OBSERVATION OF 5TH SOUND IN A PLANAR SUPERFLUID HE-4 FILM

We report an observation of a superfluid helium-4 film sound mode consisting predominantly of a temperature wave, as in second sound, but with the normal-fluid component clamped. Our measurements indicate that this wave, called fifth sound, results in a contribution to the velocity of propagation which varies as C5=(ρnρ)12C2, as predicted by the two-fluid theory. © 1979 The American Physical Society.