73 research outputs found
Visualizing Pure Quantum Turbulence in Superfluid He: Andreev Reflection and its Spectral Properties
Superfluid He-B in the zero-temperature limit offers a unique means of
studying quantum turbulence by the Andreev reflection of quasiparticle
excitations by the vortex flow fields. We validate the experimental
visualization of turbulence in He-B by showing the relation between the
vortex-line density and the Andreev reflectance of the vortex tangle in the
first simulations of the Andreev reflectance by a realistic 3D vortex tangle,
and comparing the results with the first experimental measurements able to
probe quantum turbulence on length scales smaller than the inter-vortex
separation.Comment: 5 pages, 4 figures, and Supplemental Material (2 pages, 2 figures
Crossover from hydrodynamic to acoustic drag on quartz tuning forks in normal and superfluid 4He
We present measurements of the drag forces on quartz tuning forks oscillating at low velocities in normal and superfluid 4He. We have investigated the dissipative drag over a wide range of frequencies, from 6.5 to 600 kHz, by using arrays of forks with varying prong lengths and by exciting the forks in their fundamental and first overtone modes. At low frequencies the behavior is dominated by laminar hydrodynamic drag, governed by the fluid viscosity. At higher frequencies acoustic drag is dominant and is described well by a three-dimensional model of sound emission
Turbulent drag on a low-frequency vibrating grid in superfluid He-4 at very low temperatures
We present measurements of the dissipative turbulent drag on a vibrating grid in superfluid He-4 over a wide range of (low) frequencies. At high velocities, the dissipative drag is independent of frequency and is approximately the same as that measured in normal liquid He-4. We present measurements on a similar grid in superfluid He-3-B at low temperatures which shows an almost identical turbulent drag coefficient at low frequencies. However, the turbulent drag in He-3-B is substantially higher at higher frequencies. We also present measurements of the inertial drag coefficient for grid turbulence in He-4. The inertial drag coefficient is significantly reduced by turbulence in both superfluid and normal liquid He-4
Measurements of vortex line density generated by a quartz tuning fork in superfluid 4 He
We present proof-of-concept measurements of the vortex line density generated by a quartz tuning fork resonator probed by the attenuation of second sound in superfluid 4He at 1.6 K. The force–velocity response of a quartz tuning fork operating at a frequency of 31 kHz exhibited the onset of extra damping at a velocity of 0.5 ms−1. Attenuation of the 5th resonant mode of second sound was observed at the same velocity, indicating the production of vortex lines. Our measurements demonstrate that an increase of the drag coefficient corresponds to the development of quantum turbulence
Melting curve of He: no sign of the supersolid transition down to 10 mK
We have measured the melting curve of He in the temperature range from 10
to 400 mK with the accuracy of about 0.5 bar. Crystals of different
quality show the expected -dependence in the range from 80 to 400 mK
without any sign of the supersolid transition, and the coefficient is in
excellent agreement with available data on the sound velocity in liquid He
and on the Debye temperature of solid He. Below 80 mK we have observed a
small deviation from -dependence which however cannot be attributed to the
supersolid transition because instead of decrease the entropy of the solid
rather remains constant, about Comment: 4 pages, 2 figures, published in Physical Review Letter
Quasiparticle transport in a two-dimensional boundary superfluid
The B phase of superfluid 3He can be cooled into the "pure" superfluid
regime, characterised by negligible thermal quasiparticle density. Here, the
bulk superfluid is bounded by a two-dimensional quantum well at the boundaries
of the container, where creating quasiparticles requires much less energy. In
this Article, we carry out experiments where we create a non-equilibrium state
within the quantum well and show that the induced quasiparticle currents flow
diffusively in the two-dimensional system. We conclude that the bulk of
superfluid 3He is wrapped by an independent two-dimensional superfluid that
interacts with mechanical probes instead of the bulk superfluid, only providing
access to the bulk superfluid if given a sudden burst of energy. That is,
superfluid 3He at the lowest temperatures and applied energies is
thermo-mechanically two dimensional. Our work opens this two-dimensional
quantum condensate and the interface it forms between the observer and the bulk
superfluid for exploration, and provides the possibility of creating
two-dimensional condensates of arbitrary topology.Comment: 11 pages, 9 figure
A low-frequency, high-amplitude, torsional oscillator for turbulence studies in quantum fluids
We describe a new type of torsional oscillator, suitable for studies of quantum fluids at frequencies of ∼ 100 Hz, but capable of reaching high velocities of up to several cm\,s−1. This requires the oscillator amplitude to exceed 100 μm, which is much too large for a conventional capacitor-driven device. We describe the new geometry for the oscillator, discuss its design, and report our initial tests of its performance
Anomalous damping of a low frequency vibrating wire in superfluid He-3-B due to vortex shielding
We have investigated the behaviour of a large vibrating wire resonator in the B-phase of superfluid He-3 at zero pressure and at temperatures below 200 mu K. The vibrating wire has a low resonant frequency of around 60 Hz. At low velocities the motion of the wire is impeded by its intrinsic (vacuum) damping and by the scattering of thermal quasiparticle excitations. At higher velocities we would normally expect the motion to be further damped by the creation of quasiparticles from pair-breaking. However, for a range of temperatures, as we increase the driving force we observe a sudden decrease in the damping of the wire. This results from a reduction in the thermal damping arising from the presence of quantum vortex lines generated by the wire. These vortex lines Andreev-reflect low energy excitations and thus partially shield the wire from incident thermal quasiparticles
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