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  • 1
    Electronic Resource
    Electronic Resource
    Coherent and dissipative wave packet dynamics in cyclic model systems with four equivalent potential minima (1994)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 100 (1994), S. 9007-9017 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The dynamics of cyclic systems with four equivalent potential minima is studied here from two different points of view. The solution of the time-dependent Schrödinger equation provides insight into the coherent wave packet motion. The resulting reaction mechanism involves relocalization between opposite, not neighboring potential minima. The inclusion of an environment within a density matrix description leads to dissipation and therefore to a transition from coherent to incoherent dynamics. The theoretical considerations are applied to a simple model of the cyclic motion of a proton in a molecular framework.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.466705
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  • 2
    Electronic Resource
    Electronic Resource
    Properties of the two-dimensional electron gas in modulation-doped GaInAs(P)/InP structures grown by low-pressure metalorganic vapor-phase epitaxy (1989)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 66 (1989), S. 697-703 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: This paper is concerned with the characteristics of the two-dimensional electron gas (2DEG) at modulation-doped GaInAs/InP and GaInAsP/InP heterojunctions and GaInAs/InP single-quantum-well structures grown by low-pressure metalorganic vapor-phase epitaxy (LP-MOVPE) at 20 hPa. Using trimethyl-In, trimethyl-Ga, AsH3, and PH3 as source materials, this process permits the deposition of very uniform films with abrupt interfaces and doping profiles. We give the first detailed analysis of modulation-doped structures by classical Hall effect, Shubnikov–de Haas, and quantum Hall-effect measurements. Study of the scattering mechanisms of the 2DEG in modulation-doped GaInAs/InP and GaInAsP/InP structures reveals a small interface roughness and a low impurity content at the interfaces. Electron mobilities at 4.2 K up to 189 000 cm2/V s for a GaInAs/InP and 72 000 cm2/V s for a GaInAsP/InP heterojunction as well as values as high as 170 000 cm2/V s for a GaInAs/InP quantum well structure demonstrate the excellent control of the growth process and the quality of the material attainable by LP-MOVPE.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.343540
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  • 3
    Electronic Resource
    Electronic Resource
    Schottky barrier height enhancement on n-In0.53Ga0.47As (1992)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 72 (1992), S. 2347-2355 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Schottky barrier height enhancement on n-InGaAs is studied on structures with thin surface layers of different compositions. Counter-doped p+-InGaAs layers, as well as layers of n- and p-InP, n-GaAs, and n-InGaP of different thicknesses and dopant densities, respectively, were used to enhance the barrier. Titanium was used as a barrier metal to prepare Schottky diodes of different areas and the barrier height is analyzed by current-voltage measurements. It is observed that the barrier height enhancement by p+-InGaAs layers increases with the layer thickness and dopant density, respectively, and effective barrier heights up to 0.63–0.68 eV, i.e., higher values than previously reported, have been measured. The barrier height enhancement by counter-doped p+-InGaAs layers on n-InGaAs can be described by the two-carrier model. Schottky diodes with extremely low reverse current densities have been prepared, JR(1 V) =4.5×10−6 A/cm2. It is shown that lattice-matched InP surface layers can be used as an alternative to enhance the barrier height on n-InGaAs. The barrier height increases with the layer thickness up to φB=0.53–0.55 eV, i.e., up to values previously reported as barrier heights on thick n-InP. Additional barrier enhancement can be achieved by counter doping of the InP surface layer and barrier heights of 0.66 eV have been obtained by p-InP surface layers on n-InGaAs. On structures with barrier-enhanced n-GaAs layers, a remarkable decrease of the reverse current density is observed if the layer thickness is reduced to the critical layer thickness, but the barrier height is very low due to the small n-GaAs thickness. For structures with slightly lattice-mismatched n-InGaP layers (xGaP=0.11) measured barrier heights are similar to those for n-InP enhancement layers of the same thicknesses.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.351576
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  • 4
    Electronic Resource
    Electronic Resource
    Optimization of modulation-doped Ga1−xInxAs/InP heterostructures towards extremely high mobilities (1993)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 73 (1993), S. 4489-4493 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: This paper presents a study of the electrical and structural properties of inverted modulation-doped GaInAs/InP heterostructures grown by low-pressure metalorganic vapor phase epitaxy. First, the thickness of the GaInAs layer was optimized in lattice-matched samples to find the smallest thickness in which high Hall mobility is observed. Next, in a section closest to the InP the In content was varied. A steady increase of mobility with indium composition was observed. A maximum of 450 000 and 15 500 cm2/V s was obtained for a 10-nm-thick Ga1−xInxAs layer with x=0.77 at 6 and 300 K, respectively. Channels with higher indium content exceed the critical thickness and mobility drops off sharply. The decreasing mobility correlates with the formation of misfit dislocations at the interface indicating increasing scattering processes of the GaInAs layer.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.352789
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  • 5
    Electronic Resource
    Electronic Resource
    Hydrogen transfer in double minimum potential: Kinetic properties derived from quantum dynamics (1987)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 86 (1987), S. 784-801 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The interaction of hydrogen transfer in a double minimum potential with a condensed phase environment is studied. For a symmetric double minimum system, the tunneling motion in the vibrational ground state is retarded efficiently by fluctuation as well as by rearrangement of the lattice consisting of harmonic oscillators. Environmental displacements with large inertia cause dynamic asymmetry by failing to cooperate with the transfer motion and favor a thermally activated process even at low temperatures. To describe such processes, an effective Hamiltonian is derived, which consists of a leading term referring to a one-dimensional transfer motion along an asymmetric potential profile and of a random perturbation term linear in the transfer coordinate. The power spectral density is derived for the perturbation given as a superposition of the time-dependent quantum mechanical expectation values of the vibrational displacements in the environment. A master equation treatment is proposed to describe the kinetic properties and is applied to a model for benzoic acid dimers in the crystalline state. The model reproduces the full temperature dependence of the observed NMR T1 data for (C6H5COOH)2 and (C6H5COOD)2 with plausible parameters and relates the temperature-dependent apparent activation energy to the energy level scheme of the transfer motion.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.452281
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  • 6
    Electronic Resource
    Electronic Resource
    Brillouin light scattering from nematic polymer solutions (1992)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 97 (1992), S. 568-571 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A multipass tandem Fabry–Perot interferometer was used to measure the Brillouin light scattering in nematic polymer solutions of poly-γ-benzyl glutamate (PBG). The longitudinal acoustic phonon frequency and linewidth were determined as a function of the angle between the phonon wave vector and the nematic director. We have observed an anisotropy of 22% in sound velocity and 40% in the Brillouin linewidth in PBG nematic solutions. Our analysis indicates that elastic relaxation due to polymer concentration fluctuations is important. However, the proposed second sound wave could not be determined due to the experimental limit of the present technique.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.463551
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  • 7
    Electronic Resource
    Electronic Resource
    Hydrogen bond dynamics in isotopically substituted benzoic acid dimers (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 93 (1990), S. 1502-1520 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The hydrogen pair transfer in the hydrogen-bonded dimers of benzoic acid and its carboxyl-deuterated species is investigated in the solid. Measurements are reported for the temperature-dependent NMR relaxation time T1 in single crystals containing dimers with one or two carboxylic deuterons. Combined with previous data, the temperature dependence of the measurements is analyzed in terms of a master-equation description for a one- or two-dimensional quantum-mechanical model of the transfer motion. The description by a one-dimensional model is found to be inadequate as it yields unrealistic isotope effects in the model parameters. On the other hand, reasonable results are obtained for a two-dimensional model which includes, apart from the transfer motion of the hydrogens, a heavy atom mode with properties suggested by x-ray structural data. This model explains the thermal activation of the transfer process mainly as a result of promotion of tunneling by heavy atom rocking. Activation energies remain considerably smaller than the barrier height and indicate predominance of tunneling even at room temperature.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.459129
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  • 8
    Electronic Resource
    Electronic Resource
    Transitions induced in a double minimum system by interaction with a quantum mechanical heat bath (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 93 (1990), S. 5518-5532 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A quantum mechanical treatment of a double minimum system interacting with a heat bath is presented for the purpose of interpreting experimental data on transfer kinetics in condensed hydrogen-bonded systems. The model describes the transfer motion in one or two dimensions. The heat bath is represented by a set of harmonic oscillators and the interaction by a term linear in the system coordinates and in the bath coordinates. Extending an earlier random field approach, the present treatment consistently accounts for the quantum nature of the total system. With crystalline benzoic acid dimer used as an example, the master equation for the populations of the energy levels of the hydrogen transfer motion is derived. Transition probabilities consistent with the principle of detailed balance are obtained, based on a representation with explicit off-diagonal tunnel interactions for pairs of states localized on different sides of the barrier and with diagonal terms describing the rearrangement of the heat bath as a consequence of the tunneling motion. The activation of the double minimum transfer process with increasing temperature is related to the excitation of the local vibrations in the two potential wells.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.459621
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  • 9
    Electronic Resource
    Electronic Resource
    Microwave and millimeter-wave spectra, electric dipole moment, and internal rotation effects of methyl hydroperoxide (1992)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 97 (1992), S. 6168-6180 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The microwave spectrum of methyl hydroperoxide which has been investigated in the range of 12–307 GHz is dominated by the internal rotation of the OH group. The molecule exists in a gauche equilibrium configuration. Pure rotational transitions as well as much stronger torsional–rotational transitions have been assigned. A model of the overall rotation for a molecule with two strongly coupled tunneling states has been used for the analysis of all assigned transitions. Quantum-mechanical tunneling between the two equivalent gauche minima splits the ground torsional state by 448.7603(1) GHz. Additional small splittings of the microwave transitions have been attributed to the internal rotation of the CH3 group with a threefold barrier of 1120(10) cm−1. The permanent electric dipole components have been estimated from Stark splittings. Ab initio calculations of the molecular structure and of the potential functions for internal rotations of the OH and CH3 groups complement the experimental study.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.463725
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  • 10
    Electronic Resource
    Electronic Resource
    Quasiresonance of long life (1986)
    College Park, Md. : American Institute of Physics (AIP)
    Journal of Mathematical Physics 27 (1986), S. 238-248 
    Details
    ISSN: 1089-7658
    Source: AIP Digital Archive
    Topics: Mathematics , Physics
    Notes: Schrödinger's equation for two-body scattering is analyzed in the tunneling range of energy for arbitrary, analytic potentials with one barrier. An exact scattering representation is given which furnishes a rigorous and efficient basis for the determination of all the eigenvalues of very long life and the associated linewidths and resonant responses. The approach is direct and simple enough to avoid computation or estimation of wave functions and asymptotic or perturbation expansions.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.527368
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