Current-induced hydrogen tautomerization and conductance switching of naphthalocyanine molecules

The bistability in the position of the two hydrogen atoms in the inner cavity of single free-base naphthalocyanine molecules constitutes a two-level system that was manipulated and probed by low-temperature scanning tunneling microscopy. When adsorbed on an ultrathin insulating film, the molecules can be switched in a controlled fashion between the two states by excitation induced by the inelastic tunneling current. The tautomerization reaction can be probed by resonant tunneling through the molecule and is expressed as considerable changes in the conductivity of the molecule. We also demonstrated a coupling of the switching process so that the charge injection in one molecule induced tautomerization in an adjacent molecule.     

Complexity in strongly correlated electronic systems

A wide variety of experimental results and theoretical investigations in recent years have convincingly demonstrated that several transition metal oxides and other materials have dominant states that are not spatially homogeneous. This occurs in cases in which several physical interactions-spin, charge, lattice, and/or orbital-are simultaneously active. This phenomenon causes interesting effects, such as colossal magnetoresistance, and it also appears crucial to understand the high-temperature superconductors. The spontaneous emergence of electronic nanometer-scale structures in transition metal oxides, and the existence of many competing states, are properties often associated with complex matter where nonlinearities dominate, such as soft materials and biological systems. This electronic complexity could have potential consequences for applications of correlated electronic materials, because not only charge (semiconducting electronic), or charge and spin (spintronics) are of relevance, but in addition the lattice and orbital degrees of freedom are active, leading to giant responses to small perturbations. Moreover, several metallic and insulating phases compete, increasing the potential for novel behavior.     

The response of electrons to structural changes

The properties of a molecule are determined by the distribution of its electrons. This distribution can be described by the charge density, which is readily obtained from the wave functions derived by ab initio molecular orbital calculations. The charge density may be analyzed in a number of different fashions to give information about the effects of substituents, structural changes, and electronic excitation on the properties of molecules; one common procedure makes use of projection density or charge difference plots. Charge density also may be partitioned among atoms, and by numerical integration over appropriate volume elements one may obtain atomic charges, dipoles, kinetic energies, and other properties of the atoms in a molecule. Many chemical phenomena have been analyzed in terms of charge densities.     

Superconducting pair correlations in an amorphous insulating nanohoneycomb film

The Cooper pairing mechanism that binds single electrons to form pairs in metals allows electrons to circumvent the exclusion principle and condense into a single superconducting or zero-resistance state. We present results from an amorphous bismuth film system patterned with a nanohoneycomb array of holes, which undergoes a thickness-tuned insulator-superconductor transition. The insulating films exhibit activated resistances and magnetoresistance oscillations dictated by the superconducting flux quantum h/2e. This 2e period is direct evidence indicating that Cooper pairing is also responsible for electrically insulating behavior.     

Force microscopy with light-atom probes

The charge distribution in atoms with closed electron shells is spherically symmetric, whereas atoms with partially filled shells can form covalent bonds with pointed lobes of increased charge density. Covalent bonding in the bulk can also affect surface atoms, leading to four tiny humps spaced by less than 100 picometers in the charge density of adatoms on a (001) tungsten surface. We imaged these charge distributions by means of atomic force microscopy with the use of a light-atom probe (a graphite atom), which directly measured high-order force derivatives of its interaction with a tungsten tip. This process revealed features with a lateral distance of only 77 picometers.     

绝缘接头或绝缘法兰绝缘性的讨论

绝缘接头或绝缘法兰是管道阴极保护中必不可少的重要管件，但在现场安装时都曾出现过绝缘性达不到替代洒求值的问题。对此现象进行了剖析，指出影响电绝缘性的主要原因是绝缘连接头的绝缘材料制造，装配等方面的质量问题。     

Controlled atomic doping of a single C60 molecule

We report a method for controllably attaching an arbitrary number of charge dopant atoms directly to a single, isolated molecule. Charge-donating K atoms adsorbed on a silver surface were reversibly attached to a C60 molecule by moving it over K atoms with a scanning tunneling microscope tip. Spectroscopic measurements reveal that each attached K atom donates a constant amount of charge (approximately 0.6 electron charge) to the C60 host, thereby enabling its molecular electronic structure to be precisely and reversibly tuned.     

Josephson junctions with tunable weak links

The electrical properties of organic molecular crystals, such as polyacenes or C60, can be tuned from insulating to superconducting by application of an electric field. By structuring the gate electrode of such a field-effect switch, the charge carrier density, and therefore also the superfluid density, can be modulated. Hence, weak links that behave like Josephson junctions can be fabricated between two superconducting regions. The coupling between the superconducting regions can be tuned and controlled over a wide range by the applied gate bias. Such devices might be used in superconducting circuits, and they are a useful scientific tool to study superconducting material parameters, such as the superconducting gap, as a function of carrier concentration or transition temperature.     

绝缘性陶瓷材料Si_3N_4电火花加工稳定性实验研究

用TiN蒸镀膜作辅助电极.在不同脉宽系数下对绝缘性陶瓷材料Si3N4的电火花加工稳定性进行 了实验研究,得出了脉宽系数越小,加工越稳定的结论。     

Surface charge--induced ordering of the au(111) surface

Synchrotron surface x-ray scattering (SXS) studies have been carried out at the Au(lll)/electrolyte interface to determine the influence of surface charge on the microscopic arrangement of gold surface atoms. At the electrochemical interface, the surface charge density can be continuously varied by controlling the applied potential. The top layer of gold atoms undergoes a reversible phase transition between the (1 x 1) bulk termination and a (23 x radical3) reconstructed phase on changing the electrode potential. In order to differentiate the respective roles of surface charge and adsorbates, studies were carried out in 0.1 M NaF, NaCl, and NaBr solutions. The phase transition occurs at an induced surface charge density of 0.07 +/- 0.02 electron per atom in all three solutions.     

The future of attosecond spectroscopy

Attoscience is the study of physical processes that occur in less than a fraction of a cycle of visible light, in times less than a quadrillionth of a second. The motion of electrons inside atoms and molecules that are undergoing photoionization or chemical change falls within this time scale, as does the plasma motion that causes the reflectivity of metals. The techniques to study motion on this scale are based on careful control of strong-field laser-atom interactions. These techniques and new research opportunities in attosecond spectroscopy are reviewed.     

The Resonating Valence Bond State in La2CuO4 and Superconductivity

The oxide superconductors, particularly those recently discovered that are based on La(2)CuO(4), have a set of peculiarities that suggest a common, unique mechanism: they tend in every case to occur near a metal-insulator transition into an odd-electron insulator with peculiar magnetic properties. This insulating phase is proposed to be the long-sought "resonating-valence-bond" state or "quantum spin liquid" hypothesized in 1973. This insulating magnetic phase is favored by low spin, low dimensionality, and magnetic frustration. The preexisting magnetic singlet pairs of the insulating state become charged superconducting pairs when the insulator is doped sufficiently strongly. The mechanism for superconductivity is hence predominantly electronic and magnetic, although weak phonon interactions may favor the state. Many unusual properties are predicted, especially of the insulating state.     

An experimental approach to the percolation of sticky nanotubes

Percolation is a statistical concept that describes the formation of an infinite cluster of connected particles or pathways. Lowering the percolation threshold is a critical issue to achieve light and low-cost conductive composites made of an insulating matrix loaded with conductive particles. This has interest for applications where charge dissipation and electrical conductivity are sought in films, coatings, paints, or composite materials. One route to decreasing the loading required for percolation is to use rod-like particles. Theoretical predictions indicate that this may also be achieved by altering the interaction potential between the particles. Although percolation may not always respond monotonically to interactions, the use of adhesive rods can be expected to be an ideal combination. By using a system made of carbon nanotubes in an aqueous surfactant solution, we find that very small attraction can markedly lower the percolation threshold. The strength of this effect can thereby have direct technological interest and explain the large variability of experimental results in the literature dealing with the electrical behavior of composites loaded with conducting rods.     

石墨烯基碱金属原子有效电荷变化规律

应用S.Yu.Davydov提出的石墨烯态密度模型,求出吸附在石墨烯上的碱金属原子的有效电荷数,研究了吸附原子的电子能级、能级移动量、有效电荷数随金属原子元素的变化以及有效电荷数随电子能量的变化规律.结果表明:(1)被吸附的碱金属原子的电子能级和能级移动量随原子序数的变化为非线性,在Li,Na,K,Rb,Se,Fr这6种碱金属原子中,以Na原子的值为最小,其原因在于碱金属原子的电离能以及石墨烯与吸附原子的相互作用能均随原子序数的增大而减小;(2)石墨烯能带电子和吸附原子的局域态电子对有效电荷的贡献以及总有效电荷数,均随原子序数的增加而非线性地减小.其中,能带电子对有效电荷的贡献与电子能量无关,而吸附原子局域态电子的贡献与总有效电荷数和电子能量都有关,且随电子能量的变化有明显的局域特点,最可几电子能量随原子序数的增大而增大.     

Orbital reconstruction and covalent bonding at an oxide interface

Orbital reconstructions and covalent bonding must be considered as important factors in the rational design of oxide heterostructures with engineered physical properties. We have investigated the interface between high-temperature superconducting (Y,Ca)Ba(2)Cu3O7 and metallic La(0.67)Ca(0.33)MnO3 by resonant x-ray spectroscopy. A charge of about -0.2 electron is transferred from Mn to Cu ions across the interface and induces a major reconstruction of the orbital occupation and orbital symmetry in the interfacial CuO2 layers. In particular, the Cu d(3z(2)-r(2)) orbital, which is fully occupied and electronically inactive in the bulk, is partially occupied at the interface. Supported by exact-diagonalization calculations, these data indicate the formation of a strong chemical bond between Cu and Mn atoms across the interface. Orbital reconstructions and associated covalent bonding are thus important factors in determining the physical properties of oxide heterostructures.     

Electromechanical resonators from graphene sheets

Nanoelectromechanical systems were fabricated from single- and multilayer graphene sheets by mechanically exfoliating thin sheets from graphite over trenches in silicon oxide. Vibrations with fundamental resonant frequencies in the megahertz range are actuated either optically or electrically and detected optically by interferometry. We demonstrate room-temperature charge sensitivities down to 8 x 10(-4) electrons per root hertz. The thinnest resonator consists of a single suspended layer of atoms and represents the ultimate limit of two-dimensional nanoelectromechanical systems.     

Potential energy fields about nitrogen in choline and ethanolamine: biological function at cellular surfaces

Partial charge distribution on first and second neighbor atoms to nitrogen in choline and ethanolamine have been calculated. Coulombic and steric parameters were then utilized to evaluate the interaction of a negative test charge with the two molecules. Both the position and the magnitude of the maximum of interaction energy in the two systems were significantly different. The results suggest that ethanolamine interacts more strongly with anions than choline does. This is due principally to steric repulsion of the negative charge by the methyl groups in choline.     

茶树短穗扦插2夹1膜覆盖保温技术研究

[目的]研究茶树短穗扦插2夹1膜覆盖保温技术。[方法]在原有双层覆盖的基础上,加盖1层塑料薄膜,形成2层塑料薄膜中间1层遮阳网——2夹1膜的覆盖保温方式。[结果]越冬期间,2夹1膜覆盖日最低温度平均值为2.32℃,高于无膜覆盖2.29℃,高于双膜覆盖1.79℃;区试期间,极端最低温度无膜覆盖为-11.51℃,双膜覆盖为-9.47℃,2夹1膜覆盖为-6.47℃;2夹1膜覆盖日平均温度为10.30℃,高于无膜覆盖4.93℃,高于双膜覆盖1.78℃;2夹1膜日最高温度平均值为28.12℃,高于双膜覆盖4.30℃,高于无膜覆盖14.30℃;采用2夹1膜覆盖技术,育苗成活率比双膜覆盖高4.48百分点,增值12 750元/hm2。[结论]2夹1膜具有很好的保温、增温能力;可消除越冬时的冻土抬苗现象,提高育苗的安全系数。     

Observation of giant diffusivity along dislocation cores

Diffusion of atoms in a crystalline lattice is a thermally activated process that can be strongly accelerated by defects such as grain boundaries or dislocations. When carried by dislocations, this elemental mechanism is known as "pipe diffusion." Pipe diffusion has been used to explain abnormal diffusion, Cottrell atmospheres, and dislocation-precipitate interactions during creep, although this rests more on conjecture than on direct demonstration. The motion of dislocations between silicon nanoprecipitates in an aluminum thin film was recently observed and controlled via in situ transmission electron microscopy. We observed the pipe diffusion phenomenon and measured the diffusivity along a single dislocation line. It is found that dislocations accelerate the diffusion of impurities by almost three orders of magnitude as compared with bulk diffusion.     

Chemistry. Superacids--it's al lot about anions

Superacids are powerful agents for protonating weak bases, with applications ranging from synthetic chemistry to batteries and fuel cells. DesMarteau explains in his Perspective that many superacids have undesirable properties, because after proton donation, the resulting anion may interact strongly with the protonated base or the solvent. Reed et al. report the discovery of a superacid that overcomes this problem. In the anion, charge is distributed efficiently over many atoms, reducing unwanted interactions.