Publications

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Many problems in science and engineering require the solution of a long sequence of slowly changing linear systems. We propose and analyze two methods that significantly reduce the total number of matrix-vector products required to solve all systems. We consider the general case where both the matrix and right-hand side change, and we make no assumptions regarding the change in the right-hand sides. Furthermore, we consider general nonsingular matrices, and we do not assume that all matrices are pairwise close or that the sequence of matrices converges to a particular matrix. Our methods work well under these general assumptions, and hence form a significant advancement with respect to related work in this area. We can reduce the cost of solving subsequent systems in the sequence by recycling selected subspaces generated for previous systems. We consider two approaches that allow for the continuous improvement of the recycled subspace at low cost. We consider both Hermitian and non-Hermitian problems, and we analyze our algorithms both theoretically and numerically to illustrate the effects of subspace recycling. We also demonstrate the effectiveness of our algorithms for a range of applications from computational mechanics, materials science, and computational physics. © 2006 Society for Industrial and Applied Mathematics.

We discuss computing first derivatives for models based on elements, such as large-scale finite-element PDE discretizations, implemented in the C++ programming language. We use a hybrid technique of automatic differentiation (AD) and manual assembly, with local element-level derivatives computed via AD and manually summed into the global derivative. C++ templating and operator overloading work well for both forward- and reverse-mode derivative computations. We found that AD derivative computations compared favorably in time to finite differencing for a scalable finite-element discretization of a convection-diffusion problem in two dimensions. © Springer-Verlag Berlin Heidelberg 2006.

Microsystems are potentially exposed to laser irradiation during processing, diagnostic measurements, and, in some cases, device operation. The behavior of the components in an optical MEMS device that are irradiated by a laser needs to be optimized for reliable operation. Utilizing numerical simulations facilitates design and optimization. This paper reports on experimental and numerical investigations of the thermomechanical response of polycrystalline silicon microcantilevers that are 250 {micro}m wide, 500 {micro}m long, and 2.25 {micro}m thick when heated by an 808 nm laser. At laser powers above 400 mW significant deflection is observed during the laser pulse using a white light interferometer. Permanent deformation is detected at laser powers above 650 mW in the experiments. Numerical calculations using a coupled physics finite element code, Calagio, agree qualitatively with the experimental results. Both the experimental and numerical results reveal that the initial stress state is very significant. Microcantilevers deflect in the direction of their initial deformation upon irradiation with a laser.

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We have investigated the valley splitting of two-dimensional electrons in high-quality Si/Si{sub 1-x}Ge{sub x} heterostructures under tilted magnetic fields. For all the samples in our study, the valley splitting at filling factor {nu} = 3 ({Delta}{sub 3}) is significantly different before and after the coincidence angle, at which energy levels cross at the Fermi level. On both sides of the coincidence, a linear dependence of {Delta}{sub 3} on the electron density was observed, while the slope of these two configurations differs by more than a factor of 2. We argue that screening of the Coulomb interaction from the low-lying filled levels, which also explains the observed spin-dependent resistivity, is responsible for the large difference of {Delta}{sub 3} before and after the coincidence.

Electrical contacts to semiconductors play a key role in electronics. For nanoscale electronic devices, particularly those employing novel low-dimensionality materials, contacts are expected to play an even more important role. Here we show that for quasi-one-dimensional structures such as nanotubes and nanowires, side contact with the metal only leads to weak band re-alignment, in contrast to bulk metal-semiconductor contacts. Schottky barriers are much reduced compared with the bulk limit, and should facilitate the formation of good contacts. However, the conventional strategy of heavily doping the semiconductor to obtain ohmic contacts breaks down as the nanowire diameter is reduced. The issue of Fermi level pinning is also discussed, and it is demonstrated that the unique density of states of quasi-one-dimensional structures make them less sensitive to this effect. Our results agree with recent experimental work, and should apply to a broad range of quasi-one-dimensional materials.

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Surfpack is a general-purpose software library of multidimensional function approximation methods for applications such as data visualization, data mining, sensitivity analysis, uncertainty quantification, and numerical optimization. Surfpack is primarily intended for use on sparse, irregularly-spaced, n-dimensional data sets where classical function approximation methods are not applicable. Surfpack is under development at Sandia National Laboratories, with a public release of Surfpack version 1.0 in August 2006. This paper provides an overview of Surfpack's function approximation methods along with some of its software design attributes. In addition, this paper provides some simple examples to illustrate the utility of Surfpack for data trend analysis, data visualization, and optimization. Copyright © 2006 by the American Institute of Aeronautics and Astronautics, Inc.

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HfB{sub 2} and ZrB{sub 2} are of interest for thermal protection materials because of favorable thermal stability, mechanical properties, and oxidation resistance. We have made dense diboride ceramics with 2 to 20 % SiC by hot pressing at 2000 C and 5000 psi. High-resolution transmission electron microscopy (TEM) shows very thin grain boundary phases that suggest liquid phase sintering. Fracture toughness measurements give RT values of 4 to 6 MPam{sup 1/2}. Four-pt flexure strengths measured in air up to 1450 C were as high as 450-500 MPa. Thermal diffusivities were measured to 2000 C for ZrB{sub 2} and HfB{sub 2} ceramics with SiC contents from 2 to 20%. Thermal conductivities were calculated from thermal diffusivities and measured heat capacities. Thermal diffusivities were modeled using different two-phase composite models. These materials exhibit excellent high temperature properties and are attractive for further development for thermal protection systems.

In evaluating new high-speed network interfaces, the usual metrics of latency and bandwidth are commonly measured and reported. There are numerous other message passing characteristics that can have a dramatic effect on application performance that should be analyzed when evaluating a new interconnect. One such metric is overhead, which dictates the networks ability to allow the application to perform non-message passing work while a transfer is taking place. A method for measuring overhead, and hence calculating application availability, is presented. Results for several next-generation network interfaces are also presented. © Springer-Verlag Berlin Heidelberg 2006.

The development and verification of a one-dimensional constant density material thermal response code with ablation is presented. The implicit time integrator, control volume finite element spatial discretization, and Newton's method for nonlinear iteration on the entire system of equations have been implemented and verified for variable material properties, Q* ablation, and thermochemical ablation problems. Timing studies were performed, and when accuracy is considered the method developed in this study exhibits significant time savings over the property lagging approach. In addition, maximizing the Newton solver's convergence rate by including sensitivities to the surface recession rate reduces the overall computational time when compared to excluding recession rate sensitivites.

As the specimen gets smaller and thinner, traditional strain measurement method using the strain gage is impossible. In this paper, the strain is measured using non-contact laser interferometry method. Two markers are placed on the LIGA specimens along the loading direction to reflect the laser beams to generate the interferometric fringe patterns. The markers are generated using micro-hardness indentation for the LIGA specimens. A pair of CCD cameras is used to capture the interferometric fringes during each step of the loading along the longitudinal direction. Fast Fourier Transform (FFT) is then applied to calculate the frequency and phase shift of the fringes. The displacement and strain can be obtained from the phase shift of the fringe pattern. This ISDG strain measurement technique is further developed by using multi markers to obtain fringes during the whole loading when the specimen undergoes larger motion. Biaxial strain measurement using ISDG is also developed to obtain both Young's modulus and Poisson's ratio simultaneously. A third marker is located orthogonal to the first pair of markers along the loading direction. Two pairs of CCD cameras are used to acquire the digital images of the interferometric fringes patterns along both longitudinal and transverse directions. The stress-strain curves as well as the material properties are very consistent from the different tests using ISDG. Copyright © 2006 by ASME.

The telecommunication network is recognized by the federal government as one of the critical national infrastructures that must be maintained and protected against debilitating attacks. We have previously shown how failures in the telecommunication network can quickly lead to telecommunication congestion and to extended delays in successful call completion. However, even if the telecom network remains fully operational, the special telecommunication demands that materialize at times of emergencies, and dynamically change based on subscriber behavior, can also adversely affect the performance of the overall telecommunication network. The Network Simulation Modeling and Analysis Research Tool (N-SMART) has been developed by Bell Labs as part of its work with the National Infrastructure Simulation and Analysis Center. This center is a joint program at Sandia National Laboratories and Los Alamos National Laboratory, funded and managed by the Department of Homeland Security's (DHS) Preparedness Directorate. N-SMART is a discrete event (call level) telecom model that simulates capacities, blocking levels, retrials, and time to complete calls for both wireline and wireless networks. N-SMART supports the capability of simulating subscriber reattempt behaviour under various scenarios. Using this capability we show how the network can be adversely impacted by sudden changes in subscriber behavior. We also explore potential solutions and ways of mitigating those impacts.

Micromirrors arrays can be used to correct residual wavefront aberrations in certain optical systems. The ability to correct Zernike aberrations using arrays of pistononly and arrays of piston-tip-tilt micromirror arrays are compared. Our micromirror fabrication program is discussed. © 2006 Optical Society of America.

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A particle filter based algorithm was developed to track vehicles in a network of roads under the assumption of sporadic and non-persistent sensor data. It is assumed we have a number of sensors that provide position and velocity information only, which are scattered at possibly uneven intervals throughout the road system of interest. Further, the sensor ranges do not overlap, meaning we do not have constant eyes on target. The algorithm was based on the particle filter, but differed from the classical particle filter in two fundamental ways. First, particle weights are not used. Instead, a correspondence function is calculated only when a sensor is tripped, giving weight to the validity of the sensor report. Potentially this results in a computational savings. Second, we do not periodically resample particles. Results demonstrate the approach can effectively track multiple targets in simulations with sparse surveillance. © 2006 IEEE.

We have developed a bioparticle detection platform which combines insulatorbased dielectrophoretic (iDEP) concentration with impedance feedback. The system continuously and selectively accumulates particles while electrical responses of the suspension at the trapping site are recorded. The operating conditions for trapping are determined by the physical and electrical properties of the target particle type. Recordings of phase offset, relative to the reference sensing signal, act as the principal monitoring indicators. These measurements enable us to detect the presence and the approximate concentration of biological contaminants in a sample. This study is the first to illustrate the potential of iDEP concentration in conjunction with impedance measurements. The results obtained from fluorescent beads and viable B. subtilis spores demonstrate the feasibility of using iDEP concentration with active impedance monitoring to detect biological pathogens collected from dilute samples. © 2006 Society for Chemistry and Micro-Nano Systems.

We report a novel means of fractionating proteins based on their voltage-dependent electromigration through nanopores of a polymer membrane. The nanoporous membranes were fabricated in situ in channels of a microchip using photopolymerization. The pores (1-10 nm) are small enough that proteins are excluded from passage with low applied electric fields, but increasing the field enables proteins to pass through. The magnitude of field required for a change in exclusion behavior is protein-specific with a correlation to protein size. Passage of proteins through the pores at higher field strengths could be attributed to partial unfolding or deformation of proteins due to the driving force of the applied field. The field-dependent exclusion mechanism could be useful as a multifaceted fractionation tool with single membranes or a network of membranes. Another exciting possibility is characterizing protein conformation, folding and stability based on field-dependent transport through nanopores. © 2006 Society for Chemistry and Micro-Nano Systems.