J. M. Kyynäräinen

A cryopump-operated rotating nuclear demagnetization cryostat for research on superfluid3He

We have constructed a rotating submillikelvin cryostat in which the dilution refrigerator can be operated in a continuous mode using two cryopumps. The4He pot is also equipped with similar pumps. The dilution unit is employed to precool a nuclear stage with 32 mol of copper in an 8-T field. In the design, special care was taken to make fast rotations possible. The apparatus is currently used to measure collective mode phenomena in superfluid3He-B.

Zero-sound attenuation in rotating and stationary3He-A and3He-B

We report on measurements of zero-sound attenuation in rotating and stationary3He-A and3He-B, in magnetic fields up to 350 mT. Strong and highly nonlinear rotation speed dependencies of sound amplitudes have been observed in both phases. The data gives information on vortex types and core sizes, although the analysis is not straightforward. The anomalous attenuation in3He-B at 200 mT near the AB transition, both in the stationary and in the rotating state, is interpreted to arise from the distortion of the energy gap of the B phase. Excess attenuation during the AB phase change was observed. Evidence for soft vortex cores in3He-B is presented. In addition, a critical velocity in the vortex free state, related to a textural transition, and the vortex creation times have been measured in3He-B. Furthermore, a metastable structure, possibly a new vortex state, has been observed in3He-B by rotating the sample through the A → B transition.

The Re-Entrant Normal Flapping Mode in the Dipolar Boundary Layers of 3He-A

Zero sound experiments on superfluid 3He-A at low pressures ( 0.3 T) show an increase in the attenuation at low temperatures (≤ 0.4 mK) which has been a puzzle until the present experiment. Coupling of the re-entrant normal flapping (NFL) mode to zero sound in a geometry where sound propagates in the direction of the magnetic field is shown to arise from a boundary layer at the walls whose thickness is about the dipolar coherence length ξD of 3He-A. We estimate the value of ξD at low temperatures and at low and intermediate pressures. We have also studied the nonlinearity of sound propagation near the NFL mode and found a reversal of it when passing the mode.

Ultrasonic investigation of rotating superfluid 3He

The coupling of the real squashing collective mode to the zero sound in rotating superfluid 3He‐B has been investigated. In a magnetic field rotation enhances the coupling of the non‐zero mJ substrates, and the five or sometimes six‐fold line splitting becomes observable even when H is parallel to Ω and to the direction of sound propagation. Equilibrium vortex lattices and vortex free states can be distinguished by their characteristic absorption spectra. The data are compared with microscopic calculations and a recent phenomenological theory. In the A‐phase the anisotropy of the sound attenuation serves as a powerful means to detect the 1 texture rearrangement when a vortex lattice is formed; measurements have been made in zero and in weak (1.7 mT) magnetic fields.

Zero sound experiments on the distorted b phase and in the BA transition region at low pressures

Abstract Zero sound at 8.9 and 26.8 MHz, propagating parallel with the magnetic field, shows an attenuationmaximumalongtheTB A line in a field of∼ 2 kG at low pressures. This is in accordance with the theoretical prediction that a critical magnetic field separates two types of B phases, with and without nodes in the energy gap at the transition line. The B → A phase change takes place via an intermediate state, with excess sound attenuation both at 8.9 and 26.8 MHz.

Wave acoustics for propagation of ultrasound along a vortex array in superfluid 3He-A

A wave-acoustics theory has been developed to describe the propagation of zero sound parallel to vortex lines in rotating 3 He-A. We show that a diffraction «shadow» is formed in which the wave amplitude is suppressed by interference around vortices. This phenomenon contributes to the experimentally observed effect of rotation on the sound amplitude and exceeds the attenuation at large core radii. The dependence of the diffraction contribution on the angular velocity changes drastically at the transition from vortices with a finite core to foreless vortices when the magnetic field is decreased to zero

Ultrasonic investigation of rotating3He-B in strong magnetic fields

We have studied ultrasonic attenuation in rotating 3He-B at low pressures (< 10 bar) in magnetic fields up to 300 mT. In a strong field, the energy gap Δ in 3He-B is anisotropic, and the attenuation α of zero sound depends on the orientation of Δ. Because vortices and the counterflow between the normal and superfluid components of the liquid turn the anisotropy axis of Δ, a clear dependence of α on the rotation velocity Ω can be observed. The magnitude of this response is not a linear, sometimes not even a monotonic function of Ω. Its dependence on the magnetic field is anomalous around H = 200 mT. We have also observed a metastable state in rotating 3He-B, which can be created by rotating the sample while cooling through the AB-transition. The lifetime of this state is on the order of 10 minutes.

Ultrasonic investigations of vortex textures in 3HeA

Abstract Zero sound attenuation has been used to probe vortex textures in rotating 3HeA. The highly nonlinear rotation speed dependencies of the measured sound amplitudes can be explained partly as being due to sonic velocity gradients in the vortex cores. Calculations of sound propagation in the geometrical limit have been made to extract information on the types of vortex cores and their sizes using our experimental data.

Rotating 3HeA2in the vicinity of the transition to the Al phase

Abstract We show that, owing to a decrease in the dipolar energy, the soft cores of the nonsingular, doubly quantized vortices in 3HeA2 expand and become ellipsoidal upon approaching the A1 phase. Results of zero sound experiments in rotating 3HeA2, very near the A2A1 transition, support this conclusion. The experimental data was analyzed by a geometrical acoustics method, using theoretically calculated attenuation and velocity parameters.

Ultrasonic experiments on rotating superfluid 3HeA1

Abstract The first experiments on rotating 3HeA1 have been done using pulsed zero sound transmission techniques at several pressures in magnetic fields up to 1 T. Preliminary analysis of our data by a geometrical acoustics method suggests that the vortex cores in 3HeA1 are larger than in 3HeA2. This result is based on the strong and highly non-linear rotation dependence of sound attenuation in 3HeAl. Conclusions about the type of vortices in 3HeA1 cannot be made at present owing to lack of theory.