Evidence of phase transition in the break up of Kr-projectile nuclei at 0.95A GeV

Projectile fragmentation mechanism and the possible liquid gas phase transition have been studied by extracting the critical exponents using cluster approximation technique. A ‘toy model’, schematically accounting for pre-equilibrium, has been developed and various moments as well as conditional moments have been evaluated with the data obtained from the toy model to see the effect of mass conservation constraint. An exponent δ, related to third order moment, has been evaluated to obtain a set of values of γ, β and τ that follow the corresponding scaling relation. The values of γ, β and τ so obtained are found to be 1.34±0.19, 0.54±0.15 and 2.31±0.06 respectively.     

Effect of calcium deficiency on the critical behavior near the paramagnetic to ferromagnetic phase transition temperature in La0.8Ca0.2MnO3 oxides

The critical behavior associated with the magnetic phase transition has been investigated by magnetization isotherms in La_0.8Ca_0.2MnO₃ and La_0.8Ca_0.1□_0.1MnO₃ (□ is the calcium deficiency). The critical exponents are estimated by various techniques such as the Modified Arrott plot, Kouvel–Fisher plot and critical isotherm technique. The critical exponents values for La_0.8Ca_0.2MnO₃ are very close to those found out by the 3D-Ising model (β=0.328, γ=1.180, and δ=4.826 at an average T_C=181.676 K). Moreover, the estimated critical exponents of La_0.8Ca_0.1□_0.1MnO₃ are consistent with the prediction of the 3D-Heisenberg model (β=0.357, γ=1.167, and δ=4.802 at an average T_C=178.182 K). We noted that the critical exponents γ are almost similar to the value of the mean-field theory which can be explained by the existence of a long-range dipole–dipole interaction. Following the Harris criterion, we deduced that the disorder in our case is relevant, which can be the cause of the change in the universality class.     

Monte Carlo study of the critical behavior and magnetic properties of La2/3Ca1/3MnO3 thin films

Critical exponents offer important information concerning the interaction mechanisms near the paramagnetic to ferromagnetic transition. In this work a Monte Carlo-Metropolis simulation of the critical behavior in La_2/3Ca_1/3MnO₃ thin films is addressed. Canonical ensemble averages for magnetization per site, magnetic susceptibility and specific heat of stoichiometric manganite within a three-dimensional classical Heisenberg model with nearest magnetic neighbor interactions are computed. The La_2/3Ca_1/3MnO₃ thin films were simulated addressing the thickness influence and thermal dependence. In the model, Mn magnetic ions are distributed on a simple cubic lattice according to the perovskite structure of this manganite. Ferromagnetic coupling for the bonds Mn³⁺-Mn³⁺(e_g-e_g′), Mn³⁺-Mn⁴⁺(e_g-d³) and Mn³⁺-Mn⁴⁺(e_g′-d³) were taken into account. On the basis of finite-size scaling theory, our best estimates of critical exponents, linked to the ferromagnetic to paramagnetic transition, for the correlation length, specific heat, magnetization and susceptibility are, respectively: v=0.56±0.01, α=0.16±0.03, β=0.34±0.04γ and γ=1.17±0.05. These theoretical results are consistent with the Rushbrooke equalitiy α+2β+γ=2.     

Novel scaling behavior of the Ising model on curved surfaces

We demonstrate the nontrivial scaling behavior of Ising models defined on (i) a donut-shaped surface and (ii) a curved surface with a constant negative curvature. By performing Monte Carlo simulations, we find that the former model has two distinct critical temperatures at which both the specific heat C(T) and magnetic susceptibility χ(T) show sharp peaks. The critical exponents associated with the two critical temperatures are evaluated by the finite-size scaling analysis; the result reveals that the values of these exponents vary depending on the temperature range under consideration. In the case of the latter model, it is found that static and dynamic critical exponents deviate from those of the Ising model on a flat plane; this is a direct consequence of the constant negative curvature of the underlying surface.     

Finite-size scaling analysis of the critical behavior of a general epidemic process in 2D

We investigate the critical behavior of a stochastic lattice model describing a General Epidemic Process. By means of a Monte Carlo procedure, we simulate the model on a regular square lattice and follow the spreading of an epidemic process with immunization. A finite size scaling analysis is employed to determine the critical point as well as some critical exponents. We show that the usual scaling analysis of the order parameter moment ratio does not provide an accurate estimate of the critical point. Precise estimates of the critical quantities are obtained from data of the order parameter variation rate and its fluctuations. Our numerical results corroborate that this model belongs to the dynamic isotropic percolation universality class. We also check the validity of the hyperscaling relation and present data collapse curves which reinforce the accuracy of the estimated critical parameters.     

Study on the dynamic critical behavior of a ferroelectric heterostructure

A Landau–Devonshire theory added in Landau–Khalatnikov dynamic equation has been used firstly to explore the dynamic critical behavior of a ferroelectric heterostructure composed of two different ferroelectric films. Two identical surface transition layers within each film are assumed, and an antiferroelectric interfacial coupling between two materials is considered. One interfacial parameter β is introduced to describe the differences of physical characteristics between two constituent films, which can reflect more realistic dynamic mechanism. It is found that the ferroelectric heterostructure may exhibit multi-loop hysteresis loop and four peaks of dielectric susceptibility if the appropriate values of parameter β, antiferroelectric interfacial coupling and size of the system are selected. We obtain the critical behavior of the appearance in multi-loop hysteresis loops and four peaks of dielectric susceptibility by equilibrium action of parameter β and antiferroelectric interfacial coupling, which will provide theoretical guiding for designing the multi-state memory and miniaturized device in future.     

Critical behavior of a two-species reaction-diffusion problem in 2D

In this work we study the critical behavior of a model that simulates the propagation of an epidemic process over a population. We simulate the model on two-dimensional finite lattices to determine the critical density of the diffusive population. A finite size scaling analysis is employed to determine the order parameter and correlation length critical exponents.     

On the application of the critical minimum energy subspace method to disordered systems

We discuss the recent application to strongly disordered systems of the Critical Minimum Energy Subspace (CrMES) method, used to limit the energy subspace of the Wang-Landau sampling. We compare with our results on the 3D random field Ising model obtained by a multi-range Wang-Landau simulation over the whole energy range. We point out some problems that may arise when applying the CrMES scheme to models having a complex free energy landscape.     

Critical behavior in Co-doped manganites La0.67Pb0.33Mn1−xCoxO3 (0≤x≤0.08)

The critical properties of perovskite manganite La_0.67Pb_0.33Mn_1−xCo_xO₃ (0≤x≤0.08) around the paramagnetic-ferromagnetic phase transition are investigated through various techniques such as the modified Arrott plot, Kouvel-Fisher method and critical isotherm analysis. Though the nature of this transition was found to be in second order, the estimated critical exponents β (0.233≤β≤0.368), γ (1.03≤γ≤1.40) and δ (4.32≤δ≤5.54) are in between the theoretically predicted values for three-dimensional Heisenberg and tricritical mean-field model. This model suggests the coexistence of the short-range and long-range ferromagnetic orders around the critical temperature. The values of the critical exponents obtained from different methods and the well-obeyed scaling behavior confirm that the calculated exponents are unambiguous and purely intrinsic to the system.     

Dependency of percolation critical exponents on the exponent of power law size distribution

The standard percolation theory uses objects of the same size. Moreover, it has long been observed that the percolation properties of the systems with a finite distribution of sizes are controlled by an effective size and consequently, the universality of the percolation theory is still valid. In this study, the effect of power law size distribution on the critical exponents of the percolation theory of the two dimensional models is investigated. Two different object shapes i.e., stick-shaped and square are considered. These two shapes are the representative of the fractures in fracture reservoirs and the sandbodies in clastic reservoirs. The finite size scaling arguments are used for the connectivity to determine the dependency of the critical exponents on the power law exponent. In particular, the deviations of percolation exponents from their universal values as well as the connectivity behavior of such systems are investigated numerically. As a result, this extends the applicability of the conventional percolation approach to study the connectivity of systems with a very broad size distribution.     

Wang-Landau study of the critical behavior of the bimodal 3D random field Ising model

We apply the Wang-Landau method to the study of the critical behavior of the three-dimensional random field Ising model with a bimodal probability distribution. For high values of the random field intensity we find that the energy probability distribution at the transition temperature is double peaked, suggesting that the phase transition is of first order. On the other hand, the transition looks continuous for low values of the field intensity. In spite of the large sample to sample fluctuations observed, the double peak in the probability distribution is always present for high fields.     

Gelation and dynamic mechanical behavior of liquid crystalline networks

The dynamic mechanical behavior of polyurethane networks based on liquid crystalline (LC) diol, 6,6′-[ethylenebis(1,4-phenyleneoxy)]dihexanol (D), 2(4)-methyl-1,3-phenylene diisocyanate (DI), and poly(oxypropylene)triol (T) at the stoichiometric molar ratio of isocyanate (NCO) and hydroxy (OH) groups was studied. Samples were prepared at various initial molar ratios of the reactive groups [OH]_T/[NCO]_DI/[OH]_D from 1/1/0 to 1/40/39. The gelation studies during the curing reaction in the LC and isotropic states showed that the critical gel structure at the sougel transition exhibits a power-law mechanical behavior; the relaxation exponent in the LC state is always higher than that in the isotropic state. From viscoelastic results, it follows that formation of the LC mesophase enhances the connectivity of the molecular structure at the gel point. Introduction of chemical cross-links in fully cured networks reduces the flexibility of the elastically active network chains (EANCs) and inhibits conformational arrangements required for LC ordering. Increasing the content of LC diol in networks, which leads to an increase of the length of EANCs, increases the concentration of permanent physical interactions (trapped entanglements) in the systems; the intensity of the slow relaxation process in the rubbery region also increases.     

Impact of vacancy and Na substitutions on the critical magnetic behavior in polycrystalline La0.8Pb0.2MnO3

We present the effect of vacancy and Na doping on the critical behavior in La_0.8Pb_0.2MnO₃ via dc magnetization measurements. Critical exponents values for La_0.8Pb_0.2MnO₃ match well with those predicted for the three-dimensional Heisenberg model. The vacancy in La_0.8Pb_0.1□_0.1MnO₃ does not alter the Heisenberg universality class. However, the critical isotherm exponent is slightly larger than predicted consistent with the presence of a Griffiths phase. For La_0.8Pb_0.1Na_0.1MnO₃, we found that the critical exponents values are in a close agreement with those predicted for the mean-field theory. The reason for this behavior may be long-range interactions between spins in this system.     

Cooperative influence of H2 and C2H4 molecules on theb-a anda-X transitions in the O2 molecule in a ternary complex

Moments of the b-a and a-X transitions induced in an oxygen molecule by collision in the O₂ + C₂H₄ + H₂ complex are calculated by the semiempirical MINDO/3 method with allowance for the spin-orbital interaction. On the basis of two collisional models it is shown that the cooperative influence of C₂H₄ and H₂ molecules can be both enhancing and weakening as compared with their individual influence. Here, the relative location of the oxygen molecule and its surroundings plays an important role. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 67, No. 4, pp. 453–456, July–August, 2000.     

Phase transitions and critical phenomena in a three-dimensional site-diluted Potts model

The phase transitions and critical phenomena in the three-dimensional (3D) site-diluted q-state Potts models on a simple cubic lattice are explored. We systematically study the phase transitions of the models for q=3 and q=4 on the basis of Wolff high-effective algorithm by the Monte–Carlo (MC) method. The calculations are carried out for systems with periodic boundary conditions and spin concentrations p=1.00–0.65. It is shown that introducing of weak disorder (p∼0.95) into the system is sufficient to change the first order phase transition into a second order one for the 3D 3-state Potts model, while for the 3D 4-state Potts model, such a phase transformation occurs when introducing strong disorder (p∼0.65). Results for 3D pure 3-state and 4-state Potts models (p=1.00) agree with conclusions of mean field theory. The static critical exponents of the specific heat α, susceptibility γ, magnetization β, and the exponent of the correlation radius ν are calculated for the samples on the basis of finite-size scaling theory.     

Photo-induced electrical instability in hydrogenated amorphous silicon based thin-film transistors and the effect of its phase transition

We investigate the origin of photo-induced electrical instability in hydrogenated amorphous silicon based thin-film transistors (a-Si:H TFTs). Photo instability alone was accompanied by a positive shift in the threshold voltage (V_TH) caused by photo irradiation, and even larger positive or negative shift further exacerbated the instability caused by photo-induced electrical bias stress. Such phenomena can occur as a result of extended charge trapping and/or the creation of defect-states at the semiconductor/dielectric interface or in the gate dielectric. The mechanism for such is difficult to describe through chemical interactions of electron-donating and -withdrawing molecules that exhibit a shift in V_TH in only one direction. We also prove that a transition from an amorphous to a protocrystalline phase improves the photo-induced electrical stability. Such results originate from a reduction in the density of the localized states in protocrystalline-Si:H films relative to that of a-Si:H. We believe that this study provides significant information on the device physics of optoelectronics, which commonly exhibit photo-induced instability and charge transport, as a result of prolonged exposure to photo irradiation.     

Excess molar heat capacity of isobutyric acid–water binary liquid mixtures near and far away from the critical temperature

The heat capacity of the liquid–liquid mixture isobutyric acid–water has been measured for the first time near and far away from its critical point using an adiabatic calorimeter. The measurements were performed at atmospheric pressure, in the one phase region as a function of three temperatures: (1) T − T_C = 0.055 °C, (2) T − T_C = 3.055 °C, (3) T − T_C = 8.055 °C and of the composition X in acid (IA). The heat capacity C_p decreases rapidly when X increases at the used temperatures. Near the critical composition, C_p is not affected by the correlation of the concentration fluctuations.

The molar excess heat capacity of the system under investigation was analysed along the phase diagram and considered as a structural transformation effect.     

Crossover of critical behavior in La0.7Ca0.3Mn1−xTixO3

Critical behavior of Ti-doped La_0.7Ca_0.3MnO₃ ceramics are studied using magnetization methods. The results show that the paramagnetic–ferromagnetic transition is first order for the undoped sample, then for low-doping samples the mean-field model is suitable, and finally a 3D Heisenberg model is satisfied for higher doping samples. These findings demonstrate that the critical behavior of the magnetic transition for manganites is sensitive to Mn-site doping.     

Phase equilibria in Hg-He mixtures near the critical point of almost pure Hg

Abstract

The paper presents new high pressure - high temperature experiments on the phase behavior of mercury-helium mixtures. Two distinct fluid phases are shown to exist in Hg-He-mixtures at temperatures and pressures higher than the critical temperature and pressure of pure mercury. Very accurate thermoelectric properties in the immediate vicinity of the critical point of pure fluid mercury show that profound chnages in the electronic structure in that region are manifest in a correspondingly strong thermodynamic state-dependence of the effective interparticle interaction. The latter noticeably influences the features of the phase behavior of pure fluid Hg and the Hg-He-mixtures.     

Phase diagrams of noisy traffic states in the presence of a bottleneck

The traffic states are investigated for the noisy traffic flow in the presence of a bottleneck under the open boundary condition. The phase diagrams of the different kinds of congested traffic are presented for different values of sensitivity (the inverse of delay time). It is shown that the five different types of traffic states appear below the critical point: the free traffic with a pinned localized cluster, the weak standing shock, the moving localized cluster, the oscillatory congested traffic, and the strong standing shock. Above the critical point, the moving localized cluster and oscillatory congested traffic do not occur. Also, in the traffic flow without noise, the moving localized cluster and oscillatory congested traffic do not appear.