K. Obara

Turbulence in boundary flow of superfluid 4He triggered by free vortex rings.

The transition to turbulence in the boundary flow of superfluid 4He is investigated using a vortex-free vibrating wire. At high wire vibration velocities, we found that stable alternating flow around the wire enters a turbulent phase triggered by free vortex rings. Numerical simulations of vortex dynamics demonstrate that vortex rings can attach to the surface of an oscillating obstacle and expand unstably due to the boundary flow of the superfluid, forming turbulence. Experimental investigations indicate that the turbulent phase continues even after stopping the injection of vortex rings, which is also confirmed by the simulations.

Motions of quantized vortices attached to a boundary in alternating currents of superfluid {sup 4}He

The motions of superfluid vortices attached to a boundary are investigated in alternating currents by using a vibrating wire. The attached vortices appear to form a layer on the wire and enhance the mass of the wire, even for low velocity currents. In turbulence, chaotic motions of vortices such as entanglement and reconnection reduce the thickness of the layer in spite of the fact that the vortices unstably expand. When turbulence subsides, the attached vortices appear to shrink, with the degree of shrinking influenced by thermal excitations in the superfluid.

Study on the Turbulent Flow of Superfluid 4He Generated by a Vibrating Wire

We have studied the flow of superfluid 4He generated by a vibrating wire. As the drive force increases, the velocity of the wire grows in the laminar‐flow regime, until it suddenly drops at the onset of the turbulent‐flow regime. As the drive force decreases, the turbulence disappears at a critical velocity. This result suggests that the vortices on the wire are confined within a finite size, even in turbulence. We have measured the critical velocity of seven vibrating wires, whose resonance frequencies range from 0.5 kHz to 9 kHz, at 1.4 K and found that the critical velocity is almost constant below an oscillation frequency of 2 kHz and increases above this frequency. We have also observed the response of a vibrating wire in superfluid 4He at a low temperature of 30 mK. We find that the resonance frequency jumps upward at the same moment as the entry of the flow to a turbulent state. The frequency jump may be caused by vortex dynamics such as expansion, entanglement, and reconnection occurring in the tu...

Hydrodynamic Property of Oscillating Superfluid 3He in Aerogel

The investigation of the superfluidity of liquid 3He in aerogel of 97.5% and 98.5% porosities using the fourth sound resonance technique revealed two distinct observations. First, the superfluid transition temperature TC and the superfluid density ρs/ρ of 3He in aerogel are greatly suppressed. Second, the sound attenuation does not depend on temperature at higher temperatures, but monotonically diminishes with decreasing temperature at lower temperatures.

Pulsed NMR Measurements in Superfluid 3He in Aerogel of 97.5 % Porosity

Aerogel is made of thin SiO2 strands of a few nanometer diameter. Since the coherence length of superfluid 3He is much longer than the silica strand diameter and is nearly the same as the mean distance between silica strands, aerogel gives us a chance to study the effects of an impurity in superfluid 3He. To investigate what superfluid states are formed in aerogel, we performed a pulsed NMR experiment. Both the A‐like and B‐like phases show a tipping angle dependent frequency shift in the FID signal after an rf pulse. The dependence in the A‐like phase is well explained by an expectation based on the “robust phase” introduced by Fomin, while the FID frequencies in the B‐like phase behave similarly to those observed in the bulk B phase in a slab geometry with the initial condition of a non‐Leggett configuration.

NMR study of superfluid phases of 3He confined in 97.5% porous silica aerogel

We have studied the scattering effect from aerogel strands on superfluid phases of 3He by a cw NMR method at 920 kHz. Liquid 3He at a pressure of 13 bar was confined in 97.5% porous aerogel from the same batch as that of a recent 4th sound study. The NMR experiment was performed in a magnetic field of 28.4 mT down to 0.3 mK. As temperature decreased, the NMR resonant frequency increased below 0.76 mK. The temperature of 0.76 mK agrees with the superfluid transition temperature Taerogelc observed in the 4th sound study at the same pressure. Below Taerogelc the behavior of thefrequency shift as a function of temperature indicates that there is no phasetransition to the other superfluid phase down to about 0.4 Taerogelc. Owing to a very large surface solid 3He magnetization, we could not determine the superfluid phase of 3He in the aerogel in the magnetization measurement.

Vortex emission by a low-frequency vibrating wire in superfluid 3He-B

We report the investigation of vortex emission by a vibrating wire in superfluid 3He-B at a pressure of 28 bar. We used two vibrating wires with 47 Hz and 183 Hz resonance frequencies as a generator and a detector of vortices. The onset velocity of vortex emission for the 183 Hz vibrating wire is higher than a pair-breaking velocity, indicating that pair breaking causes vortex generation. In contrast, the onset velocity for the 47 Hz vibrating wire is lower than the pair-breaking velocity. This result suggests that another mechanism of vortex emission arises for the lower-frequency vibrating wire: pair breaking due to local flow enhanced by protuberances on the surface of the wire, or instability of remanent vortices attached to the wire by vibration.

A‐B Phase Transition and Pinning of Phase Boundary of Superfluid 3He in Aerogel

Phase transition in superfluid 3He in aerogel has been studied by NMR. Above 19 bar, we have clearly observed the A‐like and B‐like phases by following changes in the NMR lineshapes and resonance frequencies. There is a wide temperature region in which the A‐like phase and the B‐like phase coexist, extending from near the superfluid transition temperature Tcaero to the lowest temperature of coexistence, TABaero, below which only the B‐like phase exists. There are two temperature regions, only in which the phase conversion occurs. Both regions are a few tens of μK wide, the upper region being just below Tcaero and the lower one just above TABaero. In cooling down and warming up with the two phases in coexistence, no phase conversion occurs between the two regions. The phase boundary between the A‐like phase and B‐like phase cannot move in aerogel due to strong pinning by inhomogenities of aerogel.

Vortex emissions from quantum turbulence generated by vibrating wire in superfluid 4He at finite temperature

An oscillating object immersed in superfluid helium generates quantum turbulence, emitting quantized vortices to its surroundings. We report vortex emissions in directions parallel and perpendicular to the oscillating motion of a thin wire used as a turbulence generator. Two vibrating wires are used to detect the vortex emissions. We use superfluid 4He as a medium, with the temperature set to 1.25 K, at which a small amount of normal fluid component is present. In this setup, only vortex loops with sizes larger than a certain loop diameter D = 42 μm can be detected. In the perpendicular direction, vortex loops are detected when the oscillation amplitude is comparable with D. In the parallel direction, however, no vortex loops are detected at the same amplitude, suggesting an anisotropic emission of vortex loops.

Anomalous sound absorption of finite amplitude sound in liquid 4He

We report the nonlinear and unsteady response of the finite amplitude pressure wave in superfluid 4He, using a classical vibrating liquid column standing wave resonator. The frequency was kept at the resonance frequency of the liquid column, so that the maximum of the pressure field always came to the surface of the driver. As a result, in the low driving-amplitude regime, I/O relations were linear. However, in the high driving-amplitude regime, anomalous responses were found; The amplitude of the signals suddenly reduced and gradually recovered; they were repeated with random intervals. Moreover, in the highly suppressed region in the waveform, we found a generation of new type of turbulence. These absorptions are expected to be the result of the vapor bubble ejection from the surface of the driver transducer.