X-ray studies of low-temperature solid ⁴He

Recent measurements have found nonclassical 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 (dimensions∼millimeters) form. Scanning with a small (down to 10×10 μ m²) beam, we resolve a mosaic structure within these crystals, which is consistent with 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.