Determination of surface state parameters from surface photovoltage transients

Photo-stimulated electron transitions from the surface states into the conduction band which are involved in surface photovoltage spectroscopy are analyzed in terms of phenomenological surface state parameters. The surface state parameters are determined frolm photovoltage transients on the basis of relationships derived for a general case where the mechanism of electron transients is not specified, for the case where the surface states are in equilibrium with the bulk and the case where the surface states are not in equilibrium with tpe bulk. The procedure is illustrated utilizing experimental data obtained on CdS surfaces.     

Size-dependent dispersion characteristics in piezoelectric nanoplates with surface effects

In this paper, surface effects on the dispersion characteristics of elastic waves propagating in an infinite piezoelectric nanoplate are investigated by using the surface piezoelectricity model. Based on the surface piezoelectric constitutive theory, the presence of surface stresses and surface electric displacements exerting on the boundary conditions of the piezoelectric nanoplate is taken into account in the modified mechanical and electric equilibrium relations. The partial wave technique is employed to obtain the general solutions of governing equations, and the dispersion relations with surface effects are expressed in an explicit closed form. The impacts of surface piezoelectricity, residual surface stress and plate thickness on the propagation properties of elastic waves are analyzed in detail. Numerical results show that the dispersion behaviors in piezoelectric nanoplates are size-dependent, and there exists a critical plate thickness above which the surface effects may vanish.     

Ag（110）表面声子谱的分析型EAM模型计算

应用晶格动力学，结合分析型EAM模型，计算了Ag(110)表面声子频率和振动极化方向，通过比较弛豫与未弛豫表面的计算结果，发现弛豫后的结果与He原子散射实验数据符合得很好，表面振动的极化局域特征符合表面的二维对称性，沿方向的S2和S3表面模也和实验结果—致. 关键词： 表面声子谱 弛豫 EAM 极化     

Simulation of elastic displacement of surface during laser short pulse heating of gold

The laser short pulse heating initiates nonequilibrium heating of the substrate material, which in turn results in the thermal stresses developing in the region below the surface. The surface temperature can be measured possibly through the monitoring of the resulting surface displacement. This requires in detail investigation into the surface displacement and surface temperature rises across the heated spot during the laser short pulse heating process. In the present study, the laser short pulse heating of gold surface is considered and the temperature rise at the surface and elastic displacement of the surface are investigated. The spatial and temporal distributions of surface displacement and surface temperature are predicted and the elastic response of the substrate material due to temperature rise is explored. It is found that the temporal and spatial distributions of the surface displacement do not follow the temperature rise at the surface. Consequently, care should be taken when measuring the temperature rise at the surface by means of monitoring the surface displacement during a laser short pulse heating process.     

Critical behavior of a spherical model with a free surface

Critical phenomena ind-dimensional ferromagnetic spherical models on hypercubic lattices with free surfaces are studied. The surface specific heat and surface susceptibilities are obtained. The exponents characterizing the divergence of these surface quantities at the bulk critical temperature are found to satisfy recently proposed scaling relations. The variation of the susceptibility with distance from the surface is also discussed. The author's recent scaling theory for surface properties is investigated in detail, and found to give an exact representation for the free energy of a three-dimensional spherical model of finite thickness in finite bulk and surface magnetic fields. A scaling form for the surface free energy is derived.     

Study of a surface critical phenomenon associated with surface segregation in Cu---Ag alloys

This paper presents a modified regular solution formalism for surface segregation in substitutional alloys which distinguishes between bond energies at the surface and in the bulk of a solid solution. The differences between surface and bulk energies are ascribed to strain energy effects associated with misfitting solutes. Experimental measurements of surface composition on (111) surfaces of Cu---Ag alloys dilute in Ag are reported and show the existence of a surface phase transition, which manifests itself as an abrupt surface composition change at temperatures below 788 K. Similar measurements on (100) surfaces show no comparable critical phenomena down to temperatures as low as 523 K, indicating a strong anisotropic behavior of the surface phase transition. The data are compared with the model to obtain model-based estimates of the surface critical temperatures and the surface regular solution constants for both surface orientations.     

Back-propagating surface polaritons of a one-dimensional photonic crystal containing single negative metamaterials

The existence of the surface polaritons at the interface separating a semi-infinite uniform left-handed metamaterial and a one-dimensional photonic crystal composed of alternating layers of two kinds of single-negative materials is theoretically investigated. The dispersion characteristics of the surface polaritons are analyzed and demonstrated that in the presence of metamaterial, the surface polaritons are sensitive to light polarization, so that there exist only backward TM-polarized (or TE-polarized) kind of the surface polaritons depending on the ratio of the thicknesses of the two periodic stacking layers. The existence regions of the surface polariton modes are determined for both TM-polarized and TE-polarized surface polariton modes.     

真空中激光触发脉冲电压下绝缘材料闪络特性

搭建了真空中脉冲电压下激光触发沿面闪络试验平台,在试验平台上进行常用开关绝缘介质尼龙、聚碳酸酯和Al2O3陶瓷的激光触发沿面闪络特性试验,探讨了真空中激光触发沿面闪络的机理。试验结果表明:聚碳酸酯试品的自闪络电压最高;在真空中脉冲电压下的激光触发沿面闪络试验中,随着激光能量密度的增大,3种材料的时延和抖动均减小;532 nm和1 064 nm波长激光触发的条件下,3种材料的抖动均在1 ns左右,聚碳酸酯抖动较小;532 nm波长激光触发的时延小于1 064 nm波长激光触发的时延。     

Anisotropic surface stress in polymer crystals: Example of polyethylene

The model of sharp folds at the surface of lamellar single crystals of polymers lead to high surface stress. The layer of folds is analogous to an epitaxial layer. Weak strains of the crystalline layers, when they are possible, relax these surface stresses. The principal properties of the most studied polymer, polyethylene, which are not observed in nonpolymeric materials, are shown to be deformation modes for relaxing the surface stress. From experimental observations it is inferred that in polyethylene, in the (110) and (100) fold domains, there are, respectively, an extension and a contraction along the growth faces. The observed twinning and martensitic modes appearing in polyethylene single crystals are the modes activated by the tensile surface stress due to the intrafold interaction. The idea proposed in this paper that surface stress activates some deformation modes can be easily extended to other lamellar polymer crystals.     

The study of guided waves in surfaces and thin supported films using surface Brillouin scattering and acoustic microscopy

This paper reviews the use of surface Brillouin scattering (SBS) and acoustic microscopy (AM) in studying the surface dynamics of solids in order to obtain information about the near-surface elastic properties of solids and thin supported films. The vibrational modes that are probed by these means include Rayleigh surface and pseudo-surface acoustic waves, longitudinal lateral waves (surface skimming bulk longitudinal waves) and various thin film guided modes, such as Sezawa and Love waves. SBS is the inelastic scattering of light, mediated by thermodynamic fluctuations in the surface elevation and near surface elastic strains. The scattering cross-section is conveniently expressed in terms of Fourier domain elastodynamic Green's functions. AM depends on the insonification of a surface through a coupling fluid, and the resulting excitation and subsequent decay of the various surface modes. The complex reflectivity of the fluid-loaded surface, and the line and point force surface Green's functions are invoked in the interpretation of different modalities of AM, yielding much the same information about the surface dynamics. The focus in this paper is on the Green's function approach. A number of illustrative examples, drawn from the authors' research, are provided.     

Waves in an active medium guided by a reactive surface with reactance of gain modulation

The characteristics of surface waves (i) in a passive medium supported by a passive surface with reactance modulation and an active surface with the modulation of either the reactance or the negative resistance, and (ii) in an active medium supported by a passive surface with a reactance modulation are investigated with emphasis on the band regions. The periodic variation is sinusoidal and is in the propagation direction. For a passive medium terminated by a passive surface with a reactance modulation, there are stop bands in frequency for the surface waves and the structure of the first two stop bands is analyzed. For a passive medium terminated by an active surface and for an active medium terminated by a reactance-modulated passive surface, the characteristics of the absolute instabilities occurring in the first-order band are examined. The nature of the convective instability taking place in the second-order band for an active medium terminated by a reactance-modulated surface is discussed. Analytical expressions for the frequency shift of the second-order band are deduced. The Floquet theory is used to obtain the exact dispersion relation in the form of a continued fraction which is analyzed by singular expansions.     

A molecular dynamics study of effective parameters on nano-droplet surface tension

The main purpose of this paper is to numerically study the effect of droplet radius, temperature, and surface wettability on droplet surface tension. Moreover, the validity of Young-Laplace equation (Y-L) for nano-droplet is examined. Simulations of droplet surrounded by its vapor and droplet on solid surface are carried out and the results are compared to each other in order to comprehend the role of surface wettability on droplet surface tension. The pair potential for the liquid-liquid and liquid-solid interaction is considered using Lennard-Jones model. Different numbers of atoms and surface wettabilities are employed to generate droplet of different radiuses. In addition, contact angle of droplet on solid surface is computed. Pressure tensor and density profile is locally calculated. Furthermore, liquid pressure is evaluated far from the interface using the virial theorem and gas pressure is obtained using an equation of state. In order to calculate the surface tension, two different approaches are employed; Young-Laplace equation and direct molecular dynamics (MD) simulation. The surface tension increases with increase in droplet radius and it is seen that the surface wettability does not directly influence the surface tension.     

Curvature dependence of surface phonon and plasmon frequencies

Non-retarded surface collective excitations (optical phonons and plasmons) are investigated in solids of general shape, and their polarization eigenmodes and frequencies are found. The cases of prolate and oblate spheroids and rotational paraboloids are studied in detail. The coupling of a charged particle to surface excitations is treated in the Hamiltonian formalism. Its spatial variation is studied, especially its dependence on the relative particle position with respect to the local surface curvature, which determines the contribution of each surface mode to the overall excitation spectrum in charged-particle — surface scattering.     

Influence of surface orientation and defects on early-stage oxidation and ultrathin oxide growth on pure copper

We investigate oxidation and oxide growth on single-crystal copper surfaces using reactive molecular dynamics simulation. The kinetics of surface oxide growth are strongly correlated with the microstructure of the metal substrates. Simulating oxide layer growth along the (100), (110), and (111) orientations of crystalline copper, oxidation characteristics are investigated at temperatures of 300?K and 600?K. The oxidation kinetics are found to strongly depend on the surface orientation, ambient temperature, and surface defects. The effect of surface morphology on oxidation characteristics is analyzed by comparing oxygen adsorption on various sites and the structure factor. The surface oxide formed on (100) retains the initial crystal structure in the 300–600?K range. The (100) surface shows the highest oxidation rate at both temperature conditions but saturates, facilitating oxygen adsorption on hollow sites. The oxidation kinetics of the (100) orientation are found to be not significantly affected by surface defects. (110) shows modest oxidation at 300?K but the highest oxidation is observed at 600?K. By surface disorder and reconstruction, the oxide layer is produced continuously. The (111) surface is sensitive to ambient temperature and surface defects, showing that surface reconstruction is a key element for further oxidation. The charge distribution of oxidized Cu atoms indicates multiple groups of stoichiometric oxides, while the fraction of CuO-like characteristics increases significantly on the (110) and (111) orientations at higher temperature (600?K). The energetics and mechanisms of oxidation on Cu metal substrates at the nanoscale are discussed in detail, and comparisons with available experimental and other theoretical studies are presented wherever possible.     

Accounting for the local surface geometry in the classical theory of spin waves

The effect of the surface energy on the formation of the spin spectrum is considered. Different aspects are examined of the effect of the surface on the energy spectrum of the system associated with structural features of the surface structure. The calculations are carried out for the spectrum of elementary excitations of a ferromagnetic system that is bounded by a surface. Calculations are conducted following the standard method of the Hamiltonian diagonalization. Relationships are obtained that completely determine the surface spectrum in the zero approximation in the spin-spin interaction.     

不稳定表面粗糙目标的太赫兹波段散射特性分析

研究了不同粗糙度下的非均匀和分区均匀不稳定表面粗糙导体目标在太赫兹波段的散射特性。区别于采用经验公式的建模方法,提出把随机粗糙面的建模理念应用到太赫兹波段的非均匀和分区均匀不稳定表面粗糙导体目标的建模中,用描述粗糙面的均方根高度和相关长度两个物理量来调节目标表面的粗糙度变化。提出表面粗糙目标的分类形式并给出具体模型,然后用随机高斯粗糙面来模拟非均匀和分区均匀不稳定粗糙目标的表面,再采用物理光学和等效电流相结合的方法来进行仿真计算,分别对不同入射角、不同频率和不同粗糙度的非均匀和分区均匀不稳定表面粗糙导体目标的太赫兹波散射特性进行分析,最后得出了相关的结论。     

A method for the absolute measurement of anisotropic surface free energies

Using the vector method of Cahn and Hoffman, equations are derived that relate the anisotropic surface free energy to the shapes of grain-boundary grooves at the solid-vapor, solidsolvent, solid-solid, or solid-melt interface in either a gravitational field or a temperature gradient. The components of the vector function ξ at any point on the surface of the groove are given in terms of distances and areas directly measurable on micrographs. The surface free energy is the component of ξ along the surface normal. The experimental methods now in use for measuring absolute surface energy in the isotropic case, can be employed without modification to yield data for the evaluation of anisotropic surface free energy and surface torques. Procedures for using the symmetry properties of crystals to remove the indeterminacy and reduce the amount of data gathering for anisotropic crystals are developed.     

Experimental Investigation of Subcooled Flow Boiling in an Inclined Channel with a Hot Spot

In this study, the effects of surface roughness, fluid velocity, and surface inclination on hot spot subcooled flow boiling are investigated experimentally. The experimental set-up consists of a circular heater that is placed on the lower wall of a channel. Based on the experimental data, two new independent empirical correlations are presented. The experimental results show that by increasing the surface roughness and fluid velocity, the surface heat fluxes increase. Inclination of the surface in either direction yields a higher heat transfer coefficient in comparison to its horizontal position when the surface is smooth and lower for the rough surface.     

修正反射系数的Kirchhoff近似方法计算高斯起伏冰面三维声散射

针对随机起伏冰面的声散射计算问题,利用修正反射系数的Kirchhoff近似方法计算了高斯起伏冰面的三维声散射。在计算模型中引入了冰面局部统计平均反射系数的概念,将二维高斯起伏冰面的散射分为相干散射和非相干散射,分别得到两类散射成分的散射系数公式,计算了高斯起伏冰面三维声散射的散射强度。分析了散射强度与随机起伏冰面的均方根高度、声波入射角度及频率的关系。通过实验室水池中高斯起伏冰面的散射强度测量实验,对理论模型的计算结果进行了验证。将实验结果分别与采用冰面局部统计平均反射系数的模型计算结果和文献中采用平整冰面镜反射系数的模型计算结果进行了对比,采用冰面局部统计平均反射系数的模型计算结果与实验测量值吻合较好。      

Virtual excitation type surface polaritons on anisotropic media

We discuss the nature of a new type of surface polariton which occurs on anisotropic media, involving a photon coupled to a surface virtual excitation. Dispersion curves are calculated for α-quartz, where both real excitation type and virtual excitation type surface polaritons are predicted. The dispersion curves for virtual excitation surface polaritons are found to remain at small wavevector, and the endpoints of the dispersion curves terminate on the bulk polariton dispersion curves in the two media. The virtual excitation surface polaritons which occur on gyromagnetic and gyrodielectric media are also noted.