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[en] The magnetic order-disorder layering transitions of a spin-1 Ising model are investigated, under the effect of a variable surface crystal field Δs, 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 Δk=Δs/kα, 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 k0 layers. This reentrant phenomenon is absent for the remaining N-k0 layers, but the cricritical 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 Δs and selected values of the parameter α are established
[en] The effect of a variable surface transverse magnetic field Ωs, 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)=Ωs/zα, (z=1, triple dot, 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
[en] In this paper, we propose a stochastic Cellular Automata (CA) model to study traffic flow at a single-lane urban roundabout (resp. traffic circle) of N entry points (resp. exit points), the entry points are controlled by rates α_1 and α_2 while the removal rates from the exit points are denoted by β. The traffic is controlled by a self-organized scheme. Based on computer simulation, density profiles, global density and current are calculated in terms of rates. Furthermore, the phase diagrams for roundabout as well as traffic circle are constructed. It has turned out that the phase diagrams consist essentially of two phases namely free flow and jamming. It is noted that the typology of the phase diagrams of the roundabout is not similar to it in the traffic circle. Furthermore, we have compared the performance of the two systems in terms of the geometrical properties and the number of entry points. - Highlights: • Simulations are conducted to study traffic flow at a single-lane urban roundabout as well as traffic circle. • Phase diagrams for roundabout as well as traffic circle are constructed. • The performance of the two systems are compared in terms of the geometrical properties and the number of entry points
[en] The effects of alternating transverse fields Ωa and Ωb on the critical behavior of an alternating spin-1 Ising superlattice are studied within an effective field theory with a probability distribution technique that accounts for the single-site spin correlation. Critical temperatures are calculated as a function of the thickness of the superlattice and the strength of the transverse field. Depending on the values of the transverse fields Ωa and Ωb, the critical temperature can increase or decrease with increasing the thickness of the film, such result is not obtained in the uniform transverse field case (Ωa = Ωb). Furthermore, for each thickness L of the film, a long range ordered phase persist at low temperature for selected values of the transverse field Ωa and arbitrary values of Ωb. The effects of interlayer and intralayer exchange interactions are also examined
[en] Using mean-field theory, we have studied the effect of quantum transverse anisotropies with RKKY interaction on the multi-layer transition and magnetic properties of the spin-1 Blume—Capel model of a system formed by two magnetic multi-layer materials, of different thicknesses, separated by a non-magnetic spacer of thickness M. It is found that the multilayer magnetic order—disorder transition temperature depends strongly on the value of the transverse anisotropy. The multilayer transition temperature decreases when increasing the transverse anisotropy. Furthermore, there exists a critical quantum transverse anisotropy ΔxL beyond which the separate transitions occur in the two magnetic layers. The critical transverse anisotropy ΔxL decreases (increases) on increasing the non-magnetic spacer of thickness M (on increasing the crystal field), and ΔxL undergoes oscillations as a function of the Fermi level. (condensed matter: electronic structure, electrical, magnetic, and optical properties)
[en] Quantum key distribution based on encoding in three-dimensional systems in the presence of several eavesdroppers is proposed. This extends the BB84 protocol in the presence of many eavesdroppers where two-level quantum systems (qubits) are replaced by three-level systems (qutrits). We discuss the scenarios involving two, three and four complementary bases. We derive the explicit form of Alice and Bob mutual information and the information gained by each eavesdropper. In particular, we show that, in the presence of only one eavesdropper, the protocol involving four bases is safer than the other ones. However, for two eavesdroppers, the security is strongly dependent on the attack probabilities. The effect of a large number of eavesdroppers is also investigated.
[en] The control of vehicles in urban traffic is a requirement to maximize the flow and to ensure the safety of traffic. Using the cellular automata Nagel–Schreckenberg (NaSch) model within a parallel dynamic update, we studied the effect of the intersection of two symmetrical roads, with typical periodic boundary conditions. It is found that the fundamental diagram depends strongly on the probability P of priority and the probability P1 of changing the road at the intersection. Beside the free flow, the platoon and the jamming phases, the fundamental diagram exhibits a fourth new phase occurring for any value of P ≠ 0.5, which disappears gradually as one increases the probability P, and disappears completely for P = 0.5. The effects of the braking probability Pb on the fundamental diagram and space time structures are also computed for different values of maximal velocities. (papers)
[en] The perovskite type oxide SrHfO had a huge scientist interest for the past few years thanks to its properties, which allowed it to be applied in different area, in our case we focused on the photovoltaic field application and it is known that this technology has been based on the use of semiconductors with a specific gap value since its birth, which indicates that the gap value is an important element who influences on the efficiency of panels. The aim of our work is based on reducing the gap value by applying different percentage of doping SrHfOS (x = 0%, 8% and 16%) and the determination of electronic and optical properties of all percentage of S using density functional theory (DFT). As a result we reduced the gap value from 5.60 eV corresponding to 0% of S to 2.09 eV corresponding to 16% of S and the band gap is changed from an indirect band gap equivalent to 0% of S to a direct band gap for 8% and 16% of S.
[en] According to first-principles density functional calculations, we have investigated the magnetic properties of Mn-doped GaN with defects, Ga1−x−yVGxMny N1−z−tVNzOt with Mn substituted at Ga sites, nitrogen vacancies VN, gallium vacancies VG and oxygen substituted at nitrogen sites. The magnetic interaction in Mn-doped GaN favours the ferromagnetic coupling via the double exchange mechanism. The ground state is found to be well described by a model based on a Mn3+−d5 in a high spin state coupled via a double exchange to a partially delocalized hole accommodated in the 2p states of neighbouring nitrogen ions. The effect of defects on ferromagnetic coupling is investigated. It is found that in the presence of donor defects, such as oxygen substituted at nitrogen sites, nitrogen vacancy antiferromagnetic interactions appear, while in the case of Ga vacancies, the interactions remain ferromagnetic; in the case of acceptor defects like Mg and Zn codoping, ferromagnetism is stabilized. The formation energies of these defects are computed. Furthermore, the half-metallic behaviours appear in some studied compounds. (condensed matter: structural, mechanical, and thermal properties)
[en] Using Monte Carlo simulations, magnetic properties of the ferromagnetic nanoparticles of Ising spin-1 are investigated in the framework of the Ising model. The system is considered to have a Rubik’s cube structure composed of nanocubes having an equivalent exchange coupling, while, between adjacent nanocubes, the exchange coupling is assumed to vary. Both size effects and system parameters’ influence on phase diagrams of the nanosystem are studied. Thus, the magnetic properties of the system such as the critical temperature, the magnetization, and the coercive field are computed.