Parametric Generation of Second 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 narrow temperature range in the vicinity of $T_{\lambda }$. The temperature dependence of the threshold FS amplitude is found to be in a good quantitative agreement with theory [V. L. Pokrovskii and I. M. Khalatnikov, Sov. Phys. JETP 44, 1036 (1976)] and corrected for a finite geometry. Strong amplitude fluctuations and two types of the SS spectra are observed above the bifurcation. The latter effect is explained by the discreteness of the wave vector space and the strong temperature dependence of the SS dissipation length.