The effect of surface solid on the propagation of sound through superfluid4He in Grafoil

Abstract

The velocity and attenuation of sound propagating through superfluid4He confined to Grafoil pores have been measured. The data are correlated with an adsorption isotherm of surface solid4He, grown from the pressurized liquid phase. The adsorption isotherm was obtained simultaneously in the same cell. A definite relationship exists between changes of the acoustic properties and growth of the solid layer, as suggested by Maynardet al. The sound is sensitive not only to thickness changes, but also to structural phase transitions of the solid layer. The results imply that in contrast to bulk4He, melting and freezing in thin solid layers is not a wavelike phenomenon. Instead it is proposed that coupling of sound to the surface is achieved through an excitation of a Rayleigh wave.