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arxiv: 2511.19746 · v2 · submitted 2025-11-24 · ❄️ cond-mat.mtrl-sci

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

Pith reviewed 2026-05-17 05:18 UTC · model grok-4.3

classification ❄️ cond-mat.mtrl-sci
keywords amorphous silicontwo-level systemsinternal frictionmoment tensor potentialactivation-relaxation techniquemachine-learned potentialbond exchangedissipation
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The pith

DFT-trained moment tensor potential finds twice as many complex two-level systems in amorphous silicon.

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

This paper combines a machine-learned moment tensor potential trained on density functional theory data with the activation-relaxation technique to locate and classify two-level systems in amorphous silicon. It shows that overall material properties such as radial distribution functions, defect levels, and internal friction match both experimental measurements and results from a modified Stillinger-Weber potential. The atomistic details differ, however, with complex two-level systems based on Wooten-Winer-Weaire bond exchange appearing roughly twice as often. These systems also tend to remain isolated and oscillate independently rather than interacting. Understanding these differences matters because two-level systems control low-temperature dissipation in amorphous materials used in sensors and quantum technologies.

Core claim

The central claim is that MTP-based models of amorphous silicon recover experimental structural and dissipative properties yet produce a higher density of complex two-level systems, specifically those involving Wooten-Winer-Weaire bond exchanges, at about twice the rate found with a modified Stillinger-Weber potential, while the density of simpler bond-hopping two-level systems remains comparable and the identified systems mostly act as isolated, independent oscillators.

What carries the argument

Moment Tensor Potential trained on DFT data, paired with the Activation-Relaxation Technique nouveau to sample and classify two-level systems according to their atomic rearrangement mechanisms.

If this is right

  • Average structural and dissipative properties including radial distribution functions and internal friction remain consistent with experimental data.
  • The density of simple bond-hopping two-level systems stays similar across the two potentials.
  • Two-level systems appear mostly isolated and oscillate independently in the MTP models.
  • More complex two-level systems involving bond exchange are roughly twice as common as in the Stillinger-Weber models.

Where Pith is reading between the lines

These are editorial extensions of the paper, not claims the author makes directly.

  • The method could be applied to other amorphous solids to predict how their low-temperature dissipation scales with preparation history.
  • Verification against direct quantum calculations on the discovered complex TLS configurations would test whether the potential captures tunneling rates correctly.
  • Isolated TLS behavior may simplify models of decoherence in devices that use amorphous silicon layers at millikelvin temperatures.

Load-bearing premise

The moment tensor potential accurately reproduces the energy barriers and atomic configurations of rare, low-energy two-level system events that were never included in its training data.

What would settle it

Direct density functional theory calculations of energy barriers and relaxed configurations for the specific two-level system geometries identified by the MTP-ARTn search, to test whether they agree with the potential's predictions.

Figures

Figures reproduced from arXiv: 2511.19746 by Carl L\'evesque, Fran\c{c}ois Schiettekatte, Normand Mousseau, Renaude Girard.

Figure 1
Figure 1. Figure 1: FIG. 1. Representation of the configurational energy of a TLS [PITH_FULL_IMAGE:figures/full_fig_p002_1.png] view at source ↗
Figure 2
Figure 2. Figure 2: FIG. 2. Attempt frequency, or prefactor, given by the har [PITH_FULL_IMAGE:figures/full_fig_p004_2.png] view at source ↗
Figure 3
Figure 3. Figure 3: FIG. 3. Top panel: The mean radial distribution function [PITH_FULL_IMAGE:figures/full_fig_p005_3.png] view at source ↗
Figure 4
Figure 4. Figure 4: FIG. 4. Energy asymmetry as a function of the forward energy [PITH_FULL_IMAGE:figures/full_fig_p007_4.png] view at source ↗
Figure 6
Figure 6. Figure 6: FIG. 6. Histogram of the density of TLS events per 1000 [PITH_FULL_IMAGE:figures/full_fig_p008_6.png] view at source ↗
Figure 8
Figure 8. Figure 8: FIG. 8. Density of TLS species per 1000 atoms in term of [PITH_FULL_IMAGE:figures/full_fig_p008_8.png] view at source ↗
Figure 9
Figure 9. Figure 9: FIG. 9. Loss angle contribution for MTP, mSW (recomputed [PITH_FULL_IMAGE:figures/full_fig_p009_9.png] view at source ↗
read the original 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.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit. Tearing a paper down is the easy half of reading it; the pith above is the substance, this is the friction.

Referee Report

2 major / 2 minor

Summary. The manuscript uses a DFT-trained Moment Tensor Potential (MTP) for amorphous silicon together with the Activation-Relaxation Technique nouveau (ARTn) to locate and classify two-level systems (TLS). It reports that MTP-generated samples reproduce experimental radial distribution functions and internal-friction values at a level comparable to a modified Stillinger-Weber potential, yet yield a similar density of bond-hopping TLS but roughly twice as many complex Wooten-Winer-Weaire bond-exchange TLS; the MTP TLS are also described as predominantly isolated and independently oscillating.

Significance. If the central claims are substantiated, the work would demonstrate that machine-learned potentials can resolve a richer set of TLS mechanisms than empirical potentials while still matching key experimental observables, thereby strengthening atomistic interpretations of dissipation in amorphous silicon and related glasses.

major comments (2)
  1. [MTP Training subsection] MTP Training subsection: the potential is stated to be trained on DFT data tailored to amorphous silicon, yet no explicit validation or augmentation with low-energy TLS configurations, near-degenerate minima, or saddle-point structures is described. Because the ARTn search and subsequent TLS counting rest on accurate relative energies of these rare events, even modest barrier errors (~0.05–0.1 eV) could change the sampled ensemble and directly affect the reported factor-of-two difference in complex TLS density.
  2. [TLS Statistics and Classification section] TLS Statistics and Classification section: TLS densities, type fractions, and the claim that MTP TLS are “mostly isolated” are presented without error bars, without the number of independent samples or total TLS events counted, and without convergence tests versus system size or ARTn search parameters. These omissions undermine quantitative comparison of the bond-hopping versus Wooten-Winer-Weaire populations.
minor comments (2)
  1. [Figures 1–3] Figure captions and axis labels should explicitly state whether error bands or standard deviations from multiple samples are shown; currently the RDF and internal-friction plots appear to lack them.
  2. [Methods] The precise geometric criteria used to classify a TLS as “bond-hopping” versus “Wooten-Winer-Weaire bond exchange” should be stated in a dedicated methods paragraph or table to allow reproducibility.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We are grateful to the referee for their detailed and constructive comments, which have helped us improve the clarity and rigor of our manuscript. Below, we respond to each major comment in turn. We have made revisions to the manuscript to incorporate additional information and clarifications as detailed in our responses.

read point-by-point responses
  1. Referee: MTP Training subsection: the potential is stated to be trained on DFT data tailored to amorphous silicon, yet no explicit validation or augmentation with low-energy TLS configurations, near-degenerate minima, or saddle-point structures is described. Because the ARTn search and subsequent TLS counting rest on accurate relative energies of these rare events, even modest barrier errors (~0.05–0.1 eV) could change the sampled ensemble and directly affect the reported factor-of-two difference in complex TLS density.

    Authors: We acknowledge the importance of validating the MTP on configurations relevant to TLS, such as low-energy barriers and saddle points. Our training dataset includes a wide range of amorphous silicon structures generated via DFT, encompassing various local environments and energies that are representative of the material. While specific augmentation with TLS minima was not performed, the potential reproduces key experimental observables like the radial distribution function and internal friction values. To address this comment, we will revise the MTP Training subsection to include a discussion of the training data's coverage of low-energy configurations and any relevant validation metrics. We will also note that small barrier errors, if present, are unlikely to alter the qualitative finding of increased complex TLS density, as this difference is observed consistently. revision: yes

  2. Referee: TLS Statistics and Classification section: TLS densities, type fractions, and the claim that MTP TLS are “mostly isolated” are presented without error bars, without the number of independent samples or total TLS events counted, and without convergence tests versus system size or ARTn search parameters. These omissions undermine quantitative comparison of the bond-hopping versus Wooten-Winer-Weaire populations.

    Authors: We agree that providing statistical details is essential for the credibility of our quantitative claims. In the revised version, we will add the number of independent amorphous samples generated and analyzed, the total count of TLS identified across all samples, and error bars derived from the standard deviation across samples for the TLS densities and type fractions. Additionally, we will include a short paragraph on convergence tests with respect to system size and ARTn search parameters, confirming that the reported trends are stable. These additions will support the comparison between MTP and Stillinger-Weber potentials and the assertion that MTP TLS are predominantly isolated. revision: yes

Circularity Check

0 steps flagged

No significant circularity; central claims are direct simulation outputs

full rationale

The paper generates amorphous silicon samples with an externally trained MTP (on DFT data) and a modified SW potential, then applies the ARTn method to locate and classify TLS events. Reported quantities such as TLS densities for bond-hopping versus Wooten-Winer-Weaire mechanisms and the conclusion that MTP TLSs are mostly isolated are obtained as direct counts and statistics from these independent runs. No step defines a TLS density or isolation metric in terms of the training parameters or prior results by construction, and the comparative claims do not reduce to a self-citation chain or fitted-input renaming. The methodology is therefore self-contained against external benchmarks.

Axiom & Free-Parameter Ledger

0 free parameters · 1 axioms · 0 invented entities

The work relies on the assumption that the MTP potential energy surface is sufficiently accurate for TLS identification even though training data details are not supplied; no new physical entities are postulated.

axioms (1)
  • domain assumption The MTP reproduces DFT energies and forces with sufficient accuracy for locating low-energy TLS configurations.
    Invoked when claiming that MTP-based TLS statistics are physically meaningful.

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