Collective Modes and Nonlinear Acoustics in Superfluid 3He-B

TL;DR

This paper explores the relationship between collisionless sound propagation, attenuation, and collective modes in superfluid 3He-B, emphasizing the nonlinear acoustic response and resonance phenomena involving specific collective modes.

Contribution

It develops a comprehensive theory of nonlinear acoustic response in superfluid 3He-B, including three-wave resonance and higher harmonic generation involving J=2+/- modes.

Findings

Nonlinear absorption and velocity anomalies are predicted based on well-understood material properties.

The theory links collective modes to high-frequency acoustic responses.

Nonlinear coupling between sound waves and collective modes is characterized.

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.