Molecular dynamics simulations of atomic assembly in the process of GaN film growth

Molecular dynamics simulations using a Coulomb–Buckingham potential have been used to investigate the process of wurtzite GaN films growth, including the appearance of films in early stage, regulation of growth, structure of the surface and the dynamic features. The simulations show that the N atoms and Ga atoms are absorbed on the lattice of substrate and take on a distinct sandwich structure. Time evolution of the mean square displacements and diffusion coefficient of the deposited atoms are observed, the results show that the clusters will become stable with the increase of time steps and the atoms reach the initial stable state after 25 ps; N atoms reach the equilibrium positions more quickly than Ga atoms. It is proved by radial distribution function and the ratio of vacancy of every deposited layer that the crystalline characters of the films will become better as the time steps increase and weaker from bottom to top.     

Density functional theory prediction for diffusion of lithium on boron-doped graphene surface

The density functional theory (DFT) investigation shows that graphene has changed from semimetal to semiconductor with the increasing number of doped boron atoms. Lithium and boron atoms acted as charge contributors and recipients, which attracted to each other. Further investigations show that, the potential barrier for lithium diffusion on boron-doped graphene is higher than that of intrinsic graphene. The potential barrier is up to 0.22 eV when six boron atoms doped (B₆C₂₆), which is the lowest potential barrier in all the doped graphene. The potential barrier is dramatically affected by the surface structure of graphene.     

玻色-爱因斯坦凝聚系统中原子的空间混沌分布

研究了非对称周期势阱中玻色-爱因斯坦凝聚原子的空间混沌分布结构. 在凝聚体相位为常数的情况下, 凝聚体内部不存在原子流,凝聚原子的空间分布结构可以用一个无阻尼双驱动Duffing方程描述. 理论分析给出了原子间呈排斥作用系统的Mel'nikov混沌判据.数值模拟结果显示,化学势的增大能够对原子混沌分布产生明显的抑制作用,甚至使混沌完全消失. 对于原子间呈吸引作用的系统,在一定参数条件下,调节光格势强度比可以使凝聚原子由周期状态进入到空间混沌分布状态,随着化学势的增大这种空间混沌分布被完全抑制. 关键词： 玻色-爱因斯坦凝聚 Mel'nikov函数 混沌     

Microtrap on a concave grating reflector for atom trapping

We propose a novel scheme of optical confinement for atoms by using a concave grating reflector.The two-dimension grating structure with a concave surface shape exhibits strong focusing ability under radially polarized illumination.Especially,the light intensity at the focal point is about 100 times higher than that of the incident light.Such a focusing optical field reflected from the curved grating structure can provide a deep potential to trap cold atoms.We discuss the feasibility of the structure serving as an optical dipole trap.Our results are as follows.(i) Van der Waals attraction potential to the surface of the structure has a low effect on trapped atoms,(ⅱ) The maximum trapping potential is ~1.14 mK in the optical trap,which is high enough to trap cold ~(87)Rb atoms from a standard magneto-optical trap with a temperature of 120 μK,and the maximum photon scattering rate is lower than 1/s.(ⅲ) Such a microtrap array can also manipulate and control cold molecules,or microscopic particles.     

氢在Nd晶体中行为的分子动力学模拟

由三维Mobius反演变换所得的金属Nd原子和H原子间的相互作用势和组合规则的方法得到的Nd H原子间的相互作用势 ,利用正则系统分子动力学算法研究了在一定加载应力强度因子K =0 .6MPam下 ,氢在Nd晶体中的行为。模拟结果表明 ,氢在Nd晶体裂尖富集成许多氢原子团或氢气团。这可用来部分地解释NdFeB稀土永磁体吸氢后的氢爆行为。     

间隙原子对ZnNb2O6光电特性影响的第一性原理研究

基于密度泛函理论第一性原理,研究Zn、Nb、O间隙原子对ZnNb2 O6体系光电特性的影响.分析显示:间隙原子对体系晶格畸变的影响与间隙原子几何尺寸有关.缺陷结构中,由于间隙原子电负性存在差异,也是产生晶格畸变的因素.光电特性分析显示:含有Zn、Nb间隙原子的体系表现为n型简并半导体.且Nbi表现出较强的介电效应,主要...     

Structure and dynamics of graphite-supported bimetallic nanoclusters

Molecular dynamics simulations are used to analyze the structure and dynamics of isolated bimetallic nanoclusters of 343 (Cu-Ni) and 1000 atoms (Cu-Ni and Pt-Au) deposited on a graphite substrate. The metal-metal interactions are modeled with the many-body Sutton-Chen potential, and a Lennard-Jones potential is used to describe the metal-carbon interactions. The nanocluster melting temperature is determined from caloric and heat capacity curves, and the atomic distribution is studied layer-by-layer as a function of temperature in a direction perpendicular to the substrate plane. Changes in the nanocluster shape as temperature increases are monitored through deformation parameters that show clear evidence of structural and melting transitions as well as of atomic surface diffusion in the cluster. Dynamic properties such as atomic and whole-cluster diffusion, and the motion of the metal atoms at the interface metal/graphite are characterized as a function of temperature.     

单晶硅纳米切削中C–C键断裂对金刚石刀具磨损的影响

本文基于分子动力学方法模拟金刚石刀具纳米切削单晶硅, 从刀具的弹塑性变形、C–C键断裂对碳原子结构的影响以及金刚石刀具的石墨化磨损等方面对金刚石刀具的磨损进行分析, 采用配位数法和6元环法表征刀具上的磨损碳原子. 模拟结果表明: 在纳米切削过程中, 金刚石刀具表层C–C键的断裂使其两端碳原子由sp³杂化转变为sp²杂化, 同时, 表面上的杂化结构发生变化的碳原子与其第一近邻的sp²杂化碳原子所构成的区域发生平整, 由金刚石的立体网状结构转变为石墨的平面结构, 导致金刚石刀具发生磨损; 刀具表面低配位数碳原子的重构使其近邻区域产生扭曲变形, C–C键键能随之减弱, 在高温和高剪切应力的作用下, 极易发生断裂; 在切削刃的棱边上, 由于表面碳原子的配位严重不足, 断开较少的C–C键就可以使表面6 元环中碳原子的配位数都小于4, 导致金刚石刀具发生石墨化磨损.     

基于内核构建的Cu-Au-Pd团簇稳定结构优化

具有特殊催化、磁性和化学活性的三元合金团簇已成为基础科学研究的热点问题.确定其稳定结构是研究团簇性质的重要前提.针对大尺寸Cu-Au-Pd团簇结构优化,提出了内核构建的方法改进了自适应免疫优化算法的效率(称为AIOA-IC算法).采用基于紧束缚势二阶矩近似的多体Gupta势函数来描述三元合金团簇原子间相互作用.为测试算法效率优化了原子数为60的Ag-Pd-Pt团簇稳定结构.结果显示新得到的结构比文献报道的团簇结构势能量值更低,由此可知AIOA-IC算法具有更强的势能面搜索能力.运用该算法研究了38及55原子Cu-Au-Pd团簇的稳定结构.所研究的38原子Cu-Au-Pd团簇包含了五折叠、六折叠和截角八面体结构,并且原子成分比例影响了团簇的结构类型.而55原子Cu-Au-Pd团簇均为完整二十面体结构,序列参数显示Cu,Au和Pd原子分层现象明显.对于147原子Cu_(12)Au_(93)Pd_(42)团簇完整二十面体结构,中心原子为Au,内层和次外层分别被12个Cu原子和42个Pd原子占据,最外层则被92个Au原子占满.通过原子半径及表面能分析了Cu,Pd和Au原子分别倾向于分布在内层、次外层和最外层的规律.     

Optimal asymmetric 1 → 4 quantum cloning in arbitrary dimension

A Monte Carlo method has been performed to simulate the structure evolution of Ni–Cu alloy nanorods encapsulated in the carbon nanotube and removed from the carbon nanotube. The Sutton-Chen many-body potential and Lennard-Jones potential are used to describe the metal-metal and metal-carbon interactions, respectively. The studies show that all Ni–Cu atoms in carbon nanotube are arranged in a series of concentric cylindrical layers even they have different fractions, and Cu atoms are apt to stay at the surface layers. If the carbon nanotube was removed, Ni–Cu alloy nanorod would turn into a cluster with nickel core and copper shell. The physical origin for such structure formation and evolution toward core-shell motifs are discussed. The bond pair analysis shows that the nanorods and clusters are amorphous structures dominated by the rhombohedral structure and mixed up with some local short-range order.     

低能Cu原子簇沉积薄膜的分子动力学模拟

用分子动力学计算机模拟研究了能量为5—20eV/atorn,结构为正二十面体的(Cu)₁₃原子簇在Cu（001）表面的沉积过程.采用紧束缚势同Moliers势的结合描述Cu原子间相互作用通过原子簇-衬底相互作用的“快照”研究沉积的动态过程．结果表明,当入射能量较低时,轰击弛豫后,入射原子簇在衬底表面发生重构,生成很好的外延层,靶没有任何损伤．随着轰击能量的增加,原子簇原子穿入靶的深度增加.当入射能量达到20eV/atom时,原子簇完全穿入靶并开始造成辐照损伤,表面出现空位,靶内产生间     

Control of tripod-scheme cold-atom wavepackets by manipulating a non-Abelian vector potential

Tripod-scheme cold atoms interacting with laser beams have attracted considerable interest for their role in synthesizing effective non-Abelian vector potentials. Such effective vector potentials can be exploited to realize an all-optical imprinting of geometric phases onto matter waves. By working on carefully designed extensions of our previous work, we show that coherent lattice structure of cold-atom sub-wavepackets can be formed and that the non-Abelian Aharonov-Bohm effect can be easily manifested via the translational motion of cold atoms. We also show that by changing the frame of reference, effects due to a non-Abelian vector potential may be connected with a simple dynamical phase effect, and that under certain conditions it can be understood as an Abelian geometric phase in a different frame of reference. Results should help design better schemes for the control of cold-atom matter waves.     

Dynamics of cold atoms in a quadrupole magnetic trap with an orbiting potential

We study the dynamics of atoms confined to a quadrupole magnetic trap with an orbiting potential. For typical values of the experimental parameters of the trap, the rotating magnetic field is shown to produce high-frequency modulation of atomic momenta with an amplitude comparable to the widths of the momentum distributions for the lowest oscillation states of atoms in the time-averaged potential. We find the quantum-statistical momentum and position distributions of atoms and show that at temperatures much higher than the effective vibrational temperature of the atoms in the trap the quantum-statistical momentum and position distributions are Gaussian. We also establish that at temperatures comparable to the effective vibrational temperature of the atoms the quantum-statistical momentum distribution has an annular structure in the trap’s symmetry plane, which is due to the deep modulation of the atomic momenta caused by the rotating magnetic field. Zh. éksp. Teor. Fiz. 114, 23–36 (July 1998)     

熔融Cu55团簇在铜块体中凝固过程的分子动力学模拟

应用基于嵌入原子势函数的分子动力学方法,模拟了嵌入在具有面心立方结构同质块体中的熔融Cu55团簇在不同急冷温度下微观结构的演变情况.通过计算熔融Cu55团簇的均方位移和原子平均能量随时间步的变化,并应用键对分析技术,分析了急冷温度对熔融Cu55团簇结构变化的影响.研究结果表明,由于受到块体结构的影响,在所研究的急冷温度范围内,熔融Cu55团簇在凝固过程中形成了以面心立方结构为主的微观结构.结晶过程是原子不断交换其位置的过程,团簇原子位置的重排敏感于温度的变化.随着急冷温度的升高,原子的扩散范围增大.在100,300和500 K三个较低的温度下有利于形成稳定的面心立方结构,但当急冷到100 K时,团簇中的原子在没有找到其最佳位置之前就已经完成晶化.在急冷到500 K时,团簇中的原子在块体中扩散充分,与块体中的原子形成理想的面心立方结构.在700,900和1100 K三个较高的温度上,局域结构表现为随时间步波动性变化.     

Four-parameter analytical local model potential for atoms

Analytical local model potential for modeling the interaction in an atom reduces the computational effort in electronic structure calculations significantly. A new four-parameter analytical local model potential is proposed for atoms Li through Lr, and the values of four parameters are shell-independent and obtained by fitting the results of X_a method. At the same time, the energy eigenvalues, the radial wave functions and the total energies of electrons are obtained by solving the radial Schr?dinger equation with a new form of potential function by Numerov's numerical method. The results show that our new form of potential function is suitable for high, medium and low Z atoms. A comparison among the new potential function and other analytical potential functions shows the greater flexibility and greater accuracy of the present new potential function.     

分子动力学模拟研究熔态硅的局部结构

采用Tersoff势,修正试用不同的粒子间相互作用距离R,S的取值,进行了液态硅的分子动力学模拟.模拟的结果表明,修正Tersoff势下得到的径向分布函数能与X射线衍射、中子散射实验相一致.模拟得到在液态硅中,Si的配位数为6.9,键长为0.254nm.分子动力学模拟表明,液态硅中Si原子间联接成一种网络状结构,但大多数Si原子与其近邻Si原子仍保持近似于正四面体的局部构型.键角概率分布出现两个峰值~57°和~102°.通过键序参量分析,得到在液态硅近邻结构中,正四面体构型约占82%,键取向波动方差为5. 关键词：     

两原子自发辐射线型的一般理论

研究了两个双能级原子与真空腔场具有不同相互作用强度时的自发辐射线型。在一般情况下,两原子系统的“真空场拉比劈裂”存在12峰结构。单原子的双峰结构和两等同原子的6峰结构可作为特例得出。     

熔融Cu55团簇在铜块体中凝固过程的分子动力学模拟

应用基于嵌入原子势函数的分子动力学方法,模拟了嵌入在具有面心立方结构同质块体中的熔融Cu₅₅团簇在不同急冷温度下微观结构的演变情况.通过计算熔融Cu₅₅团簇的均方位移和原子平均能量随时间步的变化,并应用键对分析技术,分析了急冷温度对熔融Cu₅₅团簇结构变化的影响.研究结果表明,由于受到块体结构的影响,在所研究的急冷温度范围内,熔融Cu₅₅团簇在凝固过程中形成了以面心立方结构为主的微观结构.结晶过程是原子不断交换其位置的过程,团簇原子位置的重排敏感于温度的变化.随着急冷温度的升高,原子的扩散范围增大.在100,300和500 K三个较低的温度下有利于形成稳定的面心立方结构,但当急冷到100 K时,团簇中的原子在没有找到其最佳位置之前就已经完成晶化.在急冷到500 K时,团簇中的原子在块体中扩散充分,与块体中的原子形成理想的面心立方结构.在700,900和1100 K三个较高的温度上,局域结构表现为随时间步波动性变化. 关键词： 团簇 分子动力学 计算机模拟 凝固     

双层h-BN/Graphene结构稳定性及其掺杂特性的第一性原理研究

采用基于密度泛函理论的第一性原理计算方法研究了双层h-BN/Graphene的稳定性及其掺杂特性.研究发现,双层h-BN/Graphene能带结构在K点处有一个小的带隙,在费米能处有类Graphene的线性色散关系.通过施加应变和掺杂来调节带隙,发现掺杂后费米能级附近引入的新能级,主要是N原子的贡献,掺杂后的Na原子和N,C之间存在电荷转移,材料转变为金属性.电荷的转移、载流子密度的增加,在电子元器件中有重要的应用前景.     

平面团簇稳定结构的蒙特卡罗树搜索

以平面团簇为例提出了一种结合结构识别和蒙特卡罗树技术搜索稳定结构的新方法.体系原子之间的相互作用由两类模型势能函数来描述:Lennard-Jones二体势函数与基于Lennard-Jones势的三体势函数.考虑可能的三角晶格碎片作为候选结构,引入编号策略对结构进行快速识别,并运用蒙特卡罗树搜索研究稳定结构随着原子数增大的演化过程;对于能量较低的候选结构,进一步采取局域优化来获得对应体系的稳定结构.计算表明,Lennard-Jones二体势函数对应的三角晶格团簇更稳定;在特定的参数下,三体势函数对应的六角晶格团簇更稳定.结合结构识别和蒙特卡罗树搜索可以对候选结构空间进行高效扫描,在较短时间内更容易搜索到稳定的团簇结构,并可以与第一原理计算结合实现材料的结构预测.