Electronic read-out of a single nuclear spin using a molecular spin transistor

Quantum control of individual spins in condensed-matter devices is an emerging field with a wide range of applications, from nanospintronics to quantum computing. The electron, possessing spin and orbital degrees of freedom, is conventionally used as the carrier of quantum information in proposed devices. However, electrons couple strongly to the environment, and so have very short relaxation and coherence times. It is therefore extremely difficult to achieve quantum coherence and stable entanglement of electron spins. Alternative concepts propose nuclear spins as the building blocks for quantum computing, because such spins are extremely well isolated from the environment and less prone to decoherence. However, weak coupling comes at a price: it remains challenging to address and manipulate individual nuclear spins. Here we show that the nuclear spin of an individual metal atom embedded in a single-molecule magnet can be read out electronically. The observed long lifetimes (tens of seconds) and relaxation characteristics of nuclear spin at the single-atom scale open the way to a completely new world of devices in which quantum logic may be implemented.     

Electrical detection of the spin resonance of a single electron in a silicon field-effect transistor

The ability to manipulate and monitor a single-electron spin using electron spin resonance is a long-sought goal. Such control would be invaluable for nanoscopic spin electronics, quantum information processing using individual electron spin qubits and magnetic resonance imaging of single molecules. There have been several examples of magnetic resonance detection of a single-electron spin in solids. Spin resonance of a nitrogen-vacancy defect centre in diamond has been detected optically, and spin precession of a localized electron spin on a surface was detected using scanning tunnelling microscopy. Spins in semiconductors are particularly attractive for study because of their very long decoherence times. Here we demonstrate electrical sensing of the magnetic resonance spin-flips of a single electron paramagnetic spin centre, formed by a defect in the gate oxide of a standard silicon transistor. The spin orientation is converted to electric charge, which we measure as a change in the source/drain channel current. Our set-up may facilitate the direct study of the physics of spin decoherence, and has the practical advantage of being composed of test transistors in a conventional, commercial, silicon integrated circuit. It is well known from the rich literature of magnetic resonance studies that there sometimes exist structural paramagnetic defects near the Si/SiO2 interface. For a small transistor, there might be only one isolated trap state that is within a tunnelling distance of the channel, and that has a charging energy close to the Fermi level.     

Single-electron transistor of a single organic molecule with access to several redox states

A combination of classical Coulomb charging, electronic level spacings, spin, and vibrational modes determines the single-electron transfer reactions through nanoscale systems connected to external electrodes by tunnelling barriers. Coulomb charging effects have been shown to dominate such transport in semiconductor quantum dots, metallic and semiconducting nanoparticles, carbon nanotubes, and single molecules. Recently, transport has been shown to be also influenced by spin--through the Kondo effect--for both nanotubes and single molecules, as well as by vibrational fine structure. Here we describe a single-electron transistor where the electronic levels of a single pi-conjugated molecule in several distinct charged states control the transport properties. The molecular electronic levels extracted from the single-electron-transistor measurements are strongly perturbed compared to those of the molecule in solution, leading to a very significant reduction of the gap between the highest occupied molecular orbital and the lowest unoccupied molecular orbital. We suggest, and verify by simple model calculations, that this surprising effect could be caused by image charges generated in the source and drain electrodes resulting in a strong localization of the charges on the molecule.     

单目CCD离面位移测量

以针孔成像原理为基础,利用结构表面的离面位移与所成图像中散斑颗粒的应变的线性对应关系,通过计算散斑应变场的方法来计算结构表面的离面位移场,对其理论进行数字模拟分析.数字模拟结果表明,该方法可进行离面位移场测量,测量精度高,与理论相符.还用该方法进行了离面位移试验,试验结果及分析表明,噪音对该方法的影响很小,离面位移测量精度为毫米,可应用于土木工程结构试验中.     

Driven coherent oscillations of a single electron spin in a quantum dot

The ability to control the quantum state of a single electron spin in a quantum dot is at the heart of recent developments towards a scalable spin-based quantum computer. In combination with the recently demonstrated controlled exchange gate between two neighbouring spins, driven coherent single spin rotations would permit universal quantum operations. Here, we report the experimental realization of single electron spin rotations in a double quantum dot. First, we apply a continuous-wave oscillating magnetic field, generated on-chip, and observe electron spin resonance in spin-dependent transport measurements through the two dots. Next, we coherently control the quantum state of the electron spin by applying short bursts of the oscillating magnetic field and observe about eight oscillations of the spin state (so-called Rabi oscillations) during a microsecond burst. These results demonstrate the feasibility of operating single-electron spins in a quantum dot as quantum bits.     

Kondo resonance in a single-molecule transistor

When an individual molecule, nanocrystal, nanotube or lithographically defined quantum dot is attached to metallic electrodes via tunnel barriers, electron transport is dominated by single-electron charging and energy-level quantization. As the coupling to the electrodes increases, higher-order tunnelling and correlated electron motion give rise to new phenomena, including the Kondo resonance. To date, all of the studies of Kondo phenomena in quantum dots have been performed on systems where precise control over the spin degrees of freedom is difficult. Molecules incorporating transition-metal atoms provide powerful new systems in this regard, because the spin and orbital degrees of freedom can be controlled through well-defined chemistry. Here we report the observation of the Kondo effect in single-molecule transistors, where an individual divanadium molecule serves as a spin impurity. We find that the Kondo resonance can be tuned reversibly using the gate voltage to alter the charge and spin state of the molecule. The resonance persists at temperatures up to 30 K and when the energy separation between the molecular state and the Fermi level of the metal exceeds 100 meV.     

一种利用单馈源实现双波束的理论模型

给出了在一个角锥喇叭馈源上实现有一定夹角双波束的理论模型,并分析了该模型的E面,H面远场及两波束夹角的形成因素。     

Nanomechanical oscillations in a single-C60 transistor

The motion of electrons through quantum dots is strongly modified by single-electron charging and the quantization of energy levels. Much effort has been directed towards extending studies of electron transport to chemical nanostructures, including molecules, nanocrystals and nanotubes. Here we report the fabrication of single-molecule transistors based on individual C60 molecules connected to gold electrodes. We perform transport measurements that provide evidence for a coupling between the centre-of-mass motion of the C60 molecules and single-electron hopping--a conduction mechanism that has not been observed previously in quantum dot studies. The coupling is manifest as quantized nano-mechanical oscillations of the C60 molecule against the gold surface, with a frequency of about 1.2 THz. This value is in good agreement with a simple theoretical estimate based on van der Waals and electrostatic interactions between C60 molecules and gold electrodes.     

Detection of magnetic circular dichroism using a transmission electron microscope

A material is said to exhibit dichroism if its photon absorption spectrum depends on the polarization of the incident radiation. In the case of X-ray magnetic circular dichroism (XMCD), the absorption cross-section of a ferromagnet or a paramagnet in a magnetic field changes when the helicity of a circularly polarized photon is reversed relative to the magnetization direction. Although similarities between X-ray absorption and electron energy-loss spectroscopy in a transmission electron microscope (TEM) have long been recognized, it has been assumed that extending such equivalence to circular dichroism would require the electron beam in the TEM to be spin-polarized. Recently, it was argued on theoretical grounds that this assumption is probably wrong. Here we report the direct experimental detection of magnetic circular dichroism in a TEM. We compare our measurements of electron energy-loss magnetic chiral dichroism (EMCD) with XMCD spectra obtained from the same specimen that, together with theoretical calculations, show that chiral atomic transitions in a specimen are accessible with inelastic electron scattering under particular scattering conditions. This finding could have important consequences for the study of magnetism on the nanometre and subnanometre scales, as EMCD offers the potential for such spatial resolution down to the nanometre scale while providing depth information--in contrast to X-ray methods, which are mainly surface-sensitive.     

基于加权平均PN互相关的DTMB单频网频谱感知

该文首次研究了中国地面数字电视广播传输标准(DTMB)单频网(SFN)的频谱感知问题,提出一种基于DT-MB信号独特帧结构的加权平均(WA)伪随机(PN)序列互相关的频谱感知算法。该算法充分考虑了单频网信道具有典型人为强多径的鲜明特征,以对多个互相关峰值进行加权平均处理结果作为频谱感知的判决统计量,与传统的基于PN互相关的仅取相关峰最大值处理作为判决统计量的方法相比,其检测性能从理论上分析更适合于单频网信道下DTMB信号的频谱感知。仿真结果表明,该算法在典型单频网信道(如广电7和广电8信道)下可获得显著的性能改进。     

Measurement of the displacement field of dislocations to 0.03 A by electron microscopy

Defects and their associated long-range strain fields are of considerable importance in many areas of materials science. For example, a major challenge facing the semiconductor industry is to understand the influence of defects on device operation, a task made difficult by the fact that their interactions with charge carriers can occur far from defect cores, where the influence of the defect is subtle and difficult to quantify. The accurate measurement of strain around defects would therefore allow more detailed understanding of how strain fields affect small structures-in particular their electronic, mechanical and chemical properties--and how such fields are modified when confined to nanometre-sized volumes. Here we report the measurement of displacements around an edge dislocation in silicon using a combination of high-resolution electron microscopy and image analysis inherited from optical interferometry. The agreement of our observations with anisotropic elastic theory calculations is better than 0.03 A. Indeed, the results can be considered as an experimental verification of anisotropic theory at the near-atomic scale. With the development of nanostructured materials and devices, we expect the use of electron microscopy as a metrological tool for strain analysis to become of increasing importance.     

基于单层小波变换的自适应压缩感知图像处理

文章在未知二维图像的稀疏度的情况下,提出了基于单层小波变换的自适应压缩感知算法,保留其中的低频系数,只针对高频系数进行测量。在小波变换把二维图像分成低低、低高、高低和高高的4块之后,利用稀疏度自适应匹配追踪算法,分别对其中包含在低高块中的列、高低块中的行、高高块整体中的那些高频系数进行恢复,再进行小波逆变换重构图像。仿真结果表明,与原来的单层小波变换的非自适应压缩感知算法相比,该算法解决了稀疏度未知情况下的图像恢复问题,而且重构图像质量也得到很好的保证,例如在相同的采样率下,新算法与原算法之间的PSNR相差不过2dB。     

High-quality electron beams from a laser wakefield accelerator using plasma-channel guiding

Laser-driven accelerators, in which particles are accelerated by the electric field of a plasma wave (the wakefield) driven by an intense laser, have demonstrated accelerating electric fields of hundreds of GV m(-1) (refs 1-3). These fields are thousands of times greater than those achievable in conventional radio-frequency accelerators, spurring interest in laser accelerators as compact next-generation sources of energetic electrons and radiation. To date, however, acceleration distances have been severely limited by the lack of a controllable method for extending the propagation distance of the focused laser pulse. The ensuing short acceleration distance results in low-energy beams with 100 per cent electron energy spread, which limits potential applications. Here we demonstrate a laser accelerator that produces electron beams with an energy spread of a few per cent, low emittance and increased energy (more than 10(9) electrons above 80 MeV). Our technique involves the use of a preformed plasma density channel to guide a relativistically intense laser, resulting in a longer propagation distance. The results open the way for compact and tunable high-brightness sources of electrons and radiation.     

用杆件截面递增迭代法对位移约束桁架进行优化设计

采用杆件截面递增迭代法,对具有位移约束的桁架进行优化设计.在迭代公式中,杆件截面增量的权重采用杆件各自的应变比能,保证了迭代收敛的稳定性.为提高优化设计的收敛速度,提出一种迭代加速因子,它既保证了优化设计解的精度又提高了收敛速度.通过对三杆平面桁架、n节间平面桁架进行优化设计研究,验证了该方法的有效性,为大型结构优化设计奠定了基础.     

Remote sensing of algal blooms using a turbidity-free function for near-infrared and red signals

HARMFUL ALGAL BLOOM ACTS UPON IMPORTANT ENVIRON-MENTAL PROBLEMS IN COASTAL WATERS.A UNIFIED AND IM-PLEMENTING METHOD FOR SURVEY AND MONITORING ALGALBLOOMS HAS BEEN DEVELOPED SINCE LAST DECADE.HOWEVER THERE IS A DATA GAP OF ABOUT TENS YEARS IN THE PERIOD W…     

冰川变化遥感监测的研究进展

总结分析冰川变化遥感监测中使用的影像数据和方法,探讨全球范围内冰川变化遥感监测最新研究成果,发现分辨率较高时间序列较长的Landsat影像适合大区域长周期的冰川变化研究.ASTER影像在近几年冰川变化研究中发挥着重要作用,而分辨率达到米级的影像数据适合面积小、精度要求高的冰川变化研究.冰川边界的提取通常采用比值阈值法,同时也应尝试其他方法,以选择最佳的方案.全球范围内,现代冰川退缩速率不尽相同,新几内亚与非洲地区冰川退缩最为严重,反应了低纬度地区的小冰川对气候变化响应更为敏感;斯堪的纳维亚半岛与两极地区冰川规模大,冰川变化相对较小.     

基于数字投影仪的三维轮廓检测系统

通过对DLP(digital lighting process)数字光处理器投影仪的原理分析,建立了基于透视变换的投影仪原理模型.模型中建立了投影点与空间投影直线的关系,得到光栅平面在空间中的表达式,在此基础上设计编码光栅对空间进行划分.由于编码光栅是一系列的矩形光栅,所以通过编码光栅不同的排列组合对空间进行编码划分,确定空间中每一个光栅面的次序.再由测量系统的空间几何结构关系,得到被测物体表面点的三维坐标,实现了基于数字投影仪的物体表面三维信息测量.实验证明,利用数字光处理器投影仪可以方便地实现编码模式,基于本方法的三维轮廓检测系统结构简单,适合于各种高度变化的物体,可以快速、方便地进行三维信息重构.     

基于离散多小波变换的遥感图像融合方法

以不同分辨率的遥感图像为对象,基于Chui—Lian（CL）离散多小波变换的特性,提出了一种新的图像融合方法．该方法将两幅不同的源图像分别进行预处理和多小波分解得到各个分解图像,然后对分解图像分别采用基于区域特征的融合方法,得到混合的分解系数,通过多小波重构和后处理算法从而获得融合图像．该方法能够为图像融合提供一种比传统的小波变换更加精确的融合方法．实验结果证明采用这种方法可以得到更好的融合效果,不仅能够完好地显示源图像各自的信息,而且能更好地将源图像的细节融合在一起．     

基于改进的最小二乘支持向量机的高光谱遥感图像分类

支持向量机因其适用高维特征、小样本与不确定性问题的优越性,是一种极具潜力的高光谱遥感分类方法.核函数是支持向量机的核心,核函数分为局部核函数与全局核函数两大类,不同的核函数将产生不同的分类效果.核函数也是支持向量机理论中比较难理解的一部分.在基本核函数中引入光谱匹配识别中的典型方法--光谱角度匹配法(SAM法),兼顾到光谱亮度与光谱向量方向的距离测度,结合最小二乘支持向量机,通过与传统SVM分类方法的比较,证明这种方法的有效性.     

基于激光遥测方法的某在役风电塔现场实测分析

风力发电塔作为一类特殊的高柔结构,顶部支撑的风机在工作状态下对塔身产生动态激励,目前的风电塔振动实测通常采用接触式传感器.为发展风电塔现场非接触测试技术,本文对激光遥测方法进行阐述,利用加速度计和激光遥测设备对某1.5 MW风电塔进行环境脉动激励下的动力实测,对两种仪器的现场使用及测试结果进行对比,分析遥测方法的优势.利用得到的多组实测信号,分别采用峰值拾取法和随机子空间识别法进行分析处理,获得该风电塔的自振频率.