Photon-graviton pair conversion

We consider the conversion of gravitons and photons as a four-wave mixing process. A nonlinear coupled systems of equations involving two gravitons and two photons is obtained, and the energy exchange between the different degrees of freedom is found. The scattering amplitudes are obtained, from which a crossection for incoherent processes can be found. An analytical example is given, and applications to the early Universe are discussed.Comment: 5 pages, slightly modified as compared to v1, to appear in Class. Quantum Grav. as a Letter to the Edito

Finite Element Analysis of Experimentally Tested Concrete Slabs Subjected to Airblast

Since the last century, concrete has been used to protect structures against intentional or accidental detonation of explosives. Recently, as concerns about terrorist activities and accidents in plants using explosives increase worldwide, the study of the behaviour of this type of material and any civil or military structure under the influence of explosions has increased. Among the lethal effects of explosive devices, which cause greater loads in structural elements is the airblast effect. For this reason, this paper presents a series of airblast finite element (FEM) simulations developed in Abaqus/Explicit®. To validate the computational method, such simulations are geometrically and structurally kept similar to full-scale tests conducted in a blast test area of the Science and Technology Aerospace Department (Brazilian Air Force). Both simulations and tests consisted of seven reinforced concrete slabs with compressive strengths of about 40 to 60 MPa, variable steel reinforcement areas, slab dimensions measuring 1×1 m, and subjected to 2.7 kg of non-confined plastic bonded explosive. The results demonstrated that FEM simulations can predict the rupture of the tested slabs and how the effect occurs, showing a valid method to investigating the response of RC slabs when compared to expensive field tests. Differences in displacements were observed between the results of FEM simulations and blast field tests, mainly caused by the sensitivity of the case studied, limits of computational capacity, and intrinsic variations in the materials and sensors used in the field tests. However, these differences showed an order of magnitude compatible with the safety coefficients used with RC, demonstrating that the method can be used for the design of RC slabs under the effect of airblast

Pressure and chemical substitution effects in the local atomic structure of BaFe2As2

The effects of K and Co substitutions and quasi-hydrostatic applied pressure (P<9 GPa) in the local atomic structure of BaFe2As2, Ba(Fe{0.937}Co{0.063})2As2 and Ba{0.85}K{0.15}Fe2As2 superconductors were investigated by extended x-ray absorption fine structure (EXAFS) measurements in the As K absorption edge. The As-Fe bond length is found to be slightly reduced (<~ 0.01 Angstroms) by both Co and K substitutions, without any observable increment in the corresponding Debye Waller factor. Also, this bond is shown to be compressible (k = 3.3(3)x10^{-3} GPa^{-1}). The observed contractions of As-Fe bond under pressure and chemical substitutions are likely related with a reduction of the local Fe magnetic moments, and should be an important tuning parameter in the phase diagrams of the Fe-based superconductors.Comment: 7 pages, 6 figure