Dilute helium mixtures at low temperatures : properties and cooling methods

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Verkkokirja (2116 KB, 31 s.)
TKK dissertations, 176
This thesis describes experimental work on dilute mixtures of ³He in 4He, mainly at millikelvin temperatures. The isotopic helium mixture has the unique property of remaining a miscible liquid down to the absolute zero temperature. In the mK regime, it consists of two very different components: perfectly superfluid 4He, and a weakly interacting degenerate Fermi liquid of ³He, predicted by theory to undergo transition to the superfluid state at an extremely low temperature. To discover that transition, new ways of cooling helium mixtures need to be developed, as it is not likely that the conventional method of nuclear demagnetization of copper can be improved to reach helium temperatures notably below the temperatures of order 0.1 mK attained so far. Adiabatic melting of 4He in the presence of liquid ³He is probably the most promising method of cooling helium mixtures to microkelvin temperatures. It produces helium mixture colder than the initial temperature as a direct consequence of the mixing of the isotopes. The starting configuration is attainable by pressurizing liquid helium mixture to its solidification pressure at a temperature below some tens of mK, as only 4He then enters the solid phase. This thesis describes an experiment in which the method of adiabatic melting was, for the first time, realized at sub-millikelvin temperatures, where the superfluidity of the pure ³He phase enables, in principle, a drastic decrease of temperature. In the experiments, cooling from initial temperatures between 0.3 and 0.9 mK was detected, temperature reduction remaining below a factor of two for recognized reasons of a technical rather than fundamental nature. A capacitive differential pressure transducer, constructed for the experiment, was used for high accuracy measurements of the temperature dependence of the melting pressure of helium mixtures with several ³He concentrations. The melting pressure is suitable for thermometry and carries information on the interactions of ³He particles in the mixture. Also, the response of a quartz tuning fork immersed in helium was studied. Its sensitivity to the properties of the surrounding fluid was utilized to determine the saturation concentration of dilute ³He across the entire accessible pressure range. The tuning fork was found to exhibit a complex pattern of anomalies attributed to resonant modes of second sound, or concentration waves, inside its cylindrical container.
helium mixture, superfluid, tuning fork, melting pressure, adiabatic melting, second sound
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    DOI: 10.1023/A:1021468614550. View at publisher
  • Journal of Low Temperature Physics, volume 146, numbers 1-2, pages 71-83.
    DOI: 10.1007/s10909-006-9267-8. View at publisher
  • Journal of Low Temperature Physics, volume 148, numbers 5-6, pages 725-729.
    DOI: 10.1007/s10909-007-9443-5. View at publisher
  • Journal of Low Temperature Physics, volume 150, numbers 3-4, pages 181-186.
    DOI: 10.1007/s10909-007-9535-2. View at publisher
  • Journal of Low Temperature Physics, volume 150, numbers 3-4, pages 555-560.
    DOI: 10.1007/s10909-007-9583-7. View at publisher
  • Physical Review B, volume 78, number 6, 064509.
    DOI: 10.1103/PhysRevB.78.064509. View at publisher
  • [Publication 7]: Elias Pentti, Juho Rysti, Anssi Salmela, Alexander Sebedash, and Juha Tuoriniemi. 2009. Studies on helium liquids by vibrating wires and quartz tuning forks. Espoo, Finland. 36 pages. Helsinki University of Technology, Low Temperature Laboratory Publications, Report TKK-KYL-021. © 2009 by authors.
  • [Errata file]: Errata of publications 2 and 6