Attenuation and dispersion of first sound in 3He-4He mixtures near T(lambda)

The attenuation α, the velocity u, and the dispersion D = u(ω)-u(0) of first sound have been determined in 3He-4He mixtures (molar 3He concentrations X3 = 0, 0.070, 0.194, 0.377, and 0.517) at frequencies between 2.3 kHz and 627 kHz, and in the temperature range LT-T(lambda) between 1 μK and 20 mK. From measured velocities we evaluate the thermodynamic velocity u(0). The attenuation and the dispersion are considerably reduced when the 3He concentration is increased. They are interpreted as arising from a relaxation process occurring only below T(lambda), and a fluctuation process occurring on both sides of the lambda-transition. For T > T(lambda), our absorption and dispersion data for all ω and X3 can be scaled. This scaling analysis shows that the time characterizing the critical attenuation and dispersion at T > T(lambda) has the same temperature and concentration dependence as the relaxation time at T < T(lambda); these two times differ at most by a constant multiplicative factor. (orig./WL)