A. Benyoussef

Electronic structure of acceptor defects in (Zn,Mn)O and (Zn,Mn)(O,N)

Using first-principles density functional calculations, we study the electronic structure and magnetic properties of Mn-doped ZnO, wurtzite crystal structure, with various defects. This allows to understand and to explain the half-metallicity and the ferromagnetism stability, observed in Mn-doped ZnO with acceptor defects like Zn vacancies. The calculations were performed using the Korringa–Kohn–Rostoker method combined with the coherent potential approximation. Hydrogenation effects in (Zn,Mn)O and (Zn,Mn)(O,N) is also investigated with and without defects. This work presents detailed information about total, atom, and vacancy projected density of states functions, and magnetic moment for different atoms and defects in Mn-doped ZnO and N-codoped (Zn,Mn)O. The Curie temperature TC is evaluated by using the mean field approximation. We show also that higher values of TC are attained for high concentration of vacancy defects sites in (Zn,Mn)O and for small concentration of vacancy defects sites in (Zn,Mn)(O...

Tricritical behaviour in diluted mixed spin-1 and spin-12 on square lattice

Abstract An effective-field theory with correlations is developed for a diluted mixed sping- 1 2 and spin-1 Ising ferromagnetic system with a crystal-field interaction D in a square lattice (Z = 4). The phase diagrams in temperature-concentration of magnetic atoms (p) and in temperature-crystal field interaction planes exhibit a variety of interesting phenomena such as second order phase transitions, tricritical points and first order phase transitions. The reentrant phenomena are also observed.

First-principles prediction of the magnetism of 4f rare-earth-metal-doped wurtzite zinc oxide

Abstract Electronic structure and magnetic properties of wurtzite ZnO semiconductor doped with rare earth (RE=La, Ce, Pr, Pm, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm and Yb) atoms were studied using spin-polarized density functional theory based on the full-potential linear augmented plane wave (FP-LAPW) method as implemented in the Wien2k code. In this approach the generalized gradient approximation (GGA) was used for the exchange-correlation (XC) potential. Our results showed that the substitution of RE ions in ZnO induced spins polarized localized states in the band gap. Moreover, the studied DMSs compounds retained half metallicity at dopant concentration x =0.625% for most of the studied elements, with 100% spin polarization at the Fermi level ( E F ). The total magnetic moments of these compounds existed due to RE 4f states present at E F , while small induced magnetic moments existed on other non-magnetic atoms as well. Finally, the energy difference between far and near configurations was investigated. It was found that the room temperature ferromagnetism was possible for RE-doped ZnO at near configuration. Since the RE-RE separation was long enough (far configuration) for magnetic coupling, the system became paramagnetic or antiferromagnetic ground state.

Edge wetting of an Ising three-dimensional system.

The effect of edge on wetting and layering transitions of a three-dimensional spin-1/2 Ising model is investigated, in the presence of longitudinal and surface magnetic fields, using mean field theory and Monte Carlo simulations. For T=0, the ground state phase diagram shows that there exist only three allowed transitions, namely, surface and bulk transition, surface transition, and bulk transition. However, there exist a surface intralayering temperature T(s)(L), above which the surface and the intralayering surface transitions occur. While the bulk layering and intralayering transitions appear above another finite temperature T(b)(L)(>or=T(s)(L)). These surface and bulk intralayering transitions are not seen in the perfect surfaces case. Numerical values of T(s)(L) and T(b)(L), computed by Monte Carlo method are found to be smaller than those obtained using mean field theory. However, the results predicted by the two methods become similar, and are exactly those given by the ground state phase diagram, for very low temperatures. On the other hand, the behavior of the local magnetizations as a function of the external magnetic field, shows that the transitions are of the first order type. T(s)(L) and T(b)(L) decrease when increasing the system size and/or the surface magnetic field. In particular, T(b)(L) reaches the wetting temperature T(w) for sufficiently large system sizes.

Diluted mixed spin-1 and spin-12 on honeycomb lattice

Abstract Using an effective field theory with correlation we study a diluted mixed spin ferromagnetic system consisting of spin - 1 2 and spin -1 with a crystal field interaction D . The influence of D on magnetic properties of the honeycomb lattice is discussed and a variety of interesting phase diagrams (in T/J, p ) and ( T/J, D/J ) planes are given.

Order–disorder layering transitions of a spin-1 Ising model in a variable crystal field

Abstract The magnetic order–disorder layering transitions of a spin-1 Ising model are investigated, under the effect of a variable surface crystal field Δ s , using the mean-field theory. Each layer k , of the film formed with N layers, disorders at a finite surface crystal field distributed according to the law Δ k = Δ s / k α , k =1,2,…, N and α being a positive constant. We have established the temperature-crystal field phase diagrams and found a constant tricritical point and a reentrant phenomenon for the first k 0 layers. This reentrant phenomenon is absent for the remaining N − k 0 layers, but the tricritical points subsist and depend not only on the film thickness but also on the exponent α . On the other hand, the thermal behaviour of the surface magnetisation for a fixed value of the surface crystal field Δ s and selected values of the parameter α are established.

Anisotropy effect on two-dimensional cellular-automaton traffic flow with periodic and open boundaries

Using computer simulations we investigate, in a version of the Biham-Middleton-Levine model with random sequential update on a square lattice, the anisotropy effect of the probabilities of the change of the motion directions of cars, from up to right (p(ur)) and from right to up (p(ru)), on the dynamical jamming transition and velocities under periodic boundaries on one hand and the phase diagram under open boundaries on the other hand. However, in the former case, the sharp jamming transition appears only for p(ur)=0=p(ru)=0 (i.e., when the cars alter their motion directions). In the open boundary conditions, it is found that the first-order line transition between jamming and moving phases is curved. Hence, by increasing the anisotropy, the moving phase region expands as well as the contraction of the jamming and maximal current phases takes place. Moreover, in the anisotropic case, the transition between the jamming phase (or moving phase) and the maximal current phase is of second order while in the isotropic case, and when each car changes its direction of motion at every time step (p(ru)=p(ur)=1), the transition is of first order. Furthermore, in the maximal current phase, the density profile decays with an exponent gamma approximately 1/4.

Order-disorder layering transitions in a variable transverse field

Abstract The effect of a variable surface transverse magnetic field Ω s , on the order–disorder layering transitions of an Ising spin − 1 2 model is investigated using mean field theory (MF) and finite cluster approximation (FC). For a film with finite thickness formed with N layers, each layer z transits at a fixed temperature from the ordered to the disordered phase, under the effect of the transverse magnetic field Ω(z)=Ω s /z α , (z=1, …, N) . These order–disorder layering transitions are absent for α =0 even if increasing the temperature and/or the surface transverse magnetic field. On the other hand, the dependency of longitudinal and transverse magnetisations on temperature and surface transverse magnetic field are illustrated for a fixed value of the exponent α .

Phase diagrams of a ferromagnetic amorphous bilayer system

Abstract Using the effective-field theory, we study phase diagrams of a ferromagnetic amorphous bilayer system, consisting of two monolayers (A and B) with different spins ( S A = 1 2 and S B = 1 2 , 1) and different interaction constants coupled together with an interlayer coupling. The effects of amorphization in the monolayer B and in the interlayer coupling are investigated. The influence of the crystal-field interaction D , in the case S A = 1 2 and S B =1, is also studied. A number of interesting phenomena are obtained such as the dependence of the tricritical behavior on amorphization. Other phenomena have been given in a variety of phase diagrams. The temperature dependence of the total magnetization is also examined.

Optoelectronic response of spinels CdX2O4 with X = (Al, Ga, In) through the modified Becke–Johnson functional

Structural, electronic, and optical properties of CdAl2O4, CdGa2O4, and CdIn2O4 are investigated using the first principles technique based on the new potential approximation known as the Tran–Blaha modified Becke–Johnson exchange potential approximation (TB-mBJ). We discuss the improvements in the band gap determination using TB-mBJ compared to the standard generalized gradient approximation in density functional theory. The calculated equilibrium lattice parameters are in reasonable agreement with the experimental results. Electronic properties have been studied through the calculation of band structure and density of states. Based on our electronic structure obtained using the mBJ method, we have calculated various optical properties, including the complex dielectric function e(ω), complex index of refraction n(ω), reflectivity coefficient R(ω), absorption coefficient α(ω), and the electron energy-loss function L(ω) as functions of the photon energy. We chose these three representative oxides spinel du...