碳纳米管封装表面异构Si20融化的结构演变

采用分子动力学方法模拟一种硅的特殊结构（表面异构的硅十二面体结构）填充到扶手型单壁纳米管组成的复合结构的加热过程,通过可视化,键角分布,二面角分布等分析方法来研究这种结构在碳纳米管中的稳定性和结构演变情况。研究发现这种结构在碳纳米管中是非常稳定的,并且随着温度的升高,硅纳米团簇的四面体结构开始减少,近邻原子数目有所增加,但不超过8个。该复合结构是由二十个四面体组成的正十二面体,通过模拟分析可知这种结构具有相当高的稳定性,一部分原因是正四面体的单臂纳米管比较稳定,对十二面体结构的硅起了一定的保护作用;另一部分原因是Si20的正十二面体本身具有较高稳定度,这一点我们通过可视化软件发现这种团簇是缩成一团而并不是从中间裂开观察得到。     

Molecular dynamics study of structural evolution of aluminum during rapid quenching under different pressures

We present a molecular dynamics simulation for liquid Al during the rapid quenching under different pressures. The pair analysis technique and the probabilities of bond energy distribution of inherent structures have been employed to reveal the structural characteristics of liquid and glassy Al. During the liquid-glass quenching process, the bond pairs representing the degree of icosahedral short-range ordering are largely enhanced, whereas the bond pairs being related to fcc and hcp crystalline order increase at first then decrease. The pressure effect on various bond pairs for liquids is larger than for glasses. Two kinds of bond pairs, which exist in large proportion in the amorphous, are demonstrated for the transformation from 1431 to 1541 bond pairs when decreasing temperature or increasing pressure below glass transformation temperature (T_g). Although the sum of these two pairs keeps unchanged below T_g, the role of them is like a bridge which links another two different kinds of bonded pairs, icosahedral ordering and crystalline ordering pairs.     

Kinetic details of crystallization in supercooled liquid Pb during the isothermal relaxation

The kinetic details of crystallization in supercooled liquid Pb during the isothermal relaxation process have been investigated by molecular dynamics simulations, and the microstructure evolution analyzed by the cluster-type index method (CTIM) and the tracing method. It has been found that, the dynamic features are consistently correlated with the microstructure evolution and the crystallization characteristics in the mean square displacement (MSD) and the non-Gaussian parameter (NGP): the β relaxation regime corresponds to the minor structural rearrangement because of the “cage effect”, and the atoms attempt to escape from the “cages”; the α relaxation regime is related to a more diffusive movement of atoms, and the appearance of the second plateau in MSD and the non-zero plateau in NGP corresponds to the completion of crystallization. In addition, three distinct stages of nucleation, growth of nuclei and coarsening of crystallites in the crystallization process have been clearly revealed.     

液态Ni原子团簇演变的计算机模拟

采用常温常压分子动力学模拟技术,模拟了液态Ni中原子团簇在快速凝固条件下的演变过程,模型采用TB作用势.采用偶分布函数、键对和多面体等结构参数来描述快速凝固条件下团簇种类和数量的变化,并将团簇结构可视化.在2 000 K下,液态Ni中团簇数量较少,由一定数量的1551、1441及1661键对所形成的缺陷二十面体构成;在快速冷却条件下,团簇的数量随温度的降低不断增加,且出现由12个1551键型所形成的完整二十面体团簇,体系最终形成了由二十面体和缺陷二十面体团簇网络所组成的非晶结构.     

贵金属Au冷却过程中结构及能量变化的分子动力学计算机模拟

通过分子动力学方法,研究了不同冷速下贵金属Ａｕ在温度２０００～３００Ｋ的冷却过程中微观结构的变化特点,结果发现,冷却速度对Ａｕ的微观结构产生重要影响,采用偶关联函数和键对分析技术对原子局域团簇结果者分析,并考察了冷却过程中原子势能随温度的变化,比较了Ａｕ的微观结构转变与能量变化的对应关系,从能量转化的角度对冷却过程中Ａｕ的结构变化进行了说明。     

耐蚀合金Au3Cu高温冷却过程中能量及结构转变的分子动力学模拟

用分子动学模拟方法对液态Au3Cu冷却过程进行了研究,考察了不同冷却速度下Au3Cu结构变化特点,原子间相互作用势采用F-S多体势,结构分析采用键取向序和对分析技术。计算结果表明,冷却速度对液态Au3Cu能量及结构转变有重要影响,给出了不同冷却速度下液态Au3Cu结构转变的微观信息。     

Study of the structure factor anisotropy and long range correlations of ferrofluids in the dilute low-coupling regime

Dipolar soft-sphere (DSS) fluids in the dilute low-coupling regime are studied via Molecular Dynamic simulations and the extension of a theoretical formalism previously used for dipolar hard spheres in which new terms for the virial expansion of the radial distribution function corresponding to the three-particle contribution are presented and tested for the zero and non-zero magnetic field case. A thorough comparison with simulations shows that the extended formalism is able to account for the structure factors of DSS with and without externally applied magnetic fields in the dilute low-coupling regime: quantitative agreement between theory and simulations is found for dipolar coupling parameters λ?2, and volume fraction φ?0.25. When λ>1 the new added term to the virial expansion is observed to play a crucial role in order to match quantitatively theory and simulations at zero field. In the presence of an external magnetic field our tests show that further improvements are needed and only new terms with Langevin function dependences can significatively contribute to improve the predictions for the dilute low-coupling regime. Numerical simulations show that despite that the ferrofluids considered here are in the dilute low-coupling regime, when an external field is applied, important correlations along the parallel direction to the field and depletion phenomena along the perpendicular direction are observed in the averaged density surrounding a particle.     

Atomistic simulation analysis of the effects of void interaction on void growth and coalescence in a metallic system

Material deformation caused by the interaction between defects is a significant factor of material fracture failure. The present study employs molecular dynamics simulations of single-void and double-void crystalline Ni atomic systems to investigate inter-void interactions. Furthermore, simulations showing the evolution of dislocations for three different crystallographic orientations are conducted to study the void growth and coalescence. The simulations also consider the effect of the radius of the secondary void on dislocation evolution. The results show that double-void systems are more prone to yield than single-void systems. Further microstructural analysis indicates that the interaction between voids is realized by dislocation reactions. The simulation results of the dislocation evolution of the three orientations reveal that a relationship exists between the evolution of the dislocation density and the stress-strain curve. At the initial stage of dislocation, the dislocation grows slowly, and consists of Shockley partial dislocation. The dislocation growth rate then increases significantly in the sharply declining stage of the stress-strain curve, where most of dislocations are Shockley partial dislocation. Analysis of the dislocation length during the overall simulation indicates that the dislocation length of the [110] orientation is the longest, followed by that of the [111] orientation and the [100] orientation, which has the shortest dislocation length.     

Simulation Study on the Structure and Dynamics of Water in Sodium Tetrafluoroborate/Water

应用分子动力学模拟方法研究了室温条件下四氟硼酸钠(NaBF₄)/水混合体系中水分子的微观结构、IR光谱以及转动动力学. 考察了混合物体系中水分子的摩尔分数浓度分别为6.25%、25.0%、50.0%、75.0%、90.0% 和99.6%时体系的结构和动力学性质. 研究显示在不同水分子含量的混合物中水分子以自由分子存在,随着混合物中水分子摩尔分数的增加,水分子的转动和弯曲振动带红移,而O-H伸缩振动蓝移,混合物中水分子内和分子间的氢键和非谐性相互作用增强,分子平动和转动变得困难和缓慢,研究结果与实验观测一致.     

Simulating the stopping dynamics of highly charged ions in an ultra-cold, strongly coupled plasma

We introduce a method for stopping highly charged ions (HCIs) in a laser-cooled one-component plasma (OCP) of ²⁴Mg⁺ ions and present results on stopping times derived from realistic molecular dynamics simulations of the complete stopping process. This stopping scheme can provide ultra-cold highly charged ions for future in-trap precision mass measurements. The choice of an ultra-cold ion plasma as a stopping medium is governed by the almost negligible charge exchange of the HCI with the laser-cooled ions and the very low temperatures which can be reached. In our analysis we focus on the stability and fast recooling of the plasma – two features essential for the experimental realization of this stopping scheme.      

Time-dependent analysis of tunneling effect in the formation of ultracold molecules via photoassociation of laser-cooled atoms

The paper contains a time-dependent investigation of the tunneling effect observed in the photoassociation spectrum of Cs₂ and attributed to the 0_g ^-(6s, 6p _3/2) double well. When by photoassociation of two cold cesium atoms a vibrational level of the outer well is populated, tunneling is an efficient mechanism for transferring the population to the inner well (R < 15a ₀), where spontaneous emission may lead to formation of cold molecules in low vibrational levels of the a ³Σ⁺ _u(6s, 6s) electronic state. This tunneling effect is analyzed by wavepackets propagation, first considering the double well potential alone, and following a packet made by a superposition of states initially located at large distances. Characteristic times for the vibration dynamics, corresponding to a beating phenomenon between the two wells, to partial “revival” at large distances, and to maxima in the population localized in the inner well are reported and discussed. Second, we simulate the two-channels a ³Σ⁺ _u(6s, 6s)↦0_g ^-(6s, 6p _3/2) photoassociation at detunings around 2.9 cm^-1: the inner well can be populated either by the excitation of a vibrational level of the external well (resonant excitation), or by tuning the photoassociation laser at the energy of the inner well level which displays tunneling (“off-resonance excitation”). In the first case the photoassociation is efficient, while the tunneling probability is small; in the second, the tunneling probability is large, so that despite the poor efficiency of the photoassociation process, more population can be transferred to the inner well. This second choice is shown to be very sensitive to the laser intensity, which could be used to control the population of the inner well and hence the formation of ultracold molecules in low vibrational levels. Received 19 April 2002 Published online 1st October 2002 RID="a" ID="a"e-mail: francoise.masnou@lac.u-psud.fr