|M.Sc Student||Lior Embon|
|Subject||Crystal Growth and Search for Supersolidity in BCC Solid|
|Department||Department of Physics||Supervisor||Professor Emeritus Polturak Emil|
|Full Thesis text|
Supersolidity is a state in which a solid supports superflow through the bulk, while, at the same time, maintaining its crystalline form. This bizarre and intriguing phenomenon was predicted to occur in solid 4He some thirty years ago, but the first apparent evidence for it was only found in 2004. After the initial discovery, a great deal of effort, both theoretical and experimental, was invested in the subject. The effect has been replicated many times, but despite the abundance of accumulated information, there is still no complete and agreed explanation for this phenomenon .
The main experimental method used in our research, and in this field in general, is the torsional oscillator technique. In this method, the dependence of the resonant frequency of an oscillator on its oscillating mass is utilized, and the oscillator acts as a highly sensitive micro-balance. The technique involves oscillating a solid Helium sample, grown in a cell with constant volume. By tracking changes in the resonant frequency, one can extract information regarding the solid sample .
Experiments have shown a sudden increase in the resonant frequency below a temperature of about 250mK. It is as if some of the sample's mass has vanished. Since the cell is sealed, and there was no real change in mass, the interpretation is that some fraction of the solid has become super-flowing, and thus decoupled from the oscillating sample. This effect, the non-classical inertia of rotation, was interpreted as a transition to a new phase of Helium - "supersolid ".
In this work we have studied solid Helium samples in the BCC phase, which exists at a narrow temperature span around 1.5K. We found a very similar effect to the one described above, but at temperatures ten times higher than ever before. We show that this phenomenon is highly connected to the quality of the crystal, and that it is probably the outcome of some sort of an order-disorder transition .