High-frequency nanometre-scale vibration in 'quiescent' flagellar axonemes

The movement of cilia and flagella is based on the interaction between dynein arms and microtubules coupled with ATP hydrolysis. Although it is established that dynein arms cause adjacent microtubules to slide, little is known about the elementary process underlying the force production. To look more closely at the mechano-chemical conversion mechanism, we recently developed an optical method for measuring a nanometre-scale displacement with a time-resolution better than 1 ms. We now report the detection of high frequency (approximately 300 Hz) vibration of sub-nanometre amplitude in non-beating flagellar axonemes. This vibration could reflect the movement of individual activated dynein arms.     

Tracking kinesin-driven movements with nanometre-scale precision

Several enzyme complexes drive cellular movements by coupling free energy-liberating chemical reactions to the production of mechanical work. A key goal in the study of these systems is to characterize at the molecular level mechanical events associated with individual reaction steps in the catalytic cycles of single enzyme molecules. Ideally, one would like to measure movements driven by single (or a few) enzyme molecules with sufficient temporal resolution and spatial precision that these events can be directly observed. Kinesin, a force-generating ATPase involved in microtubule-based intracellular organelle transport, will drive the unidirectional movement of microscopic plastic beads along microtubules in vitro. Under certain conditions, a few (less than or equal to 10) kinesin molecules may be sufficient to drive either bead movement or organelle transport. Here we describe a method for determining precise positional information from light-microscope images. The method is applied to measure kinesin-driven bead movements in vitro with a precision of 1-2 nm. Our measurements reveal basic mechanical features of kinesin-driven movements along the microtubule lattice, and place significant constraints on possible molecular mechanisms of movement.     

Mesoscopic fast ion conduction in nanometre-scale planar heterostructures

Ion conduction is of prime importance for solid-state reactions in ionic systems, and for devices such as high-temperature batteries and fuel cells, chemical filters and sensors. Ionic conductivity in solid electrolytes can be improved by dissolving appropriate impurities into the structure or by introducing interfaces that cause the redistribution of ions in the space-charge regions. Heterojunctions in two-phase systems should be particularly efficient at improving ionic conduction, and a qualitatively different conductivity behaviour is expected when interface spacing is comparable to or smaller than the width of the space-charge regions in comparatively large crystals. Here we report the preparation, by molecular-beam epitaxy, of defined heterolayered films composed of CaF2 and BaF2 that exhibit ionic conductivity (parallel to the interfaces) increasing proportionally with interface density--for interfacial spacing greater than 50 nanometres. The results are in excellent agreement with semi-infinite space-charge calculations, assuming a redistribution of fluoride ions at the interfaces. If the spacing is reduced further, the boundary zones overlap and the predicted mesoscopic size effect is observed. At this point, the single layers lose their individuality and an artificial ionically conducting material with anomalous transport properties is generated. Our results should lead to fundamental insight into ionic contact processes and to tailored ionic conductors of potential relevance for medium-temperature applications.     

Controlled multiple quantum coherences of nuclear spins in a nanometre-scale device

The analytical technique of nuclear magnetic resonance (NMR) is based on coherent quantum mechanical superposition of nuclear spin states. Recently, NMR has received considerable renewed interest in the context of quantum computation and information processing, which require controlled coherent qubit operations. However, standard NMR is not suitable for the implementation of realistic scalable devices, which would require all-electrical control and the means to detect microscopic quantities of coherent nuclear spins. Here we present a self-contained NMR semiconductor device that can control nuclear spins in a nanometre-scale region. Our approach enables the direct detection of (otherwise invisible) multiple quantum coherences between levels separated by more than one quantum of spin angular momentum. This microscopic high sensitivity NMR technique is especially suitable for probing materials whose nuclei contain multiple spin levels, and may form the basis of a versatile multiple qubit device.     

有机分子电子输运性质的研究

利用从头计算理论和弹性散射格林函数的方法,对有机分子1,4-苯二硫酚（benzene-1,4-dithiol）和对苯二甲氰（1,4-phenylene diisocyanide）的电子输运性质进行了理论研究．结果显示两种分子都具有电导的平台效应和电流的线性响应,但分子与电极的耦合程度不同,电流的开启位置和电导平台的宽度与高度也有明显的区别．     

电子回旋共振加热期间的电子热输运和约束

采用ONETWO软件对在HL-2A托卡马克上电子回旋共振加热的实验进行分析,研究了ECRH加热期间的电子热输运和约束行为.ECRH引起了等离子体的共振区域电子温度上升,电子热输运系数变大.同时共振区域能量的增加使能量约束变坏,约束模式由一般模式(O模)转变为低约束模式(L模),能量约束时间下降.这些结果将对HL-2A中进一步提高安全运行和电子热输运的研究提供参考.     

Electronic measurement and control of spin transport in silicon

The spin lifetime and diffusion length of electrons are transport parameters that define the scale of coherence in spintronic devices and circuits. As these parameters are many orders of magnitude larger in semiconductors than in metals, semiconductors could be the most suitable for spintronics. So far, spin transport has only been measured in direct-bandgap semiconductors or in combination with magnetic semiconductors, excluding a wide range of non-magnetic semiconductors with indirect bandgaps. Most notable in this group is silicon, Si, which (in addition to its market entrenchment in electronics) has long been predicted a superior semiconductor for spintronics with enhanced lifetime and transport length due to low spin-orbit scattering and lattice inversion symmetry. Despite this promise, a demonstration of coherent spin transport in Si has remained elusive, because most experiments focused on magnetoresistive devices; these methods fail because of a fundamental impedance mismatch between ferromagnetic metal and semiconductor, and measurements are obscured by other magnetoelectronic effects. Here we demonstrate conduction-band spin transport across 10 mum undoped Si in a device that operates by spin-dependent ballistic hot-electron filtering through ferromagnetic thin films for both spin injection and spin detection. As it is not based on magnetoresistance, the hot-electron spin injection and spin detection avoids impedance mismatch issues and prevents interference from parasitic effects. The clean collector current shows independent magnetic and electrical control of spin precession, and thus confirms spin coherent drift in the conduction band of silicon.     

In-Sn20合金熔体电输运性质的研究

文章以直流四电极法和微差法分别测量了In-Sn20合金熔体的电导率σ和热电势S随温度的变化,并对不同温度下的熔体进行了X射线衍射实验。结果表明,合金熔体的电导率和热电势均在远高于液相线几百度的温区内发生了突变,且2种电子输运性质的突变温度一致,表明合金熔体在升温过程中可能发生了某种结构转变。利用Fiber-Ziman理论及赝势模型,计算并分析了合金熔体的电子状态密度N(EF)及其随能量的变化率dN(EF)/dE,发现N(EF)和dN(EF)/dE在一定温度范围内也随温度发生了突变。     

糖尿病患者红细胞膜葡萄糖运输速率的变化

测定了１８例糖尿病患者（其中Ⅰ型５例，Ⅱ型１３例）红细胞膜上葡萄糖跨膜运输速率的变化，结果显示患者的葡萄糖输出速率略有减小，但变化不显著，虽然在１３例Ⅱ型糖尿病患者中的２例显示出最大运输速度（Ｖｍａｘ）和半饱和浓度（Ｋｍ）有变化的趋势．对９例Ⅱ型糖尿病患者红细胞的葡萄地跨膜输入测定表明，患者的葡萄糖跨膜向内运输速率较正常的显著降低．葡萄糖向内和向外运输速率变化的差异表明在糖尿病患者红细胞膜上葡萄糖运输蛋白的外侧葡萄糖结合位点发生了某种变化．     

A nanometre-scale electronic switch consisting of a metal cluster and redox-addressable groups

So-called bottom-up fabrication methods aim to assemble and integrate molecular components exhibiting specific functions into electronic devices that are orders of magnitude smaller than can be fabricated by lithographic techniques. Fundamental to the success of the bottom-up approach is the ability to control electron transport across molecular components. Organic molecules containing redox centres-chemical species whose oxidation number, and hence electronic structure, can be changed reversibly-support resonant tunnelling and display promising functional behaviour when sandwiched as molecular layers between electrical contacts, but their integration into more complex assemblies remains challenging. For this reason, functionalized metal nanoparticles have attracted much interest: they exhibit single-electron characteristics (such as quantized capacitance charging) and can be organized through simple self-assembly methods into well ordered structures, with the nanoparticles at controlled locations. Here we report scanning tunnelling microscopy measurements showing that organic molecules containing redox centres can be used to attach metal nanoparticles to electrode surfaces and so control the electron transport between them. Our system consists of gold nanoclusters a few nanometres across and functionalized with polymethylene chains that carry a central, reversibly reducible bipyridinium moiety. We expect that the ability to electronically contact metal nanoparticles via redox-active molecules, and to alter profoundly their tunnelling properties by charge injection into these molecules, can form the basis for a range of nanoscale electronic switches.     

拓扑绝缘体与低维超导体的电输运特性

拓扑绝缘体这种具有新奇量子特性的物质和具有百余年研究历史的超导体都是当前凝聚态物理学领域的研究热点．本文简要介绍了拓扑绝缘体、超导体的研究背景和基本特性,重点回顾了拓扑绝缘体、超导薄膜及纳米桥、超导纳米线以及纳米尺度下拓扑绝缘体．超导异质结构的电输运特性．并对该领域的进一步发展做出了展望．     

Electronic spin transport and spin precession in single graphene layers at room temperature

Electronic transport in single or a few layers of graphene is the subject of intense interest at present. The specific band structure of graphene, with its unique valley structure and Dirac neutrality point separating hole states from electron states, has led to the observation of new electronic transport phenomena such as anomalously quantized Hall effects, absence of weak localization and the existence of a minimum conductivity. In addition to dissipative transport, supercurrent transport has also been observed. Graphene might also be a promising material for spintronics and related applications, such as the realization of spin qubits, owing to the low intrinsic spin orbit interaction, as well as the low hyperfine interaction of the electron spins with the carbon nuclei. Here we report the observation of spin transport, as well as Larmor spin precession, over micrometre-scale distances in single graphene layers. The 'non-local' spin valve geometry was used in these experiments, employing four-terminal contact geometries with ferromagnetic cobalt electrodes making contact with the graphene sheet through a thin oxide layer. We observe clear bipolar (changing from positive to negative sign) spin signals that reflect the magnetization direction of all four electrodes, indicating that spin coherence extends underneath all of the contacts. No significant changes in the spin signals occur between 4.2 K, 77 K and room temperature. We extract a spin relaxation length between 1.5 and 2 mum at room temperature, only weakly dependent on charge density. The spin polarization of the ferromagnetic contacts is calculated from the measurements to be around ten per cent.     

与非共线铁磁引线耦合的双量子点环中电子的输运

用非平衡格林函数理论研究与两个铁磁引线连接的平行连耦合双量子点系统中电子输运性质。两个铁磁引线的磁矩相互成任意角度,即磁矩是非共线性的。通过调节磁矩间的角度,可以控制不同自旋方向电子的输运性质,从而能充分操作通过整个系统的电流和隧穿磁阻。两个铁磁弓I线磁矩间的相对角度对量子点中不同自旋电子的占据数有显著的影响,是决定电流和隧穿磁阻性质的内在机制。量子点间的耦合强度、量子点与引线的耦合强度等也会对系统的电流有明显的作用,从而提供了丰富的电流操控手段。本文研究的系统可以用作自旋阀,在自旋电子学器件中有实际的应用价值。     

An equivalent 1D nanochannel model to describe ion transport in multilayered graphene membranes

Multilayered graphene-based membranes are promising for a variety of applications related to ion or molecule transport, such as energy storage and water treatment. However, the complex three-dimensional cascading nanoslit-like structure embedded in the membrane makes it difficult to interpret and rationalize experimental results, quantitatively compare with the traditional membrane systems, and quantitatively design new membrane structures. In this paper, systematic numerical simulations were performed to establish an equivalent onedimensional(1 D) nanochannel model to represent the structure of multilayered graphene membranes. We have established a quantitative relationship between effective diffusion length Leffand cross-section area Aeffof the1 D model and our recently developed two dimensional(2 D) representative microstructure for graphene membranes. We find that only in the cases of a relatively large lateral size L( ~100 nm) and a small slit size h( 2 nm), the effective diffusion length Leffand Aeffcan be calculated by an over-simplified but often used model. Otherwise, they show complex dependence on all three structural parameters of the 2 D structural model.Our equivalent 1 D nano-channel model can reproduce experimental results very well except for h 0.5 nm. The discrepancy could be attributed to the anomalous behaviour of molecules under nano-confinement that is not considered in our simulations. This model can also be extended to multilayered membranes assembled by other2 D materials.     

金属与聚乙炔环三端连接的电子输运性质

基于Landauer-Buttiker理论,用求解散射波函数的方法,研究了金属与环状聚乙炔链三端连接的电子输运性质.结果表明,在给定入射端的条件下,电导在正负能量上是对称的,当3个端口满足旋转对称性时,2个出射端的电导相同.电导随着导线与聚乙炔链间耦合强度的不同而呈现不同的性质,弱耦合时电导突变明显;耦合增强后,电导趋于稳定.各端的电导跟电子在该端口碳原子上的局域密度有关,随着导线与碳链间耦合强度的增大,局域密度随能量的振荡会减弱.