K. Torizuka

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.

Experimental investigation of two-phonon absorption by the real squashing mode in superfluid 3He-B

We report on investigations of a nonlinear parametric excitation of the real squashing (RSQ) collective mode in superfluid 3He-B. Two-phonon absorption (TPA) at fs + fp = frsq has been observed by two coincident zero sound pulses, with the signal and the pump wave frequencies at fs and fp, respectively. We have also discovered and studied the Zeeman splitting of TPA in a magnetic field, which is a characteristic feature of the J = 2+ mode in the linear response regime. The dependence of the mode frequency on the magnitude of the wave vector has been investigated; this can be done easily in parametric measurements only. In our earlier, preliminary observations, the coupling of the RSQ mode by TPA was weaker than theory predicted; the discrepancy has now been systematically studied and clarified.

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.

Experiments on nonlinear acoustics in3He-B

Observations and applications of nonlinear acoustic phenomena in superfluid3He-B are reported. Two-phonon absorption (TPA) by the real squashing (rsq) mode has been detected under several experimental conditions below p = 3.5 bar, using two coincident sound pulses. The attenuation peak height has been investigated as a function of the energy densities of the two sound waves. We discovered the five-fold Zeeman splitting of TPA by parallel sound pulses in an applied magnetic field and the two-fold dispersion splitting due to the finite wave vector of the mode when the two sound pulses are mutually perpendicular. The dispersion relation of the real squashing mode has been investigated at zero pressure and in zero magnetic field by exciting the mode with two parallel, perpendicular, or antiparallel sound waves. Experimental values for the parameters that determine the collective-mode velocities have been extracted from the positions of the observed attenuation maxima. An anomalous structure has been observed in the attenuation and phase velocity spectra of a single high-intensity sound wave near the threshold for pair breaking by two phonons; in an applied magnetic field the phase velocity anomaly splits into a triplet.

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.

Nonlinear acoustical investigations of3He-B near the two-phonon pair breaking edge

Abstract We report on experiments using energetic ultrasound pulses on superfluid 3 He-B. Near the two-phonon pair breaking edge, at hf=Δ(T) , an anomaly in ultrasonic attenuation has been detected. This feature grows with increasing acoustical intensity, and a new peak develops in the attenuation spectrum. The anomaly, coupling nonlinearly to ultrasound, can be observed also in the phase velocity of sound; in a magnetic field the anomaly splits to at least three features.

Nonlinear acoustic spectroscopy on the real squashing collective mode in3He-B

Abstract We have excited the real squashing (RSQ) collective modes in3He-B with two simultaneous ultrasound pulses, yielding two-phonon absorption (TPA). We also detected TPA using single-frequency high-intensity sound pulses. The Zeeman splitting of the parametrically excited RSQ mode has been demonstrated. We checked the dispersion relations of the J=2+ modes in zero magnetic field and extracted the collective mode velocities directly from experiments.

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 3He-A vortices in low magnetic fields

Abstract We report ultrasonic measurements on rotating 3He-A at 26.7 bar pressure and at magnetic fields below 12.0 mT. We observed two distinct types of vortices: the attenuation of zero sound by vortices which were created at small fields is higher than the attenuation by vortices created at high fields. The first order transition between the two vortex states was observed at Hc∼HD (≌3 mT). The low field structure can be identified with type-I vortices proposed by Fujita et al. and the high field one with that of Seppalaand Volovik and initially discovered in NMR experiments. The structural change may be viewed as a topological transition, because the dˆ-vector distribution is different for the dipole-locked vortices in low fields from that for the dipole-unlocked vortices in high fields. In addition, a continuous change between coreless vortices and vortices with a well-defined core was observed at low enough fields H′ ≪Hc. The vortices in 3He-A can, therefore, be characterized by three magnetic fields: H′, Hc, and Hcl. The last is the catastrophe field at which the dipole-locked vortices become unstable.