13. Tutkimustietoportaalin artikkelit / Articles in the Research information portal

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Yliopiston tutkimustietojärjestelmään tallennetut avoimet julkaisut sekä EU-rahoitteisten projektien tutkimustuotokset.
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Browsing 13. Tutkimustietoportaalin artikkelit / Articles in the Research information portal by Issue Date

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  • Electrons and positrons in metal vacancies
    (1975-11-15) Manninen, M.; Nieminen, R.; Hautojärvi, P.; Arponen, J.
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    The electron density profiles at monovacancies of simple metals are calculated by the self-consistent Kohn-Sham method and by a number of statistical methods. The metal is described by a uniform positive background charge together with an interacting electron gas, and the vacancy is approximated as a spherical hole in the background. The Kohn-Sham electron density inside the vacancy is found to be in average 15 of the density in the bulk material. Of the various statistical methods, the simple Thomas-Fermi approximation is found to describe best the average electron density over the whole metallic density range when compared to the Kohn-Sham results. The energies of vacancy formation are calculated by using the Kohn-Sham electron densities together with three lattice models, and reasonable numerical success is achieved for alkali metals. In the case of polyvalent metals the results are not satisfactory even if the uniform background were replaced by point ions or if the electron-ion interactions were described by Ashcroft empty-core pseudopotentials. The lifetimes of a positron trapped at the vacancies of several metals are calculated by using both the Kohn-Sham and the Thomas-Fermi electron densities. The results for most metals are in agreement with experimental values. The angular-correlation curve of the positron in aluminum vacancy is calculated directly from the Kohn-Sham one-electron wave functions. The result agrees with the curve calculated from the so-called mixed-density approximation and also with the experimental result.
  • Temperature dependence of positron trapping at voids in metals
    (1979-02-01) Nieminen, R. M.; Laakkonen, J.; Hautojärvi, P.; Vehanen, A.
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    We report positron-lifetime measurements in void-containing aluminum samples, which show strong temperature dependence for the positron trapping probability. A theory is presented for the positron motion and trapping in a three-dimensional array of large voids, which compares favorably with the experimental data. It is shown that at low temperatures the trapping is transition limited and strongly temperature dependent with a crossover to diffusion-limited and weakly-temperature-dependent behavior at high temperatures.
  • Density-functional theory of positronium and electron bubbles in helium fluids
    (1980-05-01) Nieminen, R. M.; Välimaa, I.; Manninen, M.; Hautojärvi, P.
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    The density-functional method is applied to excess electrons and positronium atoms in helium fluids. The self-trapping is investigated in a fully self-consistent fashion, and formulas are given for the particle energy and Ps pick-off annihilation rate in quasifree as well as localized states. The numerical results compare well with experimental data. However, the need for a more sophisticated treatment of threshold effects near the onset of bubble formation is indicated.
  • Atoms embedded in an electron gas
    (1981) Puska, M. J.; Nieminen, R. M.; Manninen, M.
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    Energies of atoms, H through Ar, embedded in a homogeneous electron gas are calculated within the density-functional scheme as a function of the electron-gas density. The energy-versus-density curves and the induced densities of states are analyzed and discussed in terms of the interaction properties of an atom with its environment. The low-density limit of the immersion energy is related to the electron-atom scattering length. The results should prove useful in detailed investigations of the recently suggested "quasiatom" or "effective-medium" approaches to chemical binding. The lowest-order estimates of the binding energies of diatomic molecules and chemisorbed atoms are obtained.
  • Embedded-atom calculations of Auger and x-ray photoemission shifts for metallic elements
    (1982-01-01) Nieminen, R. M.; Puska, M. J.
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    Change in self-consistent-field energy density-functional calculations are reported for Auger and core-level binding-energy shifts in sp-bonded metals. The basic model, atom in jellium vacancy, gives good agreement with experiment, especially in the Auger case. The chemical and relaxation contributions to the shifts are discussed, and the extra-atomic response is analyzed in detail, both in position and energy space. The adequacy of the "excited-atom" approach to the energy shifts is discussed.
  • Flow of 3He-B through narrow channels
    (1982-03) Manninen, M. T.; Pekola, J. P.
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    The critical current Jc of superfluid 3He-B through 0.8-μm-diam channels has been measured. For small currents the pressure difference ΔP=0 along the flow channels within the resolution, implying small or zero dissipation. ΔP grows rapidly with increasing current above Jc; a clear transition to dissipative flow is thus observed. The temperature dependence of Jc indicates that the superfluid density and the critical temperature are reduced inside the narrow flow channels.
  • NMR Experiments on Rotating Superfluid 3He-A: Evidence for Vorticity
    (1982-06) Hakonen, P. J.; Ikkala, O. T.; Islander, S. T.; Lounasmaa, O. V.; Markkula, T. K.; Roubeau, P.; Saloheimo, K. M.; Volovik, G. E.; Andronikashvili, E. L.; Garibashvili, D. I.; Tsakadze, J. S.
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    Experiments on rotating superfluid 3He-A in an open cylindrical geometry show a change in the NMR line shape as a result of rotation: The amplitude of the peak decreases in proportion to f(T)g(Ω), where Ω is the angular velocity of rotation; at the same time the line broadens. Near Tc, f(T) is a linear function of 1−TTc. At small velocities g(Ω)∝Ω. These observations are consistent with the existence of vortices in rotating 3He-A.
  • Experiments on Vortices in Rotating Superfluid 3He-A
    (1982-10) Hakonen, P. J.; Ikkala, O. T.; Islander, S. T.
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    A satellite peak has been observed in the NMR spectrum of rotating He3-A; the peak intensity depends linearly on Ω at the high angular velocities, Ω=0.6-1.5 rad/s, needed to resolve it. The frequency shift of the satellite is independent of Ω. These results strongly suggest the existence of vortices in rotating He3-A with the vortex density proportional to Ω. Another satellite peak also has been observed which probably is due to solitons.
  • Positron surface states on clean and oxidized Al and in surface vacancies
    (1983-01-24) Nieminen, R. M.; Puska, M. J.
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    This Letter reports on the first discrete-lattice calculation of positron surface states on the surfaces of Al. The authors reproduce the observed values and anisotropy of the binding energies on clean surfaces, and predict the surface-state lifetimes. The temperature-independent lateral diffusion constant is calculated. Monovacancies on surfaces are predicted not to trap positrons. The effect of ordered chemisorbed monolayers of oxygen is investigated: Oxidation makes the surface state unstable with respect to positronium emission.
  • Atoms embedded in an electron gas
    (1983-05-15) Puska, M. J.; Nieminen, R. M.
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    The Fermi-level scattering phase shifts and the transport cross sections are reported for atoms embedded in a homogeneous electron gas. The applications of the results are discussed, using the electronic stopping power for slow ions and impurity resistivity as examples.
  • Collision effects in velocity-selective optical pumping of sodium
    (1983-08-01) Aminoff, C. G.; Javanainen, J.; Kaivola, M.
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    We report on a quantitative experimental investigation of velocity-changing collisions by means of velocity-selective optical pumping (VSOP). We have calculated the VSOP line shape for an atom with hyperfine structure with the use of two phenomenological kernels for the collision effects: the Keilson-Storer kernel, and a two-term kernel consisting of a broad Keilson-Storer part and a narrower Gaussian component. Corrections were included to account for the finite absorption in the sample and the backward reflection of the pumping beam. The experiments were carried out in sodium vapor with neon as the perturber gas. The D1 line of sodium was used for optical pumping, and the orientation of the ground state was detected. Free parameters of the theory were determined by fitting the predicted line shapes to experimental curves. The Keilson-Storer kernel proved unsatisfactory, but the two-term kernel reproduced well the observed line shapes over the entire collision profiles in the neon pressure range 0-57 mtorr. In an independent experiment using rapidly modulated VSOP we also measured directly the cross section of velocity-changing collisions: =(1.13±0.10)×10-14 cm2. The large weight obtained for the narrow Gaussian from the fits, as well as the collision cross section which is three times as large as the cross section deduced from tabulated gas kinetic radii, may indicate the presence of collisions with relatively small velocity changes in addition to hard-sphere encounters.
  • Quantum motion of chemisorbed hydrogen on Ni surfaces
    (1983-09-19) Puska, M. J.; Nieminen, R. M.; Manninen, Matti; Chakraborty, Bulbul; Holloway, S.; Nørskov, J. K.
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    Quantum mechanical energy levels and wave functions have been calculated for the motion of chemisorbed hydrogen atoms on Ni surfaces. The results show considerable quantum effects for the adatom in both the ground and the excited states. The description of the adparticles as being delocalized along the surface offers a novel interpretation of several phenomena, in particular the vibrational excitations.
  • Magnetic Vortices in Rotating Superfluid 3He-B
    (1983-10) Hakonen, P. J.; Krusius, M.; Salomaa, M. M.; Simola, J. T.; Bunkov, Yu. M.; Mineev, V. P.; Volovik, G. E.
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    Rotating superfluid He3-B is found to possess a new contribution to the NMR frequency shift, which changes sign on reversal of either the angular velocity of rotation or the magnetic field. For p=29.3 bars this gyromagnetic effect shows a discontinuity in magnitude at the first-order phase-transition temperature TTc=0.6, at which a change in the vortex-core structure takes place. These observations support the conclusion that the vortex core possesses a spontaneous intrinsic magnetization.
  • Repulsive interaction of the helium atom with a metal surface
    (1984-02-15) Manninen, Matti; Nørskov, J. K.; Puska, M. J.; Umrigar, Cyrus
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    The repulsive part of the helium scattering potential at a surface is approximately proportional to the surface electron density. The proportionality coefficient is shown to be a well-defined quantity, which can be related to the electron-helium scattering length. The spread in the values of the proportionality constant suggested in the literature is shown to be due to different definitions of the coefficient or due to inadequate calculational methods. The value calculated using the local density approximation with a self-interaction correction is in very good agreement with the electron-scattering-length measurements.
  • Muon states in uniaxially strained iron
    (1984-04-01) Jena, P.; Manninen, Matti; Nieminen, R. M.; Puska, M. J.
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    Effects of lattice relaxation, quantum motion, and uniaxial strain on the internal field at a positive-muon site in iron have been calculated. The uniaxial strain gives rise to a statistical shift of the muon population at interstitial sites. The effect of the population shift is found to be primarily responsible for the observed changes in the muon-precession frequency. The theory also predicts a 1T temperature dependence of the frequency shifts.
  • Theory of hydrogen and helium impurities in metals
    (1984-05-15) Puska, M. J.; Nieminen, R. M.
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    A powerful computational scheme is presented for calculating the static properties of light interstitials in metallic hosts. The method entails (i) the construction of the potential-energy field using the quasiatom concept, (ii) the wave-mechanical solution of the impurity distribution ("zero-point motion"), (iii) calculation of the forces exerted on the adjacent host atoms and their displacements, and (iv) iteration to self-consistency. We investigate self-trapping phenomena in bcc and fcc metals in detail, and calculate both the ground and low-lying excited states. Implications of the wave-mechanical or band picture to diffusion mechanisms and inelastic scattering experiments are discussed. Impurities treated are +, H, D, T, and He, and particular attention is paid to isotope effects among the hydrogenic impurities. It is argued that especially for + and H the quantum nature of the impurity is crucial. The calculated results are in agreement with a wealth of experimental data.
  • Persistent-current experiments on superfluid 3He-B and 3He-A
    (1984-07-02) Pekola, J. P.; Simola, J. T.; Nummila, K. K.; Lounasmaa, O. V.; Packard, R. E.
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    We have investigated persistent flow of superfluid 3He with an ac gyroscope filled with 20-μm powder. In 3He-B, currents circulate undiminished for 48 h at least; this implies a viscosity 12 orders of magnitude lower than in the normal fluid. In 3He-A, the current does not persist. The observed critical velocity in 3He-B at P12 bars there are two regimes in the B phase: For example, at 29.3 bars the ultimate critical velocities are 5.4 and 7.8 mm/s, respectively.
  • Electronically induced trapping of hydrogen by impurities in niobium
    (1984-07-15) Manninen, Matti; Puska, M. J.; Nieminen, R. M.; Jena, P.
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    The binding energies of hydrogen and its isotopes to substitutional impurities Ti, Cr, and V in niobium have been calculated. The hydrogen-metal interaction is based on the effective-medium theory. The wave mechanics of the hydrogenic interstitials are explicity dealt with, and the lattice distortion created by the hydrogen is incorporated through the method of lattice statics. The difference in the electronic structure between impurity and host atoms is shown to be largely responsible for the binding of hydrogen to the impurities. The results are in agreement with recent inelastic neutron scattering experiments.
  • Comment on the positron surface-state lifetime
    (1984-09-24) Nieminen, R. M.; Puska, M. J.; Manninen, Matti
     Comment/debate
  • Comment on "Nucleation of He3-B from the a phase: A cosmic-ray effect?"
    (1985) Hakonen, P. J.; Krusius, M.; Salomaa, M. M.; Simola, J. T.
     Letter