Observation of Macroscopic Structural Fluctuations in bcc Solid 4He

TL;DR

This study reveals macroscopic structural fluctuations in bcc solid 4He, involving grain rotations and transformations, triggered by vibrations, with no similar effects observed in the hcp phase.

Contribution

It provides the first evidence of dynamic, solid-solid transformations and grain rotations in bcc helium-4, linked to vibrational triggers and observed via neutron diffraction.

Findings

Macroscopic grain rotations occur in bcc 4He.

Structural changes are triggered by vibrations.

Effects are absent in hcp phase.

Abstract

We report neutron diffraction studies of low density bcc and hcp solid 4He. In the bcc phase, we observed a continuous dynamical behaviour involving macroscopic structural changes of the solid. The dynamical behaviour takes place in a cell full of solid, and therefore represents a solidsolid transformation. The structural changes are consistent with a gradual rotation of macroscopic grains separated by low angle grain boundaries. We suggest that these changes are triggered by random momentary vibrations of the experimental system. An analysis of Laue diffraction patterns indicates that in some cases these structural changes, once initiated by a momentary impulse, seem to proceed at a constant rate over times approaching an hour. The energy associated with these macroscopic changes appears to be on the order of kT. Under similar conditions (temperature and pressure), these effects were absent in the hcp phase.