An activation-relaxation technique study of two-level system impact on internal dissipation using DFT-based moment tensor potential

TL;DR

This work investigates atomistic mechanisms of two-level systems (TLS) in amorphous silicon as a source of coating noise in gravitational-wave detectors. It combines a DFT-trained, universal Moment Tensor Potential (MTP) with Activation-Relaxation Technique nouveau (ARTn) to map the energy landscape and identify TLS, and compares results to an empirical Stillinger-Weber model. The study finds a higher TLS density and a shift toward more complex, bond-exchange TLSs with the MTP, while the loss angle predicted by the MTP landscape closely matches experimental dissipation data, outperforming the empirical model. The findings underscore the importance of using accurate, DFT-based potentials to capture the atomistic details of TLS-driven dissipation, informing strategies to mitigate coating noise in gravitational-wave detectors.

Abstract

We use a recently-developed machine-learned Moment Tensor Potential (MTP) trained on data generated with the density functional theory (DFT) and tailored to amorphous silicon coupled with the Activation-Relaxation Technique nouveau (ARTn) to identify and classify two-level systems (TLS). The samples generated using MTP recover experimental results and provide average structural and dissipative properties similar to those obtained with a modified Stillinger-Weber potential, including radial distribution function, defect concentration and internal friction. Atomistic details, however, are significantly different, including the density and type of TLS. In particular, we find that while the density of TLS involving a bond-hopping mechanism is similar for the two potentials, more complex TLSs, such as those involving a Wooten-Winer-Weaire bond exchange, are about twice as common. Analysis also shows that TLSs, for MTP-based models, are mostly isolated and oscillate independently from each other.

Figures (9)

• Figure 1: Representation of the configurational energy of a TLS as a function of its reaction coordinate. This TLS consists of two minima, $A$ and $B$, separated by a saddle point $S$ surrounded by high-energy barriers. $\Delta$ indicates the asymmetry between the two minima and $V$ the mean energy barrier.
• Figure 2: Attempt frequency, or prefactor, given by the harmonic transition theory, as a function of energy barrier for all events with a barrier below 5 eV found in representative 1000-atom a-Si configuration prepared as discussed in the text. The green rectangle indicates the data range studied in the present work and the black line the constant prefactor usually used for a-Si
• Figure 3: Top panel: The mean radial distribution function (RDF) for the MTP ( total of 28 samples), the mSW (total of 200 samples) models and from diffraction experiments (EXP) laaziri_high_1999. Bottom panel: The mean angular distribution function for the two sets of models.
• Figure 4: Energy asymmetry as a function of the forward energy barrier for events corresponding to the TLS definition criteria. The doted lines delimit the cone of the acceptable asymmetry as a function of energy barrier. Top panel: results generated in this study; bottom panel, TLS generated using mSW and taken from Ref. levesque_internal_2022.
• Figure 5: Mean square displacement between the initial and final states of TLSs as a function of the forward energy barrier. Symbols are colored according to the number of atoms moving by more than 0.1 Å between the two states. Top panel: results generated in this study; bottom panel, TLS generated using mSW and taken from Ref. levesque_internal_2022.