|M.Sc Student||Livne Ethan|
|Subject||Listening to Friction in a Quantum Solid|
|Department||Department of Physics||Supervisor||Professor Emeritus Emil Polturak|
|Full Thesis text|
Past experiments by our group have shown that crystal grains of solid 4He can slide past each other along the grain boundaries separating them. In this work, we characterize a macroscopic flow of solid hcp 4He composed of such grains. The solid is grown inside an annular channel which is mounted on a Torsional Oscillator. In order to better understand the interactions between 4He crystal grains, we employ an in-situ measurement using a sensitive "microphone" embedded in the wall of the channel. This microphone was designed and constructed in our lab. Flow is detected by counting the vibrations induced by the helium atoms moving past the microphone. Such vibrations were detected only at T=0.5K , our lowest temperature. At this temperature, the dissipation arising from friction between the moving and static solid, is zero within our accuracy. Our results indicate that the surface of the moving solid is the (0001) basal plane of the hcp structure. At T=0.5K, we found that for speeds <7μm/sec, the solid flows without detectable friction. At larger speeds, friction sets in, in a way characteristic of a critical velocity. The friction between the moving and static solid increases with temperature. Above 1K, we estimate the friction force per unit area in our experiment at 6*10-5Pa, four orders of magnitude smaller than the critical shear stress for solid 4He. This value is nine orders of magnitude smaller than the lowest friction force measured to date, and may allow us to search for Quantum mechanisms of friction, for example due to phonon-phonon interaction.