Kinetic analysis of the cross reaction between dithioester and alkoxyamine by a Monte Carlo simulation

A model reaction of dithioester and alkoxyamine is proposed to probe the reversible addition–fragmentation chain transfer (RAFT) process. The kinetics of the model reaction is analyzed and compared with that of pure alkoxyamine homolysis with a Monte Carlo simulation. Although the pure alkoxyamine obeys the law of persistent radical effect, the model reaction results in higher concentration of the persistent radical during the main period of the reaction. However, for a very fast RAFT process or a very low addition rate constant, the time dependence of the persistent radical concentration is quite close to that of pure alkoxyamine. Furthermore, the cross termination between the intermediate and alkyl radicals causes a retardation effect for the model reaction when the intermediate is relatively long‐lived. The Monte Carlo simulation indicates that it is feasible to measure the individual rate constants of the RAFT process, such as the rate constant of addition, with a large excess of alkoxyamine. In addition, the special feature of the system with different leaving groups in the alkoxyamine and dithioester is also discussed. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 374–387, 2007     

Visualization of a Phase Object by Two-Wave Coupling in a Photorefractive Bismuth Silicon Oxide Crystal

We propose a method to visualize a phase object using a locally-controllable image amplification scheme we developed earlier which is based on two-wave coupling in a bismuth silicon oxide (Bi₁₂SiO₂₀, or BSO) crystal. In this method the BSO crystal is used as a phase plate in the configuration of Zernike’s phase contrast imaging system. The principle of the phase-visualization with two-wave coupling is presented. An experimental result to verify the proposed method is shown.     

Domain Patterns in the Microwave-Induced Zero-Resistance State

It has been proposed that the microwave-induced “zero-resistance” phenomenon, observed in a GaAs two-dimensional electron system at low temperatures in moderate magnetic fields, results from a state with multiple domains, in which a large local electric field E(r) is oriented in different directions. We explore here the questions of what may determine the domain arrangement in a given sample, what do the domains look like in representative cases, and what may be the consequences of domain-wall localization on the macroscopic dc conductance. We consider both effects of sample boundaries and effects of disorder, in a simple model, which has a constant Hall conductivity, and is characterized by a Lyapunov functional.     

Analysis of persistent photoconductivity due to potential barriers

Persistent photoconductivity has been seen in thin silicon resistors fabricated with SIMOX material at temperatures between 60 and 220 K. This effect has been attributed to the depletion of carriers near the interface between the top silicon layer and the buried oxide, which is due to the large number of surface traps at this interface. The depletion of carriers is accompanied by a built-in field on the order of 10,000 V/cm, which causes a potential barrier that is nearly a quarter of the energy gap of silicon. The theory of the recombination kinetics of majority carriers with minority carriers trapped at the interface on the other side of a potential barrier is studied. Both the possibilities of tunneling and thermal activation have been considered. The results show that thermal activation dominates at the temperatures of our measurements in SIMOX material, while at lower temperatures tunneling would dominate.     

基于持续同调性的人脑认知过程格式塔模式识别拓扑分析

信息通信技术和神经科学的融合发展预示了脑对脑无线通信的可能性与巨大潜力。将持续同调分析方法与脑电图（EEG）结合，提取了在格式塔完形（Gestalt）认知测试中大脑对不同轮廓和形状的神经反应的生理学特征。实验结果表明，当被试者观察随机序列图像（random sequence diagram，RSD）时，其大脑额叶涉及的活动区域多于其观察有序格式塔图像（Gestalt image，GST）。同时，RSD诱发的EEG信号在几个频带上与GST的持续同调熵（persistent entropy，PE）有着显著不同，这表明人类对形状和轮廓的认知过程，可以通过拓扑分析在一定程度上实现分类区分。该方法可以在保留原生信号的整体和局部特征的前提下实现神经信号的数字化。总的来说，通过对EEG信号的持续同源性特征评估量化了认知过程神经信号的相关性，提供了实现B2BC中神经信号数字化的可行方法。     

蓝宝石邻晶面衬底MBE生长GaN薄膜的瞬态光电导弛豫特性研究

在实验优化MBE工艺条件的基础上，采用蓝宝石(0001)邻晶面衬底制备出了具有较高质量的GaN薄膜.XRD分析表明邻晶面衬底生长的GaN薄膜晶体结构质量明显提高，AFM表征结果显示邻晶面生长的样品表面形貌显著改善.蓝宝石衬底GaN薄膜的瞬态光电导弛豫特性对比实验研究发现，常规衬底生长的GaN薄膜光电导弛豫特性出现双分子复合、单分子复合和弛豫振荡三个过程，持续时间分别为0.91，7.7和35.5ms；蓝宝石邻晶面衬底生长的GaN薄膜光电导弛豫过程主要是双分子复合和单分子复合过程，持续时间分别为0.78和14ms.理论分析表明MBE生长GaN薄膜的持续光电导效应主要起源于本生位错缺陷引发的深能级. 关键词： 邻晶面蓝宝石衬底 GaN薄膜 瞬态光电导 弛豫特性     

存在自旋轨道耦合的介观小环中的持续自旋流

文章作者研究了存在自旋轨道耦合的介观小环的平衡态性质.此前人们已经知道,在有磁通穿过的介观小环中,绕环运动的电子会产生一附加的Berry相位而导致持续电流;同样地,在仅有自旋轨道耦合的体系中,电子绕环运动也应当会产生附加的自旋Berry相位,进而驱动持续自旋流.文章作者通过对一个有正常区和自旋轨道耦合区的复合小环的计算,结果表明,无电流伴随的纯持续自旋流的确存在.文章作者指出,这持续自旋流描述真实的自旋运动,并且它能被实验观测.     

Changes in Electrical Characteristics of ZnO Thin Films Due to Environmental Factors

The impact of light and controlled gas ambient on the electrical characteristics of ZnO:P grown by pulsed laser deposition (PLD) is investigated with temperature-dependent Hall-effect and photo-Hall-effect using above-bandgap light. Exposure to blue/ultraviolet (UV) light results in long-lived persistent photoconductivity (PPC) effects dominated by electron conduction. However, these persistent effects can be largely reversed by exposing the sample to a controlled ambient of dry O₂ gas. These O₂-induced changes in the electronic properties persist in vacuum up to at least 400 K. Exposure to dry N₂ gas following blue/UV light has no effect on the observed PPC characteristics. The implications of these effects on the preparation of p-type ZnO will be discussed.     

Photon-induced switching and tunneling phenomena in a YBCO thin film junction

In the persistent photoconductivity (PPC) phenomenon, illumination of a YBa₂Cu₃O_6.5 thin film junction with a 1mW He−Ne laser leads to the decrease of the critical voltage (similar to the threshold voltage). The decrease of the critical voltage was reversed by illumination with incandescent light. The critical voltage across the junction was experimentally decreased and increased by alternating illumination between He−Ne laser and incandescent light. We also observed visible quenching of the photo-induced state using a 5mW He−Ne laser. Finally, the threshold behavior of the junction was destroyed by illuminating it with incandescent light.     

Temperature dependence of dc photoconductivity in CdTe thin films

The temperature dependence of dc photoconductivity in the measuring range 303–417 K has been studied in CdTe thin films having thicknesst < 4000 Å. The photoactivation energy decreases in dark which is explained on the basis of grain boundary (GB) effect. The current lost to recombination at GB space charge region causes a negative effect on the photosensitivity of the films. A decrease in photosensitivity with increase in temperature is attributed to the reduction of photoexcitation process. It is observed that the minority carrier lifetime varies inversely with light intensity which supports the sublinear relationship of photoconductivity with the intensity of light and thereby confirms the defect-controlled photoconductivity in CdTe thin films