关于铁电相变的一个唯象讨论

从不可逆过程热力学的角度研究了铁电相变中的不可逆性.一级铁电相变中的热滞及铁电体的多畴结构,可以在最小熵产生原理的基础上得到说明.并得出结论,热滞并不是一级铁电相变体系的内禀性质,体系表面的有限性与热滞是有关的. 关键词： 热滞 畴构型 不可逆性 最小熵产生     

The thermodynamic functions of SbSBr crystal

Using density functional theory methods, the phonon density of states, Helmholtz free energy, internal energy, and entropy of ferroelectric and paraelectric phases are investigated. The temperature dependence of the free energy indicates that vibrational entropy contributes to the destabilization of the ferroelectric phase. The vibrational entropy of Sb, S, and Br atoms is attributed to the stabilization of ferroelectric SbSBr at the temperature T _c. Calculations indicated that SbSBr in ferroelectric phase become more stable than in paraelectric phase at temperatures lower than 22.8 K. The calculated temperature of ferroelectric phase transition is in good agreement with the experimental data.     

Order-parameter coupling in the improper ferroelectric lawsonite

Low-temperature specific heat and thermal expansion measurements are used to study the hydrogen-based ferroelectric lawsonite over the temperature range 1.8 K ≤ T ≤ 300 K. The second-order phase transition near 125 K is detected in the experiments, and the low-temperature phase is determined to be improper ferroelectric and co-elastic. In the ferroelectric phase T ≤ 125 K, the spontaneous polarization P(s) is proportional to (1) the volume strain e(s), and (2) the excess entropy ΔS(e). These proportionalities confirm the improper character of the ferroelectric phase transition. We develop a structural model that allows the off-centering of hydrogen positions to generate the spontaneous polarization. In the low-temperature limit we detect a Schottky anomaly (two-level system) with an energy gap of Δ ～ 0.5 meV.     

The thermodynamic functions of ferroelectric and paraelectric SbSI

A first-principle method is used to calculate phonon density of states, Helmholtz free energy, internal energy, and entropy for ferroelectric and paraelectric SbSI. Theoretical phase transition temperature was obtained using the difference of the Helmholtz free energy, internal energy, and entropy term between ferroelectric and paraelectric phases on temperature. The obtained value is in reasonable agreement with the experimental second-order phase transition temperature T_c2 = 233 K.     

The role of the spatial distribution of local polarization perturbations in the posistor effect onset

The electrical resistance of polycrystalline ferroelectric semiconductors is defined by the potential barriers due to the existence of local charged surface states at crystallite boundaries. The barrier screening depends on the state of the ferroelectric system and is maximal during spontaneous-polarization switching. It is shown in this paper that the local perturbation of the ferroelectric system, resulting from the repolarization and appearing as a domain wall between the regions with different polarization directions, has a zigzag configuration. The electric field in the vicinity of the zigzag domain wall is stabilized and coincides with the coercive field, which provides low potential barriers in the ferroelectric phase compared with the paraelectric phase. The repolarization processes become inefficient in the potential barrier screening at the transition from the ferroelectric to the paraelectric phase. As a result, a sharp increase in the electrical resistance is observed at the ferroelectric-paraelectric phase transition, called the posistor effect.     

Entropy production in nonequilibrium systems at stationary states

We present a stochastic approach to nonequilibrium thermodynamics based on the expression of the entropy production rate advanced by Schnakenberg for systems described by a master equation. From the microscopic Schnakenberg expression we get the macroscopic bilinear form for the entropy production rate in terms of fluxes and forces. This is performed by placing the system in contact with two reservoirs with distinct sets of thermodynamic fields and by assuming an appropriate form for the transition rate. The approach is applied to an interacting lattice gas model in contact with two heat and particle reservoirs. On a square lattice, a continuous symmetry breaking phase transition takes place such that at the nonequilibrium ordered phase a heat flow sets in even when the temperatures of the reservoirs are the same. The entropy production rate is found to have a singularity at the critical point of the linear-logarithm type.     

Hydrogen-bond topology and the ice VII/VIII and ice Ih/XI proton-ordering phase transitions

The existence of an ice Ih/XI proton-ordering transition to a low-temperature ferroelectric phase has sparked considerable debate in the literature. Electronic density functional theory calculations, extended using graph invariants, confirm that a transition to a low-temperature ferroelectric phase should occur. The predicted transition at 98 K is in qualitative agreement with the observed transition at 72 K, and the low-temperature phase is the ferroelectric phase determined in diffraction experiments. The theoretical methods used to predict the phase transition are validated by comparing their prediction to the well-characterized ice VII/VIII proton-ordering transition.     

Influence of the geometry of a porous network on the phase transition in a ferroelectric embedded in a porous matrix

A ferroelectric phase transition in a system of electrically interacting small particles is considered. The size effects in a single particle are described in terms of the Landau phenomenological model. The interaction between particles is reduced to a dipole-dipole interaction. It is shown that the interparticle interaction can lead to a substantial increase in the phase transition temperature as compared to the temperature of the ferroelectric transition in a single small particle.     

A New Model of Ferroelectric Phase Transition with Neglectable Tunneling Effect

Due to the obvious deviations of the existing theoretical models from the experimental results of ferroelectric phase transition, a new model is proposed on the basis of the coupling between spontaneous polarization and spontaneous strain in ferroelectrics. The spontaneous polarization and specific heat of ferroelectric phase transition predicted by the model are in better agreement with the corresponding data of triglyceride sulfate, a typical ferroelectric.In addition, the model predicts a new type of ferroelectric in which a phase transition and a phase-like transition coexist.     

Specific features of the thermal physical properties of relaxor ceramics based on lead zirconate titanate

The heat capacity and thermal expansion of ferroelectric relaxors based on lead zirconate titanate are studied near the diffuse phase transition. It is shown that no spontaneous phase transitions from the paraelectric phase to the ferroelectric phase and from the relaxor state to the normal ferroelectric state occur in an ensemble of nanometer-sized polar regions. It is noted that the transitions can be caused only by external electric fields or storage for a fairly long time.     

Slow stage in the evolution of an incommensurate ferroelectric superlattice

The nature of the slow evolution of a soliton system in an incommensurate phase of a ferroelectric is investigated experimentally. It is shown that the duration of the time interval in which the anomalous permittivity and the corresponding soliton spacings are governed by a logarithmic law increases from a few minutes to several hours as the ferroelectric phase transition is approached. Fiz. Tverd. Tela (St. Petersburg) 40, 2101–2102 (November 1998)     

Entanglement, fidelity, and quantum phase transition in antiferromagnetic-ferromagnetic alternating Heisenberg chain

The fidelity and entanglement entropy in an antiferromagnetic-ferromagnetic alternating Heisenberg chain are investigated by using the method of density-matrix renormalization-group. The effects of anisotropy on fidelity and entanglement entropy are investigated. The relations between fidelity, entanglement entropy and quantum phase transition are analyzed. It is found that the quantum phase transition point can be well characterized by both the ground-state entropy and fidelity for large system.     

基于伊辛模型的Fe_(0.5)Mn_(0.1)Al_(0.4)合金磁化强度和磁熵变蒙特卡洛模拟

基于伊辛模型的单自旋反转蒙特卡洛算法,考虑了粒子间的最近邻以及次近邻相互作用,研究了无序Fe0.5Mn0.1Al0.4合金的磁化强度和磁熵变.首先,强调了粒子间的次近邻相关作用对体系的磁性和热力学性质的影响,明确了次近邻相互作用系数,证实了低温合金阻挫的存在;其次,研究了在相变温度处(不同磁场下)磁化强度随外加磁场(温度)的变化情况以及磁性粒子对磁化强度的贡献,发现反铁磁性粒子Mn在低温区对Fe0.5Mn0.1Al0.4合金的相变起了主要作用,而高温区体系的相变是由铁磁性粒子Fe贡献的;最后,分析了体系在相变温度处磁熵变数值随外加磁场的变化情况以及磁熵变在不同的外磁场下随温度的变化情况,当外加磁场H=0.14(a.u.)时,Mn粒子在冻结温度处的平均磁化强度为零,体系处于最无序的状态,对应的磁熵变ΔS(0.1,0.14)达到了正向最大值,极值的位置对应于体系的相变温度.     

基于伊辛模型的Fe0.5Mn0.1Al0.4 合金磁化强度和磁熵变蒙特卡洛模拟

基于伊辛模型的单自旋反转蒙特卡洛算法,考虑了粒子间的最近邻以及次近邻相互作用,研究了无序 合金的磁化强度和磁熵变。首先,强调了粒子间的次近邻相关作用对体系的磁性和热力学性质的影响,明确了次近邻相互作用系数,证实了低温合金阻挫的存在;其次,研究了在相变温度处(不同磁场下)磁化强度随外加磁场（温度）的变化情况以及磁性粒子对磁化强度的贡献,发现反铁磁性粒子Mn在低温区对 合金的相变起了主要作用,而高温区体系的相变是由铁磁性粒子Fe贡献的;最后,分析了体系在相变温度处磁熵变数值随外加磁场的变化情况以及磁熵变在不同的外磁场下随温度的变化情况,当外加磁场h=0.14时,Mn粒子在冻结温度处的平均磁化强度为零,体系处于最无序的状态,对应的磁熵变 达到了正向最大值,极值的位置对应于体系的相变温度。     

Ising model for a ferroelectric phase transition in a system of interacting small particles

An approach based on the Ising model has been proposed for describing a ferroelectric phase transition in a system of interacting identical small particles. It has been found that the shift of the phase transition temperature with respect to the transition point in a bulk sample is affected by both the size effects due to the smallness of the particles and their interaction with each other. The behavior of the dependence of the phase transition temperature on the distance between particles is determined by the nature of the interparticle interaction. An analysis has demonstrated that the interaction between small particles should be taken into account in the interpretation of the ferroelectric properties of nanocomposite materials.     

Domain wall motion in aperiodic crystal systems and magnetoelectrics

Three classes in the family of aperiodic crystal systems are the displacive modulated phases, the incommensurate composites and quasicrystals. In each of these families one can have a phase transition incommensurate-incommensurate within the incommensurate phase. This transition can be described as a transition from a smooth dependence on the parameters to a discontinuous one. This transition usually is accompanied by the occurrence of a soft excitation that can be considered as a soft phason. The character of the phason and the phase transition is discussed for an example from each class. Finally the situation in a more complicated system with two different subsystems is considered: the case of coupling of a ferroelectric and antiferromagnetic phase.     

Entropy Production Rate of Non-equilibrium Systems from the Fokker-Planck Equation

The entropy production rate of non-equilibrium systems is studied via the Fokker-Planck equation. This approach, based on the entropy production rate equation given by Schnakenberg from a master equation, requires information on the transition rate of the system under study. We obtain the transition rate from the conditional probability extracted from the Fokker-Planck equation and then derive a new and more operable expression for the entropy production rate. A few examples are presented as applications of our approach.     

Probing the material properties and phase transitions of ferroelectric liquid crystals by determination of the Landau potential

The full Landau potential of several, widely varying ferroelectric liquid-crystalline materials has been experimentally determined. Tilt angle and polarisation data is analysed across the SmA to SmC transition for varying applied electric-field amplitudes, allowing the determination of all the coefficients of the generalised Landau model of ferroelectric liquid crystals. The materials investigated encompass different materials, including low-polarisation mixtures to high-polarisation single-component materials. The materials also possess a variation in the order of the SmA to SmC phase transition from strongly first order to strongly second order. The effects of both the polarisation and order of phase transition of the system are discussed with respect to the various terms of the generalised Landau model. Further, the mechanisms behind the difference between a first- and second-order phase transition are discussed with respect to the Landau potential and the second Landau coefficient b .     

Origin of ferroelectricity in high-T(c) magnetic ferroelectric CuO

Cupric oxide is a unique magnetic ferroelectric material with a transition temperature significantly higher than the boiling point of liquid nitrogen. However, the mechanism of high-T(c) multiferroicity in CuO remains puzzling. In this Letter, we clarify the mechanism of high-T(c) multiferroicity in CuO by using combined first-principles calculations and an effective hamiltonian model. We find that CuO contains two magnetic sublattices, with strong intrasublattice interactions and weakly frustrated intersublattice interactions. The weak spin frustration leads to incommensurate spin excitations that dramatically enhance the entropy of the multiferroic phase and eventually stabilize that phase in CuO.