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  • 1
    ISSN: 1420-9098
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology
    Description / Table of Contents: Resume Chez les Guêpes et les Frelons, les cellules sont hexagonales lorsqu'elles sont situées dans les zones centrales du rayon. Les cellules périphériques, comme toutes les cellules au début de leur construction, ont des parois arrondies plutôt que angulaires. La transformation d'une cellule en forme de gobelet, à parois arrondies, en une cellule hexagonale avec un angle de 120° entre les parois est partiellement due à des processus de transformation des parois cellulaires existantes au moment où de nouvelles parois leur sont attachées depuis le côté extérieur. Le développement de la construction des cellules initiales aboutit à 6 parois dans les rayons construits autour d'une seule cellule centrale ou à 5,5 parois lorsqui'il y a 2 cellules centrales contiguës. Cependant, dans les constructions ultérieures, ce type d'évolution des parois diminue graduellement à mesure que de nouveaux anneaux de cellules sont ajoutés, pour atteindre une valeur moyenne de 3 nouvelles parois par cellule, indépendamment du fait que le rayon s'est construit autour d'une ou de deux cellules centrales. Les résultats présentés dans cet article sur l'utilisation plus efficiente des parois dans les rayons plus grands apportent une confirmation du caractère économique de l'utilisation des matériaux dans la construction du rayon par les Vespinæ.
    Notes: Summary The individual cell in the hornet or wasp comb is hexagonal in cross-section when it is one of the inner “ring” of cells. On the other hand, peripheral cells, like all cells built initially, have rounded rather than angled walls. This transformation from a goblet-shaped cell with rounded walls to an hexagonally-walled cell with an angle of 120° between each two walls is partly acheived by additional processing of the existing cell walls as new walls (between old and new cells) are accreted onto them from the outside. The comb cells are essentially hexagonal so that the “expenditure” in the building of the initial cell(s) is either 6 walls (in combs built around a single central cell) or 5.5 walls (where there are two abutting, central cells). With subsequent building, however, this “wall expenditure” diminishes gradually as more rings of cells are laid down, approaching an average value of 3 new walls per cell regardless of whether the comb is built around one or rather two central cells. Presented data on more efficient wall utilization in the larger combs offer further confirmation of the frugality of comb construction by Vespinæ.
    Type of Medium: Electronic Resource
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  • 2
    Electronic Resource
    Electronic Resource
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 65 (1994), S. 2223-2225 
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: We report on theoretical and experimental results on a novel metal-semiconductor-metal (MSM) photodetector with a backgate provided by a p-doped layer. The backgate allows extremely short sweep-out times for the holes, due to strong electric fields normal to the surface. Thus, long tails due to slow moving holes and screening of the external drift fields by hole space charge accumulation at high optical power, which lead to a degradation of the time response of conventional MSM photodetectors, are avoided. The high frequency performance measured up to 8 GHz in the time and frequency domain showed a significant reduction of the bandwidth limiting hole tail compared to standard MSM photodetectors. © 1994 American Institute of Physics.
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  • 3
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 98 (1993), S. 8277-8283 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Diffusion of ammonia was studied over the smooth Re(001) surface utilizing optical second harmonic diffraction from surface coverage grating. The decay of the first order diffraction peak as a function of surface temperature for different initial coverages was measured and then simulated by numerical solution of Fick's second diffusion equation, employing a coverage dependent diffusion constant. For the first time the second order diffracted second harmonic signal was monitored during the diffusion process, supporting the calculated initial coverage profile and the diffusion model. The resulting diffusivity D(θ)=D0exp[−Em(θ)/RT], is defined by a barrier for diffusion Em(θ)=E0−ωZθ, with activation energy at zero coverage E0=3.4±0.6 kcal/mol, D0=2.8×10−3 cm2 sec−1 and the repulsion energy between the pair of nearest neighbor molecules ω=0.2±0.06 kcal/mol. The magnitude and effect of the repulsive interactions on diffusion are compared to their strong effect on the activation energy for desorption.
    Type of Medium: Electronic Resource
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  • 4
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 96 (1992), S. 4040-4043 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Real time monitoring of laser induced thermal desorption of NH3 from Re(0001) at ns time scale was performed utilizing optical SHG. Coverage dependent desorption kinetic parameters determined under equilibrium conditions were found to correctly describe the 8 orders of magnitude faster desorption rates. The role of strong repulsive interactions is discussed.
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  • 5
    Electronic Resource
    Electronic Resource
    New York, NY : American Institute of Physics (AIP)
    Physics of Fluids 2 (1990), S. 1376-1383 
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: In this paper, the direct observation of wake-field self-focusing of an electron beam in plasma is reported. The dynamics of beam self-pinching and the fast collisionless evolution of a Bennett-like, near-equilibrium profile are examined theoretically and computationally. The experimental results are compared to predictions of the analysis and discussed in the context of application to the plasma lens and the plasma wake-field accelerator.
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  • 6
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 70 (1991), S. 2435-2448 
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: We present a comprehensive theoretical and experimental analysis of the current response of GaAs metal-semiconductor-metal Schottky photodiodes exposed to 70 fs optical pulses. Theoretical simulations of the carrier transport in these structures by a self-consistent two-dimensional Monte Carlo calculation reveal the strong influence of the distance between the finger electrodes, the external voltage, the GaAs layer thickness and the excitation intensity on the response time and the corresponding frequency bandwidth of these photodetectors. For many experimental conditions, the model demonstrates a clear temporal separation of the electron and hole contributions to the output current due to the different mobilities of the two carrier types. For a diode with an electrode separation of 0.5 μm, an electric-field strength above 10 kV/cm and low intensity of the incident light the theory predicts a pulse rise time below 2 ps, an initial rapid decay as short as 5 ps associated with the electron sweep out and a subsequent slower tail attributed to the hole current. For weaker electric fields and/or higher light intensities a significant slowing down of the detector speed is predicted because of effective screening of the electric field by the photoexcited carriers. Heterostructure layer-based devices are shown to provide superior performance compared to diodes manufactured on bulk substrates. Experimental data obtained by photoconductive or electro-optic sampling on diodes with electrode separation between 0.5 and 1.2 μm agree fairly well with the theoretical predictions.
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  • 7
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The temporal evolution of the photocurrent in interdigitated GaAs metal-semiconductor- metal Schottky photodiodes is directly measured in the time domain by photoconductive and electro-optic sampling with subpicosecond resolution. Excellent agreement is found between experiment and theoretical data obtained by two-dimensional self-consistent Monte Carlo calculations.
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  • 8
    Electronic Resource
    Electronic Resource
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 58 (1991), S. 1881-1883 
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: We investigate the dependence of carrier lifetimes in radiation-damaged, GaAs on proton implantation dose by means of time-resolved reflectivity and photoconductivity experiments with subpicosecond resolution. The carrier lifetimes decrease with increasing implantation dose at low implantation levels whereas beyond the "amorphization dose'' a saturation at 0.5 ps can be observed due to a saturation of the defect density.
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  • 9
    Electronic Resource
    Electronic Resource
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 63 (1993), S. 2780-2782 
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The photocurrent response of a GaAs metal-semiconductor-metal (MSM) photodetector was measured after excitation with two femtosecond pulses having a variable delay Δt of 0 ps≤Δt≤100 ps. At low excitation densities the influence of the first pulse on the pulse shape of the second is negligible for Δt≥20 ps. This corresponds to a resolvable pulse train of 50 GHz repetition rate for the detectors used in our experiments. The influence of space charge effects at higher excitation density and/or lower bias could be shown.
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  • 10
    Electronic Resource
    Electronic Resource
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 58 (1991), S. 2503-2505 
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The response of GaAs metal-semiconductor-metal (MSM) photodiodes at low temperatures has been investigated in the time domain by photoconductive sampling. The dependence of the response time on temperature for T(approximately-greater-than)50 K can be described by phonon mediated intervalley and intravalley scattering. The pulse width drops from 10.8 ps at 300 K to 5.6 ps at 70 K and then grows rapidly with decreasing temperature below 50 K. These results demonstrate that the response is limited by the electron/hole transport in the semiconductor rather than by external capacitances or inductances.
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