Computer simulation studies of magnetic phase transitions

The advancements which have been made in the use of computer simulations to study magnetic-phase transitions and critical phenomena are reviewed. We describe how the use of a combination of sophisticated Monte Carlo simulation algorithms and reweighting (histogram) techniques have allowed the determination of the static critical behavior with unprecedented precision. The study of “dynamic” critical behavior in simple spin models by both Monte Carlo and spin dynamics methods is also reviewed. Recent estimates for dynamic critical exponents are given including those for true dynamics.     

The effects of a laser field on phase transitions in liquid crystals

We investigate theoretically some phase transitions in liquid crystals in the presence of a laser beam. We found, in non-absorbing nematics, a laser-induced one-way transition from a paranematic to a nematic phase. In absorbing nematics we found, in addition to this transition, a one-way transition from a nematic to a paranematic phase with increasing laser intensity. Further, we found a reentrant nematic or a reentrant paranematic via paranematic or nematic phase respectively. In the case of smectic A, laser absorption results in a coupling between the positional and orientational orders. As a result, the smectic A to nematic transition can change from second order to first order and the smectic C to smectic A transition can become first-order in the field of a laser.     

Thermodynamic properties at constant volume around the solid–liquid phase transition in single metals by using molecular dynamics

Molecular dynamics simulations were performed for eight different metals to calculate their constant volume heat capacity and latent heat in both liquid and solid phases. The atomic interaction for the simulations is taken as modeled by the n-body semi-empirical Gupta potential. The per atom energies of the simulation as a function of the temperature are recognized as the caloric curves of the systems and therefore the slopes of these curves represent the constant volume heat capacities. The values obtained in the simulation for the constant volume heat capacity are in good agreement with the Dulong and Petit law for solids at high temperature, which indicates that the equipartition of energy is well recovered in the simulations. The maximum deviation from this law occurs for metals with the slightest atomic masses. The obtained values for the constant volume heat capacities in the liquid phase are systematically smaller than those in the solid phase, this being physically correct.     

Thermodynamics and phase transitions in two-dimensional Yukawa systems

The results of numerical simulations of strongly-coupled two-dimensional dissipative Yukawa systems are presented. The thermodynamic characteristics of these systems were studied, namely the internal energy, the specific heat and the entropy. For the first time, it is discovered that the considered characteristics have two singular points on the melting line; one of these points corresponds to the first-order phase transition from crystal to the hexatic phase, and another point corresponds to the second-order phase transition from the hexatic phase to the isotropic liquid. The obtained results are compared to the existing numerical and analytical data.     

Influence of pressure on the electric-field-induced phase transitions in liquid crystals

Within the framework of Landau-de-Gennes formulation, we analyse the effect of pressure on electric-field-induced phase transitions in a liquid crystal which shows spontaneously an isotropic-smectic A transition. Inferring from the experimental pressure dependences on the layer spacing in smectic A phase, as well as the nematic-smectic A metastable temperature T^*_AN, we incorporated the pressure dependence in the free energy through (the surface energy term) and the coupling between the quadrupolar nematic ordering Q_ij and the smectic order parameter ψ. From the S-T phase diagram, we found that the stability of field-induced nematic phase increases with pressure, whereas the discontinuity of the transition decreases. Also, the region where paranematic phase transits directly to smectic A phase increases with pressure.     

X-ray investigation of dialkyldimethylammonium bromides

Abstract

A Series of dialkyldimethylammonium bromides (DDAB), (C _n H_2n+1)zN⁺(CH₃)₂Br^?with n = 10, 12, 14, 16 and 18 were investigated using X-ray diffraction. Measurements were performed by the photographic Guinier method as well as by diffractometer on samples aligned on a glass substrate. For a sample with n=12, X-ray high-pressure measurements were also performed which allowed us to confirm the destabilization under pressure of the phase existing on heating above the first transition point. For all the studied compounds, except for that with n =16, strong hysteresis and rich polymorphism were observed. A smectic T phase - high-temperature liquid crystal was confirmed for compounds with n= 16 and 18. All the remaining alkylammonⁱum salts have smectic an E high-temperature liquid crystal phase.     

Texture transitions in binary mixtures of 6OBAC with compounds of its homologous series

Recently the authors have observed in compounds of the 4,n-alkyloxybenzoic acid series, with the homologous index n ranging from 6 to 9, a texture transition in the nematic range which subdivides the nematic phase in two sub-phases displaying different textures in polarised light analysis. To investigate a persistence of texture transitions in nematic phases, we prepared binary mixtures of 4,6-alkyloxybenzoic acid (6OBAC) with other members (7-, 8-, 9-, 12-, 16OBAC) of its homologous series. Binary mixtures exhibit a broadening in the temperature ranges of both smectic and nematic phases. A nematic temperature range of 75°C is observed. In the nematic phase, in spite of the microscopic disorder introduced by mixing two components, the polarised light optics analysis of the liquid crystal cells reveals a texture transition. In the case of the binary mixture of 6OBAC with 12OBAC and with 16OBAC, that is of compounds with monomers of rather different lengths, the texture transition temperature is not homogeneous in the cell, probably due to a local variation in the relative concentrations of compounds.     

Microkinetics of the first- and second-order phase transitions

A model of the phase transition in a lattice of interacting nodes, in which each node is a statistical system with internal structure, is introduced. Configuration entropy of microscopic states of the node is defined as a basic parameter of the model. In the frame of the model the first- and second-order phase transitions are considered in details. The distinction between them on the microscopic level is analyzed. Phase diagrams have been calculated in the mean-field approximation. Changes of the phase diagrams and modifications of phase transitions under external pressure and irradiation are investigated in the frame of the microkinetic approach. Results are referred to real systems.     

The role of elastic strains at non-ferroelastic displacive phase transitions

The role of elastic strains at structural phase transitions is illustrated within the Landau theory and its first-order corrections due to critical fluctuations and defects are described. The Landau theory is sufficient to demonstrate the impossibility of bulk nucleation in a supercooled symmetrical phase and the absence of heterophase fluctuations in solids. The critical fluctuations are known to convert a second-order transition in an Ising-like system in a solid to a first-order one. Close to the mean-field tricritical point the effect can be described, for displacive systems, within a first-order perturbation theory and takes place for the Heisenberg systems as well. The influence of defects on these transitions is mediated essentially by the elastic strains. Defects smear the transition. For the “random local field” defects and an incommensurate (Heisenberg-like) transition this effect is so strong that first-order perturbation theory leads to a divergence.     

Undulation textures at the phase transitions of some alkyloxybenzoic acids

We observed undulated smectic textures for some compounds of the 4,n-alkyloxybenzoic (nOBAC) acid series, at transitions between the smectic and the isotropic phase and between the smectic and nematic phase. Studied compounds were 12OBAC, 16OBAC and binary mixtures of 12- and 16OBAC. The undulations are dressing a usual Schlieren texture. An interesting fingerprint pattern is observed at the smectic-nematic in the case of the binary mixture, approximately 1 : 1 in weight.     

Quantum entanglement and quantum phase transitions in frustrated spin ladder with ferromagnetic interchain exchanges

Using the density matrix renormalization group technique, the quantum phase transitions in a frustrated spin-1/2 ladder with ferromagnetic interchain exchanges are investigated. According to our results, the two-rung entanglement is capable of describing the quantum phase transitions. Furthermore, its first order derivative is shown to be more sensitive than order parameters for determining phase boundary. Therefore, the intermediate columnar dimerized phase region is determined definitely.     

Various types of phase transitions in the AdS soliton background

We study the basic holographic insulator and superconductor phase transition in the AdS soliton background by generalizing the spontaneous breaking of a global U(1)$\mathrm{U}(1)$ symmetry to occur via Stückelberg mechanism. We construct the soliton solutions with backreaction and examine the effects of the backreaction on the condensation of the scalar hair in the generalized Stückelberg Lagrangian. We disclose rich physics in various phase transitions. In addition to the AdS soliton configuration, we also examine the property of the phase transition in the AdS black hole background.     

Structural phase transitions and elastic properties of zirconia

The structural phase transformations in various phases (monoclinic, ortho I, ortho II, and tetragonal) of zirconia (ZrO₂) have been investigated using an effective interionic interaction potential. The cohesive energy, the equation of state, and the elastic properties of these phases have also been studied and found to reproduce well the experimentally observed data for almost all the phases of zirconia ceramics.     

Identification of first- and second-order magnetic phase transitions in ferromagnetic perovskites

A criterion used for the determination of first- and second-order magnetic phase transitions from purely magnetic methods is applied to manganese perovskites of formula La_2/3(Ca_1−xSr_x)_1/3MnO₃. A crossover from first- to second-order character at a tolerance factor t=0.92 is found, which also brings about several variations in other physical properties. At t=0.92 a change from orthorhombic to rhombohedral symmetry also takes place. The impossibility of establishing static cooperative Jahn–Teller distortions in the rhombohedral symmetry is suggested as being responsible for the observed behaviour.     

On the theory of transient nucleation at the intermediate stage of phase transitions

The evolution of a system of growing aggregates in a macroscopically homogeneous medium with account of both the reduction in metastability and the continuing initiation of new nuclei is studied. The corresponding integro-differential model describing the intermediate stage of phase transitions is solved analytically for arbitrary nucleation kinetics and growth rates of nuclei. An exact solution of the Fokker–Planck equation is found with allowance for the diffusivity along the axis of nucleus radii. In limiting cases of purely kinetic and mixed kinetic-diffusion rates of crystal growth for a special form of diffusivity, the obtained solutions transform to earlier known expressions.     

Mpemba effect and phase transitions in the adiabatic cooling of water before freezing

In this paper, an accurate experimental investigation of the Mpemba effect (that is, of the fact that initially hot water freezes before the colder one) is carried out, showing that, in the adiabatic cooling of water, relevant roles are played by supercooling, and by phase transitions which take place at 6±1 ^°C,3.5±0.5 ^°C and 1.3±0.6 ^°C. The last transition, which occurs with the non-negligible probability of 0.21 with respect to the total number of runs performed, has not been detected earlier. On the basis of our experimental results, we can present a thorough theoretical analysis of supercooling and of such phase transitions, which are interpreted in terms of the different ordering of molecule clusters in water.     

Photoacoustic detection of phase transitions at low temperatures in CsPbCl3 crystals

Phase transitions in CsPbCl₃ crystals have been explored by the photoacoustic (PA) method in the wide temperature range of 150–300?K. The PA signals, induced by ns-pulsed laser excitation of electronic states and detected with a piezoelectric transducer, are thermoelastic ultrasonic waves lasting for hundreds of microseconds. The wavelet analysis of the PA wave has clarified that the thermoelastic wave includes several MHz components. The PA signals have been made use of in order to examine phase transitions at low temperatures in the crystal. Changes of PA signals indicate that two phase transitions take place at low temperatures around 200?K. The types of phase transitions are discussed taking into account recent experimental results.