Kinetics of the Crystal 4He Growth at High Overpressurizations

Abstract

The growth of a helium crystal is studied within the temperature range of 0.4–1.7 K at the overpressurizations up to 15 mbar. The measurements are carried out in the opaque and optical containers. In the first case the growth rate is evaluated from the rate of pressure change inside the container during crystal growth. This yields the growth rate averaged over all crystallographic directions. In the optical container the direct filming of a crystal growth process complements the pressure measurements. At high temperatures the absolute values of the kinetic growth coefficient K for the bcc-phase are measured. For the hcp-phase, at high overpressurizations above both roughening transitions the growth rate anisotropy is observed. At high overpressurizations the point of the first roughening transition is found to shift towards lower temperature while the temperature of the second roughening transition increases. Below the second roughening transition, two growth regimes are found. The first one (normal) is typical for a crystal close to the phase equilibrium. The second regime occurs at high overpressurizations. In this case the crystal growth time decreases by two orders of the magnitude in comparison with the normal one. The phase diagram separating regions of the fast and slow growth of a ⁴He crystal is determined. The lifetime of the metastable state under fast kinetics is estimated.