The effect of different cooling rates on two-level system distributions*

Gravitational wave detectors like LIGO and Virgo use amorphous oxide optical coatings on the mirrors used to reflect LASER beams. The thermal noise from these coatings can be a major limit to the sensitivity of these detectors. It has been shown that annealing can lower the room temperature mechanical loss (which is directly related to thermal noise) of these coatings. In this talk, I will present results from atomic simulations aimed to understand this observation. We generate atomic models of amorphous Ta2O5 and TiO2-doped Ta2O5 using the melt-quench molecular dynamics method. By crudely assuming that fast and slow cooling rates might generate structures similar to those of unannealed and annealed thin films, we generate fast cooled and slow computer models. We use these models to find the two-level system (TLSs) in the underlying potential energy landscape (PEL); and the TLSs are subsequently used to compute the mechanical loss. I will present how changing the cooling rate affects the composition of the two-level systems as well as their barrier heights and asymmetries. Based on these calculations, we make some speculations about how annealing might alter the mechanical loss, if the analogy between slow quenching and annealing is true.