R. Baierlein et al., EXPERIMENTAL-DETERMINATION OF SOME NONLINEAR ELEMENTS OF 3RD SOUND DYNAMICS, Journal of low temperature physics, 108(1-2), 1997, pp. 31-52
The frequency shifts of third sound resonances during free decay are o
bserved to have a quadratic dependence on the amplitude of the wave mo
tion. An unambiguous measurement of amplitude allows for quantitative
comparisons to predictions for the frequency shifts based on a variety
of nonlinear influences acting within the superfluid He-4 film and re
sonator. Dispersive terms play a minor role in this analysis. We concl
ude that nonlinear terms dictated by a straight forward application of
classical hydrodynamics are applicable to superfluid helium films.