Parametric Generation of Second Sound by First Sound in Superfluid Helium

TL;DR

This paper reports the first experimental observation of parametric generation of second sound by first sound in superfluid helium, confirming long-standing theoretical predictions and exploring the effects of geometry and temperature on the process.

Contribution

It provides the first experimental evidence of parametric second sound generation in superfluid helium and validates theoretical models including finite geometry effects.

Findings

Threshold FS amplitude matches theoretical predictions.

Observation of strong amplitude fluctuations and two SS spectral types.

Quantitative explanation of spectral features based on wave vector discreteness and dissipation length.

Abstract

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 the theory suggested long time ago 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 quantitatively explained by the discreteness of the wave vector space and the strong temperature dependence of the SS dissipation length.