C轴LiNbO_3晶体中的旋转孪晶

晶体中的孪晶,按其孪生对称元素一般分为反映和旋转孪晶两类.在LiNbO3晶体中,多数孪晶是反映孪晶. 本文所述的LiNbO3孪晶与常见的孪晶不同。它是从固-液同成分配方熔体中沿C轴方向提拉的.新晶是在原晶脊的晶面开始成核,沿着该晶面延伸发育,并与C轴斜交贯穿晶体(见图1). 这种少见孪晶的孪生对称元素是什么?它究竟属于哪类孪晶?我们采用X射线背射劳厄法和腐蚀法.对这些问题进行了比较详细的分析与讨论. 一、实 验 沿垂直拉晶方向将孪晶棒平行切割成三块晶片(图1中的α,β.γ), 每片都有原晶和新晶两部分。并分别用互和立表示.在各片I部分…     

8次、12次对称及有关准晶的发现──王宁、陈焕获第二届吴健雄物理奖项目

在５次对称及有关准晶发现之后，王 宁和陈 焕等相继在一些急冷合金中发现具有８次及１２次旋转对称的准晶．进一步肯定周期性平移不是晶体的必要条件．准周期性晶体（即准晶）可以有５，８，１０，１２次等过去所谓的“不允许”的旋转对称．这些准晶中的结构单元如正三角形、正方形、４５°菱形等，都存在于有关的晶体结构中．在传统的晶体中，它们是周期性排列，而在准晶中呈准周期排列．从这个角度来看，准晶的生成及其向有关晶体的转变就显得很自然了．     

基于N次图像旋转法的两平晶三面互检技术研究

光学平面面形的绝对检验技术规避了干涉仪参考面形精度的制约,能够有效提高平晶面形的检测精度。采用N次图像旋转法的两平晶三面互检的绝对检验技术,求解待测平晶的三维绝对面形分布,结果中包含了中频波段的信息。利用递推公式构造旋转变化项的N次虚拟旋转结果,求和平均后得到旋转变化项,叠加旋转不变项结果后得到待测波面面形。推导了算法的理论误差,针对旋转角度进行优化并增加虚拟旋转次数,提高了算法精度,优化后的仿真结果的残差波面的均方根值精度为0.14 nm。对150 mm口径平晶进行两平晶三面互检实验,并将实验结果与传统三面互检法结果进行比对,均方根值偏差小于0.5 nm,验证了算法的准确性。     

连铸低碳钢一次枝晶演变数值模拟及其受力分析

连铸坯初始凝固坯壳一次枝晶生长过程中受力冲击是导致其不均匀生长的重要原因.本文采用元胞自动机法模拟连铸低碳钢(Fe-0.6 wt.%C)方坯初始凝固一次枝晶生长演变,并基于材料力学理论简化其为悬臂梁,计算实际浇铸熔体冲击下一次枝晶受力.研究表明,2—10 K过冷度范围,枝晶尖端溶质浓度随过冷度增加稳步提升,过冷度每升高2 K,尖端最大浓度增加约0.07%.过冷度2—6 K范围,枝晶臂长度随过冷度上升显著增加;6—10 K范围,枝晶臂长度增幅稳定,生长速率约为0.08 mm·s~(-1).枝晶间熔体流速0.13—0.33 m·s~(-1)时,过冷度增加使枝晶臂根部受力减轻,而恒定过冷度时根部受力则随一次枝晶长大持续增加.过冷度低于6K或一次枝晶生长1 s后,枝晶臂根部弯曲应力均超过临界断裂强度,极易诱发断裂而形成初始缺陷.     

十次对称准晶的光电导率

讨论了十次对称准晶的光电导率，推导了十次对称准晶在周期方向和准周期平面不同的光电导率的计算公式，并以此计算了十次对称准晶Al₆₅Co₁₇Cu₁₈的光电导率，得到了与实验值相当符合的结果. 关键词： 准晶 光电导率     

热扰动对过冷熔体中二次枝晶生长影响的相场法模拟

用相场法模拟了过冷纯物质二次枝晶的生长过程，并定量地研究了过冷度、各向异性、热扰动振幅等参数对二次枝晶的影响. 通过加入热扰动，相场法能更真实地模拟过冷熔体中二次枝晶的生长，计算得到的二次枝晶间距和幅值与由Wentzel_Kramers_Brillouin方法得到的结果符合较好. 模拟结果表明，过冷度和各向异性对二次枝晶有较大影响. 当过冷度增大时，二次枝晶间距随之减小；当各向异性增大时，二次枝晶间距随之增大，但二次枝晶幅值则随之减小. 热扰动振幅主要影响二次枝晶幅值，而对二次枝晶间距影响较弱. 关键词： 相场法 热扰动 枝晶生长 二次枝晶     

γ-TiAl金属间化合物面缺陷能的分子动力学研究

运用分子动力学方法，对γ-TiAl金属间化合物的面缺陷能(层错能和孪晶能)进行了研究. 计算得到γ-TiAl不同滑移系(或孪生系)的整体堆垛层错能曲线，结果表明，γ-TiAl较一般fcc晶体结构的金属可动滑移系(孪生系)的数量减少，在外界条件下呈脆性. 研究孪生系(1/6)〈112〉{111}的弛豫的整体堆垛层错(GSF)能和整体孪晶(GTF)能曲线，对不稳定层错能γ_usf、稳定层错能γ_sf和不稳定孪晶能γ_usf值进行分析，可以预知， γ-TiAl的主要变形机理为孪生系(1/6)〈112〉{111}的孪生和普通滑移系(1/6)〈110〉{111}的滑移，以及超滑移系(1/2)〈011〉{111}的滑移. 关键词： γ-TiAl')" href="#">γ-TiAl 堆垛层错能 孪晶能 分子动力学     

五次对称与Ti-Ni-V合金中的准晶

一、准晶的发现及研究现状 近百年来,人们对凝聚态的深入研究表明,固态基本可分为晶态和非晶态两大类.当用一束X射线或电子束照射到固态上时,就会发现,只有晶体才会产生明锐的布喇格衍射,而非晶固体和液体只能产生很弥散的衍射.其原因就在于晶体中原子在三维空间中的排列具有平移周期性,从而保证了高度的相干性;而非晶固体和液体中原子排列则基本处于无序(或短程有序)状态,不具备这种空间相干性,从而只能得 到如我们在X射线或电子衍射实验中所常见到的那种弥散的衍射峰或环.-正是由于这种平移周期性的制约,使得五次旋转对称在晶体中成为不…     

一种三次非旋转对称的相位板的检测系统设计

作为零位干涉检测方法中非常有前途的一种方法.计算全息可以用于非旋转对称的非球面的检测.以三次相位板为例,阐述了利用计算全息图检测非旋转对称的非球面的基本原理.分析并推导了三次相位传播过程引入的高阶波像差的理论公式,给出了三次相位板的检测系统的没计结果.详细讨论了计算全息图衍射级次的分离以及计算全息图的二元化,给出了振幅型的计算全息图的图样.计算全息图的刻线最小问隔是40μm,计算全息图的制作精度对检测结果的波前误差的影响仅仅为0.005λ.对检测系统作了详细的公差分析,结果表明所有调整公差对整个检测系统的影响和方根值为83.954 nm.     

准晶切割投影图的计算与图示

基于准晶结构的投影描述法，发展了一种准晶的切割投影图的计算方法，并将这种方法用于十次准晶。首先用计算机计算了完整十次准晶平行于准周期平面的切割投影图，然后对沿周期方向有无限长直位错的十次准晶，分别计算了仅引入相位子应变和同时引入相位子、声子应变的平行于准周期平面的切割投影图。     

Molecular dynamics simulation of twinning in devitrite,Na2Ca3Si6O16

Reports of Type II twins are quite rare for most crystal structures. When they do occur, they are usually one of a number of possible twinning modes observed in a particular material. However, for the triclinic phase devitrite, Na₂Ca₃Si₆O₁₆, which nucleates from commercial soda?lime?silica float glass subjected to suitable heat treatments, the only reported twinning mode to date is a Type II twinning mode. In this study, this Type II twinning mode is first examined by molecular dynamics simulation to determine the lowest energy configuration of perfect twin boundaries for the twin mode. This is then compared with the lowest energy configurations of perfect twin boundaries found for six possible Type I twinning modes for devitrite for which the formal deformation twinning shear is less than 0.6. The most favourable twin plane configuration for the Type II twinning crystallography is shown to produce reasonably low twin boundary energies and sensible predictions for the optimum locations of the twin plane, K ₁, and the [1?0?0] rotation axis, η ₁, about which the 180° Type II twinning operation takes place. By comparison, all the Type I twinning modes were found to have very energetically unstable atomic configurations, and for each of these twinning modes, the lowest energy configurations found all led to high effective K ₁ twin boundary energies relative to perfect crystal. These results therefore provide a rationale for the experimental observation of the particular Type II twinning mode seen in devitrite.     

Energy estimates of twinning processes in cubic single crystals

The role of twinning processes giving rise to the fragmentation and rotation of a structure caused by single-crystal deformation, as well as the joining of grains during the manufacturing of optical ceramics, is investigated. Models describing the twins in the single crystals under study, which are formed by rotating a cubic lattice about the threefold symmetry axis through an angle of 60°, are constructed. The twin formation energies are estimated.     

Dense Coding Process with Imperfect Encoding Operations

We study dense coding under the condition that the sender’s encoding operations be imperfect. In order to formally describe the effect of the imperfect encoding operations, we use four kinds of quantum noise processes. In this way, the imperfect operation is the corresponding perfect operation followed by a quantum noise process. We show the relation among the average probability of decoding the correct information, the non-maximally entangled state, the imperfect encoding operations, and the receiver’s measurement basis.     

Birefringence of PrAlO3 in the vicinity of 118 K

We have measured the birefringence of PrAlO₃ from 300 to 77 K in several different crystalline orientations. The 205 K first order and 151 K second order structural phase transitions are clearly evident as are the effects of twinning below 151 K which take the form of regular oscillations. Near 118 K we have observed anomalies in the amplitude and frequency of the twinning effect which may relate to the Anderson-Blount phase transition reported by Fleury. Direct evidence of the 118 K transition was also obtained when birefringence measurements were taken in a geometry chosen to suppress the twinning effect.     

三量子比特纠缠Greenberger-Horne-Zeilinger态和W态的量子线路实现

两个简单的量子线路被提出分别用来制备三量子比特Greenberger-Horne-Zeilinger(GHZ)态和W态。众所周知,任意的多量子比特门都可以由受控非门和单量子比特门复合而成。同样,我们发现三量子比特GHZ态和W态也可以由受控非门和单量子比特门来制备。因此,从量子计算的角度来看我们的方案十分重要。由于在整个过程只用到了单量子比特操作和双量子比特操作,所以我们的方案在实验中很容易实现。     

Investigation of the electromechanical effect in twinning of bismuth crystals in the temperature range 77–530°K

A study was made of the influence of high-density current pulses on twinning of bismuth single crystals by a concentrated load at temperatures of 77–530° K. Twinning was stimulated by current pulses throughout the investigated temperature range and at high temperatures these pulses widened the interval of twinning temperatures. The electroplastic effect in twinning appeared most clearly at low temperatures. The temperature dependences of the quantitative characteristics of twinning had three regions in which there were considerable differences in the qualitative pattern of the distribution of twins originating from indentations.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 7, pp. 29–33, July, 1980.     

基于非对称量子通道受控QOT量子投票协议

提出一种量子投票协议, 协议基于非对称量子通道受控量子局域幺正操作隐形传输(quantum operation teleportation, QOT). 由公正机构CA提供的零知识证明的量子身份认证, 保证选民身份认证的匿名性. 计票机构Bob制造高维Greenberger-Horne-Zeilinger 纠缠态建立一个高维量子通信信道. 选民对低维的量子选票进行局域幺正操作的量子投票, 是通过非对称基的测量和监票机构Charlie的辅助测量隐形传输的. Bob在Charlie帮助下可以通过幺正操作结果得到投票结果. 与其他一般的QOT量子投票协议相比, 该协议利用量子信息与传输的量子信道不同维, 使单粒子信息不能被窃取、防止伪造.选举过程由于有Charlie的监督, 使得投票公正和不可抵赖.由于量子局域幺正操作隐形传输的成功概率是1, 使量子投票的可靠性得以保证. 关键词： 量子投票 高维GHZ纠缠态 非对称基测量 量子操作隐形传输     

Quantum Teleportation with Remote Rotation on a GHZ State

This study proposes a pioneering protocol for teleporting an arbitrary single particle state and simultaneously performing a rotation operation on that particle. There are protocols for either only teleporting particles or only remotely controlling quantum particles. If one has to remotely control a teleported quantum, then he/she has to first do the quantum teleportation and then perform the remote control on the teleported quantum. Both operations were done separately on two sets of entanglements. However, this intuitive solution is inefficient because many resources are wasted. Therefore, the study attempts to complete both operations using only one Greenberger-Horne-Zeilinger (GHZ) state.     

Theoretical analysis of aliasing noises in cold atom Mach-Zehnder interferometers

We present a theoretical analysis of aliasing noises that might appear in cold atom Mach-Zehnder interferometers used for the measurement of various physical quantities. We focus more specifically on single cold atom gyroscopes. To evaluate the level of aliasing noises, we have developed a model based on the power spectral densities of the different identified noise sources as input parameters and which makes use of a servo-loop to realize a precise measurement of the rotation rate. The model allows one to take into account different modes of operation, like a continuous as well as a pulsed or even a multi-ball operation. For monokinetic atoms, we show that the intermodulation noise can be completely filtered out with a continuous mode of operation and an optimum modulation scheme for any modulation frequency but also with a pulsed operation however only for specific launching frequencies. In the case of a real continuous atomic beam having a velocity distribution, it comes out that a high attenuation can be reached which indicates clearly the potential stability improvement that can be expected from a continuous operation.     

旋转对二维正方晶格介质柱内空结构光子晶体禁带的影响

采用平面波展开的方法计算了三种旋转操作下二维正方晶格各向异性材料(Te)介质柱内空结构光子晶体TE,TM模式能带.讨论了三种旋转操作对TE,TM模式带隙及完全光子禁带的影响.发现TM模式高频带隙与结构的旋转对称性有着密切的关系.而TE模式的带隙不仅受到晶体旋转对称性的影响同时也受到介质在x-y平面分布情况的影响. 关键词： 二维光子晶体 内空结构 旋转操作 光子带隙