Probing collective modes of superfluid 3He-B with zero sound

The authors review what is currently known experimentally about the collisionless collective mode spectrum of the order parameter of the B phase of superfluid 3He. The most powerful probe of this spectrum has proved to be longitudinal zero sound and the knowledge of the remaining modes comes about through mode coupling effects. The mode coupling leads to high attenuations over much of the range of interest and negates the use of the standard (phase sensitive) pulse transmission technique; this necessitates the use of acoustic impedance methods. The acoustic and cryogenic techniques are discussed in some detail. 29 references, 7 figures, 1 table