表面Cu原子间相互作用对Cu(001)表面跳跃扩散行为的影响

采用嵌入原子方法的原子间相互作用势，利用准静态分子动力学模拟研究了Cu原子在Cu(001)表面吸附所导致的基体晶格畸变以及对其附近的另一个吸附原子自扩散行为的影响.研究结果表明，吸附原子的存在可以导致多达10层的Cu基体晶格产生畸变.两个吸附原子所产生的晶格畸变应力场之间的相互作用，可以导致吸附原子运动活性的增加.通过比较同一路径上往返跳跃扩散势垒的差异发现，在原子间相互作用势的有效距离之外，两个吸附原子的扩散行为可以认为是存在晶格畸变应力场相互作用的两个独立吸附原子的扩散；在原子间相互作用势的有效距离之 关键词： 表面吸附原子 晶格畸变 表面二聚体 扩散     

Estimate of volume changes in the diffusion zone

We obtain an analytic solution to the coupled problem of the diffusion interaction of two media, represented as two semi-infinite solid phase materials. The mathematical model demonstrates that the reason for the appearance of stresses and deformations in the diffusion zone is not only the differing atomic volumes of the materials but also the difference in their partial diffusion coefficients. It is shown that the character and size of the stresses and deformations are determined by the elastic properties and the atomic volumes of the interacting components. Physics Institute of Strength and Materials Production. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 5, pp. 19–27, May, 1997.     

Cu(111)三维表面岛对表面原子扩散影响的分子动力学研究

采用嵌入原子方法的原子间相互作用势，利用分子动力学方法计算了同质外延生长中不同层数的三维Cu(111)表面岛上表面原子扩散激活能，分析了三维表面岛的层数对表面原子交换扩散和跳跃扩散势垒的影响. 研究结果表明，二维Enrilich-Schwoebel(ES)势垒小于三维ES势垒，且三维ES势垒不随表面岛层数的增加而显著变化. 对于侧向表面为(100)的表面岛，表面原子沿〈011〉方向上的扩散行为，随表面岛层数增加而逐渐变化；在表面岛层数达到3层时，扩散路径上的势垒变化趋于稳定，表面原子扩散以下坡扩散为主. 对于侧面取向为(111)的表面岛，当表面岛层数大于3层后，开始呈现上坡扩散的可能. 关键词： 表面原子 扩散 分子动力学模拟     

Atomic scatterin G by a resonant standing light wave

The theory of atomic scattering by a resonant standing light wave is developed. It is shown that, if the natural width of atomic transition is larger than the recoil energy and the interaction time exeeds the spontaneous decay time, the atomic motion is described by the kinetic equation for atomic distribution function. The latter is a Fokker-Planck type equation and includes the light pressure force and momentum diffusion tensor. It is found that in a strong wave the maximum value of the force is limited, whereas the diffusion tensor increases proportional to wave intensity. It is concluded that for high intensities of a standing wave, it is the atomic momentum diffusion that is responsible for scattering.     

扫描隧道显微镜针尖调制的薄膜表面的原子扩散

采用原子尺度的模拟方法,探讨了在零偏压下扫描隧道显微镜(STM)针尖调制的金属表面岛上原子运动及岛边的层间质量输运. 研究结果显示STM的移动对岛上及岛边的原子扩散有重要的影响. 针尖与吸附原子的交互作用及岛和基体中强的形状变化影响了岛上吸附原子的跳跃扩散及岛边的跳下扩散和交换扩散过程. 研究发现,通过调节针尖与基体的垂直距离及针尖与吸附原子的水平距离,可以降低岛上吸附原子的跳跃扩散能垒及岛边的跳下扩散和交换扩散能垒,从而实现薄膜由三维生长模式向二维生长模式的转变. 关键词： 扫描隧道显微镜 原子运动 质量输运     

Measuring the tilt of primordial gravitational-wave power spectrum from observations

Size effect on thermodynamics and diffusion of deuterium in nano-sized vanadium(V) layers is studied. Critical temperature(Tc)for deuterium phase transition is found to decrease with the inverse thickness of V layers and the thermodynamic factor increases as V thickness decreases. These effects are related to the deuterium-deuterium(D-D) interaction change versus V thickness,which experimentally proves that the D-D interaction plays the main contribution to the previously observed V size effect on deuterium chemical diffusion coefficients(D_c). The self-diffusion coefficients(D_s) are obtained through correcting D_c with the thermodynamic factors. It is found that the D_s are similar in 14 and 28 monolayers of V while slightly larger D_s are observed at high concentrations in 14 atomic layers. The weak site blocking effect in the interface is argued to be the main contribution to the observed size effect on D_s.     

Acousto-optic modulation in magnetised diffusive semiconductors

The modulation of an intense electromagnetic beam induced by the acousto-optic (AO) effect has been analysed in a transversely magnetised semiconductor-plasma medium. The effect of carrier diffusion on the threshold field and gain profile of the modulated wave has been extremely investigated using coupled mode theory. The origin of the AO interaction is assumed to lie in the induced nonlinear diffusion current density of the medium. By considering the modulation process as a four wave parametric interaction an expression for effective third-order AO susceptibility describing the phenomena has been deduced. The modulation is greatly modified by propagation characteristics such as dispersion and diffraction due to dielectric relaxation of the acoustic mode. The threshold pump field and the steady state growth rates are estimated from the effective third-order polarisation in the plasma medium. Analytical estimation reveals that in the presence of enhanced diffusion due to excess charge carriers the modulated beam can be effectively amplified in a dispersionless acoustic wave regime. The presence of an external dc magnetic field is found to be favourable for the onset of diffusion induced modulational amplification of the modulated wave in heavily doped regime. Received 5 November 2001     

吖啶酮生物碱的核磁共振碳谱模拟研究

采用表征分子内部化学微环境及原子所处杂化状态的结构描述子：原子电性作用矢量(AEIV)和原子杂化状态指数(AHSI),对42个吖啶酮生物碱分子792个共振碳原子进行结构表征,以多元线性回归技术建立¹³C核磁共振化学位移定量结构波谱关系模型,所得回归模型的复相关系数为R=0.957,标准偏差为SD=12.247. 采用留一法交互检验的结果为R_cv=0.956,标准偏差SD_cv=12.331. 对外部样本集预测结果表明,AEIV和AHSI具有表征能力强、物化意义明确等优点,所建模型具有良好的稳定性和估计能力.     

Reaction kinetics of the Pt, O2(g)|c-ZrO2 system: precursor-mediated adsorption

A micro kinetic model of the Pt, O₂(g)|c-zirconia electrode/electrolyte system was developed in state space form (model M3). The oxygen adsorption/desorption process was modeled as a precursor-mediated surface reaction. The surface diffusion of atomic oxygen and the electrochemical reduction of atomic oxygen near the three-phase boundary (tpb) were considered. It was shown that the simulated charge-transfer behavior of M3 is significantly different from models with ordinary Langmuir kinetics (model M2). The electrochemical rate constant was estimated from selected experimental data as k₁₀=(6.05±0.25)·10⁶ m³/(mol·s). From experimental results it was concluded that only one adsorbed oxygen species is relevant for the dynamic behavior. In porous Pt electrodes binary gas phase diffusion of oxygen in O₂/N₂ gas mixtures becomes relevant at oxygen partial pressures below 10⁻³ atm. The general procedure for state and parameter estimation can be well adopted for the investigation of further reaction mechanisms.     

Static investigation of the small clusters on the Cu(111) and Au(111) surfaces

The diffusion properties of small clusters Ag_n, Cu_n, and Au_n on the Cu(111) and Au(111) surfaces were studied using the molecular statics (MS) in order to understand the atomistic processes underlying the motion. In this work, the atomic interaction potential is modeled by a semi-empirical Embedded Atom Method (EAM), while the drag method is used to determine the static activation energy for each diffusion path. The presented results indicate that the dimer can diffuse on the (111) surface via the zig-zag and concerted motion mechanisms. The trimer diffuses according to the concerted motion mechanism. For the tetramer diffusion, the mechanism that consumes the least amount of energy is the zig-zag motion, in which only two atoms are needed for the diffusion process at one time. This allows finding a static activation energy smaller compared to the trimer diffusion.     

Pseudo-binary and pseudo-ternary diffusion couple methods for estimation of the diffusion coefficients in multicomponent systems and high entropy alloys

The benefits of using the pseudo-binary and pseudo-ternary diffusion couple methods in multicomponent inhomogeneous systems are demonstrated by estimating different types of composition-dependent diffusion coefficients. These are important for understanding the basic atomic mechanism of diffusion and complex compositional evolutions. These were otherwise considered impossible during the last many decades. Without any options previously, sometimes the average values over a composition range of random choice were estimated, which are not the material constants but depend on the composition range and also the end member compositions. The steps and analyses for utilising the pseudo-binary and pseudo-ternary methods are first described in the Ni-Co-Fe-Mo system by producing the ideal diffusion profiles fulfilling the concepts behind these methods. Following, the discussion is extended to the systems related to medium (Ni-Co-Cr) and high (Ni-Co-Fe-Mn-Al) entropy alloys. In fact, this is the first report showing a correct experimental method that should be followed for the estimation of the interdiffusion and intrinsic diffusion coefficients in inhomogeneous high entropy alloys. In the end, the limitations of following these methods because of the generation of non-ideal diffusion profiles are discussed based on experimental results. The steps are also suggested to avoid such complications. These methods are easy to adopt for research engineers. Most importantly, these give an opportunity to validate the data estimated following newly proposed numerical methods by different groups with experimentally estimated diffusion coefficients, which were not possible earlier.     

Surface and transport properties of Au-In liquid alloys

Thermodynamic quantities on Au-In liquid alloys have been used as the input data for the interaction parameter calculations in the framework of the complex formation model (CFM). Once the interaction energies are computed the surface (surface tension and surface composition) and transport properties (chemical diffusion and viscosity) as well as the microscopic functions (concentration fluctuations in the long-wavelength limit and chemical short-range order parameter) have been calculated. The concentration and temperature dependent surface tension values have been compared with our new set of experimental data, obtained by the large drop method in the temperature range of T = 1273-1493 K. The anomalous change of surface tension for some alloy compositions may be attributed to a retention of order in the Au-In melts which is similar to the atomic arrangement in solid Au-In.     

Comparison of reactivity on step and terrace sites of Pd (3 3 2) surface for the dissociative adsorption of hydrogen: A quantum chemical molecular dynamics study

The notion of “active sites” is fundamental to heterogeneous catalysis. However, the exact nature of the active sites, and hence the mechanism by which they act, are still largely a matter of speculation. In this study, we have presented a systematic quantum chemical molecular dynamics (QCMD) calculations for the interaction of hydrogen on different step and terrace sites of the Pd (3 3 2) surface. Finally the dissociative adsorption of hydrogen on step and terrace as well as the influence of surface hydrogen vacancy for the dissociative adsorption of hydrogen has been investigated through QCMD. This is a state-of-the-art method for calculating the interaction of atoms and molecules with metal surfaces. It is found that fully hydrogen covered (saturated) step sites can dissociate hydrogen moderately and that a monovacancy surface is suitable for significant dissociative adsorption of hydrogen. However in terrace site of the surface we have found that dissociation of hydrogen takes place only on Pd sites where the metal atom is not bound to any pre-adsorbed hydrogen atoms. Furthermore, from the molecular dynamics and electronic structure calculations, we identify a number of consequences for the interpretation and modeling of diffusion experiments demonstrating the coverage and directional dependence of atomic hydrogen diffusion on stepped palladium surface.     

Hydrogen effusion: a probe for surface desorption and diffusion

Hydrogen effusion results are discussed for hydrogenated amorphous silicon (a-Si:H) and related alloys as well as for crystalline silicon (c-Si). It is demonstrated that depending on the microstructure of the material, hydrogen effusion gives information on hydrogen diffusion or surface desorption. The results suggest for compact a-Si:H and for ion implanted c-Si a similar hydrogen diffusion process, which is a trap limited motion of atomic hydrogen. Hydrogen effusion from defect-free c-Si and from void-rich amorphous semiconductors is limited by surface desorption. Both hydrogen diffusion and desorption depend on the Fermi energy if hydrogen bonds to the host material are broken.     

Dynamic collective displacements of atoms in metals and their role in the vacancy mechanism of diffusion

The phenomenon of dynamic collective displacements of atoms in face-centered cubic crystals has been revealed using molecular dynamics method. This phenomenon plays an important role in the vacancy mechanism of diffusion. The vacancy mechanism is provided by the collision of two regions of collective atomic displacements that move a migrating atom and a vacancy toward each other. The collective thermal atomic displacements from crystal lattice sites occur as a result of the nonuniform momentum distribution of atoms according to the Maxwellian distribution. Owing to their statistical nature, the degree of correlation of the atomic displacements depends neither on the temperature nor on the interatomic interaction potential.     

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.     

Scattering of atoms by light

This review discusses the latest theoretical and experimental achievements in the resonant light pressure acting on the translational motion of atoms. Alongside with the effects due to the spontaneous light pressure (atomic deflection, velocity bunching, cooling), various manifestations of the effects of induced light pressure are considered in detail. This paper provides the theory and experiments of atoms scattering by a standing light wave under the conditions of coherent and non-coherent interaction, diffraction and interference of atomic beams. The problems where atomic motion along two trajectories and Landau-Zener transitions between them are essential, are studied. The kinetic phenomena (scattering, cooling, channeling) due to the motion of the particles exposed to gradient force and also friction and diffusion caused by spontaneous emission are considered. The influence of the recoil effect under spontaneous emission of atoms on non-linear polarization phenomena is discussed.     

One-dimensional diffusion of vacancies on Sr/Si(100)-c(2×4) surface

An Sr/Si(100)-c(2×4) surface is investigated by high-resolution scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS). The semiconductor property of this surface is confirmed by STS. The STM images of this surface shows that it is bias-voltage dependent and an atomic resolution image can be obtained at an empty state under a bias voltage of 1.5 V. Furthermore, one-dimensional (1D) diffusion of vacancies can be found in the room-temperature STM images. Sr vacancies diffuse along the valley channels, which are constructed by silicon dimers in the surface. Weak interaction between Sr and silicon dimers, low metal coverage, surface vacancy, and energy of thermal fluctuation at room temperature all contribute to this 1D diffusion.     

Electric dipolar interaction assisted growth of single crystalline organic thin films

We report on a forest-like-to-desert-like pattern evolution in the growth of an organic thin film observed by using an atomic force microscope. We use a modified diffusion limited aggregation model to simulate the growth process and are able to reproduce the experimental patterns. The energy of electric dipole interaction is calculated and determined to be the driving force for the pattern formation and evolution. Based on these results, single crystalline films are obtained by enhancing the electric dipole interaction while limiting effects of other growth parameters.     

Mathematical modeling of the population dynamics of “dark” states of a three-level system

Mathematical modeling of the population dynamics is performed for states of a three-level system (atom) with a V-type configuration transforming a light pulse. It is assumed that the excited eigenstates of the atom are degenerate and coupled by coherent interaction, one of the states being radiating (radiative), while the other state is nonradiating (??dark??). The population dynamics of atomic states is described on the basis of numerical solutions of equations for the matrix elements of the density operator. The dependence of the efficiency of population of the atomic dark state from the values of the parameters of an irradiation pulse and from the ratio of the period of population oscillations of excited atomic states (caused by their coherent interaction) to the lifetime of the atomic radiating state is determined. Typical examples of the time dependence of the population of states of the atom considered are presented for the cases of irradiation by a short (as compared to the lifetime of the radiating state) sinusoidal light pulse and by a long rectangular light pulse with the resonance carrier frequency.