UNIVERSAL BROADENING OF FREQUENCY-SPECTRA IN PARAMETRICALLY DRIVEN SYSTEMS

We report the experimental observation of the broadening of the freque ncy spectra of parametrically generated second sound waves by first so und in superfluid He-4. The first experimental evidence of Gaussian st atistics of amplitudes of parametrically generated waves is presented. Using the kinetic theory of parametric excitation we suggest that the broadening and exponential tails of spectra result from four-wave res onance interaction in a close analogy with similar-evidence in paramet rically generated spin waves. Universality of the exponential tails in the frequency spectra is suggested.