Design of a pulsed spatial neutron magnetic spin resonator

It is well known that upon passage through a spatially alternating transverse magnetic field, produced by a meander-shaped conducting foil, in its rest frame each neutron creates its individual frequency which depends on its velocity and the period of the meander. A resonant spin flip process takes place, if this frequency equals the Larmor frequency determined by a homogeneous vertical field. Clearly, this effect can be used to monochromatise a polarised neutron beam. Here we propose a novel design of such a magnetic resonator consisting of a sequence of separate compact modules, which provide high homogeneity of the transversal field oscillations and allow rapid beam chopping since they meet the specifications of fast electronic switching. The wavelength resolution of this resonator device can be changed in an instant and likewise an arbitrary amplitude modulation of the transversal field can be established, which is required for an efficient suppression of subsidiary maxima of the wavelength-dependent spin flip probability.     

Resonant excitation of intense acoustic waves in crystals

The resonant excitation of an intense elastic wave through nonspecular reflection of a special pump wave in a crystal is described. The choice of the plane and angle of incidence is dictated by the requirement that the excited reflected wave be close to the bulk eigenmode with its energy flow along a free boundary. The resonance parameters have been found for a medium with an arbitrary anisotropy. General relations are concretized for monoclinic, rhombic, and hexagonal systems. A criterion is formulated for an optimal selection of crystals in which the resonant reflection is close to the conversion one, when almost all of the energy from the incident beam of the pump wave falls into the near-surface narrow high-intensity reflected beam. Estimates and illustrations are given for such crystals as an example. The intensity of the reflected beam increases with its narrowing, but its diffraction divergence also increases with this narrowing. Nevertheless, the intensity of the beam can be increased by a factor of 5–10 at sufficiently high frequencies while keeping its divergence at an acceptable level. Amplification by two orders of magnitude can be achieved by compressing the beam in two dimensions through its double reflection.     

Fast Computation of Fresnel Holograms Employing Difference

We propose an approximation method that can calculate the Fresnel hologram 16 times faster than the conventional method. To compute the hologram, an object is assumed to be a collection of self-illuminated points and the fringes from each object point are superposed. The distance between object point and sampling point on the hologram is used to obtain the phase of the light. Since a sampled hologram usually has small pixel intervals, the difference of the distance values between adjacent pixels is also small and its n-th order difference can be assumed to be constant. Therefore, the distance value at a certain pixel can be obtained from its neighbor with simple additions. The distance error can be reduced less that one wavelength with practical parameters. A hologram, which has a horizontal parallax only, 1.3 Mega-pixels and 1,000 object points, can be calculated in less than one second with a personal computer.     

The structure of a strange attractor

A few examples are given of an iterative map of the horseshoe type for which everything can be expressed in elementary functions. The strange attractor can be an analytic curve or a set of parallel lines with a Cantor set as its cross-section.     

一般多值相位光栅菲涅耳衍射光场的分析

基于光栅结构与距离的关系,给出了一种求解一般多值相位光栅菲涅耳衍射光场的解析表达式利用该表达式可以直接分析一般多值相位光栅的分数泰伯数效应及在阵列照明中的应用,克服了间接分析法只能分析某些类型的泰伯阵列照明器的不足之处.     

日间恒星实时探测的视频图像处理

为了解决船载光学测量设备日间测星数量偏少的问题,研究了暗小目标电视探测技术,以保证惯导系统全天时不间断航向校准。在视频图像处理中,采用中值滤波方法提高信噪比,同时采用基于模糊理论的最大模糊熵阈值分割方法自行构建隶属度函数,自适应求取阈值,实现了星体目标检测与识别。实验结果表明：在原光学设备光学系统不变的情况下,日间可探测的恒星由大于等于3.0星等、小于等于5颗,提高到了大于等于4.5星等,大于等于250颗,满足了惯导系统全天时不间断航向校准的要求。该成果可以推广应用于电视弱小目标探测领域。     

积与积减原理

文章分析了重力势能、引力势能、电荷势能、化学势能、热量势能、质量积、动量积等多种势能,发现它们均可表达为一种守恒广延量和对应的强度量的乘积,因此可将其统一定义为"积".基于积这一概念,文章得到了孤立系统内守恒广延量传递过程的积减原理,即孤立系统内进行的守恒广延量传递过程中系统的积总是减小的.进一步,文章还基于积的概念发展了孤立系统和封闭系统的势平衡判据,发现孤立系统达到势平衡状态时,系统的积达到最小值(最小积原理);当封闭系统达到势平衡状态时,系统的准自由积达到最小值(最小准自由积原理).上述结论应用于传热学中即可得到热量传递过程的(火积)减原理及相应的热平衡判据.与热力学中的核心概念熵相对应,由于物理量(火积)可以描述传热过程的不可逆性,作为传热过程的优化准则,度量系统的无序度,并给出系统的热平衡判据,因此(火积)是传热学中的核心概念. 关键词： 势能 积 积减原理 平衡判据     

Information encryption through dyadic permutations

Dyadic displacements of an image can be interpreted as a special type of permutations of its pixel addresses. Owing to this property, they can be used to encrypt the information contained in the pictures without information loss or damage. The information recovery can be performed successfully by using dyadic correlations.     

Exactly solvable one-dimensional inhomogeneous models

We present a simple way of constructing one-dimensional inhomogeneous models (random or quasiperiodic) which can be solved exactly. We treat the example of an Ising chain in a varying magnetic field, but our procedure can easily be extended to other one-dimensional inhomogeneous models. For all the models we can construct, the free energy and its derivatives with respect to temperature can be computed exactly at one particular temperature.     

Detailed nuclear structure studies far from stability

State-of-the-art spectroscopy of nuclei far from stability has achieved an extraordinary level of sophistication and detail in the last ten years. In principle, if a state can be populated, it can be characterized by its energy, spin, parity, and major decay paths. Sometimes its lifetime can be measured. In practice, one is confronted with enormous complexity. To convert raw spectroscopic data into nuclear structure data involves a complex process of disentangling gamma rays and conversion electrons into decay schemes. Specifically, coincidence techniques, especially coincidence intensities, play a crucial role in this process. Recent examples and methods from work done at UNISOR are presented.     

Quantitative measurements in thermo-fluid dynamics based on colour processing

Colour can be expressed as a weighted combination of three attributes: hue, intensity, and saturation. Non-coherent light reflected by thermo-sensitive liquid crystals holds a variable hue, moving in a generally narrow temperature interval and also depending on its inclination with respect to the plane of the crystals and on the characteristics of the impinging light. In experimental practice it is not feasible to ensure uniform lighting over an extensive area and its entire view under the same angle. Thus, the acquired hue field is non-uniform even if the liquid crystal sheet is isothermal. However, by means of proper filtering and calibration of the colour attribute, this optical technique, besides being non-intrusive and inexpensive, is capable of mapping the temperature with an accuracy better than 5% of its measuring-range amplitude. A similar method can be applied for measuring the thickness of a thin liquid film. In this case, the colour attribute to be processed is its intensity. In fact, the light transmitted through a dyed liquid decreases with an increasing thickness of the layer. Again, a perfectly uniform light source is unattainable and the recorded intensity field is non-homogeneous even if the liquid free surface is flat. Nevertheless, the film thickness can be determined by this colour-processing procedure with an accuracy better than 8% of the measuring-range amplitude, which is dictated by the utilised dyestuff concentration. Further thermo-fluid dynamic measurements performed over extensive areas could be handled with analogous methodologies. Surface temperature by emitted infrared waves and void fraction in ducts by light absorption are particular examples.     

Dynamic vortex-antivortex interaction in a single cross-tie wall

A fascinating property of micromagnetism comes from the possibility to control the domain and vortex configuration through the sample shape and size. For instance, in a rectangular platelet a configuration containing a stable combination of vortices and an antivortex can be created. Such a single cross-tie wall can be understood as being a coupled micromagnetic system with three static solitons. Here we report on its magnetization dynamics including the vortex-antivortex interactions. The spectrum of eigenmodes is investigated as well as the effect of different vortex core orientations. We show that the vortex dynamics can be used to identify the core configuration, which is not directly accessible to x-ray microscopy because of its limited spatial resolution.     

The theory of liquids at equilibrium

No rigorous calculation of the properties of a liquid has yet been made from a knowledge of the forces between its molecules. This review describes the method by which most solutions have been sought and the progress that has been made in the last five years.

The equilibrium properties of a simple liquid can be expressed in terms of the pair distribution function g(r), which measures the probability of finding a second molecule at distance r from a first. Kirkwood, and Born and Green showed that an approximate calculation of g(r) can be made by solving an integral equation. This equation, its solutions and their imperfections are described briefly.

More recently it has been found convenient to write the correlation function, h(r)=g(r) -1, as the sum of two terms, a direct term, c(r), and an indirect term. Four recent approximations can be described and compared by the form they assume for the direct correlation function, c(r). The most promising of these is the approximation of Percus and Yevick and its quantitative predictions are discussed in some detail.     

Light-induced atomic elevator in optical lattices

It is shown how an atomic elevator that can elevate falling cold atoms in a vertical optical lattice can be created. The effect appears near resonance owing to the nonlinear interaction between the electronic and mechanical degrees of freedom of an atom, which is responsible for its random walk in rigid optical lattices without any modulation and additional action. Numerical experiments involving spontaneous emission demonstrate that random walk of atoms and light-induced atomic elevator can be observed in a real experiment.     

Numerical realization of coaxial electric arcs

Basing on computational results a possibility is proposed for realizing an arc discharge at coaxial cathodes. Such a discharge can be used at a relatively high current, which will enable an increase in its thermal effect on the article to be processed.     

Anomalous magnetoelectric and thermoelectric properties of phase-separated materials of La---Mn---O type

A theory describing the mixed electronic-impurity separation into high-conduction and almost-insulating phases in giant magnetoresistance materials and HTSC is given. If the high-conduction phase is multiple-connected, and, hence, the sample as a whole is insulating, it can be converted into a high-conduction one by an external magnetic field or an increase in temperature. The total thermopower can be determined by the thermopower of the almost-insulating phase and for this reason can differ in its sign from that for the uniform state.     

Dominoes in carbon nanotubes

We demonstrate by molecular dynamics simulations that the domino process can be developed in single-walled carbon nanotubes (SWCNTs). Once a section of a SWCNT with an appropriate diameter (>3.5 nm) is collapsed, the successive collapse of the neighboring portions can generate a domino wave along the longitudinal direction of the tube. The wave is driven by van der Waals potential energy and its natural speed can be up to 1 km/s. Molecules inside the SWCNT can be accelerated by the domino wave and finally shot out. The finding shows for the first time that a SWCNT can be an energy supplier, which provides opportunities for designing new concept (domino-driven) nanoelectromechanical system devices.     

Remarks on the entropy of 3-manifolds

We give a simple combinatoric proof of an exponential upper bound on the number of distinct 3-manifolds that can be constructed by successively identifying nearest neighbour pairs of triangles in the boundary of a simplicial 3-ball and show that all closed simplicial manifolds that can be constructed in this manner are homeomorphic to S3. We discuss the problem of proving that all 3-dimensional simplicial spheres can be obtained by this construction and give an example of a simplicial 3-ball whose boundary triangles can be identified pairwise such that no triangle is identified with any of its neighbours and the resulting 3-dimensional simplicial complex is a simply connected 3-manifold.