Origin of the second peak in the mechanical loss function of amorphous silica

Abstract

The thermal noise in amorphous oxides is the limiting factor for gravitational wave detectors and other high-precision optical devices. Through the fluctuation-dissipation theorem, the thermal noise is directly connected to the internal friction (Q −1 ). Computational calculations of Q −1 that use a two-level system (TLS) model have previously been performed for several coating materials, facilitating the search for coatings with lower thermal noise. However, they are based on a historical approximation made within the TLS model that treats the TLS distribution as uncorrelated, which has limited the predictive power of the model. In this paper, we demonstrate that this approximation limits the physical description of amorphous oxides using the TLS model and a fully correlated distribution must be used to calculate high-temperature behavior. Not only does using a correlated distribution improve the theoretical standing of the TLS model, calculations of Q −1 using a fully correlated distribution reproduce and uncover the physical mechanisms of a second peak observed in measurements of ion-beam sputtered (IBS) amorphous silica. We also explore the details of the thermal activation of TLS's, and analyze the atomic transitions that contribute to Q −1 in amorphous silica.

Temperature dependence of hypersound attenuation in silica films via picosecond acoustics

“…Recent numerical works pro- vide new insights into the nature of the relaxing entities in SiO 2 glass. 54,55 They would mainly involve rotations of the oxygen bonds over several connected tetrahedrons. These motions might indeed be promoted adjacent to the network vacancies.…”

Annealing‐Induced Changes in the Atomic Structure of Amorphous Silica, Germania, and Tantala Using Accelerated Molecular Dynamics

“…[ 48,59,68 ] Modeled TLSs in and are also shown to involve several tens of atoms extended over nanometer(s). [ 69,70 ] This indicates that the change in medium‐range order observed in this work could be important to understand the TLSs and relaxation processes in these oxides.…”

Development of mirror coatings for gravitational-wave detectors

“…For example, it appears that changes in the medium range atomic ordering in Ta 2 O 5 are strongly correlated with changes in the mechanical loss [17]. Computational atomic modelling studies have begun to obtain good agreement with experimental measurements [18,19].…”