Superflow in quantum solids

by 1979- Clark, Anthony C.

iii The understanding of the apparent superfluid nature of solid 4He is incomplete. There are many theoretical models proposing defect-based mechanisms for the nonclassical rotational inertia (NCRI) that is observed in torsional oscillator measurements. It is clear that further experimental work is necessary to determine if any of the present models are applicable to solid 4He. These issues have been addressed in two parts. First, the behavior of solid hydrogen under rotation was investigated in order to determine if the possible supersolid phase is unique to 4He. Although there are clear indications of quantum mechanical motion of residual ortho-hydrogen impurities within the solid, NCRI is not observed in any solid H2 samples. Second, modifications to the traditional experimental apparatus were employed in order to make it possible to grow solid 4He at a fixed point on the solid-liquid coexistence curve within a torsional oscillator for the first time. A vast improvement in the overall reproducibility of the data is consistent with the formation of single crystals within the sample cell. The onset of NCRI is consistently found to be 79mK in helium crystals containing only ~1ppb of isotopic impurities. These results demonstrate that the supersolid-like behavior of solid 4He is not associated with superfluid liquid flowing along grain boundaries. Further results suggest that vortices exist in the solid, and that the onset of NCRI corresponds to the temperature at which their motion is suppressed. Below ~35mK the vortices are strongly pinned.