NONLINEAR ACOUSTIC EFFECTS IN SUPERFLUID HE-3-B

We consider the nonlinear interaction of zero sound with the collective modes of the order-parameter in superfluid He-3-B. The approximate particle-hole symmetry of the He-3-Fermi liquid determines selection rules for the linear and nonlinear coupling of zero sound to the collective modes. Starting from the quasiclassical theory of superfluid He-3, we have shown that the coupling strengths have a simple representation in terms of Feynman diagrams. We predict measurable two-phonon absorption and nonlinear-Raman scattering by the J = 2+ (real squashing) modes at low pressures. Recent observations of two-phonon absorption by a group in Helsinki are compared to the theoretical predictions. Two-phonon absorption can be used to determine the dispersion of the J = 2+ modes.