Electrically controlled metal-insulator transition process in VO2 thin films

We report the evolution process of VO(2) thin films from the insulating phase to the metallic phase under current injection for the two-electrode-based thin-film devices. Based on electrical characterization and Raman microscopic detection, it was found that there exist two critical current densities, based on which the insulator-to-metal transition process can be divided into three stages. In stage I with low current injection, the VO(2) film in the insulating (semiconducting) phase acts as a resistor until the first critical current density, above which the insulator-metal transition is a percolation process with metallic rutile and insulating monoclinic phases coexisting (stage II); while beyond a second critical current density, a filamentary current path with pure metallic phase is formed with the remaining part outside of the current path receding back to the pure insulating phase (stage III). We confirm that a critical current density is required for the onset of electrically induced insulator-to-metal transition in VO(2) thin films.     

Neuronal-like Irregular Spiking Dynamics in Highly Volatile Memristive Intermediate-scale AgPt-Nanoparticle Assemblies

Neuromorphic computing seeks functional materials capable of emulating brain-like dynamics to solve computational problems with time and energy efficiency, outclassing current transistor-based hardware architectures. Major efforts are focused on integrating memristive devices into highly regular circuits (i.e., crossbar arrays), where the information representation in individual memristive devices is closely oriented toward the behavior of artificial neurons. However, artificial neurons are rather rigid mathematical concepts than realistic projections of complex neuronal dynamics. Neuroscience suggests that highly efficient information representation on the level of individual neurons relies on dynamical features such as excitatory and inhibitory contributions, irregularity of firing patterns, and temporal correlations. Here, a conductive atomic force microscopy approach is applied to probe the memristive dynamics of nanoscale assemblies of AgPt-nanoparticles at the stability border of the conducting state, where physical forces causing the formation and decay of filamentary structures appear to be balanced. This unveils a dynamic regime, where the memristive response is governed by irregular firing patterns. The significance of such a dynamical regime is motivated by close similarities to excitation and inhibition-governed behavior in biological neuronal systems, which is crucial to tune biological neuronal systems into a state most suitable for information representation and computation.     

110 GHz微波电离大气产生等离子体过程的理论研究

将描述电磁波的Maxwell方程组和简化的等离子体流体方程组耦合数值求解,对110 GHz微波电离大气产生等离子体的过程进行了理论研究. 研究发现:在高气压下等离子体成丝状;中等气压下等离子体先成丝状,在向微波源移动的过程中逐渐向连续的等离子体区域过渡;低气压下电离产生连续的等离子体区域. 不同气压下等离子体区域都向微波源方向移动. 初始电子数密度分布只影响放电初始阶段的等离子体区域形状,不会影响成丝与否. 等离子体区域在垂直于电场方向和平行于电场方向的移动规律不同. 当电场平行于计算平面时,由于沿着电场方向等离子体两端存在强场区,等离子体区域被拉长,在较低的气压下会出现等离子体丝阵. 关键词： 110 GHz微波 大气电离 等离子体丝阵     

On the possibility of harmonic operation of cyclotron waveparametric amplifiers

Cyclotron wave amplifiers at the harmonics of the electron cyclotron frequency are investigated. Since the waves on the beam are electrostatic, harmonics are strongly excited in nonrelativistic beams if they are rotating rather than filamentary. These modes at the harmonics can couple to input Cuccia couplers, and pump fields which drive parametric amplification, in very much the same way as they do on filamentary beams at the cyclotron frequency. Harmonic cyclotron wave amplifiers have the possibility of giving rise to a new class of devices at millimeter wave frequencies     

下电极对ZnO薄膜电阻开关特性的影响

本文采用直流磁控溅射法在三种不同的下电极(BEs)上制备了ZnO薄膜, 获得了W/ZnO/BEs存储器结构. 研究了不同的下电极材料对器件电阻开关特性的影响. 研究结果表明, 以不同下电极所制备的器件都具有单极性电阻开关特性. 在低阻态时, ZnO薄膜的导电机理为欧姆传导, 而高阻态时薄膜的导电机理为空间电荷限制电流. 不同下电极与ZnO薄膜之间的肖特基势垒高度对电阻开关过程中的操作电压有较大的影响, 并基于导电细丝模型对不同下电极上ZnO薄膜的低阻态阻值及reset电流的变化进行了解释. 关键词： ZnO薄膜 电阻开关 下电极     

DYNAMICS IN A DRIVEN DISORDERED VORTEX LATTICE: A NUMERICAL STUDY

Using a repulsive vortex-vortex interaction model, we numerically investigate the dynamics of vortices in a driven disordered two-dimensional superconductor. At low driving forces and temperatures, we have seen the evidence of plastic and filamentary flow. At high driving forces and very low temperatures, the evidence of an ordering of the moving vortices is seen in the flux flow regime. Our results are in agreement with the previous simulation results and recent experiments.     

Soliton–like pulse dynamics in add–drop optical filters based on nonlinear fiber grating couplers

The dynamics characteristics of picosecond soliton-like pulses propagating through add–drop optical filters, based on lossless uniform and Gaussian nonlinear fiber grating couplers, are numerically investigated under different power levels. To obtain the temporal profiles, the nonlinear coupled-mode equations, that model these devices, are solved numerically. These devices can perform the extraction (drop) of an optical signal, since there is the reflection of a pulse previously switched between adjacent waveguides. The extraction efficiency, recurrently better at low power levels, is subordinated to the matching between the associated spectra of the input pulses and the hybrid devices analyzed. The switching effects, as well the gratings responsiveness, impact on the outputs profiles, since diverse temporal shapes can be obtained. The results reinforce the importance and the potential of properly engineered structures for all-optical processing requirements.     

Dependence of the yield stress of filamentary sodium chloride crystals on the starting dislocation structure

The onset of plastic flow of filamentary NaCl crystals with the simplest types of starting dislocation structure was studied under conditions which completely exclude the effect of stress concentration in the clamps of the microdeformation machine. It was established that the growth dislocations, having an outlet to the side boundaries, do not exclude the appearance of a sharp projection of the flow on the stretching diagram, though they reduce its height. The flow projection remains in samples with traces of microdeformation, formed by random effects during manipulation; in addition, the behavior of the filamentary crystals at the yield stress depends on the nature of these traces.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 4, pp. 20–24, April, 1986.     

Periodic orbit analysis at the onset of the unstable dimension variability and at the blowout bifurcation

Many chaotic dynamical systems of physical interest present a strong form of nonhyperbolicity called unstable dimension variability (UDV), for which the chaotic invariant set contains periodic orbits possessing different numbers of unstable eigendirections. The onset of UDV is usually related to the loss of transversal stability of an unstable fixed point embedded in the chaotic set. In this paper, we present a new mechanism for the onset of UDV, whereby the period of the unstable orbits losing transversal stability tends to infinity as we approach the onset of UDV. This mechanism is unveiled by means of a periodic orbit analysis of the invariant chaotic attractor for two model dynamical systems with phase spaces of low dimensionality, and seems to depend heavily on the chaotic dynamics in the invariant set. We also described, for these systems, the blowout bifurcation (for which the chaotic set as a whole loses transversal stability) and its relation with the situation where the effects of UDV are the most intense. For the latter point, we found that chaotic trajectories off, but very close to, the invariant set exhibit the same scaling characteristic of the so-called on-off intermittency.     

Chemical transients in closed chaotic flows: the role of effective dimensions

We investigate chemical activity in hydrodynamical flows in closed containers. In contrast to open flows, in closed flows the chemical field does not show a well-defined fractal property; nevertheless, there is a transient filamentary structure present. We show that the effect of the filamentary patterns on the chemical activity can be modeled by the use of time-dependent effective dimensions. We derive a new chemical rate equation, which turns out to be coupled to the dynamics of the effective dimension, and predicts an exponential convergence. Previous results concerning activity in open flows are special cases of this new rate equation.     

Study on the transition from filamentary discharge to diffuse discharge by using a dielectric barrier surface discharge device

Discharge characteristics have been investigated in different gases under different pressures using a dielectric barrier surface discharge device. Electrical measurements and optical emission spectroscopy are used to study the discharge, and the results obtained show that the discharges in atmospheric pressure helium and in low-pressure air are diffuse, while that in high-pressure air is filamentary. With decreasing pressure, the discharge in air can transit from filamentary to diffuse one. The results also indicate that corona discharge around the stripe electrode is important for the diffuse discharge. The spectral intensity of N电介质 表面放电 扩散放电 发射光谱学dielectric barrier surface discharge, diffuse discharge, optical emission spectroscopyProject supported by the National Natural Science Foundation of China (Grant Nos 10575027 and 10647123), the National Science Foundation of Hebei Province, China (Grant No A2007000134), the Education Department of Hebei Province, China (Grant No 2006106),2006-10-24Discharge characteristics have been investigated in different gases under different pressures using a dielectric barrier surface discharge device. Electrical measurements and optical emission spectroscopy are used to study the discharge, and the results obtained show that the discharges in atmospheric pressure helium and in low-pressure air are diffuse, while that in high-pressure air is filamentary. With decreasing pressure, the discharge in air can transit from filamentary to diffuse one. The results also indicate that corona discharge around the stripe electrode is important for the diffuse discharge. The spectral intensity of N＋ （391.4nm） relative to N2 （337.1 nm） is measured during the transition from diffuse to filamentary discharge. It is shown that relative spectral intensity increases during the discharge transition. This phenomenon implies that the averaged electron energy in diffuse discharge is higher than that in the filamentary discharge.     

On certain properties of filamentary RF discharge

Results of studying the plasma-formation dynamics of the capacitive rf discharge in inert gases and in the air at atmospheric pressure are presented. Measurements have been conducted using an original technique for determining the sizes of luminous objects with an emission-line spectrum. The conditions for a transition from a torch discharge to a filamentary one are found. A qualitative model of the formation of a filament discharge taking into account its inherent electromagnetic field is offered.     

Charge trapping phenomena in high-efficiency metal-oxide-silicon light-emitting diodes with ion-implanted oxide

This work is a comparative study of the processes of charge trapping in silicon dioxide layers doped with different rare-earth (RE) impurities (Gd, Tb, Er) as well as with Ge. Diode SiO₂-Si structures incorporating such oxide layers exhibit efficient electroluminescence (EL) in the spectral range of UV to IR. Ion implantation was performed over a wide dose range with the implant profiles peaking in the middle of the oxide. Charge trapping was studied using an electron injection technique in constant current regime with simultaneous measurements of the EL intensity (ELI). High-frequency C/V characteristics were used to monitor the net charge in the oxides.Analysis of the charge trapping and the variation of the EL intensity during electron injection shows that the current density range can be divided in three portions: (i) low injection level, where electron/hole capture at traps with large capture cross-sections and low ELI occurs; (ii) medium injection level corresponding to the main operation mode of the devices (odd hole trapping depending on the injected current level is observed); and (iii) high injection level (electrical quenching of the EL that correlates with electron capture at traps of extremely small capture cross-sections takes place). The nature of specific hole trapping at the medium injection level in RE-doped devices is discussed. Mechanisms of EL quenching at the high injection level are proposed.     

Nonvolatile unipolar resistive switching in ultrathin films of graphene and carbon nanotubes

We report unipolar resistive switching in ultrathin films of chemically produced graphene (reduced graphene oxide) and multiwalled carbon nanotubes. The two-terminal devices with yield >99% are made at room temperature by forming continuous films of graphene of thickness ∼20 nm on indium tin oxide coated glass electrode, followed by metal (Au or Al) deposition on the film. These memory devices are nonvolatile, rewritable with ON/OFF ratios up to ∼ 10⁵ and switching times up to 10 μs. The devices made of MWNT films are rewritable with ON/OFF ratios up to ∼400. The resistive switching mechanism is proposed to be nanogap formation and filamentary conduction paths.     

有机铬富勒烯衍生物的激发态与光限幅特性

应用倍频ｎｓ／ｐｓＮｄ：ＹＡＧ脉冲激光系统,在波长为５３２ｎｍ,脉冲宽度为２１ｐｓ的条件下,研究了新型有机铬富勒烯衍生物的激发态吸收与光限幅特性,其光限幅特性优于富勒烯甲苯溶液;并应用单重态激发态吸收理论对实验结果进行了分析,实验结果与理论结果基本一致。     

介质阻挡放电丝模式和均匀模式转化的特性

利用水电极介质阻挡放电装置，采用电学方法和发射光谱，研究了空气中介质阻挡放电从微放电丝模式向均匀放电模式转化的过程. 结果表明，大气压下增大外加电压或者电压一定减小气压，放电都能够从微放电丝模式过渡到均匀模式. 高气压下放电为流光击穿而低气压下为辉光放电. 利用放电发射光谱，研究了高能电子比例随实验参数的变化. 结果表明气压减小时高能电子比例增大，电压增加时高能电子减少. 利用壁电荷理论对以上实验结果进行了定性分析. 结果对介质阻挡均匀放电的深入研究具有重要价值.     

介质阻挡放电丝模式和均匀模式转化的特性

利用水电极介质阻挡放电装置,采用电学方法和发射光谱,研究了空气中介质阻挡放电从微放电丝模式向均匀放电模式转化的过程. 结果表明,大气压下增大外加电压或者电压一定减小气压,放电都能够从微放电丝模式过渡到均匀模式. 高气压下放电为流光击穿而低气压下为辉光放电. 利用放电发射光谱,研究了高能电子比例随实验参数的变化. 结果表明气压减小时高能电子比例增大,电压增加时高能电子减少. 利用壁电荷理论对以上实验结果进行了定性分析. 结果对介质阻挡均匀放电的深入研究具有重要价值. 关键词： 介质阻挡放电 光学发射谱 微放电丝 均匀放电模式     

Phase stability of gyroklystron amplifier

Expressions are derived for RF phase stability in conventional klystrons and gyroklystrons. Phase noise is found to depend on electron energy in a completely different way for the two types of devices, due to the inherent differences in gun dynamics and interaction mechanisms. In general, phase stability is better in gyroklystrons operating at voltage ⩽100 kV, while klystrons have somewhat better phase stability at higher operating voltage. An experimental study of phase stability in a four-cavity, 7.5-kV conventional klystron and in a three-cavity, 30-kV gyroklystron confirmed theoretical predictions. Phase sensitivity in the conventional klystron was 13° per percent change in voltage, and in gyroklystron was 4° per percent change in voltage. The magnitude and frequency of the measured phase jitter was well correlated with the droop and ripple on the beam voltage. Phase sensitivity to other parameters such as input power, magnetic field strength, and input frequency were also studied experimentally     

Sintering dynamics and thermal stability of novel configurations of Ag clusters

Monte Carlo simulations with second-moment approximation of tight-binding potential were applied to study the sintering dynamics and thermal stability for novel configurations of Ag clusters. Simulations under elevated temperatures utilizing various configurations indicated that sintering processes were strongly affected by temperature and initial design configurations. Ag clusters re-aligned themselves at the onset of sintering, forming clear necks of varying stabilities and different matter diffusion routes between clusters due to differences in initial design configurations. Notably, different Ag cluster design configurations displayed variable melting temperatures. The methodical simulation of design configurations can elucidate strategies to maintain desirable nanocluster structure during sintering processes.     

A vertical transport geometry for electrical spin injection and extraction in Si

Schottky barriers formed between ferromagnetic metal and Semiconductor are of particular interest for spin injection and detection experiments. Here, we investigate electrical spin polarized carrier injection and extraction in Si using a Co/Si/Ni vertical structure built on a 250 nm thick Si membrane. Current–voltage measurements performed on the devices at low temperatures showed evidence of the conduction being dominated by thermionic field emission, which is believed to be the key to spin injection using Schottky junctions. This, however, proved inconclusive as our devices did not show any magnetoresistance signal even at low temperatures. We attribute this partially to the high resistance-area product in our Schottky contacts at spin injection biases. We show the potential of this vertical spin-device for future experiments by numerical simulation. The results reveal that by growing a thin highly doped Ge layer at the Schottky junctions the resistance-area products could be tuned to obtain high magnetoresistance.