12. Artikkelit / Articles

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  • Electrons and positrons in metal vacancies
    (1975) Manninen, M.; Nieminen, Risto M.; Hautojärvi, P.; Arponen, J.
     School of Science | 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) Nieminen, Risto M.; Laakkonen, J.; Hautojärvi, P.; Vehanen, A.
     School of Science | 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) Nieminen, Risto M.; Välimaa, I.; Manninen, M.; Hautojärvi, P.
     School of Science | 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: Immersion energies
    (1981) Puska, Martti J.; Nieminen, Risto M.; Manninen, M.
     School of Science | 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.
  • Comparison of the National Bureau of Standards and the Helsinki Temperature Scales and its Effect on the Heat Capacity of Liquid 3He below 10 mK
    (1981) Lhota, E.; Manninen, M. T.; Pekola, Jukka P.; Soinne, A. T.; Soulen, R. J.
     School of Science | A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    The Helsinki temperature scale, used earlier in measurements of the heat capacity of liquid 3He (1-10 mK), is compared with the National Bureau of Standards (NBS) noise and nuclear-orientation temperature scale. The superfluid transition temperature (Tc) of 3He at zero pressure and the superconductive transition temperatures of tungsten and beryllium were used as fixed points. Tc on the NBS scale was found to be 1.025 ± 0.02 mK, in close agreement with the Helsinki value 1.04 mK. The results support the Helsinki data on the heat capacity of 3He.
  • Critical current of 3He-A in narrow channels
    (1982) Manninen, M. T.; Pekola, Jukka P.; Sharma, R. G.; Tagirov, M. S.
     School of Science | A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    The critical current Jc of superfluid 3He-A in 0.8-μm-diam channels has been measured by the observation of the pressure difference along the channels versus the mass current. During warming Jc was found to decrease by about 30% at TBA(cyl) and by another 30% at TBA; TBA(cyl) is the reduced B→A transition temperature in the narrow flow channels, with TBA(cyl)TBA=0.92 at 27.4 bars. Above TBA a second dissipative mechanism was observed at lower currents. These features are believed to be associated with the ends of the channels.
  • Experiments on Vortices in Rotating Superfluid 3He-A
    (1982) Hakonen, Pertti J.; Ikkala, Olli T.; Islander, S. T.
     School of Science | A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    A satellite peak has been observed in the NMR spectrum of rotating 3He-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 3He-A with the vortex density proportional to Ω. Another satellite peak also has been observed which probably is due to solitons.
  • Embedded-atom calculations of Auger and x-ray photoemission shifts for metallic elements
    (1982) Nieminen, Risto M.; Puska, Martti J.
     School of Science | 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) Manninen, M. T.; Pekola, Jukka P.
     School of Science | 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) Hakonen, Pertti J.; Ikkala, Olli 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.
     School of Science | 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−T/Tc. At small velocities g(Ω)∝Ω. These observations are consistent with the existence of vortices in rotating 3He-A.
  • Search for the ac Josephson effect in superfluid 3He
    (1983) Lounasmaa, O. V.; Manninen, M. T.; Nenonen, S. A.; Pekola, Jukka P.; Sharma, R. G.; Tagirov, M. S.
     School of Science | A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    Experiments testing for the existence of the ac Josephson effect in superfluid 3He, analogous to phenomena observed in superconducting microbridges, have been performed. Small holes were employed as the weak link between two reservoirs filled with 3He; several different orifice geometries were tried. Simple model calculations suggest that steps in the flow characteristics should be observable with our resolution when an ac pressure modulation is applied across the weak link. We found that such effects do not exist for the parameter values used in our experiments.
  • Magnetic Vortices in Rotating Superfluid 3He-B
    (1983) Hakonen, Pertti J.; Krusius, M.; Salomaa, M. M.; Simola, J. T.; Bunkov, Yu. M.; Mineev, V. P.; Volovik, G. E.
     School of Science | A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    Rotating superfluid 3He-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 T/Tc=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.
  • Collision effects in velocity-selective optical pumping of sodium
    (1983) Aminoff, C. G.; Javanainen, Juha; Kaivola, Matti
     School of Science | 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: σ=(1.13±0.10)×10exp−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.
  • Positron Surface States on Clean and Oxidized Al and in Surface Vacancies
    (1983) Nieminen, Risto M.; Puska, Martti J.
     School of Science | 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.
  • Quantum Motion of Chemisorbed Hydrogen on Ni Surfaces
    (1983) Puska, Martti J.; Nieminen, Risto M.; Manninen, M.; Chakraborty, Bulbul; Holloway, S.; Nørskov, J. K.
     School of Science | 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.
  • Atoms embedded in an electron gas: Phase shifts and cross sections
    (1983) Puska, Martti J.; Nieminen, Risto M.
     School of Science | 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.
  • Continuous Vortices with Broken Symmetry in Rotating Superfluid 3He-A
    (1984) Seppälä, H. K.; Hakonen, Pertti J.; Krusius, M.; Ohmi, T.; Salomaa, M. M.; Simola, J. T.; Volovik, G. E.
     School of Science | A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    New NMR measurements are reported on continuous 3He-A vortices in tilted magnetic fields. We introduce a symmetry classification of the continuous vortices with broken axial symmetry. It is found that the discrete internal symmetry may in addition be broken in two inequivalent ways, producing two different continuous vortices. Although NMR may not distinguish between these two vortices, the observed vortex satellite peak is well accounted for by spin waves localized in the soft core of such vortices.
  • Persistent-Current Experiments on Superfluid 3He-B and 3He-A
    (1984) Pekola, Jukka P.; Simola, J. T.; Nummila, K. K.; Lounasmaa, O. V.; Packard, R. E.
     School of Science | 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 P<~12 bars is independent of temperature. At P>12 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.
  • Repulsive interaction of the helium atom with a metal surface
    (1984) Manninen, M.; Nørskov, J. K.; Puska, Martti J.; Umrigar, Cyrus
     School of Science | 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) Jena, P.; Manninen, M.; Nieminen, Risto M.; Puska, Martti J.
     School of Science | 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.