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From sound-velocity measurements we found that the presence of helium in the pores of Vycor increased the relaxation rates of two-level systems in the glass. Superfluid ⁴He caused a much larger increase than either solid ⁴He or liquid ³He. The results are interpreted in terms of additional thermal phonons in the helium, with fourth-sound modes possibly causing the faster relaxation for superfluid ⁴He.
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Beamish et al. (1984) studied this question.
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