Observation of dispersion in the J=2+ collective modes of 3He-B by nonlinear acoustic spectroscopy

Parametric excitation yields new results, which cannot be obtained by conventional linear spectroscopy, on the collective modes in superfluid 3He-B. In our ultrasonic experiments, performed in a chamber with four quartz crystals, we have excited the real squashing (J=2+) modes by two simultaneous sound waves which are mutually either parallel, orthogonal, or antiparallel. Qualitatively these results are in agreement with the theoretical dispersion relation of the J=2+ modes, and quantitatively we have been able to extract the collective-mode velocities from the line splittings in zero magnetic field