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  • 1
    Digitale Medien
    Digitale Medien
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 112 (2000), S. 4326-4333 
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: We introduce a potential model for MgCO3 and CdCO3 which reproduces experimental structural parameters and solution energies. We employed atomistic simulation techniques to model the absorption and segregation of magnesium and cadmium ions to the low-index surfaces of calcite. Both magnesium and cadmium ions absorb at the surfaces from solution. The absorption energies are surface dependent, due to distinct relaxations of the different surfaces, and ion dependent with the smaller sized magnesium ion showing larger relaxations. The absorption energies are larger for magnesium than for cadmium, mainly due to the closer coordination of the water molecules to the magnesium substituted surfaces. The cadmium ions segregate to the bulk crystal more easily than the magnesium ions which prefer to remain at the surface. However, segregation energies for both cations in second and further layers of some surfaces are positive, indicating that once a calcium carbonate layer has overgrown the substituted surface layer, segregation to the bulk is energetically possible. © 2000 American Institute of Physics.
    Materialart: Digitale Medien
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  • 2
    Digitale Medien
    Digitale Medien
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 58 (1985), S. 3150-3153 
    ISSN: 1089-7550
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: Anomalous Hall effects, i.e., peaks in the temperature dependence of the Hall coefficient and Hall mobility, have been observed in liquid-phase-epitaxy (LPE) Hg0.8 Cd0.2Te films. After establishing that surface effects are not the cause of the anomalies and that our LPE films are indeed n type, we propose an inhomogeneity model with a network of extended p-type inclusions in an n-type matrix. Good fits to the experimental data have been obtained by computer simulation based on this model.
    Materialart: Digitale Medien
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  • 3
    Digitale Medien
    Digitale Medien
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 74 (1993), S. 5874-5876 
    ISSN: 1089-7550
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: It is shown that the forward voltage of heterostructure transistors can be modified by varying the thickness of the undoped GaAs layer beneath the gate or by low energy ion implantation of a surface layer. This suggests that the effective barrier height can be controlled by either technique.
    Materialart: Digitale Medien
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  • 4
    ISSN: 1089-7674
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: The predictions of gyrokinetic and gyrofluid simulations of ion-temperature-gradient (ITG) instability and turbulence in tokamak plasmas as well as some tokamak plasma thermal transport models, which have been widely used for predicting the performance of the proposed International Thermonuclear Experimental Reactor (ITER) tokamak [Plasma Physics and Controlled Nuclear Fusion Research, 1996 (International Atomic Energy Agency, Vienna, 1997), Vol. 1, p. 3], are compared. These comparisons provide information on effects of differences in the physics content of the various models and on the fusion-relevant figures of merit of plasma performance predicted by the models. Many of the comparisons are undertaken for a simplified plasma model and geometry which is an idealization of the plasma conditions and geometry in a Doublet III-D [Plasma Physics and Controlled Nuclear Fusion Research, 1986 (International Atomic Energy Agency, Vienna, 1987), Vol. 1, p. 159] high confinement (H-mode) experiment. Most of the models show good agreements in their predictions and assumptions for the linear growth rates and frequencies. There are some differences associated with different equilibria. However, there are significant differences in the transport levels between the models. The causes of some of the differences are examined in some detail, with particular attention to numerical convergence in the turbulence simulations (with respect to simulation mesh size, system size and, for particle-based simulations, the particle number). The implications for predictions of fusion plasma performance are also discussed. © 2000 American Institute of Physics.
    Materialart: Digitale Medien
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  • 5
    Digitale Medien
    Digitale Medien
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 9 (2002), S. 3440-3448 
    ISSN: 1089-7674
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: A comparison of the Columbia Linear Machine experiment [A. K. Sen, J. Chen, and M. Mauel, Phys. Rev. Lett. 66, 429 (1991)] with three-dimensional nonlinear electrostatic gyrokinetic simulation results is given, as well as comparisons with linear theory and simple theoretical predictions of nonlinear saturation. Linear theory is in reasonable agreement with the both experimental and simulation results. An analysis of the radial mode width which is localized by variation in the ion temperature gradient is given and compares well with both experiment and simulation. The theoretical estimate of the nonlinear saturation level is within a factor of 2 of what is observed in experiment and in simulation. In addition, both experiment and simulation show excitation of longer wavelength modes with poloidal mode numbers smaller than the linearly most unstable mode. © 2002 American Institute of Physics.
    Materialart: Digitale Medien
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  • 6
    Digitale Medien
    Digitale Medien
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 1 (1994), S. 1461-1468 
    ISSN: 1089-7674
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: The gyrokinetic and gyrofluid models show the most promise for large scale simulations of tokamak microturbulence. This paper discusses detailed comparisons of these two complementary approaches. Past comparisons with linear theory have been fairly good, therefore the emphasis here is on nonlinear comparisons. Simulations include simple two-dimensional slab test cases, turbulent three-dimensional slab cases, and toroidal cases, each modeling the nonlinear evolution of the ion temperature gradient instability. There is good agreement in both turbulent and coherent nonlinear slab comparisons in terms of the ion heat flux, both in magnitude and scaling with magnetic shear. However, the nonlinear saturation level for ||Φ|| in the slab comparisons shows differences of approximately 40%. Preliminary toroidal comparisons show agreement within 50%, in terms of ion heat flux and saturation level.
    Materialart: Digitale Medien
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  • 7
    Digitale Medien
    Digitale Medien
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 3 (1996), S. 1959-1966 
    ISSN: 1089-7674
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: Improvements in tokamak transport have recently been obtained in a variety of operational modes through the formation of transport barriers, or good confinement radial zones. Here global nonlinear three-dimensional toroidal gyrokinetic simulation is used to study three effects that are linearly stabilizing and may cause the formation of transport barriers, namely, sheared toroidal rotation, reversed magnetic shear, and peaked density profiles. The effect of toroidal shear flow on ion heat diffusivity is found to be relatively weak compared to mixing-length expectations based on linear calculations. In contrast, it is found that weak or negative magnetic shear (s〈1/2) in combination with a peaked density profile relative to the temperature profile greatly suppresses ion-temperature-gradient-driven turbulence in the central region of global nonlinear simulations. Similar features are seen experimentally in reversed magnetic shear tokamak plasmas. There is some nonlocal penetration (∼20–30ρi) of the turbulence into the subcritical core region. © 1996 American Institute of Physics.
    Materialart: Digitale Medien
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  • 8
    Digitale Medien
    Digitale Medien
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 2 (1995), S. 2231-2235 
    ISSN: 1089-7674
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: Progress toward a detailed understanding of the transport in full-volume gyrokinetic simulations of tokamaks is described. The transition between the two asymptotic regimes (large and small) of scaling of the heat flux with system size a/ρg reported earlier is explained, along with the approximate size at which the transition occurs. The larger systems have transport close to that predicted by the simple standard estimates for transport by drift-wave turbulence (viz., Bohm or gyro-Bohm) in scaling with a/ρg, temperature, magnetic field, ion mass, safety factor, and minor radius, but lying much closer to Bohm, which seems the result better supported theoretically. The characteristic downshift in the 〈kθ〉 spectrum observed previously in going from the linear to the turbulent phase is consistent with the numerically inferred coupling coefficients Mkpq of a reduced description of the system. An explanation of the downshift is given from the resemblance of the reduced system to the Hasegawa–Mima or Terry–Horton systems. These manifest an analogous downshift in slab geometry, and have Mkpq resembling those inferred from the gyrokinetic (GK) data. © 1995 American Institute of Physics.
    Materialart: Digitale Medien
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  • 9
    Digitale Medien
    Digitale Medien
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 2 (1995), S. 1217-1228 
    ISSN: 1089-7674
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: A comprehensive study of transport in full-volume gyrokinetic (GK) simulations of ion temperature gradient driven turbulence in core tokamak plasmas is presented. Though this "gyrokinetic tokamak'' is much simpler than experimental tokamaks, such simplicity is an asset, because a dependable nonlinear transport theory for such systems should be more attainable. Toward this end, two related lines of inquiry are pursued. (1) The scalings of GK tokamaks with respect to important system parameters are studied. In contrast to real machines, the scalings of larger GK systems (a/ρs(approximately-greater-than)64) with minor radius, with current, and with a/ρs are roughly consistent with the approximate theoretical expectations for electrostatic turbulent transport which exist as yet. Smaller systems manifest quite different scalings. (2) With the goal of developing a first-principles theory of GK transport, the GK data are used to infer the underlying transport physics. The data indicate that, of the many modes k present in the simulation, only a modest number (Nk∼10) of k dominate the transport, and for each, only a handful (Np∼5) of couplings to other modes p appear to be significant, implying that the essential transport physics may be described by a far simpler system than would have been expected on the basis of earlier nonlinear theory alone. Part of this analysis is the inference of the coupling coefficients Mkpq governing the nonlinear mode interactions, whose measurement from tokamak simulation data are presented here for the first time. © 1995 American Institute of Physics.
    Materialart: Digitale Medien
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  • 10
    ISSN: 1089-7550
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: In situ reflection high-energy electron diffraction and cross-sectional and plan-view transmission electron microscopy have been used to investigate the initial stages of InSb growth on GaAs(001), by molecular-beam epitaxy. Growth of the InSb commences with the formation of rectangular-based islands, having flat tops and sloping sides, with facets on certain planes of types {111} and {113}. The islands show near normal lattice spacings, with no significant straining. As deposition proceeds, islands coalesce and, after the equivalent of 40 monolayers of deposition, form a connected network. Complete coverage of the GaAs substrate is achieved after (approximately-equal-to)300 monolayers of deposition. This places a lower limit on the thickness of InSb layers, which may be considered in the design of optoelectronic devices.
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