Limits on transverse zero sound in Fermi liquid 3 He

Landau predicted that transverse sound propagates in a Fermi liquid with sufficiently strong interactions such as for liquid 3 He, unlike a classical fluid that cannot support shear oscillations. Previous attempts to observe this unique collective mode yielded inconclusive results. We microfabricated acoustic cavities with a micrometer-scale path length that is suitable for direct detection of this sound mode and used the interference fringes from our acoustic Fabry-Pérot cavities to determine both real and imaginary parts of the acoustic impedance. No interference was observed. Either transverse sound does not exist or its attenuation must be very high, well above 2000 per centimeter. We discuss a theoretical framework for such high attenuation.