Interference effect in combined elastic and inelastic scattering of an energetic electron reflected from a disordered medium with generation of a surface plasmon

Quantum interference in combined elastic and inelastic scattering of an energetic electron with excitation of a surface plasmon leads to a change in the shape of the corresponding peak in the electron-energyloss spectrum. The plasmon generation is suppressed near the frequency \({{\omega _p } \mathord{\left/ {\vphantom {{\omega _p } {\sqrt 2 }}} \right. \kern-\nulldelimiterspace} {\sqrt 2 }}\). The suppression increases with increasing surface-plasmon wave length, because the interference of the energetic-electron scattering processes differing in the sequential order of elastic and inelastic scattering becomes progressively more destructive. The decrease in the height of the surface-plasmon peak in the electron-energy-loss spectrum leads to a non-dissipative broadening in this peak. Quantum interference also causes a specific feature to occur in the azimuthangle dependence of the spectral intensity as the electron energy loss increases in the immediate vicinity of the surface-plasmon peak.     

具有不规则自由表面的粘弹性介质中声波模拟的有限差分算法

声波在具有不规则自由表面的粘弹性介质传播的数值模拟工作在地震勘探,地震预测中非常重要,特别是当模型有起伏的自由界面和较强的衰减特性时更是如此。基于镜像方法和直接方法,本文发展了一种二维有限差分算法,可以模拟不规则自由表面引起的声波散射问题。该方法将自由表面条件与速度应力方程结合求解粘弹性波动方程,在垂直和水平自由段及拐点处设置相应的边界条件。该方法假设自由表面穿过剪切应力和相应参数的网格点。为了提高计算精度,分别计算了水平和垂直方向上应力镜像值,而对质点速度,采用先水平方向后垂直方向分析进行镜像计算和更新。将粘弹性水平自由表面镜像方法和不同倾斜度平滑自由表面的改进方法的数值结果进行对比,并验证了算法的精度。     

Surface plasma waves over bismuth-vacuum interface

A surface plasma wave (SPW) over bismuth-vacuum interface has a signature of mass anisotropy of free electrons. For SPW propagation along the trigonal axis there is no birefringence. The frequency cutoff of SPW lies in the far infrared region and can be accessed using free electron laser. The damping rate of waves at low temperatures is low. The surface plasma wave may be excited by an electron beam of current ∼100 mA propagating parallel to the interface in its close proximity.     

The group velocity variation of Lamb wave in fiber reinforced composite plate

Experimentally measured Lamb wave group velocities in composite materials with anisotropic characteristics are not the same as the theoretical group velocities which is calculated with the Lamb wave dispersion equation. This discrepancy arises from the fact that the angle between the group velocity direction and the phase velocity direction in anisotropic materials exists. Wave propagation in a composite material with anisotropic characteristics should be considered with respect to magnitude correction in addition to direction correction. In this study, S0 mode phase velocity dispersion curves are depicted with the variation of degree with respect to the fiber direction using a Lamb wave dispersion relation in the unidirectional, bidirectional, and quasi-isotropic composite plates. Slowness surface is sketched by the reciprocal value of the phase velocity curves. The magnitude and direction of the group velocity could be calculated from the slowness surface. The recalculated group velocities with consideration of the magnitude and direction from the slowness surface are compared with experimentally measured group velocities. The proposed method shows good agreements with theoretical and experimental results.     

Viscous compressible hydrodynamics at planes,spheres and cylinders with finite surface slip

We consider the linearized time-dependent Navier-Stokes equation including finite compressibility and viscosity. We first constitute the Green's function, from which we derive the flow profiles and response functions for a plane, a sphere and a cylinder for arbitrary surface slip length. For high driving frequency the flow pattern is dominated by the diffusion of vorticity and compression, for low frequency compression propagates in the form of sound waves which are exponentially damped at a screening length larger than the sound wave length. The crossover between the diffusive and propagative compression regimes occurs at the fluid's intrinsic frequency w \omega ∼ c ² r₀ \rho_{0}^{}/h \eta , with c the speed of sound, r₀ \rho_{0}^{} the fluid density and h \eta the viscosity. In the propagative regime the hydrodynamic response function of spheres and cylinders exhibits a high-frequency resonance when the particle size is of the order of the sound wave length. A distinct low-frequency resonance occurs at the boundary between the propagative and diffusive regimes. Those resonant features should be detectable experimentally by tracking the diffusion of particles, as well as by measuring the fluctuation spectrum or the response spectrum of trapped particles. Since the response function depends sensitively on the slip length, in principle the slip length can be deduced from an experimentally measured response function.     

On the definition of a diffusion constant in a nonequilibrium system

A diffusion constant for electrons in a current-carrying semiconductor can be unambiguously defined in nearly uniform systems. For frequency-dependent density gradients it is $$D_{\alpha \beta } (\omega ) \equiv \int\limits_0^\infty {dt e^{i\omega t} \overline {\Delta \upsilon _\alpha (t)\Delta \upsilon _\beta (0),} } $$ where \(\overline {\Delta \upsilon _\alpha (t)\Delta \upsilon _\beta (0)} \) is the velocity correlation function with respect to the steady state in a bias field. This result has been elucidated in the relaxation approximation by different approaches to the diffusion problem. Essential for its derivation is a statistical independence assumption of space and velocities, and in order to get a classical diffusion law of Fick's type certain velocities have to be distributed according to the steady state in a bias field. Diffusion constant and noise temperature are discussed for a few band structures in the relaxation approximation.     

Propagation of sound and spectrally resolved rayleigh scattering in weakly ionized plasmas

The scattering mechanisms possible in weakly ionized plasmas are reviewed. The different cases can be discerned by means of the magnitude of three characteristic parameters: 2π/ωτ _c being the ratio of scattering time and mean free collision time, y=1/kλ _c being the scattering parameter defined as the ratio of scattering length and mean free collision path, and χω/c _{s, T} ² being the ratio between the product of thermometric conductivity and scattering frequency to the square of the adiabatic or isothermal sound velocity. For \({{2\pi } \mathord{\left/ {\vphantom {{2\pi } {\omega \tau _c<< 1}}} \right. \kern-0em} {\omega \tau _c<< 1}}\) quantum mechanical formulae have to be used, whereas in the opposite case classical treatments are applicable. The classical methods are Boltzmann equation formalisms ify?1, and fluid dynamics ify?1. In the fluid dynamical case there appear two waves for low frequencies, \({{\chi \omega } \mathord{\left/ {\vphantom {{\chi \omega } {c_{s, T}^2 }}} \right. \kern-0em} {c_{s, T}^2 }}<< 1\) , an adiabatic one which can propagate with weak damping and an isobaric one which cannot propagate, both wave types yielding together three scattered lines. In the opposite case of high frequencies, \({{\chi \omega } \mathord{\left/ {\vphantom {{\chi \omega } {c_{s, T}^2 }}} \right. \kern-0em} {c_{s, T}^2 }} > > 1\) , the scattering behavior is different from ordinary hydrodynamics. Here also do exist two types of waves, isochoric and isothermal, but none of them can propagate. Since the intensity of the scattered isochoric wave can be neglected, there is only one scattered line. Local temperatures and particle densities can be determined from the scattered spectrum. On the other hand, transport coefficients like shear and bulk viscosity as well as thermometric conductivity can be derived from sound absorption or Rayleigh scattering experiments if an independent temperature measurement is performed at the same time. The general formalism is applied to a weakly ionized hydrogen arc plasma.     

空隙波的传播与两相流相态非均质性研究

以每秒高达30000次的采集频率,应用电导探针测量气液两相流中空隙波的传播,对探针信号进行多尺度分析和频谱分析获得空隙波的波速,发现空隙波具有不同的波速并在稳定性曲线所包覆的拇指型区域内呈离散分布,甚至可逾越稳定性曲线。应用高速数字摄像对两相流的相态结构进行了观测,发现空隙波波速的多样性和随机的不稳定性根源于气液两相流复杂的细观非均质形态。     

岩石物理模型约束下声波测井数据评价及处理*

声波测井数据是石油天然气勘探开发中最为重要的基础资料,其数据质量受井壁环境、频散等因素影响较大,且部分钻井并未采集声波。这对后续开展速度分析、储层识别和烃类检测都带来较大影响。因此需要对声波测井数据进行环境校正、补齐、预测等处理。目前,主要依据研究区域内已钻井的统计规律对声波测井数据进行评估和预测。而在深水区,因钻井距离远、地质条件差异较大等原因,不同地区的声波变化较大,依据统计方法评估数据存在较大的多解性。本文提出采用岩石物理模型来评估声波质量和预测声波数据。基于Xu&White岩石物理模型,采用密度曲线反演计算泥岩孔隙度,改善了岩石物理模拟在储层段和非储层段的拟合效果,且在多口井的适用性更好。基于新方法,本文建立了可在区域内稳定适用的岩石物理模型,可计算多口井在不同岩性、不同深度下的速度,为在不同地区和不同深度的声波质量评估和预测提供依据,指导后期储层预测和含气性检测。     

Scattering of a Rayleigh surface acoustic wave by a small-size inhomogeneity in a solid half-space

We use the Born approximation of the perturbation method to solve the problem of scattering of a harmonic Rayleigh surface acoustic wave by a weak-contrast inhomogeneity that is small compared with the wavelength and is located in a solid half-space near its boundary. The material of the inhomogeneity differs from the material of the half-space only in its density. The Rayleigh wave incident on the inhomogeneity is excited by a monochromatic surface force source acting normally to the half-space boundary. We derive expressions for the displacement fields in the scattered spherical compressional and shear (SV- and SH-polarized) waves. Scattering of the Rayleigh wave into a Rayleigh wave is studied in detail. We find expressions for the vertical and horizontal components of the displacement vector in the scattered Rayleigh wave as well as its radiated power. It is shown that the field of the scattered surface wave is mainly formed by vertical oscillations of the inhomogeneity in the field of the incident wave. In this case, the radiated power for the scattered Rayleigh wave formed by vertical motion of the inhomogeneity in the incident-wave field depends on the depth of the inhomogeneity as the fourth power of the function describing the well-known depth dependence of the vertical displacements in the Rayleigh surface wave. Correspondingly, the dependence of the radiated power for the scattered Rayleigh wave formed by horizontal motion of the inhomogeneity depends on its location depth as the fourth power of the depth dependence of the horizontal displacements in the Rayleigh surface wave. We perform calculations of the ratio between the powers of the scattered and incident Rayleigh waves for different ratios between the velocities of the compressional and shear waves in a solid. It is shown that the radiated power for the scattered surface wave decreases sharply with increasing depth of the subsurface-inhomogeneity location. Thus, the scattering of a Rayleigh wave into a Rayleigh wave is fairly efficient only when the location depth of the inhomogeneity does not exceed about one-third of the wavelength of the shear wave in an elastic medium.     

激光衍射法测量液体表面波的波长并确定其属性

设计并搭建了一套简易的激光衍射实验装置,通过该装置作者测量了水的表面波的波长,并确定了水的表面波主要成份是毛细成分。     

利用谐波光谱的伴线结构测量自生磁场

在飞秒激光与固体靶相互作用中,利用OMA光学多道分析谱仪,在靶前表面激光镜反方向测量了激光的二次谐波(2ω₀)光谱和三次谐波(3ω₀)光谱,观测到了红移的2ω₀光谱和3ω₀光谱的伴线结构。在激光功率密度为～1018 W·cm-2的条件下,通过2ω₀和3ω₀谐波光谱的伴线结构,回推出激光与等离子体相互作用中产生的自生磁场均小于1 MGs。随着激光功率密度的增大,谐波谱红移峰向长波方向移动,光谱同时发生展宽。分析认为,等离子体临界面的迅速膨胀是导致二次谐波和三次谐波红移的主要原因。随着预脉冲功率密度的增大,临界面膨胀速度增大,导致了谐波光谱峰更大的红移。自生磁场的测量为诊断临界面的运动方向和速度提供了新的依据。     

Gravity wave turbulence in a laboratory flume

We present an experimental study of the statistics of surface gravity wave turbulence in a flume of a horizontal size 12 x 6 m. For a wide range of amplitudes the wave energy spectrum was found to scale as Eomega approximately omega(-nu) in a frequency range of up to one decade. However, nu appears to be nonuniversal: it depends on the wave intensity and ranges from about 6 to 4. We discuss our results in the context of existing theories and argue that at low wave amplitudes the wave statistics is affected by the flume finite size, and at high amplitudes the wave breaking effect dominates.     

Effect of cavity dissipation on the emission spectrum of an atom interacting with a field in the dispersive approximation

The emission spectrum of a two-level atom interacting dispersively with a single mode radiation field in the dissipative cavity is investigated. A general expression for the emission spectrum is derived. The numerical results for the initial field in coherent state are calculated. It is found that the spectrum structure is influenced significantly by the cavity damping constant \kappa , and the spectrum structure is dependent on the interaction time T when the cavity dissipation is present. Only one peak located at Ω_a appears in the atomic spectra for larger T.     

Properties of surface waves in an elastic cylinder filled with a liquid

The properties of harmonic surface waves in an elastic cylinder filled with a liquid are studied. The case of elastic material for which the shear wave velocity is higher than the sound velocity in a liquid is considered. The wave motion is described based on the complete system of equations of the dynamic theory of elasticity and the equation of motion of an ideal compressible liquid. The asymptotic analysis of the dispersion equation in the region of large wave numbers and qualitative analysis of the dispersion spectrum showed that in such a waveguiding system there exist two surface waves, the Stoneley and the Rayleigh waves. The lowest normal wave forms the Stoneley wave on the internal surface of the cylinder. In this waveguide phase, velocities of all normal waves, except for the lowest one, have the velocity of sound in the liquid as their limit. Therefore, the Rayleigh wave on the external surface of the cylinder is formed by all normal waves in the range of frequencies and wave numbers in which phase velocities of normal waves of the composite waveguide and the lowest normal wave of the elastic hollow cylinder coincide.     

水下声辐射激励水表面声波的特征参数测量

采用激光干涉方法对水下声辐射激励水表面声波的特征参数频率和振幅进行了测量研究。从理论上对水表面声波激光相干测量信号的频谱构成进行了分析,在此基础上提出了水表面声波两个重要声学参数频率和振幅的解算方法,并通过数值仿真进行了验证。搭建了一套简单的激光干涉测量实验系统,对不同频率和声压激励的水表面声波进行了测量实验,验证了水表面声波频率和振幅解调方法的准确性。对水表面声波横向传播的振幅衰减现象进行了初步的实验研究,结果表明水表面声波的频率越高,振幅的横向衰减越快。研究表明激光相干检测方法能够准确地实现水表面声波振幅和频率的测量。      

FREQUENCY DISPERSION CHARACTERISTICS OF PHASE VELOCITIES IN SURFACE WAVE FOR ROTATIONAL COMPONENTS OF SEISMIC MOTION

When rotational components of ground motion produced by seismic surface waves are computed, the phase velocities must always be dealt with in earthquake engineering. In this paper, appropriate methods are presented to obtain the calculation formulas for the phase velocities of surface waves by applying the theory of elastic wave propagation. Frequency dispersion characteristics of phase velocities are discussed. The rocking component around a horizontal axis and the torsional component around a vertical axis, which are generated, respectively, by the Rayleigh and Love waves, are reasonably given. A procedure is developed to calculate the time histories of these rotational components.     

The effect of ocean wave on the vertical spatial correlation of ocean ambient noise

The effect of the correlation function of noise sources derived from the ocean wave spectrum on the vertical spatial correlation of ocean ambient noise is investigated. The spatial correlation models of ocean ambient noise usually assume that the surface noise sources are uncorrelated. This assumption can be used to explain some physical phenomena, but it is not consistent with the real situation. Considering the relation between the ocean wave motion and the ambient noise generated by wind, the spectrum of ocean wave is introduced to calculate the vertical correlation of ocean ambient noise as the correlation function of noise sources by using the Kuperman-Ingenito(K/I) noise model. The comparison of the simulations and the experimental data shows that the simulations of vertical correlation of ambient noise have some differences with the experimental data by assuming the noise sources are uncorrelated and the simulations of vertical correlation of ambient noise have a good agreement with the experimental data by using the correlation function of noise sources derived from the ocean wave spectrum under the situation of high wind speed.