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Physical Review B

Vortex generation induced by low-frequency wire vibration in superfluid H3e-B

Y. Nago, M. Inui, Ryusuke Kado, K. Obara, Hideo Yano, Osamu Ishikawa, T. Hata

Published Dec 14, 2010• 11 citations
Publisher

Abstract

We report the generation of a quantized vortex in superfluid $^{3}\text{H}\text{e-B}$. Oscillating objects can generate vortices due to breaking of Cooper pairs in superfluid $^{3}\text{H}\text{e}$, 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 $^{3}\text{H}\text{e}$ 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 $^{3}\text{H}\text{e}$. 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.

DOI
10.1103/physrevb.82.224511

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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.
An oscillating object can stretch quantized vortices attached to it in superfluids due to the relative superflow, steadily generating quantum turbulence, even in the zero-temperature limit. We report the emission and propagation of quantized vortices from quantum turbulence generated in superfluid 4{}^{4}4He at low temperatures. A vortex-free vibrating wire enables us to detect the first collision of vortex rings and therefore to measure the time-of-flight of a vortex emitted from a generator to a detector. The detection times from the start of turbulence generation exhibit an exponential distribution, suggesting that the detection is a Poisson process. Vortices are emitted continuously, but each vortex has a random flight velocity and direction. We estimated the nondetection time and mean detection period from the distribution for two flight distances. By estimating the flight velocity, we find that only vortices with velocities lower than the detector velocity can be detected, even if the sizes of the emitted vortices are smaller than the wire thickness or the vibration amplitude. The ratio of the detection rate as a function of vortex velocity suggests anisotropic emission of vortices from the quantum turbulence.
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 observed that the motion of a vibrating-wire resonator in superfluid 3B^{3}\mathit{B}3B at temperatures below Tc{\mathrm{T}}_{\mathrm{c}}Tc​/8 is associated with a sharp critical velocity, vc{\mathrm{v}}_{\mathrm{c}}vc​, above which very high dissipation sets in. We identify this velocity with the Landau critical velocity for pair breaking. That the observed value of vc{\mathrm{v}}_{\mathrm{c}}vc​ is a factor of 2 lower than the critical velocity expected from the BCS value of the gap we take as evidence that the gap is suppressed by about 50% of its bulk value near a moving boundary.
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 first observation of quantized circulation in superfluid 3B^{3}\mathit{B}3B. The apparatus consists of a straight vibrating wire immersed in liquid 3He^{3}\mathrm{He}3He, which is cooled by a rotating nuclear demagnetization cryostat. The experiment is carried out at about 250 \ensuremath{\mu}K. The superfluid at this temperature is in the ballistic quasiparticle regime. Circulation around the wire is found to be stable only when it takes on the values -h/2m3{\mathit{m}}_{3}m3​, 0, and +h/2m3{\mathit{m}}_{3}m3​, where h is Planck's constant and m3{\mathit{m}}_{3}m3​ is the mass of the 3He^{3}\mathrm{He}3He atom. This experiment confirms that superfluid 3B^{3}\mathit{B}3B is a Cooper-paired superfluid with a macroscopic quantum wave function.