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  • American Institute of Physics (AIP)  (1.624)
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  • 1
    Digitale Medien
    Digitale Medien
    A fiber drop analyzer: A new analytical instrument for the individual, sequential, or collective measurement of the physical and chemical properties of liquids (1992)
    [S.l.] : American Institute of Physics (AIP)
    Review of Scientific Instruments 63 (1992), S. 3431-3454 
    Details
    ISSN: 1089-7623
    Quelle: AIP Digital Archive
    Thema: Physik , Elektrotechnik, Elektronik, Nachrichtentechnik
    Notizen: A new instrumental method has been devised for the individual, sequential, or collective measurement of the physical and chemical properties of liquids. The instrumental theory for the fiber drop analyzer (FDA) has been developed for the measurement of surface tension, viscosity, refractive index, and chemical composition of a liquid. An empirical theory is suggested for the measurement of pH in a limited range. The analytical theory for the fiber drop analyzer has been established, a prototype constructed and tested for measuring individually surface tension, viscosity, refractive index, and the chemical composition on a restricted set of test solution. The instrument is shown to have the capability to simultaneously measure the above measurands, but in addition can in individual measurement procedures, measure all these quantities. The instrument perhaps is also potentially capable of measuring specific gravity and pH in its existing form, and other optical properties of liquids with some basic modification. The laboratory FDA has been used to test a series of samples from a large cane sugar manufacturer's process and these measurements demonstrate that this technology has the potential to be used as a remote optrode industrial process monitor for sucrose manufacture and very possibly, elsewhere in other industrial applications.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.1143748
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  • 2
    Digitale Medien
    Digitale Medien
    Investigation of global Alfvén instabilities in the Tokamak Fusion Test Reactor (1992)
    New York, NY : American Institute of Physics (AIP)
    Physics of Fluids 4 (1992), S. 2122-2126 
    Details
    ISSN: 1089-7666
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: Toroidal Alfvén eigenmodes (TAE) were excited by the energetic neutral beam ions tangentially injected into plasmas at low magnetic field in the Tokamak Fusion Test Reactor (TFTR) [Proceedings of the 11th International Conference on Plasma Physics and Controlled Fusion Research (IAEA, Vienna, 1987), Vol. 1, p. 51]. The injection velocities were comparable to the Alfvén speed. The modes were identified by measurements from Mirnov coils and beam emission spectroscopy (BES). TAE modes appear in bursts whose repetition rate increases with beam power. The neutron emission rate exhibits sawtoothlike behavior and the crashes always coincide with TAE bursts. This indicates ejection of fast ions from the plasma until these modes are stabilized. The dynamics of growth and stabilization were investigated at various plasma currents and magnetic fields. The results indicate that the instability can effectively clamp the number of energetic ions in the plasmas. The observed instability threshold is discussed in light of recent theories. In addition to these TAE modes, intermittent oscillations at three times the fundamental TAE frequency were observed by Mirnov coils, but no corresponding signal was found in BES. It appears that these high-frequency oscillations do not have a direct effect on the plasma neutron source strength.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.860462
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  • 3
    Digitale Medien
    Digitale Medien
    Pulsed high-energy γ-rays from the radio pulsar PSRI706–44 (1992)
    [s.l.] : Nature Publishing Group
    Nature 359 (1992), S. 615-616 
    Details
    ISSN: 1476-4687
    Quelle: Nature Archives 1869 - 2009
    Thema: Biologie , Chemie und Pharmazie , Medizin , Allgemeine Naturwissenschaft , Physik
    Notizen: [Auszug] Following the SAS-2 and COS-B high-energy y-ray observations of the Crab (0531 + 21) and Vela (0833-45) pulsars1'2, searches were made for additional y-ray pulsars3'5. No other positive results were then found, but calculations have indicated that y-ray emission is likely from other ...
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1038/359615a0
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  • 4
    Digitale Medien
    Digitale Medien
    Arsenic cluster dynamics in doped GaAs (1992)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 72 (1992), S. 3509-3513 
    Details
    ISSN: 1089-7550
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: We have studied the formation of As precipitates in doped GaAs structures that were grown by molecular beam epitaxy at low substrate temperatures and subsequently annealed. We find that the As precipitates form preferentially on the n side of such fabricated GaAs pn junctions. As the coarsening process proceeds, there is a gradual increase in the amount of As in precipitates in the n-GaAs region and a decrease in the p-GaAs region; the depletion region between the pn junction becomes free of As precipitates. These observations can be understood qualitatively based on the charge states of the As interstitial and using thermodynamic arguments in which the crystal attempts to minimize the chemical potential during the anneal. The presence of the excess As results in a stable Be profile even to anneals of 950 °C. Finally, a temperature cycling technique to grow arbitrarily thick GaAs epilayers containing As precipitates was demonstrated.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.352326
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  • 5
    Digitale Medien
    Digitale Medien
    Plasmon excitation and self-coupling in a bi-periodically modulated two-dimensional electron gas (1992)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 71 (1992), S. 6049-6061 
    Details
    ISSN: 1089-7550
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: We report far infrared (FIR) studies of plasmons in spatially modulated two-dimensional electron gases (2DEGs) in AlGaAs/GaAs heterostructures using biased overlaid metal gratings, including interdigitated gratings, both as optical couplers and as spatially modulating gates. Comparison of the experimental results with the predictions of scattering matrix calculations of the FIR response of a modulated 2DEG in the presence of a perfectly conducting lamellar grating allow us to deduce the spatial variation of the number density distribution in the 2DEG as a function of grating bias. For the interdigitated grating gates, the 2DEG can be modulated at a period of twice that of the grating fingers by differentially biasing alternate fingers; 2D plasmon resonances have been observed at half-integral values of the grating wave vector G, corresponding to the electrically induced periodicity of the 2DEG modulation itself acting as an optical coupler in addition to the metal grating. The observed G/2 plasmon frequencies decrease with increasing amplitude of the 2DEG number density modulation, in quantitative agreement with those obtained from scattering matrix calculations of the optical response of a modulated 2DEG under a perfectly conducting lamellar grating; calculations of the oscillating charge density profiles show that this occurs because, as the modulation amplitude increases, the oscillation becomes localized in regions of low 2DEG number density which are also under one of the sets of grating fingers, and is therefore better screened.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.350462
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  • 6
    Digitale Medien
    Digitale Medien
    Extending J walking to quantum systems: Applications to atomic clusters (1992)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 97 (1992), S. 5713-5731 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: The J-walking (or jump-walking) method is extended to quantum systems by incorporating it into the Fourier path integral Monte Carlo methodology. J walking can greatly reduce systematic errors due to quasiergodicity, or the incomplete sampling of configuration space in Monte Carlo simulations. As in the classical case, quantum J walking uses a jumping scheme to overcome configurational barriers. It couples the usual Metropolis sampling to a distribution generated at a higher temperature where the sampling is sufficiently ergodic. The J-walker distributions used in quantum J walking can be either quantum or classical, with classical distributions having the advantage of lower storage requirements, but the disadvantage of being slightly more computationally intensive and having a more limited useful temperature range. The basic techniques are illustrated first on a simple one-dimensional double well potential based on a quartic polynomial. The suitability of J walking for typical multidimensional quantum Monte Carlo systems is then shown by applying the method to a multiparticle cluster system consisting of rare gas atoms bound by pairwise Lennard-Jones potentials. Different degrees of quantum behavior are considered by examining both argon and neon clusters. Remarkable improvements in the convergence rate for the cluster energy and heat capacity, analogous to those found in classical systems, are found for temperatures near the cluster transition regions.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.463756
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  • 7
    Digitale Medien
    Digitale Medien
    Observation of the focusing of x-ray transition radiation using cylindrical optics (1992)
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 61 (1992), S. 1019-1021 
    Details
    ISSN: 1077-3118
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: We have measured the profile of x rays generated by a transition radiator and focused by simple cylindrical optics. Soft x rays with photon energies between 1 and 4 keV were generated by a 93-MeV electron beam striking a stack of eight foils of 3.5-μm-thick mylar. These x rays were emitted in an annular cone and were collected by a quartz cylinder which focused the x rays to a 0.5-mm-diam spot at a distance of 1.35 m from the transition radiator.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.107702
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  • 8
    Digitale Medien
    Digitale Medien
    Organometallic vapor phase epitaxial growth and characterization of GaInP/AlInP quantum wells on 10°-off GaAs substrates using ethyldimethylindium (1992)
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 60 (1992), S. 1007-1009 
    Details
    ISSN: 1077-3118
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: GaInP/AlInP quantum wells (QWs) have been grown on (100) and 10°-off (100) GaAs substrates in an organometallic vapor phase epitaxy (OMVPE) reactor at reduced pressure. Photoluminescence (PL) studies revealed an increase in peak energy and narrowing in linewidth for the 10°-off QWs, due to the suppression of the formation of ordered microstructures during the OMVPE process. Low temperature PL results of GaInP/AlInP QWs as thin as 13 A(ring) are presented for the first time. The 10°-off QW showed a dominant peak at 551 nm, the shortest wavelength ever reported in GaInP/AlInP QWs. The origin of PL for the 13-A(ring) QWs is, however, different from that for thicker QWs as a result of the transfer of electrons to localized states in the barrier layer.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.106504
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  • 9
    Digitale Medien
    Digitale Medien
    Low threshold, 633 nm, single tensile-strained quantum well Ga0.6In0.4P/(AlxGa1−x)0.5In0.5P laser (1992)
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 60 (1992), S. 1927-1929 
    Details
    ISSN: 1077-3118
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: Low threshold, 633 nm diode lasers are demonstrated. These devices contain a single, 80 A(ring), tensile-strained Ga0.6In0.4P quantum well (QW) active region, and Al0.5In0.5P cladding layers, grown by low-pressure organometallic vapor phase epitaxy on misoriented substrates. Operating characteristics are compared with low-threshold, 680 nm compressively strained Ga0.4In0.6P QW lasers. Although performance is not as good as for the 680 nm devices, the 633 nm lasers have characteristic temperature T0∼60 K and low pulsed threshold current density (400 A/cm2). These improved characteristics are believed due to the incorporation of a single, tensile-strained QW, along with (Al0.6Ga0.4)0.5In0.5P confining layers, which offer increased electron confinement.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.107153
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  • 10
    Digitale Medien
    Digitale Medien
    Ion cyclotron range of frequencies stabilization of sawteeth on Tokamak Fusion Test Reactor (1992)
    New York, NY : American Institute of Physics (AIP)
    Physics of Fluids 4 (1992), S. 2155-2164 
    Details
    ISSN: 1089-7666
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: Results obtained from experiments utilizing high-power ion cyclotron range of frequencies (ICRF) heating to stabilize sawtooth oscillations on Tokamak Fusion Test Reactor (TFTR) [Hawryluk et al., Plasma Phys. Controlled Fusion 33, 1509 (1991)] are reviewed. The key observations include existence of a minimum ICRF power required to achieve stabilization, a dependence of the stabilization threshold on the relative size of the ICRF power deposition profile to the q=1 volume, and a peaking of the equilibrium pressure and current profiles during sawtooth-free phases of the discharges. In addition, preliminary measurements of the poloidal magnetic field profile indicate that q on axis decreases to a value of 0.55±0.15 after a sawtooth-stabilized period of ∼0.5 sec has transpired. The results are discussed in the context of theory, which suggests that the fast ions produced by the ICRF heating suppress sawteeth by stabilizing the m=1 magnetohydrodynamic (MHD) instabilities believed to be the trigger for the sawtooth oscillations. Though qualitative agreement is found between the observations and the theory, further refinement of the theory coupled with more accurate measurements of experimental profiles will be required in order to complete quantitative comparisons.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.860484
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