Electrical activity and precipitation behavior of copper in gallium arsenide

The electrical properties and preferred lattice site of Cu in GaAs were investigated combining electrical and optical measurements with ion beam and structural analysis. From this comprehensive study it was determined that Cu introduces two levels in the band gap, that the concentration of electrically active centers introduced by Cu diffusion is considerably smaller than the total Cu concentration, that this ratio of electrically active to total Cu concentration depends strongly on the cooling speed after diffusion, and that the portion of Cu that remains electrically inactive forms Cu-Ga precipitates.     

Current spreading and series resistance of proton-implanted vertical-cavity top-surface-emitting lasers

In this paper, the current flow through the whole volume of the proton-implanted Vertical-Cavity top-Surface-Emitting Lasers (VCSELs) is analysed in detail. A simple approximate analytical relation was derived for a radial distribution of the current density entering active regions of those lasers. This distribution is nearly uniform in the case of VCSELs with a very small active region, but is becoming more and more non-uniform with an increase in its size. In VCSELs with very large active regions, current is flowing practically only within a narrow annular area close to the active-region perimeter. The VCSEL series electrical resistance is determined as a function of its active-region radius.     

A study of Eu2O3, Sc2O3 co-doped tungsten matrix impregnated cathode

Eu₂O₃ and Sc₂O₃ co-doped W matrix impregnated cathodes have been prepared by the powder metallurgy method. The constitution of active elements on activated cathode surface is analyzed by in-situ Auger electron spectroscopy. It is found that although Eu exists in the matrix, no Eu is found on the cathode surface due to the formation of a stable Eu containing compound. Sc, Ba and O diffuse to the surface of the cathode and form an active surface layer during the activation period whereas the stable Eu-compound cannot liberate free Eu, which can diffuse from the cathode to the surface. The active substance of Sc, Ba and O on the cathode surface contribute to the emission property.     

Field-effect transistor based on a combination of nanometer film and undoped semiconductor

The metal oxide nanometer film semiconductor field-effect transistor (MONSFET) is reported. In this device, a combination of undoped semiconductor and nanometer film serves as the active layer. When a negative gate-source voltage is applied, electrons from the nanometer film enter into the semiconductor layer to form the conducting channel, and the drain current increases without saturation. This structure makes more materials available for the active layer, and thus suggests a new route to enrich the applications as well as to enhance the performances.     

Unique properties of microwave in interlayer exchange-coupled trilayer ferromagnetic films associated with negative imaginary part of permeability

Based upon Landau-Lifshitz equation and Maxwell's equations, we theoretically investigated properties of normally incident microwave propagation in interlayer exchange-coupled trilayer ferromagnetic film. It is found that, near resonance frequency of optic mode, imaginary part of permeability of one ferromagnetic layer is smaller than zero unusually, i.e., the ferromagnetic layer may be taken as an active medium. Thus a number of unique electromagnetic properties are presented, such as, the ferromagnetic layer becomes a left-handed material near low side of the resonance frequency of optic mode, and both phase velocity and time-averaged Poynting flow of the usually defined forward wave are negative simultaneously near high side of the resonance frequency. This work provides a feasible active medium to demonstrate the unique microwave properties.     

Separation of chiral molecules by temperature programmed desorption

The Monte Carlo simulation method was used to model thermal desorption of a pair of enantiomers from a solid surface with a chiral periodic pattern of active sites. The main objective of the study was to determine the optimal number of the active sites and their spatial distribution within the unit cell of the surface to achieve the most efficient separation of the enantiomers. For that purpose we tested the series of chiral patterns which were found previously for the equilibrium adsorption. Temperature programmed desorption spectra were calculated using a square lattice of adsorption sites in which the active sites were distributed spatially according to the candidate patterns. Additionally, influence of relaxation of the adsorbed layer on the relative shift of the TPD peaks of the enantiomers was assessed and the key factors affecting the chiral separation were identified.     

High-power Nd:YAG planar waveguide laser with YAG and Al2O3 claddings

Thermal effects in Nd:YAG planar waveguide lasers with non-symmetrical claddings are discussed. The heat generated in the active core can be removed more efficiently by directly contacting the active core to the heat sink. Several cladding materials are compared to optimize the heat removal. Furthermore, uniform pumping is achieved with oblique edge-pumping technique. Using quasi-CW pumping at 1 KHz repetition rate, an average output power of 280 W with a slope efficiency of 38% is obtained with a positive unstable resonator.     

Evidence of surface active sites on NaY zeolite by a model reaction

Owing to the development of a new test reaction, namely the isomerization of 1-dodecene, it becomes possible to characterize the activity of cationic zeolites under conditions close to those of industrial adsorption and separation processes (temperature around 150-200 °C and liquid phase). Indeed, 1-dodecene is highly active and still in a liquid state at 150 °C. Furthermore, by comparing the reactivity of NaY before and after treatments applied to reduce its activity ((i) passivation of the external surface by deposition of TetraEthylOrthoSiloxane (TEOS) and (ii) washing the zeolite with a basic or neutral solution), we are able to propose a nature and localization for the residual active sites of this zeolite. Indeed, the evolution of the NaY activity in their function indicates that the active sites are located both at the external and internal surfaces of NaY and that two types of sites can be described: OH groups and structure defects.     

The plasmon and distribution effects between incident light and active layer in PtOx-type super-resolution near-field structure

The plasmon and distribution effects of collective localized surface plasmons between incident light and active layer of PtO_x-type super resolution near-field structure (super-RENS) have been studied using finite-difference time-domain method. Four types of distribution of Pt nanoparticles, i.e., type A, B, C, and D in active layer are investigated. We find that type C and D in active layer can provide higher field intensity in a wider range of particle size when the particle sizes are varied, and the out-going filed emerging from the active layer exhibit smaller spot size than those of type A and B. Type B, C and D also provide the additional path longer than that of type A, and excite more evanescent field which located in the far edge of the bubble from the optical axis of the incident beam. Results show that the type C structure is the best choice in the view point of designing the PtO_x-type super-RENS. This study provides new information to design a super-RENS with superior resolution as well as other applications in nano photonic devices.     

Simulations on false gain in recombination-pumped soft-X-ray lasers

-1 in the case of plasmas with short active medium lengths. The false gain in the case of fiber targets is found to be of equal magnitude to that for slabs in the case of plasmas with less than 0.1 cm active medium lengths. Calculations for slab targets predict that adopting a tolerance of ±1 cm^-1 for gain will severely restrict the time and the active medium length of the plasma that can be used for error-free observations, while those for fiber targets are found to be considerably relaxed. The effects of false gain in the 54.2 Å Na Balmer α laser is also investigated, again revealing the importance of this phenomena under optimum gain conditions. Received: 10 December 1996/Revised version: 12 March 1997     

Passivation of laser induced defects in silicon by low energy hydrogen ion implantation

It is well established that the pulsed-laser annealing of as-implanted Si introduces electrically active defects. The aim of this paper is to show that these defects can be neutralized by low-energy hydrogen ion implantation. The techniques used for the sample characterization are current-voltage measurements and capacitance transient spectroscopy.     

Elongation rate measurements of nanofilms by microbump method induced by laser pulse

A laser pulse-induced microbump method that aims to measure the elongation rate of nanofilms is proposed. The sample structure is designed as “substrate/active layer/nanofilm” and the laser pulse is used as energy source to heat the active layer and to create microbump. These cause the nanofilm to expand and elongate. The surface area and length change of nanofilm is calculated by measuring the deflections and diameters of the microbumps, as well as to obtain the elongation rate of the nanofilms. A series of microbumps with different deflections are obtained. The deflections are measured precisely by atomic force microscopy (AFM) by taking AgO_x and ZnS-SiO₂ as the active layer and nanofilm, respectively, and by controlling pulse laser parameters. The line elongation rate and plane elongation rate of ZnS-SiO₂ nanofilm are measured at thickness of only 10 nm. Results show that both the two elongation rates linearly increases with laser power from 3.2 to 5.2 mW. Plane elongation rate is a little higher than the line elongation rate at the same laser power. The rupture at 5.4 mW laser power corresponds to fracture strength of the film. The maximum line elongation rate and plane elongation rate are 13.241% and 19.766%, respectively. This method applies a reproducible and efficient method for its applications in the near future.     

Filter based non-invasive control of chaos in Buck converter

A non-invasive method for controlling chaos in the voltage-mode Buck converter is proposed by using a hybrid active filter based feedback controller in this Letter. The harmonic balance method is applied to obtaining the bifurcation-point equations of the controlled system. Hence, a stability-boundary diagram is constructed, through which the control parameters are chosen correctly. The results of simulation and experiment are given after all.     

Effects of disease characteristics and population distribution on dynamics of epidemic spreading among residential sites

We investigate the spreading processes of epidemic diseases among many residential sites for different disease characteristics and different population distributions by constructing and solving a set of integrodifferential equations for the evolutions of position-dependent infected and infective rates, taking into account the infection processes both within a single site and among different sites. In a spreading process the states of an individual include susceptible (S), incubative (I), active (A) and recovered (R) states. Although the transition from S to I mainly depends on the active rate, the susceptible rate and the connectivity among individuals, the transitions from I to A and from A to R are determined by intrinsic characteristics of disease development in individuals. We adopt incubation and infection periods to describe the intrinsic features of the disease. By numerically solving the equations we find that the asymptotic behavior of the spreading crucially depends on the infection period and the population under affection of an active individual. Other factors, such as the structure of network and the popular distribution, play less important roles. The study may provide useful information for analyzing the key parameters affecting the dynamics and the asymptotic behavior.     

Active absorption to reduce the noise transmitted out of an enclosure

This paper investigates the potential of active absorbers for reducing low-frequency noise transmission through an enclosure. Active absorbers are intended to obtain a purely real prescribed impedance at the front face of a porous layer. This is achieved by an active control system which cancels the acoustic pressure at the rear face. The test bench was a simplified enclosure: a rigid-wall cavity coupled to a baffled elastic plate. The modeling of the system was based on an analytical modal approach. The purpose of this simulation was first to calculate the optimal impedance, providing maximal reduction in radiated power, and then to define a sub-optimal strategy for actual absorber production. Two 3-cell configurations were implemented on the test bench. Active control used a multichannel feedforward algorithm. In line with prediction, the absorbers provided a 5.5 dB overall reduction while covering only 2% of the cavity surface.     

A high-throughput search for direct methanol fuel cell anode electrocatalysts of type PtxBiyPbz

We used a high-throughput method to screen for direct methanol fuel cell anode electrocatalysts in the Pt-Bi-Pb system. Previous studies showed that PtBi and PtPb (both NiAs structure type) were active electrocatalysts for the oxidation of formic acid, but only PtPb was active in oxidizing methanol. We synthesized thin films with continuous composition spreads of the three elements by magnetron sputtering at deposition temperatures from ambient to 510 °C. A fluorescence method was then used to identify compositions that were active toward methanol oxidation. Only films deposited between temperatures of 160 and 400 °C showed electrocatalytic activity. The areas that were active for methanol oxidation showed predominantly the NiAs structure type according to XRD, with optimal activity for compositions near PtBi_0.01Pb_0.53.     

Comment on: “Synchronization of two chaotic nonlinear gyros using active control” [Phys. Lett. A 343 (2005) 153]

A modification to the control law presented in [Y. Lei, W. Xu, H. Zheng, Synchronization of two chaotic nonlinear gyros using active control, Phys. Lett. A 343 (2005) 153] is proposed.     

Surface defects in semiconductor lasers studied with cross-sectional scanning tunneling microscopy

Cross-sectional scanning tunneling microscopy is used to study defects on the surface of semiconductor laser devices. Step defects across the active region caused by the cleave process are identified. Curved blocking layers used in buried heterostructure lasers are shown to induce strain in the layers above them. Devices are also studied whilst powered to look at how the devices change during operation, with a numerical model that confirms the observed behavior. Whilst powered, low-doped blocking layers adjacent to the active region are found to change in real time, with dopant diffusion and the formation of surface states. A tunneling model which allows the inclusion of surface states and tip-induced band bending is applied to analyze the effects on the tunneling current, confirming that the doping concentration is reducing and defect surface states are being formed.     

Acceleration and ejection of interacting ring vortices by radial flow

Exact solution of two-dimensional hydrodynamic equations for symmetrical configuration of four point vortices in the presence of radial flow is found. This solution describes the dynamics of a dipole toroidal vortex (consisting of two counter-rotating vortex rings) in such a flow. It is shown that in a convergent flow the ring vortices are compressed and ejected with acceleration along the symmetry axes of the system. Possible application to the problem of jets formation in active galaxy nuclei is considered.     

Dynamics and mechanics of motor-filament systems

Motivated by the cytoskeleton of eukaryotic cells, we develop a general framework for describing the large-scale dynamics of an active filament network. In the cytoskeleton, active cross-links are formed by motor proteins that are able to induce relative motion between filaments. Starting from pair-wise interactions of filaments via such active processes, our framework is based on momentum conservation and an analysis of the momentum flux. This allows us to calculate the stresses in the filament network generated by the action of motor proteins. We derive effective theories for the filament dynamics which can be related to continuum theories of active polar gels. As an example, we discuss the stability of homogenous isotropic filament distributions in two spatial dimensions.