Self-consistent ion transport simulation in carbon nanotube channels

We propose a method to self-consistently deal with polarisation effects in Monte Carlo particle simulations of charge transport. The systems of interest were membrane structures with a narrow (4–8 Å) carbon nanotube (CNT) channel in an aqueous environment. Due to computational limitations for Molecular Dynamics (MD) simulations, we extended the Transport Monte Carlo known from semiconductor simulations to ionic transport in water as a background medium. This method has been used successfully to compute transport rates of ions in biological channels but polarization effects on protein walls cannot be easily included self-consistently, due to the complexity of the structure. Since CNTs have a regular structure, it is practical to employ a self-consistent scheme that accounts for the charge redistribution on the channel wall when an external bias is applied or when the electrical field of a passing ion is screened out. Previous work has shown that this is necessary and the computationally efficient tight-binding (TB) approach developed there [1] is combined with transport Monte Carlo simulations in this work.     

Electro-thermal simulation based on coupled Boltzmann transport equations for electrons and phonons

To study the thermal effect in nano-transistors, a simulator solving self-consistently the Boltzmann transport equations for both electrons and phonons has been developed. It has been used to investigate the self-heating effects in a 20 nm-long double-gate MOSFET (Fig. 1). A Monte Carlo solver for electrons is coupled with a direct solver for the steady-state phonon transport. The latter is based on the relaxation time approximation. This method is particularly efficient to provide a deep insight of the out-of-equilibrium thermal dissipation occurring at the nanometer scale when the device length is smaller than the mean free path of both charge and thermal carriers. It allows us to evaluate accurately the phonon emission and absorption spectra in both real and energy spaces.     

Efficient three‐dimensional parallel simulations of PHEMTs

An efficient 3D semiconductor device simulator is presented for a memory distributed multiprocessor environment using the drift–diffusion (D–D) approach for carrier transport. The current continuity equation and the Poisson equation, required to be solved iteratively in the D–D approach, are discretized using a finite element method (FEM) on an unstructured tetrahedral mesh. Parallel algorithms are employed to speed up the solution. The simulator has been applied to study a pseudomorphic high electron mobility transistor (PHEMT). We have carried out a careful calibration against experimental I–V characteristics of the 120 nm PHEMT achieving an excellent agreement. A simplification of the device buffer, which effectively reduces the mesh size, is investigated in order to speed up the simulations. The 3D device FEM simulator has achieved almost a linear parallel scalability for up to eight processors. Copyright © 2005 John Wiley & Sons, Ltd.     

Electron Density Calculation Using the Contact Block Reduction Method

A novel efficient method to calculate the ballistic quantum transport in two or three dimensional multi-terminal devices of arbitrary geometry, potential profile and number of leads has been presented. In this work we show that the method termed contact block reduction (CBR) allows calculation of not only the transmission function, but the electron density and density matrix of the device as well. The calculation costs are mainly determined by the size of the contact boundaries of the device.     

Simulating digital exposure of xerographic photoreceptors using the domain-decomposition method

In digital electrophotography, the photoreceptor (PR) is discharged by exposing it to a high-intensity laser beam from an imager for a very short duration in an imagewise fashion. The resultant latent image on the PR surface is a function of the PR properties, the imager properties, as well as the spatial and temporal exposure sequence. A generalized computational model for discharge based on charge generation, injection, and transport in a PR has been discussed previously (Proc. SPIE, vol. 2658, p. 112, 1996). In this paper, the use of the domain-decomposition method to enable an efficient simulation of a latent image for wide-area exposures (such as wide lines and solid areas) is discussed. A photo-induced discharge curve (PIDC) with a multibeam imager is simulated using this algorithm. The simulation results show that laser exposure sequence can create an inhomogeneous field on the generator layer, which may lead to varying amounts of charge injected at the generator layer for the various scan lines. However, the impact on the discharged image is reduced since charges in transit appear to reequilibrate spatially. The photo-induced discharge is somewhat less efficient (10-15 V for exposures greater than 5 ergs/cm/sup 2/ for results presented here) due to a two-beam interlaced exposure sequence compared to the case when all the pixels are exposed simultaneously. Simulation results for discharge of lines show that narrow positive lines grow due to the field dependence of collection efficiency during charge generation and subsequent spreading of charges in transit. A single-pixel positive line, for instance, can grow as much as 40%. The effect is less pronounced for wide lines.     

Low-temperature enhancement of the thermoelectric Seebeck coefficient in gated 2D semiconductor nanomembranes

An increasing need for effective thermal sensors, together with dwindling energy resources, have created renewed interests in thermoelectric (TE), or solid-state, energy conversion and refrigeration using semiconductor based nanostructures. Effective control of electron and phonon transport due to confinement, interface, and quantum effects has made nanostructures a good way to achieve more efficient thermoelectric energy conversion. Theoretically, a narrow delta-function shaped transport distribution function (TDF) is believed to provide the highest Seebeck coefficient, but has proven difficult to achieve in practice. We propose a novel approach to achieving a narrow window-shaped TDF through a combination of a step-like 2-dimensional density-of-states (DOS) and inelastic optical phonon scattering. A shift in the onset of scattering with respect to the step-like DOS creates a TDF which peaks over a narrow band of energies. We perform a numerical simulation of carrier transport in silicon nanoribbons based on numerically solving the coupled Schrödinger-Poisson equations together with transport in the semi-classical Boltzmann formalism. Our calculations confirm that inelastic scattering of electrons, combined with the step-like DOS in 2-dimensional nanostructures leads to the formation of a narrow window-function shaped TDF and results in enhancement of Seebeck coefficient beyond what was already achieved through confinement alone. A further analysis on maximizing this enhancement by tuning the material properties is also presented.     

The influence of the zigzag and armchair leads on quantum transport through the graphene based quantum rings

The conductivity of a graphene ring with two semi-infinite, armchair and zigzag leads has been investigated. We have performed numerical calculations based on the nearest neighbor tight-binding Hamiltonian and Dirac point approximation. A Non-Equilibrium Green’s Function (NEGF) approach has been employed to calculate the electric current under an applied bias voltage, in this two terminal mesoscopic system. We have studied the effect of the external magnetic field on transport characteristics of this graphene-based quantum ring. Coherent transport features of the system have been studied. It was shown that there is significant distinction between the I–V characteristics (and also the Aharonov-Bohm effect) of the graphene rings depending on the edge structure of the leads.     

Design of optimal electromagnets of magnetic-levitation and lateral-stabilization systems for ground transportation based on solving inverse problems

Scientific and technical works on development of transport with magnetic levitation of the vehicle have been reviewed and the main stages of design of such transport have been analyzed. An iterative algorithm for design of the elecFtromagnets used in systems of magnetic levitation and lateral stabilization of high-speed ground transportation has been constructed. The algorithm is based on solving the inverse problems of the theory of an electromagnetic field. Relationships for calculating the initial values of the sought parameters, which are very close to the exact solution, have been suggested. An additional stage of minimizing the electromagnet mass is introduced into the algorithm; as a result, the useful weight of a vehicle can be increased. In addition, the necessary and sufficient conditions for the object of design to have minimal mass were used. An example of implementation of the algorithm during design of a levitation electromagnet has been considered. The influence of eddy currents induced in the ferromagnetic rail during movement of a vehicle on the levitation force has been determined. A coefficient that allows one to take into account this effect has been determined. To justify the formula for calculations of a ferrorail width, the dependences of levitation-force components that arise upon shifting the electromagnet in respect to the ferrorail axis have been obtained. The results of investigations show that the methodology based on solving the inverse problems in designing electromagnets of levitation and lateral-stabilization systems is highly efficient. The proposed algorithm for optimal design of the electromagnets can be used in the design of similar devices in electric-instrument engineering.     

III–VI Chalcogenide Semiconductor Crystals for Broadband Tunable THz Sources and Sensors

The layered chalcogenide semiconductor GaSe has been grown under various crystal growth conditions for optimum performance for tunable terahertz (THz) wave generation and broadband THz detection. Low-temperature photoluminescence (PL), Raman spectroscopy, optical absorption/transmission, electrical charge transport property measurements, and THz time-domain spectroscopy (TDS) have been used to characterize the grown crystals. It is observed that indium doping enhances hardness of the grown GaSe crystals, which is very useful for processing and fabricating large-area devices. GaSe crystals have demonstrated promising characteristics with good optical quality (absorption coefficient les0.1 cm^-1 in the spectral range of 0.62-18 mum), high dark resistivity (ges10⁹ Omega cm), wide bandgap (2.01 eV at 300 K), good anisotropic (parand perp) electrical transport properties (mu_e/h, tau_e/h, and mutau_e/h) and long-term stability. The THz emission measurements have shown that the GaSe crystals are highly efficient for broadband tunable THz sources (up to 40 THz), and sensors (up to 100 THz). Additionally, new THz frequencies (0.1-3 THz) have been observed for the first time from an anisotropic binary and a ternary semiconductor crystal. Details of characterizations as well as optimum crystal growth conditions including simulation and computer modeling are described in this paper.     

Algorithms for Itinerary Planning in Multimodal Transportation Networks

The itinerary planning problem in an urban public transport system constitutes a common routing and scheduling decision faced by travelers. The objective of this paper is to present a new formulation and an algorithm for solving the itinerary planning problem, i.e., determination of the itinerary that lexicographically optimizes a set of criteria (i.e., total travel time, number of transfers, and total walking and waiting time) while departing from the origin and arriving at the destination within specified time windows. Based on the proposed formulation, the itinerary planning problem is expressed as a shortest path problem in a multimodal time-schedule network with time windows and time-dependent travel times. A dynamic programming-based algorithm has been developed for the solution of the emerging problem. The special case of the problem involving a mandatory visit at an intermediate stop within a given time window is formulated as two nested itinerary planning problems which are solved by the aforementioned algorithm. The proposed algorithm has been integrated in a Web-based journey planning system, whereas its performance has been assessed by solving real-life itinerary planning problems defined on the Athens urban public transport network, providing fast and accurate solutions.     

Percolation current in organic semiconductors

The charge transport in organic semiconductors has been investigated theoritically. A n analytical model describing the hopping current is developed. This model is based on VRH (variable range hopping) theory and percolation theory. The results of calculation are compared to the prediction of Mott’s formalism, which has been widely used to describe hopping transport in amorphous semiconductors. The model is also found to be in good agreement with recent experimental data.     

An atomistic quantum transport solver with dephasing for field-effect transistors

Extended Hückel theory (EHT) along with NEGF (Non-equilibrium Green’s function formalism) has been used for modeling coherent transport through molecules. Incorporating dephasing has been proposed to theoretically reproduce experimental characteristics for such devices. These elastic and inelastic dephasing effects are expected to be important in quantum devices with the feature size around 10 nm, and hence an efficient and versatile solver is needed. This model should have flexibility to be applied to a wide range of nano-scale devices, along with 3D electrostatics, for arbitrary shaped contacts and surface roughness. We report one such EHT-NEGF solver with dephasing by self-consistent Born approximation (SCBA). 3D electrostatics is included using a finite-element scheme. The model is applied to a single wall carbon nanotube (CNT) cross-bar structure with a C₆₀ molecule as the active channel. Without dephasing, a negative differential resistance (NDR) peak appears when the C₆₀ lowest unoccupied molecular orbital level crosses a van Hove singularity in the 1D density of states of the metallic CNTs acting as contacts. This NDR diminishes with increasing dephasing in the channel as expected.     

A Roadside ITS Data Bus Prototype for Intelligent Highways

Intelligent transport systems (ITSs) usually include three principal elements: vehicle, driver, and road (or, more generally, the environment). The use of ITS data bus (IDB) has been proposed to build sharable standard interfaces for in-vehicle information systems. Despite broadband communication technologies, such as dedicated short-range communication (DSRC), which have been developed to provide high-quality roadside-vehicle communication services for intelligent highways, the existing IDB model has not paid enough attention to the demands of information exchanges between the roadside and onboard units. In this paper, a new model of the roadside IDB (RIDB) is proposed to improve the existing IDB architecture. The physical layer, data-link layer, and application layer of the new model are also discussed. A prototype system of the RIDB, which is based on the wireless 802.11b protocol, has been developed in a test site. The experimental results demonstrated that the RIDB is feasible to provide high-quality roadside-vehicle and roadside-roadside communication services. Other potential applications of the IDB, such as probe cars and intersection collision prevention, are also discussed. The RIDB proposed in this paper is potentially useful for the construction of an intelligent transport infrastructure.     

Graphical tactile displays for visually-impaired people.

This paper presents an up-to-date survey of graphical tactile displays. These devices provide information through the sense of touch. At best, they should display both text and graphics (text may be considered a type of graphic). Graphs made with shapeable sheets result in bulky items awkward to store and transport; their production is expensive and time-consuming and they deteriorate quickly. Research is ongoing for a refreshable tactile display that acts as an output device for a computer or other information source and can present the information in text and graphics. The work in this field has branched into diverse areas, from physiological studies to technological aspects and challenges. Moreover, interest in these devices is now being shown by other fields such as virtual reality, minimally invasive surgery and teleoperation. It is attracting more and more people, research and money. Many proposals have been put forward, several of them succeeding in the task of presenting tactile information. However, most are research prototypes and very expensive to produce commercially. Thus the goal of an efficient low-cost tactile display for visually-impaired people has not yet been reached.     

A Non Perturbative Model of Surface Roughness Scattering for Monte Carlo Simulation of Relaxed Silicon n-MOSFETs

In the present paper the problem of interface roughness scattering is treated non-perturbatively by incorporating the effects of scattering as a boundary condition for the Boltzmann transport equation (BTE). This provides a basis for the development of a semi-classical model of the interface roughness scattering mechanism applicable to the Monte Carlo (MC) simulations of electron transport in the Si n-MOSFET. The model is based on the Boltzmann-Fuchs method for the solution of the BTE. This method has shown to be computationally efficient and is naturally suited to the semi-classical spirit of the BTE and its direct solution via the MC method.     

Fabrication and excitation testing of a fully superconducting brushless generator with magnetic flux pump

Development of a highly efficient excitation system is important in enhancing the potential of superconducting ac generators. A new conceptual design in which electricity is generated with a brushless exciter system using a superconducting dynamo (the magnetic flux pump) has been proposed. Research on a fully superconducting generator using both a superconducting armature and a superconducting field winding has been under way for the past 7 years. In this paper an experimental model of the machine is described and test results on brushless excitation with the magnetic flux pump, with the machine operating as a fully superconducting brushless generator with an estimated capacity of 20 kVA, are presented. © 1997 Scripta Technica, Inc. Electr Eng Jpn. 118 (4): 35–45, 1997     

Binary-Representation-Based Genetic Algorithm for Aircraft Arrival Sequencing and Scheduling

Arrival sequencing and scheduling (ASS) at airports is an NP-hard problem. Much effort has been made to use permutation-representation-based genetic algorithms (GAs) to tackle this problem, whereas this paper attempts to design an efficient GA based on a binary representation of arriving queues. Rather than using the order and/or arriving time of each aircraft in the queue to construct chromosomes for GAs, this paper uses the neighboring relationship between each pair of aircraft, and the resulted chromosome is a 0-1-valued matrix. A big advantage of this binary representation is a highly efficient uniform crossover operator, which is normally not applicable to those permutation representations. The strategy of receding horizon control (RHC) is also integrated into the new GA to attack the dynamic ASS problem. An extensive comparative simulation study shows that the binary-representation-based GA outperforms the permutation-representation-based GA.     

梯级水电站优化调度的改进社会情感优化算法

探索新的调度模型求解方法一直是水库优化调度研究的热点之一。社会情感优化算法（SEOA）是一种新兴的启发式智能优化算法,但目前在水库优化调度中未见应用。将SEOA应用于梯级水电站发电优化调度中,并针对算法初始种群随机生成造成的初始解代表性低,引入了初始种群均匀设计,针对部分个体过早收敛导致的种群活力低、算法易于局部收敛,制定了种群淘汰策略,从而建立了改进社会情感优化算法（改进SEOA）。实例表明,在梯级水电站发电优化调度模型的求解中,改进SEOA搜索效率高、寻优能力强、稳定性好。     

EE-BESD: molecular FET modeling for efficient and effective nanocomputing design

Molecular transistor is a good candidate as substitute of CMOS device due to small size, expected good performance and suitability to be included in high density-circuits. To date a lot of effort has been carried out to understand the conduction properties in molecular devices. However, minor effort has been devoted to reduce their computational complexity to obtain a compact molecular model. First-principle based methods frequently used are highly computational demanding for a single device, thus they are not suitable for complex circuit design. In this paper we present an accurate and at the same time computationally efficient method (named Efficient and Effective model based on Broadening level, Evaluation of peaks, SCF and discrete levels, ee-besd) to calculate the electron transport characteristics of molecular transistors in presence of applied bias and gate voltages. The results obtained show a remarkable improvement in terms of computational time with respect to existing approaches, while maintaining a very good accuracy. Finally, the ee-besd model has been embedded in a circuit level simulator in order to show its functionalities and, particularly, its computational cost. This is shown to be affordable even for circuits based on a high number of devices.     

提高输电线路索道运输工效的几点措施

索道作为一种高效、环保的运输方式,在输电线路施工中普遍应用。如何提高索道运输工效,文章分析了造成索道运输工效低下的原因,并有针对性提出了改进措施。