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  • 1
    Electronic Resource
    Electronic Resource
    Determination of roughness correlations in multilayer films for x-ray mirrors (1991)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 69 (1991), S. 1411-1424 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Interfacial roughness in multilayer films may be random or correlated, i.e., replicated from layer to layer. It is shown that these can be separated and quantified using x-ray diffraction rocking curves and a straightforward analysis. The lateral correlation length along the interfaces can additionally be determined. A quantitative evaluation for W/C multilayers shows that correlated roughness contributes significantly to the total roughness, even at length scales that are surprisingly short, of the order 2–6 nm.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.347281
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  • 2
    Electronic Resource
    Electronic Resource
    Interstitial and substitutional Zn in InP and InGaAsP (1989)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 65 (1989), S. 553-560 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Closed-ampoule Zn diffusion in InP results in a net acceptor concentration that is much smaller than the Zn concentration. After subsequent annealing of InP in an atmosphere without Zn, the Zn and net acceptor concentrations have become almost identical, due to a decreased Zn concentration and an increased net acceptor concentration. The annealing treatment gives rise to a decreased contact resistivity. The diffusion depth has not changed. Annealing with a SiN cap on the InP surface does not have this effect on the concentrations. These annealing effects also take place in InGaAsP on InP layers. The results can be explained quantitatively by assuming that Zn is incorporated as both substitutional acceptors and interstitial donors and that only the interstitial Zn is driven out by the annealing, owing to its large diffusion coefficient. Profiles calculated with this interstitial-substitutional model can be fitted to experimental profiles assuming Zn to diffuse as singly ionized interstitial donors. This model can also describe earlier reported results on Zn diffusion in n-type InP for which a profile cutoff is found at a depth where the acceptor concentration equals the background donor concentration and in which the acceptor solubility is higher than in undoped InP.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.343140
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  • 3
    Electronic Resource
    Electronic Resource
    Measurement and calculation of spontaneous recombination current and optical gain in GaAs-AlGaAs quantum-well structures (1991)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 70 (1991), S. 1144-1156 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Experimental determinations have been made of the peak optical gain as a function of spontaneous recombination current density for GaAs quantum wells of width 25 and 58 A(ring) bounded by AlGaAs barriers. These data were obtained from measurements of spontaneous emission spectra, observed through narrow windows in the 50-μm-wide contact stripes of oxide isolated lasers, using only a single reference value of the optical absorption coefficient above the band edge to calibrate the measurements in absolute units. These results are in good agreement with gain-current curves calculated using a model which includes unintentional monolayer well width fluctuations, band-gap narrowing and intraband carrier-carrier scattering. The characteristic intraband scattering time is calculated from first principles as a function of electron energy and carrier density on the basis of a 2-dimensional Auger-type process. This lifetime gives a much better representation of our observed spontaneous spectra than a lifetime which is simply dependent upon carrier density. The comparison between experiment and model calculation involves no adjustable parameters. For the 58-A(ring)-wide wells there is a difference between the experimental and calculated gain-current curves at low values of gain. We show that this is a consequence of applying the Einstein relations to a broadened spectrum in the process of deriving the gain from the observed spontaneous emission spectrum. A direct comparison of the shapes of experimental and calculated spontaneous emission spectra at several injection levels provides a more rigorous, yet equally valid, verification of the computer model.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.349622
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  • 4
    Electronic Resource
    Electronic Resource
    The dynamics of shock accelerated light and heavy gas cylinders (1993)
    New York, NY : American Institute of Physics (AIP)
    Physics of Fluids 5 (1993), S. 2239-2247 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Experiments have been carried out in which a cylindrical volume of a gas, that is either lighter or heavier than its surroundings, is impulsively accelerated by a weak shock wave. Laminar jets of helium or sulphur hexafluoride (SF6) are used to produce the cylinders, and planar laser-induced fluorescence is used to visualize the flow. It is found that the vorticity deposited on the boundary of the SF6 cylinder by the interaction with the shock wave, separates from the heavy gas to form a pair of vortices, which subsequently wrap the SF6 around them. This process is quite different from what is observed in the light gas experiments, which showed a small amount of helium to remain with the vorticity, eventually becoming part of the vortex cores. Centrifugal forces combined with differences in the rates of the diffusion of vorticity in the two gases are given as possible reasons for these differences. Measurement of the initial downstream velocity for a heavy gas cylinder is found to agree well with a theory based on two simple models. But, because diffusion causes the light gas jet density to be significantly greater than that of pure helium, the theory overpredicts the measured velocity of the light gas experiments. The final translational velocities for both light and heavy gas experiments are not accurately predicted by the model, and measurements of the vortex spacing are found to be significantly larger than those indicated by this theory. These differences are likely caused by the theory's inability to accurately describe the viscous nonuniform flow.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.858562
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  • 5
    Electronic Resource
    Electronic Resource
    A round turbulent jet produced by an oscillating diaphragm (1996)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Fluids 8 (1996), S. 2484-2495 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: A round turbulent water jet produced normal to, and at the center of a submerged, resonantly driven diaphragm is investigated experimentally. The jet which is formed without mass injection and is comprised entirely of radially entrained fluid, is present only when the excitation amplitude exceeds a given threshold. Above this excitation level, a small cluster of cavitation bubbles appears near the center of the diaphragm. The bubbles grow, apparently collapse, and then disappear during each oscillation cycle. It is conjectured that the jet is synthesized by time-periodic coalescence of vortex rings that are produced by secondary flow around the bubbles or by the collapse of the bubbles. It is remarkable that even though the jet results from a strong time-periodic excitation and its time-periodic features are detected throughout the present range of measurements, the time-averaged jet structure is similar to that of a conventional turbulent round jet in that the increase in its width and in the inverse of its centerline velocity are both linear functions of the distance from the actuator. In contrast to conventional jets, the present synthetic jets can be manipulated on relatively short time scales that are comparable to the excitation period. © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.869040
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  • 6
    Electronic Resource
    Electronic Resource
    Experimental study of incompressible Richtmyer–Meshkov instability (1996)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Fluids 8 (1996), S. 405-415 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The Richtmyer–Meshkov instability of a two-liquid system is investigated experimentally. These experiments utilize a novel technique that circumvents many of the experimental difficulties that have previously limited the study of Richtmyer–Meshkov instability. The instability is generated by vertically accelerating a tank containing two stratified liquids by bouncing it off of a fixed coil spring. A controlled two-dimensional sinusoidal initial shape is given to the interface by oscillating the container in the horizontal direction to produce standing waves. The motion of the interface is recorded during the experiments using standard video photography. Instability growth rates are measured and compared with existing linear theory. Disagreement between measured growth rates and the theory are accredited to the finite bounce length. When the linear stability theory is modified to account for an acceleration pulse of finite duration, much better agreement is attained. Late time growth curves of many different experiments seem to collapse to a single curve when correlated with the circulation deposited by the impulsive acceleration. A theory based on modeling the late time evolution of the instability using a row of vortices is developed. The growth curve given by this model has similar shape to those measured, but underestimates the late-time growth rate. © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.868794
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  • 7
    Electronic Resource
    Electronic Resource
    Experimental study of Rayleigh–Taylor instability: Low Atwood number liquid systems with single-mode initial perturbations (2001)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Fluids 13 (2001), S. 1263-1273 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Single-mode Rayleigh–Taylor instability is experimentally studied in low Atwood number fluid systems. The fluids are contained in a tank that travels vertically on a linear rail system. A single-mode initial perturbation is given to the initially stably stratified interface by gently oscillating the tank in the horizontal direction to form standing internal waves. A weight and pulley system is used to accelerate the fluids downward in excess of the earth's gravitational acceleration. Weight ranging from 90 to 450 kg produces body forces acting upward on the fluid system equivalent to those produced by a gravitational force of 0.33–1.35 times the earth's gravity. Two fluid combinations are investigated: A miscible system consisting of a salt water solution and a water–alcohol solution; and an immiscible system consisting of a salt solution and heptane to which surfactant has been added to reduce the interfacial tension. The instability is visualized using planar laser-induced fluorescence and is recorded using a video camera that travels with the fluid system. The growth in amplitude of the instability is determined from the digital images and the body forces on the fluid system are measured using accelerometers mounted on the tank. Measurements of the initial growth rate are found to agree well with linear stability theory. The average of the late-time bubble and spike velocities is observed to be constant and described by Uave=0.22(πAG/k(1+A)+πAG/k(1−A)), where A is the Atwood number, k is the wave number, and G is the apparent gravity of the fluid system (i.e., the fluid system acceleration minus the earth's gravity). © 2001 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1359762
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  • 8
    Electronic Resource
    Electronic Resource
    A membraneless experiment for the study of Richtmyer–Meshkov instability of a shock-accelerated gas interface (1997)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Fluids 9 (1997), S. 3078-3085 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Previous Richtmyer–Meshkov instability experiments carried out in shock tubes have been hampered by the need to separate the two gases with a thin plastic membrane. As a result, many of these experiments have had poor agreement with the linear stability theory of Richtmyer [Commun. Pure Appl. Math. 23, 297 (1960)]. This limitation has been removed in the present investigation by the use of a novel technique in which the interface is formed by flowing light (N2) and heavy (SF6) gases from opposite ends of a vertical shock tube. Both gases exit the shock tube through slots in the test section walls leaving behind a flat motionless interface which is then given a sinusoidal initial shape by gently oscillating the shock tube at a prescribed frequency in the horizontal direction. A weak shock wave (Ms=1.10), generated in the shock tube, impacts the interface and produces the instability. Photographs of the interface, which is visualized by seeding the heavy gas with a water droplet fog and illuminating it with a strobe light source, provide particularly clear views of the developing instability far into the nonlinear regime. In addition, amplitude measurements obtained from these photographs are found to be in good agreement with Richtmyer's theory. © 1997 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.869416
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  • 9
    Electronic Resource
    Electronic Resource
    Modeling of steady-state field distributions in blocked impurity band detectors (2000)
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 77 (2000), S. 4389-4391 
    Details
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Steady-state electric field and carrier distributions are calculated for blocked impurity band (BIB) detectors in the absence of external illumination. The results illustrate the role of space charge in the blocking layer. Calculations are presented for Ge:Ga, though the input is easily modified for other materials. The numerical model allows for observation of the effect of spatial doping variations and interface gradients. The BIB field distribution is highly dependent on the purity and compensation in the blocking layer, as well as interface sharpness. In some cases, space charge effects can cause a collapse of the field at the blocker/absorber interface or the contact and resultant low field regions in the blocking layer. © 2000 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1336558
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  • 10
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    Silencing of  -1,3-glucanase genes in tobacco correlates with an increased abundance of RNA degradation intermediates (1998)
    Oxford University Press
    Details
    Publication Date: 1998-11-01
    Print ISSN: 0305-1048
    Electronic ISSN: 1362-4962
    Topics: Biology
    Published by Oxford University Press
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