The effect of air-anneal on two-level tunneling systems of ion-beam sputtered amorphous SiO₂

Reducing mechanical loss, or internal friction, in amorphous dielectric coating materials is practically important in many present-day applications, such as mirror coatings of gravitational-wave detectors or dielectric insulating layer in superconducting quantum bits. It is known that mechanical loss is dominated by atomic tunneling through potential barriers of double-well potentials below ~10 K and by thermal activation over the same potential barrier landscape. It has been found that extended air anneal reduces near-room temperature mechanical loss of ion-beam sputtered amorphous SiO₂. We measured elastic properties of ion-beam sputtered amorphous SiO₂ from 0.3 K to room temperature that includes internal friction, shear modulus and the relative change of speed of sound, and found that internal friction increases slightly accompanied by a large drop of thermal activation peak at higher temperatures. We discuss possible mechanisms for our observations.