Thermal motion induced resonant forbidden reflections in wurtzite GaN

We report on an experimental study of forbidden reflections in GaN (wurtzite structure) by resonant X-ray scattering at the Gallium K edge. Resonant reflections are explained by the coherent sum of a Thermal Motion Induced (TMI) scattering amplitude and a temperature independent term. We show that the shape of the TMI energy spectrum is the same for a number of reflections that are exactly forbidden by spacegroup symmetry, as well as one that relies on approximate cancellation due to special atomic sites. In addition to demonstrating new selection rules, several non-trivial aspects of the theory of TMI scattering in wurtzite crystals are quantitatively verified, including dependence on temperature, energy, azimuthal angle and polarization. The temperature-dependent and temperature independent spectra of GaN are very similar to those found in ZnO, suggesting strong similarities in the anisotropy of their electronic states. This is also supported by the strong linear dichroism observed in GaN. TMI spectra are determined by the evolution of the electronic anisotropy with nuclear position, and are likely to be extremely valuable for developing theories of electronic properties at elevated temperatures.     

Ab initio calculations of the forbidden Bragg reflections energy spectra in wurtzites versus temperature

Thermal-motion induced (TMI) scattering is caused by the influence of atomic displacements on electronic states in crystals and strongly depends on temperature. It corresponds to dipole-dipole resonant x-ray scattering, but is usually accompanied by dipole-quadrupole scattering. The phenomenological theory supposes the dipole-quadrupole term to be temperature independent (TI). As a result, the transformation of the energy spectra with temperature observed experimentally in ZnO and GaN corresponds to the interference between the TMI and TI terms. In the present paper the direct confirmation of this theoretical prediction is given. Ab initio molecular dynamics was used to simulate the sets of atomic sites at various temperatures followed by quantum mechanical calculation of resonant Bragg reflection energy spectra. The results of simulation are in excellent coincidence with experimental energy spectra of forbidden reflections and confirm the earlier phenomenological conjecture about the interference between the TI dipole-quadrupole and TMI dipole-dipole contributions to the resonant atomic factor.     

Phase determination by wavelength-modulated diffraction. I. Centrosymmetric case

Wavelength-modulated diffraction was developed by Iwasaki, Yurugi & Yoshimura [Acta Cryst. (1999), A55, 864-870] as a method for phase determination, in which the intensity of Bragg reflections is recorded using radiation whose wavelength is changing continually over a range in the vicinity of the absorption edge of an atom in the crystal. Using a ferrocene derivative crystal (chemical formula C36H32O7Fe, space group P2(1)/a) with the Fe atoms chosen as anomalous scatterers, measurements were made of the intensity gradient dI/d lambda of the reflections with an imaging plate as a detector on a synchrotron radiation source at Ritsumeikan University. In the case of a centrosymmetric crystal, the phase of the structure factor could be derived by measuring only the sign of dI/d lambda at one wavelength in the range. Of 104 reflections measured, the correct phase was assigned to 101 reflections. A discussion is given on the errors involved and on the limits of application of the method.     

X-ray spectroscopy of thermally distorted electronic states in crystals

A new type of x-ray spectroscopy is proposed which can detect the thermal-motion-induced distortions of atomic electronic states in crystals. It is shown that those distortions can cause extra Bragg reflections (so-called forbidden reflections) and that their intensity should grow with increasing temperature. The reason is that the thermal displacements, which change the symmetry of atomic environment, can modify the tensor amplitude of x-ray resonant scattering. In the first approximation, the structure factor of extra reflections is proportional to the reflection vector H and to the mean-square thermal displacement for optical phonons. It is demonstrated that the forbidden resonant reflections, observed recently in Ge, could be caused by the thermal motion. Pis’ma Zh. éksp. Teor. Fiz. 69, No. 12, 885–889 (25 June 1999) Published in English in the original Russian journal. Edited by Steve Torstveit.     

空间辐射地面模拟装置(SESRI)同步加速器注入设计和模拟研究

国家"十二五"工程将在哈尔滨工业大学建造一台空间辐射地面模拟装置（SESRI）,该装置能够模拟空间辐照环境,对研究离子辐照材料、生命体等具有重要意义。SESRI主要由双ECR离子源,直线注入器,同步环和3个高能实验终端组成。周长为43.9 m的同步环作为装置的核心部分,能够向3个实验终端输送离子种类多、能量范围广的粒子束。为了保证环内有足够的粒子数,注入系统的设计至关重要,SESRI同步环采用多圈注入方案,并利用ACCSIM程序模拟粒子真实运动情况。结果表明,粒子的注入效率为85.5%,离子数增益可达17.1,可满足设计要求。Space Environment Simulation and Research Infrastructure (SESRI), which is one of the large-scale scientific projects proposed in National Twelfth Five-Year Plan of China, will be constructed in Harbin Institution of Technology. The SESRI project dedicated to space radiation consists of two ECR ion sources, a high intensity ion linac, a synchrotron and 3 research terminals. As the key part of the complex, a 43.9 m synchrotron can provide broadest energy range and variable ion beam for each terminal. To obtain required intensity in the synchrotron, the injection system is significant. A multi-turn injection scheme is adopted in the synchrotron and the injection process is simulated by ACCSIM with multi-particle tracking method. The results show that the injection efficiency and intensity gain factor can reach 85.5% and 17.1 respectively. The multi-turn injection system can meet the design requirement.     

On the theory of radiation of an infinitely long charged current-carrying thread in a uniaxial transparent crystal

The problem relating to the synchrotron radiation of a charged current-carrying thread in a uniaxial transparent crystal is considered here using the Sitenko-Kolomensky method. The formulas for the intensity of the synchrotron radiation and the limiting values of the formulas giving the intensity of the Cerenkov radiation of a thread moving uniformly and rectilinearly parallel to the optical axis of a crystal have been derived.     

On the use of relative line intensities of forbidden and allowed components of several HeI lines for electric field measurements

In this Letter we propose a method for electric field measurements based on intensity ratio of forbidden and allowed helium lines. HeI 402.6 nm, HeI 447.1 nm, and HeI 492.1 lines and their forbidden components are used for the measurement of electric field strength in the cathode region of the abnormal glow discharge in helium-hydrogen mixture. Electric filed strength was independently determined using polarization spectroscopy of Hβ line and then compared with electric filed strength obtained from relative intensity of HeI forbidden and allowed lines.     

Single crystal diffraction under high pressure with synchrotron radiation and a diamond anvil cell

Abstract

A single crystal study on AlPO₄ was performed at 2.90 GPa with synchrotron radiation with a wavelength of only 0.54 ?. The diffracted intensity was high enough to measure even weak reflections with sufficient counting statistics. However, the search for the reflections needed to setup the orientation matrix required a lot of beamtime. A feasibility study was carried out using a proportional area counter to reduce this search time. The results demonstrate that such counters can considerably reduce the time needed for the orientation of the crystal and the data collection.

Presented at the IUCr Workshop on ‘Synchrotron Radiation Instrumentation for High Pressure Crystallography’. Daresbury Laboratory 20-21 July 1991     

Coupling between spin and orbital degrees of freedom in KCuF3

We present the results of resonant x-ray scattering experiments on KCuF3. Structurally forbidden reflections, corresponding to magnetic and 3d-orbital long-range order, have been observed. Integrated intensities have been measured as a function of incident energy, polarization, azimuthal angle, and temperature. The results give evidence for a strong coupling between orbital and spin degrees of freedom. The interplay between magnetic and orbital order parameters is revealed by the temperature dependence of the intensity of orbital Bragg peaks.     

Characterization of wax as a potential diffraction intensity standard for macromolecular crystallography beamlines

A number of commercially available waxes in the form of thin disc samples have been investigated as possible diffraction intensity standards for macromolecular crystallography synchrotron beamlines. Synchrotron X‐ray powder diffraction measurements show that beeswax offers the best performance of these waxes owing to its polycrystallinity. Crystallographic lattice parameters and diffraction intensities were examined between 281 and 309 K, and show stable and predictable thermal behaviour. Using an X‐ray beam of known incident flux at λ = 1 Å, the diffraction power of two strong Bragg reflections for beeswax were quantified as a function of sample thickness and normalized to 10¹⁰ photons s^?1. To demonstrate its feasibility as a diffraction intensity standard, test measurements were then performed on a new third‐generation macromolecular crystallography synchrotron beamline.     

基于柯西分布的视频图像序列背景建模和运动目标检测

提出了一种用于视觉监视系统的基于柯西分布的发光模型的光照不变变化检测方法.假定视频图像序列中每个背景图像像素点灰度观测值的时序变化由白噪声引起,利用建立的初始化背景高斯统计模型对每帧图像进行归一化,得到了背景像灰度比值的分布符合标准柯西分布的结论,解决了柯西分布的模型参量估计问题.在变化检测的基础上,YCbCr颜色空间的亮度、色调和饱和度被用来识别和消除由阴影和反光等引起的变化区域.结果表明,提出的背景建模方法对场景中各种光线变化、小的背景扰动等噪声具有稳健性,可以较为可靠地检测前景目标,识别和去除阴影和反光.     

Topics in Coherent Synchrotron Radiation: Experimental Consequences of Radiation Impedance

Coherent synchrotron radiation (CSR) is now an established source of radiation in the terahertz region, with flux capabilities many orders of magnitude higher than conventional SR sources. Accordingly, CSR is playing an ever-increasing role in the field of Far-IR spectroscopy at synchrotron facilities around the world. However, with the improvement in spectroscopic resolution, the incident CSR spectra from several facilities are displaying intensity profiles that appear to be modulated by periodic peaks or deep indentations, while theory predicts a featureless distribution. After eliminating the usual suspects, like multiple reflections from windows, etc., one looks for a deeper explanation, such as the role played by the radiation impedance. This impedance is largely defined by the vacuum chamber in the region of the bend magnets, but probably with some effects from the straight sections as well.     

Interaction between solid nickel and a tin–indium eutectic melt,and the nature of eutectic alloys

Structural transformations during reactions between nickel and an indium–tin eutectic melt are studied via the diffraction of high-energy synchrotron radiation. It is shown that InNi intermetallic compound is the first to form after melting. No phases belonging to a Ni–Sn system are observed. A hypothesis concerning the nanocluster structure of the eutectic melt is proposed: Structural formations with forbidden five-fold, seven-fold, and higher-order rotational axes of symmetry lie at the heart of nanosized clusters.     

Double diffraction spots in RHEED patterns from clean Ge(111) and Si(001) surfaces

In RHEED patterns from clean Ge(111) and Si(001) surfaces, extra diffraction spots have been observed with superlattice reflection spots due to Ge(111) 2 × 8 and Si(001) 2 × 1 surface structures. The extra spots have not been found out in many previous LEED and RHEED patterns of clean Ge(111) and Si(001) surfaces. When the Ge(111) and Si(001) samples were rotated about an axis normal to the surfaces so as to vary the incident direction of the primary electron beam, the intensity of the extra spots showed a remarkable dependence upon the incident direction and they became invisible in some incident directions, in spite of the experimental condition that an Ewald sphere intersected reciprocal lattice rods of the extra spots. In this study, the extra spots are understood as forbidden reflection spots resulting from double diffraction of superlattice reflections of the surface structures, and the remarkable dependence of their intensity upon the incident direction is explained in terms of excitation of the surface wave of the superlattice reflections. These results suggest that the intensity of diffraction spots in RHEED patterns may be greatly influenced by the surface wave excitation of fundamental and superlattice reflections.     

X-ray double phase retarders to compensate for off-axis aberration

An X-ray double phase retarder system composed of two transmission-type phase retarders is proposed and developed in order to compensate for off-axis aberration (phase-shift inhomogeneity due to angular divergence of incident X-rays). The scattering planes of the two phase retarders are set to be inclined by 45 degrees with respect to the plane of incident polarization, but the two phase retarders give Bragg reflections in opposite directions. By using this X-ray optical system, vertically polarized X-rays with a 0.99 degree of linear polarization were obtained from horizontally polarized synchrotron radiation with a horizontal beam divergence of 20 arcsec (0.1 mrad). This value is favorably compared with the value of 0.87 which was obtained using a conventional single phase retarder of identical total thickness, 627 microns. The comparison was made at the nickel K-absorption edge (8333 eV) with the condition that 47% of incident X-rays were transmitted through the two phase retarder crystals. The crystals were (100)-oriented diamond plates giving asymmetric 111 Laue reflections.     

About forbidden and weak reflections

Reflections forbidden under the single scattering approximation are not expected to remain extinct for crystal thicknesses for which multiple scattering becomes important, except for reflections of the Gj?nnes-Moodie type. However, it has been observed that in many crystals with the incident beam along a zone axis, such as diamond-like crystals along the [110] zone, reflections forbidden under the single scattering approximation remain very weak up to large thicknesses. This is hard to explain in terms of many-beam dynamical scattering in Fourier space. The picture becomes clear if one describes the scattering in real space in terms of the channelling of the electrons along the atom columns parallel to the zone axis. In that case the exit wave of each atom column can be described by the S-state model, which is radially symmetric around the centre of the atom column. As a consequence the exit wave shows the same symmetry as the projected potential, so that the reflections forbidden under the single scattering approximation remain extinct. This condition only breaks down when the crystal thickness becomes so large that the S-state model becomes invalid, which is a function of the distance between neighbouring atom columns and/or the tilt from the exact zone axis. The sensitivity for small tilts is also in agreement with very old observations that have not been explained thus far.     

Taking into account channels of focused intensity losses for refractive optics elements

Methods are proposed for taking into account intensity losses due to random deviations of composition profiles and due to small-angle scattering effects in amorphous materials and wide-angle scattering effects in polycrystalline materials. Experimental data are presented on the study of polycrystalline-diamond lenses using a synchrotron radiation beam.     

Coplanar monolithic resonator for X-ray and synchrotron radiation

A scheme of a three-block monolithic X-ray resonator with Si \((04\bar 4,40\bar 4)\)-configuration single crystal for NiKα₂ and synchrotron radiation is proposed and realized experimentally. Specialties and advantages of the proposed scheme are given. It is shown that using asymmetric Bragg reflections makes possible to solve one of the basic problems of X-ray resonators, that of the input of radiation into the resonator and creation of the lossless cycle. The technology of fabrication of the resonator is described and the problems of raising the efficiency of its operation are considered.     

Crystal structure of NaNO3 at high pressure

Abstract

Effect of high pressure on the crystal structure of rhombohedral NaNO₃ was investigated by X-ray diffraction of single crystals mounted in a miniature diamond-anvil cell on synchrotron radiation source. Diffraction intensity measurements were made at three pressures across a suggested transition pressure 4.3 GPa. No change was observed in an overall distribution of reflections in the reciprocal space with increasing pressure, but there was a systematic variation in diffraction intensity for particular groups of reflections. An analysis based on the structure factor calculation showed that a structure change induced by pressure is mainly a rotation of the nitrate groups in the alternate layers along the threefold axis in opposite directions. Least-squares refinement of the atomic positional parameters yielded the angle of the rotation to be 4.3 and 7.0 deg at pressures of 4.4 and 5.0 GPa, respectively. It has also been shown that the positions of the sodium and nitrogen atoms are slightly displaced along the axis, resulting in the formation of dipoles in the high pressure phase.     

一种简便的研究一维光子晶体禁带特征的新方法

利用光在一维光子晶体中传播的色散关系,提出了一种禁带边缘分析法,以研究一维光子晶体禁带特征的方法.该方法利用色散关系引入一个确定禁带边缘位置的函数F,F =0为禁带边缘的位置.通过绘出函数F的图线,由F =0确定禁带边缘的位置,进而分析得到禁带和允许带的特征.它避免了常用的特征矩阵法中对反射率复杂的数值计算,并能得到与特征矩阵法相同的结果.