Reversible, nanometer-scale conductance transitions in an organic complex

Reversible conductance transitions are demonstrated on the molecular scale in a complex of 3-nitrobenzal malononitrile and 1, 4-phenylenediamine, by application of local electric field pulses. Both macroscopic and local current-voltage (I/V) measurements show similar electrical bistability behavior. The mechanism of the electrical bistability is discussed.     

Optical bistability in fluorescein dyes

Optical bistability has been observed in highly concentrated fluorescein dye solutions and in thin (1 m) doped polymeric films. At concentrations larger than 10^–5 mole/l dye dimers are formed. For fluorescein dye, the dimer-monomer equilibrium constant is 10⁵ l/mole so that most of the dye species are in the dimer form. At 480 nm the dimer absorption cross section is 10^–18 cm²/molecule, while that for the dye monomer molecule is 7.6×10^–17 cm²/molecule. Upon laser excitation dimers dissociate to form monomers thus providing a highly nonlinear laser induced absorption. This high nonlinear absorption coefficient can be utilized for optically bistable response of the dye system.Optical bistability was observed by placing dye solutions or dye thin films inside a Fabry-Perot resonator and exciting it with 480 nm dye laser pulses of 10 ns duration. The effect is more pronounced in 10^–4 mole/l fluorescein than in 10^–6 mole/l fluorescein in which dimer formation is not that efficient.In disodium fluorescein no significant dimer formation is observed even at 10^–3 mole/l dye concentration. The observed bistability both in solution and in thin films can be explained in terms of recent models for optical bistability in nonlinearly absorbing molecular systems.     

基于Rotaxane类分子的稳定、重复、可反复擦写的纳米信息存储

Rotaxane类分子在溶液中可以发生可逆的分子构型改变,并随之引起分子电导特性的转变,在纳米电子器件和分子存储器件中具有潜在的应用前景.但是还不能确定这类分子在固体薄膜中是否具有类似于在溶液中的结构与电导转变,需要对Rotaxane类分子固态薄膜进行深入的结构和特性研究.文章作者在一类Rotaxane分子H1和H2的固态薄膜上获得了纳米尺度的电导转变和稳定、重复的、近于单分子尺度的纳米级存储;同时,成功地在H2分子薄膜上实现了信息记录点的可反复擦写.另外,在单个分子和亚分子的水平上直接观察到了Rotaxane分子在外电场诱导下分子结构的可逆变化以及随之发生的相应电导特性的可逆转变, 证实了Rotaxane分子在固态薄膜中的可逆结构和电导转变.     

基于Rotaxane类分子的稳定、重复、可反复擦写的纳米信息存储

Rotaxane类分子在溶液中可以发生可逆的分子构型改变,并随之引起分子电导特性的转变,在纳米电子器件和分子存储器件中具有潜在的应用前景.但是还不能确定这类分子在固体薄膜中是否具有类似于在溶液中的结构与电导转变,需要对Rotaxane类分子固态薄膜进行深入的结构和特性研究.文章作者在一类Rotaxane分子H1和H2的固态薄膜上获得了纳米尺度的电导转变和稳定、重复的、近于单分子尺度的纳米级存储;同时,成功地在H2分子薄膜上实现了信息记录点的可反复擦写.另外,在单个分子和亚分子的水平上直接观察到了Rotax     

Tubulin bistability and polymorphic dynamics of microtubules

Based on the hypothesis that the GDP-tubulin dimer is a conformationally bistable molecule-rapidly fluctuating between a discrete curved and a straight state-we develop a model for polymorphic dynamics of the microtubule lattice. We show that GDP-tubulin bistability consistently explains unusual dynamic fluctuations, the apparent length-stiffness relation of grafted taxol-stabilized microtubules, and the curved-helical appearance of microtubules in general. When clamped by one end the microtubules undergo an unusual zero energy motion-in its effect reminiscent of a limited rotational hinge. We conclude that microtubules exist in highly cooperative energy-degenerate helical states and discuss possible implications in vivo.     

ULTRAHIGH DATA DENSITY STORAGE WITH SCANNING TUNNELING MICROSCOPY

Ultrahigh density data storage devices made by scanning probe techniques based on various recording media and their corresponding recording mechanisms, have attracted much attention recently, since they ensure a high data density in a non-volatile, erasable form in some kinds of ways. It is of particular interest to employ organic polymers with novel functional properties within a single molecule (or a single molecular complex) for fabricating electronic devices on a single molecular scale. Here, it is reported that a new process for ultrahigh density and erasable data storage, namely, molecular bistability on an organic charge transfer complex of 3-nitrobenzal malononitrile and 1,4-phenylenediamine (NBMN-pDA) switched by a scanning tunneling microscope (STM). Data density exceeds 10¹³ bits/cm² with a writing time per bit of ～1μs. Current-voltage (I/V) measurements before and after the voltage pulse from the STM tip, together with optical absorption spectroscopy and macroscopic four-probe I/V measurements demonstrate that the writing mechanism is conductance transition in the organic complex. This mechanism offers an attractive combination of ultrahigh data density coupled with high speed. The ultimate bit density achievable appears to be limited only by the size of the organic complex, which is less than 1nm in our case, corresponding to 10¹⁴ bits/cm². We believe that provided the lifetime can be improved, molecular bistability may represent a practical route for ultrahigh density data storage devices.     

Electrical bistable devices using composites of zinc sulfide nanoparticles and poly-(N-vinylcarbazole)

N-dodecanethiol capped zinc sulfide(Zn S) nanocrystals were synthesized by the one-pot approach and blended with poly(N-vinylcarbazole)(PVK) to fabricate electrical bistable devices. The corresponding devices did exhibit electrical bistability and negative differential resistance(NDR) effects. A large ON/OFF current ratio of 104 at negative voltages was obtained by applying different amplitudes of sweeping voltage. The observed conductance switching and the negative differential resistance are attributed to the electric-field-induced charge transfer between the nanocrystals and the polymer,and the charge trapping/detrapping in the nanocrystals.     

Laser-driven blackbody radiator with bistability

We report a particular construction of a laser-driven blackbody radiator with bistability mode based on efficient light-into-heat conversion of a rare earth system. The laser-induced thermal avalanche nonlinearity and the internal stimulative feedback mechanisms are revealed to interpret the typical S-pattern bistability. The standard blackbody radiation and the sizable bistability mode are experimentally demonstrated through ultrabroadband thermal spectra measurements of ZrO₂:Yb–Tm nanophase compounds. Such a noncontact, laser-driven scheme for microscale blackbody radiation has attractive applications for compact standard spectra source and broadband spectra switching in the on-chip all-optical systems.     

Electric field gradients in niobium pentachloride dimer

Electric field gradient tensors at the bridging chlorine sites in niobium pentachloride dimer have been evaluated using point-charge model with de Wette’s planewise summation method for rapid convergence of the lattice sums. The results are discussed based on the experimental results and the existing theory on dimeric molecules.     

Optical bistability in semiconductors induced by thermal effects

We investigate thermally induced optical bistability (OB) in semiconductors using a cw-Argon-ion-laser beam pulsed by an electro-optic modulator. We obtain different types of OB due to absorptive and refractive changes with switching times in the order of ms. The different processes are modeled using a simple rate equation for the lattice temperature. Good agreement between theoretical predictions and the measurements are clearly to be seen.     

Controllable optical bistability of Bose—Einstein condensate in an optical cavity with Kerr medium

We study the optical bistability for a Bose-Einstein condensate of atoms in a driven optical cavity with Kerr medium. We find that both the threshold point of optical bistability transition and the width of optical bistability hysteresis can be controlled by appropriately adjusting the Kerr interaction between the photons. In particular, we show that the optical bistability will disappear when the Kerr interaction exceeds a critical value.     

Photothermal and electrooptic nonlinearities in ZnSe and ZnTe and their application in bistable devices

In this paper we present results on ZnSe and ZnTe optically bistable elements in connection with electrooptic effects. The behaviour of optical hysteresis under an applied electric field is studied and the electrical characteristics under illumination with monochromatic laser radiation are investigated. Influences of optical bistability on the photocurrent are measured and an attempt is made to explain the unusual behaviour of the current with respect to the absorbed amount of light, namely a switching down in photocurrent when the absorptive switching takes place. Towards this aim we report here for the first time on spatially resolved measurements of photoconductivity in ZnSe to investigate the influence of Schottky contacts on photothermal electrooptic bistability. Furthermore, we report also to our knowledge for the first time on the influence of the 3D-Stank effect on optical bistability.     

射频超导谐振腔中的电磁场研究

由高纯铌制成的射频超导谐振腔在高场时会出现Q值下降现象，这是高场下超导腔表面局部磁场增强所致. 为研究高场下超导腔表面的电磁场特性，用单cell超导腔设计了腔表面峰值磁场的研究实验. 通过对单cell超导腔表面峰值磁场的测量，可以确定超导腔焊缝及iris附近表面是否存在缺陷，以便进一步进行腔的处理来改进腔的性能.     

用于射频超导腔制造的大晶粒铌材热导研究（英）

大晶粒铌材由于声子峰效应在2 K温度具有良好的热导,这有助于提高超导腔的热稳定性。对宁夏东方钽业股份有限公司（OTIC）生产的大晶粒铌材在不同热处理条件下的热导和晶格缺陷开展了研究,结果表明经过超过800 ℃的热处理,OTIC的大晶粒铌材能够恢复声子峰,这与之前DESY的测量结果不同。认为用其加工的超导腔有望具备较好热稳定性。     

Controllable optical bistability of Bose-Einstein condensate in an optical cavity with a Kerr medium

We study the optical bistability for a Bose-Einstein condensate of atoms in a driven optical cavity with a Kerr medium. We find that both the threshold point of optical bistability transition and the width of optical bistability hysteresis can be controlled by appropriately adjusting the Kerr interaction between the photons. In particular, we show that the optical bistability will disappear when the Kerr interaction exceeds a critical value.     

Nature of the eigenstates in the miniband of random dimer-barrier superlattices

The dc conductance, the universal quantum fluctuations and the resistance distribution are numerically investigated in dimer semiconductor superlattices by means of the transfer matrix formalism. We are interested in the GaAs/Al_x Ga _1 − xAs layers, having identical thickness, where the aluminium concentration x takes, at random, two different values, with the constraint that one of them appears only in pairs, i.e. the random dimer barrier (RDB). These systems exhibit a miniband of extended states, around a critical energy, lying to the typical structure of the dimer cell. The states close to this resonant energy consist of weakly localized states, while in band tails i.e. for negligible conductance, the states are strongly localized. This is evidence of the suppression of localization in the RDB superlattices. The nature of the transition between these two regimes is quantitatively investigated through relevant physical quantities. The model is, hence, clearly and statistically examined.     

用于射频超导腔制造的大晶粒铌材热导研究（英）

大晶粒铌材由于声子峰效应在2 K温度具有良好的热导,这有助于提高超导腔的热稳定性。对宁夏东方钽业股份有限公司（OTIC）生产的大晶粒铌材在不同热处理条件下的热导和晶格缺陷开展了研究,结果表明经过超过800 ℃的热处理,OTIC的大晶粒铌材能够恢复声子峰,这与之前DESY的测量结果不同。认为用其加工的超导腔有望具备较好热稳定性。     

Electron microscope study of precipitation in a niobium-containing (Nd,Dy)-Fe-B sintered magnet

The microstructure of a niobium-containing (Nd, Dy)-Fe-B alloy has been investigated using transmission electron microscopy, STEM and SEM X-ray microanalysis and optical microscopy, Magnetic measurements showed an increase in coercivity when niobium was added to a (Nd, Dy)-Fe-B magnet. The microstructure was found to be similar to that of a ternary Nd-Fe-B magnet but with two additional phases, a Laves Fe₂Nb compound and a finely dispersed niobium containing coherent precipitate found in the magnetically hard phase. Lorentz microscopy indicated domain wall interactions with the fine precipitates which may be responsible for the enhanced coercivity of the niobium doped magnet.     

The peculiarities of the tunneling conductance and phonon induced features in tetrahedrically bonded semiconductors under pressure

Abstract

Summary The 30 kbar high pressure unit for low temperatures and the digital processing have been applied to the tunneling investigations of Ge and GaAs. The obtained results include the values of the mode Gruneisen constants for zone boundary phonons in <100> direction in Ge, the irregular optical phonons behavior in GaAs, the bistability of tunneling voltage-current curve for GaAs at elevated pressure and the observation of optically induced oscillations on the tunneling conductance of GaAs Schottky barrier. The semi-quantitative model involving the conductance band structure change under pressure and DX-centers in GaAs(Te) is used for the tentative explanation.     

Synthesis of niobium oxide nanopowders by field-crystallization-assisted anodization

Herein, for the first time, we demonstrate that a large quantity of niobium oxide nanopowders consisting of amorphous round shape nanoparticles and highly crystalline needle shape nanoparticles can be prepared by anodization of a niobium foil in ethylene glycol containing NH₄F at a high voltage. FE-SEM, TEM, XRD measurements were performed to characterize the anodically prepared nanopowders and to reveal the formation mechanism. We ascribed the formation of niobium oxide nanopowders on the foil to field-crystallization induced breakdown of oxide film and chemical dissolution.