Disordered cellular automaton traffic flow model: Phase separated state, density waves and self organized criticality

We suggest a disordered traffic flow model that captures many features of traffic flow. It is an extension of the Nagel-Schreckenberg (NaSch) stochastic cellular automata for single line vehicular traffic model. It incorporates random acceleration and deceleration terms that may be greater than one unit. Our model leads under its intrinsic dynamics, for high values of braking probability $p$, to a constant flow at intermediate densities without introducing any spatial inhomogeneities. For a system of fast drivers $p\to 0$, the model exhibits a density wave behavior that was observed in car following models with optimal velocity. The gap of the disordered model we present exhibits, for high values of $p$ and random deceleration, at a critical density, a power law distribution which is a hall mark of a self organized criticality phenomena.Comment: 23 pages, 14 figure

Disordered Cellular automata traffic flow models

In this paper, we extend the Nagel-Schreckenberg (NaSch) model by introducing disordered acceleration and deceleration terms. The disorder leads to segregated states where the flow is constant at intermediate densities for high values of breaking probability p. Within the model we present a density wave behavior appears below a critical value of p. Such result was found in car following models with an optimal velocity. The behavior of the gap distribution shows that the traffic exhibits a self organized criticality for high values of p and random deceleration.In this paper, we extend the Nagel-Schreckenberg (NaSch) model by introducing disordered acceleration and deceleration terms. The disorder leads to segregated states where the flow is constant at intermediate densities for high values of breaking probability p. Within the model we present a density wave behavior appears below a critical value of p. Such result was found in car following models with an optimal velocity. The behavior of the gap distribution shows that the traffic exhibits a self organized criticality for high values of p and random deceleration

Numerical study of rice-pile model

A one-dimensional model of a rice-pile is numerically studied for different driving mechanisms. We found that for a sufficiently large system, there is a sharp transition between the trivial behaviour of a 1D BTW model and self-organized critical (SOC) behaviour. Depending on the driving mechanism, the self-organized critical rice-pile model belongs to two different universality classes.A one-dimensional model of a rice-pile is numerically studied for different driving mechanisms. We found that for a sufficiently large system, there is a sharp transition between the trivial behaviour of a 1D BTW model and self-organized critical (SOC) behaviour. Depending on the driving mechanism, the self-organized critical rice-pile model belongs to two different universality classes

Phase diagrams and magnetic behavior of films with amorphization and anisotropy in surfaces

The phase diagrams and magnetic behavior of thin films with two amorphous surfaces are investigated by the use of the effective field theory with correlations. The transition temperature dependence of the exchange integral at surfaces, coupling between surface and nearest-layers, film thickness, and structural fluctuations are studied. Some interesting phenomena can occur as wetting phenomena and compensation point.The phase diagrams and magnetic behavior of thin films with two amorphous surfaces are investigated by the use of the effective field theory with correlations. The transition temperature dependence of the exchange integral at surfaces, coupling between surface and nearest-layers, film thickness, and structural fluctuations are studied. Some interesting phenomena can occur as wetting phenomena and compensation point

NUMERCAL STUDY OF A FOUR COMPONENTS SYSTEM

We have investigated numerically a statistical model of four component systems, which exhibit two critical temperatures, called the Ashkin-Teller model (ATM). The effects of, the anisotropy coupling, the single ion potential field and the mixed spin on the structure of the phase diagram have been studied. The model presents a rich variety of phase transitions which meet on tricritical or multicritical points. Different partially ordered phases with a partially broken symmetry appears at high temperatures. Their region of stability and their structure depend on the phase parameter space. The nature of critical lines which bound these partially ordered phases depends on the coupling parameters and the crystalline anisotropy".We have investigated numerically a statistical model of four component systems, which exhibit two critical temperatures, called the Ashkin-Teller model (ATM). The effects of, the anisotropy coupling, the single ion potential field and the mixed spin on the structure of the phase diagram have been studied. The model presents a rich variety of phase transitions which meet on tricritical or multicritical points. Different partially ordered phases with a partially broken symmetry appears at high temperatures. Their region of stability and their structure depend on the phase parameter space. The nature of critical lines which bound these partially ordered phases depends on the coupling parameters and the crystalline anisotropy

Monte Carlo study of phase transitions and magnetic properties of LaMnO3

On the basis of Mean Field Approximation (MFA), Monte Carlo Simulations (MCS) and ab initio calculations we have studied the phase diagrams and magnetic properties of the bulk perovskite, LaMnO3 using Ising model Hamiltonian. It is shown that the antiferromagnetic coupling between next neighbors Mn ions is reponsible for a series of magnetic phase transitions. The transition temperature and the critical exponents obtained, in the framework of Monte Carlo simulations, using the experimental values of the exchange couplings and magnetic anisotropy are in agreement with the experimental ones. The exchange couplings deduced from ab initio calculations lead, by using Monte Carlo simulations, to a quantitative agreement with the experimental transition temperatures

Magnetization and ordering temperature of films and multilayers

We investigate, in this paper, a number of magnetic properties of single and multilayer thin film systems within the Ising model by application of mean field, finite cluster approximations as well as by Monte Carlo simulations. The magnetization profiles and the magnetic ordering temperature are calculated for different magnetic systems. The influence of corrugation and disorder at the surface, on the critical behavior of ferromagnetic Ising film is also studied. It is found that the critical surface exponent of the magnetization 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 depends on the concentration of the strong interaction, while such critical exponent is independent on the concentration. Moreover, in the case of corrugated surface, the effective exponent for a given layer, is a function of the number of steps at the surface. The probability of a magnetic ground state is larger for low spatial dimensionality of an extended system, or lower for local symmetry of a given site in the atomic lattice. Consequently, the magnetic properties are usually more pronounced at the surface of a bulk magnet as compared to the bulk interior. The phase diagram and the characteristic behaviors of the surface magnetization, are investigated for amorphous or cristalline surfaces. Indeed, the size effects become more relevant at low temperature depending on film thickness.We investigate, in this paper, a number of magnetic properties of single and multilayer thin film systems within the Ising model by application of mean field, finite cluster approximations as well as by Monte Carlo simulations. The magnetization profiles and the magnetic ordering temperature are calculated for different magnetic systems. The influence of corrugation and disorder at the surface, on the critical behavior of ferromagnetic Ising film is also studied. It is found that the critical surface exponent of the magnetization 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 depends on the concentration of the strong interaction, while such critical exponent is independent on the concentration. Moreover, in the case of corrugated surface, the effective exponent for a given layer, is a function of the number of steps at the surface. The probability of a magnetic ground state is larger for low spatial dimensionality of an extended system, or lower for local symmetry of a given site in the atomic lattice. Consequently, the magnetic properties are usually more pronounced at the surface of a bulk magnet as compared to the bulk interior. The phase diagram and the characteristic behaviors of the surface magnetization, are investigated for amorphous or cristalline surfaces. Indeed, the size effects become more relevant at low temperature depending on film thickness

Ab initio and Effective field Study of V doped TiO2

Using the effective field theory (EFT), the Curie temperature of V- doped rutile TiO2 has been determined. The exchange coupling used in the calculation has been deduced from first principal calculation based on the density functional theory with the local spin density approximation (LSDA). We also showed that V- doped rutile TiO2 is half metallic and the hybridization is between the V 3d states and the nearest-neighbouring O 2p states. The results from ab initio calculation showed that for weak holes concentrations the ferromagnetism is due to the double exchange, while, for higher hole concentration it comes from the RKKY exchange interaction. In addition, we show via effective field theory calculations that the transition from the nonmagnetic (NM) to the ferromagnetic (FM) phase is marked by the magnetic percolation. Magnetization as a function of the temperature for different values of the hole concentration, nc, is also given