A broad generalization of memristors--a recently postulated circuit element--to an interesting class of nonlinear dynamical systems called memristive systems is introduced. These systems are unconventional in the sense that while they behave like resistive devices, they can be endowed with a rather exotic variety of dynamic characteristics. While possessing memory and exhibiting small-signal inductive or capacitive effects, they are incapable of energy discharge and they introduce no phase shift between the input and output waveforms. This zero-crossing property gives rise to a Lissajous figure which always passes through the origin. Memristive systems are hysteretic in the sense that their Lissajous figures vary with the excitation frequency. At very low frequencies, memristive systems are indistinguishable from nonlinear resistors while at extremely high frequencies, they reduce to linear resistors. These anomalous properties have misled and prevented the identification of many memristive devices and systems-including the thermistor, the Hodgkin-Huxley membrane circuit model, and the discharge tubes. Generic properties of memristive systems are derived and a canonic dynamical system model is presented along with an explicit algorithm for identifying the model parameters and functions.
Abstract A simple molding process carried out within the confines of a chromatographic column has been used for the preparation of macroporous poly(glycidyl methacrylate‐ co ‐ethylene dimethacrylate) and poly(styrene‐ co ‐divinylbenzene) rods. The novel monolithic separation media that are obtained are useful for the HPLC separation of biological and synthetic polymers. The presence of large pores with a diameter of about 1 μm makes the molded rod columns easily permeable to eluents. Therefore, the back pressure of these columns is modest even at high flow rates. In contrast to the conventional HPLC columns packed with beads, all of the mobile phase flows through the continuous monolithic medium. As a result of this total convection, the efficiency of the molded media is almost independent of the flow rate. This improves significantly the separation ability of the rod columns and very fast separations of macromolecules such as peptides, proteins, and synthetic polymers have been demonstrated.
Abstract Flux measurements of greenhouse gases provide information on the metabolic activity of ecosystems. And they are useful for parameterizing and validating remote sensing and biogeochemical models that are used to assess water and carbon fluxes across space and time. I discuss the road trip by which we, a community of greenhouse gas flux data producers and consumers, have traveled over the last four decades. A subset of us started working on this topic in the 1980s as independent researchers. Then, we adopted the nascent eddy covariance method to measure greenhouse gas fluxes directly over managed and natural ecosystems. During this journey, our ability and capacity to measure greenhouse gas fluxes co‐evolved with the microprocessor revolution that produced our sensors, computers, and data storage systems. As time passed, we built the software and social and intellectual capital needed to conduct this work more extensively, which formed networks of observation sites. This capital was required to fund salaries for those doing the field work, to construct towers and to purchase sensors. Separate infusions of capital were needed to operate a data system that shared, vetted and distributed flux and meteorological data and site metadata. Accomplishments include producing over 4,000 site‐years of greenhouse gas flux data from hundreds of sites spanning the globe. Now, we are using these data to do next generation science on the broad topic of biosphere‐atmosphere interactions, like building high‐resolution, data‐informed global flux maps and studying trends on gas fluxes in a warming world with more CO 2 .
I begin by introducing Virtual Design Apprenticeship (VDA), a learning model — built on a solid foundation of education principles and theories — that promotes learning of design skills via overlay design tools. In VDA, when an individual needs to learn a new design skill or paradigm she is provided accessible, concrete examples that have been annotated with design rationale. These annotations make expert thinking visible and allow the novice to immediately use, and gradually understand, new best practices. By combining abstract rationale with concrete design instances, annotated artifacts become more useful than either could be alone. I describe the essential components of the VDA framework: annotated design artifacts, a repository of carefully chosen annotated examples, and a community of experts and learners. I walk the reader through an example of how VDA scaffolds learners as they move from a novice's understanding of a design space towards that of an expert. Within the context of this example, I present a set of design principles that guide the creation of VDA design tools — user interfaces built to mediate an individual's interactions with the three core VDA components. While VDA is applicable to most design fields, I narrow the scope of consideration to one particular domain of design by focusing in-depth on the instructional design difficulties that university-level faculty members face and how the VDA approach can address them. These instructors face precisely the type of design paradigm shift that VDA was developed to ease as they attempt to move away from traditional, lecture-based pedagogical methods and towards more modern, learner-centered techniques. I engaged with these instructors and a curriculum design research group in a six-year period of contextual inquiry. Findings from this study influenced my formulation of the VDA framework and the design of PACT, a design tool that leverages the learning principle of making thinking visible to assist novices as they transition from concrete to abstract reasoning about curriculum design. The central focus of PACT is the incorporation of annotated references to pedagogical design patterns — abstract representations of best practices in instructional design. I discuss the iterative design and implementation of PACT in detail, highlighting the ways in which it embodies the VDA design principles for promoting learning of instructional design via overlay design tools. Next, I study the challenges of converting abstract best practices and design patterns into concrete annotations that can be applied directly to content. My solution, the PACT Annotation Schema, is a formal mechanism for generating tags and pattern annotations from freeform pattern text. Formal representations of patterns are far more useful than generic references, both as scaffolds for learning and for structuring user interactions with design artifacts. Using this schema, I have generated the PACT Annotation Library, a collection of 56 tags and 74 pattern annotations based on the work of the Pedagogical Patterns Project. Visual representations of these formal annotations are the centerpiece of PACT's user interface. The PACT tool was evaluated in two distinct stages. First, I present a formative study conducted with early, prototype versions of the PACT tool. This study examines the utility of PACT for expert curriculum designers and curriculum research groups, using a sample annotation process — and reflection on the outcomes of that process — to demonstrate that my approach is feasible and useful for those groups. I then present a summative user study of the utility of PACT for novice learner-centered curriculum designers. I demonstrate PACT's significant impact on how novice designers learn from expert-generated examples, how they perceive the credibility of those examples, and the quality of curriculum designs those novices can produce. These findings show that the VDA approach to learning works and that the PACT overlay curriculum design tool is a successful realization of VDA's design principles. (Abstract shortened by UMI.)
The gapped symmetric phase of the Affleck-Kennedy-Lieb-Tasaki (AKLT) model exhibits fractionalized spins at the ends of an open chain. We show that breaking SU(2) symmetry and applying a global spin-lowering dissipator achieves synchronization of these fractionalized spins. Additional local dissipators ensure convergence to the ground state manifold. In order to understand which aspects of this synchronization are robust within the entire Haldane-gap phase, we reduce the biquadratic term which eliminates the need for an external field but destabilizes synchronization. Within the ground state subspace, stability is regained using only the global lowering dissipator. These results demonstrate that fractionalized degrees of freedom can be synchronized in extended systems with a significant degree of robustness arising from topological protection. \rev{A direct consequence is that permutation symmetries are not required for the dynamics to be synchronized, representing a clear advantage of topological synchronization compared to synchronization induced by permutation symmetries.
The passive state of iron in borate buffer solutions containing EDTA (ethylenediaminetetraacetic acid, disodium salt, C10H14N2Na2O8) ranging in pH from 8.15 to 12.87 at ambient temperature, has been explored using electrochemical impedance spectroscopy (EIS) and steady-state polarization methods. EDTA prevents the formation of the outer layer of the passive film, thereby permitting interrogation of the barrier layer alone. It has been found that, in the passive state, the impedance is only weakly-dependent on the solution pH and film formation voltage, but at high pH (pH > 12) and especially at high voltages, the impedance becomes very pH- and voltagedependent. Under the steady-state conditions achieved in this work, passive iron is shown to satisfy the conditions of linearity, causality, stability, and finiteness, as required by Linear Systems Theory, on the basis of Kramers-Kronig (K-K) transformation of experimental impedance data. The experimental data have been interpreted in terms of the Point Defect Model (PDM), the predictions of which for an -type, semi-conducting passive film have been experimentally observed. An impedance model for passive iron, based on the PDM, has been developed and optimized on the impedance data to yield values for fundamental parameters (transfer coefficients and standard rate constants) for the interfacial reactions occurring in the barrier oxide layer. We conclude that the dominant defect(s) in the barrier layer of the passive film on iron must be the oxygen vacancy or cation interstitial, with the latter being favored by the values of the kinetic parameters.
Abstract: Koiter’s linear shell theory applies to isotropic elastic materials and to anisotropic materials that exhibit reflection symmetry of the elastic properties with respect to the shell midsurface. To the extent that such shells are exceptional, classical linear shell theory is incomplete. This lacuna is addressed here through a systematic procedure, applicable equally to all kinds of material symmetry, which entails an expansion of the potential energy of the shell in powers of its thickness in a manner reminiscent of Koiter’s work. The variables are the displacement field of the shell midsurface and certain director fields that arise in the course of the expansion procedure. The directors are constrained in accordance with necessary conditions arising in the exact three-dimensional theory, yielding the optimal expression for the potential energy among those of third order in the small thickness. For materials lacking reflection symmetry, it is found that the strain energy of the shell is sensitive to tangential gradients of strain in addition to the usual strain and bending strain of the conventional theory.
We report a new system for the silylation of aryl C-H bonds. The combination of [Ir(cod)(OMe)]<sub>2</sub> and 2,9-Me<sub>2</sub>-phenanthroline (2,9-Me<sub>2</sub>-phen) catalyzes the silylation of arenes at lower temperatures and with faster rates than those reported previously, when the hydrogen byproduct is removed, and with high functional group tolerance and regioselectivity. Inhibition of reactions by the H<sub>2</sub> byproduct is shown to limit the silylation of aryl C-H bonds in the presence of the most active catalysts, thereby masking their high activity. Analysis of initial rates uncovered the high reactivity of the catalyst containing the sterically hindered 2,9-Me<sub>2</sub>-phen ligand but accompanying rapid inhibition by hydrogen. With this catalyst, under a flow of nitrogen to remove hydrogen, electron-rich arenes, including those containing sensitive functional groups, undergo silylation in high yield for the first time, and arenes that underwent silylation with prior catalysts react over much shorter times with lower catalyst loadings. The synthetic value of this methodology is demonstrated by the preparation of key intermediates in the synthesis of medicinally important compounds in concise sequences comprising silylation and functionalization. Mechanistic studies demonstrate that the cleavage of the aryl C-H bond is reversible and that the higher rates observed with the 2,9-Me<sub>2</sub>-phen ligand are due to a more thermodynamically favorable oxidative addition of aryl C-H bonds.
We study Deinococcus radiodurans, a nonpathogenic prokaryote able to withstand high doses of ionizing radiation, for bioremediation and biostabilization of heavy metals and actinides to limit the migration of these contaminants. Our goal is to develop a system for D. radiodurans to bioprecipitate actinides similar to that of a genetically engineered strain of Pseudomonas aeruginosa capable of polyphosphate accumulation, inducible degradation and secretion, and UO22+ precipitation. The radioresistance of D. radiodurans to ionizing gamma radiation has been well studied, yet is not well understood1. Several DNA repair mechanisms have been discovered, yet survivability studies are typically done under nutrient-rich conditions at a single growth stage. Although a minimal irradiation medium has been described2, a clear study of the effect of different variables on survivability in a nutrient-starved environment has not been performed. We have shown that D. radiodurans grown in typical TGY complex media displays increasing radioresistance with increasing age of the culture at the time of harvest; however, the dose below which there is little loss of reproductive viability does not change significantly with growth phase. Higher irradiation rates in nutrient-starved medium display higher survivability. These results indicate that there is a threshold below which the radiation dose is not sufficient to overcome the cells' passive defense mechanisms, such as radical scavenging by carotenoids in the cell wall. Additional radiation resistance is then due to active resistance induced by cell damage, which can be related to aging. Using the 88" cyclotron at the Lawrence Berkeley National Laboratory, we also studied the effects of light-ion irradiation inaqueous suspension and show increasing lethality corresponding to increasing linear energy transfer (LET) values of the radiation. Previous studies only examined heavy ion effects on cells supported on solid medium3. To better understand the chemistry of non-engineered D. radiodurans, we studied the interaction of strain R1 with UO22+ in dilute salt solution. R1 sorbs uranyl more than two orders of magnitude less than the engineered P. aeruginosa. We are increasing the ability of D. radiodurans to sequester uranium via metabolic engineering. Chemical studies of the cell-uranyl binding strength and pH optima support spectroscopic data indicating that a carboxyl surface group, consistent with known characteristics of D. radiodurans' S-layer4, interacts with and binds the uranyl. Studies including Infrared Spectroscopy, Laser Fluorescence Spectroscopy, and Extended X-ray Absorption Fine Structure Spectroscopy (EXAFS) are underway to further elucidate the mechanism of uranyl complexation to the cell surface and to further characterize a strain provided that has been engineered to accumulate polyphosphate.
