טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
Ph.D Thesis
Ph.D StudentOri Scaly
SubjectInvestigation of Internal Friction Mechanisms in
hcp 4He Solid using a Torsional
Oscillator
DepartmentDepartment of Physics
Supervisor Professor Emeritus Polturak Emil
Full Thesis textFull thesis text - English Version


Abstract

Until present, solid Helium was at the heart of an ongoing search for super solid
state of matter. Experiments conducted on Helium solid grown inside a torsional oscillator, showed results of what was presumed at first as evidence of super solid phase, but later turned out to result from the elastic properties of Helium at low temperatures.
These experiments inspired our group at 2008 to design and conduct similar
experiment but at temperatures an order of magnitude higher, between 0.4K and 2.5K.
Results of experiments done here at the Technion discovered an effect suggesting that Helium solid cannot be treated as a classical rigid body. This effect suggested that in Helium solid neighboring grains can move with a relative velocity between them. This effect was called “the decoupled mass effect”.
In order to test this model of relative motion between neighboring grains, we
designed a new torsional oscillator with a sensitive microphone that was embedded in-situ.
The microphone was designed to measure sound waves, i.e. vibrations, which supposed to be emitted during the relative motion. The microphone is designed to be extremely sensitive, with capability to detect vibration as small as 0.3.
We performed our experiments between 0.4K and 2.5K, and grew in our torsional
oscillator samples prepared from pure 4He as single crystals or as a polycrystal.
We discovered that the “decoupled mass effect” is actually a consequence of the
torsional oscillator design, in particularly of the connection between the oscillator and the filling line that supplies the Helium. This understanding, unfortunate as it was, also gave us a new tool for research of internal friction in solid Helium. We discovered that the main mechanism for internal friction in Helium solid, in our temperature range, is dislocation motion. The amount of structural disorder in a sense of investigating a polycrystal or a single crystals seems less important for the internal friction.