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Physical Review Letters

Repetitive Single Vortex-Loop Creation by a Vibrating Wire in Superfluid 3He−B

D. I. Bradley

Published Feb 7, 2000• 47 citations
Publisher

Abstract

Spectacular features are observed on the velocity-force characteristics of a vibrating wire resonator in superfluid 3He-B at ultralow temperatures. Both plateaus and discontinuities are seen in the characteristics. The plateaus seem to have two separate critical velocities where first some "event" occurs, which causes the wire to lose energy and slow down, followed by a second lower critical velocity where the event decouples. It is suggested that these events are due to vortex-loop creation at protuberances on the vibrating wire. This opens up the possibility of controlling the creation of vorticity through specially prepared protuberances.

DOI
10.1103/physrevlett.84.1252

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Comment on “Repetitive Single Vortex-Loop Creation by a Vibrating Wire in Superfluid H3e−B ”

Michael Niemetz, W. Schoepe, M. Krusius

A Comment on the Letter by D. I. Bradley, Phys. Rev. Lett. 84, 1252 (2000). The authors of the Letter offer a Reply.Received 18 January 2001DOI:https://doi.org/10.1103/PhysRevLett.87.059601©2001 American Physical Society

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Comment on "Repetitive single vortex-loop creation by a vibrating wire in superfluid He-3-B" - Reply.

D. I. Bradley, G. R. Pickett

•Jul 1, 2001•0 citations
Spectacular features are observed on the velocity-force characteristics of a vibrating wire resonator in superfluid 3He-B at ultralow temperatures. Both plateaus and discontinuities are seen in the characteristics. The plateaus seem to have two separate critical velocities where first some "event" occurs, which causes the wire to lose energy and slow down, followed by a second lower critical velocity where the event decouples. It is suggested that these events are due to vortex-loop creation at protuberances on the vibrating wire. This opens up the possibility of controlling the creation of vorticity through specially prepared protuberances.
We have investigated the behaviour of a large vibrating wire resonator in the B-phase of superfluid 3 He at zero pressure and at temperatures below 200 μK. 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.
Probing Bogoliubov Quasiparticles in Superfluid 3^33 3 He with a ‘Vibrating-Wire Like’ MEMS Device
We have measured the interaction between superfluid 3He–B and a micro-machined goalpost-shaped device at temperatures below 0.2Tc. The measured damping follows well the theory developed for vibrating wires, in which the Andreev reflection of quasiparticles in the flow field around the moving structure leads to a nonlinear frictional force. At low velocities, the damping force is proportional to velocity, while it tends to saturate for larger excitations. Above a velocity of 2.6 mm s−1, the damping abruptly increases, which is interpreted in terms of Cooper-pair breaking. Interestingly, this critical velocity is significantly lower than that reported with other mechanical probes immersed in superfluid 3He. Furthermore, we report on a nonlinear resonance shape for large motion amplitudes that we interpret as an inertial effect due to quasiparticle friction, but other mechanisms could possibly be invoked as well.
This thesis describes experiments conducted on B phase 3He in the μK temperature regime to investigate the turbulent properties of quantised vortices created by driving a vibrating wire resonator above its superfluid pair breaking critical velocity. By operating several resonators as highly sensitive vortex detectors the localised effects of vortex lines have been measured. The results have shown the vortices being of greater density in the directions of the generating wire motion and the rate of production being dependent upon the generator wire velocity. The rate at which the vortices decay spatially has been shown to approximate to a simple exponential and the decay length of this exponential has been measured. Using a thermal quasiparticle beam emitted by a black body radiator, the temperature dependence of the vorticity has been investigated for several quasiparticle beam temperatures between 177μK and 275μK and several bulk superfluid temperatures between 171μK and 236μK. The vortex line density of the turbulence has been calculated using a mathematical model developed here at Lancaster and shows some dependence on the temperature of the superfluid. The method of using a quasiparticle beam emitted from a black body radiator may be developed to allow a higher resolution of investigation of vortex creation and decay.
We report the generation of a quantized vortex in superfluid 3He-B^{3}\text{H}\text{e-B}3He-B. Oscillating objects can generate vortices due to breaking of Cooper pairs in superfluid 3He^{3}\text{H}\text{e}3He, resulting that the threshold velocity for vortex generation is higher than the pair-breaking velocity. Using very-low-frequency vibrating wires with smooth surfaces, however, we find that the threshold velocity of vortex generation is lower than the pair-breaking velocity. The threshold velocity remains constant with decreasing 3He^{3}\text{H}\text{e}3He pressure until the pair-breaking velocity equals the vortex-generation threshold velocity at a low pressure. These results indicate that another mechanism that is independent of pressure generates vortices for the low-frequency vibrating wire in superfluid 3He^{3}\text{H}\text{e}3He. The threshold velocity observed in experiments turns out to be nearly equal to the velocity of the instability of the vortex lines attached to the wire, which is characterized by only the circulation quantum.