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  • 1
    ISSN: 1434-6036
    Keywords: 36.40.+d ; 33.70.Jg ; 67.40.Yv
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract We examine the effects of impurities (“doping”) and rotational excitation on the structural and energetic properties of helium clusters. Quantum Monte Carlo (QMC) techniques are used to study ground and rotationally excited states of pure and doped clusters. We use exponentially correlated wave functions and treat the molecular impurities as rigid. Whereas pure HeN show essentially monotonic decay of density from a central maximum value, addition of impurities induces local ordering of He to an extent dependent on the impurity-He binding. Rotational excitation of HeN gives rise to extremely large centrifugal distortions. The location of impurities also appears to change upon rotational excitation. The implications of these distortions on impurity spectra are discussed for SF6HeN, and compared to recent experimental results.
    Type of Medium: Electronic Resource
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  • 2
    Electronic Resource
    Electronic Resource
    [s.l.] : Macmillian Magazines Ltd.
    Nature 408 (2000), S. 339-342 
    ISSN: 1476-4687
    Source: Nature Archives 1869 - 2009
    Topics: Biology , Chemistry and Pharmacology , Medicine , Natural Sciences in General , Physics
    Notes: [Auszug] Various physical implementations of quantum computers are being investigated, although the requirements that must be met to make such devices a reality in the laboratory at present involve capabilities well beyond the state of the art. Recent solid-state approaches have used quantum dots, ...
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  • 3
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 93 (1990), S. 746-759 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Theoretical methods are developed for determining the collective vibrational excitation spectra of quantum clusters, and applied to clusters of 4He. A quantum liquid drop model gives excitation energies in terms of two-point ground state density correlations, which are evaluated from microscopic calculations of the ground state wave functions. An alternative approach based on a Bijl–Feynman ansatz for the excited states also yields excitation energies in terms of ground state correlations, without the imposition of a sharp liquid surface. These methods are applied to the calculation of the excitation spectra of 4HeN , N=20, 70, and 240 clusters. The relationship between the collective spectra of these clusters and that of bulk fluid He II, and the significance of an observed roton minimum in the spectrum for N≥70 are discussed.
    Type of Medium: Electronic Resource
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  • 4
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 90 (1989), S. 2758-2767 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Self-consistent-field (SCF) methods are developed for solution of multidimensional diffusion problems. Time-dependent self-consistent-field (TDSCF) equations are derived for the Smoluchowski diffusion equation, and are applied to a two-dimensional barrier crossing problem. This is compared to both time-dependent and time-independent SCF approximations derived for the Schrödinger equation in imaginary time, which is obtained by transformation of the diffusion equation. Results for the model problem show that the TDSCF approximation for the original diffusion equation is accurate, efficient, and readily implementable in higher dimensions. Applications to diffusion problems in condensed media are noted.
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  • 5
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 84 (1986), S. 5181-5195 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A quantum mechanical theory is presented for the low temperature diffusion of atomic hydrogen on metal surfaces, based on a band model for the hydrogen motion. The theory is applicable to the diffusion of many interacting particles obeying quantum statistics. At low coverage the hydrogen band motion is limited by collisions between adsorbates, giving rise to a decrease of the diffusion constant with concentration. Other aspects of the hydrogen–hydrogen interaction are introduced to explain the coverage dependence at higher adsorbate concentrations. Comparison with recent low temperature diffusion measurements for H, D, and T on W(110) show that the above model reproduces satisfactorily the main features of the experimental coverage dependence of diffusion. The usefulness and limitations of band treatment for heavy particle diffusion are discussed in the light of these results.
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  • 6
    Electronic Resource
    Electronic Resource
    Springer
    The European physical journal 20 (1991), S. 223-226 
    ISSN: 1434-6079
    Keywords: 36.40 + d ; 61.20Ja ; 61.25.Em ; 67.90. + z
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract The ground state energies and structural properties of small (H2) N ,N=2−7, are calculated using the variational Monte Carlo method. These wavefunctions include both short- and long-range correlation effects that are important in the binding of van der Waals clusters. We have investigated these clusters using shadow wavefunctions and found that the coupling to shadow variables raises the energy in all cases, implying that the ground states of these small clusters are properly described as quantum liquids rather than solid structures.
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  • 7
    Electronic Resource
    Electronic Resource
    Springer
    The European physical journal 31 (1994), S. 75-84 
    ISSN: 1434-6079
    Keywords: 34.40.+d ; 61.25.Bi ; 67.40.Yv
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract We review recently developed theories of molecular interactions with helium clusters. Ground state energies and structures are obtained from Monte Carlo and density functional calculations for a variety of impurities and a range of cluster sizes. Of particular interest are chemical potentials, location of the impurity, and shape of the helium density surrounding it. Included in the work summarized here are results for light molecules (H2 and D2) which interact weakly with He and results for heavier molecules (Cl2 and SF6) which interact more strongly with He. Finally, theoretical and experimental results are compared for SF6 in helium clusters.
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  • 8
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 102 (1995), S. 2290-2290 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
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  • 9
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 96 (1992), S. 2953-2965 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Variational Monte Carlo techniques are employed in studying 4He clusters, with and without an H2 impurity. We find that a novel, yet simple, analytic nuclear wave-function form, derived from a numerical H2He wave function, yields high accuracy in computed ground-state energies of 4HeN. For the clusters studied here, three to twenty atoms, energies range from 94% to 90% of the exact values. Density profiles and distributions of particle separation are also computed. For reasonable computational cost (e.g., 〈20 Cray/X-MP14 minutes for the largest cluster), density profiles are determined for the first time to high statistical accuracy to within 0.5 A(ring) or less of the cluster center. The density profile of He3 is found to possess a uniquely pronounced peak at the cluster center resulting from contributions of near-collinear atomic arrangements. We also study the effect of substituting an He by H2, using modified wave functions containing products of pairwise He–H2 terms. For all cluster sizes studied, we find a lowering of the total energy upon exchanging an He for an H2. The exchange energy increases in magnitude with increasing cluster size, yet is still well below bulk estimates at N=20. Size comparisons with the pure helium clusters show very little change upon He/H2 exchange, e.g., the rms radii differ by ≤2% for N〉3. Density profiles and bond distributions show noticeable differentiation between H2 and He. For N≥4, the peak in the H2 density profile is not at the cluster but does remain inside the cluster. This peak is most pronounced for H2He13 implying an enhanced resistance to H2 penetration for He13.
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  • 10
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 93 (1990), S. 6738-6751 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The ground and several l=0 breathing mode vibrational excited state wave functions of HeN clusters are determined for N=3–5, 20, 70, and 240, using the variational Monte Carlo method. These wave functions incorporate one-, two-, and three-particle correlation effects and give binding energies, density profiles, and vibrational excitation energies accurately. The larger clusters have liquid-like structure, characterized by a pair distribution function showing approximately two coordination shells. The smallest clusters (N=3, 4) have extensively delocalized structures, which on average are equilateral triangular and tetrahedral, respectively. The N=5 cluster has a totally symmetric average structure, which can only be described in terms of a quantum liquid. No molecular structure, whether rigid or floppy can be assigned in this case. The relative importance of various correlation effects in clusters of different sizes is analyzed and discussed. These wave functions are completely analytical and are convenient as importance functions in diffusion and Green's function Monte Carlo calculations.
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