Results from the ORPHEUS dark matter detector

The ORPHEUS dark matter detector consists of 450 g superconducting tin granules with diameters of 28 and 36 μm. The detector has been operating in the shallow site of the underground laboratory in Bern (70 m.w.e). First preliminary results on WIMP detection sensitivity will be presented.     

Least-squares integration of one-dimensional codistributions with application to approximate feedback linearization

We study the problem of approximating one-dimensional nonintegrable codistributions by integrable ones and apply the resulting approximations to approximate feedback linearization of single-input systems. The approach derived in this paper allows a linearizable nonlinear system to be found that is close to the given system in a least-squares (L ₂) sense. A linearly controllable single-input affine nonlinear system is feedback linearizable if and only if its characteristic distribution is involutive (hence integrable) or, equivalently, any characteristic one-form (a one-form that annihilates the characteristic distribution) is integrable. We study the problem of finding (least-squares approximate) integrating factors that make a fixed characteristic one-form close to being exact in anL ₂ sense. A given one-form can be decomposed into exact and inexact parts using the Hodge decomposition. We derive an upper bound on the size of the inexact part of a scaled characteristic one-form and show that a least-squares integrating factor provides the minimum value for this upper bound. We also consider higher-order approximate integrating factors that scale a nonintegrable one-form in a way that the scaled form is closer to being integrable inL ₂ together with some derivatives and derive similar bounds for the inexact part. This allows a linearizable nonlinear system that is close to the given system in a least-squares (L ₂) sense together with some derivatives to be found. The Sobolev embedding techniques allow us to obtain an upper bound on the uniform (L _∞) distance between the nonlinear system and its linearizable approximation. This research was supported in part by NSF under Grant PYI ECS-9396296, by AFOSR under Grant AFOSR F49620-94-1-0183, and by a grant from the Hughes Aircraft Company.     

Forward-bias conduction of Schottky diodes on polysilicon thinfilms

An analysis of forward I-V characteristics for Al Schottky contacts to polycrystalline thin films is presented. Experimental results for lateral structures with various n^- doping levels show the expected twofold exponential characteristics representing a transition from electrode-limited to bulk-limited conduction. At higher doping levels the bulk-limited characteristic is not a perfect exponential. The thermionic emission theory has been modified to include crystallite resistivity between grain boundaries and successfully matches experimental data     

High quality gate dielectrics formed by rapid thermal chemical vapor deposition of silane and nitrous oxide

In the present study, we have performed electrical characterization of oxides deposited via rapid thermal chemical vapor deposition using SiH₄ and N₂O. We have investigated the effect of temperature, pressure, and SiH₄ to N₂O ratio on the electrical and material properties of as-deposited films. We have found that as-deposited oxides deposited at low temperatures, low pressures, and with a low silane to nitrous oxide ratio of ～0.5% give good material and electrical properties. The as-deposited films are stoichiometric in nature and have high deposition rates. As-deposited films had very low D_it values, high breakdown fields, and excellent subthreshold swing. The leakage currents and metal oxide semiconductor field effect transistor current drive, although lower than thermal oxides, were found to be quite acceptable. We have also investigated the thickness dependence of the films and found that as the film thickness is reduced below 50Å, the reliability improves for all oxides including the silicon-rich deposited oxides.     

Using formalized temporal message-flow diagrams

Temporal message-flow diagrams (TMFDs), alternatively called sequence charts, interaction diagrams, event traces, or actor diagrams, are illustrations of a system's global message-passing activity over time, and a pictorial aid to understanding the system's behavior. They are widely used for requirements and documentation for network protocols and object-oriented applications. We present a general formalism for TMFDs, describe a suite of tools we have designed that employs this formalism, and present our experiences with these tools. The formalism and tools described serve to support and broaden the use of TMFDs in developing communicating systems.     

Mapping and Modelling Single Scan Track Formation in Direct Metal Selective Laser Melting

When a laser beam scans the surface of a metallic powder bed, the resulting track may be continuous with a crescent shape cross-section, continuous with an elliptical section, discontinuously irregularly broken, discontinuously balled or only partially melted. This paper reports experimental observations of what CO₂laser powers and scan speeds lead to what type of track, for M2 and H13 tools steels and 314S stainless steel powder beds. It also presents measurements of bed physical properties relevant to predicting the behaviour and the results of modelling that agree with the experiments. The results are placed in the broader context of selective laser melting build strategies