Sapphire whispering gallery mode resonators at milli-kelvin temperature

Abstract

Single crystal sapphire exhibits extremely high electrical and mechanical Q-factors (~109 at 4K), which are important characteristics for electromechanical experiments at the quantum limit. We report the first cool down of a bulk sapphire sample below super fluid liquid helium temperature (1.6K) to as low as 25mK. The electromagnetic properties were characterized at microwave frequencies over a broad range of excitation powers. At high powers we report the observation of thermal bistability in millikelvin sapphire due to the material properties such as the T3 dependence on thermal conductivity and the ultra-low dielectric loss tangent of the material. We identify "magic temperatures" between..