Excitonic effects in the optical properties of CdSe nanowires

Using a first-principle approach beyond density functional theory we calculate the electronic and optical properties of small diameter CdSe nanowires.Our results demonstrate how some approximations commonly used in bulk systems fail at this nano-scale level and how indispensable it is to include crystal local fields and excitonic effects to predict the unique optical properties of nanowires. From our results, we then construct a simple model that describes the optical gap as a function of the diameter of the wire, that turns out to be in excellent agreement with experiments for intermediate and large diameters.Comment: submitte

Construction of the B88 exchange-energy functional in two dimensions

We construct a generalized-gradient approximation for the exchange-energy density of finite two-dimensional systems. Guided by non-empirical principles, we include the proper small-gradient limit and the proper tail for the exchange-hole potential. The observed performance is superior to that of the two-dimensional local-density approximation, which underlines the usefulness of the approach in practical applications

Child with multiple ecchymoses … is it abuse?

Caso clínico: Os maus tratos infantis são um problema de saúde pública que pode, em algumas situações, ser mimetizado por patologia da hemóstase. Pretende-se alertar para esta problemática através da descrição do caso clínico de uma criança com quadro de discrasia hemorrágica (equimoses dispersas), em que se levantou a suspeita de abuso físico. Da avaliação realizada, incluindo o estudo da coagulação, conclui-se tratar--se de Doença de von Willebrand tipo I grave, não se tendo provado disfunção social. Comentários: Perante uma suspeita de maus tratos baseada em sinais de discrasia é absolutamente necessário excluir uma alteração da hemostase. A avaliação laboratorial deve ter em conta a história pessoal e familiar de discrasia, as alterações do exame objetivo e a prevalência das patologias. A confirmação diagnóstica de uma patologia da hemóstase não exclui, contudo, um mau trato físico concomitante devendo ser mantida a vigilância da situação socia

Two-dimensional dispersion analytical approach: Eddy diffusivities depending on source distance

An analytical air quality dispersion approach based on the steadystate two-dimensional advection-diffusion equation is presented. The solution employs a spectral method and is analytical in the sense that no approximation is made along its derivation. The approach is valid for homogeneous turbulence and for situations of uniform mean wind speed, and for practical purposes, to elevated releases that occur in neutral stability conditions without strong buoyancy. To simulate and compare the results of this approach against observed ground-level crosswind-integrated concentration two eddy diffusivities are considered. The first eddy diffusivity depends on the distance from the source while the second one assumes a constant value independent of the source distance. It is found that the memory effect contained in the eddy diffusivity, which is a function of downwind distance from the source, allows a better description of the turbulent dispersion of atmospheric contaminants released by an elevated continuous point source

On the violation of a local form of the Lieb-Oxford bound

In the framework of density-functional theory, several popular density functionals for exchange and correlation have been constructed to satisfy a local form of the Lieb-Oxford bound. In its original global expression, the bound represents a rigorous lower limit for the indirect Coulomb interaction energy. Here we employ exact-exchange calculations for the G2 test set to show that the local form of the bound is violated in an extensive range of both the dimensionless gradient and the average electron density. Hence, the results demonstrate the severity in the usage of the local form of the bound in functional development. On the other hand, our results suggest alternative ways to construct accurate density functionals for the exchange energy.Comment: (Submitted on 27 April 2012

Mechanical and tribological characterization of four commercial braking formulation materials

Organic friction materials for brake linings, namely brake pads are polymer matrix composites usually containing numerous macroscopic as well as microscopic constituents such as filamentary reinforcements, solid lubricants, abrasives and fillers. Their main objective of the these composite system are that a constant and stable coefficient of friction (COF) is provided irrespective of environmental conditions such as pressure, sliding velocity, temperature, humidity etc. Brake pads performance should be maintained over a wide range of stress conditions. Although the wear of the brake pad is inevitable, it should be minimized as far as possible. While cast iron has been the dominant material for brake discs, brake pad materials are constantly evolving into more complex composites. The main objectives of the present work is to characterize the physical and the tribological behavior of four commercial braking pads material formulations. For all compositions the studied factors are normal pressure and sliding speed, considering one braking-test protocol designed with temperature and friction load data acquisition. A double pad-on-disc tribological test configuration was used to determine tribological properties of brake pad for heavy vehicles commercial. Three wear tests via increased severity of rubbing conditions for a constant braking distance.The authors acknowledge Auto Travões Viseu-recondicionamento de Travões e embraiagens Lda the support through the provision of the frictrion linning materials presented in the study