Abstract
In pure superfluid 3He–B at ultra-low temperatures, the quartz tuning fork oscillator response is expected to saturate when the dissipation caused by the superfluid medium becomes substantially smaller than the internal dissipation of the oscillator. However, even with a small amount of 4He covering the surfaces, we have observed saturation already at significantly higher temperatures than anticipated, where we have other indicators to prove that the 3He liquid is still cooling. We found that this anomalous behavior has a rather strong pressure dependence, and it practically disappears above the crystallization pressure of 4He. We also observed a maximum in the fork resonance frequency at temperatures where the transition in quasiparticle flow from the hydrodynamic to the ballistic regime is expected. We suggest that such anomalous features derive from the superfluid 4He film on the oscillator surface.
Highlights
Quartz tuning forks (QTFs) are used for temperature, pressure, viscosity and turbulence measurements in normal and superfluid helium [1,2,3,4]
We have observed a saturation in the temperature dependence of the resonance width of quartz tuning fork oscillators in two independent experiments in 3He systems with surfaces coated by 4He
The temperature indicated by the QTF resonance width saturates to a value at which the cooling power of the helium isotope mixing process would still be significantly larger than the external heat leak
Summary
Quartz tuning forks (QTFs) are used for temperature, pressure, viscosity and turbulence measurements in normal and superfluid helium [1,2,3,4]. We have two independent experiments that demonstrate similar saturation behavior: one is a nafen-filled 3He cell with surfaces coated with approximately 3 atomic layers of 4He, and the other an adiabatic melting cell that contains a saturated 3He–4He mixture at 4He crystallization pressure, where we expect a much thicker equilibrium film. In both experiments, we have observed a maximum in the resonance frequency at temperatures where the quasiparticle flow regime changes from the hydrodynamic to the ballistic one. We focus on reporting our experimental observations, while the detailed explanation on the origin of the effects remains a task for the future
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