The theoretical study of oscillating objects immersed in liquid 3He

Helium, as the only known substance that remains in liquid form at absolute zero temperature, is a central subject of study in the field of condensed matter physics. Liquid helium possesses many unique and interesting features, most famously a superfluid state.

At a sufficiently low temperature, 3He goes through a phase transition into a superfluid state where it flows with zero viscosity. Before transitioning to the superfluid state 3He is said to be in normal state. It's behaviour can then be described using Landau's Fermi liquid theory. Describing the dynamics of an interacting many-body system is in general immensely difficult. Such problems are however central to most fields of physics. In the case of 3He, due to the properties of 3He atoms, at low temperatures it is sufficient to study the collective excitations of the 3He particles. This idea is the foundation of Fermi liquid theory. The collective excitations are called quasiparticles. Under suitable conditions quasiparticles are very long lived and their collisions rare.

My first subject of research is to study the motion of a thin film of 3He on top of an oscillating planar substrate. Thin film in this case means that the top surface of the liquid film has an observable effect. Fermi liquid has its own unique set of sound modes called zero sound, that propagate even in the absence of quasiparticle collisions. The oscillating substrate generates transverse sound waves that travel through the fluid film. I study the propagation of these waves theoretically. This kind of study has previously only been done for a thick film.

My second research project concerns experiments done using a moving wire immersed in superfluid 3He. If the flow of a superfluid exceeds a critical value, the superfluid state is broken down. This value is called the Landau critical velocity. For a wire moving through a superfluid, this breakdown means that if the wire is moving quickly enough, the superfluid should begin to suddenly resist its movement. Experimentally it has been found however, that for vibrating wires this resistance begins at unexpected velocity values. For wires moving steadily through a superfluid, this sudden onset of resistance is not detected [1]. We attempt to explain these phenomena.



12.10.2017 Juri Kuorelahti