|Ph.D Student||Eyal Anna|
|Subject||Macroscopic Mobility of Solid 4He above 1k|
|Department||Department of Physics||Supervisor||Professor Emeritus Emil Polturak|
|Full Thesis text|
Solid helium at low temperature was considered as a suitable candidate to exhibit supersolidity since the end of the 1960s, yet only in 2004 the first possible experimental evidence of this state was reported. Those first experiments studied solid 4He contained inside a cell which was part of a torsional oscillator. At temperatures below 0.2K, a small portion of the solid apparently became decoupled from the walls of the cell, as expected from a frictionless supersolid. These initial experiments stimulated an intense activity, both experimental and theoretical. The results to date, however, are confusing, leading to more questions than answers. The only issue on which there is consensus is that the phenomenon is more complicated than originally thought.
We have carried out similar torsional oscillator experiments on solid 4He, but at temperatures almost an order of magnitude higher, between 1.1K and 1.9K. We grew single crystals inside our oscillator using pure 4He or 3He-4He mixtures containing 100 ppm 3He. Crystals were grown from the liquid at a constant temperature and pressure. Once the cell was full of solid, the crystals were disordered. We discovered that disordering the crystals causes a partial decoupling of the solid helium mass from the oscillator, an effect similar to what was observed by others at temperatures below 0.2K. We found that this effect exists both in the bcc and in the hcp crystalline phases of solid 4He, and its magnitude does not depend strongly on the temperature. At these temperatures, the decoupled mass fraction of the crystals reached a limiting value of around 35%. The fact that a similar phenomenon can be induced at practically any temperature suggests that we are not dealing with a usual phase transition.
In a follow-up experiment, we compared the properties of samples grown with the rotation axis of the oscillator at different orientations with respect to gravity. Gravity determines the growth direction of the crystal. 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 a solid could offer one possible explanation of this anisotropy, as well as for the rest of our results. However, the dissipationless nature of the mass flow remains a puzzle.