Investigation of internal friction mechanisms in hcp 4He solid using a Torsional Oscillator

Until recently, solid Helium was at the heart of an ongoing search for Super Solid state of matter. Experiments conducted on solid Helium grown inside a torsional oscillator, showed effects of what was presumed as evidence of the super-solid phase.  Later on, these effects turned out to result not from supersolidity, but rather from the elastic properties of solid Helium at low temperatures. 

These initial experiments inspired our group at 2008 to design and conduct similar experiments but at temperatures an order of magnitude higher, between 0.4K and 2.5K. These experiments discovered an effect suggesting that Helium solid cannot be treated as a classical rigid body. This effect suggested that in solid Helium, neighboring grains can move relative to each other with some velocity. This effect was called “the decoupled mass effect”.

Continuing this research we designed and built a new torsional oscillator with an in-situ sensitive microphone designed to measure extremely weak sound waves. We grew in our torsional oscillator samples of pure (_^4)He as single crystals or as a polycrystals. We measured the internal friction in these samples between 0.4K and 2.5K.

We discovered that the “decoupled mass effect” is actually a consequence of the torsional oscillator design, in particularly the connection between the oscillator and the filling line that supplies the Helium. This understanding gave us a new tool for research on the internal friction in solid Helium. We discovered that the main mechanism of internal friction in our temperature range is dislocation motion. The type of structural disorder reflected in the difference between polycrystals and single crystals does not affect the internal friction.