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  • American Institute of Physics (AIP)  (9)
  • Cambridge University Press  (2)
  • 1990-1994  (11)
  • 1993  (11)
  • 1
    Electronic Resource
    Electronic Resource
    Ambipolar diffusion and carrier lifetime measurements in all-binary (InAs)2(GaAs)5 strained quantum wells grown on GaAs (1993)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 74 (1993), S. 1868-1873 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The in-well ambipolar diffusion coefficients and carrier lifetimes in ordered all-binary quantum wells, composed of (InAs)2(GaAs)5 short-period strained-layer superlattices (SPSLSs) grown on [001]-oriented GaAs substrates, are measured using picosecond optically induced transient grating and pump-probe techniques. Quantum wells containing SPSLSs may be expected to exhibit higher in-plane hole mobilities compared to InGaAs ternary alloy quantum wells because of a larger average indium content and reduced disorder scattering in the SPSLS structures. The results obtained in the SPSLSs are compared to those obtained in InGaAs alloy quantum wells of comparable well width and confinement energies. This indicates that, for a given SPSLS and its equivalent alloy, the ambipolar diffusion coefficients are comparable while the carrier lifetimes in the SPSLSs are longer. The longer carrier lifetimes in the SPSLSs suggest that these binary structures possess fewer nonradiative recombination centers than the alloys. The absence of a dramatic improvement in the transport properties, however, may indicate that imperfect interfaces in the SPSLSs may be offsetting their possible advantages, in spite of the fact that optical measurements in the SPSLSs indicate high quality and x-ray and transmission electron microscopic measurements demonstrate the binary nature of these structures.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.355301
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  • 2
    Electronic Resource
    Electronic Resource
    Magnitude, origin, and evolution of piezoelectric optical nonlinearities in strained [111]B InGaAs/GaAs quantum wells (1993)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 73 (1993), S. 7767-7774 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The magnitudes of excitonic nonlinearities were compared at 12 K in InGaAs/GaAs multiple quantum well structures with growth directions oriented along the [100] and [111] crystal axes by measuring both the steady-state and time-resolved differential transmission spectra. As expected, the spectra for the [100] sample are indicative of excitonic bleaching at all times and for all excitation levels, and a carrier recombination time of 0.8 ns and a nonlinear cross section (change in absorption coefficient per carrier pair) of ∼8×10−14 cm2 are extracted for the [100] sample. By comparison, for low excitation levels, the spectra for the [111] sample are consistent with a blueshift of the exciton, indicating a screening of the strain-induced piezoelectric field. At higher excitation levels, the spectra are dominated by excitonic bleaching. Under identical 1 ps pulsed excitation conditions, the magnitudes of the changes in the absorption coefficient caused by screening in the [111] sample are comparable to those measured for bleaching in the [100] sample. By contrast, the steady-state changes in the absorption coefficient caused by screening in the [111] sample are an order of magnitude larger than the changes caused by bleaching in the [100] sample. It was demonstrated that the larger steady-state response for the [111] sample is caused by carrier accumulation over the longer (density-dependent) lifetime for that sample and that it is not the result of a larger nonlinear cross section. The slow, nonexponential, density-dependent recombination rates measured for the [111] sample are consistent with carrier escape and drift to screen the entire multiple quantum well structure and are not consistent with screening within the individual quantum wells.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.353976
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  • 3
    Electronic Resource
    Electronic Resource
    Enhanced ambipolar in-plane transport in an InAs/GaAs hetero-n-i-p-i (1993)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 73 (1993), S. 3860-3866 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: In-plane transport in an InAs/GaAs semiconductor hetero-n-i-p-i has been investigated using picosecond transient grating techniques and an order-of-magnitude enhancement of the ambipolar transport relative to that measured in a similar undoped sample has been demonstrated. Both the magnitude and the density dependence of this enhanced transport are consistent with an additional driving force that is associated with an in-plane modulation of the screened n-i-p-i field. This modulation is the result of the spatial separation by perpendicular transport of electrons and holes that also have an in-plane density modulation.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.352897
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  • 4
    Electronic Resource
    Electronic Resource
    Dwell time in doped double-barrier heterostructures (1993)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 74 (1993), S. 1855-1861 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: A time-dependent Schrödinger equation has been solved numerically for a double-barrier and a quantum-well resonant tunnelling structure. Special emphasis has been paid to the system where barriers are doped specially by negative delta-function potentials (δ potentials) which broaden the widths of the resonances and in turn decrease the dwell times. The strengths of the delta functions could be such that they may form bound states in the barrier regions, but the states bound to δ potentials are very shallow. Delta-function potentials are replaced by equivalent barriers of different heights and widths which are easy to incorporate into the numerical calculation of the propagation of the wave packet, and the corresponding physical structures can be conveniently fabricated. It is found that for a certain strength of the δ potential or parametric value of the equivalent barriers in the barriers of the resonant tunneling structure, there are three resonance states very close together. The square of the wave functions trapped in the well region for the states oscillates in time for a broad wave packet in k space, whereas the wave function trapped in the whole structure decays exponentially. The oscillating part has a resemblance with the quantum beats. There are no oscillations for a narrow wave packet in k space.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.354793
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  • 5
    Electronic Resource
    Electronic Resource
    Theoretical study of small Ar*n clusters in low-lying electronic states (1993)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 98 (1993), S. 6406-6416 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Rydberg excited rare gas atom clusters are treated by the diatomics-in-ionic-systems (DIIS) method and by a simple model approach. According to the results of the calculations, metastable clusters Ar*n (n≥2) are not formed, most probably, from the lowest excited state (4s), but can be formed from higher excited states such as 4p. DIIS calculations of small Arn van der Waals clusters provide the excitation energy dependence on the number n of atoms. The model calculation shows that in the Ar13 van der Waals clusters the excited electron is pushed away from the inner cluster region to the outside region.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.464805
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  • 6
    Electronic Resource
    Electronic Resource
    The effect of surface passivation on rotationally inelastic scattering: N2 scattered from W(110), W(110)–(2×2)N, W(110)–(1×1)H, and Pt(111) (1993)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 99 (1993), S. 7076-7089 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The effects of surface reactivity on rotationally inelastic scattering has been examined by comparing the scattering of N2 from the reactive W(110) surface and from the passivated N and H reconstructions, W(110)–(2×2)N and W(110)–(1×1)H, as well as the nonreactive Pt(111) surface. The translational energy, rotational state population distributions, and angular momentum alignment of N2 scattered from these surfaces have been measured as a function of incident energy Ei, incident angle θi, and exit angle θf. The trends in the final translational and rotational energy correlate well with the average atomic mass of each surface. However, some of the greater rotational excitation for N2 scattered from W(110) compared to W(110)–(1×1)H can be attributed to a more anisotropic potential with respect to initial molecular orientation for N2–W(110) compared to N2–W(110)–(1×1)H or N2–Pt(111). The rotational distributions indicate that parallel momentum is not conserved for N2 scattered from W(110) or from W(110)–(2×2)N, but that parallel momentum is partially conserved for N2 scattered from W(110)–(1×1)H. Furthermore, measurements of angular momentum alignment show that significant in-plane forces are present, and that these forces are due to the repulsive part of the N2–surface potential of each system. The results are consistent with the N2–W(110) potential being highly corrugated with respect to both initial molecular orientation and impact parameter and with this corrugation being reduced dramatically by passivating the surface with the adsorption of hydrogen.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.465426
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  • 7
    Electronic Resource
    Electronic Resource
    A finite-time variational method for determining optimal paths and obtaining bounds on free energy changes from computer simulations (1993)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 99 (1993), S. 6856-6864 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: This paper elaborates on the method introduced by Reinhardt and Hunter [J. Chem. Phys. 97, 1599 (1992)] for determining upper and lower bounds on free energy changes and systematically improving these bounds by variational optimization of the Hamiltonian switching path connecting the two states of interest. The advantages, conditions of applicability, and potential pitfalls of this finite-time variational method are illustrated in detail in a Monte Carlo calculation of the free energy change accompanying the reversal of magnetic field in a two-dimensional Ising model. The power of its flexibility in an inhomogeneous system is demonstrated by computing the boundary tension between domains of positive and negative magnetization in the Ising model. Its success in a problem involving molecular dynamics and continuous potentials is illustrated by computing the free energy difference between a Lennard-Jones fluid and a soft-sphere fluid. Finally, its connection with the method of dynamically modified slow growth is discussed.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.465830
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  • 8
    Electronic Resource
    Electronic Resource
    Integration of low-temperature GaAs on Si substrates (1993)
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 62 (1993), S. 255-257 
    Details
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: GaAs was grown on Si substrates by molecular-beam epitaxy at low substrate temperatures. Femtosecond time-resolved reflectivity investigations revealed a significant reduction of carrier lifetime in GaAs epilayers from 14 ps to 〈0.5 ps as the growth temperature was reduced from 400 to 150 °C. Photoconductors were fabricated on the epilayers, and electro-optic sampling was used to confirm the electrical response times of 〈1 ps. The responsivity is comparable to that for low-substrate-temperature GaAs grown on GaAs substrates, with a photogenerated carrier mobility of ∼100–300 cm2/V s. The low growth temperatures allow ultrafast GaAs-based photodetector incorporation into Si-based integrated circuits for novel optoelectronic applications.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.109620
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  • 9
    Electronic Resource
    Electronic Resource
    Nonlinear wave coupling and subharmonic resonance in planar jet shear layer transition (1993)
    New York, NY : American Institute of Physics (AIP)
    Physics of Fluids 5 (1993), S. 630-646 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: An experimental investigation of the nonlinear spectral development associated with the transition of an initially laminar planar jet shear layer is presented. The local shear layer spectral dynamics are modeled as a nonlinear system containing both linear and quadratically nonlinear system elements. A novel digital signal processing technique is applied which allows the linear and quadratically nonlinear wave coupling coefficients that characterize the local spectral dynamics to be estimated directly from time-series velocity fluctuation data. The method utilized is noniterative and explicitly includes fourth-order spectral moments. From the linear coupling coefficient, the local spatial growth rate and dispersion relation may be obtained. Of particular interest in the work reported is the associated nonlinear wave coupling coefficient which provides a measure of the efficiency of local nonlinear three-wave coupling. These show that the jet shear layer exhibits a strong preference for difference mode interactions. With the estimation of the related nonlinear power transfer function, the total, linear, and quadratic nonlinear spectral energy transfer may be locally estimated. When these measures are used in conjunction with the local quadratic bicoherency and local linear–quadratic coupling bicoherency, the local nonlinear spectral dynamics of the flow may be completely and unambiguously quantified. Particular emphasis is placed upon discerning the mechanism which gives rise to subharmonic resonance with the fundamental. Results show that the amplification of the subharmonic occurs via a parametric resonance with the fundamental rather than by direct quadratic difference interaction. That is, the subharmonic is boosted by a weakly nonlinear mechanism involving a linear growth rate modification due to a resonant phase lock with the fundamental instability.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.858649
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  • 10
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    Flow generated by a small oscillating cylinder in a mixing layer (1993)
    Cambridge University Press
    Details
    Publication Date: 1993-03-01
    Description: We study the flow generated by a small circular cylinder in a mixing layer. The cylinder is executing an oscillatory translation whose frequency is within the range of unstable frequencies of the shear layer. The smallness of the cylinder is measured by the ratio of its radius to the characteristic thickness of the layer. This (small) ratio serves as the expansion parameter for our theory; the flow naturally divides into inner and outer regions. The former is in the immediate vicinity of the cylinder and the latter is the far field which contains the instability waves. The solution to this problem is obtained by the method of matched asymptotic expansion. One objective is to study the dependence of this solution on various parameters such as the frequency of oscillation, velocity ratio, etc., and thus shed light on the associated receptivity, Other objectives deal with a restatement of causality and with the hydrodynamic field near the streamwise location of the cylinder. We find that receptivity is a strong function of frequency and velocity ratio and that the local hydrodynamic field may be quite large. Causality is restated in terms of the well-known exponential integral. © 1993, Cambridge University Press. All rights reserved.
    Print ISSN: 0022-1120
    Electronic ISSN: 1469-7645
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics , Physics
    Published by Cambridge University Press
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