Results 11 - 20 of 152
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[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] Phase diagram of the q state antiferromagnetic Potts model on a square lattice is investigated in a neighborhood of the critical point h = 4, T = 0. Using a constructive criterion of uniqueness introduced by Dobrushin, Kolafa, and Shlosman, for the antiferromagnetic Ising model, we show that the activity, ac(q) > 1/(q - 1) and the slope increases from zero (q = 2) to infinity (q = is proportional). The phase diagram bulges near the mentioned critical point for 2 < q < is proportional. (author). 5 refs, 6 figs
[en] Using numerical simulations we study the effect of the jumping rate on the density, current and phase diagram in the open boundaries case, and on the velocities of particles in the case of one and two species of particles on a ring, of one dimensional asymmetric exclusion model. However, on the one hand, a passage from discontinuous to continuous transition occurs by decreasing the jumping rate in the open boundaries, on the other hand, for two species of particles on a ring the velocity of particle 1 increases while the velocity of particle 2 decreases as the jumping rate increases. (author)
[en] The effect of the Dzyaloshinskii-Moriya interaction on the stability of the Neel phase and the energy gap for the XXZ Heisenberg model on a d-dimensional hypercubic lattice is investigated using the linear spin-wave theory. In one-dimension, the disordered phase disappears above a critical value of the spin-axis anisotropy. In two-dimension, the instability of the Neel order occurs below a critical value of the easy-axis anisotropy. We find that for all d-dimensional systems, the energy gap vanishes above a critical value of the DM interaction. (author)
[en] The (1+1)-dimensional Hamiltonian q-state ferromagnetic Potts model in an external magnetic field H is studied using finite lattice extrapolation techniques. The possible phases and their boundaries are determined from the ground state energy and the gap in the excitation energy, for arbitrary values of q and both for positive and negative field. We found that, for H positive, there is a critical value qc, where a first order transition line appears for q>qc, starting at the zero field transition point and terminating at a critical point in (h/λ, H/λ) plane, which separates the disordered phase and the ordered phase in the direction of H. For h negative a critical transition line separating the disordered phase and the others (q-1) ordered phases becomes first order for q>qc. (author). 12 refs, 2 figs
[en] The effects of amorphized film and diluted surface on phase diagrams and magnetic properties of a spin-1/2 Ising film are investigated within a finite cluster approximation. Depending on the value of the concentration p of the dilution couplings at the surface, it is found that there exists a critical value pc, when increasing the thickness of the film the critical temperature Tc decreases for p greater than pc, while it increases for p less than pc. Moreover, the critical concentration depends on the amorphization δ. Indeed, pc increasing with increasing δ. Besides, the phase diagram represented in (Tc,α), where α represents the strength of the strong coupling at the surface, shows that there exists a critical value of the amorphization δc=δc(p) for p not= 1, for δ≤δc(p) a long range order occurs for any value of α at low temperature, while for δ>δc(p) an order-disorder transition occurs at T=0 and α=αc(δ). Magnetization is calculated as a function of the amorphization δ, the concentration p and the strength α of the strong coupling. (author)
[en] The influence of corrugation and disorder at the surface on the critical behaviour of a ferromagnetic spin-1/2 Ising film is investigated using mean-field theory and finite cluster approximation. It is found that the critical surface exponent β1 follows closely the one of a perfect surface, in the two cases: corrugated surface and random equiprobable coupling surface. However, in the case of flat surface with random interactions the surface critical exponent β1 depends on the concentration p of the strong interaction for p>pc=0,5, while for p≤pc, such critical exponent is independent on the value of p and is equal to the one of the perfect surface. Moreover, in the case of corrugated surface, the effective exponent for a layer z, βeffJ(z,n), is calculated as a function of the number of steps at the surface. (author)
[en] The effect of the chain and the dimer anisotropies on the ground state energy and the energy gap of the spin-1/2 quasi-one-dimensional antiferromagnetic Heisenberg model is investigated using a mean field theory. The dependence of the magnetization and the effective hopping parameters on the anisotropy αxy(=Jxyperpendicular/Jxyparallel) are presented for several values of the chain anisotropy. However, such a system exhibits a transition from antiferromagnetic ordered to disordered phases for arbitrary chain anisotropy and dimer anisotropy. (author). 22 refs, 11 figs
[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] The effect of the chain and perpendicular anisotropies on the energy gap for odd number of coupled quantum spin-1/2 antiferromagnetic anisotropic Heisenberg chains is investigated using a spin wave theory. The energy gap opens above a critical anisotropic value. The known results of the isotropic case have been obtained. (author). 11 refs, 4 figs