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There should be two ways to describe the flat rotation curves of galaxies and cluster of galaxies. Either one can introduce a dark matter component for the matter filling the halo, or by modifying the gravity theory and give the flat rotation curve a geometrical nature. Here we adopt an $$f(R)$$ model suitable for describing the effect. After matching the solution with the exterior solution, the effective density, radial and tangential pressures are obtained. Then the energy conditions and lensing effect is investigated. [ABSTRACT FROM AUTHOR]

Using a completely covariant approach, we discuss the role of boundary conditions (BCs) and the corresponding Gibbons-Hawking-York (GHY) terms in f(R)-gravity in arbitrary dimensions. Following the Ostrogradsky approach, we can introduce a scalar field in the framework of Brans-Dicke formalism to the system to have consistent BCs by considering appropriate GHY terms. In addition to the Dirichlet BC, the GHY terms for both Neumann and two types of mixed BCs are derived. We show the remarkable result that the f(R)-gravity is itself compatible with one type of mixed BCs, in D dimension, i.e. it doesn't require any GHY term. For each BC, we rewrite the GHY term in terms of Arnowit-Deser-Misner (ADM) variables. [ABSTRACT FROM AUTHOR]

We have considered the perturbation equations governing the growth of fluctuations during inflation in generalized scalar tensor theory f(R,ϕ). We have found that the scalar metric perturbations at very early times are negligible compared to the scalar field perturbation, just like general relativity. At sufficiently early times, when the physical momentum of perturbation mode, q / a is much larger than the Hubble parameter H, i.e. q/a≫H, we have obtained the metric and scalar field perturbation in the form of WKB solutions up to an undetermined coefficient. Then we have quantized the scalar fluctuations and expanded the metric and the scalar field perturbations with the help of annihilation and creation operators of the scalar field perturbation. The standard commutation relations of annihilation and creation operators fix the unknown coefficient. Going over to the gauge invariant quantities which are conserved beyond the horizon, we have obtained the initial condition of the generalized Mukhanov-Sasaki equation. Then a similar procedure is performed for the case of tensor metric perturbation. As an example of the generalized Mukhanov-Sasaki equation and its initial condition, we have proposed a power-law functional form as f(R,ϕ)=f0Rmϕn and obtained an exact inflationary solution. In this background, then we have discussed how the scalar and tensor fluctuations grow. [ABSTRACT FROM AUTHOR]

Herein, by using first-principles calculations, we demonstrate a two-dimensional (2D) of XSb (X = Si, Ge, and Sn) monolayers that have a honey-like crystal structure. The structural, mechanical, electronic, thermoelectric efficiency, and optical properties of XSb monolayers are studied. Ab initio molecular dynamic simulations and phonon dispersion calculations suggests their good thermal and dynamical stabilities. The mechanical properties of XSb monolayers shows that the monolayers are considerably softer than graphene, and their in-plane stiffness decreases from SiSb to SnSb. Our results shows that the single layers of SiSb, GeSb and SnSb are semiconductor with band gap of 1.48, 0.77 and 0.73 eV, respectively. The optical analysis illustrate that the first absorption peaks of the SiSb, GeSb and SnSb monolayers along the in-plane polarization are located in visible range of light which may serve as a promising candidate to design advanced optoelectronic devices. Thermoelectric properties of the XSb monolayers, including Seebeck coefficient, electrical conductivity, electronic thermal conductivity, power factor and figure of merit are calculated as a function of doping level at temperatures of 300 K and 800 K. Between the studied two-dimensional materials (2DM), SiSb single layer may be the most promising candidate for application in the thermoelectric generators., Competing Interests: The authors declare that there are no conflicts of interest regarding the publication of this paper., (This journal is © The Royal Society of Chemistry.)

High purity micron and sub-micron scale nickel materials have been mass-produced by the nickel carbonyl process for many decades. The same technology is being explored for mass production of nano-scale nickel products. The paper presents an overview of nickel carbonyl technology suitable for mass production of nano-scale nickel powders with surface areas exceeding 100 m2/g, and various three-dimensional nickel based micro structures. The industrial processes are based on the homogeneous decomposition of carbonyl in the free space of a CVD reactor and on the heterogeneous decomposition of carbonyl on a variety of substrates, including polymer foams and carbon fibres.