Negative normal restitution coefficient found in simulation of nanocluster collisions

The oblique impacts of nanoclusters are studied theoretically and by means of molecular dynamics. In simulations we explore two models--Lennard-Jones clusters and particles with covalently bonded atoms. In contrast with the case of macroscopic bodies, the standard definition of the normal restitution coefficient yields for this coefficient negative values for oblique collisions of nanoclusters. We explain this effect and propose a proper definition of the restitution coefficient which is always positive. We develop a theory of an oblique impact based on a continuum model of particles. A surprisingly good agreement between the macroscopic theory and simulations leads to the conclusion that macroscopic concepts of elasticity, bulk viscosity, and surface tension remain valid for nanoparticles of a few hundred atoms.     

Dipolar correlations and the dielectric permittivity of water

The static dielectric properties of liquid and solid water are investigated within linear response theory in the context of ab initio molecular dynamics. Using maximally localized Wannier functions to treat the macroscopic polarization we formulate a first-principles, parameter-free, generalization of Kirkwood's phenomenological theory. Our calculated static permittivity is in good agreement with experiment. Two effects of the hydrogen bonds, i.e., a significant increase of the average local moment and a local alignment of the molecular dipoles, contribute in almost equal measure to the unusually large dielectric constant of water.     

Intrinsic dielectric loss in crystals

We review the current theory of intrinsic dielectric loss, that is the loss in a perfect crystal due to anharmonic interaction of a.c. electric field with the phonon system of the crystal. Both ordinary dielectrics and displacive ferroelectrics are considered. The theory predicts dependence of the loss on frequency ω and temperature T. This dependence is very sensitive to the symmetry of the crystal.

For ordinary dielectrics, the results are presented and tabulated for all 32 symmetry groups, except for non-symmorphic groups.

The existing experimental date are analysed and explained on the basis of the theory.     

Superfluidity of indirect excitons in a quantum dot

The superfluidity and Bose–Einstein condensation of indirect excitons in coupled quantum dots or trapped atoms with induced dipole momenta (or dipole molecules) are studied by path-integral Monte Carlo simulations. The temperature dependencies of superfluid and Bose-condensed fraction are calculated at different strengths of interaction. Using Kosterlitz–Thouless recursion relations, we also predict the critical temperature in a macroscopic system of indirect excitons.     

On the theory of thermodynamic properties of geometrically confined disordered ferroelectrics

We propose a theoretical approach to calculate the thermodynamic properties of thin films fabricated from disordered ferroelectrics. To calculate the above thermodynamic properties, we use so-called random field method, modified for the case of thin films. The essence of modification is the altering of the interaction between impurity dipoles by geometrical confinement. We show that in thin films the ferroelectric phase transition is inhibited as compared to the case of bulk samples. Our theory is generalizable to ferroelectrics of other shapes as well as magnets and multiferroics.     

金属纳米锥表面吸附分子分布的数值模拟

在均匀外电场中,将吸附在由过渡金属基底生长的纳米锥颗粒表面的CO分子等效为偶极子,在考虑偶极子与局域电场,偶极子之间色散力及偶极子与锥表面原子之间三种相互作用的情况下,给出各相互作用能的数学模型,并用Monte Carlo方法进行数值模拟,得到纳米锥颗粒表面吸附CO分子的空间分布构型.结果表明,在这些相互作用下,纳米锥表面吸附CO分子产生局部凝聚,且随着纳米锥角的变小,吸附在锥顶部的分子更加密集,导致吸附分子间相互作用更强,为解释纳米结构表面吸附体系的异常红外效应提供依据.     

Determination of the of rate cross slip of screw dislocations

The rate for cross slip of screw dislocations during annihilation of screw dipoles in copper is determined by molecular dynamics simulations. The temperature dependence of the rate is seen to obey an Arrhenius behavior in the investigated temperature range: 225-375 K. The activation energy and the effective attempt frequency can therefore be extracted from the simulations. The transition state energy for the annihilation process is calculated by identifying the transition state using the nudged elastic band path technique. The two activation energies agree very well, indicating that transition state theory is applicable for this type of process.     

铁电相变中极化与介电性的随机场效应

基于随机场Ising模型描述的有序无序相变中偶极子在电场作用下的反转运动, 研究了有序无序相变过程中电场与极化强度的关系. 认为tanh(x) 的函数关系与位移型二阶铁电相变极化强度随电场变化的实验结果完全相同. 由此得出基本结论: 偶极子的集体转向造成了极化强度的增大并等同于内电场的增加. 通过区分光学模和偶极子对介电隔离率的贡献, 考虑偶极子极化对介电常数复数形式的频率色散关系, 发现从高斯分布的居里温度可以导出二阶铁电相变过程中介电常数与温度和频率的色散关系. 关键词： 铁电相变 极化强度 随机场     

拉曼光活性(ROA)经典理论模型研究

拉曼光活性（ＲＯＡ）经典理论模型研究马树国吴国祯（清华大学物理系北京１０００８４）ＡＣｌａｓｉｃａｌＴｈｅｏｒｙｆｏｒｔｈｅＲａｍａｎＯｐｔｉｃａｌＡｃｔｉｖｉｔｙＭａＳｈｕｇｕｏ，ＷｕＧｕｏｚｈｅｎ（ＤｅｐａｒｔｍｅｎｔｏｆＰｈｙｓｉｃｓ，Ｔｓｉｎ...     

Quasi-coarse-grained dynamics: modelling of metallic materials at mesoscales

A computationally efficient modelling method called quasi-coarse-grained dynamics (QCGD) is developed to expand the capabilities of molecular dynamics (MD) simulations to model behaviour of metallic materials at the mesoscales. This mesoscale method is based on solving the equations of motion for a chosen set of representative atoms from an atomistic microstructure and using scaling relationships for the atomic-scale interatomic potentials in MD simulations to define the interactions between representative atoms. The scaling relationships retain the atomic-scale degrees of freedom and therefore energetics of the representative atoms as would be predicted in MD simulations. The total energetics of the system is retained by scaling the energetics and the atomic-scale degrees of freedom of these representative atoms to account for the missing atoms in the microstructure. This scaling of the energetics renders improved time steps for the QCGD simulations. The success of the QCGD method is demonstrated by the prediction of the structural energetics, high-temperature thermodynamics, deformation behaviour of interfaces, phase transformation behaviour, plastic deformation behaviour, heat generation during plastic deformation, as well as the wave propagation behaviour, as would be predicted using MD simulations for a reduced number of representative atoms. The reduced number of atoms and the improved time steps enables the modelling of metallic materials at the mesoscale in extreme environments.     

Spontaneous emission of an atom in the presence of an interface between two dielectrics

The rate of spontaneous emission of an atom in the presence of an interface between two dielectrics was calculated using the quantum theory of electromagnetic radiation. Spatial distribution of the field for quasi-continuous spectrum inside an infinite cavity with ideally conducting walls was determined for multiple values of real refractive indices. Spontaneous emission in the continuous spectrum of the field was calculated using the known spatial Green’s function for a one-dimensional dielectric interface. The rate of spontaneous emission of an atom may either be higher or much lower than that in the free space, depending on the refractive indices and the distance between the atom and the interface between dielectrics.     

介电极化新概念--边界屏蔽理论

熟知的介电极化只是一种快效应,还有一种慢效应。慢极化效应使电介质的极化偏离了麦克斯韦方程组描述的规律,引入边界屏蔽电荷的概述２来描述介质的极化过程,可以固避电介质内部复杂的电位移运动,从而能用屏蔽电荷激发的量子统计理论来说明驻极体和铁电体的许多性质。     

Rapidly calculating the partition function of macroscopic systems

It has remained an open problem to accurately compute the partition function of macroscopic systems since the establishment of statistical physics. A rapid method approaching this problem was presented and was strictly tested by molecular dynamic(MD) simulations on Ar atoms in both dense gaseous and liquid states. The outcomes from the method on the internal energy and the work of isothermal expansion(and therefore the free energy) are in good agreement with the MD simulations, suggesting the method would be immediately applied in vast areas.     

Griffiths phase manifestation in disordered dielectrics

We predict the existence of a Griffiths phase in dielectrics with a concentrational crossover between dipole glass (electric analog of spin glass) and ferroelectricity. Particular representatives of the above substances are KTaO₃:Li, Nb, Na, or relaxor ferroelectrics like Pb_1–xLa_xZr_0.65Ti_0.35O₃. Since this phase exists above the ferroelectric phase-transition temperature (but below that temperature for ordered substances), we call it a “para-glass phase”. We assert that the difference between paraelectric and para-glass phases in the above substances is the existence of clusters (inherent to the “ordinary” Griffiths phase of Ising magnets) of correlated dipoles. We show that randomness plays a decisive role in the Griffiths (para-glass) phase formation: this phase does not exist in a mean field approximation. To investigate the Griffiths phase properties, we calculate the density of Yang-Lee (YL) zeros in the partition function and find that it has “tails” inherent to the Griffiths phase in the above temperature interval. We perform calculations on the basis of our self-consistent equation for the long-range order parameter in an external electric field. This equation has been derived in the framework of the random field theory. The latter automatically incorporates both short-range (due to indirect interaction via transverse optical phonons of the host lattice) and long-range (ordinary dipole-dipole) interactions between impurity dipoles, so that the problem of long-range interaction considerations does not appear in it. Received 17 May 2000     

Simulation of domain patterns in BaTiO3

Computer simulations of domain structure were performed within the continuous phenomenological time-dependent Ginzburg–Landau–Devonshire model including electrostatic long-range interactions. Calculations are done on cube or rectangle area blocks with periodic boundary conditions, employing the previously proposed method consisting in eliminating the elastic field using Euler's equations and solving the kinetic equations in Fourier space. The authors demonstrate that both strong anisotropy of the Ginzburg gradient interaction and realistic estimation of elastic and electrostatic long-range interactions are crucial for correct domain wall properties of BaTiO₃-type ferroelectrics. Domain architecture obtained from simulations performed with the authors' model parameters for BaTiO₃ is found to be in reasonable agreement with experiment.     

Self-Consistent Study of Conjugated Aromatic Molecular Transistors

We study the current through conjugated aromatic molecular transistors modulated by a transverse field. The selfconsistent calculation is realized with density function theory through the standard quantum chemistry software Gaussian03 and the non-equilibrium Green＇s function formalism. The calculated I - V curves controlled by the transverse field present the characteristics of different organic molecular transistors, the transverse field effect of which is improved by the substitutions of nitrogen atoms or fluorine atoms. On the other hand, the asymmetry of molecular configurations to the axis connecting two sulfur atoms is in favor of realizing the transverse field modulation. Suitably designed conjugated aromatic molecular transistors possess different I - V characteristics, some of them are similar to those of metal-oxide-semiconductor field-effect transistors （MOSFET）. Some of the calculated molecular devices may work as elements in graphene electronics. Our results present the richness and flexibility of molecular transistors, which describe the colorful prospect of next generation devices.     

水滴撞击结冰过程的分子动力学模拟

利用三维分子动力学模拟方法,研究了纳米尺度水滴撞击冷壁面的结冰过程.数值模拟中,统计系统采用微正则系综,势能函数选用TIP4P/ice模型,温度校正使用速度定标法,牛顿运动方程的求解采用文莱特算法,水滴内部结冰过程则通过统计垂直方向水分子温度分布来判定.研究发现,当冷壁面温度降低时,水滴完全结冰的时间减小,但水滴降至壁面温度的时间却增大;同时随着壁面亲水性降低,水滴内部热传递速度减慢(尤其是冷壁面与水滴底端分子层间),水滴内部温度趋于均匀,但水滴完全结冰时间延长.     

Unusual domain evolution in semiconducting ferroelectrics: A phase field study

The effect of electrical conductivity on the domain evolution of semiconducting ferroelectrics is investigated using a phase field model which includes the drift of space charges. Phase field simulations show that the tail-to-tail 90° charged domain wall appears during the domain formation in the semiconducting ferroelectrics at zero field, which is prohibited in common insulating ferroelectrics. Due to the screening of polarization charges, the domain switching takes place through the motion of head-to-head 180° charged domain wall in the semiconducting single-domain ferroelectrics subjected to an electric field. Comparing to the insulating ferroelectrics, the semiconducting ferroelectrics have a lower speed of domain evolution due to the decrease of mobility of charged domain walls. The response of semiconducting ferroelectrics to a mechanical load is also found different from that of insulating ferroelectrics.     

Continuous description of a grain boundary in forsterite from atomic scale simulations: the role of disclinations

We present continuous modelling at inter-atomic scale of a high-angle symmetric tilt boundary in forsterite. The model is grounded in periodic arrays of dislocation and disclination dipoles built on information gathered from discrete atomistic configurations generated by molecular dynamics simulations. The displacement, distortion (strain and rotation), curvature, dislocation and disclination density fields are determined in the boundary area using finite difference and interpolation techniques between atomic sites. The distortion fields of the O, Si and Mg sub-lattices are detailed to compare their roles in the accommodation of lattice incompatibility along the boundary. It is shown that the strain and curvature fields associated with the dislocation and disclination fields in the ‘skeleton’ O and Si sub-lattices accommodate the tilt incompatibility, whereas the elastic strain and rotation fields of the Mg sub-lattice are essentially compatible and induce stresses balancing the incompatibility stresses in the overall equilibrium.