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

TL;DR

This study investigates how crystal symmetry (bcc vs. hcp) affects high-temperature mobility in solid helium-4, revealing anisotropic behavior in hcp crystals linked to dislocation dynamics, unlike isotropic bcc crystals.

Contribution

It demonstrates that crystal symmetry influences high-temperature mobility and decoupled mass fraction in solid helium-4, with hcp crystals showing orientation-dependent behavior.

Findings

Decoupled mass fraction in polycrystals is about 1%.

bcc crystals show isotropic decoupled mass fraction.

hcp crystals exhibit orientation-dependent decoupled mass.

Abstract

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.