Facet growth of 4He crystals at mK temperatures

Our results on the growth of 4He crystals at mK temperatures show that remarkable shortcomings in the theoretical understanding appear also when dynamic properties of facets are investigated. Typically, growth of facets at low temperatures is assigned to the presence of screw dislocations (see FIG.). Our results indicate [PRL76, 4187 (1996)], however, that slow, steady growth is possible also in crystals with no screw dislocations at all along the c-axis. This finding calls for novel mechanisms of growth in quantum crystals.

 

FIGURE  Facet velocity v as a function of driving pressure p-peq for a 4He crystal with a dislocation density of 5 cm-2. The data at T  = 100, 150, and 200 mK can be fitted quite well using the classical (p-peq)2 power law. At T  = 2 - 20 mK, on the other hand, the dependence on pressure is nearly linear within our resolution. In all sets of data, deviations from the initial pressure dependence are observed at large velocities. 

The linear pressure dependence can be explained by inertial effects at low temperatures (T = 2 - 20 mK) as the speed of an individual step approaches the speed of sound. The tendency towards saturation in velocity is an indication of localization of elementary kinks (and, thereby, steps). By including these phenomena in a modified  theory of spiral growth, all our data are well accounted for as shown by the fitted curves.

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