Observation of Fifth Sound in a Planar SuperfluidHe4Film

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 $C_5={\left(\frac{{\rho }_n}{\rho }\right)}^{\frac{1}{2}}{C}_2$, as predicted by the two-fluid theory.