Sound propagation in liquid He in impurity-helium solids

Abstract

The observed features of the attenuation of ultrasound in Im–He samples created after the introduction of impurity particles (D₂,N₂, Ne, Kr) in a volume of helium II show that a porous substance consisting of a loosely interconnected continuous network is created. It is formed by impurity particles encapsulated in solidified helium. The propagation of ordinary sound in these porous samples is similar to the fast sound mode in light aerogels. The temperature dependence of the attenuation for different Im–He samples is investigated. It is established that the character of the attenuation in D₂–He samples is considerably different from that in heavier Im–He solids (Im=N₂, Ne, Kr). Analysis of the attenuation leads to the conclusion that Im–He samples have a wide distribution of pores, from 8 nm to 800 nm. The study of ultrasound in helium in Im–He samples near the λ point shows the presence of broadening in the attenuation peak as compared with bulk liquid helium. The suppression of Tc is very small, ≤ 0.2 mK.