Inhomogeneity of local temperature in small clusters in microcanonical equilibrium

Overall homogeneity of temperature is a condition for thermal equilibrium, but, as is demonstrated by classical molecular dynamics simulations, the local temperatures of atoms in small, isolated crystalline clusters in microcanonical equilibrium are not uniform. The effective temperature determined from individual atomic velocity decreases with distance from the cluster center. It is argued that these effects are due to the conservation of angular and translational momentum. A general microcanonical expression is derived for the spatial dependence of the statistics of the kinetic energies of individual atoms; this fits the numerical observations well.     

Factorization of wave functions of the quantum integrable particle systems

The relation between the characteristics of the equilibrium configurations of the classical Calogero-Moser integrable systems and properties of the ground state of their quantum analogs is found. It is shown that under the condition of factorization of the wave function of these systems the coordinates of classical particles at equilibrium are zeroes of the polynomial solutions of the second-order linear differential equation. It turns out that, under these conditions, the dependence of classical and quantum minimal energies on the parameters of the interaction potential is the same.     

Effective potential for water vapour

A suitable temperature dependence is introduced into the force parameters of the Lennard-Jones potential by taking the free-energy average of the Rowlinson potential. The resulting temperature-dependent parameters are used to calculate the effective potential, which is plotted for water vapour at a few different temperatures. The temperature-dependence of the second virial coefficient is shown for the cases of temperature-independent and temperature-dependent parameters of which the latter agrees very well with experimental results.     

不同退火温度下晶化硅薄膜的电学输运性质

采用等离子体化学气相沉积技术制备氢化非晶硅薄膜,经过不同温度下的热退火处理,使薄膜由非晶结构向晶化结构转变,得到含有纳米晶粒的晶化硅薄膜.在晶化过程中,采用Raman技术对样品的结构进行表征.通过变温电导率的测试,对薄膜的电学输运性质进行了分析.研究结果表明：退火温度为700 ℃时,样品中开始有纳米晶形成,随着退火温度的增加,样品的晶化比增大,在1000 ℃时,薄膜的晶化比达到90%以上.在700 ℃退火时,薄膜中晶化成分较低,载流子的传输特性主要受到与硅悬挂键有关的缺陷态影响,表现为带尾定域态的跳跃电导 关键词： 氢化非晶硅 退火 纳米硅 电输运     

金红石结构TiO2晶体点缺陷形成能的经验途径计算

基于经验参数化途径 ,通过对晶体结构、晶格形成能、介电性质和弹性实验数据拟合确定金红石结构氧化物晶体TiO2 的电子壳模型参数和非Coulomb互作用势参数 .计算点缺陷形成能 .论证Schottky缺陷是金红石结构TiO2中的本征缺陷 .     

A kinetic model for the premelting of a crystalline structure

An analytical kinetic approach to examine the premelting phenomenon is suggested by using a first passage time analysis. Premelting is considered to occur when the time of formation of a Frenkel type defect in the surface monolayer becomes sufficiently small. The mean time of defect formation on the surface lattice, i.e., the mean time necessary for a selected (surface-located) molecule to leave its lattice site and form a Frenkel defect, is calculated by using a first passage time analysis. The model is illustrated by numerical calculations for a crystalline structure composed of molecules interacting via the Lennard-Jones (LJ) potential. The lattice vectors in the plane parallel to the free surface of the crystal were assumed to be equal (to the lattice parameter) and the angle between them was varied. The model predictions of the Tammann temperature (of premelting) are very sensitive to the parameters of the LJ potential. In all the cases considered, the temperature dependence of the mean first passage time has two clearly distinct regimes: at low temperatures the dependence is sharp and at high temperatures it is weak.     

On the impact of stress on intrinsic defect formation during single crystal silicon growth

The incorporation of intrinsic point defects during silicon crystal growth from the melt is discussed using most recent data for intrinsic point defect thermal equilibrium concentration and diffusivity. It is shown that by taking into account the impact of stress on the thermal equilibrium concentration of vacancies and self-interstitials, the intrinsic point defect properties derived from self- and metal-diffusion experiments can be used to model point defect incorporation and clustering during silicon crystal growth from the melt. The solution is however not unique and a large uncertainty still remains on the real thermodynamic parameters of both intrinsic point defects. Only comparison with a wide range of experimental data from crystal pulling experiments—taking into account also the stress effects—will allow to further narrow down the uncertainty ranges on the intrinsic point defect properties in silicon.     

The effect of crystal equilibrium condition on the lattice dynamical model parameters of a body centred cubic metal

Using barium, for which results that compare favourably with experiment have been obtained, the effect on the lattice dynamical model parameters of a body centred cubic metal of explicitly imposing the crystal equilibrium condition on various ion-ion coupling schemes when used with the modified electron-ion interaction scheme of Bhatia has been studied. When the equilibrium condition is not used, the calculated model parameters lose their physical significance with the result that at equilibrium, the sum of the ionic and electronic pressures is not zero. This inconsistency is removed when the equilibrium condition is explicitly imposed on the models. It is not meaningful to compare the two results with and without the equilibrium condition imposed, not only because the imposition of the equilibrium condition reduces the number of independent parameters of the models by one, but also because the results apply to different states of the crystal. Also the comparison of calculated model parameters and measured quantities such as phonon frequencies make sense only when both are determined for the same state of the crystal. This condition is satisfied only when the equilibrium condition, which has unfortunately been ignored in most lattice dynamical studies, is explicitly imposed on lattice dynamical models.     

Site preference of iron,gallium and aluminum in quasibinary mixed garnets

The temperature dependence of the equilibrium distribution of Ga resp. Al substitutions over the a-and d-sites in single crystal yttrium iron garnet has been determined from magnetization measurements for substitution levels 0 < z < 1.5 and equilibrium temperatures between 773 and 1773 K. Including literature data derived by different experimental methods for as grown samples of various substituted garnet systems with 0 < z < 5 it is shown that the cation distribution observed on single crystals and polycrystalline samples can be well described by a thermodynamic equilibrium distribution model with two stabilizing energies as fitting parameters. The model includes the influence of the level of substitution and of the actual distribution on the effective site stabilizing energy. By a least error fit to the experimental data the two fitting parameters are determined in terms of the site stabilizing energies for the end member garnets with substitution levels z → 0 and z → 5, respectively: 0.272eV and 0.230eV for Ga substituted yttrium iron garnet: 0.178eV and 0.132eV for Al substituted yttrium iron garnet; 0.26 eV and 0.11 eV for Ga substituted yttrium aluminum garnet; 0.20 eV and 0.15eV for Ga substituted europium iron garnet.     

Temperature dependence of host-associated luminescence from YAG transparent ceramic material

We have prepared yttrium aluminum garnet (YAG) in the form of a transparent optical ceramic and measured its spectroscopic properties as a function of temperature. The measurements reveal the presence of at least three distinct types of luminescent defects. These defects are both distinct in nature and more abundant in number than in the corresponding single crystal. While all three emit at low temperatures, only one persists all the way up to and above room temperature. We have demonstrated that energy transfer processes take place between some of the centers. And finally, from the temperature-dependent shapes of the emission and excitation spectra, we have derived an energy level schematic that accurately describes the transfer processes that occur upon stimulation of the various centers.     

Lattice statics calculations for a vacancy in magnesium

Using the Green function method of lattice statics, the lattice distortion in the neighborhood of a vacancy in magnesium has been calculated in three different axially-symmetric (AS) host lattice models and also in a model derived from an empirical interatomic pair potential. The variations in the lattice distortion and the relaxation energy of the vacancy are studied as the size of the defect space is allowed to vary from two to four surrounding neighbors. The perfect static lattice Green functions are computed up to 19 neighbors in the 4 models and the values obtained are shown to be not very sensitive to the model chosen. The lattice relaxation is found to be negligible in all models except in a four-neighbor AS model. The relaxation energies in these four models are computed to be 0.0027 eV, 0.034 eV, 0.28 eV and 0.0069 eV respectively. Results for the monovacancy formation energy, the elastic dipole tensor and the volume change of the crystal due to the vacancy are also presented.     

体材料结晶能力的理论预测

引入评价晶体材料缺陷的残缺度及描述晶面表面势场的结晶势两个概念,针对Ni,Cu,Al,Ar单质在不同温度条件下的再结晶过程进行了大量的分子动力学模拟,发现残缺度随结晶势的增大而单调地减小,即结晶势越大则形成单晶体能力越强.由于结晶势能够唯一地确定结晶能力并且其计算简单,因此可从理论上快速方便地预测材料形成单晶体的能力. 关键词： 单晶 结晶 分子动力学 材料设计     

Temperature-dependence of phonons,solid state properties and liquid structure of noble metals: A comparison of pair-potentials

Two groups of effective pair-potentials are studied from the viewpoint of their suitability in being able to describe solid state properties and liquid state structure of noble metals Cu, Ag and Au over a wide temperature range. Since the effective pair-potentials are usually empirical in nature, with parameters obtained by fitting to some reference state properties, the objective of the present study is to determine whether a particular parametrization scheme has any definite advantage over another. We consider Morse potentials with parameters determined by equilibrium lattice parameter, cohesive/sublimation energies as well as bulk modulus values of the solid at low/room temperatures. The other group of potentials considered is Erkoç potentials, where the parameters were determined first by studying dimers and further modified using bulk stability condition and bulk cohesive energy values. The potentials were then used to study the energetics of microclusters containing 3–7 atoms. Quasiharmonic results for the solid obtained at different temperatures and Monte Carlo simulation for the liquid state show that phonon spectra, thermal expansion, temperature-dependence of specific heats and liquid structure are much better described by the latter group. The first group of potentials may have an advantage in reproducing the temperature-dependence of elastic constants and bulk moduli, since they are based on room temperature values of these properties, which show only weak temperature-dependence in general for all metals. It is argued that potentials based on parameters fitted to the properties at a single volume are less versatile in capturing the temperature-dependence of various thermodynamic properties over a wide range. Potentials capable of reproducing the energetics of clusters of different co-ordination numbers and volumes per atom may fare better in this regard.     

Si molecular beam Epitaxy: A model for temperature dependent incorporation probabilities and depth distributions of dopants exhibiting strong surface segregation

A model describing the incorporation of thermal dopants into single crystal films grown by molecular beam epitaxy (MBE) is presented. The model is general, accounts for dopant surface segregation during deposition, and allows dopant incorporation probabilities and depth profiles to be calculated as a function of film growth conditions (e.g. deposition rate, dopant beam flux, and growth temperature T_s). Input data to the model include thermodynamic parameters such as the free energy of segregation and dopant-surface binding energies together with kinetic parameters such as incident fluxes and dopant diffusivities. The model is applied here to Si MBE in which common dopants are typically characterized by strong surface segregation and temperature-dependent incorporation probabilities σ. Calculated values of σ(T_s) and calculated depth profiles were found to agree very well with available experimental data for both group-III acceptors and group-V donors in Si. In addition, the model predicts, in agreement with limited experimental data, that a growth parameter range exists in which abrupt doping profiles can be obtained, even for dopants which exhibit strong surface segregation. Finally, transition temperatures from equilibrium to kinetically-limited segregation are determined for several dopants.     

Equilibrium melting temperature as a result of induction time measurements: Isotactic polypropylene

A method for the determination of equilibrium melting temperature from induction time measurements is suggested. Theory of the induction time, t _i (most probable period from the beginning of isothermal crystallization to the instant when a stable crystal nucleus starts growing) involves parameters that influence the nucleation-crystallization process, such as specific interfacial free-energy parameter, specific surface energies of a growing nucleus, enthalpy of crystal melting, diffusion activation energy, undercooling and the equilibrium melting temperature, T_m°. An extrapolation method exploiting the aspect of the induction time that it increases to infinity, that is, 1/t _i decreases to zero at the equilibrium melting temperature, cannot be used to calculate the equilibrium melting temperature. High- or low-temperature approximations of the basic equation yield some simplifications that make it possible to find its parameters via the best fit of the equation with experimental data. This procedure can yield also the value of the equilibrium melting temperature if the measured data are sufficiently precise. Applying that procedure to crystallization data of isotactic polypropylene, we obtained the values of the equilibrium melting temperatures 199.5°C (high-temperature approximation) and 212.7°C (low-temperature approximation). A more detailed discussion of the procedure suggests that from both these reasonable values, the higher one is more justified. This result agrees well with higher T_m° data reported in the literature.     

Surface Morphologies: Transient and Equilibrium Shapes

Many different well defined surface morphologies of crystalline materials have been studied in the past 50 years. One of the important and most interesting objectives has been to elucidate the physical origin of the anisotropic surface free energy and, in more recent time, the energetics of surface defects, such as monatomic steps and kinks, which are thermally generated at steps. Changes in surface morphology related to phase transitions, such as faceting or surface roughening, have also attracted much attention. Kinetic experiments dealing with non-equilibrium structures and transient shapes, have lead to quantitative transport properties, for example, surface diffusion coefficients. Fully equilibrated structures, such as 2D and 3D equilibrium crystal shapes, have been imaged to extract the orientation dependent step and surface free energies, even at various temperatures. New imaging techniques with atomic resolution (notably scanning tunneling microscopy) applied to transient as well as equilibrium structures have provided more data of increased accuracy in kinetics and defect/surface energetics. The present paper is an attempt of a brief review of this rapidly advancing field, emphasizing more recent results.     

Local field correction effect on inelastic Coulomb scattering lifetime of two-dimensional quasiparticles at low temperatures

We have investigated the effect of local field correction on the inelastic Coulomb scattering lifetime of high mobility quasiparticles in a quantum layer at low temperatures. By replacing temperature-dependent dynamic dielectric function for the zero-temperature one in calculations, we have considered our improved zero-temperature STLS local field correction for low temperatures in lifetime calculations and compared the results with those obtained from the RPA and Hubbard approximation. It has been found that the quasiparticle lifetime decreases by including the STLS local field factor in all temperatures, energies and layer thicknesses.     

Effective potential for ammonia vapour

The modified free-energy averaged potential for water vapour is extended to NH₃ vapour. The resulting temperature-dependent parameters are used to compute the second virial coefficient of NH₃ vapour which agrees with experimental results particularly at higher temperatures.     

Spatially Periodic Inhomogeneous States in a Nonlinear Crystal with a Nonlinear Defect

Spatially periodic inhomogeneous stationary states are shown to exist near a thin defect layer with nonlinear properties separating nonlinear Kerr-type crystals. The contacts of nonlinear self-focusing and defocusing crystals have been analyzed. The spatial field distribution obeys a time-independent nonlinear Schrödinger equation with a nonlinear (relative to the field) potential modeling the thin defect layer with nonlinear properties. Both symmetric and asymmetric states relative to the defect plane are shown to exist. It has been established that new states emerge in a self-focusing crystal, whose existence is attributable to the defect nonlinearity and which do not emerge in the case of a linear defect. The dispersion relations defining the energy of spatially periodic inhomogeneous stationary states have been derived. The expressions for the energies of such states have been derived in an explicit analytical form in special cases. The conditions for the existence of periodic states and their localization, depending on the defect and medium characteristics, have been determined.     

高温条件下Ga3PO7晶体热学及声表面波性质的理论研究

高温压电晶体是许多机电器件必需的一种多功能材料, Ga₃PO₇晶体的居里温度高达1364 ℃, 可应用于高温极限条件. 但是预测高温极限条件下晶体的结构以及物理性质的问题采用实验研究的手段非常困难, 而理论上的预测未见研究. 本文采用密度泛函-准谐振近似理论计算了温度在0-1200 ℃范围内Ga₃PO₇ 晶体的结构常数和热学性质, 结果表明Ga₃PO₇晶体的晶格常数a和c随温度的升高呈线性增大, 且c方向受温度影响更为显著; 晶体的密度随温度的升高而减小, 计算的a 和c方向平均热膨胀系数分别为1.67×10^-6 K^-1和3.58×10^-6 K^-1, 高温区定压热容为2.067 J/g·K, 与实验值一致. 计算了从常温到高温下该晶体的弹性常数以及体弹性模量的变化, 研究了高温条件下的声表面波特性, 发现随着温度的升高, 声表面波速度浮动较小, 而机电耦合系数略有增大; 在传播角为151° 时该晶体具有较好的温度稳定性且机电耦合系数达到最大值, 这表明Ga₃PO₇ 晶体是一种有望应用于高温环境下的压电晶体.