Frequency-dependent drag from quantum turbulence produced by quartz tuning forks in superfluid He4

We have measured the drag force from quantum turbulence on a series of quartz tuning forks in superfluid helium. The tuning forks were custom made from a 75-μm-thick wafer. They have identical prong widths and prong spacings, but different lengths to give different resonant frequencies. We have used both the fundamental and overtone flexure modes to probe the turbulent drag over a broad range of frequencies f=ω/2π from 6.5 to 300 kHz. Optical measurements show that the velocity profiles of the flexure modes are well described by a cantilever beam model. The critical velocity for the onset of quantum turbulence at low temperatures is measured to be vc≈0.7κω−−−−−√ where κ is the circulation quantum. The drag from quantum turbulence shows a small frequency dependence when plotted against the scaled velocity v/v

A quasiparticle detector for imaging quantum turbulence in superfluid 3He-B

We describe the development of a two-dimensional quasiparticle detector for use in visualising quantum turbulence in superfluid 3He-B at ultra-low temperatures. The detector consists of a 5×5 matrix of pixels, each a 1mm diameter hole in a copper block containing aminiature quartz tuning fork. The damping on each fork provides a measure of the local quasiparticle flux. The detector is illuminated by a beam of ballistic quasiparticles generated from a nearby black-body radiator. A comparison of the damping on the different forks provides a measure of the cross-sectional profile of the beam. Further, we generate a tangle of vortices (quantum turbulence) in the path of the beam using a vibrating wire resonator. The vortices cast a shadow onto the face of the detector due to the Andreev reflection of quasiparticles in the beam. This allows us to image the vortices and to investigate their dynamics. Here we give details of the design and construction of the detector and show some preliminary results for one row of pixels which demonstrates its successful application tomeasuring quasiparticle beams and quantum turbulence

Producing and imaging quantum turbulence via pair-breaking in superfluid \textsuperscript{3}He-B

Destroying superfluidity is a fundamental process and in fermionic superfluid such as \textsuperscript{3}He-B it splits Cooper pairs into thermal excitations, quasiparticles. At the lowest temperatures, a gas of these quasiparticle excitations is tenuous enough for the propagation to be ballistic. We describe here an exploitation of the ballistic quasiparticles as the ``photons’’ to observe the local destruction of superfluid \textsuperscript{3}He-B by a mechanical resonator. We use a 5 by 5 pixel quasiparticle camera to image an emergence of quasiparticle excitations and a tangle of quantized vortices accompanying the pair-breaking. The detected quantum tangle is asymmetric around the mechanical resonator and is governed by the stability of vortices on the resonator surface. The vortex distribution shows that a conventional production of a quantum tangle via repetitive emission of vortex rings starts on the top surface of the generator and spreads around whole surface at high velocity when escaping vortex rings get re-trapped by the moving resonator

Measuring the Prong Velocity of Quartz Tuning Forks Used to Probe Quantum Fluids

Recently, quartz tuning forks have been used to probe the dynamics of quantum fluids. For many of these measurements it is important to know the velocity amplitude of the tips of the vibrating fork prongs. We have used different techniques to establish, with an accuracy of a few percent, the relationship between the electrical and mechanical properties of several commercial quartz tuning forks with fundamental resonant frequency similar to 32 kHz. The velocity is usually inferred from an electro-mechanical calibration that models a quartz prong as a clamped, rectangular cantilever beam. We have tested the accuracy of this calibration using three methods: measurement of the amplitude at which the fork prongs touch each other; direct optical measurement of the moving fork prongs using strobe microscopy; and a Michelson interferometry technique operating with a 670 nm laser. All three methods yield consistent results. The velocity so determined is found to be 10% lower than that of the standard electro-mechanical calibration