Fifth Sound in SuperfluidHe4andHe3-He4Mixtures

Abstract

A new thermal mode, fifth sound, has been observed in a superleak partially filled with superfluid helium. The velocity of the mode in ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ and ${}_{}{}^3{}_{}{}^{}\mathrm{He}$-${}_{}{}^4{}_{}{}^{}\mathrm{He}$ mixtures is in good agreement with the theoretical value ${c_5}^2=\left(\frac{{\rho }_n}{\rho }\right){c_2}^2$. The index of refraction is found to be a strong function of the fractional filling of the superleak.