Pulsed nuclear magnetic resonance measurements of the transverse frequency and magnetization of superfluid He in a 98.2% porous aerogel are observed at temperatures reduced significantly below the bulk He superfluid transition and are discussed in terms of an isotropic impurity scattering model. Magnetization measurements suggest an equal-spin pairing superfluid. For NMR tipping angles, @~4 0, the shifts drop abruptly to zero, unlike the known dependence of either the~He-A or He-B superfluid phases.

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The effect of impurity scattering on the phase diagram of pure superfluid 3He has been investigated by acoustic techniques near the bulk polycritical pressure. Impurities were introduced with a 98% porous silica aerogel. In zero applied field, the equilibrium phase is a B phase, consistent with predictions from isotropic scattering. The superfluid transition in a magnetic field at 25 bars is from normal to A phase and is independent of the magnetic field up to 2.9 kG. A first-order phase transition between A and B superfluids is observed with surprisingly strong supercooling in both zero and nonzero applied field.

Acoustic waves provide a powerful tool for studying the structure of matter. For example, the speed, attenuation and dispersion of acoustic waves can give useful information on molecular forces and the microscopic mechanisms of absorption and scattering of acoustic energy. In solids, both compression and shear waves occurÐlongitudinal and transverse sound, respectively. But normal liquids do not support shear forces and consequently transverse waves do not propagate in liquids, with one notable exception. In 1957 Landau predicted 1 that the quantum-liquid phase of helium-3 might support transverse sound waves at suf®ciently low temperatures, the restoring forces for shear waves being supplied by the collective quantum behaviour of the particles in the¯uid. Such shear waves will involve displacements of the¯uid transverse to the direction of propagation, and so de®ne a polarization direction similar to that of electromagnetic waves. Here we con®rm experimentally the existence of transverse sound waves in super¯uid 3 He-B by observing the rotation of the polarization of these waves in the presence of a magnetic ®eld. This phenomenon is the acoustic analogue of the magneto-optic Faraday effect, whereby the polarization direction of an electromagnetic wave is rotated by a magnetic ®eld applied along the propagation direction.Super¯uidity in 3 He results from the binding of the 3 He particles with nuclear spin s 1=2 into molecules called``Cooper pairs'' with binding energy 2¢ (refs 2±5). The pairs undergo a type of Bose± Einstein condensation, which has a close analogy to the Bardeen± Cooper±Schrieffer condensation 6 phenomenon associated with superconductivity in metals. One important difference is that the pairs that form the condensate in 3 He have total spin S 1 and an orbital wavefunction with relative angular momentum L 1 (`p-wave'). This is in contrast to superconductors which are formed with Cooper pairs of electrons having S 0 and L 0 (`s-wave') or, as is the case of high-temperature superconductors, S 0 and L 2 (`d-wave'). In super¯uid 3 He, the spin and orbital angular momentum vectors are locked at a ®xed angle to one another. This is called broken relative spin±orbit symmetry 2,3 . The equilibrium super¯uid state is described as a condensate of Cooper pairs with a total angular momentum J L S 0. In addition, the Cooper pairs can be resonantly excited by sound waves to quantum states with total angular momentum J 2 (ref. 7). This is reminiscent of diatomic molecules which have similar excited states.The above description applies to the B-phase of super¯uid 3 He, the most stable phase at low pressure. The acoustic Faraday effect occurs in 3 He-B as a consequence of spontaneously broken relative spin±orbit symmetry 8 . An applied ®eld magnetically polarizes the spins of the Cooper pairs which, through coupling to their orbital motion, rotates the polarization of transverse sound. The rotational excitations of Cooper pairs are essential 8 to our observation of the propagation of transverse acoustic waves in 3...

High-frequency ( approximately 15 MHz) acoustics were performed on 3He in 98% porous silica aerogel using an acoustic cavity technique. Measurements of the sound attenuation in the normal Fermi liquid and superfluid display behavior quite different from the bulk owing to strong elastic scattering of quasiparticles. The transition from first-to-zero sound is completely obscured with a quasiparticle mean-free path estimated to be in the range of 200-300 nm. No collective mode attenuation peak was observed at or below the superfluid transition.