Quantum Crystal Oscillations in a Superfluid Liquid

We study the hydrodynamics of quantum ⁴He crystal oscillations in a superfluid liquid with involving the dynamics of atomically rough surfaces. It is shown that, due to enhancement of the kinetic growth coefficient as the temperature lowers, the reaction force of the liquid applied to the ⁴He crystal changes its character from the inertial to the viscous one vanishing as T→0. The model is confirmed by the experiments on the oscillations of the ⁴He crystal within the temperature range 0.54–1.43 K at frequencies 484 and 211 Hz. New type of a hydrodynamic instability is found. The instability occurs provided that the oscillation amplitude of the velocity becomes higher than ～3 cm/s.