Sound propagation in liquid He in impurity-helium solids

The observed features of the attenuation of ultrasound in Im-He samples cre ated after the introduction of impurity particles (D-2,N-2, Ne, Kr) in a vo lume of helium II show that a porous substance consisting of a loosely inte rconnected continuous network is created. It is formed by impurity particle s encapsulated in solidified helium. The propagation of ordinary sound in t hese porous samples is similar to the fast sound mode in light aerogels. Th e temperature dependence of the attenuation for different Im-He samples is investigated. It is established that the character of the attenuation in D- 2-He samples is considerably different from that in heavier Im-He solids (I m=N-2, 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 stu dy of ultrasound in helium in Im-He samples near the lambda point shows the presence of broadening in the attenuation peak as compared with bulk liqui d helium. The suppression of T-c is very small, less than or equal to 0.2 m K. (C) 2000 American Institute of Physics. [S1063-777X(00)00309-1].