New methods of measurements in superfluid helium

Abstract

In this thesis we use quartz tuning fork resonators to probe properties of normal and superfluid 4He and 3He. Our main goal is to study both quantum turbulence and acoustic emission of tuning forks in liquid helium. By employing a multi-frequency lock-in amplifier we contrast single and multi- frequency methods of measuring tuning forks in the linear regime. In the non-linear response of tuning forks during turbulence we create multi-frequency excitations called intermodulation products which are used to find the non-linear forces that created them. We apply this technique to quantum turbulence in superfluid 4He-II and find that the retarding in-phase force on the fork increases at a critical velocity for turbulence nucleation. We also observe that the out-of-phase non-linear force increases, which we attribute to energy loss via vortex ring emission by the fork. Superfluid 3He is a fermionic condensate of Cooper pairs of 3He atoms. At ultra-low temperatures of 120 μK thermally excited unpaired quasiparticles travel ballistically through the condensate. We beam quasiparticles from a black body source towards a 5 × 5-pixel camera and observe that the excitations follow photonic-like trajectories. We apply the source-camera configuration to non-invasively detect and even image quantum vortices, that are topological defects in the superfluid. Lastly, we explore the frequency dependent damping of quartz tuning forks in liquid 3He. We find that at high frequencies the fork damping is governed by acoustic emission. Furthermore, we show that existing models developed for sound emission in 4He can be used to predict observed acoustic damping in 3He. The results also suggest that devices for 3He experiments should be placed in cavities or designed to operate at low frequencies.