Viscous Cosmology

We discuss the possibility to implement a viscous cosmological model, attributing to the dark matter component a behaviour described by bulk viscosity. Since bulk viscosity implies negative pressure, this rises the possibility to unify the dark sector. At the same time, the presence of dissipative effects may alleviate the so called small scale problems in the $\Lambda$CDM model. While the unified viscous description for the dark sector does not lead to consistent results, the non-linear behaviour indeed improves the situation with respect to the standard cosmological model.Comment: Latex file, 7 pages, 6 figures. To appear in the proceedings of the XIIth International Conference on Gravitation, Astrophysics and Cosmology, June 28-July 5, 2015, PFUR, Moscow, Russi

Evidence of surface transport and weak anti-localization in single crystal of Bi2Te2Se topological insulator

Topological insulators are known to their metallic surface states, a result of strong-spin-orbital coupling, that show unique surface transport phenomenon. But these surface transports are buried in presence of metallic bulk conduction. We synthesized very high quality Bi$_2$Te$_2$Se single crystals by modified Bridgman method, that possess high bulk resistivity of $>$20~$\Omega$cm below 20~K, whereas the bulk is mostly inactive and surface transport dominates. Temperature dependence resistivity follows the activation law like a gap semiconductor in temperature range 20-300~K. We designed a special measurement geometry, which aims to extract the surface transport from the bulk. This special geometry is applied to measure the resistance and found that Bi$_2$Te$_2$Se single crystal exhibits a cross over from bulk to surface conduction at 20~K. Simultaneously, the material also shows strong evidence of weak anti-localization in magneto-transport due to the protection against scattering by conducting surface states. This novel simple geometry is an easy route to find the evidence of surface transport in topological insulators, which are the promising materials for future spintronic applications.Comment: 6 Pages, 4 Figure

Spin-phonon coupling in Gd(Co1/2Mn1/2)O3 perovskite

We have investigated the temperature-dependent Raman-active phonons and the magnetic properties of Gd(Co1/2Mn1/2)O3 perovskite ceramics in the temperature range from 40 K to 300 K. The samples crystallized in an orthorhombic distorted simple perovskite, whose symmetry belongs to the Pnma space group. The data reveals spin-phonon coupling near the ferromagnetic transition occurring at around 120 K. The correlation of the Raman and magnetization data suggests that the structural order influences the magnitude of the spin-phonon coupling.Comment: 3 Figures, suplementary materia

Charge renormalization and phase separation in colloidal suspensions

We explore the effects of counterion condensation on fluid-fluid phase separation in charged colloidal suspensions. It is found that formation of double layers around the colloidal particles stabilizes suspensions against phase separation. Addition of salt, however, produces an instability which, in principle, can lead to a fluid-fluid separation. The instability, however, is so weak that it should be impossible to observe a fully equilibrated coexistence experimentally.Comment: 7 pages, Europhysics Letters (in press

MODELING AND SIMULATION OF COMPRESSION-IGNITION INTERNAL COMBUSTION ENGINES’ EMISSIONS PRODUCED BY DIESEL AND BIODIESEL MIXTURES

Biofuels have been identified as possible solutions to the problems caused by the usage of fossil fuels in energy production. Although they generally produce fewer emissions, there are indications that engines powered with biodiesel mixtures emit pollutants such as nitrogen oxides in greater quantities than when powered by fossil diesel. So, further investigation on the emissions produced by these two fuels is needed, with the goal of best knowing what kind of harm to the environment each one of those is causing.  One of the best tools available for expanding any subject’s comprehension, without spending lots of resources, are mathematical models. In order to better understand the relations between the fuel used to power a compression-ignition internal combustion engine (ICO) and the emissions produced as subproducts of the thermodynamic process, this paper aims at developing a mathematical model of the production of emissions according to the fuel mixture used. The main goal is to develop a simple model, from the point of view of chemical kinetics, but with the support of well-collected experimental data, and methods of mathematical model adjustments and validations, to make the model describe the reality of the phenomena with satisfactory precision. This Mathematical Model is completely implemented using FORTRAN® Language. There are 2 sorts of data: one used to calibrate and adjust the model’s constants so the model can properly describe the reality of the events, and the other as the basis of comparison for the validation of the model after adjustments and calibrations. With this work, it is expected that the knowledge about how the use of these fuels impact global emissions, and how it is possible to optimize our energy production by using the best mixture of fuels at the optimal point between net power outtake and net emissions produced