We show theoretically that an electric current in a high-mobility quasi-two-dimensional electron layer induces a significant drift of excitons in an adjacent layer through interlayer Coulomb interaction. The exciton gas is shown to drift with a velocity which can be a significant fraction of the electron drift velocity at low temperatures. The estimated drift length is of the order of micrometers or larger during the typical exciton lifetime for GaAs/Al{sub x}Ga{sub 1-x} double quantum wells. A possible enhancement of the exciton radiative lifetime due to the drift is discussed.
The U.S. Department of Energy's (DOE's) GeoPowering the West (GPW) program works with the U.S. geothermal industry, power companies, industrial and residential consumers, and federal, state, and local officials to provide technical and institutional support and limited, cost-shared funding to state-level activities. By demonstrating the benefits of geothermal energy, GPW increases state and regional awareness of opportunities to enhance local economies and strengthen our nation's energy security while minimizing environmental impact. By identifying barriers to development and working with others to eliminate them, GPW helps a state or region create a regulatory and economic environment that is more favorable for geothermal and other renewable energy development. Electricity is produced using expanding steam or very hot water from the underground reservoir to spin a conventional turbine-generator. Geothermal power plants operate at high capacity factors (70-100%), with availability factors typically greater than 95%. Geothermal plants are among the cleanest sources of electric power available. Direct use applications directly pipe hot water from geothermal resources to provide heat for industrial processes, crop drying, greenhouses, aquaculture, recreation, sidewalk snow-melting, and buildings. Geothermal district heating systems supply heat to multiple buildings through a network of pipes carrying the hot geothermal water.
The effects of wall interaction on combustion and soot formation processes of a diesel fuel jet were investigated in an optically-accessible constant-volume combustion vessel at experimental conditions typical of a diesel engine. At identical ambient and injector conditions, soot processes were studied in free jets, plane wall jets, and 'confined' wall jets (a box-shaped geometry simulating secondary interaction with adjacent walls and jets in an engine). The investigation showed that soot levels are significantly lower in a plane wall jet compared to a free jet. At some operating conditions, sooting free jets become soot-free as plane wall jets. Possible mechanisms to explain the reduced or delayed soot formation upon wall interaction include an increased fuel-air mixing rate and a wall-jet-cooling effect. However, in a confined-jet configuration, there is an opposite trend in soot formation. Jet confinement causes combustion gases to be redirected towards the incoming jet, causing the lift-off length to shorten and soot to increase. This effect can be avoided by ending fuel injection prior to the time of significant interaction with redirected combustion gases. For a fixed confined-wall geometry, an increase in ambient gas density delays jet interaction, allowing longer injection durations with no increase in soot. Jet interaction with redirected combustion products may also be avoided using reduced ambient oxygen concentration because of an increased ignition delay. Although simplified geometries were employed, the identification of important mechanisms affecting soot formation after the time of wall interaction is expected to be useful for understanding these processes in more complex and realistic diesel engine geometries.
One challenge faced by engineers today is replicating an operating environment such as transportation in a test lab. This paper focuses on the process of identifying sine-on-random content in an aircraft transportation environment, although the methodology can be applied to other events. The ultimate goal of this effort was to develop an automated way to identify significant peaks in the PSDs of the operating data, catalog the peaks, and determine whether each peak was sinusoidal or random in nature. This information helps design a test environment that accurately reflects the operating environment. A series of Matlab functions have been developed to achieve this goal with a relatively high degree of accuracy. The software is able to distinguish between sine-on-random and random-on-random peaks in most cases. This paper describes the approach taken for converting the time history segments to the frequency domain, identifying peaks from the resulting PSD, and filtering the time histories to determine the peak amplitude and characteristics. This approach is validated through some contrived data, and then applied to actual test data. Observations and conclusions, including limitations of this process, are also presented.
Dielectric spectroscopy, lattice structure, and thermal properties have revealed the relaxor dielectric response of Ba-substituted lead zirconate/titanate (PZT) having the composition (Pb0.71Ba0.29) (Zr0.71Ti0.29)O3 and containing 2 at. % Bi as an additive. The relaxor behavior is attributed to the compositional disorder introduced by the substitution of Ba2+ at the A site and Bi3+/5+ at the B site (and possibly A site) of the ABO3 PZT host lattice. Analysis of the results gives clear evidence for the nucleation of polar nanodomains at a temperature much higher than the peak (Tm) in the dielectric susceptibility. These nanodomains grow in size as their correlation length increases with decreasing temperature, and ultimately their dipolar fluctuations slow down below Tm leading to the formation of the relaxor state. The influences of hydrostatic pressure on the dielectric susceptibility and the dynamics of the relaxation of the polar nanodomains were investigated and can be understood in terms of the decrease in the size of the nanodomains with pressure. The influence of dc electrical bias on the susceptibility was also investigated. Physical models of the relaxor response of this material are discussed.
Preliminary investigation areas (PIA) for a potential repository of high-level radioactive waste must be evaluated by NUMO with regard to a number of qualifying factors. One of these factors is related to earthquakes and fault activity. This study develops a spatial statistical assessment method that can be applied to the active faults in Japan to perform such screening evaluations. This analysis uses the distribution of seismicity near faults to define the width of the associated process zone. This concept is based on previous observations of aftershock earthquakes clustered near active faults and on the assumption that such seismic activity is indicative of fracturing and associated impacts on bedrock integrity. Preliminary analyses of aggregate data for all of Japan confirmed that the frequency of earthquakes is higher near active faults. Data used in the analysis were obtained from NUMO and consist of three primary sources: (1) active fault attributes compiled in a spreadsheet, (2) earthquake hypocenter data, and (3) active fault locations. Examination of these data revealed several limitations with regard to the ability to associate fault attributes from the spreadsheet to locations of individual fault trace segments. In particular, there was no direct link between attributes of the active faults in the spreadsheet and the active fault locations in the GIS database. In addition, the hypocenter location resolution in the pre-1983 data was less accurate than for later data. These pre-1983 hypocenters were eliminated from further analysis.
The Arsenic Water Technology Partnership (AWTP) program is a multi-year program funded by a congressional appropriation through the Department of Energy to develop and test innovative technologies that have the potential to reduce the costs of arsenic removal from drinking water. The AWTP members include Sandia National Laboratories, the American Water Works Association (Awwa) Research Foundation and WERC (A Consortium for Environmental Education and Technology Development). The program is designed to move technologies from bench-scale tests to field demonstrations. The Awwa Research Foundation is managing bench-scale research programs; Sandia National Laboratories is conducting the pilot demonstration program and WERC will evaluate the economic feasibility of the technologies investigated and conduct technology transfer activities. The objective of the Sandia Arsenic Treatment Technology Demonstration project (SATTD) is the field demonstration testing of both commercial and innovative technologies. The scope for this work includes: (1) Identification of sites for pilot demonstrations; (2) Accelerated identification of candidate technologies through Vendor Forums, proof-of-principle laboratory and local pilot-scale studies, collaboration with the Awwa Research Foundation bench-scale research program and consultation with relevant advisory panels; and (3) Pilot testing multiple technologies at several sites throughout the country, gathering information on: (a) Performance, as measured by arsenic removal; (b) Costs, including capital and Operation and Maintenance (O&M) costs; (c) O&M requirements, including personnel requirements, and level of operator training; and (d) Waste residuals generation. The New Mexico Environment Department has identified over 90 public water systems that currently exceed the 10 {micro}g/L MCL for arsenic. The Sandia Arsenic Treatment Technology Demonstration project is currently operating pilots at three sites in New Mexico. The cities of Socorro, Anthony, and Rio Rancho vary in population, water chemistry, and source of arsenic. Figure 1 shows the locations of each city. The following pages summarize the work being performed at each site. At each site, the owners (e.g. city utility) provide access to the site, water, electricity, means to discharge treated water, and daily operational checks. Daily checks include filling out a logsheet with information on the flow rates, pressure drops, flow adjustments (when needed), and notification of Sandia personnel if a leak is present. Sandia owns all equipment and is responsible for the disposal of spent media and other waste streams. Sandia also performs all field tests and collects water samples for laboratory analysis.
We present theoretical performance estimates for nanotube optoelectronic devices under bias. Current-voltage characteristics of illuminated nanotube p-n junctions are calculated using a self-consistent nonequilibrium Green's function approach. Energy conversion rates reaching tens of percent are predicted for incident photon energies near the band gap energy. In addition, the energy conversion rate increases as the diameter of the nanotube is reduced, even though the quantum efficiency shows little dependence on nanotube radius. These results indicate that the quantum efficiency is not a limiting factor for use of nanotubes in optoelectronics.
Tabu search is one of the most effective heuristics for locating high-quality solutions to a diverse array of NP-hard combinatorial optimization problems. Despite the widespread success of tabu search, researchers have a poor understanding of many key theoretical aspects of this algorithm, including models of the high-level run-time dynamics and identification of those search space features that influence problem difficulty. We consider these questions in the context of the job-shop scheduling problem (JSP), a domain where tabu search algorithms have been shown to be remarkably effective. Previously, we demonstrated that the mean distance between random local optima and the nearest optimal solution is highly correlated with problem difficulty for a well-known tabu search algorithm for the JSP introduced by Taillard. In this paper, we discuss various shortcomings of this measure and develop a new model of problem difficulty that corrects these deficiencies. We show that Taillard's algorithm can be modeled with high fidelity as a simple variant of a straightforward random walk. The random walk model accounts for nearly all of the variability in the cost required to locate both optimal and sub-optimal solutions to random JSPs, and provides an explanation for differences in the difficulty of random versus structured JSPs. Finally, we discuss and empirically substantiate two novel predictions regarding tabu search algorithm behavior. First, the method for constructing the initial solution is highly unlikely to impact the performance of tabu search. Second, tabu tenure should be selected to be as small as possible while simultaneously avoiding search stagnation; values larger than necessary lead to significant degradations in performance.