Browsing by Author "Nummila, K. K."

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  • Observation of nuclear ferromagnetic ordering in silver at negative nanokelvin temperatures
    (1992) Hakonen, Pertti J.; Nummila, K. K.; Vuorinen, R. T.; Lounasmaa, O. V.
     School of Science | A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    The ferromagnetically ordered state in the nuclear spin system of silver has been reached at negative absolute temperatures by adiabatic nuclear demagnetization at entropies below 0.82 ln2. The ordering, caused by the antiferromagnetic Ruderman-Kittel interaction, was observed below -1.9 nK as a saturation of susceptibility close to -1 and as an increase of the NMR frequencies. Comparison with recent mean-field calculations by Viertiö and Oja suggests a domain configuration. The phase diagram of silver nuclei at T<0 is outlined in the magnetic field versus entropy plane.
  • 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.
  • 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.
  • Phase Diagram of the First-Order Vortex-Core Transition in Superfluid 3He-B
    (1984) Pekola, Jukka P.; Simola, J. T.; Hakonen, Pertti J.; Krusius, M.; Lounasmaa, O. V.; Nummila, K. K.; Mamniashvili, G.; Packard, R. E.; Volovik, G. E.
     School of Science | A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    The phase diagram of quantized vortices in 3He-B has been obtained with two techniques. Our NMR measurements show that magnetization displays a pressure-dependent first-order phase change at TV(P). Our gyroscopic experiments reveal a transition curve TG(P) which marks an abrupt change in the critical velocity. Both curves show a weak magnetic field dependence, run similarly in the (P, T) plane, and seem to mark the same vortex-core transition.
  • Spin dynamics in highly polarized silver at negative absolute temperatures
    (1992) Hakonen, Pertti J.; Nummila, K. K.; Vuorinen, R. T.
     School of Science | A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    We have studied the spin dynamics of highly polarized silver nuclei at negative absolute temperatures up to inverted polarizations p≃-70%. The measured NMR spectra at B=0.4 and 0.8 mT display a clear inverted suppression-enhancement effect, arising from an interaction between the two spin species, 107Ag and 109Ag. The averaged interaction-field description accounts well for the intensity ratio of the isotopic NMR lines. The frequency difference of the absorption maxima has been used to study the merging and repulsion of the NMR modes at large negative polarizations. At B=0.2 mT, our spectra display only a single exchange-merged line, indicating that the two isotopes act as like spins. The transition from a single NMR line to two separate lines proceeds in a similar fashion as at positive temperatures. No effects attributable to the tendency of ferromagnetic alignment of spins at T<0 were observed.
  • Surface spin waves in 3He-A, a probe for vortex phenomena in narrow gaps
    (1987-02) Hakonen, P. J.; Nummila, K. K.; Simola, J.T.; Skrbek, L.; Mamniashvili, G.
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    We report measurements on a new collective spin-wave mode trapped by the textural boundary layers of 3He-A within a stack of thin Mylar plates. The surface mode was seen as a new peak in the cw NMR spectrum measured at H0=284 Oe. Rotation of the sample, with orthogonal to the gaps, increased the spectral weight of the surface mode, indicating an increase in the textural boundary layers caused by a counterflow-induced transition. This phenomenon was used to study vortex creation and persistent currents.
  • Surface spin waves in 3He-A, a probe for vortex phenomena in narrow gaps
    (1987) Hakonen, Pertti J.; Nummila, K. K.; Simola, J. T.; Skrbek, L.; Mamniashvili, G.
     School of Science | A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    We report measurements on a new collective spin-wave mode trapped by the textural boundary layers of 3 He-A within a stack of thin Mylar plates. The surface mode was seen as a new peak in the cw NMR spectrum measured at H0=284 Oe. Rotation of the sample, with Ω orthogonal to the gaps, increased the spectral weight of the surface mode, indicating an increase in the textural boundary layers caused by a counterflow-induced transition. This phenomenon was used to study vortex creation and persistent currents.
  • Vortex-free state of 3He-B in a rotating cylinder
    (1987-08) Hakonen, P. J.; Nummila, K. K.
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    Vortex-free states have been observed in B3 at rotation speeds up to 0.9 rad/s in a cylinder of 5-mm diameter. The measured cw NMR spectra, reflecting the order-parameter structure, respond very differently to rotation in opposite directions. The sign of this asymmetry is not linked to the direction of the applied magnetic field, as expected for a gyromagnetic effect, but varies randomly from one cooldown to another.
  • Vortex-free state of 3He-B in a rotating cylinder
    (1987) Hakonen, Pertti J.; Nummila, K. K.
     School of Science | A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    Vortex-free states have been observed in 3B at rotation speeds up to 0.9 rad/s in a cylinder of 5-mm diameter. The measured cw NMR spectra, reflecting the order-parameter structure, respond very differently to rotation in opposite directions. The sign of this asymmetry is not linked to the direction of the applied magnetic field, as expected for a gyromagnetic effect, but varies randomly from one cooldown to another.