The phase diagrams of a spin-1 transverse Ising model

The phase diagrams of the spin-1 transverse Ising model with the presence of a crystal field is investigated by using an effective-field theory (EFT). We give a method to calculate the Gibbs free energy numerically at finite temperature within the EFT. The first-order transition lines are obtained by comparing the Gibbs free energy. The phase diagrams and the Gibbs free energy are also compared with those given using the mean-field theory (MFT).     

Fast ion transport in solids induced by an electric field

A theoretical model is suggested to describe the abrupt transition, induced by an electric field, of a cation sublattice in the ionic crystal bulk, as a result of which the crystal passes into the superionic conductivity state. The concepts which underlie the model are that the Frenkel defect interactions play an essential part in crystal, and the electric field greatly influences the defect formation energy. Expressions for the free energy of disordering, for the interstitial population in the crystal as a function of the strength of the electric field applied and for the critical value of the electric field which induces phase transitions have been obtained in terms of the mean field approximation. The first experimental data on the discovery and analysis of the jump in the ionic conductivity induced by an electric field, described previously, are discussed.     

A Thermodynamic Model of Nucleation of p-Terphenyl Crystals with Surface Energy Anisotropy at the Liquid–Air Interface

Physics of the Solid State - An analysis of the variation of Gibbs free energy ΔG upon the formation of a plane nucleus of the p-terphenyl crystal at the liquid–air interface is...     

液晶在电场和剪切耦合作用下的流变学行为

本文通过理论和实验对液晶 5CB在剪切和电场耦合作用下流变行为进行了研究. 采用液晶连续理论, 建立了包括界面锚定能, 弹性自由能, 介电自由能和流动能在内的系统 Gibbs自由能公式, 通过最小化系统自由能的方法求解液晶在剪切和电场耦合作用下的取向分布及其黏度变化, 从分子基础模型上揭示了液晶在耦合作用下的流变行为、微观机理及其影响规律, 并通过流变测试对此进行验证. 对比分析了理论和试验结果的误差和原因, 发现界面锚定效应对于液晶分子的取向和黏度具有重要影响. 理论和试验结果均表明, 液晶在电场作用下具有明显的电黏效应, 表现出非牛顿流变行为, 其黏度值由剪切和电场的竞争和耦合作用共同决定. 在外电场作用下液晶的黏度可以增加到初始值的 4倍左右, 液晶这种其自身黏度可随着外场 (例如运动速度) 改变的特性在一定的条件下可以自适应地满足不同工况对黏度的要求, 这对实现智能摩擦润滑具有重要的意义. 关键词： 液晶 流变行为 电黏效应 耦合作用     

Modeling of silicon-nanocrystal formation in amorphous silicon/silicon dioxide multilayer structure

The formation process of silicon-nanocrystals （Si-NCs） in the amorphous silicon/silicon dioxide （a-Si/SiO2） multilayer structure during thermal annealing is theoretically studied with a modified model based on the Gibbs free energy variation. In this model, the concept of average effective interfacial free energy variation is introduced and the whole formation process consisting of nucleation and subsequent growth is considered. The calculating results indicate that there is a lower limit of the silicon layer thickness for forming Si-NCs in a-Si/SiO2 multilayer, and the oxide interfaces cannot constrain their lateral growth. Furthermore, by comparing the results for a-Si/SiO2 and a-Si/SiNx multilayers, it is found that the constraint on the crystal growth from the dielectric interfaces depends on the difference between interfacial free energies.     

Temperature-accelerated method for exploring polymorphism in molecular crystals based on free energy

The ability of certain organic molecules to form multiple crystal structures, known as polymorphism, has important ramifications for pharmaceuticals and high energy materials. Here, we introduce an efficient molecular dynamics method for rapidly identifying and thermodynamically ranking polymorphs. The new method employs high temperature and adiabatic decoupling to the simulation cell parameters in order to sample the Gibbs free energy of the polymorphs. Polymorphism in solid benzene is revisited, and a resolution to a long-standing controversy concerning the benzene II structure is proposed.     

Interfacial Free Energy at the Metallic Crystal–Melt Interface

The technique for estimating the interfacial free energy of transition-metal nanocrystals and its anisotropy at the interface with their melts has been developed. The expression for the coordinate of the Gibbs’ interface, which takes into account the size dependence, has been derived. The interfacial free energy of crystal faces at the interface with the related melts of monomorphic 4d and 5d metals decreases nonlinearly with a decrease in the nanocrystal size and, at a certain size, disappears. At the nanocrystal radius of more than 10 nm, the interfacial free energy of the faces approaches that for a macrocrystal. The temperature dependence of the interfacial free energy at the crystal–melt interface is almost linear. The technique developed is shown to be in agreement with the known experimental data for mono- and polycrystals and applicable for estimating the orientational, temperature, and size dependences of the interfacial free energy at the interfaces of nano-, micro-, and macrocrystals with their melts.     

THE INTERACTION OF Au AND THE Si/SiO2 INTERFACE DEFECT Hit(0.494)

The defects at the Si/SiO₂ interface have been studied by the deep-level transient spectroscopy (DLTS) technique in p-type MOS structures with and without gold diffusion. The experimental results show that the interaction of gold and Si/SiO₂ interface defect,H_it(0.494), results in the formation of a new interface de-fect, Au-H_it(0.445). Just like the interface defect, H_it(0.494), the new interface defect possesses a few interesting properties, for example, when the gate voltage applied across the MOS structure reduces the energy interval between Fermi-level and Si valence band of the Si surface to values smaller than the hole ionization Gibbs free energy of the defect, a sharp DLTS peak is still observable; and the hole apparent activation energy increases with the decrease of the Si surface potential barrier height. These properties can be successfully explained with the transition energy band model of the Si/SiO₂ interface.     

The interaction of a screw dislocation with point defects in bcc iron

In this study, we calculate the interaction energy of intrinsic point defects vacancies and interstitials) with screw dislocations in body-centered cubic iron. First (we calculate the dipole tensor of a defect in the bulk crystal using molecular statics. Using a formulation based on linear elasticity theory, we calculate the interaction energy of the defect and the dislocation using both isotropic and anisotropic strain fields. Second, we perform atomistic calculations using molecular statics methods to directly calculate the interaction energy. Results from these two methods are compared. We verify that continuum methods alone are unable to correctly predict the interactions of defects and dislocations near the core. Although anisotropic theory agrees qualitatively with atomistics far from the core, it cannot predict which dumbbell orientations are stable and any continuum calculations must be used with caution. Spontaneous absorption by the core of both vacancies and dumbbells is seen. This paper demonstrates and discusses the differences between continuum and atomistic calculations of interaction energy between a dislocation core and a point defect.     

Anisotropy of Interfaces in an Ordered HCP Binary Alloy

A multiple-order-parameter mean-field theory of ordering on a binary hexagonal- close-packed (HCP) crystal structure is developed, and adapted to provide a continuum formulation that incorporates the underlying symmetries of the HCP crystal in both the bulk and gradient energy terms of the free energy. The work is an extension of the previous treatment by Braun et al. [Phil. Trans. Roy. Soc. Lond. A 355:1787 (1997)] of order–disorder transitions on a face-centered-cubic crystal (FCC) lattice. The theory is used to compute the orientation dependence of the structure and energy of interphase and antiphase boundaries in ordering to the Cd₃Mg and CdMg structures, which are the HCP analogs of Cu₃Au and CuAu structures in FCC. As in the corresponding FCC case, the multiple order parameters do not form a vector. Anisotropy is a natural consequence of the underlying crystal symmetries and the multiple-order-parameter continuum formation presented here. The isotropy transverse to the sixfold axis expected for a scalar order parameter is not found.     

关于Young-Laplace公式的吉布斯自由能证明

对于为论证球形液滴附加压强的Young-Laplace公式而设计的一个理想实验,有文献试图借助吉布斯自由能函数进行证明,本文给出符合这一条件的证明方法.     

Thermodynamically equilibrium roton states of nanoparticles in molten and vapour phases

We show a possibility for a thermodynamically equilibrium nanocrystalline structure consisting of nanosized solid inclusions to appear in a melt just beyond the melting curve. Thermodynamic stability of the nanocrystalline structure in the melt results from the free energy lowering due to rotational motion of nanoparticles. The main contribution to the reduction of the free energy of the system is due to an increase in the rotational entropy and change in formation energy of nanocrystals, i.e. the nanocrystalline structure in the melt, like vacancies in a crystal, is an equilibrium defect structure of the melt. It is demonstrated that similar nanocrystalline structures can also appear in the vapour phase in the form of liquid nanodrops and in liquid solutions, e.g. in He II.     

Experimental study of thermodynamics of plastic deformation by infrared thermography

A method is proposed for calculating the entropy of plastic deformation from IR scanning data and for determining the refined thermodynamic parameter (Gibbs free energy) of the system under quasistatic straining. The method is based on the assumption that the specific heat of the material does not depend on the stored defect concentration and elastic strain.     

Spontaneous ionization of bound exciton complexes

Within the framework of the effective mass method a formula for the computation of the probability of the following non-radiative transition in a semiconductor- or insulator crystal was deduced: two holes and one electron or two electrons and one hole are trapped on a crystal lattice defect, one electron recombines with one hole and the energy released thereby is taken up by the remaining quasiparticle which is thus set free and a free carier is formed. The deduced formula is applied to a Cu₂O-crystal with a copper-ion vacancy as the crystal lattice defect.The author wishes to express his gratitute to Dr. M. Trlifaj for pointing out this interesting problem and for his valuable comments.     

Energy model for the Zr-based metallic glass alloy melt with clusters

An energy model for the melt of bulk metallic glass (BMG) with clusters was established, the Gibbs free energy and interfacial energy for the Zr-Al-Ni ternary alloy melt with Zr2Ni clusters were calculated, and the effects of the clusters on the Gibbs free energy, interfacial energy and nucleation rate were analyzed. The results showed that the existence of the clusters in the Zr-Al-Ni ternary alloy melt enables the Gibbs free energy to decrease in the composition range where bulk metallic glass forms easily, makes the interfacial energy increase and changes the distribution of the interfacial energy with the alloy composition. Because of the clusters in the melt, the Gibbs free energy of the Zr66Al8Ni26 alloy melt decreases about 0.3-1 kJ/mol and the interfacial energy between the melt and crystal nucleus increases about 0.016 J/m2. The nucleation rate of the undercooled Zr66Al8Ni26 alloy melt decreases evidently under the influence of the clusters on Gibbs free energy and the interfacial energy, and the maximum of the nucleation rate in the melt with the Zr2Ni clusters is only about 107 mol-1·s-1.     

THE INTERACTION OF Au AND THE Si/SiO2 INTERFACE DEFECT Hit(0.494)

The defects at the Si/SiO₂ interface have been studied by the deep-level transient spectroscopy (DLTS) technique in p-type MOS structures with and without gold diffusion. The experimental results show that the interaction of gold and Si/SiO₂ interface defect,H_it(0.494), results in the formation of a new interface de-fect, Au-H_it(0.445). Just like the interface defect, H_it(0.494), the new interface defect possesses a few interesting properties, for example, when the gate voltage applied across the MOS structure reduces the energy interval between Fermi-level and Si valence band of the Si surface to values smaller than the hole ionization Gibbs free energy of the defect, a sharp DLTS peak is still observable; and the hole apparent activation energy increases with the decrease of the Si surface potential barrier height. These properties can be successfully explained with the transition energy band model of the Si/SiO₂ interface.     

Explicit Gibbs free energy equation of state for solids

Semi-empirical equations of state (EOS) are used for interpolation and extrapolation of experimental data and/or electronic structure calculations. For calculation of phase equilibria, it is preferable to use an explicit Gibbs free energy EOS, that is, to express the Gibbs free energy directly as a function of the pressure and temperature. Existing explicit Gibbs free energy EOS formulations often give unphysical predictions at high pressures. The origins of these problems are internal inconsistencies and uncontrolled extrapolations. A set of conditions is put forward, that should be fulfilled by semi-empirical EOS formulations in order to constrain them to known physical behaviour, e.g., to the Thomas-Fermi and quasi-harmonic models at high pressures. A new alternative integration path is devised that eliminates the need for the problematic extrapolation of the heat capacity to high temperatures at low pressures. Based on these developments, a new explicit Gibbs free energy EOS is formulated which is suitable for computational applications. The new EOS may be fitted to represent the thermophysical properties of solids with a reasonably small number of adjustable parameters. A sample application for MgO is presented.     

Size effect on thermodynamic properties of free nanocrystals

We demonstrate that the discrete character of the vibrational spectrum of a small crystal accounts for size dependence of its thermodynamic properties and melting temperature. Using a self-consistent statistical method [Phys. Rev. B 66, 054302 (2002)] we derive the Gibbs free energy of free nanocrystalline plates and calculate the thermodynamic parameters as functions of plate thickness for Cu.     

Canonical versus Grand-Canonical Free Energies and Phase Diagrams of a Bipolaronic Superconductor Model

We continue the discussion of a bipolaronic superconductor (resp. an anisotropic antiferromagnet in quasispin formulation) as formulated in a previous work, based on a quantum-statistical, microscopic mean-field model. The grand-canonical thermodynamic limit is compared with the canonical thermodynamic limit in terms of a net of perturbations, becoming singular in the infinite lattice limit. A generalized thermostatistical framework is elaborated which covers model potentials with infinite parts. The function of the limiting free energy density in selecting the (stable) phases with broken symmetry is graphically illustrated. The phase diagrams for the two types of ensembles are shown to differ in the region where both the gauge symmetry and the invariance under sublattice exchange are broken. In particular, the type of the phase transitions, the order of the critical points, and the shape of some phase boundaries are found to depend on the ensemble, which clarifies certain controversial topics for these models. The uniqueness of the limiting Gibbs states with free boundary conditions in all thermodynamic phase regions is proved, and their decomposition into pure phase states in terms of a symmetric measure is evaluated. The field operators of the condensed particles are determined in the representations over the limiting Gibbs states.