The influence of small concentrations of 3He impurities on the velocity of first sound in He <scp>ii</scp> at low temperatures

Precise relative measurements of the velocity of first sound in solutions containing 0.05, 0.155 0.51% 3He have been made in the temperature range 0.1–0.7° K. It was found that the nature of the temperature dependence of the velocity changes with changing 3He concentration. A maximum velocity (the magnitude of which increases with decreasing 3He concentration) is found in very weak solutions, while the velocity falls monotonically with increasing temperature in more concentrated solutions. The dependences obtained cannot be explained within the framework of existing theoretical concepts.

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