Acoustic studies of the mobility of bcc 4He–He II phase boundary

Measurements of the second sound reflection coefficient at the bcc 4He–He II phase boundary are used to obtain information on the kinetic coefficient of crystal growth. This coefficient increases with decreasing temperature, which is a characteristic feature of a rough interface in the case of helium. The coefficient of transformation of the first sound into second is measured, and absolute values of temperature oscillations at the interface are determined. The obtained results are correctly interpreted by the Castaing–Nozieres theory. First and second sound velocities in He II are measured along the melting curve. It is shown that in a wide temperature range the experimental results on the growth rate of helium crystals may be quantitatively described by the diffusion growth model if the coefficient of roton self-diffusion is used as the effective diffusion coefficient. It is found that in the metastability region, the amplitudes of the transmitted first sound and of the second sound appearing at the interface increase sharply with decreasing temperature.

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