Ultrafast Magnetization Manipulation Using Single Femtosecond Light and Hot‐Electron Pulses

Current‐induced magnetization manipulation is a key issue for spintronic applications. This manipulation must be fast, deterministic, and nondestructive in order to function in device applications. Therefore, single‐ electronic‐pulse‐driven deterministic switching of the magnetization on the picosecond timescale represents a major step toward future developments of ultrafast spintronic systems. Here, the ultrafast magnetization dynamics in engineered Gd_x [FeCo]_1?x ‐based structures are studied to compare the effect of femtosecond laser and hot‐electron pulses. It is demonstrated that a single femtosecond hot‐electron pulse causes deterministic magnetization reversal in either Gd‐rich and FeCo‐rich alloys similarly to a femtosecond laser pulse. In addition, it is shown that the limiting factor of such manipulation for perpendicular magnetized films arises from the formation of a multidomain state due to dipolar interactions. By performing time‐resolved measurements under various magnetic fields, it is demonstrated that the same magnetization dynamics are observed for both light and hot‐electron excitation, and that the full magnetization reversal takes place within 40 ps. The efficiency of the ultrafast current‐induced magnetization manipulation is enhanced due to the ballistic transport of hot electrons before reaching the GdFeCo magnetic layer.     

Performance analysis of data intensive cloud systems based on data management and replication: a survey

As we delve deeper into the ‘Digital Age’, we witness an explosive growth in the volume, velocity, and variety of the data available on the Internet. For example, in 2012 about 2.5 quintillion bytes of data was created on a daily basis that originated from myriad of sources and applications including mobile devices, sensors, individual archives, social networks, Internet of Things, enterprises, cameras, software logs, etc. Such ‘Data Explosions’ has led to one of the most challenging research issues of the current Information and Communication Technology era: how to optimally manage (e.g., store, replicated, filter, and the like) such large amount of data and identify new ways to analyze large amounts of data for unlocking information. It is clear that such large data streams cannot be managed by setting up on-premises enterprise database systems as it leads to a large up-front cost in buying and administering the hardware and software systems. Therefore, next generation data management systems must be deployed on cloud. The cloud computing paradigm provides scalable and elastic resources, such as data and services accessible over the Internet Every Cloud Service Provider must assure that data is efficiently processed and distributed in a way that does not compromise end-users’ Quality of Service (QoS) in terms of data availability, data search delay, data analysis delay, and the like. In the aforementioned perspective, data replication is used in the cloud for improving the performance (e.g., read and write delay) of applications that access data. Through replication a data intensive application or system can achieve high availability, better fault tolerance, and data recovery. In this paper, we survey data management and replication approaches (from 2007 to 2011) that are developed by both industrial and research communities. The focus of the survey is to discuss and characterize the existing approaches of data replication and management that tackle the resource usage and QoS provisioning with different levels of efficiencies. Moreover, the breakdown of both influential expressions (data replication and management) to provide different QoS attributes is deliberated. Furthermore, the performance advantages and disadvantages of data replication and management approaches in the cloud computing environments are analyzed. Open issues and future challenges related to data consistency, scalability, load balancing, processing and placement are also reported.     

Feasibility-Guided Constraint-Handling Techniques for Engineering Optimization Problems

The particle swarm optimization (PSO) algorithm is an established nature-inspired population-based meta-heuristic that replicates the synchronizing movements of birds and fish. PSO is essentially an unconstrained algorithm and requires constraint handling techniques (CHTs) to solve constrained optimization problems (COPs). For this purpose, we integrate two CHTs, the superiority of feasibility (SF) and the violation constraint-handling (VCH), with a PSO. These CHTs distinguish feasible solutions from infeasible ones. Moreover, in SF, the selection of infeasible solutions is based on their degree of constraint violations, whereas in VCH, the number of constraint violations by an infeasible solution is of more importance. Therefore, a PSO is adapted for constrained optimization, yielding two constrained variants, denoted SF-PSO and VCH-PSO. Both SF-PSO and VCH-PSO are evaluated with respect to five engineering problems: the Himmelblau’s nonlinear optimization, the welded beam design, the spring design, the pressure vessel design, and the three-bar truss design. The simulation results show that both algorithms are consistent in terms of their solutions to these problems, including their different available versions. Comparison of the SF-PSO and the VCH-PSO with other existing algorithms on the tested problems shows that the proposed algorithms have lower computational cost in terms of the number of function evaluations used. We also report our disagreement with some unjust comparisons made by other researchers regarding the tested problems and their different variants.     

Spectrum-aware Beaconless Geographical Routing Protocol for Cognitive Radio Enabled Vehicular Networks

The FCC and ETSI have allocated spectrum in the 5.9 GHz band for intelligent transportation systems. However, this spectrum supports short-range transmissions (up to 1000 m) and limited bandwidth (up to 75 MHz), which are not enough to meet the increasing demand for in-car infotainment services. In this paper, we propose a distributed routing protocol for vehicular ad hoc networks, where cognitive radio enabled vehicles (CRVs) dynamically share the TV-band channels. In the proposed protocol, CRVs jointly select relay nodes, channels, transmission powers, and transmission rates so that their total transmission rates are maximized while meeting their rate demands and power constraints. This selection process is carefully executed so that ongoing communications between primary radios (PRs) and between other CRVs are not disrupted. Once the relay nodes are selected, they continue to relay more messages as long as they stay in a predefined forwarding area. By doing so, the overhead for selecting relay nodes can be substantially reduced. Channels, powers, and rates are changed on a per-packet and per-hop basis so that the proposed protocol can efficiently adapt to spectrum dynamics. Simulation results show that our protocol increases the end-to-end network throughput by up to 250 % and decreases the end-to-end delay by up to 400 % compared with other geographical routing protocols.     

Effect of low segregation coefficient on Ga‐doped multicrystalline silicon solar cell performance

High‐quality Ga‐doped ingots are grown in different casting furnaces at optimized growth parameters; 3·5 kg ingots exhibit normal distribution of diffusion lengths along their height with very high diffusion lengths at the center of the ingot. Effective lifetimes as high as 1·1 ms are realized in 10 Ω cm Ga‐doped wafers after proper P‐diffusion and hydrogen passivation. Average effective lifetimes above 400 µs are also realized after P‐diffusion and hydrogen passivation for Ga‐doped wafers cut from 75 kg ingot where the response to P‐diffusion and hydrogen passivation is pronounced. High effective lifetimes are realized over the whole ingot with minimum values of 20 µs at the top of the ingot, indicating the possible use of about 85% of the ingot for solar cell production. Conversion efficiencies above 15·5% were realized in utilizing more than 80% of the ingot. High efficiencies of about 16% were realized in wafers with resistivities higher than 5 Ω cm p ‐type multicrystalline silicon wafers. Copyright © 2005 John Wiley & Sons, Ltd.     

Impact of Selective Grain Refinement on Superplastic Deformation: Finite Element Analysis

Recently, Friction Stir Processing has been introduced as an effective tool to refine the grain structure of sheet metals and enhance their superplasticity. A possible application of friction stir processing is selective grain refinement to enhance deformation uniformity and minimize localized thinning. This selective treatment will result in a sheet with grain size gradient. Very limited studies have been directed toward investigating the effect of grain size gradient on the superplastic deformation during SPF. In this work, Finite Element simulations for the free bulging of a dome made of 7075Al alloy were conducted to examine the effects of initial grain size gradient within the sheet on SPF characteristics. The results clearly demonstrate that selective grain refinement can be utilized to eliminate severe thinning and improve the integrity of the superplastic formed part. This article was presented at the AeroMat Conference, International Symposium on Superplasticity and Superplastic Forming (SPF) held in Baltimore, MD, June 25-28, 2007.     

Competitive flow control in general multi-node multi-link communication networks

In this paper, we consider the flow control in a general multi-node multi-link communication network with competing users. Each user has a source node, a destination node, and an existing route for its data flow over any set of links in the network from its source to its destination node. The flow rate for each user is a control variable that is determined by optimizing a user-specific utility function which combines maximizing the flow rate and minimizing the network congestion for that user. A preference parameter in the utility function allows each user to adjust the trade-off between these two objectives. Since all users share the same network resources and are only interested in optimizing their own utility functions, the Nash equilibrium of game theory represents a reasonable solution concept for this multi-user general network. The existence and uniqueness of such an equilibrium is therefore very important for the network to admit an enforceable flow configuration. In this paper, we derive an expression for the Nash equilibrium and prove its uniqueness. We illustrate the results with an example and discuss some properties and observations related to the network performance when in the Nash equilibrium. Copyright © 2007 John Wiley & Sons, Ltd.     

Coping with Global Environmental Change,Disasters and Security: Threats,Challenges, Vulnerabilities and Risks

Since the publication of the third assessment report of The Intergovernmental Panel on Climate Change, vulnerability to climate change has become an important research question. Vulnerability assessment on the urban scale has become a major issue. This paper describes a conceptual framework for modelling vulnerability at the urban scale, the Climate Change Vulnerability Assessment model. The model is applied to Shanghai, a typical geographically vulnerable and rapidly-urbanizing case study area. Using Arc-GIS, a vulnerability map was created for understanding the spatial dynamics of climate change vulnerability in Shanghai. An additional process, combined with the weighting coefficients, produced different vulnerable areas. Based on the vulnerability map, we located several high risk areas. The vulnerability of each area was assessed. Identifying the risks in each case and associating them with a specific region can be useful for decision makers.     

Field experience on solar electric power systems and their potential in Palestine

Palestine has a large number of rural small villages far from the national electric grids. Electrical loads in such villages are mostly small and can be covered by means of photovoltaic (PV) generators, which are economically more feasible than extending the electric grid or using diesel electric generators. Since PV has been rarely used in Palestine, this paper is devoted to investigating the potential of PV applications in Palestine, identifying the barriers for prevalence of PV applications as in other countries and demonstrating the reliability and feasibility of utilizing PV systems by presenting the test results of a PV system by supplying a rural clinic with its power demands. A method for designing the PV power system respecting the local environmental conditions is presented in this paper. The results of the measurements carried out over two years verify the reliability of the applied method. The illustrated test results show how far the PV-power generation can be matched with load demands and state of battery charge even during periods of low solar radiation. This could be achieved by respecting the local weather parameters in the illustrated sizing method. Long term field experience in designing, testing and operation of PV projects outside Palestine is presented in this paper.     

Solar powered induction motor-driven water pump operating on a desert well, simulation and field tests

A photovoltaic-powered water pumping system, employing an induction motor pump, capable of supplying a daily average of 50 m³ at 37-m head has been developed. The system was installed on a desert well in Jordan, where: the average solar radiation amount to 5.5 kW h/m³/day, to provide the Bedouins living in the well area with drinking water.A mathematical model to enable testing the system performance by computer simulation was developed. This model allows the representation of motor torque in function of speed (and slip) at different supply frequencies, as well as the flow rate and efficiency of the system in function of supply frequency and pumping head.Prior to its installation on the desert well, the system performance, in accordance with frequency and head, was thoroughly tested in the laboratory. As illustrated in this paper, simulation and laboratory testing results are well matched. At constant pumping head, the flow rate is proportional to the supply frequency of the motor. At constant flow rate, the pumping head is proportional to the supply frequency squared only in the range below the peak efficiency of the pump. For higher flow rate values, a special algorithm based on the experimental results could be developed.Higher system efficiency is achievable at higher frequency. It is advisable to operate the motor pump at the nominal frequency, flow rate and head corresponding to maximum efficiency.Long-term field testing of the system shows that it is reliable and has an overall efficiency exceeding 3%, which is comparable to the highest efficiencies reported elsewhere for solar powered pumps.