990 research outputs found

    Electrical properties of epoxies and film resistors

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    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

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    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^{-2}), the nonmonotonic temperature dependence (∌50−300\sim 50-300 mK) of the resistivity is consistent with temperature dependent screening of residual impurities. At a fixed temperature of TT= 50 mK, the conductivity vs. density data indicates an inhomogeneity driven percolation-type transition to an insulating state at a critical density of 3.8×1093.8\times 10^9 cm−2^{-2}.Comment: accepted for publication in PR

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

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    We report on the temperature dependence of the mobility, ÎŒ\mu, of the two-dimensional electron gas in a variable density AlGaN/GaN field effect transistor, with carrier densities ranging from 0.4×1012\times10^{12} cm−2^{-2} to 3.0×1012\times10^{12} cm−2^{-2} and a peak mobility of 80,000 cm2^{2}/Vs. Between 20 K and 50 K we observe a linear dependence ÎŒac−1=α\mu_{ac}^{-1} = \alphaT indicating that acoustic phonon scattering dominates the temperature dependence of the mobility, with α\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

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    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 αso\alpha_{so} ∌\sim 6×\times 10−13^{-13}eVm, while the conduction band spin-orbit splitting energy amounts to Δso\Delta_{so} ∌\sim 0.3meV at ne_e=1×1016\times10^{16}m−2^{-2}.Comment: Accepted for publication in PR

    Quelle rĂ©gulation pour l’arrĂȘt d’un protocole de recherche clinique de thĂ©rapie gĂ©nique somatique ? État des lieux auprĂšs des cliniciens-chercheurs europĂ©ens

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    Depuis 2002, le dĂ©bat sur les risques associĂ©s Ă  la thĂ©rapie gĂ©nique est initiĂ© suite Ă  l’annonce que deux enfants inclus dans un essai thĂ©rapeutique impliquant une thĂ©rapie gĂ©nique ont dĂ©veloppĂ© des effets indĂ©sirables important. En Janvier 2005, le dĂ©bat sur les risques reprit suite Ă  l’interruption du protocole sur les enfants bulle du Pr Fischer Ă  l’hĂŽpital Necker de Paris. Nous avons donc Ă©tudiĂ© le processus impliquĂ© ainsi que la rĂ©flexion Ă©thique associĂ©e aux dĂ©cisions d’arrĂȘt de protocole de recherche. Notre travail a Ă©tĂ© menĂ© par une Ă©quipe pluridisciplinaire combinant chercheurs en santĂ©, gĂ©nĂ©ticiens et Ă©thiciens. Nous avons Ă©tudiĂ© la participation des chercheurs, des patients, des institutions officielles, des comitĂ©s d’éthique ainsi que des associations de patients dans le processus de dĂ©cision d’interruption d’un protocole de recherche.Nous avons Ă©galement analysĂ© les critĂšres jugĂ©s les plus pertinents dans l’arrĂȘt d’un protocole de recherche. Enfin nous avons analysĂ© le point de vue des personnes directement impliquĂ©es dans la thĂ©rapie gĂ©nique au moyen d’un questionnaire. Toutes les personnes contactĂ©es ont prĂ©sentĂ© un poster de recherche au congrĂšs de la SociĂ©tĂ© EuropĂ©enne de ThĂ©rapie GĂ©nique. 62 personnes d’autant d’équipes de recherche diffĂ©rentes, de 17 pays, sur les 350 contactĂ©s ont rĂ©pondu. Selon eux, la dĂ©cision d’arrĂȘt d’un protocole de recherche doit ĂȘtre prise suite Ă  une consultation des chercheurs, des patients, du ministĂšre de tutelle, d’une agence nationale de rĂ©gulation ou d’un comitĂ© d’éthique ; la lĂ©gitimitĂ© Ă©tant accordĂ©e Ă  des dĂ©cisions prises en commun par les chercheurs, les patients et les comitĂ©s d’éthique. Les incidents sĂ©rieux et de façon plus surprenante, les incidents moins graves sont jugĂ©s comme Ă©tant des critĂšres suffisants pour interrompre un essai. Nous avons fini par analyser les consĂ©quences Ă©thiques, telles que balance bĂ©nĂ©fice/risque, processus de rĂ©gulation ou responsabilitĂ©, de ces critĂšres sur l’arrĂȘt 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 HĂŽpital 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

    Impact of spin-orbit coupling on quantum Hall nematic phases

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    Anisotropic charge transport is observed in a two-dimensional (2D) hole system in a perpendicular magnetic field at filling factors nu=7/2, nu=11/2, and nu=13/2 at low temperature. In stark contrast, the transport at nu=9/2 is isotropic for all temperatures. Isotropic hole transport at nu=7/2 is restored for sufficiently low 2D densities or an asymmetric confining potential. The density and symmetry dependences of the observed anisotropies suggest that strong spin-orbit coupling in the hole system contributes to the unusual transport behavior.Comment: 4 pages, 4 figure

    Bias-Dependent Generation and Quenching of Defects in Pentacene

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    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

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

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    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
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