Defence of dissertation in the field of engineering physics, Tapio Riekki, MSc

Reaching for record-low temperatures in liquid helium

The title of the dissertation is: "Adiabatic melting experiment: ultra-low temperatures in helium mixtures"

Mixture of the two stable helium isotopes, He-3 and He-4, is a fascinating subject to study. It is a mixture of two fundamentally different quantum mechanical particles, fermions and bosons, whose properties need to be understood if we wish to uncover the mysteries from quantum computers to neutron starts. Liquid helium mixture represents a model system where such particles can be studied in a clean and controlled environment.

A peculiar phenomenon that occurs at sufficiently low temperature (about 1 Kelvin in pure He-4 and 0.001 Kelvin in pure He-3) is superfluidity. Superfluid is a quantum mechanically organized fluid that can, for example, flow without friction through even the smallest holes. In this thesis, we developed and studied a novel cooling method that aimed to cool helium mixture to such a low temperature that both isotopes would be in superfluid state at the same time. The required temperature was anticipated to be below 0,0001 Kelvin.

The method presented in the thesis is based on melting of solid He-4 and its mixing to He-3. We study the properties of the melting method and seek out the optimal experimental parameters to reach as low temperature as possible. We also study the difficulties in thermometry and heat leaks at these ultra-low temperatures, and develop a computational thermal model to better understand the thermodynamics of the novel cooling technique. With the gained information, we can give suggestions on how to improve the cooling method in the future.

Opponent is Professor Richard Haley, Lancaster University, UK

Custos is Professor Pertti Hakonen, Aalto University School of Science, Department of Applied Physics

Supervising professor is Professor Pertti Hakonen, Aalto University School of Science, Department of Applied Physics

The public defense will be organized via remote technology:  Electronic dissertation

The dissertation is publicly displayed as online display 10 days before the defence at

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