9,840 research outputs found

    The 13C nuclear magnetic resonance in graphite intercalation compounds

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    The (13)C NMR chemical shifts of graphite intercalation compounds were calculated. For acceptor types, the shifts come mainly from the paramagnetic (Ramsey) intra-atomic terms. They are related to the gross features of the two-dimensional band structures. The calculated anisotropy is about -140 ppm and is independent of the finer details such as charge transfer. For donor types, the carbon 2p pi orbitals are spin-polarized because of mixing with metal conduction electrons, thus there is an additional dipolar contribution which may be correlated with the electronic specific heat. The general agreement with experimental data is satisfactory

    Sound velocity anisotropy in cubic crystals

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    Simple analytical expressions may be derived for sound velocities in cubic crystals by using lattice harmonics or functions which are invariant under the crystal symmetry operations. These expressions are in good agreement with the exact results for typical crystals such as metallic iron and potassium fluoride

    User manual of the CATSS system (version 1.0) communication analysis tool for space station

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    The Communication Analysis Tool for the Space Station (CATSS) is a FORTRAN language software package capable of predicting the communications links performance for the Space Station (SS) communication and tracking (C & T) system. An interactive software package was currently developed to run on the DEC/VAX computers. The CATSS models and evaluates the various C & T links of the SS, which includes the modulation schemes such as Binary-Phase-Shift-Keying (BPSK), BPSK with Direct Sequence Spread Spectrum (PN/BPSK), and M-ary Frequency-Shift-Keying with Frequency Hopping (FH/MFSK). Optical Space Communication link is also included. CATSS is a C & T system engineering tool used to predict and analyze the system performance for different link environment. Identification of system weaknesses is achieved through evaluation of performance with varying system parameters. System tradeoff for different values of system parameters are made based on the performance prediction

    Hong Kong secondary students' perspectives on selecting test difficultly level and learner washback: Effects of a graded approach to assessment

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    This sequential mixed-methods study investigates washback on learning in a high-stakes school exit examination by examining learner perceptions and reported behaviours in relation to learners’ beliefs and language learning experience, the role of other stakeholders in the washback mechanism, and socio-educational forces. The focus is the graded approach of the Hong Kong Diploma of Secondary Education English Language Examination (HKDSE-English), incorporated in 2012, that allows test-takers to choose between easier and more difficult sections for reading and listening-integrated skills papers. Inductive coding of focus groups involving 12 secondary students fed into the development of the Washback on Students’ Learning questionnaire, which was administered to another 150 learners. Exploratory factor analyses of identified washback effects revealed four major types straddling different settings (classrooms, tutorial schools, learners’ personal environment), and seven categories of mediating variables pertaining to learners themselves, other stakeholders, and societal influences. Simultaneous multiple regressions identified influential clusters of mediating variables and showed the strongest predictors for each macro-level washback type varied. At least one intrinsic and one extrinsic factor category significantly contributed to all types, reaffirming learner washback as a socially situated, negotiated construct. Implications related to the consequences, use, and fairness of the graded approach are discussed

    All-optical high speed NOR gate based on two photon absorption in silicon wire waveguides

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    We demonstrate for the first time an all-optical logic NOR gate in submicron size silicon wire waveguides. High speed operation at equivalent 80Gbps data rate was achieved using pump induced non-degenerate two-photon absorption inside the waveguides. The device requires low pulse energy (few pJ) for logic gate operation. (C) 2005 Optical Society of America

    Endohedral terthiophene in zigzag carbon nanotubes: Density functional calculations

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    The inclusion and encapsulation of terthiophene (T3) molecules inside zigzag single-walled carbon nanotubes (CNTs) is addressed by density functional calculations. We consider the T3 molecule inside five semiconducting CNTs with diameters ranging from 9.6 to 12.7 Ang. Our results show that the T3 inclusion process is exothermic for CNTs with diameters larger than 9.5 Ang. The highest energy gain is found to be of 2 eV, decreasing as the CNT diameter increases. This notable effect of stabilization is attributed to the positively charged CNT inner space, as induced by its curvature, which is able to accommodate the neutral T3 molecule. The band structure of the T3@CNT system shows that T3 preserves its electronic identity inside the CNTs, superimposing their molecular orbitals onto the empty CNT band structure without hybridization. Our results predict that the electronic states added by the T3 molecules would give rise to optical effects and nonradiative relaxation from excited states.Comment: 5 pages, 5 figures, 1 table, accepted in PR

    Re-entrant Layer-by-Layer Etching of GaAs(001)

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    We report the first observation of re-entrant layer-by-layer etching based on {\it in situ\/} reflection high-energy electron-diffraction measurements. With AsBr3_3 used to etch GaAs(001), sustained specular-beam intensity oscillations are seen at high substrate temperatures, a decaying intensity with no oscillations at intermediate temperatures, but oscillations reappearing at still lower temperatures. Simulations of an atomistic model for the etching kinetics reproduce the temperature ranges of these three regimes and support an interpretation of the origin of this phenomenon as the site-selectivity of the etching process combined with activation barriers to interlayer adatom migration.Comment: 11 pages, REVTeX 3.0. Physical Review Letters, in press
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