Showing papers by "United States Department of Energy published in 1991"

Two-dimensional vortices in a stack of thin superconducting films: A model for high-temperature superconducting multilayers

Abstract: The structure of vortices within an infinite stack of thin superconducting layers is considered and examined in detail in the limit of zero interlayer Josephson coupling The basic building block for the description of three-dimensional (3D) vortex lines is shown to be the 2D pancake vortex, which is a vortex located in only one of the layers; the other layers contain no vortices, but have an important effect in screening the magnetic field generated by currents in the first layer It is shown that 3D vortex lines can be built up by superposing the contributions of stacks of 2D pancake vortices Thermal excitation is shown to break up a single 3D vortex line at a temperature corresponding to the Kosterlitz-Thouless temperature of a single superconducting layer The effect of thermally induced decoupling of the 2D vortex solids in different layers, corresponding to melting only in the direction perpendicular to the layers, is also considered It is shown that Josephson coupling can be neglected in the high-temperature superconductors only under very stringent conditions Although these conditions evidently are not met in ${\mathrm{Bi}}_{2}$${\mathrm{Sr}}_{2}$${\mathrm{CaCu}}_{2}$${\mathrm{O}}_{8}$ and ${\mathrm{Tl}}_{2}$${\mathrm{Ba}}_{2}$${\mathrm{Ca}}_{1}$${\mathrm{Cu}}_{2}$${\mathrm{O}}_{8}$, they should be satisfied in superconducting-insulating multilayer systems, such as ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$/${\mathrm{PrBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$

Plant Lipids: Metabolism, Mutants, and Membranes

Abstract: The mechanisms that regulate plant lipid metabolism determine the dietary and industrial value of storage oils found in economically important species and may control the ability of many plants to survive exposure to temperature extremes. Many of the problems researchers have in defining the pathways, enzymes, and genes involved in plant lipid metabolism appear to be amenable to analysis by genetic approaches. Mutants with alterations in membrane lipid composition have also been used to study the structural and adaptive roles of lipids. The application of genetic engineering methods affords opportunities for researchers to apply knowledge gained about plant lipid metabolism toward enhanced use of plant oils as abundant and renewable sources of reduced carbon.

Model for the reversible magnetization of high- kappa type-II superconductors: Application to high-Tc superconductors.

Abstract: A simple theoretical model is proposed for the reversible magnetization of type-II superconductors as a function of the applied field H for the entire field region between ${\mathit{H}}_{\mathit{c}1}$ and ${\mathit{H}}_{\mathit{c}2}$. For H\ensuremath{\simeq}${\mathit{H}}_{\mathit{c}1}$, the theory reduces to a variational model, from which ${\mathit{H}}_{\mathit{c}1}$ can be accurately computed in the Ginzburg-Landau regime. In calculating the free energy, we include, in addition to the supercurrent kinetic energy and the magnetic-field energy, the kinetic-energy and the condensation-energy terms arising from suppression of the order parameter in the vortex core. The model is further extended to include anisotropy by introducing an effective-mass tensor for the case when H is parallel to one of the principal axes. The theory is compared to reversible-magnetization data on a ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7}$ single crystal. The method permits an accurate determination of ${\mathit{H}}_{\mathit{c}2}$ versus temperature from measurements of the magnetization versus temperature at fixed magnetic field and explains why the measurements have different slopes in different fields, contrary to what might have been expected from the linear Abrikosov formula near ${\mathit{H}}_{\mathit{c}2}$. The deduced ${\mathit{dH}}_{\mathit{c}2}$/dT is (-1.65\ifmmode\pm\else\textpm\fi{}0.23) T/K for H parallel to the c axis near ${\mathit{T}}_{\mathit{c}}$, implying ${\ensuremath{\xi}}_{\mathit{a}\mathit{b}}$(0)=(17\ifmmode\pm\else\textpm\fi{}1) \AA{}.

Structure of the (√3 × √3 )R30° Ag/Si(111) surface from first-principles calculations

Abstract: The structure of (\ensuremath{\surd}3 \ifmmode\times\else\texttimes\fi{} \ensuremath{\surd}3 ) Ag/Si(111) is investigated using first-principles total-energy calculations. The lowest-energy configuration consists of a top layer of Ag atoms arranged as ``honeycomb chained trimers'' lying above a distorted ``missing top layer'' Si(111) substrate. We find that the honeycomb structure observed in scanning-tunneling-microscope images arises from the electronic charge density on an empty surface band near the Fermi level.

Reversibly immobilized biological materials in monolayer films on electrodes

Abstract: Methods and techniques are described for reversibly binding charged biological particles in a fluid medium to an electrode surface. The methods are useful in a variety of applications. The biological materials may include microbes, proteins, and viruses. The electrode surface may consist of reversibly electroactive materials such as polyvinylferrocene, silicon-linked ferrocene or quinone.

Melamine-formaldehyde aerogels

Abstract: Organic aerogels that are transparent and essentially colorless are prepa from the aqueous, sol-gel polymerization of melamine with formaldehyde. The melamine-formaldehyde (MF) aerogels have low densities, high surface areas, continuous porsity, ultrafine cell/pore sizes, and optical clarity.

Multiple landscape scales: An intersite comparison

Abstract: Vegetation transect data from three locations were analyzed to determine if multiple scales of pattern could be detected. The sites included a semiarid grassland in New Mexico, a series of calcareous openings in a deciduous forest in Tennessee, and a shrub-steppe system in Washington. The data were explored with four statistical techniques. A scale of pattern was accepted if detected by more than one analytical method or located by a single method in multiple taxa. The analyses indicated 3–5 scales of pattern on all three sites, as predicted by Hierarchy Theory.

Limitations of the London model for the reversible magnetization of type-II superconductors.

Abstract: We point out that the London model for the reversible magnetization {ital M} of high-{kappa} type-II superconductors in the intermediate-field region is quantitatively incorrect, and we discuss the implications on our understanding of the mixed state of type-II superconductors. We also show that the apparently linear dependence of {ital M} vs ln{ital H} in the intermediate-field region, a behavior that has been observed experimentally, can be obtained from a previously proposed variational model.

Eutectic spacing selection in lead-based alloy systems

Abstract: Directional solidification studies have been carried out in Pb-Au, Pb-Pd, Pd-Cd, and Pb-Sn systems to characterize the variation in eutectic spacing with velocity. In the Pb-Cd, Pb-Sn, and Pb-Pd systems, statistical distributions of spacings at each velocity were determined and a significant spread was observed. The smallest and the largest spacings, along with the average spacing, have been characterized. A broad distribution curve has been observed for low velocities which becomes sharper as the velocities increase. A comparison with the theoretical model shows that the average spacings are consistently larger than the spacings predicted by the minimum undercooling criterion, whereas the smallest observed spacing corresponds to the theoretical extremum value. Dynamical studies have been carried out to examine the spacing selection process at a given velocity by initially starting with a higher or a lower average spacing. The final spacing distribution at a given velocity was found to be the same irrespective of the initial starting condition which establishes that a definite distribution of spacings exists in eutectic systems without any hysteresis effect and that dynamical effects are not responsible for the existence of a band of stable spacing.

Integration of photoactive and electroactive components with vertical cavity surface emitting lasers

Abstract: A monolithically integrated optoelectronic device is provided which integrates a vertical cavity surface emitting laser and either a photosensitive or an electrosensitive device either as input or output to the vertical cavity surface emitting laser either in parallel or series connection Both vertical and side-by-side arrangements are disclosed, and optical and electronic feedback means are provided Arrays of these devices can be configured to enable optical computing and neural network applications

A Method for Estimating Effective Porosity and Ground‐Water Velocity

Abstract: Estimates of ground-water velocity, based either on Darcy's law or on the single-well drift and pumpback tracer method, require prior knowledge of effective porosity. That is, after field data have been collected, the equation for ground-water velocity, using either method, still contains the two unknowns, velocity and porosity. If the local hydraulic gradient is known and if a drift and pumpback tracer test is conducted at a well whose hydraulic conductivity has been determined, two independent functional relationships between velocity and porosity are established. By treating these functions as nonlinear simultaneous equations, a unique solution for the local velocity and porosity can be obtained.

Multiple-frequency acoustic wave devices for chemical sensing and materials characterization in both gas and liquid phase

Abstract: A chemical sensor (1) includes two or more pairs of interdigital electrodes (10) having different periodicities Each pair is comprised of a first electrode (10a) and a second electrode (10b) The electrodes are patterned on a surface of a piezoelectric substrate (12) Each pair of electrodes may launch and receive various acoustic waves (AW), including a surface acoustic wave (SAW), and may also launch and receive several acoustic plate modes (APMs) The frequencies associated with each are functions of the transducer periodicity as well as the velocity of the particular AW in the chosen substrate material An AW interaction region (13) exists between each pair of electrodes Circuitry (20, 40) is used to launch, receive, and monitor the propagation characteristics of the AWs and may be configured in an intermittent measurement fashion or in a continuous measurement fashion Perturbations to the AW velocity and attenuation are recorded at several frequencies and provide the sensor response

Coherence delay augmented laser beam homogenizer

Abstract: The geometrical restrictions on a laser beam homogenizer are relaxed by ug a coherence delay line to separate a coherent input beam into several components each having a path length difference equal to a multiple of the coherence length with respect to the other components. The components recombine incoherently at the output of the homogenizer, and the resultant beam has a more uniform spatial intensity suitable for microlithography and laser pantogography. Also disclosed is a variable aperture homogenizer, and a liquid filled homogenizer.

Minefield reconnaissance and detector system

Abstract: A multi-sensor system (10) for detecting the presence of objects on the surface of the ground or buried just under the surface, such as anti-personnel or anti-tank mines or the like. A remote sensor platform (12) has a plurality of metal detector sensors (22) and a plurality of short pulse radar sensors (24). The remote sensor platform (12) is remotely controlled from a processing and control unit (14) and signals from the remote sensor platform (12) are sent to the processing and control unit (14) where they are individually evaluated in separate data analysis subprocess steps (34, 36) to obtain a probability "score" for each of the pluralities of sensors (22, 24). These probability scores are combined in a fusion subprocess step (38) by comparing score sets to a probability table (130) which is derived based upon the historical incidence of object present conditions given that score set. A decision making rule is applied to provide an output which is optionally provided to a marker subprocess (40) for controlling a marker device (76) to mark the location of found objects.

Measurements of Aerosol Particle Sizes from a Direct Injection Nebulizer

Abstract: Photographs and laser-scattering measurements showed that a direct injection nebulizer (DIN) produced a finer aerosol with a narrower drop size distribution than that for a conventional glass concentric nebulizer when both were operated at the same liquid and nebulizer gas flow rates. The droplets from the DIN were slightly larger, however, than those leaving a Scott-type spray chamber with a glass concentric nebulizer. The droplets from the DIN had a Sauter mean diameter of ∼7 μm and a 90% mass diameter of 11–16 μm. The droplet size distribution became narrower and shifted to lower diameters as the aerosol gas flow rate increased.