Nonparabolicity effects and the spin-split electron dwell time in symmetric III-V double-barrier structures

We start from the fourth-order nonparabolic and anisotropic conduction band bulk dispersion relation to obtain a one-band effective Hamiltonian, which we apply to an AlGaSb symmetric double-barrier structure with resonant energies significantly (more than 200 meV) above the well bottom. The spin-splitting is described by the k³ Dresselhaus spin-orbit coupling term modifying only the effective mass of the spin eigenstates in the investigated structure. Apart from the bulk-like resonant energy shift due to the band nonparabolicity, we obtain a substantial shift depending on the choice of boundary conditions for the envelope functions at interfaces between different materials. The shift of resonant energy levels leads to the change of spin-splitting and the magnitude of the dwell times. We attempt to explain the influence of both the nonparabolicity and boundary conditions choice by introducing various effective masses.     

Approximate solutions for heavily loaded Markov-modulated queues

The existing exact solutions for Markov-modulated queues are computationally intensive and prone to numerical problems when the number of states of the Markovian environment becomes large. To address this problem, a simple geometric approximation is proposed. It uses the dominant eigenvalue of the characteristic matrix polynomial, together with the associated left eigenvector. That approximation is shown to be asymptotically exact in heavy traffic. In other cases, its accuracy is examined numerically.     

Activated carbon from agave wastes (agave tequilana) for supercapacitors via potentiostatic floating test

To prepare an efficient supercapacitor, an activated carbon from agave wastes was prepared and their electrochemical performance was evaluated as a novel electrode for supercapacitor. The carbon was prepared by two thermal pyrolysis processes under nitrogen atmosphere. The first pyrolysis was achieved at 500 °C until the charring of the bagasse; in the second pyrolysis step, the char was impregnated with different mass ratios of KOH (1:2–1:4) and thermally treated at 800 or 900 °C, for 1 h under N₂ flow. The textural analysis showed that the activated carbon had a specific surface area of 1462 m² g^?1 and depicted a type I isotherm (IUPAC) characteristic of a microporous carbon. Raman spectroscopy and XRD measurements confirm that the activated carbon contains a small graphitization degree and a disordered structure. The electrochemical study of the symmetric carbon supercapacitor was carried out in 1 M Li₂SO₄ solution as the electrolyte. The electrochemical performance of the coin cell supercapacitor was evaluated under an accelerated aging floating test consisting of potentiostatic steps at different voltages (1.5, 1.6 and 1.8 V) for 10 h followed by galvanostatic charge/discharge sequences, and the overall procedure summarized a floating time up to 200 h. The highest capacitance was observed at a floating voltage of 1.5 V, with a large initial specific capacitance of 297 F g^?1.

     

On the propagation of updates in distributed replicated systems

We analyse the performance and dependability of protocols which implement ‘sequence consistency’ in distributed replicated systems. Sites send messages to each other from time to time, passing information about the update requests that have been received. The recipient of a message is chosen according to some probability distribution which may depend on the sender. The random variables of main interest are (a) the interval between receiving an update request and being able to execute it on the original site, subject to the consistency requirement, and (b) the time it takes to bring all sites to a consistent state after the arrival of an update.     

Service center trade-offs between customer impatience and power consumption

We consider the problem of managing a service center where it is desirable to keep power consumption low. Customers defect (i.e. leave the system) if they have to wait too long before starting service. A block of servers is designated as ‘reserve’. The reserves are powered up when the number of jobs in the system is sufficiently high, and are powered down when that number is sufficiently low. Powering up takes an interval of time during which the reserves consume power but do not serve jobs. The objective function to be minimized takes into account both the number of servers that are powered up and the number of jobs that are lost through defections. The question of how to choose the number of reserves, and the up and down thresholds, is answered by analyzing a suitable queueing model. Heuristics and numerical results are also presented.     

Prospects for geoinformatics-based precision farming in the Savanna River basin,Nigeria

A geoinformatics-based land suitability assessment approach was used to examine the prospects for precision farming in the Savanna River basin in Nigeria. The procedure involved the overlay analysis of land use, land cover characteristics interpreted on Landsat Thematic Mapper (TM) imagery and physico-chemical soil properties and nutrient attributes in line with FAO suitability guidelines and models. The empirical analysis revealed areas of high, moderate and marginal suitability for the cultivation of maize, yam, cassava and oil palm in the basin. The critical advantages of geoinformation science technology for precision farming in a developing economy such as Nigeria were highlighted.     

Characteristics of beechnut oil (Fagus orientalis lipsky) of turkish origin

The characteristics of beechnut oil (Fagus orientalis Lipsky) of Turkish origin were investigated. The fatty acid composition was determined by capillary gas chromatography. The oil showed the characteristics of a semidrying oil.     

A comparative assessment of the economics and proliferation resistance of advanced nuclear energy systems

Three advanced nuclear energy systems and their fuel cycles are compared with respect to both their relative economic potential and proliferation resistance. These systems include the light-water reactor (LWR) once-through cycle, the breeder and light-water reactor fuel cycles on plutonium recycle and, lastly, an advanced converter-breeder system on a denatured thorium cycle. To quantify the relative proliferation risk of these systems, a multi-attribute decision analysis method was applied. Economic potential was measured on the basis of an electricity generation cost model. Results indicate that the plutonium recycle LWR-breeder option can be made sufficiently proliferation resistant if methods that denature the plutonium and/or do not separate out a pure plutonium stream are utilized. The denatured thorium cycle shows no obvious proliferation advantage and exhibits some economic penalty when compared to the other systems. It is therefore recommended that further research and development concentrate on the development of diversion-resistant technology for the mixed-oxide, plutonium-based LWR-breeder system.     

Spectral expansion solution for a class of Markov models: application and comparison with the matrix-geometric method

Many two-dimensional Markov models whose state space is a semi-infinite strip (i.e. finite in one dimension and infinite in the other) can be solved efficiently by means of spectral expansion. This method and its application are described in the context of an M/M//N queue with general breakdowns and repairs. The results of experiments aimed at evaluating the relative merits of the spectral expansion and the matrix-geometric solutions are also presented.