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  • 1990-1994  (5)
  • 1985-1989  (7)
  • 1993  (5)
  • 1989  (7)
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  • 1990-1994  (5)
  • 1985-1989  (7)
Year
  • 1
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Results are reported from an investigation of the effects of selected processing parameters on the morphology and properties of YBa2Cu3O7−δ (YBCO) superconducting thin films grown directly on polycrystalline silver substrates by chemical-vapor deposition (CVD). These results were achieved through a set of experimental studies which examined: (i) recrystallization mechanisms of polycrystalline silver and their effect on the deposition of YBCO thin films; and (ii) CVD processing conditions leading to the growth of high-quality YBCO films. The samples were analyzed using dynamic impedance, four-point resistivity probe, x-ray diffraction, Rutherford backscattering, and scanning electron microscopy. These studies showed that substrate temperature played a critical role not only in the formation of YBCO films, but also in the recrystallization of silver substrates, which in turn greatly influenced film growth. The studies also led to the identification of a two-stage processing scheme for the growth of YBCO films on silver. The first processing stage consisted of a substrate conditioning cycle which involved a 10 min ramping from room temperature to deposition temperature where the substrates were held for an additional 10 min in a flow of 70 sccm O2 at a reactor working pressure of 2 Torr. The second processing stage involved actual film deposition at 760–800 °C for 3–10 min (depending on desired film thickness) in a mixed flow of 70 sccm O2 and 210 sccm N2O at a reactor working pressure of 4 Torr. Samples thus produced were highly oriented along the c axis perpendicular to the substrate with a zero resistance transition temperature of 87 K and a critical current density of 2×104 A/cm2 (77 K, B=0). The films had a thickness of 200–700 nm depending on the length of the growth cycle, which corresponded to the growth rates in the range 65–130 nm/min. A growth mechanism for YBCO on polycrystalline silver, which emphasized the role of silver recrystallization, was consequently proposed and discussed.
    Type of Medium: Electronic Resource
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  • 2
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 99 (1993), S. 9428-9437 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Classical Monte Carlo and molecular dynamics (MD) simulations were carried out to investigate the structures, the infrared spectra, and the rigid–nonrigid transitions of small methanol clusters (CH3OH)n for n=3–6. The study was motivated by experimental results for these clusters from size specific infrared (IR) dissociation spectroscopy. The MD simulations revealed the following transitions: The trimer passes from a rigid ring configuration into a series of nonrigid open chain structures starting at 197 K. For n=4 and 5 such transitions occur between rings and rapidly fluctuating ring structures at T=357 and 243 K, respectively. For n=6 first a pure isomeric transition between the two energetically lowest isomers of S6 and C2 symmetry is found at 35 K, and then a similar transition to a nonrigid behavior as is observed for n=4 and 5 is seen at 197 K. The measured spectra display in all cases the rigid lowest energy configurations.
    Type of Medium: Electronic Resource
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  • 3
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 91 (1989), S. 6477-6493 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: In order to address the problem of three-body interactions in gas–surface scattering, we considered the collision of a He atom with the (0001) surface of graphite coated by a monolayer of Xe. To eliminate the uncertainties connected with errors in the two-body He–Xe interaction, we determined the latter by crossed-beam differential collision cross-section measurements performed at two energies (67.2 and 22.35 meV). These scattering data together with room-temperature bulk diffusion data are then fitted with a Hartree–Fock–dispersion–type function to yield an interaction potential that explains most of the properties of this system within the experimental errors and represents an improvement on previously published He–Xe potentials. Helium diffraction measurements are then carried out from the Xe overlayer and the dependence of the specular intensity from the angle of incidence is carefully determined. Further, a He–surface potential is constructed by adding together the following terms: (1) the He–Xe pairwise sum, (2) the long-range He–(0001)C interaction, (3) the three-body contribution generated by the Axilrod–Teller–Muto term, (4) the so-called surface-mediated three-body interaction He–Xe–(0001)C first considered by A. D. McLachlan [Mol. Phys. 7, 381 (1964)], and finally (5) a small correction which is meant to take into account the nonstationary nature of the surface. Using this potential, well-converged close-coupling scattering calculations are carried out, and their results compared with the data. In general, good agreement is obtained. The agreement can, however, be improved by (a) an increase of about 30% in the contribution of three-body forces, (b) the lowering of the He–graphite long-range attraction coefficient by about 15%, or (c) a reduction of the two-body interaction well depth of 1.6% (the experimental error) together with any combination of the factors under (a) and (b) reduced by an adequate amount. Elimination of the contribution of the graphite surface by studying Xe multilayers is hindered by the uncertainties in the "thermal correction'' [point (5) above] which, due to the multilayer increased "softness,'' becomes an appreciable source of uncertainty.
    Type of Medium: Electronic Resource
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  • 4
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: A technologically useful chemical vapor deposition process with high growth rate ((approximately-greater-than)4 μm/h) was developed for the epitaxial growth of YBa2Cu3O7−x (YBCO) thin films. Even at the high growth rate used in this process, a spiral growth mechanism was observed and the films grown had electrical (Tc=92 K, Jc of 2×106A/cm2 at 77 K) and structural properties equal to films produced by physical vapor deposition.
    Type of Medium: Electronic Resource
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  • 5
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 98 (1993), S. 9425-9431 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Infrared photodissociation spectra of the mixed complexes C2H4–CH3COCH3 and C2H4–(CH3COCH3)2 have been observed after size selecting them by scattering from a helium beam combined with mass spectrometric detection. The excitation of the ν7-out of plane symmetric wagging mode of ethene near its gas phase frequency at 949.3 cm−1 with a cw-CO2 laser leads to a characteristic depletion of the cluster beam. The dissociation spectrum of the 1:1 complex can be explained by two peaks at 950.8 and 961.6 cm−1. Calculations of minimum energy configurations and band shifts based on an empirical site–site potential show that these frequencies can be attributed to the absorption of two different isomers. They correspond to the two different binding patterns of the H atoms of ethene to the O atom of acetone and those of acetone to the C–C group of ethene, respectively. For the 1:2 complex, a large peak at 958.5 cm−1 and a smaller one around 940.5 cm−1 are found which can be explained in a similar manner by several isomers found in the structure calculations.
    Type of Medium: Electronic Resource
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  • 6
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    Unknown
    Berlin : Periodicals Archive Online (PAO)
    Deutsche Zeitschrift für Philosophie. 37:3 (1989) 257 
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  • 7
    ISSN: 1432-1106
    Keywords: Cutaneous EPSPs ; Fictive locomotion ; Cat
    Source: Springer Online Journal Archives 1860-2000
    Topics: Medicine
    Notes: Summary We examined modulation of transmission in short-latency, distal hindlimb cutaneous reflex pathways during fictive locomotion in 19 decerebrate cats. Fictive stepping was produced either by electrical stimulation of the mesencephalic locomotor region (MLR) or by administration of Nialamide and 1-DOPA to acutely spinalized animals. Postsynaptic potentials (PSPs) produced by electrical stimulation of low threshold afferents (〈 2.5 times threshold) in the superficial peroneal (SP), sural, saphenous or medial plantar nerves were recorded intracellularly from various extensor (n = 28) and flexor (n = 24) motoneurons and averaged throughout the step cycle, together with voltage responses to intrasomatic constant current pulses (in order to monitor relative cell input resistance). Each motoneuron studied displayed rhythmic background oscillations in membrane potential and correlated variations in input resistance. The average input resistance of extensor motoneurons was lowest during mid-flexion, when the cells were relatively hyperpolarized and silent. Conversely, average input resistance of flexor motoneurons was highest during mid-flexion, when they were depolarized and active. The amplitude of the minimum-latency excitatory components of PSPs produced by cutaneous nerve stimulation were measured from computer averaged records representing six subdivisions of the fictive step cycle. Oligosynaptic EPSP components were consistently modulated only in the superficial peroneal responses in flexor motoneurons, which exhibited enhanced amplitude during the flexion phase. With the other skin nerves tested (sural, saphenous, and plantar), no consistent patterns of modulation were observed during fictive locomotion. We conclude that transmission through some, but not all, oligosynaptic excitatory cutaneous pathways is enhanced by premotoneuronal mechanisms during the flexion phase of fictive stepping in several cat hindlimb motor nuclei. The present results suggest that the patterns of interaction between the locomotor central pattern generator and excitatory cutaneous reflex pathways depend on the source of afferent input and on the identity of the target motoneuron population.
    Type of Medium: Electronic Resource
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  • 8
    ISSN: 1436-6304
    Source: Springer Online Journal Archives 1860-2000
    Topics: Mathematics , Economics
    Type of Medium: Electronic Resource
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  • 9
    Electronic Resource
    Electronic Resource
    Springer
    OR spectrum 11 (1989), S. 191-195 
    ISSN: 1436-6304
    Source: Springer Online Journal Archives 1860-2000
    Topics: Mathematics , Economics
    Description / Table of Contents: Summary The technique of designing and constructing simulation models is difficult and requires a lot of knowledge and experience. In general, users of simulation models don't have this expert knowledge. Up to now, it has not been possible to incorporate the expert knowledge necessary for the overall modelling within an expert system which would then in turn make the techniques accessible to the inexperienced user. In general, the close cooperation among experts in the fields of application and simulation techniques remains — for the time being — the only guarantee for successfully scrutinizing a model study. It is, however, possible to combine expert systems and simulation models when using carefully chosen, closely defined fields of application. It is here that an important area of interest and study lies. There are three conceivable alternatives: 1. To provide a simulation model with an intelligent environment which supports the user in constructing as well as evaluating the results made. 2. An expert system contains a simulation model as a possible component. If the expert system needs information that is not directly available, a simulation model is accessed which in turn sends the necessary data back to the expert system. 3. A simulation model contains an expert system as a component. In this case, the model has specific strategies at its disposal that are to be taken into consideration. If the model reaches a point where decisions have to be made, the simulation model turns to the expert system in order to receive information on relevant decisions. These relevant decisions are then used to determine the further course of the model. All three alternatives assume that the expert system as well as the simulation system have an open architecture. The open architecture is a prerequisite for the possibility of coupling the two. Up to now, this requirement has rarely been achieved.
    Notes: Zusammenfassung Der Entwurf und der Aufbau von Simulationsmodellen ist eine schwierige Technik, die sehr viel Wissen und Erfahrung voraussetzt. Die Anwender von Simulationsmodellen verfügen in der Regel über dieses Expertenwissen nicht. Bisher ist es nicht möglich, das für die Modellierung insgesamt erforderliche Expertenwissen in ein Expertensystem zu inkorporieren um es auf diese Weise dem in der Simulationstechnik unerfahrenen Anwender zugänglich zu machen. Im allgemeinen bleibt die enge Zusammenarbeit zwischen Fachleuten auf dem Anwendungsgebiet und der Simulationstechnik bis auf weiteres die einzige Garantie für eine erfolgreiche Modelluntersuchung. In ausgewählten, sehr engen abgegrenzten Anwendungsgebieten ist es jedoch möglich, Expertensysteme und Simulationsmodelle zu vereinen. Hier entwickelt sich ein bedeutendes Arbeits- und Aufgabenfeld. Es sind drei Alternativen vorstellbar: 1. Ein Simulationsmodell erhält eine intelligente Umgebung, die den Anwender beim Modellaufbau und bei der Ergebnisauswertung unterstützt. 2. Ein Expertensystem enthält als eine mögliche Komponente ein Simulationsmodell. Falls das Expertensystem Informationen benötigt, die nicht direkt verfügbar sind, wird ein Simulationsmodell angestoßen, das die erforderlichen Daten an das Expertensystem zurückliefert. 3. Ein Simulationsmodell enthält als Komponente ein Expertensystem. In diesem Fall enthält das Modell Strategien, die zu berücksichtigen sind. Falls das Modell an einen Punkt kommt, an dem Entscheidungen getroffen werden müssen, wendet sich das Simulationsmodell an das Expertensystem und läßt sich die maßgebenden, den weiteren Modellablauf bestimmenden Entscheidungen mitteilen. Alle drei Alternativen verlangen vom Expertensystem und dem Simulationssystem eine offene Architektur. Die offene Architektur ist Voraussetzung dafür, daß sich beide Systeme koppeln lassen. Diese Forderung ist bisher kaum erfüllt.
    Type of Medium: Electronic Resource
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  • 10
    Electronic Resource
    Electronic Resource
    Springer
    General relativity and gravitation 25 (1993), S. 373-397 
    ISSN: 1572-9532
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract We describe an algorithm for solving 1 + 1-systems that are in symmetric hyperbolic form. It is applied to spherically symmetric deviations from ak = 0, radiation filled Isotropic universe. We compare the solution to the full Einstein equations with those of the linearized equations. For small enough initial data the evolutions are indistinguishable. However, for large data, i.e., for initial density contrasts in the 1 percent range, trapped surfaces appear and singularities form.
    Type of Medium: Electronic Resource
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