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Abstract:
We consider the nonlinear interaction of zero sound with the collective modes of the order-parameter in
superfluid 3He-B. The approximate particle-hole symmetry of the 3He-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 3He, we have shown that the coupling strenghts have a
simple representation in terms of Feynman diagrams. We predict measurable two-photon 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.
Invited Paper: Proceedings of the Körber Symposium on Superfluid 3He in Rotation, Helsinki, 1992
Paper: [PDF]
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