Sapphire whispering gallery mode resonators at millikelvin temperature

Abstract

Single crystal sapphire exhibits extremely high electrical and mechanical Q-factors (∼10 9 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 T 3 dependence on thermal conductivity and the ultra-low dielectric loss tangent of the material. We identify "magic temperatures" betwe..