Showing papers by "Alcatel-Lucent published in 1987"

Fundamental Studies of the Chemisorption of Organosulfur Compounds on Au( 111). Implications for Molecular Self-Assembly on Gold Surfaces

Abstract: Studies of the adsorption of methanethiol and dimethyl disulfide on an Au(111) surface under UHV conditions are described. Both adsorbates bind strongly, with the bonding of the disulfide being greatly favored. It is found that, under these conditions, the disulfide bond is dissociated to give a stable surface thiolate. Adsorption of methanethiol does not involve cleavage of the S-H bond. The implications of these results for solution adsorption experiments and the thermodynamics characterizing monolayer formation are discussed.

Spontaneously organized molecular assemblies. 3. Preparation and properties of solution adsorbed monolayers of organic disulfides on gold surfaces

Abstract: This paper shows that stable, oriented, polyfunctional organic monolayers can be prepared by the spontaneous organization of structurally complex organic disulfides on polycrystalline gold substrates. Chemisorption proceeds to very high coverages, approaching that equivalent to the bulk-phase densities of the adsorbate molecules. The bonding to the surface is also highly specific, inasmuch as the chemisorption of the disulfide moiety is favored greatly over a wide range of other functionality. This latter feature allows the ready preparation of a broad variety of organic surfaces with well-defined microscopic and macroscopic characteristics. Several representative examples of monolayer films are described, their chemical and thermal properties explored, and their structures characterized by several techniques including infrared and photoelectron spectroscopies.

Bulk superconductivity at 36 K in La1.8Sr0.2CuO4.

Abstract: We report the results of resistivity and magnetic susceptibility measurements in La/sub 2-//sub x/Sr/sub x/CuO/sub 4/ for xless than or equal to0.3. The x = 0.2 sample shows a superconducting transition at 36.2 K with a width of 1.4 K. The associated dc diamagnetic susceptibility (Meissner effect) is a large fraction (60%--70%) of the ideal value. We estimate the density of states from critical-field and resistivity data and suggest, by analogy to BaPb/sub 1-//sub x/Bi/sub x/O/sub 3/, that conventional phonon-mediated superconductivity can account for the high T/sub c/ in this class of materials.

Absolute calibration of small‐angle neutron scattering data

Abstract: Absolute calibration forms a valuable diagnostic tool in small-angle neutron scattering (SANS) experiments, and allows the parameters of a given model to be restricted to the set which reproduces the observed intensity. Discrepancies between the observed and calculated intensities may arise from potential artifacts or even new physical processes and absolute calibration methods are useful in delineating these circumstances. General methods which are available for absolute scaling are discussed along with estimates of the degree of internal consistency which may be achieved between the various standards. In order to minimize the time devoted to calibration in a given experimental program, emphasis is placed on developing a set of precalibrated strongly scattering standards which may be run in the chosen experimental geometry. Comparison of such a set developed at the National Center for Small-Angle Scattering Research (Oak Ridge) with independent determinations by SANS users indicates consistency to within ± 5%.

Evidence for weak link and anisotropy limitations on the transport critical current in bulk polycrystalline Y1Ba2Cu3Ox

Abstract: Measurements of the transport critical‐current density (Jc), magnetization Jc, and magnetoresistance in a number of bulk sintered samples of Y1Ba2Cu3Ox from several different laboratories indicate that the transport Jc is limited by weak‐link regions between high Jc regions. The weak‐link Jc has a Josephson character, decreasing by two orders of magnitude as the magnetic field is increased from 0.1 to 10 mT at 77 K. An examination of the grain‐boundary region in Y1Ba2Cu3Ox shows no observable impurities or second phases to the scale of the [001] lattice planes (∼12 A). The effect of intrinsic conduction anisotropy is discussed. A current‐transfer model is proposed in which weak conduction along the c axis plays a role in limiting Jc at grain boundaries. Orienting the grains in the powder state during processing may result in enhanced transport Jc in bulk conductors.

Depolarization- and transmitter-induced changes in intracellular Ca2+ of rat cerebellar granule cells in explant cultures

Abstract: Digital imaging of the Ca indicator fura-2 has been used to study the responses of developing granule cells in culture to depolarization and transmitter action. Unstimulated cells bathed in Krebs saline exhibited cytoplasmic Ca ion concentrations, [Ca2+], that were generally in the 30–60 nM range. Exposure of cells to high-potassium (25 mM) saline depolarized the membrane potential and produced an immediate rise in [Ca2+] that recovered within 2–3 min in normal saline. The response grew progressively larger over the first 20 d in culture. Transient increases in [Ca2+] to levels greater than 1 microM were observed after 12–14 d in vitro, at which time the cells displayed intense electrical activity when exposed to high K. At this stage, the increases were attenuated by blocking action potential activity with TTX. In TTX- treated or immature cells, in which the transient phase of the Ca change was relatively small, a second exposure to high K typically produced a much larger Ca response that the initial exposure. The duration of this facilitation of the response persisted for periods longer than 5 min. Application of the neurotransmitter GABA induced a transient increase in membrane conductance, with a reversal potential near resting potential (approx. -60 mV), and caused an intracellular Ca2+ increase that outlasted the exposure to GABA by several minutes. Glutamate, or kainate, induced an increase in membrane conductance but with a reversal potential more positive than spike threshold. These agents also elevated intracellular Ca2+, but unlike the case with GABA, this Ca response reversed rapidly upon removal of the transmitter. The facilitatory effect of repeated exposures to high-K saline, as well as the persistent Ca elevation following a brief GABA application, suggests that granule cells possess the capability of displaying activity-dependent changes in Ca levels in culture.

Superconductivity in YBa2Cu3O7 single crystals

Abstract: Increased understanding of fundamental properties of the new cuprate superconductors depends on advances in the preparation of quality materials for study. Early work focused on the study of sintered polycrystalline samples. In some cases, grain growth during sintering yielded individual grains large enough (˜80 μm) for single crystal X-ray determination1. Larger single crystals allow measurements of physical properties including measurements of anisotropic behaviour. The anisotropy of upper critical fields in YBa2Cu3O7 single crystals has been reported by lye et al.2 who found critical fields characteristic of a quasi-two-dimensional superconductor, which are highest when the field is orientated perpendicular to the c-axis. Dinger et al.3 have measured critical fields and critical currents of single crystals and find anisotropies of 10 and greater3. These studies contain little detail on the crystal growth of YBa2Cu3O7. Here we report the growth of single crystals as large as 4 mm of YBa2Cu3O7 and DyBa2Cu3O7 and confirm superconductivity in them by magnetic and electrical measurements. Single crystals of other rare earth barium cuprates are also obtained although we have yet to optimize growth conditions for these phases.

An efficient block-matching algorithm for motion-compensated coding

Abstract: We present an efficient search technique which minimizes the computations necessary for estimating the motion in video-sequences by the block matching method We also discuss the theoretical basis for conducting such a reduced search by our technique We then present two algorithms which employ the proposed technique for estimating the motion typical of video-conferencing environment Next, the results of computer simulations on a real video-sequence are included which demonstrate the effectiveness of the proposed technique Finally, the results of a study of statistical properties of block motion-compensated frame difference signals are also summarized, to assist in future choice of a coding strategy for such signals

Local versus global torque optimization of redundant manipulators

Abstract: This paper compares local and global optimization methods for minimizing torque loading at the joints of a redundant manipulator in a least squares sense. Simulations show that the local methods show an unexpected instability for relatively long trajectories. The global method, however, was able to find a stable solution which outperformed the local algorithms at all times.

Crystal structure of the high-temperature superconductor La1.85Sr

Abstract: The crystal structure of La/sub 1.85/Sr/sub 0.15/CuO/sub 4/ has been determined at 300, 60, and 10 K by neutron diffraction powder profile analysis. The structure is of the tetragonal K/sub 2/NiF/sub 4/ type at ambient temperature, but undergoes an orthorhombic distortion near 200 K which buckles the copper-oxygen planes. The crystal structure of the orthorhombic phase is not significantly different above and below T/sub c/. The copper-oxygen coordination polyhedron changes shape in a subtle manner at the tetragonal-to-orthorhombic phase transition, but is unchanged at T/sub c/.

Wide‐frequency specific heat spectrometer

Abstract: We have developed a technique to measure the frequency‐dependent specific heat of a liquid over a frequency range exceeding five decades, 0.01–3000 Hz. The measurement is not performed adiabatically, but rather relies on heat diffusion from a thin, planar heater immersed in the liquid sample. The spectrometer has been used to study organic liquids near the glass transition.

Feed horn for a telecommunications antenna

Abstract: A feed horn comprising a radiating portion having a circular aperture, a transition formed by progressively varying waveguide sections and a flange which provides a coupling with a rectangular waveguide is fabricated from moldable material and designed to be fixed in a support. The radiating portion of the horn is formed by a cylindrical flange having at least two concentric grooves which form two traps.

Nonzero isotope effect in La 1.85 Sr 0.15 CuO 4

Abstract: The effect on the superconducting transition temperature of exchanging /sup 18/O for /sup 16/O in La/sub 1.85/Sr/sub 0.15/CuO/sub 4/ has been studied. The concentration of the two isotopes has been determined by Rutherford backscattering spectroscopy and Raman spectroscopy confirmed the mass-related phonon frequency shift. The /sup 18/O substitution is found to be the same on the two O sites. Magnetic measurements of the transition temperature reveal a nonzero isotope effect (..cap alpha.. = 0.16 +- 0.02). The exponent ..cap alpha.. has also been calculated within a conventional electron-phonon interaction model and was found to be significantly larger.

Superconducting Y‐Ba‐Cu‐O oxide films by sputtering

Abstract: We have prepared superconducting thin films of Y‐Ba‐Cu‐O with Tc onsets above 95 K by both diode and magnetron sputtering. Films with full superconductivity (R=0) at 85 K have been produced by dc magnetron sputtering. The compositions of the films are fairly uniform across an area 50 mm in diameter and through the film thickness. Structural properties of the films were studied by x‐ray diffraction. Critical current densities in the range of 3000 to 104 A/cm2 have been measured at 4.2 K.

Electroabsorption in GaAs/AlGaAs coupled quantum well waveguides

Abstract: A novel GaAs/AlGaAs coupled quantum well structure, consisting of two 46 A wells separated by a 11.5 A barrier, was embedded in a leaky waveguide. Polarization anisotropy and quantum‐confined Stark effect absorption changes are observed, and a 14:1 modulator is demonstrated operating at the peak of the first light‐hole transition. Although the lowest energy transitions behave as predicted by tunneling resonance calculations, higher energy states exhibit unusual behavior due to valence‐band mixing.

Viscoelasticity of homogeneous polymer melts near a critical point

Abstract: The anomalous contribution to the shear stress caused by critical fluctuations in homogeneous polymer melts is investigated. Mean field expressions for the critical contributions to the stress tensor, dynamic shear modulus, and nonlinear steady shear viscosity are derived. These expressions are applied to the case of a homogeneous diblock copolymer near its microphase separation transition. The low frequency scaling behavior of the storage modulus is found to be G’(ω)∼ω2a−5/2, while the loss modulus is predicted to scale like G‘(ω)∼ωa−3/2. The dimensionless parameter a is defined by a=2(χN)s−2χN, where χN is the product of the Flory interaction parameter and the number of Kuhn segments per chain, and (χN)s is the value of χN on the spinodal computed by Leibler. The linear steady shear viscosity is also found to be singular as a→0. A comparison is made between the present theoretical predictions and the recent experiments of Bates on 1,4/1,2 polybutadiene block copolymers.

Absence of many states in realistic spin glasses

Abstract: Based on a simple scaling ansatz, the authors argue that no pure thermodynamic states other than the paramagnetic state and a pair of states in zero magnetic field which are related by a global spin flip can exist in short-range Ising spin glass models in any dimension. An analogous result should hold for XY and Heisenberg spin glasses, as well as for square-integrable long-range interactions.

Structural properties of Ba2RCu3O7 high-Tc superconductors

Abstract: The crystal chemistry of several Ba/sub 2/RCu/sub 3/O/sub 7/ high-T/sub c/ superconductors (R = Y, Pr, Sm, Eu, Gd, Dy, Ho, Er, Tm) has been studied by x-ray diffraction on twinned crystals. All samples except the Pr compound show superconductivity around 90 K. From the variation of the interatomic distances with different rare-earth ions, we infer the presence of a (CuO/sub 2/-BaO-CuOD'Alembertian-BaO-CuO/sub 2/) building unit.

Low‐loss InGaAs/InP multiple quantum well optical electroabsorption waveguide modulator

Abstract: An optical electroabsorption waveguide modulator is described based on the quantum‐confined Stark effect in an InGaAs/InP multiple quantum well waveguide. The optical modulator has a high on/off ratio (47:1) with very low insertion loss (2.9 dB) and a 3‐dB modulation bandwidth of 3.0 GHz at 0.1 mW optical input power.

Melt Rheology of Filled Thermoplastics

Abstract: 206

Low internal loss separate confinement heterostructure InGaAs/InGaAsP quantum well laser

Abstract: The structure and properties of a high performance InGaAs/InGaAsP separate confinement heterostructure multiple quantum well laser operating at 1.54 μm are described. The laser has low threshold current (18 mA), high quantum efficiency (22% facet), high cw output power (42 mW/facet), and weak dependence of the threshold currents and efficiencies on cavity length.

Controlled suppression and enhancement of the photoactivity of titanium dioxide (rutile) pigment

Abstract: The variation in the quantum yield of a photoreaction of an organic adsorbate on an n-TiO/sub 2/ pigment particle is considered. The yield depends on the ratio of the rate of oxidation of the organic adsorbate by holes to the sum of the surface and bulk electron-hole recombination rates. It is shown that at low bulk-recombination rate the quantum efficiency is controlled by the surface density of electrons. This density is determined by the barrier height, i.e., the Fermi level of the particles. Reduction of the particles raises their Fermi level, correspondingly increases the height of the potential barriers that repel electrons from the surface, reduces the rate of surface recombination, and thereby increases the quantum yield. Correspondingly, oxidation of the particles lowers their Fermi level and thereby the quantum yield. In reduced particles, i.e., when the surface-recombination rate is low, bulk defects dominate the recombination process. In this case the photoactivity of the particles decreases upon ball milling under clean conditions and increases upon removal of the lattice defects through etching by boiling mineral acids. When combined, oxidation/reduction and mechanical damage/etching allow controlled variation of the photoactivity of the 0.2-..mu..m particles by 2 orders of magnitude.

Mechanisms of buried oxide formation by ion implantation

Abstract: We have studied the process of buried oxide formation as a function of implantation and annealing conditions. Concentrating on substoichiometric implants (<1×1018 O/cm2), we varied the implantation energies from 100 keV to 1 MeV. Some apparent precipitation of SiO2 similar to that observed in Czochralski‐grown silicon occurs on implantation. This means that formation of the buried oxide layer and perfection of the overlying crystalline Si layer depend more strongly on the substrate temperature during the implant than on the annealing temperature.