Public defence in Engineering Physics, M.Sc. (Tech) Timo Kamppinen

 Title of the doctoral thesis: Nanoelectromechanical resonators and nanoconfinement in quantum fluids

Opponent: Director of Research Eddy Collin, CNRS, France
Custos: Professor Pertti Hakonen, Aalto University School of Science, Department of Applied Physics

The doctoral thesis is publicly displayed 10 days before the defence in the publication archive Aaltodoc of Aalto University

Electronic thesis

Public defence announcement: 

Quantum fluids are fluids whose characteristics are affected by their quantum statistics. Examples of quantum fluids are superfluids which exhibit dissipationless superflow, such as the fermionic helium-3 and the bosonic helium-4 at low temperatures near the absolute zero. Nano-sized sensors and nano-structured confinement in the quantum fluids open up new avenues for probing and exploiting novel phenomena. 

In this thesis, nanoelectromechanical probes of the quantum fluids are developed. The nanodevices are characterized in vacuum, helium-4 gas and superfluid helium-4. It is shown that the aluminum devices host two-level systems that tunnel between the states by the laws of quantum mechanics. Evidence is provided by the characteristic signatures that the tunneling two-level systems produce in the response of the devices at temperatures near the absolute zero. The tunneling two-level systems relax via vibrations of the device lattice, where the number of lattice modes is dimensionally reduced by the miniature size of the nanodevices. Beyond nanoelectromechanical resonators, the gained understanding of tunneling two-level systems can be utilized for optimizing the performance of quantum computers. In helium-4, the devices show extremely high sensitivity to particles in the gaseous phase and to quasiparticles in the superfluid phase. In addition to fast and accurate thermometry, the high sensitivity of the devices could be utilized for studying quantized vortices in superfluids. 

In this thesis, the polar phase of superfluid helium-3 is also studied. The polar phase is stabilized in commercially available nano-structured material called nafen, which consists of nearly parallel strands of aluminum oxide. The nafen strands pin quantized vortices strongly, allowing accurate determination of the density of vortices created during the phase transition to the superfluid state, and it is shown that the number of vortices can be suppressed by applying a bias magnetic field. The finding provides insights on reaching defect-free states in quantum phase transitions. It is experimentally shown that the amplitude of the superfluid gap in the polar phase follows the theoretical expectations, and that the superconducting transition temperature is only slightly suppressed compared to the bulk value. The result helps identifying structures where unconventional superconductivity could be materialized, perhaps even at room temperature.

Contact details of the doctoral student: [email protected], 0440130099