Superflow as a tool for studying superfluid 3He

Jennings, Ash and Zmeev, Dmitry and Haley, Richard (2022) Superflow as a tool for studying superfluid 3He. PhD thesis, Lancaster University.

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Abstract

In this thesis, we use mechanical oscillators to induce fluid flow in the superfluid 3He in order to investigate the properties of several superfluid phases. In particular, we investigate the B phase below temperatures of 230 μK and superfluid 3He confined in an anisotropic aerogel. Superfluid 3He is a fermionic superfluid consisting of Cooper pairs. If the superfluid flows above a critical velocity called the Landau velocity, the Cooper pairs break and form into quasiparticles. When an oscillating object goes faster than the critical velocity, it experiences a large increase in the drag force on the object. In this work, we move a device in superfluid 3He-B with constant velocity rather than oscillating it. We see only a small increase in damping for velocities above the critical velocity, instead of the large increase. After subtracting the results of thermally excited quasiparticles, we demonstrate that the small increase in damping is due to expelling quasiparticles occupying surface-bound-states on the wire surface. By monitoring the thermometer response to the movements, we measure the Kapitza resistance. We also observe the damping of wires oscillating in superfluid 3He-B with a diameter approaching the coherence length. The damping is much smaller than expected from considering the collisions of thermally excited quasiparticles. Lastly, we built a new device for exploring the superfluid phases of 3He confined inside an anisotropic aerogel. The aerogel has 10 nm strands aligned parallel with a mean distance between the strands of 100 nm, close to the coherence length of the superfluid. The device can be moved and oscillated, inducing flow. By observing the resonance frequency and the damping of the device, one can measure the superfluid fraction and observe other phenomena in the superfluid. Sharp changes in the velocity of measurements below the aerogel transition temperature have been found using this technique.