Results 1 - 10 of 127
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[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 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] 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 cationic substitutions effects at different concentrations, within the ZnxCd1-xCr2Se4 lattice (0.35≤x≤0.58), on the electronic transfer in chromium's ion are investigated. For this purpose, after taking into account the exchange integrals, we use the Anderson-Kanamori theory of superexchange to determine the transfer integrals bσσ and bπσ. Using the conventional molecular orbital model, the covalency-mixing parameters λσ and λπ, the electron spin transfer coefficients fi(i=σ, π, s) and the total charge C are evaluated. The bσσ and bσπ variation with x, agrees with the passage of the system from ferromagnetic to antiferromagnetic order. For the compound presenting re-entrant behaviour (0.45≤x≤0.58), fs exhibits anomalies. The C increasing with x is consistent with the expected decrease in covalency of the non-magnetic cation in a tetrahedral site
[en] The magnetic properties of ferrimagnetic mixed-spin Ising model in the checkerboard lattice are studied using Monte Carlo simulations. The variation of total magnetization and magnetic susceptibility with the crystal field has been established. A transition from an order to a disordered phase has been obtained in some critical value of the physical variables. The reduced transition temperature is obtained for different exchange interactions. The magnetic hysteresis cycles have been established. The multiples hysteresis cycle in checkerboard lattice are obtained. The multiples hysteresis cycle have been established. The ferrimagnetic mixed-spin Ising model in checkerboard lattice is very interesting from the experimental point of view. The mixed spins system have many technological applications such as in domain opto-electronics, memory, nanomedicine and nano-biological systems. The obtained results show that that crystal field induce long-range spin–spin correlations even bellow the reduced transition temperature
[en] Magnetic, electronic and structural properties of titanium dioxide material with different structural defects are studied using the first-principles ab-initio calculations and the Korringa–Kohn–Rostoker method (KKR) combined with the coherent potential approximation (CPA) method in connection with the local density approximation (LDA). We investigated all structural defects in rutile TiO_2 such as Titanium interstitial (Ti_i), Titanium anti-sites (Ti_o), Titanium vacancies (V_T_i), Oxygen interstitial (O_i), Oxygen anti-sites (O_T_i) and oxygen vacancies (V_o). Mechanisms of hybridization and interaction between magnetic atoms are investigated. The transition temperature is computed using the Mean Field Approximation (MFA).Magnetic stability energy of ferromagnetic and disordered local moment states is calculated to determine the most stable state. Titanium anti-sites have a half-metallic aspect. We also studied the change type caused by structural defects in this material. - Highlights: • Green function technique is used to study disordered systems. • We used DFT to study electronic structure of TiO_2 perturbed by defects. • TiO_2 with titanium antisite defect posesses a magnetic behavior. • The transition temperature is computed using the Mean Field Approximation.
[en] In this paper, we aimed to study the effect of doping of the compound CdTe with the cobalt impurity (Co), as well as the vacancy defects in Cd sites. On one hand, this leads to the investigation of the magnetic properties and the Faraday rotation effect for the studied alloy, doped with different concentrations of cobalt (0.01, 0.05, 0.1, 0.15, 0.2, 0.25). On the other hand, we have created 0.01 of vacancy defects in Cd sites. Then, we raised the vacancy defect concentration to 0.05, keeping the same concentrations of cobalt. As a result, we have investigated that there is a magnetism appearing with the cobalt doping, while the vacancy defects in Cd sites affect the stability of the magnetic states. In general, it improves the ferromagnetic state that will be well explained in the discussion. Our calculations were performed using the KKR-CPA method within the spin-polarized density functional theory (DFT) with the local density approximation (LDA). The curves of the density of states (DOS) illustrate the results of this study which has been discussed, analyzed, and explained below. In addition, the energy of each case was calculated and given in the tables below for both the DLM (half of the cobalt spins are up while the other half are down) and the ferromagnetic state in order to confirm which one of them is stable.
[en] The kinetics of the semi-infinite Ising model in the presence of a time-dependent oscillating external field is studied within the framework of the mean-field approach. We use the Glauber-type stochastic dynamics to describe the evolution time of this system. We present a variety of phase in a semi-infinite cubic ferromagnet with spin-1 in two different, planes; phase diagrams contain (BF, S 1P, SP) phase, (BF, S 1F, SP) phase, completely ordered phase (BF, S 1F, SF) and completely disordered phase (BP, S 1P, SP), which strongly depend on interaction parameters. The nature first and second orders of the transitions is characterized by studying the thermal behaviors of the dynamic magnetizations. Furthermore, the system exhibits dynamical tricritical phenomenon and the reentrant behaviors. The magnetizations and phase space trajectories are given and discussed.
[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] The effects of two different single-ion anisotropies on the magnetization of the mixed (7/2, 5/2) Ising ferrimagnetic system on a square lattice are investigated by the use of Monte Carlo simulation. The ground-state phase diagram is obtained. Compensation points are detected for certain values of the crystal fields. Different types of magnetization curves are available.