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  • 1
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 116 (2002), S. 1250-1258 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Control of near-degeneracy effects and dynamical correlation in atoms and molecules is within sight, thanks to an economical method that mixes configuration interaction (CI) and density functional theory (DFT). The influence of the size of the configuration-space has been studied for light systems including elements of the first and second period of the Periodic Table. © 2002 American Institute of Physics.
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  • 2
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 115 (2001), S. 6827-6833 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Careful calculations are performed to obtain the radial density–density response function for the He and the Be series. This is also done along the adiabatic connection of the density functional theory (as the system evolves from the real, physical system to the Kohn–Sham one). In this process the electron density is kept constant, while the strength of the interaction between electrons changes. The response functions are analyzed in terms of their eigenvalues and eigenfunctions. The latter change only little along this process. The absolute value of the eigenvalues is in general reduced by the interaction: A screening effect is present. For the near-degenerate systems, we notice that the opposite effect can appear (antiscreening). © 2001 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 115 (2001), S. 7678-7684 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A condensed polynomial model, that captures the main features of high- or low-pressure catalytic oscillations, is used to simulate spatiotemporal patterns in a cylindrical catalytic surface. This model includes a single autocatalytic variable (activator) and a slow changing and localized inhibitor subject to a global interaction mechanism which maintains the spatial average of the activator at the set point. While for very short (small length L) or very narrow (small perimeter P) cylinders the pattern preserves the structures of the corresponding one-dimensional problems (a ring or a wire), two-dimensional patterns emerge for comparable L and P showing a large multiplicity of spatiotemporal behavior because of a very high sensivity to initial conditions. The effect of kinetic parameters and system size is studied. Approximate solutions for the bifurcation from one- to two-dimension patterns are derived. © 2001 American Institute of Physics.
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  • 4
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 103 (1995), S. 691-697 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: To get an idea of the qualitative behavior of an optimal Jastrow factor we have considered a Hylleraas-type wave function of very high accuracy for the helium atom. Owing to the lack of nodes this wave function can be easily interpreted as a general type of Jastrow factor. As a result we obtained a simple parameter dependent ansatz for a Jastrow factor, which incorporates the essential features of the observed behavior. We have optimized the parameters with respect to the variance of the local energy, using variational Monte Carlo techniques, for the atoms He through Ne and for the simple molecules H2, LiH, and Li2. Finally, we compare our approach with other types of Jastrow factors discussed in the literature. © 1995 American Institute of Physics.
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  • 5
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 95 (1991), S. 1928-1942 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Relativistic spherically averaged numerical all-electron densities ρ were computed for the atoms Be–Ba, B–Tl, C–Pb, Cu–Au, and Zn–Hg. The Laplacian of these densities is not able to resolve the valence shell from the inner shells in case of heavy atoms, starting with the fourth row. The distribution of the local kinetic energy Ekin shows a valence maximum even for these heavy atoms, unfortunately, in a region of negative kinetic energy; i.e., nonclassically allowed. The quantity −||∇ρ||/ρ was also investigated. For all computed atoms, the −||∇ρ||/ρ diagrams are capable of describing the complete shell structure. −||∇ρ||/ρ is sensitive to basis set quality: poor Gaussian basis sets exhibit spurious oscillations and a premature onset of the linear decay. For the atoms B–Tl, Ba, Au, Hg, and Pb, nonrelativistic numerical calculations were performed to examine the effect of the relativity on the aforementioned quantities. Tests with pseudopotential densities reveal that for pseudopotential calculations, it is advisable to use at least two outermost shells in order to reproduce the all-electron values of ∇ 2ρ, Ekin, and −||∇ρ||/ρ in the valence region.
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  • 6
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 110 (1999), S. 2828-2835 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Switching on the electron–electron interaction connects the Kohn–Sham to the physical system. The correlation energy, the only unknown energy component in this process, is determined at fixed density, using a technique based on the Lieb Legendre transform definition of the universal density functional. Results are shown for this adiabatic coupling process for He,Ne8+,Be,Ne6+ as well as for the exponential densities nN,ζ(r)=N(ζ3/π)e−2ζr (N=2 or 4; ζ≥1; for N=4 degeneracy is present and 1D and 3P are analyzed). The data are fitted to a rational approximant and appear to be in good agreement with those given by the less computationally demanding Harris–Jones adiabatic connection. © 1999 American Institute of Physics.
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  • 7
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 68 (1990), S. 4133-4140 
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Ohmic contacts to p-GaAs, Zn doped at 5×1018 and 1×1019 cm−3, with the best specific contact resistance of 1.2×10−5 and 2.4×10−6 Ω cm2, respectively, have been formed with deposited layers of Ti and Pt. These layers, which were sequentially evaporated and then rapid thermally processed at 450 °C for 30 s, contained an interfacial layer constituted mainly of the TiAs phase adjacent to the substrate and the TixGa1−x solid solution in between it and the Ti layer. In addition, a significant amount of the Pt3Ti intermetallic was found at the Ti/Pt interface. The same metallization scheme, applied to 1×1018 cm−3 Zn-doped GaAs, produced a Schottky contact for the as-deposited and heat-treated samples. The temperature dependence characteristic of the specific contact resistance of the as-deposited Pt/Ti contacts to 5×1018 and 1×1019 cm−3 Zn-doped GaAs revealed a thermionic emission dominated carrier transport mechanism with an apparent barrier height of about 0.118 and 0.115 eV, respectively. This mechanism remained as the dominated one for the heat-treated contacts to the lower doped substrate. The contact resistance of the heat-treated contacts to the more heavily doped substrate, however, revealed a weaker temperature dependence. This indicates a conversion to a mixture of thermionic and field-emission carrier transport mechanisms across the interfacial barrier, and a reduced barrier height to a minimum value of 0.068 eV measured after sintering at 450 °C.
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  • 8
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 69 (1991), S. 2276-2279 
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Two different metallization schemes, AuBe(80 nm) and Pt(75 nm)/Ti(50 nm), were investigated as potential ohmic contacts for highly carbon-doped p-AlxGa1−xAs. The best contact resistance values of 0.015 and 0.025 Ω mm were achieved by applying the Pt/Ti scheme onto 1×1020 and 1×1019 cm−3 C-doped AlGaAs, respectively, followed by rapid thermal processing at 450 °C. The AuBe contacts yielded their lowest values of 0.025 and 0.05 Ω mm for the same doping levels as a result of rapid thermal processing at 425 °C. The heat treatment at 450 °C caused only limited reactions in both the Pt/Ti and Ti/AlGaAs interfaces and did not lead to significant degradation of the stable microstructure.
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  • 9
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The temperature dependence of the resistance in the Pt(60 nm)/Ti(50 nm) nonalloyed ohmic contacts to p-InAs (Zn doped 1×1018 to 1×1019 cm−3 ) induced by rapid thermal processing in the temperature range of 300–600 °C was studied. The ohmic nature of these contacts was attributed to both the low metal-semiconductor interfacial barriers and to the heavily doped semiconductor contacting layers. A phenomenological model was used to fit the measured temperature dependence contact resistance. The results indicated conversion from thermionic emission as the dominant carriers transport mechanism across the interfacial barrier for the as-deposited sample to a combination of thermionic and field emission mechanism for the heat-treated samples.
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  • 10
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 97 (1992), S. 459-463 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The usual form of nonlocal pseudopotentials which project the wave function on real spherical harmonics is inconvenient in quantum Monte Carlo calculations because the projection integrals are still functions of r. It is thus necessary to calculate the integral in every point of the simulation. We circumvent this problem by transforming the pseudopotential to a form where the projection occurs on Cartesian Gaussian-type functions which makes the projection integrals independent of the electron coordinates. We applied our method to two-valence electron systems using the pure diffusion method, where the approximation in the pseudopotential is the only one present.
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