Papers by Keyword: Phase Transition

Abstract: The phase transitions and critical phenomena in three-dimensional (3D) site-diluted 3-and 4-state Potts models is investigated by Monte-Carlo method based on the highly efficient Wolff algorithm. The systems with linear sizes L=20-44 at spin concentrations p=1.00, 0.95, 0.90, 0.80, 0.70, 0.65 are explored. The second-order phase transition is shown to occur in the three-dimensional 3-state Potts model with nonmagnetic impurities. In the 4-state Potts model there are observed first-order phase transitions in weakly diluted state, when the model is strongly diluted the first-order phase transitions change to the second-order one. On the basis of the finite-size scaling theory static critical exponents of specific heat α, susceptibility γ , magnetization β, and exponent of correlation radius ν for the systems under study are calculated.

Abstract: Results of structural neutron diffraction study and data on the transport and magnetic properties (the linear and nonlinear (second and third order) susceptibilities) are presented for the La_1-xSr_xCoO₃ and La_1-xCa_xCoO₃ at x = 0.15 single crystals. It is founf that these compounds are insulators and exhibit the rhombohedral R3-c space group. According to the magnetic measurements, the development of the ferromagnetic (F) clustered state proceeds into three stages. On cooling, during the first stage, the F clusters nucleate at the preferential sites that are likely produced by variation in oxygen and doping stoichiometry. The second stage is characterized by sharp increasing the concetraion of the isolated F clusters on cooling. This is the process of the homogeneous nucleation. A coalescence of the isolated F clusters into some large-scale complexes containig some ammount of the domains is associated with the third stage. Note that it is the typical behavior observed at a first order phase transition. The data allow one to determined reliably the temperature boundaries of the stages.

Abstract: We introduce and investigate a few models of three-dimensional frustrated magnets withpossible new pseudo-universal behavior appearing upon a phase transition of weak first order.We an-alyze the different broken symmetry scenarios implemented in frustrated XY and Heisenberg magnetsto compare with our results.

Abstract: High-frequency dynamic magnetic properties of quasi-two-dimensional organic conductors (BEDT-TTF)₂IBr₂ of α'- and β-phases have been studied by Electron Spin Resonance (ESR) spectroscopy. The heating of crystals in the α'-phase is accompanied by their phase transition to the β-phase, so, the Dyson distortion of the ESR line is appeared. Contributions from the crystal field and symmetry of the system as well as the orbital magnetic moment admixture to the total magnetic moment of charge carriers have been judged by the comparison of the orientation and temperature dependencies of the ESR spectra for the α'- and β-phases. Experimental data have proved localized states of charge carriers either in α'- or β-phases.

Abstract: The problem of discontinuous phase transition in the dynamic spin-fluctuation theory is resolved by taking into account large anharmonic spin fluctuations and nonlocality of the mean Green function. The extended theory is applied to the calculation of magnetic properties of iron.

Abstract: We study the critical behavior of three-dimensional antiferromagnet Heisenberg model with nearest-neighbor (J) and next-nearest-neighbor (J₁) interactions by the Monte Carlo method using a high-effective replica exchange algorithm. Here is calculated a full set of main static critical exponents for values R =J₁/J= 0.0; 0.025; 0.05; 0.075; 0.1; 0.115 using the finite-size scaling theory. A phase diagram of dependency of the critical temperature on a relation between nearest-neighbor and next-nearest-neighbor R is plotted.

Abstract: We investigated the phase transition and the isotope effect in squaric acid (H₂C₄O₄, abbreviated H₂SQ), a hydrogen-bonded dielectric material. Using first-principles calculation, we found that Jahn-Teller distortion of the unit structure (C₄H₄O₄) was the major driving force for the phase transition in the H₂SQ crystal. In order to elucidate the isotope effect on the phase transition in deuterated squaric acid (D₂SQ), we employed the multi-component molecular orbital (MC_MO) method, which directly takes into account the quantum effects of protons and deuterons. Using this model, we successfully predicted the difference between the phase transition temperature of H₂SQ and that of D₂SQ to be 192K, which is in reasonable agreement with the experimental value of 145 K. We found that the isotope effect in the H₂SQ/D₂SQ system was based more on shrinking distribution of the deuteron wave rather than that of the proton wave. When the MC_MO method was coupled with adequate cluster models, first-principles calculations were effective to determining the origin of the phase transition and the H/D isotope effect in hydrogen-bonded dielectric materials.

Abstract: This article reviews the theories and experiments on the macroscopic/nanoscopic scales, which indicate that nearly free electrons/holes appear at polarization discontinuities as a result of not only extrinsic mechanisms such as imperfections but also intrinsic mechanisms. We examine the consistency of these theories and experiments with conventional ones. Such electrons/holes lead to various novel properties of ferroelectrics and provide new insight into ferroelectricity, including fundamental issues such as the origin of ferroelectricity. This origin appears naturally compatible with the existence of multiferroicity.

Abstract: Nano-capsules with organic-inorganic composite PMMA-SiO2 as the shell and paraffin as the core have been largely and rapidly synthesized by the cooperative reaction of interfacial hydrolysis and polymerization. Ammonium persulfate as the initiator and catalyst was used to make the polymerization and hydrolysis process occurring simultaneously and to improve the uniformity and controllability of capsule morphology. The prepared homogeneous core-shell structures of capsules with the grain size around 100 nm have small volume change and stable thermodynamic characteristics. The cooperative reactions of polymerization and hydrolysis process provide a convenient and efficient method for preparing homogeneous and stable organic-inorganic composite capsules.

Abstract: This paper is devoted to studying the desulfurization and structural transformation behavior of lead sulfate in alkaline medium. The effects of roasting temperature, roasting time, and raw material ratio on the desulfurization and phase transition behavior of lead sulfate in sodium hydroxide molten salt was systematically investigated through a series of experiments and analyses. The results show that lead sulfate is gradually transformed to Pb₅O₄SO₄, α-PbO, β-PbO, and finally to Pb₃O₄ during roasting in sodium hydroxide molten salt at temperatures of 250°C to 550°C. At the conditions of roasting temperature 450°C, roasting time 40 minutes, molar ratio of lead sulfate to sodium hydroxide 1 : 2.6, lead sulfate can be completely converted to near-pure phase β-PbO powder. This paper provides a simple and efficient method for the recycling of waste lead acid battery, which consists mainly of lead sulfate.