Electron vortex beams with high quanta of orbital angular momentum

Electron beams with helical wavefronts carrying orbital angular momentum are expected to provide new capabilities for electron microscopy and other applications. We used nanofabricated diffraction holograms in an electron microscope to produce multiple electron vortex beams with well-defined topological charge. Beams carrying quantized amounts of orbital angular momentum (up to 100?) per electron were observed. We describe how the electrons can exhibit such orbital motion in free space in the absence of any confining potential or external field, and discuss how these beams can be applied to improved electron microscopy of magnetic and biological specimens.     

Closed-shell molecules that ionize more readily than cesium

We report a class of molecules with extremely low ionization enthalpies, one member of which has been determined to have a gas-phase ionization energy (onset, 3.51 electron volts) lower than that of the cesium atom (which has the lowest gas-phase ionization energy of the elements) or of any other known closed-shell molecule or neutral transient species reported. The molecules are dimetal complexes with the general formula M2(hpp)4 (where M is Cr, Mo, or W, and hpp is the anion of 1,3,4,6,7,8-hexahydro-2H-pyrimido[1,2-a]pyrimidine), structurally characterized in the solid state, spectroscopically characterized in the gas phase, and modeled with theoretical computations. The low-energy ionization of each molecule corresponds to the removal of an electron from the delta bonding orbital of the quadruple metal-metal bond, and a strong interaction of this orbital with a filled orbital on the hpp ligands largely accounts for the low ionization energies.     

Orbital physics in transition-metal oxides

An electron in a solid, that is, bound to or nearly localized on the specific atomic site, has three attributes: charge, spin, and orbital. The orbital represents the shape of the electron cloud in solid. In transition-metal oxides with anisotropic-shaped d-orbital electrons, the Coulomb interaction between the electrons (strong electron correlation effect) is of importance for understanding their metal-insulator transitions and properties such as high-temperature superconductivity and colossal magnetoresistance. The orbital degree of freedom occasionally plays an important role in these phenomena, and its correlation and/or order-disorder transition causes a variety of phenomena through strong coupling with charge, spin, and lattice dynamics. An overview is given here on this "orbital physics," which will be a key concept for the science and technology of correlated electrons.     

Electronically induced atom motion in engineered CoCun nanostructures

We have measured the quantum yield for exciting the motion of a single Co atom in CoCu(n) linear molecules constructed on a Cu(111) surface. The Co atom switched between two lattice positions during electron excitation from the tip of a scanning tunneling microscope. The tip location with highest probability for inducing motion was consistent with the position of an active state identified through electronic structure calculations. Atom motion within the molecule decreased with increased molecular length and reflected the corresponding variation in electronic structure.     

Synthesized light transients

Manipulation of electron dynamics calls for electromagnetic forces that can be confined to and controlled over sub-femtosecond time intervals. Tailored transients of light fields can provide these forces. We report on the generation of subcycle field transients spanning the infrared, visible, and ultraviolet frequency regimes with a 1.5-octave three-channel optical field synthesizer and their attosecond sampling. To demonstrate applicability, we field-ionized krypton atoms within a single wave crest and launched a valence-shell electron wavepacket with a well-defined initial phase. Half-cycle field excitation and attosecond probing revealed fine details of atomic-scale electron motion, such as the instantaneous rate of tunneling, the initial charge distribution of a valence-shell wavepacket, the attosecond dynamic shift (instantaneous ac Stark shift) of its energy levels, and its few-femtosecond coherent oscillations.     

紫草素衍生物定量构效关系的量子化学研究

为探讨紫草素衍生物结构与其抗植物病毒活性之间的关系,采用Gaussian 03程序量子化学的从头算法对9种紫草素衍生物进行了量子化学计算。将得到的分子空间结构、轨道能量、轨道组成、净电荷等性质进行分析,并与实验得到的生物活性参数进行对比研究。结果表明:紫草素衍生物的抗植物病毒活性与化合物的前沿分子轨道LUMO能级存在负相关性(R2=0.626 6);此类化合物与受体作用时,可能与受体发生电子转移,形成电子配合物,从而发挥药效;萘醌环结构应为此类化合物的活性关键部位,R基团为有效的结构修饰点。     

Tunneling into a single magnetic atom: spectroscopic evidence of the kondo resonance

The Kondo effect arises from the quantum mechanical interplay between the electrons of a host metal and a magnetic impurity and is predicted to result in local charge and spin variations around the magnetic impurity. A cryogenic scanning tunneling microscope was used to spatially resolve the electronic properties of individual magnetic atoms displaying the Kondo effect. Spectroscopic measurements performed on individual cobalt atoms on the surface of gold show an energetically narrow feature that is identified as the Kondo resonance-the predicted response of a Kondo impurity. Unexpected structure in the Kondo resonance is shown to arise from quantum mechanical interference between the d orbital and conduction electron channels for an electron tunneling into a magnetic atom in a metallic host.     

Experiments on the structure of an individual elementary particle

Research begun in the early 1970s on geonium is reviewed. Genoium is a man-made atom, created at liquid-helium temperature in ultrahigh vacuum from an individual electron in magnetic and electric trapping fields. For this atom the electron gyromagnetic ratio g = 2. 000 000 000 110(60) has been measured in microwave spectroscopy experiments after subtraction of quantum electrodynamics shifts. The g - g(Dirac) = 11 x 10(-11) excess over the value g(Dirac) = 2 for the theoretical Dirac point electron suggests for the electron of nature a corresponding excess radius R(e) - R(Dirac) over the Dirac radius R(Dirac) = 0 and a spatial structure. From a plot of measured g and R values for the near-Dirac particles electron, proton, triton, and helium-3 an electron radius R(e) approximately 10(-20) centimeter is extrapolated. In a speculation, the triton-proton-quark model has been extended to the electron, to a succession of subquarks, and finally to the "cosmon." Rapid decay of a cosmon-anticosmon pair created from the "nothing" state in a spontaneous quantum jump initiated the Big Bang.     

Development of one-dimensional band structure in artificial gold chains

The ability of a scanning tunneling microscope to manipulate single atoms is used to build well-defined gold chains on NiAl(110). The electronic properties of the one-dimensional chains are dominated by an unoccupied electron band, gradually developing from a single atomic orbital present in a gold atom. Spatially resolved conductance measurements along a 20-atom chain provide the dispersion relation, effective mass, and density of states of the free electron-like band. These experiments demonstrate a strategy for probing the interrelation between geometric structure, elemental composition, and electronic properties in metallic nanostructures.     

H 2O和H 2 S与双卤分子形成的卤键的理论研究

用量子化学从头算方法对H2O和H2S与卤素分子(X—Y)形成的卤键O/S…X—Y进行了理论研究.计算结果表明,卤键的形成导致X—Y键长增大与伸缩振动频率红移;电子密度拓扑(AIM)分析表明,复合物中的卤键为闭壳层相互作用;自然键轨道(NBO)分析表明,卤键O/S…X—Y形成时,强的分子间超共轭n(O/S)→σ*(X—Y)引起的电荷转移是X—Y键伸缩振动频率红移的重要原因.     

梯形振荡水翼水动力学特性的三维数值模拟研究

振荡水翼式潮流能采集系统一般通过二维水翼水动力学模型建立其关于俯仰与升沉方向两自由度的运动方程。但水翼三维模型与二维模型在诱导阻力计算上有着一定的差别,而诱导阻力又直接影响到系统的能量转化效率,因此有必要对振荡水翼三维的水动力学特性进行研究。本文根据叶素理论,以梯形翼作为三维水翼模型的代表,建立了梯形振荡翼的水动力学模型和运动方程。对该模型采用三维非结构网格和动网格技术进行数值模拟,着重探讨梯形比对其运动状态和能量转化效率的影响。结果表明:采用动网格模拟三维梯形振荡水翼的水动力学特性,得到的水翼运动状态与二维的相同;振荡水翼的诱导阻力会改变其稳定振荡频率和升沉运动幅值,直接影响了系统的能量采集效率;随着梯形比的降低,系统的能量转化效率先上升后降低,在梯形比为0.5时最大。本文的研究成果将有助于完善振荡水翼的结构设计方法。     

Submicrometer ferromagnetic NOT gate and shift register

An all-metallic submicrometer device is demonstrated experimentally at room temperature that performs logical NOT operations on magnetic logic signals. When this two-terminal ferromagnetic structure is incorporated into a magnetic feedback loop, the junction performs a frequency division operation on an applied oscillating magnetic field. Up to 11 of these junctions are then directly linked together to create a magnetic shift register.     

磁场和LO声子效应对量子盘中强耦合磁极化子qubit的影响

采用Lee-Low-Pines-Huybrechts变分法研究了外磁场、LO声子效应和量子点厚度对量子盘中电子-LO声子强耦合磁极化子的振动频率和qubit的影响,推导出磁极化子的振动频率λ,量子比特振荡的周期T0和电子的几率分布ρ的表达式.数值计算结果表明,磁极化子基态的振动频率大于激发态的振动频率,λ随ωc和α的增加而增大,随L的增加而减小;T0随受限强度ω0和量子盘厚度L的增加而增大,随电子-声子耦合强度α的增加而减小;ρ随ω0的增加而减小,随L的增加而振荡减小;T0和ρ随ωc和L的变化规律受ω0和α的影响显著.     

Cassini discovers a kinematic spiral ring around Saturn

Since the time of the Voyager flybys of Saturn in 1980-1981, Saturn's eccentric F ring has been known to be accompanied on either side by faint strands of material. New Cassini observations show that these strands, initially interpreted as concentric ring segments, are in fact connected and form a single one-arm trailing spiral winding at least three times around Saturn. The spiral rotates around Saturn with the orbital motion of its constituent particles. This structure is likely the result of differential orbital motion stretching an initial cloud of particles scattered from the dense core of the F ring. Different scenarios of formation, implying ringlet-satellite interactions, are explored. A recently discovered moon candidate, S/2004 S6, is on an orbit that crosses the F-ring core at the intersection of the spiral with the ring, which suggests a dynamical connection between S/2004 S6 and the spiral.     

射频电磁波生物学效应实验系统的研制

设计和研制了一套用于研究电磁波非热效应的实验系统。该系统综合了工程上的TEM小室、开阔场地测量屏蔽室和微波暗室的优点，能产生一个具有极端的宽带特征、分布均匀、易于测量的电磁场区域，对周围的人或设备无害或无干扰。     

Laser-induced electron tunneling and diffraction

Molecular structure is usually determined by measuring the diffraction pattern the molecule impresses on x-rays or electrons. We used a laser field to extract electrons from the molecule itself, accelerate them, and in some cases force them to recollide with and diffract from the parent ion, all within a fraction of a laser period. Here, we show that the momentum distribution of the extracted electron carries the fingerprint of the highest occupied molecular orbital, whereas the elastically scattered electrons reveal the position of the nuclear components of the molecule. Thus, in one comprehensive technology, the photoelectrons give detailed information about the electronic orbital and the position of the nuclei.     

Control of electron localization in molecular dissociation

We demonstrated how the subcycle evolution of the electric field of light can be used to control the motion of bound electrons. Results are presented for the dissociative ionization of deuterium molecules (D2 --> D+ + D), where asymmetric ejection of the ionic fragment reveals that light-driven intramolecular electronic motion before dissociation localizes the electron on one of the two D+ ions in a controlled way. The results extend subfemtosecond electron control to molecules and provide evidence of its usefulness in controlling reaction dynamics.     

Spot-scan imaging in transmission electron microscopy.

The determination of the structure of proteins and other organic materials by transmission electron microscopy is a rapidly developing field. Obtaining high-resolution images of these radiation-sensitive specimens has, until recently, been problematic. The development of spot-scan imaging, in which the electron beam is focused to a spot with a diameter of about 1000 angstroms and moved over the specimen to record the image, has overcome some of the most severe problems, which result from beam-induced motion of the specimen and its image. Elimination of this motion greatly enhances the contrast of high-resolution features of the image and promises a significant increase in the speed with which future structural work can be accomplished.     

Nuclei-induced frequency focusing of electron spin coherence

The hyperfine interaction of an electron with the nuclei is considered as the primary obstacle to coherent control of the electron spin in semiconductor quantum dots. We show, however, that the nuclei in singly charged quantum dots act constructively by focusing the electron spin precession about a magnetic field into well-defined modes synchronized with a laser pulse protocol. In a dot with a synchronized electron, the light-stimulated fluctuations of the hyperfine nuclear field acting on the electron are suppressed. The information about electron spin precession is imprinted in the nuclei and thereby can be stored for tens of minutes in darkness. The frequency focusing drives an electron spin ensemble into dephasing-free subspaces with the potential to realize single frequency precession of the entire ensemble.     

Metastable oxygen emission bands

Recombination of ground-state oxygen atoms populates six different bound electronic states of molecular oxygen. Of the six optical transitions expected between the three upper states at 4 to 4.5 electron volts and the two lowest states, five have been observed in the afterglow of a conventional helium-oxygen microwave discharge in both 16O(2) and (18)O(2), three of them for the first time in gas-phase spectra. Generation of these emissions from oxygen atoms in a system free of molecular oxygen establishes that atom recombination is the production mechanism.