1202.1392 (Anna Eyal et al.)

BCC vs. HCP - The Effect of Crystal Symmetry on the High Temperature
Mobility of Solid $^4$He    [PDF]

Anna Eyal, Emil Polturak

We report results of torsional oscillator (TO) experiments on solid $^4$He at
temperatures above 1K. We have previously found that single crystals, once
disordered, show some mobility (decoupled mass) even at these rather high
temperatures. The decoupled mass fraction with single crystals is typically 20-
30%. In the present work we performed similar measurements on polycrystalline
solid samples. The decoupled mass with polycrystals is much smaller, $\sim$ 1%,
similar to what is observed by other groups. In particular, we compared the
properties of samples grown with the TO's rotation axis at different
orientations with respect to gravity. We found that the decoupled mass fraction
of bcc samples is independent of the angle between the rotation axis and
gravity. In contrast, hcp samples showed a significant difference in the
fraction of decoupled mass as the angle between the rotation axis and gravity
was varied between zero and 85 degrees. Dislocation dynamics in the solid
offers one possible explanation of this anisotropy.

View original: http://arxiv.org/abs/1202.1392