GROWTH SHAPE OF HE-3 NEEDLE CRYSTALS

We present experiments on the growth shape of He-3 needle crystals. Th ese crystals show a surprisingly weak sidebranching. Growth velocities v and the tip radii rho have been measured, and vrho2 is found to be roughly constant. We also measured the surface tension anisotropy. We compare our results with the microscopic solvability theory, which nee ds to be modified due to the existence of a thermal (Kapitza) resistan ce R(K) at the interface. By studying the linear stability of a spheri cal seed, we show that R(K) can be taken into account by modifying the ratio of the bulk heat conductivities. The selected values v and rho at the tip are found, in satisfactory agreement with the modified theo ry. Some qualitative arguments are also presented which may explain th e weakness of the sidebranching instability.