Analysis of two-level systems and mechanical loss of amorphous coatings by non-cage-breaking and cage-breaking transitions

Thermal noise from amorphous mirror coatings is one of the limiting noise sources for the laser interferometer gravitational-wave observatory (LIGO).[CQG 25.11,114041] In order to better understand the origin of the mechanical loss in amorphous coatings, their energy landscapes have been characterized by the correlated parameter distributions of two-level systems (TLS). By applying the concept of oxygen cage [J. Chem. Phys. 129, 164507 (2008)], the TLSs we find for pure and doped tantala can be separated into two groups named non-cage-breaking transitions and cage-breaking transitions. Non-cage-breaking transitions only have small structure changes, while cage-breaking transitions involve at least one large atom-atom pair distance change between the initial and final atomic structures. The further mechanical loss calculation of ZrO2-doped Ta2O5 models establish a connection between the atomic structure changes and mechanical loss, which shows that the non-cage-breaking transitions have higher averaged mechanical loss and are responsible for the large mechanical loss peak near 40K, while the cage-breaking transitions are the majority of transitions and mainly contributed to the loss peak near 120K.