Evolution of ion-irradiated point defect concentration by cluster dynamics simulation

The relationship between ions irradiation and the induced microstructures(point defects, dislocations, clusters, etc.)could be better analyzed and explained by simulation. The mean field rate theory and cluster dynamics are used to simulate the effect of implanted Fe on the point defects concentration quantitatively. It is found that the depth distribution of point defect concentration is relatively gentle than that of damage calculated by SRIM software. Specifically, the damage rate and point defect concentration increase by 1.5 times and 0.6 times from depth of 120 nm to 825 nm, respectively. With the consideration of implanted Fe ions, which effectively act as interstitial atoms at the depth of high ion implantation rate, the vacancy concentration C_v decreases significantly after reaching the peak value, while the interstitial atom concentration C_i increases significantly after decline of the previous stage. At the peak depth of ion implantation, C_v dropped by 86%, and C_i increased by 6.2 times. Therefore, the implanted ions should be considered into the point defects concentration under high dose of heavy ion irradiation, which may help predict the concentration distribution of defect clusters, further analyzing the evolution behavior of solute precipitation.     

Line defect dynamics around a colloidal particle

We study experimentally the dynamics of a topological defect located around a colloidal particle suspended in a thermotropic nematic liquid crystal. The considered defect consists of a disclination loop encircling the particle at the equator. Under specific conditions, it is shown that this disclination continuously shrinks to a hedgehog defect located in the immediate vicinity of the particle. This phenomenon corresponds to a transition between an elastic quadrupolar configuration and an elastic dipolar configuration. We performed a basic numerical calculation to get an estimate of the dissipated energy during the transition; we compare the results with theoretical predictions that describe the elastic energy of particles surrounded by defects. Received 21 December 2001     

Non-linear dynamics of a driven two-level tunnelling defect

With increasing external-field amplitude the dynamics of a driven two-level system shows a cross-over from the weak-coupling or linear-response behaviour to a non-linear regime which is characterised by the appearance of Rabi oscillations, a field-dependent dynamical susceptibility, saturation of energy dissipation and additional frequencies in the correlation function. The system any initial distribution will relax towards a stationary state with a time-dependent polarisation vector. These features are derived from the propagator of the two-level system which is calculated using a perturbative scheme with a well-defined small parameter; the roatating-wave approximation of the Bloch equations is avoided. the dynamical behaviour is discussed by means of correlation and response functions.     

High-frequency subsurface and bulk dynamics of liquid indium

We have performed bulk and surface-sensitive inelastic x-ray scattering experiments on liquid indium with 3 meV energy resolution. The experimental data are well reproduced within a generalized hydrodynamic model including structural and microscopic relaxation processes. We find a longitudinal viscosity of 22 mPa s in the near-surface region compared to 7.4 mPa s in the bulk. The origin of the increase is associated with a slowing down of the collective dynamics in a subsurface region of 4.6 nm.     

Clustering and cooperative dynamics in a reactive system

We study the dependence of the dynamics on the size of particle clusters that grow by stepwise aggregation in a reactive epoxy-amine mixture. The data reveal the cluster property involved in the glasslike arrest and its quantitative link with the structural relaxation time. We find that the number-average cluster size xn governs the formation of a glassy phase as distinct from a gel phase, and that xn correlates to the size of the "cooperatively rearranging regions" postulated by the Adam-Gibbs model for glass forming liquids. These results suggest that the step polymerization process generates clusters that behave much like dynamical heterogeneities observed in supercooled liquids.     

Detection of a small defect on a rough surface

We report experimental results that show that the far-field correlation function is sensitive to a small local change in rough-surface geometry, where the speckle spatial correlation rather than the sample ensemble average is adopted. The angular cross-correlation function of the far-field speckles scattered by one-dimensional random rough surfaces is measured when a polarized beam of light is incident upon the rough surface from vacuum, where one part of the surface used is a thin dielectric film deposited upon a glass substrate and the other part is identical to the first except for a localized defect. We envisage application of this sensitivity property to inspection of a sample with a defect by means of speckle mapping.     

Effects of a one-dimensional surface defect on designer surface plasmon polaritons

By using a finite difference time domain(FDTD) method,the effects of a one-dimensional(1D) surface defect on designer surface plasmon polaritons(designer SPPs) supported by a 1D metallic grating in THz domain are investigated.When the size of the defect is in a special range which is not too large,the designer SPPs reflected and scattered by the defect are weak enough to be neglected.The defect only induces a disturbance in the energy distribution of the designer SPP supported by the whole defect grating.If the defect size exceeds the said range,the reflecting and scattering are dominant in the influences of the defect on designer SPPs.Our analysis opens opportunities to control and direct designer SPPs by introducing a 1D defect,especially in low frequency domain.     

Lattice dynamics of defect crystals with singular defect submatrix

Formulae for calculating Green's functions of defect lattices can be simplified if the submatrix characterising the defect is singular. From this point of view the model of central forces is investigated, and a special case of vibrations of a crystal with a substitutional defect where the change of long range Coulomb forces is considered.     

分子动力学研究表面缺陷对硅纳米线杨氏模量的影响

硅纳米线因受量子尺寸效应与表面效应的影响而具有奇特的力、电及其耦合特性,成为了纳米电子器件的核心构件.然而在硅纳米线的制备过程中,表面产生缺陷不可避免.因此本文采用分子动力学方法着重研究了表面缺陷浓度对不同横截面形状(正方形、六角形和三角形)的[110]晶向和[111]晶向硅纳米线杨氏模量的影响.研究结果表明,当硅纳米线仅有单一表面缺陷时,不同晶向硅纳米线的杨氏模量均随表面缺陷浓度增加而迅速单调减小.当表面缺陷浓度为10%时,杨氏模量的减小幅度在10%-20%之间,减小幅度的差异与硅纳米线的晶向以及横截面形状密切相关.当存在多个表面缺陷时,杨氏模量随着缺陷浓度的增加表现出了不同程度的波动趋势.三角形截面硅纳米线的杨氏模量波动幅度最大,正方形截面的波动较小,即表面缺陷分布的不同对正方形截面硅纳米线的杨氏模量影响较小,这表明表面缺陷的影响与其分布及硅纳米线的横截面形状密切相关.通过与实验结果对比,本文的研究结果揭示了表面缺陷是导致硅纳米线杨氏模量实验值变小的重要因素,因此在表征硅纳米线的力学性能时,需要考虑表面缺陷的影响.     

Trapping of a diffusing adatom by a substitutional surface defect

We propose a simple model to explain qualitatively the results of a recent field-ion-microscopy (FIM) study in which a substitutional Ir atom in the Rh(001) surface traps diffusing Rh adatoms [G.L. Kellogg, Phys. Rev. Lett., to be published]. The proposed explanation is supported by an embedded-atom-method investigation of the effect of a substitutional Pt atom in the Pd(001) surface on Pd adatom diffusion. We find a smaller energy barrier for a single adatom to diffuse around the defect than away from it, i.e., the adatom is effectively trapped by this defect in qualitative agreement with the FIM study. We also find that Pd adatom clusters are more strongly bound at the defect than away from it, indicating that such a defect can act as a nucleation site for cluster growth.     

Effect of fused silica subsurface defect site density on light intensification

The effect of defect density on the modulation of incident laser waves is investigated. First, based on the actual defect distribution in the subsurface of fused silica, a three-dimensional (3D) grid model of defect sites is constructed. The 3D finite-difference time-domain method is developed to solve the Maxwell equations. Then the electrical field intensity in the vicinity of the defect sites in the subsurface of fused silica is numerically calculated. The relationships between the maximal electrical field intensity in fused silica and the geometry of the defect sites are given. The simulated results reveal that the modulation becomes more remarkable with an increase of the defect density. In addition, the effect of the distribution mode of defects on modulation is discussed. Meanwhile, the underlying physical mechanism is analyzed in detail.     

Chemical stability and surface stoichiometry of vanadium oxide phases studied by reactive molecular dynamics simulations

Compositional stability of various vanadium oxides and oxide growth on vanadium surfaces have been studied using reactive molecular dynamics simulation methods. Vanadium dioxide (VO₂), sesquioxide (V₂O₃), pentoxide (V₂O₅), and hexavanadium tridecaoxide (V₆O₁₃) are studied in bulk crystalline and thin film structures, investigating charge distribution and pair distribution functions of particle interactions. The stability is estimated to be pentoxide, hexavanadium tridecaoxide, sesquioxide, and dioxide respectively in decreasing order in thin film structures. We then analyze oxide growth kinetics on vanadium (100) and (110) surfaces. The oxidation rate, stoichiometry, charge distribution, and the effect of surface orientation on kinetic phenomena are noted. In the early stages of surface oxidation of our simulation configurations, sesquioxide is found to be the dominant component. The modeling and simulation results are compared with experiments where available.     

Long-range reactive dynamics in myoglobin

We report the complete vibrational spectrum of the probe nucleus 57Fe at the oxygen-binding site of the protein myoglobin. The Fe-pyrrole nitrogen stretching modes of the heme group, identified here, probe asymmetric interactions with the protein environment. Collective oscillations of the polypeptide, rather than localized heme vibrations, dominate the low frequency region. We conclude that the heme "doming" mode is significantly delocalized, so that distant sites respond to oxygen binding on vibrational time scales. This has ramifications for understanding long-range interactions in biomolecules, such as those that mediate cooperativity in allosteric proteins.     

Lattice dynamics of an isotopic defect

Using a spectrum ～Ω ²√Ω _m ² ?Ω ² for the frequenciesΩ of the ideal crystal simple analytic expressions are derived for the response function, phonon scattering cross section and related quantities. The interpretation of resonances and localized vibrations as damped and undamped harmonic oscillators is discussed in detail.     

光学材料亚表面缺陷处强激光电磁场分布的3维模拟

利用时域有限差分法求解麦克斯韦旋度方程,研究了光学材料表面缺陷对入射激光场的调制作用,建立了亚表面缺陷的3维模型。以长方体表面缺陷为例,对缺陷附近的光场分布进行了3维数值计算,给出了电场强度3维分布图和缺陷不同尺寸时的最大电场强度。研究表明,相对于2维简化情况,3维表面缺陷对强激光电磁场的调制作用更加明显。     

光学材料亚表面缺陷处强激光电磁场分布的3维模拟

 利用时域有限差分法求解麦克斯韦旋度方程,研究了光学材料表面缺陷对入射激光场的调制作用,建立了亚表面缺陷的3维模型。以长方体表面缺陷为例,对缺陷附近的光场分布进行了3维数值计算,给出了电场强度3维分布图和缺陷不同尺寸时的最大电场强度。研究表明,相对于2维简化情况,3维表面缺陷对强激光电磁场的调制作用更加明显。     

Evolution of a two-parameter chaotic dynamics from universal attractors

We observed endlessly repeating sequences of the period-doubling universal route to chaos followed by ordered states in the dynamics of a two-parameter 1-D system. The coalescence of these sequences are reported together with measurements of a strange attractor which evolves from this overlap.