Electrical properties of epoxies and film resistors

The reliability of hybrid microcircuits has been enhanced in recent years by the use of organic adhesives as a replacement for solder and eutectics. The epoxies have been the most effective and widely used material for this application. Methods for measuring the electrical and mechanical properties of epoxies are developed. Data are given for selected conductive adhesives at high and low frequencies. The temperature coefficients of resistance of thick film resistors are presented

Transport and percolation in a low-density high-mobility two-dimensional hole system

We present a study of the temperature and density dependence of the resistivity of an extremely high quality two-dimensional hole system grown on the (100) surface of GaAs. For high densities in the metallic regime (p\agt 4 \times 10^{9} cm$^{-2}$), the nonmonotonic temperature dependence ($\sim 50-300$ mK) of the resistivity is consistent with temperature dependent screening of residual impurities. At a fixed temperature of $T$= 50 mK, the conductivity vs. density data indicates an inhomogeneity driven percolation-type transition to an insulating state at a critical density of $3.8\times 10^9$ cm$^{-2}$.Comment: accepted for publication in PR

Acoustic phonon scattering in a low density, high mobility AlGaN/GaN field effect transistor

We report on the temperature dependence of the mobility, $\mu$, of the two-dimensional electron gas in a variable density AlGaN/GaN field effect transistor, with carrier densities ranging from 0.4$\times10^{12}$ cm$^{-2}$ to 3.0$\times10^{12}$ cm$^{-2}$ and a peak mobility of 80,000 cm$^{2}$/Vs. Between 20 K and 50 K we observe a linear dependence $\mu_{ac}^{-1} = \alpha$T indicating that acoustic phonon scattering dominates the temperature dependence of the mobility, with $\alpha$ being a monotonically increasing function of decreasing 2D electron density. This behavior is contrary to predictions of scattering in a degenerate electron gas, but consistent with calculations which account for thermal broadening and the temperature dependence of the electron screening. Our data imply a deformation potential D = 12-15 eV.Comment: 3 pages, 2 figures, RevTeX. Submitted to Appl Phys Let

Large Bychkov-Rashba spin-orbit coupling in high-mobility GaN/AlGaN heterostructures

We present low temperature magnetoconductivity measurements of a density-tunable and high mobility two-dimensional electron gas confined in the wide bandgap GaN/AlGaN system. We observed pronounced anti-localization minima in the low-field conductivity, indicating the presence of strong spin-orbit coupling. Density dependent measurements of magnetoconductivity indicate that the coupling is mainly due to the Bychkov-Rashba mechanism. In addition, we have derived a closed-form expression for the magnetoconductivity, allowing us to extract reliable transport parameters for our devices. The Rashba spin-orbit coupling constant is $\alpha_{so}$ $\sim$ 6$\times$ 10$^{-13}$eVm, while the conduction band spin-orbit splitting energy amounts to $\Delta_{so}$ $\sim$ 0.3meV at n$_e$=1$\times10^{16}$m$^{-2}$.Comment: Accepted for publication in PR

Bias-Dependent Generation and Quenching of Defects in Pentacene

We describe a defect in pentacene single crystals that is created by bias stress and persists at room temperature for an hour in the dark but only seconds with 420nm illumination. The defect gives rise to a hole trap at Ev + 0.38eV and causes metastable transport effects at room temperature. Creation and decay rates of the hole trap have a 0.67eV activation energy with a small (108 s-1) prefactor, suggesting that atomic motion plays a key role in the generation and quenching process.Comment: 10 pages, 3 figure

Quelle régulation pour l’arrêt d’un protocole de recherche clinique de thérapie génique somatique ? État des lieux auprès des cliniciens-chercheurs européens

Depuis 2002, le débat sur les risques associés à la thérapie génique est initié suite à l’annonce que deux enfants inclus dans un essai thérapeutique impliquant une thérapie génique ont développé des effets indésirables important. En Janvier 2005, le débat sur les risques reprit suite à l’interruption du protocole sur les enfants bulle du Pr Fischer à l’hôpital Necker de Paris. Nous avons donc étudié le processus impliqué ainsi que la réflexion éthique associée aux décisions d’arrêt de protocole de recherche. Notre travail a été mené par une équipe pluridisciplinaire combinant chercheurs en santé, généticiens et éthiciens. Nous avons étudié la participation des chercheurs, des patients, des institutions officielles, des comités d’éthique ainsi que des associations de patients dans le processus de décision d’interruption d’un protocole de recherche.Nous avons également analysé les critères jugés les plus pertinents dans l’arrêt d’un protocole de recherche. Enfin nous avons analysé le point de vue des personnes directement impliquées dans la thérapie génique au moyen d’un questionnaire. Toutes les personnes contactées ont présenté un poster de recherche au congrès de la Société Européenne de Thérapie Génique. 62 personnes d’autant d’équipes de recherche différentes, de 17 pays, sur les 350 contactés ont répondu. Selon eux, la décision d’arrêt d’un protocole de recherche doit être prise suite à une consultation des chercheurs, des patients, du ministère de tutelle, d’une agence nationale de régulation ou d’un comité d’éthique ; la légitimité étant accordée à des décisions prises en commun par les chercheurs, les patients et les comités d’éthique. Les incidents sérieux et de façon plus surprenante, les incidents moins graves sont jugés comme étant des critères suffisants pour interrompre un essai. Nous avons fini par analyser les conséquences éthiques, telles que balance bénéfice/risque, processus de régulation ou responsabilité, de ces critères sur l’arrêt d’un protocole de recherche.In 2002, the debate on the risks of gene therapy was initiated following the annoucement that two children included in a clinical trial developed serious adverse effects. In January 2005, the debate was reignited following the interruption of the “bubble kids protocol” at the Hôpital Necker in Paris. We have thus investigated the ethical stakes involved in decisions to stop protocols. This work was carried out by a multidisciplinary team combining ethics researchers and geneticists. We studied the specific participation of researchers, patients, official institution, ethics committees and patient associations in the processes that can lead to an interruption of trial.We also analysed the criterion judged most relevant for halting a trial. Finally, we analyzed the perspective of the actors implicated directly in the provision of gene therapy, by means of a questionnaire. All the individuals contacted had presented a scientific poster at the European Society of Gene Therapy. 62 out of 350 persons, from 17 countries, responded to our questionnaire. According to these respondants, decisions to stop a trial should be taken after consultation with researchers, patients, the ministry, national agencies or ethics committees. Legitimacy was accorded to joint decision-making by researchers, patients and committees. Serious incidents, and surprisingly less serious incidents, clearly emerge as criterion for stopping a trial. We conclude by analyzing the ethical consequences, such as risk/benefit ratios, regulatory processes and responsibility, associated with these criterions and decisions to stop a trial

Quantum theory of dissipation of a harmonic oscillator coupled to a nonequilibrium bath; Wigner-Weisskopf decay and physical spectra

We extend the quantum theory of dissipation in the context of system-reservoir model, where the reservoir in question is kept in a nonequilibrium condition. Based on a systematic separation of time scales involved in the dynamics, appropriate generalizations of the fluctuation-dissipation and Einstein's relations have been pointed out. We show that the Wigner-Weisskopf decay of the system mode results in a rate constant which depending on the relaxation of nonequilibrium bath is dynamically modified. We also calculate the time-dependent spectra of a cavity mode with a suitable gain when the cavity is kept in contact with a nonequilibrium bath.Comment: Plain Latex, 28 pages, 2 PS figure

Worldwide food recall patterns over an eleven month period: A country perspective.

<p>Abstract</p> <p>Background</p> <p>Following the World Health Organization Forum in November 2007, the Beijing Declaration recognized the importance of food safety along with the rights of all individuals to a safe and adequate diet. The aim of this study is to retrospectively analyze the patterns in food alert and recall by countries to identify the principal hazard generators and gatekeepers of food safety in the eleven months leading up to the Declaration.</p> <p>Methods</p> <p>The food recall data set was collected by the Laboratory of the Government Chemist (LGC, UK) over the period from January to November 2007. Statistics were computed with the focus reporting patterns by the 117 countries. The complexity of the recorded interrelations was depicted as a network constructed from structural properties contained in the data. The analysed network properties included degrees, weighted degrees, modularity and <it>k</it>-core decomposition. Network analyses of the reports, based on 'country making report' (<it>detector</it>) and 'country reported on' (<it>transgressor</it>), revealed that the network is organized around a dominant core.</p> <p>Results</p> <p>Ten countries were reported for sixty per cent of all faulty products marketed, with the top 5 countries having received between 100 to 281 reports. Further analysis of the dominant core revealed that out of the top five transgressors three made no reports (in the order China > Turkey > Iran). The top ten detectors account for three quarters of reports with three > 300 (Italy: 406, Germany: 340, United Kingdom: 322).</p> <p>Conclusion</p> <p>Of the 117 countries studied, the vast majority of food reports are made by 10 countries, with EU countries predominating. The majority of the faulty foodstuffs originate in ten countries with four major producers making no reports. This pattern is very distant from that proposed by the Beijing Declaration which urges all countries to take responsibility for the provision of safe and adequate diets for their nationals.</p

Longer fixation duration while viewing face images

The spatio-temporal properties of saccadic eye movements can be influenced by the cognitive demand and the characteristics of the observed scene. Probably due to its crucial role in social communication, it is argued that face perception may involve different cognitive processes compared with non-face object or scene perception. In this study, we investigated whether and how face and natural scene images can influence the patterns of visuomotor activity. We recorded monkeys’ saccadic eye movements as they freely viewed monkey face and natural scene images. The face and natural scene images attracted similar number of fixations, but viewing of faces was accompanied by longer fixations compared with natural scenes. These longer fixations were dependent on the context of facial features. The duration of fixations directed at facial contours decreased when the face images were scrambled, and increased at the later stage of normal face viewing. The results suggest that face and natural scene images can generate different patterns of visuomotor activity. The extra fixation duration on faces may be correlated with the detailed analysis of facial features