Note: The intermodulation lockin analyzer

Nonlinear systems can be probed by driving them with two or more pure tones while measuring the intermodulation products of the drive tones in the response. We describe a digital lockin analyzer which is designed explicitly for this purpose. The analyzer is implemented on a field-programmable gate array, providing speed in analysis, real-time feedback, and stability in operation. The use of the analyzer is demonstrated for intermodulation atomic force microscopy. A generalization of the intermodulation spectral technique to arbitrary drive waveforms is discussed.     

Re-examining the causal structure of information technology impact research

About 20 years ago, Markus and Robey noted that most research on IT impacts had been guided by deterministic perspectives and had neglected to use an emergent perspective, which could account for contradictory findings. They further observed that most research in this area had been carried out using variance theories at the expense of process theories. Finally, they suggested that more emphasis on multilevel theory building would likely improve empirical reliability. In this paper, we reiterate the observations and suggestions made by Markus and Robey on the causal structure of IT impact theories and carry out an analysis of empirical research published in four major IS journals, Management Information Systems Quarterly (MISQ), Information Systems Research (ISR), the European Journal of Information Systems (EJIS), and Information and Organization (I&O), to assess compliance with those recommendations. Our final sample consisted of 161 theory-driven articles, accounting for approximately 21% of all the empirical articles published in these journals. Our results first reveal that 91% of the studies in MISQ, ISR, and EJIS focused on deterministic theories, while 63% of those in I&O adopted an emergent perspective. Furthermore, 91% of the articles in MISQ, ISR, and EJIS adopted a variance model; this compares with 71% from I&O that applied a process model. Lastly, mixed levels of analysis were found in 14% of all the surveyed articles. Implications of these findings for future research are discussed.     

AVCOL: Availability-aware information aggregation in large distributed systems under uncollaborative behavior

Aggregation of system-wide information in large-scale distributed systems, such as p2p systems and Grids, can be unfairly influenced by nodes that are selfish, colluding with each other, or are offline most of the time. We present AVCOL, which uses probabilistic and gossip-style techniques to provide availability-aware aggregation. Concretely, AVCOL is the first aggregation system that: (1) implements any (arbitrary) global predicate that explicitly specifies any node’s probability of inclusion in the global aggregate, as a mathematical function of that node’s availability (i.e., percentage time online); (2) probabilistically tolerates large numbers of selfish nodes and large groups of colluders; and (3) scales well with hundreds to thousands of nodes. AVCOL uses several unique design decisions: per-aggregation tree construction where nodes are allowed a limited but flexible probabilistic choice of parents or children, probabilistic aggregation along trees, and auditing of nodes both during aggregation as well as in gossip-style (i.e., periodically). We have implemented AVCOL, and we experimentally evaluated it using real-life churn traces. Our evaluation and our mathematical analysis show that AVCOL satisfies arbitrary predicates, scales well, and withstands a variety of selfish and colluding attacks.     

Analysis of the inappropriate utilization of the emergency service of a third-level hospital

OBJECTIVE: a) To find the reasons for inadequate use of Hospital Casualty Departments (HCDs); b) to discover the general population's levels of information concerning non-hospital Emergency Services (NESs); c) to analyse the relationship between the type of Primary Care Centre (PCC), i.e. whether reformed or not, and inadequate use of HCDs. DESIGN: A crossover study. SETTING: The Germans Trias i Pujol Hospital Casualty Department, Badalona. PATIENTS: A random sample of 321 patients over 14 years old, who were attended for pathologies considered emergencies not requiring hospital treatment. MEASUREMENTS AND MAIN RESULTS: The variables gathered were: age, gender, where the patient came from, reason for hospital use, knowledge of NESs and subsequent referral. 83.2% of patients attended on their own initiative. The main reasons given were "advantages of a hospital" and "the persistence of symptoms." 49% of the patients stated they knew of the NESs. 80% of the patients proceeded from PCCs which had not been reformed. There were no statistically significant differences (p > 0.05) when the variables "reason for use" and "knowledge of NESs" were compared in relation to where the patient came from (i.e. reformed or non-reformed PCC). CONCLUSIONS: General practitioners in reformed PCCs are less likely to refer to HCDs. Knowledge and use of NESs should be actively supported.     

Supervisory control using variable lookahead policies

This paper deals with the efficient on-line calculation of supervisory controls for discrete event systems (DES's) in the framework of limited lookahead control policies (or LLPs) that we introduced in previous papers. In the LLP scheme, the control action after a given trace of events has been executed is calculated on-line on the basis of anN-step ahead projection of the behavior of the DES. To compute these controls, one must calculate after the execution of each event the supremal controllable sublanguage of a finite language with respect to another finite larger language. In our previous work, we showed how the required supremal controllable sublanguage calculation can be performed by using a backward dynamic programming algorithm over the nodes of the tree representation of these two languages. In this paper, we pursue the same approach for the calculation of LLP controls, but instead we adopt a forward calculation procedure over theN-level tree of interest. This forward procedure improves upon previous work by avoiding the explicit consideration of all the nodes of theN-level tree, while still permitting tree-to-tree recursiveness as enabled events are executed by the system. The forward search ends whenever a control decision can be made unambiguously or whenever the boundary of theN-level tree is reached, whichever comes first. This motivates the name Variable Lookahead Policy (or VLP) for this implementation of the LLP supervisory control scheme. This paper presents a general VLP algorithm and studies the properties of several special cases of it. The paper also discusses the implementation of the VLP algorithms and presents computational results regarding the application of these algorithms to a time-varying DES.     

Forward scattering of a Gaussian beam by a nonabsorbing sphere

The forward scattering of a Gaussian laser beam by a spherical particle located along the beam axis is analyzed with the generalized Lorenz-Mie theory (GLMT) and with diffraction theory. Forwardscattering and near-forward-scattering profiles from electrodynamically levitated droplets, 51.6 μm in diameter, are also presented and compared with GLMT-based predictions. The total intensity in the forward direction, formed by the superposition of the incident and the scattered fields, is found to correlate with the particle-extinction cross section, the particle diameter, and the beam width. Based on comparison with the GLMT, the diffraction solution is accurate when beam widths that are approximately greater than or equal to the particle diameter are considered and when large particles that have an extinction efficiency near the asymptotic value of 2 are considered. However, diffraction fails to describe the forward intensity for more tightly focused beams. The experimental observations, which are in good agreement with GLMT-based predictions, reveal that the total intensity profile about the forward direction is quite sensitive to particle axial position within a Gaussian beam. These finite beam effects are significant when the ratio of the beam to the particle diameter is less than approximately 5:1. For larger beam-to-particle-diameter ratios, the total field in the forward direction is dominated by the incident beam.     

Interest of molecular functionalization for electrochemical storage

Despite great interests in electrochemical energy storage systems for numerous applications, considerable challenges remain to be overcome. Among the various approaches to improving the stability, safety, performance, and cost of these systems, molecular functionalization has recently been proved an attractive method that allows the tuning of material surface reactivity while retaining the properties of the bulk material. For this purpose, the reduction of aryldiazonium salt, which is a versatile method, is considered suitable; it forms robust covalent bonds with the material surface, however, with the formation of multilayer structures and sp³ defects (for carbon substrate) that can be detrimental to the electronic conductivity. Alternatively, non-covalent molecular functionalization based on π–π interactions using aromatic ring units has been proposed. In this review, the various advances in molecular functionalization concerning the current limitations in lithium-ion batteries and electrochemical capacitors are discussed. According to the targeted applications and required properties, both covalent and non-covalent functionalization methods have proved to be very efficient and versatile. Fundamental aspects to achieve a better understanding of the functionalization reactions as well as molecular layer properties and their effects on the electrochemical performance are also discussed. Finally, perspectives are proposed for future implementation of molecular functionalization in the field of electrochemical storage.

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