PARAMETRIC GENERATION OF 2ND SOUND BY FIRST SOUND IN SUPERFLUID-HELIUM

We report the first experimental observation of parametric generation of second sound (SS) by first sound (FS) in superfluid helium in a nar row temperature range in the vicinity of T-lambda. The temperature dep endence of the threshold FS amplitude is found to be in a good quantit ative agreement with theory [V. L. Pokrovskii and I.M. Khalatnikov, So v. Phys. JETP 44, 1036 (1976)] and corrected for a finite geometry. St rong amplitude fluctuations and two types of the SS spectra are observ ed above the bifurcation. The latter effect is explained by the discre teness of the wave vector space and the strong temperature dependence of the SS dissipation length.