Helical signature motif in the fibre diffraction patterns of random-walk chains

Helical geometry is one of the most dominant structural features in polypeptides, nucleic acids, carbohydrates and long-chain polymers in general. Structures of a large number of helical biomolecules and important non-biological polymers have been determined by fibre diffraction and helical diffraction theory. This paper reports on a study of fibre diffraction patterns calculated for two different on-lattice random-walk models of polymers, with no built-in helical features. It is noted that such on-lattice random walks are natural models for polymers with fixed monomer geometries and inter-monomer angles. The presence of layer-line intensities is observed, characteristic of fibre diffraction patterns from helices with an integral number of units per turn. It is shown that under certain circumstances, fibre diffraction patterns of helical objects may be difficult to distinguish from cylindrically-averaged fibre diffraction patterns of random walks on lattices with fixed angles. A simple correspondence is demonstrated between the parameters of a helix and a random-walk chain with equivalent fibre diffraction patterns. These results call for a critical examination of the way the helical diffraction theory is typically used: certain structures that have been modelled as helical might, under some circumstances, be more naturally described as random-walk chains with no preferred conformation even on the shortest length-scale and in the context of a fibre.     

Young's fringes analysis with a computer image-processing system

A two-dimensional Fourier transform analysis is presented for numerically processing the Young's fringes diffraction pattern from a double- exposure speckle photograph. The fringe spacing and orientation are determined using only one Young's fringes pattern without any other diffraction halo patterns. This algorithm is based on the 2-D FFT of the fringe pattern with 256 × 256 pixels. Therefore, it enables automatic analysis to be performed on specklegram with noisy and poor fringe patterns. The measurement of deformation by the laser speckle method can be carried out with high accuracy, reliability and speed.     

Revised interpretations of the available x-ray data for B-DNA

Comparisons are made between calculations based on the double-helix and side-by-side models of DNA and data taken from the available x-ray diffraction measurements of the B form of DNA. The SBS model invoked here is an improvement on the one which was published earlier. In our estimation the fibre diffraction patterns do not significantly favour either model. However, the DH model is incompatible with the on-axis form of the cylindrically-averaged Patterson function, whereas the SBS model is compatible (this result is relatively insensitive to appreciable modifications to the SBS structure)—the crucial question is whether the quality of the data is sufficient to make this finding significant. It is shown that the maximum expected effect on the fibre pattern, of exchanging A-T base pairs for G-C base pairs, is less than the probable experimental error.     

Relativistic perihelion precession of orbits of Venus and the Earth

Among all the theories proposed to explain the “anomalous” perihelion precession of Mercury’s orbit first announced in 1859 by Le Verrier, the general theory of relativity proposed by Einstein in November 1915 alone could calculate Mercury’s “anomalous” precession with the precision demanded by observational accuracy. Since Mercury’s precession was a directly derived result of the full general theory, it was viewed by Einstein as the most critical test of general relativity from amongst the three tests he proposed. With the advent of the space age, the level of observational accuracy has improved further and it is now possible to detect this precession for other planetary orbits of the solar system — viz., Venus and the Earth. This conclusively proved that the phenomenon of “anomalous” perihelion precession of planetary orbits is a relativistic effect. Our previous papers presented the mathematical model and the computed value of the relativistic perihelion precession of Mercury’s orbit using an alternate relativistic gravitational model, which is a remodeled form of Einstein’s relativity theories, and which retained only experimentally proven principles. In addition this model has the benefit of data from almost a century of relativity experimentation, including those that have become possible with the advent of the space age. Using this model, we present in this paper the computed values of the relativistic precession of Venus and the Earth, which compare well with the predictions of general relativity and are also in agreement with the observed values within the range of uncertainty.      

Mapping fibre orientation in complex-shaped biological systems with micrometre resolution by scanning X-ray microdiffraction

A fully automated procedure to extract and to image local fibre orientation in biological tissues from scanning X-ray diffraction is presented. The preferred chitin fibre orientation in the flow sensing system of crickets is determined with high spatial resolution by applying synchrotron radiation based X-ray microbeam diffraction in conjunction with advanced sample sectioning using a UV micro-laser. The data analysis is based on an automated detection of azimuthal diffraction maxima after 2D convolution filtering (smoothing) of the 2D diffraction patterns. Under the assumption of crystallographic fibre symmetry around the morphological fibre axis, the evaluation method allows mapping the three-dimensional orientation of the fibre axes in space. The resulting two-dimensional maps of the local fibre orientations - together with the complex shape of the flow sensing system - may be useful for a better understanding of the mechanical optimization of such tissues.     

静电场对α-LiIO3单晶透射光的影响

沿α-LiIO₃单晶z轴向施加静电场。当沿z轴方向通光时,几乎观察不出晶体透射像的变化;当偏离z轴方向通光时,透射像上显现出一些花纹。本文还作了空间频谱分析,显微观察和光衍射。 关键词：     

Model analysis of supermode generation in active 5-core optical fibre

In the paper, the phase-locked emitters in multi-core optical fibres for high power fibre lasers are presented. The influence of the normalized frequency and diameters of the cores on the shape of the pattern in the Fraunhofer diffraction region has been analysed. The simulation of coupling coefficient between cores influences on phase-locking and in the consequence on the far-field pattern of the fibre laser, based on the analyzed multi-core optical fibre, were performed. In the analyzed fibre laser, while exchanging power of the generated radiation between two adjacent cores on the fibre length (L = 5 m) within the range of 4–10%, the radiation becomes phased. Having satisfied this condition, in the far-field low-divergence, high-power laser beam (supermode) is attained. 5-core double clad optical fibre doped with neodymium ions was fabricated. Luminescence spectra of the manufactured fibre were measured.     

The generation of intense, transform-limited laser pulses with tunable duration from 6 to 30 fs in a differentially pumped hollow fibre

We have investigated experimentally the energy transmission and spectral broadening of 30-fs, 700-μJ laser pulses in a neon-filled, 250-μm inner diameter hollow fibre. We implement a differentially pumped fibre, where a vacuum is maintained at the fibre entrance, and compare this to a statically filled fibre. We obtain significantly higher transmission and increased spectral broadening in the differentially pumped case due to a reduction of ionisation defocusing at the fibre entrance. This arrangement provides a method for the generation of near-transform- limited pulses with smoothly varying pulse duration whilst maintaining constant pulse energy, by simple adjustment of the gas pressure. Compression of ∼450-μJ pulses from the differentially pumped fibre to a duration of 6.5 fs has been achieved for pulses with spectra spanning 650–900 nm, by use of negatively dispersive chirped mirrors. PACS 42.65.Re; 42.65.Jx; 42.65.-k     

The Al-Pd-Mn quasicrystalline approximant -phase revisited

A detailed investigation of the Fourier space of several Al-Pd-Mn samples with composition Al-72.6 at. %, Pd-22.9 at. %, Mn-4.5 at. % is reported. In the phase diagram of the Al-Pd-Mn ternary alloy, this composition corresponds to the so-called ξ' phase which was described as an icosahedral quasicrystalline approximant. By re-examining the Fourier space by means of X-ray diffraction (powder patterns and single crystal precession patterns), complex structures in close relation with the ξ'-phase have been observed. These long-range order complex structures are described as resulting from a periodic perturbation of the ξ' structure along the c direction. Two states with periodicities c (3 + τ) and c (5 + τ) have been observed in this study (τ: golden mean). Structural models based on periodic arrangements of “defects” layers separating layers of phase are proposed. These two states are certainly intermediate states between the phase and the metastable decagonal quasicrystalline phase. Received 11 April 2002 / Received in final form 24 June 2002 Published online 17 September 2002     

Study of condensed nitrogen by μSR method

The temperature dependences of parameters of the muon spin relaxation in liquid and crystalline nitrogen have been studied. It has been established that in condensed nitrogen there takes place a fast depolarization of muons. An anomalous behaviour of the amplitude and phase of muon precession is found in the vicinity of the orientation phase transition in solid nitrogen. It has been shown that muon spin relaxation parameters in nitrogen do not change at reduction of the oxygen impurity content from 0.7·10⁻⁴ to 10⁻⁶. The fast depolarization of muons in condensed nitrogen is apparently due to the formation of muonium atoms. To explain the phenomena observed, a model of the muonium chemical reaction is proposed. The initial phase of the muon precession has been measured as a function of the perpendicular magnetic field to determine the state of short-lived muonium in nitrogen. It has been determined that muonium in nitrogen is in an excited state. Consideration of the nuclear hyperfine interaction of muonium in condensed nitrogen makes it possible to give a qualitative explanation for the temperature dependence of the initial amplitude of the muon precession.     

数显德拜相测量仪的研制

为测量X射线粉末衍射照片德拜弧,进而计算布拉格角,制作了一种数字显示测量仪器。该仪器用数字读出方式显示测量结果,适用于近代物理X光实验教学。     

Enhancement of measurement sensitivity in determination of changes in the shape of a wavefront by holographic lateral-shearing interferometry

A method for increasing measurement sensitivity in the visualization of dynamics of changes in wavefront shape with the use of holographic lateral-shearing interferometry is considered. At the first stage of this method, a photograph of a reference shear interferogram obtained with tuning to closely spaced fringes is recorded under linear conditions. After chemical treatment, the photograph of the reference shear interferogram is installed in exactly its initial position. At the second stage, a series of photographs of lateral-shear interferograms of the wavefront under study is recorded under nonlinear conditions using the photograph of the reference interferogram. The photographs of the lateral-shear interferograms are optically processed by two coherent beams with selection of the highest diffraction orders, which makes it possible to visualize the dynamic component of the wavefront deformation with an increase in the measurement sensitivity (the static component is excluded in this case). The effect of the degree of nonlinearity of photograph recording on the range in which the measurement sensitivity can be controlled during the optical processing of a pair of matched photographs or one double-exposure photograph of interferograms recorded at different values of a lateral shift is analyzed.     

On the possibility of recording the sagnac effect using π-mesons

We propose to measure the Sagnac effect for counterpropagating de Broglie waves of π⁺ or⁻ mesons whose spin is equal to zero. The latter circumstance allows us to avoid the nonreciprocal phase shift of counterpropagating waves, which is related to both the Thomas precession (the effect of the special relativity theory) and the Larmor precession of the spin of elementary particles due to the presence of magnetic fields. Among the advantages of using π mesons, we should mention their electric charge, which allows us to use the well-known devices for focusing, separation, and joining the particle beam, and their comparatively long lifetime. A modulation method for processing the interference signal is proposed. The maximum sensitivity of the above method, which is limited by the shot noise, is estimated. Institute of Applied Physics of the Russian Academy of Sciences, Nizhny Novgorod, Russia. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 41, No. 6, pp. 767–774, June, 1998.     

金属有机物化学气相沉积生长的$lt;i$gt;a$lt;/i$gt;(1120)面GaN三角坑缺陷的消除研究

用金属有机物化学气相沉积方法在r面蓝宝石上生长了非极性a面GaN薄膜,通过采用AlGaN多量子阱插入层,得到了高质量的非极性GaN材料. 用原子力显微镜和高分辨X射线衍射仪研究了a面GaN的表面形貌和结晶质量,发现非极性材料上典型的三角坑缺陷被消除,（1120）面X射线双晶摇摆曲线的半峰宽为680″. 关键词： GaN 原子力显微镜 高分辨X射线衍射仪 非极性     

Revealing macrodefects in periodic structures of the transmission type in white light on the basis of shift of images

The possibility of revealing macrodefects in periodic structures of the transmission type in white light on the basis of shift of images of the structures under investigation is shown. A simple scheme of visualization of macrodefects on the basis of obtaining shifted images of structures by using a plane mirror placed behind the periodic structure is described. A method for revealing macrodefects in periodic structures with the possibility of increasing the sensitivity of measurements is proposed. The method is based on making a photograph of a periodic structure with subsequent optical processing of this structure and its photograph, which are optically conjugate and shifted with respect to each other. An increase in the sensitivity of this method is achieved owing to separation of complex conjugate orders of diffraction of light by the structure and its photograph. The methods proposed do not require using reference periodic structures. Experimental results of testing of the methods for visualization of macrodefects in cross-shaped metal grids are presented.     

Average unit cell approach to diffraction analysis of some aperiodic structures — decorated fibonacci chain

The average unit cell approach has been successfully used to construct the probability distributions of atomic distances from the reference lattice points and to calculate the diffraction patterns for decorated Fibonacci chains with non-Fibonacci type of decoration. General method able to distinguish between various quasilattice decorations has been elaborated and tested for decorated Fibonacci chains. Financial support from the Polish Committee for Sci. Research under grant No. 2 P03B 041 16 is acknowledged. Presented at the DI-CRM Workshop held in Prague, 18–21 June 2000.     

Structure of boundary diffraction wave revisited

The physically appealing boundary diffraction wave theory which suggests that diffraction patterns arise due to interference of an undisturbed (geometrical) wave and the boundary diffraction wave generated by edge of the diffracting aperture, simplifies the solution of diffraction problems by reducing the Fresnel–Kirchhoff surface integral into a line integral over the illuminated boundary of the diffracting aperture. The present work reports experimental investigations carried out on the structure of the boundary diffraction wave. It has been shown that the boundary diffraction wave is continuous behind the diffracting aperture and apparently there does not exist any discontinuity at the geometrically light to shadow transition boundary, as is required by the theory. PACS 42.25.-p; 42.25.Fx; 42.25.Hz     

Structural properties of vapor-grown C60 crystals

Vapor grown crystals of C₆₀ with thin flat triangular, rhombic or trapezoidal shapes of size to 1×2×0.001 mm as well as prismastic crystals typically 0.5×0.4×0.2 mm have been grown using a high temperature vapor transport method. Room temperature X-ray precession photography shows these crystals exhibit diffraction patterns consistent with those for either (a) single or (b) twinned crystals of the previously reported face-centered cubic structure or (c) a more complex cell of hexagonal symmetry with a=10.010(2) Å and c=49.064(11) Å. This latter from actually is a multiple twin containing both the face-centered cubic and the close-packed hexagonal structure types. The sharp diffraction maxima for either the single or the twinned crystals demonstrate that they consist of large coherent domains and are essentially free of planar defects parallel to the twin planes.     

μ− SR in semiconductorsSR in semiconductors

μ⁻ SR experiments have been performed on Si between room temperature and 6 K. The amplitude of the muon spin precession signal in an applied magnetic field of 0.04 T decreased below 30 K. A zero-field measurement at 6 K revealed a μ⁻ spin precession frequency of 650 MHz. The muonic atom represents an aluminium acceptor in the silicon matrix, its electronic state is responsible for the μSR signal. A possible influence of the γ recoil produced by the X-ray cascade is discussed.     

Demonstration of fundamental mode only propagation in highly multimode fibre for high power EDFAs

The use of short lengths of large core phosphate glass fibre, doped with high concentrations of Er or Er:Yb represents an attractive route to achieving high power erbium doped fibre amplifiers (EDFAs) and lasers (EDFLs). With the aim of investigating the potential of achieving diffraction limited output from such large core fibres, we present experimental results of fundamental mode propagation through a 20 cm length of passive 300 μm core multimode fibre when the input is a well-aligned Gaussian beam. Through careful control of fibre geometry, input beam parameters and alignment, we measured an output M² of 1.1 ± 0.05. The fibre had a numerical aperture of 0.389, implying a V number of 236.8. To our knowledge, this is the largest core fibre through which diffraction limited fundamental mode propagation has been demonstrated. Although the results presented here relate to undoped fibre, they do provide the practical basis for a new generation of EDFAs and EDFLs.