S. Naji

First principle calculations for improving desorption temperature in Mg16H32 doped with Ca, Sr and Ba elements

Using ab initio calculations, we predict the improvement of the desorption temperature and the hydrogen storage properties of doped Mg-based hydrides such as, Mg15AMH32 (AM = Ca, Sr and Ba) as a super cell 2 × 2 × 2 of MgH2. In particular, the electronic structure has been obtained numerically using the all-electron full-potential local-orbital minimum-basis scheme FPLO9·00-34. Then, we discuss the formation energy calculations in terms of the material stabilities and the hydrogen storage thermodynamic properties improvements. Among others, we find that the stability and the temperature of desorption decrease without reducing significantly the high storage capacity of hydrogen. Moreover, it has been observed that such a doping procedure does not affect the electronic behavior as seen in MgH2, including the insulator state in contrast with the transition metal hydrides, which modify the electronic structure of pure MgH2.

Interdistance Effects on Flat and Buckled Silicene Like-bilayers

Using ab intio numerical calculations based on the all-electron full-potential local-orbital minimum-basis scheme FPLO9.00-34, we discuss the interdistance effect on the energy gap of two parallel layers of the silicone systems. The like- bilayer systems we dealt with here are relying on a dynamic monolayer of silicene located at distance d along the normal direction z forming with a static one a (AA) stacking arrangement. In particular, we investigate the effect of the dynamic layer by varying the distance d starting from a distance around the bond length of Van der Waals. More precisely, we consider the flat and two buckled geometries in (AA) arrangements. The flat geometry is associated with the usual (AA) configuration appearing in the pure graphene material. For buckled geometry, we can distinguish two configurations. The first one corresponds to the usual buckled configuration that keeps the same vertical distance between the two layers atoms while the remaining one is obtained by reversing one silicene layer. We show that the band gap can be opened by simply varying the distance, starting around a Van der Waals distance, between two parallel silicene for flat and buckled geometries due to an electronic transition of electrons living in pz orbital states. Furthermore, we study the stability between the buckled and the flat configuration in the mono and bilayer system.

New statistical lattice model with double honeycomb symmetry

Inspired from the connection between Lie symmetries and two-dimensional materials, we propose a new statistical lattice model based on a double hexagonal structure appearing in the G2 symmetry. We first construct an Ising-1/2 model, with spin values σ = ±1, exhibiting such a symmetry. The corresponding ground state shows the ferromagnetic, the antiferromagnetic, the partial ferrimagnetic and the topological ferrimagnetic phases depending on the exchange couplings. Then, we examine the phase diagrams and the magnetization using the mean field approximation (MFA). Among others, it has been suggested that the present model could be localized between systems involving the triangular and the single hexagonal lattice geometries.

ELECTRONIC STRUCTURE OF GRAPHENE AND GERMANENE BASED ON DOUBLE HEXAGONAL STRUCTURE

In this paper, we study the electronic structure of monolayer materials based on a double hexagonal geometry with (1×1) and superstructures. Inspired from the two-dimensional root system of an exceptional Lie algebra called G2, this hexagonal atomic configuration involves two hexagons of unequal side length at angle 30°. The principal unit hexagonal cell contains twelve atoms instead of the usual configuration involving only six ones relying only on the (1×1) superstructure. Using ab initio calculations based on FPLO9.00-34 code, we investigate numerically the graphene and the germanene with the double hexagonal geometry. In particular, we find that the usual electronic properties and the lattice parameters of such materials are modified. More precisely, the lattice parameters are increased. It has been shown that, in the single hexagonal geometry, the grapheme and the germanene behave as a gapless semiconductor and a semi-metallic, respectively. In double hexagonal geometry however, both materials becomes metallic.

The investigations of electronic structure, optical and magnetic properties of MgB 2 nanosheets

First principles calculations were performed to study the electronic structures of Magnesium diboride (MgB2) bulk and nanosheet with different layer and thickness in order to understand the influence of layer number and thickness on electronic and optical properties for this compound, using the Full Potential Linearized Augmented Plane Waves (FP-LAPW) method with the Generalized Gradient Approximation (GGA) implemented in Wien2k code. It is shown that the MgB2 is superconductor with Tc=39K, however MgB2 sheet become a nonmagnetic semiconductor and good transparent in the visible range, the absorption coefficient decrease and the superconductor properties appears again with increasing number of layers.