X-ray Studies of Structure and Defects in Solid 4He from 50 mK to Melting

TL;DR

This study uses x-ray synchrotron radiation to analyze the structure and defects in solid 4He at very low temperatures, finding no direct evidence of a supersolid transition but revealing complex defect structures.

Contribution

It provides detailed x-ray analysis of defect structures in solid 4He at millikelvin temperatures, linking defect characteristics to NCRI phenomena.

Findings

No signs of supersolid transition in lattice parameters or peak intensities.

Presence of small angle grain boundaries within large crystals.

Mosaic structure shifts over time, indicating dynamic defect behavior.

Abstract

Recent measurements have found non-classical rotational inertia (NCRI) in solid 4He starting at T ~ 200 mK, leading to speculation that a supersolid state may exist in these materials. Differences in the NCRI fraction due to the growth method and annealing history imply that defects play an important role in the effect. Using x-ray synchrotron radiation, we have studied the nature of the crystals and the properties of the defects in solid 4He at temperatures down to 50 mK. Measurements of peak intensities and lattice parameters do not show indications of the supersolid transition. Using growth methods similar to those of groups measuring the NCRI we find that large crystals form. Scanning with a small (down to 10 x 10 um2) beam, we resolve a mosaic structure within these crystals consistent with numerous small angle grain boundaries. The mosaic shows significant shifts over time even at temperatures far from melting. We discuss the relevance of these defects to the NCRI observations.