441 research outputs found

    Spin and energy transfer in nanocrystals without transport of charge

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    We describe a mechanism of spin transfer between individual quantum dots that does not require tunneling. Incident circularly-polarized photons create inter-band excitons with non-zero electron spin in the first quantum dot. When the quantum-dot pair is properly designed, this excitation can be transferred to the neighboring dot via the Coulomb interaction with either {\it conservation} or {\it flipping} of the electron spin. The second dot can radiate circularly-polarized photons at lower energy. Selection rules for spin transfer are determined by the resonant conditions and by the strong spin-orbit interaction in the valence band of nanocrystals. Coulomb-induced energy and spin transfer in pairs and chains of dots can become very efficient under resonant conditions. The electron can preserve its spin orientation even in randomly-oriented nanocrystals.Comment: 13 pages, 3 figure

    Organic film thickness influence on the bias stress instability in Sexithiophene Field Effect Transistors

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    In this paper, the dynamics of bias stress phenomenon in Sexithiophene (T6) Field Effect Transistors (FETs) has been investigated. T6 FETs have been fabricated by vacuum depositing films with thickness from 10 nm to 130 nm on Si/SiO2 substrates. After the T6 film structural analysis by X-Ray diffraction and the FET electrical investigation focused on carrier mobility evaluation, bias stress instability parameters have been estimated and discussed in the context of existing models. By increasing the film thickness, a clear correlation between the stress parameters and the structural properties of the organic layer has been highlighted. Conversely, the mobility values result almost thickness independent

    Signals of Supersymmetric Flavour Models in B Physics

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    If the mechanism of Supersymmetry breaking is not flavour blind, some flavour symmetry is likely to be needed to prevent excessive flavour changing neutral current effects. We discuss two flavour models (based respectively on a U(2) and on a SU(3) horizontal symmetry) providing a good fit to fermion masses and mixings and particularly constraining the supersymmetry soft breaking terms. We show that, while reproducing successfully the Standard Model fit of the unitarity triangle, it is possible to obtain sizable deviations from the Standard Model predictions for three very clean B-physics observables: the time dependent CP asymmetries in Bd→J/ψK0B_d \to J/\psi K^0 and in Bs→J/ψϕB_s \to J/\psi \phi and the Bs−BˉsB_s-\bar{B}_s mass difference. Our analysis exhibits with two explicit realizations that in supersymmetric theories with a new flavour structure in addition to the Yukawa matrices there exist concrete potentialities for revealing supersymmetry indirectly in theoretically clean BB-physics observables.Comment: 18 pages, 7 figures, 3 of which in colo

    Knowledge dating and knowledge sharing in ad-hoc transient communities

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    This chapter will focus on the question of what rules and policies are conducive to the emergence of an adequate support infrastructure in the non-formal settings of a Learning Network. Once such an infrastructure is in place, a variety of ways again embedded in rules and policies may be used to guarantee its continued exis-tence. This was the subject of the previous chapter. Here the focus is on the phase of the emergence of social interaction and the formation of an incipient social in-frastructure. It is in this stage that the lone learner makes acquaintance with his or her online peers. More specifically, since learning through the exchange of knowl-edge is the ultimate objective of any Learning Network, we will present guidelines for effective and efficient knowledge dating in Learning Networks; ultimately, this should lead to knowledge sharing. During our discussion, we will introduce the notion of ad-hoc transient communities, temporal online gatherings of people fo-cused on a particular issue, which precisely because of their focus and transience help build a social infrastructure in a Learning Network. Ad-hoc transient com-munities may be seen as a knowledge dating and sharing service, offered in the context of a Learning Network

    Production and Decay of D_1(2420)^0 and D_2^*(2460)^0

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    We have investigated D+π−D^{+}\pi^{-} and D∗+π−D^{*+}\pi^{-} final states and observed the two established L=1L=1 charmed mesons, the D1(2420)0D_1(2420)^0 with mass 2421−2−2+1+22421^{+1+2}_{-2-2} MeV/c2^{2} and width 20−5−3+6+320^{+6+3}_{-5-3} MeV/c2^{2} and the D2∗(2460)0D_2^*(2460)^0 with mass 2465±3±32465 \pm 3 \pm 3 MeV/c2^{2} and width 28−7−6+8+628^{+8+6}_{-7-6} MeV/c2^{2}. Properties of these final states, including their decay angular distributions and spin-parity assignments, have been studied. We identify these two mesons as the jlight=3/2j_{light}=3/2 doublet predicted by HQET. We also obtain constraints on {\footnotesize ΓS/(ΓS+ΓD)\Gamma_S/(\Gamma_S + \Gamma_D)} as a function of the cosine of the relative phase of the two amplitudes in the D1(2420)0D_1(2420)^0 decay.Comment: 15 pages in REVTEX format. hardcopies with figures can be obtained by sending mail to: [email protected]

    Measurement of the branching fraction for ΄(1S)→τ+τ−\Upsilon (1S) \to \tau^+ \tau^-

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    We have studied the leptonic decay of the ΄(1S)\Upsilon (1S) resonance into tau pairs using the CLEO II detector. A clean sample of tau pair events is identified via events containing two charged particles where exactly one of the particles is an identified electron. We find B(΄(1S)→τ+τ−)=(2.61 ± 0.12 +0.09−0.13)B(\Upsilon(1S) \to \tau^+ \tau^-) = (2.61~\pm~0.12~{+0.09\atop{-0.13}})%. The result is consistent with expectations from lepton universality.Comment: 9 pages, RevTeX, two Postscript figures available upon request, CLNS 94/1297, CLEO 94-20 (submitted to Physics Letters B

    Measurement of the Decay Asymmetry Parameters in Λc+→Λπ+\Lambda_c^+ \to \Lambda\pi^+ and Λc+→Σ+π0\Lambda_c^+ \to \Sigma^+\pi^0

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    We have measured the weak decay asymmetry parameters (\aLC ) for two \LC\ decay modes. Our measurements are \aLC = -0.94^{+0.21+0.12}_{-0.06-0.06} for the decay mode Λc+→Λπ+\Lambda_c^+ \to \Lambda\pi^+ and \aLC = -0.45\pm 0.31 \pm 0.06 for the decay mode Λc→Σ+π0\Lambda_c \to \Sigma^+\pi^0 . By combining these measurements with the previously measured decay rates, we have extracted the parity-violating and parity-conserving amplitudes. These amplitudes are used to test models of nonleptonic charmed baryon decay.Comment: 11 pages including the figures. Uses REVTEX and psfig macros. Figures as uuencoded postscript. Also available as http://w4.lns.cornell.edu/public/CLNS/1995/CLNS95-1319.p

    Observation of the Ξc+\Xi_c^+ Charmed Baryon Decays to ÎŁ+K−π+\Sigma^+ K^-\pi^+, ÎŁ+Kˉ∗0\Sigma^+ \bar{K}^{*0}, and ΛK−π+π+\Lambda K^-\pi^+\pi^+

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    We have observed two new decay modes of the charmed baryon Ξc+\Xi_c^+ into ÎŁ+K−π+\Sigma^+ K^-\pi^+ and ÎŁ+Kˉ∗0\Sigma^+ \bar{K}^{*0} using data collected with the CLEO II detector. We also present the first measurement of the branching fraction for the previously observed decay mode Ξc+→ΛK−π+π+\Xi_c^+\to\Lambda K^-\pi^+\pi^+. The branching fractions for these three modes relative to Ξc+→Ξ−π+π+\Xi_c^+\to\Xi^-\pi^+\pi^+ are measured to be 1.18±0.26±0.171.18 \pm 0.26 \pm 0.17, 0.92±0.27±0.140.92 \pm 0.27 \pm 0.14, and 0.58±0.16±0.070.58 \pm 0.16 \pm 0.07, respectively.Comment: 12 page uuencoded postscript file, postscript file also available through http://w4.lns.cornell.edu/public/CLN
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