controls for shared evolutionary history. In these models, relevant correlations are usually assessed through the high posterior density interval of their marginal distributions. However, the selected correlations alone may not provide the full picture regarding trait relationships. Their association structure, expressed through a graph that encodes partial correlations, can in contrast highlight sparsity patterns featuring direct associations between traits. In order to develop a model-based method to identify this association structure we explore the use of Gaussian graphical models (GGM) for covariance selection. We model the precision matrix with a G-Wishart conjugate prior, which results in sparse precision estimates. Furthermore the model naturally allows for Bayes Factor tests of association between the traits, with no additional computation required. We evaluate our approach through Monte Carlo simulations and applications that examine the association structure and evolutionary correlations of phenotypic traits in Darwin's finches and genomic and phenotypic traits in prokaryotes. Our approach provides accurate graph estimates and lower errors for the precision and correlation parameter estimates, particularly for conditionally independent traits, which are the target for sparsity in GGMs.Comment: 24 pages, 4 figure

LaMMos - Latching Mechanism based on Motorized-screw for Reconfigurable Robots and Exoskeleton Suits

Reconfigurable robots refer to a category of robots that their components (individual joints and links) can be assembled in multiple configurations and geometries. Most of existing latching mechanisms are based on physical tools such as hooks, cages or magnets, which limit the payload capacity. Therefore, robots re- quire a latching mechanism which can help to reconfigure itself without sacrificing the payload capability. This paper presents a latching mechanism based on the flexible screw attaching principle. In which, actuators are used to move the robot links and joints while connecting them with a motorized-screw and dis- connecting them by unfastening the screw. The brackets used in our mechanism configuration helps to hold maximum force up to 5000N. The LaMMos - Latching Mechanism based on Motorized- screw has been applied to the DeWaLoP - Developing Water Loss Prevention in-pipe robot. It helps the robot to shrink its body to crawl into the pipe with minimum diameter, by recon- figuring the leg positions. And it helps to recover the legs positions to original status once the robot is inside the pipe. Also, LaMMos add stiffness to the robot legs by dynamically integrate them to the structure. Additionally, we present an application of the LaMMos mechanism to exoskeleton suits, for easing the mo- tors from the joints when carrying heavy weights for long periods of time. This mechanism offers many interesting opportunities for robotics research in terms of functionality, pay- load and size.Comment: 14 pages, 15 figure

Soybean growth and production under straw of maize, Urochloa brizantha, Conyza spp. and Digitaria insularis

Weeds can cause serious damage during soybean development, due to allelopathy, competition for water, light and nutrients. It is necessary to investigate the influence of straw, of weeds Conyza spp. and Digitaria insularis, in soybean growth, production and composition and grains. If there is influence of allelopathic compounds at the crop. The aim of this study was to evaluate the influence of straw of maize and Urochloa brizantha (A.Rich.) R.D.Webster crops and Conyza spp., D. insularis, on growth, production and composition of grains produced by soybean. Treatments consisted of the control (absence of straw), maize straw, Urochloa brizantha straw, Conyza spp. straw (500, 1000, 1500 or 2000 kg ha-1) and D. insularis straw (500, 1000, 1500 or 2000 kg ha-1). The chlorophyll index, height of plants and insertion of the first pod, stem diameter at the collar and at 5 cm from the collar, root dry mass, number of pods and grains, weight of total grains, weight of 100 grains, protein and nitrogen (N), catalase and peroxidase contents in grains were evaluated. There was no difference between treatments for plant height, first pod height and chlorophyll index, as well as for total pods and 100 grain weight and protein content, N content and peroxidase and carboxylase enzymatic activity of the grains produced. For stem diameter, a higher value was found for the treatment with maize straw compared to the control (no straw). For dry root matter, treatments without straw and with Conyza spp. straw up to 1500 kg ha-1, differed from the treatment with maize straw. Even in some respects they provided beneficial effects compared to the absence of straw, which indicates the importance of crop residues. No allelopathic effects of weeds were observed on the growth and development of soybean. Conyza spp., D. insularis, maize or U. brizantha straws do not negatively affect soybean growth, production and grain composition

Public awareness and engagement in relation to the coastal oil spill in northeast Brazil

Social media data is a rich source of information to assess human activities in catastrophic events. Here, we use social media data to understand how the 2019 Brazilian oil spill influenced social attitudes. Data were collected from the globally popular Instagram platform between August 1, 2019 and March 1, 2020. First, we manually identified the 5 most popular (portuguese language) hashtags related to the oil spill #oleonononordeste;#desastreambiental;#ma rsemoleo;#sosnordeste;#ma rsempetroleo. In the sequence, we collected information on captions, post metadata and users associated with posts retrieved using the selected hashtags. We identified a total of 7,413 posts. These posts were grouped in topics: government (47.76%), protest (24.37%), volunteers (24.45%), biodiversity (0.003%), origin (0.006%), tourism (0.008%) and others (0.016%). All topics had the peak of posts in October and November 2019. Nevertheless, interest in the oil spill was temporary, with most posts appearing in the 2-4 months after the beginning of the disaster. Our findings illustrate the enormous potential of using social media data for understanding and monitoring human engagement with environmental disasters, but also suggest that conservationists and environmental groups may only have a limited 'window of opportunity' to engage and mobilize public support for remediation and restoration efforts.Peer reviewe