T. Hata

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.

Magnetic properties of solid 3He down to 0.3 mK

Nuclear magnetization of solid 3He has been studied by static magnetization measurements from 10 mK down to 0.3 mK for molar volumes ranging from V = 24.14 to 21.02 cm3/mole in the bcc phase and from V= 19.83 to 19.26 cm3/mole in the hcp phase. In the bcc phase, both the antiferromagnetic transition temperature TN and the reciprocal of the maximum magnetization Vmax−1 at TN vary in proportion to V16.5±1, and the magnetization below TN is constant. The magnetization reduced by Mmax is found to be represented by a universal function of the reduced temperature T/TN. In the hcp phase, the magnetization can approximately be represented by Curies law, and the estimated Weiss temperatures are below 50 ΜK. We also observed that the boundary magnetism of liquid 3He depends considerably on pressure. The transition temperature of solid 3He to the antiferromagnetic phase coexisting with liquid in a restricted geometry is 15% higher than that of the bulk solid on the melting curve.

Cooling of3He-4He dilute solution down to 97 μK. Thermal boundary resistance between dilute solution and metal powder

We have cooled a3He-4He dilute solution down to 97 μK, which is the lowest temperature ever been achieved in a dilute mixture. However, there is no sign of the superfluid transition of3He quasiparticles in the solution. In the sub-millikelvin region, we have measured the thermal boundary resistance between the solutions and sintered metal powder as a function of temperature T. We find that the thermal boundary resistence is proportional to T−2 below 1 mK and that the resistance shows a strong dependence on magnetic fields below 0.1 T. These results suggest that the magnetic coupling is dominant in this temperature region. We have also estimated the heat leak into the dilute solution. It is found that the heat leak is proportional to the power of one third of inverse time, and the main source of the heat leak is ascribed to the viscous movement of3He quasiparticles.

A–BTransition of Superfluid 3He with a Film Geometry

We have performedcwNMR experiments on superfluid3He confined to a parallel-plate geometry with a μm scale spacing for a wide pressure range. A static field was applied parallel or perpendicular to the plate surface. The spectra of two absorption signals, a main and a satellite, have been observed below the superfluid transition temperature in a parallel field. As the temperature decreased, the main signal decreased with shifts to higher frequencies, and the satellite grew with shifts to much higher frequencies. From the temperature dependence of these signals and the result in the perpendicular field, it is confirmed that the main signal and the satellite correspond to the A phase signal (ABM state) and the B phase signal (BW state), respectively. The temperature dependence of the two signals indicates that a phase transition from the A phase to the B phase occurs with decreasing temperature. By analyzing these signals, we determine A–B transition temperatures experimentaly. TheA–Btransition temperature normalized by the superfluid transition temperature is 0.95 at 20 bar, and decreased further to 0.70 at 0 bar for a thickness of 0.88 μm for pure3He. The values of TAB/TCwere slightly elevated when covering the surface with 4.5 layers of4He film, which suggests that this transition is also influenced by the surface condition.

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.

Surface Effects of 4He Coating on the Viscosity and the Slip Length of Normal and Superfluid 3He

We have measured the viscosity, η, and the slip length, ζ, of normal and superfluid3He using a torsional oscillator with a thick sample space. We coated the oscillating surface with 2.5 layers of4He film to study how the4He thin film changes the scattering mechanism of3He quasiparticles at the cell wall at 5 bar and 21 bar. In the normal phase, the temperature dependence of the viscosity was changed a little by the4He film at 21 bar but no change was observed at 5 bar. The slip length was enhanced by4He coating at 5 bar. This enhancement indicates the increase of specularity of3He quasiparticles scattering at the oscillating surface. On the other hand, a reduction of the slip length was observed at 21 bar. In the superfluid phase, the temperature dependence of ζ supports the existence of Andreev reflection even with4He film on the surface at 5 bar and 21 bar.

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.

Sound Experiments on Superfluidity of 3He in Highly Porous Aerogels

We have carried out sound experiments on superfluid 3He in three highly porous aerogels with different porosities. Two of the acoustic cells contain aerogels inside the pores in roughly sintered silver powder to avoid the vibration of the aerogel. In these acoustic cells we have detected fourth sound, and extracted the superfluid density from the fourth sound velocity. The effect of the sintered silver on superfluid 3He was examined by using another acoustic cell which contains the sintered silver without aerogel. The size of the pores in the sintered silver was large enough not to show the size effect of superfluid 3He and small enough to observe fourth sound of 3He. In another cell without sintered silver, we have observed second-sound-like signal. The superfluid transition temperatures of 3He are suppressed more in higher density aerogel. The aerogel density dependence of the suppression of the superfluid transition temperature of 3He in aerogel can be explained qualitatively by the simple s-wave scattering approximation. However, the superfluid density shows quite different pressure-dependence in different porous aerogels. The reason of this phenomenon is not understood yet.