The doctoral dissertations of the former Helsinki University of Technology (TKK) and Aalto University Schools of Technology (CHEM, ELEC, ENG, SCI) published in electronic format are available in the electronic publications archive of Aalto University - Aaltodoc.
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Dilute Helium Mixtures at Low Temperatures: Properties and Cooling Methods

Elias Pentti

Dissertation for the degree of Doctor of Science in Technology to be presented with due permission of the Faculty of Information and Natural Sciences for public examination and debate in Auditorium AS1 at Helsinki University of Technology (Espoo, Finland) on the 31st of August, 2009, at 12 noon.

Overview in PDF format (ISBN 978-952-248-021-7)   [2116 KB]
Dissertation is also available in print (ISBN 978-952-248-020-0)

Abstract

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.

This thesis consists of an overview and of the following 7 publications:

  1. Juha Tuoriniemi, Juha Martikainen, Elias Pentti, Alexander Sebedash, Sergey Boldarev, and George Pickett. 2002. Towards superfluidity of 3He diluted by 4He. Journal of Low Temperature Physics, volume 129, numbers 5-6, pages 531-545. DOI: 10.1023/A:1021468614550.
  2. E. Pentti, J. Tuoriniemi, A. Salmela, and A. Sebedash. 2007. Melting pressure thermometry of the saturated helium mixture at millikelvin temperatures. Journal of Low Temperature Physics, volume 146, numbers 1-2, pages 71-83. DOI: 10.1007/s10909-006-9267-8.
  3. A. P. Sebedash, J. T. Tuoriniemi, S. T. Boldarev, E. M. M. Pentti, and A. J. Salmela. 2007. Adiabatic melting of 4He crystal in superfluid 3He at sub-millikelvin temperatures. Journal of Low Temperature Physics, volume 148, numbers 5-6, pages 725-729. DOI: 10.1007/s10909-007-9443-5.
  4. A. P. Sebedash, J. T. Tuoriniemi, E. M. M. Pentti, and A. J. Salmela. 2008. Osmotic pressure of 3He-4He solutions at 25.3 bar and low temperatures. Journal of Low Temperature Physics, volume 150, numbers 3-4, pages 181-186. DOI: 10.1007/s10909-007-9535-2.
  5. E. M. Pentti, J. T. Tuoriniemi, A. J. Salmela, and A. P. Sebedash. 2008. Quartz tuning fork in helium. Journal of Low Temperature Physics, volume 150, numbers 3-4, pages 555-560. DOI: 10.1007/s10909-007-9583-7.
  6. Elias M. Pentti, Juha T. Tuoriniemi, Anssi J. Salmela, and Alexander P. Sebedash. 2008. Solubility of 3He in 4He at millikelvin temperatures up to the melting pressure measured by a quartz tuning fork. Physical Review B, volume 78, number 6, 064509. DOI: 10.1103/PhysRevB.78.064509. © 2008 American Physical Society. By permission.
  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 of publications 2 and 6

Keywords: helium mixture, superfluid, tuning fork, melting pressure, adiabatic melting, second sound

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© 2009 Helsinki University of Technology

Last update 2011-05-26