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Abstract:
We discuss the relationship of collisionless sound propagation and attenuation to the order parameter
collective modes of superfluid 3He-B. These modes, which reflect the symmetries of the normal
state as well as the broken gauge and relative rotational symmetries of the condensate, play a central
role in the high-frequency acoustic response of the superfluid. We review the theory of acoustic
spectroscopy based on the linear coupling of the J=2± modes. We develop the theory of
the nonlinear acoustic response of superfluid 3He-B for the case of a three-wave resonance
between two sound waves of different frequencies and the J=2± modes, as well as the
generation of higher harmonic sound waves. The J=2+ modes couple nonlinearly to two
zero-sound waves. The predictions for the nonlinear absorption and velocity anomalies resulting from
the J=2+ modes depend on relatively well understood material properties of 3He.
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