Nature of the short wavelength excitations in vitreous silica: An X-Ray brillouin scattering study

Spontaneous emission in photonic crystals with anisotropic three-dimensional dispersion relation is studied. If the upper level is below a characteristic frequency omega(1), or above omega(2), or between omega(1) and omega(2), the radiation is a localized field with a frequency in the band gap, or a propagating field with a frquency in the band, or a diffusion field, respectively. An analytical expression for the Lamb shift is obtained. The Lamb shift for the current case is small compared to that in an ordinary vacuum or in one- or two-dimensional photonic crystals due to lower density of states.     

Focal Shift of Paraxial Gaussian Beams in a Left-Handed Material Slab Lens

Focal shift is inevitable in conventional lens systems due to the Fresnel number and angular aperture. In this Letter, we demonstrate that there is no focal shift when a paraxial Gaussian beam passes through a left-handed material slab lens without absorption or gain. However, the effect is exhibited in the presence of absorption or gain, and becomes larger as the absorption or gain increases. When the absorption is equal to the gain, the phenomenon of the focal shift caused by the gain is more obvious. In addition, the field distribution is not affected by the absorption or gain and always remains Gaussian both in internal and external focus planes.     

Loschmidt's and Zermelo's paradoxes do not exist

A strict operational (i.e., informational) analysis of the meaning of preparing a system to realize the paradoxes of Loschmidt or Zermelo is made. Where reversal or recurrence are operationally realizable, no contradiction with the irreversible nature of macroscopic operations occurs. Paradox results either from neglecting irreversible phenomena in the means for preparing a reversed state, or from confusing elements or ensembles, which are meaningful in microstate language but meaningless operationally, with preparable macrostates, whoserepresentation in microstate language is an ensemble whose very definition is incompatible with that of any paradox-generating element or ensemble,Supported in part by NSF Grant No. GN 534.1.     

Determination of the 2D amplitude-phase structure of an optical field and the transfer function in the case of a convolution-like effect of a medium

A solution of the phase problem in optics as applied to the simultaneous detection and analysis of the phase-amplitude structure of image-forming or image-transmitting 2D optical fields and the phase-amplitude structure of probed media or objects, transfer or instrumental functions of signal-transmitting media, or field-or image-forming systems is considered. The effect of media or objects is described by the operation of convolution. The essence of the method applied is the introduction of two additional modulators, which in some way perform the function of visualizing the phase information. Optical schemes of two types are considered. In both cases, the first additional modulation precedes the action of a medium or an object. The second additional modulation takes place either in the plane immediately behind the probed medium (first type of scheme) or in the plane of spatial frequencies formed by the optical system (second type of scheme). In the first variant, the plane of detection is that of the spatial frequencies; in the second variant, it is the plane of the image formation. The resulting intensity distributions yield a solution to the problem.     

Damping properties of layered cylindrical shells,vibrating in axially symmetric modes

The damping of cylindrical shells coated with unconstrained layers of viscoelastic material either on one side of the shell (inside or outside) or on both sides is estimated. The basic equations of motion are derived which describe harmonic forced flexural damped vibrations in axisymmetric modes. For pure sinusoidal modes expressions for the overall loss factors are given. The damping properties of cylindrical shells of finite length, coated on the inside or outside, or on both sides (symmetrically or unsymmetrically) are compared. Classical thin shell theory is used for the analysis. It is shown how two-layered damped shells differ from two-layered damped beams. The extent of damping reduction in shells resulting from the fact that the shell cross-section is closed is discussed.     

高能飞秒激光系统中空间滤波器的研究和设计

 由于透镜传输时间延迟效应（PTD），飞秒激光脉冲经过空间滤波器产生脉冲波前变形。讨论了用双胶合透镜或双分离透镜代替空间滤波器中的单透镜，以消除或减小PTD效应，设计了适用于飞秒激光装置的空间滤波器。     

鋼鐵中碳、氮的擴散、脱溶和沉澱

一般认为钢的变脆例如蓝脆和回火脆都与钢中的扩散和沉淀有关系。本文尝试用内耗测量的方法研究碳与氮在α-铁及碳素钢中的扩散、脱溶和沉淀,从而进一步地了解钢的变脆的机构。实验的结果指出,碳在α-铁中的扩散不受自身浓度、合金元素和沉淀历史的影响。氮的情形与碳显著不同,合金元素使氮的扩散变慢,在沉淀初期使氮的扩散加速。较有系统地研究了碳、氮在加工后的α-铁中脱溶时所引起的内耗峰(当振动频率约为每秒1周时出现在250℃左右),发现了高温淬火在含碳或氮较多的试样中所产生的内应力也可以引起这个内耗峰。实验指出:①这个脱溶内耗峰的出现条件与钢的蓝脆的出现条件相同,都是一种应变时效或淬火时效的现象;②与这个内耗峰有关的碳、氮是处于原子的状态。这些联系使我们认为钢的蓝脆是一种应变脱溶或淬火脱溶的过程,是由于原子状态的碳或氮聚集于钢中的内应力区域或晶体缺陷(例如原子脱节)中所引起来的。关于这方面的深入研究正在继续进行中。根据本实验及以前关于钢铁中碳氮的扩散、脱溶和沉淀所得的结果,对于钢的回火脆的机构也提出了一种初步的看法。回火脆是一种回火沉淀的过程,引起回火脆的沉淀似乎是氮化物而不是碳化物。     

Phase integral theory,coupled wave equations,and mode conversion

Phase integral or WKB theory is applied to multicomponent wave equations, i.e., wave equations in which the wave field is a vector, spinor, or tensor of some kind. Specific examples of physical interest often have special features that simplify their analysis, when compared with the general theory. The case of coupled channel equations in atomic or molecular scattering theory in the Born-Oppenheimer approximation is examined in this context. The problem of mode conversion, also called surface jumping or Landau-Zener-Stuckelberg transitions, is examined in the multidimensional case, and cast into normal form. The group theoretical principles of the normal form transformation are laid out, and shown to involve both the Lorentz group and the symplectic group.     

枸杞子四种原性状的FTIR光谱法鉴别

本文利用傅里叶变换红外（FTIR）光谱法无损快速鉴别了枸杞了的四种原性状。结果表明：不同颜色、不同味道、不同质地及不同形状的枸杞子的红外光谱谱图较为特征,各种枸杞子的红外官能团吸收均遵循一定的变化规律。该方法快速、简便、直观、样品不需要分离提取。     

Anisotropic charge transfer rates in the scattering of oriented atoms or molecules from surfaces

Atoms or molecules scattered from surfaces can undergo charge transfer to or from the surface during the scattering event. It is proposed that when the incident particle is “prepared” in an excited or ionic state which is oriented with respect to the surface, observable anisotropies in charge transfer rates involving the oriented state should occur. The effect is illustrated with two examples.     

激光传输模拟中基于衍射过程FFT算法的参量优化

为实现采用相位屏法正确模拟准直激光束经湍流大气的传输特性,根据湍流特性、抽样定理以及两屏间衍射过程FFT算法本身所决定的相邻两屏网格间距之间的关系,得出相位屏间距和网格间距确定网格数目最优取值的解析表达式,数值模拟高斯光束经自由空间和湍流大气传输的远场分布,网格数目取解析式求得的最优值848时,光束经自由空间传输的模拟结果与ABCD定律求得的解析结果完全一致;网格数目取836或860时,等效于光束通过偶数相位屏时被正透镜或负透镜阵列会聚或发散,削弱或夸大湍流大气对光束的影响,模拟结果出现严重偏差。     

Comparison of Pareto and tapered Pareto distributions for environmental phenomena

The Pareto distribution is often used to describe environmental phenomena such as the sizes of earthquakes or wildfires, or the interevent times or distances between such environmental disturbances. Because it is heavy-tailed, the Pareto distribution, or power-law distribution as it is occasionally called, suggests that a higher frequency of extremely large values occur compared to other, more familiar distributions such as the normal, exponential, or uniform. However, an alternative distribution called the tapered Pareto has been shown in some cases to fit as well or better to data than the Pareto distribution, and the tapered Pareto distribution is not heavy-tailed, suggesting a far lower frequency of extreme events. Even with rather large datasets, it is often quite difficult to distinguish which of these distributions is preferable, as they only differ markedly in the extreme upper tail where few, if any, observations are recorded. This article reviews the evidence and arguments related to these two competing distributions, especially in the context of earthquakes and wildfires.     

Electrical conducting properties of proton-conducting terbium-doped strontium cerate membrane

The electrical conduction behavior of SrCe_0.95Tb_0.05O_3−δ (SCTb) was investigated in different gases at high temperatures. In air, oxygen or nitrogen SCTb shows small electronic-hole conduction below 800°C and oxygen ionic conduction over 800°C with activation energy about 30 kJ/mol and 164–181 kJ/mol respectively. SCTb becomes a protonic conductor in hydrogen or methane in 500–900°C, with the proton conductivity in the range of 10⁻³–10⁻² S/cm, two to three orders of magnitude higher than electronic or oxygen ionic conductivity of SCTb in air or oxygen. The activation energy for protonic conduction in SCTb is 49 kJ/mol in methane and 54 kJ/mol in hydrogen. The electrical conductivity of SCTb in water vapor-saturated nitrogen, air or oxygen is higher than in corresponding gas without water vapor. Presence of water vapor does not affect the electrical conduction of SCTb in hydrogen or methane. Gas permeation measurements show that SCTb membrane is impermeable to hydrogen when the membrane is exposed to hydrogen or methane upstream and nitrogen or oxygen downstream. These results confirm that SCTb is a pure protonic conductor with very low electronic and oxygen ionic conductivity. SCTb will find applications as a high temperature electrolyte in fuel cells or hydrogen sensors.     

Effect of longitudinal or inplane deformation and inertia on the large amplitude flexural vibrations of slender beams and thin plates

Large amplitude flexural vibrations of slender beams, and thin circular and rectangular plates have been studied when a compatible longitudinal or inplane mode is coupled with the fundamental flexural mode. It is shown that the effect of longitudinal or inplane deformation and inertia is to reduce the non-linearity in the flexural frequency-amplitude relationship. Further, for slender beams and thin plates, the effect of longitudinal or inplane inertia is negligible.     

Analysis of sound propagation in a fluid through a screen of scatterers

A multiple scattering analysis in a nonviscous fluid is developed in detail in order to predict the coherent sound motion in the presence of disordered heterogeneities, such as particles, fibers, bubbles, or contrast agents. Scatterers can be homogeneous, layered, shell-like with encapsulated liquids or gas, nonabsorbing, or absorbing, and can take a wide variety of shapes. A priori imposed limitations or physical assumptions are absent in the derivation, whether they concern the expected response of the fluid-scatterer mixture, the scatterer size relative to wavelength, or the scatterer concentration or the screen thickness. However, as in any multiple scattering formulation, a closure assumption is invoked. Closed-form results for the backscattered and forward-scattered wave motions on either side of the screen of scatterers are obtained. The fluid-scatterer mixture is shown to behave as an effective dissipative medium from the standpoint of the coherent motion. It is found that the effective medium is fully described once two parameters are determined: the effective wave number and the reflection coefficient for the associated half-space screen. Remarkably, both parameters depend only on the far-field scattering properties of a single scatterer.     

部分相干平顶光束在增益或损耗介质中传输的M2因子

 根据光束在介质中传输强度二阶矩计算公式,推导出部分相干平顶光束在增益或损耗介质中传输的M2因子解析表达式,并将高斯 谢尔模型光束在增益或损耗介质中的传输作为特例统一于一般表达式中。研究结果表明：部分相干平顶光束在增益或损耗介质中传输时的M2因子与光束传输距离、光束相干长度、光束阶数及介质的特性有关。光束在增益或损耗介质中传输的这种特性为应用和控制光束传输提供了理论依据。     

Phase diagram of La2-xSrxCuO4 probed in the infared: imprints of charge stripe excitations

While there is increasing evidence for antiferromagnetic (AF) ordering in the Cu-O planes of high-T(c) superconductors, either static or fluctuating, there is no direct evidence so far for the charge stripes that should separate the AF domains. By investigating the optical response of La2-xSrxCuO4 for 0相似文献   

Correlation between the sub-structure parameters and the manufacturing technologies of metal threads in historical textiles using X-ray line profile analysis

Micro-structure can talk when documentation is missing. In ancient Roman or medieval periods, kings, queens, or just rich people decorated their clothes or even their horse covers richly with miniature jewels or metal threads. The origin or the fabrication techniques of these ancient threads is often unknown. Thirteen thread samples made of gold or gilt silver manufactured during the last sixteen hundred years are investigated for the micro-structure in terms of dislocation density, crystallite size, and planar defects. In a few cases, these features are compared with sub-structure of similar metallic threads prepared in modern, twentieth century workshops. The sub-structure is determined by X-ray line profile analysis, using high resolution diffractograms with negligible instrumental broadening. On the basis of the sub-structure parameters, we attempt to assess the metal-threads manufacturing procedures on samples stemming from the fourth century A.D. until now.     

Effect of variable gravitational field on thermal instability of a rotating fluid layer with magnetic field in porous medium

The effect of variable gravitational field on thermal instability of a rotating fluid layer in the presence of magnetic field in porous medium is investigated. It is found that the system is stable when gravity is decreasing upwards. The principle of exchange of stability is valid in the absence of rotation and magnetic field when gravity increases upwards. In the stationary convection, rotation has stabilizing or destabilizing effect depending upon whether gravity is increasing or decreasing upwards. The medium permeability and magnetic field have stabilizing or destabilizing effect depending upon condition.     

Fractals,clusters, and order-statistics: A new relation between probability and entropy

The relationship between fractals and clusters and order-statistics is developed. The initial distributions for order-statistics coincide with homogeneous power laws, used in fractal geometry and clustering to generate self-similar objects. The entropy reduction is defined in terms of the number of particles or events from the top or the bottom of the ordered set. Expressions for the joint and conditional distributions of order-statistics are given in terms of the entropy differences of the interval. Statistical equivalence principles are given in which the probability of the entropy reduction being less (or greater) than the index of the order-statistic is the same as the probability of that order-statistic being greater (or less) than a given value.