The Transverse Acoustic Response of Superfluid HELIUM-3-BORON

Collisionless longitudinal sound has been a powerful tool for probing the superfluid states of ^3 He; most notable has been the study of the so called squashing modes of superfluid ^3He -B. A prediction, for both normal fluid and superfluid ^3He, is for the propagation of a collisionless transverse sound mode. The study of this mode in the normal fluid has been problematic: it travels only slightly faster than the Fermi velocity and is very highly attenuated. Although early theoretical results suggested that transverse sound would not propagate in the superfluid, recent theoretical work has predicted that this mode should indeed propagate, at sufficiently high frequencies and low temperatures, due to the interaction with the imaginary squashing mode. In the present work the transverse acoustic response has been measured in the B phase of superfluid ^3He. These measurements were performed on a short path length (30.5 microns) acoustic cavity, using a continuous wave, single ended, acoustic impedance technique. Simultaneous measurements were made of the longitudinal acoustic response, on an adjacent acoustic cavity of similar geometry. Both sound modes were excited at a frequency of 61 MHz. With this arrangement, well understood features in the longitudinal acoustic response were used as fiducial points for the study of heretofore ambiguous or unobserved features in the transverse acoustic response. As predicted by recent theoretical calculations, the transverse acoustic response was markedly different when the sound frequency was greater than the imaginary squashing mode frequency, as compared to when the sound frequency was less than the imaginary squashing mode frequency. At lower pressures the transverse acoustic response clearly exhibited the signatures of an evolving standing wave pattern (with the transverse sound velocity much less than the longitudinal sound velocity), and as such provides convincing evidence of a propagating transverse mode.