Critical flow velocity in super fluid 3He-B

Skyba, Maros and Haley, Richard and Tsepelin, Viktor (2016) Critical flow velocity in super fluid 3He-B. PhD thesis, Lancaster University.

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The experiments detailed within this thesis have measured the distortion of the superfluid energy gap in high magnetic field and the dissipation for an object in uniform linear motion through superfluid 3He-B. The latter experiments led to an astonishing discovery of no discontinuity in the dissipation for an object in uniform linear motion at the Landau critical velocity. The experiments were performed in a “Lancaster style” nested experimental cell at ultra-low temperatures within the ballistic limit. In the first set of experiments we studied two almost identical quartz tuning fork resonators with different vibration directions with respect to the vertical magnetic field. One vibrated along the field direction and the other vibrated in the horizontal plane. Our measurements have shown that the critical velocity for the vertical fork decreases significantly with increasing field, dropping to almost 60% of its original value as the highest field is approached. However, there is very little change of the critical velocity for the horizontal fork. Our data shows good agreement with theoretical predictions and previous experiments using vibrating wires. During measurements at high magnetic fields, 300 mT to 330 mT, we observed discontinuities in the velocity response for very small changes in driving force. This behaviour might be due to vortex generation around the vibrating object and a subsequent shielding effect, previously observed in the response of a large vibrating wire (with diameter of 100 µm, similar to a typical fork dimension). Intriguingly the detailed behaviour also appears to depend on the orientation of the tuning fork with respect to the magnetic field direction. The second set of experiments used a novel measurement tool referred to as the “flopper”. The idea behind the development of the flopper was to have a low frequency device with low Q-factor that could be moved in an arbitrary fashion. The flopper is a large 25 × 9 mm goalpost-shaped NbTi vibrating wire. With AC current the wire can be driven at its resonance frequency or by using a DC linear stroke it can be moved over a controlled distance within the cell. By adding a high frequency “probe” signal on top of the DC signal we calibrated the position of the flopper with respect to the cell. We performed DC strokes of the flopper over short distances within the cell at various velocities. This led to us discovering that the dissipation of uniform linear motion at velocities exceeding the Landau velocity did not show any discontinuity. Since the critical Landau velocity is so fundamental in the understanding of superfluidity, this was a considerable surprise. The comparisons between AC and DC motion led to the development of a model to describe the dissipation processes in 3He-B.

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01 Aug 2016 10:00
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04 Jan 2024 00:03