Results 21 - 30 of 45
Results 21 - 30 of 45. Search took: 0.018 seconds
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[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] The effect of the interplane and intraplane anisotropies on the energy gap for odd number of coupled quantum spin-1/2 antiferromagnetic anisotropic Heisenberg planes is investigated using a linear spin wave theory. However, in the isotropic case, such system exhibits a long range order, and no energy gap, while in the anisotropic case, the energy gap opens above a critical anisotropic value. The known results of a single plane have been obtained. (author)
[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.
[en] The first-principles density functional calculation is used to investigate the electronic structures and magnetic properties of Mn-doped and N-co-doped ZnO nanofilms. The band structure calculation shows that the band gaps of ZnO films with 2, 4, and 6 layers are larger than the band gap of the bulk with wurtzite structure and decrease with the increase of film thickness. However, the four-layer ZnO nanofilms exhibit ferromagnetic phases for Mn concentrations less than 24% and 12% for Mn-doping performed in the whole layers and two layers of the film respectively, while they exhibit spin glass phases for higher Mn concentrations. It is also found, on the one hand, that the spin glass phase turns into the ferromagnetic one, with the substitution of nitrogen atoms for oxygen atoms, for nitrogen concentrations higher than 16% and 5% for Mn-doping performed in the whole layers and two layers of the film respectively. On the other hand, the spin-glass state is more stable for ZnO bulk containing 5% of Mn impurities, while the ferromagnetic phase is stable by introducing the p-type carriers into the bulk system. Moreover, it is shown that using the effective field theory for ferromagnetic system, the Curie temperature is close to the room temperature for the undamped Ruderman—Kittel—Kasuya—Yoshida (RKKY) interaction
[en] A Monte Carlo (MC) simulation has been used to study the magnetic properties of an antiferromagnetic surface which is coupled ferromagnetically to a ferromagnetic layer. The influence of different exchange interactions on the phase diagram of the surface has been investigated. We have found that in the ordered phase, depending on the values of the exchange interactions, the antiferromagnetic surface can change to ferromagnetic or ferrimagnetic phases. Furthermore, in the case of a bilayer surface, the layering antiferromagnetic-ferromagnetic or ferrimagnetic transitions occur
[en] The effect of a random transverse field (RTF) on the wetting and layering transitions of a spin-1/2 Ising model, in the presence of bulk and surface fields, is studied within an effective field theory by using the differential operator technique. Indeed, the dependencies of the wetting temperature and wetting transverse field on the probability of the presence of a transverse field are established. For specific values of the surface field we show the existence of a critical probability p, above which wetting and layering transitions disappear. (author)
[en] In this paper, we used the ab-initio calculations, based on the Korringa-Kohn-Rostoker (KKR) method combined with the coherent potential approximation (CPA), to simulate the magnetic properties of ZnO, doped and co-doped with manganese and carbon, respectively. For this purpose, we have used two different approximations: the Local Density Approximation (LDA) and the Local Density Approximation-Self-Interaction Correction (LDA-SIC). Numerical results are presented for the compound Zn1 − 0.06Mn0.06O1−xCx when doping and co-doping is performed with Mn and C as doping elements. Total and partial DOSs are given for different concentrations using the two approximations, LDA and LDA-SIC. It is found that for 6% with doping by Mn the system becomes magnetic. The co-doping with carbon changes the behavior of the system : it becomes also magnetic for 4, 6 and 10% concentrations within both, LDA and LDA-SIC approximations. Furthermore, we have discussed the type of mechanism of exchange interaction and found that the double exchange is responsible for the appearing magnetism in the system, within the LDA and p-d interaction for LDA-SIC approximation. For 10% of carbon, we have found that the critical temperature approaches 280 K in the LDA approximation solely; and is about 305 K in the LDA-SIC approximation.
[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 correlations. 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 persists 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. (author)