Sound propagation in liquid He impurity-helium solids

The observed features of attenuation of ultrasound in Im-He samples created after 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 this porous samples is similar to the fast sound mode in light aerogels. The temperature dependence of attenuation for different Im-He samples is investigated. It is established that the character of attenuation in D2-He samples is considerably different from that in heavier Im-He solids (Im=N2,Ne,Kr). Analysis of attenuation lets us conclude that Im-He samples have a wide distribution of pores from 8nm 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, ≤ 0.2mK