Showing papers by "Georgia Institute of Technology published in 1998"

Carbon nanotube quantum resistors

Abstract: The conductance of multiwalled carbon nanotubes (MWNTs) was found to be quantized. The experimental method involved measuring the conductance of nanotubes by replacing the tip of a scanning probe microscope with a nanotube fiber, which could be lowered into a liquid metal to establish a gentle electrical contact with a nanotube at the tip of the fiber. The conductance of arc-produced MWNTs is one unit of the conductance quantum G0 5 2e 2 /h 5 (12.9 kilohms) ‐1 . The nanotubes conduct current ballistically and do not dissipate heat. The nanotubes, which are typically 15 nanometers wide and 4 micrometers long, are several orders of magnitude greater in size and stability than other typical room-temperature quantum conductors. Extremely high stable current densities, J . 10 7 amperes per square centimeter, have been attained.

GeneMark.hmm: New solutions for gene finding

Abstract: The number of completely sequenced bacterial genomes has been growing fast. There are computer methods available for finding genes but yet there is a need for more accurate algorithms. The GeneMark. hmm algorithm presented here was designed to improve the gene prediction quality in terms of finding exact gene boundaries. The idea was to embed the GeneMark models into naturally derived hidden Markov model framework with gene boundaries modeled as transitions between hidden states. We also used the specially derived ribosome binding site pattern to refine predictions of translation initiation codons. The algorithm was evaluated on several test sets including 10 complete bacterial genomes. It was shown that the new algorithm is significantly more accurate than GeneMark in exact gene prediction. Interestingly, the high gene finding accuracy was observed even in the case when Markov models of order zero, one and two were used. We present the analysis of false positive and false negative predictions with the caution that these categories are not precisely defined if the public database annotation is used as a control.

The formation and evolution of synthetic jets

Abstract: A nominally plane turbulent jet is synthesized by the interactions of a train of counter-rotating vortex pairs that are formed at the edge of an orifice by the time-periodic motion of a flexible diaphragm in a sealed cavity. Even though the jet is formed without net mass injection, the hydrodynamic impulse of the ejected fluid and thus the momentum of the ensuing jet are nonzero. Successive vortex pairs are not subjected to pairing or other subharmonic interactions. Each vortex of the pair develops a spanwise instability and ultimately undergoes transition to turbulence, slows down, loses its coherence and becomes indistinguishable from the mean jet flow. The trajectories of vortex pairs at a given formation frequency scale with the length of the ejected fluid slug regardless of the magnitude of the formation impulse and, near the jet exit plane, their celerity decreases monotonically with streamwise distance while the local mean velocity of the ensuing jet increases. In the far field, the synthetic jet i...

Gold nanoelectrodes of varied size: Transition to molecule-like charging

Abstract: A transition from metal-like double-layer capacitive charging to redox-like charging was observed in electrochemical ensemble Coulomb staircase experiments on solutions of gold nanoparticles of varied core size. The monodisperse gold nanoparticles are stabilized by short-chain alkanethiolate monolayers and have 8 to 38 kilodaltons core mass (1.1 to 1.9 nanometers in diameter). Larger cores display Coulomb staircase responses consistent with double-layer charging of metal-electrolyte interfaces, whereas smaller core nanoparticles exhibit redox chemical character, including a large central gap. The change in behavior is consistent with new near-infrared spectroscopic data showing an emerging gap between the highest occupied and lowest unoccupied orbitals of 0.4 to 0.9 electron volt.

Overview of radiolocation in CDMA cellular systems

Abstract: Applications for the location of subscribers of wireless services continue to expand. Consequently, location techniques for wireless technologies are being investigated. With code-division multiple access (CDMA) being deployed by a variety of cellular and PCS providers, developing an approach for location in CDMA networks is imperative. This article discusses the applications of location technology, the methods available for its implementation in CDMA networks, and the problems that are encountered when using CDMA networks for positioning.

Communication with Chaotic Lasers

Abstract: Recent experiments with chaotic electronic circuits have shown the possibility of communication with chaos The experimental demonstration of chaotic communication with an optical system is described An erbium-doped fiber ring laser (EDFRL) was used to produce chaotic light with a wavelength of 153 micrometers A small 10-megahertz message was embedded in the larger chaotic carrier and transmitted to a receiver system where the message was recovered from the chaos Chaotic optical waveforms can thus be used to communicate masked information at high bandwidths

Geometric compression through topological surgery

Abstract: The abundance and importance of complex 3-D data bases in major industry segments, the affordability of interactive 3-D rendering for office and consumer use, and the exploitation of the Internet to distribute and share 3-D data have intensified the need for an effective 3-D geometric compression technique that would significantly reduce the time required to transmit 3-D models over digital communication channels, and the amount of memory or disk space required to store the models. Because the prevalent representation of 3-D models for graphics purposes is polyhedral and because polyhedral models are in general triangulated for rendering, this article introduces a new compressed representation for complex triangulated models and simple, yet efficient, compression and decompression algorithms. In this scheme, vertex positions are quantized within the desired accuracy, a vertex spanning tree is used to predict the position of each vertex from 2,3, or 4 of its ancestors in the tree, and the correction vectors are entropy encoded. Properties, such as normals, colors, and texture coordinates, are compressed in a similar manner. The connectivity is encoded with no loss of information to an average of less than two bits per triangle. The vertex spanning tree and a small set of jump edges are used to split the model into a simple polygon. A triangle spanning tree and a sequence of marching bits are used to encode the triangulation of the polygon. Our approach improves on Michael Deering's pioneering results by exploiting the geometric coherence of several ancestors in the vertex spanning tree, preserving the connectivity with no loss of information, avoiding vertex repetitions, and using about three fewer bits for the connectivity. However, since decompression requires random access to all vertices, this method must be modified for hardware rendering with limited onboard memory. Finally, we demonstrate implementation results for a variety of VRML models with up to two orders of magnitude compression.

A low-voltage, low quiescent current, low drop-out regulator

Abstract: The demand for low-voltage, low drop-out (LDO) regulators is increasing because of the growing demand for portable electronics, i.e., cellular phones, pagers, laptops, etc. LDO's are used coherently with dc-dc converters as well as standalone parts. In power supply systems, they are typically cascaded onto switching regulators to suppress noise and provide a low noise output. The need for low voltage is innate to portable low power devices and corroborated by lower breakdown voltages resulting from reductions in feature size. Low quiescent current in a battery-operated system is an intrinsic performance parameter because it partially determines battery life. This paper discusses some techniques that enable the practical realizations of low quiescent current LDO's at low voltages and in existing technologies. The proposed circuit exploits the frequency response dependence on load-current to minimize quiescent current flow. Moreover, the output current capabilities of MOS power transistors are enhanced and drop-out voltages are decreased for a given device size. Other applications, like dc-dc converters, can also reap the benefits of these enhanced MOS devices. An LDO prototype incorporating the aforementioned techniques was fabricated. The circuit was operable down to input voltages of 1 V with a zero-load quiescent current flow of 23 /spl mu/A. Moreover, the regulator provided 18 and 50 mA of output current at input voltages of 1 and 1.2 V, respectively.

Isolation and Selected Properties of a 10.4 kDa Gold:Glutathione Cluster Compound

Abstract: An unprecedented small thioaurite cluster compound (with metallic Au0 core) has been isolated in high yield by decomposition of polymeric Au(I)SG compounds, where GSH is the ubiquitous tripeptide glutathione, N-γ-glutamyl-cysteinyl-glycine The Au:SG clusters appear to share the high stability and robustness of their hydrophobic n-alkyl analogues but are highly water soluble The most abundant cluster produced by these methods can be easily separated from its homologues by gel electrophoresis Its total molecular weight is ca 104 kDa, and the mass of its strongly bound inorganic core is 56 kDa, suggesting the composition Au28(SG)16 This composition is also consistent with the X-ray diffraction pattern of the crystalline molecular solid Distinct features in the optical absorption spectroscopy are inherently different from either larger clusters or smaller gold cluster compounds The compound is optically active, as evidenced by circular dichroism in the near-IR, visible, and near-UV regions The 13C N

Constructive incremental learning from only local information

Abstract: We introduce a constructive, incremental learning system for regression problems that models data by means of spatially localized linear models. In contrast to other approaches, the size and shape of the receptive field of each locally linear model, as well as the parameters of the locally linear model itself, are learned independently, that is, without the need for competition or any other kind of communication. Independent learning is accomplished by incrementally minimizing a weighted local cross-validation error. As a result, we obtain a learning system that can allocate resources as needed while dealing with the bias-variance dilemma in a principled way. The spatial localization of the linear models increases robustness toward negative interference. Our learning system can be interpreted as a nonparametric adaptive bandwidth smoother, as a mixture of experts where the experts are trained in isolation, and as a learning system that profits from combining independent expert knowledge on the same problem. This article illustrates the potential learning capabilities of purely local learning and offers an interesting and powerful approach to learning with receptive fields.

Oscillatory Shear Stress Stimulates Adhesion Molecule Expression in Cultured Human Endothelium

Abstract: —Low and oscillatory shear stresses are major features of the hemodynamic environment of sites opposite arterial flow dividers that are predisposed to atherosclerosis. Atherosclerosis is a focal inflammatory disease characterized initially by the recruitment of mononuclear cells into the arterial wall. The specific characteristics of the hemodynamic environment that facilitate the generation of arterial inflammatory responses in the presence of, for example, hyperlipidemia are unknown. We show here that prolonged oscillatory shear stress induces expression of endothelial cell leukocyte adhesion molecules, which are centrally important in mediating leukocyte localization into the arterial wall. Vascular cell adhesion molecule-1 was upregulated an average 9-fold relative to endothelial monolayers in static culture. Intercellular adhesion molecule-1 and E-selectin exhibited 11-fold and 7.5-fold increases, respectively. Upregulation of these adhesion molecules was associated with enhanced monocyte adh...

MITOMAP: a human mitochondrial genome database—2004 update

Abstract: MITOMAP (http://www.MITOMAP.org), a database for the human mitochondrial genome, has grown rapidly in data content over the past several years as interest in the role of mitochondrial DNA (mtDNA) variation in human origins, forensics, degenerative diseases, cancer and aging has increased dramatically. To accommodate this information explosion, MITOMAP has implemented a new relational database and an improved search engine, and all programs have been rewritten. System administrative changes have been made to improve security and efficiency, and to make MITOMAP compatible with a new automatic mtDNA sequence analyzer known as Mitomaster.

Subscriber location in CDMA cellular networks

Abstract: Subscriber radio location techniques are investigated for code-division multiple-access (CDMA) cellular networks. Two methods are considered for radio location: measured times of arrival (ToA) and angles of arrival (AoA). The ToA measurements are obtained from the code tracking loop in the CDMA receiver, and the AoA measurements at a base station (BS) are assumed to be made with an antenna array. The performance of the two methods is evaluated for both ranging and two-dimensional (2-D) location, while varying the propagation conditions and the number of BS's used for the location estimate.

Comparison of response surface and kriging models for multidisciplinary design optimization

Abstract: In this paper, we compare and contrast the use of second-order response surface models and kriging models for approximating non-random, deterministic computer analyses. After reviewing the response surface method for constructing polynomial approximations, kriging is presented as an alternative approximation method for the design and analysis of computer experiments. Both methods are applied to the multidisciplinary design of an aerospike nozzle which consists of a computational fluid dynamics model and a finite-element model. Error analysis of the response surface and kriging models is performed along with a graphical comparison of the approximations, and four optimization problems m formulated and solved using both sets of approximation models. The second-order response surface models and kriging models-using a constant underlying global model and a Gaussian correlation function-yield comparable results.

Novel multilevel inverter carrier-based PWM methods

Abstract: The advent of the transformerless multilevel inverter topology has brought forth various pulsewidth modulation (PWM) schemes as a means to control the switching of the active devices in each of the multiple voltage levels in the inverter. An analysis of how existing multilevel carrier-based PWM affects switch utilization for the different levels of a diode-clamped inverter is conducted. Two novel carrier-based multilevel PWM schemes are presented which help to optimize or balance the switch utilization in multilevel inverters. A 10 kW prototype six-level diode-clamped inverter has been built and controlled with the novel PWM strategies proposed in this paper to act as a voltage-source inverter for a motor drive.

A local boundary integral equation (LBIE) method in computational mechanics, and a meshless discretization approach

Abstract: The Galerkin finite element method (GFEM) owes its popularity to the local nature of nodal basis functions, i.e., the nodal basis function, when viewed globally, is non-zero only over a patch of elements connecting the node in question to its immediately neighboring nodes. The boundary element method (BEM), on the other hand, reduces the dimensionality of the problem by one, through involving the trial functions and their derivatives, only in the integrals over the global boundary of the domain; whereas, the GFEM involves the integration of the “energy” corresponding to the trial function over a patch of elements immediately surrounding the node. The GFEM leads to banded, sparse and symmetric matrices; the BEM based on the global boundary integral equation (GBIE) leads to full and unsymmetrical matrices. Because of the seemingly insurmountable difficulties associated with the automatic generation of element-meshes in GFEM, especially for 3-D problems, there has been a considerable interest in element free Galerkin methods (EFGM) in recent literature. However, the EFGMs still involve domain integrals over shadow elements and lead to difficulties in enforcing essential boundary conditions and in treating nonlinear problems. The object of the present paper is to present a new method that combines the advantageous features of all the three methods: GFEM, BEM and EFGM. It is a meshless method. It involves only boundary integration, however, over a local boundary centered at the node in question; it poses no difficulties in satisfying essential boundary conditions; it leads to banded and sparse system matrices; it uses the moving least squares (MLS) approximations. The method is based on a Local Boundary Integral Equation (LBIE) approach, which is quite general and easily applicable to nonlinear problems, and non-homogeneous domains. The concept of a “companion solution” is introduced so that the LBIE for the value of trial solution at the source point, inside the domain Ω of the given problem, involves only the trial function in the integral over the local boundary Ω s of a sub-domain Ω s centered at the node in question. This is in contrast to the traditional GBIE which involves the trial function as well as its gradient over the global boundary Γ of Ω. For source points that lie on Γ, the integrals over Ω s involve, on the other hand, both the trial function and its gradient. It is shown that the satisfaction of the essential as well as natural boundary conditions is quite simple and algorithmically very efficient in the present LBIE approach. In the example problems dealing with Laplace and Poisson's equations, high rates of convergence for the Sobolev norms ||·||0 and ||·||1 have been found. In essence, the present EF-LBIE (Element Free-Local Boundary Integral Equation) approach is found to be a simple, efficient, and attractive alternative to the EFG methods that have been extensively popularized in recent literature.

Kinetically Controlled Growth and Shape Formation Mechanism of Platinum Nanoparticles

Abstract: Recently, we have been able to synthesize platinum colloidal nanoparticles of different shapes (Science, 1996, 272, 1924). In this report, we present transmission electron microscopic (TEM) results on the time-dependent shape distribution of platinum nanoparticles during the growth period and its dependence on the concentration of the capping polymer as well as the pH of the solution. The results suggest a shape-controlled growth mechanism in which the difference between the rate of the catalytic reduction process of Pt2+ on the {111} and {100} faces, the competition between the Pt2+ reduction and the capping process on the different nanoparticle surfaces, and the concentration-dependent buffer action of the polymer itself all control the final distribution of the different shapes observed.

Spatiotemporal evolution of ventricular fibrillation

Abstract: Sudden cardiac death is the leading cause of death in the industrialized world, with the majority of such tragedies being due to ventricular fibrillation1. Ventricular fibrillation is a frenzied and irregular disturbance of the heart rhythm that quickly renders the heart incapable of sustaining life. Rotors, electrophysiological structures that emit rotating spiral waves, occur in several systems that all share with the heart the functional properties of excitability and refractoriness. These re-entrant waves, seen in numerical solutions of simplified models of cardiac tissue2, may occur during ventricular tachycardias3,4. It has been difficult to detect such forms of re-entry in fibrillating mammalian ventricles5,6,7,8. Here we show that, in isolated perfused dog hearts, high spatial and temporal resolution mapping of optical transmembrane potentials can easily detect transiently erupting rotors during the early phase of ventricular fibrillation. This activity is characterized by a relatively high spatiotemporal cross-correlation. During this early fibrillatory interval, frequent wavefront collisions and wavebreak generation9 are also dominant features. Interestingly, this spatiotemporal pattern undergoes an evolution to a less highly spatially correlated mechanism that lacks the epicardial manifestations of rotors despite continued myocardial perfusion.

A modified collocation method and a penalty formulation for enforcing the essential boundary conditions in the element free Galerkin method

Abstract: The Element free Galerkin method, which is based on the Moving Least Squares approximation, requires only nodal data and no element connectivity, and therefore is more flexible than the conventional finite element method. Direct imposition of essential boundary conditions for the element free Galerkin (EFG) method is always difficult because the shape functions from the Moving Least Squares approximation do not have the delta function property. In the prior literature, a direct collocation of the fictitious nodal values u used as undetermined coefficients in the MLS approximation, uh(x) [uh(x)=Φ·u], was used to enforce the essential boundary conditions. A modified collocation method using the actual nodal values of the trial function uh(x) is presented here, to enforce the essential boundary conditions. This modified collocation method is more consistent with the variational basis of the EFG method. Alternatively, a penalty formulation for easily imposing the essential boundary conditions in the EFG method with the MLS approximation is also presented. The present penalty formulation yields a symmetric positive definite system stiffness matrix. Numerical examples show that the present penalty method does not exhibit any volumetric locking and retains high rates of convergence for both displacements and strain energy. The penalty method is easy to implement as compared to the Lagrange multiplier method, which increases the number of degrees of freedom and yields a non-positive definite system matrix.

Designing 1-V op amps using standard digital CMOS technology

Abstract: This paper addresses the difficulty of designing 1-V capable analog circuits in standard digital complementary metal-oxide-semiconductor (CMOS) technology, Design techniques for facilitating 1-V operation are discussed and 1-V analog building block circuits are presented. Most of these circuits use the bulk-driving technique to circumvent the metal-oxide-semiconductor field-effect transistor turn-on (threshold) voltage requirement. Finally, techniques are combined within a 1-V CMOS operational amplifier with rail-to-rail input and output ranges. While consuming 300 /spl mu/W, the 1-V rail-to-rail CMOS op amp achieves 1.3-MHz unity-gain frequency and 57/spl deg/ phase margin for a 22-pF load capacitance.

Learning and Performance Orientation of Salespeople: The Role of Supervisors:

Abstract: The recent work of Sujan, Weitz, and Kumar (1994) brings into sharp focus two distinct goal orientations of salespeople: learning and performance. In this article, the authors make two primary cont...

The role of equivalence ratio oscillations in driving combustion instabilities in low NOx gas turbines

Abstract: This paper presents a theoretical investigation of combustion instabilities in low NOX gas turbines (LNGT) that burn fuel in a lean premixed mode. It is shown that these instabilities may be caused by interactions of combustor pressure oscillations with the reactants' supply rates, producing equivalence ratio perturbations in the inlet duct. These perturbations are convected by the mean flow to the combustor where they produce large-amplitude heat-release oscillations that drive combustor pressure oscillations. It is shown in this study that in contrast to earlier analyses, which assumed a uniform instantaneous heat release throughout the flame region, the heat release within the flame may exhibit strong spatial dependence that can significantly affect the combustor stability. The proposed instability mechanism is incorporated into a model that is used to predict LNGT stability limits. The model results show that LNGT are highly prone to combustion instabilities, especially under lean operating conditions, and that the regions of instability can be approximately described in terms of a ratio of the reactants' convective time from the fuel injector to the combustor and the period of the oscillations (with some modifications that account for the structure of the combustion region). Significantly, the developed model's predictions are in good agreement with available experimental data, strongly suggesting that the proposed mechanism and the developed model properly account for the essential physics of the problem.

The B-DNA Dodecamer at High Resolution Reveals a Spine of Water on Sodium†,‡

Abstract: We describe a very accurate addition (called structure X here) to the B-DNA dodecamer family of X-ray structures. Our results confirm the observation of Drew and Dickerson [(1981) J. Mol. Biol. 151, 535−556] that the spine of hydration in AT tract DNA is two layers deep. However, our results suggest that the primary spine is partially occupied by sodium ions. We suggest that many sequence-dependent features of DNA conformation are mediated by site specific binding of cations. For example, preferential localization of cations, as described here within the minor groove of structure X, is probably the structural origin of AT tract bending and groove narrowing. The secondary spine, which does not interact directly with the DNA, is as geometrically regular as the primary spine, providing a model for transmission of sequence information into solvent regions. A fully hydrated magnesium ion located in the major groove of structure X appears to pull cytosine bases partially out from the helical stack, exposing π-s...

The evolution of slow dispersal rates: a reaction diffusion model

Abstract: We consider n phenotypes of a species in a continuous but heterogeneous environment. It is assumed that the phenotypes differ only in their diffusion rates. With haploid genetics and a small rate of mutation, it is shown that the only nontrivial equilibrium is a population dominated by the slowest diffusing phenotype. We also prove that if there are only two possible phenotypes, then this equilibrium is a global attractor and conjecture that this is true in general. Numerical simulations supporting this conjecture and suggesting that this is a robust phenomenon are also discussed.

Nonlinear adaptive flight control using neural networks

Abstract: The effectiveness of a controller architecture, which combines adaptive feedforward neural networks with feedback linearization, has been demonstrated on a variety of flight vehicles. The boundedness of tracking error and control signals is guaranteed. The architecture can accommodate both linear-in-the-parameters networks, as well as single-hidden-layer perceptron neural networks. Both theoretical and experimental research is planned to expand and improve the applicability of the approach, and to demonstrate practical utility in the areas of cost reduction and improved flight safety.