Abstract
The velocity and attenuation of ultrasound passing through porous impurity-helium solids immersed in liquid 4He have been measured in the temperature range 1.2–2.3 K. These solids were formed by injecting a mixture of impurity (Ne or N 2) and helium gases into superfluid 4He. We found that the speed of sound at low temperatures is close to that of first sound in bulk helium but decreases more rapidly with increasing temperature, similar to behavior observed in aerogel. The attenuation of sound in helium in the impurity-helium solids is bigger than in bulk helium and increases rapidly with temperature up to ∼1.8 K , after which a crossover to a much weaker temperature dependence was observed.
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