Pressure dependence of the sound velocity of 4He in aerogel

Abstract

Acoustic properties have been studied in 4He–aerogel system. The longitudinal ultrasound velocity and attenuation have been measured in 92.6 and 94.0% porous aerogels at elevating pressures from the saturated vapor pressure to the melting point for both normal and superfluid phase. The temperature dependence of the sound velocity was similar to that of bulk liquid. The superfluid transition temperature was determined by a dip in velocity and a sharp peak in attenuation. The sound velocity was analyzed using a hydrodynamic theory for both phases. The pressure and temperature dependencies of the sound velocity in the normal phase can be rather well explained by a simple hydodynamic theory. However, at the lowest temperature where no normal component is expected, the sound velocity behavior differs significantly from the case of bulk case. It is shown that coupling mechanism between acoustic phonons in the liquid and aerogel matrix other than viscosity is necessar to elucidate the sound velocity below 1 K.